Laserfiche WebLink
but is not particulazly applicable to the rational method and land use at the Logan Wash <br />Mine. <br />Table 3.1 shows that the 30 minute 100-year storm produces a rainfall intensity of 2.6 <br />inches/hour. This event appeazs to closely approximate the event in July 2004 at the <br />mine. However, long-term stormwater control of more frequent storms is considered a <br />more practical approach, especially because the mine is currently being reclaimed and <br />closed. Reclamation will include revegetation of all mine benches and roads, including <br />the inter-bench stormwater channels. In addition, the construction of large conveyance <br />channels associated with a large peak discharge and a more infrequent storm event are <br />not only more costly, but may not be feasible in some azeas due to limited space (narrow <br />roads and shallow bedrock) and may not be compatible with the reclamation grading <br />plan. Based on these considerations, the selected design storm is the10-year event. <br />Storm intensity, I, was therefore calculated by using Figure 4.0 or Figure 4.1 and <br />applying the total time of concentration to storm duration for the subbasins) being <br />evaluated. The resulting I value corresponds to the intersection of the storm duration <br />time in minutes and the curve representing the 10-yeaz return period. <br />2.4 Peak Discharge <br />Peak dischazge, as derived by the rational method, was first calculated as an estimate of <br />discharge from a subbasin for the use of better approximating travel times for the <br />subbasin (Table 1.0). A second and final calculation of peak discharge was performed <br />once total travel times were estimated. <br />Peak discharge rates were initially calculated for each individual subbasin for the <br />purposes of estimating runoff into an interbasin channel so that channel travel times <br />could be estimated. These discharge values were based on a storm intensity associated <br />with each subbasin's t~ value for overland flow only and were then used to estimate flow <br />velocity and travel time within each interbasin channel. Each subbasin's t~ value was <br />used in Figure 4.0 as storm duration time to estimate the storm intensity. Given the <br />subbasin area, A, the runoff coefficient, C, and the storm intensity, I, the peak discharge, <br />Q (Qini[ial), ~'as calculated for each subbasin or subbasins at the mine for the 10-year <br />storm event. These dischazge data are shown in Table 1.0. <br />The process was repeated after new total travel times were calculated by summing the <br />maximum t~ (overland flow) value for subbasins contributing flow to a common channel, <br />and the calculated channel flow time. The total peak discharge to be received by each <br />drainage structure was estimated by summing C•A values for subbasins that mutually <br />contribute to the channel being evaluated, and multiplying by the storm intensity as <br />determined from the total travel time and Figure 4.0. The resulting peak discharge rate <br />for each of the drainage structures on the mine site is shown in Table 2.0. <br />As noted in Table 2.0, drainage structure DS2 and D57 have been separated into northern <br />and southern segments to assess peak discharge for each segment. As previously noted, <br />Western Water & Land Inc. 10 <br />