3.8 Sediment Models
Open channel hydraulic models often ignore the effect of sediment transported by the stream or assume that the sediment has no significant effect on the hydraulic computations. This assumption is usually acceptable; however, certain situations require that sediment transport be considered. For example, modeling a river that is obviously in distress, such as rapid and frequent channel shifts, great scour or deposition during flood events, extensive downed trees in the channel, and headcutting (channel erosion in an upstream direction) on the main channel or tributaries, may require that the sediment transport characteristics of the river be evaluated in conjunction with the hydraulics. Major man-made modifications, such as extensive channelization, reservoir construction, and flow diversions, can result in significant effects on the river's sediment regime, as well as to the stream's hydrology and hydraulics. Chapters 11 and 13 further discuss the interrelationship between the hydrologic/hydraulic and sediment regimes.
Sediment studies can range from qualitative "impact"-type studies to major sediment transport, scour, and deposition analyses performed by computer simulation. References on this subject are available in Chapter 11. In addition to the data needed for steady or unsteady flow hydraulic studies, use of a sediment analysis program requires a description of the bed material (grain sizes) throughout the reaches under study, as well as the definition of the sediment load curve (discharge versus sediment carried) for the main river and all tributaries carrying significant sediment and water inflow. The grain size distribution of each load curve is also needed. Sediment transport studies are more data intensive than pure hydraulic studies and will normally give less precise results. Trends in deposition and erosion with rough estimates of channel geometry changes in the reach over time are often the main information derived from such studies.
The following section describes two popular sediment transport models used in the United States. Neither program is intended for detailed scour analysis through bridges. Bridge scour incorporates contraction, pier, and abutment scour components that may be computed with FHWA procedures by HEC-RAS. These procedures are discussed in Chapter 13. These two programs could be used to determine the overall reach scour (for the reach containing the bridge), with HEC-RAS performing the detailed bridge scour analysis. HEC-RAS also allows the computation of the sediment transport relationships for either the steady or unsteady flow version. Six different sediment transport equations are available in the program to compute the water discharge versus sediment discharge relationship for any cross section or over a specified reach. This feature is further discussed in Chapter 13.
HEC-6
The Corps of Engineers' program HEC-6, Scour and Deposition in Rivers and Reservoirs (USACE, 1993), has been the most widely used sediment transport model in the United States since its initial release about three decades ago. It was created by Tony Thomas of the HEC and WES and has been upgraded and expanded several times. This program will be incorporated within HEC-RAS in the future.
The model is appropriate for one-dimensional, gradually varied, steady flow simulation. For a given flow, the standard step equations are applied to compute system hydraulics for the entire reach. Following this step, sediment transport is computed from cross section to cross section, with gains or losses to the transported sediment.
The geometry is then modified (elevations increased uniformly for deposition, decreased for scour) based on these changes, a new discharge for the next time step is applied, and the process repeated. Although the sediment computations are based on steady flow, a long-term hydrograph of discharge may be modeled in a series of time steps with the program. Decades of discharge data may be used to study changes over time. HEC-6 can be applied over long segments of rivers, ranging from 10 miles (16 km) to more than 100 miles (160 km). Sediment deposition in reservoirs has also been evaluated with the program. HEC-6 does not allow for modeling sedimentation in floodplain areas, lateral variations in channel deposition or erosion, or bankline migration.
SED2D
The SED2D program (USACE, 2000a) is an extensively rewritten and modernized version of the Sediment Transport in Unsteady, 2-Dimensional Flow, Horizontal Plane program (STUDH). STUDH and now SED2D is the sediment analysis module for the WES TABS-MD modeling system. SED2D can be used as a one- or two-dimensional model to analyze steady state or dynamic flow situations. The program can determine the exchange of material between the moving water and the stream bed. Bed shear stress due to currents or shear stress for combined currents and wind waves can be calculated. Both clay and sand can be analyzed, but only a single effective grain size can be used during each simulation. Thus, separate simulations are needed for each effective grain size.
The program does not compute water surface elevations or velocities. These values are usually computed externally with the RMA2 program. SED2D may be applied to clay- or sand-bed streams where the velocities are two-dimensional in the horizontal plane (depth-average velocity). It is typically used in sediment scour, transport, and deposition studies near major obstructions to river flow, such as navigation dams and bridge crossings. SED2D permits the evaluation of complex river and reservoir geometry and has good output-visualization graphics. It has extensive data requirements and is very computationally intensive.
