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CONSULTING ENGINEERING & LAND .. SURVEYING
4444 CARTER CREEKPKWY,SUITE10B-BRYAN. TEXAS 77802 .(409) 846-2688 (409) 846-3094
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January 31,.1994
Mr. David. Pullen,P.E.
City Engineer
DepartmentOfDeveloplnent Services
P.O. Box 9960
College Station, Texas. 77842-0960
RE: Detention Pond Facilities Associated With The Wilson Subdivision~
College Station, Brazos County, Texas.
Dear David,
Based on the assumption that the above Inelltioned. detention facility may be over-sized or may not be
required as part of stipulations required in the drainage ordinance, we feel it necessary that we review the
project once again to>determine if indeed it is possible for . us to scale the facility down or even eliIninate
the detention facilitieselltirely.
In reviewing the contents of the "Drainage(~olnpuL:1.lions For Wilson Subdivision" as submitted to the
City for staff review and consideration, it can be seen .t11at the volulne ofstol1n.water run-off in both the
predeveloplnentand postdeveloplneutconditions is relatively minute... The predeveloplnent and post
developInentstormwaterrun-off peak. discharge rates.3re tahulatedas follows:
Stonn
Frequency
PredevelopIuent PostdevelopInent
Discharge Dischlrrge
Increase
(Volulue)
Increase
(Percentage)
2-year:
5-year:
10-year:
25-year:
50-year:
100-year:
1.28c.f.s.
1.90c.f.s.
2.13 c.f.s.
2.43 c.f.s.
2.75c.f.s.
3.10c.f.s.
~.l)4c.f.s.
3.01 c.f.s.
3.37c.f.s.
3.87 c.f.s.
4.38 c.f.s.
4.93 c.Ls.
0.75 c.f.s.
t.12 c.f.s.
t.24c.f.s.
1.44 c.f.s.
1.64 c.f.s.
1.83 c.f.s.
59 %
59 %,
58%
59%
60.%
59 Ok)
FrOIn. the approved McClure Engineeringtsdrainage report forSpring.BrookSubdivision it can be. seen
that thethreebarrcllOxJ o reinforced concrete box culvert is the controlling structure for Spring Brook
Subdivision and the area iInlnediatelytrihutary to Spring Brook Subdivision. l]lcWilson 1.00 ALTe
Subdivision lies within the basin tributary to the SaIne box culvert structure.
Basically speakingtheareaupstremnand adjacent to Ole suhjectbox culvert functions as a detention pond
with the cui vertstructurelnetering the slonnwater run-off to areas downstream towards Carters Creek.
The tilneofconcentratiouforthebasin trihutnry to the. box culvert including Spring Brook. Subdivisnon is
approximalely38millUtcs(O.63.l1ours) [orthc.posldcvcloprnent conditions [or Spring Brook Subdivision.
The Tilne of concentractiollfortheWiIson Subdivision relative to the subject box culvert is. esliInated to
" .' .;., ..'1
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be 15 minutes (0.25 hours). Comparing the twoaforemeIltioned times of concentration it can be seen that
the run-off from the . Wilson Subdivision will. reach. the. box culvert structure . approximately 23 minutes
prior totherun-off frolnthe. Spring Brook Subdivision reaches the same location. This. 23 minute . lag
time >would certainly allow the run-off from the Wilson Subdivision topassthrougbthe box culvert
structure prior to. the arrival of the run-off frolntheSpring . Brook Subdivision.. It. is with this reasoning
that on-site detention within the Wilsol1Subdivision appears to be counter productive to the storm water
management plan as the.. lag created by the. metered discharge from the Wilson Subdivision would
coincide with the time of concentration for the Spring Brook Subdivision producing a higher rate of.. run-
offal tbeculvert structure.
We recommend based on the above presentation tbaton-site detention be removed from the scope of
deveIopmentoftheWilsonSubdivision as the metered discharge from the detention facilitywould serve to
compound the impact of developlnent upon the basin tributary to the aforementioned culvert structure at
Texas Highway 6.
I hope you will find items contained herein adequate for your approval of the deletion of the detention
facility from theWilson..Subdivision,. boweverif you.have allY. additional questions or probleIns..with same
items, pleaseconmct me at your earliest convenience (409) 846-2688.
7.-3#4 ':7
DonaldD. Garrett,P.E.,.R.P.L.S.
Garrett Engineering
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Copie: Tom Wilson
Ray Patranela
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WILSON PLUMBING COMPANY, INC. 106 Walton Dr. P.O. Box 9083 College Station, TX 77840
Texas Master Plumbing License 14526 (409) 696-4219