FEIS - Tahoe Regional Planning Agency

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RESPONSE TO COMMENTS ON THE DEIS B o u l d e r B a y C o m m u n i t y E n h a n c e m e n t P r o g r a m P r o j e c t E I S Table 8.5-1 Comparison of total Runoff Volumes for Various Designs and Storms for Project Area BMP Designs Project Area BMPs/SWMP EC Existing Conditions E20 Existing Conditions (20 yr Design)*** C20 Alternative C (20 yr Design) C100 Alternative C (100 yr Design) BMP Capacity (CF) 500 22,647 39,079 58,152 LID elements (green roofs, pervious pavers, cisterns) (CF)** none Notes: none none 12,838 Total Capacity 500 22,647 39,079 70,990 20 yr - 1 hr storm Volume (CF) 39,075 39,075 39,075 39,075 Storm Volume Runoff (CF) 38,575 16,428 -4* -31,915* 50 yr - 1 hr storm Volume (CF) 48,844 48,844 48,844 48,844 Storm Volume Runoff (CF) 48,344 26,197 9,765 -22,146* 100 yr - 1 hr Storm Volume (CF) 60,566 60,567 60,567 60,567 Storm Volume Runoff (CF) 60,066 37,920 21,488 -10,423* * A negative storm volume runoff represents excess design capacity for the storm event. ** For C100, an estimate of capacity for the LID strategies is included for comparison purposes. The actual capacity varies for the loading calculations depending on antecedent moisture due to previous weather. *** E20 results in runoff for the 20-year storm due to the contribution of NDOT and Washoe County ROW. E20 does not include capacity for theses surfaces. Appendix AB of the <strong>FEIS</strong> provides the detailed results and conclusions from the LSPC modeling exercises. The results and conclusions are summarized in the follow bullets: • Dry Water Years (1993-1994 and 2007-2008) – o Implementation of C20 SWMP reduces loading compared to EC in both dry years by roughly 50 percent. o Implementation of C100 SWMP contains the stormwater runoff completely such that there is minimal to no discharges to down-gradient drainage and stormwater systems and ultimately Lake <strong>Tahoe</strong>. PAGE 8- 14 HAUGE BRUECK ASSOCIATES SEPTEMBER 8 , 2010
RESPONSE TO COMMENTS ON THE DEIS B o u l d e r B a y C o m m u n i t y E n h a n c e m e n t P r o g r a m P r o j e c t E I S o Stormwater runoff from the project area occurs in 6-7 days under E20 conditions and 2-6 days for C20 conditions and does not occur for C100 conditions. o Total sediment, fine sediment and nutrient loads are presented in Tables 2 and 3 of Appendix AB for a variety of wet and dry water year conditions. Field monitoring of runoff from disturb soils indicates FSP load is 50 percent of TSS loads from granitic soils and JBR data (DEIS Appendix P) reported levels as high as 90%. For modeling and reporting purposes for the supplemental analysis, FSP of less than 20 microns are reported as 60 to 90 percent of the TSS load. The FSP load of less than 20 microns includes FSP loads of less than 16 microns. For E20, C20 and C100, a dry year is forecasted to contribute 4,374 lbs, 1,714 lbs and 134 lbs of FSP, respectively. o The C100 SWMP will reduce total sediment loading by 97 to 100 percent as compared to E20 SWMP. • Wet Water Years/Worst-Case Scenarios (1994-1995 and 2005-2006) – o E20 conditions result in possible discharge of approximately 17,000 to 32,000 lbs of total sediment (note that the DEIS reports approximately 34,000 lbs based on grab sample extrapolations) and 14,000 to 29,000 lbs of FSP leaving the project area. o Implementation of C20 reduces the landing compared to E20 by roughly 23 to 43 percent to ranges of 13,000 to 19,000 lbs of total sediment and 8,000 to 18,000 lbs of FSP. o Implementation of C100 reduces loading compared to E20 by roughly 88 to 92 percent to ranges of 1,400 to 3,900 lbs total sediment and 800 to 3,500 lbs of FSP. o Stormwater runoff from the project area occurs on 34 to 42 days under E20 conditions, 16 to 27 days for C20 conditions and 3 to 5 days for C100 conditions. o Stormwater runoff from the project area occurs under C100 conditions only for a substantial rain-on-snow event of 5.37 inches (New Year’s Eve 2005) and after sequential 2 inch rain-on snow events (January 2005). o Data shows that in wetter years, which represent worst-case scenarios, total sediment output from C100 SWMP is over an order of magnitude (10X) less than those produced by E20. Please see Appendix AB for supplemental analysis for additional quantification of surface water quality benefits, notably predicted load reductions for total sediment, fine sediment and nutrients. The analyses for Impacts HYDRO-1 and HYDRO-3 are presented in the format determined and reviewed by TRPA Staff. The supplemental water quality analysis does not change the analysis for HYDRO-1 and HYDRO-3 but provides additional quantified results that support the conclusions that potential impacts discussed in HYDRO-1 and HYDRO-3 are reduced to a level of less than significant. SEPTEMBER 8 , 2010 HAUGE BRUECK ASSOCIATES PAGE 8- 15
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REVISIONS TO THE DEIS B o u l d e r
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FEIS - Tahoe Regional Planning Agency

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