Laserfiche WebLink
Base Flow <br />As described in the AWA report, the most likely timing of the PMP would be for it to <br />occur in the latter half of July, August, or September. This timing corresponds to the period <br />when the flow of Ute Creek and its tributaries is declining from the peak of the snowmelt <br />season. The base flow during this time period was estimated based primarily on the gaged <br />streamflow data from the Williams Fork River near Leal gage. Streamflow data from this <br />gage for the period 1947 to 2002 (56 years) was adjusted by adding historic diversion by <br />Denver through the Jones Pass Tunnel to estimate the virgin flow for the tributary basin. Unit <br />discharge values were then calculated based on a basin area of 89.5 square miles. <br />Consistent with the timing of the design storm, the average unit discharge for July and <br />August for each year of data was calculated. The maximum average value over the 56 year <br />period occurred in 1983 with a value of 5.47 cfs per square mile. The average July /August <br />flow over the 56 -year period was 1.82 cfs per square mile. It is evident that the Henderson <br />Mill tributary basins are lower in elevation than the Leal gage basin, and have a lower <br />average annual precipitation amount. To account for this, the unit discharge at Leal was <br />adjusted down based on the average annual precipitation as shown on the Colorado Climate <br />Center /USGS map Colorado Average Annual Precipitation 1951 -1980. The adjusted unit <br />discharge that was used in our analysis for the Henderson Mill basins was 3.61 cfs per <br />square mile. This would be representative of a very wet year base flow during the late <br />summer months. <br />Antecedent Conditions <br />I TPT <br />TOTAL <br />2.54 <br />11.52 <br />1,626 <br />7,373 <br />0.18 <br />50 I <br />The antecedent conditions of the watershed (i.e. conditions before the PMP storm) <br />can have a marked effect on the flood runoff volume. To ascertain that the residual or <br />freeboard storage capacity of the tailing pond is conservatively sized, it was assumed that <br />conditions prior to the PMP were such that the initial abstractions and soil moisture holding <br />capacities were fulfilled thus leaving more rainfall available for runoff. To do so, the initial <br />9 <br />