1990-01-09_REVISION - M1988112 (6)

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?U 61/0? 1?•22 ~ 3 '+N= '95 e?i? SF'ti UEpt~ 05 BAftI.E MOUNTAIN RESOURCES, SAN LUIS PROJECT CMLRU AUFQUACY LETTER RESPONSES Page ]i of 28 19. What effect will the additional 80 cfs diverted to the north by the east diversion ditch have on the drainageway receiving this flow? Are any channel"improvements proposed for this north drainage to assure stable conditions during the 1017-yr, 24-hr storm runoff event? ' RCS?U48SC; An analysis of the north drainage area was performed using the ; computer program WASHED. In the analysis, the discharge Prom the east ' diversion ditch (Drainage Area D}, which is anticipated to be 80 cfs r'rom the lOD-y r, 24-hr storm, was routed through the dry inage. A peak discharge of 97 cfs was predicted. As shown on the enclosed calculations the peak flow for the existing channel is routed before the peak flow for the east diversion ditches resultiml ir, only 17 ~ cfs of flow in the existing channel at the time of pas•: tq of the peak flow from the east diversion. Tha north drainage channel will be riprapped to stabilizE~ the channel during the 1C0-yr, Z4 hr event. based on the geometry of the existing drainage, a riprap D50 of 0.7?. ft is required to si.abili7e the channel. Cxisting cna~irel characteristics and ripra;> sizing is pre~:er,ted 1n Attach~~ent 3. Riprap bedding material is described in I*.etn 43. Haw was the SC5 curve number of 70 derived? What soil anj vegetation conditions were assumed fn the derivation? +'",uCti~F.~ The SCS curvy number of 70 was chosen to be a conservttive number for runoff from the site to allow for conservative runoff control desions. Based on a rangeland classification with an estimated 60X cover and a hydrologic soli yroup classification of 8 (is discussed in the approved permit, Exhthits [ and J), the curve number could rangy hetweert approxirtlately 46 and 70 (Figures S-3 and S-4 from Peak Ftnus in Colorado, U.S. Soil Conservation Service, 1584). 70 was cho:;en as the most conservative number. ?? A velocity of 4.5 fps will result in the south diversion ditch under design conditions. A velocity of 3.1 fps will result in the east diversion ditch. Wi11 any attempt be made to improve channel bottom conditions in these diversions and reduce possible sedimentation downstream? 1"he Division recommends the diversions be .seeded after construction for erosion control. ~2~'S?ONSE: The diversion ditches will be constructed primarily 1n •:he cemented gravels of the Santa re formation. Locally, gravel particles within the Santa Fe range from 0.75" to 3" in diameter. In addition, the channel will be seeded after Construction to aid in erosion contro' Natural armoring of the ditch bottom, coupled with the recommertde reseeding, is expected to maintain the stability of tt~e diversion ditch and control erosion.