FEIS - Tahoe Regional Planning Agency

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
reader. The calculations that were performed are summarized in Tables 4.3-5 through
4.3-12. DEIS Appendix G provides the supporting data. The data that were used are
based on the contributing areas, impervious surface areas, and the treatment
capacities proposed. These numbers and calculations vary in accordance with the
characteristics of the Alternative. The LID strategy assumptions used for the
calculations are outlined in DEIS Appendix R.
Comment 335-ab: Comment Summary - App. P should describe the certainty of the estimates based on
the limited sampling, and how the events compare to 20-, 50-, and 100-year events.
DEIS Appendix P presents the results from the pre-project monitoring completed for
purposes of characterizing the existing conditions of the project area and to
appropriately design and size the proposed stormwater treatment systems. The storm
events are summarized on pages 4.3-5 to 4.3-7 of the DEIS and detailed in Appendix
P.
Comment 335-ac: Comment Summary - Clarify fine sediment sampling methods and results.
As stated in DEIS Appendix J and reiterated in the DEIS hydrology analysis, the
loading estimates should be used with caution because of the small sample size. See
Appendix AB for supplemental surface water quality analysis that further supports
the level of impact conclusions for HYDRO-1 and HYDRO-3. Automated samplers
are not currently installed. The range is large because the loading estimates are
presented by "event basis", meaning that 253 lb/day was measured for one storm
event and 9,947 lbs/day was measured for a separate event. The range provided in
the DEIS states the range calculated for the 6 events. See Appendix AB for annual
loading estimates. The 34,450 lbs/yr is within the predicted range of 12,245 lbs of
total sediment in a dry water year (1993-94) to 52,825 lbs of total sediment in a wet
water year (1994-1995). Additional estimates for other water year scenarios are listed
in Tables 2 and 3. The fine sediment analysis for the events did differ slightly as
explained in the JBR report. There is no codified regulation that requires analysis of
fine sediment at this time. The analyses were completed voluntarily to characterize
the project area and appropriately design and size stormwater treatment systems.
Comment 335-ad: Comment Summary - Define the terms "effective coverage", "Minimum capacity
(TRPA Coverage): 100% of 50-year/1-hr storm” and “Minimum Capacity (TMDL
Reduction Coverage): 100% of 100yr/1hr storm" in relation to TRPA and other
agency regulations.
As stated on page 4.3-29: Effective coverage is defined as subset of total impervious
area that is hydrologically-connected via sheet flow or discrete conveyance to a
drainage system or receiving body of water (Washington State University 2005). This
is a standard term used when discussing strategies for direct reductions in runoff
contributions or attenuation of peak runoff volumes. TRPA does not regulate
effective coverage. Assuming that the commenter is referring to DEIS Appendix R,
Minimum Capacity (TRPA Coverage) means based on TRPA defined land coverage
and Minimum Capacity (TMDL Coverage Reductions) means that LID effective
coverage reductions are included. The calculations in support of the 100yr/1hr storm
volume capture are summarized in Table 4.3-12.
Comment 335-ae: Comment Summary - Provide data supporting the performance of pervious pavement,
the sweeper plan, green roofs, and SWT bio-retention systems for reducing fine
sediments and nutrients.
PAGE 8- 150 HAUGE BRUECK ASSOCIATES SEPTEMBER 8 , 2010