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<br />i <br /> <br />2b. Energy Equation: <br /> <br />y} + d1 + SO *L = Y.z2 + d2 + s F *L <br />2g 2g <br /> <br />2c. Continuity Equation: <br /> <br />q=a*v <br /> <br />Where: <br /> <br />v = velocity (ftlsec) <br />n = Manning's roughness coefficient <br />r = Hydraulic radius:: alp <br />a = Cross sectional area (square feet) <br />p = Welted perimeter (feet) <br />s = Slope of channel bed <br />SO = Friction slope or slope of the high water marks <br />g F = Acceleration due to gravity (32.2 feet per second) <br />d = Depth of flow (ft) <br />L = Reach length (ft) <br />q = Discharge (cfs) <br /> <br />And: Subscripts 1 and 2 refer to upstream and downstream locations on the <br />channel respectively. <br /> <br />3. OBTAINING FIELD DATA. <br /> <br />Field data must be obtained as soon after the flood peak as possible. Ideally, it <br />should be obtained within 24 to 48 hours after the crest. Data must be collected before <br />further rainfall wipes out the traces of the flood crest. <br /> <br />3.1 SELECTING THE REACH. The first step is to select the reach of channel. <br />This step is highly important and, will have much to do with the accuracy of the final peak <br />discharge computation. The following are several factors to look for when choosing a <br />slope-area reach: <br /> <br />3.1.1 IT SHOULD HAVE GOOD HIGH WATER MARKS. - Clearly defined <br />high water marks are necessary to accurately define the energy grade line through the <br />reach. Selection of the high water marks will be discussed later in this section. <br /> <br />3.1.2 IT MUST HAVE AN ABSENCE OF BACKWATER CONDITIONS.- <br /> <br />1-2 <br />