HomeMy WebLinkAboutTR 35337 HYDROLOGY REPORT-023 Spyglass Ranch Tract 35337
Hydrology Study
scenarios in order to determine the required interim and ultimate
condition facilities. In short, the developed condition storm flows will be
discharged to the matching existing drainage course under the Stage 1
condition and conveyed to the west to join the Wasson Canyon Creek.
Under the Ultimate condition, the developed condition storm flows from this
drainage area will connect to the South Shore storm drain system to be
conveyed to the South Shore drainage basin.
Area "D" and Basin #3
Area "D" is a significant drainage area in the post-developed condition
that is comprised of one-quarter acre residential development in the
south-easterly vicinity of the site. The primary comparable drainage area
in the pre-developed, or existing condition is Area 9. This area is entirely
tributary to Basin #3.
Since, Area "D" is impacted differently under each of the Stage 1 and
Ultimate scenarios, this study evaluates the drainage area under both
scenarios in order to determine the more critical condition. In short, the
Stage 1 developed condition will generate a higher tributary area than the
Ultimate developed condition will generate at the drainage area's outlet
point. Therefore, the Stage 1 scenario drainage area is used in modeling
the unit hydrograph for Area D (UHD) that is used in the basin routing
analysis.
The unit hydrograph analysis for this sub-area is evaluated for two
areas: (1) Unit Hydrograph "D" (UHD) for the Stage 1 Grading Design, and
(2) UHD for the Stage 1 Grading Design Tributary to Basin #3. UHD under
the first condition generates a higher tributary area to the adjacent
downstream development primarily due to the offsite grading in that
vicinity. Accordingly, the first hydrograph is used in comparison with the
Existing Condition Unit Hydrograph 9 (UH9) in order to determine the total
required mitigation. The second hydrograph is used in the basin routing
analysis to determine the detention basin performance. Exhibits for each
of the hydrographs are provided in the Exhibits section of this report.
Table 10 provides a summary of the existing vs. proposed hydrograph
results and the corresponding Q reduction required in order to match the
historic flows.
Table 10: Unit Hydrograph Summary - 10-Year Storm Frequency
Duration QPK (cfs) UH9' QPK (cfs) UHD2 Required QPK (cfs)
(hr) (Existing) (Proposed) reduction
6 32.8 35.0 2.2
24 10.7 10.7 NIL
Notes:
1. UH9 = Unit hydrograph for Area 9.
2. UHD = Unit hydrograph for Area I'D" under Stage 1 Grading Design.
R: \265324\Final\llydro1ogy\Rcporl\324 Report 13ody.doex - 19 -