HomeMy WebLinkAboutTR 35337 HYDROLOGY REPORT-021 Spyglass Ranch Tract 35337
Hydrology Study
Per the PM 30985 grading and improvement plans, the approved design Q
for the existing RCB culvert is 185 cfs in the Spyglass Ranch pre—developed
condition. In the Spyglass Ranch post—developed condition, the total Q
delivered to the existing RCB culvert is 181 cfs, which is within the
approved design Q of 185 cfs. Table 9 summarizes the total resulting flows
entering the existing RCB culvert in the proposed condition.
Table 9: Post—developed Q Summary to Existing RCB Culvert
Hydrology Q
Runoff Source Description Map Node (cfs)
Number
PM 30985 Onsite Runoff n/a 40
Spyglass Area "A" (Basin #2 outlet)' 0.120 73
Spyglass Area KB" via PM 30985 0.202 43
Spyglass Minor Runoff 207;209 3
Street runoff (catch basins at PM 30985) n/a 18
Street runoff 144 4
Total Q to RCB Culvert: 181
Notes:
1. Q shown from Basin #2 outlet is post mitigation.
Area KB"
Area "B" is a significant drainage area in the post—developed condition
that is comprised of a multi—family parcel and an offsite tributary area.
The closest comparable drainage area in the pre—developed, or existing
condition based upon the drainage outlet is Area 2. One may observe that
Area 2 is much larger than Area "B". This is due to the proposed onsite
grading that causes a large portion of the existing drainage area that is
tributary to PM 30985 to be redirected to the Spyglass Ranch drainage
basin and away from the existing Area 2 outlet point at the common
boundary.
The post—developed runoff from this area will drain onto PM 30985, as it
does in the existing condition. PM 30985 is to provide a storm drain inlet
structure to intercept this runoff. The peak flow rate in the developed
condition that will discharge to PM 30985 at the common boundary is 43
cfs, which is significantly less than both the historic flow rate at that
location, and the approved flow rate anticipated by PM 30985 of 128 cfs.
Therefore, no mitigation is required at this location. Tables 2 and 3 provide
a complete summary of the rational method analysis for each drainage
area.
The low flows that require water quality treatment for this area
originate primarily from the multi—family building pad. Since the site plan
is not available at this time, the impervious fraction cannot be calculated
accurately. Accordingly, the applicable BMP devices will be designed and
installed in accordance with NPDES guidelines at the time the site plan is
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