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Optimizing Bedload Using Two-Dimensional Sediment Transport Modeling for Split Flow Condition In-Stream Restoration Designs: A Case Study on the Gwynns Falls Tributary at Kingsbury +

Brian Tetrick
WSP-USA
Baltimore, MD

Authors: Brian Tetrick, PE, WSP-WSA
Justin Lennon, PE, WSP-USA
Mike Coco, PE, ENV-SP, WSP-USA
Dennis Genito, Baltimore County DEPS

Channel bedload sediment dynamics play a key role in the sustainability of streams. The balance of bedload can create or damage aquatic habitat and stabilize or destabilize the channel. This presentation demonstrates the use of two-dimensional hydraulic modeling as a powerful tool for projecting the transport and distribution of bedload sediment and its application in the design of a stream restoration project to divert sediment to protect infrastructure.

Baltimore County Department of Environmental Protection and Sustainability (DEPS) identified a tributary to Gwynns Falls as a priority restoration project.  Working with DEPS, the design team faced unique challenges requiring a flow split to maintain baseflow through a road culvert while removing high flow stresses threatening existing infrastructure.  Designing the split was easily resolved with a 2D modeling approach, however, concerns over long-term aggradation of the remaining baseflow channel required implementation of a 2D sediment transport model to optimize the design.

We developed a dynamic 2D sediment transport model that balanced the erosion of upstream seed bedload sediments with transport through the restoration design and the flow split.  We iteratively modeled modifications to the mainstem alignment and the divergence of the bypass channel to develop an alignment that optimized sediment movement across a wide range of flows.

The resulting optimal bypass channel location was upstream of where we initially anticipated from visual cues and design experience, demonstrating that the two-dimensional analysis and sediment transport was instrumental in creating a data-driven flow split location. The backwater conditions from the roadway and confluence with Gwynns Falls resulted in flow dynamics that were not inherently obvious.

The first year of monitoring results show no discernable deposition through the baseflow channel downstream of the bypass. The pools and entrance to the bypass channel have deposition in the form of gravel bars and aggradation within the pools, indicating that the transport of sediment is behaving as modeled.