A Practical Approach to Floodplain Sedimentation Modeling Using HEC-RAS and Associated Nutrient Retention
Craig Carson
McCormick Taylor
Baltimore, MD
Authors: Josh Smalley, Scott Lowe, Mike Miller, McCormick Taylor
Channel-floodplain reconnection is an important component of stream restoration due to its interconnected processes, especially storage and reduced downstream flux of water, sediment and nutrients. Deposition of sediment in the floodplain can represent a significant proportion, or even equal streambank erosion in the watershed. Difficulties in modeling floodplain sedimentation have led to a lack of quantitative goals associated with long term storage of nutrients provided by floodplain sedimentation. Without quantitative goals and measurements projects are unlikely to maximize the benefits associated with increased floodplain sedimentation. To provide a practical method for determining floodplain sedimentation, freely available one-dimensional hydraulic modeling software (HEC-RAS) output was processed through spreadsheet analysis to provide estimates which are comparable to actual results.
The floodplain sedimentation analysis utilized 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 an effective, whereas projects which only provide stabilization or floodplain reconnection without consideration of floodplain dynamics are unlikely to promote significant floodplain deposition.
This presentation focuses on (1) methods used to model and predict sediment deposition on the floodplain and (2) annualization of predicted sedimentation to provide comparable results. The suspended sediment modeling approach described above provides a framework for quantitative evaluation of channel-floodplain reconnection projects.
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