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< SLOPE-AREA METHOD MEASUREMENTS 1. INTRODUCTION. The slope-area method is a technique used to determine the peak discharge of a channel after the flood crest has passed. Other methods include the calculation of peak discharge using the head loss through various width contractions, such as culverts and bridges and flow over weirs and dams. The slope-area method is most widely used by the U.S. Geological Survey (U.S.G.S.) and other cooperating agencies in determining the peak discharge at a stream gage when no measurement was made. The U.S.G.S. also uses the slope area method when conducting Manning's "n" verification studies, in which the discharge is measured and the calculations are used to solve for the roughness coefficient. Slope-areas are made by the Corps of Engineers when estimates of peak flow are needed at ungaged sites in order to analyze a flood problem or design a project. 2. BACKGROUND. The discharge is computed using the slope-area method, which is based upon a uniform flow equation involving channel characteristics, water surface profiles and a roughness coefficient. An assumption is made that the flow in the reach at the time of the instantaneous peak discharge is uniform, or that the flow depth is the same at every section of the channel at that instant. In the solution of the equation for discharge, it is also assumed that the entire cross sectional area is effective, or that there is no zero flow or reverse flow in any cross section. A slope-area estimate of peak discharge makes use of the Manning equation, the energy equation and the continuity equation. They are solved simultaneously for discharge for the reach of. channel being analyzed. Those equations are listed below: 2a. Manning Equation: v = 1.486 * r % * S y, n 1-1