ECOHYDRAULIC MODELING: DEVELOPING A PHABSIM MODULE FOR HEC-RAS 2D

Urban streams are susceptible to declining water quality and rapid geomorphic changes degrading physical habitat. Stream restoration goals are to rehabilitate stream health closer to natural conditions though common restoration practices generally lack consideration of biological integrity. An increasingly popular solution to this issue is the use of ecohydraulic modeling. This process typically involves 2D hydraulic modeling integrating Habitat Suitability Criteria (HSC) for target aquatic species among different flow stages to assess habitat suitability for existing and proposed restoration conditions. Habitat suitability is quantified by applying a PHABSIM modeling approach, based on species preferences for stream velocity, water depth, and bed substrate where model output is a composite Weighted Usable Area (WUA). Currently available freeware, River2D, performs 2D hydraulic modeling and computation of WUA. However, the engineering standard for 2D hydraulic modeling is the HEC-RAS model which lacks PHABSIM capabilities. The objective of this research project was to develop a modeling routine for HEC-RAS 2D that incorporates the PHABSIM capability. To accomplish this objective, a reach of Beaver Creek near Knoxville, Tennessee at low flow and bankfull conditions was modeled using HEC-RAS 2D to obtain velocity and depth outputs per mesh cell. Additionally, substrate data was included by importing a substrate raster into HEC-RAS, and HSC data for three fish species with different habitat preferences was utilized. To calculate WUA, RASter Calculator was used to create a function that multiplies the surface area of each mesh cell by its corresponding composite suitability value, producing a WUA per cell output raster. Outputs were summed to a total WUA for the entire reach. Results were then compared to composite suitability and WUA results from a River2D model developed for the same channel segment by Schwartz et al. (2015). The composite suitability maps for HEC-RAS corresponded well to fish habitat preferences during both high and low flow. Composite suitability differed slightly between HEC-RAS 2D and River2D, which may be attributed to different velocity patterns across the channel segment between model outputs as well as a difference in substrate classification during model development. Overall, a PHABSIM module can be feasibly implemented into HEC-RAS 2D.