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Shipley's Choice Dam Rehabilitation

Location: Anne Arundel County, Maryland Client: Anne Arundel County

JMT provided design services for the removal of a twenty foot high failing stormwater management facility dam and the restoration of the area to a natural channel and riparian floodplain system.

This project resulted from a Consent Decree issued by the Maryland Department of the Environment to remove a failing dam that spanned the Bear Branch (tributary to the Severn River) stream valley. In conjunction with the dam removal, JMT's design improved ecological function of the downstream riparian corridor, re-established fish passage, and mitigated for large increases in streamflow due to removal of the dam. Other design challenges include the replacement of a downstream 66-inch culvert with a 10'X7' box culvert to safely pass the 50-year flood event, and providing long-term geomorphic stability at that location.

JMT's design strategy sought to preserve or improve stream ecology and wildlife habitat in conjunction with the profound increases in hydraulic stresses expected with the removal of a large on-line dam and the replacement of a significantly undersized culvert. Our design included restoration of 3,700 linear feet of Bear Branch to protect and enhance the existing expansive wetlands immediately upstream of the dam and to reconnect incised reaches of the stream to an active floodplain. A major design objective was to transition the abrupt elevation and slope differences through the dam footprint and re-connect the floodplain areas without utilizing "hard armoring" techniques. Thus, the design focused on enhancing the existing reaches using native coastal plain materials such as sand substrate and buried logs harvested from the project site. The design also created and enhanced existing micro-channels in the floodplain areas to create a highly-attached, ana-branching riparian wetland stream system.

JMT performed Rosgen stream inventories and assessments, geomorphic surveys and analysis, and a sediment mobility study of the sand bed system. JMT also prepared hydrologic models utilizing GISHydro and TR20, hydraulic models for existing and proposed conditions utilizing HEC-RAS, floodplain studies and mapping, site survey and topographic mapping, wetland delineation and joint permit application, forest stand delineation/forest conservation plan, geotechnical dam breach studies and analysis, structural culvert design, utility relocation and design, and construction documents, estimates and specifications.

Other design elements included the relocation of an existing 16-inch gravity sewer line, 16-inch waterline, and 16-inch sanitary force main from the vicinity of the existing culvert in order to improve fish passage through the proposed culvert. This was challenging due to the size of the utilities and relocations occurring at the bottom of a 100 foot deep ravine, with existing valves on the water and sewer force main a considerable distance away. JMT designed the relocated gravity trunk sewer out of the way from stream erosion while maintaining accessibility for maintenance staff. The water main and sewer force main relocations were designed in their permanent locations using restrained joint pipe, pipe supports and other protection methods while culvert and road building construction activities take place. Inflatable line stops were required to minimize shut-down times of both utilities and to prevent pipe collapse at the higher portions of the system caused by a sudden drop in water column pressure between the nearest valve and the relocation. JMT recommended night-time work in conjunction with wetwell storage relocation for the sewer force main.

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