HomeMy WebLinkAboutTR 35337 HYDROLOGY REPORT-027 Spyglass Ranch Tract 35337
Hydrology Study
Table 15: Basin #3 Detention Basin Route Summary
Duration QPK (cfs) UHD QPK (cfs) QPK (cfs) Max Water
(hr) into basin UHD out of reduction Depth in
basin Basin (ft)
6 31.0 16.3 14.7 4.6
24 9.5 8.4 1.1 1 3.4
Notes:
1. UHD = unit hydrograph for Area "D"
2. Q is 10—Year Strom Frequency.
Area "D" contains minor sub—areas 11D.18" and "D.19," which are located
along the east Spyglass Ranch boundary, that are comprised of natural
area and manufactured slope area. These minor sub—areas are correlated
to existing Areas 6, 7, and 8 for purposes of comparison. These areas are
not tributary to Basin #3, as is the main portion of Area "D." These areas
will drain to the east onto the adjacent downstream property, Tract 32013.
The minor developed condition sub—areas produce a lower tributary
drainage area when combined and compared to the correlating existing
condition drainage areas under both the Stage 1 and Ultimate scenarios.
The rational analysis indicates that the total developed condition peak flow
rate of Sub—Areas "D.18" and ,D.19" is less than the total pre—developed
condition peak flow rate of Areas 6, 7, and 8. It follows, consequently,
that no storm mitigation is required for these sub—areas.
Area "E"and Basin-4
Area "E" is a major drainage area in the post—developed condition that
is comprised of one—quarter acre residential development, multi—family
development, and commercial land use. The comparable drainage area in
the pre—developed, or existing condition is Area 12. The first flush, or low
storm flows for Area "E" will be designed to bifurcate into Basin #1 for
water quality treatment. Because there is a favorable percolation rate at
this site, Basin #1 is designed as an infiltration basin. The percolation
rate based upon the percolation evaluation that was performed at the basin
site is 5 minutes—per—inch.' The corresponding design infiltration rate
based upon RCFCD guidelines is 2.37 inches—per—hour. The complete basin
design worksheets are provided in the WQMP.
The high storm flows are designed to discharge into the existing 54"
culvert located in Camino Del Norte. The post—developed tributary area is
significantly less than the pre—developed area to the same outlet point.
The rational analysis indicates that the total developed condition peak flow
rate of Area "E" is less than the total pre—developed condition peak flow
rate of Area 12. By in large, this is due to the reduced drainage area in
the post—developed condition that is being conveyed to the existing 54"
culvert. It follows, consequently, that the capacity of the existing culvert
will not be exceeded and therefore no storm mitigation is required for this
t Ref: "Limited Percolation Evaluation," by GeoTek, Inc. prepared July 22, 2009.
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