HomeMy WebLinkAboutTR 35337 HYDROLOGY REPORT-017 Spyglass Ranch Tract 35337
Hydrology Study
The following sections provide a brief description of each drainage area
and the drainage basins along with the general results and computation
summaries. The hydrology computations and output files are provided in
the Appendix of this report.
Area "A" and Basin #2
Area "A" is a major drainage area in the post-developed condition that
is comprised of about half of the west side of the site. The closest
comparable drainage area in the pre-developed, or existing condition based
upon the drainage outlet is Area 1. One may observe that Area 1 is much
smaller than Area "A". This is due to the proposed onsite grading that
causes a large portion of the existing drainage area that is tributary to
the adjacent downstream property to the west, PM 30985, to be redirected
to the Spyglass Ranch drainage basin, which is near the existing Area 1
outlet point. This localized diversion is not an issue for three reasons: (1)
the onsite basin is designed to detain the increased runoff from the
drainage area, (2) the reduced runoff received to PM 30985 does not create
a potential adverse condition, and (3) the runoff from this area is
discharged to the Wasson Canyon Basin, a publically owned drainage facility.
Area "A" is completely tributary to Basin #2. The higher storm runoff
that enters the basin from this drainage area is mitigated at a ratio of
about 0.6 to 1 - in other words the basin can detain about 60% of the
volumetric flow rate delivered to it. The runoff from this area will drain
through the detention basin and into the existing 61x5' RCB culvert through
the basin's outlet pipe located in Camino Del Norte. The basin is sized to
retard the necessary flow such that the approved overall total flow rate of
the existing RCB culvert is not exceeded. Basin #2 is also designed as a
water quality facility that will treat the first flush flows from the entire
tributary area as required by NPDES guidelines.
Table 4 provides a summary of Basin #2 Volume vs. Depth relationship,
as utilized in the routing analysis. Table 5 provides a similar summary of
Basin #2 Volume vs. Depth relationship, except as related to the required
water quality treatment.
Table 4: Basin #2 - Volume Vs. Depth
Contour Contour Volume Total Total Basin
Elevation Area (CF) Volume Volume Depth
(SF) (CF) (Acre-FT) (FT)
1300.00 55600 27275 287028 6.5893 8.00
1299.50 53500 26225 259753 5.9631 7.50
1299.00 51400 25200 233528 5.3611 7.00
1298.50 49400 24175 208328 4.7826 6.50
1298.00 47300 23150 184153 4.2276 6.00
1297.50 45300 22150 161003 3.6961 E5.50
1297.00 43300 21175 138853 3.1876 5.00
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