A HISTORY OF MARSH EXTENT IN BACK BAY
an exploration of changes in wave climate and ecosystem quality in Back Bay National Wildlife Refuge using historic marsh extents
GOAL
Image from Back Bay National Wildlife Refuge Society, 2024
Marsh loss in Back Bay over the last century is striking and has had a significant measurable impact on wave climate. As marshes are lost, fetch lengthens, which allows more intense waves. As the City of Virginia Beach considers implementing marsh terracing restoration systems in Back Bay National Wildlife Refuge to reestablish marshes in shallow areas, the administration must consider a historic perspective of the dynamic natural landscape. This report will outline and compare wave climates in Back Bay over time to analyze the proposed restoration initiative and the evolution of the Bay's ecosystem services.
BACKGROUND
Established in June of 1938 by the United States Fish and Wildlife Service to protect migratory wildfowl, Back Bay National Wildlife Refuge has experienced many changes over the last century.
1930s
As part of the New Deal Program, the Civilian Conservation Corps constructed dunes along the barrier islands. As a result, much of the ocean washover was cut off and the watershed began to shift from a brackish to an oligohaline system.
1950s
The freshwater system of the bay became more established, allowing for underwater grasses and a vibrant bass fishery to emerge. The photo below from Lynnhaven River Now's 2022 "Nature Notes" shows a recent submerged aquatic vegetation restoration project taking place in Back Bay.
Wild celery (photo by Sara Sweeten via the Princess Anne Independent News, 2020)
1960-80s
In the latter half of the twentieth century, the water quality in Back Bay dwindled due to runoff from nearby agriculture and development, which crushed submerged aquatic vegetation growth. Subsequently, the largemouth bass fishery collapsed and waterfowl struggled to find food. In response, Back Bay National Wildlife Refuge created an impoundment complex, a series of connected water pools whose levels can be adjusted to cater to the needs of birds visiting the refuge throughout the year, to accommodate wildfowl (US Fish and Wildlife Service).
The map above shows the current extent of the refuge, from the U.S. Fish and Wildlife Service (2012).
1965 - 1973
The City of Virginia Beach attempted various saltwater pumping initiatives in an attempt to decrease the turbidity in the bay to encourage SAV recovery via flocculation. Saltwater pumping ceased in 1987 because SAV populations did not respond; gradually the salinity decreased and stabilized at the oligohaline levels they are today (Pomposini, 2017).
1980s
U.S. Fish and Wildlife Service began a land acquisition program in order to buffer against development and therefore reduce erosion and runoff.
Underwater grass beds are returning, as are populations of largemouth bass and other fish. Waterbirds are now able to use both the impoundment complex and the Back Bay for habitat. The health of the bay may not be pristine but recent improvements are encouraging.
Geography and Ecology of the Southern Watershed
The two most unique features of Back Bay are its wind-driven tides and oligohaline environment (Virginia Department of Conservation and Recreation, 2021). Being long, narrow, and severed from other bodies of water with no northern outlet, wind pushes water up the system from south to north. It is also likely that the strength of the wind in influencing tidal events and surges in water level increased over history due to loss of submerged aquatic vegetation. This is because submerged grasses attenuate wave energy (Henderson, 2019).
Hydrologic Threats to Back Bay
Other than pollution and runoff, the wildfowl haven of Back Bay is threatened by marsh loss and sea level rise.
Sea Level Rise
The oligohaline environment of the bay is maintained by the barrier islands.
A break in these islands, whether gradually by sea level rise or rapidly by an extreme storm event, could compromise the ecology of the entire watershed by allowing salty ocean water into the closed system.
MARSHES
As referenced in the timeline, Back Bay has experienced many changes to the quality and category of its ecosystem. As part of maintaining the now-freshwater system in Back Bay, water quality must improve.
This is a goal that marshes can assist in achieving: marshes store carbon, remove excessive nutrients, and reduce the sediment load of the water (Maryland Department of the Environment, n.d.).
In addition to water quality benefits, marshes also reduce wave energy. Calmer waters associated with greater marsh extent will allow for more sunlight penetration, perhaps helping submerged aquatic vegetation in the refuge rebound.
This report will offer a historic glimpse of marsh extent and wave climate in Back Bay in order to emphasize and analyze the importance of marsh restoration initiatives for reducing wind-driven wave energy.
LITERATURE REVIEW
This literature review examines the history of Back Bay’s marsh extent and the ecosystem services offered by submerged aquatic vegetation and marshes to provide support for the project. Listed below are some key takeaways.
- Back Bay has experienced many changes in its ecological makeup since the early twentieth century.
- Marsh vegetation is positively correlated with shoreline stabilization and could even raise shoreline elevations to balance the effects of sea level rise.
- Areas of low wave energy, which could be created with marsh terracing, can allow for the more efficient growth of submerged aquatic vegetation.
- Submerged aquatic vegetation and marshes can have positive impacts on ecosystem and water quality.
Simulated plants used in an MIT study of the protective benefits of marsh grasses. Photo credited to Xiaoxia Zhang, retrieved from MIT News Office.
WAVE ENERGY CALCULATIONS
The aim of this research is to calculate wave metrics and compare wave climates in Back Bay over time to provide context for and analyze Virginia Beach’s restoration initiative. To achieve this objective, significant wave height and peak wave period were calculated for 10 and 25-year return periods in different scenarios. The scenarios were based on fetch lines drawn on three different resources: a t-chart from 1890, the current marsh extent of Back Bay from Nearmap aerial imagery, and plans for the restoration project.
Current Marsh Extent
For the calculations of significant wave height and peak wave period on the current marsh extent, we based the measurements for the fetch lines on aerial imagery from Nearmap from Thursday, October 19th, 2019. This ensured that the measurements would be accurate and fairly recent.
Average Depth
The average depth for each site was retrieved from 2019 - 2020 NOAA NGS Topobathy Lidar for Coastal VA, NC, and SC from NOAA National Geodetic Survey and determined to be about one foot. We then added storm surge approximations for 10 and 25 year storms to the one foot constant.
Using NOAA’s National Hurricane Center’s Storm Surge Risk map, we determined storm surge for a 10 year wind in Back Bay to be 1 foot and storm surge for a 25 year wind in Back Bay to be 1.5 feet. These numbers were determined because they are less than the expected surge for a Category 1 hurricane, as were the wind speeds.
Based on that reasoning, we determined the water depth for 10 year return period wind was 0.6 meters and that the water depth for 25 year return period winds were 0.8 meters.
Southern Site
The Southern Site's fetch lines were drawn and measured every five degrees. After performing a calculation of significant wave height and peak wave period on each line, we averaged the results.
We only included the fetch lines for this site that would interact with the island area of Back Bay in order to give a better perspective of the change in marshland. However, this could have resulted in lower numbers than what is actually representative of the area.
We drew our fetch through the small islands in the middle of Bonney Cove in order to make sure the results emphasized what the loss of those islands could do to wave energy in Back Bay.
Fetch lines for the southern site based on a real imagery from 2019
Ten year return period: the mean significant wave height was 0.4 meters; the mean peak wave period was 2.6 seconds.
Twenty-five year return period: the mean significant wave height was 0.5 meters; the mean peak wave period was 2.5 seconds.
Interior Site
The Interior Site's fetch lines were drawn and measured every five degrees. After performing a calculation of significant wave height and peak wave period on each line, we averaged the results.
Internal site fetch based on arial imagery from 2019
Ten year return period: the mean significant wave height was 0.3 meters; the mean peak wave period was 1.9 seconds.
Twenty-five year return period: the mean significant wave height was 0.4 meters; the mean peak wave period was 2.0 seconds.
Northern Site
The Northern Site's fetch lines were drawn and measured every ten degrees. This is different from the other sites because the fetch lines were much longer and there was more open water available. After performing a calculation of significant wave height and peak wave period on each line, we averaged the results.
Northern Site fetch based on aerial imagery from 2019
Ten year return period: the mean significant wave height was 0.4 meters; the mean peak wave period was 2.4 seconds.
Twenty-five year return period: the mean significant wave height was 0.5 meters; the mean peak wave period was 2.5 seconds.
Back Bay in 1890
To gain an understanding of wave metrics in the past, we used a t-chart that appropriately aligned with the area of the marsh restoration project. We wanted to overlay multiple maps over the area, but this one was the most relevant one we found on the NOAA database.
T-sheet used as a reference for historic marsh extent
T-charts provide interesting information because they denote only areas that were considered significant for navigators.
We used the same depth for this scenario as for the present in order to ensure that the fetch would be the only variable changing for a more specific analysis.
It is also worth mentioning that we used a roughness constant for all of the calculations in this study, when it is very possible that submerged aquatic vegetation could have reduced the wave energy in any of the senarios as well. This discrepancy is acceptable because our study only involves the impact of fetch on wave energy and avoids confounding variables.
Southern Site
- Fetch lines drawn every five degrees based on an outline of the historic marsh extent
- Significant wave height and peak wave period calculated for each line, then averaged together.
Southern Site Fetch lines with 1890 marsh extent
Ten year return period: the mean significant wave height was 0.2 meters; the mean peak wave period was 1.6 seconds.
Twenty-five year return period: the mean significant wave height was 0.3 meters; the mean peak wave period was 1.7 seconds.
Interior Site
- Fetch lines drawn every five degrees and based on an outline of the historic marsh extent
- Significant wave height and peak wave period calculated for each line, then averaged together
Interior site fetch lines with 1890 marsh extent
Ten year return period: the mean significant wave height was 0.3 meters; the mean peak wave period was 1.8 seconds.
Twenty-five year return period: the mean significant wave height was 0.4 meters; the mean peak wave period was 1.9 seconds.
Northern Site
- Fetch lines drawn every ten degrees and based on an outline of the historic marsh extent
- Significant wave height and peak wave period calculated for each line, then averaged together
Fetch lines for the Northern Site based on historic marsh extent
Ten year return period: the mean significant wave height was 0.4 meters; the mean peak wave period was 2.4 seconds.
Twenty-five year return period: the mean significant wave height was 0.5 meters; the mean peak wave period was 2.5 seconds.
What will wave energy measurements look like with the proposed restoration project?
We used the city planning documents for the marsh restoration project to draw fetch lines and calculate the wave energy in the future if the project is implemented.
Southern Site
This site was expected to be similar to the historical case since they're closing off waves through Bonney Cove.
Ten year return period: the mean significant wave height was 0.4 meters; the mean peak wave period was 2.3 seconds.
Twenty-five year return period: the mean significant wave height was 0.5 meters; the mean peak wave period was 2.5 seconds.
Interior Site
The results from this site should reflect the distance between the chevron islands.
Ten year return period: the mean significant wave height was 0.3 meters; the mean peak wave period was 1.3 seconds.
Twenty-five year return period: the mean significant wave height was 0.2 meters; the mean peak wave period was 1.3 seconds.
Northern Site
We expected this site to have similar values to the existing scenario.
Ten year return period: the mean significant wave height was 0.4 meters; the mean peak wave period was 2.3 seconds.
Twenty-five year return period: the mean significant wave height was 0.5 meters; the mean peak wave period was 2.5 seconds.
STATISTICAL ANALYSIS
Listed in the charts below are the 10 year return period values and fetch lengths at each location in Back Bay for each scenario.
Scenario | Average Fetch Length | Average Significant Wave Height | Average Peak Wave Period |
|---|---|---|---|
1890 Marsh Extent | 0.5 kilometers | 0.2 meters | 1.6 seconds |
Present Marsh Extent | 1.4 kilometers | 0.4 meters | 2.4 seconds |
Restoration Marshes | 1.1 kilometers | 0.4 meters | 2.3 seconds |
Comparison of rounded values for a ten year return period at the Southern Site in each scenario
Back Bay Current Marsh Extent - Southern Site
Southern Site (Ten Year Return Period)
Upon a glance, the wave energy in 1890 is the lowest with a significant wave height of 0.2 meters and a peak wave period of 1.6 seconds. Based on the results of a two-proportion t-test, there is significant evidence that the wave height in 1890 is less than wave height at the in the present at the 0.05 alpha level. There is also evidence of the same trend between the two scenarios' peak wave periods.
This difference is striking compared to the minute differences between the present wave energy for the present marsh extent and the wave energy for the restoration plans. In fact, there is not significant evidence that wave height in the present is greater than wave height after the restoration project at the 0.05 alpha level. If we were to recalculate this with fetch lines drawn to the width of the bay in the present, perhaps we would see different results.
This is most likely because the restoration project is primarily geared to rebuilding the interior of Long Island and Bonney Cove as opposed to the Northern or Southern end.
Scenario | Average Fetch Length | Average Significant Wave Height | Average Peak Wave Period |
|---|---|---|---|
1890 Marsh Extent | 0.3 kilometers | 0.3 meters | 1.8 seconds |
Present Marsh Extent | 0.7 kilometers | 0.3 meters | 1.9 seconds |
Restoration Marshes | 0.1 kilometers | 0.3 meters | 1.3 seconds |
Comparison of rounded values for a ten year return period at the Interior Site in each scenario
Back Bay Current Marsh Extent - Interior Site
Interior Site (Ten Year Return Period)
The interior site's results show different trends than the southern site. The values for both mean significant wave height and mean peak wave period are very similar for the present marsh extent and marsh extent in 1890, while the restoration statistics are the lowest.
Even thought the reduction in peak wave period is impressive, there is not evidence that the wave height at the interior site after the restoration project is less than wave height at the interior site in the present at the 0.05 alpha level. However, there was statistically significant evidence that the present peak wave period at the interior site is greater than the peak wave period after the restoration project at the 0.05 alpha level. This discrepancy could have been the result of a type two error.
These results, while not completely cohesive, are a testament to the benefits of marsh terracing in reducing wave energy.
Scenario | Average Fetch Length | Average Significant Wave Height | Average Peak Wave Period |
|---|---|---|---|
1890 Marsh Extent | 1.7 kilometers | 0.3 meters | 2.1 seconds |
Present Marsh Extent | 2.1 kilometers | 0.4 meters | 2.4 seconds |
Restoration Marshes | 2.1 kilometers | 0.4 meters | 2.3 seconds |
Comparison of rounded values for a ten year return period at the Northern Site in each scenario
Back Bay Current Marsh Extent - Northern Site
Northern Site (Ten Year Return Period)
In this data set, there is not a large difference between the fetch length for the present and restoration scenarios because the restoration site does not impact the Northern end of the island. However, our data does show a small, yet notable increase in the wave energy between 1880 and the present because of the marsh loss that occurred between the two scenarios.
CONCLUSION
At each of the sites, the wave energy measurements show a low value for the 1890 scenario, a testament to the ability of marsh extent to reduce wave energy. The reductions in energy are primarily caused by decreases in wave period. As mentioned in the literature review, the necessity for marsh recuperation in Back Bay is further underscored by the fact that reduced wave energy can allow for the more efficient growth of submerged aquatic vegetation and reduction in erosion.
The restoration project shows the largest impact on the interior site. Property owners in the north will be better protected from surging waters from the south, but perhaps further efforts should be taken to construct terraces or other living shorelines around the perimeter of the bay to better protect property owners whose fetches do not directly intersect that area.
The marsh restoration project will reduce the intensity of waves coming up from the south for residents of the north end of the bay. However, the project may overlook the potential for new marsh area to restrict the flow of the water in Back Bay. Implications of venturi could cause a backup of water in the south or local increases in water speed. Effects of constriction from the project could cause flooding around the perimeter of the bay. For those two concerns, there are various areas we would like to examine further, especially including the historical context:
- What will the effect of the constriction of storm flow by the restoration be on water velocities in the area and potential flooding?
- Another next step could involve comparing wave power to the erosion thresholds of marsh perimeters (that either have or do not have vegetation).
