2008-03-27_REVISION - M1984041 - Laserfiche WebLink

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The hydrologic characteristics of the soil were based on information from the NRCS Soil Survey of Aspen-Gypsum Area, Colorado, see portion used in Appendix B. The typical Hydrologic Soil Group was "C" for the on-site basin. It was assumed that the average cover for the area would approximate a "dirt road" (Table 2-2, SCS-TR-55) and was thus assigned a Curve Number of 87. ON SITE DRAINAGE On site drainage is shown on Figure 4, the Drainage Plan. Essentially, the onsite drainage will be contained within the site by perimeter berms with an adjacent swale. Since the flows will be relatively shallow and diffuse, these minimal improvements will serve to direct the onsite drainage to the proposed retention pond. Retention (vs. detention) is proposed for this site because the nature of the materials stored will be best managed by minimizing the possibility of storm water exiting the site. Note that per the table headed "Lake Evaporation Worksheet", the expected monthly evaporation always exceeds the monthly mean precipitation. (With the possible exception of December and January where there may be no evaporation, but there probably would still be sublimation.) Therefore, it is expected that the retention pond will have a net monthly loss of volume under normal circumstances. EROSION CONTROL In general, erosion control will also be addressed by the use of perimeter berms to direct drainage to the retention pond. The retention pond has been oversized, but will need periodic monitoring to evaluate whether it should be cleaned out or re-excavated to maintain a storage volume in excess of approximately 17,000 cubic feet (the 100 year runoff volume). In addition, areas that will be minimally, or only occasionally disturbed by the composting processes should be seeded with a dry land type grass mix. SUMMARY Drainage impacts to the surrounding areas due to the proposed compost processing area can be mitigated by retaining flows on the site.