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UFL/COEL-2000/008
EVALUATION AND DETERMINATION OF EROSIONAL HOT
SPOTS AFTER BEACH FILL PLACEMENT IN LONGBOAT
KEY, FLORIDA
by
Cris Kelii Weber
Thesis
2000
EVALUATION AND DETERMINATION OF EROSIONAL HOT SPOTS AFTER
BEACH FILL PLACEMENT IN LONGBOAT KEY, FLORIDA
CRIS KELII WEBER
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF
FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2000
ACKNOWLEDGEMENTS
I extend my greatest appreciation to my supervisory committee chairman Dr.
Robert G. Dean. His support and counseling have been indispensable. I also thank Dr.
Daniel M. Hanes and Dr. Ashish J. Mehta for always finding time for my questions and
for serving on my supervisory committee.
I thank the rest of the faculty in the Civil and Coastal Engineering Department for
making this a great environment in which to learn, especially Dr. Thieke for his inspiring
insight and knowledge; Becky Hudson for being a great friend and great support, as well
as always having an answer for even my most obscure procedural questions; Dr. Ochi for
being concerned about students; and Dona, Doretha, Joann, and Lucy. You have all
made the stress involved with paperwork and departmental headaches of a graduate
student much easier to cope with. I also thank Helen Twedell and Kim in the archives
whose efforts cannot go unnoticed.
I reserve special gratitude for the support I have received from SHOALS and John
E. Chance. Thanks to Jeff Lillycrop and Wade Jumonville for having made this study
opportunity possible for me; to Jen Irish and Jennifer McClung for always helping me
with questions and information; to Doug Delcambre for just being himself; and to
everyone else who has helped me along the way.
For the friends I have acquired here at the University of Florida, ya'll have made
this an exciting and unforgettable experience. Thanks to Jamie MacMahan, (we
may not agree on anything, but we still manage to have fun), Dave Altman, Sean
Mulcahy, Heather Sumerall, Vadim Alymov, Al Browder, Kevin Barry, Lisa Heckman,
all of the Statistics crew, and special thanks to Jenn S. You are my sunshine!!
Finally, I would like to thank my parents (Leslie Johnston and Bill Weber) for
always believing in me; my sister Dawn Prince for always giving me encouragement and
support, (including the newest additions to her crew, my nieces, Ku'uleinakili and
Kapo'oleilalani), as well as the rest of the family who have stood by my side. I greatly
appreciate all your confidence, blessings, and best wishes.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS .......................................................................................... ii
LIST O F TA B LES ....................................................................................................... vi
LIST OF FIGURES .................................................................................................... vii
CHAPTER 1 INTRODUCTION............................................................................... 1
1.1 G general D description ........................................................................................ 1
1.2 O objective ......................................................................................................... 2
CHAPTER 2 LITERATURE REVIEW ................................................................ 4
2.1 Beach Nourishment Characteristics ............................................................ 4
2.2 Potential Mechanisms for EHS.................................................................. 7
2.3 Observed Erosional Hot Spots.................................................................. 16
CHAPTER 3 BEACH NOURISHMENT PROJECT, LONGBOAT KEY, FLORIDA. 25
3.1 Site O overview ................................................................................................ 25
3.2 Nourishment Performance .......................................................................... 36
CHAPTER 4 SURVEY DATA ANALYSIS .............................................................. 44
4.1 Objective of Analysis.................................................... .............................. 44
4.2 Survey D ata Sources ...................................................... ............................. 44
4.3 M methodology ................................................................................................. 47
CHAPTER 5 SEDIMENT ANALYSIS .................................................................. 62
5.1 D ata Source................................................................................................... 62
5.2 Longboat Key Sand Characteristics............................................. ........... .... 62
CHAPTER 6 EVALUATION OF EROSIONAL HOT SPOTS ON LONGBOAT KEY
......................................................................................................................................... 7 1
CHAPTER 7 SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS ............ 77
iv
7.1 Summary and Conclusions ....................................................... ................. 77
7.2 R ecom m endations............................................................................................... 80
APPENDIX A DNR PROFILES ................................................................ ................... 81
APPENDIX B SEDIMENT DATA................................................... ................... 87
APPENDIX C POWER SPECTRUM DENSITY PLOTS................................... ......... 93
C.1 Islander Club Segment Power Spectrum Density .............................................. 93
C.2 Bayport Segment Power Spectrum Density....................................................... 96
APPENDIX D PROFILE COMPARISONS ALONG TRANSECTS .......................... 99
D .1 B ayport Profiles ............................................................................................... 100
D .2 Islander Profiles ............................................................................................... 102
R E FE R EN C E S ............................................................................................................... 104
v
LIST OF TABLES
2.1 Comparison of Tax Revenue for Different Government Levels ............................... 6
2.2 Recognized Causes of Erosional Hot Spots.......................................... ............... 9
3.1 Reach Segments with Corresponding DNR Monument Range............................. 28
3.2 Erosional Trends by Reach Designation........................... .................................. 29
3.3 Statistical Wave Summary at Station 41 (WIS)................................... ............ .. 33
3.4 Littoral Drift Estimates for Three Sections Along Longboat Key (ATM 1995)....... 34
3.5 Significant Storm Events and Summarized Wave and Wind Data (ATM 1995)...... 38
4.1 Department of Environmental Protection Survey Data for Longboat Key, FL......... 46
4.2 SHOALS Lidar Data for Longboat Key, FL ...................................... ............. .. 46
5.1 U.S. Standard Series Testing Sieves.................................................. .............. 64
5.2 Summary recommended A values (units of A parameter are m 1/3) ....................... 68
B.1 Longboat Key 1-Year Sediment Composite of Percent Finer............................... 87
LIST OF FIGURES
2.1 Effect of nourishment material scale parameter, AF, on width of resulting dry beach.
Four examples of decreasing AF with same added volume per unit beach length
(D ean 199 1). ........................................................................................................ 11
2.2 Planform of additional dry beach width resulting from variability in alongshore
sedim ent size (Bridges, 1995) ............................................................................. 11
2.3 Plan view of nourishment in front of armored shoreline creating headland effect
(L iotta 1999) ....................................................................................................... 15
2.4 Lowering of profiles at a seawall (Liotta 1999)................................... ............ .. 16
2.5 Location of Ocean City, Maryland ................................................................... 18
2.6 Comparison of potential longshore sand transport rates....................................... 21
2.7 Location of Broward County, Florida.............................................................. 23
3.1 Location of Longboat Key in Manatee and Sarasota Counties with the project
delineated reaches ............................................................................................... 26
3.2 Shoreline change in feet per year for Manatee County, Florida............................ 29
3.3 Shoreline change in feet per year for Sarasota County, Florida ............................ 30
3.4 Longshore orientation of primary axis alignment (ATM 1991)............................. 31
3.5 Percent occurrence for directional wave spectrum (ATM 1991)................................ 32
3.6 Percent occurrence for directional energy spectrum (ATM 1991)........................ 32
3.7 1993 Pre-construction sediment budget estimate for Longboat Key, FL .................. 35
3.8 1993-95 Post-construction sediment budget estimate for Longboat Key, FL ......... 36
3.9 Shoreline advancement from preconstruction to 6-month post construction survey
and 1 year post construction survey (ATM 1995) ............................................... 40
3.10 Location of sub-sections in identified erosional hot spots on Longboat Key, FL ... 41
3.11 Contour map of Bayport segment of the EHS on Longboat Key derived from
SH O A LS data, 0994 ........................................................................................... 42
3.12 Contour map of Islander segment of the EHS on Longboat Key derived from
SH O A LS data, 0994 ........................................................................................... 43
4.1 Geometry for the magnitude of the deviation....................................... ........... .. 50
4.2 Intersection of least-square contour lines and profile transect lines for Bayport
segment, monuments T-1 through R-R6........................................................ 51
4.3 Magnitude of deviations from least-square line of the 2-meter contour with respect to
DNR benchmarks, SHOALS data 0994......................................................... 52
4.4 Magnitude of deviations from least-square line of the 2-meter contour with respect to
DNR benchmarks, SHOALS data 1294......................................................... 52
4.5 Magnitude of deviations from least-square line of the 3-meter contour with respect to
DNR benchmarks, SHOALS data 0994 and 1294.............................................. 53
4.6 Magnitude of deviations from least-square line of the 4-meter contour with respect to
DNR benchmarks, SHOALS data, 0994 and 1294............................................ 54
4.7 Magnitude of deviations from least-square line of the 5-meter contour with respect to
DNR benchmarks, SHOALS data, 0994 and 1294............................... ........... 54
4.8 Magnitude of deviations from least-square line of the 6-meter contour with respect to
DNR benchmarks, SHOALS data, 0994 and 1294............................................. 55
4.9 Magnitudes of deviations from least-square line of the shoreline with respect to DNR
benchmarks, DNR profile transect data, 0394 and 0894........................................ 56
4.10 Magnitude of deviations from least-square line of the 2-meter contour with respect
to DNR benchmarks, SHOALS data, 0994 and 1294......................................... 57
4.11 Magnitude of deviations from least-square line of the 3-meter contour with respect
to DNR benchmarks, SHOALS data, 0994 and 1294........................................ 57
4.12 Magnitude of deviations from least-square line of the 4-meter contour with respect
to DNR benchmarks, SHOALS data, 0994 and 1294......................................... 58
4.13 Magnitude of deviations from least-square line of the 5-meter contour with respect
to DNR benchmarks, SHOALS data, 0994 and 1294.......................................... 58
4.14 Magnitude of deviations from least-square line of the 6-meter contour with respect
to DNR benchmarks, SHOALS data, 0994 and 1294......................................... 59
4.15 Power Spectrum Density for 2-meter contour of SHOALS data, 0994................... 60
5.1 Composite sediment distribution for 1-year monitoring Survey ........................... 65
5.2 Cross-shore mean sediment diameter distribution along DNR benchmark
designations......................................................................................................... 66
5.3 Comparison of ideal profiles with different mean diameters to actual collected profile
at bench ark R -13 .................................................................................................... 69
5.4 Comparison of ideal profiles with different mean diameters to actual collected profile
at bench ark R-14.............................................................................................. 69
6.1 Comparison ofprenourishment and post nourishment nearshore profiles for Bayport
segm ent ............................................................................................................... 74
6.2 Comparison ofpre-nourishment and post-nourishment nearshore profiles for Bayport
segm ent ............................................................................................................... 75
6.3 Comparison ofpre-nourishment and post-nourishment nearshore profiles for Bayport
segm ent ............................................................................................................... 75
6.4 Comparison ofpre-nourishment and post-nourishment nearshore profiles for Bayport
segm ent ............................................................................................................... 75
6.5 Comparison ofpre-nourishment and post-nourishment nearshore profiles for Bayport
segm ent ............................................................................................................... 76
6.6 Comparison of pre-nourishment and post-nourishment nearshore profiles for Bayport
segm ent .......................................................................................... ... .................. 76
A. 1 DNR 0894 profile transect monument T- ........................................... ........... ... 81
A.2 DNR 0894 profile transect monument R-2......................................... ........... ... 81
A.3 DNR 0894 profile transect monument R-3 ......................................... ........... ... 82
A.4 DNR 0894 profile transect monument R-4......................................... ............ .. 82
A.5 DNR 0894 profile transect monument T-5 ......................................... ............ .. 82
A.6 DNR 0894 profile transect monument R-6......................................... ........... ... 83
A.7 DNR 0894 profile transect monument R-7......................................... ........... ... 83
A.8 DNR 0894 profile transect monument R-8......................................... ........... ... 83
A.9 DNR 0894 profile transect monument R-9......................................... ........... ... 84
A.10 DNR 0894 profile transect monument R-10.................................... ........... .. 84
A.11 DNR 0894 profile transect monument R-11........................................................... 84
A.12 DNR 0894 profile transect monument R-12.................................... ........... .. 85
A.13 DNR 0894 profile transect monument R-13.................................... ........... .. 85
A.14 DNR 0894 profile transect monument R-14...................................... ............ 85
A.15 DNR 0894 profile transect monument R-15...................................... ............ 86
B. 1 Sediment distribution of depth contours (feet, NGVD) from 1-year monitoring
survey D N R T- .................................................................................................. 89
B.2 Sediment distribution of depth contours (feet, NGVD) from 1-year monitoring
survey DN R R -4 ................................................................................................. 90
B.3 Sediment distribution of depth contours (feet, NGVD) from 1-year monitoring
survey D N R R-6.5 .............................................................................................. 90
B.4 Sediment distribution of depth contours (feet, NGVD) from 1-year monitoring
survey D N R R -9 ................................................................................................. 91
B.5 Sediment distribution of depth contours (feet, NGVD) from 1-year monitoring
survey D N R R -11.5 ............................................................................................ 91
B.6 Sediment distribution of depth contours (feet, NGVD) from 1-year monitoring
survey D N R R -14 ............................................................................................... 92
B.7 Sediment distribution of depth contours (feet, NGVD) from 1-year monitoring
survey D N R R -16.5 ............................................................................................ 92
C. 1 Power Spectrum Density for shoreline contour of DNR 0894.............................. 93
C.2 Islander Club segment power spectrum density for 2-meter contour of SHOALS
data, 0994 .................................................................................................. .......... 94
C.3 Islander Club segment power spectrum density for 3-meter contour of SHOALS
data, 0994 ................................................... ....................................... .............. .... 94
C.4 Islander Club segment power spectrum density for 4-meter contour of SHOALS
data, 0994 ............................................................................................................ 95
C.5 Islander Club segment power spectrum density for 5-meter contour of SHOALS
data, 0994 ............................................................................................................ 95
C.6 Islander Club segment power spectrum density for 6-meter contour of SHOALS
data, 0994 ............................................................................................................ 96
C.7 Bayport segment power spectrum density for 2-meter contour of SHOALS data,
0 9 94 ........................................................................................................................... 9 6
C.8 Bayport segment power spectrum density for 3-meter contour of SHOALS data,
0 994 ........................................................................................................................... 9 7
C.9 Bayport segment power spectrum density for 4-meter contour of SHOALS data,
0 9 9 4 ........................................................................................................................... 9 7
C. 10 Bayport segment power spectrum density for 5-meter contour of SHOALS data,
0 9 94 ........................................................................................................................... 9 8
C. 11 Bayport segment power spectrum density for 6-meter contour of SHOALS data,
0 9 94 ........................................................................................................................... 9 8
D.1 Location and orientation of profile transect data for Longboat Key, Florida from T-1
through T -15 ....................................................................................................... 99
D.2 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Bayport segment (DNR Monument R-l)............................................... 100
D.3 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Bayport segment (DNR Monument R-2)............................................... 100
D.4 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Bayport segment (DNR Monument R-3)............................................... 101
D.5 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Bayport segment (DNR Monument R-4)............................................... 101
D.6 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Bayport segment (DNR Monument T-5) ............................................... 101
D.7 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Bayport segment (DNR Monument R-7)............................................... 102
D.8 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Islander segment (DNR Monument R-12)............................................. 102
D.9 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Islander segment (DNR Monument R-13)............................................. 102
D.10 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Islander segment (DNR Monument R-14)............................................. 103
D. 11 Comparison between the SHOALS data collected in 0994 and the data collected in
1294 for Islander segment (DNR Monument R-15)............................................. 103
Abstract of Thesis Presented to the Graduate School of the University of Florida in
Partial Fulfillment of the Requirements for the Degree of Master of Science
EVALUATION AND DETERMINATION OF EROSIONAL HOT SPOTS AFTER
BEACH FILL PLACEMENT IN LONGBOAT KEY, FLORIDA
By
Cris Kelii Weber
August 2000
Chair: Dr. Robert G. Dean
Major Department: Civil and Coastal Engineering
Erosional hot spots (EHS) are areas whose shorelines perform significantly worse
than expectations or than adjacent beaches. Erosional hot spots may occur on natural or
nourished beaches. The purpose of this thesis is to develop a viable relationship between
the placement of beach nourishment or beach revitalization and instabilities that may
occur, such as erosional hot spots, during the equilibration period after placement,
especially within the alongshore region.
The Army Corps of Engineers, in conjunction with other state agencies and local
entities, conducted many beach nourishment projects. During such projects, EHS may
occur, but the exact causes of these instabilities are still poorly understood. Multiple
causes may compound the effect if several causes exist in the same region. Through
detailed analysis of the mechanisms involved, it may be possible to avoid or minimize
EHS for some of the particular causes during construction.
Longboat Key, which includes portions of Manatee and Sarasota counties on the
western coast of Florida, has experienced several EHS. Longboat Key serves as the
geographic area for analysis focusing on the EHS that developed after the 1993
nourishment project. The EHS, which developed in the central portion of the project,
experienced approximately 30% more shoreline recession than did the adjacent
nourishment areas. It is necessary to examine the bathymetry in the area for which the
nourishment was established, as well as other possible variables such as bottom
morphology, residual bathymetry, sediment transport characteristics, extreme weather
conditions, and dredge selectivity.
The Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS)
system, developed by the U. S. Army Corps of Engineers Waterways Experiment Station,
conducted several high-density lidar surveys directly after the 1993 sand placement on
Longboat Key and conducted several bathymetric surveys since then. The SHOALS
system is an airborne surveying system based on lidar (light detection and ranging)
technology. The analysis of these data-intensive surveys is used to help determine the
mechanisms that lead to erosional hot spots; to assist in optimizing nourishment strategies
in order to avoid the creation of an EHS; and to mitigate against the need to renourish a
project prematurely because of the development of an EHS.
The aim of this thesis is to present a simple analysis method to confirm and
evaluate the existence of erosional hot spots that formed after the beach nourishment
project conducted in the fall of 1993 on the island of Longboat Key, Florida. The
development of a process to determine the causes) of erosional hotspots from the
Longboat Key data may be used to ascertain areas where an EHS may have a tendency to
form and, in some areas, may contribute to the avoidance of or minimization of EHS
during construction of a nourishment project. This could reduce the costs of regional
sediment budgets by predicting when an area has a predisposition for instability.
CHAPTER 1
INTRODUCTION
1.1 General Description
The field of coastal engineering is quite young when compared with many other
engineering disciplines. Consequently, historic engineering methodologies applied to
coastal regions have been approached with little to no knowledge of the physical
processes intrinsic and unique to the coastal environment. This type of engineering
proceeded without consideration of the localized adverse effects or the possible long-term
ramifications to the large-scale environment.
It has only been in the past half century that a more thorough knowledge of the
governing processes involved with the coastal environment has been gained. It is this
continual development of fundamental principles involving coastal processes that creates
opportunities for improvements that protect, predict, and restore coastlines from both
natural erosion and erosion caused by human modifications. The realization that beaches
are a valuable natural resource for public and private interests has led to an intense
reevaluation of the uses for these coastal zones in order to maximize efficiency of
dwindling resources, protect against unexpected storm events, and eliminate mistakes
made in past attempts to control the shorelines.
One of the most recent advances in shoreline protection is the technique of beach
nourishment. This method of erosion control optimally places large quantities of proper
quality and compatible sand in the nearshore region to advance the shoreline seaward.
Beach nourishment is the main solution for stabilizing and controlling beach erosion in
the state of Florida. Historical methods of erosion control include groins, jetties, and
seawalls. But where each one of these hard structures may be adequate in limited
instances, they are generally associated with adverse erosional effects, usually due to
inappropriate design considerations. Conversely, the soft approach of beach nourishment
has limited downdrift impacts and will generally be beneficial for beaches adjacent to the
nourishment sites. It is noted that a combination of both hard and soft structures can
represent effective solutions to shoreline erosion.
The probable lifespan of a nourishment project must be calculated and may be
anywhere from five to fifty years, depending on design requirements. The estimated time
for periodic maintenance of a nourishment project also must be calculated, but it also
may be necessary if the project is observed to erode more quickly than expected. If the
nourishment loses sediment too rapidly, it may be considered a failure regardless of the
causes. It is also possible for a nourishment project to be considered a success even
though localized erosion has caused a portion to not perform as expected. This latter case
may be considered an erosional hot spot.
1.2 Objective
An erosional hot spot, or EHS, is an area that erodes more rapidly than adjacent
beaches or performs worse than expectations. The mechanisms involved with the
formation of erosional hotspots may be difficult to identify because only limited research
has been conducted on their causes. The term hotspot is currently used as a research
expression and erosional hotspots have been accepted only recently as features that must
be understood in order to protect inland property from extreme erosion and mitigate
against excessive costs for repairing nourishment projects. For this reason the amount of
available literature related to erosional hotspots is increasing rapidly.
Hotspots may occur on natural beaches, as well as nourished beaches, and the
erosion may be chronic or sporadic. It is possible that an occurrence of an EHS is simply
a result of a shoreline reacting to the localized wave climate, such as under extreme storm
conditions, transporting the sediment offshore or passing the sediment elsewhere in the
coastal system thereby narrowing the available dry beach width at a specific location. An
EHS may also be the result of a permanent change in the subaqueous morphology, caused
by human alterations in a coastal region, thus changing the bathymetry to a new
equilibrium state.
It is in the area of human modifications to the natural environment where
proactive engineering has the greatest potential. By understanding natural coastal
processes, through field studies, computer models, and laboratory experiments more
experience and better judgment will lead to improved engineering methods. In order to
develop a comprehensive relationship between an erosional hotspot and the conditions
present during its initial formation, it becomes necessary to examine the relevant factors
including the natural bathymetry in the area where the nourishment was conducted, as
well as other possible variables such as bottom geomorphology, residual bathymetry,
sediment characteristics, and extreme current or weather conditions.
The objective of this thesis is to use simple analysis methods to confirm and
evaluate the existence of an erosional hot spot that formed after the beach nourishment
project conducted in the fall of 1993 on the island of Longboat Key, Florida.
CHAPTER 2
LITERATURE REVIEW
2.1 Beach Nourishment Characteristics
Before the advent of beach nourishment as a coastal engineering method, hard
structures such as jetties, groins, and seawalls were thought to be the most effective
means to control the dynamic character of the coastal zone. As with most hard structures,
these may create as many problems as they solve. These structures have an important
function in coastal engineering, but without proper implementation, they generally have a
detrimental effect that is a tendency to interfere with or interrupt the natural sediment
transport. This creates a deficit of sediment on the downdrift shoreline of these structures
and a surplus of sediment updrift.
2.1.1 Positive and Negative Influences
Beach nourishment has become one of the most effective coastal engineering
approaches used at the water-land interface for erosion control. This soft-structure
approach, contrary to its hard-structure counter parts, generally has beneficial effects on
downdrift beaches. Beach nourishment, or beach fill, is the placement of large quantities
of good quality sediment on an eroding beach to advance the shoreline seaward of its
present location. The nature of beach nourishment usually benefits adjacent beaches as
the project equilibrates. Nourishment begins as a protrusion of sediment along the
shoreline that is out of equilibrium, cross-shore and alongshore, with its surrounding
environment. Oblique waves, tides, and alongshore currents then act to return this
protuberance to the natural morphology. The spreading-out losses (inherent with beach
fill) carry sand away from the alongshore ends of the placement area making this
sediment available to adjacent beaches. The additional beach width creates a near-natural
aesthetic feature while providing protection against storms, expanding recreational
facilities and development opportunities, and possibly producing a viable nesting habitat
for some species of sea turtles.
Unfortunately, the diffusion of beach nourishment sediment alongshore, coupled
with the possible cross-shore equilibration, reduces the available dry beach width, and the
project tends to revert to its prenourishment equilibrium where erosion is still the
concern. The time necessary for onshore/offshore equilibration is a few months to
several years, depending on wave climate and storm events. However, depending on the
project length and other factors, the alongshore sediment equilibration time scale is
several years to several decades. Therefore during nourishment project planning, it
becomes essential to design an appropriate renourishment schedule to replenish the areas
expected to be affected by the sediment equilibration process.
The initial cost requirements in conjunction with the need for continual
monitoring and possible remediation measures represent the largest drawbacks for beach
nourishment. Although the cost would be much less expensive, the investment in a
nourishment project with a short length becomes unreasonable due to the rapid return to
its natural state. The longer nourishment projects perform exponentially better than do
localized nourishments, but also require a much larger initial economic investment. This
combination of economic factors generally limits beach nourishment as a viable solution
to shoreline erosion in areas that have high property values, that are important tourist
locations, or that are sensitive environmental regions. In some cases it is possible to limit
costs if beach nourishment projects coincide with and use material from existing
maintenance dredging of inlets, ebb shoals, and flood shoals.
2.1.2 Economics of Beach Nourishment
Coastal resources generate considerable economic returns for local communities,
state agencies, and the Federal government. These returns allow communities to develop
through increased job markets. The state agencies profit through property and other
taxes. The Federal government benefits from the increase in tourism from abroad. Table
2.1 shows the economic importance of revenues generated by beaches for the Federal
government and for the governments of the State of Florida (Stronge 1998).
Table 2.1 Comparison of Tax Revenue for Different Government Levels
Level of Government Revenues
Local $ 320 million
State $ 266 million
Federal $ 429 million
Beaches are ranked first in tourist destinations in America with over 85% of the
associated revenues generated in coastal states (Houston 1996). The estimated impact of
Florida's beaches on property values in the state is approximately $16 billion. A study
conducted by Stronge (1998) in the State of Florida, suggests that every million dollars
spent for increasing property values through coastal improvements generates an increase
of about 30% in direct expenditures leading to a $9 billion dollar impact on the state
economy, and creating 250,000 jobs. The beach nourishment conducted at Miami Beach
from 1976-1981 illustrates the economic benefits of a successful project. The total cost
of the nourishment project was approximately $52 million, but the annual amount of
money spent by foreign tourists investing in these beaches is over $2 billion. This
suggests that every $1 invested per year into beach improvement realizes a $700 return in
foreign capital alone.
Although there are probably more tourist visits to recreational beaches than to all
National Park Service and Bureau of Land Management areas combined, the amount of
money allocated to beach preservation is disproportionate to that in the international
community. Germany, for example, has spent nearly $3.3 billion over four decades on
beach protection (Houston 1996). This amounts to five times more than corresponding
U.S. expenditures to protect a coastline that is five percent of that in America. Japan has
a shore protection budget that exceeds $1.5 billon in a year, whereas the United States
has spent only $15 million a year over the past forty years for beach protection (Houston
1996). Spain is conducting a five-year coastal improvement scenario to restore beaches
(as well as to create new ones) that exceeds the United States' efforts over the past half
century. Because pressure is increasing to develop the coastal region for recreation and
commercial uses, it is necessary to obtain proficient skills in the emerging field of coastal
engineering (Houston 1996).
2.2 Potential Mechanisms for EHS
Because the erosion of coastal regions has a direct correlation with economic
success, it has become increasingly necessary to predict, as well as possible, the evolution
of a beach planform after a beach fill project. One of the greatest concerns is the
development of erosional hot spots, during and after nourishment. If an EHS develops,
the mobilization of equipment and personnel for remediation increases the overall cost of
the nourishment project. Another concern is that although a nourishment project may
perform well on average, a localized erosional hot spot may cause the public to perceive
the project as performing poorly or as a failure. This sensitivity to public concern may be
diminished by educating sponsors, the public, property owners, and relevant agencies
about likely EHSs and possible maintenance before construction (Dean and Campbell,
1999). Through proper analysis of initial investigative data, the potential for EHS
development may be greatly reduced through preemptive beach nourishment design.
Although public concern for the performance of beach nourishment projects
argues for the dissemination of design procedures, many of the reasons for EHSs are still
poorly understood. Because the recognition of erosional hot spots is quite recent, all of
the literature, with the exception of a few studies, has been published within the past
decade. Table 2.2 lists some of the recognized causes for erosional hot spots, as well as
the tendency for an associated erosional cold spot. An erosional cold spot (ECS) is
simply the converse of an EHS, a localized accretion or advancement of the shoreline.
This list represents many of the causes, but is not a complete list. Each of the types
mentioned may be categorized in terms of four headings: nonuniform wave conditions
along the shore, preexisting natural or constructed structures, nonuniform sediment sizes
along the shoreline, and sediment transport into a borrow pit (Dean et al., 1998). The
causes identified in Table 2.2 are a general listing, but each has been determined on a
site-to-site basis. Although many forms of potential causes have been listed, this thesis
will examine only an abbreviated list of potential causes of the erosional hot spots that
developed at Longboat Key, Florida after the beach nourishment project was conducted
in 1993-1994.
Table 2.2 Recognized Causes of Erosional Hot Spots
Type Cause Related To Associated ECS?
1 Dredge Selectivity Sand Size No
2 Residual Structure Induced Slope Pre-Existing Structure No
3 Borrow Pit Location Wave Transformation Yes
4 Breaks in Bars Wave Concentration Possibly
5 Mechanically Place Fill Less Fill Placed No
Profile Lowering in front of
6 Pre-Existing Structure No
Seawalls
7 Headlands Pre-Existing Structure Yes
8 Residual Bathymetry Wave Transformation Yes
Less Fill Available to
9 Losses Over or Through Reefs No
beaches
Wave Focusing to Offshore
10 Wave Focusing Yes
Translation of Beach
2.2.1 Dredge Selectivity
Dredge selectivity causes erosional hot spots through the distribution of
nonuniform sediment sizes along a nourishment project. Once a borrow area is permitted
as a source, dredge operators are allowed to use any sediment in that area. As it is
common for dredge operators to know the distribution of sand sizes in the borrow area,
they have discretion as to which sediment is used for each section of the nourishment.
Because the dredge site usually contains portions of fine sediment and coarse sediment,
in order to reduce operating costs, dredge contractors may identify locations in a
designated borrow site for placement of the finer sand. When there is a long distance
between the dredge site and the placement site of the dredged material, less energy is
necessary to pump finer sand, thus requiring fewer booster stations, thereby reducing the
overall cost of the project (Bridges, 1995). This may lead to fill areas located farther
from the dredge site containing fine sediment and areas closer to the dredge site
containing coarse sediment, thus creating the potential for localized erosion and greater
profile equilibration due to longshore sediment variability.
The placement of different-size sediment along a project has ramifications on how
the nourishment will behave. For example, it has been demonstrated by Dean (1991) that
an area with finer than native added sediment will equilibrate more and have less dry
beach width per volume placed, than a similar area with coarser than native fill sediment.
This leads to an increase in shoreline recession, or the dry beach width containing fine
sediment fill will be narrower than that using coarse sediment. Figure 2.1 illustrates that
for finer sand, the slope of the profile is less than that for coarser sand, and that the dry
beach width is narrower. The profile scale parameter for the fill conditions is represented
by AF, and the scale parameter for the native conditions is symbolized by AN. Similarly
the DF and the DN denote the fill and the native sediment sizes, respectively. Figure 2.2
is a plan view of the erosional potential of dredge selectivity during equilibration with the
fine sediment placement bordered on both sides by coarse sediment.
92.4m F-
8 1_.Sm
S h. 6m
45.3m
S Ih. = 6m
Non-intersecting Profiles
AN' AF 0.lTmi3
I 5m.9m
Non-Intersecting Profiles'
AN O.lm1' AF= 0.09m 13
a 6m
Limiting Case of Nourishment Advancement, il
Non-Intersecting Profiles, AN. O.1m'13AF= 0.085m
100 200 300 400
OFFSHORE DISTANCE
Figure 2.1 Effect of nourishment material scale parameter, AF, on width of resulting dry
beach. Four examples of decreasing AF with same added volume per unit beach length
(Dean 1991).
- Borrow Site A
30.00 -
20.00-
S Fill I
rnrw
LONGSHORE DISTANCE
(KILOMETERS)
Figure 2.2 Planform of additional dry beach width resulting from variability in
alongshore sediment size (Bridges, 1995).
Borrow Site B -.
8.00
I
500
(m)
200
6.00
2.2.2 Anomalous Bathymetry
Another possible cause of erosional hotspots is called residual bathymetry. First
identified by Dean and Yoo (1992), this process was recognized while researching a
numerical method to represent wave refraction and shoaling near nourishment projects.
This phenomenon occurs when beach nourishment sediments are placed to a depth that
extends beyond the depth of closure, or to an extent where wave action no longer affects
the mobilization of beach fill. At this depth of no wave-induced sediment motion, the
hydrodynamic effects of the bathymetry could be changed due to the irregular placement
of sediment. The subsequent residual bathymetry can cause changes in the wave
refraction and shoaling which in turn will change the form of the shoreline. Dean and
Yoo (1992) illustrated that for an offshore contour alignment of AyR, the displacement of
the shoreline, Ays, about its mean alignment is
Ay, = AYR[1- (2.1)
where CI is the wave phase speed at the outer depths of the nourishment and C. is the
wave celerity at the depth of closure. The effects on the shoreline due to the offshore
geomorphology becomes a much greater concern because of the assumption that the
alongshore transport will normalize the irregularities in the nearshore region. But beyond
the depth of closure, the wave energy is insufficient to smooth the bathymetric anomalies.
There are several possible causes of irregular bathymetry during beach
nourishment projects (Bridges, 1995). When sediment is hydraulically placed, the dredge
will pump sand onto the beach for a given amount of time, and then move the discharge
point to a different segment of the beach. This process can create anomalies in beach fill
volumetric distribution that must be redistributed using land-based equipment.
Unfortunately, any subaqueous perturbations remain irregular, causing the shoreline to be
affected by the geometry of the new bathymetry. This leads to the potential of the
shoreline being permanently altered while the wave action attempts to level the below-
water sediment structure, and it is these sustained perturbations which have the ability to
create localized shoreline erosion.
Another source capable of altering the natural geomorphology of a beach profile
is the placement of dredged material (Bridges, 1995). If a channel must be dredged in
order to maintain navigation safety, or an inlet must be cleared to maintain its intended
function or location, the dredged material may sometimes be placed in adjacent waters.
Over an extended period of time, the sediment will accumulate to a level where the
shoreline will reorganize itself to match the irregularities created by such placement; and
if the degree of these irregularities reaches a point outside the natural fluctuations of the
normal shoreline, an erosional hotspot may form.
Refraction and possibly diffraction associated with another form of bottom
irregularity, borrow sites, also has been shown to cause erosional hotspots (Dean and
Dalrymple, 1998). The borrow pit acts very similar to a breakwater as the wave field
landward of the pit is reduced. The sheltering of the shoreline leeward of the borrow pit
may cause sand to be deposited, forming a cuspate feature. The sediment accumulated in
these salients is drawn from adjacent beaches, particularly if the transport direction
changes systematically. The high wave set-up that would occur on neighboring beaches
compounded with the smaller set-up behind the borrow pit would drive the currents into
the sheltered area. Conversely, the borrow pit could cause the refraction effects to
change the wave direction of the adjacent beaches, removing sand, and leaving a
sheltered area landward of the pit. Previous studies of the Longboat Key erosional hot
spot, did not determine the effects of the borrow locations to have a considerable impact
on the shoreline.
2.2.3 Headland Effects
Artificial headlands create a scenario where an armored area along a shoreline is
unable to retreat as long-term background erosion occurs. Seawalls, revetments, or other
shore parallel hard structures that are constructed to protect valuable coastal properties,
protrude farther seaward than retreating adjacent beaches. A nourishment project may be
designed to advance the shoreline uniformly seaward. This causes the fill in front of the
armored section to extend farther than adjacent fill sections creating a headland effect
(see Figure 2.3). The wave energy then redistributes this bulge to its prenourishment
position, creating an apparent erosional hot spot in the location of the armored section,
and an erosional cold spot on either side of the protuberance.
"Spreading-out Post-Nourishment
Losses Shoreline Position
Unnatural
Protrusion
Protrusion Receded Shoreline
Design Template -- Position
DSYi - ----- - ----- --
Pre-Nourishment
High Value Structure Shoreline Position
Figure 2.3 Plan view of nourishment in front of armored shoreline creating headland
effect (Liotta 1999)
2.2.4 Profile Lowering in Front of a Seawall
Another possible effect of armoring is that the profile in front of an armored area
will be lowered because the shoreline is unable to retreat with the rest of the shoreline. A
seawall or other shore parallel hard structure serves to protect a sandy shoreline by
armoring it against erosional retreat. As the shoreline erodes, the equilibrium beach
profile in front of the seawall becomes lower than adjacent beaches. Then the lowered
profile in front of the armored section requires more volume of placed sediment to create
an incipient beach. But if adequate fill is not accounted for in the initial design analysis,
a large portion of the volume intended to widen the beach must be used to create the
incipient beach (Bridges 1995). Figure 2.4 shows the process of an eroding beach and
the associated profile lowering that occurs. The dry beach width becomes narrower until
there is only an incipient beach remaining. Once the encroachment exceeds the incipient
beach location, a virtual profile origin is created. This virtual origin is the position of the
origin if there were no seawall present, and as background erosion continues, the amount
of sediment in front of the armored section of beach also will continue to decrease (Dean
1991).
Knowledge of this characteristic of sediment behavior is vital for correct beach
nourishment design. The volume of sand necessary to create an incipient beach at an
armored shoreline must be taken into account and added to the amount of volume
necessary to create the proper width. This allows for the shoreline of adjacent beaches to
have the same dry beach width as that in front of the seawall. Otherwise, a portion of the
material intended to create the dry beach width will be used instead to create the incipient
beach, thereby leading to an apparent erosional hot spot.
Profile Virtual
0.00
^ \ 88 \__o
Existing Profile, t.
Profile /
Receded, t. + t
Seawall
Figure 2.4 Lowering of profiles at a seawall (Liotta 1999)
2.3 Observed Erosional Hot Spots
Coastal erosion has been of great concern for several decades, but erosional hot
spots have only recently become areas of interest. It is vital that we develop procedures
to identify erosional hot spots and understand the underlying causes of hot spots. With a
greater understanding of the characteristics of a beach fill project, mitigation of EHS may
be accounted for during initial design, reducing the overall cost of beach nourishment and
increasing project life spans. The following are examples of erosional hot spots that have
occurred in locations along the east coast of the United States.
2.3.1 Ocean City, Maryland
Ocean City, Maryland is located on Fenwick Island along the mid-Atlantic coast
on the Delaware-Maryland-Virginia Delmarva peninsula, Figure 2.5. A nourishment
project conducted at Ocean City encompassed cooperation from the states of Maryland
and Delaware, and the Federal government. The project consisted of several phases of
nourishment with Maryland placing approximately 2-million cubic meters during the
summer of 1988 for recreational purposes. In conjunction with Maryland, the State of
Delaware placed 333,500 cubic yards during 1988, and the Federal portion of the project
placed roughly 3.8 million cubic yards during the summers of 1990 and 1991 for storm
protection.
18
.. CAPE HENLOPEN
Indian RiXvr
.ELAWARE inlet
E C, Dlhony e. ch
1 .iStudy
nAreo
poy OOcwn Crty
Sep VA AAasot que
J slond AIsTloC
CAPE: C o '2 3 5OC CAlI
SE j F i s h i n g
J \ 'cinyPoint
., V hcPENINSULA .
Figure 2.5 Location of Ocean City, Maryland
The collected data consisted of 22 sled survey lines conducted approximately
three times a year, nearshore water level measurements, and aerial photography (Stauble
1994). After Federal placement of sediment on the completion of the north section of the
project, several severe storms hit the Ocean City region. These storms caused significant
erosion along portions of the entire length of the project that required redistribution of
sand through mechanical methods.
The erosional hot spots identified by Stauble (1994) were identified from
shoreline position change, sediment volume change, and percent of fill remaining.
During the State fill portion, two hot spots of erosion were recognized, and during the
analysis after completion of the Federal portion, two additional hot spots were identified.
Although storm-induced subaerial beach erosion occurred as expected, the four regions of
identified EHS resulted in large volume losses from the foreshore to the depth of closure
and a shoreline retreat in the areas where the hot spots were evident. The analysis of
percent fill remaining showed a correlation of hot spots and cold spots similar in location
to those determined through shoreline position and volume changes.
Stauble concluded that the nourishment project provides overall protection for the
shoreline and that most of the sediment used for beach fill remained inside the cross-
shore limits of the project after the occurrences of the major storms. But the EHS areas
that developed in this project had higher erosion rates and showed less recovery than the
rest of the project. He suggested that erosion at the southern end of the project stemmed
from a groin located in this vicinity, while the EHS in the middle of the project may be
correlated to wave focusing induced by shoreface-attached shoals.
In contrast to the empirical analysis used by Stauble in identifying the creation of
erosional hot spots at Ocean City, Smith and Smith (1997), used a numerical model to
discern the effects of irregular bathymetry on the development of the EHS. The model
used Hindcast data from the Wave Information Study (WIS), (US Army Corps of
Engineers, 1989), to describe the offshore wave climate, and the monochromatic wave
propagation model REF/DIF 1 to characterize the nearshore irregular bathymetry. The
average potential longshore sand transport rates were determined by using the WIS and
REF/DIF data as input for a modified version of the SMS utility NSTRAN.
REF/DIF is a phase-resolving parabolic refraction-diffraction model for ocean
surface wave propagation. It was originally developed by Jim Kirby and Tony Dalrymple
in 1982 and eventually led to the development of REFDIF 1, which is a monochromatic
wave model. The SMS utility NSTRAN was used to estimate potential longshore sand
transport volumes and rates with inputs of nearshore wave conditions, an offshore time
series, and location and depth of nearshore reference line (Smith et al., 1997). The model
uses sand transport calculations derived from the longshore wave energy flux (Smith et
al., 1997) using the sediment transport characteristic, Q:
Q= K g H s2a) (2.2)
16 -1a y2.386
where
Q= potential longshore transport
K= nondimensional empirical sand transport coefficient (K=0.77)
p= density of water (g/cm3)
ps= density of sand (quarts sand, p=2.65 g/cm3)
a'= volume of solids/total volume (accounts for sand porosity, a'=0.6)
g= acceleration due to gravity (m2/s)
y= breaking wave index (y= 0.78)
Hb= significant wave height at breaking, m
ab= breaking wave angle.
The most difficult aspect of numerical modeling arises from the limitations that
must be applied to simplify the complex processes involved in the coastal region. The
NSTRAN model has several drawbacks. The model does not take into consideration the
size or shape of the sediment used in the nourishment project; instead it uses an empirical
coefficient to determine the longshore potential transport. This assumes a uniform
distribution of sediment that leads to the necessity to interpret trends rather than
quantitative results in the model output. NSTRAN also assumes that an unlimited supply
of sand is available for longshore transport, there is no cross-shore transport occurs, and
there are no littoral barriers located in the region, such as the groins in the north section
of the project, or the inlet at the south.
The comparison of the data obtained with the numerical model approach with that
obtained by the empirically observed method show the discrepancy that is possible in
interpretation. The numerical model claims to be capable of identification of erosional
and depositional trends along the length of the Ocean City nourishment project with close
correlation to those determined by Stauble, but as Figure 2.5 shows, there appears to be
an oscillatory trend in the data, but quantification of localized erosion does not seem
quite as intuitive. The arrows on the top portion of the graph represent the hot spots
(filled arrows) and cold spots (hollow arrows) identified by Stauble (1994), and the
arrows on the bottom represent those hot spots (filled) and cold spots (hollow)
determined by Smith and Smith (1997).
2[ i
, t r 5 t ,
0 5 10 15 20
Distance North of O.C. Inlet (km)
-. ......NSTRAN Filtered
Figure 2.6 Comparison of potential longshore sand transport rates
2.3.2 Broward County, Florida
Broward County is located along the Atlantic southeast coast of Florida and
covers roughly 24 miles of shoreline that the Federal government performed three
nourishment projects along since 1971. Raichle, et al. (1998), described a study to
identify erosional hot spots in Broward County, and through an extensive study of coastal
processes in the region, developed five diagnostic parameters to identify erosional hot
spots.
The most apparent symptom of an erosional hot spot is determined by the beach
fill width. By identifying a minimum beach width standard, the identification of a hot
area may be more precisely classified. Erosion rates are the most dynamic symptom of
localized nourishment erosion. The volumetric and planimetric erosion rates may be
considered indicative of possible hot spots if they exceed an established threshold.
Through the consideration of nourishment profile evolution, areas of potentially high
erosion may be identified. Pressure from developing coastal communities or industries
may lead to "over-eroded" profiles that may be considered not only symptoms of but also
causes of erosional hot spots (Raichle et al., 1998). The analysis of historical shoreline
positions compared with current positions can further lead to understanding the
background processes present in a region. And finally, the wave climate and currents,
which are the main forces involved with shoreline evolution, must be considered as
physical settings. While each of these parameters may be considered causes of erosional
hot spots, they must be determined based on site-specific analysis.
Gulf of Mexico
Atlantic
Ocean
Florida Broward County
Figure 2.7 Location of Broward County, Florida
The Hollywood/Hallandale beach fill project that exhibits signs of EHSs was
analyzed using the diagnostic parameters (Raichle et al., 1998). The causes of the
erosion were determined as amplified beach fill diffusion, encroachment of upland
development, and sediment starvation. The project had a locally wide nourished beach
that led to the rapid diffusion of the project sediment. The exceedance of the local
maximum fill capacity, due to a nearshore reef, in conjunction with upland encroachment
created a narrow beach width. Additionally, the Port Everglades entrance, which
interrupts the net sediment supply to this beach, compounds the sediment starvation
problem.
The shoreline located within the John U. Lloyd nourishment project was also
analyzed using the diagnostic parameters. The hot spot that developed was determined to
be a caused by the direct influence of the Port Everglades inlet entrance to the north of
24
the project. The inlet amplified wave energy interfering with the natural sediment
transport conditions eliminating incoming sand supply and creating a sink to which
sediment was lost. The historical research showed the erosion process has been evident
since the construction of the jetties in 1926 (Raichle et al.).
CHAPTER 3
BEACH NOURISHMENT PROJECT, LONGBOAT KEY, FLORIDA
3.1 Site Overview
In the spring and summer of 1993, over two million cubic meters of beach quality
sand were placed as a beach nourishment project on Longboat Key, Florida. In addition
to the placement of beachfill, 5,751 tons of derelict concrete, rock, and wooden coastal
structures were removed (ATM 1995). This nourishment project restored over fourteen
kilometers of severely eroded Gulf Coast beaches on Longboat Key, extending north
from New Pass to Longboat Pass. The borrow material was excavated from the ebb
shoals of these two inlets. Longboat Key is an excellent example of unexpected localized
beach erosion, and will serve as the basis for analysis focusing on the EHSs that
developed after this beach nourishment project.
3.1.1 Site Background
Longboat Key extends approximately 14 kilometers along the southwest coast of
Florida and is located approximately seven miles south of Tampa Bay. It is bounded to
the north by Longboat Pass with New Pass on the south. The northern 7 kilometers of
Longboat key are located in Manatee County and the southern seven kilometers are
located in Sarasota County. Figure 3.1 illustrates the location of Manatee and Sarasota
Counties in relation to the State of Florida, and also presents an expanded view of
Longboat Key with the reach designations as determined in the initial project design.
The divisions of reaches are characterized by beach areas with similar beach erosion,
morphological and upland characteristics, existing beach widths, existing erosion
control structures, and comparable types of land use.
Atlantic
SOcean
Gulf of Mexico
,. Florida
Figure 3.1 Location of Longboat Key in Manatee and Sarasota Counties with the project
delineated reaches
The island of Longboat Key has experienced substantial shoreline changes
throughout its documented history. The wave climate, the bounding inlets, shoreline
hardening structures, and the offshore bathymetry have created the current sedimentation
patterns. The barrier island contains beach ridges that range in height from 5 to 10 feet in
elevation, overwash terraces, and small dunes. New Pass and Longboat Pass have both
been federally maintained navigation channels since 1964 and 1977, respectively (ATM
1991). Both inlets act as significant barriers to the longshore sediment transport along
Longboat Key. The Longboat Pass shoal system acts as a sediment sink containing in
excess of 5 million cubic yards of sand, and both shoal systems create modifications of
the local wave climatology (ATM 1991).
The north end of the island experienced severe erosion in the late 1960's after the
occurrence of several hurricanes. Due to the erosional trend from these storms, in
conjunction with rapid development of the area, extensive shoreline armoring structures
were constructed. These structures created almost 2 kilometers of armoring along
Manatee County shoreline and over three kilometers of structural hardening along the
Sarasota County segment of the island.
3.1.2 Erosional Trends
The Florida Department of Environmental Protection has developed an extensive
database on the position of shorelines in the State of Florida, extending, on average, over
the past 120 years. Dean et al. (1998) utilized this information to assemble a report with
a complete listing of shoreline changes through out the 24 counties comprising the sandy
beaches of Florida. Figure 3.2 and 3.3 show the estimated shoreline changes for
Longboat Key in Manatee County and Sarasota County, respectively. Table 3.1
delineates the Reach segments in each county with the corresponding DNR monument
numbers.
The historic data collected for Manatee County encompass the period of 1874 to
1986 and the recent time domain covers 1974 though 1986. The historic shoreline data
for Sarasota County spans from 1883-1994 and the recent period includes the years 1972
to 1994. The data suggests an average positive change in the shoreline position of 0.66
ft/yr for the historic and 2.14 ft/yr for the recent period in the Manatee County portion of
the island. Similarly the data suggests an average negative change in the shoreline
position of -2.23 ft/ yr for historic and -3.73 ft/yr for the recent period in Sarasota
County.
Table 3.1 Reach Segments with Corresponding DNR Monument Range
MANATEE COUNTY SARASOTA COUNTY
Reach A R42-R48 Reach F R-1-R-6
Reach B R-48-R-51 Reach G R-6-R-17
Reach C R-51-R-56 Reach H R-17-R-22
Reach D R-56-R-R62 Reach I R-22-R-27
Reach E R-62-R-67 Reach J R-27-R-29
The overall average erosional trend implied by the data for Longboat Key has the
shoreline receding at a rate of -0.87 ft/yr and -0.96 ft/yr for the historic and recent data
respectively. It is apparent from the Figures 3.2 and 3.3 that there is an erosional trend on
the southern section of Longboat Key during recent and historic times. Table 3.2
illustrates the average erosion rates as computed for each of the reach sections designated
in the initial nourishment design. It is evident that an erosional trend exists in the
southern section of Longboat Key. It should be noted the 1994 data included the effect of
the nourishment.
Table 3.2 Erosional Trends by Reach Designation
Reach Historic (ft/yr) Recent (ft/yr) Reach Historic (ft/yr) Recent
(ft/yr)
A 1.46 13.44 F -0.78 -1.31
B 3.57 -4.93 G -1.67 -4.12
C 2.00 -6.86 H -4.18 -5.22
D -0.67 2.17 I -4.78 -4.30
E -1.96 -0.48 J 1.50 -3.70
Manatee County Shoreline Change
South from Longboat Pass --
45
35
25 ---- 1874-1986 Shoreline
S__ Change, Historic (ft/yr)
C 15
1 1974-1986 Shoreline
o 5 --- ---- Change, Recent (ft/yr)
-15
DNR Monument
Figure 3.2 Shoreline change in feet per year for Manatee County, Florida
Sarasota County Shoreline Change
South to New Pass --
45
35
25
15
15
-5
-15
DNR Monument
a 1883-1994 Shoreline
Change, Historic (ft/yr)
* 1972-1994 Shoreline
Change, Recent (ft/yr)
Figure 3.3 Shoreline change in feet per year for Sarasota County, Florida
3.1.3 Local Wave History
The average shoreline alignment of Longboat Key is approximately 326 degrees
with an associated shore normal direction of 236 degrees, as shown in Figure 3.4. With
this alignment, Longboat Key is more predominately impacted by wave energy directed
from 145 degrees to 315 degrees. The alongshore energy associated with the 315 degree
and 157.5 degree angles are discounted due to the refraction of waves by the complex
nature of the bathymetry adjacent to these shorelines, and consequently the longshore
sediment transport from these directions are assumed to be negligible (ATM 1991).
Northern Sectors
Southerly Sectors
Figure 3.4 Longshore orientation of primary axis alignment (ATM 1991)
In order to generate a 20-year summary of existing wave conditions at Longboat
Key, Applied Technology and Management (1991) used wave data taken from the
USACE Wave Information Study (WIS) Gulf of Mexico Hindcasting Model. Station 41,
located approximately 20 miles northwest of Longboat Pass, was employed to provide
the joint frequency distribution of wave height and period for each of sixteen directional
sectors, as well as the directional wave spectrum (Figure 3.5) and directional energy
spectrum (Figure 3.6). Twelve wave conditions were analyzed to determine the
representational average sea and swell conditions for the six directional sectors impacting
the Longboat Key shoreline. Sea conditions were defined as the significant wave heights
32
for waves with less than 6.0 -second peak periods, whereas swell conditions were defined
as significant wave heights for peak wave periods of 6.0-seconds or greater. Table 3.3
shows the average significant wave heights and peak wave periods with their associated
percentage of annual occurrence.
Directional Spectrum
WIS Slaton 41, Gulf of Mexico 27N 83W
-- Impact Shoreline of Longboat Key -
0
o -
Azimuth of Sector (Degrees)
Figure 3.5 Percent occurrence for directional wave spectrum (ATM 1991)
Directional Energy Spectrum
WIS Stlan 41, Gulf of Mexi 27N 83W
s - ~Impact Shoreine of Longboat Key
Sl. *K
I
a . 8 *. n iit4 IM 5,8 1.5 .005 80 0487 e 75*5 3e sae*
Azimuth of Sscor (Dogre.a)
Figure 3.6 Percent occurrence for directional energy spectrum (ATM 1991)
Table 3.3 Statistical Wave Summary at Station 41 (WIS)
Shore Normal Wave Hs Tp Percent Weighted Total
(Degrees Direction (ft) (sec) Occurrence Energy (% Energy (%
Perpendicular) (Degrees Occurrence) Occurrence
North)
550 180
Sea 2.6 4.5 2.70 18.25 5.71
Swell 5.7 7.2 0.08 2.69 0.81
320 2020
Sea 2.6 4.6 2.50 16.90 5.29
Swell 4.1 7.5 0.11 1.85 0.58
100 225
Sea 2.5 4.7 4.40 27.50 8.61
Swell 4.4 7.5 0.16 3.10 0.97
-57 292
Sea 2.8 5.1 6.40 50.18 15.70
Swell 5.0 7.5 2.40 60.00 18.78
-34 270
Sea 2.5 4.8 8.40 52.50 16.43
Swell 4.8 7.6 1.70 39.17 12.26
-12 247~
Sea 2.6 4.8 5.10 34.48 10.79
Swell 5.1 7.6 0.50 13.01 4.07
Sum= 319.52
3.1.4 Local Sediment Transport
An analysis of volumetric sand losses performed by Applied Technology and
Management (1991) show a net loss of 86,300 cubic yards per year for the entire island
of Longboat Key. ATM also determined the existence of a nodal point near R-51 in
Manatee County. These losses point to erosion in response to a deficiency in the littoral
material needed to overcome background erosion rates. It is apparent that Longboat Pass
and New Pass ebb shoals ineffectively transport sediment across the inlets.
As described in ATM (1992), Walton (1976) predicted transport rates along the
shoreline as a function of shoreline orientation. For Longboat Key, the shore normal
angles were determined for three sections. The northern section extends from Longboat
Pass south to DNR monument R-57 with a shore-normal angle of 226 degrees. The
middle section lies from DNR monument R-54 south to R-14 with an angle of 238
degrees. The southern section has a shore-normal angle of 227 degrees and extends from
DNR monument R-13 to New Pass. The summary of littoral drift estimates is presented
in Table 3.4. The method used for analysis provides rough estimates for the littoral drift,
but does not consider localized bathymetric changes or nearshore features such as ebb or
tidal shoals near the inlets (ATM 1992).
Table 3.4 Littoral Drift Estimates for Three Sections Along Longboat Key (ATM 1995)
Total Northerly Total Southerly Net Transport and
Shorline Section
Transport (cy/yr) Transport (cy/yr) Directions (cy/yr)
Northern 80,200 113,800 30-45,000 South
Middle 91,250 125,925 30-45,000 South
Southern 80,200 113,800 30-50,000 South
Coastal Planning and Engineering (1995) also performed a pre- and post-
construction littoral budget analysis. The methodology assumes total sand volume
remains constant above the depth of closure and educated guesses are made for the net
movement of sand. They also make the assumption that there exists a nodal point
between DNR monuments R-45 and R-46 as determined by ATM (1994). Figure 3.7
illustrates the estimated sediment transport for the pre-construction at Longboat Key in
1993 (cubic yards per year), and Figure 3.8 shows the estimated post-construction
sediment transport for 1993-95 (cubic yards per year), as determined by CP&E (1995).
Most offshore movement occurred between the 6-month and 1-year time frame.
Pass
T1
Manatee Co.
1.7 T
0+6.
3.-4 -
0.0o
n
C
-11.1 L
0
Sarasota Co.
-21.4
/
0
-4
/
-20.3
60.0
Y
- ""'Y New Pass
77S
Legend
Littoral Drift Quantity Mechanical Placement
Figure 3.7 1993 Pre-construction sediment budget estimate for Longboat Key, FL
17.3
17.0
V'\ \
38.0 ongboat Pass 38.t0
|fq ,!m ^I. Sarasota Co. \T
S Manatee Co.
-39.0 1 [f \---.
-68.0
32.0 -35.0 R -
Lo Ao
68.0 1%
g /
9.- 30.0 Y
+106.0 3 0 .
o 'S r- ." -
N/ -42.0 I
Sarasota Co. jNew Pa
New Pass
Manatee Co. 72.0 ,
Legend
SLittoral Drift Quantity Mechanical Placement
Figure 3.8 1993-95 Post-construction sediment budget estimate for Longboat Key, FL
3.2 Nourishment Performance
During the six-month period following the placement of the beach fill on
Longboat Key, the morphology changed dramatically from conditions immediately
following construction. The mid-key region immediately south of the Manatee
County/Sarasota County line developed a seriously eroded segment of shoreline evident
as a decrease in actual dry beach width. This eroded area has been identified as
containing three erosional hot spots. It is believed that the 1993 "Storm of the Century"
and the 1994-95 winter storm conditions are possibly responsible for these EHS (ATM
1995).
3.2.1 Extreme Wave Conditions
The "Storm of the Century" impacted the island on March 12-14, 1993. As this
storm occurred just prior to or during the nourishment construction, it primarily caused
extreme changes to the pre-construction survey profiles. It is estimated that 1.1 million
cubic yards of sediment was eroded from the upland section of the beach to a seaward
depth of-10 feet, NGVD (ATM 1995). This seaward transfer of sediment gave rise to
the decreased upland beach width.
The worst erosion caused by the storm took place on the submerged portion of the
profiles at the mid-key locations. The full volume of sediment lost to the storm could not
be placed on the beach during nourishment due to the permit restrictions on available fill.
This caused the beach profile between R-2 and R-14, in Sarasota County, to have a
sediment deficiency post-construction. As the profiles equilibrated offshore to fill the
major morphologic changes caused by the storm, the upland dry beach width was
diminished and the existing shoreline hardening structures were exposed (ATM 1995).
Table 3.5 lists a summary of the wave data collected for significant storm events at
Longboat Key from March 1993 to August 1994.
Although a portion of the project shoreline receded to pre-project positions,
Applied Technology and Management (1995) determined the average overall project
performance was at least as good or better than expected, and that less than 17 percent of
the project length was considered as performing poorly. ATM was also able to account
for over 90 percent of the originally placed sand volume, inside the project limits, with
most of the sediment volume within the nearshore, active portion of the profile. A bar
formation developed in this nearshore region allowing the stored sediment to be available
for onshore transport and act as limited protection against storm conditions.
Table 3.5 Significant Storm Events and Summarized Wave and Wind Data (ATM 1995)
Wave Data
Storm Tide Duration Average Wind
Year Date of Storm ft) Height (ft) (hrs) Period Directio
(sec) n (deg)
1993 March 12-14 4.8 Hs> 16.4 24 14 250
Oct 30-31 2.0 Hs> 9.8 39 8-10 180-270
Hmo> 4.9 34 8-10
Hmx= 5.6
Dec 14 1.6 Hmo > 3.3 24 9
Hmx = 4.9
1994 Jan 2-4 1.0 Hmo > 3.3 26 10
Hmx = 7.2
Mar 2-3 2.0 Hmo > 3.3 40 7
Hmx = 6.2
Notes: 1. Storm tide shown is the sum of astronomical tide level and meteorological
effects, but does not include wave setup.
2. Wave height sources:
Hs = Significant wave height from an offshore NOAA buoy 42-003 (25.9N
85.9W)
Hmo = Significant wave height from local nearshore LBK2 pressure gauge
(except from March 2-3 from Siesta Key pressure gauge
Hmx = Maximum significant wave height from local nearshore pressure
gauge.
3. Shoreline normal is 236 degrees.
3.2.2 Location of Erosional Hot Spot
The monitoring of sand gains and losses were conducted immediately after
construction in 1993, and 6-month post-nourishment. Project monitoring surveys were
then conducted annually starting with the 1-year post-construction survey. Applied
Technology & Management (1995) determined that initially the shoreline advanced an
average of 110 feet overall on Longboat Key. This average advancement represented the
unequilibrated shoreline location immediately after the nourishment placement. In the
comparison of the 6-month and 1-year monitoring surveys ATM assessed the shoreline
both accreted and eroded with changes ranging from -41 feet to +35 feet. The
comparison of the alongshore variation in shoreline advancement between the shoreline
pre-nourishment and 1-year post nourishment surveys, and advancement between pre-
nourishment and 6-month post nourishment surveys are presented in Figure 3.9.
Following the March 1993 "Storm of the Century" and the 1994-95 winter storm
conditions, the central segment of the barrier island experienced a severely eroded section
of shoreline immediately south of the Manatee County/Sarasota County line. This area
was considered to be the location of an erosional hot spot. This segment stretching from
DNR monument R-66 in the north to R-16 in the south was divided into three sub-
sections: the Bayport section extends from R-2 to R-5, the Diplomat Beach sections
ranges from R-7 to R-11, and the Islander Club Area is located between R-13 and R-14,
as shown in Figure 3.10. The Bayport EHS is approximately 1000 meters long, the
Diplomat Beach EHS is roughly 1200 meters alongshore, and the Islander Club EHS has
a length just over 1 km.
UNIT FILL
VOLUME PLACED CY/FT
200 100 0
Shoreline Changes
6-Month to 1-Year Survevs
Accretion
Erosion
R M
.5 *?
Transition
*R dI
'Rri
R;es
*A-o
a -
-6-Month -
/./ Survey
/- 1 Y'ear *
Surv -
,. '
SBeachfill wa ,
placed above
+3ft. as .r
6 supplement to
the USACOE t
project
Figure 3.9 Shoreline advancement from preconstruction to 6-month post construction
survey and 1 year post construction survey (ATM 1995)
SHORELINE
ACCRETION FT.
100
a1l
4
Sarasoata Bay
Islander Club/ S
Figure 3.10 Location of sub-sections in identified erosional hot spots on Longboat Key,
FL
Figures 3.11 and 3.13 illustrate the bathymetry of each subdivision for SHOALS
data collected during the September 1994 survey used for analysis. The multiple bar
system is apparent along 90% of the shoreline. The profile comparison between the
SHOALS data collected in 0994 and the data collected in 1294 for these two subsections
are presented in appendix D.
3/ AV R-7
/ "
0" /' 1-/
Figure 3.11 Contour map of Bayport segment of the EHS on Longboat Key derived from
SHOALS data, 0994
R-11
R-12
3 2 R-13
*R-14
-/, '2 R-15
7 /
Figure 3.12 Contour map of Islander segment of the EHS on Longboat Key derived from
SHOALS data, 0994
CHAPTER 4
SURVEY DATA ANALYSIS
4.1 Objective of Analysis
The objective of the analysis of the data collected for Longboat Key is to confirm
the existence of an erosional hot spot that formed after the nourishment project that was
conducted in the fall of 1993 and to evaluate the causess. By using a relatively simple
analysis method, a quick and readily available system can be utilized to determine the
location of erosional hot spot locations.
Through the use of commercially available software programs, an analysis of the
survey data has been performed in order to evaluate erosional hotspots at Longboat Key,
Florida. The use of a simple approach was utilized in hopes that it would lead to
correlations that estimate the impacts of EHS on shorelines due to irregular bathymetry.
4.2 Survey Data Sources
Construction of the Longboat Key Beach Restoration Project occurred between
February and August of 1993. In accordance with the conditions of the construction
permit granted by the Florida Department of Natural Resources (FDNR), Division of
Beaches and Shores, Applied Technology and Management conducted the post-
construction, 6-month post-construction, and 1-year post-construction project monitoring
surveys. Subsequent to the 1-year post construction survey, Coastal Planning and
Engineering conducted the annual project monitoring surveys for the Town of Longboat
Key. The SHOALS lidar system also contributed to the collection of nearshore
bathymetric data for the FDNR-DBS.
The spatial coverage area of Longboat Key is extensive when all the survey data
are combined. The profile transects begin from the upland dunes and extend beyond the
depth of closure with the lidar data filling in the nearshore region between transects. The
dense coverage of the SHOALS data provides great detail in rendering the bathymetry
that existed at the time of each survey. The nearshore area of coverage creates a uniform
horizontal grid with soundings every 4 square meters. The limiting characteristic of the
lidar data is that it begins in approximately 1-meter depth and only extends to a depth of
8-meters. The limiting nature of the profile transects is the distance between each profile.
For purposes of analysis, all horizontal and vertical datum were converted to
match those of the SHOALS data, with a horizontal datum of in NAD 83, FI-W, meters,
and a vertical datum of NGVD 29, meters.
4.2.1 Longboat Key Beach Profiles
The project profile transect data are available through the Office of Beaches and
Coastal Systems, (OBCS), of the Florida Department of Environmental Protection,
(FDEP). This branch of the FDEP has developed an internet website that contains
historical shoreline trends, nearshore and offshore bathymetry, profile information,
general coastal regulations, and extensive information and descriptions of projects in
Florida. The profile information obtained from the DEP consisted of survey transects
spaced at approximately 300-meter intervals along the project area, marked by the
FDNR-DBS monuments with a given azimuth extending to varying offshore distances.
Project survey data were collected by the private contractors and submitted to the
Town of Longboat Key and to the Florida DEP. The data utilized for analysis available
through the DEP online database are presented in Table 4.1.
Table 4.1 Department of Environmental Protection Survey Data for Longboat Key, FL
Survey Date Survey Type Survey Datum
Lbk0193 Profile and Azimuth NAD27 (79), NGVD 29 (feet)
Lbk0394 Profile and Azimuth NAD27 (79), NGVD 29 (feet)
Lbk0894 Profile and Azimuth NAD27 (79), NGVD 29 (feet)
4.2.2 Dense Bathymetric Data
In addition to the profile data obtained from the DEP website, the SHOALS lidar
system conducted several high-density bathymetric surveys of the nearshore region along
Longboat Key. The SHOALS system is an airborne surveying system based on lidar
technology. Lidar is an acronym for Light Detection and Ranging. At the time of the
surveys, SHOALS consisted of a laser transmitter/receiver capable of measuring 200
soundings per second. The SHOALS survey data available for analysis are presented in
Table 4.2.
Table 4.2 SHOALS Lidar Data for Longboat Key, FL
Survey Date Survey Type Survey Datum
9403.xyz Northing, Easting, Elevation NAD 83, Fl-W, NGVD 29 (m)
9409.xyz Northing, Easting, Elevation NAD 83, Fl-W, NGVD 29 (m)
9412.xyz Northing, Easting, Elevation NAD 83, Fl-W, NGVD 29 (m)
The system operated from a Bell 212 helicopter, flying at 200 to 1000 meters
altitude with a ground speed of 0 to 180 km/hr. The Bell 212 is provided by the National
Oceanic and Atmospheric Administration (NOAA), Aircraft Operations Center, through
a Memorandum of Agreement. The SHOALS system also includes a ground-based data
processing system for maintaining post-processed data accuracy. Since the collection of
the Longboat Key data, the SHOALS system has been modified for use with a fixed wing
airborne platform and is capable of collecting data at a rate of 400 soundings per second.
4.3 Methodology
The main analysis tool for evaluating the EHS on Longboat Key was the Surface-
Water Modeling System (SMS) version 7.0 beta. The designers of the software have
worked with SHOALS personnel to develop a package specifically created to handle
dense bathymetric data as collected by lidar systems. The result is an application called
the SHOALS toolbox that is an integrated part of SMS program.
SMS is a comprehensive graphical user environment for 2-dimensional numerical
modeling. It was developed by the Environmental Modeling Research Laboratory at
Brigham Young University in cooperation with the U.S. Army Corps of Engineers
Waterways Experiment Station (USACE-WES) and the Federal Highways
Administration (FHWA).
The most notable characteristic of the SMS software is the ease with which the
user can analyze and manipulate large data sets. A large SHOALS data set may consist
of 1-million or more soundings for a project area. Among the many capabilities of the
SHOALS toolbox application are the ability to examine multiple data sets
simultaneously, create and read-in profile transects, produce contour representations of
xyz data, create individual data sets along specific contours, and generate graphs of
profile data.
4.3.1 SHOALS Bathymetric Data
The lidar data initially extended a distance of approximately 10 km north from
New Pass along the Longboat Key nearshore area and consisted of nearly 750 thousand
soundings per survey. In order to evaluate the EHS in the mid-key region, the initial
survey data were divided into sections that coincided with the areas considered to have
experienced excessive erosion, Bayport Beach, Diplomat Beach, and Islander Club.
Each of the subsets were then divided such that the amount of data in
consideration was about 4 times the alongshore distance of the hot spots. The Bayport
segment is approximately 2 km and encompasses DNR monuments T1 to R7, the
Diplomat section is just over 2 km from monuments T5 to R13, and the Islander Club
division is about 1500 meters long from monuments R11 to R16, reference Figure 3.10.
The naming conventions are used to identify analysis regions with areas as set by
previous studies of this EHS.
Once the regional subsets were created, an x,y data set was then generated for
each depth contour. The contours were produced at 1-meter intervals throughout the
depth of available data, roughly -1-meter depth to -7-meter depth. The SMS software
interpolated, using a triangular irregular network (TIN), across any holidays present in
the lidar data to create a smooth contour. If the holiday in the data set was large enough
to create extreme irregularities in the contour, data were deleted from that section of the
contour data subset.
In order to evaluate the presence of erosional hot spots, each of the x,y contour
data sets was fitted with a Least Square regression line. The deviation of every data point
from the least square regression line was determined using the methodology illustrated in
Figure 4.1. The equation for a line, y = mx + b, is used to project the actual data point,
(xl, yi), onto the least square line in both the x and y direction, resulting with the
coordinates (x2, yi) and (xi, y2), respectively. From triangle ABC, it is apparent that
r = (4.1)
where x = (xi-x2), and y = (yl-y2). Using triangle ABD,
sin = (4.2)
therefore,
r = y sin = (yI Y2)sin[tan-Y -'2)) (4.3)
where r is the magnitude of the deviation from the data point to the least-square line.
Because negative values are more intuitive when considering erosion, the analysis
program was written so that a large negative magnitude or negative trend for the
deviation would represent an erosional trend for the contour data set. Therefore by
examining consistencies in cross-shore deviations, the method correlates offshore
bathymetry to known estimated locations of EHS.
(xp,y2)
b Least Squares Line
D
Y
(x1,yX) /
A -Data Point (x2,Yl)
Figure 4.1 Geometry for the magnitude of the deviation
4.3.2 DEP Profile Transects
The methodology used for analysis with the lidar data was also used with the DEP
profile transects. Due to the spacing of the transects, the analysis becomes much less
accurate. The single line profile surveys, spaced at wide intervals, create a large section
of alongshore distance that the SMS program must necessarily interpolate across. These
interpolations greatly effect the reliability of results for small-scale processes. Over a 1-
km stretch of alongshore distance, such as necessary for considering a subsection of the
EHS at Longboat Key, approximately 5 points would define the contours for that section.
4.4 Results
Because the data points were numbered sequentially for the deviations analysis,
the spatial orientation was altered. In order to create a point of reference, a line was
obtained following from the DNR benchmark along the profile transect azimuth. This
line was then crossed with the least-square best-fit line to determine a point of
intersection for each of the DNR monuments and for each of the contour intervals. This
intersection point was then sorted along the point number axis to determine the
approximate location in the data subset of the benchmark. Figure 4.1 illustrates an
example of the location for each monument and contour in the Bayport segment.
T-1
R-2
R-3
R-4
R-5
R-6
Figure 4.2 Intersection of least-square contour lines and profile transect lines for Bayport
segment, monuments T-1 through R-R6
4.4.1 Islander Club Segment
The results from the analysis of the deviations from a least-square best-fit line in
the Islander Club region seem to indicate a clear pattern. There appears to be a trend
indicating that the deviations in the depth contours are correlated to the location of the
known EHS for this segment. Figures 4.3 and 4.4 show the 2-meter depth contour for the
Island Club Area for SHOALS surveys collected in 9409 and 9412, respectively.
Applied Technology & Management determined in the -1-year post-construction survey
that the EHS for this section of Longboat Key was located between monuments R-13 and
R-14 (reference Figure 3.9). It can be seen from these figures that the largest occurring
deviations seem to be centered about benchmark R13, indicating the possible erosional
trend. The multiple lines in Figure 4.4 represent the double bar formation of the
bathymetry during the December lidar survey. The x,y subset appears as three
52
consecutive shore-parallel lines (Appendix A). The erosional hot spot appears between
points number 45 to 75, approximately 300 m.
Deviation of Data From Let Sq line 2m
R11 I
............ ........... ......
...........: ............ ......
......... .i.......... ......
............ ......... . -
P1?
... ... ..'
. .-- ..-
SR'I
----I **---- -- -* ** *y
Possible EHS
i,
0 10 20 30 40 50 60 70 80
point nurmtLer
Figure 4.3 Magnitude of deviations from least-square line of the 2-meter contour with
respect to DNR benchmarks, SHOALS data 0994
Deia[i'on of DsEa Frorm L.t q line 2m
R11
--n------- --
--ll -i
................ ..............
------------ ----- .--------
------------------------ -----
R12
.- .--.---..------.--------
-- Offshore Contc
i Possible EHS
\ Middlk Conti
S.. ..--------........----...--
I------------- i------- -
I; \
t I "
R13
ur
----------------&--- ------ --
.. ... . .,....-------- .- -- --
,ur i t
-Nearshore Contour
0 20 40 60 80 100 120
poinr number-
Figure 4.4 Magnitude of deviations from least-square line of the 2-meter contour with
respect to DNR benchmarks, SHOALS data 1294
R14
i--C--------
---------
----------
----------
I I I m I
4
II
.. .. .. ..
. . . .
. . . .
. . . . . .
I
I
53
The same trend can be seen in each of the contours out to a depth of 6-meter.
Beyond that depth the contours become too irregular to establish any relevant
correlations. The amount of scatter from the least-square line reflects the more
anomalous bathymetry found offshore. The estimated location of the EHS can be
determined from the point where the deviations become negative values, as a negative
deviation represents a landward displacement of the contour. In Figure 4.5 the 3-meter
deviations show a negative trend located between point numbers 20 and 70. The 4-meter
contour and the 5-meter contour, Figures 4.6 and 4.7, seem to suggest a pattern between
the R-12 and R-14 monument lines. Figure 4.8, for the 6-meter contour, begins to show
the high frequency of deviations, but the deviations still show an evident trend of
landward displacement in the general vicinity of the estimated EHS, especially in the
0994 data.
Dev.i 3rion of Data From Lst Si line 3m
R11
....-......-.
R12
----------- ------
-.---------- ------
R13
--------- ---------
R14
129
j........ ------
R15
4------- ---------------
- - - -
-20 u1
0 20 40 60 80 100 120
pointnumber
Figure 4.5 Magnitude of deviations from least-square line of the 3-meter contour with
respect to DNR benchmarks, SHOALS data 0994 and 1294
30
o
0 20
1o
0
-10
Deviation of Data From Lst Sq line 4m
P1
V.
------------
: . . . .. .. .
R12 R13
-------------C-----------i---------
................----------------------
........ ..... .... ..
.. ... ... ... .. ....... ...
...... .. ... ......
.................
ii i
R14
--------i------
.... .. . .
-0994W
. .I _.
R15
S. ....... ................
-1294
............ ...........
.. . . .
0 20 40 60 80 100 120
point number
Figure 4.6 Magnitude of deviations from least-square line of the 4-meter contour with
respect to DNR benchmarks, SHOALS data, 0994 and 1294
Deviation of Data From Lst Sq line 5m
,n .
..-...12./-.194
1l94
---- --- ---
.099II
R11
F--
2---------- ----- -
T ------ --
R12
---13
... ........
R13
-P14-----
R14
A---- - -
------ - - -
---------------------
-------------------
Rj5
20 40 60 80 100 120
point number
Figure 4.7 Magnitude of deviations from least-square line of the 5-meter contour with
respect to DNR benchmarks, SHOALS data, 0994 and 1294
De'-ation of Data From Lst Sq line 6m
200
150
100
50
0
-50
-100ci
I n
Ru1
...............
----------V.:r
. . . . . .
. . . . . . . .
P12
.*i-'- ** ----------
.... ...........
i :
R13
- 1294 :.
-. ........-- ..... ;.
S ..* :
0994
R14
..............
*,- ;.. ... .. .
*} .. ... ... .
0 50 100 150 'CO 250 300
point number
Figure 4.8 Magnitude of deviations from least-square line of the 6-meter contour with
respect to DNR benchmarks, SHOALS data, 0994 and 1294
Figure 4.9 presents for the shoreline deviations based on the DNR profile transect
data collected in March and August of 1994. The negative trends in the deviations are
much less evident, but there are traces of an erosional inclination in the March data
centered near monument R-13. The August data profiles show a slight erosional trend
between monuments R-13 and R-14. Although these trends are somewhat more
ambiguous than the offshore profiles, they appear to support the cross-shore pattern in the
estimated location of the EHS. Although the shoreline deviations are suspect due to the
triangulation performed by the SMS modeling software to interpolate between actual data
points, as mentioned previously. Where the profile data is very sparse, such as with the
DNR profile transects, a lower degree of accuracy in the analysis is achieved.
R: 5
"' ................
}* --- --!---------
,. ........ .........
... .... .. .....
i i .
*ii i* *- -- *-
Deviation of Dara From Lst Sq line: Om
60
50
40
30
20
10
0
-10
-20
-30
4-n
R-11
--I,
.... .........
--- ::---;--------
~---C--------
----------- -
... ... ... ..
R-12
----------
0 10 20 30 40 50 60 70 SO 90 100
point number
Figure 4.9 Magnitudes of deviations from least-square line of the shoreline with respect
to DNR benchmarks, DNR profile transect data, 0394 and 0894
4.4.2 Bayport Segment
The EHS located in the Bayport segment of Longboat Key was identified in the 1-
year post-construction survey by ATM (1995) as having localized erosion extending from
benchmark T-1 to benchmark R-5 (reference Figure 3.9). The results of the deviation
analysis for the contours of 2-meter depth to the 6-meter depth are presented in Figures
4.10 through 4.14. The most apparent trend in the negative deviations seem to occur near
DNR benchmark T-1. In Figure 4.10 for the 2-meter depth contour, the trend occurs for
the data collected in the September survey. It is also present in the deviations analyzed
for the 4, 5, and 6-meter depth contours, Figures 4.12 through 4.14, respectively. These
Figures also suggest a landward trend near benchmark R-5. This trend is also noticeable
in Figure 4.10 and 4.11 for the 2 and 3-meter contours.
* *
*
f I I I I
R-13 R-14 R-15
'* ...... .. ..... . .... ...
--- -- -- - ....... ........ ........
------------------- ---..
... ....... ........... .......
0394
57
Figure 4.11 shows an erosional trend beginning just north of monument T- and
continuing for approximately 900 meters through the majority of the data set.
Deviarion of Data From Lst Sq line 2m
T-1
----..- -----. .........
..... -.. ....
I,
R-2
9412
.... ...........
..'.. i.' --" -.
09
R-3 R-5
-.-- - .- ..- -.-- -
." ,, ... .. .. ... I
........ ...... '....... . ..
., .. ..... .... .. ........i. ...i
___ ___ __ i _____
R-6
S.. .....
.......----------
0 20 40 60 80 100 120
point number
Figure 4.10 Magnitude of deviations from least-square line of the 2-meter contour with
respect to DNR benchmarks, SHOALS data, 0994 and 1294
Deviation of Data From Lst Sq. line' 3m
25
15 |- ----------------- ------ i--- ---------------
T.I I -2 R-3 R-
20 .. . .............. ...... ................. . ... ..... .............. ............. ...........
10 --- -- .............
|. ...... I .. .' .! ......../:...:
.. ............. .. .... :... .
-I)
-5 ...... ... ...... .......... ....... .; .". .
............................... ......
-15, i
-10................. --------------..............
O 20 40 60 80 100 120
point number
Figure 4.11 Magnitude of deviations from least-square line of the 3-meter contour with
respect to DNR benchmarks, SHOALS data, 0994 and 1294
-..,...
_...;~
_....:.
-
.
1 : :
,,.....
20
15
10
03
-25
-C
T,
E -15
-20
-25
-'n
I
Deviation of Data From Lst Sq line. 4m
T-1
---------------
S ..... . .............. ..
...... ..............,
S ----------
20 40
R-2 R-3 R-4 R-5 : R-6
..... ...... .................. ......
-9409
9412
--- ---- -.. ........ .. ......-- .. ... ....--, -
80 100
120 140
point number
Figure 4.12 Magnitude of deviations from least-square line of the 4-meter contour with
respect to DNR benchmarks, SHOALS data, 0994 and 1294
T-1 R-2
. . .. .. .. . . . . . . . .. . .. .
. . . . . . . ... .
-- ---------- -----
------ --------- ----- ----
--------------- -- -----------------
-- - ------ -- -------------------
R-3
-9412
..............
-9409
'\
------- -------
-------------
Deviation of Data From Lst Sq line 5m
R-4
...... ..
- --- ----
- ------ --
R-6
---- -
^-c~.
-- ---
. . . . . .
...............
---------------
R-5
..A..... -
--- ---i-
point number
Figure 4.13 Magnitude of deviations from least-square line of the 5-meter contour with
respect to DNR benchmarks, SHOALS data, 0994 and 1294
Af
-40 L
0
50
40
30
C
S20
'0
"-10
-20
-30
-40
AeN
I
Deviation of Data From Lst Sq. line: 6m
100 1 I I
T-1: R-2 P-3 .4 R-5 iR-6
6 0 ------------- ---- ------ - -- ------ "-- ------ --------- ------ ---- ------- -- --- - --------- -- 1z
-80
60 .. .. .... ....... .........i. ........... ....... .. ... ........... ......
Sc .24 a [149412
c 40 i i. ........ --- - -h -i- s"h'
29 llh'tJ- ............ ". i
-80 ........ .... ........--- ----
0 50 100 150 200 250
2 0o - .. . .... . . .. I .. .. .. . . . .
460 ,1 ...... .............. .... ...... .......... .... .... .. ..... .. .................
-60 r---- -i--- ----------- -i-- --L ------
-80 r Dni
0 50 100 150 200 250
point number
Figure 4.14 Magnitude of deviations from least-square line of the 6-meter contour with
respect to DNR benchmarks, SHOALS data, 0994 and 1294
As with the Islander Club data set, the figures begin to show a high degree of
scatter in the deviations near the 6-meter water depth. The analysis shows a higher
degree of fluctuation than the Islander section, but there does appear to be an evidence of
an erosional trend as represented by the deviations in the estimated location of the EHS.
4.4.3 Power Spectral Density
Another analysis was performed on the contour subsets using a Fast Fourier
Transform algorithm. The deviations measured from the least-square line were
transformed and a power spectrum was generated for the data. This is illustrated for the
2-meter contour of the SHOALS data collected in 0994, Figure 4.9. The graphs of the
power spectrum densities for the remainder of the contour subsets may be found in
Appendix C.
60
Pyyvs Wave Number. 2 m Contour
5000 -----------.------:--- -- -I----|-- ----i
5000
.4500...................... .............................
2500 ............ . . ................ ....... .. ..... ... ..... .....
S3500-
E2500
2 1 0 0 0 --......... ................ I . -- -.- I-- - - ......... .
1500 [ .j.\./ .............. ................................................... ...... ......... .
0 0005 0.01 0015 002 0025 003 0.035 004 0045 0 05
11m
Figure 4.15 Power Spectrum Density for 2-meter contour of SHOALS data, 0994
Ideally the power spectrum generates the amplitude of the fluctuation of the
deviations and the wavelength of the fundamental period of erosion. The wavelength,
given on the x-axis in terms of 1/L, is the length of the alongshore distance of the
erosional wave, where L is the unit of measure. This term may be transformed into the
wave number by using a factor of 2n.
WaveNumber = -- (4.4)
L
where L is the location of the peak value on the x-axis. The power spectrum density (y-
axis) would also need to be adjusted by dividing by 2i. The amplitude of the fluctuation
for the data about the least-square line is given by the equation
ymax (4.5)
where Pyymax is the magnitude of the fundamental wavelength, measured in m2-m. The
FFT was calculated for the magnitude of the deviations by,
Y = fft (r,n) (4.6)
and the power spectrum density was calculated using,
Pyy = Y conj(Y) (47)
PYY = Y (4.7)
n
where n is equal to the number of points in the fft analysis based on a power of 2 series.
The 512-point analysis was used for this analysis. Varying the values of n, the
fundamental wavelength remained unchanged, but the maximum value of the Pyy
spectrum increased by an approximate factor of 3 for each additional increment of n. For
the majority of the Pyy plots, the energy was almost completely contained in the lowest
fundamental wavelength. For Figure 4.15, following the described methodology for the
wavelength, the alongshore distance of the fundamental length estimated by the Fast-
Fourier transform for the deviations is approximately 1/L, or 1/0.0025 which equals 400
meters. The average distance of the estimated alongshore deviations wavelength for the
Islander Club section is approximately 750 meters, and for the Bayport section, it is
approximately 1200 meters.
CHAPTER 5
SEDIMENT ANALYSIS
5.1 Data Source
A sediment analysis was performed to determine if dredge selectivity was a
probable cause of the localized erosion that developed in the mid-key section of
Longboat Key. The data source for the sediment analysis is taken from the 1-year post-
construction sand samples collected by Applied Technology and Management as
presented in Appendix B (1995). The area of interest for sediment analysis is contained
within the EHS located in the mid-key region of Longboat Key and includes data for
particular DNR monuments from R-47 to R-65 in Manatee County and DNR monuments
T-1 to R-28 in Sarasota County. The sediment samples were taken at +6, +3, 0, -3, -6, -
12, and -15 feet (NGVD) along the designated transects spaced approximately 1000
meters apart.
5.2 Longboat Key Sand Characteristics
In the design of beach nourishment projects, it is important to estimate the dry
beach width after profile equilibration. Most nourishment profiles are constructed at
slopes considerably steeper than equilibrium and the equilibration process occurs on the
order of several years. The equilibration process consists of a redistribution of the fill
sand across the active profile out to the depth of closure. The performance of a beach fill,
in terms of the dry beach width relative to the volume of sand placed in a project area, is
a function of the compatibility of the fill sediment with the native sand. Based on
equilibrium beach profile concepts, it is evident that since profiles composed of coarser
sediments assume steeper profiles, beach fills using coarser sand will require less
sediment to provide the same equilibrium dry beach width than fills using sediment that
is finer than the native sand. A beach segment constructed with finer than native
sediments could therefore result in the formation of an EHS.
Results of the sieve analysis conducted for the 1-year monitoring survey are
presented in percent passing through specified sieve numbers. The data utilized for
comparison in the vicinity of the erosional hot spot includes sediment data for DNR
monuments T-l, R-4, R-6.5, R-9, R-l 1.5, R-14, and R-16.5 in Sarasota County. Table
5.1 shows the United States standard sieve numbers and corresponding information. A
sieve number is approximately the number of square openings per inch, and the
millimeter dimension is the length of the inside of the square opening in the screen. This
square dimension of a sieve mesh is not necessarily the maximum dimension of the
particle that can get through the opening, so these millimeter sizes must be understood as
nominal approximations of the sediment size.
There are several methods for quantifying sediment by the size of the individual
particles. The Wentworth Scales classifies sediment by size, in millimeters, based on
powers of two. Using this scale, sand is characterized as granular particles between
0.0625 mm and 2 mm in diameter. An alternate method for classification of sediment
size is the Phi Scale. The phi (4) size relates to the grain size by
( = -log2 d
(5.1)
such that 2"0=d, where d is the sediment diameter measured in millimeters (Dean and
Dalrymple, 1998).
Table 5.1 U.S. Standard Series Testing Sieves
Nominal M m Nominal
Maximum
Sieve Wire Phi Units,
Sieve Designation Individual D
Opening, Diameter, <
___ _in. Opening mm
in. mm
Standard Alternative
12.5 mm 1/2 in. 0.5000 13.310 mm 2.670 -3.64
9.5 mm 3/8 in. 0.3750 10.160 mm 2.270 -3.25
2.0 mm No. 10 0.0787 2.215 mm 0.900 -1.00
1.0 mm No. 18 0.0394 1.135 mm 0.580 0.00
500.0 pim No. 35 0.0197 585.000 pm 0.340 1.00
355.0 im No. 45 0.0139 425.000 pm 0.247 +1.49
180.0 pm No. 80 0.0070 227.000 pm 0.131 +2.47
125.0 pmr No. 120 0.0049 163.000 pim 0.091 +3.00
75.0 jpm No. 200 0.0029 103.000 um 0.053 +3.74
Standard sieve openings usually vary by 1.19 from one opening to the next larger
(by the fourth root of 2, or 0.25-phi intervals), although the range of sieve sizes used and
the interval between selected sieves may vary as required. The sieve analysis data
presented in Appendix B illustrates the grain size diameter measured in millimeters and
phi units because of the atypical sieve numbers used in the original analysis of percent
finer by weight sediment passing.
65
5.2.1 Sediment Distribution Plots
Two borrow sites were used for the nourishment project on Longboat Key. The
Longboat Pass borrow site to the north was used for the northern segment, from
Longboat Pass to the Manatee / Sarasota County line. The New Pass borrow site was
used from the county line south to New Pass. Figure 5.1 illustrates the composite
sediment distribution of all the profiles collected across the area of the EHS. The
sediment distribution plots for each of the DNR monuments listed in the composite plot
are available in Appendix B. The composite mean grain diameter, or do is 0.17 mm, as
estimated from Figure 5.1. This value is of interest due to the relationship to the native
mean diameter, dN and the fill mean diameter, dF. For the collections of sediment
statistical parameters, Applied Technology and Management (1995) determined the for
the area which corresponds to the location of the EHS, that the mean diameter for the
native sand was 0.19 mm and the 1-year South Beach mean sediment diameter was 0.17.
1-Year Sediment Composite -*-T-1
itii 1 I.;
10 1
--- R-4
R-6.5
-A- R-9
-K--R-11.5
-- R-14
-- R-16.5
0.1 0.01
Grain Size, mm
Figure 5.1 Composite sediment distribution for 1-year monitoring Survey
S100
: 80
60
I. 40
20
0
0
c~i~c~
Figure 5.2 shows the cross-shore distribution of the mean sediment diameter versus the
alongshore DNR benchmark designations. The high dso for monument R-11.5 can be
seen in the sediment distribution plot in appendix B.
Mean Diameter vs DNR Monument
----zero
14 -*-three
1.4
E six
E 1.2
S--- nine
as 1 -K-twelve
0 0.8 --fifteen
0.6
S 0.4
S0.2
0
T-1 R4 R6.5 R9 R11.5 R16.5
DNR Monument
Figure 5.2 Cross-shore mean sediment diameter distribution along DNR benchmark
designations
It was determined that the beach nourishment construction for the Manatee
County segment would use sediment from the Longboat Pass ebb shoal and the Sarasota
County segment would use sediment from New Pass ebb shoal. The fact that the mean
diameter of the New Pass borrow site was 0.22 mm would lead to the expectation that the
fill sediment diameter was coarser than the native. But the location of the EHS occurs at
the maximum northern extent of the New Pass sediment fill limits. It is then possible that
the finer sediment size located in Sarasota County at the mid-key erosional hot spot
region has a correlation to dredge selectivity.
ATM (1995) noted that the lower mean grain size of the 1-year post-nourishment
south beach segment might be an anomaly. The 6-month sediment samples were
collected immediately following a storm, but the 1-year post-construction sediment was
not impacted by any significant wave conditions. An analysis performed by Mote Marine
Laboratory (Truitt 1994) of the sediment collected by the ATM post-construction surveys
determined that sediment within the hot spots were significantly finer than outside.
Specifically, within the erosional hot spot area, two-thirds of the sand was finer that the
project design average.
5.2.2 Comparison of Profiles
It has been shown that by using linear wave theory and a simple wave breaking
model, Equilibrium Beach Profiles may be represented by the form
h =A y n (5.2)
where A represents a sediment scale parameter and depends on the sediment size. Dean
(1977) used a least square fit procedure and found the central value of the exponent n to
be 2/3 for the case of wave energy dissipation per unit volume as the dominant force.
This can be interpreted as showing the wave energy dissipation per water volume
destabilizes beach sediment through turbulence from breaking waves, and the resulting
dynamic equilibrium is a balance between constructive and destructive forces (Coastal
Engineering Manual).
There are two inherent limitations of the equation h = A y 2/3. First, the slope of
the beach profile at the water line (y=0) is infinite. Second, the beach profile form is
monotonic; i.e., it cannot represent bars.
The surface water modeling system has a function which allows for varying either
the sediment diameter, d, or the sediment scale parameter, A in order to compare an
actual profile transect to an ideal equilibrium profile. The ideal equilibrium beach
profiles in the SMS program uses the h(y) = A y 2/3 where h(y) is the distance from the
coastline, A is calculated from the grain size, and y is the water depth. Table 5.2 presents
a version of the A versus d relationship for grain sizes that are typical of beach sands.
Table 5.2 Summary recommended A values (units of A parameter are m 1/3)
d
.00 .01 .02 .03 .04 .05 .06 .07 .08 .09
(mm)
0.1 0.063 0.0672 0.0714 0.0756 0.0798 0.084 0.0872 0.0904 0.0936 0.0968
0.2 0.1 0.103 0.106 0.109 0.112 0.115 0.117 0.119 0.121 0.123
0.3 0.125 0.127 0.129 0.131 0.133 0.135 0.137 0.139 0.141 0.143
0.4 0.145 0.1466 0.1482 0.1498 0.1514 0.153 0.1546 0.1562 0.1578 0.1594
0.5 0.161 0.1622 0.1634 0.1646 0.1658 0.167 0.1682 0.1694 0.1706 0.1718
0.6 0.173 0.1742 0.1754 0.1766 0.1778 0.179 0.1802 0.1814 0.1826 0.1838
0.7 0.185 0.1859 0.1868 0.1877 0.1886 0.1895 0.1904 0.1913 0.1922 0.1931
0.8 0.194 0.1948 0.1956 0.1964 0.1972 0.198 0.1988 0.1996 0.2004 0.2012
0.9 0.202 0.2028 0.2036 0.2044 0.2052 0.206 0.2068 0.2076 0.2084 0.2092
1.0 0.21 0.2108 0.2116 0.2124 0.2132 0.214 0.2148 0.2156 0.2164 0.2172
The following profiles, Figures 5.3 and 5.4, illustrate the ideal profile dependency
with varying diameters of sediment. The actual profiles are taken from data collected by
DNR in September of 1994 at benchmark R-13 and R-14, which is the location of the
Islander Club EHS in Sarasota County. The dso estimates are determined from the SMS
program with associated A parameters as found in Table 5.2. The mean sediment
diameters are used because of the range of dso values obtained from the composite
sediment distribution graph for the area encompassing the EHS.
I-.o \ : ;C ----d5(, ^= U [17
ce K_... .. -_ __ _-d5E= q(.20 mi_
- .3.0- -- - --- ----- \ \ ^ = ^.
-6.0 ,-- -- - -- -- ^ -- -- - -- -_--..^S :: ^ -
-3.0 v- -
n.90------------------------------------ ------^----
-5.0.-
7.0----- Mmen t R- 13--
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500
Offshore Distance (m)
Figure 5.3 Comparison of ideal profiles with different mean diameters to actual collected
profile at benchmark R-13
DNR Profile Transects from 0894
4.0
3.0
2.0--
1.0
0.0
-4.0 N 1-- -
-7.0
37.0--------------------------------------- --------------------
-8.0 --- --------------------------- ---------
-9.0----- --------------------------- ------- ---- ------
-2.o--
-9.0 '-Nv l~omme it -14- -- "- ---
-11.0----------------- -----------
-12.0
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500
Offshore Distance (m)
Figure 5.4 Comparison of ideal profiles with different mean diameters to actual collected
profile at benchmark R-14
The transition from a steep profile to a flatter profile is apparent with decreasing
sediment diameter size. The graphs are unable to illustrate, but definitely imply the
detrimental effect on the dry beach width of finer than native sediment. These idealized
profiles are forced to begin at the 0.0-meter depth. If this were a free boundary, all other
parameters remaining constant, the steep profiles would obviously create more available
dry beach width, which is expected.
There are models available that illustrate how susceptible sediment is to
remaining in suspension, especially finer sediment. Dean's model (Dean 1973) on
sediment transport demonstrates how smaller diameter particles will have a slower fall
velocity, which will greatly effect the concentration of suspended sand. His model also
demonstrates one of the methods that can lead to the formation of erosional hotspots due
to the placement of beach fill that is finer than the native sediment. The finer sand is
much more likely to remain in suspension; and those particles that have settled out of
suspension are more likely to be mobilized back into suspension by the action of the
waves. The suspended sediment then becomes more likely to be transported out of a
specific region, or into a different part of the coastal system from energy supplied by
wave action. This loss of available sediment potentially gives rise to the formation of an
EHS.
CHAPTER 6
EVALUATION OF EROSIONAL HOT SPOTS ON LONGBOAT KEY
This chapter presents an evaluation of the potential causes of the erosional hot
spot on Longboat Key. Several potential causes of the EHS identified in the mid-key
region have been examined in previous chapters. The possibility of effects from
anomalous bathymetry was studied in the dense lidar data (Figures 3.11 and 3.12) and the
DNR profile transects (Appendix A), but no apparent correlation to the loss of available
dry beach width was determined. The effect of any irregular bathymetry on the EHS
within the depth of closure, approximately 6-meters, seems negligible compared to the
nearshore effects of sediment movement caused by other potential causes.
ATM (1995) determined that the harsh winter storm season of 1994-95 led to a
sediment deficiency in the pre-project beach profiles of the nearshore region that was
unable to be compensated for by the available fill material during construction. Although
the harsh winter climate could have caused changes in the beach profile bathymetry, it is
unlikely that it would have caused permanent morphological changes. The winter storm
season could be a potential factor in the creation of the EHS with the offshore movement
of sediment, but there was no similar extreme change in the profiles on the adjacent
islands of Anna Maria Key and Lido Key, and therefore unlikely as a sole cause for the
localized erosion.
Sediment analyses were performed to determine if dredge selectivity could have
been a causative factor in the formation of the EHS. The comparison of profiles suggest
a possibility that finer than native sediment was placed along the central section of the
island, but there is no direct evidence of a definitive association. The sediment analysis
actually predicted coarser than native.sediment placement for the fill material. The
coarser sediment fill would lead to an expectation of a steeper than native profile. There
were some ambiguities in the collection of the sediment data and the subsequent results
from the data. The spacing between the profiles of collected data, the collection of
sediment data immediately following a heavy storm, and unavailable records of the
dredge logs for sediment placement contribute to the uncertainty of possible dredge
selectivity.
The extensive armoring present in the region of the EHS, headland effects and the
associated profile lowering have had a notable impact on the available dry beach width
and the EHS development. In the 1500-meter long Bayport Beach section of the EHS,
only monuments R4 through R-5 are unarmored. Vertical concrete seawalls and rock
revetments front DNR monuments T-l through R-4. Almost all of the residences and
hotels along the Diplomat Beach segment, R6-R-11, have vertical and/or steep sloped
concrete seawall armoring, except for a small gap north of R-10. The Islander Club
segment has approximately 60% armoring, with an exception just south of R-11 and
between R-12 and R-13. At its most seaward extent, this section of armoring, T-1 to T-
15 extends into the active profile almost 60 meters beyond the expected equilibrated
shoreline (ATM 1995). This protrusion into the active region of the shoreline in
conjunction with a severe winter storm season could cause the severe loss of dry beach
width seen in this section of the nourishment project.
A potential cause of the erosional hot spot not mentioned in Table 2.2 that bears
consideration is artificial profile steepening. This is the most probable cause of the EHS
on Longboat Key. It suggests that the removal of the derelict groins caused an artificial
steepening of the beach profile.
The aerial photographs taken for the Coastal Construction Control Line (1974)
show the groins field extending from Monument R-67 in Manatee County to Monument
R-11 in Sarasota County. Several groins could also be seen in front of monuments R-13
and R-14, where the length of the groins extend offshore approximately 30 meters. A
total of 105 H-pile groins were removed between R-67 and R-7. Figures 6.1 through 6.5
show the offshore profiles for the Bayport section of Longboat key, using data obtained
by the FDEP in a pre-construction survey conducted in January 1993 and the post-
construction survey conducted on August 1994. It is apparent that after the removal of
the derelict groins, the beach transitioned to a milder equilibrium profile more consistent
with that associated with the sediment grain size. In Figures 6.1 through 6.6, the overall
profile becomes much milder from the pre-construction to post-construction surveys, and
the loss of upland sediment volume would contribute to the apparent loss of available dry
beach width.
From this general overview it is obvious that several causative factors maybe
partially responsible for the erosional hot spot. The most dominant factor is the removal
of the derelict groins. This caused a significant shift in the nearshore profiles for the
region considered and resulted in substantial loss of dry beach width. The equilibration
of the sediment to resemble the milder profile associated with the actual sediment grain
size reduced the amount of available upland sediment, thereby creating a landward
movement in the shoreline. The severe winter climate also seems to have had a
considerable effect on the nearshore sediment patterns. The movement of large quantities
of sediment from the upland section of the beach profile to the bar system offshore also
contributed to a landward movement of the shoreline. Although the sediment volume
added was still present in the nearshore region, the displacement of the sand caused the
shoreline to retreat to a near pre-project location. Thus, it appears that several of the
potential causes of the erosional hot spot culminated in an unfortunate coincidence and
created an environment leading to the localized erosion of the available dry beach width
in the central portion of the beach nourishment project on Longboat Key.
5.0 DNR Monument T-1
4.0 --19 J
E 3.0
2.0
1.0
1 0.0------
v -1.0 -------
S
t -3.0rsegment
i -4.0-- ---- ----- -
o-5,0-- _
n-6.0
-7.0
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
Distance Offshore (m)
Figure 6.1 Comparison of prenourishment and post nourishment nearshore profiles for
Bayport segment
5.0 DNR Monument R-2
4.0
0.0 -089
-0-\--\\-- --------|---------------
0:2 0 ---- - - - - - - - - - - -
-1.0 ----- --------------
-2.0 --"
-3.0
-5.0 -,--- -- --
-6.0 ------------------------------------------
-7.0---- ----------
-R fl
E
e
v
a
t
o0
n
(m)
10 20 30 40 50 60 70 80 90 100
Distance Offshore (m)
110 120 130 140 150 160 170 180 190 200
Figure 6.2 Comparison of pre-nourishment and post-nourishment nearshore profiles for
Bayport segment
DNR Monument R-3
4.0 ---
2.0 ",
1.0
v -1.0-
a .2.0 - -- ---------------------
t -3.0
i -4.0
S-- ----.----------------------------
0.50 -- -- --------=^ -----'-^'-Z-------
n -6.0 ----------------
m). 1
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130
Distance Offshore (m)
140 150 160 170 180 190 200
Figure 6.3 Comparison of pre-nourishment and post-nourishment nearshore profiles for
Bayport segment
DNR Monument R-4
0
0 0193
0 ---- -,-- ,- -089
2 ------------------ "S---------- "^ ,------------------------------------------------y- -----., ----------
- "" -39 -. .
2 -------------------------------- ------------
2l----I---------------------------------I---------I-~-I-I------I--I--I
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Distance Offshore (m)
150 160 170 180 190 200
Figure 6.4 Comparison of pre-nourishment and post-nourishment nearshore profiles for
Bayport segment
-30 -20 -10
5.(
4.1
E 3.
2.t
1.1
e 0.
V -1.
a -2.0
t -3.0
i -4.1
o-5.(
n -6.0
-7.
(M)8.
(
E
e
v
a
t.
o
n-
(m)
5.0 DNR Monument R-5
5.0 ---------------------------------
4.0
2.0
1.0---
0.0 ------------------------
-1.0------------------------
-2.0
-3.0 0 -
-4.0- -
-5.0-------------------------- ------------
-6.0--
-7.0
a n
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
Distance Offshore (m)
Figure 6.5 Comparison of pre-nourishment and post-nourishment nearshore profiles for
Bayport segment
5. DNR Monument R-6
5.0---- ---------------------- --------------- ------------------- ------------------- -------
4.0 01
v -1.0 ------
0..0 ----_=s ------------------------------
a ---------2.0--------
Distance Offshore (m)
Figure 6.6 Comparison of pre-nourishment and post-nourishment nearshore profiles for
Bayport segment
CHAPTER 7
SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS
7.1 Summary and Conclusions
There are many analytical and numerical methods capable of modeling large-scale
coastal processes. Given the appropriate boundary conditions and initial wave
parameters, these methods can accurately determine general evolution trends of beach
nourishment projects. However, for small-scale processes, such as an erosional hot spot,
there still remains an inability to model and predict localized anomalies to the degree
required. Many different types of erosional hot spots have been identified and several
have been discussed as possibly pertaining to Longboat Key: dredge selectivity,
headland effects, profile lowering, and anomalous bathymetry.
The main area of interest was the EHS that developed after the 1993 beach
nourishment project in the mid-key region, from DNR monuments R-1 to R-14 in
Sarasota County. This area is of concern because it had a water line that receded
landward to a pre-project position within less than 1-year after completion of the
nourishment project. The landward recession of the land-water interface created
narrower than expected beaches in several locations.
A sediment analysis was performed in order to determine if dredge selectivity,
due to the placement of fine sediment, was a probable cause for the formation of the
EHS. Although a slightly finer than native sediment diameter appeared to exist for the 1-
year post-construction sediment data, the analysis does not provide a convincing
correlation. Two typical profiles were examined from the southernmost EHS in the
Islander Club area, R13 and R-14. The analysis confirmed the finer grain sizes used for
fill sediment would cause a milder equilibrium profile, and thus could be a factor in the
formation of the EHS. However the equation used for the analysis, h = A y2/3, applies to
equilibrated profiles, whereas the profiles examined were still in the equilibration phase.
Also, this idealized form is strictly valid for monotonic beach slopes, where there are no
bars present.
The time involved for the equilibration process is necessary for consideration.
Since total equilibration can range from several years to perhaps ten years, it is probable
that the beach profiles collected during the 1-year post-construction survey were still in
an equilibrating state, resulting in beach profiles with steeper slopes.
The offshore movement of the sediment, forming a two bar system along most of
the EHS area, appears to be a contributing cause of the apparent loss of dry beach width.
This shift of offshore sand could have been caused by the harsh winter storm conditions
that occurred during and after the placement of the fill material. However it is more
likely the derelict groins along the mid-key region were causing an artificial beach-
steepening effect. Once these structures were removed, the beach profile equilibrated to
a more natural form for the sediment grain size. Due to the extensive armoring present in
the region of the EHS, headland effects and the associated profile lowering have also had
an impact on the available dry beach width and the EHS development in the central
section of the island. The combination of these causative factors has apparently created a
situation that has led to extensive loss of available dry beach width through localized
erosion on Longboat Key.
In the 1-year post-construction monitoring survey, Applied Technology and
Management initially recognized the existence of an erosional hot spot in the mid-key
region of Longboat Key (ATM 1995). Further analysis showed the EHS at mid-key to
contain three subsections: Bayport Beach, Diplomat Beach, and Islander Club. The
analyses of the deviations from the least-square lines were conducted for the Bayport
segment and the Islander Club segment.
The analysis of the deviations in the contours illustrated an apparent correlation
between the location of expected erosional trends and detected erosional trends. The
EHS located in the Islander Club and Bayport sub-sections were examined in detail
measuring the magnitude of the deviations from the least-square best-fit line and
determining the parameters of the power spectrum density. The Islander Club segment
had a much shorter alongshore distance estimated for the erosional hot spot, between
monuments R-13 and R-14. This allowed the analysis to have a more concise location
for comparison. The Bayport segment had a much longer alongshore estimated distance
for the EHS, benchmarks T-l through R-5, so the actual location was more ambiguous.
The evidence of the analysis of the deviations supports the conclusion of an erosional
trend in this section.
Although this analysis showed positive results for deviations estimating the
erosional trends in these mid-key sub-sections, the data became too irregular offshore to
account for any possible residual bathymetry. The limitations in the offshore depth of the
available data made it difficult to draw a correspondence between the presence of any
anomalous bathymetry and the development of the erosional hot spot. The time history
used for comparison limits the analysis. The only available high-density lidar data
available for the mid-key section is for post-nourishment conditions. Another limiting
factor of the analysis is the relatively short alongshore distance of the EHS.
Determination of trends in this small-scale region has inherently limited parameters
because of the many variables associated with the dynamic nearshore region.
7.2 Recommendations
It is apparent that more research is necessary to study the developmental
processes and causes of erosional hot spots. In order to better understand the processes
involved in small-scale profile evolution, more detailed models and analysis procedures
are also necessary. The development of an EHS is a dynamic three-dimensional process
and a more precise method for determining correlations between bathymetry and its
effects on shoreline evolution are necessary. In order to create an accurate diagnostic
method, all pertinent factors must be taken into account. These factors include, non-
uniform alongshore and cross-shore grain size, profile equilibration time, an extensive
data set available analysis, and improvements to or creation of, numerical models capable
of integrating all these factors. By improving our ability to model a physical process, the
ability to improve project performance, longevity, and cost effectiveness is also
increased.
APPENDIX A
PROFILES
A.1 DNR Profiles
The DNR transect profiles were originally collected in the horizontal datum
NAD27 and vertical datum NGVD 29, feet. The CORPSCON coordinate conversion
program was used to convert them into NAD83, FL-W, NGVD, meters, horizontal and
vertical, respectively. The average sediment size used for calculation of the ideal profile
has dso = 0.17 mm, and the equation is h =A y 23
DNR Profile Transects from 0894
15.0
10.0
6.0
D 0.0
p -5.0
h -10.0
-15.0
-20.0
-25.0
-400 -200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800
Offshore Distance
Figure A.1 DNR 0894 profile transect monument T-1
ONR Profile Transacts from 0894
15.0
10.0
5.0
S0.0
p -5.0
t -10.0
-15.0
-20.0
-250
-600 -400 -200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800
Offshore Distance
Figure A.2 DNR 0894 profile transect monument R-2
82
DNR Profile Transacts from 0894
15.0
10.0
5.0
D 0.0
e
p -5.0
t -10.0
h
-15.0
-20.0
-25.0
-400 -200
200 400 600 800 1000 1200
Offshore Distance
1400 1600 1800 2000 2200 2400
Figure A.4 DNR 0894 profile transect monument R-4.
DNR Profile Transects from 0894
-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Offshore Distance
Figure A.3 DNR 0894 profile transect monument R-3
DNR Profile Transects from 0894
-400
15.0
10.0
5.0
D 0.0
e
p -5.0
t -10.0
h
-15.0
-20.0
.g ; n
15.0
10.0
5.0
D 00
e
p -5.0
t -10.0
h
-15.0
-20.0
-.,; n
-300 -200 -100 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Offshore Distance
Figure A.5 DNR 0894 profile transect monument T-5
1
--
15.0 DNR Profile Transects from 0894
10.0
5.0
D 0.0
p -5.0
t -10.0
h
-15.0
-20.0
-25.0
-300 -200 -100 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 100
Offshore Distance
Figure A.6 DNR 0894 profile transect monument R-6
150 DNR Profile Transects from 0894
10.0
5.0
e -5.0
P-10.0
h -15.0
-0 00
-25.0
-30.0
-400 -200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Offshore Distance
Figure A.7 DNR 0894 profile transect monument R-7
DNR Profile Transects from 0894
15.0
10.0
5.0
D 0.0
p -5.0
t -10.0
h
.15.0
-20 0
-25 0
-200 -100 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900
Offshore Distance
Figure A.8 DNR 0894 profile transect monument R-8
84
DNR Profile Transects from 0894
15.0
10.0
50
0 0.0
e
p -5.0
t -10.0
.15.0
-20.0
-25.0
-300 -200 -100 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 15 1600 1700 1800 1900 2000
Offshore Distance
Figure A.9 DNR 0894 profile transect monument R-9
DNR Profile Transects from 0894
10.0
5.0
0.0
e -5.0
Figure A. 10 DNR 0894 profile transect monument R-10
-10.0
h -15.0
-20.0
-25.0
-400 -200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 260 2800
Offshore Distance
Figure A.10 DNR 0894 profile transect monument R-10
DNR Profile Transects from 0894
10.0
50
0.0
D
e -50
t -10.0
h -15.0
-20.0
-25.0
-400 -200 0 200 400 600 80 1000 1200 1400 1600 1800 2000 2200 2400 2600
Offshore Distance
Figure A.11 DNR 0894 profile transect monument R-11
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