Tidal marsh restoration at Blackwater National Wildlife Refuge, Maryland: A case study in thin-layer placement

Tidal marsh loss at Blackwater National Wildlife Refuge (NWR) has been a major concern of refuge managers in recent decades. The approximately 2,035 hectares (5,028 acres) of tidal marsh that have converted to open water in Blackwater NWR since 1938 (Scott et al. 2009) represent one of the most significant areas of marsh conversion within the Chesapeake Bay. In 2013, a suite of climate adaptation strategies focused on sea level rise was developed for Blackwater NWR and surrounding areas of Dorchester County by the Blackwater Climate Adaptation Project (BCAP). The BCAP is a collaboration of The Conservation Fund, Audubon Maryland-DC, and the U.S. Fish and Wildlife Service, assisted by the Maryland Department of Natural Resources (MD DNR), U.S. Geological Survey, and others. In 2016, the BCAP implemented a thin-layer placement (TLP) project at Shorter’s Wharf in Blackwater NWR on 16 hectares (40 acres) of subsiding and fragmenting tidal marsh dominated by Schoenoplectus americanus, Spartina alterniflora, and Spartina patens. The purpose of the project was to increase the 16 hectares’ (40 acres’) resiliency to climate-driven sea level rise and storm impacts. The project built up the marsh elevation by applying thin layers of sediment dredged from the adjacent Blackwater River. The sediment enhancement was designed to extend the longevity of the marsh and increase its resiliency by raising its surface elevation in relation to the tidal regime and to return the habitat to its prior high-marsh condition with S. patens dominating. The colonization of this site by saltmarsh sparrow would be an indicator of success in reaching this goal. Dredging operations in November and December 2016 placed approximately 19,900 cubic meters (26,000 cubic yards) of sediment on the project site. Post-restoration elevations obtained one year after material placement indicated that, although the target elevations were achieved in 78% of the surveyed placement area, the material was not distributed uniformly. Coarser material tended to stack up at the discharge location while the grain size declined and the slopes flattened toward the periphery of the discharge area. In 2017, natural vegetation had regenerated through the placed sediment with vigorous regrowth of S. americanus and S. alterniflora . This regrowth was supplemented with hand-planting of more than 200,000 plugs of S. patens. Vegetation monitoring is ongoing to determine the plant composition evolution within the placement site. Pre-dredge and post-dredge bathymetric surveys reveal 70% accretion nearly two years after dredging within the borrow area footprint.

Can sea level rise help us restore coastal wetlands? The hydrologic restoration of the Slop Bowl, Brazoria National Wildlife Refuge, Texas

The Slop Bowl marsh in the Brazoria National Wildlife Refuge provides extraordinarily high quality, heavily used bird habitat. Much of this habitat has experienced hypersaline conditions due to both hydrologic alteration by humans and a rapidly and changing physical environment over the past several decades. Oil and natural gas extraction activities have resulted in excavation and channelization along pipelines and hydrologic obstruction by an access road. In addition, subsidence along growth faults has altered hydrologic pathways and lowered surface elevations in the center of the marsh. Our objective was to understand the underlying processes that contribute to hypersaline conditions and to evaluate possible restoration alternatives to reduce the severity of those conditions. Accordingly, we conducted extensive field and hydrologic modeling efforts, and identified the past, present, and future of this marsh habitat under a baseline scenario. We then compared various restoration action scenarios against this baseline. We found that, beginning in about 15 years, relative sea level rise will improve the hydrologic conditions by enhancing tidal flushing. However, if fill material is continually added to elevate the obstructing road as the sea rises, this hydrologic relief may never be realized. Moreover, we found that if a drought occurs during this critical period, a difference of only a few centimeters in the relative water level and road elevation, or changes in marsh surface elevations driven by fault motion and subsidence, may have catastrophic consequences. The modeling also suggests that several potential interventions can bridge this gap over the next 15 years and beyond. Actions that improve tidal circulation, reduce salinity, and enhance marsh accretion are being developed by the project team to enhance and restore habitat in the near term. The most optimal approaches evaluated thus far include the installation of culverts at critical locations, the excavation of a small channel, the modification of flow pathways, and the beneficial use of sediments and vegetative plantings. We conclude that, under specific circumstances or at unique locations such as the Slop Bowl marsh, sea level rise can be leveraged to improve coastal wetland health.

Restoration of estuarine wetlands using thin cover placement: A pilot application in Brunswick, Georgia

This paper presents the design concepts and basis for using a thin layer cover (TLC) of sand to restore historically impacted wetlands in Georgia’s Brunswick estuary. The project site is a mix of tidal creeks, marshes, brackish estuary, and an adjacent upland area that has been affected by historical industrial operations. A pilot project to test cover placement methodology and performance in advance of future full-scale TLC implementation was completed in 2018. It involved placing 6-9 inches of material in a 2/3-acre marsh area. Two material types — sand and higher organic content fines — were tested. The contractor, Sevenson Environmental Services, identified the appropriate equipment, means, and methods to hydraulically convey and place the TLC material within the pilot area in accordance with stated performance objectives. A mat-based access road was installed to enable equipment to move the pipeline and spray nozzle for fine placement control within the pilot marsh area. The thin cover placed in the field ranged from 6-12 inches thick (versus the design thickness of 6-9 inches) to meet the minimum required thickness and account for over placement. A 30- to 45-degree spray yielded the best distribution of materials for the equipment used. Placement of sandy material was faster and more uniform than fines due to the material’s enhanced settling characteristics and ease of distribution. A modified topsoil-fines mix with a baffle plate eventually permitted optimal placement of fines within the study area while maintaining the target organic content. Turbidity in the water discharged from the pilot area was minimized by environmental controls (e.g. perimeter hay bales) installed by the contractor. The mat-based access road initially experienced some settlement due to loading on the soft sediments and marsh root mat; the road required restoration following project completion. Physical and vegetative monitoring conducted in six-month increments over a two-year period indicated strong natural recolonization of vegetation and the re-establishment of benthic species including fiddler crab. This paper presents lessons learned, design implications, and best management practices for future thin cover placement projects in estuarine settings.

Enhancing marsh elevation using sediment augmentation: A case study from southern California, USA

Tidal marshes are an important component of estuaries that provide habitat for fish and wildlife, protection from flooding, recreation opportunities, and can improve water quality. Critical to maintaining these functions is vertical accretion, a key mechanism by which tidal marshes build elevation relative to local sea level. The beneficial use of dredged material to build marsh elevations in response to accelerating sea level rise has gained attention as a management action to prevent habitat loss over the coming decades. In January 2016, a sediment augmentation project using local dredged material was undertaken at Seal Beach National Wildlife Refuge in Anaheim Bay, California, USA, to benefit tidal marsh habitat and the listed species it supports. The application process added 12,900 cubic meters of sediment with an initial, average 22-cm gain in elevation over a 3.2-hectare site. Due to sediment characteristics and higher than anticipated elevations in some areas, vegetation colonization did not occur at the expected rate; therefore, adaptive management measures were undertaken to improve hydrology of the site and facilitate vegetation colonization. More case studies that test and monitor sea level adaptation actions are needed to assist in the planning and implementation of climate-resilient projects to prevent coastal habitat loss over the coming century.

Evaluating direct and strategic placement of dredged material for marsh restoration through model simulations

Dredged material can be used for marsh restoration by depositing it on the marsh surface (thin-layer placement), by releasing it at the mouth of channels and allowing tidal currents to transport it onto the marsh platform (channel seeding), or by creating new marshes over shallow areas of open water. We investigate the efficacy of these different methods using a comprehensive 2D marsh evolution model that simulates tidal dynamics, vegetation processes, bank and wave erosion, and ponding. Total marsh area is assessed over 50 years in an idealized microtidal marsh under different relative sea level rise (RSLR) scenarios. For a given volume of total sediment added, the frequency of deposition is relatively unimportant in maximizing total marsh area, but the spatial allocation of the dredged material is crucial. For a given volume of sediment, thin-layer deposition is most effective at preserving total marsh area, especially at high rates of RSLR. Channel seeding is less efficient, but it could still provide benefits if larger amounts of sediment are deposited every 1-2 years. Marsh creation is also beneficial, because it not only increases the marsh area, but additionally slows the erosion of the existing marsh. The 2D model is highly computationally efficient and thus suited to explore many scenarios when evaluating a restoration project. Coupling the model with a cost assessment of the different restoration techniques would provide a tool to optimize marsh restoration.

Thin layer placement for marsh enhancement: Planning, design, construction, and monitoring considerations

Thin layer placement (TLP) is the purposeful placement of thin layers of sediment in an environmentally acceptable manner to achieve a target elevation or thickness. TLP is used for a variety of purposes, such as sediment management, beneficial use of dredged material (DM), and ecological enhancement. The term “thin” is used to distinguish TLP from other methods of sediment placement in which sediments are applied in layers on the order of several meters thick. In this paper, DM disposal refers to the deposition of sediment in a location and manner where no beneficial use is attained; with DM placement the sediment is used to benefit society and the environment. The application of thin layers of sediment has advantages over more traditional, thicker sediment applications in environments where these thicker layers pose potential challenges to natural resources, infrastructure, navigation, or other assets. Although TLP projects are most often conducted in wetlands, there are open-water applications as well. But because TLP is relatively early in its development, there is a dearth of design and construction information and guidance available to practitioners. This paper provides a high-level summary of pending national TLP guidance being developed by the authors on behalf of the U.S. Army Corps of Engineers’ Engineer Research and Development Center (USACE ERDC).

Engineering and design of the Lightning Point Shoreline Restoration Project in Bayou La Batre, Alabama

Lightning Point, located in Alabama at the confluence of the Bayou La Batre navigation channel and Mississippi Sound, is a culturally and ecologically valuable site with an extensive history of shoreline erosion. Between 1916 and 2019, the shoreline experienced approximately 750 to 1,000 ft of shoreline retreat as a result of severe weather events and anthropogenic causes such as shoreline modification and response efforts related to the Deepwater Horizon oil spill. Moffatt & Nichol worked with The Nature Conservancy to restore the lost habitat and resources through ecology-based engineering and design. The Lightning Point Shoreline Restoration Project is a 1-milelong living shoreline that includes approximately 4,700 ft of segmented, overlapping breakwaters, 40 acres of marsh and upland habitat creation, and 10,000 linear feet of tidal creeks. The project was designed to include a diversity of habitat types (subtidal, intertidal, higher scrub-shrub) and to serve as a resilient restoration solution capable of adapting in the face of sea level rise and increasing storm activity.

Incorporation of coarse-grained dredged material into marsh and shoreline restoration projects in coastal New Jersey

Millions of cubic yards of sediment are dredged every year in coastal New Jersey for the operation and maintenance of an extensive marine transportation system stretching from the New Jersey Harbor south along the Atlantic Coast from Sandy Hook to Cape May and north up the Delaware River. Dredged material from these public and private projects has been managed using a variety of placement approaches and technologies, from open-water disposal to landfilling to construction materials. For the past several decades, the State of New Jersey has advocated for and implemented a policy of beneficial use of dredged material rather than its disposal. The New Jersey Department of Transportation’s Office of Maritime Resources (NJDOT/OMR) is the lead state agency for research and implementation of beneficial use statewide. NJDOT/ OMR is also responsible for the recovery of the 200-mile network of shallow-draft navigation channels along the Atlantic coast of New Jersey that was damaged by a series of severe coastal storms, most notably Superstorm Sandy in 2012. For the past decade, considerable effort has been made to develop methods that use clean dredged material from the Atlantic region to rebuild and improve coastal features such as marshes, dunes, and beaches, thereby retaining the sediment in the ecosystem. Although there have been a number of successful beneficial use projects, concerns remain about the long-term sustainability of the program due to high cost, timelines, scalability, habitat sensitivity, resiliency, aesthetics, and other factors. This paper explores some of these issues and proposes solutions. It focuses on the use of available coarse-grained material as a way to provide resiliency to these restored features while increasing scale and efficiency, protecting aesthetics, and providing increased habitat value.

Evolution of a nourished sand beach under low wave energy in Thailand

Sand beaches are essential for coastal tourism in Thailand, but erosion narrowed some beaches significantly over the years. Pattaya is a famous resort near Bangkok in the upper Gulf of Thailand. The Pattaya beach is microtidal with the average tidal range of 1.5 m. The average significant wave height is 0.2 m and the wave energy is low. The beach was widened by placing 130 m3/m of medium sand along the shoreline length of 2.8 km between two terminal groins constructed in 2018. The bathymetry and topography were measured in 2015, 2019, and 2020. Approximately 14% of the placed sand in the water depth less than 2 m was lost after one year, as may be expected for nourished beaches. The bathymetry change in the water depth of 2-4 m varied alongshore. The sand volume change in this offshore zone beyond the surf zone was as large as that in the landward sand placement zone. The assumption of negligible profile changes seaward of a closure depth is not applicable to this beach during 2015-2020.

Rip current rescues on unguarded beaches

Rip currents are the greatest danger at surf beaches. Professional lifeguards rescue tens of thousands of people every year at U.S. beaches, but only a small percentage of the nation’s beaches are guarded. Oftentimes it is a young person who is caught in a rip current, and a bystander will attempt a rescue without a flotation device. The U.S. Lifesaving Association strongly suggests that this kind of rescue should not be undertaken because too often the rescuer will drown. Some coastal towns such as Cocoa Beach in Florida are now posting ring buoys on their unguarded beaches with the warning to throw, but not to go into the water. Ring buoys of two different weights were tested for efficiency when thrown in terms of distance and accuracy. The participants threw the ring buoys two different ways: one way of their choosing (un-instructed) and second by Red Cross recommendation (instructed). The buoyancy was also tested for each buoy. While these flotation devices have some merit, they clearly have limitations.