Fine-Grained Sediment Dynamics Downstream from a Dam

The processes controlling fine sediment deposition, storage, and transport within laterally constricted pools of the North Fork Cache la Poudre River, northern CO, were investigated following the 1996 release of approximately 7,000 m³ of sediment. The release provided an opportunity to develop a field data set of fine sediment dynamics and to test the predictions of three different one- and two-dimensional sediment transport and hydraulic models against the field observations. Sediment transport within the North Fork following the release is similar, on a smaller scale, to transport of sediment following dam removal. Sediment from the release filled pools up to 3.5 m deep, and defined the starting condition for the modeling. During an experimental release of water from the reservoir in March 1997, sediment was scoured from the thalweg of pools and transported by a combination of translation and dispersion. Some sediment became deposited as higher lateral channel bars following thalweg scouring as sediment moved from pool to pool downstream during subsequent flow releases. At high discharges, the development of a strong shear zone prevented degradation of sediment deposits within the eddy. One-dimensional HEC-6 results provided the greatest agreement between predicted and measured pool bed elevation change. Results from the two-dimensional model RMA2 substantially improved the representation of eddy pool hydraulics, and provided input for bed load and total load computations which accurately delineated areas of scour and deposition. Less energetic fluvial systems could result in longer-term deposition of sediment than was experienced on the North Fork.