HomeMy WebLinkAbout37 Development Permit 255 Southwood Section 30DRAINAGE COMPUTATIONS
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For
Tract 11 8 11 and Tract 11 C 11
Southwood Valley -Section 30
College Station, Brazos County, Texas
Prepared By:
Garrett Engineer in g
4444 Carter Creek Parkway -Suite 108
Bryan , Texas 77802
Telephone : (409) 846-2688
*July, 1994 *
DRAINAGE COMPUTATIONS
For
Tract "B" & T rac t "C" Southwood Valley -Section 30
College Station. B razos Counry, T exas
* Jul y, 199 4 *
SUMMARY
111e project i s located adjac ent to W elsh Avenu e and B al co nes Ddve in Coll ege Station , Brazos
County, T exas. Tract "D" h as bee n sub divided into 4 l ots and Tract "C" h as bee n subdivided into
5 l ots. The av erage l ot size i s 1.01 acres in Tract "B " and the lots in Tract "C" average 0 .9 5 acres.
Detwee n Tracts "B" and "C" th ere ex i sts a drainage channel that co ntinues into B ee Cree k . T11 e
upstrea m draina ge area for thi s drainage channel i s 49.9 4 ac res as show n on th e topographic map
of College St atio n, located in the Appendix sectio n of thi s repoit. Of this 4 9.94 acres, 20.22 acr es
are pervious, and 29.72 acres are impervi ous. The run-off coefficient for this upstream area is
0.75. 111e maximum tim e of concentraUon i s 47.88 minutes, following the patJ1 as shown on th e
aforementioned topograpl1i c map of College Station.
From the computaUons on th e f ollowing pa ge, based on th e upstrea m drainage area as previously
di sc ussed, th e r el ative predevelopment: peak discha rge rates ("Q") were dete rmined. 111e RaUonal
M etJ1o d was utrn zed to co mpute th e es tim ated pea k pred eve l op ment discharge rates. 111e equaUon
that r epres ents IJ1 e RaUonal M eth od i s as follows: Q = CI A, wher e "Q" i s th e peak discharge rate
in c ubic feet per seco nd , "C" i s th e predeve l opme nt run -off coe ffi ci ent, "I" i s th e storm inte nsity in
inches per hour, and "A" i s th e area of th e up strea m drain age basin in ac r es.
Due to th e upst r ea m run off, a 60" pipe c ul ve 1t will be placed at th e beginning of th e previous ly
di sc ussed drai nage channel that i s loca ted betwee n Tra ct "B" and Trac t "C." This 60" pipe will
ca1Ty flows up to and including th e 25 year stor m . A co ncr ete drain age swa l e will be pl ace d over
th e pipe culvert to carry anything ove r tJ1 e 25 yea r storm . The difference in tJ1 e 25-year and 100-
year storm i s 3 1.04 cubic fee t per seco nd . By the co mputati o ns on tJ1 e following pag e, tJ1 e 2.0 foot
deep trapezoidal swa l e will fl ow 0.8 4 feet deep during th e JOO year storm . The swal e will be IO'
wide, flow 5 .36' wide, have a botto m width of 2', and have sides l opes of 2: I. At thi s geometry, th e
swa l e will caJTy 31.32 cubic fee t per seco nd .
Determine Total Pre -Developm en t
Peak Storm Water Discharge Rate s
Tributary Area ("A"): 49 .94
Pervious Area :
Impervious A rea:
Run-Off Coefficient ("Cwt"):
Time Of Concentration ("T/c"):
Hourly Intensity Rates ("I"):
Peak Discharge Rate ("O"):
Flow Calcu lations for Swale :
Acres
20.22 Acres c = 0.40
c = 0.98 29.72 Acres
0.75
Lengt h Slope Velocit y
(Feet) (Ft I Ft) (Ft I Sec)
Woodland 580 0 .004 0 .26
Street 1 550 0 .007 1.94
Street 2 410 0 .005 1.38
Street 3 130 0 .012 3.27
Total Travel Time : 47 .88 Minutes
2-Year: 2.54 Inches I Hour
5-Year: 3 .21 Inches I Hour
10-Year: 3 .69 Inches I Hour
25-Year: 4 .26 Inches I Hour
50-Year: 4 .86 Inches I Hour
100-Year: 5 .09 Inches I Hour
2-Year: 94.47 Cubic Feet I Second
5-Year: 119 .36 Cubic Feet I Second
10-Year: 137.30 Cubic Feet I Second
25-Year: 158.39 Cubic Feet I Second.
50-Year: 180.85 Cubic Feet I Second
100-Year: 189.43 Cubic Feet I Second
W : 8 .72 Feet
D: 0 .84 Feet
B: 2 Fe et
S: 0 .0179 Ft /Ft
0 : 31 .32 Cubic Feet I Second
Time
(Min)
37.54
4.73
4 .94
0.66
DRAINAGE COMPUTATIONS
For
Tract 11 8 11 and Tract 11 C 11
Southwood Valley -Section 30
College Station , Brazos County, Texas
Prepared By:
Garrett Engineer in g
4444 Carter Creek Parkway -Suite 108
Bryan , Texas 77802
Telephone : (409) 846 -2688
*July,1994*
DRAINAGE COMPUTATIONS
Fo r
T ract "B" & T ract "C" South wood Vall ey -Sectio n 30
Coll ege Stati o n, B ra zos Co un ty, Texas
*Jul y, 1994 *
SUMMARY
Th e proj ect is located adj ace nt to W elsh Ave nu e and B alcones D rive in C oll ege Statio n, Brazos
County , Texas. Trac t "B " h as been s ub di vided into 4 lots and T ract "C" h as been subdivided into
5 lots. The aver age lot si ze i s 1.0 1 ac res in T rac t "Il " and tJ1 e lots in Trac t "C" average 0 .95 acres .
B etween T racts "B" an d "C" th ere ex i sts a drain age channe l tJ1 at co ntinu es into Bee C reek. The
u pstream drai nage area fo r tJ1i s drai nage c ha nn el i s 49 .94 acres as show n o n the to pogra pllic ma p
of Coll ege Statio n, located in tJ1 e Appe ndix sect1o n of this re po rt. Of Uli s 4 9 .94 ac res, 20.22 ac res
are pervi o us, and 29.72 ac res are impe rvi ous. The run-off coefficient fo r thi s up s trea m area is
0.75. The maxi mum ti me of concentra ti o n is 4 7.88 mi nu tes, foll owin 1e path as how o n th e
afore me nti o ned to pograplli c ma p of Co ll ege Sta ti o n.
From th e computaOo ns o n th e fo ll owin g page, b ase d o n tJ1 e up strea m drain age area as pre vi o usly
disc ussed , the relative predevelo pm e nt peak disc harge rates ("Q") were de tennined . The R ational
Method w as uOli zed to compu te th e es tjn rn ted pea k prede ve lopm e nt disch arge rates. 111e eq uaOo n
tl iat re presents th e R a Oo nal Me tJ1 od i s as fo ll ows: Q = CIA, wh ere "Q" i s th e peak disc h arge rate
i n c ubi c feet per second , "C" is th e predevelop me nt run -off coeffi cient, "I " i s t11 e storm inte nsi ty in
in ches pe r hour , a nd "A" is th e area of th e u pstrea m drai nage b as in i n acres .
D ue to th e u pstre am run off, a 60" pipe c ul vert will be placed at tJ1e beginning of tJ1 e previ o usly
di scussed drain age cha nn e l th at is located be tween Tract "Il " and T rac t "C ." The up s tream wa te r
elevati o n at the c ul ve it is 278.52 ms!. At tJ1 e outl e t e nd of th e cul vert , th e 25 year w ate r surface
elevaOo n is 276 .20 msl, a nd tJ1 e 25 year peak d isc h arge rate is 15 8.39 cubi c feet pe r second . 111e
di ffe re nce betwee n these tw o water surface eleva ti ons i s 2.32 feet By Ul e compu ta Oo ns o n th e
fo ll owin g p age, th e nlinimum required hea d for a 250 foot lo ng, 60 in c h di a meter concre te pipe,
a nd 160 .29 cubi c feet pe r seco nd disc harge is 2.3 1 fee t, th us makin g th e e xi stin g he ad of 2 .32 fee t
accep t a bl e.
I
I
I
11
I I
I
I
I
,
I
I
'
' I
' I
-.
Determine Total Pre-Development
Peak St orm Water Discharge Rates
Tributary Area ("A"): 49 .94 Acres
Pervious Area : 20 .22 Acres
Acres
c = 0.40
c = 0 .98 Impervious A rea : 29.72
Run-Off Coefficient ("Cwt"):
Time Of Co ncentration ("T/c"):
Hourly Intensity Rates ("I"):
Peak Discha rge Rate ("Q "):
Head Requirement ("H "):
D :
L:
Q (25):
H:
0 .75
Length Slope Velocit y
(Feet) (Ft I Ft) (Ft I Sec)
Woodland 580 0.004 0 .26
Street 1 550 0 .007 1.94
Stre et 2 410 0.005 1.38
Stre et 3 130 0.012 3.27
Total Travel Time :
2-Year: 2 .54
5-Year: 3.2 1
10-Year: 3 .69
25-Year: 4.26
50-Year: 4 .86
1 00-Year: 5 .09
2-Year: 94.47
5-Year: 119.36
10-Year: 137.3 0
25-Year: 158.39
50-Year: 180.85
100-Year: 189.43
5 Feet
250 Feet
47 .88 Minutes
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Cubic Feet I Second
Cubic Feet I Second
Cubic Feet I Second
Cubic Feet I Second
Cubic Feet I Second
Cubic Feet I Second
158.39 Cubic Feet I S econd
2.31 Fe et
)(
Time
(Min)
37.54
4 .73
4 .94
0 .66
22 October, 1996
Mr. Steve Homeyer
Engineer
City of College Station
P.O . Box 9960
College Station , Texas 77842-9960
RE : Your Letter of 10/14/96
Dear Steve :
I am in receipt of your letter of 10/14/96. Our intentions are to move
equ ipment to lots 81 and 84 within the next 10 days . Presently, the equipment
is operating at the corner of Hwy . 30 and the East Bypass and should be freed
up within that time period . Also , thank you for the reminder about the silt screen .
This will also be addressed .
Thank you for your i nterest.
Sincerely ,
<Mf\_----
Jeffrey L. McDowell
Rf.Qlll tl( Texas Heritage REAL TORS•
4101 S. Texas Avenue, Suite B
Bryan, Texas 77802
Office: (409) 846-4500
Fax: (409) 846-9378
Each Office Independently Owned and Operated
GARRETT ENGINEERING
COllSUL TlllG EllGIHEERJllG & LAllD SURVEVltlG
4444 CARTER CREEK PARKWAY, SUITE 108 -BRYAN , TEXAS 77102 -(409) 14'-2681 -(409) 84'-1094
TO: 6-r! Pr /5. Dale: 7 -27 -9;/
RE:
We Are S e ndin g You : ___!!:_ HcrewiU1 __ Under Se parate Cover The Following:
Copies Date Description
_&_ 7-Z-7 .2/v/i,4~ a;E/faS {)& ~5d~ fe,e 5°0cJl?IW''10) u~
I 7-?7 ~,J4~ 6~7/o./s-RRPCJ.er'
R emark s :
cc:
DEVELOPMENT PERMIT
PERMIT NO. 255R-EXT
FOR AREAS INSIDE THE SPECIAL FLOOD HAZARD AREA
RE : CHAPTER 13 OF THE COLLEGE STATION CITY CODE
SITE LEGAL DESCRIPTION:
Southwood Valley -Section 30; Lots CS & Bl
SITE ADDRESS:
701 Balcones
OWNER:
Jim Woods I J eff McDowell
DRAINAGE BASIN:
Bee Creek -Tributary B
TYPE OF DEVELOPMENT:
RECE\VED MAY 2 3 \996
~~
This permit is an extension of a previous permit and is valid for grading of lot CS only. The
Contractor shall provide erosion control in accordance with the approved erosion control plans.
The Contractor shall take all necessary precautions to prevent silt and debris from leaving the
immediate construction site and entering city streets or area creeks in accordance with the City
of College Station Drainage Policy and Design Criteria.
Although construction has been completed on lot Bl, the Contractor shall assure that
establishment of vegetation occurs on both lots prior to removal of any silt fencing or hay bales
used for temporary erosion control. Contractor shall be responsible for any damage to adjacent
properties, city streets or infrastructure due to heavy machinery and/or equipment as well as
erosion, siltation or sedimentation resulting from the permitted work.
This permit expires six months from the date of issuance.
In accordance with Chapter 13 of the Code of Ordinances of the City of College Station,
measures shall be taken to insure that debris from construction, erosion, and sedimentation shall
not be deposited in city streets, or existing drainage facilities .
I hereby grant this permit for development of an area outside the special flood hazard area.
All development shall be in accordance with the plans and specifications submitted to and
approved by the City Engineer in the development permit application for the above named project
and aii of ihe codes and ordinances of the City of College Station that appiy .
Date /
f) /<W/qt,
Co Date
~erJAgent ? /'UJ(9~
Date
DEVELOPMENT PERMIT
PERMIT N O. 255R
F OR AREAS INSIDE THE SPECIAL FLOO D HAZARD AREA
RE: CHAPTER 13 OF THE COLLEGE STATION CITY CODE
SITE LEGAL DESCRIPTIO N:
Lots Bl-B4 & Lot C5; Southwood Valley -Section 30
SITE AD DRESS:
701 Balcones
OWNER:
Jim Woods I Jeff McDowell
DRAINAGE BASIN:
Bee Creek -Tributary B
TYPE O F DEVELO PMENT:
This permit is valid for site clearing in lots Bl -B4 & lot
This is to facilitate surveying for these lots. Permit also
sewer improvements between lots Bl and C5. Contractor
kYoads Sa:d Je
~ Q,,.s,'J
~)rVf/);.__ t\ ~ ~ve
to assure silt and sedimentation does not enter Bee ---·
migration occurs, contractor shall be responsible for cleanup downs
The Contractor shall provide erosion control in accordance with the approved erosion control
plans. The Contractor shall take all necessary precautions to prevent silt and debris from
leaving the immediate construction site and entering city streets or area creeks in accordance
with the City of College Station Drainage Policy and Design Criteria. Contractor shall assure
that establishment of vegetation occurs prior to removal of any silt fencing of hay bales used for
temporary erosion control. Contractor shall be responsible for any damage to adjacent
properties, city streets or infrastructure due to heavy machinery and/or equipment as well as
erosion, siltation or sedimentation resulting from the permitted work.
This permit expires six months from the date of issuance.
In accordance with Chapter 13 of the Code of Ordinances of t he City of College Station,
measures shall be taken to insure that debris from construction, erosion, and sedimentation shall
not be deposited in city streets, or existing drainage facilities .
I hereby grant this permit for development of an area outside the special flood hazard area .
development shall be in accordance with the plans an d specifications submitted to and
roved by the City Engineer in the development permit app lication for the above named project
all of the codes and rdinances of the City of College Station that apply .
Date
~1ii,/1tf-I
q I 1(o /qcJ
Contractor Date
~~'t: o,f,t-/ rf
Date ; Ofler/ Agent
May 9, 1996
Mr. Jim Woods
JHW General Contractors, Inc.
2114 Southwood Drive
College Station , TX 77840
RE: REIMBURSEMENT OF FEE FOR DDP REVIEW
Dear Mr. Woods :
This letter is to reiterate the City's reimbursement of money we collected from you
earlier this week. The grading work on Lot CS, for which you are requesting an
extension, was included in a drainage report we previously reviewed and approved.
Therefore the DDP is being extended without further review .
Since no further review is needed , the money you paid in advance for such a
review is being refunded. A check for $100 will be mailed as soon as it is
available . Please sign and return the enclosed DDP extension, making a copy for .
your own records. You may contact me at 764-3570 if you have any questions .
Sincerely,
Samantha Smith
Engineering Assistant
CITY OF COLLEGE STATION
Pos t Office Box 9960 1101 Texas Avenue
Colle ge Station, Texas 77842-9960
(409) 764-3500
August 3, 1995
J H W General Contractors
2114 Southwood Dr.
College Station , TX 77840
Attn : Jim Woods
RE : SOUTHWOOD VALLEY, SECT. 30 PUNCH LIST
Dear Mr. Woods::
Following is the City Inspector's list of items for Southwood Valley, Section 30 which are
still in need of attention .
I . Build concrete sloped headwal l & flume -section "B"
2 . Place TCS # 018 rock rubble rip-rap
3. Build concrete swale -section "A"
4 . Fin ish grading & seedin g
5. Remove existing handrail
This project has not been finished as of this date , and because of its location, (near the
creek), must be completed prior to our rainy season . We request that these above items
be corrected within four ( 4) weeks from today's date . Your promptness is greatly
apprec iat ed .
VM :djd
cc : Kent Laza , City Engineer
Shirley Volk, Development Coordinator
Jerry Jones, Engineering Technici a n
J fite
(,
Home o f Texas A&M University
VJ bz9cls >J~
J chd ene-<;
< ""s f uA-.-""' ,,12
N 1_ [l'Je, ~ k,s
DEVELOPMENT PERMIT
PERMIT NO. 255
FOR AREAS INSIDE THE SPECIAL FLOOD HAZARD AREA
RE : CHAPTER 13 OF THE COLLEGE STATION CITY CODE
SITE LEGAL DESCRIPTION:
Lots Bl-B4 & Lot C5; 5'011thwood Valley -Secbon 30
SITE ADDRESS:
701 Balcones
OWNER:
Jim Woods I Jeff McDowell
DRAINAGE BASIN:
Bee Creek -Tributary B
TYPE OF DEVELOPMENT:
This permit is valid.for site clearing in Lots Bl -B4 & Lot C5 of Southwood Valley -Section 30.
This is to.facilitate surveying.for these Lots.
The Contractor shall provide erosion control in accordance with the approved erosion control
plans. The Contractor shall take all necessmy precautions to prevent silt and debris from
leaving the immediate construcbon site and entering city streets or area creeks in accordance
with the City l?f College Station Drainage Policy and Design Criteria. Contractor shall assure
that establishment of vege tabon occurs prior to removal of any silt fencing of hay bales used for
tempormy erosion control. Contractor shall be responsible for any damage to adjacent
properties, city streets or infrastructure due to heavy machinefy and/or equipment as well as
erosion, siltation or sedimentation resulting.from the permitted work.
This permit expires six months.from the date of issuance.
In accordance with Chapter 13 of the Code of Ordinances of the City of College Station,
measures shall be taken to insure that debris from construction, erosion, and sedimentation shall
not be deposited in city streets, or existing drainage facilities .
I hereby grant this permit for development of an area outside the special flood hazard area.
development shall be in accordance with the plans and specifications submitted to and
roved by the City Engineer in the development permit application for the above named project
all of the c/.de and ordinances of the City of College Station that apply.
9/B/C?:f
Date
Figure XII
Development Permit
City of College Station, Texas
t
Site Legal Description: 4rs p/, /3t( / ,ltir C-5'-
.J ~ E.S +\. v.:>DvOI
Site Owner: ....)e:ffrici L.... M c.,Oo wC'\.--'-
Architect/ /t
Engineer: &,4tgg,r.,-~,
Contractor : J t-\w \·1.J L-
Date Application Filed : __ t/-1--"'-=Z=---_,9'--,/~--
1
Address : 4101 Te:''iCA> AJE. -Sur% ..B f>vt..J~,"Ty
Telephone : -+di -e,4(.. -4St>C>
Address: -------------
Telephone No : (gq.J ~ i li1t7
Approved: ___________ _
Application is hereby made for the following development specific waterway alterat~ons:
!l?ehcll-rn,w Fote l1/M~~ t/4::::-:/ 1# /Gr> 21-~</ 7 br ~~ ~ 5edn1Je<:>-P i'At.hd -s e_--~f)
Application Fee _.-~~ b{c., t~J-~/-fa{JiU~~~D. _I 1.
Signed Cert1ficat1ons a CJ ().) ~~ ~~
Drainage and erosion control plan , wit h supporting Drainage R epo rt two (l) ~o;fes each .
Site and Construction Plans, with supporting Drainage Report two (2) copies each .
O t her: ------------------------------
ACKNOWLEDGMENTS:
r),(;.;, L <Acw)
/
, des ig n en g ineer, hereb y acknowledge or affirm that:
The information and conclusions conta ined in the above plans and supporting documents comply
with the current requirements of the City of College Station, Texas City Code, Chapter 13 and its
associated Drainage Policy and Des ig n Standards.
As a condition of approval of this permit application, I agree to ~n struct the improvements
proposed in this application according to these documents and the requirements of Chapter 13 of
the College Station City Code . _.../-, . -ti_•~
~Yvt~---u~-;-
Property Owner(s) Contractor
Figure XII Continued
CERTIFICATIONS : (for proposed alterations within designated flood hazard areas .)
A I , J rfAoi~ -k,../i.J , certify that any nonresidential structure on or propo~ed
to be on this site as part of this application is designated to prevent damage to the structure or its
contents ·z sult offlooding from the 100 ye ar storm
~r ~ l'.Ac~J -D-at-e~~~-~z=--~~r./'--~~~~~~~~-
B . I , -:Dw1titl& ~l./1J , certify that the finished floor el evation of the lowest
floor, including any basement, of any residential structure , proposed a s p art of this application is
at or above the base flood elevation established in the latest Federal Insurance Administration
Flood Hazard Study and maps , as amended .
~ ~ 0i,;ldh-) 9 -2-'lc/
C Eng ineer Date
C. I , ~Atrf FC-tf../,.f , c e rti fy that the alterations or d evelopment covered by
this permit shall not diminish the flood-carryin g ca pacity of the waterway adjoining or crossing
this permitted sit e a nd that su c h alterations or deve lopment are con sist e nt with require ments of
the City of College Station Cit y Code, Ch a pter l 3 concerning encroachm ents of flood ways and of
floodw a y frin :es /
_;= {~ ~ UJ~M~ --'--9-_z-__,<Jt/~------c ~n ee r Date
I, ~NM ,J f!,.. lt1!-l , do certify th at the proposed alterati ons do not raise the leve l
of the 100 year flood above elevation establish e d in the latest Federal Insurance Administration
Flood Hazard Sry .
~~ (,..__,) fttr:k,/r ) __ o/1_-z_-_9c/ _____ _
Engineer Date
Conditions or comments as part of approval:
---------~---------~
In accordance with Chapter 13 of the Code of Ordinances of the City o f College Station ,
measures shall be t a ken to insure that debris from construction erosion a:-i d sedimentation shall , ,
not be deposited in city streets , or existing drainag e facilities .
I hereby grant this permit for development. All dev elopment shall be in a cc ordance with the plans
and specifications submitted to and approved by the City Engineer for the above named project.
All of the applicable codes and ordinances of the City of College Stati o n s1 all apply .
fom1..,/a ~'dpcrmilll 0·91
DRAINAGE COMPUTATIONS
For
Marekwoods Apartments
To Be Located On Lot C3 & lot C4 Of A Replat Of
A Replat Of Tract "B" & Tract "C" -Southwood Valley -Section 30
College Station, Brazos County, Texas
Prepared By :
Garrett Engineer ing
4444 Carter Creek Parkway -Suite 108
Bryan , Texas 77802
Telephone : (409) 846-2688
*May, 1994 *
DRAINAGE COMPUTATIONS
For
Marekwoods Apartme nts
To Be Loc at ed
Lot C3 & Lot C4
OJ A Repl at OJ Tract "B" & Trac t "C"
So uthwood Vall ey - Secti on 30
Coll ege Stati o n, Brazo s Coun ty, T e xas
*M ay, 1994 *
SUMMARY
T he proj ect co ns ists of t11 e co ns tru c ti o n o f two a partme nt buildings, assoc ia ted s id e walks, parkin g areas
and drives to be located on Lot C3 & Lot C4 of a Replate of Tract "B" & Trac t "C" -So uthwo od Valle y
Secti o n 30, Coll ege Station , Braz os County, Texas. C urrentl y th e s ubj ect s ite is co ve red by woodlands
with Bee Creek crossin g th e re ar of srun e prop erty . Drainage s tructures a t Wel sh Av enu e and at F.M .
2 8 18 "mete r" s to rm water run-off in Bee C reek , making th e run of Bee Creek imm ediate ly adj ace nt to th e
proj ect function as a large or reg ion al de tenti o n po nd durin g th e s torm s of lesser re turn frequ e ncies. The
in creased run-off du e to t11 e con stru c ti o n of th e proj ec t would ra ise th e curre nt s tonnwater eleva tion s in
Bee Creek very littl e . Du e to th e prox c imity of th e proj ec t s ite to a primary channe l (B ee Creek) It is
anticipa ted th a t de tention fac ilites will n o t be requ ired .
C omputation s supportin g t11 e a bo ve s ta te ments are as follow s:
From t11 e compu tation s on page 1 based o n t11 e proj ec t area o f 1.68 acres as shown on th e afore mention ed
Site & Gradin g Pl a n, th e predevelo pm e nt run-off coeffici ent be in g as s um ed a t 0.40 a nd a minimum tim e
of concentrati o n of 10 minutes tl1 e relative predevelo pm e nt peak di scharge rates ("Q") were de tennined .
Th e inten s iti es sho wn on page 1 were taken fr om "F igure III-1 Int ens ity Duration Freciu e nc y Curves F or
The City Of Coll ei:e Station"as containe d in th e "Drain a ge Po licy and Design Standards for th e City of
Coll e ge Station" ass umin g same minimum tim e of concentrati o n of 10 minutes. The R a tional Me thod was
utilized to compute t11 e es tim a ted peak prede velo pm e nt di scharge rates. T he e quation th a t re presents ili e
R a ti o nal Me t110d is a s foll ows : Q = CIA , wh e re; "Q" is t11 e pe ak di scharge rate in cubic fe et per second .
"C" is th e run-o ff coeffici e nt, in th e prede vel o pm e nt conditi on "C" is as s um ed to be 0.40, "I " is th e stonn
inten s ity in in ches pe r hour a nd "A" whi ch is t11 e area of tl1 e s ite in acres.
F rom th e compu ta tion s on page 2 based on th e proj ect area o f 1.68 acres, th e pos td eve lo pm e nt run-off
coeffici ent be in g ass um ed a t 0 .75 and a minimum tim e of c oncentr atio n of 10 minutes th e re lativ e
pre developm e nt peak di scharge rates ("Q ") w ere de te rmin ed. T he intens iti es s hown on page 1 we re tak e n
fr o m "Figure III-1 Intens ity Duration Frequ enc y C urv e s For T he City Of Coll e ge Sta tion"as contained in
th e "Drainage Poli c y ru1d Des ii:-n Standard s for ili e C ity of Co ll e fl e Station " a ss umin g sam e minimum tim e
o f co ncentra ti o n of 10 minutes . Th e R a ti o nal Me t11 od was utili zed to co mpute ili e es tim ate d peak po st
developm e nt di scharge rates. The equ ation tliat re presents th e Rational Me iliod is as fo llow s: Q = CIA ,
wh e re; "Q" is t11 e peak di scharge rate in cubi c fee t pe r seco nd. "C" is t11 e run-off coeffic ie nt, in ili e po st
developm e nt condition "C" is ass um ed to be 0 .75, "I" is th e stonn inte nsi ty in in c hes pe r ho ur a nd "A "
whi c h is th e area o f tl1 e s ite in acres.
Th e compari so n be tw ee n peak predevelo pm e nt d isc harge rates a nd pos t develo pm ent di sc harge rates is
included as a n ai d to t11 e des ig ne r to es ta bli sh a pre liminary "tar get" as to th e volwn e n ecessary in th e
pro po sed de te ntion facility s hould a de te nti o n fac ili ty be deemed necess ary. Thi s pre liminary es tim a te is
arrived at by s impl y de te rminin g t11 e vo lum e ge nerated fr om th e diffe re nce o f t11 e predevelo pm e nt and po st
development hydrographs (see pages 5-7 for pred eve lopment and post development hydrographs). Please
note that the post development hydrograph shown on t11ese same graphs assumes no detention .
Page 3 and page 4 are tabulations of t11e predevelopment hydrograph and t11e post development
hydrograph bas ed on tlle computations perform ed on pag e 1 and page 2. The post development
hydrograph assumes no detention to facilitate direct comparison in tlle pre and post development
conditions.
The hydrographs are triangular in shape and are based on U1e standard SCS unit hydrograph witll time to
peak set equal to the time of concentration and the total time base set at 2.67 times tlle time of
concentration. Please note that a 26.7 minute total storm event duration wa<; utilized exclusively
tllroughout tllis report due totally to tlle small physical size of tlle project
Based On tlle stonn that produces tlle high es t inte si ty of run-off ( tlle storm equal in duration to tlle time
of concentraction on tlle site (10 minutes) it can be see n by rel ating tlle volume of run-off produc ed by tlle
100 -year storm as derived on pa ge 2 (5971 cubic feet of strorage would be required) . Noting tllat the
approximate area of tlle 100 year flood plain betwee n Welch Avenue and F.M. 2818 is 640,000 square feet
a relation between t11e increase deptll in tlle flood plain elevation can realized as follows:
5971 cubic feet I 640,000 square feet => 0.01 feet (approximate change in flood plain elevation).
Determine Total Pre-Development
Peak Storm Water Discharge Rates
Tributary Area ("A"): 1.68 Acres
Pervious Area : 1.68 Acres
Imperv ious Area : 0 .00 Acres
Run-Off Coefficient ("Cwt"): 0.40
Time Of Concentration ("T/c "):
Woodlands: Low Elevation :
High Elevation :
Distance (Feet ):
Slope (% Grade):
Veloc ity ("Vw "):
Time :
Pastures: Low Elevation:
High Elevation :
Distance (Feet):
Slope (%Grade):
Velocity ("Vp "):
Time :
Pavements: Low E levation :
High Elevation :
Distance (Feet):
Slope(% Grad e):
Velocity ("Vpave"):
Time :
Total Travel T i me:
Hourly Intensity Rates ("I"): 2-Year: 5 .20
5-Year : 7 .69
10-Year: 8.63
25-Year: 9.86
50-Year : 11 .15
100-Year: 12.60
Peak Discharge Rate ("Q "): 2-Year: 3.49
5-Year: 5.17
10-Year: 5 .80
25-Year: 6 .63
50-Year: 7.49
100-Year: 8.47
C= 0.40
C= 0.98
0 .00
0.00
0 .00
0.00
0.00 Feet I Second
0 .00 Minutes
0.00
0.00
0.00
0 .00
0 .00 Feet I Second
0.00 Minutes
0 .00
0 .00
0 .00
0 .00
0 .00 Feet I Second
0.00 Minutes
10.00 Minutes
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Cubic Feet I Second
Cubic Feet I Second
Cubic Feet I Second
Cubic Feet I Second
Cub ic Feet I Second
Cubic Feet I Second
Determine Post-development Peak
Storm Water Dischar e No Detention
Tributary Area ("A"): 1.68 Acres
Pervious Area : 0.66 Acres
Imp ervio us Area: 1.02 Acres
Run-Off C oefficient ("Cwt"): 0.75
T ime Of Conc entration ("T/c"): 10 Minutes (Min)
Hourly Inte nsity Rates ("I"): 2-Year: 5.2 0
5-Yea r: 7.70
10-Year: 8.60
25-Year: 9.90
50-Year : 11 .20
100-Year: 12.60
C= 0.40
C = 0.98
Inches I Hour
Inche s I Hou r
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Peak Disc harge Rate ("O"): 2-Yea r: 6.57 Cubic Fe et I Sec ond
5-Year: 9.73 Cubic Feet I Sec ond
10-Year: 10.8 7 Cubic Feet I Sec o nd
25-Yea r: 12.5 1 Cubi c Feet I Second
50-Year: 14 .15 Cubic Feet I Sec o nd
100-Year: 15 .92 Cubic Feet I Se co nd
Comparison Of Predevelopment And
Post-develo ment Peak Dischar e Rates
Prede velopment Post -Develop ment Inc rease
2-Year: 3.49 Ft3/sec 6 .57 Ft3/sec 3 .08 Ft3/se c
5-Yea r : 5 .17 Ft3/sec 9 . 73 Ft3/sec 4 .56 Ft3/sec
10-Year: 5.80 Ft3/sec 10.87 Ft3/sec 5.06 Ft3/sec
25-Year: 6.63 Ft3/sec 12.5 1 Ft3/sec 5.88 Ft3/sec
50-Year: 7.49 Ft3/sec 14 .15 Ft3/sec 6.66 Ft3/sec
100-Year: 8.4 7 Ft3/sec 15.92 Ft3/sec 7.45 Ft3/sec
Prel iminary Determination Of
Detention Pond Volume
2-Year: 3.08 Ft3/sec x (26 .7 Min . x 60 Sec . I 2 ) = 2,464 Cubic
5-Yea r: 4.56 Ft 3/sec x (26 .7 Min . x 60 Se c . I 2 ) = 3 ,653 Cubic
10-Year: 5.06 Ft3/sec x (26 .7 Min . x 60 Sec . I 2 ) = 4,057 Cubic
25-Yea r: 5.88 Ft 3/sec x (26.7 Min. x 60 Sec . I 2 ) = 4,7 12 C ub ic
50-Yea r: 6.66 Ft3/sec x (26.7 Min . x 60 Sec . I 2 ) = 5,336 Cubic
100-Yea r: 7.45 Ft3/sec x (26.7 Min . x 60 Sec . I 2 ) = 5,971 Cubic
Feet
Feet
Feet
Feet
Feet
Feet
Pre-Development Storm Intensity Rates (Inches Per Hour) Time 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year (Minutes) Storm Storm Storm Storm Storm Storm 0 0.00 0.00 0.00 0.00 0.00 0.00 1 0.35 0.52 0.58 0.66 0.75 0.85 2 0.70 1.03 1.16 1.33 1.50 1.69 3 1.05 1.55 1.74 1.99 2.25 2.54 4 1.40 2.07 2.32 2.65 3.00 3.39 5 1.75 2.58 2.90 3.31 3.75 4.23 6 2.10 3.10 3.48 3.98 4.49 5.08 7 2.45 3.62 4.06 4.64 5.24 5.93 8 2.80 4.14 4.64 5.30 5.99 6.77 9 3.14 4.65 5.22 5.96 6.74 7.62 -----__ , Peak=> L 10 3.49 5.17 5.80 6.63 7.49 8.47 --. ----11 3.29 4.86 5.45 6.23 7.04 7.96 12 3.08 4.55 5.11 5.83 6.59 7.45 13 2.87 4.24 4.76 5.44 6.15 6.95 14 2.66 3.93 4.41 5.04 5.70 6.44 15 2.45 3.62 4.07 4.64 5.25 5.93 16 2.24 3.31 3.72 4.25 4.80 5.43 17 2.03 3.00 3.37 3.85 4.35 4.92 18 1.82 2.69 3.02 3.45 3.90 4.41 19 1.61 2.38 2.68 3.06 3.45 3.90 20 1.40 2.07 2.33 2.66 3.01 3.40 21 1.19 1.76 1.98 2.26 2.56 2.89 22 0.98 1.45 1.63 1.87 2.11 2.38 23 0.77 1.15 1.29 1.47 1.66 1.88 24 0.56 0.84 0.94 1.07 1.21 1.37 25 0.36 0.53 0.59 0.67 0.76 0.86 26 0.15 0.22 0.24 0.28 0.31 0.35 27 0.00 0.00 0.00 0.00 0.00 0.00 3
Post-Development Storm Intensity Rates {Inches Per Hour) Time 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year (Minutes) Storm Storm Storm Storm Storm Storm 0 0.00 0.00 0.00 0.00 0.00 0.00 1 0.66 0.97 1.09 1.25 1.42 1.59 2 1.31 1.95 2.17 2.50 2.83 3.18 3 1.97 2.92 3.26 3.75 4.25 4.78 4 2.63 3.89 4.35 5.00 5.66 6.37 5 3.29 4.86 5.43 6.25 7.08 7.96 6 3.94 5.84 6.52 7.51 8.49 9.55 7 4.60 6.81 7.61 8.76 9.91 11.14 8 5.26 7.78 8.69 10.01 11.32 12.74 9 5.91 8.76 9.78 11.26 12.74 14.33 l10 ~ n -. ----Peak=> 6.57 9.73 10.87 12.51 14.15 15.92 ~ ---11 6.18 9.15 10.22 11.76 13.30 14.97 12 5.78 8.56 9.57 11.01 12.46 14.01 13 5.39 7.98 8.91 10.26 11.61 13.06 14 5.00 7.40 8.26 9.51 10.76 12.11 15 4.60 6.82 7.61 8.76 9.92 11.15 16 4.21 6.23 6.96 8.02 9.07 10.20 17 3.82 5.65 6.31 7.27 8.22 9.25 18 3.42 5.07 5.66 6.52 7.37 8.29 19 3.03 4.49 5.01 5.77 6.53 7.34 20 2.64 3.90 4.36 5.02 5.68 6.39 21 2.24 3.32 3.71 4.27 4.83 5.43 22 1.85 2.74 3.06 3.52 3.98 4.48 23 1.46 2.16 2.41 2.77 3.14 3.53 24 1.06 1.57 1.76 2.02 2.29 2.57 25 0.67 0.99 1.11 1.27 1.44 1.62 26 0.28 0.41 0.46 0.52 0.59 0.67 27 0.00 0.00 0.00 0.00 0.00 0.00 4
...
Cl>
CL. -Cl>
Cl> u..
0 .......
·-"O .0 c
::::J 0 u 0 ....... Cl>
Cl> <I)
0) ...
0 .c
0
U) a
...
Total Pre-Development Hydrograph Vs. Total Post-
Development Hydrograph For 2-Year Storm Event
: 6 .00------+-----1-t--------+------+----+------;
"$ .2 5.00 ------+---~~-1-------------+------+-------;
0 ....... ~ ~ 4 .oo------+--1----t------+----~---+----+-----l
::::J 0
() 0 ....... Cl> 3.00 +----~---7'"'----l---~-...:---+------.3io~----+------1
Cl> <I)
0) 5 2 .00+--~~-+-:;~---4~---+----=""""7--+----"'~--+----~
.c
0 '3 1.00
0.00 ~----+---~i------+-----+----..,_; .... ---4
10 .00
9.00
8.00
7.00
6.00
5.00
4.00
3.00
2.00
l.00
0 .00
0
0 5 10 15 20 25
Time (Minutes)
Pre-Development Hydrograph ----Total Post -Development Hydrograph
Total Pre-Development Vs. Total Post-Development (No
Detention) Peak Discharge Rates 5-Year Storm Event
5 10 15 20 25
Time (Minutes)
30
Pre-Development Hydrograph ----Post-Development Outflow Hydrograph
With out Detention
3(
....
QI
Q.. -QI
QI u..
0 .......
·-"O .0 c ;:, 0
(.) 0
-QI QI Cl)
O> ....
0
~
0
U)
0
....
QI
Q.. -QI
QI u..
0 .......
·-"O .0 c ;:, 0
(.) 0
-QI QI Cl)
O> ....
0
~
0
U)
0
12 .00
10 .00
8.00
6 .00
4 .00
2 .00
0 .00
0
Total Pre-Development Vs . Total Post -Development (N o
Detention) Peak Discharge Rates 10-Year Storm Event
5 10 15 20 25
Time (M inutes)
30
Pre-Development Hydrograph Post -Developm e nt Outflow Hydrograph
With out Detention
14 .00
12 .00
10 .00
8 .00
6 .00
4 .00
2 .00
0 .00
0
Total Pre-Development Vs. Total Post-Development (No
Detention) Peak Discharge Rates 25-Year Storm Event
5 10 15 20 25
Time (M inutes)
30
Pre -Development Hydrograph Post -Developm ent Outflow Hydrogra ph
Wi th out Detention
16 .00 -Q)
14.00 0.. -Q) 12.00 Q)
LL.
0 ....... 10.00 ·-"O .0 c :::> 0 8 .00 () 0 ......., Q)
6 .00 Q) Cl)
{)) -4 .00 0 ..c.
0 2 .00 '1) a
0 .00
0
Total Pre-Development Vs. Total Post-Development (No
Detention) Peak Discharge Rates 50-Year Storm Event
5 10 15 20 25
Time (Minutes)
30
---Pre -Development Hydrograph ----Post -Dev elopment Outflow Hyd ro graph
With out Det entio n
16 .00 -Q)
14.00 0.. -Q) 12 .00 Q)
LL.
0 ....... 10 .00 ·-"O .0 c
:::> 0 8.00 () 0 ......., Q)
6.00 Q) Cl)
{)) -4 .00 0 ..c.
0 2 .00 '1) a
0 .00
0
Total Pre-Development Vs. Total Post-Development (No
Detention) Peak Discharge Rates 100-Year Storm Event
5 10 15 20 25
Time (Minutes)
30
----Pre-Dev elop ment Hyd rograph ----Po st-Development Outflo w Hydrograph
With o ut Detenti o n
7