EROSIONAL HOT SPOTS
Robert G. Dean
November 19, 1999
Florida Department of Environmental Protection
Office of Beaches & Coastal Systems
3900 Commonwealth Boulevard
Tallahassee, Florida 32399-3000
EROSIONAL HOT SPOTS
November 19, 1999
Florida Department of Environmental Protection
Office of Beaches & Coastal Systems
3900 Commonwealth Boulevard
Tallahassee, Florida 32399-3000
Robert G. Dean, Roberto Liotta and Guillermo Sim6n
Coastal and Oceanographic Engineering Department
University of Florida
345 Weil Hall
Gainesville, FL 32611
This report presents the results of a study of erosionall hot spots" (EHS) with particular
reference to these features in nourished projects. EHSs are areas that erode more rapidly than
anticipated in design or more rapidly than adjacent portions of the nourished project. Most beach
nourishment projects experience one or more EHSs. These features are important as their
remediation may require mobilization earlier than otherwise required or may focus adverse public
attention on an otherwise successful project. The twelve types of EHSs identified to date are
reviewed according to their causes. The generic causes include non uniform distribution of wave
characteristics (height and direction) along the project, nonuniform distribution of sand sizes along
the project and the effects of either natural or constructed features (structures) which result in a
difficulty in maintaining the desired beach width.
The performances of three beach nourishment projects in Florida are reviewed with a focus
on the associated locations and causes of EHSs. The projects reviewed include: the Miami Beach
portion of the Dade County Project initially nourished from 1976 to 1981, the Anna Maria Key
Project constructed in 1993 and the Delray Beach Project initially nourished in 1973. Two EHSs
were identified in each of these three projects although one of the EHSs was found to be present in
some surveys and not present in others. In two of the projects, the EHSs were located near the ends
of the projects and could be interpreted as the results of spreading losses. An alternate or contributing
explanation is that the borrow pits modify the waves and thus cause uneven longshore patterns of
wave height and direction and shoreline change on those portions of the projects which are
approximately landward of the ends of the borrow pits. For one EHS it appears that the proximity
of the borrow pit may be a contributing factor.
Systematic methods of identifying, interpreting, mitigating and avoiding EHSs are presented.
In general the best approach to avoiding EHSs is through an understanding of their potential causes
and the associated processes and to "exercise", in the design phase, the available methods of
predicting EHSs. If the analyses indicate the likelihood of one or more EHSs, the design should be
modified to the extent possible to minimize/eliminate the EHS. In some cases, the appropriate
remedy may simply be the placement of a greater nourishment volume density or the careful use of
structures. EHS causes due to construction methods should be avoided whenever possible.
Preliminary numerical modeling was carried out to evaluate the shoreline effects that would
occur due to wave reflection from a borrow pit. This portion of the investigation was prompted, in
part, by a rather remarkable documented occurrence of salients and associated EHSs which
developed in the lee of borrow areas at Grand Isle, LA and, in part, by the patterns of the EHSs at
two of the projects investigated here. Relative to the case of no wave reflection from a borrow pit,
the effects include shoreline advancement and recession on the updrift and downdrift shorelines in
the general vicinity of the borrow pit, respectively. The results would depend on the directional
distribution of wave energy including sediment transport reversals. Based on the modeling results,
it is recommended that further examination of wave reflection as a potential major contributor to
EHSs be conducted, and if found to be significant, methodology be developed to evaluate reflection
during the beach nourishment design and evaluation process.
TABLE OF CONTENTS
EXECUTIVE SUMMARY ..... ........ ....................................... ii
LIST OF FIGURES ........................................................... v
LIST OF TABLES ..... ...................................................... ix
1 INTRODUCTION ....................................................... 1
2 BACKGROUND ....................................................... 1
2.1 General .......................................................... 2
2.1.1 Dredge Selectivity ............................................ 2
2.1.2 Residual Structure Induced Slope ............................... 4
2.1.3 Borrow Pit Location .......................................... 6
2.1.4 Breaks in Bars ............................................... 8
2.1.5 Mechanically Placed Fill ........................ ........ ...... 8
2.1.6 Profile Lowering in Front of Seawalls ........................... 8
2.1.7 Headlands .................................................12
2.1.8 Residual Bathymetry ........................................ 12
2.1.9 Permanent Offshore Losses of Nourished Sediment ............... 14
2.1.10 Wave Focusing Due to Offshore Translation of Beach ............. 15
2.1.11 Wave Focusing Due to Seaward Bathymetry .................... 15
2.1.12 Borrow Pit Located Within Active Profile Zone .................. 15
3 RESULTS .................................. ........................ 18
3.1 General ...........................................................18
3.2 Case Studies ...................................................... 18
3.2.1 Dade County Project ......................................... 18
184.108.40.206 General ......................................... .18
220.127.116.11 Erosional Hot Spots in the Dade County Project ......... 23
3.2.2 Anna Maria Key ........................................... 26
18.104.22.168 General ........................................... 26
22.214.171.124 Erosional Hot Spots in the Manatee County Project ...... 29
3.2.3 Delray Beach Nourishment Project............................. 29
126.96.36.199 General Background ................................29
188.8.131.52 Erosional Hot Spots in the Delray Beach Nourishment
Project ................................ ......... .34
184.108.40.206 Sand Size Characteristics .............. .............. 40
220.127.116.11 Modeling of the Delray Beach Nourishment Project ...... 40
4 EROSIONAL HOT SPOTS: IDENTIFICATION, INTERPRETATION, MITIGA-
TION AND AVOIDANCE MEASURES ................................... 45
4.1 Identification of Erosional Hot Spots ................................. 45
4.2 Interpretation of Erosional Hot Spots .................................45
4.3 Mitigation of and/or Remedial Measures for Erosional Hot Spots ........... 48
4.3.1 Types 1 and 9 EHSs ....................................... 49
4.3.2 Type 2 EHS ............................................. 49
4.3.3 Types 3,4,8 and 12 EHSs ....................... ...... ..... 49
4.3.4 Type 5 EHS ............................................ 50
4.3.5 Type 6 EHS ............................................ 50
4.3.6 Type 7 EHS ............................................... 50
4.3.7 Types 10 and 11 EHS ....................................... 50
4.4 Avoidance of Erosional Hot Spots in Design and Construction ............. 50
5 CAUSES OF EROSIONAL HOT SPOTS INVESTIGATED HERE .............. 51
6 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS .................. 56
6.1 Summary ............................................... ...... .... 56
6.2 Conclusions ................................................ ...... 57
6.3 Recommendations .............................................. 57
7 REFERENCES ........................................................ 57
LIST OF FIGURES
2.1 Equilibrated Beach Profile With the Same Volumes of Coarser and Fine
Sediments .......................................................... 3
2.2 Example Illustrating Additional Dry Beach Width Variation With Sediment
Size and Nourishment Density. h. = 20 ft, B = 6 ft, DN = 0.2 mm, DF = 0.275
mm,D 2 = 0.2 mm,DF = 0.14 mm ........................................ 4
2.3 Effect of a Groin Field Causing a Locally Steeper Profile ...................... 5
2.4 Two Salients (ECSs) and Three EHSs Caused by Wave Transformation Over
Two Borrow Pits Off Grand Isle, LA (Combe and Solieau, 1987) ................ 6
2.5 Contours of Diffraction Coefficient for Single Pit with a/L = 1.0, b/L = 0.5, d/h
= 3, Kh = 0.167, and 0 = 0. Region Shown Is 4L Upstream and 8L
Downstream Measured from the Front of the Pit, and + 4L in the Transverse
Direction Measured from the Pit Center. From McDougal et al. (1996) .......... .7
2.6 Localized Erosion Due to Break in Bar. Note Waves Impacting Shoreline
Through Break in Bar on Left of Photograph and Wide Berm on Right of
Photograph. Fire Island, NY ............................................ 9
2.7 Schematic of Greater Wave Energy Propagating Through Break in the Bar
Causing Localized Shoreline Erosion .................................... 10
2.8 Illustration of Greater Volume Density to Achieve Design Template Through
Hydraulic Placement Compared to Truck Placement ......................... 11
2.9 Pre- and Post-Nourishment Profiles at Midtown Beach, FL. Note Presence of
Seawall Extending to a Depth of Approximately 5 ft on the Pre-Nourished
(October, 1995)Profile ............................................... 11
2.10 Illustration of Volume Required to Form Incipient Dry Beach .................. 12
2.11 Illustration of Redistribution of Sand Placed Seaward of Artificial Headland ...... 13
2.12 Effect of "Residual Bathymetry" in Depths Greater than the Closure Depth
Impressing the Form of the Residual Bathymetry on the Equilibrated
Shoreline ........................................................ 14
2.13 Illustration of Coarser Sand "Perching" a Wider Beach in the Presence of an
Offshore Reef Control ................................... ... ....... 15
2.14 Illustration of Wave Characteristics Varying with Offshore Location ............ 16
2.15 Nearshore Bathymetry Causes Non-Uniform Wave Climate Along Shoreline.
Cause of Type 11 EHS ............................................... 17
2.16 Borrow Pit Within Active Nearshore Zone Induces Sediment Transport Into
Borrow Pit. Cause of Type 12 EHS ....................... ............... 18
3.1 Dade County Location ................. ............................... 19
3.2 Borrow Site Location for Dade County Beach Nourishment Project ............. 19
3.3 Pre- and Post-Nourishment Sand Grain Size Distribution, Dade County
Nourishment Project ................................................. 21
3.4 Cross-Shore Distribution of Average Median Grain Size, Based on Sampling
Along Eight Profiles (Charles, 1994) ................................... 21
3.5 (Panel a) Annual Shoreline Change Rate for the Period 1992 to 1996. (Panel
b) Shoreline Changes from 1962 to 1996. (Panel c) Number of Years for the
Shoreline to Reach the 1962 Positions Based on 1992 to 1996 Erosion Rates
(Note: In Panel c, Only the Points Characterized by an Erosion Rate Have Been
Included) ..................... ................. .. ......... .... ..... 22
3.6 Total Volume Changes and Volumes Added Relative to 1962 Between Bakers
Haulover Inlet and Government Cut (Based on Shoreline Changes) ............. 23
3.7 Comparison Between Volume Changes per Unit Length Based on Profile
Changes and Shoreline Changes (1980-1996) ............................... 24
3.8a Dade County. Comparison Between the Constructed Line (1992) and the
Average Shoreline Position (1867 to 1936), from Monument R-27 to R-37 ....... 25
3.8b Dade County. Comparison Between the Constructed Line (1992) and the
Average Shoreline Position (1867 to 1936), from Monument R-53 to R-61 ....... 25
3.9 Manatee County Location ............................................. 26
3.10 Limits of the Manatee County, Shore Protection Project ...................... 27
3.11 Grain Size Distribution for Native and Borrow Area Sediments With and
Without Carbonates .................. ............................... 27
3.12 Bathymetry off Anna Maria Key, Showing Location of Borrow Pit .............. 28
3.13 Shoreline Position for Different Periods Relative to August 1993 ............... 29
3.14 Location Map for Delray Beach, Florida (From Beachler and Mann, 1996) ....... 30
3.15 Placement of Sand for the Different Nourishments (From Beachler and Mann,
1996) ............................................................. 31
3.16 Volumes of Sand Placed Along the Project for Each of the Nourishments ........ 33
3.17 Cumulative Volume of Sand Place Along the Project Since 1973 and Since 1974 .. 33
3.18 Location of the Borrow Area Relative to the Fill Area ........................ 35
3.19 Mean High Water Shoreline Changes Computed by Beachler (1993): a)
Between 1974 to 1990, and b) Between 1973 and 1990 ....................... 36
3.20 Volume Changes Computed by Beachler (1993) Between 1974 and 1990 ........ 37
3.21 Comparison Between Mean High Water Shoreline Changes from 1974 to 1990
and Mean High Water Shoreline Changes from 1974 to 1995 (Beachler and
Mann, 1996) ....................................................... 37
3.22 Location of Erosional Hot Spots and Cold Spots for 1975 to 1990, Using: a)
Shoreline Change Differences, and b) Volume Change Differences. The Area
Shown Encompasses the Project Limits ................................. 38
3.23 Location of Erosional Hot Spots and Cold Spots for 1975 to 1998, Using: a)
Shoreline Change Differences, and b) Volume Change Differences. The Area
Shown Encompasses the Project Limits ................................. 39
3.24 Longshore Distribution of the Sediment Size: a) After Second Renourishment,
and b) After Third Renourishment ...................................... 41
3.25 Sediment Size Variation with Time: a) After Second Renourishment, and b)
After Third Renourishment ............................................ 42
3.26 Comparison Between Predicted and Measured NGVD Shoreline Changes from
1975 to 1990 ....................................................... 43
3.27 Comparison Between Predicted and Measured NGVD Shoreline Changes from
1975 to 1998 .......................................... .......... 43
3.28 Comparison Between Predicted and Volumetric Profile Changes from 1975 to
1990 ...... ....................................... ................44
3.29 Comparison Between Predicted and Measured Volumetric Profile Changes
from 1975 to 1998 ............... ................................. 44
5.1 Miami Beach Nourishment Project and Volumetric Change Rates .............. 52
5.2 Relative Locations of Beach Nourishment Projects and Associated Borrow
Areas, Anna Maria Key and Delray Beach ............................... 53
5.3 Effect on Beach Nourishment Evolution Due to Wave Reflection from Borrow
Pit. Wave Transmission Coefficient is 0.76 Immediately Landward of Borrow
Pit. Upper Panel Presents Initial Nourished Planform, Symmetric Evolution
Without Reflection and Evolution as Affected by Reflection. Lower Panel is
Difference Between Affected and Unaffected Evolutions ..................... 54
5.4 Effect on Beach Nourishment Evolution Due to Wave Reflection from Borrow
Pit. Wave Transmission Coefficient is 0.76 Immediately Landward of Borrow
Pit and Tapers to 1.0 Within a One-Half Mile Distance Either Side of Borrow
Pit. Upper Panel Presents Initial Nourished Planform, Symmetric Evolution
Without Reflection and Evolution as Affected by Reflection. Lower Panel is
Difference Between Affected and Unaffected Evolutions ..................... 54
5.5 Anna Maria Key Beach Nourishment Project Showing Relative Longshore
Location of EHSs and Borrow Area ..................................... 55
5.6 Pre- and Post-Nourishment Profiles Anna Maria Key, Monument R-26 .......... 56
LIST OF TABLES
2.1 Possible Causes of Erosional Hot Spots .................................. 3
3.1 Characteristics of the Five Phases of the Dade County Beach Nourishment Project 20
3.2 Volume of Sand Placed in the Delray Beach Nourishment Project .............. 32
4.1 List of Possible Combination of Erosional Hot Spots ......................... 46
4.2 List of Possible Causes of Case a Hot Spots ............................... 47
4.3 List of the Possible Causes of Case b Hot Spots ........................... 47
4.4 List of the Possible Causes of Case c Hot Spots ........................... 47
4.5 Mitigation and/or Remedial Measures for EHS Types ........................ 48
EROSIONAL HOT SPOTS
Generically speaking, there are three options to coping with shoreline erosion: beach
nourishment, coastal armoring and retreat and, of course, combinations of these three. Of these
options, along a developed shoreline where recreation, storm protection and ecology are significant
factors, beach nourishment has become the alternative of choice. This alternative preserves the
shoreline in a near natural condition, provides habitats to various biological species including
threatened and endangered sea turtles, maintains the aesthetics, provides recreational opportunities
and also provides significant storm damage reduction during periods of elevated water levels and
high waves induced by extreme meteorological events. This report addresses erosional hot spots
(EHS), an aspect of beach nourishment which has only been recognized as of significant design and
performance concern within only the last five years or so. EHSs are defined as areas which erode
more rapidly than anticipated during design or more rapidly than the adjacent portions of the beach.
When viewed within the context of beach nourishment projects, erosional hot spots present two
major difficulties. The first is that the presence of a more rapidly eroding area than the remainder of
the project may require early equipment mobilization to address this local phenomenon and thus
increase the overall cost of the beach nourishment project. The second is that, although the overall
project may be performing well in terms of maintaining an average design width, the public may
focus on and interpret the erosional hot spots as an indication of generally poor project performance.
The purpose of this report is to investigate erosional hot spots in general and to carry out and
interpret case studies of beach nourishment projects within which erosional hot spots have occurred.
The intended result of this investigation is an improved understanding of erosional hot spots leading
to recommendations for identifying, remedying, and/or avoiding these features. The best basis for
anticipating and remedying erosional hot spots after their occurrence or avoiding them in the design
and construction phases in future projects is through an understanding of the processes which result
in their formation.
Due to their origins, some of the figures and tables presented in this report are in the metric
system whereas others are in the Customary Units (English) using feet, miles, etc. Quantities in the
text are usually presented in Customary Units with the metric equivalents following in parentheses.
However, when referring to quantities from graphs, for reference purposes, the units in the graph are
stated first followed by the quantity expressed in the second type of units in parentheses. Note that
1 m = 3.28 ft, 1 m3 = 1.31 yd3 and 1 m3/m = 0.4 yd3/ft.
Because of the infancy of study related to erosional hot spots, the literature is fairly sparse.
With the exception of one or two possible causes of erosional hot spots, all of the related literature
has been published in the 1990's.
In his Master's thesis, Bridges (1995) identified eight possible causes of erosional hot spots.
These were based on consideration of the associated processes, limited wave tank tests and
preliminary numerical modeling. In addition to these eight possible causes, four more have been
identified resulting in a total of twelve possible causes of erosional hot spots. Raichle et al. (1998)
examined various causes of EHSs in Broward County and presented a structured diagnostic analysis
to identify the causes) of a particular EHS. In particular offshore sand losses over or through reefs
acting as a "control" were identified. A cause, identified herein, is a different wave climate at the
more seaward location of the nourished beach. Smith and Ebersole (1997) have examined EHSs at
Ocean City, MD and concluded that wave transformation over pre-existing offshore bathymetry may
cause EHSs. Finally, the location of the borrow pit within the active sediment zone can induce
sediment flows into the borrow pit resulting in an EHS. Later sections of this report will review
screening methods to determined the most probable cause of a particular erosional hot spot. The
sections below review the twelve possible causes of erosional hot spots.
The following sections will demonstrate that EHSs can occur as a result of non-uniform wave
conditions along the shoreline, pre-existing natural or constructed structures, non-uniform sediment
sizes along the shoreline and sediment transport into a borrow pit. Table 2.1 lists the twelve
identified types of erosional hot spots and classifies them according to these four generic causes. In
cases in which the EHS is due to a localized gradient in longshore sediment transport, there will
usually be one or two adjacent erosional cold spots (ECSs) where advancement/accretion of the
shoreline is evident. Table 2.1 also indicates which of the types of EHS will have one or more
associated ECSs. The following sections discuss the mechanisms and causes associated with the
twelve individual EHSs.
2.1.1 Dredge Selectivity
This potential cause of an erosional hot spot is related to different sediment sizes placed
along a particular project during the nourishment process. Borrow areas usually contain sub-areas
of larger and finer sediments. Information describing this distribution of sediment sizes is usually
known to the dredge contractor through the general availability of the sand search results. With the
latitude for the dredger to select the use and/or sequencing of use of the sub-areas for beach
nourishment, the dredger is likely to choose to pump the finer sediment to those greater distances
of the project due to the lesser costs of pumping fine sediment and the possible avoidance of the need
to employ a booster pump. As illustrated in Figures 2.1 and 2.2, substantially more amounts of finer
sediment are required to advance the shoreline to a particular width than for coarser sediments.
Figure 2.2 is based on equilibrium beach profile concepts and illustrates the relationships between
equilibrated dry beach width versus volume density added for three nourishment sand sizes. It is seen
that for the conditions of this example and a typical volume density placed of 100 yd3/ft (250 m3/m)
and a native sand size of 0.2 mm, use of nourishment sands of 0.275 mm, 0.20 mm and 0.14 mm will
result in equilibrated dry beach widths of 205 ft (62.5 m), 105 ft (32.0 m) and 0 ft, respectively.
Table 2.1. Possible Causes of Erosional Hot Spots
Type Cause Related to Associated ECS?
1 Dredge Selectivity Sand Size No
2 Residual Structure Induced Slope Pre-Existing Structure No
3 Borrow Pit Location Wave Transformation Yes
4 Breaks in Bars Wave Concentration Possibly
5 Mechanically Placed Fill Less Fill Placed No
6 Profile Lowering in Front of Seawalls Pre-Existing Structure No
7 Headlands Pre-Existing Structure Yes
8 Residual Bathymetry Wave Transformation Yes
9 Losses Over or Through Reefs Less Fill Available to No
10 Wave Focusing Due to Offshore Wave Focusing Yes
Translation of Beach
11 Wave Focusing Due to Offshore Wave Focusing Yes
12 Borrow Pit Located Within Active Profile Trapping of Sediment No
Zone Within Borrow Pit
---- Nourishment With Finer Sediments
/ Nourishment With Coarser Sediments
k "- ",..
Figure 2.1. Equilibrated Beach Profile With the Same Volumes of Coarser and Fine Sediments.
Range for Beach Nourishment Projects
0 100 200 300 400 500 600 700 800
Nourishment Density (yd3/ft)
Example Illustrating Additional Dry Beach Width Variation With Sediment Size and
Nourishment Density. h. = 20 ft, B = 6 ft, DN = 0.2 mm, DF = 0.275 mm, Dp = 0.2
mm,DF =0.14 mm
2.1.2 Residual Structure Induced Slope
This potential cause pertains to shorelines along which groins have been constructed for
stabilization purposes. The effects of the groins are well known and include trapping sediment from
longshore transport although the erosional processes which were responsible for the installation of
the groins continue seaward of the groin ends. This results in a profile as illustrated qualitatively in
Figure 2.3. If standard methodology relating the changes in shoreline position to nourishment volume
density is used, the result will be that the required volume is under-predicted resulting in a narrower
equilibrated dry beach width than anticipated in the design. The reason is that the pre-nourishment
shoreline was held in an artificially advanced position by the groins.
AFIAN =2 12
SAFIAN .= 1.-O------ ..... .... ........ ..
......... ... .. .
Intersecting ANon-inectng A = 0.8 ...
.. . ../ "i . .7 ''. . . . . . . . .L ". . . . . . . . .:. . . .
a) Plan View
Profile With Groin Field
S .Profile Without Groin Field
b) Cross-Sectional View
Figure 2.3. Effect of a Groin Field Causing a Locally Steeper Profile.
2.1.3 Borrow Pit Location
This particular potential cause of erosional hot spots is the most studied of any of the
identified potential causes. Borrow pit locations can interact with waves and cause a modification
of the wave patterns along the shoreline and thus a nonuniform quasi equilibrium beach planform.
An example of the planform that resulted from two nearshore borrow pits at Grand Isle, Louisiana
is shown in Figure 2.4 where three EHSs and two ECSs are evident. The ECSs are located
immediately landward of the borrow pits. There are four separate wave transformation processes
which can be caused by the borrow pit: wave refraction, wave diffraction, wave reflection and wave
dissipation. Gravens and Rosati (1994) have examined the EHSs at Grand Isle and have concluded
that the relevant process is due to wave refraction which resulted in low wave energy at the shoreline
directly landward of the pits and higher wave energy adjacent to the pits. This results in larger wave
setup at the shoreline on the two sides of the two pits which causes currents to be directed from the
adjacent beaches to the areas behind the pits resulting in sediment transport and localized deposition
which forms the salients. Of course, the effect of a redistribution of wave energy along the shoreline
results only in a rearrangement of the available sand resources. Thus, associated with an erosional
cold spot landward of a borrow pit will be two erosional hot spots adjacent to this deposition as
shown in Figure 2.4. However, there is the possibility that wave reflection from and/or dissipation
due to a borrow pit will also result in an overall reduction in the wave energy reaching the shoreline.
Horikawa et al. (1977) has carried out calculations and model studies of the effects of dredged
borrow areas and has concluded that they result in shoreline advancement landward of the pits. Only
wave refraction was considered in the calculations. It is not clear how the calculations suggested
formation of a salient, since wave set-up was not considered. Motyka and Willis (1974) carried out
Figure 2.4. Two Salients (ECSs) and Three EHSs Caused by Wave Transformation Over Two
Borrow Pits Off Grand Isle, LA (Combe and Solieau, 1987).
calculations of the effects of dredged holes and predicted beach recession seaward of the dredged
holes which is the expected result if only wave refraction and normal wave incidence are considered.
Kojima et al. (1986) examined the effects of dredge pits off the coast of Japan and examined wind
and wave data, shoreline change history, offshore mining activities, monitoring of dredge pits and
sand tracers. They concluded that the offshore mining was likely a contributor to some of the
observed beach erosion. Price et al. (1978) have examined further the effects of mining offshore of
the England coast using rods driven into the sea bed for observation of bed elevation changes by
divers, tracer studies and numerical modeling. It was concluded that mining in water depths greater
than 46 ft (14 m) to 59 ft (18 m) caused little adverse effects off the south and east coasts of England.
McDougal et al. (1996) carried out numerical modeling of offshore pits and their effects on the wave
transformation processes. He found that a substantial proportion of the incident wave energy could
be diffracted and reflected seaward from the borrow pits depending on the dimensions of the pits
relative to the wave length. Figure 2.5 is a particular example from their study.
In summary, the wave transformation processes associated with a borrow pit depression are
complicated and the available documentation suggests that, landward of the borrow pits, it is
possible for either shoreline advancement or recession to occur. Only the recent publication by
McDougal et al. (1996) has considered the possible role of wave reflection by the pit. As will be
discussed later, it is probable that wave reflection has played a role in some of the EHSs examined
in this report.
Figure 2.5. Contours of Diffraction Coefficient for Single Pit with a/L = 1.0, b/L = 0.5, d/h = 3, Kh
= 0.167, and 6 = 0. Region Shown Is 4L Upstream and 8L Downstream Measured from the Front
of the Pit, and + 4L in the Transverse Direction Measured from the Pit Center. From McDougal et
2.1.4 Breaks in Bars
Along many shorelines, shore parallel bars are present. Occasionally, these bars will be
discontinuous with either reasonably narrow breaks in the bars such as might be associated with rip
currents or longer sections in which the bar is not present. These breaks in bars, in contrast to the
effects of borrow pits, allow greater wave energy to penetrate to those shorelines landward of these
breaks. This greater wave energy can cause redistribution of the sand resources along the shoreline
and an uneven pattern of shoreline change. This uneven pattern will cause erosion in the lee of
breaks in the bars and may result in deposition on either side of the breaks. Figure 2.6 presents a
photograph from Fire Island, NY of a beach affected by a break in the bar. Figure 2.7 is an idealized
portrayal of this effect.
2.1.5 Mechanically Placed Fill
Beach nourishment projects can be placed by either hydraulic means such as with a pipeline
dredge or mechanically, an example of which would be truck haul of the material to the nourishment
site. Due to the fluid nature of material placed as a slurry from a pipeline, a substantial portion of the
material thus placed flows seaward beyond the toe of the design construction template. This can
result in a significant volumetric overfill of the planned project. It is not unusual for the amount of
material actually placed by hydraulic means to exceed that required by the design template by
approximately 20% to 30%. Additionally, hydraulically placed material will generally have a smaller
void ratio than truck haul material resulting in less mass placed per unit placed volume. Thus, for
the same design template, material which is mechanically placed usually has associated with it less
volume density along the project area. In some cases projects may include both hydraulically
placed and mechanically placed nourishment in different sections of the project.
As profile equilibration of the project proceeds, those areas where mechanical placement of
the material was carried out will equilibrate with a lesser dry beach width due to the lesser
consolidated volume densities than the adjacent beaches. Thus there will be the appearance of an
erosional hot spot in those areas where mechanical fill placement occurred. Figure 2.8 illustrates
these profiles schematically.
2.1.6. Profile Lowering in Front of Seawalls
Some beach nourishment projects include sections where, prior to project construction,
erosion had occurred to the degree that no dry beach was present immediately fronting a seawall. In
some unusual cases a substantial water depth may occur adjacent to the seawall. An example is
provided by portions of the Midtown Beach project in the Town of Palm Beach prior to the 1995
nourishment where NGVD depths were 3 ft to 4 ft (0.9 m to 1.24 m) in places, see Figure 2.9. In
these cases, application of the usual design relationships between shoreline advancement and
required volumes will under estimate the volume density required resulting in a smaller equilibrated
beach width than anticipated which may be considered as an EHS. In particular, a threshold volume
is required for the formation of an incipient dry beach. Figure 2.10 illustrates this cause.
Figure 2.6. Localized Erosion Due to Break in Bar. Note Waves Impacting Shoreline Through Break in Bar on Left of Photograph and
Wide Berm on Right of Photograph. Fire Island, NY.
""~ ~C**1~4CnLU-LI*~'~ ~.-plwi~i~e~P sarry*qpg. r*X
- Diffracted Wave Front
Figure 2.7. Schematic of Greater Wave Energy Propagating Through Break in the Bar Causing
Localized Shoreline Erosion.
Truck Placement Profile
i ./-- Hydraulic Placement Profile
Design Profile Template -J
Figure 2.8. Illustration of Greater Volume Density to Achieve Design Template Through Hydraulic
Placement Compared to Truck Placement.
S--- October 1995
> :.......... April 1996
S 0 ---- August1996
.... ... .......... ...... .. . .. ........... ........ ..................... .. ..... ... .............
2 0o -. * '-, .......... . . ...... .............. .. -- .... ... ...... ; ... .. .. ... .
> -30 .-......................................
0 1000 2000 3000 4000
Seaward Distance from Monument (ft)
Figure 2.9. Pre- and Post-Nourishment Profiles at Midtown Beach, FL. Note Presence of Seawall
Extending to a Depth of Approximately 5 ft on the Pre-Nourished (October, 1995) Profile.
-Volume Required for Formation of Incipient Beach
SProfile Associated With Incipient Beach
Figure 2.10. Illustration of Volume Required to Form Incipient Dry Beach.
This potential cause of an EHS includes those situations where, through coastal armoring and
general long-term erosional processes, a portion of the shoreline has been maintained seaward of
those segments where the shoreline was allowed to recede. Thus the armored sections of the
shoreline represent artificial headlands which may extend seaward by tens of feet relative to the
adjacent shoreline. In the design of beach nourishment projects, it is natural to plan to widen the
beach, more or less uniformly, along the entire project. However, it should be recognized that
widening the shoreline fronting these artificial headlands by the same amount as the adjacent beaches
will represent an unnatural protuberance or "bulge in the shoreline which will erode more rapidly
with the sand transported to adjacent beaches. This situation of the erosion fronting the seawalls
could be represented in design or analysis as an additional beach nourishment project with the length
of the seawall and a width equal to the additional width of the nourished shoreline fronting the
seawall relative to the adjacent nourished shoreline. Thus, unless the seawall is very long, the
shoreline fronting the seawall will erode quite rapidly with the sand transferred to the adjacent
beaches resulting in one EHS and two adjacent ECSs. Figure 2.11 illustrates this situation and the
associated longshore redistribution of sediment.
2.1.8 Residual Bathymetry
Residual bathymetry pertains to the situation shown in Figure 2.12 in which the nourishment
project results in filling to depths greater than the depth of closure such that as the project evolves
both in planform and profile there will be residual bathymetry remaining in the area seaward of the
project evolution (depths greater than closure depths). Unless this residual bathymetry is comprised
of contours which are straight and parallel to the shoreline, they will influence the wave patterns
through refraction. This refraction will cause an uneven distribution of wave heights and wave angles
along the shoreline resulting in a quasi equilibrium shoreline which tends to "mimic" the shapes of
the residual contours. Dean and Yoo (1993) have shown the quasi equilibrium shoreline planform
is related to the residual contours by the relationship
- Initially Nourished Shoreline
Nourished Shoreline At Some
Figure 2.11. Illustration of Redistribution of Sand Placed Seaward of Artificial Headland.
Closure Depth, h = h.
I \ Residual Ba
I I h, h.
Figure 2.12. Effect of "Residual Bathymetry" in Depths Greater than the Closure Depth Impressing
the Form of the Residual Bathymetry on the Equilibrated Shoreline.
in which the subscripts "1" and "2" apply at the outer limit of the dredge placement and at the depth
of limiting motion, respectively and y, and y, represent the distances to the outward contour and the
shoreline, respectively and C, and C2 are the wave celebrities at the associated locations. Dalrymple
(1995) has also investigated residual bathymetry and has found similar results.
2.1.9 Permanent Offshore Losses of Nourished Sediment
Raichle et al. (1997) studied the performance of beach nourishment projects in Broward
County and concluded that the losses of sediment placed can be accounted for only if transport and
permanent losses of sand are recognized to the beach system over or through gaps in offshore, shore
parallel reefs. Under the scenario of restoring a beach to an earlier alignment and the use of
nourishment sand compatible with the native, it would appear that the original profile would be
restored and such losses would be minimal. However, if nourishment material finer than the native
were used, attempts to restore the original beach position would result in a milder beach profile with
possible associated losses over the shore parallel reef. Also attempts to advance the shoreline
seaward of its historic position with compatible sand could result in losses over seaward reefs. In
such cases, the use of coarser sand which would equilibrate to a steeper slope would reduce offshore
sand losses. Figure 2.13 illustrates the effects of coarser sand "perching" a wider beach in the
presence of an offshore reef control.
\ "- Nourishment With Finer Sediment
Nourishment With Coarser Sediment
Figure 2.13. Illustration of Coarser Sand "Perching" a Wider Beach in the Presence of an Offshore
2.1.10 Wave Focusing Due to Offshore Translation of Beach
Irregular offshore bathymetry can result in focusing of wave energy at different locations in
the nearshore region. If the beach is translated to a more seaward location, then the longshore
distribution of wave energy levels will be altered such that at one longshore location, the wave
energy may be increased and at another, the wave energy may be decreased. This effect can result
in the occurrence of erosional hot spots and erosional cold spots at locations of concentration and
reduction of wave energy, respectively. This type EHS is illustrated in Figure 2.14.
2.1.11 Wave Focusing Due to Seaward Bathymetry
Wave transformation over pre-nourishment may cause focusing and erosional hot spots that
were not apparent in pre-project conditions due to the lack of a beach adjacent to a seawall or other
shore protective structure such as shoreline stabilization by groins. Smith and Ebersole (1997) have
conducted numerical modeling of the beach nourishment project at Ocean City, MD and correlated
observed EHSs with gradients in calculated transport. Figure 2.15 illustrates this possibility.
2.1.12 Borrow Pit Located Within Active Profile Zone
If the borrow pit is located so far landward that it could either induce seaward sediment
transport into the pit or trap sediment during normal seasonal or storm related cross-shore transport,
an EHS landward of the pit will result. Figure 2.16 illustrates this generic cause.
I Isolines of Relatively
S- Low Wave Height
Figure 2.14. Illustration of Wave Characteristics Varying with Offshore Location.
Isolines of Relative
Large Wave Height
Figure 2.15. Nearshore Bathymetry Causes Non-Uniform Wave Climate Along Shoreline. Cause
of Type 11 EHS.
Due to \
- Post-Nourished Profile
Figure 2.16. Borrow Pit Within Active Nearshore Zone Induces Sediment Transport Into Borrow
Pit. Cause of Type 12 EHS.
The results presented herein are based, in part, on case studies and, in part, on examining the
processes associated with each of the twelve potential causes of erosional hot spots as discussed in
the last section and listed in Table 2.1. This section describes the results of examining three case
studies of beach nourishment projects in which erosional hot spots have occurred. These include the
Miami Beach portion of the Dade County project, the Manatee County project and the Delray Beach
project. The characteristics of each of these projects and the associated EHSs are discussed in the
3.2 Case Studies
3.2.1 Dade County Project
The Miami Beach portion of the Dade County Project, also called the Miami Beach Project,
was constructed during the period 1976 to 1981 and comprised the placement of approximately 10
million yd3 (7.7 million m3) between Bakers Haulover at the north to Government Cut at the south,
a distance of approximately 10 miles (16 km)and encompassing DEP Monuments R-27 at the north
through R-74 at the south, see Figure 3.1. Wiegel (1992) has provided an extensive review of the
history of this project. Because of its size, the project was carried out in five successive stages over
the five year nourishment period. The borrow areas illustrated in Figure 3.2 were located between
shore parallel reefs. The characteristics of the five phases of the initial nourishment are shown in
Table 3.1. Part of the need for nourishment was due to encroachment of the hotel complexes along
the beach. It was found after construction of many of the hotels that their clientele preferred a deck
Figure 3.1. Dade County Location.
Figure 3.2. Borrow Site Location for Dade County Beach Nourishment Project.
and swimming pool rather than, or in addition to, the ocean for recreation, and thus the hotel
facilities were extended seaward and much of the active beach was encapsulated by a seawall.
Additionally, the erosion along Miami Beach was exacerbated by the cutting of Bakers Haulover
Inlet in 1918 which interfered with the net longshore sediment transport from north to south. Finally
the major hurricane of 1926 caused significant damage to the beaches along this portion of Dade
Table 3.1. Characteristics of the Five Phases of the Dade County Beach Nourishment Project
Phase Period of Work Area Encompassed Volume of Unit
No. (Streets) Sand Placed Cost
1 May 1977 September 1978 from 80' to 96" 2,940,000 1.95
+ Haulover Beach Park
2 August 1978 1979 from 63d to 80h 1,530,000 1.87
3 August 1978 1980 from 36t to 63rd 3,177,100 2.66
4 May 1980 October 1981 from 16h to 36"t 2,200,000 4.95
5 October 1981 January 1982 from Government Cut to 16' 2,400,000 9.00
Sediment characteristics along the Miami Beach Project were documented by the U.S. Army
Corps of Engineers both prior to and after beach nourishment and in the borrow area. These results
are shown in Figure 3.3 where it can be seen that the median diameter of the native beach sands was
approximately 0.33 mm, and that of the borrow area and post-nourishment beach were 0.27 mm and
0.34 mm, respectively. It is known from personal observations, that substantial amounts of very fine
material were placed along the southern portions of this beach nourishment project. This fine
material included approximately 10% silt and clay. Charles (1994) documented sand sizes along the
Miami Beach area as shown in Figure 3.4. The gradual decrease in sediment size with offshore
distance from the shoreline is expected and the increase in sediment size farther seaward is due to
more calcium carbonate (shells and coral fragments) in the offshore region which may include
contributions from local reefs.
The Dade County Project has been extremely successful. Figure 3.5a presents the longshore
distribution of shoreline change rate based on data from 1992 to 1996 along the entire project length.
The shoreline position in 1996 relative to 1962 is shown in Panel b of Figure 3.5 The information
in Panels a and b was combined to estimate the number of years required for the shoreline to recede
back to the 1962 position as presented in Figure 3.5c. It is seen from Panel b that erosion was
documented at only 24 of the 48 total monuments. Of the 24 monuments which exhibited erosion,
at only two of the monuments are the shorelines projected to erode back to the 1962 location in a
time period of less than 10 years. For six of the monuments, the shoreline location is projected to be
back to the 1962 location in less than 20 years. A total of nine of the monuments would require more
than 30 years to erode back to the 1962 shoreline position and at three monuments more than 40
years would be required for the shoreline to return to its location in 1962.
DAOC CO. LLC.
cIIIsIp ease 141
mmmunM - - -s -
-e Y IIIl I.W I
/A 0 I C. .
N I SMIM 1.NI -
-: -o - I.w
I--- ^- ---- ---20
w t sul I. Lu
-*4 *4 I U 3 4
gRAIN SIIZl ( UNITt)
Figure 3.3. Pre- and Post-Nourishment Sand Grain Size Distribution, Dade County Nourishment
Oho. lslfaihahMlrt. 100
Figure 3.4. Cross-Shore Distribution of Average Median Grain
Eight Profiles (Charles, 1994).
Size, Based on Sampling Along
... ............... ..i........... .............. .. .........
.L -. i.. iv ......... .. .... ; ....... .. ........ ...... ........ ................ ..... ..
0 .I.I t l --
25 30 35 4 45 50 55 60 85 70 75
20..-- ..-..-......- .-... ........ .... ....... .....--. .. ............
25 30 35 40 45 50 55 60 66 70 75
DEP Monumer No.
Figure 3.5. (Panel a) Annual Shoreline Change Rate for the Period 1992 to 1996. (Panel b)
Shoreline Changes from 1962 to 1996. (Panel c) Number of Years for the Shoreline to Reach the
1962 Positions Based on 1992 to 1996 Erosion Rates (Note: In Panel c, Only the Points
Characterized by an Erosion Rate Have Been Included).
Figure 3.6 presents the history of nourishment volumes for the Miami Beach project from
1962 to 1996. Also presented is the history of documented volumes remaining in the project area.
Considering the time period from 1962 to 1996, a duration of 34 years, a total of 13.9 million yd3
(10.6 million m3) of sand has been added with a documented 12.5 million yd3 (9.6 million m3)
remaining in 1996. Thus there has been a loss of only 1.4 million yd3 (1.1 million m3) of sand from
the total placed representing approximately 10% of the total. In some respects this should not be
surprising. With the long recurved jetty at Bakers Haulover and the very long jetty at Government
Cut, there is very little possibility for loss of good quality material.
12x 1 .
*----* Volume Changes
6 1 1975 i 1 1 j
.4 ...................... .... ....... .. I ............. ........ ........... .............1
4 ..... ^ .............. ...................... . ... ...... .... ........ .............................................. ................ ...... .............
198019662 1965 1970 1975 1980 1985 1990 1995 1998
Figure 3.6. Total Volume Changes and Volumes Added Relative to 1962 Between Bakers Haulover
Inlet and Government Cut (Based on Shoreline Changes).
18.104.22.168 Erosional Hot Spots in the Dade County Project
There are two EHSs along the Dade County Project. The northern EHS is located in the
approximate beach segment from R-28 to R-38 and the southern EHS extends from R-48 to R-61.
Figure 3.7 presents the volume change rates between 1980 and 1996 based on shoreline
change rates and profile change rates. The volume changes determined from shoreline changes were
based on the simple relationship
AV = Ay (h. + B)
--- Based on Profile Changes
Based on Shoreline Changes
. ..... ....... .......... ....... .
.30 35 40 45 50 55 60 65 70 75
DEP Morenur No.
Figure 3.7. Comparison Between Volume Changes per Unit Length Based on Profile Changes and
Shoreline Changes (1980-1996).
where AV is the volume density change (volume per unit beach length), Ay is the change of shoreline
position, h. is the so-called depth of closure and B is the berm height (h. + B was taken as 23 ft
(7.0 m) for this example). The reason that the volume losses associated with the shoreline change
rates are greater than with the volume change rates computed directly from the profiles is that part
of the shoreline change rates are due to profile equilibration. It is evident from Figure 3.7 that the
volume loss rates in the region R-28 to R-38 and in the region R-48 to R-61 generally exceed those
in the remainder of the area. It is also noted that nourishment was placed between R-65 and R-74 in
Both EHSs identified above are in the vicinity where an encroachment on the historically
active beach has occurred, thus these EHSs maybe, at least in part, Type 7 EHSs (Table 2.1). Figures
3.8a and 3.8b present plan views of the 1992 seawall lines relative to the shoreline positions
averaged over the available data from 1867 to 1936. Referring to the northern EHS (Figure 3.8a),
the construction line is seaward of the average shoreline position between R-29 and R-36 which
coincides approximately with the northern EHS appearing in Figure 3.7. Referring to Figure 3.8b,
the construction line is seaward of the average historical shoreline between R-56 to R-61. Again, this
corresponds approximately to the location of the southern EHS. It is also likely that the combination
of the net southerly longshore sediment transport and its interruption by Bakers Haulover Inlet is a
contributing factor to the northerly EHS from Monument R-28 to R-38.
In summary, the two EHSs in the Dade County Beach Nourishment Project appear to be of
Type 7, that is their areas encroach locally beyond the historical shorelines which represent artificial
headlands. Each of these EHSs has two associated ECSs (Figure 3.7). This cause of these EHSs has
been identified previously through the studies of Coastal Systems International (1997). The northerly
EHS may be augmented by the proximity to Bakers Haulover Inlet.
,,... i .-....... .... .i ...
-eo.o ---- -, -
I -* Canrulanljn*
DEP Mamuent Na
Figure 3.8a. Dade County. Comparison Between the Constructed Line (1992) and the Average
Shoreline Position (1867 to 1936), from Monument R-27 to R-37.
DEP Monumnt No.
Figure 3.8b. Dade County. Comparison Between the Constructed Line (1992) and the Average
Shoreline Position (1867 to 1936), from Monument R-53 to R-61
3.2.2 Anna Maria Key
Anna Maria Key is the northern most island in Manatee County extending from Passage Key
Inlet at the north to Longboat Pass at the south, see Figure 3.9. This island is 7.2 miles (11.6 km) in
length and the 1993 nourishment project is 4.2 miles (6.8 km) in length with an additional one-half
mile (0.7 km) transition at the southern end of the project. The nourishment comprised the placement
of 2,028,000 yd3 (1,552,000 m3) between DNR Monuments R-12 and R-33A with an additional
180,000 yd3 (140,000 m3) transition between Monuments R-33A and R-35, see Figure 3.10. The
average nourishment density for this project was 89 yd3/ft (225 m3/m). Both the native and the
borrow materials included considerable quantities of shell. The composite native grain size including
the shell material was 0.36 mm. After the shell had been removed by dissolution with acid, the
remaining native sediment size was 0.17 mm. The documented composite borrow area material was
0.30 mm which was reduced to 0.12 mm after the calcarious material had been removed by acid
treatment. These results are presented in Figure 3.11 indicating reasonably compatible nourishment
sediments. As shown in Figure 3.12, the borrow areas were located at distances ranging from 1,600
feet to 2,600 feet (490 m to 790 m) from the shoreline and extended from Monument R-24 to R-34.5.
Figure 3.13 shows seven distributions of shoreline changes ranging from August 1993 to February
1998 where it is seen that the areas of greatest recession occur near the two ends of the project.
Figure 3.9. Manatee County Location.
MANATEE COUNTY- 9 9i it 19 iT T ."" it m m i ".
SHORE PTECTION PRECT
COMPOSITE GRAM SIZE
- - - -III fi-- -l IIIIII-- 1111 ------
0 I641 A-A ES AT 1
I CIP LA ON S W 0 RE
C0 S _ -ATS
--- - 0
S I l II
- !/ 1* (
f i /
lU I lill .ii| LUL ilm .u L Ji il L iW W LLi 11 Ill il Ii .IIIIo
1PI4I ?IRAIN, SJZIE ,
MI U.U 4.0 20 1.0 0., O.Z5
Figure 3.10. Limits of the Manatee County,
Shore Protection Project.
Figure 3.11. Grain Size Distribution for Native
and Borrow Area Sediments With and Without
-- -- -- -- -- --- ---
Passage Key Inlet
0. a -22
Figure 3.12. Bathymetry off Anna Maria Key, Showing Location of Borrow Pit.
0 ............ ... i ........... .... .... .. .. .......... ..... ....
0 Passage K....... y I l..t ....... .....Longba.t P. .
05 10 15 20 25 30 35 40 45
DNR Monumert No.
Figure 3.13. Shoreline Position for Different Periods Relative to August 1993.
22.214.171.124 Erosional Hot Spots in the Manatee County Project
Inspection of Figure 3.13 demonstrates that there are two EHS "lobes", one located near the
northern end of the project and a more extensive lobe near the southern end of the project. The
shapes of these lobes conform qualitatively to those expected from simple beach planform evolution
theory, that is the ends of the project erode more rapidly due to the transition to the unnourished
adjacent beaches. This type of an EHS should be predicted through the application of simple
calculations or considerably more complicated numerical models. However, the magnitudes of the
shoreline recessions in the vicinities of these two EHSs are considerably greater than expected,
especially for the southerly EHS and it is possible interaction with the borrow pit also contributes
to these EHSs. This possibility will be examined in greater detail later.
3.2.3 Delray Beach Nourishment Project
126.96.36.199 General Background
The Delray Beach nourishment project is located in Palm Beach County and extends from
DNR Monument 175 to Monument 189, a distance of approximately 2.7 miles (4.3 km), see Figures
3.14 and 3.15. The Delray Beach project was first nourished in 1973 after the State Coastal Highway
A1A had been threatened repeatedly and damaged occasionally by erosion. Early attempts to protect
__ .n BEACH
BROWARO COUNTY s . jo
Figure 3.14. Location Map for Delray Beach, Florida. (From Beachler and Mann, 1996).
R 176 A
R 178 3
1500 0 1500
GRAPHIC SCALE IN FEET R 179 m
ALANIC AV ENUE
R 180 o
/ R 182 ; l i
R 88 A z
A FOEP SURVEY
MONUMENT R 19
Figure 3.15. Placement of Sand for the Different Nourishments. (From Beachler and Mann, 1996).
the roadway with an articulated interlocking concrete block revetment were not successful. During
most of the year, there was little to no dry beach at the toe of the revetment and under the action of
waves, this revetment failed twice leading to considerable litigation and the eventual decision to
construct the Delray Beach nourishment project.
The Delray Beach nourishment project is one of the two most well-documented long-term
beach nourishment projects in the State of Florida and in the nation. (The other is the Jupiter Island
project, a privately funded endeavor. The initial nourishment of this project was also in 1973). The
Delray Beach project has been monitored on a nearly annual basis, originally by Arthur Strock and
Associates and later 1984 by Coastal Planning and Engineering, Inc.
The intervals between renourishments have been increasing even though the volume
remaining in the nourishment area has been increasing. Referring to Table 3.2, it is seen that the first
renourishment occurred in 1978, five years after the original nourishment. This is consistent with
understanding of the mechanisms governing the evolution of nourished beaches in response to
multiple nourishments. The second renourishment occurred six years after the first renourishment.
The third and, to date, last renourishment occurred in 1992, eight years after the second
renourishment. Figure 3.15 shows the alongshore extent of these various nourishment events. The
1973 nourishment encompassed the entire 2.7 mile (4.3 km) length of the project whereas the 1978
nourishment occurred in two segments located near the southerly limits of the project and near the
northern middle of the project. The 1984 project again was placed along the entire 2.7 mile (4.3 km)
project length. The final renourishment in 1992 extended from the southern limits of the project to
somewhat north of the middle of the project. The locations of the placements of the renourishments
were established on the basis of project monitoring which indicated those sections of the project in
greatest need of renourishment. Figure 3.16 presents the alongshore distribution of volume additions
in these four nourishment events. Because there was no documentation of the placement distribution
of the material in the 1984 renourishment, it was considered to be uniform along the entire project
length. Figure 3.17 presents the cumulative volume of sand placed along the entire project since
1973 and since 1974. It is seen that somewhat more than half of the material has been placed in the
southern half of the project area.
Table 3.2. Volume of Sand Placed in the Delray Beach Nourishment Project
Period of Construction Length Encompassed Volume Placed Cumulative Volume
(miles) (x 103 yd3) (x 103 yd3)
July August, 1973 2.65 1,630 1,630
February May, 1978 1.23 (north section) 530 2,160
0.57 (south section) 170 2,330
September- October, 1984 2.65 1,300 3,630
November December, 1992 1.95 1,020 4,650
Source: Coastal Planning and Engineering, 1997.
\ t ; f : I ^ ^-
1 ______' /" i' '* _--
i| ..........,. .. .... r .. . . . ..... . . .....- ,
I I r i' t
t I P 100 t ,4
a I / I . s i io -; L s j .l
-2.50 O2.00 -1.50 .1.00 -50 0.00 0.50 1.00 1.50 200 250
Longh DiotncM pon
Figure 3.16. Volumes of Sand Placed Along the Project for Each of the Nourishments.
--Volus Pua ics 1573 '
S-Vlmane Placed inie 1174
I I a a | \ i
I fV J ; !
-2.50 *.0 -1JO -1.00 -.50 0.00 0.50 1.00 1.50 2.00 2.50
tLaghnm Dmsen |m"I
Figure 3.17. Cumulative Volume of Sand Place Along the Project Since 1973 and Since 1974.
The borrow area for the Delray Beach nourishment project is located approximately 2,500
ft (760 m) offshore as shown in Figure 3.18. There is a "no dredging zone" near the north end of the
borrow area approximately 1,000 ft (305 m) in length to avoid a sewage outfall. The effective wave
height and period for this area as given by Dean and Grant (1989) are 1.4 ft and 6.5 secs,
respectively. The vertical dimension of the active profile given in the same reference is 23.5 ft.
As summarized in Table 3.2, there have been four nourishments at Delray Beach totaling
4,653,000 yd3 (3,560,000 m3) placed in the Delray Beach project. As of 1997, there were more than
3,000,000 yd3 (2,300,000 m3)of this amount remaining within the project area.
Several previous studies have focused on the performance of the Delray Beach nourishment
project. Beachler, (1993) presented the two plots in Figure 3.19. Panel a shows the shoreline changes
between 1974 and 1990. The patterns of shoreline change document that there have been substantial
benefits from the 1978 and 1984 beach nourishment events within the fill area and also outside of
the fill area. Near the ends of and inside of the project limits, as might be expected, there is less
benefit due to the spreading losses associated with the fill. Panel b of Figure 3.19 presents the
shoreline changes from 1973 to 1990 which differ from the results in Panel a by including the effects
of the original nourishment. Again, the benefits of the project extend well outside of the project
limits. The volume changes at each monument from 1974 to 1990 as published by Beachler (1993)
are presented in Figure 3.20. There is one monument (R-186) within this fill area where the volume
changes are negative. This appears to be an erosional hot spot and will be discussed later in this
Beachler and Mann (1996) presented the mean high water line changes from 1974 to 1990
and 1974 to 1995 as shown in Figure 3.21, the latter period encompassing the 1992 nourishment
event. Again reflecting the effects of spreading near the ends of the project, it is seen that the project
benefits extend well beyond the project limits. The relatively poor performance at R-186 is also
evident from this plot.
188.8.131.52 Erosional Hot Spots in the Delray Beach Nourishment Project
Two periods are examined in the present report: 1975 to 1990 and 1975 to 1998. Based on
which of these two periods is considered, it appears that there are either one or two erosional hot
spots. The erosional hot spot that is consistently present for both periods is in the vicinity of DNR
Monument R-186. Additionally for the 1975 to 1990 period an erosional hot spot occurs in the
vicinity of R-180. Figures 3.22 and 3.23 present these results for 1975 to 1990 and 1975 to 1998,
respectively. These figures will be discussed in detail later and present the differences between the
calculated and measured volume and shoreline changes. The upper panel in each of these figures
presents the shoreline changes and the lower panel presents the volume changes. Comparing the
location of the consistent erosional hot spot near R-186 with the location of the no dredging zone
(Figure 3.18), it is unlikely that these two are related. However, the location of the southerly limit
of the borrow area may be related to the EHS in the vicinity of R-186.
--- ---aw com UmIT
Y 778 000 _______ | U
I* Ilro aIM AMA
W ay b\lh SOUTH A mrer
Figure 3.18. Location of the Borrow Area Relative to the Fill Area.
1974 1990 SHORELINE CHANGES
SHORELiNE CHANGE !FEZT)
FILL AREA =-
1170 R175 Also Als alt0 RISS
1973 1990 SHORELINE
SHORELINE CHANGE (FEET)
0 b) 4 FiLL AREA
ISO r"" -,* -- ------ 1--
R171 R177 A183 R189 R196
Figure 3.19. Mean High Water Shoreline Changes Computed by Beachler (1993): a) Between 1974
to 1990, and b) Between 1973 and 1990.
50 -- ---*
VOLUME CHANGE IN CUBIC YAROS(Thousandsl
o20 I------_U U
-20 ......*----*....... ..- .. ....... ...... ..........
A170 1175 A180 A8iS R190
Figure 3.20. Volume Changes Computed by Beachler (1993) Between 1974 and 1990.
165 187 19 171 173 175 177 17?9 181 13 185 17 189 191 193 195 197 199 201
186 168 170 172 174 176 178 160 182 184 186 1M 190 192 194 189 198 200
1 1174 T0 10 1974TO1995
Figure 3.21. Comparison Between Mean High Water Shoreline Changes from 1974 to 1990 and
Mean High Water Shoreline Changes from 1974 to 1995 (Beachler and Mann, 1996)
. I .
Erosonal htot identlcalM n (1975-1990)
Ersonl hot spot Idnlca on (1975-1 90)
- LIIhO DWM
Figure 3.22. Location of Erosional Hot Spots and Cold Spots for 1975 to 1990, Using: a) Shoreline
Change Differences, and b) Volume Change Differences. The Area Shown Encompasses the Project
ErosionaI hot spot Identification (1975-19N8)
Erosnal hot spot mntMcaton (1975-1998)
b) _H_ __ ___ __
---- -------- i-- --- ---# --
-------- ---- ---- -- w -------- /\ \ = -- ---
Figure 3.23. Location of Erosional Hot Spots and Cold Spots for 1975 to 1998, Using: a) Shoreline
Change Differences, and b) Volume Change Differences. The Area Shown Encompasses the Project
184.108.40.206 Sand Size Characteristics
Sand size characteristics along the Delray Beach nourishment project were reasonably well-
documented after the second renourishment in 1984. Figure 3.24 presents the longshore distribution
of sand size after the second and third renourishments. In general is seen that the sand size tends to
coarsen as a function of time. In particular, one month after the second renourishment, the average
grain size was approximately 0.25 mm, but after 27 months had coarsened to approximately 0.35
mm, a size which was maintained until 52 months after the second renourishment. Similarly, but to
a lesser extent, the same variation with time was documented after the third renourishment in
December of 1992 in which the average grain size was approximately 0.28 mm some 12 months
after nourishment and then gradually increased but with some fluctuations until 36 months when the
size again had decreased back to approximately the original value after 12 months. Comparing
Panels a and b, it is possible that the sand had coarsened substantially at the 12 month documentation
after the third renourishment. Figure 3.25 presents the variation in sand size with time at the
individual monuments. Panel a shows the evolution of sand size at four monuments after the second
renourishment and Panel b presents the same information after the third renourishment.
220.127.116.11 Modeling of the Delray Beach Nourishment Project.
The numerical model DNRBS developed for the Division of Beaches and Shores by Dean
and Grant (1989) was modified to account for multiple nourishments and applied to the Delray
Beach nourishment project. The model allows as input the actual distribution of placement densities
along the shoreline for the various renourishment events. The results of modeling will be presented
for the periods 1975 to 1990 and 1975 to 1998. Figure 3.26 presents the results for the period 1975
to 1990. This figure presents the longshore distribution of measured and predicted shoreline changes
and the difference between the two. Note that the measured and predicted plots represent the total
shoreline change over the time period represented without the added beach width. Stated differently,
these changes are those with the cumulative additional beach width due to nourishment removed.
From Figure 3.26, it is seen that the differences between the predicted and measured shoreline
changes are generally within 20 meters (66 ft) and within the project area all of the differences
between predicted and measured shoreline changes are less than 20 meters (66 ft). This is considered
reasonably good agreement.
The corresponding results between predicted and measured shoreline changes from 1975 to
1998 are presented in Figure 3.27 where again it is seen that, except in the vicinity of Monument R-
186, all of the changes are significantly less than 20 meters (66 ft). In evaluating the significance of
a 20 meter (66 ft) change, it is worthwhile to note that the recession near the central part of the
project for the period 1975 to 1990 was on the order of 80 meters (260 ft) and for the period 1975
to 1998 was on the order of 70 meters (230 ft). Thus, a difference of 20 meters (66 ft) is on the order
of 25% to 30% of the total change.
The same calculations described for the shoreline changes in the preceding paragraph were
carried out for the volume density changes and are presented in Figures 3.28 and 3.29 for the two
periods of interest. Again the results are quite similar. The maximum changes within the project area
Sedment Size Variation along Delray Beach Project
2nd Renourshment (October, 1964)
Loanghw Dalnc p
Sediment Size Varition along Delay Bach Project
3rd Renourihment (December, 1992)
-2 -1.75 -1.5 -1.2 -0.75 -o0. .0.25 0 0.25 05 0.75 1 1.25 1.5 1.75 2
Longeore ODIse ton]
Figure 3.24. Longshore Distribution of the Sediment Size: a) After Second Renourishment, and b)
After Third Renourishment.
- 1 adrh
-*- 27 moan
- * a monll
-- man4 .
Men Grain Size Evolution (1984 Renourishment)
0 6 12 18 24 30 36 42 49 54 50
Monmw arM NourWlinm
Mean Grain Size Evolution (1992 Rnourishment)
0.300 /* ,... R-180
S0.25 .. .___________.. _
0 6 12 11 24 30 36 42 48 64 60
Moniu aIor Nouishhm
Figure 3.25. Sediment Size Variation with Time: a) After Second Renourishment, and b) After
Shoresne changes without added beach wdth
between 1/14/75 and 1/15/90
-PoMdcId -M--esmud -- -Dw Imnce
Figure 3.26. Comparison Between Predicted and Measured NGVD Shoreline Changes from 1975
Shorin changes without" added beach wkdth
between 1/14/75 and 1/15/98
- Prdicted -- Muwuned .----n DWlm -mi
Figure 3.27. Comparison Between Predicted and Measured NGVD Shoreline Changes from 1975
Volume chang "without volume placed
between 1/14/75 and 1/15/90
i A A A .,
,,ilt W E S
t '* p
I- -- -
r~* r1 EU 3 1 1 9g n9 Pi
-Pmdctd --uhsnd ---...-.Dlic
Figure 3.28. Comparison Between Predicted and Volumetric Profile Changes from 1975 to 1990.
Volume changes "wthour volume placed
tween 1/14/75 and 1/15/98
S- m nIem Pal
-Prndcb --Masurd -...... -Oln
Figure 3.29. Comparison Between Predicted and Measured Volumetric Profile Changes from 1975
are on the order of 600 to 800 m3/m (240 to 320 yd3/ft) whereas the difference between the predicted
and measured changes are generally less than 200 m3/m (80 yd3/ft) Thus the relative differences are
approximately the same. Additionally, the maximum difference occurs in the results from 1975 to
1998 in the vicinity of R-186 where the differences exceed 200 m3/m (80 yd3/ft), representing
approximately 25% to 33% of the maximum changes.
4 EROSIONAL HOT SPOTS: IDENTIFICATION, INTERPRETATION,
MITIGATION AND AVOIDANCE MEASURES
Erosional hot spots can be identified in terms of either volumetric or shoreline changes. An
erosional hot spot is an area which exhibits greater recession or volumetric loss than anticipated in
design or than occurs on beaches adjacent to the erosional hot spots. Thus, erosional hot spots can
be considered as unexpected changes in shoreline position or volume. As described in Section 2,
there are a number of possible types and causes of EHSs, the effects of which could be predicted
if our understanding of the processes and capabilities to calculate these processes were sufficiently
advanced. Some of these types can be predicted using available technology whereas others must
await further improvements in methodology. As the state-of-the-art in beach nourishment technology
develops, it is expected that the numbers and areas which qualify as erosional hot spots will be
reduced due to our capabilities to predict and thus avoid unanticipated shoreline and volumetric
4.1 Identification of Erosional Hot Spots
One general approach to identifying erosional hot spots would be to conduct numerical
modeling of the shoreline and volumetric changes and to compare the longshore distribution of
shoreline and/or volumetric changes with the changes documented through monitoring and to
quantify the deviation between the numerical model predictions and the measured changes. Those
areas which exceed a certain multiple of the standard deviation between the measured and predicted
changes and which extend over a significant length of shoreline could qualify as erosional hot spots.
For purposes here, this procedure will be demonstrated for the Delray Beach nourishment project
for the periods 1975 to 1990 and 1975 to 1998. Erosional hot spots will be considered as those areas
which exceed one standard deviation of the difference between the measured and predicted shoreline
or volume changes and which persist in an alongshore direction for a distance greater than 1,000 feet.
It is seen from Figure 3.23 that for the period 1975 to 1990 based on volume changes (Panel b) there
are two erosional hot spots, one near the north-central portion of the project and one near the
southern end of the project (R-186). Based on shoreline changes (Panel a) only the EHS near R-186
qualifies. For the period 1975-1998 only the EHS at R-186 qualifies based on differences in
shoreline and volumetric predictions and measurements (Figure 3.24).
4.2 Interpretation of Erosional Hot Spots
In general, the best avoidance and mitigation strategies for erosional hot spots are based on
an interpretation of the causes of the EHSs which leads to an understanding and thus a reasonable
approach to their mitigation and/or avoidance. In the following sections a screening approach will
be introduced to assist in the interpretation of the erosional hot spot types and causes.
There are two useful discriminators in identifying the causes of erosional hot spots. These
include a comparison of the volumetric and shoreline changes at the erosional hot spot and a
comparison of the volumetric changes at the erosional hot spots and volumetric changes on the
adjacent beaches. Other useful information is the detailed bathymetry in the vicinity of the erosional
hot spots. Table 4.1 summarizes the possibilities of interest identified as Cases a, b and c. In Case
a there are both volumetric losses and shoreline recession which exceed either those of the adjacent
area or those anticipated in the design process. In Case b the volumetric changes are not significantly
different than anticipated or in the adjacent project areas, however the shoreline recession is greater.
Finally in Case c, the volumetric changes are greater than those in the adjacent project areas,
however the shoreline changes are not.
Table 4.1. List of Possible Combination of Erosional Hot Spots
Yes Case a Case c
No Case b
*Differences Based on Shoreline Changes.
**Differences Based on Volume Changes.
Table 4.2 evaluates the three possible causes of Case a hot spots. The first, Subcase a-1, is
spreading out losses at the end of a nourished area. This appears to be a substantial contributor to
the cause of the two erosional hot spots in the Manatee County project. The recommended method
of study is through planform evolution models and the remedy would be to simply anticipate this
occurrence in the design phase and perhaps to place more sand in the vicinity of the ends of the
project and/or possibly to add transitions (tapers) to the ends of the project. The second subcase is
of a borrow pit located within the nearshore zone, and the method of study would be through wave
refraction, reflection, diffraction and dissipation models, and the remedies would be to chose borrow
pits either farther from the shoreline and/or shallower with more gradual bathymetric changes such
that the effects would be lessened. This type is illustrated by the EHSs and ECSs at Grand Isle, LA
(Figure 2.4). The third and final Case a (Subcase a-3) is residual bathymetry resulting from the
nourishment construction. A method of study would be through wave refraction models and the
remedy is simply to avoid the residual bathymetry through ensuring that nourishment placement
results in reasonably uniform alongshore bathymetry.
Table 4.2. List of Possible Causes of Case a Hot Spots
Sub-case Possible Causes Method of Study Remedies
a-1 "Spreading Out Losses" at Planform Evolution Anticipate in Design
the End of a Nourished Area Models
a-2 Borrow Pit Located Inside Wave Refraction and Choose Borrow Pits
the Nearshore Zone (Too Diffraction Models and Farther (and Deeper)
Close to the Shoreline) Consideration of Seasonal Offshore
a-3 Residual Bathymetry Post- Wave Refraction Models Avoid the Residual
Nourishment Construction Bathymetry (Uniform
Case b EHSs are those in which the effect is locally present in the additional dry beach width
but not in the volume density. As shown in Table 4.3, this is indicative of either a sediment size
which is locally smaller than on the adjacent shoreline (Type 1) or profile lowering in front of a
seawall (Type 6).
Table 4.3. List of the Possible Causes of Case b Hot Spots
Sub-case Possible Causes Method of Study Remedies
b-1 Sediment Size Smaller Than Plot Sediment Size Add More Sand of
Adjacent Areas Distribution Along Compatible Size
the Project Area
b-2 Coastal Structures, Seawalls Profile Configuration Add More Sand of
and/or Groins Compatible Size
As shown in Table 4.4, the only cause recognized for a Case c EHS is the possible local
nourishment with a smaller volume density of sand which is coarser than that placed on the adjacent
beaches. This case is expected to be fairly rare.
Table 4.4. List of the Possible Causes of Case c Hot Spots
Sub-case Possible Causes Method of Study Remedies
c-1 Coarser Sand Profile Configuration May Not Be Required
4.3 Mitigation of and/or Remedial Measures for Erosional Hot Spots
Given the occurrence of an EHS, its mitigation in future renourishment programs or as a
special effort are of interest. As discussed previously, the best mitigation and or remedial measures
are based on an identification of the cause and type of the EHS and an interpretation and
understanding of the associated processes. The following paragraphs and Table 4.5 reference the
various measures that should be considered in mitigating and/or remediating EHSs. If various
constraints preclude the use of these measures for some particular cases, the remaining approach is
to advise the Client and/or Sponsor of the causes and likely future scenarios for the occurrence of
the EHS. A second approach may be to place a greater nourishment volume density in those areas
where EHSs are anticipated.
Table 4.5. Mitigation and/or Remedial Measures for EHS Types
Type Cause Mitigation and/or Remedial Measures
1 Dredge Selectivity
Use coarser sand.
9 Losses Over or Through Reefs
a) Recognize volume requirements,
2 Residual Structure Induced Slope
b) Employ stabilizing structures
Borrow Pit Causes Wave a) Reduce bathymetric nonuniformities
3 Transformation (Types 3 and 8).
b) Recognize temporary nature of effect or
4 Break in Bars if effect is unacceptable, fill break in bar
8 Residual Bathymetry c) Nourish with enough sand such that
effects are acceptable (Types 3, 4 and 8).
12 Borrow Pit Within Active Zone d) Possibly use coarser sediment in future
renourishments (Type 12)
5 Mechanically Placed Fill Ensure that volume density is consistent with
6 Profile Lowering in Front of Recognize volumetric density needs in
7 Headlands Recognize effect and if unacceptable,
consider use of structures as in 2(b).
10 Wave Focusing Due to Offshore
Translation of Beach_ Accept effect. No known remediation or
11 Wave Focusing Due to Pre- mitigation.
4.3.1 Types 1 and 9 EHSs
These types of EHS are due to the use of sand which is either locally finer than used for the
adjacent sections of the project (Type 1) or the use of sand which is too fine for the offshore reef
controls and the desired beach width (Type 9). The mitigation/remedial mitigation measure is to
employ coarser sand in subsequent renourishment phases of the project. For the Type 1 EHS, coarser
sand placed at the EHS will result in a more continuous distribution of sand size and associated
beach width. For the Type 9 EHS, a coarser sand will result in a steeper beach profile and thus a
wider equilibrated dry beach width with the offshore reef as a control.
4.3.2 Type 2 EHS
The appropriate measure for the Type 2 EHS is to anticipate in subsequent renourishments,
the additional volume density requirements to achieve the desired beach width. If the desired
planform alignment is not in equilibrium, that is, not approximately parallel to the nearshore
contours and/or the historic shoreline alignment, it may be worthwhile to consider low profile groins
or detached breakwaters as stabilizing structures. Low profile groins are most effective in areas
where a substantial gross longshore sediment transport is present.
4.3.3 Types 3,4,8 and 12 EHSs
These types of EHSs are due to bathymetric anomalies, caused by either the nourishment
project (Types 3, 8 and 12) or perhaps naturally occurring (Type 4).
For a Type 3 EHS, it could be possible during subsequent nourishments to extract sand,
essentially "trimming" the edges of the borrow pit such that it causes less longshore variation of
wave characteristics along the shoreline. Computational procedures are not readily available in those
cases where wave reflection is significant. It would probably be economically impractical to fill the
borrow pits, even though this is a technically effective alternative.
For a Type 4 EHS, which is considered to be natural and unrelated to the nourishment
project, it is probably most appropriate to recognize the probability of the occurrence through study
of the historical shoreline positions and to convey this information to the sponsor and regulatory
agencies. The nourishment volumetric density should be sufficiently large so that the natural
shoreline fluctuations associated with gaps in bars are acceptable.
During subsequent renourishments, sand could be placed to minimize residual bathymetry
causing Type 8 EHS.
For a Type 12 EHS, coarse sand could be used in subsequent renourishments which would
locate the toe of the fill at a more landward location for a desired shoreline displacement and thus
farther from the borrow pit.
4.3.4 Type 5 EHS
As noted earlier, this type of EHS is due to a lesser consolidated volumetric density due to
mechanical placement allowing a steeper slope than hydraulic placement and thus smaller dry beach
width after equilibration for the mechanically placed fill. The remedy is to simply ensure alongshore
uniformity in consolidated volumetric density during subsequent renourishments.
4.3.5 Type 6 EHS
This type of EHS is due to the lack of recognition of the additional placement volumetric
requirement to achieve the design dry beach width. The remedy is to provide an appropriate
volumetric density in subsequent renourishments. It should be recognized that this type of EHS may
also contain elements of the Type 7 EHS (headlands) and thus additional (structural) types of
remedies/mitigation or acceptance of the resulting dry beach width may be appropriate.
4.3.6 Type 7 EHS
Remediation of this type EHS requires either the acceptance of a narrower beach or the use
of some stabilization devices to maintain the shoreline at the desired location. As discussed, these
could include low profile groins or detached breakwaters. Low profile groins would be more
effective on shorelines with a reasonably large gross longshore sediment transport.
4.3.7 Types 10 and 11 EHS
There is no known practical solution to wave focusing due to a seaward displacement of the
shoreline or to pre-existing bathymetry. The most effective solution appears to be the application of
a sufficiently large nourishment volume density that the nonuniformities in the shoreline position
would be acceptable.
4.4 Avoidance of Erosional Hot Spots in Design and Construction
The three preceding sections on identification, interpretation and remediation of EHSs
provide the basis for their avoidance in the design and construction phases. In general, the best
approach is to predict the response of the project using the best analysis tools available including the
range from simple to complex numerical models. The design should also include recognition of the
effects identified in Table 2.1 and Table 4.5 which can lead to EHSs. If one or more EHSs are
identified by this process, approaches to their minimization or ideally elimination should be
developed as indicated in Section 4.3 and Table 4.5.
In this manner, with a clear, directed and documented effort to identify and avoid EHSs in
the design and construction phases, the identification of an EHS through monitoring after project
construction will signal the need for improved understanding of the causes) and development of new
tools. The pre-project identification and avoidance measures will provide guidance in this effort.
Obviously, care should also be taken to ensure that the project is constructed according to
specifications and thus that no EHSs are caused by construction methods or placement which are not
in accord with design specifications.
5. CAUSES OF EROSIONAL HOT SPOTS INVESTIGATED HERE
There have been six EHSs identified in the three nourishment projects examined here. It is
of interest to investigate which of the types and causes in Table 2.1 (or Table 4.5) are more likely
The EHSs in the Miami Beach project have been identified as Type 7, that is, a headland due
to encroachment of a seawall and the fill beyond the historic water line. Figures 3.5a and 3.5b have
shown that the location of the nourished shorelines of 1992 (at the northern and southern EHSs) are
generally seaward of the average shoreline from 1867 to 1936. However, it is also very likely that
the northern EHS is in part due to the net southerly longshore sediment transport and the associated
downdrift effect of Bakers Haulover Inlet. Figure 5.1 presents a juxtaposition of the erosional zones
and borrow areas for the Miami Beach project. It is seen that there is no obvious correlation between
It was noticed in examining the EHSs on Delray Beach and Anna Maria Key that the EHSs
occurred near the ends of the projects with the greatest effects near the southern ends of the projects.
Figure 5.2 presents the locations of the borrow pits in relation to the nourishment projects and the
identified EHSs in these two projects.
In order to test in an approximate manner the possibility of wave reflection from a borrow
pit causing the EHSs through gradients in longshore sediment transport, approximate calculations
were carried out using a modified version of the program DNRBS discussed earlier. Two
calculations were conducted as described in the following paragraphs.
All three calculations used the same wave input: Deep water wave height = 2 ft (0.61 m),
wave period = 6 sec, wave obliquity = 100, nourishment project length = 2.4 miles (3.8 km),
nourished beach width = 100 feet (30.5 m) and simulation time = 1 year. In Simulation 1, the wave
height immediately landward of the borrow pit was reduced by 24%. In Simulation 2, the wave
height immediately landward of the borrow pit was also reduced by 24% and the reduction tapered
to zero at a distance of one-half mile (0.8 km) from the ends of the project. Simulation 2 was
intended to represent, in a very approximate manner, the results of variability in wave direction or
borrow pits that exhibited a gradual variation in depth in the longshore direction. The results from
the two simulations are discussed below.
The evolutions from Simulation 1 are presented in Figure 5.3 where calculations are shown
in the upper panel for the case of no wave reflection and with the reflection described. The lower
panel presents the difference between the evolution affected by evolution and the unaffected
evolution cases presented in the upper panel. The overall effects of wave reflection are seen to be
accretion on the updrift side of the project area (left side in the plot) and erosion on the downdrift
side. The effects of the borrow pit are reasonably localized since the waves change from being
affected by the borrow pit to no change in one computational cell.
The results from Simulation 2 are presented in Figure 5.4 in the same format as for
Simulation 1. The main differences are that because the reduction in wave height landward of the
pit occurs over a one-half mile (0.8 km, 10 computational cells) distance rather than in one
computational cell, the effects are broader and less extreme.
Bakers Haulover Inlet
B- orrow Area
(a) Miami Beach and Borrow Areas
(b) Volume Change Rates per Unit Length of Beach Based on Profile
Changes and Shoreline Changes. (1980-1996)
Figure 5.1. Miami Beach Nourishment Project and Volumetric Change Rates.
Passage Key Inlet
@R-33A sTom CV WTsmaV uMff
*R-39 al ,r
*R-40 g-- ---" *o s ar'S"
a) Anna Maria Key Project b) Delray Beach Project
Figure 5.2. Relative Locations of Beach Nourishment Projects and Associated Borrow Areas, Anna Maria Key and Delray Beach.
Alongshore Distance (1000's feet)
Alongshore Distance (1000's of feet)
Figure 5.3. Effect on Beach Nourishment Evolution Due to Wave Reflection from Borrow Pit.
Wave Transmission Coefficient is 0.76 Immediately Landward of Borrow Pit. Upper Panel Presents
Initial Nourished Planform, Symmetric Evolution Without Reflection and Evolution as Affected by
Reflection. Lower Panel is Difference Between Affected and Unaffected Evolutions.
Alongshore Distance (1000's feet)
-------0---- -- ...-.. .. .. 0 ....
.. . . . . .. .
. . . . .. .. . . . . . .
S50 60 70 80
Alongshore Distance (l00's of fet)
Figure 5.4. Effect on Beach Nourishment Evolution Due to Wave Reflection from Borrow Pit.
Wave Transmission Coefficient is 0.76 Immediately Landward of Borrow Pit and Tapers to 1.0
Within a One-Half Mile Distance Either Side of Borrow Pit. Upper Panel Presents Initial Nourished
Planform, Symmetric Evolution Without Reflection and Evolution as Affected by Reflection. Lower
Panel is Difference Between Affected and Unaffected Evolution.
S*Extent of Nourishment **:** -
.a" nd Borr.owPit... .....Pit .. ... -
. . . . .
The effects shown in Figures 5.3 and 5.4 are somewhat similar to the EHSs identified here
for the Delray Beach and Anna Maria Key nourishment projects.
It is noted from Figure 5.2 that the location of the borrow pit is relatively close to shore for
the Anna Maria Key project and may affect the 6 m contour. To investigate this possibility further,
Figure 5.5 juxtaposes the shoreline changes with the location of the borrow pit. It is clear that the
southern EHS is more extreme than the northerly EHS thus suggesting a relationship. Figure 5.6
presents a cross-section at Monument R-26 before (Dec. 1992) and after nourishment (Oct. 1993 and
May 1994). It is seen that the landward limit of the borrow pit is located in a water depth of
approximately 19 ft (5.8 m). By comparison, the depth of closure recommended by Dean and Grant
(1989) for this area is 14.6 ft (4.5 m).
Borrow Pit Location.
b) Shoreline Changes.
Figure 5.5. Anna Maria Key Beach Nourishment Project Showing Relative Longshore Location of
EHSs and Borrow Area.
1 0 ..... ...... .........................
-- ---- October 1993
g 0 ... ...... ............. ............. ---- May 1994
,-20 .... .
-30 .................................................. . .. .
0 1000 2000 3000
Distance From Monument (ft)
Figure 5.6. Pre- and Post-Nourishment Profiles Anna Maria Key, Monument R-26.
6 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
Erosional hot spots (EHS) are localized areas which erode more rapidly than the adjacent
sections of beach or more rapidly than anticipated in the design phase of the nourishment project.
These features are of interest as they can require early mobilization to renourish a project that is
generally performing well and/or they can provide a negative public focus on a relatively small
section of an otherwise successful project. EHSs can be due to the manner in which the project was
designed and constructed or can be due to natural processes. Some types of EHSs can also occur on
Methods are recommended for identifying, interpreting, mitigating and avoiding erosional
hot spots. For some types for which remediation or avoidance is not practical, the only approach is
to anticipate and accept the effect and possibly to apply a large volume density such that an EHS will
not threaten upland structures or compromise significantly the effectiveness of the project. The use
of stabilization structures (low profile groins or detached breakwaters) may be appropriate in some
I I I
As the causes and mechanisms of EHSs become better understood, their occurrences on
nourished beaches should decrease through more effective design and, for those that occur,
approaches to their mitigation will be more evident. More prevalent use of existing design
methodologies and development of new methodologies are required to reduce the occurrence of
EHSs on nourished projects. Of the twelve types of EHSs now identified, it is believed that
application of readily available design methodology would lead to either the avoidance or
minimization for seven of the types. Although some of the methods require the use of reasonably
advanced numerical models, for others, simple models/considerations are adequate to indicate the
tendency for an EHS in the design phase. It appears that wave reflection from borrow pits may be
a significant cause of EHSs.
Based on the results of the preliminary modeling of the effects of wave reflection, it is
recommended that further examination of wave reflection as a potential major contributor to EHSs
be conducted, and if found to be significant, methodology be developed to evaluate the effect of
reflection during the beach nourishment design and evaluation process.
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Louisiana," Proceedings, Coastal Sediments '87, ASCE, New Orleans, LA, pp. 1232-1242.
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National Conference on Beach Preservation Technology, Florida Shore and Beach Preservation
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Coastal Research, Vol. 7, No. 1, Winter, 1991, pp. 53-84.
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