San Jacinto River Sediment Transport & Scour Analysis

San Jacinto, CA

PACE was contracted to perform a streambed adjustment analysis for the City of San Jacinto.  Services included river field reconnaissance and assessment, floodplain model review and river hydraulics characteristics, sediment data evaluation, sediment mass balance analysis, HEC-6T fluvial model development, sediment transport function selection, upstream sediment delivery, local scour analysis, and streambed response/ adjustment assessment.

The City of San Jacinto is currently planning to construct earthen material levee improvements along a five mile stretch of the San Jacinto River to protect adjacent property from 100-year flood levels. A sediment transport and scour analysis was performed to determine the toe-down depth of the levee revetment and fluvial considerations associate with armoring structures, levee lining, and bridge protection measures and the change in conditions between existing and proposed conditions including the impacts of a proposed in-channel detention basin downstream of the project reach. The analysis was performed based on the goals of providing the design recommendation of the river levee revetment toe-down requirements, understanding the general fluvial response of the river system, and developing an accurate fluvial model to quantify (1) long-term scour, (2) general scour, and (3) local scour, and (4) identifying the effects of an in-river sediment basin and the amount of sediment trapping that can be achieved. The project analyses were performed on a qualitative level to understand the geomorphic characteristics of the existing river corridor, and second, a quantitative analysis involving detailed engineering hydraulic/fluvial modeling.

A moveable bed/sediment routing model was developed along the defined portions of the river system to evaluate changes in the streambed during a storm event to evaluate general scour and the long-term scour.  An initial quantitative assessment for the natural river stability was evaluated through a steady state sediment continuity / mass balance analysis.   The routing model estimated the response of the channel invert during the passage of the storm hydrograph for different return periods.  A HEC-6 model was developed and included three primary areas of input (1) channel geometry data, (2) sediment data, and (3) discharge / flow hydrograph, as well as the boundary conditions.  The sediment inflow hydrograph was applied to the various sediment categories based upon the grain size distribution. The HEC-6T model was developed following the general recommended procedures outlined in ACOE TD-13.