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channel time in drainage structure DS6 is estimated to be 0.29 minutes. The larger time of subbasin A6 was added to the estimated channel flow time to arrive at a total travel time of 28.80 minutes. Subbasins A7 and A10 simultaneously contribute runoff to drainage structure DS7. The southern portion of DS7 will consist of a v-ditch along the inside edge of the Lower Bench Road (subbasin A10). The inlet time for subbasin A7 is 10.74 minutes and the inlet time for A10 is 11.60 minutes. The estimated channel time in the southern portion (on the mine road) of drainage structure DS7 is estimated to be 3.37 minutes. The larger time of subbasin A10 was added to the estimated channel flow time to arrive at a total travel time of 14.97 minutes. On the Lower Bench, subbasin A10 intersects subbasin A9. Runoff to DS7 north of this point is from the northern portion of subbasin A7, the slope between the Upper and Lower Bench, including discharge from DP1 and DP2 on the Upper Bench. Therefore, the channel design of the northern portion of DS7 (on the Lower Bench) will include the increased discharge from these sources. The inlet time for subbasin A7 is 10.74 minutes. The estimated channel time in the northern portion (on the Lower Bench) of drainage structure DS7 is estimated to be 1.99 minutes. The total travel time to DP4 is therefore 12.73 minutes. subbasin A8 contributes runoff to drainage DS8. The inlet time for subbasin A8 is 36.17 minutes and the channel time in DS8 is 3.15 minutes. The total travel time is 39.32 minutes. Subbasin A9 contributes runoff to drainage DS9. The inlet time for subbasin A9 is 19.76 minutes and the channel time in DS9 is 1.11 minutes. The total travel time is 20.87 minutes. 2.3 Rainfall Intensity The selection of the precipitation event (rainfall intensity) for the runoff calculations was conducted by considering local county stormwater guidance, and conventional procedures for the determination ofintensity-duration-frequency (IDF) relationships as reported by the National Atmospheric and Oceanic Administration (NOAA 1973). This document is commonly referred to as NOAA Atlas 2. 2.3.1 County Guidance Although the mine site is located just east of the Mesa County boundary in Garfield County, the City of Grand Junction and Mesa County Stormwater Management Manual (SWMM) (SWMM 1996) was readily available, comprehensive, and considered a relevant source of information for the area. The manual indicates that the required design storm frequency for most urban applications is a 100-year storm. It should be noted that Western Water S Land Inc.