Runoff Hydrology in a Constructed Flow-Through Wetland Treating Storm Flows from East Fork Little Miami River in Southwest Ohio

Abstract Submission: In response to recurring harmful algal blooms in Harsha Lake, Ohio, a flow-through riverine wetland complex was constructed in the floodplain of the East Fork Little Miami River. The 5.5 acre constructed wetland consists of three contiguous treatment zones: a wintering pool, a detention basin developed with a retired drinking water reservoir, and a highly sinuous wetland. An inlet and outlet channel connect the wetland system to the river and offload flows during elevated river stage. The system was designed to treat 15-30 storm events/year. Inflows have occurred seasonally at approximately 8 events/year – less frequent than predicted due to inlet sedimentation in excess of one foot (which highlights long-term maintenance needs for these systems) and drought conditions. Site-wide continuous hydrology data have been recorded for the first 1.5 years after wetland construction. Aerial surface mapping and water level data were used to develop a stage-storage model of the wetland. However, stage-storage calculations are complicated by the complex wetland hydrology, e.g. multiple flow paths, and backwater conditions at the inlet and outlet when river stage is elevated. To improve confidence in the stage-storage model, the same data are being used to develop and calibrate a HEC-RAS 2D unsteady flow model. It is anticipated that the HEC-RAS model will aid in quantifying storm flow dynamics, enabling the stage-storage model to be refined, thereby increasing confidence in pollutant load estimates for the wetland. This presentation will provide insight into methods, challenges, and successes of quantifying complex hydrology for a riverine wetland system.

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