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A Practical Approach to Floodplain Sedimentation Modeling Using HEC-RAS and Associated Nutrient Retention

Mike Miller
McCormick Taylor
Harrisburg, PA

Authors:  Josh Smalley, Scott Lowe, Mike Miller

Channel-floodplain reconnection is an important component of stream restoration based on the multitude of associated geomorphic, water quality, and ecologic benefits.  Deposition of sediment in the floodplain can represent a significant proportion of sediment in the watershed, even equal to streambank erosion.  However, few floodplain reconnection projects are quantitatively designed to determine the amount of sediment that could potentially be stored.  The fourth principle of Process-based Principles for Restoring River Ecosystems (Beechie et al. 2010) states to be explicit about expected outcomes of a restoration project.  Projects are unlikely to maximize benefits associated with channel-floodplain reconnection without quantitative design goals and practical methods to quantitatively evaluate potential sedimentation. 

Practical methods for designing and evaluating channel-floodplain reconnection have been developed using one-dimensional hydraulic modeling software (HEC-RAS) output that is processed through spreadsheet analysis to provide estimates that are comparable with actual results. The floodplain sedimentation analysis utilizes relationships from Asselman and van Wijngaarden 2002, Middelkoop and van der Perk 1998, and Julien 2010. The Asselman and van Wijngaarden equations set the theoretical framework for sedimentation in floodplains. The Middelkoop and van der Perk and Julien relationships provide modifications to account for unique characteristics of floodplain cells and a more thorough evaluation of storm hydrographs.

With the theoretical framework for floodplain sedimentation set up this process can be applied to a range of flow conditions which allows for calibration based on measured site conditions.  Wider application of this framework also provides a method for annualizing expected floodplain sedimentation and accompanying nutrient retention on an annual basis to provide comparable metrics.

Case studies were investigated to determine the applicability of this method to a range of projects.  Results of this analysis suggest that restoration with floodplain sedimentation as a primary goal can be effective, whereas projects which only provide stabilization or floodplain reconnection without consideration of floodplain dynamics are unlikely to promote significant floodplain deposition.  The suspended sediment modeling approach described above provides a framework for quantitative evaluation of channel-floodplain reconnection projects.