Citation
Erosional hot spots

Material Information

Title:
Erosional hot spots
Series Title:
UFLCOEL-99021
Creator:
Dean, Robert G ( Robert George ), 1930-
Liotta, Roberto
Simón, Guillermo
Florida -- Office of Beaches and Coastal Systems
Place of Publication:
Gainesville Fla
Publisher:
Coastal & Oceanographic Engineering Program, University of Florida
Publication Date:
Language:
English
Physical Description:
ix, 60 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Beach erosion -- Florida ( lcsh )
Beach nourishment -- Florida ( lcsh )
Genre:
government publication (state, provincial, terriorial, dependent) ( marcgt )
bibliography ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references (leaves 57-60).
General Note:
"November 19, 1999."
Statement of Responsibility:
prepared for Florida Department of Environmental Protection, Office of Beaches & Coastal Systems ... prepared by Robert G. Dean, Roberto Liotta and Guillermo Simón.

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University of Florida
Holding Location:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
49575831 ( OCLC )

Full Text
UFL/COEL-99/021

EROSIONAL HOT SPOTS
by
Robert G. Dean Roberto Liotta and
Guillermo Sim6n

November 19, 1999
Prepared for:
Florida Department of Environmental Protection Office of Beaches & Coastal Systems 3900 Commonwealth Boulevard Tallahassee, Florida 32399-3000




EROSIONAL HOT SPOTS
November 19, 1999
Prepared for:
Florida Department of Environmental Protection
Office of Beaches & Coastal Systems
3900 Commonwealth Boulevard Tallahassee, Florida 32399-3000
Prepared by:
Robert G. Dean, Roberto Liotta and Guillermo Sim6n Coastal and Oceanographic Engineering Department
University of Florida
345 Weil Hall
Gainesville, FL 32611




EXECUTIVE SUMMARY

This report presents the results of a study of "erosional 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. EIIS 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
I INTRODUCTION ........................................................ 1
2 BACKGROUND .......................................................... 1
2.1 G eneral ............................................................ 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 M echanically Placed Fill ....................................... 8
2.1.6 Profile Lowering in Front of Seawalls ........................... 8
2.1.7 H eadlands ................................................. 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 RE SU LTS .............................................................. 18
3.1 G eneral ........................................................... 18
3.2 Case Studies ....................................................... 18
3.2.1 Dade County Project ......................................... 18
3.2.1.1 G eneral ........................................... 18
3.2.1.2 Erosional Hot Spots in the Dade County Project ......... 23
3.2.2 Anna M aria Key ............................................ 26
3.2.2.1 G eneral ........................................... 26
3.2.2.3 Erosional Hot Spots in the Manatee County Project ...... 29
3.2.3 Delray Beach Nourishment Project ............................. 29
3.2.3.1 General Background ................................ 29
3.2.3.2 Erosional Hot Spots in the Delray Beach Nourishment
Project ............................................ 34
3.2.3.3 Sand Size Characteristics ............................ 40
3.2.3.4 Modeling of the Delray Beach Nourishment Project ...... 40




4 EROSIONAL HOT SPOTS: IDENTIFICATION, INTERPRETATION, MITIGATION 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

FIGURE PAGE
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, D FI= 0.275
minD 2= 0.2 mm,D F3=0.14 mm F.................... 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, cl/h
= 3, i'h = 0.167, and 0 = 00. 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 TypelIlIEHS............................................. 17
2.16 Borrow Pit Within Active Nearshore Zone Induces Sediment Transport Into
Borrow Pit. Cause of Type 12 EIS .................................... 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) .......................................................2
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
M ann, 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

TABLE PAGE
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

1 INTRODUCTION
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 I in = 3.28 ft, I in 3 = 1.31 yd3 and I m3/m z 0.4 yd 3/ft.
2 BACKGROUND
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. Raichie et al. (1998) examined various causes of EHSs in Broward County and presented a structured diagnostic analysis to identify the cause(s) 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 EFISs at Ocean City, MD and concluded that wave transformation over pre-existing offshore bathymetry may cause EFISs. 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.
2.1 General
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 EFIS 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 EFIS will have one or more associated ECSs. The following sections discuss the mechanisms and causes associated with the twelve individual EI{Ss.
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 mmn, 0.20 mm and 0. 14 mm will result in equilibrated dry beach widths of 205 ft (62.5 in), 105 ft (32.0 mn) and 0 ft, respectively.




Table 2.1. Possible Causes of Erosional Hot Spots Type Cause ] Related to J[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
Beaches
10 Wave Focusing Due to Offshore Wave Focusing Yes
Translation of Beach
I1I Wave Focusing Due to Offshore Wave Focusing Yes
B athymetry ________________12 Borrow Pit Located Within Active Profile Trapping of Sediment No
___Zone IWithin Borrow Pit _______

Nourishment With Finer Sediments Nourishment With Coarser Sediments
B

IS{h

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 (yd3lft)

Figure 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, DF1 = 0.275 mm, D = 0.2 mm,DF3 =0.14mm

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.

600

bUU V..

AFIAN=l1.2
SAFIAN=I'2O -, !:
.. . .. F ... N . . . . . .. . . . ... .. .. .
Intersecting Non-In secting : "
AFIAN =0.8 -..~.
.. ... .............. .. .. .. .. ... .. .. .8 .... . . .. . . .
.. .. .. .. . .. . .. .. . . .. ...... .... . .. . . .
....... .. ..
. I I I I " I. .. . .




Groin

a) Plan View Profile With Groin Field
----------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 mn) to 59 ft (18 mn) 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, bfL =0.5, d/h =3, Kh = 0. 167, and 0 =00*. 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).




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.




Shoreline

Bar

Indentation in Shoreline

Diffracted Wave Front \- Bar

Figure 2.7. Schematic of Greater Wave Energy Propagating Through Break in the Bar Causing Localized Shoreline Erosion.




Pre-Nourished Profile

Truck Placement Profile
,,- 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.

U
0

-20
-30

. . . . . . . . . . . . ... . . . . . . . . . . . i. . . . . . . .. .
October 1995
; i ~.......... Ad19
........ ....... ....................................... ....Apr1996
August 1996
.. .. . . . . ... . .. . . . . . . .. .. . . .. . . . . . . . . .. .. .. . .. :. . . . . ..
....... .....:
. . . . .. .\. . . . .. . ... . . . . . . .. .. . . .. .. . .. . .. . . . . . . . . . . . . . . . .
IIIIIiI
1000 2000 3000
Seaward Distance from Monument (ft)

4000

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.
11




Seawall

Initial 7Profile
Figure 2.10. Illustration of Volume Required to Form Incipient Dry Beach.
2.1.7 Headlands
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
1 /Later Time
Original Shoreline
,,,, .x'kTransport
Seawall
i
I i
*:1
I
i Transport
I I!
I
I
Figure 2.11. Illustration of Redistribution of Sand Placed Seaward of Artificial Headland.




Pre-Nourished Shoreline Closure Depth, h = h.
I Residual Bathymetry,
I h> h.
I j
Shoreline Immfdiately J/
After Nourishing
I I
Shoreline Aft+
Planform Equiibratidn
4 I"
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.
[: I
in which the subscripts 1" and "2" apply at the outer lim-it of the dredge placement and at the depth of limiting motion, respectively and yi and y., represent the distances to the outward contour and the shoreline, respectively and C, and C2 are the wave celerities 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.
Pre-Nourished Profile
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 Reef Control.
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.




Pre-Nourished Shoreline

Post-Nourished Shoreline Isolines of Rela 1Low Wave Heig
, /

tively
ht

I IN %.

v~
\ \
N'
,I
N.I /

-~, ,

I
I
*1 I
I I I I
I
1
I
I
I
I
I
I
*1
I
I
1 I 1 1
I
I
I

Figure 2.14. Illustration of Wave Characteristics Varying with Offshore Location.

/
I j
I I
' \

Isolines of Relative LargeWave Height




Pre-Nourished Shoreline
Post-Nourished Shoreline

0
I0

/

Large Wave Height Vave Refraction

Figure 2.15. Nearshore Bathymetry Causes Non-Uniform Wave Climate Along Shoreline. Cause of Type 11 EHS.

'C CD
CD

4
CD

Zone of Due to

I,
-4, '-I

^%,NO\




Pre-Nourished Profile
- Post-Nourished Profile

Figure 2.16. Borrow Pit Within Active Nearshore Zone Induces Sediment Transport Into Borrow Pit. Cause of Type 12 ELIS.
3 RESULTS
3.1 General
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 sections below.
3.2 Case Studies
3.2.1 Dade County Project
3.2.1.1 General
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 yd' (7.7 million in') 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 pooi 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 County.
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
______ __ ___ __ ___ __ ___ __ ___ __ __ ___ __ ___ __ ___ __(yd') ($Iyd3)
1 May 1977 September 1978 from 80~' to 96'" 2,940,000 1.95
+ Haulover Beach Park
2 August 1978 1979 from 63rdto 80'h 1,530,000 1.87
3 August 1978 -1980 from 36dlto 63d 3,177,100 2.66
4 May 1980 October 1981 from 16h to 36d' 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.




DADE CO. .L C. fl 1 41 l .
RENOUNSHMrENT
COMPOM enAIN Ut
- magUTI- -- KAN
mNAMIII SIR *;1.29
S W Ill S W I l .0
saii Mam 10 /A A, 1. C. ., t
wM OI SIZE 1.71 1
Pill SMFIM 1.M6 1
S- Pa P|mom -- /"- l
S-
r eson
----
M lAR 1i11 llI I 1111 i l I Ill s I li I i ll 1 u I III IIII Alls sl e ii ll el e t
*- -4 0 I a a 4
WMAIN SIZE ( UNITS

Figure 3.3. Pre- and Post-Nourishment Sand Grain Size Distribution, Dade County Nourishment Project.

ownm utMo..w mwabee a e

Figure 3.4. Cross-Shore Distribution of Average Median Grain Eight Profiles (Charles, 1994).

Size, Based on Sampling Along




10
(92-96) 4

J100
~50 '0

wp 40 ,30 20 10
0

30 35 40 45 so 55 60 65

5

30 35 40 45 s0 55
DEP Monumeit No.

60 65

70 7

70 75

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).

. .... ... ......... .... ................. .. .* ....... ....... ......
- .-. *.. .-..- . -...-.-- -.-...- .-..... 40t..i...........
.- .. ....... ..
. .. .. .... .. ..... . ... ... .. .... .. ..... .... ... ... .. .. .. .. ..




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 yd 3 (10.6 million in3) of sand has been added with a documented 12.5 million yd 3 (9.6 million Mn3) remaining in 1996. Thus there has been a loss of only 1.4 million yd 3 (1.1 million Mn3) 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.

. .X10

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).
3.2.1.2 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-6 1.
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)

... .. .. ... ... ... . .. . .. . ... ....I.... ....

....... .... ... I .. .. ..... ........ .. .. .. ..
............. II : ... ......
I..... ............ ....................V.. ... .........
.. . .. . .. .. .. ... .... ... I. .. .. .. .. .. ... .. . ... .. . .. .. ..- ... .... ..




-o Based on Profile Changes I
25 30 3 5 40 45 50 55 60 65 70 75 DEP M n 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 1992.
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.




W00

4 0 ... "
_ I ," I..** *** .. i I _
20.0 i "
... ... .. ..........
0.0
T ~~31 M 2 4 3
___I __; "-_ _.20.0 ........_.. .....
-40.0
-W.0
.100.0
C ...canwwrucon n
-120,0
-140.0
DEP Ltnmwmflt No.
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 Monummnt 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

3.2.2.1 General
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 kin) in length and the 1993 nourishment project is 4.2 miles (6.8 kin) in length with an additional one-half mile (0.7 kmn) transition at the southern end of the project. The nourishment comprised the placement of 2,028,000 yd 3 (1,552,000 in3) between DNR Monuments R-12 and R-33A with an additional 180,000 yd' (140,000 in3) transition between Monuments R-33A and R-35, see Figure 3.10. The average nourishment density for this project was 89 yd 3/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 in) 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.

FLORIDA

County

..^ 0




Figure 3.10. Limits of the Manatee County, Shore Protection Project.

MNATEE COUNTY r i " "1 1 '
SHORE PROTECTION PROJT
COMPOSITE GRAIN SIZE
DISTRIBUTIONS
a~rue ----------A :
-**T1II AHH I - ------ -. -g NAT VIE BEA -NH- ES I C 90ATE 10
m can LA o sv R ORG W W RE
70
Co
ICLPLAON S CO W RIINA -- l
---- -- --- - -7o- ------ - 0
: / _,o;
- - 0
" *4
/
/1 /5
IiiY 0.lI l I I I I l l I I l l I l ll 1 1 I 1 l l t I I I t l l I 1 n

PHI RAIN SJZE

.u U 2U V 1.0 0. 05
MILLIMETERS

U.IZD 0U03

Figure 3.11. Grain Size Distribution for Native and Borrow Area Sediments With and Without Carbonates.

-4 -3




Passage Key Inlet

Longboat Pass
Figure 3.12. Bathymetry off Anna Maria Key, Showing Location of Borrow Pit.




DNR Momrxwrt No.

Figure 3.13. Shoreline Position for Different Periods Relative to August 1993.
3.2.2.3 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
3.2.3.1 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 kcm), see Figures 3.14 and 3.15. The Delray Beach project was first nourished in 1973 after the State Coastal Highway AlIA had been threatened repeatedly and damaged occasionally by erosion. Early attempts to protect

g 0 1.10
1-20




BROWAR COUNY
BRWR& CUT

c'Y or
DELRAY BEACH
N.T.S. *
Figure 3.14. Location Map for Delray Beach, Florida. (From Beachler and Mann, 1996).
Figure 3.14. Location Map for Delray Beach, Florida. (From Beachler and Mann, 1996).




I
/ R 175A
R 176 A R 177
0 in R 178
1500 0 1500 "
GRAPHIC SCALE IN FEET R 179
Z
0
ATLANTIC AVENUJE R 180 -z ( I
0: ix
Uas
R 181
R 182
12R 183
R 84 I
LLo 5
5 o 11 1A
s ,
187
I A
LINTON BLVD. 8
R 88.A z
R
R 191 A
LEGEND
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 10 yd3) (x 10 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.




A G
2 -1 I, I
I j!i
-2.50 -2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 2.00 2.50
Loshor e Distan |k)
Figure 3.16. Volumes of Sand Placed Along the Project for Each of the Nourishments.
-V ne Placed 1sin1e 1973
-Vokne Placed sice 1974
a A A A a && & & A
a a 3 g ; g a a
-2.S 4-00 -1.50 -1.00 -0.50 0.0 0.50 1.00 1.50 2.00 2.50
Longihem DMs hm1
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 mn) 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 yd' (3,560,000 in') placed in the Delray Beach project. As of 1997, there were more than 3,000,000 yd 3 (2,300,000 m')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 report.
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.
3.2.3.2 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.




Tows of
- V- --.-A7 OR COASMCr'V LIMIT
Cit of ANO P Jer Ler
asTMeaTw AM I jNo dredging rA. zon
I-7
BORROW AMA
%J1
SOWNY CosOOMI 4Ln"
City of
OD so Both $OW A9OCr
Yiur 7 8 t he B A a o
Figure 3.18. Location of the Borrow Area Relative to the Fill Area.




1974 1990 SHORELINE CHANGES
SHORELINE CHANGE 'FEiET

200
150

- FILL AREA =- >

1001
so ..

A170 R17S R180 AlS
DNR MONUMENT
1973 1990 SHORELINE
(AVERAGE)

SHOREUNE CHANGE (FEET)
200 1 "4=l FILL AREA =4

RIS0 RISS CHANGES

R171 R177 R183 R189 R196
DNR MONUMENT

Figure 3.19. Mean High Water Shoreline Changes Computed by Beachler (1993): a) Between 1974 to 1990, and b) Between 1973 and 1990.

01
RI

5

T

R201

R201

0 = R165

G




VOLUME CHANGE IN CUBIC YARDS(Thousandsl

R170 R17S l180 R185 R190
DNR MONUMENT

R R20
RtSS R20

Figure 3.20. Volume Changes Computed by Beachler (1993) Between 1974 and 1990.

165 167 169 171 173 175 177 179 181 183 185 187 189 191 193 15 197 190 201
166 16 170 172 174 176 178 180 152 184 186 188 190 192 194 196 198 200
FDEP MONMENT
m 1974TO0NO 1974TO 1995

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)

40- FILL AREA
..... j...i........l.l ... ... ..... .. ..........

I

.80

SR165$

!
!




Erosional hot spo identiication (1975-1990)

nO-e obw M

EMoslonal hot spot identification (1975-1990)

LOhwe DbinM m

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 Limits.




Erosional hot spot identification (1975-1998)

Erosional hot spot idniliUcation (1975-1908)
-a 0\ -is '0\ 2
F5 20 50
- O W
;. ;.,, ; ., ; I
Lnanm un. Dio pn

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 Limits.

L-40- DbW- 11M




3.2.3.3 Sand Size Characteristics

Sand size characteristics along the Delray Beach nourishment project were reasonably welldocumented 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.
3.2.3.4 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 R186, 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




Sediment Size Variation along Deiray Beach Project
2nd Renourishnment (October, 1984)

LoIhoe Dims lkm

Sediment Size Variation along Deiray Beach Project
3rd Renourishment (December, 1992)

-2 .1.75 -1.5 -1.25 -1 -0.75 -0.5 -025 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Lngswore DitM, 9an]
Figure 3.24. Longshore Distribution of the Sediment Size: a) After Second Renourishment, and b) After Third Renourishment.

-1 monlh
*-. 15mn
-* 27 moEh
- 41 monts
* a monle

S12 monhs 24- -mat
- -3monhs
- 49 months




Mean Grain Size Evolution (1984 Renourishment)

--- -177
*-.- R-100
- R-184
T-1817

0 6 12 18 24 30 36 42 48 54 60
Mon w altr Nourhrnt

Mean Grain Size Evolution (1992 Renourilshment)

" i '" ". T .- R-I77
0.275 W
'*" R-154
2 . --T-187
oa ... .i I
0.250
0.225
0200
0.175
0 6 12 18 24 30 36 42 48 54 60
Month alter NowuiWmt
Figure 3.25. Sediment Size Variation with Time: a) After Second Renourishment, and b) After Third Renourishment.
42




Shoreline changes *without" added beach width
between 1/14/75 and 1/15/90

LegheW dlane pe]
- Praied Measumed ...... Difernc [p-m]

Figure 3.26. Comparison Between Predicted and Measured NGVD Shoreline Changes from 1975 to 1990.
Shoreline changes "without" added beach width between 1/14/75 and 1/15/98

LPngshoeted atue ifrm [p
- Prelced Measured ..*****Delference [p"m

Figure 3.27. Comparison Between Predicted and Measured NGVD Shoreline Changes from 1975 to 1998.




Volume changes withoutr" volume placed between 1/14175 and 1/15/90
100-
FIN a
- Pr:Cled Meaured ...... ******
I
AL LA& a A A AL LA A A A ALAL A

Figure 3.28. Comparison Between Predicted and Volumetric Profile Changes from 1975 to 1990.
Volume changes "withour volume placed between 1114/75 and 1/15/98 100
-40
it. iti
-Predcled -IMesred *...... Dftnce
Figure 3.29. Comparison Between Predicted and Measured Volumetric Profile Changes from 1975 to 1998.
44




are on the order of 600 to 800 m3/m (240 to 320 yd~ft) whereas the difference between the predicted and measured changes are generally less than 200 m3/m (80 yd3lft) 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 yd 3/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 changes.
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). B ased 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
CYes No
Yes Case a Cs
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-i1, 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 Profile Changes
a-3 Residual Bathymetry Post- Wave Refraction Models Avoid the Residual
Nourishment Construction Bathymetry (Uniform
Volume Density)
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-i 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 jMitigation 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
Transformation (Types 3 and 8).
b) Recognize temporary nature of effect or 4 Break in Bars if effect is unacceptable, fill break in bar
(Type 4).
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
adjacent beaches.
6 Profile Lowering in Front of Recognize volumetric density needs in
Seawalls design.
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.
Existing Bathymetry




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 cause(s) 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 responsible.
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 EELS 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 the two.
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 in), wave period = 6 sec, wave obliquity = 100, nourishment project length = 2.4 miles (3.8 kin), 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 kin) 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 kin, 10 computational cells) distance rather than in one computational cell, the effects are broader and less extreme.




j Bakers -laulover lolet

Borrow Area (Typical)

I:

(b) Volume Change Rates per Unit Length of Beach Based on Profile Changes and Shoreline Changes. (1980-1996)

(a) Miami Beach and Borrow Areas

Figure 5.1. Miami Beach Nourishment Project and Volumetric Change Rates.




Passage Key Inlel
,\- eA

guat AVA RwVAOJr LMTr as a wa r mr r
marmm A

,R.20
R-21 a $ -22
-23
-24
-25
O R-26 T 7r0 nm
-27
*28
-29
*T-30 OR-31
-R-32
oR-33A s oum coUrwrC aw uMtr
*R-34
*R-35
-36
*R-37
*R-38
*R-39 ar of
~SOUTN AMltr
*R-40 J w en s r r
- ,R-41 I r';7Longboat Pass
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.




120 100
80 S60
40 20 00 0 6 -20
- -40
-60
z 60 a 40
2
t 20
.20 -20
840
. -60
5

50

60 70
Alongshore Distance (1000's feet)

60
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.

60
40 t- 20
0
-20 -0

60 7
Alongshore Distance (1000's feet)

5 . 0

Xi 50 60 70 so
Alongshore Distance (1000s of feet)
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.

50




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 in). By comparison, the depth of closure recommended by Dean and Grant (1989) for this area is 14.6 ft (4.5 in).

PSUaa Key L.*W

LOPSm

a) Nourishment Project, Showing Borrow Pit Location.

b) Shoreline Changes. Figure 5.5. Anna Maria Key Beach Nourishment Project Showing Relative Longshore Location of EHSs and Borrow Area.




10 . . ..
December 1992
-------- October 1993
................... ....... M ay 1994 .....
................. ... "' . ................................ ...... .... .. ...............
c -10 ,
-30 ............ ................... ................... .. ..? ;-". ... ......... ......
-40
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
6.1 Summary
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 natural beaches.
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 cases.




6.2 Conclusions

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.
6.3 Recommendations
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.
7 REFERENCES
Arthur V. Strock & Associates, Inc., 1973, "Beach Restoration Project for the City of Delray Beach,
Analysis and Construction Report," Deerfield Beach, FL.
Arthur V. Strock & Associates, Inc., 1979, "City of Delray Beach, Beach Maintenance Nourishment
Project, 12 Month Follow-Up Study," Deerfield Beach, FL.
Arthur V. Strock & Associates, Inc., 198 1, "City of Delray Beach, Beach Maintenance Nourishment
Project, 24 Month Follow-Up Study," Deerfield Beach, FL.
Arthur V. Strock & Associates, Inc., 1984, "City of Delray Beach, Beach Maintenance Nourishment
Project, 60 Month Follow-Up Study," Deerfield Beach, FL.
Arthur V. Strock & Associates, Inc., 1983, "Schematic Design Report, 1984 Delray Beach
Maintenance Nourishment Project," Deerfield Beach, FL.
Beachler, K.E., 1993, "The Positive Impacts to Neighboring Beaches from the Delray Beach
Nourishment Program," Proceedings, Beach Preservation Technology 1993, Florida Shore &
Beach Preservation Association, St. Petersburg, Florida, pp. 223-238.




Beachler, K.E. and Mann, D.W., 1996, "Long Range Positive Effects of the Delray Beach
Nourishment Program," Proceedings, 25' International Conference on Coastal Engineering,
ASCE, Orlando, pp. 4613-4620.
Bridges, M.H., 1995, "Analysis of the Processes Creating Erosional Hot Spots in Beach
Nourishment Projects," M.S. Thesis, Coastal and Oceanographic Engineering Department,
University of Florida, Gainesville.
Charles, L.L., 1994, "Application of Equilibrium Beach Profile Concepts to Florida's East Coast,"
M.S. Thesis, UFL/COEL-94/016, Coastal & Oceanographic Engineering Department, University
of Florida, Gainesville, FL.
Coastal and Oceanographic Engineering Laboratory, 1973, "Performance Predictions of Planned
Beach Fill at Delray Beach, Florida," Report No. UF/COEL-73/7, University of Florida,
Gainesville, FL.
Coastal Planning and Engineering, Inc., 1985, "Dune and Vegetation Study for the City of Delray
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