For decades, Maryland’s Bird River has suffered from excess sedimentation. Since the river is used by commercial and recreational boaters, it now must be routinely dredged. The dredged sediment is delivered through pipes to a nearby Dredge Material Containment Facility (DMCF). When Baltimore County needed to enlarge the Bird River DMCF, they were required to create and restore a minimum of 70,566 square feet of forested, non-tidal wetlands to mitigate the associated negative impacts. The mitigation site was chosen because of its proximity to both Bird River and the DMCF.
Working closely with the County, Biohabitats created a wetland mitigation design to restore more than 86,600 square feet of forested wetland and preserve nearly 38,000 square feet of recently developing wetlands on a site near both the Bird River and the DMCF. The design, which created three wetland “cells” of varying size, included a series of carbon-rich sand berms (terraces of sand that are above water level) and riffle grade controls (shallow landforms in flowing water). These features work to control the maximum pooling depth, increase water contact with soil and vegetation, and allow the water to spread across the wetlands and slowly seep through the sand berms.
The vegetation strategy included retaining and making use of as many hydrophytic plants (only grow in saturated soil) within the project footprint as possible. This included preserving areas of developing wetlands and allowing succeeding areas of wet-adapted tree saplings, such as sweet gum. Native seed mixes, ecologically appropriate to the region and compatible with existing vegetation were planted to enhance diversity and allow the wetlands to self-organize.
A 2:1 ratio (the repaired wetland must be twice the size of the impacted wetland) was necessary with this mitigation project. Though Biohabitats’ design provided the County with a cost-effective solution that exceeded this requirement, outside construction implementation did not comply with Biohabitats’ plans, and the constructed mitigation site was too small.
Biohabitats was brought back in to oversee and recommend construction modifications. Biohabitats retained a surveyor to perform an as-built survey, allowing all involved to understand where the design and construction project diverged. As a result of this survey, Biohabitats developed a set of recommendations which were approved by the County and implemented. Biohabitats then initiated post construction monitoring of wetland vegetation establishment, groundwater conditions, and hydric soil development.
TAGS
Owner: Baltimore County DEPS
Bioregion: Chesapeake/Delaware Bays
Ecoregion: Chesapeake-Pamlico Lowlands and Tidal Marshes
Physiographic province: Coastal Plain
Watershed: Gunpowder River-Chesapeake Bay