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Appendix A SYLVESTER GULCH DRAINAGE CALCULATIONS: ROAD EXTENSION November 2001 CALCULATIONS: The drainage for the upper Sylvester Gulch was divided into two subdrainages as shown on the attached sketch. The total drainage azea for the new road design is 1.36 square miles. The upper basin is 0.83 square miles and the lower basin is 0.53 squaze miles. The HEC-1 hydrograph model developed by the U.S. Army Corps was used for estimating peak runoff from the 24-how Type I I Distribution for the 100 year and 25 yeaz events. Total precipitation of 3-inches for the 100 year event and 2.5 inches for the 25 yeaz event were used based on NOAA maps. The Runoff Curve Number (RCN) method was used for estimation of runoff. Slopes on the north side of the drainage are bedrock with minimal vegetation and little to no natural detention, these were modeled as soils group D with RCN of 90. Slopes on the southerly side of the drainage aze deeper soils with moderate drainage, fair to good vegetative cover, and some natwal detention, RCN used was 50. An overall RCN of 60 was used. The lag times were estimated using velocity charts for slope and vegetative cover and checked using the SCS Curve number method. Lag time for the upper basin was estimated at 0.9 hows and for the lower basin 0.6 hows. Model results were as follows: Upper basin: 100-yeaz event peak flow of 43 cfs. Upper basin: 25-yeaz event peak flow of 17 cfs. Upper basin combined with lower basin: 100-yeaz event peak flow of 70 cfs. Upper basin combined with lower basin: 25-yeaz event peak flow of 27 cfs. DESIGN: For open channel design we recommend design for the 100-yeaz event. The proposed average channel design is 2-foot bottom width and 2-foot water depth with side slopes of 2:1 in fill and 1.5:1 in cut. At the lowest slope of 7-percent this channel will carry in the order of 100-cfs (using Manning open channel estimation). This provides some free-boazd over the estimated 100-yeaz combined peak flow.