HomeMy WebLinkAbout6 Drainage ComputationsDRAINAGE COMPUTATIONS
For
El Chico Restaurant
To Be Located Lot 5 & Lot 6
Anderson Ridge Subdivision -Phase Four
College Station, Brazos County, Texas
Prep a red By :
Garrett Engineer ing
4444 Carter Creek Parkway -Suite 108
Bryan , Texas 77802
Telephone : (409) 846-2688
*May, 1994 *
DRAINAGE COMPUTATIONS
For
El Chico Restaurant
To Be Localed
Lots &Lot 6
And erson Rid ge Subdivi s ion -Phase Four
College Station , Brazos County, Texas
*May, 1994 *
SUMMARY
The proj ect cons is ts of tb e con s tru c tion of a 6440 square foo t restaurant and as sociated s ide walks,
dri veways and parkin g areas s ite is occupied by a 9825 square foot building which will be demoli s hed .
Exis tin g pave m ent will be utilized tl1r o ug ho ut mos t of th e prope rty , howe ve r, some will be remo ved (S ee
attac hed s ite pl an at re ar of thi s presentation . At present th ere is no de te nti on facility con s tructed on-s ite .
Storm water run-off s heet drain s acro ss th e prope rty in a northweste rly direction and exits th e property via
curb leave-ou ts.
Du e to the demolition of th e e xi sting buildin g located on th e s ubject s ite and th e con s truction of the
proposed structure along with tl1 e pave ment modifi cation s th e pos tdevelopme nt p eak run-off rates will be
less th an tlle p eak run-off rates currently present o n-s ite. It is with tl1i s reali zation tbat the owner intend s
to allow run-o ff to exit tl1 e prope rty in it's current m ann er with o ut providin g on-site de tention.
C omputation s s ub s ta nti a tin g th e abo ve m enti o ns results are presente d a s fo ll o w s:
From tlle computa ti o ns on p age 1 b ased on tl1 e proj ec t area of 1.51 acres a s shown on tl1 e afore me ntion ed
Site & Gradin g Pl a n, tl1 e prede ve lo pm ent run-o ff coeffi cient be in g as s um ed a t 0 .87 and a minimum tim e
of concentrati o n o f 10 minutes tl1 e rela ti ve predevelo pm ent peak di scharge rates ("Q") we re de termin ed.
The inte nsiti es shown on page 1 were take n fr o m "F igure UT -1 Intensity Duratiou Frequ ency Curve s For
The City Of Coll e ge Station"as conta in ed in th e "Drainag e P olicy and D es ign Standard s for th e City of
Coll e ge Station " as sumin g s am e miuimum tim e of co nc e ntr ati on of 10 miuutes . The R a tional M e thod wac.;
utilized to compute tl1 e es tim a ted peak predevelo pm ent di scharge rates . The equ a tion tilat re prese nts tl1 e
R a ti o nal Me tJ10d is as follow s: Q = CIA , wh e re; "Q" is tl1 e p eak di sch ar ge rate in cubic feet p er second .
"C" is tb e run-off coeffici e n t, in tl1 e prede velo pm e nt co nditi on "C " is ass um ed to be 0 .8 7 , "I " is tl1 e s torm
inte u s ity in in ch es pe r hour and "A" whi ch i tl1 e ar ea of til e s ite in acres .
F rom tl1 e computa tion s on p age 2 b ased o n th e proj ect ar ea of 1.51 acres, th e pos td evelopme nt run-off
coeffi cient b e in g ass um ed a t 0.83 a nd a minimum tim e of co nc enlrnti o n of 10 minutes til e relative
prede velopm e nt peak di sc harge rates ("Q ") were detennined. T he iuten s iti es s hown o n page 1 were take n
fr om "F igure IH-1 Inte ns ity Du ra 1i o n F requ e ncy C urv es For T he City Of Coll e ge Stali o n"a s coutaine d in
tl1 e "Drainag e P oli c y and D es ign Standard s fo r th e C ity of C oll e ge Station" ass umin g sam e minimum tim e
of conc entrati o n of 10 minutes. The R a ti o nal Me tJ10d was utili zed to compute tl1 e es tim a ted p eak po s t
develo pm e nt di scharge rates . Th e e qu ati on that re presents th e Rational Me thod is as follow s : Q = CIA ,
wh e re; "Q" is th e peak di scharge rate in cubi c feet per seco nd. "C " is tl1 e run-off coeffici e nt, in tl1 e po st
develo pm ent conditi o n "C" is ass um ed to be 0 .83, "I" is tll e sto rm inte ns ity in inch es p e r ho ur and "A"
whi c h is tl1 e a rea o f tl1 e s ite in ac res.
The compari so n betw een pe ak predevelo pm e nt di scharge rates a nd pos t develo pm ent di scharge rates is
includ ed a s an a id to tl1 e des ig ne r to es tabli s h a pre limin ar y "ta rge t" a s to tl1 e volmn e n ecessary in tl1 e
proposed de te nti o u fa c ility s ho uld a de te nti o n fac ility be deem ed necessar y . Thi s preliminary es timate is
arri ved at by s imply de te rminin g tJ1 e vo lum e gen era ted from tJ1 e diffe ren ce of tl1 e predevelo pm e nt and po st
develop ment hydrographs (see pages 5-7 for predevelopment and post dev elopment hydro gra ph s). Please
note Urnt U1e post development hydrogra ph show n on Ules e same gra phs assumes no detention.
Pa ge 3 and pa ge 4 are tabulation s of ilie predevelopment hydrograph and the po st dev elopment
hydro gra ph based on U1e computations perform ed on page 1 and pa ge 2. The post development
hydro gra ph ass um es no dete nti on to fac ilitate direct comparison in U1e pre and post development
conditions.
The hydrograph s are triangular in shape and are based on U1 e sta ndard SCS unit hydrograph wiili time to
peak se t equal to the time of concentration and U1e total time base set at 2.67 tim es U1e time of
concentration. Please note Urnt a 26.7 minute total storm eve nt duration was utilized excl usively
U1rou ghout U1i s r eport due totally to U1 e small ph ys ical si ze of U1 e project.
A s prev iou sly stated, th e anticipated po sttl eve lopment peak run-off rates will be l ess Urnt U1e
pred evelopment peak di sc har ge rates U1 erefo r e, it is anticipated Uiat no detention facilities will be
req uired .
Determine Total Pre-Development
Peak Storm Wate r Discharge Rates
Tributary Ar ea ("A"): 1.51 Acres
Pervious A rea : 0.28 Acres
Impervio us Ar ea : 1.23 Acres
Run-Off C oefficient ("Cwt"): 0.87
Time Of Conc entration ("T/c"):
Woodland s : Low Elevation :
High Elevation :
Distance (Feet):
S lope (% G rade):
Velocity ("Vw"):
Time :
Pa stures: Low Ele vation:
High El evation :
Distance (F eet):
Sl ope (%Grade):
Velocit y ("V p "):
Time:
Pavem e nts: Low Elevation :
High Elevation :
Distance (Feet):
Slope (% Grade):
Velocit y ("Vpave"):
T ime :
Total Trav el Time:
Hourly Inte nsity Rates ("I "): 2-Yea r: 5.20
5-Yea r: 7.69
10-Year: 8.63
25-Year: 9.86
50-Yea r: 11.15
100-Year: 12.60
Peak Discharge Rate ("O"): 2-Year: 6.85
5-Year : 10 .13
10-Year: 11 .38
25-Year: 12 .99
50-Year: 14.69
100-Year: 16 .60
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
0.0 0
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 Hou r
Inches I Hour
Cubic Feet I Second
Cubic Feet I Second
Cubic Feet I Seco nd
Cubic Feet I Seco nd
Cubic Feet I Second
Cubic Feet I Seco nd
Determine Post -development Peak
Storm Water Discharge (No Detention)
T ributary Area ("A"): 1.51 Acres
Pervious Ar ea: 0.4 0 Acres
Im pervious A rea: 1.11 Acres
Run-Off C oefficient ("Cwt"): 0.83
Time Of Con centration ("T/c"): 10 Minut es (Min)
Hourly Inte nsity Rates ("I"): 2-Year: 5.20
5-Yea r: 7.70
10-Year: 8.60
25-Year: 9.9 0
50-Year: 11 .20
100-Year: 12 .60
c = 0.40
C= 0.98
Inches I Hour
Inche s I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Peak Discharge Rate ("O"): 2-Year: 6.49 Cubic Fee t I Second
5-Year: 9.61 Cubic Feet I Second
10-Year: 10 .73 Cubi c Feet I Se co nd
25-Yea r: 12 .35 C ubic Feet I Sec ond
50-Year : 13 .98 Cubic Feet I Seco nd
100-Year: 15 .72 Cubic Feet I Second
Comparison Of Predeve lopment And
Post-development Peak Discharge Rates
Prede velopme nt Post-Dev elopme nt In c rease
2-Year: 6.85 Ft3/sec 6.49 Ft3/sec -0.36 Ft3/sec
5-Year: 10 .13 Ft3/sec 9.61 Ft3 /sec -0.53 Ft3/sec
10-Year: 11 .38 Ft3/sec 10 .73 Ft3 /sec -0 .64 Ft3/sec
25-Year: 12 .99 Ft3/sec 12 .35 Ft3 /sec -0.64 Ft3/sec
50-Year: 14 .69 Ft3/sec 13.98 Ft3 /sec -0.71 Ft3/sec
100-Year: 16 .60 Ft3 /sec 15 . 72 Ft3 /sec -0.88 Ft3/sec
Prelim i nary De termination Of
Detention Pond Volume
2-Year: -0.36 Ft3/sec x (26 .7 Min . x 60 Sec. I 2 ) = -290 Cubic
5-Year: -0 .53 Ft3/sec x (26 .7 Min . x 60 Sec . I 2 ) = -422 Cubic
10-Year: -0 .64 Ft3/sec x (26 .7 Min . x 60 Sec. I 2 ) = -516 Cubic
25-Year: -0 .64 Ft3/sec x (26.7 Min . x 60 Sec. I 2 ) = -51 1 Cubic
50-Year: -0 .71 Ft3/sec x (26 .7 Min . x 60 Sec . I 2 ) = -569 Cubic
100-Year: -0 .88 Ft3/sec x (26 .7 Min . x 60 Sec . I 2 ) = -702 Cubic
Feet
Feet
Feet
Feet
Feet
Feet
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 0.65 0.96 1.07 1.24 1.40 1.57 2 1.30 1.92 2.15 2.47 2.80 3.14 3 1.95 2.88 3.22 3.71 4.19 4.72 4 2.60 3.84 4.29 4.94 5.59 6.29 5 3.24 4.80 5.37 6.18 6.99 7.86 6 3.89 5.76 6.44 7.41 8.39 9.43 7 4.54 6.73 7.51 8.65 9.78 11.01 8 5.19 7.69 8.58 9.88 11.18 12.58 5.84 8.65 9.66 11.12 12.58 14.15 110 --·---·-------Peak=> 6.49 _Q.§L. 10.73 12.35 13.98 15.72 --------11 6.10 9.03 10.09 11.61 13.14 14.78 12 5.71 8.46 9.45 10.87 12.30 13.84 13 5.32 7.88 8.80 10.13 11.46 12.90 14 4.93 7.31 8.16 9.39 10.63 11.96 15 4.55 6.73 7.52 8.65 9.79 11.02 16 4.16 6.16 6.88 7.91 8.95 10.07 17 3.77 5.58 6.23 7.18 8.12 9.13 18 3.38 5.01 5.59 6.44 7.28 8.19 19 2.99 4.43 4.95 5.70 6.44 7.25 20 2.60 3.85 4.31 4.96 5.61 6.31 21 2.21 3.28 3.66 4.22 4.77 5.37 22 1.83 2.70 3.02 3.48 3.93 4.42 23 1.44 2.13 2.38 2.74 3.10 3.48 24 1.05 1.55 1.73 2.00 2.26 2.54 25 0.66 0.98 1.09 1.26 1.42 1.60 26 0.27 0.40 0.45 0.52 0.59 0.66 27 0.00 0.00 0.00 0.00 0.00 0.00
Inflow I Outflow Simulation 2-Year Storm Event Time Inflow 11+ 12 2s/t-0 2s/t+O Outflow Depth Elevation Velocity (Minutes) (Ft3/Sec) (Ft3/Sec) (Feet) (Ft/Sec) 0 0.00 0.00 0.00 0.00 0.00 0.00 293.51 0.00 1 0.33 0.33 0.31 0.33 0.01 0.04 293.55 0.62 2 0.65 0.98 1.23 1.29 0.03 0.10 293.61 0.98 3 0.98 1.63 2.74 2.87 0.06 0.17 293.68 1.28 4 1.31 2.29 4.80 5.02 0.11 0.25 293.76 1.55 5 1.63 2.94 7.39 7.74 0.17 0.34 293.85 1.79 6 1.96 3.59 10.49 10.98 0.24 0.42 293.93 2.00 7 2.29 4.25 14.09 14.74 0.32 0.51 294.02 2.20 8 2.61 4.90 18.15 18.99 0.42 0.60 294.11 2.39 9 2.94 5.55 22.67 23.71 0.52 0.69 294.20 2.57 10 3.27 6.20 27.62 28.88 0.63 0.79 294.30 2.74 11 6.10 9.37 35.39 36.99 0.80 0.93 294.44 2.97 12 5.71 11.81 45.09 47.20 1.05 1.11 294.62 3.26 13 5.32 11.03 53.54 56.13 1.29 1.28 294.79 3.49 14 4.93 10.26 60.80 63.80 1.50 1.41 294.92 3.66 15 4.55 9.48 66.93 70.29 1.68 1.52 295.03 3.80 16 4.16 8.70 71.96 75.64 1.84 1.61 295.12 3.92 17 3.77 7.93 75.96 79.89 1.97 1.69 295.20 4.01 18 3.38 7.15 78.98 83.11 2.06 1.74 295.25 4.08 19 2.99 6.37 81.09 85.35 2.13 1.78 295.29 4.12 20 2.60 5.59 82.35 86.69 2.17 1.80 295.31 4.15 21 2.21 4.82 82.80 87.17 2.18 1.81 295.32 4.16 22 1.83 4.04 82.49 86.84 2.17 1.81 295.32 4.15 23 1.44 3.26 81.47 85.75 2.14 1.79 295.30 4.13 24 1.05 2.49 79.78 83.96 2.09 1.76 295.27 4.09 25 0.66 1.71 77.46 81.49 2.01 1.72 295.23 4.04 26 0.27 0.93 74.55 78.40 1.92 1.66 295.17 3.98 27 0.00 0.27 71.20 74.83 1.81 1.60 295.11 3.91 28 0.00 0.00 67.79 71.20 1.70 1.54 295.05 3.83 29 0.00 0.00 64.59 67.79 1.60 1.48 294.99 3.75 30 0.00 0.00 61.55 64.59 1.52 1.42 294.93 3.68