Laserfiche WebLink
30 <br />Modifications to the original R-2 Cross model were made to relax <br />some of the simplifying assumptions involved in applying Eq. 2-2 <br />(Wesche and Rechard 1980). The Manning Equation was developed as an <br />empirical description of uniform flows in open channels. In the <br />original model, slope and roughness coefficients are assumed to be <br />independent of stream discharge. However, both of these parameters <br />change as a function of discharge at most locations in natural stream <br />channels. To use this model in a more realistic manner, empirical <br />equations have been proposed which make roughness and slope functions <br />of hydraulic parameters such as Q or R (see Sect. 3.1). <br />Development of instream flow recommendations with R-2 Cross is <br />essentially the same as that with other habitat evaluation methods. <br />Habitat-discharge curves are drawn, and either a habitat retention <br />criterion [e.g., 25% reduction (or 75% retention) of a habitat <br />attribute from optimum or reference conditions] or an inflection point <br />criterion is applied. The use of this method is restricted to narrow, <br />wadable streams with low roughness (uniform or gradually varied flow). <br />2.5 WSP Hydraulic Simulation <br />This method is based on a step-backwater, hydraulic simulation <br />model, Water Surface Profile (WSP), developed by the Bureau of <br />Reclamation. The original purpose of the WSP model was to predict <br />water surface elevations for flood routing problems (the HEC-2 model <br />developed by the U.S Army Corps of Engineers and PSEUDO developed by <br />the Bureau of Land Management are other examples of this type of <br />simulation model). It has been adapted to instream flow management <br />problems in many of the same ways that R-2 Cross has been used; i.e., <br />to predict changes in physical habitat parameters such as depths, <br />velocities, and wetted perimeter with varying stream discharge (White <br />1976, Cochnauer 1976, Dooley 1976, Elser 1976, Workman 1976, Bovee and <br />Mi1hous 1978). The advantage of using this method instead of R-2 <br />Cross is that it uses a more sophisticated approach in modeling open <br />e