Morphodynamics of a Constructed Marsh: Project Greenshores, Pensacola, Florida
Chasidy Hobbs
Department of Environmental Studies
University of West Florida
11000 University Parkway
Pensacola, Florida
32514
1
Abstract
Project Greenshores, an ecosystem restoration project in Pensacola, Florida, was established on a
barred nearshore terrace to provide local marsh and seagrass habitat along the now highly
developed shoreline. The project included the placement of dredge material in a series of islands
and the establishment of submergent and emergent grasses fronted by oyster reefs.
While the
establishment of the marsh alone was considered an important success, the long-term stability of
this project, and most designed environments in general, is not clear. This study examines the
morphological response of the center island to a sequence of storm events from July 2005 to July
2006.
Morphological change was monitored using erosion pins and related to elevation,
inundation period, and vegetation density and height using correspondence analysis, a nonparametric test for association amongst nominal data. Over the course of the study, which
included Hurricanes Dennis and Katrina, the island migrated landward to the north-northwest.
The observed pattern of sedimentation and erosion and its relationship to the inundation period,
vegetation characteristics and elevation varied through the growing season.
The relatively
simple erosion-deposition couplet observed is more characteristic of a cross-island shoaling and
attenuation of wave energy during storms.
In this respect, the morphological change is likened
to that of an intertidal bar rather than that of a natural marsh. It is reasonable to expect that the
island will continue to continue migrate landward during storm events and potentially connect to
the mainland.
2
1. INTRODUCTION
Coastal wetlands are complex ecosystems that, in general, serve as a defense for
coastlines by binding deposited sediments and dissipating wave and current energies (ADAMS,
2002; BROOME, SENECA, and WOODHOUSE, 1986; LEONARD and LUTHER, 1995; SILANDER and
HALL, 1997; VAN DER WAL and PYE, 2004). Wetlands also filter freshwater flowing from land
to sea (WOLANSKI et al., 2004), are valuable nursery grounds for aquatic organisms (ADAMS,
2002; SACCO et. al., 1994), and provide food and habitat for many land and aquatic species
(WOLANSKI et al., 2004). Natural salt marshes form in low energy estuarine environments, once
sufficient accretion has occurred to allow for emergent halophytes on mudflats and/or sandflats
(DAVIDSON-ARNOTT et al., 2002). Several factors influence the morphological progression of
coastal marshes, for example: elevation (PETHICK, 1981; SENECA, BROOME,
AND
WOODHOUSE,
1985), proximity to sediment sources (STODDART, REED, and FRENCH, 1989), local plant
communities (STUMPF, 1983), tidal range (HARRISON and BLOOM, 1977), organic accumulation
rates (CALLAWAY, DELAUNE, and PATRICK, 1997; TURNER, SWENSON, and MILAN, 2000), sea
level rise (DONNELLY and BERTNESS, 2001; ORSON, PANAGEOTOU, and LAETHERMAN, 1985;
REED, 1995; STEVENSON, WARD, and KEARNEY, 1986), and inundation period (PETHICK, 1981).
Marsh accretion is usually associated with tidal creeks (LAWRENCE, ALLEN,
AND
HAVELOCK,
2004; WANG, SIKORA, and WANG, 1994; LEONARD et al., 1995; REED, 1988; TEMMERMAN et al.,
2005), which not only provide sediment but also nutrients essential for health (REED, 1988). In
marshes that lack riverine inputs, sediments are usually associated with the re-suspension of
nearby estuarine sediments, and accretion depends on the hydroperiod (CAHOON and REED,
1995; TEMMERMAN et al., 2005). In general, longer inundation periods lead to higher accretion
rates (Stevenson et al., 1986) leading to a complex feedback between sedimentation, elevation
and vegetation zonation.
From 1950 to the mid 1970s, the United States lost coastal wetlands at an average rate of
nearly 500,000 acres per year (CIUPEK, 1986). To offset the impact of this wetland loss, there
have been several initiatives including nationwide wetland restoration projects. One such
undertaking is Project Greenshores (Figure 1), one of the largest ecosystem rehabilitation
projects in northwest Florida led by Florida Department of Environmental Protection's (FDEP)
3
Ecosystem Restoration Laboratory. The project entails the establishment of an oyster reef,
seagrasses and a Spartina alterniflora salt marsh within Pensacola Bay. Greenshores was also
constructed to filter sediments and pollutants from a large stormwater outfall that drains a mixeduse development area of Pensacola, Florida, in hopes of improving the water quality of the
303(d) listed Pensacola Bay. Similar to the Ria Formosa lagoon in Portugal (NEUMEIER and
CIAVOLA, 2004) Pensacola Bay can be characterized as having little to no wave action except
during extra-tropical storms and high suspended sediment events during winter storms. Without
tidal creek systems, the source of sediment for Greenshores, comes from both runoff and
stormwater outfalls along with re-suspended sediments within the bay. Greenshores provides
essential habitat and shoreline protection during storms; however the stability of the created
marsh islands has not been investigated.
Project Greenshores is being completed in two phases. Phase I, which is the study site of
this research, was completed in the 2003 (with a small replanting event in the summer of 2004).
When Phase I of the project was started, the sub-littoral zone was a barred terrace with no
emergent or aquatic vegetation. The area offered very little valuable habitat, an element crucial
to local commercial fisheries, including the local shrimping industry, which harvests within
Pensacola Bay. As part of this phase of construction, 8 acres of salt marsh and seagrass beds
were planted on ~27,000 cubic meters of dredge material. Approximately 40,000 S. alterniflora
and 3,900 locally propagated seedlings of Ruppia maritime, were planted on 5 intertidal islands
(Islands I – V) just offshore (Figure 1). Since the islands have a maximum elevation of .37 m (at
the start of this study) and the tidal range in Pensacola Bay is ~0.50 m, the islands can be
considered intertidal features.
In addition, 7 acres of oyster reefs were constructed seaward of
the islands with 14,000 tons of Kentucky limestone, 6,000 tons of recycled concrete and 40 wave
attenuators. The surfaces of these reefs were covered with oyster shells recycled from local
restaurants. As breakwaters, these reefs not only provide protection for the newly planted
vegetation and dredge spoil, but they also create habitat for the propagation of new oysters.
While considerable effort and expense is put into the development of a constructed
wetland, little attention is paid to the long-term stability of these environments, particularly
where wetlands (such as Project Greenshores) are placed in environments that were not
4
historically wetlands. ZEDLER and CALLAWAY (2000) suggest a lack of topographic complexity,
soil organic content and nutrients, excessive sedimentation and erosion and contamination during
wetland creation as common limitations of constructed environments. Also, as suggested by
Craft, et al. (1988) it takes anywhere from 15 to 30 years for macro-organic matter nutrient pools
to develop and considerably longer for soil carbon, nitrogen and phosphorus reservoirs to
accumulate. ZEDLER and CALLAWAY (2000) further note that more science-based research and
post-construction monitoring be incorporated into restoration projects in order to allow for
quicker development of successful restoration practices, improved assessment methods and an
enhanced understanding of the functions of both natural and restored ecosystems.
In order to determine if a constructed environment is stable (and can be deemed a
success), it is important that the system and the environmental forcing (such as waves, currents
and inundation regime) be monitored. While it is generally understood that marsh plants will
influence and force the sedimentation and erosion patterns, this depends on the incident wave
and current energy, along with a complex and poorly understood feedback created by changes in
elevation, inundation period, and the distribution of vegetation. Project Greenshores is locally
considered a “success” based on the original construction goals, but the morphological stability
of the project is unclear. This paper reports on a monitoring study at Project Greenshores to
determine the degree, if any, vegetation, inundation period and elevation influence
morphological change and potentially force stability in this dredge spoil marsh.
2. METHODS
Island III was nominally chosen as the focus of this study due to its location in the center
of the project and also due to it being the largest in size. In this respect, the results of this study
are not necessarily applicable to the other islands or Phase II of the project. At the onset of the
study, a differential GPS was used to map the perimeter of Island III (which corresponds with the
vegetation perimeter), and this was used to define a systematic grid for morphological and
vegetation sampling on the island.
2.1 Morphological Response
5
Erosion pins, or perhaps more accurately called elevation rods, provided an inexpensive
and reasonable means to characterize the morphological response (patterns of erosion and
accretion) of Island III over the course of this study. Elevation rods have been used in a number
of studies to determine patterns of accretion and erosion (morphological change) in the nearshore
(HOUSER, GREENWOOD, and AAGAARD, 2006; KING, 1951), on foredunes (ARENS, 1996;ARENS
et al., 2001) and in coastal marshes (CARLING, 1982; REED, 1988). Sixty-eight elevation rods
were placed in a grid pattern throughout the island and the grid pattern was composed of 10
cross-island transects. The elevation at each elevation rod (sample points) at the beginning of the
three periods (discussed further in the vegetation analysis section) was determined using a
combination of a total station survey performed on December 13, 2005 and the morphological
change measured by the elevation rods. By the end of the study, two of the rods were bent and
six were covered with oysters, both of which complicated measurement and increased
measurement error. Due to this, data from 8 erosion pins were removed for all analyses and only
those 60 remaining sampling points will be discussed further.
2.2 Inundation
The local tidal variation was obtained from a NOAA station (Station ID: 8729840)
located approximately 1.8 km alongshore from Project Greenshores. The inundation period at
each sample point was calculated based on the relative elevation of the erosion pin and the tide
level. This method of determining inundation period represents the minimum-flooding regime,
as it does not take into account ponding effects brought on by the local microtopography (REED
and CAHOON, 1992). Wind speed and direction, and atmospheric pressure were obtained from
the same NOAA station. At the start of the study, wave heights were measured at a number of
locations at the study site, but the incident wave field generated by the oyster reefs and
breakwaters were overly complex and an appropriate spatial sampling strategy could not be
developed with available sampling equipment.
2.3 Vegetation Analysis
Given that variations in vegetation demographics will affect the pattern of erosion and
sedimentation (FRENCH, et al., 1995; LEONARD and LUTHER, 1995; REED et al., 1999), the study
6
was divided into three periods assumed to have similar vegetation characteristics: Period 1 (July
7, 2005 to November 8, 2005), characterized by summer storms and the end of the growing
season; Period 2 (November 9, 2005 to February 28, 2006), characterized by winter cold fronts
and the dieback of the Spartina from the previous period; and Period 3 (March 1, 2006 to July
19, 2006), characterized by a calm spring growing season. A non-invasive vegetation survey was
performed during each period (on September 1, 2005, January 12, 2006, and May 5, 2006). A 1
m2 frame made from pvc pipe was used for the vegetation surveys and was centered on each of
the 60 elevation rods. Vegetation density was recorded as the total number of standing shoots at
a height of 10 cm above the bed, and plant height was determined by an average of ten random
measurements from the bed to the extent of live shoot (KASWADJI, GOOSELINK, and TURNER,
1990; LASALLE, LANDIN, and SIMS, 1991; LEONARD and LUTHER 1995; NEWLING, 1981; REICE
and STIVEN, 1983; WEBB and NEWLING, 1985).
2.4 Relationship Analysis
Inverse distance weighted (IDW) interpolation was used to create raster surfaces for each
of the studied variables within each of the 3 study periods. IDW is based on the assumption that
nearby values of known points influence the value of an unknown point and the weight of this
influence diminishes as distance between points increases. The interpolated surface is therefore,
a weighted average of values based on known points. The weight each known value has on an
unknown point is defined by the power of the relationship. Small powers allow for an even
distribution of weight across all known values; as power increases more weight is put on closer
known values. IDW was chosen due to the fact that the spatial variation in the data was assumed
to be heterogeneous and unknown values were closer related to nearby known values.
Correspondence Analysis (CA) has commonly been used in the study of ecology; see for
example, GAUCH (1982). CA is a non-parametric test, which allows for several discrete variables
– nominal, ordinal, or continuous (broken into segmented ranges) – and analyzes associations
among these variables. This statistical test is a variation of Principal Component Analysis;
however, this test is tailored to categorical, rather than linear, data. CA uses Chi-square distances
(rather than Euclidian distance) to arrange the variables (beginning elevation, morphological
change, inundation period, and vegetation density and height), relative to an axis in
7
multidimensional space, according to their similarity (or association) to each other (CLAUSEN,
1998). Drawing a line from the origin to the plotted position of each variable and comparing the
angle between these plotted positions determines associations among variables. The closer the
angle is to 0 (positive correlation) or 180 (negative correlation), the stronger the relationship.
Angles near 90 and 270 show little to no relationship between variables.
Since correspondence analysis uses categorical data, morphological change was
categorized as either accretion or erosion. All other variables were split into three ranges based
on natural breaks in the histogram of all data collected for each variable: tall (> 1 m), average
(0.5 – 1 m) and short (< 0.5 m) for height; dense (> 200), average (100 – 200) and sparse (< 100)
for density; rare (< 33%), moderate (33 – 66%) and frequent (> 66%) for inundation and low (< 0.3 m), average (-0.3 – 0 m) and high (> 0 m) for beginning elevation.
3. RESULTS
Island III has a continuous ridge that runs along the center of the island (east/west and
closer to the seaward side of the island) that decreases in width at a small channel near the
middle of the island. The channel was wide at the beginning of the study and nearly split the
island in two; although, a small, vegetated area south of the channel connected both sides of the
island. The channel began to fill in with both sand-sized sediment and vegetation over the
remainder of the study, especially after Hurricane Katrina when a large sheet of plywood was
deposited in the channel and became part of the bed. Overall, the island migrated to the northnorthwest as sediments eroded from the seaward side were deposited along the back of the island
(Figure 2). As shown in Figure 3a, sediment was removed from the front (or southern edge) of
the island during Period 1, and sediment accreted on the back of the island. In total there was an
average accretion of ~3 mm during this period characterized by summer storms (including
Hurricanes Dennis and Katrina) and the end of the growing season. Period 2 had no systematic
variation in erosion and accretion (Figure 3b), although there was an average accretion of ~ 8
mm during this period of winter cold fronts. In contrast to the previous periods, during Period 3
(Figure 3c) the island lost an average of ~3 mm in elevation in response to erosion along most of
the periphery. Over the course of the study the average elevation change was 8 mm accretion.
There was no statistical difference at the 95% confidence level between the change that occurred
8
during Periods 1 and 2 (p = 0.27) nor for Periods 1 and 3 (p = 0.23). However, there was a
statistical difference between the morphological change of Periods 2 and 3 (p = 0.01).
3.1 Inundation
Period 1 was characterized by relatively high water levels, in response to the low pressure
and storm surge generated as Hurricanes Dennis, Katrina, Rita and Wilma which passed through
the area or in the Gulf of Mexico. While the prevailing (modal) wind during Periods 1 and 2
were from the north (southwest for Period 3), the strongest winds (greater than 7.5 m s-1) for all
three periods were from the southeast.
During this period of generally elevated water
levels.During Period 1, waves were observed to both break on the leading edge and propagate
over the island. Period 2 was associated with the passage of 12 cold fronts; in Pensacola Bay
winter cold fronts first bring strong southerly winds, which “push” water in from the Gulf of
Mexico and stir up available sediments through increased wave activity, which in turn increases
turbidity of Pensacola Bay. As these fronts pass winds shift from the north and depress water
levels within Pensacola Bay. Visual observations suggest that during these periods of depressed
water levels and increased turbidity, waves generally break on the island. During Period 3 there
was very little storm activity and inundation generally occurred only with tidal cycles. Periods 2
and 3 were statistically different in their beginning elevations at the 95% confidence level, and
all three periods had statistically different inundation periods.
Period 1 had the longest
inundation period (~ 82%) while Period 2 had the shortest inundation period (~ 54%) and Period
3 had an intermediate inundation period (~ 65%).
3.2 Vegetation Analysis
Since Period 1 incorporated the end of the growing season it was characterized by the
tallest vegetation; a total of 4215 plants were found with an average height of 0.52 m at the 60
sample points. While Period 2 had the greatest density of vegetation due to the fact that the
previous season’s plants were still standing and there was also new growth, particularly towards
the end of the period. Due to the dieback and new growth, Period 2 had an average plant height
of 0.25 m and a total of 8074 plants counted at the time of the survey. Period 3 had intermediate
vegetation height and density; there were a total of 6807 plants at an average of 0.4 m height.
9
There was a statistical difference between all three periods for both plant height and plant density
at the 95% confidence level.
3.4 Relationship Analysis
Correspondence Analysis was used to examine the relationship between the
morphological change (accretion/erosion), vegetation height and density, elevation and
inundation period. As noted, the closer the angle on the correspondence plot is to 0 (positive
correlation) or 180 (negative correlation), the stronger the relationship. Angles near 90 and
270 show little to no relationship between variables. As shown in Figure 4, Period 1 showed a
strong positive association between erosion, high elevation, moderate inundation and average
density. Accretion tended to occur at points that were frequently inundated but had a low density
of vegetation. There were no areas with dense vegetation so this variable is missing from the
analysis. During Period 2, erosion tended to be in areas with low elevation (leading to frequent
inundation) and sparse vegetation (Figure 5). Consistent with the results of the morphological
change, accretion was well-distributed on the island and equally distributed between areas of
differing inundation. Accretion was more closely associated with an average vegetation density,
although a moderately strong relationship was observed between sites with accretion and those
with dense vegetation. During Period 3, sites with accretion tended to be weakly associated with
high elevation, and short vegetation of average and high density (Figure 6). Consistent with
Period 2, sites with erosion tended to be associated with low elevation with sparse vegetation.
4. DISCUSSION
Prior to the establishment of Project Greenshores, the site was a barred nearshore terrace,
which suggests that there was some level of local wave energy approaching the shoreline
(MASSELINK et al., 2006) prohibiting natural marsh development. To reduce incident wave
energy, breakwaters were emplaced to increase the potential that the dredge spoil marsh would
be stable. Although the breakwaters dissipate incident wave energy through breaking, waves can
pass unbroken into the area during storm surge and refraction and diffraction around the
structures greatly complicates the wave field. This complicated wave field precluded the use of
10
the limited wave recorders available for this study, although waves were largely focused along
the southern periphery of the island. Since only sand-sized material, or similar quality and size to
the dredge material, was observed in those areas that experienced accretion it can be concluded
that the morphological change was associated with a redistribution of the island material by the
incident waves. It appears that there is no appreciable amount of fine sediment or other local
sediment introduced to the island during storm events. In this respect, the island is not truly
accreting and building as suggested by the elevation data. While it is generally assumed that
vegetation density and height contribute to sedimentation through form drag, the results of this
study suggest that erosion and sedimentation are more closely associated with both location on
the island and the levels and patterns of inundation.
Period 1 experienced ~ 3 mm accretion and had the longest inundation period (~ 82%)
with the tallest and fewest plants. Period 2, which experienced the highest accretion levels (~ 8
mm), had the lowest inundation period (~ 54%), and the densest and shortest vegetation
characteristics. Period 3, which was characterized by a general erosion pattern (~3 mm), had a
slightly higher than average inundation period (~ 65%) and medium vegetation density and
height. During Period 2, erosion was only observed in those areas that experienced frequent
inundation, while accretion tended to occur in areas of both moderate and low levels of
inundation. While the island was inundated more frequently during Period 1, the vegetation was
relatively well established compared to Period 3, which had more erosion and smaller vegetation.
In contrast, Period 2 was characterized by several cold front passages, but had the shortest
inundation period (average 54%). Most wave activity during this period was attenuated by the
fronting breakwater creating a choppy and complex wave field at the study site. In response to
the smaller waves, most of the island experienced accretion, with a spatially averaged accretion
of ~1cm.
These results support the interpretation that the pattern of erosion and accretion are
related to the transformation of sediment on the island. Sediment entrained along the leading
edge of the island is transported to the back of the island, leading to the development of the
center ridge. The transport gradients are not associated with vegetation density and height, but
11
rather are a response to wave transformation across the island. In this respect, the dredge spoil is
more akin to a slowly migrating intertidal bar than it is to a natural marsh.
As noted, this marsh was created on a barred terrace and the morphological progression
of Island III appears to be more characteristic of an intertidal bar or barrier island than that of a
typical salt marsh. Although the breakwaters dissipate some incident wave energy, they also
greatly complicate the wave field around the islands through the processes of diffraction and
refraction. This change in the incident wave field, along with natural season variability, may
explain why landward migration is not evident during the entire study. Should environmental
forcing factors influencing this island during the study period continue relatively unchanged for
years to come, it is possible that the island(s?) may connect to the mainland in the future.
Whether or not this constructed marsh is stable depends partly on ones definition of
stability. Due to the construction design and island placement, it seems natural that Island III is
migrating towards a place of equilibrium. Or, in other words, to a zone which natural historical
marshes would have existed (landward). Considering Island III accreted on average, the resource
is not being lost; it is simply migrating. Therefore, if one allows for migration within the
definition of stability, it is concluded that Island III is stable. On a final note, it is suggested that
further restoration projects be designed to more closely mimic natural shorelines in the region. If
the process of migration were not incorporated into the history of this particular created marsh it
would be several years closer to acquiring organics nutrients and developing more as a natural
marsh, rather than this process being put on hold until the point of equilibrium is reached through
migration.
12
Fig. 1. Phase I Project Greenshores, Pensacola, Florida
13
Fig. 2. Morphological change for the entire study period based on cut/fill analysis of inverse
distance weighted interpolation surfaces.
14
Fig. 3a. Morphological change during Period 1. White represents areas of accretion while black
represents areas of erosion.
Fig. 3b. Morphological change during Period 2. White represents areas of accretion while black
represents areas of erosion.
15
Fig. 3c. Morphological change during Period 3. White represents areas of accretion while black
represents areas of erosion.
Fig. 4. Correspondence Plot for Period 1.
16
Fig. 5. Correspondence Plot for Period 2.
17
Fig. 6. Correspondence Plot for Period 3.
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