North Poudre Sediment Study Final Report

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HEC-6 Robust Simulation If the entire suite of field data are utilized, such that the thickness of sediment from the release within the pools and riffles is input to the model, and deposition and erosion are allowed to occur, the model predicts 75-80% of the actual bed change that occurred (Figure 7, Simulation 4 of Table 3a). Such a high percentage with a small spread implies greater than or equal to 75% modeling accuracy for most of the pool. None of the simulations, however, detected the deposition that occurred on the exit slope of the Tick Pool at Cross Section 14. One other shortcoming of the robust simulations was that while scouring of the pool thalweg occurred and was modeled, repeat surveys indicated simultaneous deposition on the pool margins, though not simulated by HEC-6. Because it is a purely one-dimensional model, HEC-6 only predicts uniform bed changes. These limitations supported the use of a quasi-two-dimensional model to strengthen the results. CST4RS 2.0 Simulations Default simulations were not possible within GSTARS 2,0. The model settings listed in Table 2 refer to nonequilibrium conditions which are typically used for sedimentation in quiescent water, and account for the lag time between when sediment is entrained by the flow and when the transport capacity is fulfilled, Initially, Yang's 1983 (Yang et al. 1998) sediment transport equation was used to maintain consistency between the two models. GST ARS 2.0 simulated between 13-90% of the actual bed change along the North Fork (Table 3b), a wide spread indicating large variability between surveyed cross sections and model predictions at each cross section, Subsequent simulations in GST ARS 2,0 indicaled that the model was sensitive to different sediment transport equations. For example, using the Meyer-Peter Mueller formulation. and keeping all other settings constant. the model predicted 0% of the actual bed change. II