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Table 4: Potential instream flow study situations and recommended <br />hydraulic simulation approaches. , <br />Study Recomnendedl <br />Setting Approach Advantages Disadvantages Pages <br />1. River basin or <br />watershed over- <br />view studys <br />Manning equation, gra- <br />dually varied flow <br />computation procedure <br />(step-backwater) <br />Input data may be <br />estimated without <br />actual field mea- <br />surement. <br />Low accuracy. <br />Limited grange of ex- <br />trapolation <br /> <br />II. River or site <br />specific instream <br />flow studies or <br />U\ environmental im- <br />C4 pact studies. <br />Conflict with <br />other water uses <br />low. <br />A. Simple, sta- <br />ble channel <br /> <br />Mantling equation, <br />step-backwater com- <br />putation procedure, <br />one set of field <br />measurements. <br />Fair accuracy with <br />minimal amount of <br />field data collec- <br />tion <br />o. Complex, sta- Rating curve with two <br />ble channel; or more sets of field <br />configuration measurements. <br />changes ra- <br />pidly spattal'r: <br />ly. Large bed <br />streams. <br />Easier calibration <br />of field data in <br />model. Fair-to- <br />high accuracy. <br />Large range of <br />flows may be cal- <br />culated. <br />Accuracy depends on <br />cross sectional and <br />roughness variability. <br />Limited range of ex- <br />trapolation <br />subject to "two-point" <br />errors, which may be <br />substantial. Modest <br />data collection re- <br />quirement. <br />Ia-- Iy <br />14 - 15 <br />-21