Laserfiche WebLink
<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />component is capable of modeling subcritical, supercritical, and mixed flow regime water <br /> <br />surface profiles (Bruner, 1998). HEC-RAS has a graphical user interface (Gill) and requires <br /> <br />station and elevation coordinates for each cross section. Energy loss due to friction is <br /> <br />accounted for with cross-section average values for Manning n. Contraction and expansion of <br /> <br />the channel is accounted for with the inclusion of the distance between right, left and thalweg <br /> <br />points at adjacent cross sections. Simulation output can be expressed in tabular or graphical <br /> <br />format and generally consists of depth, average cross-sectional velocity, and permutations of <br /> <br />depth and velocity. <br />Because cross-sections were not specifically surveyed in the field, and to make sure <br /> <br />that wetted perimeter and wetted area measurements were based on the same channel surface, <br /> <br />HEC-RAS cross-sections were based on a Triangulated Irregular Network (TIN) representing <br /> <br />the channel bed. The TIN's used as a basis for these measurements included points that were <br /> <br />interpolated from the original survey to make sure that the TIN portrayed channel <br />characteristics as realistically as possible. <br /> <br />HEC-RAS output includes water surface elevation and average velocity for each cross- <br /> <br />section in the model. By applying the HEC-RAS calculated water surface elevations to the <br /> <br />ends of the cross-sections, a TIN can be created that represents the water surface elevation. <br /> <br /> <br />By subtracting the bathymetry TIN from the water surface elevation TIN, a line can be drawn <br /> <br /> <br />which del imitates the aerial wetted perimeter. This line can then be used to bound the 2-D <br /> <br /> <br />mesh if water surface elevations are not available for the highest discharge to be modeled. <br /> <br />There is one user-defined parameter in the HEC-RAS model that can affect the water <br /> <br />surface elevations and that is the Manning n. Manning's n is an empirically derived number <br /> <br />that represents the roughness ofthe bed in the Manning equation. As Manning n increases the <br /> <br />velocity slows and the water surface elevation increases. By calibrating the watersurface <br /> <br />23 <br />