Application of the HEC-RAS hydraulic and water quality model to support the development of the lower Minnesota River TMDL

This paper presents the latest HEC-RAS hydraulic and water quality model and its application to the Lower Minnesota River (LMNR) in support of the development of total maximum daily load (TMDL) and assessment of riverine water quality. The LMNR has been listed as impaired because of high algal biomass and dissolved oxygen depletion during low discharge periods, deposition of nutrient-rich sediment, and excessive loads of nitrogen and phosphorus to the system. These concerns have led to the development of the TMDL for establishing goals for load reduction of point and nonpoint sources and evaluation of management scenarios to improve current water quality conditions in the system. The HEC-RAS model was calibrated and validated with an extensive set of field data collected during a six-year period (2001 – 2006). The field measurements agree considerably well with the model results for flow and water quality constituents including temperature, inorganic suspended solids, algae, organic phosphorus and nitrogen, phosphate, ammonia, nitrate, and dissolved oxygen. With rare exceptions, the statistical measures of HEC-RAS model performance were excellent. The HEC-RAS model was able to support the development of the lower Minnesota River TMDL and predict the behavior of the water quality responses for external nutrient loading reduction scenarios.