Habitat Creation
Wetlands
The wetland restoration component of the Poplar Island Restoration Site will result in the construction of a tidal marsh system through the placement of dredged material into cells. These cells are component areas of the site, which are filled consecutively as dredged material needs to be deposited. Each of these cells is separated from the surrounding waterway, the Chesapeake Bay, and adjacent cells by containment dikes. These cells are filled with sediment (sand, silt, and clay materials) removed from the shipping channels in the Chesapeake Bay. The material is removed from the channel with a
clamshell dredge and transported to the site by means of a scow barge. The material from the scow is then emptied by directly pumping the material into the cell. Because the material must be pumped through pipes, the sediment material must be mixed with water to create a slurry. As the slurry material is pumped into the cell, special devices called weir boxes are used to decant or skim the water from the dredged material as the sand and silt grains settle to the bottom of the slurry. More slurry is pumped in and dewatered until enough solids are in place to meet the post-placement elevations that are required to support the selected habitat and vegetation.
The post-placement elevations of the deposited material will initially be approximately 1-2 feet higher than the target or final elevation as some short-term (1-3 months) and long-term settlement and compaction of underlying soils is expected. Settlement of the deposited dredge material typically occurs approximately 1 year following the completion of dredged material placement with some additional settlement possible after that. The final target elevations will range from +0.0 feet to + 2.5 feet Mean Low Water. This will place the wetlands platform and habitat within the tidal range, creating low and high marsh habitats.
Tidal channels will be located within the low marsh area to provide finfish and benthic habitat (shellfish, etc), enhance nutrient uptake from seawater, and drain the marsh plain each tidal cycle. Higher elevations (high marsh and upland/dune transition) will be scattered throughout the marsh creation area, mostly in areas where the placed dredged material adjoins the island and containment system, although additional high areas will be built in the central portions of the cells. This micro topography will increase the diversity and value of the marsh construction area to more closely resemble a natural tidal marsh ecosystem (in 2003, a wetland cell of approximately 20 acres was constructed and tidal flow was established with Poplar Harbor).
It is desirable to vegetate the new platform to achieve stabilization of the dredged material as quickly as possible to protect the tidal marsh area against erosion. The most logical and cost-effective way is to rely on natural recruitment from adjacent areas (e.g., seed and rootstock transport by tides, wind, and biota). But, in relatively colder climates like the mid-Atlantic, natural vegetation of these areas could take several years to fully establish. Therefore, the planting of vegetation in these newly created wetlands will accelerate the growth of vegetative cover. Planting strategies utilizing containerized seedlings or "plugs" has proven itself in past projects as the most effective method to vegetate and therefore, quickly stabilize the marsh surface.
High marsh communities are located in the range between the spring and neap high tide elevations, in other words the elevation between the highest high tides and the lowest high tides. This allows occasional saturation of these areas roughly 4 to 10 times per month. The filling operations for higher marsh communities involve piling material at above normal levels at selected locations within the containment dikes.
For this project, circular mounds of sand are placed at selected locations within the containment areas before filling the lower marsh areas. This is done to ensure better quality material, such as coarse sand, is dedicated to constructing higher wetlands and that an optimal ratio of high to low marsh area is established before building the lower wetlands.
The low marsh community is generally found in the area between mean sea level and low tide. This allows these areas to experience one saturation and one atmosphere exposure per tidal cycle, or one wet-dry cycle approximately two times per day. The salt flats, which house their own ecosystems, are interspersed among and between the high and low marsh communities. These areas are of higher salinities due to periodic inundation by salt water and evaporation.
While the final elevations of the dredged material placed in the wetland construction area can be generally predicted, the elevation differences for the different plant habitats can be small, and often overlap. Therefore, the planting scheme includes providing a mixture of species to compensate for the tidal dynamics, micro topography, and other site features that will ultimately determine the distribution of tidal marsh communities.
Given the rather viscous slurry composition of the dredged material when placed within the containment system, planting generally occurs after a period of time and in a phased approach. If rapid vegetation is desired, it is advantageous to begin planting as soon as possible. Accessing areas that are not completely drained is most easily done from a platform (e.g., flat-bottomed boat) dragged across the marsh surface. Planting units (plugs or containerized seedlings) would be transported in the boat from the nearest stable access road to the planting area. The higher, typically firmer areas are more easily accessible and could also be planted immediately. Exact timing and method for planting is determined following the completion of dredged material placement in each cell, since exact settling and drying times cannot be predicted. Typically, planting on softer soils is less productive than planting on firmer soils, however, as previously mentioned, early planting can provide a valuable head start on stabilizing and vegetating the new area.
From an accessibility standpoint, planting is most productive when the marsh substrate has stabilized - typically up to 1 year following the completion of dredged material placement in each cell. Planting units (plugs or containerized seedlings) would be spaced on 18-to-24 inch centers in selected locations to establish cover and stabilize dredged material. These planted areas would also provide a source of plant material to vegetate the remaining areas of marsh. The banks of established tidal channels would also be planted. There will also be some recruitment of vegetation (e.g., seed and rootstock) from adjacent wetland and dike areas.
To maximize the establishment of tidal marsh species, a mixture of species are planted in each habitat that overlaps adjacent habitats. Consideration is given to cost, availability, and time to plant and evaluated on a case-by-case basis for each planting effort.
Tidal Channels
The creation of tidal channels in the wetlands is important for the fisheries value of the new marsh. There was no definitive approach for the design of tidal channels, however, experience in the beginning stages of the Poplar Island project and in other projects completed over the last decade have identified features that are key to the success of channel creation. One important feature is the selection of appropriate vegetation to be planted along the channel to stabilize the banks and foster overbank usage by fish.
The initial target tidal channel configuration will consist of approximately 10-to 25-foot wide channels. The techniques used to create the channels vary depending on conditions and generally, each construction method is field determined based on specific site conditions. There are two primary techniques for the creation of the channels that are typically used: a pre-dredge material placement technique and a post-dredge material placement technique. The pre-dredge material placement technique takes advantage of the natural dewatering behaviors of dredged material. As dredged material settles and dewaters, small drainage rivulets form as the water drains up through the material and flows from the surface toward the weir boxes. After the most appropriately aligned natural rivulets are identified, they are enhanced with a marsh excavator by deepening the rivulets. This work is typically begun immediately following dredging and can be part of the dredging mobilization. In many cases, some earthwork is required later after settlement is complete to ensure tidal channels allow proper water circulation and drainage.
The post-dredge tidal channel creation technique involves excavating channels after dewatering and most settlement is complete. The advantage of this method is that the channel is excavated to its full functioning level immediately with only minor adjustments needed later to ensure flushing. This method requires waiting until after the dredged material is sufficiently stabilized, so that it can support heavy earthwork equipment.
|
