2008-11-10_ENFORCEMENT - C1981019 (2)

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AUGUST 9, 2008 THUNDERSTORM AND IMPACTS TO PROSPECT SEDIMENT POND On August 9, 2008 the Colowyo Mine received a significant thunderstorm event during the late afternoon. This resulted in a large runoff event which quickly filled the pond and subsequently activated and ultimately overtopped the Prospect Sediment Pond's emergency spillway. While there was no significant damage to either the impoundment's embankment or to downstream structures, these flows crossed State Highway 13 and moved some debris onto the roadway. As shown on Figure 1, the majority of the rainfall occurred in a one hour period. This figure also shows the cumulative rainfall recorded at each of the four meteorological sites. There was good agreement between three of the four sites, indicating that this thunderstorm was probably fairly large and widespread. The following precipitation- frequency values have been derived from the Colowyo permit documents and supplemented with data from NOAA Atlas 2, Volume III - Colorado. Colowyo Mine, Precipitation- Frequency Values, Inches of Rainfall Figure 2 has been prepared to show this same data graphically. Notice that the 10 year 24 hour design storm has a total rainfall amount of 1.80 inches. The August 9, 2008 event has been added to this dataset. Generally, it can be concluded that this thunderstorm has a return period in excess of the 50 year event. Current conditions of the Prospect Pond upstream drainage area are depicted on Figure 3. Ultimately, this pond will have an upstream drainage area of about 728 acres, although at 5 present aboutP-3-3—a—c—re-sl are contributory. There are three subwatersheds that are currently kv non - contributory until additional reclamation work is completed. CNQ l CNq l The majority of the runoff is generated from four sub�4 ershed shown as SWS2, SWS5, SWS6 and SWS10. The watersheds designated SWS2 and SWS6 are disturbed areas which have been rough graded but have not been topsoiled or seeded. These have drainage areas of 19.4 acres and 34.5 acres respectively. These two areas contribute about 44% of the total runoff. Watersheds SWS5 and SWS10 consists of recent reclamation with drainage areas of 38.6 acres and 47.0 acres respectively. These two areas contribute about 43% of the total runoff. In combination, the four watersheds generate 87% of the runoff. At the time of the event, some temporary storage was available in a sediment trap at the upstream portion of the 1 Storm Duration Return Period 1 hour 6 hour 24 hour 2 Year 0.55 0.90 1.20 10 Year 0.90 1.35 1.80 25 Year 1.00 1.70 2.05 50 Year 1.20 1.85 2.40 100 Year 1.40 2.10 2.70 50% PMP 4.20 Figure 2 has been prepared to show this same data graphically. Notice that the 10 year 24 hour design storm has a total rainfall amount of 1.80 inches. The August 9, 2008 event has been added to this dataset. Generally, it can be concluded that this thunderstorm has a return period in excess of the 50 year event. Current conditions of the Prospect Pond upstream drainage area are depicted on Figure 3. Ultimately, this pond will have an upstream drainage area of about 728 acres, although at 5 present aboutP-3-3—a—c—re-sl are contributory. There are three subwatersheds that are currently kv non - contributory until additional reclamation work is completed. CNQ l CNq l The majority of the runoff is generated from four sub�4 ershed shown as SWS2, SWS5, SWS6 and SWS10. The watersheds designated SWS2 and SWS6 are disturbed areas which have been rough graded but have not been topsoiled or seeded. These have drainage areas of 19.4 acres and 34.5 acres respectively. These two areas contribute about 44% of the total runoff. Watersheds SWS5 and SWS10 consists of recent reclamation with drainage areas of 38.6 acres and 47.0 acres respectively. These two areas contribute about 43% of the total runoff. In combination, the four watersheds generate 87% of the runoff. At the time of the event, some temporary storage was available in a sediment trap at the upstream portion of the 1