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UFL/COEL-99/007
EROSIONAL HOT SPOTS: CAUSES AND CASE STUDIES AT
DADE AND MANATEE COUNTIES
by
Roberto Liotta
Thesis
May 1999
EROSIONAL HOT SPOTS: CAUSES AND
CASE STUDIES AT DADE AND MANATEE COUNTIES
By
ROBERTO LIOTTA
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
1999
ACKNOWLEDGMENTS
I would like to express my sincere appreciation to my advisor and supervisor
committee chairman, Dr. Robert G. Dean, for his enthusiastic, constructive, and constant
support during my past two years. His continuous research for answers to littoral processes
has been a source of valuable discussions.
I would also like to thank Dr. Ashish J. Mehta and Dr. Robert J. Thieke for serving
on my committee and for being my professors in different interesting classes.
My gratitude is also extended to the staff of the Coastal and Oceanographic
Engineering Department, including Becky, Subarna, Lucy, Sandra, and Helen, who made my
presence here enjoyable.
Special thanks are for the coastal students, with whom I played exciting and
exhausting football games, especially Kevin, Vadim, Justin, Joel, Erica, Daniel and Edward.
My thanks go to Adam, who introduced me to the coastal department when I first arrived
here, and to Thanasis, Yeon Sihk, Ki Jin, and Haifeng for their precious companionship and
help when needed. Particular thanks go to Guillermo, Nicholas, Al and Jamie, with whom
I shared the same office and unforgettable moments.
I would like to thank my parents and family for their constant and precious support,
which allowed me to go ahead and complete this constructive experience. Special thanks go
to my uncle Giovanni and my aunt for their important advices and support.
Finally, I would especially like to thank Teresa for staying morally close to me during
these past two years, believing in us, and for giving me her complete support for the
realization of one of my life's goals.
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS. ii
.LIST OF TABLES. vi
LIST OF FIGURES. vii
ABSTRACT.. .. x
CHAPTERS
1 INTRODUCTION. 1
1.1 General Description. 1
1.2 Motivation and Purpose. . 2
2 LITERATURE REVIEW. 4
2.1 Introduction. 4
2.2 Potential Causes for Erosional Hot Spots. 5
2.2.1 Residual Bathymetry. 5
2.2.2 Borrow Pit Location. 6
2.2.3 Bar Breaks. 10
2.2.4 Dredge Selectivity. 11
2.2.5 Headland Effects. 12
2.2.6 Profile Lowering Adjacent to Seawalls. 13
2.2.7 Residual Structure Induced Steepened Slopes. . 15
2.2.8 Mechanical Placed Fill.. 16
3 ANALYSIS OF TWO BEACH NOURISHMENT PROJECTS. 18
3.1 The Dade County Project. 18
3.1.1 Site Locations and Study Area. . 18
3.1.2 Background (Historical Events). 18
3.1.3 Data Sources. 25
3.1.4 Sand Characteristics. . 26
3.1.5 Beach Profiles. . 28
3.1.6 Historical Shoreline Changes. . 30
3.1.7 Volumetric Changes. 34
3.1.8 Time Scale of Beach Profile Evolution. 41
3.2 Anna Maria Key, Manatee County Project. 44
3.2.1 Site Locations and Study Area. 44
3.2.2 Background. 45
3.2.3 Data Sources. 48
3.2.4 Sand Characteristics. . 49
3.2.5 Beach Profiles. . 51
3.2.6 Historical Shoreline Changes. . 52
3.2.7 Volumetric Changes. 55
3.2.8 Time Scale of Beach Profile Evolution. 60
4 HOT SPOT IDENTIFICATION, MITIGATION, AND AVOIDANCE
MEASURES. 62
4.1 Analysis for Identification of Erosional Hot Spots. 62
4.1.1 Historical Shoreline Changes. . 62
4.1.2 Historical Volume Changes. 64
4.2 Criteria to Identify the Causes of Erosional Hot Spots. 64
4.2.1 Case a. 65
4.2.2 Case b. 67
4.2.3 Case c. 68
4.3 Other Analysis Methods. . 69
4.3.1 Historical Shoreline Analysis. . 69
4.3.2 Breaking Wave Energy Density and Angle. . 70
4.4 Cases of Study. 71
4.4.1 Dade County Project Hot Spots. 71
4.4.2 Manatee County Project Hot Spots. . 79
5 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS. 89
5.1 Summary and Conclusions. 89
5.2 Recommendations. 90
APPENDICES
A PROFILES. 92
B NUMERICAL MODEL OF BEACH PLANFORM EVOLUTION. .115
REFERENCES. 120
BIOGRAPHICAL SKETCH. 124
LIST OF TABLES
Table page
3.1 Characteristics of the Five Phases of the Dade County
Beach Nourishment Project. 23
3.2 Available Survey Data for Miami Beach. 25
3.3 Tidal Data for the Littoral Segment Located Between Bakers Houlover
Inlet and Government Cut. 26
3.4 Volume Change Characteristics for Various Beach Segments, 1992-1996. 35
3.5 Volumetric Change Characteristics, from R-58 to R-65. 36
3.6 List of the Most Significant Events. 38
3.7 Hot Spot Characteristics, 1992-1996. 41
3.8 List of the Parameters k and t5so for Different Values of m. . 43
3.9 List of Survey Data Used for Anna Maria Key. 48
3.10 Tidal Data for Anna Maria Key. 49
3.11 Average Shoreline Change Rate from 8/1993 for Different Beach Segments
and Different Periods. 54
3.12 Volumetric Changes for Different Periods. 56
3.13 List of the k and ts50 Parameters for Different Values of m. . 60
4.1 List of Possible Combination of Erosional Hot Spots. 65
4.2 List of the Possible Causes of Case a Hot Spots. 65
4.3 List of the Possible Causes of Case b Hot Spots. 67
4.4 List of the Possible Causes of Case c Hot Spots. 69
vi
LIST OF FIGURES
Figure page
2.1 Effect of Wave Refraction Behind a Borrow Pit. 7
2.2 Beach Plan Shape due to the Dredged Hole. 7
2.3 Wave Height Distribution at Different Cross-Shore Distances. 9
2.4 Contours of Diffraction Coefficient for Single Pit with a/L=1.0,
b/L=0.5, d/h=3, kh=0.167, and Normal Wave Incidence, Where a, b, d, L, h,
and k are the Pit Length, Pit Width, Pit Depth, Wave Length, Seaward Water
Depth, Wave Number. 9
2.5 Isolines of Approximate Diffraction Coefficient for Normal Wave
Incidence and a Breakwater Gap Width of 2.5 Wave Lengths. . 10
2.6 Plan View of a Hot Spot due to Dredge Selectivity. 12
2.7 Plan View of Hot Spot due to Headland Effect. 14
2.8 Eroding Profile Evolution in Front of a Seawall. 14
3.1 Dade County Location. 19
3.2 Borrow Site Location for Dade County, Beach Nourishment Project.. 23
3.3 Map of the Five Different Phases of the Beach Nourishment Project.. 24
3.4 Sand Grain Size Distribution, Dade County Nourishment Project. 27
3.5 Cross-Shore Distribution of Average Median Grain Size,
Based on Sampling Along Eight Profiles. 28
3.6 Comparison Between 1980 and 1996 Average Profiles. 29
3.7 Shoreline Position for Different Periods Relative to the Shoreline
Position of 1962. 30
3.8 Annual Rate of Shoreline Change for a Five Month Period
Between 1980 and 1981. 32
3.9 (Panel a) Annual Shoreline Change Rate for the Period 1992 to 1996.
(Panel b) Shoreline Change from 1962 to 1996. (Panel c) Number
of Years for the Shoreline to Reach the 1962 Position Based on
1992 to 1996 Erosion Rate. 33
.3.10 Total Volume Changes and Volumes Added Relative to 1962 Between
Bakers Haulover Inlet and Government Cut. 37
3.11 Total Volume Changes Relative to 1980 Between
Bakers Haulover Inlet and Government Cut. 39
3.12 Comparison Between Volume Changes per Unit Length
Based on Profile Changes and Shoreline Changes. 40
3.13 Average MHW Shoreline Change (Best Fitting Curve) Between
Bakers Haulover Inlet and Monument R-65 Neglecting the 1981 Data,
Which Appears Anomalous. 44
3.14 Manatee County Location. 45
3.15 Limits of the Manatee County, Shore Protection Project. 47
3.16 Grain Size Distribution for Native and Borrow Area Sand
with and without Carbonates. 50
3.17 Comparison Between Pre- (12/1992) and Post-Nourishment (8/1993)
Average Profiles. 52
3.18 Shoreline Position for Different Periods Relative to August 1993. 53
3.19 Shoreline Change Rate for Different Time Period and Littoral Segments. 57
3.20 Cumulative Volumetric Changes in the Project Area
from 1993 to 1995(Panel a) for Each Survey Time Interval.
(Panel b) Relative to August 1993. 58
3.21 Cumulative Volumetric Changes in the Project Area
Between 1995 and 1998. 59
3.22 Average MHW Shoreline Changes (Best Fitting Curve) for Anna
Maria Key, Shore Protection Project for Different Value of m. 61
4.1 Dade County. Hot Spot Locations Based on Shoreline Change
Analysis for the 1980-1996 Time Period. 72
4.2 Dade County. Hot Spot and Cold Spot Locations Based on Volume
Change Analysis for the 1980-1996 Time Period. 72
4.3 Dade County. Hot Spot and Cold Spot Locations Based on Shoreline
Change Analysis for the 1992-1996 Time Period. 73
4.4 Dade County. Comparison Between the Measured and Predicted
(DNRBS) Shoreline Changes for the 1980-1996 Time Period. 76
4.5 Dade County. Comparison Between the Measured and Predicted
(DNRBS) Volume Change Rates for the 1980-1996 Time Period. 77
4.6 Dade County. Alongshore Sediment Size Distribution for Different
Beach Contours (Based on Sampling Along Eight Profiles). 78
4.7 Dade County. Comparison Between the Constructed Line (1992)
and the Average Shoreline Position (1867 to 1936),
from Monument R-27 to R-37. 80
4.8 Dade County. Comparison Between the Constructed Line (1992)
and the Average Shoreline Position (1867 to 1936),
from Monument R-53 to R-61. 80
4.9 Manatee County. Hot Spot Locations Based on Shoreline Change
Analysis for the 1993-1998 Time Period. 82
4.10 Manatee County. Hot Spot and Cold Spot Locations Based on
Shoreline Change Analysis for the 1993-1995 Time Period. . 83
4.11 Manatee County. Hot Spot and Cold Spot Locations Based on
Volume Change Analysis for the 1993-1995 Time Period. 85
4.12 Manatee County. Comparison Between the Measured and Predicted
(DNRBS) Shoreline Changes for the 1992-1998 Time Period. . 86
4.13 Manatee County. Comparison Between the Measured and Predicted
(DNRBS) Volume Changes for the 1992-1998 Time Period. .87
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
EROSIONAL HOT SPOTS: CAUSES AND
CASE STUDIES AT DADE AND MANATEE COUNTIES
By
Roberto Liotta
August, 1999
Chairperson: Dr. Robert G. Dean
Major Department: Coastal and Oceanographic Engineering
The occurrence of erosional hot spots within the limits of beach nourishment projects
is one of the most perplexing and expensive aspects of this soft structure engineering solution
to beach erosion. Erosional hot spots have been observed to occur in many otherwise
successful beach restoration projects and, unfortunately, contribute significantly to the
overall cost of the project and to the negative public perception of its performance. Erosional
hot spots are due to coastal processes which act on a local scale and therefore are not
predictable from a large scale analysis. In this thesis a review of the different types and
causes of erosional hot spots, either anthropogenic or natural, is presented, together with
formalized criteria to identify their presence not only within the limits of the beach
nourishment project, but also adjacent to the projects. Also, guidelines to understand the
possible causes of erosional hot spots are suggested, with the aim to simplify the diagnostic
process.
Two case studies of erosional hot spots are discussed; in the Dade and Manatee
County beach nourishment projects. In both cases, shoreline and volumetric change analyses
were conducted, showing that the formation of erosional hot spots is a highly 3-dimensional
nearshore process, which includes cross-shore and longshore sediment transport components.
The one-line numerical model DNRBS was applied to both projects, and the results show a
qualitative agreement in large scale, especially in volumetric terms, but with less agreement
on the local scale. The application of detailed refraction/diffraction models to better
understand the shoreline response to local nonuniform bathymetry, such as borrow pits, and
wave conditions may improve prediction capabilities substantially.
CHAPTER 1
INTRODUCTION
1.1 General Description
Wave action causes the most significant changes in beach morphology. Large
volumes of sand are carried offshore and onshore over a period of time, and also carried
along shore by the action of oblique waves. In the proximity of inlets, tide and current effects
also become significant. It is of critical importance to understand and to be able to manage
the dynamic coastal environment, which has experienced increasing human pressure for
shore development, recreational facilities and shore protection from storm surge damage.
Past shore protection practices have included hard structures such as groins, jetties and
seawalls along the coast, resulting in undesirable sand accumulation or erosion due to
interference with on/offshore and longshore sediment transport processes.
More recently, soft structures have been implemented to prevent coastal erosion. One
of the most effective is the beach nourishment, which consists of the placement of large
quantities of good quality sand in the nearshore system in order to create or restore a beach
system. Due to its high cost, this engineering solution is usually justified in those areas
characterized by high tourist, residential, and environmental values.
1.2 Motivation and Purpose
One of the benefits of this soft approach is that supplying additional sand does not adversely
affect adjacent beaches. Conversely, one of the most perplexing and expensive aspects of the
beach nourishment solution is the occurrence of erosional hot spots within the limits of the
project. An erosional hot spot is defined as a limited area which erodes more rapidly and/or
equilibrates with a significantly narrower beach width compared to the adjacent beaches.
Erosional hot spots have been observed to occur in many successful beach nourishment
projects and can contribute significantly to the overall cost of the project and to the negative
perception of its performance. As an example, the Dade County beach nourishment project,
constructed between 1976 and 1982 with the placement of approximately 10 million m3 of
sand on the nearshore, analyses have shown that in more than 15 years its performance has
been globally excellent, except for some local areas where the shoreline has receded faster
or equilibrated with a narrower dry beach width. Different tools are available to predict the
shoreline response to the construction of a beach nourishment project. Some tools focus more
on the beach planform evolution such as the numerical computer model DNRBS (Dean and
Grant, 1989) and Genesis (Hanson and Kraus, 1989). Others focus more on the beach profile
evolution, such as SBEACH (Larson and Kraus, 1989) and EDUNE (Kriebel and Dean,
1985). In all these models, average input characteristics are considered such as sediment
size, depth of limiting motion, wave conditions and in some cases contours, which allow
reasonable predictions of the large scale evolution of the beach system, but they are not able
to predict localized (a few hundreds of meters) shoreline and/or volume evolution.
3
One of the objectives of this thesis is to determine general criteria to identify the
presence of erosional hot spots inside or outside the limits of beach nourishment projects and
also, to propose a guideline to assist in identifying the possible causes of erosional hot spot.
This thesis provides a review of different types and potential causes of erosional hot
spots. Criteria to identify the presence of erosional hot spots and a guideline to understand
the possible causes will also be introduced. Two study cases will be analyzed and the results
discussed following previous criteria. Finally, conclusions will be drawn concerning the
causes of hot spots, and recommendations for research to provide further understanding of
the underlying processes will be proposed.
CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
Beach nourishment is one of the most non-intrusive technique used to protect
shorelines and coastal structures and it consists of the placement of good quality sand on
the beach in order to create or restore a beach platform for recreational and storm protection
purposes. One of the benefits of this soft approach is that supplying additional sand does not
adversely affect adjacent beaches. Conversely, one of the most perplexing and expensive
aspects of the beach nourishment approach is the occurrence of erosional hot spots within
the limits of the project. An erosional hot spot is defined as a limited area which erodes more
rapidly and/or equilibrated with a significantly narrower beach width compared to the
adjacent beaches. Erosional hot spots have been observed to occur in many beach
nourishment projects and contribute significantly to the overall cost of the project and to the
negative public perception of its performance.
Bridges (1995) identified at least eight different mechanisms of erosional hot spot
formation, which can be generated by pre-existing conditions or by design and/or
construction procedures. The characteristics of each different mechanism are discussed
briefly below, together with some examples of erosional hot spots.
2.2 Potential Causes for Erosional Hot Spots
The presence of erosional hot spots (EHS), as well as cold spots (ECS), within the
limits of different beach nourishment projects has been noted in many monitoring reports.
In some cases, it has been observed that more than one mechanism contributed to the
formation of hot spots. Various causes of erosional hot spots are reviewed below.
2.2.1 Residual bathymetry
This type of erosional hot spot is primarily due to wave refraction by the contours
below the depth of closure. The contours, located seaward of the depth of limiting motion,
are referred as "residual bathymetry" which, due to the refraction process, induce the wave
front to bend and consequently to shape the shoreline similarly. These contours can exist
before the construction of beach nourishment projects or can be created during the
construction process placing uneven quantities of material to depths greater than the depth
of closure. Dean and Yoo (1993) have shown that for an offshore contour alignment of AyR,
the displacement of the shoreline, Ays, about its mean alignment is
Ays = AYR 1 (2.2)
where C, and C, are the wave celebrities at the outer depths of the nourishment and at the
depth of closure, respectively. This process was first identified by Dean and Yoo (1993)
6
while they were developing a method for representing wave refraction and shoaling in the
vicinity of beach nourishment projects.
2.2.2 Borrow Pit Location
Very often the borrow pits are located immediately offshore the littoral segment to
be nourished. Depending on the distance from the shoreline the borrow pits may influence
the wave front orientation as well as the wave energy distribution along the beach through
wave refraction, reflection and diffraction. The result on the shoreline could be an erosional
hot spot or a cold spot, which is an area where the shoreline accretes due to accumulation of
sand. It is very common to see cold spots adjacent to erosional hot spots. Different studies
have been conducted in order to determine the influence of dredged holes on the shoreline.
These studies identified that the shoreline changes are caused by two different but
simultaneous processes, wave refraction and wave diffraction, and depending on which
process is predominant, the formation of an erosional hot spot and of a salient could be
observed, respectively. Motyka and Willis (1974) noted that if refraction were the only
mechanism to induce longshore currents, a significant erosion would be observed in the lee
of dredged holes. A nodal point would be located in the shoreline behind the pit. The
described mechanism is shown in Figure 2.1.
Horikawa et al. (1977) conducted computer simulations and laboratory studies of the
influence of dredged holes on the shoreline. The theoretical and laboratory investigations
both showed the formation of a salient in the area behind the borrow pit and the formation
of hot spots in the adjacent areas (see Figure 2.2).
WAVES BREAKAT
AN ANGLE. CREATINO
ALONOSHORE WAVES B AT
CURMRE TOHE ANANOE CREATING
ET A LONGSHORS
CUIRRE1TT1HE
RIGHT
SAND IS MOVED AWAY FROM THE AREA BEHIND TIB BORROW PIT
BY WAVE INDUCED LONGSHORE TRANSPORT
Figure 2.1 Effect of Wave Refraction Behind a Borrow Pit. (From Bridges, 1995).
Figure 2.2 Beach Plan Shape due to a dredged hole.
S1 Dredged Hole
Wave Direction
Original Shoreline
Position _
Erosion
Accretion
Shoreline Position due
to the Dredged Hole
8
Horikawa et al. suggested that sand accumulates behind the dredged hole where wave
action is reduced and calm water is created. The process suggested by Horikawa was
supported by the monitoring data of the 1984 Grand Isle, Louisiana nourishment project
provided by Combe and Soileau (1987), which documented the formation of two salients in
the lee of two borrow pits, and consequently the formation of erosional hot spots in the
adjacent areas. Gravens and Rosati (1994) concluded that the resulting nearshore currents are
generated by the longshore gradients in wave set up, which similarly follows the longshore
wave height distribution shown in Figure 2.3. The resulting currents and associated sediment
transports will go from regions of greater to lower wave set up, therefore, from the extremity
areas to the central area of the dredged hole lee, favoring the sediment deposition in the
central area.
McDougal, et al. (1995) investigated the effects of dredged holes on the wave field.
The presence of a partial standing wave in the region immediately seaward of the borrow pit
and a zone of lower wave height in the landward region was observed, in agreement with the
results of Horikawa et al. (Figure 2.4).
The particular location of the borrow pits as well as of an undredged area in the
general borrow area, seems to be one of the causes of the Delray Beach (Florida) hot spot.
The undredged zone is located between two borrow pits, acting like a shoal, therefore,
focusing wave energy in its lee. This process contributes to the formation of lower wave
energy area behind the adjacent borrow pits.
Note that more research is needed to better understand which combination of borrow
pit parameters (dredging depth, alongshore length, offshore distance, etc.) cause
perturbations to the shoreline.
a) Seaward of the dredged hole
b) Immediately Landward
)j
I c) Closer to the Shoreline
Center of the Dredged Hole
Longshore Distance
Figure 2.3 Wave Height Distribution at Different Cross-Shore Distances.
Figure 2.4
Contours of Diffraction Coefficient for Single Pit with a/L=1.0, b/L=0.5,
d/h=3, kh=0.167, and Normal Wave Incidence, Where a, b, d, L, h, and k
Are the Pit Length, Pit Width, Pit Depth, Wave Length, Seaward
Water Depth, Wave Number. Waves Propagate from Left to Right
(From McDougal et al.,1995).
2.2.3 Bar Breaks
An offshore bar or reef can behave as a submerged breakwater reducing the wave
energy impacting on the shoreline immediately landward. If there are breaks along the
offshore bar or reef, greater wave energy will result immediately landward of the break,
indenting the shoreline in that particular area. Breaks in bars can be a result of a natural
process or of a man-made action. Although the mechanism for bar break formation is not
completely understood, its effects on the shoreline are similar to those of gaps between
breakwaters, which have been well investigated by Penny and Price (1952).The results were
presented in plots showing the isolines of the diffraction coefficient behind the breakwater
gap for different widths and for normal wave incidence. Figure 2.5 presents an example for
a gap that is 2.5 wave lengths long.
I OOL
0.10 0.20 0.25
0 25L
.7
-I'5
Width of gap 2.5 L
Figure 2.5 Isolines of Approximate Diffraction Coefficients for Normal Wave Incidence
and a Breakwater Gap Width of 2.5 Wave Lengths
(From Penny and Price, 1952).
It is seen that the greater diffraction coefficients, which are directly related to the wave
heights, are located behind the gap and similarly behind the bar break. The larger wave
height relative to the adjacent areas will generate a longshore sediment transport directed out
of the area behind the bar break, causing the formation of a hot spot. In these cases, it is not
unusual to identify the presence of cold spots immediately beside hot spots. An example of
this formation mechanism of erosional hot spots can be seen in a littoral segment of Fire
Island, New York, where a much higher erosion rate has been experienced due to a break in
the offshore bar (Dean, 1995).
2.2.4 Dredge Selectivity
Dredge selectivity is a possible cause of an erosional hot spot which principally
depends on the construction procedure followed by the dredging contractor. The sand size
distribution in the borrow pit can be non-uniform. When there are considerable differences
between the shortest and longest required pumping distances it is less costly for the
contractor to pump the finer sand to greater distances, and the coarser to shorter distances.
The result is a non-uniform sand size distribution along the nourished shoreline. Finer sand
assumes a milder equilibrium beach profile than coarser sand. As shown by Dean (1991), a
relatively small difference in sand size can have a relatively large effect on dry beach width
for the same volume density placed. Figure 2.6 shows the plan view of a hot spot due to
dredge selectivity. Two borrow areas located at each end of the project are used to nourish
the beach. At the fill locations farthest away from the borrow pits, the dredge selected finer
sand to allow savings in operating costs resulting in the central part of the project being filled
BorrowSite A Barrow Site B -
3aoo A.. (Ulkd -t .35MM .4"d
3(100 AnumflW wilm
AtMu" fle4d wkh dA nlaC nslasiA s A
(a "n diamdIr fom OJ5mm to
$ 2000- 0.25mo)
LONGSHORE DISTANCE
a(KLoMETERS)
Figure 2.6 Plan View of a Hot Spot Due to Dredge Selectivity (From Bridges, 1995).
with finer sand than the extremities. Considering only the cross-shore response of the beach
nourishment project (neglecting the longshore response, as the transport of sand out of the
area placed), after the beach reaches equilibrium, a narrower dry beach width occurs for the
area filled with finer sand. Moreover, the equilibration process of the littoral segment with
finer sand is faster (greater erosion rate), emphasizing in this way one of the defining
characteristics of hot spots. This mechanism of formation of hot spots was recognized by
Coastal Planning and Engineering as one of the causes of the hot spots on the nourishment
projects of Longboat Key, Florida, and Captiva Island (1991), Florida.
2.2.5 Headland Effects
In an attempt to protect sections of the shoreline with particular high value (hotels
and/or their facilities) from wave attack, seawalls, revetments and other hard structures can
13
be utilized. An artificial headland can result through hardening these sections of the shoreline
as the adjacent shorelines continuing to recede. If the beach is nourished to a uniform
additional beach width (see Figure 2.7), the hardened section will actually protrude
unnaturally beyond the adjacent shorelines. The protrusion tends to be eliminated by the
wave action which redistributes the sand from the perturbation (higher wave energy density)
to the adjacent beaches (lower wave energy density). This results in the observation of an
apparent erosional hot spot with adjacent cold spots. This concept has been introduced by
Olsen Associates, Inc. (1993), and by Coastal Planning and Engineering (1991) while they
were examining hot spots on Hilton Head Island and Captiva Island, respectively. Aerial
photos and historical shoreline changes can be effective tools to check for such type of
protrusions.
2.2.6 Profile Lowering Adjacent to Seawalls
The presence of a seawall or other parallel hard structure on an eroding shoreline
eventually results in a greater water depth at the toe of the structure. Figure 2.8 presents the
evolution of an eroding profile in front of a seawall. At time to, the profile is characterized
by a certain value of dry beach width and cross-shore shape, which follows the so-called
equilibrium beach profile concept (Dean, 1991). Due to background erosion, the shoreline
will retreat in time, reaching the seawall position first and moving with the "virtual origin"
landward of the seawall. After the shoreline has reached the incipient dry beach
configuration, the profile becomes deeper and truncated in the upper part as the virtual
shoreline origin retreats behind the structure. If nourishment is required, in order to have the
same dry beach width in the armored as in the unarmored area, an additional volume of sand
"Spreading-out"
Figure 2.7 Plan View of a Hot Spot due to Headland Effect.
Figure 2.8 Eroding Profile Evolution in Front of a Seawall.
Post-Nourishment
Shoreline Position
Unnatural
Protrusion
u so Receded Shoreline
Design Template - -. Position
-e p- ----- -t----- -.. .-- -
Pre-Nourishment
Shoreline Position
High Value Structure
Seawall
15
has to be placed in front of the armored area, to create an incipient beach. If this threshold
volume of sand is not considered, a percentage of the placed sand will create the equilibrium
incipient beach, therefore, gaining a narrower dry beach width than expected. Olsen
Associates, Inc. (1993), and Coastal Planning and Engineering recognized this erosional hot
spot formation mechanism while examining hot spots in Hilton Head Island, South Carolina,
and Longboat Key, respectively.
Note that in those places where the seawall has also had a headland effect as
discussed in the previous type, a combination of these two mechanisms can occur.
2.2.7 Residual Structure Induced Steepened Slopes
In some eroding areas where the longshore sediment transport is significant, a groin
field has been employed to stabilize the shoreline due to its capacity to trap sand, resulting
in an artificially shallow region within the portion of the profile over which the groins
extend. The groins will prevent landward recession of these contours, maintaining the slope
of the upper portion of the profile constant. However, the seaward portion of the profile
which is not reached by the groin will experience erosion due to the recession of the offshore
contours. The result is that the seaward portion of the active profile will become steeper with
time. If the groin field is removed in conjunction with a beach nourishment project, the
shoreline (thus the profile) is free to move back toward its equilibrium position. Usually, the
beach is filled with sand within a short period of time after the groin field has been removed
without allowing the shoreline to retreat to its equilibrium position. If this recession is not
considered in the design process, and if the artificially advanced position is not recognized,
the volume density allocated to this segment of the shoreline will not be enough to achieve
16
the designed shoreline advancement. Therefore, this section of the beach will experience a
higher erosion rate and so will be identified as an erosional hot spot.
This hot spot formation mechanism was identified by Coastal Planning and Engineering
while investigating on the causes of hot spots in Longboat Key where a groin field was
removed before the construction of the beach nourishment.
Note that this process can also occur when the groin field is not removed, and the
nourishment is extended seaward such that the holding effect of the groins is reduced.
2.2.8 Mechanical Placed Fill
Two different methods can be used to construct a beach nourishment project:
hydraulic and mechanical. The hydraulic method consists of pumping a mixture of sand and
water onto the beach from an offshore borrow area, until the design template is reached. The
mechanical method consists of placing the fill material, which usually is dry, by trucks and
then configuring it by bulldozers until the design template is reached. The fill materials in
the two cases have different angles of repose which will influence the total volume of sand
placed; Bagnold (1954) showed that if the sand is dry its angle of repose is greater than that
of a sand-water mixture. If the fill material is placed mechanically the result is that, even
though the design template is accomplished, less sand will be placed in the profile to achieve
the design advancement, than if the material were placed hydraulically. It is not unusual for
the dredge operator to overfill the project area by up to 25% in volume, in part, to avoid the
cost of returning to refill an under filled area. Thus, after profile equilibration, those
segments where the fill was placed mechanically may recede more relative to areas where
the fill was placed hydraulically. This hot spot formation mechanism was identified by Olsen
17
Associates, Inc. (1993), while conducting studies on the causes of hot spots in Hilton Head
Island.
CHAPTER 3
ANALYSIS OF TWO BEACH NOURISHMENT PROJECTS
This chapter illustrates erosional hot spot occurrence through the analysis of two
beach nourishment projects: (1) Dade County, FL and (2) Anna Maria Key, FL.
3.1 The Dade County Project
3.1.1 Site Location and Study Limits
The study area is located in Dade County on the southeast coast of Florida (Figure
3.1) and includes the beaches of Bal Harbour, Surfside, and Miami Beach. The study area
is a sandy barrier island approximately 14.8 km (9.2 miles) long and is bounded to the north
by Bakers Haulover Inlet and to the south by Government Cut. The area of interest is shown
on National Oceanic and Atmospheric Administration (N.O.A.A.) Nautical charts from no.
11465 through no. 11468, plus others.The limits of the present study extend to the littoral
segment located between Bakers Haulover Inlet on the north and Government on the south
focusing particularly on the areas where erosional hot spots (EHS) have been recognized.
3.1.2 Background (Historical Events)
The most substantial shoreline changes in this general area have occurred as a result
of various coastal engineering projects and natural processes, including
The excavation of Government Cut at the south end of Miami Beach in 1904,
including the construction of two jetties.
The excavation of Bakers Haulover Inlet at the north end of Miami Beach in 1925,
included the construction of two jetties.
The September 1926 hurricane which impacted the Miami area.
The construction of a system of seawalls and abutting groins along the beach.
The encroachment of construction onto the active beach.
The Dade County, Florida, Beach Erosion Control and Hurricane Surge Protection
Project (1976-1981).
Renourishments in 1960-69, 1981-92 and 1994.
Each of these is discussed in the following sections.
FLORIDA
FLORIDA
0
o
Dade
County
Figure 3.1 Dade County Location.
-. .
Government Cut
The initial Government Cut federal navigation project commenced in 1904 and
included the construction of a north jetty, which was extended seaward different times in
subsequent years, reaching the present length of 1,434 m (4,688 feet) in 1929. It was repaired
and sand tightened in 1959-60 in order to reduce the deposition of sand in the entrance
channel, due to the longshore sediment transport toward the south. In 1973, another project
to further sand-tighten the north jetty was conducted. At that time, significant quantities of
sand and rock were placed along the north side of the jetty. After the beach nourishment
project was completed in 1982, another repair effort was necessary to sand-tighten the more
seaward section of the jetty.
Bakers Haulover Inlet
The Bakers Haulover Inlet project was excavated in 1925 and sponsored by local
interests. It was stabilized by two short jetties. After the south jetty was destroyed by the
1926 hurricane, it was rebuilt in 1964, as part of a Federal navigation project. In July 1975,
the south jetty was extended seaward about 224 m (735 feet) and curved southerly to deflect
alongshore currents to the south and encourage a gyre to reduce these currents and minimize
sand losses.
September 1926 Hurricane
"The September 1926 hurricane has been the most severe hurricane that impacted
Miami Beach since records have been kept. The barrier island was inundated by a
combination of astronomical tide, storm surge, wave setup and wave runup, with water
21
depths over the island up to 1.0 m (3 feet), and with water ponded in a few places near the
ocean to an elevation of from 3 m (10 to 11 feet) above MLW. During the hurricane, a large
amount of sand was transported up to 300 m (1,000 feet) inland from the beach, covering the
streets of the city with 1 m (3 feet) of sand" (Wiegel, 1992). It was also during this hurricane
that the north and south jetties of Bakers Haulover Inlet were destroyed and the bridge,
located landward of the jetty, was left standing 49 m (160 feet) offshore as a result of
erosion. Later the bridge was relocated farther landward.
Construction of Shoreline Stabilization Structures
The development of the Miami Beach area resulted in many hotels and private
construction located close to the shoreline. In some cases, hotels expanded their facilities
seaward of the existing MHW. An almost continuous line of seawalls was built to protect
these facilities from wave attack. These seawalls survived surprisingly well during the 1926
hurricane. After the 1926 hurricane, in order to stabilize the shoreline, a system of
perpendicular groins was built from Bakers Haulover Inlet to Lummus Park, located
approximately 2 km north of Government Cut. Most of these groins were sponsored by
private interests. Many extended up to 50 m ( 164 ft) seaward of the MHW line. By 1975,
more than 14.6 km (48,000 linear feet) of seawalls and numerous groins have been
constructed.
Dade County. Florida. Beach Erosion and Hurricane Surge Protection Project
The Beach Erosion Control and Hurricane Surge Protection Project was authorized
by Congress in 1968 and was constructed from 1976-1981.This project was designed for
22
beach erosion control and also to provide hurricane protection for a storm of intensity similar
to that which had occurred in 1926. Moreover, it provided a recreational beach along Dade
County. The design increased the beach width by 75-90 m (250-300 feet) and included a
dune with an elevation of 3.5 m (11.5 feet) above MLW. The design life of the project was
50 years, with estimated renourishment on the order of 145,160 m3/year (191,000 yd3/year)
for the reach between Government Cut and Bakers Haulover Inlet, amounting to 9.35
m3/year per meter (3.75 ydc/year per foot), (R. L. Wiegel, 1992). The total amount of fill was
approximately 10.5 million m3 (13.9 million yd3), including the 1975 Bal Harbour fill. The
sand was dredged from nearby offshore borrow areas and pumped to the beach, as shown
in Figure 3.2. The seawalls and the abutting groins were not removed but were covered by
the fill.
Local interests had placed sand south of Bakers Haulover Inlet on six occasions
between September 1960 and August 1969, averaging 23,180 m3 /year (30,500 yd3/year).
Also in July 1975, local interests contracted for the placement of 1.235 million m3 (1.625
million yd3) of beach fill along the 1.37 km (0.85 mile) Bal Harbour reach; at the same time,
as noted previously, the south jetty of Bakers Haulover Inlet was extended approximately
224 m (735 feet).
Due to the magnitude of the complete Dade County project, it was realized in five
different phases and contracts (Figure 3.3), which are summarized in Table 3.1. Note that the
cost per cubic yard of the nourishment increased in time.
Table 3.1
Characteristics of the Five Phases of the Dade County Beach Nourishment Project.
Phase Period of Work Area Encompassed Volume of Unit Cost
no. (Streets) Sand Placed ($/yd3)
(yd3)
1 May 1977 September.1978 from 80t to 96h 2,940,000 1.95
+ Haulover Beach Park
2 August 1978 1979 from 63rd to 80h 1,530,000 1.87
3 August 1978 1980 from 36h to 63rd 3,177,100 2.66
4 May 1980 October 1981 from 16th to 36h 2,200,000 4.95
5 October 1981 January 1982 from Govern. Cut to 16t 2,400,000 9.00
The total amount of sand placed was 10.6 million m3 (13.9 million yd3), including 1.26
million m3 (1.65 million yd3) of 1975 Bal Harbour fill.
Figure 3.2 Borrow Site Location for Dade County, Beach Nourishment Project.
Renourishments
As noted previously, before the Dade County Beach Erosion and Hurricane Surge
Protection Project was constructed, local interests placed sand south of Bakers Haulover Inlet
between 1960 and 1969, averaging 23,300 m'/year (30,500 yd3/year). After the project was
constructed, in some particular areas, renourishment projects were required: 122,200 m3
(160,000 yd3) between 63rd and 71st Streets. (R-46 and R-41 Monuments) and 53,500 m3
(70,000 yd3) between 27th and 34h Streets (approximately between R-61 and R-58) were
placed in 1985, and 175,700 m3 (230,000 yd3) placed at Bal Harbour Beach in 1990.
Figure 3.3 Map of the Five Different Phases of the Beach Nourishment Project.
SBker H.auwr Cat
R-30
R-40
R-50 PHIASEt
R-60
Ilk rPASE
R-70 Slh PASIE
Scale:
0 2 4 6 8
Miles
Dade County, FL
25
Moreover, 91,700 m3 (120,000 yd3) of sand were placed between R-55 and R-56 in 1994
(Dade County Regional Sediment Budget, 1997). The total renourishment up to 1998 has
been 653,000 m3 (855,000 yd3).
3.1.3 Data Sources
Miami Beach has been one of the most important tourist areas in Florida since the
beginning of the century. Profiles in this study area have been surveyed several times and
most of the data are available on the internet at the State of Florida, Department of
Environmental Protection (DEP) web page, www2.dep.state.fl.us/water/beaches.
The data available are summarized in Table 3.2.
Table 3.2
Available Survey Data for Miami Beach.
Date Source Average Offshore Data Description
Extent of Data (m)
1962 DEP' Shoreline Position
1969 UF Project 6906 400 Beach Profiles
1975 DEP Shoreline Position
November 1980 DEP 450 Beach Profiles
March 1981 CCCL" Shoreline Position
February 1986 DEP 100 Beach Profiles"**
August 1992"' DEP Water Line NGVD Beach Profiles*".
June 1996 DEP 700 Beach Profiles
DEP Department of Environmental Protection.
CCCL Coastal Construction Control Line Photos.
After Hurricane Andrew
Wading profiles
The beach profile data of 1986 and 1992 did not extend very far seaward (wading profiles),
26
while the data of 1980 and 1996 extended seaward to the depth of closure, 4.6 m (>15 ft).
It must be noted that in 1992 some monuments were moved seaward from their original
locations. For calculation purposes it has been assumed that only the cross-shore coordinates
of the profiles from different years changed. The data obtained from the sources listed above
were used to compute MHW shoreline changes as well as volumetric changes. Cross-sections
are plotted and compared to determine long-term changes that have occurred over the 1980-
1996 time period. Table 3.3 lists the tidal data for the area of interest (Balsillie, 1987).
Table 3.3
Tidal Data for the Littoral Segment Located between Bakers Houlover Inlet and
Government Cut.
Datum Elevation above NGVD in feet (m)
Mean Higher High Water (MHHW) +1.76 (+0.536)
Mean High Water (MHW) +1.69 (+0.515)
Mean Tide Level (MTL) +0.44 (+0.134)
Mean Low Water (MLW) -0.72 (-0.219)
Mean Lower Low Water (MLLW) -0.90 (-0.274)
3.1.4 Sand Characteristics
In 1977-78, before the placement of fill, the Jacksonville District of the U.S. Army
Corps of Engineers (USA/CESAJ) took "native beach" sand samples along profile lines 0.5
miles apart. After project completion, other samples were taken. Figure 3.4 shows the
comparison between the grain size distribution of native, borrow area and project (after.
placement) sand. The mean grain sizes are, 1.59 (d=0.33 mm, native), 1.71 4 (d=0.30 mm,
borrow area) and (=1.46 (d=0.36 mm, completed project'). The Corps reported
(USA/CESAJ, General Design Memorandum Addendum III, Sept. 1986) that "all three
27
composite distributions are poorly sorted. The native beach curve is better sorted than the
project borrow and post-nourishment curves. The post-nourishment material is slightly better
sorted than the in-place material" (p. A-1). Also they commented that "The visual estimate
of shell content for the native beach was 23 percent. The in place borrow material was 34
percent shell and the post nourishment beach was 39 percent shell. The higher percentage of
shell in the fill material reflects the high carbonate content of the sand in the borrow areas"
(p. A-i).
cADE CO. B. p il.s witin the ara of interest (Chleste
RENOURISHMENT
COMPOSITE 3 GRAI S hor
DISTRIBUTIONS -
0-0 -MTIK IEKM /
1977-78
MAN QUWIN SIZE 1.69 0 - -
SPHI SOrING D 1.04
BMW rli hig values o d c7 mm, ad t
2/A A, at C. 0 E i f s t v
HEA ,mRAIN SI h o1.71
1982 $0017
f4[M GBAIN SIZE 1.46 _ _/ 0 o_
MI SWI11 /01
-- -+ 0
-4 -3 -2 -I 1 I Z 3
GRAIN SIZE (6i UNITS)
Figure 3.4 Sand Grain Size Distribution, Dade County Nourishment Project.
Figure 3.5 presents the cross-shore distribution of average median grain size (d50)
computed by averaging data of 8 profiles within the area of interest (Charles, 1994). Note
the relative high values ofd50 close to the shoreline, over 0.7 mm, and the decrease to around
0.2 mm at a seaward distance of 100 m. Still farther seaward the value ofdso increases again
reaching almost 0.45 mm at 300 m offshore.
0. -
0.7
0.6
0.4
30.3.5 B h
0.2 "
0.1
10 50 100 150 200 2 O 300
Offfhor z f sance fro m Storelne ( mm)
Figure 3.5 Cross-Shore Distribution of Average Median Grain Size, Based on Sampling
Along Eight Profiles (Charles, 1994).
3.1.5 Beach Profilks
Comparative profiles have been plotted for the area of interest at each monument and
are presented in Appendix A. The 1980 and 1996 data extend farther offshore than the 1969,
1986 and 1992 data. The efe beach fill project was on average 1:20 as shown by the
1980 beach profiles from Monument R-27 to R-65. The 1996 profiles are steeper in the
foreshore zone followed by a trough at approximately -2.0 m depth (6.0 ft) and by a milder
slope over the rest of the profile reaching the depth of closure, which is on average 5.0 m (16
ft). In most of the profiles from Monument R-27 to R-49 and from R-58 to R-62 it appears
that between 1980 and 1996, as a result of the profile equilibration process, the sand has
migrated seaward from the foreshore, causing a recession of the shoreline, and deposited on
the bar but still inside the nearshore zone (see Figure 3.6, panel a and c). In the 1996 profiles
from Monument R-50 to R-58 the sand from the foreshore zone has migrated cross-shore out
of the nearshore zone or has migrated longshore impounding downdrift near the north jetty
29
of Government Cut (see Figure 3.6, panel d). The 1996 beach profiles from Monument R-60
to R-74 have almost the same shape as the previous 1992 profiles, as shown in Appendix A.
Compared to the 1980, 1986 and 1992 data, there has been deposition on the foreshore zone
and also on the bar causing an accretion of the shoreline.
Panel a of Figure 3.6 compares the average profiles of 1980 and 1996 computed for the
reach between Bakers Haulover Inlet (Monument R-27) and Monument R-65, and panel b
between R-65 and Government Cut (Monument R-74). Also, panel a shows that the 1996
profile has adjusted toward a new equilibrium. The significant difference between the 1980
and 1996 profiles in panel b is primarily due to the construction of 5t" Phase of the
Nourishment Project.
4 4
-4 .. .......... i .... : N i . -4 . . .. N .' ......... ...
-from Monum. R-27 o R65. -from Monum. R-66 to R74.
0 100 200 300 400 0 100 200 300 400
4 c). 4
c) d)
0 MHW 0 M
-4. ... ... i .4 .............. -2.
om Monum. R-27 to R-49 a '
S-from Monym. R-58 to -62. -from Monum. R-50 lo R58.
0 100 200 300 400 0 100 200 300 400
Offshore Distance from Monument (m) Offshore Distance from Monument (m)
Figure 3.6 Comparison Between 1980 and 1996 Average Profiles. (Panel a) from Bakers
Haulover Inlet to Monument R-65. (Panel b) from Monument R-66 to
Government Cut. (Panel c) from Monument R-27 to R-49 and from
R-58 to R-62. (Panel d) from Monument R-50 to R-58.
3.1.6 Historical Shoreline Changes
Figure 3.7 presents the shoreline positions for different years relative to the 1962
shoreline, before the Beach Erosion Control and Hurricane Protection Project was initiated.
Several results are evident from Figure 3.7:
25 30 35 40 45 50 55
DEP Monument No.
60 65 70
Figure 3.7 Shoreline Positions for Different Periods Relative to the Shoreline Position of
1962.
Between Monument R-27 and R-33, which corresponds approximately to Bal
Harbour Beach, the 1975 shoreline position is located seaward of the 1962 shoreline, due to
the beach fill placed between Monuments R-27 and R-31 by local interests in 1975. Also,
the 1975 shoreline has advanced seaward along the remainder of the study area, but less
than in the Bal Harbour area and immediately north of Government Cut, where the net
longshore sediment transport was impounded by the north jetty. This general advance of the
L -x-X --e X x *-- '* X X X- -Xx K--r x xI x
L Bskrs HPlltmrInlrt
'""
G rnmemt CuQ
31
1975 shoreline position outside the area of nourishment in 1975 is probably due to the
natural recovery following the Ash Wednesday storm of March 1962.
The 1980 data show the shoreline position after the beach fill was placed from
Monument R-31 to R-65. It is seen that the average shoreline advance was approximately
125 m (410 ft). The fifth phase of the beach nourishment project, approximately from
Monument R-65 to Government Cut, had not yet commenced.
The 1981 data, as the 1980 data, show that the fifth phase of the project had not
commenced. Figure 3.8 presents the annual rate of shoreline change between November
1980 and March 1981. In only 5 months the shoreline receded on average 13.4 m (44.0 ft),
equivalent to an annual rate of 32.1 m/year (105.3 ft/year). This is a result of the beach fill
profiles adjusting rapidly to reach a new equilibrium. Between Monuments R-65 and R-66
the effect of the beach fill spreading out is also evident.
The 1992 data (see Figure 3.7) show that the shoreline receded from its position in
1981 except for the reach between Monument R-65 and Government Cut due to the shoreline
advancement associated with the fifth phase of the beach nourishment project. Considering
only the beach area between Bakers Haulover Inlet and Monument R-65, the annual rate of
shoreline change on average is -0.9 m/year (-2.95 ft/year), which clearly shows that the beach
profiles have almost reached equilibrium. A spreading effect due to the fifth phase of the
nourishment can be noted updrift (north) of Monument R-65; the shoreline advanced instead
of receding.
Comparison of the data of 1996 to that of 1992 shows that the shoreline continued
to recede between Bakers Haulover Inlet and Monument R-60 on average of-2.4 m/year (-
32
9.68 ft/year), while advancing from Monument R-60 to R-70 on average of 1.7 m/year (5.58
ft/year).
Figure 3.9 presents the annual rate of shoreline change from 1992 to 1996 and the
shoreline change from 1962 to 1996. The shoreline advancement between 1962 and 1996 is
on average 98.3 m (322.4 ft) with a peak of 189.8 m (622.5 ft) at Monument R-67 and a
minimum of 39.1 m (128.2 ft) at Monument R-36. Considering the annual erosional rate of
1992-1996, which is -2.4 m/year (-7.87 ft/year), it would take approximately 41 years before
the average shoreline again reaches the 1962 shoreline position, which implies a good
performance of the complete project. However, in some locations it would take only 9-12
years to reach the 1962 shoreline position, such as at Monuments R-34, R-36, R-58 and R-
59 (see Figure 3.9, Panel c). These areas are referred as Erosional Hot Spots (EHS) with the
most severe EHS at Monuments R-36 and R-59.
100
1981
50-
L Sakr H-ulovy r Inlt :Gom er
-100 -
25 30 35 40 45 50 55 60 65 70 75
DEP Monunent No.
Figure 3.8 Annual Rate of Shoreline Change for a Five Month Period between 1980 and
1981 (Note: This Is Primarily the Result of Profile Equilibration).
40k-
10o
'25 30 35 40 45 50 55
DEP Monument No.
C)
60 65 70 75
Figure 3.9 (Panel a) Annual Shoreline Change Rate for the Period 1992 to 1996.
(Panel b) Shoreline Change from 1962 to 1996. (Panel c) Number
of Years for the Shoreline to Reach the 1962 Position Based on 1992
to 1996 Erosion Rate (Note: In Panel c, Only the Points Characterized
by an Erosion Rate Have Been Included).
* .* *
*
**
* ~ *
**
3.1.7 Volumetric Changes
The net sediment transport along the east coast of Florida is toward the south, as
inferred from observations of sand impoundment at jetties and groins. Various estimates of
the longshore sediment transport rate are available for the Miami Beach area. Before the
beach nourishment and hurricane surge protection project started, the U.S. Army Corps of
Engineers (USACE) estimated that the net transport rate was 15,280 m3/year (20,000
yd3/year) towards the south and the magnitude was affected by the presence of groins along
the beach (USA/CESAJ, General Design Memorandum Phase I, July 1974). The Corps also
estimated that the historic alongshore net transport rate would be 165,788 m3/year (217,000
yd3/year) towards the south if there were no barriers (groins) to affect the transport, with
146,688 m3/year (192,000 yd3/year) towards the north and 312,476 m3/year (409,000
yd3/year) towards the south. The Coastal Engineering Research Center (CERC) later
calculated a net transport of 72,580 m3/year (95,000 yd3/year) towards the south, based on
the Wave Information Study (WIS) Phase III wave data, with a gross annual transport of
168,080 m3/year (220,000 yd3/year) (USA/CESAJ, General Design Memorandum
Addendum II, June 1984). A University of Florida study, reported by Wiegel (1992),
estimated a net transport of 179,540 m3/year (235,000 yd3/year) towards the south, with
142,868 m3/year (187,000 yd'/year) to the north and 322,408 m'/year (422,000 yd/year) to
the south, consistent with the previous estimates of the USACE. Coastal Systems
International Inc. in the Dade County Regional Sediment Budget, 1997, estimated a
southerly net sediment transport of 3,820 m3/year (5,000 yd3/year). This latter estimate is not
affected by groins.
35
Data are available relative to the shoreline positions immediately after the beach
restoration projects for only the first and second phases while those relative to the third
through the fifth phases are available only for later periods.
Appendix B presents plots of the distribution of the annual rate of volumetric change
per unit longshore distance along the beach for different time intervals. Only the volumetric
changes between 1980 and 1996 have been computed from the profile data, because they
extend farther than the depth of closure. The other volumetric changes are estimated from
shoreline changes, using a berm elevation of 2.0 m (6.50 ft) and a depth of closure of 5.0 m
(16.4 ft). It has been assumed that the profiles recede or advance without changing form
(Bruun, 1954, and Dean, 1977, 1991). It should be noted that, based on shoreline changes,
from 1992 to 1996, the beach between Bakers Haulover Inlet and Government Cut has lost
on average 8.56 m3/m per year (3.41 yd3/ft per year), which in four years equals 507,200 m3
(663,900 yd3), (Table 3.4). Because these results are based on shoreline changes, they are
overestimates due to the effects of profile equilibration, which consists in a transport of sand
from the upper to the lower (submerged) part of the profile reaching a more natural form,
associated with the mean grain size used in the project.
Table 3.4
Volume Change Characteristics for Various Beach Segments, 1992-1996.
Monuments Annual Rate of Volume Annual Total Volume Total Volume Change
1992-1996 Change per Unit Distance Change between 1992 and 1996
(m3/m per year) (m3/year) (m3)
R-27 R-74 -8.56 -126,800 -507,200
R-27 R-60 -16.59 -170,922 -683,688
R-60 R-74 +9.80 +44,122 +176,488
(Based on Shoreline Changes)
36
It appears that a fraction of the sand lost between R-27 and R-60, approximately
26%, has been deposited between R-60 and R-74 creating a very wide dry beach.
Volumetric changes for the reach between Monuments R-58 and R-65 are shown in
Table 3.5, based on losses to the -2 m (-6 ft) and -4 m (-12 ft) contours (USA/CESAJ,
General Design Memorandum Addendum 111, 1986). The volumetric changes show that most
losses occurred in the nearshore region (landward of the 2 m (6 ft) contour). This suggests
that the apparent loss of sand was principally due to slope adjustment not for Monument R-
58 to R-60. Additionally, none of the profiles experienced erosion for the -2 m (-6 ft) to -4
m (-12 ft) depth range indicating continuing profile equilibration.
Table 3.5
Volumetric Change Characteristics, from R-58 to R-65 for Various Depth Ranges,
1981-1986.
DEP Monument No. Volumetric Changes, yd3/ft from 1981 to 1986
Berm to MLW MLWto -6ft -6ft to -12ft Berm to -12ft
R-58 -31.7' -5.3 +4.6' -32.4
R-59 -51.5 -7.8 0.0 -59.3
R-60 -34.4 -6.1 +6.5 -34.0
R-61 -16.1 +5.6 +13.9 +3.4
R-63 -5.2 +11.9 +59.6 +66.3
R-64 -11.1 +24.1 +12.0 +25.0
R-65 +42.8 +9.8 +24.3 +76.9
(+) indicates gain of material; (-) indicates loss of material;
Total volume changes between Bakers Haulover Inlet and Government Cut relative
to 1962 have been calculated based on shoreline changes and are presented in Figure 3.10.
The most significant events which occurred in the area between Bakers Haulover Inlet and
Government Cut are summarized in Table 3.6. In 1975, the project area had experienced an
37
apparent increase in volume of 2,169,000 m3 due to (1) the Bal Harbour beach nourishment
projects between 1962 and 1975 (181,400 m3), (2) the Bal Harbour fill of 1,235,000 m3 in
July 1975, (3) the extension of the south jetty of Bakers Haulover Inlet and of the north jetty
of Government Cut, and (4) the natural recovery of sand following the 1962 Ash Wednesday
storm. During the next five years, the first four phases of the nourishment project were
completed and the volume change in 1980 was 10,312,800 m3. The apparent significant sand
losses which occurred between November 1980 and March 1981 are due in large part to a
rapid adjustment of the nourished profiles toward a new equilibrium. The fifth and last phase
of the nourishment project was completed in 1982. From 1982 to 1992, renourishment was
required in some particular areas (Wiegel, 1992):
x106
*----* Volume Changes
Volume Added
1 0 ..... ..... .... ... ... . ... ... .. ... ........ ... ..... .. ;, ; ........
0 1
i " i- I n :
I f
19601962 1965 1970 1975 1980 1985 1990 1995 1998
5 I 1 1 1/ f 1 i i
Time
Figure 3.10 Total Volume Changes and Volumes Added Relative to 1962 Between
Bakers Haulover Inlet and Government Cut (Based on Shoreline
Changes).
Table 3.6
List of the Most Significant Events.
3/1962 Ash Wednesday Storm
62-69 Nourishment Projects (213,500 yd3)
1973 Extension of the North Jetty of Government Cut
7/1975 Extension of the South Jetty of Bakers Haulover Inlet
1975 Bal Harbour Beach Fill (1,625,000 yd3)
5/1977 Phase I of the Beach Nourishment Project (2,940,000 yd3)
8/1978 Phase II (1,530,000 yd3)
8/1978 Phase III (3,177,100 yd3)
5/1980 Phase IV (2,200,000 yd3)
10/1981 Phase V (2,400,000 yd3)
1981-1992 Renourishments (460,000 yd3)
8/1992 Hurricane Andrew
1994 Renourishment (120,000 yd3)
122,200 m3 (160,000 yd3) between 63rd and 71st Streets (T-46 and R-41 Monuments); 53,500
m3 (70,000 yd3) between 27" and 34th Streets (approximately between R-61 and R-58) were
placed after about 6 years, and 175,700 m3 (230,000 yd3) placed at Bal Harbour Beach after
about 15 years. Moreover, 91,700 m3 (120,000 yd3) of sand were placed between R-55 and
R-56 in 1994 (Dade County Regional Sediment Budget, 1997). The total renourishment has
been 624,500 m3 (817,200 yd3). The combined renourishment rate is 67,700 m'/year (88,600
yd3/year). Up to 1996, the total volume change is 9,599,000 m3 (12,564,000 yd3) and the
total fill placed is 10,700,000 m3 (14,000,000 yd3) including the renourishment fills.
Therefore, the total volume loss over the 1962-1996 period is 1,101,000 m3 (1,441,000 yd3)
resulting in a volume loss rate of 32,600 m3/year (42,400 yd3/year).It can be noted that
39
considering only the volumetric changes from 1992 to 1996 the volume loss is 507,200 m3
(663,800 yd3) and the corresponding volume loss rate is 126,800 m'/year (166,000 yd/year)
which is still lower than the estimated renourishment rate of 145,200 m3 /year (191,000
yd3/year). The total volume change between 1980 and 1996 based on profile changes (Figure
3.11) is positive while that based on shoreline change is negative (Figure 3.10). This is due
to the fact that not all of the shoreline recession is caused by volumetric losses, but a major
part is due to slope adjustment.
x 10
2.5 i i u J ---------
.----. Volume Changes
4-* Volume Added
.10 Renourishment j
1 ....... S t h p h a s oe .. .... ....... ....... ........... ) "* ............ .................... .. ....................... .
0.5
SBased on Profile Changes
1980 1985 1990 1995 1996
Time
Figure 3.11 Total Volume Changes Relative to 1980 Between Bakers Haulover Inlet and
Government Cut (Based on Profile Changes).
Figure 3.12 presents the volumetric changes per unit length between 1980 and 1996
based on shoreline changes and profile changes. The total volumetric changes (after
subtracting the volume added), based on profile changes, is -508,000 m3 (-665,000 yd3). The
related volume change rate is -31,700 m /year (-41,500 yd3/year), which is consistent with
40
the long-term volume change rate obtained from shoreline data between 1962 and 1996. The
volume change rate based on shoreline data of 1980 and 1996 is -186,900 m3/year (-244,600
yd3/year), which is almost 6 times greater than the previous value and is believed to be a
substantial overestimation.
80 i --- i \ i i i i -
-- Based on Profile Changes
Based on Shoreline Changes
4 60 ....... ....- .. .... ..-. ... ...... .. .
40
S\0
Nourished
In 1982
1-20
Bakers Haulover Inlet Government Cut
-40 I Ii -
25 30 35 40 45 50 55 60 65 70 75
DEP Monument No.
Figure 3.12 Comparison Between Volume Changes per Unit Length Based on
Profile Changes and Shoreline Changes (1980-1996).
Volumetric losses could occur through the south jetty at Bakers Haulover Inlet,
through the north jetty at Government Cut, and landward or seaward of the project.
Additionally, some settling (consolidation) of the placed material may have occurred.
Although some losses may occur through the jetties, this sand transport pathway is believed
to be relatively minor. It is known from personal inspection that some of the material placed
contained a substantial fraction of silt and clay. This would have been carried out of the
41
active profile by suspension, as was evident by the "milky" color of the water for several
years after nourishment. My interpretation is that the two major contributors of loss are due
to fine sediment suspension and consolidation (loss of volume but not of sand). If this
interpretation is correct the volumetric loss rates should decrease with time.
Two erosional hot spots are located in the area of study: the first is between
monuments R-55 and R-60 and the second between monuments R-33 and R-37. The
characteristics of these two hot spots are summarized in Table 3.7.
Table 3.7
Hot Spot Characteristics, 1992-1996.
Hot Spot Average Annual Rate of Volume Maximum Annual Rate of Total Volume
Change (m'/m per year) Volume Change (m3/mper year) Change (m3)
R-55 R-60 -20.4 -39.2 (R-58) -148,200
R-33 R-37 -20.6 -32.4 (R-36) -126,300
(Based on Shoreline Changes)
3.1.8 Time Scale of Beach Profile Evolution
An equilibrium beach profile represents a dynamic balance of constructive and
destructive forces acting upon the beach. A change in one of these two forces will result in
a disequilibrium. Beach nourishment projects represent a perturbation on the actual littoral
systems which overtime will tend to eliminate the perturbation or at least to smooth its form.
After nourishment, the beach profiles are artificially steeper than equilibrium, reducing the
volume of water over which the incident wave energy can be dissipated. The wave energy
dissipation per unit volume will be greater than the equilibrium value causing an increase of
the total destructive forces over the constructive forces for a period of time until the
evolution of the beach profile brings the competing forces back into balance. During this
42
period of time a seaward redistribution of the sediments will take place along the profile.
The time scale of beach profile evolution may vary significantly. Under different conditions,
some beaches reach equilibrium very quickly (3 years), others very slowly (10 years).
In an attempt to understand the causes of and quantify the different time scale, we
examine the following equation expressing the time rate of change of the shoreline position
Y
y
dt -k(y eq)m (3.1)
where, k is the decay parameter, yeq is the shoreline displacement after the profile has
achieved total equilibrium. With the initial condition that for y(t=0)=yo, the solution of Eq.
(3.1) is given by
y(t) = Yeq + (Y Yeq)e-kt for m=l (3.2)
and
(Y0 Yeq )m-1 --11
y(t) = Yeq+ k(- Y eq )m-1 + for m arbitrary (3.3)
e k(yo keq)(M- t+ 1
From Eq. (3.3), it can be shown that
1
t = K(y ye)(m 1) (3.4)
Equation (3.2) represents the linear solution while Equation (3.3) is the non -linear
solution. Note that this analytical model does not perform well for beach accretion, since no
constructive mechanism has been considered. The decay parameter k has been estimated for
the linear (m=l) and non-linear (m=2, m=3) cases for the littoral segment between Bakers
Haulover Inlet and Monument R-65. Figure 3.13 presents the best fit curves of the average
measured MHW shoreline changes determined by using the least squares method. It also
shows how the equilibrium process is rapid during the first years (especially for m=3) and
then decreases with time after the perturbation has been reduced by the waves. The different
values ofk and time scales required for an equilibration of 50% to the native profile (t50o)
have been summarized in Table 3.8. The k and t50% values have also been computed
neglecting the 1981 data, which appears anomalous. Based on the comparison in Figure 3.13,
it is not possible to judge which value ofm is more appropriate.
Table 3.8
List of the Parameters k and to,, for Different Values ofm.
Data from 1981 to 1996 Data from 1986 to 1996
m=l m=2 m=3 m=1 m=2 m=3
k (years-) 0.0828 0.0033 0.0039 0.0788 0.0029 0.0034
tso, (years) 8.4 6.4 8.1 8.8 7.3 9.3
Time (year)
Figure 3.13 Average MHW Shoreline Changes (Best Fitting Curve) for the Between Bakers
Haulover Inlet and Monument R-65 Neglecting the 1981 Data, Which
Appears Anomalous.
3.2 Anna Maria Key, Manatee County
3.2.1 Site Location and Study Area
Anna Maria Key is located in Manatee County approximately on the central west
coast of Florida (Figure 3.14) along the Gulf of Mexico and includes the beaches of Holmes
Beach and Bradenton Beach. Anna Maria Key is a sandy barrier island approximately 11.5
km (7.2 miles) long and is bounded on the north by Passage Key Inlet and on the south by
Longboat Pass. The barrier island is separated from the mainland by Sarasota Bay. Maps of
the area of interest can be found on N.O.A.A.'s nautical charts no. 11414, 11424, and 11425.
45
The limits of the area of interest extend along the reach of Gulf shoreline between
DNR (Department of Natural Resources) Monument R-9 and R-41 (Figure 3.15), focusing
successively on those areas where EHS have been recognized.
Manatee -
County
FLORIDA -
Figure 3.14 Manatee County Location.
3.2.2 Background
The two inlets which bound Anna Maria Key are natural inlets. Passage Key Inlet
connects the southerly part of Tampa Bay with the Gulf of Mexico while Longboat Pass
connects Sarasota Bay with the Gulf of Mexico.
The most significant shoreline changes which occurred in Anna Maria Key are due
to different events:
The construction of shoreline stabilization structures.
The construction of the Manatee County, Florida, Shore Protection Project in 1993.
Hurricane Opal in October 1995.
Each of these is discussed in the following sections.
Construction of Shoreline Stabilization Structures
The littoral segment included in the 4.2 mile nourishment project was heavily
armored with revetments, seawalls, groins, and bulkheads by local interests first and public
projects after. In the early 1950's, the District of Anna Maria Island Erosion Prevention
installed about 100 stone groins varying in length from 15.2 to 21.3 m (50 to 70 feet)
(USACE, 1991). In 1959, the state road department constructed an additional 20 concrete
groins along the southern end of Anna Maria Key at Coquina Beach. In the subsequent years,
two permeable groins, respectively 140 and 97 m (460 and 320 feet) long, were built in an
attempt to stabilize the shoreline.
Manatee County, Florida, Shore Protection Project
The Manatee County, Florida, Shore Protection Project was authorized by the Public
Law 89-298 in 1965 and consisted originally in the restoration of 5.1 km (3.2 miles) of the
Gulf beach of Anna Maria Key. In 1989, the authorized project was modified to provide for
restoration of 6.7 km (4.2 miles) of shoreline and construction of a 0.8 km (0.5 mile) beach
fill transition zone at the southern end of the project area in order to reduce the effects of the
spreading out losses (Figure 3.15).
The purpose of this project was to provide storm damage reduction to upland
structure improvements threatened by beach erosion. A volume of 1,550,000 m3 (2,028,000
yd3) of sand was placed between Monument R-12 and R-33A while a volume of 137,000 m3
(180,000 yd3) was placed in the transition zone between approximately Monument R-33A
and R-35. The total amount of fill was 1,687,000 m3 (2,208,000 yd3), corresponding to an
average density of 223.4 m3/m (89.0 yd'/ft). The project limits extend from Monument R-12
47
in the north to R-35 in the south.Periodic required renourishments of approximately 508,000
m3 (664,800 yd3) were estimated every 9 years, corresponding to a renourishment rate of
56,500 m3/year (73,900 yd3/year). The sand was pumped from borrow areas located
approximately 500-800 m offshore of the 1992 shoreline position, as is evident from the R-
25 through R-34 profile surveys. The design of the cross-section increased the berm width
by approximately 23 m (75 feet) at an elevation of 1.9 m ( 6.2 feet) above MLW and
included seaward slopes of 1:11 from the berm to the MLW shoreline position, and of 1:27
until intersecting the existing bottom profile.
GULF
OF
MEXICO
TRANSITION ZONE
SCALE IN FEET
Figure 3.15 Limits of the Manatee County, Shore Protection Project.
Hurricane Opal (1995)
When Hurricane Opal passed 325 nautical miles (602 km) west of Manatee County
in October 1995, it was a category 4 hurricane with highest sustained winds of 150 mph
(67.1 m/s). Wave heights of 6.9 m (22.6 ft) with a dominant wave period of nearly 13
48
seconds were recorded by the National Data Buoy Center buoy number 42003, located
approximately 210 nautical miles west of Manatee County. The Corps of Engineers (1996)
reported that Hurricane Opal produced an estimated storm surge of 0.3 1.0 m (1-3 feet),
strong winds and wave action. These conditions, combined with two higher than normal tidal
events resulted in waves overtopping the beach berm, flooding the back area of the project
and transporting sand to the back beach or offshore. Based on observations, the shoreline
retreated an average of approximately 9.1-15.2 m (30-50 feet). The Corps also reported that
the southern area of the project between Monument R-24 and R-33, was particularly affected.
3.2.3 Data Sources
Various monitoring surveys have been conducted on Anna Maria Key. The data used
in this study are listed in Table 3.9 All the dara are included in the State of Florida, DEP
database which is available on the internet at the website www2.dep.fl.us/water/beaches/.
Table 3.9
List of Survey Data Used for Anna Maria Key.
Date Source Average Offshore Data Description
Extend of Data (m)
December 1992 DEP' 680 Beach Profiles
August 1993 DEP 740 Beach Profiles
October 1993 DEP 900 Beach Profiles
February 1994 DEP 700 Beach Profiles
May 1994 DEP 820 Beach Profiles
February 1995 DEP 320 Beach Profiles
July 1997 DEP Shoreline Position
February 1998 DEP 50 Beach Profiles*
Department Of Environmental Protection
SWading Profiles
49
The list presented above does not encompass all of the available data for the study
area, but encompasses only the recent data set for the Manatee County, Shore Protection
Project.
It must be noted that monuments of some survey lines were moved during the years
from their original position (12/1992). All the measurements have been referred to the
monument positions of December 1992.
The data listed above were used to compute MHW shoreline changes as well as
volumetric changes. The tidal data for Anna Maria Key are listed in Table 3.10, (Balsillie,
1987).
Table 3.10
Tidal Data for Anna Maria Key, (Balsillie, 1987).
Datum Elevation above NGVD in feet (m)
Mean Higher High Water (MHHW) +1.44 (+0.439)
Mean High Water (MHW) +1.11 (+0.338)
Mean Tide Level (MTL) +0.40 (+0.122)
Mean Low Water (MLW) -0.31 (-0.094)
Mean Lower Low Water (MLLW) -0.80 (-0.244)
3.2.4 Sand Characteristics
In 1988, the Jacksonville District of the U.S. Army Corps of Engineers collected
surface samples on 10 profile lines spaced 915 m (3,000 feet) apart along the project area.
Samples were collected along each profile line at three-foot increments of elevation above
the MLW, and five-foot increments below the MLW. Also in 1988, The Corps drilled twenty
five borings in the principal alongshore borrow area. The composite mean grain size of the
50
native beach and borrow area are 1.47 4 (0.36 mm) and 1.76 4 (0.30 mm), respectively. The
two composite distributions are poorly sorted with a composite sorting value of 1.47 for the
native sand and 1.51 for the borrow area sand. The mean value of the visual estimate of the
shell content for both the native and the borrow area samples was 27 percent and 21 percent,
respectively. Subsequently, an acid treatment was applied to all the samples in order to
determine the contribution of the carbonate fraction (shells) to the grain size distribution.
After removing the carbonate fraction, the composite mean grain size and the sorting were
2.52 ) (0.17 mm) and 0.47 (well sorted) for the native sand and 3.01 4 (0.12 mm) and 0.46
(well sorted) for the borrow area sand. The shell content was found to be 27% by weight for
the native sand and 25.8% for the borrow area sand. Figure 3.16 shows the composite
cumulative mean grain size distributions for the native beach and the principal alongshore
borrow areas before and after the acid treatment.
Figure 3.16 Grain Size Distribution for Native and Borrow Area Sand
With and Without Carbonates.
L -.- - - i -- i
Wih a---Wi--outCarbonates.
It must be noted that the ratio between the mean grain size of the borrow area and the
native sediment is 0.70, which results in a greater volume of fill in order to construct the
design berm width.
3.2.5 Beach Profiles
Comparative profiles of the area of interest have been plotted at each Monument and
are presented in Appendix A. The slope of the constructed beach was on average 1:11 from
the berm to the MLW shoreline position, and 1:27 from MLW shoreline position to the
intersection with the existing bottom profile. Figure 3.17 presents the average profile pre-
(12/1992) and post-nourishment (8/1993) along with the 2/1995 survey. After 1.5 year more
than 3/4 of the original beach width remains in place. It is interesting to note that after 1.5
year, the slope of the upper part of the profile (from the berm to -2.0 m) is steeper than the
pre-and post construction slope, followed by a trough at approximately -2.0 m depth and
again by a steeper slope in the lower part of the profile reaching the limiting depth of
motion, which is on average 6.0 m.
In most of the profiles it has been seen that a portion of the sand has migrated from
the foreshore seaward to the shore face as a result of the profile equilibration process, causing
the expected recession of the shoreline.
It is of interest to note that the mean grain size of the sand used to nourish the 4.2
mile beach of Anna Maria Key (0.12 mm) was smaller than the mean grain size of the native
beach (0.17 mm); therefore, a milder slope of the profiles would have been expected after
1.5 year, both in the foreshore and in the shore face.
100 150 200
Offshore Distance (m)
Figure 3.17 Comparison Between Pre- (12/1992) and Post-Nourishment (8/1993)
Average Profiles.
3.2.6 Historical Shoreline Changes
Dean et al. (1998) calculated the long term (1874-1974) and short term (1974-1986)
average shoreline change rate for Anna Maria Key, which are -0.36 m/year and +0.90
m/year, respectively; considering only the shoreline segment included in the nourishment,
the average shoreline change rates are -0.61 m/year and -0.13 m/year.
Figure 3.18 presents the shoreline changes after the construction of the shore
protection project, where the shoreline position of August 1993 has been taken as a
reference, avoiding in this way the effects on the shoreline due to the several coastal
structures (groins, seawalls, and revetments) existing before the nourishment.
25
DNR Monument No.
Figure 3.18 Shoreline Position for Different Periods Relative to August 1993.
The 1998 data show that in the project area (R-12 to R-35) after 4.5 years the
shoreline receded on average 17.1 m. The average shoreline recession of the center segment
of the nourished area (R-20 to R-26) is 8.2 m, while those of the northern (R-12 to R-19) and
of the southern part (R-27 to R-35) are 20.0 m and 23.0 m, respectively. Greater shoreline
recession on the extremities of the project is in qualitative agreement with the planform
evolution theory. Table 3.11 summarizes the shoreline changes and the shoreline change rate
for the different periods.
Table 3.11
Average Shoreline Change Rate from 8/1993 for Different Beach Segments and Different
Periods.
Time Average Shoreline Change Rate (m/year)
from R-9 to R-41 from R-9 to R-12 from R-12 to R-35 from R-35 to R-41
Project Area
10/1993 +36.5 +27.7 +42.2 +27.8
2/1994 -6.8 +1.9 -7.4 -6.6
5/1994 -5.1 +4.5 -6.8 -0.3
2/1995 -4.6 -1.6 -5.1 -2.5
7/1997 -1.9 +0.9 -3.4 +2.7
2/1998 -2.9 -1.5 -4.0 +0.9
Note that + is accretion while is erosion.
Some observations are evident from the above table. The Anna Maria Key shoreline
has experienced a surprising accretion in the period between August and October 1993,
resulting in a potential accretion rate of 36.5 m/year in the whole area from Monument R-9
to Monument R-41. After this initial anomalous behavior, the average shoreline change rate
of the project area follows a consistent negative trend from higher to smaller values, as
expected. The reduction of the erosion rates with time is principally due to two different
factors, (1) the equilibrium profile process and (2) the diffusion process at the boundaries of
the nourished area.
In the beach segments adjacent to the project area (from R-9 to R-12, and from R-35
to R-41), the shoreline has experienced alternating recession and accretion. Accretion would
have been expected due to the spreading losses from the nourishment. Generally, all the
shoreline changes, with the exception of those of 10/1993, seem to be nearly symmetric
55
relative to Monument R-23 which is located approximately at the middle of the nourished
area, according to the expected beach planform response. The shoreline changes are quite
small in the center of the project due to the small planform gradients.
Average shoreline change rates have been calculated for the project (Figure 3.19,
Panel b) and adjacent areas (Figure 3.19, Panel a and c). Initially, in the nourished area, the
average shoreline change rate decreases with time (according to the beach fill response
theory), while in the last period from July 1997 to February 1998 it increases again, but is
still negative (recession). On the other hand, in the adjacent beaches the average shoreline
change rate fluctuates from positive to negative values, possible due to seasonal effects.
3.2.7 Volumetric Changes
The Corps of Engineers (1991) suggested a divergent littoral drift regime for Anna
Maria Key with the existence of a nodal point approximately in the middle of the barrier
island shoreline. However, inspection of one of the permeable groins located near the nodal
point showed that a southerly littoral drift was dominant. An exception to this is the northern
part of the island. Walton (1976) estimated a weak littoral drift directed toward the south.
The volumetric changes for the period between December 1992 and February 1995
have been calculated based on profile changes, while those between February 1995 and
February 1998 are based on shoreline changes. The computations for these profiles measured
in the area where the alongshore borrow area is located have been carried out to the limit of
the dredged area due to the project construction, assuming that at that point the cross-shore
sediment transport is small. Table 3.12 lists the volumetric changes for different periods and
littoral segments.
Table 3.12
Volumetric Changes for Different Periods.
Date Volumetric Changes from R- Volumetric Changes from R-
9 to R-41 (m') 12 to R-35 (m)
12/1992 8/1993 +1,770,000' (+83,000)" +1,737,200' (+50,200)"
8/1993 10/1993 +161,300 +114,300
10/1993 2/1994 -329,400 -295,500
2/1994 5/1994 +355,000 +251,100
5/1994 2/1995 -220,200 -210,700
8/1993 2/1995 -33,300 -140,300
SBeach Fill, 1,687,000 m3
SDifference Between Measured and Placed Volumes.
It is seen that the volumetric changes oscillate with time. The area has experienced
subsequent periods of accretion and erosion. The volume oscillations suggest possible
systematic survey errors.
Figure 3.20, panel a, shows the cumulative volumetric changes from 1993 to 1995.
Their fluctuations vary from positive to negative values up to a magnitude of 20% of the
original beach fill. These fluctuations may be related to seasonal effects, since during the
winters the area experiences erosion while during the spring and summer it recovers. This
particular behavior would be expected of the shoreline changes but unlikely of the volumetric
changes. During the winter seasons the sand is forced by the more energetic waves to migrate
from the beach face to the shore face region to form a bar, but the sand should still remain
within the active profile system. Figure 3.20 panel b indicates that for the surveyed period
from August 1993 to February 1995, the area within the original project lost an estimated
140,300 m3 of sand. This represents approximately 8% of the original beach fill.
From R-9 To R-12
E40
20
10 ccr. Wo
*10- V
-20 -
S1993 1994 1995 1 19996 7 1998 1999
From R-12 To R-35 (Project Anm)
40 \ -- -
10 .. ,
-20 .
193 1994 1995 1996 1997 1998 1999
From R-35 To R-41
20-
1 0 .- . ..... . ..... . . . ...". .. . .. ...e .. -
S-10 O i -
S193 1994 1995 1996 1997 1998 1999
Time (years)
Figure 3.19 Shoreline Change Rate for Different Time Periods and Littoral Segments.
The erosion rate associated with this volume losses is 93,500 m/year, which is 65%
greater than the estimated renourishment rate (56,500 m3). As of February 1995, the project
has performed very well in terms of volume losses. It is seen also that most of the volume
loss occurs in the southern half of the project site, adjacent to Longboat Pass, which can
represent a sink of sand for the littoral system.
Figure 3.21 presents the cumulative volumetric changes between 1995 and 1998
based on shoreline changes and not on the actual volumetric changes at each profile. A berm
height of 1.52 m and a depth of closure of 6.0 m have been used. It is seen that from 1995
x105
From R-12 to R-35
3
2 P aG e K ey .I d ..... ... .. ....... ......... ..... .. ... ... .. ..... .. .... .........
.,-.
S ... -....... ....... .. ......... . ..-. .... ...-....... .................... ....... - 4 ..... ........
0
-1
----- 81199310 101993
+---- 101993 to 21994
2/1994 to 5/1994
----* 5/1994 to 2/1995
-3
5 10 15 20 25 30 35 40 4!
DNR Monument No.
x 10
.5 ..... ........ .... ......... .. ........ ... ........ T ... .... .... ... .............
PamsgeKe eyInlet Lngboat Pao
S ... ... ... .. ............ ..... .. ....... .. . ... .. ............ I. .. ...........
. .. .. .. I .. .. ........ ... .. . ...... : ::......... .......... ...... . .......... I ............ ... ...................
.5-
i -i
n __ *---... ...
1.5
-1
5 10 15 20
25
DNR Monument No.
30 35 40 45
Figure 3.20 Cumulative Volumetric Changes in the Project Area from 1993 to 1995.
(Panel a), for Each Survey Time Interval. (Panel b), Relative to August 1993.
ci
E
0
6
> _
E
-
---- 8/1993 to 10/1993
+ --- 8 993to 2/1994 ............................ . ............. .......... .............. .............
8/1993 to 5/1994
*-- 8/1993 to 2/1995
1' ' 'I
-
59
the middle segment of the nourished area (form R-17 to R-25) does not experience particular
erosion, as do the two extremities. In July 1997, the volumetric losses reached almost
290,000 m3 while in February 1998 the losses were approximately 550,000 m3. The total
volume losses from August 1993 to February 1998 (4.5 years) are approximately 690,300
m3 (41% of the original fill placed), corresponding to an overall erosion rate of 153,400
m'/year, which is greater than the estimated renourishment rate.
It must be noted that the volume losses based on shoreline changes seem to be an
overestimation. A significant percentage of the measured shoreline changes, especially in the
time period immediately after the nourishment, are not due to volume losses but due to
profile equilibration. As of February 1998, 59% of the volume of sand placed in the original
shoreline segment is still in place.
Volume Changes between 2/1995 and 2/1998, from R-12 to R-35
25
DNR Monument No.
Figure 3.21 Cumulative Volumetric Changes in the Project Area Between 1995 and 1998.
60
3.2.8 Time Scale of Beach Profile Evolution
An estimation of the decay parameter k have been done for the linear and non-linear
(m=2, m=3) cases. Figure 3.21 shows the best fitting curves of the average measured MHW
shoreline changes determined by using the least squares method and it can be noted that the
equilibration process is rapid during the first years (especially for m=3) and then decreases
with time after the perturbation has been smoothed by the natural processes. The different
values ofk and time scales required for an equilibration of 50% to the native profile (ts5.)
have been summarized in Table 3.13.
Table 3.13
List of the k and t,,s Parameters for Different Values of m.
Linear Solution Non-Linear Solutions
m=l m=2 m=3
k (years-) 0.0913 0.0020 0.0027
tso. (years) 7.6 8.8 9.9
It is seen that the non-linear solutions related to non-linear transport relationships will
require more time (16 to 30 percent) in order to reach half profile equilibration. It is
encouraging that equilibration time scale for Anna Maria Key is approximately the same as
those determinated for the Dade County Project.
61
Average Shorelne Changes (MHW) from R12 to R35
100
m = 1, k= 0.09128 yr1
95 -- m = 2, k = 0.00201 yrl -
S m = 3, k = 0.00268 yr1
90
o ........ .... : -
75 .\
SN
70
65 .
60
1993 1994 1995 1996 1997 1998 1999
Time (year)
Figure 3.22 Average MHW Shoreline Changes (Best Fitting Curve) for Anna Maria Key,
Shore Protection Project for different values of m.
CHAPTER 4
HOT SPOT IDENTIFICATION, MITIGATION, AND AVOIDANCE MEASURES
A potential erosional hot spot (EHS) is defined as a local area which erodes more
rapidly and/or equilibrates with a significantly narrower beach width compared to the
adjacent beaches. It has been seen in Chapter 2 that many hot spots have been identified
within different beach nourishment projects along either the Gulf or Atlantic coast of Florida.
They are often located within beach nourishment projects which, globally, have performed
well. In this chapter some criteria to identify the existence and the causes of EHS will be
discussed. In conclusion two study cases will be analyzed based on these criteria: the Dade
County and Manatee County beach nourishment projects.
4.1 Analyses for Identification of Erosional Hot Spots
The identification of an EHS is based on historical shoreline and volumetric change
analysis. The interpretation of the results of both analyses can lead to better understand the
causes and the possible remedies to the EHSs.
4.1.1 Historical Shoreline Changes
The analysis of historical shoreline changes is a useful tool during the monitoring
phase to identify areas where potential erosional hot spots have occurred. If the erosion rate
of an area is some percentage greater than a reference value, s, characteristic of the entire
project, this area will be qualified as a potential EHS. A question occurs in defining this
reference value. When the data set available for the area of interest is complete, the reference
value could be chosen as the rms of the measured shoreline changes, Ay,,
2
S(Aym ) (4.1)
n
in which n is the number of measured data. If the available data are incomplete, s can
alternatively be computed considering the shoreline changes predicted by planform evolution
models (one-line model). In this last case, in order to avoid an under or over estimation of
the s value, an adjustment of the shoreline changes due to the cross-shore equilibration
process needs to be done (see paragraph 3.1.8). Note that in order to calculate the reference
value s, shoreline change rates can also be used. The same procedure utilized to identify EHS
can be followed to identify cold spots.
The analysis of historical shoreline changes is also a useful tool during the beach
nourishment design process in order to identify those areas that historically have greater
erosion rates, therefore those areas which normally tend to erode faster than others.
64
Note that shoreline recession is the most intuitive parameter used to recognize the
presence of an EHS, especially by the public.
4.1.2 Historical Volume Changes
The historical volume change analysis is conducted comparing beach profiles of
different years, and is extended to the entire nearshore zone from the dry beach to the depth
of limiting motion. After the construction of a beach nourishment project, an analysis of the
volumetric changes gives an estimate of the project performance regardless of the
distribution of the placed volume across the profile.
A similar analysis, as described for the shoreline changes, can be conducted for the
volumetric changes. Those areas characterized by volume change rates of some percentage
greater than a reference value, v, characteristic of the entire project area will be qualified as
an EHS or a cold spot, depending on whether the volume change rate is negative or positive,
respectively. The reference value v can be defined as in Eq. 4.1, where Aym are now the
measured volume change rates Av,. Note that no adjustment of the v value needs to be done
in order to account for the cross-shore profile equilibration.
4.2 Criteria to Identify the Causes of Erosional Hot Spot
A comparison between hot spots through shoreline change analysis and those
identified through volumetric change analysis can be conducted, and three cases can occur
(see Table 4.1), which are discussed below.
Table 4.1
List of Possible Combination of Erosional Hot Spots.
Shoreline Changes.
SVolume Changes.
4.2.1 Casea
This is the case when the presence of an EHS in a littoral segment has been identified both
by shoreline change and volumetric change analysis. If the presence of a cold spot has been
observed in areas adjacent to the EHS, the possible causes could be: "spreading out losses"
at the ends of a nourished area (sub-case a-1), presence of a borrow pit close to the shoreline
(sub-case a-2), or residual bathymetry after the construction of the nourishment project (sub-
case a-3). These sub-cases are summarized in Table 4.2.
Table 4.2
List of the Possible Causes of Case a Hot Spots.
Sub-case Possible Causes Method of Study Remedies
"Spreading Out Losses" at Planform Evolution Anticipate in
a- the End of a Nourished Area Models Design
Borrow Pit Located Inside Wave Refraction Choose Borrow
a-2 the Nearshore Zone (Too and Diffraction Pits Farther (and
close to the Shoreline) Models Deeper) Offshore
Avoid the Residual
Residual Bathymetry Post- Wave Refraction Bathymetry
Nourishment Construction Models (Uniform Volume
Density)
S.C.
S Yes No
V.C."
Yes Case a Case c
No Case b -
*
A,
----
66
Sub-case a-1 can be analyzed using analytical or numerical computer models which
predict the evolution of the nourished planform area and also predict the effects on the
adjacent areas due to the diffusion processes of the beach fill. A comparison between the
predicted and the measured planforms can show if the EHS, and consequently the cold spots,
were due to the normal response of the shoreline to the diffusion process, which takes place
under the action of the waves. This study is particularly helpful if different nourishment
projects have been constructed at different times in the same area of interest. Various
numerical models are available to conduct this studies. The "one-line" model DNRBS
(Department of Natural Resources, Beaches and Shores) developed by Dean and Grant
(1989) and described in Appendix B has been applied to two study cases and the results will
be presented later in this Chapter. Note that the magnitude of the EHS and cold spots
generated by this mechanism may decrease in time, once the anomaly is smoothed by the
action of the waves.
The effects of the presence of a borrow pit inside the nearshore zone (sub-case a-2)
have been largely discussed previously in Chapter 2. A change of the wave front orientation
results as well as of the wave energy distribution along the beach through wave refraction
and diffraction (change in wave set up distribution). Usually a formation of a salient has been
observed in the lee of the borrow pits and of hot spots in the adjacent areas (see Figure 2.2).
Wave refraction-diffraction models can be employed to better analyze the impacts of borrow
pits on the shoreline. A possible and simple remedy is to chose borrow areas outside the
nearshore region where their impact on the shoreline is minor. This may result in a greater
initial cost of the nourishment project, but will reduce renourishment costs successively.
67
Wave refraction models can also be employed to analyze EHS and cold spots due to
residual bathymetry (sub-case a-3). A remedy to this type of EHS is to exercise more care
when the beach fill material is placed on the project area, probably reducing the distance
between each pipe extension and trying to place a uniform amount of material along the
beach.
4.2.2 Case b
This is the case in which the presence of an EHS has been identified through
shoreline change analysis, whereas the results of the volumetric changes for the same littoral
segment show a stability. The possible causes could be a non uniform sediment size
distribution (sub-case b-1), the presence of coastal structures, like seawalls and groins (sub-
case b-2). These sub-cases are summarized in Table 4.3.
Table 4.3
List of the Possible Causes of Case b Hot Spots.
Sub-case Possible Causes Method of Study Remedies
Plot Sediment Size
Sediment Size Smaller lot Add More Sand of
b-1 Distribution Along
b- Than Adjacent Areas strict Alog Compatible Size
the Project Area
Coastal Structures, Seawalls Profile Add More Sand of
b-2 and/or Groins Configuration Compatible Size
In sub-case b-1, if the design parameter of the nourishment project is the volume fill
density (constant along the beach), the resulting beach width after cross-shore equilibration
will be narrower. Conversely, if the design parameter is the beach width, the resulting
68
volume fill density will be greater than in the adjacent areas with coarser sand. Probably
during this process, a residual bathymetry will be formed, which illustrates the interaction
of different causes of hot spots. A simple remedy is to add more compatible sand in those
areas characterized by smaller sediment size in order to reach, after equilibration, the same
beach width of the adjacent beaches. The application of a refraction model is suggested to
determine if the residual bathymetry has a significant impact on the shoreline evolution.
As discussed in Chapter 2, coastal structures, like seawalls and groins, can be
employed to stabilize the shoreline, however, the volume losses continue resulting in deeper
and steeper profiles. The littoral segments impacted by the presence of the structures could
exhibit hot spots if no measures of overfill are taken during the design and construction
process. In order to avoid that, a complete analysis of the beach profiles should be conducted
before nourishing, to localize those areas impacted by coastal structures. Also it is possible
to estimate the additional amount of volume required in those areas in order to create a stable
beach of the desired width.
4.2.3 Case
This is the case when the presence of an EHS has been observed only through
volumetric changes, and the shoreline is substantially stable. The possible causes are
summarized in Table 4.4.
Table 4.4
List of the Possible Causes of Case c Hot Spots.
Sub-case Possible Causes Method of Study Remedies
Profile
c-1 Seawalls fi Nourish
Configuration
Shoreline and volume change analyses conducted in areas where seawalls are present
can also show a stable shoreline position while conversely volume losses occur. The
shoreline position is maintained stable by the seawalls while the erosion process still takes
place, deepening the profiles. This case is similar to the case analyzed in the previous section
but occurs during a different time period, before the construction of the beach nourishment
project.
4.3 Other Analysis Methods
4.3.1 Historical Shoreline Analysis
In areas with a more developed tourist industry, hotels and recreational facilities were
often extended seaward of the MHW line. The "constructed line", which is considered as the
boundary of the developed areas, can be determined from surveys and/or from Coastal
Construction Control Line (CCCL) aerial photos. The average historical mean high water
shoreline is computed by averaging the positions of the shoreline before construction of
hotels and facilities (1869 to 1936) and then compared with the "constructed line". In this
way it is possible to determine those areas which have been developed seaward of the
historical mean high water shoreline. These area represents projection into the littoral system
70
which tend to be smoothed by the waves and will be characterized by greater erosion rates
(EHS) than those of the adjacent areas.
4.3.2 Breaking Wave Energy Density and Angle
Waves force the sediments to move, changing in time the configuration of the
shoreline. Utilizing numerical computer models, the average characteristics and alongshore
distribution of the parameters of the breaking waves (height and direction) can be estimated.
Also, the average alongshore breaking wave energy density distribution can be computed.
Those areas characterized by a particular high value of energy density may be defined as
potential erosional hot spots. In this phase, an important role is played by the offshore
bathymetry (residual bathymetry) through the wave refraction and diffraction process,
especially, when dredged holes were created offshore of the project area.
In addition to the breaking wave energy density, the potential longshore sediment
transport distribution can be computed utilizing the CERC formula expressed in terms of
breaking wave conditions. The sediment transport results may be interpreted to predict
shoreline behavior. The gradient of the potential longshore sediment transport is a measure
of its acceleration. For example, in those areas where the potential sediment transport
decelerates, accretion would be expected, while in those areas where it accelerates, erosion
would be expected and depending on the magnitude of the acceleration, hot areas can be
individuated.
4.4 Case Studies
Two case studies of hot spots are presented below: those in the Dade County, and
Manatee County projects. Different criteria have been applied in order to identify the
existence, and the causes of erosional hot spots after the construction of the beach
nourishment projects.
4.4.1 Dade County Project Hot Spots
Figure 4.1 shows the shoreline change rates over to the 1980-1996 period. The
reference value of erosion/accretion rate, s, is represented by the dotted line which has been
calculated following the criteria discussed in paragraph 4.1.1, and has been found to be equal
to 2.6 m/year. All the areas having erosion and accretion rates greater than s, in absolute
value, qualify as EHS and ECS, respectively. Based on this criterion the presence of three
erosional hot spots has been observed (1) from monument R-54 to R-60, (2) from R-33 to
R-37 and (3) from R-49 to R-52.
Figure 4.2 presents the volume change rates relative to the 1980-1996 period. The
reference value, v, of erosion/accretion rates per unit beach width, in terms of volumes, is
represented by the dotted line, and it has been obtained as discussed in paragraph 4.1.2 and
it has been found to be equal to 11.4 m3/m/year. Since a phase of the beach nourishment
project (from R-65 to R-74) was constructed in between the two surveys, the reference value
DEP Monument No.
Figure 4.1 Dade County. Hot Spot Locations Based on Shoreline Change Analysis
for the 1980-1996 time period.
100.0
-------1980 to 1996
.......R reference V lue,v
80.0
~40.0
i 20. 0
S........ ____________
Hot Spot 1.4m'/mln i r
-40.0 --
DEP Monument No.
Figure 4.2 Dade County. Hot Spot and Cold Spot Locations Based on Volume Change
Analysis for the 1980-1996 time period.
73
of v has been computed neglecting the data from R-65 to R-74 which are considered too
large. The areas having greater erosion/accretion rate than the reference value, which could
be considered as EHS or cold spots, are shown in Figure 4.2; these areas are, (1) in the
vicinity of Monument R-30, (2) from R-34 to R-36, and (3) in the vicinity of R-50, and (4)
from R-58 to R-59.
Figure 4.3 presents the shoreline change rates for the time period between 1992 and
1996. A running average considering five consecutive points has been done to smooth the
shoreline change values. Applying the same method used above a reference value of s equal
to 2.3 m/year has been found. Three EHS have been localized, (1) from R-32 to R-40, (2)
from R-48 to R-52, and (3) from R-55 to R-59. A cold spot has also been individuated from
R-62 to R-67.
4.0 1 ------------,-------------________ ____ ______
2.3 rnVy r Cold Spt o
3.0
........ ............ ....................................... ............ ....3 ) 3 i ....... .
2.0
1.0
0.0
-.- 1992 to 1996
S.......-- Reference Value. s
-2.0
-3.0
0 H -2.3 m/ye r
-4.0 ---- -------
DEP Monument No.
Figure 4.3 Dade County. Hot Spot and Cold Spot Locations Based on Shoreline Change
Analysis for the 1992-1996 time period.
74
Comparing the locations of the EHS identified by shoreline and volume change analyses,
they seem to match fairly well, except for the EHS located in the vicinity of Monument R-30,
which has been observed only from volume change analysis. The presence of coastal
structures in this area was particularly significant.
The numerical model DNRBS, discussed briefly in Appendix B, predicts shoreline
changes in the vicinity of a beach nourishment project. A slightly modified version of the
DNRBS model also includes the effects of subsequent multiple nourishment projects on the
same area. The DNRBS model was applied to the approximately 15 km long beach, between
Bakers Haulover Inlet and Government Cut.
In the time period from 1980 to 1996, an average of approximately 480,000 m3/year
was placed in the study area as part of the beach erosion control and hurricane surge
protection project and also, as part of the renourishment program. The DNRBS model
assumes straight and parallel shoreline and contours, and that the nearshore system has
approached a near equilibrium, the details of which present modeling techniques cannot
represent adequately (Dean and Yoo, 1989). The beach fills were placed on the idealized
1975 shoreline which is believed to be in a near equilibrium. The representative wave
characteristics for the site of interest are those recommended by Dean and Grant (1989):
effective deep water wave height, Ho = 1.0 ft (0.3 m), effective wave period, T= 6.0 s, and
ac = a, = 870. Moreover, = 1800, and a sediment transport coefficient ofK=l.1 were
chosen. The area of interest is bounded on the north and south by inlets, which are stabilized
by long jetties. At Bakers Haulover Inlet and Government Cut, it was assumed that the
sediment transport, was zero.
75
Wave refraction and diffraction processes are neglected by the numerical model, thus
the wave angle is constant along the entire project area. Figure 4.4 presents a comparison
between the 1996 measured and predicted shoreline positions relative to 1980. A general
agreement can be noted in the area between Monument R-48 and R-65, and also between R-
33 and R-45. Note that the one-line model does not account for cross-shore profile
equilibration, so the predicted shoreline changes should be adjusted for the recession due to
the cross-shore equilibration. In the southern area, from Monument R-65 to Government Cut
a significant accretion has also been predicted but with a different distribution. The Phase
5 nourishment was placed in this time period from Monument R-65 to Monument R-74.
Conversely, in the northern area, the predicted and measured 1996 shoreline position
experienced erosion since no sediment supply is allowed to bypass Bakers Haulover Inlet
and the waves generates a southern directed sediment transport.
The same analysis has been conducted in terms of volume changes. Figure 4.5
compares the 1996 volume change rates measured with those predicted, obtained
multiplying the shoreline changes by the depth of closure. Note that the differences between
the predicted and measured volume change rates are less respect to those of shoreline
changes; this is due to the fact that no adjustment is required for the cross-shore profile
equilibration process. Qualitatively agreement can be observed from R-55 to R-63, and from
R-36 to R-43. Accumulation of sand can be observed at the southern extremity of the project
area (Government Cut) due to the impoundment on the long jetty and the prementioned
nourishment, while erosion is predicted in the northern boundary due to lack of sediment
N
200.0
CI \
o -
050.0 -
for the 1980-1996 Time Period. 1980 to 1996 ed
-5
Figure 4.4 Dade County. Comparison Between the Measured and Predicted (DNRBS) Shoreline Changes
for the 1980-1996 Time Period.
N
I ~ o
"D .. o
CO
100
, 80 -..
S-- 1980 to 1996 predicted
E 60 --- 1980 to 1996 measured ;-
40
40-----------------------------/ "-"" --. ,
E. /\.
g 20---'
" *- -, -40DO """'62b 4 0) . [)0"'"'' . ----'op"100 .." 12000 14000
o -2 - ^ -- --- --------- ^ ----------------
4 -20 0"- ---------"----
-40
Longshore Distance from R-27 (m)
Figure 4.5 Dade County. Comparison Between the Measured and Predicted (DNRBS) Volume Change
Rates for the 1980-1996 Time Period.
Rates for the 1980-1996 Time Period. ,
78
supply. Globally, the results obtained from the application of the DNRBS model seem to be
questionable due to various uncertainties, as for example the wave direction relative to the
normal of the shoreline, and the sediment transport rate coming in and out, respectively, from
the northern and southern boundaries.
The sediment size distribution along the project for different water depth is plotted
in Figure 4.6 (Charles, 1994). The average sediment size decreases gradually from
approximately 0.47 mm at R-30 to 0.29 mm at the vicinity of Government Cut. It can be
noted that the sediment size distribution of the NGVD waterline presents a peak between R-
52 and R-62, which could suggest that greater waves with higher associated wave energy act
on that beach segment and that the finer sediments have been winnowed out.
30 35 40 45 50 55 60 65 70 75
DEP Monumnt No.
Figure 4.6 Dade County. Alongshore Sediment Size Distribution for Different Beach
Contours. (Based on Sampling Along Eight Profiles (Charles, 1994)).
79
Since the project area has a very high tourist value, many hotels and recreational
facilities were built very close to or seaward of the MHW contour line. The solid line
(reference) of Figures 4.7 and 4.8 is the average MHW shoreline position relative to the
period before hotels and facilities were constructed (1867 1936), while the dotted line
represents the seaward boundary of the area where the hotels and facilities are now located.
It is evident that the "construction line" is seaward of the reference shoreline position
between Monument R-30 and R-36, and between R-56 and R-60. Nourishment in the vicinity
of these projections will be smoothed by wave and current action, and will be characterized
by greater erosion rates.
Several EHS and ECS were observed along the Dade County beach. It seems
necessary to conduct a refraction study in order to obtain the breaking wave distribution, and
therefore the breaking wave energy density distribution along the area of interest, in order
to identify those areas which are characterized by higher wave action. Probably the reference
value to use, could be calculated in the same way is calculated for shoreline and volume
changes.
4.4.2 Manatee County Project Hot Spots
The shoreline changes relative to the period from 8/1993 to 2/1998 have been plotted
in Figure 4.9. The August 1993 shoreline position (post-nourishment) has been taken as
reference. The dashed line represents the reference erosion rate value, s, for the area of
interest. It has been calculated as the rms of the measured shoreline changes and it is equal
to 4.6 m/year. Two beach segments have greater erosion rates than the reference value, s, and
thus are considered EHS. These areas are located approximately at the two extremities of the
80.0 ____-- --
*g 0.0. ------------- ---- ---- ---- ---- -----------
-20.0
Co.ou L n-
-20.0
-0.0
Figure 4.7 Dade County. Comparison Between the Constructed Line (1992) and the Average
o o o o 20.0, from Mon t R-27 to R-
c -40 .0 ---------------------------- --
-100.0 -----------------------------------
.... Cons..c.on "ne |
-120.0 .---- --
-140.0 -
Figure 4.7 Dade County. Comparison Between the Constructed Line (1992) and the Average
Shoreline Position (1867 to 1936), from Monument R-27 to R-37.
-10.0
- -20.0
-30.0
DEP Monument No.
Figure 4.8 Dade County. Comparison Between the Constructed Line (1992) and the Average
Shoreline Position (1867 to 1936), from Monument R-53 to R-61.
81
filled area, approximately between Monument R-33 and R-26, and between R-16 and R-12,
with a maximum recession rate of 10 m/year and 8.5 m/year, respectively. The designed
advance of the MHW shoreline was approximately 40 m If the recession rate were constant
in time, in those areas with higher rates it would take approximately from 4 to 5 years to
reach again the pre-nourishment shoreline position. Note that the high values of erosion rates
calculated includes also the shoreline recession due to profile equilibration.
Shoreline changes relative to a shorter period of time (8/1993 to 2/1995) are
presented in Figure 4.10 The reference value of erosion/accretion rate, s, has been found
equal to 6.7 m/year. Two EHS were observed, from R-32 to R-28, and from R-16 to R-13,
and their locations approximately match with those observed from volume change analysis.
Also a cold spot was observed in the southern area of the project, from Monument R-36 to
R-35. The s value relative to the shorter period of time, 1.5 year, is greater than that relative
to a longer period of time, 4.5 years, and this shows how the profile equilibration rates
decrease with time.
The same procedure has been followed for the volumetric changes. Volume change
analysis has been conducted only for the time period from 8/1993 to 2/1995, since the 2/1998
data were wading profiles. Figure 4.11 presents the volume change rate for the project area
relative to the period between 8/1993 to 2/1995. The dashed line is the reference value of
volume erosion rate, v, for the area of interest, and it has been found to be equal to 45
m'/m/year. Three EHS have been identified, and they are located approximately between
Monument R-34 and R-33, between R-30 and R-29, and in the surrounding area of R-27.
Also a large cold spot was observed from Monument R-12 to R-10.
The DNRBS model was applied to Anna Maria Key shoreline. In the time period
Co
0n
a)
b0
6
4
2
0
-2
-4
-6
-8
-10
-12
Project Area
Longshore Distance from R-41 (m)
Figure 4.9 Manatee County. Hot Spot Locations Based on Shoreline Change Analysis for the 1993-1998 Time Period.
4.6 m/ye r 8/93 to 2/98 measured
.............- ..- ......... .--- ...- ...... .....--- .........i8/93 to 2/98 m easured-.
.- - - Reference Value, s
/ 10D 2 00 3030 4000 50 0 / 603 7000 8000 9 9O106
SHot Spt .6 m/year
--_ HotSpot
Project Area
---
o
0
(Q
CO S
Co o
^.
10
S5
a
. o
m 0
0
" -5
O
o
o -10
W}
Longshore Distance from R-41 (m)
Figure 4.10 Manatee County. Hot Spot and Cold Spot Locations Based on Shoreline Change Analysis
for the 1993-1995 Time Period.
84
from 1992 to 1998 only one beach nourishment was constructed, placing approximately
1,690,000 m3 (2,200,000 yd3), which corresponds to a fill volume density of 223 m3/m (89
yd3/ft). The pre-nourishment shoreline and contours have been considered straight, assuming
that the nearshore system has approached a near equilibrium. The representative wave
conditions for the site of interest are those recommended by Dean and Grant (1989):
effective deep water wave height, Ho = 1.6 ft (0.5 m), effective wave period, T= 6.0 s, and
wave direction, aq = a, = 87. Moreover, a shoreline orientation /,= 1800, and sediment
transport coefficients of K=1.5 and 0.77 were chosen. The beach nourishment project was
constructed with finer sand than the native (0.12 mm). At the boundaries (inlets), it has been
assumed that the shoreline changes are zero, allowing sediments to come in and come out
of the study area. Figure 4.12 shows a comparison between the measured and predicted
shoreline changes from 12/1992 and 2/1998. The significant differences in the north side of
the barrier island are due to the presence of Passage Inlet. This area is subject to seasonal
fluctuations. Different beach segments seem to behave differently than predicted. These areas
are located in the surrounding area of Monument R-32, between R-28 and R-22, between R-
20 and R-16, and the northern extremity of the project area. Conversely, the southern end is
performing better than expected. The solid horizontal dark line indicates the designed
shoreline advancement due to the nourishment.
The same analysis has been conducted in volume terms. Figure 4.13 shows the comparison
between the measured and predicted volume changes relative to the time period from 2/1993
and 2/1995. Reasonable good agreement between the predicted and measured volume
changes can be noted, except in the northern part of the project area, between R-18 and R- 1
Project Area
^ ---- _-----------------------------No
Longshore Distance from R-41 (m)
Figure 4.11 Manatee County. Hot Spot and Cold Spot Locations Based on Volume Change Analysis
for the 1993-1995 Time Period.
200
150
100
50
0
-50
-100
Project Area
0
o
5O
-n
40-
E
S30-
-
0 -20.
E
0
O -10
C.
$ -20
-30
-40
Longshore Distance from R-41 (m)
Figure 4.12 Manatee County. Comparison Between the Measured and Predicted (DNRBS) Shoreline Changes
for the 1992-1998 Time Period.
(Q
o
S0
500
400
E
( 300
S200
E
* 100
0
o-.
a 0
E
0
Project Area
Longshore Distance from R-41 (m)
Figure 4.13 Manatee County. Comparison Between the Measured and Predicted (DNRBS) Volume Changes
for the 1992-1998 Time Period.
88
where the measured volumes are considerably greater that the predicted and placed.
Note that near the extreme north and south ends of the project, the effect of the
volume sink effects of the inlets are evident. The beach segments located in the surrounding
area of Monument R-T-30, between R-28 and R-26, and from R-24 to R-19 are performing
slightly worse than expected. It has been seen in Chapter 3, that the volumetric changes in
this area oscillate significantly, suggesting possible systematic survey errors.
A refraction/diffraction study seems to be necessary in order to localize those areas
where concentration of wave energy occurs, especially in the lee of the borrow pit.
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