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Last modified
1/25/2010 7:14:55 PM
Creation date
10/5/2006 3:44:19 AM
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Template:
Floodplain Documents
County
Statewide
Basin
Statewide
Title
HEC-2 Water Surface Profiles
Date
8/1/1979
Prepared For
US
Prepared By
US Army Corps of Engineers
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />where: <br /> <br />H <br />WS <br /> <br />. <br />= total energy head <br /> <br />= water surface elevation <br /> <br />a.v2 <br />29 <br /> <br />= vel oci ty head <br /> <br />The critical water surface elevation is the elevation for which the <br />total energy head is a minimum. The critical elevation is determined <br />with an iterative procedure whereby values of WS are assumed and corre- <br />sponding values of H are determined with equation (11) until a minimum <br />value for H is reached. <br /> <br />To speed the iteration process, a parabolic interpolation procedure <br />is followed. The procedure basically involves determining values of H <br />for three values of WS that are spaced at equal ~WS intervals. The WS <br />corresponding to the minimum value for H defined by a parabola passing <br />through the three points (on the H versus WS plane) is used as the basis <br />for the next assumption of a value for WS. <br /> <br />It is presumed that critical depth has been obtained when there is <br />less than a 2.5% change in depth from one iteration to the next and <br />provided the energy head has either decreased or has not increased by <br />more than .01 feet. The tolerance of 2.5% may be changed by program input. <br /> <br />7. PROGRAM LIMITATIONS <br /> <br />The following assumptions are implicit ;n the analytical expressions <br />used in the program: <br /> <br />(1) Flow;s steady <br /> <br />(2) Flow is gradually varied <br /> <br />(3) Flow is one dimensional (i.e., velocity components in direc- <br />tions other than the direction of flow are not accounted for) <br /> <br />(4) River channels have 'small I slopes, say less than 1:10 <br /> <br />Flow is assumed to be steady because time-dependent terms are not included <br />in the energy equation (1). Flow is assumed to be gradually varied because <br />equation (1) is based on the premise that a hydrostatic pressure distribu- <br />tion exists at each cross section. Flow is assumed to be one-dimensional <br />because equation (4) is based on the premise that the total energy head <br />is the same for all points in a cross section. Small channel slopes are <br />assumed because the pressure head which is a component of WS in equation <br />(1) is represented by the water depth measured vertically. <br /> <br />9 <br />
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