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<br />DAM FAILURE ANALYSIS <br /> <br />1. General <br />The dam failure analysis portion of the HEC-l program was <br />used to model potential discharges downstream of Leyden <br />Reservoir in the event that Leyden Dam failed. The failure <br />analysis is based on an overtopping failure in which the <br />breach is simulated as a triangular or trapezoidal shaped <br />opening that grows progressively downward from the dam crest <br />with time. Flow through the breach at any instant is <br />calculated using the broad-crested weir equation. Estimates <br />of dam failure and the resulting discharges were modeled under <br />two conditions: 1) using previously calculated 10-, 50-, 100-, <br />and 500-year inflows to Leyden Reservoir, which are based on a <br />uniform infiltration rate of 0.6 inches per hour, and 2) <br />during the event of a clear weather breach. In the first <br />scenario, the analysis began with water surface level at the <br />crest of the spillway and initiated the breaching of the dam <br />when inflows caused the overtopping of the dam. In the second <br />scenario, a clear weather breach, failure occurred with a <br />static water surface elevation at the spillway crest with no <br />inflows. <br /> <br />2. Breach Parameters <br />Peak discharges for various overtopping dam failure <br />scenarios were determined using HEC-1. The scenarios involved <br />estimating a matrix of breach widths and times for the <br />breaches to develop. The breach widths (21.5, 43, 86, 129, <br />and 172 feet) ranged from one-half to four times the dam <br />height as suggested in the National Weather Service's DAMBRK <br />User's Manual. Since HEC-1 allows for a non-uniform <br />embankment profile, the top of dam was modeled at the lowest <br />height of the top of the dam. The HEC-1 models were setup to <br />begin failure once the top of the dam embankment had been <br />overtopped. Inflow hydro graphs from the 10-, 50-, 100-, and <br />500-year storms were used to simulate flow into the reservoir <br />and create the overtopping effect. <br /> <br />3. Dam Breach Results <br />The water surface elevations created by the la-year storm <br />were adequately routed through the spillway and never reached <br />the top of the dam. Consequently, no dam failure was modeled <br />for this storm. Tables 9-11 show the matrix of possible dam <br />failure widths and times and the corresponding peak failure <br />discharges for each of the other design storms. Figure 16 <br />compares loo-year flood hydrographs for a moderate breach <br />width of 86 feet (allowing one hour for breach to form) at <br />various points below Leyden Dam. Figure 17 shows flood <br />hydrographs in the event of a clear weather breach. Since the <br />model predicts the maximum water surface elevations created by <br />reservoir inflows, a graph was made plotting storm frequency <br />with maximum water surface elevation. From this plot (with <br /> <br />10 <br />