Modeling pool sediment dynamics in a mountain river

An increasingly important source of sediment into river systems is sediment that accumulates within reservoirs and is subsequently released into the downstream ecosystem. In Colorado alone, five large-scale sediment releases from reservoirs within the last decade have resulted in a host of environmental hazards, particularly the loss of aquatic biota and habitat. In September 1996 approximately 7,000 m3 of fine-grained sediment were released from Halligan Dam into the North Fork Cache la Poudre River, northern Colorado. The sediment caused extensive aggradation of the original bed, primarily in pools, and complete fish mortality for 12 km downstream from the dam; The purpose of this investigation was to evaluate the applicability of various hydraulic and sediment transport models as predictors of pool recovery. Two modeling scenarios were modeled using one- and semi-two dimensional sediment transport models, HEC-6 and GSTARS2.0. The models were calibrated against quantitative measurements of pool bed elevation obtained during field surveys. Greater than 50 percent of the actual scour and deposition within pools was modeled using HEC-6. Modeling accuracy using GSTARS2.0 was considerably more variable.; A two-dimensional, finite element hydraulic model, RMA2, improved delineation of flow hydraulics in areas of flow separation and recirculation. RMA2 results of depth-averaged velocity magnitude correspond with velocity measurements from low-velocity eddy pools. Patterns of boundary shear stress and a particle stability index accurately predict gross areas of scour and deposition, but fail to represent the simultaneous aggradation and degradation measured in pools.; Finally, a conceptual model of pool sediment dynamics serves as an alternative to the numerical modeling. Predictable sites of channel aggradation and degradation resulting from a sediment pulse are identified. Sedimentation within laterally confined pools is dependent on pool geometry, distance downstream from the dam, and the duration and magnitude of flows following the release. The sediment release from Halligan induced persistent, long-term storage of fine sediment because of an elevated channel bed and loss of channel capacity. Recognition of the hazards associated with a large influx of sediment into a river is critical for a greater understanding of the effects of sediment releases, and future management of sediment within reservoirs.