2013-10-30_REVISION - M1977310 (3)

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Whetstone Associates Technical Memorandum (d) Signed and stamped drawings are provided in the revised DDP. (e) The SWRP spillway approach, crest, and exit section will be lined with rock and gravel mulch, as described in the revised DDP. A 0.75 -inch Type II rock filter is specified for the spillway, based on the overflow velocity of 0.42 ft /sec. 6 Page 11, Figure 1. There appears to be significant Upper Diversion Structure and Middle Diversion Structure channel segments extending beyond their respective delineated sub - basins. Figure 1 also identifies an additional 0.28 acres, and 0.18 acres that appear to contribute to these structures, respectively. Please explain why sub - basins N/C and N/D don't extend to the diversion discharge point and correct Figure 1, Table 7 and the WinTR -SS hydrologic analyses as appropriate. Cotter response: The 0.28 acres and 0.18 acres shown on Figure 1 of the DDP refer to the total permitted disturbed area for the Upper and Middle Diversion Structures, respectively. The diversion structures route water across the local drainage divide into the adjacent sub -basin to the east. These extensions of the disturbed area into the adjacent sub -basin include a small section of excavated channel and the transition from the excavated channel to the natural drainage. In the Drainage Design Plan, the UDS is shown as extending approximately 60 feet into the adjacent sub -basin while the MDS is shown as extending 86 feet into the adjacent sub - basin, or 9% and 22% of the total length of the diversion structures. However, because these extensions occur at and near the local ridgeline, the added surface area in the eastern sub -basin that could contribute runoff into the diversion ditch is small. As shown in the following figure, approximately 0.033 acres would be captured by the UDS extension and 0.079 acres would be captured by the MDS extension. These areas represent 0.06% and 8.6% of the sub -basin areas modeled as reporting to the diversion structures. The WinTR -55 model was re -run using the additional 0.033 acres and 0.079 acres reporting to the UDS and MDS, respectively, and the results are presented in the October 2013 revised DDP. Peak flow through the UDS increased from 20.14 cfs to 20.15 cfs for the 10 -year 24- hour storm and from 62.36 cfs to 62.40 cfs for the 100 -year 24 -hour storm. Peak flow through the MDS increased from 0.37 cfs to 0.41 cfs for the 10 -year 24 -hour storm and from 1.13 cfs to 1.23 cfs for the 100 -year 24 -hour storm. 4148A.131030 4