Tributary to Rock Creek Restoration using Steppool Seepage Wetland System (i.e. Regenerative Stormwater Conveyance, RSC)
Project Owner’s Name/Address: Montgomery County Department of Parks and Planning (M-NCPPC) Completion Date: March 2009 Project Features:
Brightwater completed the first Regenerative Stormwater Conveyance System (RSC) design in the Maryland Piedmont in 2008 for Maryland National Park and Planning Commission (M-NCPPC). The Coastal Plain Outfall techniques successfully applied in the Maryland Coastal Plain were adapted and customized for the deeply incised and unstable tributaries to Rock Creek at Waverly-Schuylkill Park in Kensington, Montgomery County. Originally M-NCPPC retained the services of Brightwater to stabilize only a portion of the eroding stream network in the park. However, after field examination, Brightwater recommended that all of the eroding and unstable stream segments be restored. Otherwise, the restoration work would be subject to continued excess sediment from head cuts and bank erosion in the un-restored channels. M-NCPPC agreed with the recommendation.
When streams are severely eroded, traditional approaches to stabilization often include stabilizing in place with riprap or regrading the channel to widen the cross sections to create flood relief. While both of these approaches can reduce erosion, they have little benefits for aquatic habitat and can result in significant impacts to natural resources. Brightwater proposed a recently developed technique of constructing a step pool seepage wetland system that raises the incised stream back to the original flood plain elevation. While this approach was originally developed for the coastal plain region of Maryland, Brightwater and the Parks Department agreed its principals could be adapted for use in Montgomery County, Maryland, which lies in the piedmont province. |
 Photograph of Senetor Shore at the ribbon cutting ceremony The advantages of this approach are several: 1. The stream is reconnected with its floodplain allowing normal processes of overbank flooding to occur with attenuation of stormflows in a vegetated zone. This has pollutant removal benefits, along with reducing shear stress within the stream channel to allow for improved stability. 2. A seepage wetland system is created by constructing a streambed of bankrun gravel mixed with organic matter. This captures a portion of the runoff and allows it to seep into the shallow groundwater, thus reducing stormwater runoff and extending the period of base flow. 3. The seepage wetland also provides water quality treatment of the stormwater that is captured within the pools and allow to infiltrated into the substrate. 4. The constructed pools retain water within the reach for extended periods of time, which allows for improved aquatic habitat and provides water access for riparian ecological systems. 5. The pools also provide volume for allowing suspended sediments to settled out and become trapped in the substrate Design elements include design discharge determination (2, 10, and 100-year), alternatives analysis, shear stress analysis, sizing and placement of Riffle/Weir grade control structures, and vertical and horizontal alignments. Construction document preparation includes a design report, sequence of construction, plan and profile views, planting plan, sediment erosion control plans and details, specifications, and cost estimate. The HEC-RAS model was used to delineate the existing and proposed 100-year floodplain. Plans and permit applications for the Sediment Control Permit, Natural Resource Inventory and Forest Stand Delineation, Forest Conservation Plan, and Non-tidal Wetland Joint Permit. The Steppool Seepage wetland system is created by filling the incised channel with a mixture of coarse sand and woodchips to develop an underlying seepage seam. The pools were built about two feet deep, enhanced with woody debris, and held in place with stone weirs and cobble aprons. Each weir was stepped down 1’ in elevation in order to create a more consistent slope through the project areas. The channel grading was adjusted to minimize impacts to natural resources while reducing construction costs. Following construction, the pools and riparian areas were planted with native wetland and upland vegetation. The community maintained their enthusiasm for the project despite even through the disturbances during construction. Upon completion the community organized a ribbon cutting ceremony, and the Parks’ aquatic biologists helped local children release tadpools into the new habitat. The children also enjoyed using the stone weirs to cross the streams and climbing on additional rocks placed along the stream. The Parks Department is currently working with the neighbors to develop interpretive signage for the project. The project was granted an award from the Petroleum Council and the Maryland DNR, and the Award of Merit from the County Engineers Association of Maryland in 2010. |
