HomeMy WebLinkAboutDrainage Designj;ATTISENGINEERING.
ENGINEERS * CONSU LTANTS
Apri I 5, 2007
Carol Cotter, EIT
Engineering Servicess
City of College Station
College Station, Texas 77845
Re: J & S Studies Site Drainage Design Summary Letter
~ Lot 3, Block 2 of the Crescent Pointe Phase 1 Development
Dear Mrs. Cotter,
As per the Drainage Report for Lots 1 -4, Block 2 of Crescent Pointe Phase 1 Development by Kling
Engineering & Surveying dated August 11, 2006, the storm water run-off produced by the development of
Lot 3, Block 2 shall be captured and conveyed to the existing 30" HDPE storm sewer pipe on the
southern portion of the lot which subsequently outfalls to the regional detention facility (Pond lB) for this
development.
Lot 3 of Block 2 is 1.003 acres of Drainage Area lB which consists of 17.66 acres (pre-developed). This
lot consists of native trees and grassland with a general 5% grade from the north to the south. The
proposed development will consist of concrete paving, sidewalks and two office buildings to be done in
two phases. The development figures are as follows:
PROJECT HYDRAULIC DATA
Description Area 'C' Factor Weighted 'C'
Phase 1 Building 4,344 s.f.
Phase 2 Building 4,099 s.f.
All Sidewalks 985 s.f.
All Pavement & Drainage Flumes 18,341 s.f.
Landscaped I Undisturbed Areas 17,098 s.f
Total Area= 44,867 s.f.
Wtd.
Description Tc 'C' Area 2yr
J & S Studies 10 0.717 1.03 ac 4.67
*Brazos County Rainfull Intensity data used
DRAINAGE DESIGN CRITERIA
Drainage Area 1 B
Allowable Weighted 'C' Factor
=
=
17.66 ac.
0.77
0.90 0.087
0.90 0.082
0.90 0.018
0.90 0.370
0.42 0.160
Weighted 'C' = 0.717
Q=CIA -Storm Events (cfs)*
5yr lOyr 25yr 50yr
5.67 6.37 7.28 8.23
As per the drainage design report referenced above, Drainage Area IB (Developed) has a
lOOyr
8.60
Weighted 'C' Factor of 0.77. The proposed J & S Studies Medical Center's proposed Weighted 'C' is
less than allowable design at 0. 717 resulting in the design flows listed above. This illustrates that the
proposed development is within limits of the drainage design criteria as established by the drainage
design report referenced above.
Gattis Engineering J & S Studies Medical Center
.. 'It'
30" DRAINAGE PIPE CAPACITY
The run-off from the site shall be collected via two area inlet grates and discharged into the existing 30"
HDPE pipe at the rear of the property. The 30" pipe has a Manning Roughness of 0.012 and is at a
current grade of -0.99% (per referenced drainage report). With these specific hydraulic characteristics,
the pipe capacity = 44.15 cfs. Per page 7 of the Crescent Pointe Drainage Report under Storm Sewer
Facility Design table, the J & S Studies (Lot 3, Block 2) project site projected flows were taken into
account for the storm sewer design with I 00 yr Storm Event flows equaling 35.48 cfs, which is less than
capacity.
Please accept this Jetter as confirmation of compliance of the site storm water mitigation and design as
described in the Drainage Report for Lots 1 -4, Block 2 of Crescent Pointe Phase l Development by
Kling Engineering & Surveying dated August 11, 2006.
Respectfully,
GATTIS ENGINEERING, LLC
e . Gattis, PE
ic nsed Professional Engineer
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$ P 0. Box 13461 • College Station, Texas 77841 • Tel. 979.575.5022 • Fax 979.268.0151 $
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