Modeling sedimentation and scour in small urban lakes

Lower Shaker Lake is a small urban lake in the Doan Brook Watershed of Cleveland, Ohio. Due to an unusual sequence of events, a “dry dredging” project resulted in the lake being drained and dried without sediment removal. This led to questions about the sedimentation balance of the lake and questions about unusual features such as a submerged channel and sediment island. This manuscript discusses the hydraulic modeling that was conducted to answer these questions. The modeling effort was based on RMA2 hydraulic model solutions, SMS post-analysis field variable mapping and a simple experimental test developed to quantify sediment scour thresholds. Procedures and results are illustrated using Lower Shaker Lake as a case study. Results indicate that the existing sedimentation state of the lake was a dynamic equilibrium balanced between dry weather sediment accumulation and storm flow scour. Drying the lake may have disrupted this balance. Results also provided a hydraulic explanation for the existence of the sediment island that had incorrectly been assumed to be evidence of rapid sediment accumulation. The modeling approach proved to be very useful in uncovering the systems at work in Lower Shaker Lake and may be helpful in evaluating remediation strategies for other small urban lakes.