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Evaluation and determination of erosional hot spots after beach fill placement in Longboat Key, Florida

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Title:
Evaluation and determination of erosional hot spots after beach fill placement in Longboat Key, Florida
Series Title:
Evaluation and determination of erosional hot spots after beach fill placement in Longboat Key, Florida
Creator:
Weber, Cris Kelli
Place of Publication:
Gainesville, Fla.
Publisher:
Coastal & Oceanographic Engineering Dept. of Civil & Coastal Engineering, University of Florida
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English

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University of Florida
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University of Florida
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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
ACKN OW LED GEM EN TS ................................................................................................ ii
LIST OF TABLES ............................................................................................................. vi
LIST OF FIGURES .......................................................................................................... vii
CHAPTER 1 INTRODUCTION ....................................................................................... 1
1.1 General Description .............................................................................................. 1
1.2 Objective ............................................................................................................... 2
CHAPTER 2 LITERATURE REVIEW ........................................................................... 4
2.1 Beach N ourishm ent Characteristics ...................................................................... 4
2.2 Potential M echanism s for EH S ............................................................................. 7
2.3 Observed Erosional Hot Spots ............................................................................ 16
CHAPTER 3 BEACH NOURISHMENT PROJECT, LONGBOAT KEY, FLORIDA. 25
.).I Site Overview ...................................................................................................... 25
3.2 N ourishm ent Perform ance .................................................................................. 36
CHAPTER 4 SURVEY DATA ANALYSIS .................................................................. 44
4.1 Objective of Analysis .......................................................................................... 44
4.2 Survey D ata Sources ........................................................................................... 44
4.3 M ethodology ....................................................................................................... 47
CHAPTER 5 SEDIM ENT ANALYSIS .......................................................................... 62
5.1 Data 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 Sum m ary and Conclusions ................................................................................. 77
7.2 Recomm endations ............................................................................................... 80
APPEN DIX A DN R PROFILES ..................................................................................... 81
APPEN DIX B SEDIM ENT DATA ................................................................................. 87
APPENDIX C POWER SPECTRUM DENSITY PLOTS .............................................. 93
C. I Islander Club Segm ent Power Spectrum Density .............................................. 93
C.2 Bayport Segm ent Power Spectrum D ensity ....................................................... 96
APPENDIX D PROFILE COMPARISONS ALONG TRANSECTS ............................ 99
D .1 Bayport Profiles ............................................................................................... 100
D .2 Islander Profiles ............................................................................................... 102
REFEREN CES ............................................................................................................... 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 in 1/3) .......................... 68
B. I 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
(Dean 1991) ................................................................................ 11
2.2 Planform of additional dry beach width resulting from variability in alongshore
sediment size (Bridges, 1995) ............................................................ 11
2.3 Plan view of nourishment in front of armored shoreline creating headland effect
(Liotta 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 199 1) .....................31
3.5 Percent occurrence for directional wave spectrum (ATM 1991) ...................3 2
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 benchm ark R-13 ............................................................................................... 69
5.4 Comparison of ideal profiles with different mean diameters to actual collected profile
at benchm ark R 14 ............................................................................................... 69
6.1 Comparison of prenourishment and post nourishment nearshore profiles for Bayport
segm ent ..................................................................................................................... 74
6.2 Comparison of pre-nourishment and post-nourishment nearshore profiles for Bayport
segm ent ..................................................................................................................... 75
6.3 Comparison of pre-nourishment and post-nourishment nearshore profiles for Bayport
segm ent ..................................................................................................................... 7 5
6.4 Comparison of pre-nourishment and post-nourishment nearshore profiles for Bayport
segm ent ..................................................................................................................... 7 5
6.5 Comparison of pre-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 ..................................................................................................................... 7 6
A. 1 DNR 0894 profile transect monument T-1 .......................................................... 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-1 .................................................................................................... 89
B.2 Sediment distribution of depth contours (feet, NGVD) from 1-year monitoring
survey D N 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 -1 1.5 .............................................................................................. 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. I 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,
0994 ...................................................................................... 96
C.8 Bayport segment power spectrum density for 3-meter contour of SHOALS data,
0994 ...................................................................................... 97
C.9 Bayport segment power spectrum density for 4-meter contour of SHOALS data,
0994 ...................................................................................... 97
C. 10 Bayport segment power spectrum density for 5 -meter contour of SHOALS data,
0994 ...................................................................................... 98
C. 11I Bayport segment power spectrum density for 6-meter contour of SHOALS data,
0994 ...................................................................................... 98
D.l1 Location and orientation of profile transect data for Longboat Key, Florida from T-1I
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 cause(s) 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 along shore 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 governm-ent 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
S Level of Government RevenuesI
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 platform 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 (199 1) 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
8 a 1.Sm
hl. 6m
interseeling Profiles. AN' 0.1mI AF a0.14m 45.3m
- h. 6m
Non-intersecting Profiles AN= AF = 0.1Tinl 13
h. 6m
Non-Intersecting Profiles AN= -0.1m ,AF= 0.09m,/3

a 6m

Limiting Case of Nourishment Advancement, il Non-Intersecting Profiles, AN. 0.1m13,AF= 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
30.0020.c00
Fill I

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, Ay,, about its mean alignment is
AY, = AYR[ K] (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 bathymnetry. This leads to the potential of the shoreline being permanently altered while the wave action attempts to level the belowwater 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
Protrusion Receded Shoreline
Desin Template_- Position
Pre-Nourishm"e'>
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
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
nd-n X",ion
A -8oy -'., .Ocean C, y ....WA inlet
L IslonMARYL1tO lalqu
OCEA1l
e Fishing
point
- *&z~** ~ 'vChicoteaq~ue '~~d o M EE
PENINSULA
BArl
CAPE: CHARLES o 2 3 5 OCCAN
HC
\Chesapeake Day
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 crossshore 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:
2
K gHb
K=1[ ~fJ 2H.38 i(2a) (2.2)
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) 7= breaking wave index (y= 0.78) Hb= significant wave height at breaking, m C-b= 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).
4 ... - .. . ................. .... 7 .. .. . - .. T .. ...... ...... .......
~T
I Y
M 'n
~-0
h1A
-3 4 ..
0 S 10 15 20
Distance North of O.C. Inlet (kin)
........... 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 197 1. 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
Gulf of Mexico ca
.... 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- l-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
5 -ED 1874-1986 Shoreline
25
-= __ Change, Historic (ft/yr)
N 1974-1986 Shoreline o 5 Change, Recent (ft/yr)
0 5
~~-15
DNR Monument
Figure 3.2 Shoreline change in feet per year for Manatee County, Florida




Sarasota County Shoreline Change South to New Pass No

45 35 25 15
5
-5
-15
DNR Monument

O 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
WI/S Stalon 41, Gulf of Mexico 27N 83W
-- Impact Shoreline of Longboat Key
0
00 0 7 8 IS M3. 8 *1 50 03 05 7. .-S. 3 1 Azimuth of Sector (Degrees)
Figure 3.5 Percent occurrence for directional wave spectrum (ATM 1991)
Directional Energy Spectrum
WIS Staon 41, Gulf of Mexico 27N 83W
Imp act Shorei e of Longboat Key
.L 1.
I -

a..s.S.e8. 3. Psren oue fos dao e 47 sectrum (u T 1e Azimuth of Secors (Dogres)
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)
5 o5 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 2 2 5
Sea 2.5 4.7 4.40 27.50 8.61
___Swell 4.4 7.5 0.16 3.10 0.97
-7' 2920
Sea 2.8 5.1 6.40 50.18 15.70
-4 D-F Swell 5.0 7.5 2.40 60.00 18.78
Sea 2.5 4.8 8.40 52.50 16.43
-1 40Swell 4.8 7.6 1.70 39.17 12.26
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 Transpor
An analysis of volumetric sand losses performed by Applied Technology and
Management (199 1) show a net loss of 86,3 00 cubic yards per year for the entire island
of Longboat Key. ATM also determined the existence of a nodal point near R-5 1 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 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 postconstruction 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 V +6.
3.4 I

n
C
-111

Sarasota Co.

-21.4

0

/

-20.3

60.0
Y

- New Pass

Legend
I Littoral Drift Quantity Mechanical Placement
Figure 3.7 1993 Pre-construction sediment budget estimate for Longboat Key, FL

17.3 17.0




3 8 O e l 3 8 0 f
A0,(,j~ongboat Pass
f ,.T S arasota C o. -- T1
Manatee Co.
-39.0
_ _ _-68.0
32.0 -35.0 Rt)
9..- 30.0Y
+106.0q 3
S.1 "-'
X Q)
/-42.0 X
Sarasota Co. ,
SaraoT~\%j~J ~New Pass Manatee Co. 72.0 :,
Legend
Littoral Drift Quantity I 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
(fi Hegh (hs) (sec) n (deg)
1993 March 12-14 4.8 H,> 16.4 24 14 250
Oct 30-31 2.0 H,> 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 1 10 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 prenourishment and 6-month post nourishment surveys are presented in Figure 3.9.
Following the March 1993 "Storm of the Centur" and the 1994-9 5 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 subsections: 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 Survev~
Accretion
SErosion

Transition

.R61 -'A

Beachfill waplaced above +--3ft. as supplement to the USACOE if project
,R7

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




Sarasoata Bay

Bayport Beach t
Diplomat Beach ,ooo
Islander Club
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 R-7
hi ~~" 4 *,~
/3
I6
Figure 3.11 Contour map of Bayport segment of the EHS on Longboat Key derived from SHOALS data, 0994




R-11

R-12

32 R-13
*R-14
-, '2R-15
7," / \,
3
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 cause(s). 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 postconstruction, 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
Lbk0 193 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, FI-W, NGVD 29 (inm)
9409.xyz Northing, Easting, Elevation NAD 83, Fl-W, NGVD 29 (inm)
9412.xyz Northing, Easting, Elevation NAD 83, Fl-W, NGVD 29 (inm)




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 SurfaceWater 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 RI3, and the Islander Club division is about 1500 meters long from monuments RI Ito 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, (xI, yI), onto the least square line in both the x and y direction, resulting with the coordinates (x2, y') and (xl, Y2), respectively. From triangle ABC, it is apparent that r= ta- X)(4.1) where x = (xI-x2), and y = (yI-y2). Using triangle ABD, sin" = r (4.2)
Y
therefore,
r = ysiny = (Y Y2)sin[tan-(Y1 -X)]Y2))] (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.




(Xa,y2)

"Z- Least Squares Line
D
Y(I,1c
A _-Data Point (x2,Y1)
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 1km 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
77-6
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 RI 3, 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 Lst Sq. line: 2m

W II
R11
* I

I
R1B
Possible EH-S
- ----- ----------- -------J4
I I

R14
------------ - - - - --.
- - -
- -.. ... . . .
-- .---

R1)
- --

I

0 10 20 30 40 50 60 70 80
point number
Figure 4.3 Magnitude of deviations from least-square line of the 2-meter contour with
respect to DNR benchmarks, SHOALS data 0994

Deviation of Data From Lst Sq line: 2m

R11
-ll
,,.' 17h1
- - - -- . .. ."-. .
- - --x\

i i
R12
-------------------------- Offshore Contc i Possible EHS
7MiddkO Conti
I \.
-
- -I

I I
R13
ur
Nr
--------------- -------------V ,
----------------- ---------------)\ I
-Nearshore Contour

R14
-----------Y -------?- ----------.
-- - - -

"0 20 40 60 80 100 120
point number
Figure 4.4 Magnitude of deviations from least-square line of the 2-meter contour with
respect to DNR benchmarks, SHOALS data 1294

25
20 15 10
5
0
-5
-15
-20

I

I

1

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.

Deviation of Data From Lst Sq. line, 3n

R12
----------- ---------------- .......
----------- ---------------- ------

R13
.. . .. ..- --- . .....
-,-- -------

R14
129 0994

-20UI
0 20 40 60 80 100 120
point number
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

-- -- - -
.. .. . -- - - --
- - - - - - -

40
30
02
o -10
-10

R15
-- - -- -------------iil
- -- - -- -- -- -- --... .
4
- - -- - -




Deviation of Data From Lst Sq. line"4m

; 4. .i...... i......
- - - - - - - -
-- - - - -

R12 R13
..........----... .. ........----......

R14
-0994:
. . .. . - - --

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.." "

-l9
---------1--4
099
R11

T --- - -
P12

R13

'I
P1 ----- -

0 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

R15
-1294
........--------. ----------- -- -- - -
Z I -- - -

- - - -
- --------------..
A, - -- -
15




"Dev tl&n of Data'From Lst $4jin&6 6m
200 ------ R1
R1 R12 R13 314 RI5
* 'I*1
150 ---------" '
--------------- ------------- -
+--------------* .. .S
. 50 ---. .. -- I--- --4 --- -. .. ..
,' :: !",,:: I ... ,
-00 100 109 20 5I0
-150i II Ii L .iI
0(' 0 100 ,50 2.0.250.30
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 El-S. 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.




Dbviati6n of b)ata From Lst Sq. line, Om

60 50
40 30
20 10
0
-10
-20
-30 An

"R* I R-1 1
---------4 ,i
S ............
4 --'-- -- -- --------.
--_ ---- ------I I -

R-12
- -

0 10 20 30 40 50 60 70 80 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 1year 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.

T1 I I i
R-13 R- 14 R-15
----------- ----------------- ....I--------------- ---- ----- ------- -------- -------* 0894
*0394




57
Figure 4.11 shows an erosional trend beginning just north of monument T- 1 and continuing for approximately 900 meters through the majority of the data set.

20
15 10
t-O 5 >~ 0 '~-5 10
-15
-20
-25

Deviation of Data From Lst Sq. line: 2m

T-1 --:---T -----A
9x
........--- ;---------"
- - - - -- - - - -

R-2
9412
09

R-3 R-..'
-- -- -- P 1- - -

R-5
-----------------------------------------....

R-6 .-------..

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

T-1
9412
- - - -
- - --

R-2
........ --.......

R-3
i----------iiii-

R-4
i

"0 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

-- - -- -
R -- - -
- - - -
-,---------------- - - - -




Deviation of Data From Lst Sq. line: 4m An.

T-1
-- - ------------- ---- ..----- -----
- ---. .. . . .
- --K

20 40

I I I
R-2 R-3 R-4 R-5 R-6
........... .. ....... -- -- -- -- j
------------ --- ------ ---- ------- ----- -------- -------------9409"
-9412
- ~ ~ ~ ~ .. . ,- -. . . .. .. -2 -...... ------- -------- ------- -7

80 100

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

Deviation of Data From Lst Sq. line: 5m

T-1 R-2
- ------ - - -
-- - - -- - - - -- - -
- - - - - - - - - - - -

R-3
-9412 r-9409

.R-4 R-5

R-4
, --- --- - - -

----------------------------------------------------------------------- -----------0-1-..

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

30
20
0
0 '-10
0)

I

R-6




D91viation of Data From Lst Sq~. line: 6m
100
T-1: R-2 R-3 -4 R-5 R-6
6= 0--------------- ------- ------- -----------------------------20 ------ --- ---- ---------60 -- ---- ------------ --- ----- -------------------------------80,- L ___.... L____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.




Pyyvs, Wave Number 2 m Contour

5000
-3500------------ ----------------- ----------- -------------------- ..... -- -- --3~0 0 0 - --- - - - - - --I- -I- -
25
~2000 ----- -- -- ---- --------- -- ----I-----I-- -- -I--- - -I-- -
0.
S 100 0 -- - - - - -- L . ..I- - - - - -- - - - - -- - - -
500 --- ..... .. - -- -- -- --- - --I-- - -I-- --- ------ 1 -5 -
0 0.005 0.01 0.015 0.02 0.025 0.3 0.035 0.04 0.045 0.05 1rn
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 I /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 27r.

WaveNumber =2;

(4.4)

where L is the location of the peak value on the x-axis. The power spectrum density (yaxis) would also need to be adjusted by dividing by 27r. The amplitude of the fluctuation for the data about the least-square line is given by the equation
FD~M;x (4.5)
where Pyy a, is the magnitude of the fundamental wavelength, measured in m 2_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, PY = on(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 FastFourier transform for the deviations is approximately I/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 postconstruction 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-1I 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-1, R-4, R-6.5, R-9, R-I 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 min in diameter. An alternate method for classification of sediment size is the Phi Scale. The phi ( ) size relates to the grain size by

= -1092 d

(5.1)




such that 20=d, where d is the sediment diameter measured in millimeters (Dean and Dalrymple, 1998).
Table 5.1 U.S. Standard Series Testing Sieves Nominal Maximum Nominal
SeeDsgainSieve Iniiul Wire Phi Units,
Siee Dsinaton Opening, Oninua Diameter,
_________in. OeigMmITI___Standard Alternative
12.5 mm 1/2 in. 0.5000 13.3 10 mmn 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 mmn No. 18 0.0394 1.135 mm 0.580 0.00
500.0 pim No. 35 0.0197 585.000 prm 0.340 1.00
355.0 pim No. 45 0.0139 425.000 pim 0.247 +1.49
180.0 pim No. 80 0.0070 227.000 pim 0.13 1 +2.47
125.0 prm No. 120 0.0049 163.000 prm 0.091 +3.00
75.0 pim No. 200 0.0029 103.000 i 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 El-S. 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 d50, 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 I* T-11

it ~ I.;
I ~ ~II -~

10 1

-n- R-4
R-6.5
--R-9
-*K-R-1 1 .5
-R-14
--R-1 6.5

0.1 0.01

Grain Size, mm
Figure 5.1 Composite sediment distribution for 1-year monitoring Survey

.~100 S80
60
40 20
0




Figure 5.2 shows the cross-shore distribution of the mean sediment diameter versus the alongshore DNR benchmark designations. The high d50 for monument R- 11.5 can be seen in the sediment distribution plot in appendix B.
Mean Diameter vs DNR Monument
-$- zero
1.4 -4--three
E six
E 1.2-- -n e
)K~ twelve
05 0.8 -4-fifteen Z
.~0.6
C0.4
S0.2
0
T-1 R4 R6.5 R9 RI1.5 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 mmn 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 213 First, the slope of the beach profile at the water line (y=O) 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 mn 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 d50 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 d50 values obtained from the composite

sediment distribution graph for the area encompassing the EHS.




- C5 = U 17 MM
c dD (. 2
-3.0 "du--- 0.3 --- ---- -----\m-n
.4.0... .
-5.0-. .
-80 ---- Molnin_ T -_;:Z
-7.o [ NR 1nmnt 1 3 -- "10.00 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
1.0------ ---1.0- - ',
-10o
---3.__ -- =
---,C -_ ., -.- -. .
-8.0 *
-------------------------------/"
-9.0 'omme t -14 --12.0
-12.00 20 40 60 60 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 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 hotshots 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-lI through R-4. Almost all of the residences and hotels along the Diplomat Beach segment, R6-R-1 1, have vertical and/or steep sloped concrete seawall armoring, except for a small gap north of R-l10. The Islander Club segment has approximately 60% armoring, with an exception just south of R-l1 Iand between R- 12 and R- 13. At its most seaward extent, this section of armoring, T- 1 to T15 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-1 1 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 postconstruction 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 beco mes 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 3.01 .
2.0
1.0 /- -09
1 0.0 A---
V -1.0
a -2.0 t -3.0-------------i-4.0--------------------,_-.
o-5,0-- - _n -6.0
W-7.0
()8 .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 1/-'-'10191
.0
3-1.0 -/19
-2.0 "" ""'
-3.0- ---5.0------ --
-6.0 -----7.0 -----

E
I
e
v
a t.
i.
0
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
5.0
4 .0 -14 .0
2.0 ,
1,0
0.0 -.- --.-""-- --'-,- ----07 0 r- -089- 1 - -- --
v -1.0 t -3.0
i -4.0 F""
n -6.0 ...
na.7.0
m).

-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
0 089
0
0
0 -

-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.
E 3. 2.,
1.1 e 0. v -1.0 a -2.0 t -3.
S-4.
0-5.0
n -6.0
-7 0

!0




E
I
e
v
a t.
i.
0
(n)

5.0 DNR Monument R-5
4.0
3.0 - --0.0 - -- - - -
-1.0 -- - - - -
-2.0 "-
-3. 0- --4.0- --5.0 ''" / '-6.0 --7.0
a n----------------- - - - - _

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 160 190 200 Distance Offshore (m) Figure 6.5 Comparison of pre-nourishment and post-nourishment nearshore profiles for Bayport segment
DNR Monument R-6
4.0 -019
0.0 -- 7z-v -1.0 a -2.0 ,
t -3.01" i -4.0- -0-5.0
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 (n) 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 proj ect 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-proj ect 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 Y213 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 beachsteepening 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-lI 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, nonuniform 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 ds50 = 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
9
t -5.0 h -10.0
h
-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 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
-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 Transects from 0894

15.0 10.0
50 D 0.0
e
p -50 S-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 0694

-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
-g5 0

15.0
10.0 5.0 D 00
e
p -5.0
- 10.0
h
-15.0
-20.0
-2:0

.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




15.0 DNR Profile Transects from 0894
10.0 50
D 0.0
e
p -5.0
-10.0
h
-150
-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
15.0 DNR Profile Transects from 0894
10.0 5.0
e -5.0 p -10.0
0 00 .
h -15.0
-Ml)
-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
-15.0
p -5.0 t -lO.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
15.0 DNR Profile Transects from 0894
15.0 10.0 5.0 0 0.0
e
p -5.0 S-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 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
0
e -5.0 r-10.0 h .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.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 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600
Offshore Distance Figure A.11 DNR 0894 profile transect monument R-11