THE FLORIDA STATE UNIVERSITY 

COLLEGE OF SOCIAL SCIENCES 

QUANTIFICATION, ANALYSIS, AND MANAGEMENT OF INTRACOASTAL 

WATERWAY CHANNEL MARGIN EROSION IN THE GUANA TOLOMATO MATANZAS 

NATIONAL ESTUARINE RESEARCH RESERVE, FLORIDA 

By 

FRANKLIN D. PRICE 

A Thesis submitted to the 

Department of Urban and Regional Planning 

in partial fulfillment of the 

requirements for the degree of 

Master of Science 

Degree Awarded: 

Spring Semester, 2005 
The members of the Committee approve the thesis of Franklin D. Price defended on March 29, 

2005.

 

Robert Deyle 

Professor Directing Thesis

 

Sergio Fagherazzi 

Outside Committee Member 

 

John Thomas 

Committee Member 

 

The Office of Graduate Studies has verified and approved the above named committee 

members. 

ii 
ACKNOWLEDGEMENTS 

I would first like to thank my wife, Kadee, for her tolerance and assistance over the past 

three years and for being the most understanding, supportive friend and the best lover I 

could ever imagine. 

I would also like to thank my parents for having me. Now that I am finishing my 

Master’s, I cannot believe how hard it must have been to have a kid while in grad school. 

I would finally like to thank the Matanzas River for giving me the inspiration I needed to 

make myself work, my boss, Helen Light, who allowed me the time to work, my 

committee for being available to answer questions, Melanie Darst for her editing 

assistance, my brother Samuel for letting me use his boat for field work, and the helpful 

staffs of the GTMNERR, the FDOT, the FIND, the USCOE, the FDEP, the FFWCC, the 

SJRWMD, the USGS, NOAA, the St. Johns County GIS office, and Florida Sea Grant. 

iii 
TABLE OF CONTENTS 

List of Tables ................................................................................................................................. vi 

List of Figures............................................................................................................................... vii 

Abstract.......................................................................................................................................... ix 

INTRODUCTION .......................................................................................................................... 1 

1. QUANTIFICATION AND ANALYSIS OF EROSIVE TRENDS........................................... 4 

Background......................................................................................................................... 4 

Methods............................................................................................................................. 14 

Results............................................................................................................................... 24 

Discussion......................................................................................................................... 31 

Further Research ............................................................................................................... 32 

2. EROSION MANAGEMENT ALTERNATIVES ................................................................... 33 

Introduction....................................................................................................................... 33 

The Goals of Inshore Erosion Management ..................................................................... 33 

Stabilization Based Alternatives....................................................................................... 34 

Regulation Based Strategies ............................................................................................. 37 

3. POLICY GOVERNING IMPLEMENTATION OF MANAGEMENT ALTERNATIVES... 39 

Introduction....................................................................................................................... 39 

Implementation of Stabilization Plans.............................................................................. 39 

Implementation of Regulation Plans................................................................................. 40 

CONCLUSION............................................................................................................................. 45 

GLOSSARY ................................................................................................................................. 46 

APPENDIX................................................................................................................................... 47 

REFERENCES ............................................................................................................................. 64 

iv 
BIOGRAPHICAL SKETCH ........................................................................................................ 70

v 
LIST OF TABLES 

1. Summary of 1970/1971 channel margin classification..................................................... 10 

2. Indicators of factors affecting erosion and accretion........................................................ 19 

3. Summary of area eroded 1970/1971 to 2002.................................................................... 24 

4. Summary of lateral movement from 1970/1971 to 2002.................................................. 25 

5. Regression of lateral movement on significant causal factors and margin type............... 28 

6. Correlations of causal factors and channel margin movement ......................................... 30 

vi 
LIST OF FIGURES 

1. Study area – Southern portion of Guana Tolomato Matanzas National Estuarine Research 

Reserve (GTMNERR) ........................................................................................................ 2

2. Factors affecting erosion and accretion along an estuarine channel margin ...................... 4 

3. Alterations in the vicinity of Matanzas Inlet....................................................................... 9 

4. Digitizing the channel margin in the vicinity of a side channel ....................................... 16 

5. Estimation of error ............................................................................................................ 16 

6. Classification of channel margin types............................................................................. 17 

7. Methods used to measure erosion and accretion............................................................... 18 

8. Coding of exposure to waves caused by predominant winds ........................................... 20 

9. Determination of exposure to navigation channel boat wakes ......................................... 21 

10. Measurement of distance from edge of 1999 navigation channel to channel margin ...... 22 

11. Coding of curvature variable and determination of radii of curvature ............................. 23 

12. Minimum, maximum and quartile rates of lateral movement from 1970/1971 to 2002 

classified by margin type .................................................................................................. 26

13. Minimum, maximum and quartile rates of lateral movement from 1970/1971 to 2002 

classified by exposure to causal factors............................................................................ 27

14. 1970/1971 to 2002 change in area estimated from rates of lateral movement ................. 29 

15. Project Greenshores in Pensacola, Florida........................................................................ 36 

16. Boat registrations in Flagler and St. Johns Counties, 1977 to 2001 ................................. 38 

17. Index of plates................................................................................................................... 47 

18. Plate 1................................................................................................................................ 48 

19. Plate 2................................................................................................................................ 49 

vii 
20. Plate 3................................................................................................................................ 50 

21. Plate 4................................................................................................................................ 51 

22. Plate 5................................................................................................................................ 52 

23. Plate 6................................................................................................................................ 53 

24. Plate 7................................................................................................................................ 54 

25. Plate 8................................................................................................................................ 55 

26. Plate 9................................................................................................................................ 56 

27. Plate 10.............................................................................................................................. 57 

28. Plate 11.............................................................................................................................. 58 

29. Plate 12.............................................................................................................................. 59 

30. Plate 13.............................................................................................................................. 60 

31. Plate 14.............................................................................................................................. 61 

32. Plate 15.............................................................................................................................. 62 

33. Plate 16.............................................................................................................................. 63 

viii 
ABSTRACT 

The Guana Tolomato Matanzas National Estuarine Research Reserve is one of twenty-six 

such reserves in the United States established with the intent of protecting coastal estuaries. 

GIS-based analysis of aerial photographs of the southern half of the reserve reveals high rates of 

erosion along the margin of the Atlantic Intracoastal Waterway which runs through the reserve. 

From 1970/1971 to 2002 nearly 70 hectares (approximately 170 acres) of shoreline habitat were 

degraded by erosion along the 64.8 kilometers of channel margin analyzed. Wakes generated by 

vessels in the Intracoastal are hypothesized to be the primary cause of this erosion. An 

examination of the relationships between lateral movement of the channel margin and factors 

with the potential to affect erosion and accretion supports this hypothesis. Exposure to boat 

wakes was found to be the causal factor most strongly correlated with rate of lateral margin 

movement. Margin movement rates were also found to vary significantly with exposure to wind 

waves and with the type of channel margin eroded. A reduction in nearshore wave energy 

appears to be necessary to allow the recovery of impacted ecosystems. Approaches to erosion 

management based on nearshore stabilization and regulation of navigation are discussed, and the 

intricacies of the implementation of such plans are described.

ix 
INTRODUCTION 

 Coastal wetlands worldwide are increasingly valued for buffering the capacity of coastal 

storms to flood or erode uplands, filtering urban runoff, providing wildlife habitat, and 

supporting coastal fisheries (Beatley, Brower, & Schwab, 2002). Rapid development of coastal 

regions has led to the establishment of an array of local, state, and national regulatory efforts to 

protect these values. Among the national programs in the United States which support 

conservation of coastal wetlands is the National Estuarine Research Reserve (NERR) system. 

The NERR program was created by the Coastal Zone Management Act of 1972 (National 

Estuarine Research Reserve System, 2004) to encourage “long-term research, water-quality 

monitoring, education, and coastal stewardship” (Guana Tolomato Matanzas Reserve, 2004, 

February 18). The reserves, which constitute the system, are selected from areas nominated by 

states to represent distinct biogeographic regions. If chosen for incorporation into the national 

program, the reserves enter into a federal-state partnership in which the National Oceanic and 

Atmospheric Administration (NOAA) provides 50% of funding for reserve operation and a 

cohesive, system-wide management structure. State partners are responsible for providing the 

remaining 50% of funding and for management of resources and administration of programs at 

the local level. 

The federal government does not purchase or assert direct management authority over the 

reserve lands, but instead relies on the states to provide resource protection to “ensure a stable 

environment for research" (National Estuarine Research Reserve System, 2005(a)). Still, 

conservation is a significant focus of the NERR system. This is well-reflected in one of the 

stated objectives of the reserve program— to “maintain and enhance the integrity of reserve 

habitats through stewardship and restoration” (National Estuarine Research Reserve System, 

2004). The emphasis on resource protection is further emphasized in the fact that NOAA is 

authorized to withdraw the designation of a reserve if a stable research environment is not 

maintained (National Estuarine Research Reserve System, 2005(b)). With this in mind, it is 

clear that any degradation of habitat within a reserve would be an issue of concern. This study 

addresses one such issue of concern in the Guana Tolomato Matanzas National Estuarine 

Research Reserve (GTMNERR)— the issue of habitat degradation due to erosion along the 

1 
margin of the Atlantic Intracoastal Waterway (AICW). Not only is such habitat degradation an 

issue of concern given the reserve management objectives, but it also contradicts the desired 

trend of “no net loss of wetlands” supported by local regulations (Flagler County, 2004 ) and 

federal commitments (U.S. Environmental Protection Agency, 2004). 

The GTMNERR is divided into two sections, together comprising approximately 24,000 

hectares (60,000 acres) in St. Johns and Flagler counties in northeastern Florida. The Guana, 

Tolomato, and Matanzas Rivers are the major estuarine bodies of the reserve; together they form 

a string of relatively narrow “bar bounded” estuaries behind the barrier islands which line the 

Atlantic coast. This study focuses on the AICW in the southern portion of the GTMNERR 

(Figure 1). The AICW in the study area consists of a marked channel in the Matanzas River, 

portions of which have been deepened and straightened to provide for navigation, and several 

reaches of completely man-made channel. 

St. Johns County

Flagler County

Atlantic Ocean

012345 Kilometers ¢

Legend

Study area

GTMNERR

GTMNERR

GTMNERR

Figure 1: Study area – Southern portion of Guana Tolomato Matanzas National Estuarine 

Research Reserve (GTMNERR) 

2 
Habitats found in the reserve include salt marsh and mangrove tidal wetlands, oyster bars, 

estuarine lagoons, creeks and rivers, dredge spoil disposal areas, upland, and a section of the 

adjoining Atlantic Ocean. It is of interest that the mangrove wetlands in the reserve constitute 

“the northern-most extent of mangrove habitat on the east coast of the United States” (Guana 

Tolomato Matanzas Reserve, February 18, 2004). Although the reserve is described as relatively 

undeveloped, the hydrology of the estuaries has been significantly altered by human activities, 

including the construction of the AICW (Guana Tolomato Matanzas Reserve, 2004, February 18) 

which was first dredged as early as 1883 (Florida Inland Navigation District, 1967). 

Years of personal observations and a brief pilot study conducted in the fall of 2003 made 

apparent the process of erosion and subsequent habitat degradation in the GTMNERR. Although 

erosion is a natural process, the high rate of erosion observed in the GTMNERR appeared to be 

degrading natural habitats at a rate far faster than they could rebuild. Studies have shown that 

lateral erosion of salt marsh channels, such as that of the AICW and its tidal tributaries, is 

naturally offset by deposition in other areas (Letzsch & Frey, 1980). Thus, any observation of 

widespread erosion, not offset by accretion elsewhere, warrants careful examination. 

The margin of the AICW channel which runs through the reserve has eroded 

considerably over the past thirty years. As a result, productive habitats, including salt marsh, 

mangroves, and oyster bars, are being eroded and replaced by intertidal sand flats which are 

considerably less productive (Montague & Wiegert, 1990) and thus, also considerably less 

valuable from an anthropocentric perspective. The channel of the AICW in the GTMNERR is 

lined with tidal creeks, oyster bars, salt and mangrove marshes, dredge spoil islands, and 

developed uplands. The intent of this study is to quantify the extent of habitat loss due to 

channel margin erosion from 1970 to 2002, examine correlations between erosion rates and 

possible causal factors, investigate management alternatives which could be used to limit habitat 

degradation, and examine the regulatory framework surrounding the implementation of such 

alternatives. 

3 
CHAPTER 1 

QUANTIFICATION AND ANALYSIS OF EROSIVE TRENDS 

Background 

 The process of habitat loss to erosion in the GTMNERR can be described as erosion 

along the margin of a major estuarine channel which has been modified to provide for 

navigation. Sediment transport in estuaries has been widely studied, and the basic mechanics of 

it are well understood. While the majority of studies concerned with erosion and accretion 

within estuaries have focused specifically on salt marsh channels, the findings have applications 

to channels of other margin types. Along estuarine channel margins, erosion and accretion are 

regulated by the interaction of forces that add energy to the nearshore environment with variables 

which regulate the input of new sediment and which govern sediment mobility (Figure 2). 

Sediment 

Supply 

Biological 

Stabilization and 

Destabilization 

Sediment 

Type 

Waves 

and 

Wakes 

Tidal 

Currents 

Dredging 

and Filling 

Sediment 

Movement 

Sediment 

Input 

Sediment 

Mobility 

Erosion or Accretion 

of 

Channel Margin 

Figure 2: Factors affecting erosion and accretion along an estuarine channel margin 

4 
As displayed in Figure 2, the primary forces responsible for the movement of sediment are 

waves, currents, and human dredging and filling. Waves and currents affect shoreline 

sedimentary processes by adding energy to the nearshore environment. Dredging and filling 

physically add or remove sediment. Sediment supply regulates the amount of sediment available 

for accretion, and the sediment type and level of biological stabilization or destabilization govern 

the mobility of channel margin sediments. Of these factors, several can be disregarded as 

potential causes of the erosion in the GTMNERR and several are likely contributing causes. The 

role of sea level is also addressed below due to its connection to the global climate change debate 

and frequent association with coastal erosion. 

Waves and Wakes

In modern estuarine systems, the two major sources of wave energy are wind waves and 

boat wakes. Both are recognized as capable of causing significant sediment transport in a variety 

of aquatic environments. 

The shoreline impact of waves generated in estuaries is influenced by a number of 

factors, but two of the most significant of these are nearshore bathymetry and bottom friction. 

Because waves break and lose energy when they enter sufficiently shallow water, the wave 

energy at impact, and thus the erosive capacity of a wave, is significantly affected by the 

bathymetric profile of nearshore areas. A gently sloping profile allows wave energy to dissipate 

gradually because waves break farther from the shore, while a steep profile focuses a larger 

portion of the wave energy directly on the shoreline. In a similar manner, submerged bars off a 

shoreline can significantly reduce the impact of waves approaching from open water. Increased 

bottom friction also leads to a decrease in wave energy. 

Quantification of the effects of bathymetry and bottom friction in the GTMNERR is 

difficult; however, these factors are certainly of importance. Even a cursory observation of 

shorelines in the study area reveals a great deal of variation in nearshore depths and substrate 

types. The level of variation makes measurement and analysis of the role of this variation 

difficult. 

Wind waves are cited as a cause of marsh erosion in a number of studies (Day, Scarton, 

Rismondo, & Are, 1998; Doane, Wells, & Merman, 1998; Downs, Nicholls, Leatherman, & 

Hautzenroder, 1994; Phillips, 1986b; Schwimmer, 2001). Waves cause marsh erosion directly 

5 
by undermining the stabilizing root mat of smooth cordgrass, Spartina alterniflora, the dominant 

marsh vegetation in the GTMNERR. Non-marsh channel margins are eroded more gradually. 

The prediction of the extent of wind wave erosion is difficult due to the number of factors which 

influence wave energy at bank impact. These factors include wind speed, duration and fetch 

(distance that winds blows over water), as well as water depth and angle of wave impact. 

Previous studies (Hershberger & Ting, 1996) have shown that even complex models of inshore 

wave propagation can encounter considerable error. Hershberger and Ting’s research in the Gulf 

Intracoastal Waterway compared field measurements of wave height and period with those 

predicted by the U.S. Army Corps of Engineers (USCOE) Automated Coastal Engineering 

System model and found the model to over-predict waves when wind was blowing along the 

channel and under-predict waves when wind was blowing across the channel. Considerable 

expertise is necessary to accurately predict wind wave-caused erosion through the prediction of 

wave energy in a channel such as the AICW. A simpler predictor of channel margin erosion may 

be the presence or absence of exposure to causal factors such as wind waves. 

Wind wave erosion has been found to be most severe downwind of the prevailing wind 

and largest local fetch (Day et al., 1998; Doane et al., 1998; Downs et al., 1994; Schwimmer, 

2001). National Oceanographic and Atmospheric Administration data from a coastal automated 

weather station approximately 2 km east of the study area show the predominant direction of 

winds over 10 knots to be from 345o to 45o (National Oceanographic and Atmospheric 

Administration, 2003). The Beaufort wind scale defines 10 knots as the wind velocity at which 

small waves generally start to form and thus wind wave erosion can be expected to begin to 

occur. Given the results of previous studies, wind wave erosion can be expected to be most 

severe along the Matanzas River downwind of this wind direction, or along river channel 

margins facing from 90 to 300 degrees. 

Boat wakes are also widely recognized as a cause of bank erosion in inland bodies of 

water (Grossfeld, 1997; Kennish, 2002; Maynord et al., 2001; Raines, 2003; Wilcox, n.d.; 

Williams, 1993; Zabawa, Ostrom & Byrne, 1980). Factors influencing the erosive impact of 

boat wakes include the size of the wake, the water depth, the current direction and velocity, the 

morphology of the impacted bank, the presence of wind waves, and the distance of the vessel 

from the shore (Macfarlane & Renilson, 1999). The size of the wake is governed by vessel 

speed, hull form, draft, loading, and trim. Generally, fast moving vessels displacing large 

6 
volumes of water produce the largest wakes while vessels displacing less water and moving 

slowly or at planing speed produce the smallest wakes. 

Although boat wakes and wind waves affect the channel margin in a similar manner, 

wake-caused erosion can be distinguished from wind wave-caused erosion in that it occurs in 

areas sheltered from wind waves and may be most severe where the AICW channel is closest to 

the channel margin. Wakes can be expected to be a much more significant problem in the 

GTMNERR than in wider bodies of water such as the nearby St. Johns River. The relatively 

narrow channel of the AICW does not allow significant distance for wake energy to subside 

before wakes impact the margin. The narrow channel also does not provide as large a fetch for 

the development of wind waves as wider channels do; thus, ecosystems along the margins of the 

AICW are adapted to significantly lower energy levels than those along channels where larger 

wind wave propagation is possible. Personal observations supported by consultation with 

knowledgeable locals and experts in the field of coastal geomorphology (Sergio Fagherazzi, 

personal communication, 2004) have led to the hypothesis that boat wakes in the AICW are the 

primary case of erosion in the GTMNERR. 

Tidal Currents

Tidal currents are capable of causing erosion along estuarine channels in the same 

manner that riverine currents erode river banks. Generally, channels erode on the outside of 

bends, where current velocity and resultant shear stress is highest, and accrete on the point bars 

on the inside of bends. Tighter bends, with smaller radii of curvature, erode on the outside and 

accrete on the inside faster than wider bends with larger radii of curvature (Leopold, Wolman, & 

Miller, 1992). In stable systems lateral, current-induced erosion in one area is offset by accretion 

in another (Letzsch & Frey, 1980). 

Dredging and filling

Dredging and filling for navigational or other purposes can cause erosion or accretion in 

several ways. Most obviously, channels can be dredged directly through an area or existing 

channels can be filled. This results in apparent erosion or accretion in a map or aerial 

photograph. According to Brian Brodehl of the USCOE Jacksonville district (personal 

communication, September 10, 2004), dredging can also cause channel widening when a channel 

7 
is dredged to such a depth that when channel bed sediments reach their natural angle of repose, 

the bank is under cut. If the dimensions of a channel and the angle of repose of constituent 

sediments are known, it is possible to calculate the potential for erosion due to this mechanism. 

Considering that the planned dimensions for the AICW navigation channel are 125 feet wide by 

12 feet deep and that the approximate angle of repose of bed sediments is 1:2.5 (according to B. 

Brodehl.), the current mean width of the entire tidal channel, over 1000 feet, is more than 

sufficient to accommodate the construction of the channel without under cutting banks. Mr. 

Brodehl acknowledged that although the USCOE considers this calculation in the dredging of the 

AICW channel, it is likely that historically dredging efforts were not as carefully engineered. 

It is also possible for dredging to alter depth or fetch available for wave propagation, or 

to alter current direction or velocity and thus indirectly influence local erosion rates. One 

mechanism through which channel dredging may increase current velocities and increase erosion 

rates is through alteration of the tidal prism (the difference in the volume of water in a water 

body between low and high tides) (Cox, Wadsworth, & Thomson, 2003). It is likely that 

dredging associated with the creation of the AICW altered the local tidal prism, but it is difficult 

to determine the magnitude of the change or to relate this change to sedimentary processes. 

A second manner in which navigation related dredging and filling may have affected tidal 

currents, and thus affected sedimentation, is through the alteration of natural channels in the 

vicinity of Matanzas Inlet, both during the initial construction of the AICW and again in the 

1970’s. Figure 3 allows the comparison of the modern channel configuration with the 

unmodified channel, as depicted in United States Coast Survey maps created in 1867 and 1872. 

The channel to the north of the inlet was realigned during construction of the AICW channel as 

was the smaller channel running south from the inlet. The thin strip of land dividing the inlet 

and the navigation channel was also fortified to prevent tidal currents from depositing sediment 

in the channel. Together these modifications dramatically altered the natural tidal channels in 

the vicinity of the inlet and are likely to have caused substantial changes in sediment transport 

processes. 

8 
0 400 800 Meters

Channel margin from ~1870 UCS maps

AICW channel margin from 2002 photography

Back ground is 2002 SJC ph otography

Shorelines in this area fortified 

after dredging of AICW 

Matanzas Inlet

Figure 3: Alterations in the vicinity of Matanzas Inlet 

The precise effect of such alterations is again difficult to discern. However, a portion of 

the study area, from the State Road 206 Bridge to the north end of the study area, has apparently 

never been dredged (judging from the absence of dredge spoil islands) and can be viewed as a 

control for the examination of dredging impacts. 

Sediment supply

 The primary condition which must be met for accretion to occur is the existence of a 

sufficient supply of sediment. The primary sources of allocthonous sediment for most marsh 

systems are (1) riverine sources, (2) off-shore sources, (3) barrier wash-over, (4) erosion of 

coastal cliffs, and (5) wind-blown sediments. Biogenic organic aggregates provide the major 

source of autochthonous sediments (Frey & Basan, 1978). The AICW in the study area does not 

receive significant input of sediment from riverine sources or experience substantial barrier 

island wash over and is not located near any coastal cliffs. Off-shore sources, wind-blown 

sediments, and organic aggregates are most probably the major potential sources of new 

9 
sediment (personal observations, 1980-present). Dredged bed sediment also may contribute 

considerably to the sediment supply, both during the dredging process and later as dredged 

sediment is eroded from shoreline disposal areas. There is no indication of a change in sediment 

supply levels during the study period. 

Sediment type

Channel margin sediments in the study area vary from coarse oyster shells to fine organic 

sediments. According to this variation in sediment, any location along the AICW channel 

margin can be placed into one of five different categories: intertidal bars, salt marsh, sandy 

dredge spoil disposal areas, uplands, or water (the mouths of tributaries or off-channel areas 

which join the main channel). Although these categories are somewhat subjective, they can be 

viewed as an indicator of both channel margin sediment type and the level of biological 

stabilization or destabilization. Intertidal bars contain mostly disarticulated oyster shells; 

marshes contain very fine sediments; spoil is generally sandy with some shell; uplands are sandy, 

but are usually reinforced with tree roots or seawalls. The 64.8 kilometers of channel margin 

examined in this study were classified as displayed in Table 1 below. Each sedimentary class 

can be expected to respond differently to erosive forces due to both the physical structure of the 

sediment and the levels of biological stabilization or destabilization. 

 

Table 1: Summary of 1970/1971 channel margin classification 

Margin 

classification Margin length (km) Proportion of margin length 

intertidal bars 8.7 13% 

marsh 24.8 38% 

dredge spoil 19.3 30% 

upland 6.1 9% 

water 5.9 9% 

Total 64.8 100% 

10 
Biological stabilization and destabilization 

Estuarine organisms can have both positive and negative influences on the stability of 

channel margins. The major species responsible for shoreline and nearshore stabilization in the 

GTMNERR are smooth cordgrass, Spartina alterniflora, and the eastern oyster, Crassostrea 

virginica. Atlantic coast salt marshes, such those found in the GTMNERR, commonly consist of 

extremely fine unconsolidated silts and clays (Frey & Basan, 1978), the top several inches of 

which are reinforced by the rhizomatic mat of Spartina alterniflora. Personal observations 

suggest that marsh erosion in the reserve follows the general pattern described in the seminal 

work of Redfield (1972). Erosion of the sediment from under the root mat leaves the marsh 

surface unsupported and results in the mass wasting of large blocks of the vegetative mat and 

coherent sediment. This erosive process makes clear the fact that although Spartina roots do 

reinforce marsh sediments, they cannot prevent erosion in a high energy environment. 

Oysters protect fine marsh sediments from erosive forces in a manner similar to 

cordgrass, but oysters are also not immune to damage from waves. Grizzle, Adams and Walters 

(2002) conducted a study of aerial photography in the Indian River Lagoon, on Florida’s east 

coast, and observed widespread death of oyster bar margins, an occurrence they attributed to the 

action of boat wakes. Wakes have the potential to inhibit the settlement of larval oysters and 

physically move or smother adult oysters with sediment. Dead oyster bar margins similar to 

those discussed by Grizzle et al. (2002) are common in the GTMNERR. They appear much 

lighter in color than healthy bars and can be distinguished in aerial photographs. Informal 

analysis of aerial photos of the reserve suggests that all oyster bar margins with direct exposure 

to the navigation channel of the AICW could be classified as dead using Grizzle’s criteria. 

Biological destabilization, or bioturbation, involves physical disturbance of sediment by 

living organisms, such as the fiddler crab, Uca pugnax (Letzsch & Frey, 1980). Fiddler crab 

burrows penetrate marsh sediments and thus increase their susceptibility to erosion by waves or 

currents. Although native organisms such as U. pugnax may decrease the stability of sediments 

in stable ecosystems, lateral erosion and deposition are roughly equal even in their presence 

(Letzsch & Frey, 1980). Thus, in the absence of any drastic ecosystem changes, bioturbation 

should not be viewed as a primary cause of erosion. The same holds true for other examples of 

ecosystem change with the potential to alter marsh erosion rates. Periwinkle snails, Littorina sp.,

have been observed to feed on Spartina; and in the absence of predation by blue crabs, 

11 
Callinectes sapidus, populations of snails may have the potential to devastate Spartina marshes, 

destabilizing sediments and lowering deposition rates (Bertness & Silliman, 2002). Again, 

overall sedimentation rates should not be expected to change significantly unless there is a 

substantial change in trophic structure. 

Alteration of the trophic structure of an ecosystem can cause otherwise innocuous 

processes, such as bioturbation or predation, to have drastic effects on the stability of the system. 

However, determination of the role of such processes in marsh erosion is difficult due to the 

natural complexity of ecosystems and the large amount of observational data necessary to 

ascertain if a change is occurring. No major changes in trophic structure, with clear potential to 

alter erosion rates, have been reported in the GTMNERR. 

The role of sea level

Relative sea level rise can result from either an increase in eustatic sea level (the level of 

the ocean in relation to the land) or from subsidence of the land. Mean sea level is measured 

through analysis of tidal gauge data corrected to account for seasonal and interannual variation 

and change in land elevation. The lack of tidal data for any one location in the study area for any 

significant portion of the study period negates the possibility of conducting this type of analysis. 

However, a NOAA sea level trend analysis for a tidal gauge approximately 48 miles north of the 

study area reported an average rate of sea level rise of 2.43 millimeters/year, from 1928 to 1999 

(NOAA, n.d. a). This rate is at the high end of the range of the estimated current global average 

rate of sea level rise of 1.0 to 2.4 millimeters/year (NOAA, n.d. b). 

Sea level rise has been implicated as a causal factor in a number of studies of marsh 

erosion (Day et al., 1998; Downs et al., 1994; Hartig, Gornitz, Kolker, Mushacke, & Fallon, 

2002; Kastler & Wiberg, 1996; Kearney, Grace, & Stevenson, 1988; Phillips, 1986a; Reed, 

1988; Salinas, DeLaune, & Patrick, 1986). In these studies, subsidence and eustatic sea level rise 

often function in concert to increase erosion rates; but in several locations including Venice, Italy 

(Day et al., 1998) and Louisiana, (Salinas et al., 1986) subsidence appears to be the underlying 

cause of observed erosion. Subsidence is the decrease in elevation of sediments due to extraction 

of subsurface resources or geologic processes. There have been no recorded observations of 

subsidence in the reserve. 

12 
In cases of erosion exacerbated by eustatic sea level rise and those involving subsidence, 

the response of the marsh appears to be essentially the same. A simplified relationship of marsh 

elevation to relative sea level was presented by Redfield (1965), who found that if salt marsh 

surface accretion keeps pace with relative sea level rise, then the marsh will remain stable. This 

view was amended by Orson, Panageotou, and Leatherman (1985) and again by Schwimmer and 

Pizzuto (2000) who studied a rapidly eroding marsh in which the aggradation (vertical building) 

rate exceeded the rate of relative sea level rise. Schwimmer and Pizzuto propose that in the face 

of relative sea level rise a marsh can either (1) erode (retreat laterally), (2) prograde (build 

laterally), or (3) drown depending on local rates of relative sea level rise and marsh surface and 

nearshore sedimentation rates. Nearshore sedimentation is critical because it has the potential to 

alter bathymetry and thus affect the erosive impact of waves. 

A marsh shoreline erodes, in the presence of relative sea level rise, when the nearshore 

sedimentation rate is less than the local rate of relative sea level rise and the rate of marsh 

aggradation is greater than the rate of relative sea level rise (i.e. the marsh builds upward fast 

enough to stay above water but erodes laterally). A shoreline progrades, in the presence of 

relative sea level rise, when the nearshore sedimentation rate and the rate of marsh aggradation 

are both greater than the local rate of relative sea level rise (i.e. the marsh not only builds upward 

fast enough to stay above water, but high nearshore deposition rates allow it to build laterally). 

A marsh drowns when the near shore sedimentation rate and the rate of marsh aggradation are 

both less than the local rate of relative sea level rise (i.e. the marsh cannot build upward fast 

enough to stay above water). The first indicator of drowning is the deterioration of the 

vegetative marsh mat due to excessive inundation. Where mat deterioration occurs, new areas of 

open water form and are then enlarged by waves in the direction of the predominant wind 

(Stevenson, Kearney, & Pendleton, 1985). 

Considering these statements, if, in the presence of relative sea level rise, erosion is 

observed but drowning is not observed, as is the case in the GTMNERR, the marsh must be 

aggrading and erosion must therefore be caused by relatively low nearshore sedimentation rates. 

Nearshore sedimentation rates are influenced by wave climate and sediment supply (Schwimmer 

& Pizzuto, 2000), so erosion can be expected to be most severe in locations where sediment 

supply is lowest and wave energy is highest. Considering this relationship, sea level rise should 

13 
not be considered a primary cause of erosion, but rather a secondary factor with the potential to 

increase the rate of erosion due to waves or nearshore currents. 

Methods 

The commonly used methods of measuring erosion rates can be grouped into two main 

classes, on-site data collection and measurement from compiled historical sources. The main 

advantage of on-site data collection is that change rates can be measured precisely, and change 

over short time periods can be quantified accurately; however, this method requires significant 

field work and, therefore, is not well suited to research involving time constraints or large study 

areas. Measurement from historical sources is a process which allows the intrusion of 

considerable error; however, this method allows for the relatively rapid assessment of change 

over large areas and time spans and does not require intensive field work. Due to time and 

budgetary constraints and the desire to study as large an area as possible, this study used methods 

involving measurement from complied historical sources. 

Analysis of aerial photography, a method routinely used to analyze coastal geomorphic 

trends (see Cox, Wadsworth & Thomson, 2003, for a recent example), was used to measure the 

extent of erosion along the AICW in the southern section of the GTMNERR over the past thirtyone years. The Florida Department of Transportation (FDOT) Survey and Mapping Office 

provided digital versions of 1:24,000 scale, black and white aerial photography of the Flagler 

County portion of the study area taken in November and December of 1970 and in February of 

2002. FDOT also provided 1:24,000 scale, black and white photography of the St. Johns County 

portion of the study area taken in April of 1971. Photography of the St. Johns County portion of 

the study area taken in 2002 was purchased from St. Johns County. Both sets of 2002 

photography were received as digital orthophotos pre-rectified to geographic coordinate systems. 

All photo sets were scanned using a resolution of 2000 dots per inch to yield digital photo sets 

with a 0.3-meter pixel resolution. Environmental Systems Research Institute (ESRI) Arc 

Geographic Information System (GIS) software was used to georectify the 1970 and 1971 photos 

using the pre-rectified 2002 images. A minimum of 9 links was used in the rectification of each 

image and the mean root-mean-square (RMS) error for all rectified images was 3.3 meters. RMS 

error is the measure of uncertainty in geographic data promoted by the Federal Geographic Data 

Committee (FGDC, 2004). It is the square root of the mean squared differences in location 

14 
between the data set being rectified (1970 and 1971 photography) and the established data set 

used as a base for rectification (2002 photography). The FGDC does not establish a specific 

threshold by which to judge RMS error; it instead suggests that users develop a standard 

sufficient for the purposes for which the data will be used and adhere to that standard. 

Once photos were rectified, the channel margin in both the 2002 and the 1970/1971 photo 

sets were digitized to yield a digital line file. The channel margin was defined as the border of 

the major tidal channels of the GTMNERR. The margin should be distinguished from the edge 

of the navigation channel and from the channel shoreline. While the channel margin and the 

shoreline are often in the same location, there are also many locations in the study area where 

practically all of the energy generated in the channel dissipates on intertidal bars which can be 

over a hundred meters from dry land. When the shoreline and the channel margin were in the 

same place, the vegetation line was used to locate the channel margin. In unvegetated areas, 

margin definition was less precise. When gaps, such as tributary mouths or openings between 

dredge spoil islands were encountered, the shoreline was followed away from the main channel 

until the end of the erosive margin, as indicated by a white, sandy shoreline, was encountered 

(Figure 4). 

The methods used by Kastler and Wiberg (1996) were used to ascertain the uncertainty 

involved in this digitization process. Three randomly selected, vegetated, one-kilometer margin 

reaches were digitized three times each and the mean distance between each successive 

digitization was calculated. This calculation yielded a mean digitization error of 3.9 meters. 

This number was combined with the 3.3 meter RMS error as the square root of the two squared 

numbers following the methods of Gaeuman, Schmidt, and Wilcock (2003), to yield a total 

estimated error of 5.1 meters (Figure 5). 

While this error makes precise location of individual points difficult, it is essentially 

randomly distributed, so widespread changes in a single general direction are likely to be real. 

Additionally, this error only applies to vegetated margin types, including marsh, most dredge 

spoil disposal areas, and uplands. As exposure of intertidal bars varies with the tide, the degree 

to which they are distinguishable in aerial photographs varies depending on when the photos 

were taken. It was found that dead shell bars, which were significantly more common in the 

2002 photographs, were much easier to discern than live oyster bars. Water margins, found in 

the mouths of tributaries along the channel, are also imprecise. Due to these differences, 

15 
estimation of a reliable error rate in the delineation of intertidal bar and water margins is 

difficult. 

 

0 50 Meters

When a side channel was 

encountered, digitzing 

proceeded up the channel to 

the end of the light, sandy

erosive margin. 

Background is 2002 SJC photography Marsh / Backwater

Intracoastal Waterway

Tidal creek entering channel

Figure 4: Digitizing the channel margin in the vicinity of a side channel 

m

digitizing error RMS error Total estimated error

1.5)3.3()9.3(

_)_()_( _

2 2

2 2

=+

+ =

Figure 5: Estimation of error 

The channel margin lines were manually attributed as water, intertidal bars, marsh, spoil 

or upland by referencing the 1970/1971 photo set, as displayed in Figure 6. Where margin type 

was uncertain, on-site ground truthing inspections were conducted. The margin classes are 

subjective; however, they are indicators of the predominant sediment type in the area and the 

level of biological stabilization. 

16 
0 200 Meters

1971 channel margin classification

intertidal bars

marsh

spoil

upland

water

Background is 1971 FDOT photograph y

Figure 6: Classification of channel margin types 

Once digitization and margin classification were complete, change in margin location 

was assessed using two separate methods: a polygon-based analysis of change in area, and a 

point-based analysis of lateral margin movement. 

 

Polygon-based analysis

The polygon-based analysis was used to determine the total change in area of each 

margin classification as a result of erosion or accretion. Following the classification of the 

channel margin, the channel margin lines were used to create two polygons representing the 

major tidal channels in 1970/1971 and 2002. Using the methods of Gaueman et al. (2003), these 

polygons were clipped to create two new polygon files. One file contained all areas which were 

not part of the channel in 1970/1971 but were part of the channel in 2002, and thus represented 

erosion. The second file contained areas which were part of the channel in 1970/1971 but were 

not in 2002 and thus represented accretion. Examples of erosion and accretion polygons are 

displayed in Figure 7. Using GIS software, erosion polygons were manually categorized as one 

of the five margin types by spatially joining them to the file containing channel margin 

classification types. Accretion polygons, which were much less numerous, were categorized 

17 
manually while referencing the 2002 photos (Figures 18-33). The total area of erosion and 

accretion of each margin classification was calculated. 

Point-based analysis

The point-based method of analysis was structured to determine the rate of lateral erosion 

or accretion along the channel and to allow examination of rates of change in relation to a suite 

of indicator variables developed to ascertain the role of the various causal factors in erosion or 

accretion. Point files were constructed through an automated process which located points every 

10 meters along the 64.8 kilometers of digitized channel margin in the study area for both the 

1970/1971 and 2002 channel margin line files. This created two point files of approximately 

6,480 points each (examples of which are displayed in Figure 7). If the points were found to lie 

on the margins of the previously created erosion polygons, the distance from each 1970/1971 

point to the nearest point in the 2002 layer was recorded as a negative number. If the points were 

found to lie on the margins of the accretion polygons this distance was recorded as a positive 

number. 

0 10 20 Meters

1970/1971 channel margin

2002 channel margin

accretion polygon

erosion polygon

Background is 1971 FDOT photography

D D

D D

5.8 meters of accretion

3.2 meters of erosion

Intracoastal Waterway

Marsh / Backwater

Figure 7: Methods used to measure erosion and accretion 

18 
The points, at which lateral movement of the channel margin was measured, were 

classified according to their characteristics which have the potential to influence erosion and 

accretion using automated geoprocessing techniques. Each point in the 1970/1971 file was 

dichotomously classified according to its exposure to wind waves formed by prevailing winds, 

its exposure to boat wakes generated by vessels in the AICW channel, its exposure to tidal 

currents likely to cause erosion, and its location north or south of the State Road 206 Bridge (as 

an indicator of dredging activity). Points were also classified according to channel margin type. 

In addition, the width of the entire 1970/1971 channel, as well as the distance between the 2002 

channel margin and the edge of the AICW in 1999, were calculated. These indicator variables 

are summarized in Table 2. 

Table 2: Indicators of factors affecting erosion and accretion

Continuous value Variable 

mean ± stdev (min, max) Category 

Intended to 

quantify 

erosion due 

channel margin 

classification N/A 

0 (water) 

1 (intertidal bars) 

2 (marsh) 

3 (dredge spoil) 

4 (upland) 

sediment 

type and 

biological 

stabilization 

exposure to 

wind waves N/A 

1 (margin facing predominant wind direction ) 

0 (margin not facing predominant wind 

direction) 

wind waves 

exposure to 

AICW channel N/A 

1 (exposed to boat wakes generated in AICW) 

0 (unexposed to boat wakes generated in 

AICW) 

boat wakes 

entire 

1970/1971 

channel width 

(m) 

426.2 ± 687.9 (91.1, 1290.5) N/A boat wakes 

distance from 

1999 AICW 

channel (m) 

272.4 ± 711.1 (7.8, 773.0) N/A boat wakes 

exposure to 

tidal currents N/A 1 (likely to be effected by tidal currents) 

0 (unlikely to be effected by tidal currents) currents 

radius of 

curvature (m) 

of the nine 

identifiable 

bends 

789.1 ± 268.6 (439.3, 

1195.1) N/A currents 

location 

relative to 

SR206 bridge 

N/A 1 (south of bridge - along dredged channel) 

0 (north of bridge - along undredged channel) dredging 

19 
Points were coded 1, as exposed to wind waves formed by the prevailing winds, if the 

margin faced any direction from 300 to 90 degrees. Unexposed points, where the margin faced 

from 90 to 300 degrees, were coded as 0. Figure 8 displays an example of wind exposure coding 

and presents a conceptual diagram used to estimate margin angle and subsequent exposure. 

Exposure was determined based on photographs at a scale of approximately 1:5,000 so minor 

variations, on the scale of several meters, are not reflected in this variable. Such precision would 

be prohibitively time consuming and would likely exceed the precision of the digitized channel 

margins. 

0 200 Meters

exposure

0

1

Predominant winds > 10kts 

from 3450

 to 450

Channel margins facing from 

3000

 to 900 coded as exposed 

to predominant wind waves

Background is 1971 FDOT photography

Figure 8: Coding of exposure to waves caused by predominant winds 

Points were coded 1, as exposed to boat wakes, if a line drawn perpendicular to the 

centerline of the AICW navigation channel could pass through them without crossing a second 

channel margin (Figure 9). If points did not meet this condition they were coded 0 for this 

attribute. The same lines used in this test were used to measure the width of the 1970/1971 

channel. Channel width was measured perpendicular to the centerline of the AICW at 10 meter 

intervals. Since the width measurement lines usually did not intersect the points where change 

20 
was measured, channel margin points were assigned the width measurement which intersected 

the channel margin closest to their location. Points unexposed to boat wakes were excluded from 

the analyses involving channel width. Because boat wakes decay with distance from the sailing 

line of the boats which produce them, wake-caused erosion can be expected to be most severe 

when the entire tidal channel is narrower and wakes have less distance over which to dissipate 

before they impact the channel margin. 

0 200 Meters

AICW navigation channel centerline

Back ground is 1971 FDOT photo graph y

Channel margin

unexposed 

to boat wakes

Channel margin

exposed to 

boat wakes

Lines used to measure

1971 channel width

Figure 9: Determination of exposure to navigation channel boat wakes 

and measurement of 1970/1971 channel width 

The distance from the edge of the navigation channel, as defined by the location of 

United States Coast Guard navigation markers in 1999, to the channel margin was measured as 

displayed in Figure 10. This measurement serves as a third indicator of the influence of boat 

wakes on erosion rates. As with the channel width variable, points were assigned the 

measurement which intersected the channel margin closest to their location and points coded as 

unexposed to boat wakes were excluded. Erosion rates influenced by boat wakes can be 

expected to be highest where the navigation channel runs closest to the channel margin and 

wakes have the least time to dissipate. 

21 
0 200 Meters

AICW navigation channel centerline

Background is 2002 SJC ph otography

Lines used to measure 

distance from edges of 

2002 navigation channel

to channel margin

Figure 10: Measurement of distance from edge of 1999 navigation channel to channel margin 

Points were coded 1, as exposed to tidal currents, if they were on the outside of one of the 

nine identifiable bends in the AICW channel in the study area. Points on the inside of bends or 

on straight reaches of margin were coded as 0. The radius of curvature, of all identifiable bends, 

was also recorded to serve as a secondary measure of erosion caused by tidal currents. The 

radius of curvature of a bend can be interpreted as the radius of the largest circle which fits the 

curve of a bend smoothly. In this study, Arc GIS software was used to fit an arc to each curve 

and determine its radius. In addition to these two indicators of tidal current-caused erosion, the 

bathymetric cross sections of the nine bends (taken from data provided by the St. Johns Water 

Management District) were examined for signs of active outer bank erosion, mainly a significant 

increase in depth towards the outside of the bend. Such erosion can be expected to be most 

severe in the tightest bends. Figure 11 shows an example of coding of the tidal current exposure 

variable and measurement of radius of curvature. 

22 
curvature

0

1

radius = 645.3 m

D D

0 200 Meters

Back ground is 1971 FDOT photograp hy

Figure 11: Coding of curvature variable and determination of radii of curvature 

In addition to the variables discussed previously, the location of each point relative to the 

State Road 206 Bridge was also recorded as a binary variable to indicate presence or absence of 

dredging (south of bridge = 1, north = 0). 

The 1970/1971 points, where lateral margin movement was measured, were analyzed to 

detect and examine relationships between accretion or erosion rate and each of the potential 

contributing factors in Table 2. Linear regression was used to examine the correlation between 

margin movement rates and causal factors measured on a continuous scale. The Kruskal-Wallis 

test, Tukey’s studentized range test, and the Wilcoxon rank sum test were used to examine 

differences in rates of channel margin movement for independent variables measured on a 

categorical scale. All significant variables were entered into a multiple linear regression model 

to allow estimation of channel margin movement rates given different site characteristics. 

Kendall’s Tau-b was calculated to overcome the weaknesses of least-squares regression as a 

measure of correlation between binary independent variables and a continuous response. 

23 
Results 

Both polygon and point-based analysis techniques revealed high rates of channel margin 

erosion in the study area. Channel margins classified as intertidal bars, marsh, dredge spoil, and 

uplands all experienced net erosion. Channel margins classified as water were excluded from 

analyses due to the ambiguities involved in the interpretation of accretion and erosion of these 

areas. Exposure to wakes generated by vessels in the AICW was the causal factor most strongly 

correlated with higher rates of erosion. Detailed results of the polygon and point-based analyses 

further illustrate the extent of erosion and the relationship of erosion rates with causal factors.

Polygon-based analysis

The area of channel margin lost to erosion in the study area from 1970/1971 to 2002 was 

found to far exceed the area replaced through accretion. Instead of an accretion/erosion ratio of 

approximately 1 that would be expected in light of previous work (Letzsch & Frey, 1980), from 

1970/1971 to 2002 the ratio of accretion to erosion in the study area was 0.13. The total tidal 

channel area, as defined by the digitized channel margins, expanded 11.4% from 656.0 hectares 

(1621.0 acres) in 1970/1971 to 730.7 hectares (1805.7 acres) in 2002. Excluding channel 

margins attributed as water, 68.2 hectares (168.6 acres) of marsh, intertidal bars, marsh, spoil 

areas, and uplands were lost to erosion and not replaced through accretion. These findings are 

summarized in Table 3 below. 

Table 3: Summary of area eroded 1970/1971 to 2002 

Margin Area in hectares [acres] 

classification 

Proportion of 

margin length Erosion Accretion Net change 

intertidal 

bars 15% 21.0 [51.8] 0.1 [0.1] -20.9 [-51.7]

marsh 42% 26.4 [65.2] 10.2 [25.2] -16.2 [-40.1]

dredge spoil 33% 28.7 [71.0] 0.1 [0.2] -28.7 [-70.8]

upland 10% 2.7 [6.7] 0.3 [0.8] -2.4 [-6.0]

Total 100% 78.8 [219.1] 10.6 [26.2] -68.2 [-168.6]

24 
Point-based analysis

 While information concerning the area of channel margin habitat eroded is useful in the 

evaluation of the ecological impacts of erosion, the approach used in this analysis obscures the 

impacts of specific causal factors. In order to discern the relative impact of individual potential 

causes, it is necessary to eliminate the spatial variability in exposure which occurs within 

individual polygons. Comparison of points along the channel margin in 1970/1971 and 2002 

allows association of rates of change with discrete sets of causal factors. Table 4 summarizes the 

lateral erosion data obtained from point comparisons along all non-water margin types. 

Table 4: Summary of lateral movement from 1970/1971 to 2002 

Margin 

classification 

Proportion of 

margin length 

Count of 

measurement 

points 

Mean 

linear 

movement 

(m) 

intertidal bars 15% 870 -23.2 

marsh 42% 2477 -9.3 

dredge spoil 33% 1930 -16.7 

upland 10% 613 -4.6 

Total 100% 5890 -13.4 

 Comparison of Tables 3 and 4 reveals that although the largest loss to erosion in terms of 

area was from dredge spoil margins, intertidal bars were subject to a higher rate of lateral 

erosion. Upland areas experienced both the lowest level of loss of area and the lowest rate of 

lateral erosion. Marsh margins ranked third in both in terms of area eroded and lateral rate of 

movement. In Figure 12, rates of lateral erosion are depicted graphically according to margin 

classification. 

25 
-125

-100

-75

-50

-25

0

25

50

75

Intertidal

Bars (n=870)

Marsh

(n=2,477)

Dredge Spoil

(n=1,930)

Upland

(n=613)

MARGIN CLASSIFICATION

LINEAR CHANGE 1970/1971 - 2002 (m) 

Figure 12: Minimum, maximum and quartile rates of lateral movement from 1970/1971 to 2002 

classified by margin type 

A Kruskal-Wallis test showed that the rate of channel margin movement of at least one of the 

margin classifications was significantly different (p < 0.0001) than the others. A subsequent 

Tukey’s studentized range test was used to conduct pair-wise comparisons among all of the mean 

ranks of all of the margin classes. All comparisons showed significant differences (p<0.05). 

 Wilcoxon rank sum tests revealed significant differences (p < 0.0001) in the amount of 

lateral movement at points exposed and not exposed to both the AICW channel and waves 

generated by predominant winds. To discern the relative impact of these two factors, lateral 

rates of movement for points exposed to no significant causal factors, to only wind waves 

generated by predominant winds, to only boat wakes generated in the AICW channel and to both 

boat wakes and wind waves were compared. A Kruskal-Wallis test showed that the rate of 

movement for at least one of the causal factor combinations was significantly different (p < 

0.0001) than the others. A Tukey’s studentized range test revealed significant differences 

(p<0.05) in channel margin movement in all possible causal factor, pair-wise comparisons except 

26 
the “exposed to no apparent causal factors” and the “exposed to wind waves only” categories 

(see dashed box in Figure 13). The fact that this comparison was not statistically significant may 

be associated with the relatively small sample size and limited fetch available at points exposed 

only to wind waves. These points were located along secondary channels and were protected 

from boat wakes and the longer fetch found in the AICW. 

-125

-100

-75

-50

-25

0

25

50

75

100

No apparent

causal

factors

(n=277)

Wind waves

only (n=86)

Boat wakes

only

(n=3,182)

Boat wakes

and wind

waves

(n=2,345)

CAUSAL FACTOR COMBINATIONS

LINEAR CHANGE 1970/1971 - 2002 (m) 

Figure 13: Minimum, maximum and quartile rates of lateral movement from 1970/1971 to 2002 

classified by exposure to causal factors 

 In order to develop estimates of 1970/1971 to 2002 lateral movement, given variations in 

and interactions between margin type and exposure to causal factors, dichotomous variables for 

each margin type, for exposure to boat wakes generated in the AICW channel, and for exposure 

to wind waves generated by the predominant wind were entered into a least squares linear 

regression model (see Table 5). 

27 
Table 5: Regression of lateral movement on significant causal factors and margin type 

Source DF F value Pr > F 

model 5 255.69 <0.0001 

total 5889 

R-Square 0.18 

 

Variable Coefficient t value Pr > |t| 

intercept 8.30 8.57 <0.0001 

intertidal bars -17.56 -23.11 <0.0001 

marsh -5.65 -8.60 <0.0001 

spoil -10.88 -15.91 <0.0001 

exposure to wind waves -1.98 -5.04 <0.0001 

exposure to boat wakes -13.03 -16.29 <0.0001 

All coefficients in this model are significant and the very large overall F-value reveals 

that the variance explained by the model is significantly greater than the variance due to model 

error. However, the R2 value of 0.18 reveals that much of the variation in the rate of channel 

margin movement remains unexplained by the model. This unexplained variation may be due to 

the fact that the regression attempts to explain all of the variation in a continuous dependent 

variable with binary independent variables. It also could be a reflection of the use of blunt 

measures which do not capture small scale variation in bathymetry and sediment type, and thus 

do not precisely reflect the level of erosive energy at the channel margin. 

Due to this unexplained variation, the model may not be suitable for predicting 

movement at individual points; however, it can be used to estimate mean rates of lateral 

movement based on exposure to causal factors and margin type. The data in Figure 14 were 

calculated using the regression coefficients. Mean rates of lateral movement for each exposure 

and margin combination, as estimated by the regression model, were multiplied by the total 

margin length. These products were then multiplied by the proportion of the entire margin 

classified as each margin type and as exposed to each combination of causal factors. The result 

is an estimate of the change in area of each margin type associated with each causal factor. This 

figure indicates that due to the number of points susceptible to boat wake erosion, the total loss 

28 
in area associated with boat wake exposure is much greater than that associated with exposure to 

other factors. 

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

10.0

No apparent causal factors Wind waves only Boat wakes only Boat wakes and wind waves

(n=227) (n=86) (n=3,182) (n=2,345)

CAUSAL FACTOR COMBINATIONS

ESTIMATED CHANGE IN AREA (hectares) .

-123.6

-98.8

-74.1

-49.4

-24.7

0.0

24.7

ESTIMATED CHANGE IN AREA (acres) .

upland

spoil

marsh

intertidal bars

Figure 14: 1970/1971 to 2002 change in area estimated from rates of lateral movement 

In order to circumvent the problem of the low explanatory power of the regression model 

resulting from the use of binary independent variables to explain a continuous dependent 

variable, the Kendall’s Tau-b correlation coefficients were calculated for channel margin 

movement rate and all points exposed to each of the causal factor combinations (Table 6). 

29 
Table 6: Correlations of causal factors and channel margin movement 

Variable 

Kendall's 

Tau b 

coefficient p value 

exposure to wind waves -0.14 <0.0001 

exposure to boat wakes -0.20 <0.0001 

Kendall’s Tau-b can be interpreted as a measure of the percentage of variation in the dependent 

variable explained by the independent variable. While this non-parametric statistic is a more 

effective measure of correlation with binary independent variables than ordinary least-squares 

regression, it is limited in that it fails to account for interaction among the causal factors and the 

margin classification. It also does not help to overcome the coarseness in the measurement of 

the binary independent variables. These data constitute further evidence that simple exposure to 

boat wakes explains approximately 20% of the variation in channel margin movement. 

Insignificant and marginally significant variables

Simple linear regression analysis revealed no significant correlations between channel 

margin movement and any of the three continuous causal variables, radius of curvature, distance 

from the 1999 AICW channel, and 1970/1971 channel width. A Wilcoxon rank sum test 

revealed no significant difference in the rates of movement for points coded as exposed or 

unexposed to significant tidal currents. Examination of bathymetric cross sections of the nine 

identifiable bends in the study area also provided no strong evidence of tidal current caused 

erosion. 

 The binary variable involving location relative to the State Road 206 Bridge, intended as 

an indicator of the impact of dredging, was found to be related to a small (2m over study period), 

but significant difference in margin movement. However, instead of erosion being more severe 

in dredged areas, it was actually less severe. There is no obvious explanation for this finding, but 

it could be related to the difference in the sediment type and level of shoreline stabilization in 

areas north and south of the bridge. The variable was not included in the final regression model 

because, although the coefficient was significant (p<0.05) its inclusion resulted in only a 0.0025 

increase in the R2

 value. 

30 
Discussion 

Channel margin erosion in the southern half of the GTMNERR from 1970/1971 to 2002 

was severe and has most likely caused significant changes in the estuarine ecosystem. Lateral 

erosion was found to be significantly related to exposure to boat wakes generated in the AICW 

channel. Erosion was also found to be significantly related to exposure to wind waves generated 

by predominant winds; however, the effect of wind waves was significantly less than that of boat 

wakes. 

The relatively low levels of correlation between the causal factors and erosion rates 

detected using linear regression and Kendall’s Tau-b should not be viewed as an indication that 

boat wakes are not a primary cause of erosion. As stated, the reliance on binary variables and 

bluntness of measurement are two possible causes of the low R2

 of the regression model and 

these factors may also explain the relatively low Kendall’s Tau-b coefficients. 

A process of elimination of the likely causal factors also implicates boat wakes as the 

most significant cause of erosion. Because more erosion was estimated to have occurred in areas 

not exposed to waves generated by the predominant wind than in areas exposed to these wind 

waves, wind waves cannot be assumed to be the main cause of erosion. Since curvature was not 

significantly correlated with erosion rates and visual analysis of erosion patterns and bathymetric 

cross-sections did not reveal significant evidence of meander migration, currents can be rejected 

as a cause of erosion. The slightly lower rate of erosion in dredged areas south of the State Road 

206 Bridge, suggests that dredging, too, does not exacerbated erosive processes. 

Variation in sediment type, as indicated by channel margin classification, significantly 

influences erosion rates, but is not, in itself, a cause of erosion. Finally, biological change does 

not appear to be a major factor in high erosion rates because erosion was found to have occurred 

in all channel margin classification categories. This leaves boat wakes as the primary causal 

factor of erosion in the reserve. As displayed in Figure 14, above, the widespread susceptibility 

to boat wakes, alone, demonstrates the potential for vessel traffic to contribute significantly to 

erosion. 

31 
Further Research 

Further research on channel margin erosion in the GTMNERR can take two main 

directions: (1) application of the methods used in this study to other areas of the reserve or other 

time periods or (2) a more in-depth study in the current study area aimed at establishing a 

stronger correlation between causal factors and erosion rates. Work using present methods could 

make use of photography from the 1990s, 1980s, 1970s, and 1940s and possibly T-sheet maps 

created by the United States Coast Survey in the early 1870s before the AICW was created. 

Such studies could also be extended to cover the northern portion of the reserve. More in-depth 

work aimed at establishing a stronger causal correlation could involve field measurement of 

wave energy and an attempt to correlate wave energy with fetch, nearshore bathymetric profile, 

local sediment type, and erosion rates. Such a study could develop a comparison of the net 

volumes of sediment transported by relatively small but frequent wind waves and larger but less 

frequent boat wakes. 

 In light of the evidence which suggests that overall high levels of erosion in the 

GTMNERR are related to exposure to boat wakes, the following chapter discusses strategies for 

managing erosion due to wake impacts. 

32 
CHAPTER 2 

EROSION MANAGEMENT ALTERNATIVES 

Introduction 

Erosion management in coastal areas is most frequently discussed in relation to beach 

erosion. While the processes of beach and inshore channel margin erosion are similar, the low 

energy inshore environment and the distinct management goals make options which are 

generally undesirable in beach settings quite viable for inshore shoreline protection. With 

implementation in mind, methods of inshore erosion management are best grouped into two 

classes: (1) regulation-based alternatives, which address the cause of erosive forces and (2) 

stabilization-based alternatives, which address the effect of erosive forces. In the case of beach 

erosion, the human causes of erosion, frequently inlet stabilization, are often too firmly 

established to allow for regulatory management. Along inland waterways the main controllable 

cause of channel margin erosion is wake-producing vessel traffic. While dredging activity is 

also controllable, dredging does not presently appear to be a major cause of erosion in the 

GTMNERR. Regulatory options for erosion control in the GTMNERR must involve the 

regulation of navigational activities. Stabilization options include structural and non-structural 

measures. Structural stabilization includes the construction of physical structures, while 

nonstructural alternatives involve moving sediment or increasing natural stabilization. While 

structural options can exacerbate erosion and interrupt longshore sediment transport on a beach, 

they are less detrimental when used in controlling inshore erosion. The selection of erosion 

control strategies most well suited for use in an area depends mainly on the goal of erosion 

management in the locale. 

The Goals of Inshore Erosion Management 

Inshore erosion in the GTMNERR is an issue of concern because erosive processes are 

altering ecosystems of economic and environmental importance. The goal of erosion 

management activities depends to some degree on what interests are being represented. For 

33 
example, pleasure boaters, fishermen, and waterfront homeowners may each have different 

feelings from one another, regarding erosion management. In order to develop a strong erosion 

management plan, it is important to involve all concerned parties. Not only is this type of 

planning process essential to ensure public acceptance of a management plan, it will also help to 

gain the cooperation of owners of submerged land in any stabilization efforts and will ease the 

enforcement of any regulation of boating activity. Given the necessary involvement of these and 

other local interests, a suitable management goal would be to “minimize channel margin erosion 

to the extent necessary to allow the recovery of altered shoreline and nearshore ecosystems, 

while accommodating the needs of diverse local interests to the extent possible.” As evidence 

suggests, increased channel margin wave energy levels are the main cause of erosion in the 

GTMNERR; the key to limiting erosion is decreasing these energy levels. Achieving such a 

decrease on a scale as large as the entire GTMNERR is likely to involve the incorporation of 

both stabilization and regulation. 

Stabilization Based Alternatives 

 As mentioned, stabilization based options include structural and non-structural measures. 

Non-structural strategies include less permanent structures which commonly make use of 

vegetation and other naturally occurring material. Structural measures involve the construction of 

permanent stabilization works. Both types of stabilization have advantages and disadvantages. 

 Non-structural, or soft, stabilization options are probably the most widely promoted and 

possibly the most widely used means of controlling inshore erosion. They are intended to 

physically resist erosive forces by stabilizing shoreline sediments. In estuaries, some of the first 

tools of those planning soft stabilization efforts are the organisms which naturally provide 

stabilization— the marsh grass, Spartina alterniflora (Knutson & Woodhouse, 1983), and oyster 

shells (Meyer, Townsend, & Thayer, 1997). The most obvious shortcoming of these strategies is 

the fact that they rely on the same biota that are being impacted by erosion to provide stability. If 

well-established marsh grasses and oyster bars are being eroded, then it is unlikely that new 

plantings of either will establish themselves before they, too, are eroded away. Stabilizing only 

the shoreline also fails to protect nearshore habitat, such as oyster bars, from increased wave 

energy. 

34 
Two studies which recognize the impermanence of vegetative plantings in areas of 

elevated wave energy are the works of Broome, Rogers & Seneca (1992) and Rogers (1994). 

Both of these reports provide extensive discussions of the combined use of vegetation and low 

cost, wooden breakwaters in controlling erosion in a North Carolina estuary. Breakwaters are 

structures built in the water, parallel to an eroding shoreline for the purpose of reducing wave 

energy. They present the main structural option for controlling erosion in the GTMNERR. 

Erosion control structures, such as groins and jetties, which are built perpendicular to the 

shoreline, are of little use in the inshore environment because these structures are designed to 

reduce longshore transport of sediment rather than to reduce wave energy. 

With a few exceptions, breakwaters are a type of permanent stabilization, usually built 

from wood or stone. They extend from the bottom of a body of water to just below or well 

above the surface of the water. In addition to protecting shorelines, breakwaters also provide a 

hard substrate for the attachment of oysters and a refuge and foraging ground for fish. However, 

they have the potential to have unintended adverse effects on sedimentation or other ecological 

processes, and so they should be pilot tested before construction and used with caution. 

According to Dale Campbell, of the USCOE Panama City office, stone breakwaters are often 

opposed by boaters because of the potential hazards they pose to those who inadvertently leave 

the marked navigation channel (personal communication, September 10, 2004). 

One example of a nonpermanent breakwater is the new product called WhisprWave®, 

developed by Wave Dispersion Technologies, Inc.. This structure is a floating plastic 

breakwater, which serves the same purpose as a permanent breakwater but has the advantages of 

being quicker to install and easier to relocate. Such a structure would also have the advantage of 

being less damaging in case of a vessel impact. 

 An example of a large, traditional, inshore breakwater project is offered by the 11,700 

foot long stone breakwater constructed in Louisiana to protect eroding marshes bordering the 

Gulf Intracoastal Waterway from wake damage (Louisiana Coastal Wetlands Conservation and 

Restoration Task Force, 2001). An example from Florida, involving the same regulatory 

agencies which would be involved in a stabilization project in the GTMNERR, is Project Green 

Shores in Pensacola (Florida Department of Environmental Protection, 2005) (Figure 15). 

35 
Figure 15: Project Greenshores in Pensacola, Florida 

While these projects are expected to significantly decrease marsh loss to erosion, they are 

costly. The Louisiana project cost about $137 per linear foot of breakwater. Although this cost 

also includes construction of stone terraces, to reduce available fetch in a portion of the protected 

marsh, building a similar protective structure for only half of the study area would cost about $15 

million. Breakwater construction at lower cost has been shown to be possible. 

In both coastal Louisiana (Steller, 1991) and Italy’s Venice Lagoon (Scarton, Cecconi, 

Are, Day, & Rismondo, 2000), an innovative technique has been used to construct fence-type 

breakwaters from wood and brush. Both of these projects involved the construction of a sort of 

wooden bin between posts set into the bottom. In Italy, the fences were filled with willow and 

poplar bundles. In Louisiana, the fences were filled with discarded Christmas trees. This 

innovative reuse effort won a participating parish a national award for environmental 

36 
sustainability (Kratch, 1996). High levels of volunteer participation resulted in the construction 

of 7,000 meters of fence for only $190,000 in 1991. The obvious drawback of these less costly 

fences is their durability. The breakwater in Italy received severe damage during a storm and a 

large portion of the sediment which had accumulated behind it was lost. Before construction of 

this type of breakwater, careful study should be undertaken to determine if the low cost is worth 

the lower durability and resulting higher maintenance costs. It is possible that a breakwater of 

this design could be an excellent pilot project to study the effects of breakwaters on local erosion 

rates before construction of a more permanent structure. 

The high cost and potential adverse ecological impacts associated with stabilization, such 

as breakwaters, makes it unlikely that a significant portion of the GTMNERR channel margin 

environment will ever be protected by structures. This emphasizes the need for regulatory 

protective strategies. 

Regulation Based Strategies 

 Regulation-based erosion management strategies address the cause of erosion without 

permanent physical alteration of the nearshore ecosystem. These strategies focus primarily on 

the reduction of boat wakes. The role of boat wakes in channel margin erosion rates can be 

expected to increase as the level of boat traffic in the reserve increases. There has been increase 

in boat registration in St. Johns and Flagler counties of over 400% since 1977 (Figure 16). 

Boat traffic can be regulated in several ways to reduce margin erosion caused by 

wakes. As previously stated, factors influencing the erosive impact of boat wakes include the 

size of the wake, the water depth, the current direction and velocity, the morphology of the 

impacted bank, the presence of wind waves, and the distance of the vessel from the shore 

(Macfarlane & Renilson, 1999). Factors which can potentially be regulated include the distance 

of vessels from shore and factors such as vessel speed, hull form, draft, loading and trim which 

influence the size of the wake. 

Vessel speed and distance from shore are the most obvious opportunities for regulation. 

Development of precise regulations may need to be supported by additional research linking 

wakes to habitat degradation in the GTMNERR. Such research would also help to clarify which 

vessels and what activities should be regulated. For example, is it more important to regulate 

37 
less common displacement-hulled vessels that produce huge wakes or more common planing 

vessels that produce smaller wakes? Although such studies have been conducted elsewhere 

(Wilcox, n.d.), it is important to understand the impacts of the specific distribution of vessel 

types found in the GTMNERR. 

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

FY 77-78FY 78-79FY 79-80FY 80-81FY 81-82FY 82-83FY 83-84FY 84-85FY 85-86FY 86-87FY 87-88FY 88-89FY 89-90FY 90-91FY 91-92FY 92-93FY 93-94FY 94-95FY 95-96FY 96-97FY 97-98FY 98-99

20002001

VESSEL REGISTRATIONS

Flagler County commercial

St. Johns County commercial

Flagler County private

St. Johns County private

Figure 16: Boat registrations in Flagler and St. Johns Counties, 1977 to 2001 

 (Florida Statistical Abstract, 1977-2001) 

 

 Implementation of both stabilization-based plans and regulation-based plans will require 

significant planning. Before such a planning effort is undertaken, an attempt should be made to 

establish which private and governmental entities will need to be involved in the implementation 

process. 

38 
CHAPTER 3 

POLICY GOVERNING IMPLEMENTATION OF MANAGEMENT ALTERNATIVES 

Introduction 

 Given the evidence which implicates wakes from boat traffic as the primary factor in the 

observed channel margin erosion in the GTMNERR and the preceding discussion of possible 

measures which could be used to mitigate the effects of navigation on the AICW, it is useful to 

understand how such measures could be implemented. The options for implementation of 

stabilization plans and regulation plans differ considerably. Implementation of stabilization 

plans involves public participation and environmental permitting, while implementation of 

regulation is mainly a matter of law enforcement. 

Implementation of Stabilization Plans 

 Implementation of stabilization plans is complicated by the fact that the AICW in the 

GTMNERR is constructed in waters of the United States within a right-of-way, composed of 

federally owned easements on state and private land. Although shoreline protection is a 

permitted activity for property owners within the right-of-way, a large scale public margin 

stabilization project, partially on private land, would be an involved process. 

 First, the project must be permitted under applicable federal and state regulations. 

Federal permitting authority to regulate construction in waters of the AICW stems from two 

main sources, Section 404 of the Clean Water Act (2005) and Section 10 of the Rivers and 

Harbors Act (2005). The Clean Water Act regulates placement of any type of fill, including 

materials used to build stabilization works, in the navigable waters of the United States. The 

Rivers and Harbors Act applies to projects which have the potential to obstruct navigation in the 

navigable waters of the United States. The USCOE is charged with the implementation of both 

of these statutes. If federal funding is used in the completion of a project or a federal permit is 

issued, completion of an Environmental Impact Statement, in compliance with the National 

Environmental Policy Act (2005), may also be required. 

39 
The State of Florida maintains an Environmental Resource Permitting (ERP) system, 

independent of the federal system. The ERP system, which is implemented by the Florida 

Department of Environmental Protection and the Florida Water Management Districts, regulates 

most major land alterations within the state including projects involving dredging and filling of 

waters or wetlands. Statutory authorization of the ERP program to regulate activity in wetlands 

is contained in Part IV of Chapter 373 of the Florida Statutes (2004). Issuance of an ERP permit 

from the state certifies not only that a project is compliant with state wetlands regulations, but 

also that it is consistent with the goals of the Florida Coastal Zone Management Program 

(Coastal Planning and Management, 2004). Although the USCOE issued a Statewide 

Programmatic General Permit to the State of Florida to avoid duplication in federal and state 

permitting procedures, regulation of breakwaters is specifically excluded from the scope of this 

permit (State of Florida, n.d.). Separate applications to the USCOE and the state are required to 

ensure compliance with the Clean Water Act (2005) and the Rivers and Harbors Act (2005). As 

nearshore and shoreline stabilization projects have been constructed in Florida (Florida 

Department of Environmental Protection, 2005), meeting the federal and state requirements is 

clearly possible with proper project design. 

In addition to regulatory permits, proprietary authorization must also be obtained from 

involved parties. If the work is to take place within the AICW right-of-way, then the USCOE 

Real Estate Branch must authorize the use of an easement. If the work is to take place on 

submerged lands owned by the State of Florida, the state must issue a proprietary authorization 

for use as required in Chapter 253 of the Florida Statutes (2004). If the work is to take place on 

privately owned lands, consent must also be obtained from these land owners. According to 

David Roach, of the Florida Inland Navigation District (personal communication, January, 

2005), the owners of the AICW right-of-way retain all rights to the land except those interfering 

with the navigation project. Once the consent of property owners and the USCOE Real Estate 

Branch is obtained and all applicable permits are acquired, construction can begin. 

Implementation of Regulation Plans 

Implementation of new navigation regulations is mainly a matter of enforcement, which 

is discussed below under Regulation of Navigation. Before discussing the implementation of a 

40 
new set of regulations, it is important to understand the existing regulatory framework which 

establishes and supports the right to inshore navigation in the AICW. 

The Right to Inshore Navigation in Florida

In Florida, and the United States as a whole, the right to navigate is protected by 

expressions of the Public Trust Doctrine which is rooted in the Roman Code of Justinian (Robert 

Deyle, personal communication, March, 2005) under which publicly useful bodies of water are 

preserved as public property. In Florida this authority is defined in a constitutional provision (FL 

Const. art. X, § 11) which declares that land beneath navigable waters is to be publicly held, and 

not for sale (Reimer, 2001). Navigable water bodies are defined in case law as any bodies, 

which in 1845, at the time of statehood, were “capable of being used” for transportation. The 

ordinary high water line defines the extent of public ownership. In the case of saltwater bodies, 

this high water line is statutorily defined as the “mean high water line,” a location that can be 

established from local tidal gages (Reimer, 2001). The Atlantic Intracoastal Waterway, which is 

of particular concern, is a federal waterway, and so the right to navigation is based on federal 

navigational servitude as established in the Commerce Clause of the United States Constitution 

(U.S. Const. art. I, §8). As demonstrated, the right to navigate is firmly based in both the federal 

and state constitutions. 

Support of Inshore Navigation

Governmental support of navigation mainly consists of developing, marking, and 

maintaining inshore channels. The 1927 Rivers and Harbors Act (2005) assigned the federal 

government the role of constructing and maintaining the navigation channel of the Intracoastal 

Waterway (Florida Inland Navigation District, 1967a). The USCOE is assigned the 

responsibility for the physical construction work and for cooperating with state and local 

authorities in planning and project development (Florida Inland Navigation District, 2002). 

In Florida, the Florida Inland Navigation District (FIND), established by the state 

legislature as an independent special district in 1927, helps to provide necessary rights-of-ways 

and land for channel dredging spoil disposal areas, to the federal government, free of charge 

(Florida Inland Navigation District, 2002). FIND is composed of eleven Florida east coast 

counties: Duval, St. Johns, Flagler, Volusia, Brevard, Indian River, St. Lucie, Martin, Palm 

41 
Beach, Broward, and Miami-Dade. The district is governed by an eleven member Board of 

Commissioners, one from each county in the district, appointed by the Governor (Florida Inland 

Navigation District, 2002). According to Franklin Morrison, with the Jacksonville District of the 

USCOE, the local port authorities cooperate with both the USCOE and FIND in funding local 

navigational projects and the acquisition of dredge material management sites (personal 

communication, November 20, 2003). Supported by an array of state and federal legislation, the 

channel of the Intracoastal Waterway forms the backbone of inland marine navigation along 

Florida’s east coast. 

Regulation of Navigational Activities

 Once the right to navigate is established and the creation and maintenance of a waterway, 

which can support modern navigation, is ensured, the conditions are such that the environmental 

conflicts begin to occur. As boat traffic on Florida’s waterways continues to increase, the need 

to regulate boating in order to moderate environmental degradation will increase. Federal, state, 

and local authorities share an interest in the regulation of marine navigation, but the legal 

authority of these entities to enact such regulation differs substantially. 

Federal Regulation 

The ability of the federal government to regulate marine navigation is established jointly 

in the Commerce Clause of the United States Constitution, previously cited as establishing the 

federal right to navigation, and the Property Clause of the Constitution (U.S. Const. art. IV, §3). 

The Commerce Clause gives the government the right to regulate maritime activity in waters of 

the United States based on the “federal navigational servitude” and this basis for regulation has 

been supported by the courts (Gibbons v. Ogden, 1984). The authority to restrict navigation 

based on the reasoning that the U.S. has the right to make rules governing conduct on federal 

property and adjacent non-federal property, as described in the Property Clause, has also been 

upheld (McGrail v. Babbitt, 1997). The implementation of these constitutional authorities has 

been delegated to a number of federal agencies. 

Brooks (2000) discusses six federal agencies with potential power to regulate navigation 

in Florida waters, the United States Coast Guard (USCG), the USCOE, the Fish and Wildlife 

Service (USFWS), the National Marine Fisheries Service (NMFS), the National Park Service 

42 
(NPS), and NOAA. The powers of NOAA and the NPS are restricted to national marine 

sanctuaries and national parks, respectively (Brooks, 2000). Although the Fort Matanzas 

National Monument borders the study area, its small size makes any regulation within its 

boundaries insignificant in terms of protecting the resources of the GTMNERR as a whole. 

While the GTMNERR is administered jointly by NOAA and the State of Florida, it is not a 

marine sanctuary, and thus NOAA is also unlikely to have significant regulatory authority over 

activities in the reserve. The USFWS and the NMFS derive broad authority from the 

Endangered Species Act (2005) and the Marine Mammal Protection Act (2005) to regulate 

activity which may impact endangered species or marine mammals. This authority has been 

largely delegated to the State of Florida, which makes use of it in regulation of boating activities 

for the protection of the West Indian Manatee (Florida Manatee Sanctuary Act, 2004). However, 

regulation related to endangered species does not appear to have immediate application to the 

current issue of channel margin protection. The power of the USCOE to restrict navigation is 

generally limited to issues of military operations and national security (Brooks, 2000). The 

limitations on the previously discussed agencies leave the USCG as the federal body with the 

most power to affect meaningful regulation within the GTMNERR. The Coast Guard has 

authority to restrict vessels operating in the navigable waters of the United States for 

“environmental purposes” (Ports and Waterways Safety Program, 2005) and to restrict vessels 

from “safety zones” for safety or environmental purposes (Navigation and Navigable Water 

Rule, 2005). 

State Regulation 

 State regulation of inshore navigation in Florida is accomplished through the statutory 

grant of police powers and the Public Trust Doctrine, as defined in the State Constitution (FL 

Const. art. X, §11). Such regulation is implemented by the Board of Trustees of the Internal 

Improvement Trust Fund, the Florida Fish and Wildlife Conservation Commission (FWCC), and 

the Florida Department of Environmental Protection (FDEP). However, only the FWCC and 

FDEP have power to restrict navigation for environmental purposes. 

The FWCC uses its statutory authority, under the Florida Manatee Sanctuary Act (2004), 

to restrict and exclude vessels to ensure manatee protection. This authority has been upheld in 

43 
court (Marine Industries Ass’n of South Florida, Inc. v. FDEP, 1996). Perhaps this power has 

the potential to be extended to ensure the protection of other environmental entities. 

The FDEP has the authority to restrict motorized watercraft within state-defined canoe 

trails (Recreational Trails System, 2004), but at this time there are no such trails within the study 

area. 

Local Regulation 

Local governments in Florida, through exercise of their police power, have the authority 

to restrict the operation of vessels, within water bodies in their jurisdiction, through local law or 

ordinance. These local regulations cannot conflict directly with state or federal laws and cannot 

pertain to vessels operating within the AICW (Vessel Safety, 2004). 

The restriction on local laws regulating activities in the Intracoastal Waterway makes 

significant local regulation of inshore boating activity along much of Florida’s east coast very 

difficult. Particularly in regions such as northern Flagler County or southern St. Johns County, 

where practically all inshore navigation takes place in the Waterway, local governments are left 

with few means to restrict boating activity. In situations such as these, any significant restriction 

on navigation on the AICW will have to occur on the federal or state level. 

44 
CONCLUSION 

Erosion along the margin of the Atlantic Intracoastal Waterway channel in the Guana 

Tolomato Matanzas Estuarine Research Reserve is drastically altering nearshore habitat and 

resulting in substantial ecological degradation. From 1970/1971 to 2002 nearly 70 hectares (170 

acres) of nearshore intertidal bars, marsh, dredge spoil disposal areas, and uplands have been lost 

to erosion. Analysis of erosive trends suggests that increased nearshore wave energy caused by 

boat wakes is the primary cause of this erosion; however, further research may be necessary to 

accurately relate wake energy to erosion rates. Such work should be followed by the 

development and implementation of a plan intended to address the problem of erosion. 

A management plan developed to address the problem of margin erosion in the 

GTMNERR should focus primarily on minimizing nearshore wave energy levels and perhaps 

using nearshore stabilization techniques as a remediation tool where impacts have been most 

severe. Regulation of navigation on the Intracoastal Waterway would most likely be 

implemented by the State of Florida and thus involve enforcement by the Florida Fish and 

Wildlife Conservation Commission. As no clear statutory authority for such regulation exists, 

extension of existing legislation or introduction of new state legislation may be necessary. 

Implementation of the stabilization portion of such a plan would involve meeting federal and 

state permitting requirements and obtaining the consent of involved land owners. Failure to 

implement such a plan is likely to allow current rates of habitat degradation to continue and, 

therefore, has the potential to undermine the intent of the GTMNERR as a National Estuarine 

Research Reserve (National Estuarine Research Reserve System, 2005(b)). 

45 
GLOSSARY 

accretion The process of accumulation of sediment deposited by 

wind or water 

accretion polygons A closed, two-dimensional figure with at least three sides 

representing an area where accretion has occurred 

aggradation The process of a vertical increase in the elevation of a 

landform due to sediment deposition 

AICW Atlantic Intracoastal Waterway 

allochthonous sediment Sediment introduced from outside the system 

autochthonous sediment Sediment generated within the system 

bathymetric profile A cross section showing the elevation of the bottom of a 

body of water 

bioturbation Disruption caused by biological activity 

erosion Removal of sediment by wind or water 

erosion polygons A closed, two-dimensional figure with at least three sides 

that represents an area where erosion has occurred 

eustatic sea level rise A worldwide fluctuation in sea-level; as opposed to local 

or regional change 

fetch The distance wind blows across the surface of a body of 

water to generate waves 

GTMNERR Guana Tolomato Matanzas National Estuarine Research 

Reserve 

longshore sediment transport The physical movement of sediment along a shoreline by 

currents running parallel to the shore 

NERR National Estuarine Research Reserve 

organic aggregates Macroscopic particulate matter of biological origin 

predation An interaction which involves one organism eating 

another 

rhizomatic mat A structure formed by the interwoven underground stems 

of plants 

riverine Of or related to a river 

subsidence The decrease in the elevation of a landform 

surface accretion Synonymous with aggradation 

tidal prism The difference in the volume of water in a body between 

high and low tides 

trophic structure The network of interrelationships in a biological 

community based on the transfer of energy 

46 
APPENDIX 

3

8

2

1

4

5

7

6

9

11

14

10

13

12

15

16

GTMNERR

Atlantic Ocean

GTMNERR

0 1 2 3 4 5 Kilometers ¢

Figure 17: Index of plates 

47 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1970 FDOT photography

Figure 18: Plate 1 

48 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1970 FDOT photography

Figure 19: Plate 2 

49 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1970 FDOT photography

Figure 20: Plate 3 

50 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1970 and 1971 FDOT photography

Figure 21: Plate 4

51 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 22: Plate 5 

52 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1970 and 1971 FDOT photography

Figure 23: Plate 6 

53 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 24: Plate 7 

54 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 25: Plate 8 

55 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 26: Plate 9 

56 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 27: Plate 10 

 

57 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 28: Plate 11 

58 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 29: Plate 12 

59 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 30: Plate 13 

60 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 31: Plate 14 

61 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 32: Plate 15 

62 
0 100 200 300 400 500 Meters ¢

Legend

Accretion

intertidal bars

marsh

spoil

upland

Erosion

intertidal bars

marsh

spoil

upland

Background is 1971 FDOT photography

Figure 33: Plate 16 

63 
REFERENCES 

Beatley, T., Brower, D. J., & Schwab, A. K. (2002). An introduction to coastal zone 

management. Washington, DC: Island Press. 

Bertness, M., & Silliman, B. (2002). Fewer blue crabs, more periwinkle snails make salt marshes 

blue. Water Environment & Technology, 14(12), 24-25. 

Brooks, J. L. (2000). Federal, state and local authority to restrict or exclude vessels in Florida 

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69 
BIOGRAPHICAL SKETCH 

Frank Price was born in Tallahassee Memorial Hospital in 1978. In 1980 he moved to St. 

Augustine where he grew up surfing at Butler Beach and fishing in the Matanzas River. He 

moved back to Tallahassee in 1996 to earn a bachelor’s degree in Environmental Studies with a 

minor in Biology. He has been employed by Florida State University, North Carolina State 

University, and Environmental Planing and Analysis (a private environmental consulting firm). 

Currently he works for the U.S. Geological Survey in Tallahassee where he assists with 

interdisciplinary research on the Apalachicola River. Upon graduation he plans to pursue a 

career in coastal resource management in the southeastern U.S. 

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