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<br />COMPUTATION 01' PEAK I'I..OWS (Ori) AND I'LOW DEPTHS (oi) AT TYPICAL SECTIONS <br />DOWNSTREAM OF THE DAM <br /> <br />The dam failure hydrograph with peak value Qp travels downstream. creating a <br />flood with peak values and depths of Qri and oi respectively, at various <br />typical sections i = 1. 2. 3 etc. Due to the effects of side channel <br />storage. infiltration. and channel roughness. the flood hydrograph changes <br />as it proceeds downstream. The following typically occurs: <br /> <br />1. The hydrograph peak is reduced. <br /> <br />2. The hydrograph base time is increased. <br /> <br />3. The hydrograph shape becomes more rounded. <br /> <br />Because of infiltration. the volume of the flood would be expected to <br />decrease as the flood proceeds downstream. However. for ease of <br />calculation. loss of volume is neglected. assuming flood channel conditions <br />which are pre-wetted. The relationships described above are illustrated in <br />figure 6. <br /> <br />cfs <br /> <br />cfs <br /> <br /> <br />Qr; <br /> <br /> <br /> HIS Hrs <br />~ Tp ,I I. Tri ,I <br />FAILURE HYDROGRAPH ROUTED HYDROGRAPH <br /> Figure 6: Effect of Routing on Hydrograph Shape <br /> (Qp > Qri. Tp ~ Tri) <br /> <br />Many recognized methods of flood routing are available to predict routed <br />hydrograph shape. While any of these would be acceptable, the discussion <br />here will be limited to the "method of successive averages".2 This method <br />can be used to easily determine the routed hydrograph peak value, Qri <br />without the need to generate a complete hydrograph. Generally, the degree <br />of routing (i.e., flood peak reduction) is a function of the fOllowing: <br /> <br />1. Travp.l time for the hydrograph. Th. The longer the flood must travel. <br />the more the peak is reduced. <br /> <br />2. Channel slope. s. The flatter the slope the greater the travel time. <br />and the more the peak is reduced. <br /> <br />9 <br />