The coupling of a two-dimensional hydrodynamic/sediment routing model with an upland watershed erosion model in a mountain watershed, Red River, Idaho

In an effort to generate new knowledge and improved understanding of the complex interrelationships between watershed and instream parameters and the scale integrity influences on channel morphology, an integrated watershed hydrologic/sedimentation framework for mountainous watersheds is developed. This framework provides advanced analytical techniques and numerical models for simulating upland (macro level) and instream (micro level) processes in an integrated fashion. The framework is developed based on the premise that watershed-wide parameters have cumulative impacts on stream ecology and therefore, watershed modeling should facilitate integration of spatial and temporal scales in order to provide meaningful answers from the physical and statistical point of view. First, a statistical analysis is employed to classify the watershed upland and instream affecting parameters and to quantitatively describe the impacts of these parameters on stream ecology. Second, the GeoWEPP soil erosion model is employed to simulate the hydrologic, and sediment entrainment phenomena at the uplands of the Red River watershed, Idaho. Long-term averages and different frequency distributions analyses are performed to investigate the temporal variability in upland soil erosion processes. Third, a thorough investigation for the particle transit time is performed using the hypsometric curve approach and the particle virtual velocity approach. It is demonstrated that a fine sediment particle moves from the uplands to the mouth of the watershed within a relatively short period of time (few days). Fourth, this work also involves enhancing capabilities of an existing instream two-dimensional hydrodynamic/sediment transport model that was originally developed to simulate the transport of uniform sediments. The new version of the model, EnSEDZL model, incorporates the Parker and Wilcock's sediment transport equations to predict multifractional bedload transport rates. Supplemented by several empirical functions for predicting bedload transport capacity, hiding function, reference transport rate, etc. The upland soil erosion model is eventually combined with the instream numerical model by matching the return period for a rainfall storm event for the upland soil erosion processes with instream flow that has the same return period for instream sediment transport processes. Finally, modeling results are compared with 13-year detailed field data and against the predictions of commercial and private models, including the USACE models (RMA2 and SED2D) and the 3ST1D model developed in-house.