![]() If the watermark is within the floodplain overbank, separate wetted perimeters for the main channel and each overbank will need to be performed to calculate a composite n-value (see Step 4). Step 3: Use MicroStation or AutoCAD tools to measure the wetted area and wetted perimeter of the wetted cross section based on the assumed flow-depth from the watermark. ![]() Step2: Using MicroStation or AutoCAD, plot the uniform (constant) water surface on the existing cross section station based on the assumed flow-depth from the watermark. Step1:At a stream station, measure the vertical depth from the stream bed thalweg to the field watermark, which will be an assumed flow-depth. The gathering of field data is recommended at several locations when possible. The following is a basic process for calibrating a HEC-RAS model with a field located watermark at a single location. This process of calibration requires more effort than the previous and is not as precise mainly because velocity cannot be measured in the field associated with a watermark opposed to flowing water. They can be used in the main channel or more commonly in the floodplain overbanks where flowing water occurs less frequently. Roughness values can be refined by using watermarks from past storm flows found in the field, which aid in calibrating a HEC-RAS model. Simple Calibration Using Field Located Watermark Typically, the floodplain overbanks receive flow much less frequently making it difficult to gather actual flow-depth and velocity in the field. Most likely, this calibration and roughness value refinement is only for the main channel considering the need for flowing water in the field. It is good to have field measurements at a number of stream stations in order to find the most representative n-values. The n-value used to match field flow-depth results can be considered for final pre-construction model use. Step 6: In HEC-RAS, iteratively increase or decrease n-value at the specific stream station, and run HEC-RAS simulation until the HEC-RAS flow-depth matches the field measured flow-depth. Step 5: Enter the calculated flow into HEC-RAS associated with the field measured flow-depth. V= field measured velocity at specific flow-depthĪ= wetted area based on field measured flow-depth ![]() Step 4: Calculate a volume flow rate (flow) using the continuity equation considering the field measured velocity and wetted area found in MicroStation or AutoCAD. Step 3: Use MicroStation or AutoCAD tools to measure the wetted area of the cross section based on field measured flow-depth. Step 2: Using MicroStation or AutoCAD, plot the uniform (constant) water surface on the existing cross section station based on the measured flow-depth. Step1:At a stream with flowing water, measure flow-depth and corresponding velocity at a known stream station. The following is a basic procedure for calibrating a HEC-RAS model with field measured stream depth and velocity of flowing water at a single location. Appendix C: Sample Hybrid Revetment Plan Sheets & Design Cross SectionsĪppendix D: Gravel Filter Nonstandard Special Provision Appendix A - Hydraulic Model Calibration Simple Calibration Using Field Measured Flow-Depth and Velocity ![]()
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