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UFL/COEL-88/015
REVIEW OF DREDGING EFFECTS ON ADJACENT
PARK SYSTEMS
by
Robert G. Dean
with a contribution by:
Robert Dolan
Prepared for:
National Park Service
75 Spring Street, SW
Atlanta, GA 30303
December, 1988
REPORT DOCUMENTATION PAGE
1. Report No. 2. 3. Recipient Acceesion No.
4. Title and Subtitle 3. Report Date
REVIEW OF DREDGING EFFECTS ON ADJACENT PARK SYSTEMS December, 1988
6.
7. Author(s) 8. Performing Organiatioo Report No.
Robert G. Dean
with a contribution by: Robert Dolan UFL/COEL-88/015
9. Performing Organization Name and Address 10. Project/Task/Work Unit No.
Coastal and Oceanographic Engineering Department
University of Florida 11. contract or crant No.
336 Weil Hall CA-500-7-8007
Gainesville, FL 32611 13. Type of Report
12. Sponsoring Organization Name and Addres
National Park Service
Final
75 Spring Street, SW
Atlanta, GA 30303
14.
15. Supplementary Notes
16. Abstract
In today's environment, most park systems can be influenced by human
activities well outside the park boundaries. For coastal areas, dredging is known
to have the potential of altering the natural beach system many kilometers
downdrift of the activities. In some cases where previous dredging effects have
adversely impacted a park system, beach nourishment with clean sand can be a by-
product of a dredging project and can be beneficial to a park by reinstating the
natural sand flows.
This report is intended to assist park personnel in the Southeast Region to
better assess potential dredging impacts on the natural system and to present
methods of ameliorating adverse effects. The subject is introduced by four
generic examples in which dredging can play a role in the quality and behavior
of the natural system. The response of the beach system to natural forces and
to dredging activities is reviewed. Dredging techniques are discussed as well
as their potential for beneficial and adverse effects. A literature review is
presented on the biological effects of dredging. Three actual case studies are
reviewed in which dredging is being carried out adjacent to park systems in the
Southeast Region. State and federal regulations governing dredging activities
are summarized. An annotated bibliography includes contributions of dredging
activities on nearshore ecology, beach stability and sand management practices.
17. Originator's Key Words 18. Availability Statement
Beaches
Beach erosion
Beach nourishment
Dredging
Intertidal effects
19. U. S. Security Classif. of the Report 20. U. S. Security Classif. of This Page 21. No. of Pages 22. Price
Unclassified Unclassified 123
UFL/COEL-88/015
REVIEW OF DREDGING EFFECTS
ON ADJACENT PARK SYSTEMS
December, 1988
Prepared For:
National Park Service
75 Spring Street, SW
Atlanta, GA 30303
Prepared By:
Robert G. Dean
Coastal and Oceanographic Engineering Department
University of Florida
336 Weil Hall
Gainesville, FL 32611
With A Contribution By:
Robert Dolan
University of Virginia
Charlottesville, VA 22903
TABLE OF CONTENTS
PART I
INTRODUCTION AND GENERAL TYPES OF DREDGING AND COASTAL
PROBLEMS ENCOUNTERED BY THE NATIONAL PARK SERVICE 1
Introduction 2
Generic Problems 3
Generic Problem 1 Inlet Dredging 4
Problem 4
Discussion of Natural System 4
The Altered System 4
Physical Effects 4
Environmental Effects 4
Solution 5
Generic Problem 2 Channel Stabilization through Jetty
Construction 6
Problem 6
Discussion of Natural System 6
The Altered System 6
Physical Effects
Environmental Effects 7
Solution 7
Generic Problem 3 Coastal Armoring 8
Situation 8
Discussion of Natural System 8
The Altered System 8
Physical Effects 8
Environmental Effects 8
Solution 8
Generic Problem 4 Beach Nourishment Using Sand Dredged
from Offshore Dredging 10
Problem 10
Discussion of Natural System 10
The Altered System 10
Physical Effects 10
Environmental Effects 10
Solution 11
PART II
THE NATURAL BEACH SYSTEM
Geology of Barrier Islands 12
Introduction 13
Origin of Barrier Islands 13
Sea Level Rise and Barrier Islands 16
The Importance of Natural Processes 16
Beach Features and Processes 17
Longshore Sediment Transport 17
Cross-Shore Sediment Transport 19
Variation of Sand Size Across the Nearshore 22
Role of Inlets 24
PART III
THE DREDGING PROCESS 30
Introduction 31
Dredging Objectives 31
Dredging Equipment Methodologies 31
PART IV
ALTERED COASTAL SYSTEMS 38
A. GENERAL 39
Introduction 39
Modifications of Channel Entrances 39
Beach Nourishment Projects and Their Evolution 41
Case (1) Project on a Long Uninterrupted Beach 41
Case (2) Placement Immediately Downdrift of a
Littoral Barrier 43
Profile Equilibration After Nourishment 43
Relative Benefits of Offshore Sand Placement at Various
Depths 43
Need for Profile Contouring 44
B. POTENTIAL DREDGING IMPACTS 45
Physical Effects 45
Dredging to Obtain Material 45
Dredging to Increase Navigation Channel Depths 47
Environmental Effects of Beach Nourishment Projects 47
Sediment Quality 47
Emerita Talpoida (Mole Crabs) 48
Donax (Coquina Clams) 49
Ocypode Quadrate (Ghost Crab) 50
A Case Study: Panama City, FL 50
Summary Regarding Intertidal Biological Effects of Beach
Nourishment 51
Sea Turtles 52
Consolidated Beach Sediments 53
Blocking of Turtle Crawls by Dredge Pipe
Miscellaneous Effects 53
Nest Relocation Programs 54
A Case Study: Jupiter Island, Florida 54
Summary Regarding Impact of Beach Renourishment on Sea
Turtle Nesting 56
PART V
CASE STUDIES AS EXAMPLES 57
CASE STUDY I PERDIDO KEY 58
Introduction
Background 60
Rationale for Selected Plan 60
Monitoring Plan 64
Physical Monitoring 64
Performance Related Monitoring Needs 64
Profile and Planform Evolution 64
Wave Measurements 64
Wind and Precipitation Measurements 64
Vegetation Response 65
Public Interest/Education Monitoring Needs 65
Management Monitoring Needs 65
Biological Monitoring 65
Benthic Community Studies 66
Vegetation Analysis 66
Beach Mouse Population 66
CASE STUDY II OREGON INLET 66
Introduction 66
Historic Inlet Behavior 66
Present Situation 68
The National Park System and State of North Carolina
Position on Oregon Inlet 68
CASE STUDY III CUMBERLAND SOUND 71
Introduction 71
Concerns of the National Park Service 71
Monitoring Program 71
Coastal Assessment Component 73
Cumberland Sound Physical Processes Component 73
Ecological Research 73
PART VI
LEGAL AND REGULATORY ASPECTS OF DREDGING 74
Introduction 75
THE FEDERAL PROGRAM 75
Rivers and Harbors Act of 1899 75
National Environmental Policy Act of 1969 (NEPA) 75
Federal Water Pollution Control Act Amendments
of 1972 (FWPCA) 76
Marine Protection, Research and Sanctuaries Act
of 1972 76
Coastal Zone Management Act of 1972 (CZMA) 76
THE STATE
The
The
The
The
The
The
PROGRAMS
North Carolina Program
South Carolina Program
Georgia Program
Florida Program
Alabama Program
Mississippi Program
PART VII
AN INVENTORY OF NATIONAL PARK SERVICE UNITS IN THE SOUTHEAST
REGION SUSCEPTIBLE TO EROSION AND EFFECTS OF DREDGING
PART VII
SUMMARY AND CONCLUSIONS
Summary
Conclusions
REFERENCES
PART IX
ANNOTATED BIBLIOGRAPHY
Ecological Effects of Beach Nourishment
Physical Performance of Beach Nourishment Projects
Bypassing at Inlets
Effects of Inlets
General
LIST OF FIGURES
FIGURE PAGE
1. Generic Problem 1 Inlet Dredging 4
2. Generic Problem 2 Jetty Construction 6
3. Generic Problem 3 Coastal Armoring 8
4. Generic Problem 4 Beach Nourishment 10
5. The Rise of Sea Level as Obtained from Carbon 14 Dates in
Relatively Stable Areas (From Shepard, 1963). Break in Slope
Some 6400 Years Before Present (BP) May Have Provided Basis
for Barrier Island Stability 15
6. Waves Arriving Obliquely to Shoreline Cause Longshore Current
and Longshore Sediment Transport Primarily Within Surf Zone 18
7. Estimates of Net Annual Longshore Sediment Transport Along
Florida's East Coast 20
8. Normal and Storm Profiles on a Natural Shoreline 21
9. Variation of Median Sediment Size with Location Across Beach
Profile. North Jupiter Island, FL 23
10. Relationship between Spring Range Tidal Prism and Minimum
Cross-Section, Compared with Maximum Inlet Velocity of
1 m/s. Modified from O'Brien (1931) 25
11. The Balance of Forces which Maintains the Ebb Tidal Shoal
Volume in Equilibrium 27
12. Relationship Between Equilibrium Volume of Sand Stored
in Ebb Tidal Shoal and Tidal Prism (Adapted From Walton
and Adams, 1976) 28
13. The "Sand Sharing System" Comprising the Inlet, the Ebb Tidal
Shoal and Adjacent Shorelines 29
14. The Clam Shell Dredge (From Bray, 1979) 32
15. Bucket Ladder Dredge (From Richardson, 1976) 32
16. Plan View of Operation of a Pipeline Dredge (From Bray,
1979) 33
17. Self-propelled Hopper Dredge with Trailing Dragarm (From
Richardson, 1976) 33
FIGURE
18. Plain Suction Dredge (From Richardson, 1976) 35
19. Cutter Suction Dredge (with Spuds) (From Richardson, 1976) 35
20. Split Hopper Barge (Self-propelled) (From Richardson, 1976) 37
21. Example of Evolution of Initially Rectangular Nourished Beach
Planform. Example for Project Length, Q, of 6 km, Effective
Wave Height, H, of 0.6 m and Initial Nourished Beach Width of
30 m 42
22. Effect of a Dredged Borrow Area on Wave Refraction and Wave
Energy Distribution along the Shoreline 46
23. Perdido Key and Entrance Channel to Pensacola Bay 59
24. Shoreline Change Rates for Escambia County, January 1974 to
October 1984. Based on Florida DNR Surveys. Note Shoreline
Change Rates Shown Have Been Smoothed by a Five Point Running
Average 61
25. Recommended Characteristics of Nourished Profile. Illustrated
for DNR Monument No. 48 63
26. Oregon Inlet 67
27. Cross-Section of Sloping Floating Breakwater Planned for
Deployment at Oregon Inlet to Provide Shelter for Sand
Bypassing Dredge Operating in its Lee. From Inman and
Dolan (1987) 69
28. Location Map of Kings Bay Site Relative to Cumberland Island
National Seashore 72
PAGE
LIST OF TABLES
TABLE PAGE
1. SUMMARY OF SHORELINE RESPONSE TO NEW JERSEY STORM OF
NOVEMBER 6-7, 1953 (From Caldwell, 1959) 21
2. FIELD TESTS CARRIED OUT TO EVALUATE SHOREWARD SEDIMENT
TRANSPORT FROM OFFSHORE PLACEMENT 44
3. CHARACTERISTICS OF SEA TURTLE AND NESTING IN FLORIDA 52
4. HISTORY OF MAINTENANCE DREDGING PENSACOLA ENTRANCE CHANNEL
1885-1987 58
5. CHRONOLOGY OF PENSACOLA BAY ENTRANCE CHANNEL DIMENSIONS 62
6. UNITS WITHIN THE SOUTHEAST REGION SUBJECT TO SALT/BRACKISH
WATER RELATED EROSION AND ADVERSE EFFECTS OF DREDGING 81
6. (Continued) 82
viii
PART I
INTRODUCTION AND GENERAL TYPES OF DREDGING AND COASTAL PROBLEMS
ENCOUNTERED BY THE NATIONAL PARK SERVICE
PART I
INTRODUCTION AND GENERAL TYPES OF DREDGING AND COASTAL PROBLEMS
ENCOUNTERED BY THE NATIONAL PARK SERVICE
Introduction
The coastal zone is ever-dynamic responding to the forces of waves, tides,
currents and winds. Long periods of relative stability can be terminated by a
sudden storm causing both temporary and permanent changes much greater than those
occurring over many years of mild weather. Even during periods when the beach
is relatively stable, there may be a large unnoticed transport along the shore.
The coastal zone is a desirable region for habitation, recreation and
industry. Some of these uses lead to desires to alter the natural system by
various means. Such modifications could include channel deepening for
navigational purposes, dredging for beach nourishment, coastal armoring to
stabilize an eroding shoreline, etc. Engineering interaction with the coastal
zone usually causes effects which can be anticipated adequately only through a
detailed and quantitative understanding of the natural processes. Although
understanding of these processes has developed considerably over the past few
decades, our information base is still inadequate and generally unanticipated
effects of engineering interaction may occur. Some of these effects are slow
and large scale and may influence the shoreline for distances of many kilometers
from their cause.
The National Park Service (NPS) as a manager of coastal lands including
barrier islands can be impacted by a variety of modifications by adjoining
property owners. In some cases, the concern occurs on NPS property and the NPS
may initiate a study seeking appropriate remedial measures. The present report
is directed to Park resource managers with the intent of providing a familiariza-
tion of the natural processes, the range of dredging related modifications that
can occur and the potential areas of concern that should be voiced at the first
level of review by the resource managers. The following section presents several
generic problems to illustrate with greater specificity the types of modifica-
tions, impacts and the significant factors.
Generic Problems
To some degree the overall purpose of this report is to develop an
awareness for natural coastal processes and an ability to foresee the potential
problems associated with alterations to the coastal system. Later sections of
this report will discuss the processes in detail. To provide a preview of the
types and range of problems addressed in this report four generic case studies
are presented. The format is a brief presentation for each problem in which the
problem is introduced with two diagrams, a before and after, the dominant
physical and environmental impacts and focus areas warranting attention by NPS
personnel in their considerations of this type of generic problem.
A common conceptual thread in all coastal engineering problems is that of
a sediment budget. That is, the natural system has adjusted to the current
situation of sediment inflows and outflows. Any alteration of the sediment
transport processes will tip the sediment budget out of balance and cause areas
of erosion and possibly deposition.
Generic Problem 1 Inlet Dredging
Problem Channel deepening of a natural inlet is being considered. The
sand dredged will be disposed of offshore.
The Natural System The Altered System
Figure 1. Generic Problem 1 Inlet Dredging.
Discussion of Natural System In the natural system, waves arriving at
an angle to the shoreline cause a transport of sand along the shoreline. The
transport rate is Q. Upon reaching this inlet, the transport occurs over a broad
flat sand body termed an ebb tidal shoal.
The Altered System Since the shallow depths over the ebb tidal shoal are
not suitable for navigation, a channel is incised through the shoal by dredging,
thus severing the natural transport pathway or "bridge". The natural response
of the system is to rebuild the sand bridge through deposition in the channel.
This will require periodic dredging to maintain the channel depth.
Physical Effects The ultimate physical effects that can be anticipated
include erosion of the downdrift shoreline at a rate Q. In cases where transport
occurs in both directions, both shorelines may erode.
Environmental Effects The erosion will degrade the natural characteris-
tics of the beach and may destroy valuable habitat. Over long periods, the dunes
may be eroded away, along with associated vegetation, the barrier island will
be overtopped by storms causing impact to back barrier vegetation. Stable areas
for turtle nesting may be affected.
Solution Sand dredged should be returned to the adjacent beaches in
locations established through a comprehensive monitoring program. Large
quantities of sand placed at infrequent intervals may cause substantial
fluctuations of the shoreline which could impact turtle nesting.
Generic Problem 2 Channel Stabilization through Jetty Construction
Problem Jetty construction is being considered to stabilize a natural
or deepened navigational channel.
The Natural System The Altered System
Figure 2. Generic Problem 2 Jetty Construction.
Discussion of Natural System In the natural system, waves arriving at
an angle to the shoreline cause a transport of sand along the shoreline. The
transport rate is Q. Upon reaching this inlet, the transport occurs over a broad
flat sand body termed an ebb tidal shoal.
The Altered System Since the shallow depths over the ebb tidal shoal are
not suitable for navigation and dredging without jetty construction may be
considered as too temporary a solution, jetties are planned to: (1) maintain the
channel alignment, (2) to limit sand deposition from adjacent areas, and (3) to
jet the deposited sand seaward from the channel.
Physical Effects The updrift jetty will cause impoundment of the sand
arriving at the jetty. Downdrift of the inlet, the waves have the same
transporting capacity and thus will cause erosion at the same rate as the sum
of the deposition on the updrift side, and accumulation of sand farther seaward.
Environmental Effects The erosion will degrade the natural characteris-
tics of the beach and may destroy valuable habitat. Over long periods, the dunes
may be eroded away, along with associated vegetation, the barrier island will
be overtopped by storms causing impact to back barrier vegetation. Stable areas
for turtle nesting may be affected.
Solution Provision should be made for bypassing of sand around the
entrance. Ideally the sand transfer should be at fairly frequent intervals so
as to minimize the shoreline fluctuations. A degree of flexibility should be
provided for placement of sand at various locations downdrift of the entrance.
Beach surveys as part of a monitoring program would establish the optimal
bypassing locations.
Generic Problem 3 Coastal Armoring
Situation A developed area updrift of a park is affected by a long-term
erosional trend as is the park area. To stabilize the shoreline, the developed
area is considering armoring the shoreline by construction of a seawall or other
shore protection structure.
The Natural System The Altered System
Figure 3. Generic Problem 3 Coastal Armoring.
Discussion of Natural System Natural forces, assumed unknown, are causing
a large scale erosional trend. This erosion is shared by both the developed area
and Park area as indicated above.
The Altered System By armoring the developed area shoreline, and thus
preventing erosion, this source of sand to the longshore transport system is
eliminated, thus placing greater erosional pressure on the downdrift shoreline
in the amount of the deficit imposed by the armoring.
Physical Effects Increased erosion rates of the downdrift shoreline.
Environmental Effects A more rapid rate of dune loss and the associated
habitat. Generally a narrower dry sand beach and thus less favorable for
successful turtle nesting.
Solution The developed area could compare the relative economic benefits
of beach nourishment using high quality sand. If not feasible, an alternate
approach would be for the armored area to place annually sand volumes equivalent
to the reduction in supply caused by the armoring.
Generic Problem 4 Beach Nourishment Using Sand Dredged from Offshore Dredging
Problem Stabilization of an eroding beach is being considered by beach
nourishment. This comprises the removal of a large quantity of material from
an offshore area (the "borrow" area) and placement by pipeline dredge on the
beach. A coral reef is present near the borrow area.
The Natural System The Altered System
Figure 4. Generic Problem 4 Beach Nourishment.
Discussion of Natural System The natural system is experiencing a long-
term mild erosional trend.
The Altered System The completed altered system will include an offshore
deepened "borrow" area with the removed material placed on the shore.
Physical Effects Possible physical effects include local modification
of the waves affecting the shoreline due to the deepened borrow area.
Additionally, the longevity or "life" of the project may be a matter of concern
as it relates to the length of time that the benefits will occur to the area and
the associated frequency for maintenance renourishment.
Environmental Effects In most cases the primary environmental effects
are related to the quality of sediment or damage to the reefs by improper
handling of equipment such as anchors. In this case quality relates to the grain
size characteristics and color of the sediment. Ideally, the sediment should
be of about the same grain size characteristics as the sand originally occurring
on the beach. An important characteristic relating to turbidity and sedimenta-
tion in both the borrow and placement areas is the percentage of fine sediments
in the material dredged. Also too great a fine sediment content will cause a
partial cementation of the sediment placed on the beach and thus adversely affect
turtle nesting.
Solution Ensure, through an extensive sediment coring program, that the
best quality sediment available is being used and that the sediment contains less
than 5% silt and clay. Additionally, require adequate set-backs from the reef,
state-of-the-art positioning equipment on the dredge and marking of the borrow
area and reef perimeter with floating buoys for easy visual identification.
PART II
THE NATURAL BEACH SYSTEM
PART II
THE NATURAL BEACH SYSTEM
Geology of Barrier Islands
(Pages 13-16 Contributed by R. Dolan)
Introduction
The Atlantic and Gulf coastal plains that form the seaward perimeter of the
S.E. Region are relatively flat lands that slope gently seaward to a wide
submarine continental shelf. The shore zone, or interface between the land and
sea portions of the coastal plains, consists of a series of barrier islands 3
to 30 km offshore. These islands are 2 to 5 km wide, 10 to 100 km long, and low
in elevation. The highest topographic features are sand dunes usually 3 to 6 m
above sea level. The lagoons or bays on the sound side of the islands are
shallow and may have large tidal mud flats and marshes.
The storms that generate large waves are the principal agents of change on
barrier islands. Winter extratropical storms produce waves of 5 to 10 m, with
storm surges of 1 to 2 m. Hurricanes (tropical storms), which occur less
frequently, also cause major landscape changes, especially in the vicinity of
their landfall.
Extratropical and tropical storms, with their strong waves and storm surges,
often drive water and beach sands completely across the barrier island. In
contrast, during periods between storms the beaches build seaward. Thus at times
the barrier island shorelines move landward and at other times seaward, in
response to varying energy conditions. In recent decades this movement has been
mostly landward, at a rate of about 1.5 m/yr for the Atlantic coast, and somewhat
less along the Gulf coast.
Origin of Barrier Islands
Barrier island formation and migration has been a subject of debate among
earth scientists for many years. There is, however, evidence that most of the
mid-Atlantic and Gulf coast barrier islands are migrating landward. Peats and
tree stumps, remnants of forest stands on the back sides of the islands emerge
on open ocean beaches, indicating barrier island migration or transgression.
Change along and across the barrier islands is usually a function of one
or more of these factors: the amount of sediment within a coastal segment, the
magnitude of natural processes (storms), and the stability of sea level. These
factors are also directly related to the geological origin of the barrier
islands.
Sea level has oscillated several times during the past half-million years.
During the interglacial periods, continental ice melted, and the shorelines
advanced inland across the continental shelves. During the glacial periods, as
water was withdrawn from the seas and stored in the form of glacial ice, the
shorelines moved seaward across the continental shelves. This process involved
great quantities of seawater, enough to move the ocean shoreline across roughly
150 km of the coastal plain and continental shelf. When the last period of
glaciation, the Wisconsin, came to an end about 20,000 years ago, sea level was
approximately 120 m lower than it is today (Figure 5), and the shorelines of the
Atlantic and Gulf coasts were 60 to 150 km seaward of their present positions.
With the change from glacial to interglacial, the sea started to rise and
continued to rise for about 14,000 years, reaching within a few meters of the
present about 6,000 to 7,000 years ago.
As the sea rose and the shoreline moved across the continental shelf, large
masses of sand were moved with the migrating shore zone in the form of beach
deposits. Sediment that had been deposited as deltas and floodplains within
the coastal river systems was also reworked by wave action and moved along the
shore. Once sea level became fairly stable, waves, currents, and winds worked
together on the sand to form the beaches and barrier islands that rim the coast
of the S.E. Region. As long as the inshore system contained surplus sediment,
the beaches continued to build seaward until equilibrium was reached--in this
case the balance among storm and wave energy, sea level, and the amount of
sediment in the transport system.
All the evidence suggests that this equilibrium was reached about 4,000 to
5,000 years ago. At that time the barrier islands were wider--some by as much
as 2 km or more. As time passed, the complex landscape of the barrier islands
evolved. In the narrow areas, inlets breached the islands and filled in to
reform them. Long spits connected the more stable sections, such as the land
area near Cape Hatteras, where sequences of beach ridges developed, building long
1
0 C & -- 0
0.08m/1 dOyrs -
100 25
*
Lw Q 50
LU 200 m
E
Cl,
S75
A Texas Shelf
300 + Holland
x Australia
o Southwest Louisiana
Eastern Argentina
West Louisiana Shelf- 100
a Western Mexico
I I I I
5 10 15 20
THOUSANDS OF YEARS BEFORE PRESENT
Figure 5. The Rise of Sea Level as Obtained from Carbon 14 Dates in Relatively
Stable Areas (from Shepard, 1963). Break in Slope some 6400 Years
Before Present (BP) may have Provided Basis for Barrier Island Stability.
chains of Holocene barrier islands. Exceptions to this model are the more stable
"sea islands" of Georgia (Cumberland), which differ significantly from Holocene
barriers in that Holocene material is deposited on the seaward side of detached
segments of the mainland Pleistocene terrace.
Sea Level Rise and Barrier Islands
Although Holocene sea level remained fairly stable following the initial
rise during the post-Wisconsin, sea level has risen several meters in the past
2,000 years. This slow rise has resulted in the recession of shorelines and the
enlargement of bays and sounds behind the barrier islands. Over the past 200
years, the rise has been rapid, totaling slightly more than 30 cm.
The rate of barrier island recession over the last 2,000 years undoubtedly
varied as the rate in the rise of sea level changed, as the supply of sand waned,
and as the slope of the bottom of the inshore zone evolved in response to storms
and waves. Some of the eroded material has been lost into large offshore
sediment sinks, such as Diamond Shoals off Cape Hatteras. Much of it, however,
has remained within the barrier island sediment budget and has contributed to
spit growth, inlet filling, dune building, and storm-overwash deposits.
The Importance of Natural Processes
Within the coastal and marine parks of the S.E. Region, the National Park
Service has long recognized the importance of allowing natural processes to
proceed in an uninterrupted manner. However, the NPS recognizes that some of
the coastal lands now administered as parks, recreation areas, and monuments were
altered by engineering structures or sediment management practices decades before
the areas were added to the NPS system. In addition, some of the bays, lagoons,
and inlets that are now part of the NPS lands have been and continue to be part
of the vital coastal waterways. For these reasons, the NPS realizes that all
of the barrier islands, inlets, and lagoons within their jurisdiction cannot be
managed as totally "natural" systems; however, it is important to recognize, when
assessing the potential implications of a dredging project within park lands,
that the natural processes are the processes that were responsible for the
formation of the islands, and for the great natural beauty of these areas. These
processes should not be interfered with unless absolutely essential.
1
Beach Features and Processes
Longshore Sediment Transport
Generally waves approach the coastline at an angle due to the relative
location of the wave generating area. In many locations, for example, the East
coast of Florida, the predominant wave direction is seasonal due to the dominance
of various storm patterns and locations at differing times during the year.
When waves reach a sufficiently shallow depth, they break, thereby
establishing the outer limit of the "surf zone", as shown in Figure 6. The
character of the water and sediment motions within the surf zone differ greatly
from those seaward of the surf zone. Within the surf zone, the breaking waves
exert a "force" on the water causing a movement of water along the shore called
a longshoree current". These currents are apparent to the casual swimmer as he
or she is displaced along the shoreline. The magnitudes of longshore currents
are generally small, on the order of 30 cm/second, but can range up to 150
cm/second. Due to wave breaking, the water inside the surf zone is much more
turbulent and chaotic than that outside the surf zone. These two characteris-
tics, the turbulent water motions and the relatively weak longshore current are
responsible for the mobilization and transport of sediment in a longshore
direction. As can be appreciated, the magnitudes of sand transported along the
shoreline depend on the wave height and direction characteristics and can vary
considerable from place to place and can even vary from year to year at a
particular locality. Interference with the longshore sediment transport will
cause areas of accretion and erosion.
Methods exist for the calculation of longshore sediment transport based
on wave heights and directions; however, due to our imprecise understanding of
transport processes and lack of quality wave data, results of such calculations
should be considered as estimates only. Some of the best field estimates of
longshore sediment transport are based on the rates of accumulation caused by
the construction of long impermeable structures on the updrift sides of channel
entrances. Still, such data must be interpreted carefully.
The notation generally adopted for the direction of longshore sediment
transport is that positive transport is to the right as an observer faces
gure 6 Waves Arriving Obliquely to Shoreline
Cause Longshore Current and Longshore
Sediment Transport Primarily Within
Surf Zone.
seaward. At most locations, both positive and negative transport occur during
a year. The difference between the positive and negative annual transport is
termed the "net" annual transport and is either positive or negative. In some
applications, it is the "gross" transport which is the sum of the positive and
negative components (irrespective of sign) that is of importance. An example
will serve to illustrate this convention. A published estimate (Walton and Dean,
1973) of positive and negative transport rates slightly south of Cumberland
Island, GA is
Q = 370,000 m /yr
3 (1)
Q_ = 170,000 m /yr
I-l .. b".Yi ..... ... .
-.':.i! ,u ., .,.: ....
rl sc i
..:.. .. .- ,, : .. ..
..... ,, : : ,.-;~U~l.~rPI
..- i
". :...: ...{
., %
,~'." ., i
Q_ = 170,000 m3/yr
Thus the total annual southerly and northerly transport components are
370,000 m3/yr and 170,000 m3/yr, respectively. The "net" and "gross" components
are
QNET = + 200,000 m /yr (Southerly)
(2)
QGROSS = 370,000 + 170,000 = 540,000 m3/yr
In general, the net longshore sediment transport is southward along the
East coast portion of the Southeast Region. Fairly detailed estimates exist
for the East coast of Florida as shown in Figure 7.
Cross-Shore Sediment Transport
It is well-known that beaches change seasonally and with differing wave
conditions. Although beach profile changes can occur due to longshore sediment
transport, the focus here will be limited to cross-shore sediment transport.
Winter waves are usually higher and generally have shorter periods than
those occurring during the summer months. The resulting summer and winter beach
profiles differ substantially as idealized in Figure 8. The summer profile
tends to be steeper with a wider berm and the winter profile tends to be milder
in slope and to have a narrower berm. Although the range in seasonal shoreline
fluctuations is not known for many locations, it has been shown to be on the
order of 30 m at Long Island, NY (Bokuniewicz et al., 1980), 80 m at Stinson
Beach, CA (Johnson, 1971), and 10 m at Boca Raton, FL (Dewall and Richter, 1977).
Additionally, in many locations, there is a bar present in the winter profile.
At some locations, the bar is perennial.
The mechanics of cross-shore transport are not understood completely;
however storm waves of greater heights and shorter periods cause offshore
transport. Table 1 presents an example of shoreline changes that occurred in
New Jersey due to a severe storm.
If the dunes are sufficiently high during a storm to prevent overtopping,
transport may be limited to the offshore direction. However, if the storm tide
level exceeds the dune elevation or if the dunes are breached, a process called
"overwash" may occur. Overwash is the transport of water and sand over a
normally subaerial feature usually due to storm-elevated water levels and
increased wave heights. The sand deposit resulting from an overwash event is
-7-ANT K JACKSONI LRNANDINA
81/SANTA K HOM C
.. ROSA A NANDINA
'( I;"yL, ,L (.A,;,"; 460,000m 3/yr
M 0 IWALTON D
S-- S'i L ON/ I/ HAMILTONC JACKSONVILLE
GULF FRANKLIN I ;R' ST. AUGUSTINE
L-- JOHNSONVILLE
I CHRIST JOHs MARINELAND
DIXIE -i ALACHUA, PUTN
GULF O LEVY '
L '--O AYTONA
G R MARION A 400
I. -N voLUs 400,000 m3/yr
S', EW SMYRNA
I CITUS I LAKE A\
PAscO R A-- -^ PE CANAVERAL
ST-N-- \OSSCEOLA| \l
P t !o POLK 300,000m 3/yr
N \ o
I E l rTNEIA n
.-/---- -------1 RIVER" VERO BEACH
A MANATEE' ....... I OKEE \" I 200,0 00m 3/yr
^ MANATEE: HARDEE | CHBEE ST. \\\ 00,0 PIER E
I --1HIGHLAN T PIERCE
\ -DE SOTO --- -
R-O i OKEAECHO ---T JUPITER I.
HARLOTT GLADE BEE 200,000m 3yr
*(L' a PALM BEACH
LE I HENRY I PALM BEACH
\ --/ EERFIELD
COLLIER BROWAR 100,000m 3yr
S I- I BAKERS HAULOVER
PK I 74 10,lOOm3/yr
Er"O DADE MIAMI
LONROf 7B
Figure 7. Estimates of Net Annual Longshore Sediment Transport Along
Florida's East Coast.
Normal Profile
Figure 8. Normal and Storm Profiles on a Natural Shoreline.
TABLE 1
SUMMARY OF SHORELINE RESPONSE
TO NEW JERSEY STORM OF NOVEMBER 6-7, 1953
(From Caldwell, 1959)
Contour Elevation Landward Retreat of Contours (m)
(m Above Mean Low Water) Average Maximum
0 20 34
1.5 19 27
3.0 30 55
4.5 16 37
termed "washover" and may be up to 2 meters or more in thickness. As relative
sea level rises, barrier islands maintain their elevation by washover deposits
which occur in "plaques" or layers. Deposition by wind also plays a role in
increasing the elevation of barrier islands. Significant overwash may not occur
for several decades or more and with a single storm, the resulting deposits may
achieve equilibration with increased sea level which has occurred since the
previous significant overwash event.
Variation of Sand Size Across the Nearshore
If one were to collect and analyze sediment samples ranging from the dune
to water depths of 10 m or so, it would be apparent that the "forces" in this
region cause a sorting of the available sediment according to size. Although
there would be a great deal of variability in sediment size from profile to
profile, in general the sand in shallower waters is coarser than in deeper
waters. There are a number of theories to explain the details of this sorting,
the most intuitive being that the shallow water zone is an area of breaking waves
and thus if fine sediment were introduced it would be repeatedly resuspended and
could only be stable in the deeper water offshore where the near-bottom water
velocities are much weaker. This is the same argument explaining why dust
particles settle in the relatively quiet corners of rooms. Figure 9 presents
an example from Martin County, FL of sand size variation from the dune crest out
to a water depth of nearly 10 meters. It is seen that the size decreases from
0.4 mm to 0.1 mm in the deeper water. The past processes that are responsible
for the present distribution of nearshore sediments are complex and it is
possible to find nearshore deposits of coarse sediments although the decrease
in size in the offshore direction is the general case and is relevant to the
quality of nearshore material available for beach nourishment purposes.
In addition to the distribution of size discussed above, there is a general
increase in the offshore direction of the range of sizes of a sediment sample.
Due to the fairly intense turbulence in the nearshore zone and the resuspension
mechanism discussed earlier, there is almost no silt/clay content present.
However in depths exceeding 5-10 m, it is not unusual to find silt/clay
percentages of 5-10% or greater.
1
Z"
wE
C')'
wo
z L
Ocn
I-z
J EU
0 w
> I
w O
0 200
600
1000
1400
1800
DISTANCE OFFSHORE (m)
200
600
1000
1400
1800
Figure 9. Variation of Median Sediment Size with Location Across
Beach Profile. North Jupiter Island. Florida.
0.4
0.3
0.2
0.1
I I I I I I I I I I
4
Du ne 2. -
2
4
6
8
10
_1
200
-
Role of Inlets
Inlets are channels connecting outer waters to interior lagoons or bays.
Inlets are subjected to two competing forces. The rising and falling tides in
the ocean cause water to flow into the bay (flood currents) and out of the bay
(ebb currents). The volume of water entering an inlet during flood (inward)
flow or leaving during ebb (seaward) flow is termed the "tidal prism", an
important characteristic of the inlet bay system. Through refraction, waves tend
to transport sand toward inlets and to cause closure. The tidal currents through
the inlets scour excess sand from the channel maintaining it open. To understand
how these two competing forces interact, consider the simple system discussed
below. If an inlet were excavated wider than equilibrium, the scouring
velocities would decrease, sand would enter the channel and deposit thereby
decreasing the cross-sectional area with a corresponding increase in tidal
currents which would decrease the tendency for sand deposition. Conversely, if
sand were placed in an inlet in equilibrium conditions, thereby decreasing the
flow area, the velocities would increase, scouring out the excess and returning
the area to equilibrium. Thus the inlet cross-sectional area tends to be self-
equilibrating. O'Brien (1931) found the surprisingly simple relationship shown
in Figure 10 between the equilibrium cross-section of an inlet and the total
volumetric flow of water passing through an inlet during flood or ebb flow. The
dashed line in Figure 10 represents a peak velocity through the inlet of 1 m/s.
Thus it appears that in a sandy material, under equilibrium conditions, the inlet
adjusts itself such that the peak velocities in and out of the inlet are on the
order of 1 m/s. As described previously, any change in cross-sectional area to
alter this velocity will induce velocity changes to reestablish the equilibrium
area.
With the above discussion of the flow in and out of inlets and the related
scour of excess sediments, it is not surprising to find extensive sand deposits
bayward and seaward of the inlet channel. These deposits are termed flood and
ebb tidal shoals signifying the currents causing their transport to these
locations. These shoals are extremely important to the inlet stability; their
roles will be reviewed below.
The flood tidal shoals represent deposits that are relatively static and
grow to substantial volumes and, due to the usually moderate wave climate inside
10 -1 0i I .* .. ...1.111 I .1.
0 No Jetty
SA One Jetty
E 109 __ Two Jetties ._ _
CE
0l0
Z-E---- .-----
wU Corresponds to
S0 Vmax = m/s
1 Z 107.
-O'Brien's Best Fit
to Data
1 6 1 I ilI I II l I 1 I1 Ii I II
10 102 103 104 10 5 10 6
MINIMUM CROSS-SECTIONAL
FLOW AREA (m2)
Figure 10.
Relationship between Spring Range Tidal
Prism and Minimum Cross-Section, Compared
with Maximum Inlet Velocity of 1m/s. Modified
from O'Brien (1931).
the bays, may become vegetated and emerge as islands. As these features grow,
they reduce the hydraulic efficiency of the inlet and may contribute to slow
downdrift migration of the inlet to a more hydraulically favorable location or
may contribute to inlet closure and formation of a new inlet at a more
hydraulically conducive location.
The ebb tidal shoals are located in an area of much greater wave energy than
the flood tidal shoals. Breaking waves tend to drive the sand toward shore and
the ebb tidal currents induce seaward transport, Figure 11. Immediately after
formation of an inlet, the wave effect is weak due to the greater depths in the
vicinity of eventual ebb shoal formation. With continuing deposition and local
shoaling, the shoreward forces due to waves become more effective and ultimately
an equilibrium is achieved when any additional sand transported to the shoal by
the ebb tidal currents is driven back into the nearshore system by the waves,
Figure 11. In areas where a net longshore sediment transport is present, the
ebb tidal shoal and its extensions to shore provide a "sand bridge" by which the
net transport makes its way around the entrance. The shape of the ebb tidal
shoal is indicative of the relative wave energy with shoals in high energy
regions characterized by rather smooth and regular outer contours and those in
low energy regions by irregular contours. Volumes contained in these ebb tidal
shoals can be enormous. Dean and Walton (1975) developed and applied a technique
to establish the volume of sand in an ebb tidal shoal. It was found that the
Boca Grande Pass ebb tidal shoal on the West coast of Florida contained
approximately 200 million cubic yards of sand. Later Walton and Adams (1976)
calculated the ebb tidal shoal volumes for a large number of inlets and found
that the volumes correlated well with tidal prism and relative wave energy.
Figure 12 presents these results where it is noted that in accordance with
earlier discussions, the ebb shoal volumes decrease with increasing wave energy.
It is important to recognize that in the vicinity of an inlet, the ebb tidal
shoal and the adjacent beaches form components of a system in equilibrium. If,
for some reason sand is removed from one component of this system, all components
will respond to reestablish equilibrium. These components have been termed
appropriately a "sand sharing system" (Figure 13). We will see later the impact
of removing sand from the ebb tidal shoal of this "sand sharing system".
(C)
(W)
Figure 11.
Ebb Tidal Shoal
The Balance of Forces which Maintains the
Ebb Tidal Shoal Volume in Equilibrium.
Bay
Flood
t
.
.
.
,
1000
I I I l lIlll I I I i iH HII I I I |l li14
S- 00 -
_-'-
O L
0Z
0 -=
'j-J
Mildly Exposed Coasts
m Moderately Exposed Coasts-
~Highly Exposed Coasts
LU
0. -
1-
1 10 100 1000
TIDAL PRISM (m 3x 10"6)
Figure 12. Relationship Between Equilibrium Volume of Sand Stored
in Ebb Tidal Shoal and Tidal Prism (Adapted From Walton
and Adams, 1976).
'- `3
I
1-"
Figure 13. The "Sand Sharing System" Comprising the Inlet, the Ebb
Tidal Shoal and Adjacent Shorelines.
Inlet
~S~S;
.0\O
ellOQ
/'
PART III
THE DREDGING PROCESS
PART III
THE DREDGING PROCESS
Introduction
In this section we review the objectives of dredging, the equipment and
methodologies used in dredging and some of the general effects of dredging that
can adversely impact the environment.
Dredging Objectives
There are two general and fairly obvious reasons for dredging. First is
the removal of quantities of material from an area in which it is regarded as
an impediment to one or more particular activities. Examples are dredging of
a navigational channel or a marina. In this case, the placement of the material
removed is usually of secondary interest to those carrying out the dredging and
in the absence of other considerations, the material will be disposed of by the
least costly method. The second reason for dredging is to obtain material for
some use. In our context, beach nourishment is the more usual example and the
quality of the material can be of prime concern. Regardless of the reason for
dredging, removal and placement of large quantities of material will obviously
cause physical and environmental perturbations in the dredging area and in the
area where the material is placed.
Dredging Equipment Methodologies
The two general classes of dredges include mechanical dredges and hydraulic
dredges. Mechanical dredges include clam shell dredges (Figure 14) and bucket
dredges (Figure 15). Practically all large dredging projects are carried out
with hydraulic dredges; therefore, this discussion will be limited to this class.
Within the hydraulic dredge class, there are two general types of dredges,
i.e. the pipeline dredge and the hopper dredge. Pipeline dredges move very
slowly, excavating to a substantial additional depth before they leave an area,
Figure 16. This material is transported as a water-sediment mix or "slurry"
through a pipeline to the area desired. Hopper dredges load material into a hull
while underway and transport the material as a bulk cargo, Figure 17. The
original hopper dredges were designed (as their name implies) to store the
Luffing Wire
Hoist and Hold
Wires
Hoist and Hold
Winch
Aft Wire
Underwater Fairleads
Figure 14. The Clam Shell Dredge (From Bray, 1979).
Ladder
^%?O
Discharge Chute
Do
"- Bucket
Figure 15. Bucket Ladder Dredge (From Richardson, 1976).
IW~i
Plan View of Operation of a Pipeline Dredge (From Bray, 1979)
Hoppers
A7 A --
Figure 17.
Self-propelled Hopper Dredge with Trailing Dragarm
(From Richardson, 1976).
Figure 16.
dredged material in hoppers and upon reaching the disposal site, to discharge
the cargo through bottom dumping hopper doors. During the last two decades, due
to the need for beach nourishment, a number of hopper dredges have been modified
to include a capability to pump out the hulls and complete the delivery to the
beach or nearshore area via a pipeline.
Pipeline dredges are rated by the size of their discharge lines (or pipes)
and the rate of sand discharge varies substantially with pipe size. The
approximate range of pipe sizes is 15 cm to 120 cm with corresponding pumping
rates from 50 cubic meters per hour to 3,000 cubic meters per hour. Thus the
size of a project will dictate, to some degree, the size of the equipment. As
an example, a project requiring dredging of 1,000,000 cubic meters would usually
result in contracting a dredge of 60 cm diameter, i.e. approximately 1,000 hrs
of required pumping time.
The elements of a pipeline dredge include an intake pipe mounted on a
"ladder", a dredge pump and a discharge line to the placement area. The sediment
to be pumped can be mobilized by jets of water in which case the dredge is called
a "suction head" dredge (Figure 18) or if sediment mobilization is caused by a
rotating cutter head, the appellation "cutter head" dredge is used, Figure 19.
The ladder can be moved both horizontally and vertically to access more sediment
while the dredge is in a fixed location. The dredge pump is mounted on a barge
and is a centrifugal type pump with hardened elements to resist wear caused by
the pumped sand. The discharge line connects to the outlet of the pump and this
line, generally in segments of 10 m length or greater, transports the sand to
the point of delivery. If the discharge line is so long that the power supplied
by the dredge pump will not transport the slurry at sufficiently high velocities,
it may be necessary to install "booster" pumps periodically along the pipeline
with a booster pump every mile or so for smaller pipelines and booster pumps
every two to three miles for the larger pipelines. Pumping over distances in
excess of 20 km have been accomplished. It is necessary to maintain velocities
above the sediment settling values or else there is a risk of deposition
occurring in the pipeline leading to its eventual plugging. Since settling
velocity increases with size of the sediment particles, the larger the size, the
greater the required water velocity in the pipeline. Typical pipeline slurries
.Pumps, Motors, Etc.
Discharge Pipe
i Pipe
-"4- Jet Cutting Assist
YA~i7Ah~
Figure 18. Plain Suction Dredge (Swell-compensated)
(From Richardson, 1976).
Figure 19. Cutter Suction Dredge (With Spuds)
(From Richardson, 1976).
are on the order of 16% to 20% solids by volume with the remainder being water
and pipeline velocities are on the order of 5 to 8 m/s.
The "side-casting" dredge is a variation of a suction dredge which
discharges the sediment only a short distance (30 m to 100 m) to the side through
a pipe-line supported above the water surface. The relocation of sediment by
such short distances raises questions about its effectiveness, i.e. the length
of time before the sand will be redeposited in the area deepened by the dredging.
Because of these concerns, most sidecasting is carried out only for emergency
purposes.
Hopper dredges are essentially a ship hull configured as a bulk carrier.
These dredges can have bottom doors which allow release of the material carried
or more recent designs are termed "Split Hull", with large hinges fore and aft
allowing the hull to split, dropping its cargo of material, Figure 20. Loading
of hopper dredges occurs through two "drag heads" while the dredge is underway;
these drag heads are pulled along the bottom agitating and entraining the
material into a pipe which carries the slurry up and into the hull. The hull
functions as a settling basin with the sediment settling out and water and fine
sediments returning over the side. Once the dredge is fully loaded, the drag
heads are raised above the water and the material transported to the placement
site.
Plan
1~
SI %, I
\' 14norw
Iz
0
I I I ^ =__
Front Elevation
Side Elevation
Figure 20. Split Hopper Barge (Self-propelled)
(From Richardson, 1976).
' "nI"
PART IV
ALTERED COASTAL SYSTEMS
PART IV
ALTERED COASTAL SYSTEMS
A. GENERAL
Introduction
Natural beach and inlet systems may be altered in several ways, including
dredging and constructing channels at entrances, building structures along the
shoreline and nourishing beaches. Each of these alterations and their potential
impacts on the natural system will be described below.
Modifications of Channel Entrances
Modifications of natural channel entrances or construction of new entrances
have been carried out primarily for purposes of navigation and secondarily to
improve flushing and renewal of interior waters. Even those entrances that were
constructed initially for water quality improvement have been modified later for
navigational purposes. The reasons for navigational modifications include the
aforementioned shallow and energetic ebb tidal shoal which under even moderate
wave action may be treacherous or unsuitable for navigation. Even though some
ebb shoals have relatively deep natural channels incised through them, these
channels are generally circuitous and tend to migrate in an unpredictable manner,
thus contributing to the navigation jeopardy. To improve these channels for
navigation, many have been stabilized through construction of jetties which are
usually long stone structures lining the channel and extending up to several
kilometers into the sea. The term "jetty" derives from their intended function,
i.e. to constrain the seaward flows causing excess sand to be jetted offshore
by the ebb tidal currents. Jettied inlets can cause/institute changes to the
downdrift shoreline by interfering with the longshore sediment transport and by
modifying wave patterns. The greatest effect is the physical interference with
the longshore sediment transport. If no sand is bypassed around the entrance,
and if the jetties are impermeable, the updrift jetty will trap, on an annual
basis, the net longshore sediment transport rate. Not surprisingly, the annual
erosion on the downdrift beaches will occur at the same rate. If the jetties
are leaky allowing sand to flow through them and transport reversals occur, the
downdrift erosion can exceed the net longshore sediment transport. Also leaky
jetties can result in erosion of the updrift shoreline. If all of the downdrift
transport passes through the updrift jetty, the updrift shoreline will erode at
the rate of the updrift transport component. For the same scenario, the
downdrift shoreline will erode at the rate of the downdrift transport component.
From the preceding discussion, there is a clear need at modified entrances
to attempt to reinstate the sediment transport that has been interrupted by the
modifications. Unfortunately our "track record" in this regard has been much
less than exemplary. In many cases sand removed by hopper dredges for channel
maintenance has been transported offshore and deposited in water too deep to
benefit the nearshore system. Data available for the East coast of Florida shows
that within the last 5 decades or so, more than 50 million cubic meters of beach
quality sand has been disposed of in excessive water depths. Today's market
value of this sand is on the order of $250 million to $500 million. Sand
deposits as a result of channel modifications and construction should be regarded
as a valuable natural resource and not as a material to be disposed of in the
least costly manner. Returning to Florida East coast examples, it can be shown
that this 50 million cubic meters is sufficient to advance the entire 600 km East
coast shoreline seaward by 8 m. Dean (1988) has estimated that 80% of the
erosion along Florida's East coast is due to poor sand management practices,
which continue today albeit to a lesser degree.
In general, there are two approaches to maintaining longshore sediment
transport. One approach is to allow the sediment to accumulate either updrift
of the updrift jetty or in the channel and to bypass periodically, relatively
large quantities of sand. Such bypassing could be carried out annually or
biennially and could involve from hundreds of thousands of cubic meters to 2
million cubic meters in each bypassing event. This mode of bypassing is
accomplished by a rather large dredge brought to the area periodically or when
needed. The alternative approach is a "dedicated" bypass facility which
transfers sand with much greater frequency more or less as it becomes available.
The downdrift consequences of these two modes of bypassing differ markedly. In
the "batch mode" of bypassing, the downdrift shoreline will widen and narrow as
the replenishment and erosional sand waves move downdrift. This variation in
beach width may not be favorable for intertidal or nearshore fauna. Clearly in
cases where nearshore rock or reef is present and considered a valuable habitat,
the covering and uncovering of these resources may cause adverse effects. By
contrast, the more-or-less continuous bypassing mode tends to mimic the natural
processes and thus minimizes any resulting disturbances.
Some modified inlets will continue to bypass small quantities of sand
naturally whereas others will represent a complete obstruction. In many cases
the distinguishing feature is whether or not and to what depth the channel is
dredged. If the channel is not dredged, the presence of the jetties will alter
the sand transport patterns usually resulting in an increase in volume of the
ebb tidal shoal and a deflection offshore and downdrift of the ebb shoal and
associated "sand bridge" or sand bypassing bar. Thus, the bypassing efficiency
by natural forces will be decreased markedly.
Beach Nourishment Projects and Their Evolution
Beach nourishment comprises the addition of relatively large quantities of
beach quality sand. Generally, the length of time that sand remains in the area
placed is considered as a measure of the physical performance of a beach
nourishment project. The discussion on performance will be presented for two
situations: (1) a project on a long uninterrupted beach, and (2) a project
immediately downdrift of a littoral barrier.
Case (1) Project on a Long Uninterrupted Beach In this case the longevity
of a beach nourishment project is defined as the length of time that a specified
percentage of the added material remains in the area placed. The longevity can
be shown to be proportional to the square of the length, 2, of a project,
inversely proportional to the breaking wave height Hb, raised to the 5/2 power
and related to the sediment size. The half life, t of an initially
rectangular beach planform composed of medium sized sand can be shown to be
2
t = 0.17 5/2 (1)
(Hb)
in which t5 represents the time in years required for 50% of the sand to be
transported out of the region placed, is the project length in kilometers and
Hb is the effective wave height in meters. Figure 21 presents an example of the
evolution of an initially rectangular planform. Initially the sharp corners are
DISTANCE FROM ORIGINAL
SHORELINE (m)
-Original Nourished Beach Planform
-Planform After 3 Months
- 10 Months
- 7 Years
-30 Years
-130 Years
Pre-Nourished
%I'- rzy Shorelne
9 6 3 0 3 6 9
ALONGSHORE DISTANCE (km)
Figure 21.
Example of Evolution of Initially Rectangular Nourished Beach
Planform. Example for Project Length,-Z, of 6 km, Effective
Wave Height, H, of 0.6 m and Initial Nourished Beach Width
of 30 m.
rounded and changes occur rapidly. As the evolution progresses, the planform
anomaly begins to behave as a longer project and changes occur much more slowly.
In evaluating the performance of a beach nourishment project, it is
important to note that if the sediment is of good quality, although eventually
the sediment will be transported out of the region placed, it will remain within
the region of active nearshore sediment transport and will continue to provide
benefits to those areas to which it is transported.
Case (2) Placement Immediately Downdrift of a Littoral Barrier This
situation is fairly common due to the aforementioned adverse impact of inlets
modified or constructed for navigational purposes. As intuition would suggest,
if the longshore sediment transport deficit is large, the life of the beach
nourishment project will be short and in such cases, rather than considering the
longevity of the project as a measure of its performance, it may be more
appropriate to regard the nourishment as a "feeder beach" placed to reinstate
the longshore sediment transport.
Profile Equilibration After Nourishment
In addition to planform evolution, the profile will change from that
initially placed to one that approaches equilibrium with the incoming wave
characteristics and sediment size. The quality or size of sand used in
nourishment governs the shape of the equilibrium beach profile. Sand of the same
size characteristics as the original beach will have an equilibrium profile the
same as the pre-nourished beach. Sand coarser or finer than the original will
have equilibrium profiles steeper or milder, respectively, than the original
profiles.
Relative Benefits of Offshore Sand Placement at Various Depths
In some cases, it may be less expensive to place the sand in the nearshore
region than on the dry beach. Questions have arisen regarding the effectiveness
of this approach. There have been several attempts to place substantial
quantities of sand in the nearshore region and to carry out monitoring to
determine whether the sand was transported shoreward. The field test programs
and the experience with each is summarized in Table 2. As can be seen, only the
TABLE 2
FIELD TESTS CARRIED OUT
TO EVALUATE SHOREWARD SEDIMENT
TRANSPORT FROM OFFSHORE PLACEMENT
Location Water Depth Documented
(m) Movement
Toward Shore
Santa Barbara, CA 6 No
Long Beach, NJ 11 No
Atlantic City, NJ 4.5 8 No
New River Inlet, NC 2 4 Yes
placement in water depths of 2-4 m was definitively concluded to be a success
in terms of shoreward sediment transport.
Based on results such as summarized in Table 2, the state of Florida has
considered that sediment placed in water depths greater than 4 m is relatively
ineffective in nourishing the shoreline. There can be undesirable effects of
placing sand in the nearshore region. In particular, if the sand deposited
offshore is not at a uniform elevation in the longshore direction, local
sheltering can occur causing sand to accumulate on the shoreline behind those
segments with the greatest elevations and erosion of adjacent areas. Thus if
sand is to be placed offshore, it should be placed in an underwater berm of
nearly uniform elevation with a gradual decrease in elevation to the ambient
profile at the ends of the berm.
Need for Profile Contouring
Sand placed in a beach nourishment project should be configured to allow
natural processes to complete the shaping to natural profile characteristics with
the elements described previously. The underwater portion of the profile does
not present a problem as the waves will carry out the contouring. However, the
above water portion of the profile should be constructed at a sufficiently low
elevation that the run-up and overtopping due to waves can complete and "fine-
tune" the profile shaping. If the berm portion of the profile is placed too high
for waves and run-up to play a role, the resulting profile will retain an
artificial characteristic.
B. POTENTIAL DREDGING IMPACTS
Physical Effects
As described previously dredging is usually carried out to provide or remove
sediment for some purpose, such as a beach nourishment project or to provide
desired channel depths for improved navigation. Each of these two cases will
be discussed below.
Dredging to Obtain Material In this case the area from which the material
is removed (the "borrow" area) can be fairly extensive in size and on the order
of 3-6 m deeper than the ambient bottom. This anomaly can cause less damping
as the waves propagate toward shore, thereby causing slightly greater breaking
wave heights. Probably of greater importance than the net increase in wave
energy is the modified distribution of wave energy along the shoreline due to
wave refraction. The wave rays which are everywhere perpendicular to the wave
crests will tend to diffuse or spread out over the deepened area thereby
lessening the wave energy at some areas along the shoreline and increasing it
at others. The areas of wave energy increase and decrease would depend on the
wave direction as can be seen by reference to Figure 22. As there is no simple
"rule of thumb" to define the effect of such a bathymetric anomaly, wave
refraction studies should be carried out for each case to establish the potential
impact.
For purposes of later discussion, it will be of interest to comment on the
filling of the borrow depression. Although there is not a large data base
relating to this matter, borrow areas are characterized by low wave energy and
thus tend to fill with finer sediment than that removed. In areas where the
bottom is highly mobile and where concentrations of suspended sediment are small,
a greater percentage of the filling material will be from the adjacent bottom.
Zone of
Increased
Wave Energy
Zone of
Reduced
Wave Energy
Zone of
Increased
Wave Energy
Figure 22.
' Deepened
"Borrow" Area
Effect of a Dredged Borrow Area on Wave
Refraction and Wave Energy Distribution
along the Shoreline.
Dredging to Increase Navigation Channel Depths Earlier sections of this
report have discussed the "sand sharing" system composed of the ebb tidal shoal
and the adjacent shorelines. A useful basis for consideration purposes is that'
a given system in its natural condition is in equilibrium and that if changes
are made to the system, it will respond to reestablish equilibrium. Thus when
sand is removed from an ebb tidal shoal, sand will flow toward the deepened area
and a portion of this deficit will be felt at the updrift shoreline and a portion
at the downdrift shoreline. If the longshore sediment transport were nearly
unidirectional, one can simplify considerations as follows. The longshore
sediment transport tends to rebuild the ebb tidal shoal which functions as a
"sand bridge" across which this transport occurs. With the sand bridge cut
(shoal deepened), the longshore sediment transport will deposit in the cut to
reestablish the bridge. The volume of material deposited appears as a deficit
to the downdrift shoreline and results in a volumetrically equal amount of
erosion there.
The obvious appropriate approach to placement of beach quality sediment
removed from navigation channels is, through surveys of the adjacent shorelines,
to develop a basis for apportioning the high quality dredged sand on these
shorelines.
Environmental Effects of Beach Nourishment Projects
Primary potential environmental effects of beach nourishment relate to:
quality of sediment, impact of burial by the placed sediment and the more subtle
effects above water such as altering the natural dune system. The actual impact
of each of these is species dependent and to some extent locality dependent.
Sediment Quality In this section sediment quality will be considered on
a relative basis and will be quantified in terms of grain size and color. As
an ideal measure of sediment quality, the grain size distribution of the material
to be placed should match the native grain size distribution. As a more
realistic measure of good sediment quality, the general mean grain size of the
material to be placed should not be much smaller than that of the native material
and the percentage of the silt and clay fraction (the fines) should be relatively
small.
There has been considerable debate concerning the allowable percentage of
silt and clay and understandably in some project areas, the allowable limit will
be less than in others. A biological study conducted after the Miami Beach, FL
nourishment project (1976-1981) concluded that silt and clay percentages greater
than 10% could cause substantial damage to offshore coral reefs. (The silt and
clay portion of a sediment sample is that fraction with diameters less than
0.0625 mm). The Department of Environmental Regulation of the State of Florida
is currently attempting to quantify acceptable levels of silt and clay for
placement on the beach. It appears that if a value is adopted, it will be less
than or equal to 10% with 5% being a value which has been discussed considerably.
Turbidity concerns are both short-term and long-term. During placement, a small
percentage of silt and clay will generate quite visible turbidity. In some
cases, this turbidity remains confined primarily within the active surf zone,
spreading out in the longshore direction. Apart from the surf zone, the initial
turbidity is spread offshore over a wide region with generally low concentra-
tions. If silt and clay concentrations are high, the turbidity considerations
are likewise high and can present potential problems to both sessile animals
(those which cannot move) and motile animals (those which can move). Generally
fish will move away from turbidity avoiding the potential effects.
Nelson (1985) has presented an excellent review of the effects of beach
nourishment on the nearshore biota. The primary focus was on four common
nearshore organisms: (1) Emerita talpoida (mole crabs), (2) Donax (coquina
clams), (3) Ocypode (ghost crabs), and (4) Sea Turtles.
Emerita Talpoida (Mole Crabs)
This organism is a filter feeder that burrows in the lower foreshore of the
beach and can be very abundant, although the densities tend to be quite
irregular. The highly energetic swash zone appears to be the preferred
environment for E. Talpoida probably enhancing the food supply. Densities in
excess of 3,700 animals per square meter have been reported (Bowman, 1981). The
animals tend to be in greatest abundances in Florida in December to January.
E. Talpoida are very mobile and apparently have the capability to avoid
being buried by beach nourishment by leaving an area. In a project in which
956,000 m3 sand was placed on Cape Hatteras beach, Hayden and Dolan (1974) found
no dead animals and they concluded that the affected areas recovered in less than
two weeks. The sand used in this nourishment project was quite compatible with
the native sand. A second project of similar quantity (904,000 m3) at Fort
Macon, NC, was monitored by Reilly and Bellis (1978, 1983); however the sand was
taken from dredged harbor sediments and was not compatible in size characteris-
tics. Additionally the sediment was from a chemically reducing environment.
Monitoring of this latter project indicated that the E. Talpoida populations were
nonexistent in the project area during material placement but recolonized rapidly
several months later during the spring recruitment period. A delay of one month
during the recruitment period was evident. The summer after the commencement
of nourishment (the preceding December), the animal densities were the same on
the nourished and control beaches. However, there were significant differences
in the size classes with the nourishment containing exclusively juveniles. The
investigators concluded that the adult mole crabs in the vicinity of the
nourished site were killed by turbidity and that the juvenile animals had
repopulated the area from the adjacent beaches. Nelson (1985) has suggested that
the liberated hydrogen sulfide in the nourished sediments may also have
contributed to the mortality of adult animals.
In summary of the impact of beach nourishment on E. Talpoida, it is
concluded that these animals are very mobile and are able to vacate an area
unsuitable for their physiology. Moreover, with the return of favorable
conditions, they rapidly recolonize the area. If the material placed is
compatible with that originally present on the beach, effects are of quite short
duration. If poor quality sediment is used, recovery is slower, but still
relatively rapid, probably due to the high motility of these animals and the
longshore currents on the beachface.
Donax (Coquina Clams)
This genus of bivalves has two species that have been reported to be present
in the Southeast Region. The documented range of Donax Variabilis is from
Virginia Beach, VA to Mississippi. Also Donax Texasianus has been found in the
Florida panhandle.
Most Donax Variabilis migrate up and down the beach with the tide,
presumably to be in the active swash zone where the high velocities ensure ample
quantities of moving water from which these filter feeders obtain nourishment.
However some studies have reported populations that do not migrate with the tide.
The life of Donax is generally 2-3 years with one or two spawning periods per
year. Primary spawning occurs in February and in Florida a second spawning may
occur in June. The peak seasonal abundance tends to occur in June and July.
Maximum densities of Donax Texasianus in Panama City, FL was 2,050 animals/m2.
Few studies are available documenting the effects of beach nourishment on
Donax. Reilly and Bellis (1978, 1983), reporting on the effects of nourishment
on a North Carolina beach found that following a December nourishment event,
Donax were not found in the nourished area until the following July. These were
young believed to be transported in by the longshore currents and it was
suggested that the adults were killed by burial in the offshore area.
Ocypode Quadrata (Ghost Crab)
These animals burrow in the dry beach although they lay their eggs in water.
The older crabs tend to burrow higher on the beach than the young animals. Their
diet varies from dead plant and animal material to live Donax and Emerita.
Although seen frequently during the daytime, they are primarily nocturnal.
Only the studies of Reilly and Bellis (1978, 1983) have evaluated the
effects of beach nourishment on ghost crab populations. Their limited data
indicated that the summer following nourishment, there was a 50% lower
population. Their interpretation was that, since the material was placed below
a level that would cause direct burial and since the crabs could probably burrow
up through placed sand, it is likely that the reduced population was a result
of emigration of the crabs due to a reduced food supply.
A Case Study: Panama City, FL
Saloman (1976), Culter and Mahadevan (1982) and Saloman, et al. (1982) have
reported on extensive biological studies in conjunction with the 1976 nourishment
of some 300,000 cubic meters placed along the beaches of Panama City.
Saloman (1976) conducted a pre-nourishment baseline study in 1974-1975 and
documented the effects of Hurricane Eloise (September, 1975) on the biota. It
above water portion of the profile should be constructed at a sufficiently low
elevation that the run-up and overtopping due to waves can complete and "fine-
tune" the profile shaping. If the berm portion of the profile is placed too high
for waves and run-up to play a role, the resulting profile will retain an
artificial characteristic.
B. POTENTIAL DREDGING IMPACTS
Physical Effects
As described previously dredging is usually carried out to provide or remove
sediment for some purpose, such as a beach nourishment project or to provide
desired channel depths for improved navigation. Each of these two cases will
be discussed below.
Dredging to Obtain Material In this case the area from which the material
is removed (the "borrow" area) can be fairly extensive in size and on the order
of 3-6 m deeper than the ambient bottom. This anomaly can cause less damping
as the waves propagate toward shore, thereby causing slightly greater breaking
wave heights. Probably of greater importance than the net increase in wave
energy is the modified distribution of wave energy along the shoreline due to
wave refraction. The wave rays which are everywhere perpendicular to the wave
crests will tend to diffuse or spread out over the deepened area thereby
lessening the wave energy at some areas along the shoreline and increasing it
at others. The areas of wave energy increase and decrease would depend on the
wave direction as can be seen by reference to Figure 22. As there is no simple
"rule of thumb" to define the effect of such a bathymetric anomaly, wave
refraction studies should be carried out for each case to establish the potential
impact.
For purposes of later discussion, it will be of interest to comment on the
filling of the borrow depression. Although there is not a large data base
relating to this matter, borrow areas are characterized by low wave energy and
thus tend to fill with finer sediment than that removed. In areas where the
bottom is highly mobile and where concentrations of suspended sediment are small,
a greater percentage of the filling material will be from the adjacent bottom.
was found that there was no decline in the abundance of intertidal animals
following the hurricane.
Culter and Mahadevan (1982) conducted studies in 1979-1980 to examine long-
term effects of the 1976 nourishment. They concluded
"No long-term adverse environmental effects as a result
of beach nourishment could be detected within the
nearshore zone of the Panama City beaches. There were
also no adverse or stressful conditions present at the
borrow sites."
Saloman, et al. (1982) carried out a study analyzing data collected between
April 1976 and November 1977. The purpose of the study was to examine short-
term effects of offshore dredging on the benthic community. It was concluded
that there was an immediate decline in the benthic community; however, the
populations recovered rapidly and were virtually at pre-construction levels
within one year. It was noted that the borrow pits were relatively small and
no more than 5 m of sand (vertically) was removed from each pit. The pits were
located in water depths of 6 to 9 m. Initially the pits filled with material
finer than on the adjacent bottom; however, these differences tended to diminish
with further filling.
Summary Regarding Intertidal Biological Effects of Beach Nourishment
Based on a comprehensive review of published information, Nelson (1985) has
concluded that the intertidal beach organisms are well adapted to this high
energy environment including significant erosion and accretion events and
fluctuations in turbidity. During and immediately following storms, massive
erosion and deposition occur over segments of beaches long in comparison to
nourishment projects. Thus any adverse effects of beach nourishment carried out
with compatible sand tend to be short-lived as the animals can either survive
the event or are adapted to rapid lateral recolonization. Nelson notes that
although the available evidence indicates minimal and short-lived biological
effects, the present level of understanding is such that biological monitoring
programs are necessary to further document the quantitative impacts of beach
nourishment projects.
Sea Turtles
Sea turtles nest on the upper portions of the beach generally during the
months of April through September. Table 3 presents the nesting characteristics
and ranges of four species of sea turtles. The turtle nesting period coincides
approximately with the period of lowest wave activity and thus from the
standpoint of cost and least turbidity, the most desirable dredging period.
Several potential adverse effects of beach nourishment projects on sea turtles
are reviewed below.
TABLE 3
CHARACTERISTICS OF SEA TURTLES AND
NESTING IN FLORIDA*
Primary Nesting Nesting Density
Species Status Range Period (nests/km)
Loggerhead Threatened North Carolina April to 1 600
(Caretta Caretta) to Florida August
Green Sea Turtle Endangered East Central May to 1 20
(Chelonia mydas) (Florida) and Southeast September
Florida Beaches
Leatherback Endangered Puerto Rico Late Too Small
(Dermochelys and Virgin February to to Be
coriacea) Islands (Some Late July Significant
in Florida)
Hawksbill Endangered Very Few Nests June to Too Small
(Eretmochelys in the U.S. October to Be
imbricata) Significant
*This table assembled from information provided by Dr. Earl Possardt, U.S. Fish
and Wildlife Service.
Consolidated Beach Sediments Sediments dredged from an offshore borrow
area may have silt and clay content in excess of normal beach sediments which
can result in more consolidated beach material in the nesting area. Nelson and
Dickerson (1987) used a cone penetrometer to test 21 Florida natural and
nourished beaches to determine the shearing resistance of the sediments. The
cone penetrometer measures in approximate units of pounds per square inch. Of
the 11 nourished beaches tested, it was concluded that 5 were sufficiently hard
to reduce nesting. Nelson (1987) recommended tilling to a depth of 90 cm for
those nourished beaches with shearing resistances exceeding 500 pounds per square
inch.
There do not seem to be well-established acceptable limits for the silt-
clay content, but an upper limit of 5 to 10% may be appropriate. Following
construction, waves will mobilize and transport the material back and forth
across the profile with the finer and coarser sediments preferentially residing
in the offshore and onshore portions of the profile, respectively. Additionally,
material transported to the project area by the longshore processes will intermix
with the placed sands resulting in a more appropriate beach for turtle nesting.
Thus projects that are initially poorly suited for nesting will improve as they
mature.
Some studies have concluded that nourished beaches experience increased
percentages of false crawls (Mann, 1977; Fletmeyer, 1980), whereas others show
no significant difference. For those areas in which nesting appeared to be
impacted, improvements were noted in subsequent years after dredging.
Blocking of Turtle Crawls by Dredge Pipe Dredge pipe oriented parallel
to the beach may interfere with the attempts of female turtles to reach a nesting
site. To reduce this potential problem the length of dredge pipe oriented so
as to cause this problem should be kept to a minimum.
Miscellaneous Effects Additional potential impacts of beach nourishment
to sea turtles include: (1) the disturbance represented by any related activities
and lights on the beach, (2) compaction of the sediments by vehicles moving along
the beach, and (3) tire depressions which can act as impediments to the return
of hatchlings to the sea.
I
Nest Relocation Programs
If beach nourishment programs are carried out during turtle nesting season
in a nesting area, it is essential to conduct a program of locating new nests
each morning and relocate the eggs to a protected hatchery area. The hatchery
is essentially a fenced natural sand area which is protected from humans and
other predators. Care is necessary in moving and placing the eggs to avoid a
high mortality. Nelson, et al. (1987) have found the hatching success to be
above 85% if natural sand is used in the hatchery area. This is only slightly
less than in natural reference areas. Thus it can be concluded that although
further study is necessary, egg relocation to a carefully monitored hatchery area
is effective in maintaining the survival rate of hatchlings.
A Case Study: Jupiter Island, Florida
Lund (1986) has reported on a comprehensive monitoring program on Jupiter
Island to evaluate the impact of beach nourishment on sea turtle nesting. The
program was carried out each summer from 1969 to 1983 and extended from Blowing
Rocks to St. Lucie Inlet, a distance of approximately 23 kilometers. This
monitoring period encompassed major beach nourishments in 1973, 1977, 1978 and
1983, totalling 4.4 million cubic yards.
To compare the nourished and unnourished beach segments, the beach was
segmented into "South", "North" and "Fill" regions, the latter region denoting
a segment of some 8 km within which the nourishment occurred.
High erosion rates along the northern end of Jupiter Island are due to the
interruption of the longshore sediment transport by St. Lucie Inlet which was
cut in 1892. The long-term shoreline change rates vary from 9 m/year erosion
at the north end of the study area to a stable shoreline near the south end.
The "South" and "Fill" regions are within the Town of Jupiter Island. Prior to
the major nourishment projects which commenced in 1973, many shore protection
structures including seawalls, groins and revetments had been constructed to
limit erosion of the upland (Aubrey and Dekimpe, 1988). Because of the erosional
trend and the presence of the shore protection structures, the beach narrowed
significantly reducing the beach area suitable for turtle nesting.
The beach material used in nourishment was substantially finer than the
sand naturally present on the beach. The silt-clay content was sufficiently high
to result in a beach somewhat more compact and dense than optimum for nesting.
Lund's studies included documentation of the number of turtle nests per
mile along the shoreline throughout the dominant nesting of loggerhead turtles
which frequent this area. Significant findings of the study include:
(1) Prior to beach nourishment, the nesting activity in the central Fill
segment was considerably lower than in the other two segments. This was
interpreted as being due to the narrow beaches resulting from the
considerable armoring in the "Fill" segment.
(2) Following nourishment, the increase in nesting activity increased in all
three segments with the increase in the Fill segment being much greater
than in the other two (125% increase vs. an average of 28% for the other
two). However, the nesting density in the Fill segment always remained
below that in the North and South segments, and
(3) Adverse effects of nourishment included displacement from the site during
construction, difficulties in climbing up the steep erosional scarp that
develops after nourishment, and inability to excavate egg chambers in the
highly compacted fill.
As an overall summary statement, Lund documented a net beneficial effect
of beach nourishment on sea turtle nesting at Jupiter Island. Some of the
concerns discussed in (3) above are being addressed through mechanical loosening
and shaping of the beach nourishment.
Following Lund's report in 1986, additional studies have been carried out
during the Summer 1988 nesting season which followed the 1987 nourishment of
1.7 million cubic meters of sand in three segments. This fill material was
loosened by tilling, thereby facilitating turtle nesting. It was found that the
number of turtle nests in all three areas (North, South and Fill) increased
dramatically (Bill Gahagan, Personal Communication) and that the density of nests
was approximately equal in all three segments.
Summary Regarding Impact of Beach Renourishment on Sea Turtle Nesting
In summary, although much is not known regarding the detailed effects of
beach nourishment on turtle nesting, research and field programs over the last
decade have developed techniques effective in ameliorating the major potential
adverse impacts. In fact, an effective program of nest relocation during beach
nourishment projects and, where necessary, tilling of the nourished beach appear
to be effective in essentially mitigating adverse effects. Finally, in areas
where beaches have narrowed due to a beach erosion trend and the presence of
shore protection structures, the wider beaches resulting from beach nourishment
can improve substantially nesting conditions and ultimately turtle populations.
PART V
CASE STUDIES AS EXAMPLES
PART V
CASE STUDIES AS EXAMPLES
CASE STUDY I PERDIDO KEY
Introduction
The Perdido Key Gulf Island National Seashore is located on the westernmost
island in the Panhandle area of Florida. The eastern end of Perdido Key is
bounded by the entrance to Pensacola Bay, Figure 23. This entrance is a
navigation channel which has been dredged to depths of 13 m, significantly
exceeding the natural bar depth of approximately 6 m. Table 4 presents the
available history of dredging over more than one century: from 1885 to 1987.
TABLE 4
HISTORY OF MAINTENANCE DREDGING
PENSACOLA ENTRANCE CHANNEL
1885-1987
Year Volume Dredged Type Dredge Disposal Area
(m3)
1885-1975 Unknown Hopper Gulf Disposal
1975 840,000 Hopper Gulf Disposal
1981 500,000 Hopper Gulf Disposal
1983 87,000 Hopper Gulf Disposal
1984 700,000 Hopper Gulf Disposal
1985 1,860,000 Pipeline Perdido Key
1987 150,000 Hopper Gulf Disposal
Recently Pensacola Bay was designated as a homeport for the aircraft carrier
"Kittyhawk". This required an additional channel deepening with depths up to
14.6 m resulting in the attendant availability of more than 8 million cubic
meters of high quality sand. The availability of this material represented both
an opportunity and a dilemma for the National Park Service. The sections below
-S *-
0 2 4 Miles
I I
Pensacola
Perdido Key and Entrance Channel to Pensacola Bay.
Entrance Channel
\\
Gulf of Mexico
Figure 23.
detail the background of the project and the rationale leading to the implemented
plan.
Background
The net longshore sediment transport in the vicinity of Pensacola Bay
Entrance is from east to west with an estimated magnitude of approximately
200,000 m3/year. In its natural condition, depths over the ebb tidal shoal were
on the order of 6 m and formed a sand bridge from Santa Rosa Island to the
downdrift Perdido Key. Deepening as documented in Table 5 severed this bridge
and the longshore sediment transport tended to deposit in the channel,
reestablish the sand bridge and resume the natural bypassing process. Clearly
as noted before for many cases, a shallow bar which is required for bypassing
is not compatible with safe navigation. Thus the material deposited must be
removed from the channel by dredging as a periodic maintenance operation.
From the earlier discussion of natural and altered systems, it is clear that
unless the dredged material is placed on the downdrift shorelines, it will
represent a deficit and result in an erosional stress. As shown in Table 4, with
the exception of the 1985 placement of approximately 1,860,000 m3 on Perdido
Key, all placement has been at sea. As documented in Figure 24, this practice
of disposal at sea has taken a severe erosional toll on Perdido Key. This figure
presents shoreline change data collected by the Florida Department of Natural
Resources showing that over the 10 year period represented by these data, an area
extending over almost the entire Park limits was eroding at an average rate of
approximately 1.3 m/year. Using a standard rule of thumb, this is equivalent
to 100,000 m3/yr. Undoubtedly, the remainder of the deficit resulting from
interruption of the net longshore sediment transport of 200,000 m3/yr occurs by
erosion west of the western Park boundary. The nearest condominium to the west
Park boundary is clearly in jeopardy due to the erosion and erosion is evident
to substantial distances farther west.
Rationale for Selected Plan
In keeping with National Park Service policy to maintain, as near as
possible, parks in their natural condition, an overriding factor in the
considerations at Perdido Key was that the natural system had been impacted
+5.0
t
1 Mile
figure 24. Shoreline
Based on
Smoothed
Change Rates for Escambla County, January 1974
Florida DNR Survey. Note Shoreline Change Rates
by a Five Point Running Average.
I v/
SOctober 9 4.
Shown have been
FLORIDA DNR MONUMENT NUMBER
Approximate Western
| Park Boundary
West
4---
I
TABLE 5
CHRONOLOGY OF PENSACOLA BAY ENTRANCE
CHANNEL DIMENSIONS
Channel Dimensions Aut
Authorized
Year Depth (m) Width (m) or Actual
1881 7.3 24 Authorized
1885 6.9 24 Actual
1890 7.3 37 Actual
1896 9.1 91 Authorized
1902 9.1 152 Authorized
1935 9.8 152 Authorized
1959 11.3 244 Actual
Present 13.4 244 Authorized
significantly by over a century of dredging and interruption of the natural
system.
The availability of large quantities of good quality sand was viewed as an
opportunity to compensate for some of the adverse impacts to the natural system
that had occurred for more than a century. A decision was made to accept
approximately 4 million cubic meters of high quality beach sand with strict
conditions on the placement and subsequent monitoring of the project.
Experience gained through the 1985 placement was utilized in developing
requirements for the forthcoming project. Specifically, the 1985 material was
placed too high to allow normal waves to overtop the berm and contour the
profile. Additionally, this prior project had left a relatively coarse shell
residue on the berm and no vegetation program nor attempt to configure the sand
to natural berm forms had been carried out. Recommendations for the planned
project included placement over a length of approximately 6 km. The recommended
profile is as shown in Figure 25.
20
a 1 0 Natural Berm Elevation 6+ft
+5ft
5ft +4ft
z
UI 280,tt
> \
0
I 0
z
0 \ Recommended
P Nourishment Profile
-10 DNR Profile
WI Measured October 31,1984
-J
-20
0 300 600 900 1200 1500 1800
DISTANCE GULFWARD FROM DNR MONUMENT R-48 (ft)
Figure 25. Recommended Characteristics of Nourished Profile. Illustrated For DNR Monument No. 48
Monitoring Plan
The monitoring plan included both Physical and Biological components.
Physical Monitoring The physical monitoring program addressed three needs:
(1) Performance related, (2) Public information, and (3) Park management.
Performance Related Monitoring Needs
The primary performance related monitoring need is associated with the
performance and evolution of the system, especially the sand flows and beneficial
and adverse effects of the placement. Monitoring is particularly valuable to
assist in understanding the natural system and to fine-tune later maintenance
nourishment projects. Detailed needs are discussed below.
Profile and Planform Evolution Repeated profile surveys serve to document
the three-dimensional changes in the nourishment volumes. Usually sand is placed
on a profile that is steeper than the equilibrium profile. The equilibration
process occurs as a result of storms which mobilize the sediment at greater and
greater depths. Associated with this equilibration process can be a substantial
change in shoreline position that is not related to sand flow laterally along
the beach. Documented volumetric changes along with an estimate of longshore
sediment transport at one location allow determination of the rates of sand flow
as a function of alongshore distance. A sufficient number of profiles should
be measured to allow definition of anomalous features, such as the rhythmic
planform features that can be fairly accentuated at some locations; an example
is Perdido Key.
Wave Measurements The wave characteristics relevant to sediment transport
include: height (or energy), period and direction. Results obtained from a
directional wave gage provide such data and allow much better interpretation of
volumetric changes and profile adjustment. Available wave measurements also
facilitate interpretation of storm effects including any documented difference
between the effects to nourished and control areas.
Wind and Precipitation Measurements Following nourishment, generally
there will be a fairly broad expanse of dry sandy beach. Onshore winds blowing
across dry sand will tend to transport the fine fraction and deposit it as dunes.
In addition to providing a better basis for understanding this process, these
measurements along with sediment samples will assist in interpreting armoring
of the surface by the remaining larger particles, particularly shell fragments.
Vegetation Response In order to document vegetation response to
nourishment and, where carried out, the effectiveness of vegetation establishment
efforts, the monitoring should include a systematic plan for photographic
documentation, yet retain sufficient flexibility to respond to unanticipated
vegetation features of interest.
Public Interest/Education Monitoring Needs
It is anticipated that due to the substantial volume of sediment to be
placed and the obvious resulting physical changes to the system, there will be
substantial public interest in any large beach nourishment project, including
NPS rationale and justification for the placement, basis for need, etc., actual
versus anticipated consequences and modification of NPS policy as the result of
experience obtained. An adequate monitoring program will ensure a basis to
respond to this public interest consistent with NPS management policies and
responsibilities.
Management Monitoring Needs
Consistent with NPS responsibilities to manage park systems in a near-
natural state and to understand the consequences of various management
alternatives, it is essential to monitor perturbations to these systems in order
to better understand the natural system and its capacity to adjust to
anthropogenic perturbations. Knowledge gained will assist in providing guidance
to future management decisions related to beach nourishment.
Biological Monitoring
The monitoring to establish the biological effects of the beach nourishment
project includes three elements; each of these is described briefly below.
Benthic Community Studies Studies will be carried out to establish the
effect of beach nourishment project on the benthic community. Sampling will be
conducted along transects extending from the Gulf and Lagoon shores of Perdido
Key.
Vegetation Analysis Natural and revegetation success will be documented
by a combination of transects supplemented by color infrared aerial photography.
The objective will be to quantify the revegetation with time as affected by
various ground conditions including elevation, compactness, distance from
shoreline, etc.
Beach Mouse Population Effects of the nourishment project on the beach
mouse will be monitored through a series of population studies augmented by other
relevant factors including food supply and predator populations. The mouse
population studies include marking and recapture studies along selected transects
within the project area and at control transects outside the area. Predator
studies will include tracking using radio telemetry methods.
CASE STUDY II OREGON INLET
Introduction
Oregon Inlet is a natural inlet located approximately 63 km north of Cape
Hatteras in the Cape Hatteras National Seashore. This inlet connects Pamlico
Sound with the waters of the north Atlantic Ocean, Figure 26. In 1960, the
"Bonner Bridge" was constructed across Oregon Inlet thereby providing access via
State Highway NC 12 from the north to Hatteras Island. Over historic times,
the inlet had migrated fairly rapidly toward the south. With the combined
effects of construction of the fixed bridge and increasing interest in
establishing an all weather navigational channel, the stage was set for concerted
efforts to stabilize the inlet through jetty construction.
Historic Inlet Behavior
The geometric characteristics of Oregon Inlet have varied considerably over
historic times. Storms tend to widen the inlet, followed by narrowing during
periods of milder weather. The inlet center migrated southward at an average
Bar
9-
/ /
/
/ f
I
I
I I
I I I I
0 4000
Scale (ft)
Coast Guard Station
Figure 26. Oregon Inlet
rate of 20 m/year during the period 1931-1988 (Task Force, 1988). The net
southerly longshore transport is 0.5 to 1.0 million cubic meters per year (Inman
and Dolan, 1989).
Present Situation
Since the bridge construction in 1960, the inlet has continued its
relentless migration such that a spit has grown to the south under the northern
part of the elevated bridge span. This migration has caused the channel to be
dangerously close to the southern bridge abutment, where protection has been
provided by revetment construction.
The Corps attempts to maintain the inlet navigable through hopper and
sidecast dredging. From September, 1983 to February, 1988, an average of 550,000
cubic meters annually has been dredged from the inlet with the material placed
south of the inlet in water depths exceeding 6 m. Due to this substantial depth,
it is questionable whether this placement provides significant benefit to the
downdrift (south) shoreline.
The Corps of Engineers (COE) has developed a plan to stabilize Oregon Inlet
through the construction of two jetties with sand transfer accomplished by a
floating pipeline dredge which would remove accumulated sand north of the north
jetty and transfer this sand to the northern portions of Pea Island. The dredge
would operate during the summer months with protection against waves provided
by a "Sloping Floating Breakwater" (SFB), essentially a new and untried concept,
see Figure 27. The COE plan was authorized in 1970 with an estimated construc-
tion cost of approximately $50 million. Since then the estimated cost has risen
to in excess of $100 million with an annual maintenance and sand bypassing cost
of approximately $7 to $8 million.
Present concern centers on three issues: (1) the erosional threat to the
bridge, especially near the south abutment, (2) the erosional threat to the Coast
Guard station south of the inlet as shown in Figure 26, and (3) the unstable
and hazardous channel.
The National Park System and State of North Carolina Position on Oregon Inlet
NPS policy is to allow natural systems to remain in as near a natural
condition as possible. This is consistent with the State of North Carolina
1
160 Ft 15" Nystron
(Jacketed) \ Chafe ChainR
1 Shot 3" Grade 3 1 11/4 Shot -
2 3/4" Grade 3
8200 Lb Sinker
20,000 Lb Nav Moor
Figure 27. Cross Section of Sloping Floating Breakwater Planned for Deployment
at Oregon Inlet to Provide Shelter for Sand Bypassing Dredge Operating
in Its Lee. From Inman and Dolan (1987).
policy which essentially prohibits the construction of armoring in the coastal
zone. NPS recognizes the three concerns noted above and also recognizes the
uncertainties associated with the construction of a substantial jetty system and
a sand transfer system on an unprecedented scale. NPS has sought alternatives
to the jetty plan on the grounds that: (1) it is contrary to NPS and State
policy, and (2) the performance is uncertain and once constructed, the system
is there essentially forever. Although not within NPS purview, it is not clear
that the jetties are the most cost effective approach.
An approach has been sought by NPS that would be consistent with state and
NPS policies and yet accomplish common objectives. A recommendation has been
made for a two-year Demonstration Project to evaluate the effectiveness of a
"dredge only" option. In this option, flexibility would be provided to place
the sand where needed as indicated by beach monitoring. This option appears to
have a number of advantages, including:
(1) If not effective, this option could be discontinued without any lasting
impact as would be the case with the jetty option.
(2) Much could be learned about the physical system through a concerted
monitoring program during the two year demonstration project. This
information would serve to guide alternate designs or fine tune the
"dredge-only" option.
(3) Through the flexibility of sand placement, the option could address
immediately areas threatened by erosion.
(4) Through a multi-year dredging contract, the latest in dredging equipment
and technology could be brought to bear on the project, and the performance
of the contractor could be assessed on a several year basis and changes
made, if desired.
(5) Although detailed cost estimates have not been carried out, it appears that
this method is competitive. In particular the interest on the initial
investment of approximately $100 million plus the estimated annual jetty
maintenance and operating costs of the sand bypassing facility of $7 to
$8 million appear to be of the same magnitude as if not greater than the
annual cost of the dredging only option.
As of the time of writing this report (December, 1988), the U.S. Army Corps
of Engineers is still maintaining a limited navigation capability through
operation of the hopper dredge with the placement of dredged material in water
depths exceeding 6 m.
CASE STUDY III CUMBERLAND ISLAND
Introduction
Cumberland Island National Seashore is the southernmost island along the
Georgia coastline. The waterway along the southern end of Cumberland Island is
St. Marys River; the outlet to this river is protected by two long navigational
jetties. As shown in Figure 28, the community of Kings Bay, GA is on the
mainland to the west of Cumberland Island and approximately 8 km north of the
south tip of Cumberland Island. Kings Bay has been designated as a homeport for
the Ohio Class submarines.
Channel modifications necessary to accommodate these submarines include
substantial deepening, widening and lengthening of the current navigational
channel. The total initial construction dredging is in excess of ten million
cubic meters. The EIS prepared in conjunction with the project predicted an
annual maintenance dredging requirement of 1.4 million cubic yards. Later more
detailed estimates performed by the Coastal Engineering Research Center have
yielded a substantially lower value, i.e. 788,000 cubic yards per year.
Concerns of the National Park Service
Due to the large quantities of dredging being considered, the NPS has
concerns over the effects on inner and outer shorelines of Cumberland Island,
on the marsh ecosystem on the western side of Cumberland Island and on the biota
in the interior waters. These concerns led to negotiations with the Navy which
eventually culminated in a five-year comprehensive monitoring and evaluation
program. This program is reviewed briefly below.
Monitoring Program
Responsibilities for the monitoring program are shared by the Waterways
Experiment Station (WES) of the U.S. Army Corps of Engineers and the National
Park Service. The WES program includes a Coastal Assessment component and a
Benthic Community Studies Studies will be carried out to establish the
effect of beach nourishment project on the benthic community. Sampling will be
conducted along transects extending from the Gulf and Lagoon shores of Perdido
Key.
Vegetation Analysis Natural and revegetation success will be documented
by a combination of transects supplemented by color infrared aerial photography.
The objective will be to quantify the revegetation with time as affected by
various ground conditions including elevation, compactness, distance from
shoreline, etc.
Beach Mouse Population Effects of the nourishment project on the beach
mouse will be monitored through a series of population studies augmented by other
relevant factors including food supply and predator populations. The mouse
population studies include marking and recapture studies along selected transects
within the project area and at control transects outside the area. Predator
studies will include tracking using radio telemetry methods.
CASE STUDY II OREGON INLET
Introduction
Oregon Inlet is a natural inlet located approximately 63 km north of Cape
Hatteras in the Cape Hatteras National Seashore. This inlet connects Pamlico
Sound with the waters of the north Atlantic Ocean, Figure 26. In 1960, the
"Bonner Bridge" was constructed across Oregon Inlet thereby providing access via
State Highway NC 12 from the north to Hatteras Island. Over historic times,
the inlet had migrated fairly rapidly toward the south. With the combined
effects of construction of the fixed bridge and increasing interest in
establishing an all weather navigational channel, the stage was set for concerted
efforts to stabilize the inlet through jetty construction.
Historic Inlet Behavior
The geometric characteristics of Oregon Inlet have varied considerably over
historic times. Storms tend to widen the inlet, followed by narrowing during
periods of milder weather. The inlet center migrated southward at an average
OL
St. Marys Entrance
Kings E
Site
10
Scale (km)
Figure 28. Location Map of Kings Bay Site Relative to Cumberland
Island National Seashore.
0
I-Il
I I ,
0
|
Cumberland Sound Physical Processes component. The NPS is responsible for
ecological studies, primarily in Cumberland Sound. Where practical, efforts have
been made to structure all program elements such that they complement other
studies being carried out for the Navy on this project.
Coastal Assessment Component
The purpose of this component is to develop an information base that will
allow interpretation of past shoreline changes along Cumberland Island and Amelia
Island (immediately to the south of the St. Marys River Entrance). No attempt
will be made to present the component details; however, waves and tides will be
documented as the principal agents affecting the shoreline. The shoreline will
be monitored through profiling and aerial photography and sediment samples will
be taken. These results, when combined with those from a historical substudy
will provide the basis for an extrapolation subcomponent, the purpose of which
is to develop predictions of the impact of the project on the outer and inner
shorelines.
Cumberland Sound Physical Processes Component
Measurements of tides, currents and salinities and dredging in interior
waters will be combined with computational models to predict probable changes
in the physical regime of the Cumberland Sound waters. Concerns of particular
interest are the effect of channel deepening on tidal range, salinity, shoaling
patterns and mean tide range inside the sound. The possible effect of rising
sea level combined with a dredging induced increase in mean water level may
affect ability of the marshes to accrete and keep pace vertically.
Ecological Research
This component of the program is administered by the NPS and is not as
structured as the other components. The plan is to pursue initially the study
components of greatest concern and to allow the program direction to respond to
results developed. Initial efforts are focused on the effect of the Kings Bay
project on: marsh dynamics, bivalves, ground water, and manatees. In addition
a geographic information system is being developed/tailored to organize and make
all data from this study readily available.
As of the time of writing this report (December, 1988), the U.S. Army Corps
of Engineers is still maintaining a limited navigation capability through
operation of the hopper dredge with the placement of dredged material in water
depths exceeding 6 m.
CASE STUDY III CUMBERLAND ISLAND
Introduction
Cumberland Island National Seashore is the southernmost island along the
Georgia coastline. The waterway along the southern end of Cumberland Island is
St. Marys River; the outlet to this river is protected by two long navigational
jetties. As shown in Figure 28, the community of Kings Bay, GA is on the
mainland to the west of Cumberland Island and approximately 8 km north of the
south tip of Cumberland Island. Kings Bay has been designated as a homeport for
the Ohio Class submarines.
Channel modifications necessary to accommodate these submarines include
substantial deepening, widening and lengthening of the current navigational
channel. The total initial construction dredging is in excess of ten million
cubic meters. The EIS prepared in conjunction with the project predicted an
annual maintenance dredging requirement of 1.4 million cubic yards. Later more
detailed estimates performed by the Coastal Engineering Research Center have
yielded a substantially lower value, i.e. 788,000 cubic yards per year.
Concerns of the National Park Service
Due to the large quantities of dredging being considered, the NPS has
concerns over the effects on inner and outer shorelines of Cumberland Island,
on the marsh ecosystem on the western side of Cumberland Island and on the biota
in the interior waters. These concerns led to negotiations with the Navy which
eventually culminated in a five-year comprehensive monitoring and evaluation
program. This program is reviewed briefly below.
Monitoring Program
Responsibilities for the monitoring program are shared by the Waterways
Experiment Station (WES) of the U.S. Army Corps of Engineers and the National
Park Service. The WES program includes a Coastal Assessment component and a
PART VI
LEGAL AND REGULATORY ASPECTS OF DREDGING
PART VI
LEGAL AND REGULATORY ASPECTS OF DREDGING
Introduction
Dredging in the waters of the United States is regulated by both federal
and state agencies. Some of the material presented in this section is based on
the twenty-six (and still counting) review articles which Peter Graber has
published in Shore and Beach. Articles referenced are included in the
bibliography.
THE FEDERAL PROGRAM
A brief review of the evolution of the history of the federal laws may be
helpful.
Rivers and Harbors Act of 1899
The purpose of this statute was to prevent obstruction to navigation and
placed responsibility on the U.S. Army Corps of Engineers for issuing permits.
Although as noted above the concern of the original act was navigation, it was
broadened through litigation in 1970 and 1971 to require consideration of ecology
and allowed denial of a permit if the proposed project would cause ecological
damage.
National Environmental Policy Act of 1969 (NEPA)
This statute, administered by the Environmental Protection Agency, declares
"... a national policy which will encourage productive and enjoyable harmony
between man and his environment." This act formalized the change toward greater
concern for the environment and states as a goal "... a balance between
population and resource use which will permit high standards of living and a wide
sharing of life's amenities." The character of environmental impact statements
required in "major Federal actions significantly affecting the quality of the
human environment" are formalized to include, "(i) the environmental impact of
the proposed action, (ii) any adverse environmental effects which cannot be
avoided should the proposal be implemented, (iii) alternatives to the proposed
action,... ."
Federal Water Pollution Control Act Amendments of 1972 (FWPCA)
The purpose of this act also called the Clean Water Act is to "restore and
maintain the chemical, physical and biological integrity of the Nation's waters".
A system of permits is required to regulate the discharge of dredged or fill
materials into navigable waters. The Corps of Engineers is the responsible
agency to administer this program. Section 404 of this act provides for the
states to assume responsibility for permitting dredge and fill activities and
establishes requirements which these state programs must satisfy including
procedures to ensure compliance with the program.
Marine Protection, Research and Sanctuaries Act of 1972
This statute is also referred to as the Ocean Dumping Act and requires a
permit when any material is to be discharged into the territorial sea and
contiguous zone of the United States. Regulatory responsibilities are shared
by the U.S. Army Corps of Engineers for dredged material and the Environmental
Protection Agency for other materials. Criteria for permitting dumping are that
the project should not "... unreasonably degrade or endanger human health,
welfare or amenities, or the marine environment, ecological systems, or economic
potentialities."
Coastal Zone Management Act of 1972 (CZMA)
This act provides financial incentive to coastal states to develop and
adopt approved coastal zone management programs. In 1976, the federal cost
sharing of the program was increased from 66.6% to 80%. Requirement of the CZMA
are that a state program must include a designation of the states' boundaries
of the coastal zone, an inventory of the areas of particular concern, broad
guidelines on priority of uses in those areas, lists of permissible land and
water uses, etc. All states within the Southeast Region have approved programs
with the exception of Georgia.
Section 307 of the Coastal Zone Management Act of 1972 requires that
federal agencies comply with federally-approved coastal zone management programs.
This section, termed the "consistency provision", also requires that a state or
local project which affects the coastal zone must be in accordance with the
state coastal zone management program. The consistency provision encompasses
harbor development, improvement and maintenance and dredge and fill activities.
THE STATE PROGRAMS
The following paragraphs provide a brief summary of the dredging, filling
and disposal regulatory responsibilities within the six coastal states located
in the Southeast Region of the National Park Service and which have coastal park
units.
The North Carolina Program
In September 1978, North Carolina received federal approval for their
proposed Coastal Management Program. State environmental concern increased in
1969, and studies were carried out and legislation adopted to preserve the
state's coastal resources. The Coastal Wetlands Act of 1971 and Coastal Area
Management Act of 1974 provide regulatory functions. Dredging and filling of
estuarine land are regulated under the Coastal Resources Commission.
The South Carolina Program
South Carolina received federal approval for their proposed coastal zone
management program in September, 1979. Under their program, a permit must be
obtained from the South Carolina regulatory body, the Coastal Council, to "fill,
remove dredge, drain or erect any structure on or in any way alter any critical
area".
The Georgia Program
The Georgia Coastal Marshlands Act of 1970 provided that "no person shall
remove, fill, dredge or drain or otherwise alter any marshlands in the State
within the estuarine area thereof without first obtaining a permit". This
regulatory program is administered by the Coastal Marshlands Protection
Committee.
Georgia does not participate in the federal Coastal Zone Management
Program; thus dredging and disposal outside the jurisdiction of the Marshlands
Act is administered by the U.S. Army Corps of Engineers.
The Florida Program
Florida submitted its proposed Coastal Management Program to the U.S.
Office of Coastal Zone Management in February 1981 and the program was approved
in August 1981. Permits for dredging and filling in sovereign lands are
regulated by statute. This program is under the responsibility of the Department
of Florida Environmental Regulation with participation by the Department of
Natural Resources.
The Alabama Program
In 1976, Alabama established the Alabama Coastal Area Act which provides
the legal framework for the coastal zone program. The Coastal Area Board is the
responsible agency. The Coastal Area Board is responsible for issuing dredge
and dredge and fill material disposal permits.
The Mississippi Program
The Mississippi Coastal Wetlands Protection Law of 1973 formalizes public
policy as "the preservation of the natural state of the coastal wetlands...,
except where an alteration of specific wetlands would serve a higher public
interest." The Bureau of Marine Resources under the Mississippi Department of
Wildlife Conservation administers the program and is responsible for permitting
activities in the wetlands including dredging and filling.
PART VII
AN INVENTORY OF NATIONAL PARK SERVICE UNITS
IN THE SOUTHEAST REGION SUSCEPTIBLE TO EROSION
AND EFFECTS OF DREDGING
PART VII
AN INVENTORY OF NATIONAL PARK SERVICE UNITS
IN THE SOUTHEAST REGION SUSCEPTIBLE TO EROSION
AND EFFECTS OF DREDGING
Within the Southeast Region of the National Park Service there are 19 units
that are subject to erosion by salt or brackish waters. Of these, at least seven
are considered to be mildly to highly susceptible to dredging effects.
Table 6 lists the NPS units in the SE region which fall within the
classification above, i.e. either subject to erosion by salty or brackish water
or are possibly subject to adverse effects of dredging. An attempt has been made
to rank the effects of dredging as Low (L), Medium (M), High (H) and extremely
high (EH). This ranking is based on the proximity of planned or past dredging
activities and the impact on the site.
TABLE 6
UNITS WITHIN THE SOUTHEAST REGION
SUBJECT TO SALT/BRACKISH WATER RELATED
EROSION AND ADVERSE EFFECTS OF DREDGING
Susceptibility
Erosion to Dredging
Units Concerns Effects Comments
Cape Hatteras
National Seashore
(NC)
Cape Lookout National
Seashore (NC)
Fort Raleigh National
Historic Site (NC)
Fort Sumter National
Monument (SC)
Fort Pulaski National
Monument (GA)
Fort Frederica National
Monument (GA)
Cumberland Island
National Seashore (GA)
Fort Caroline National
Monument (FL)
Fort Matanzas National
Monument (FL)
Canaveral National
Seashore (FL)
EH
(Cape Hatteras
Light House)
L
M
M
M
H
M
L
M-H
Dredging Effects
High in Vicinity
of Oregon Inlet
Proximity to
Channel Dredging
Could Induce Erosion
Proximity to
Channel Dredging
Could Induce Erosion
Channel Deepening at
Kings Bay. Both Inner
and Outer Shorelines
Dredging in St. Johns
River Entrance
Possible Modifica-
tions to Matanzas
Inlet Bridge
Coastal Armoring to
North May Cause Some
Erosional Stress
Key to Ranking: L = Low; M = Medium; H = High; EH = Extremely High
TABLE 6
UNITS WITHIN THE SOUTHEAST REGION
SUBJECT TO SALT/BRACKISH WATER RELATED
EROSION AND ADVERSE EFFECTS OF DREDGING
(Continued)
Susceptibility
Erosion to Dredging
Unit Concerns Effects Comments
Biscayne National L L
Monument (FL)
Fort Jefferson National
Monument (FL)
DeSoto National
Monument (FL)
Gulf Islands National
Monument (FL)
*Santa Rosa Island
*Perdido Key
Gulf Islands National
Seashore (MS)
*Petit Bois Island
*Horn Island
*Ship Island
San Juan National
Historic Site,
Puerto Rico
Virgin Islands National
Park (St. Johns Island)
Buck Island Reef National
Monument (An Island Off
St. Croix)
Christiansted National
Proximity of Dredging
in Pensacola Entrance
Channel
Navigation Channel
Dredging from
Gulfport
El Morro Castle is
Founded on Cavernous
Limestone and is
Being Undermined
Key to Ranking: L = Low; M = Medium; H = High; EH = Extremely High
PART VIII
SUMMARY AND CONCLUSIONS
PART VIII
SUMMARY AND CONCLUSIONS
Summary
Dredging and other engineering alterations to the nearshore system have
the potential to cause physical and biological impacts to the natural beach, dune
and sand-sharing system that are both extensive spatially and long lasting over
time. The physical impacts generally occur due to the interference with the
natural sand transport system. In cases where sand is removed from this system,
the inevitable result is erosion only the distribution is unknown. In other
cases where sand is trapped (perhaps by groins) or prevented from entering the
system (by coastal armoring), the total amount of sand in the system remains the
same but is redistributed; thus there are localized areas of relative deposition
or stability and erosion. The biological impacts are generally closely related
to the quality of sand used in beach nourishment projects and the large
magnitudes of alterations that can place the system out of balance to a degree
that the biota may not be able to adapt. Examples include the use of fine
sediment that could impact offshore reefs or result in a beach too compact for
turtle nesting.
In particular instances where the natural system is out of balance due to
prior engineering actions, beach nourishment with high quality sand can be
beneficial by restoring the natural balance in the physical and biological
systems.
Conclusions
Within the general policy of the National Park Service to maintain systems
in their natural condition, each situation should be considered on a case-by-
case basis recognizing that systems may be impacted by prior engineering
alterations and that some engineering activities such as sand by-passing and
beach nourishment can exert a beneficial impact on both the (altered) physical
and biological systems.
Where systems are in their natural state, the general policy of maintaining
this natural state is appropriate and consistent with the NPS mandate as stewards
and managers of these systems for the use of and enjoyment by present and future
generations.
If consideration is given to allowing substantial dredging or construction
related activities which could affect NPS systems, it is essential that: (1)
where appropriate, the best geological, biological and engineering expertise be
used to analyze the potential impact of the activity and to identify the most
beneficial approach, and (2) that a thorough biological and physical monitoring
and analysis plan be implemented to document the impact and to improve general
understanding of the complex coastal sediment processes and biological
interactions resulting from substantial alterations to the system. Only by this
approach can NPS appropriately fulfill its responsibility to maintain the well-
being of those systems for which it is charged against the increasing pressures
for modifications which could alter these systems.
I
REFERENCES
Aubrey, D.M. and N.M. Dekimpe (1988) "Performance of Beach Nourishment at Jupiter
Island, Florida", Paper Presented at Beach Technology Conference,
Gainesville, FL, 20 pages.
Bokuniewicz, H.J., M. Zimmerman, M. Keyes, and B. McCabe (1980) "Seasonal Beach
Response at East Hampton, NY", Special Report 38, Marine Sciences Center,
State University of New York, Stony Brook, N.Y.
Bowman, M.L. (1981) "The Relationship of Emerita Talpoida to Beach Characteris-
tics", M.S. Thesis, University of Virginia, Charlottesville, Virginia,
106 pp.
Culter, J.K. and S. Mahadevan (1982) "Long-Term Effects of Beach Nourishment on
the Benthic Fauna of Panama City Beach, Florida", Miscellaneous Report
No. 82-2, U.S. Army Corps of Engineers, Coastal Engineering Research
Center, Fort Belvoir, VA.
Dean, R.G. (1977) "Equilibrium Beach Profiles: U.S. Atlantic and Gulf Coasts",
Ocean Engineering Technical Report No. 12, Department of Civil Engineering,
University of Delaware, Newark, DE.
Dean, R.G. (1983) "Principles of Beach Nourishment", in Handbook of Coastal
Processes and Erosion, CRC Press, p. 217-231.
Dean, R.G. and T.L. Walton (1975) "Sediment Transport Processes in the Vicinity
of Inlets with Special Reference to Sand Trapping", in Estuarine Research,
Volume II, Geology and Engineering, Edited by L. Eugene Cronin, Academic
Press, New York, p. 129-150.
DeWall, A.E. and J.J. Richter (1977) "Beach Nearshore Processes in Southeastern
Florida", Proceedings, ASCE Specialty Conference: Coastal Sediments '77,
p. 425-443.
Gren, G.G. (1976) "Hydraulic Dredges, Including Boosters", Proceedings of the
ASCE Specialty Conference on Dredging and Its Environmental Effects,
Mobile, Alabama, p. 115-124.
Graber, P.F. (1981) "The Law of the Coast in a Clamshell: Part II", Shore and
Beach, Vol. 49, No. 1, Jan., p. 16-20.
Graber, P.F. (1981) "The Law of the Coast in a Clamshell: Part IV The Florida
Approach", Shore and Beach, Vol. 49, No. 3, July, p. 13-20.
Graber, P.F. (1983) "The Law of the Coast in a Clamshell: Part X The North
Carolina Approach", Shore and Beach, Vol. 51, No. 1, Jan., p. 18-23.
Graber, P.F. (1984) "The Law of the Coast in a Clamshell: Part XV The South
Carolina Approach", Shore and Beach, Vol. 52, No. 2, April, p. 18-25.
Graber, P.F. (1986) "The Law of the Coast in a Clamshell: Part XII The
Mississippi Approach", Shore and Beach, Vol. 54, No. 1, Jan., p. 3-7.
Graber, P.F. (1986) "The Law of the Coast in a Clamshell: Part XXII The Georgia
Approach", Shore and Beach, Vol. 54, No. 3, July, p. 3-7.
Graber, P.F. (1988) "The Law of the Coast in a Clamshell: Part XXV The Alabama
Approach", Shore and Beach, Vol. 56, No. 2, April, p. 12-16.
Hayden, B. and R. Dolan (1974) "Impact of Beach Nourishment on Distribution of
Emerita Talpoida, the Common Mole Crab", Journal Waterways, Harbors and
Coastal Engineering Division, ASCE, Vol. 100, WW2, p. 123-132.
Inman, D.L. and R. Dolan (1987) "Discussion of the U.S. Army Corps of Engineers
Proposed Manteo (Shallowbag) Bay Project: The Stabilization of Oregon
Inlet, North Carolina," Draft Report Submitted to the National Park
Service, December.
Inman, D.L. and R. Dolan (1989) "The Outer Banks of North Carolina: Budget of
Sediment and Inlet Dynamics Along a Migrating Barrier System", Journal of
Coastal Research, Vol. 5, No. 2, p. 193-353.
Johnson, J.W. (1971) "The Significance of Seasonal Beach Changes in Tidal
Boundaries", Shore and Beach, April, p. 26-31.
Long, E.G. (1967) "Improvement of Coastal Inlets by Sidecast Dredging",
Proceedings, ASCE, Vol. 93, WW4, November, p. 185-200.
Lund, F. (1986) "Impacts of Beach Nourishment Programs Upon Marine Turtle Nesting
at Jupiter Island, Florida, 1969-1983", Report Submitted to the Town of
Jupiter Island.
Mohr, A.W. (1976) "Mechanical Dredges", Proceedings of the ASCE Specialty
Conference on Dredging and Its Environmental Effects, Mobile Alabama, p.
125-138.
Nelson, D.A. (Undated Working Draft) "The Use of Tilling to Soften Nourished
Beach Sand Consistency for Nesting Sea Turtles", U.S. Army Engineer
Waterways Experiment Station.
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