Common Modeling Mistakes Using HEC-RAS

The U.S. Army Corps of Engineers River Analysis System (HEC-RAS), developed by the Hydrologic Engineering Center, has long been the standard for one- dimensional hydraulic modeling. Despite the wide spread use of the program, even seasoned engineers can fall victim to some common modeling mistakes using HEC-RAS. Four potential modeling mistakes and oversights will be examined herein: (1.) Necessary adjustment of channel bottom elevations for internal bridge cross sections (termed BU and BD in HEC-RAS verbiage), which are copied from the upstream and downstream sections of bridges (Sections 2 and 3 of the bridge sequence) are often ignored and can result in the incorrect computation of the water surface profile within the structure and immediately upstream, (2.) Frequently ignored expansion and contraction coefficients for super-critical flow (most will use just the subcritical flow coefficients) and the drastic impact that the incorrect application can have on the location of hydraulic jumps when in the mixed flow mode, (3.) The overlooked importance of proper cross section spacing through drop structures, and (4.) The need to trim cross sections to a reasonable elevation above the water surface elevation so as not to impact the critical depth computation process is not widely understood and rarely practiced. The impacts on water surface elevation computation for these common errors are not insignificant and can be easily avoided if the engineer is aware of the correct modeling technique. In a field where calculations require accuracy to 0.01 feet, even minor errors in modeling technique can result in unnecessary high cost solutions. The following paper will document these situations through discussion of simple applications and the mechanics of calculation performed by HEC-RAS as well as presentation of methods to improve modeling accuracy. These are subtle errors that even experienced modelers may fall victim to.