Concurrent Sessions B: Modeling and Design - Restoring Fish Passage on White Marsh Run

White marsh Run, a third-order coastal plain stream draining to the tidal Bird River in Baltimore County, Maryland, has been severely impacted from historic sand and gravel mining in the stream channel and floodplain. Mining impacts include increased sediment loads, channel erosion, channel destruction, loss of riparian buffer, and a 5-foot vertical fish barrier downstream of the U.S. Route 40 crossing. Several stream restoration projects have been completed on White marsh Run, but those reaches are not accessible to anadromous fish due to the U.S. Route 40 fish barrier. It was agreed that restoring fish passage and stabilizing rapidly eroding stream banks would satisfy the mitigation requirements for streams impacted by the I-95 Express Toll Lanes, Section 100 project (12,199 linear feet). Straughan Environmental, Inc. studied the hydrologic and sediment transport regimes on Whit emarsh Run, assessed through stream gages, discharge and bedload measurements during storm events, and sediment transport modeling, and concluded that a 1,400-foot stone Riffle Grade Control (RGC) structure was the most effective means to permanently reestablish passage for anadromous and other native fish. Design constraints included a minimum flow depth and maximum flow velocity to provide fish passage during spring baseflows, structural stability during the 10-and 100-year discharges, competence and capacity to transport existing bedloads, maintenance of the existing floodplain elevation along U.S. Route 40, and strict grading limitations due to measured diesel fuel soil contamination and utility right-of-ways. Extensive hydraulic and sediment transport analyses (HEC-RAS, iSURF, various stone sizing and gradation equations) were solved iteratively to design a stable structure, constructable from a mixture of regional stone sources, that would maintain the minimum base flow depth, not exceed the maximum base flow velocity, promote surficial flow, transport bedload, include fish resting areas, and not increase the floodplain elevation over U.S. Route 40. Wetland enhancement and preservation, vernal pool construction, invasive species eradication, and native plantings are also planned on the 184-acre mitigation site.
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