Yantao Cui Stillwater Science Joanna Curran University of Virginia Engineering Pete Downs Stillwater Science Brian Graber American Rivers John Gray U.S. Geological Survey Jeffery Hartranft Pennsylvania Department of Environmental Protection Lisa Hollingsworth-Segedy American Rivers Vincent Humenay Pennsylvania Department of Environmental Protection Christopher Magirl U.S. Geological Survey Jim MacBroom Milone and MacBroom Marty Melchior Interfluve Katie Ombalski Clearwater Conservancy of North Central Pennsylvania Charles Podolak National Center for Earth-Surface Dynamics Tim Randle Bureau of Reclamation Joe Rathbun Michigan Department of Environmental Quality Matt Romkens Agricultural Research Service Gary Smillie National Park Service Sara Strassman American Rivers Tim Straub U.S. Geological Survey Illinois Marcin Whitman California Department of Fish & Game Shelby Zelonis University of South Carolina
Sediment Transport Capacity Calculations The hydraulic capacity of the stream to transport coarse sediments can be computed using a variety of equations. Sediment transport capacity can be computed at various downstream locations of interest for a range of stream flows or discharges. The required input data for the sediment transport equations include channel hydraulics and the sediment grain size. For each stream discharge of interest, the average cross-section hydraulics (depth, width, velocity, and energy slope) will have to be measured or computed using a hydraulic model or computed assuming normal depth. In order to compute the hydraulic capacity of the stream to transport reservoir sediment, use the grain size distribution representing the coarse reservoir sediments rather than the stream below the dam. Assume that the fine sediments, eroded from the reservoir, will be transported in suspension as wash load. There are many computer programs available to computer sediment transport capacity. The Bureau of Reclamation provides one such program (Huang and Bountry, 2009), which can be downloaded for free at: This program can compute sediment transport capacity using the equations listed in Table 6. Table 7. Sediment transport equations available in the SRH-Capacity program. Sediment Transport Equation Sand Gravel Sand & Gravel Mixture Engelund and Hansen (1972) √ Ackers and White (1973) √ Yang (1973) √ Yang (1979) √ Wilcock and Crowe (2003) √ √ Parker (1990) √ √ and Müller (1948) √ Yang (1984) √ Wu (2004) √ The U.S. Army of Engineers HEC-RAS (Bruner, 2016a and 2016b) can be used to compute the channel hydraulics parameters for various stream discharges of interest: The data required for the hydraulic model include the cross section coordinates of the stream channel and floodplains, estimates of channel roughness, and the downstream boundary water surface elevations associated with the stream discharges of interest.
The HEC-RAS can also compute sediment transport capacity using the following equations: Ackers and White Engelund and Hansen Larsen Meyer-Peter and Müller Geomorphic Analysis For medium and high risk dam removals, a geomorphic analysis is recommended, by an experienced geomorphologist. Initially, the geomorphic analysis utilize historic aerial photographs, geologic maps, soil maps, topographic maps, historical ground photographs and accounts, and field reconnaissance. Additional analysis may require collection and interpretation of sediment and soil samples. The geomorphic analysis will describe the physical setting of the dam, reservoir, and river channel, a description of geologic controls, significant water and sediment sources, and characterization of the river and reservoir sediment. The geomorphic analysis also will identify historic channel trends and allow for estimates of future channel evolution trends following dam removal. For dam removals with a significant reservoir drawdown and reservoir shoreline slopes, the potential for landslides during reservoir drawdown should be investigated. There are numerous references for geomorphic analysis of stream channels. The ASCE Sedimentation Engineering Manual provides two chapters: Fundamental of Fluvial Geomorphology (Biedenharn et al., 2008) Engineering Geomorphology (Schumm and Harvey, 2008) The ASCE Sediment Dynamics upon Dam Removal also provides a chapter on the geomorphic effects of dam removal (Skalak et al., 2011). Numerical Modeling, Laboratory Modeling, and Field Experiments Numerical models, laboratory models, and field experiments can be used to help understand and simulate reservoir sediment erosion and the downstream transport and deposition. If potential consequences are important, consider applying numerical and physical models and field experiments. Experiments always work best when hypotheses or predictions are made in advance to guide the measurements and interpret the results. Laboratory or physical models can provide useful qualitative predictions of complex processes such as knickpoint erosion, armoring, channel widening, braiding, some meandering, downstream transport, and deposition. There are sediment scaling issues with laboratory models that make quantitative predictions difficult. For example, the specific gravity, particle fall velocity, and the grain sizes of clay, silt, sand, and gravel cannot be scaled in the same way. Therefore, the exact physical properties of coarse and fine sediment (including cohesion) are not easily replicated in the laboratory. Bromley et al. (2011) constructed a physical model to simulate the sediment erosion in Lake