HomeMy WebLinkAboutDrainage ReportDRAINAGE REPORT
FOR
Lot 1 - Willow Run Subdivision Phase 1
John Payne Survey, A -195,
Bryan, Brazos County, Texas
February 17, 2005
OWNED BY:
Roy D. and Cindy S. Brantley
13410 Headwater Lane
College Station, Brazos County, Texas
(979)694 -0900
PREPARED BY:
Kling Engineering & Surveying
4101 S. Texas Avenue, Suite A
Bryan, TX 77802
(979) 846 -6212
Drainage Report
for Willow Run Subdivision
Lot 1 - 29.50 acres
John Payne Survey, A -195
College Station, Brazos County, Texas
February 17, 2005
GENERAL LOCATION & DESCRIPTION
Willow Run Subdivision Phase One is a rural residential development with lots ranging
in size from 4.6 acres to 29.50 acres.
Willow Run is located approximately 3800 feet southeast of Rock Prairie West and has
frontage on:
I.G.N. Road to the southwest
Capstone Drive to the southeast
Headwater Lane to the northeast (see Appendix Pg. 1)
No portion of Lot 1- (29.50 acres) lies withir , the 100 year floodplain according to the
Flood Insurance Rat Map for Brazos County Number 48041 CO200C effective date: July 2,
1992 (see Appendix Pg. 2).
The Brantley's,
creates four lots:
Lot 1 -A
Lot 1 -B
Lot 1 -C
Lot 1 -D
owners of Lot 1, have requested a replat of Lot 1 be prepared that
- 12.62 acres - Brantley homesite
- 5.10 acres - to be sold
- 6.05 acres - to be sold
- 5.74 acres - to be sold
The anticipated improvements to be placed on each of the four lots is a single family
home (minimum 2250 ft 2 ), out buildings and drives.
PURPOSE
As a requirement of the Replat, the City of College Station requested the minimum
finished floor (FF) elevations be established for each of the lots.
DRAINAGE BASIN
Hopes Creek tributary exits the lot via 3 - 84" CMP under I. &G. N. Road. The drainage
area for the tributary sub -basin is approximately 964 acres. Existing Conditions sub -basin
characteristics are as follows:
Land Use: 94% agricultural /undeveloped 6% residential development
Land Type: loamy fine sands with underlying clays, very slow permeability, mild slopes
Land Cover: 82% pasture, 4% wooded, 8% channel /pond areas, 6% residential
Willow Run is located in the Hopes Creek drainage basin. A tributary to Hopes Creek
runs through Lot 1 and all of Lot 1 drainage is to this tributary (see Appendix Pg. 3).
HYDROLOGIC ANALYSIS
The peak runoff rate leaving Lot 1 via 3 - 72" CMP at I. &G. N. Road was calculated to
be 1364 CFS at the 100 year event using SCS Unit Hydrograph Method:
DA: 964 acres
CN (AvG) # 82.16 (82)
Tc 4 hrs. 55 min. 33.24 sec. - SCS lag
Q 100 = 1364 CFS
(see Appendix Pgs 8 -10 for calculation)
HYDRAULIC ANALYSIS
A hydraulic model was created using HEC RAS which created seven sections (see
Appendix Pg. 3). The existing conditions peak Q 100 = 1364 CFS was routed through the
model.
This report uses the water surface elevation computed at sections 4, 6, 7 (see
Appendix Pgs. 3 and 4) to establish the minimum FF.
At each of the three sections, a water surface elevation was computed using Q 10 _
1364 CFS:
Sec. 4:
269.2'
Sec. 6:
270.1'
Sec. 7:
270.8'
Not fully understanding how a fully developed basin would perform, a Q 100 =1860 CFS
(fully developed basin with CN (AvG) = 90) was routed through the hydraulic model. The
maximum increase in water surface elevations at the three sections was 2.7'.
The minimum FF elevation was determined by adding three feet to the existing
conditions model:
Lot 1 -A - minimum FF = 273.8'
Lot 1 -B - minimum FF = 272.2'
Lot 1 -C - minimum FF = 273.8'
Lot 1 -D - minimum FF = 273.1'
CULVERT - I. &G.N. ROAD
There are 3-84" CMP crossing I. &G.N. Road with 12' of available head (to the top of
the road). The peak flow rate through the culverts without overtopping the road was calculated
to be:
Q um = 1644 CFS
_CONCLUSIONS
The HEC RAS model was developed using City of College Station contours to
determine the existing conditions water surface elevation. Three feet were added to the water
surface elevations to account for fully developed conditions. Minimum finished floor (FF)
elevations were developed incorporating the three foot increase.
All structures built at or above the minimum finished floor (FF) elevations established
herein are reasonably safe from flooding at the 100 year event in accordance with accepted
professional practices.
Ali.
B. J. Kli�ilg,�?
Registered P�
State of Texas
APPENDIX
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APPROXIMATE SCALE
2000 0 2000 FEET
NATIONAL FLOOD INSURANCE PROGRAM
FIRM
FLOOD INSURANCE RATE MAP
BRAZOS COUNTY,
TEXAS AND
INCORPORATED AREAS
PAW 200 OF 250
CBEE MAP INDEX FOR PANELS NOT PRINTED1
KUM
CMULGE STAIM on OF 0=3 921111
UMWAVWA119 AREAS :Bliss #zw c
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MAP NUMBER
48041C000 C
EFFECTIVE DATE:
JULY Z 1992
Fe&nd Fawsirgeticy m anqemw t A gawy
lz]? Ir" m an CIRCuld copy otaPOrbOnOf the about rateremedflood map. It
I f5c"acted uaft F-MIT On4Jne- This map dam not reflect Changes
Or 8 MOKh- Orft Which MANY 1111 been Made subsequent to the date on the
dtlfblOClL For the Pabst product Insx., Billion about National Flood insurance
Program food maps Check the FEMA Flood Map store at www . m Nc. fE me.Oc
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Type.... Runoff CN -Area Page 2.01
Name.... WR EXISTING
File.... H: \ENGINE- 1 \WILLIO- 1 \WR- PRE.PPK
Title... EXISTING CONDITION DRAINAGE AREA
RUNOFF CURVE NUMBER DATA
EXISTING CONDITION DRAINAGE AREA
-------------------------------------------------------------------- - - - - --
SIN: HOMOL0902508 Kling Engineering & Surveying
Pond Pack Ver: 8 -01 -98 (61) Compute Time: 09:16:52 Date: 02 -10 -2005
Impervious
Area
Adjustment
Adjusted
Soil /Surface Description
CN
acres
%C %UC
CN
-------------------------- - - - - --
GRASS /PASTURE
- - --
80
--- - - - - --
794.930
- - - -- - - - --
- - - - --
80.00
WOODED
83
41.800
83.00
CHANNEL
98
70.210
98.00
POND
98
2.120
98.00
RESIDENTIAL
92
54.420
92.00
COMPOSITE AREA & WEIGHTED CN - - ->
............................................
............................................
963.480
...............................
...............................
82.16 (82)
SIN: HOMOL0902508 Kling Engineering & Surveying
Pond Pack Ver: 8 -01 -98 (61) Compute Time: 09:16:52 Date: 02 -10 -2005
/ Tlpe^,_^ To Caloa Page 1,01
File...' 8:\ENGINE-1\WILLIO-1\WB-PRE.PPK
Title ... WILLOW RUN Tc EXISTING CONDITIONS
TIME OF CONCENTRATION CALCULATOR
WILLOW RUN To EXISTING CONDITIONS
------------------------------------------------------------------------
Segment #I; Tc: 3C3 Lag
Hydraulic Length II4I0.00 ft
Runoff cm 82
Slope .005000 ft/ft
xu/g.nelocity '64 ft/sec
Segment #1 Time: 4.9269 bzo
------------------------------------------------------------------------
Total To: 4,9259 hcs
o/N, HOMOL0902508 Kling Engineering a Surveying
Pond eaob Vez: 8-01-98 (61) Compute Time; 09:16;52 Date: 02-10-2005
Type.... SCS Unit Hyd. Summary Page 3.02
Name.... WR- EXISTING Tag: 100 YR Event: 100 yr
File.... H: \ENGINE- 1 \WILLIO- 1 \WR- PRE.PPK
SCS UNIT HYDROGRAPH METHOD
STORM EVENT: 100 year storm
Duration = 24.0000 hrs Rain Depth = 11.3530 in
Rain Dir = C: \HAESTAD \PPK6 \UTIL\
Rain File -ID = SCSTYPES.RNF - TypeII 24hr
Unit Hyd Type = Default Curvilinear
HYG Dir = H: \ENGINE- 1 \WILLIO ~1\
HYG File - ID = WR- PRE.HYG - WR- EXISTING 100 YR
Tc = 4.9259 hrs
Drainage Area = 963.480 acres Runoff CN= 82
Computational Time Increment = .65678 hrs
Computed Peak Time = 15.1060 hrs
Computed Peak Flow = 1363.61 cfs
Time Increment for HYG File = .0500 hrs
Peak Time, Interpolated Output = 15.1000 hrs
Peak Flow, Interpolated Output = 1363.14 cfs
DRAINAGE AREA
ID:WR
EXISTING
CN =
82
Area =
963.480 acres
S =
2.1951 in
0.25 =
.4390 in
Cumulative Runoff
--------------- - - --
9.0864 in
729.549 ac -ft
HYG Volume... 729.563 ac -ft (area under HYG curve)
* * * ** UNIT HYDROGRAPH PARAMETERS * * * **
Time Concentration, Tc = 4.92589 hrs (ID: SCS LAG -WR)
Computational Incr, Tm = .65678 hrs = 0.20000 Tp
Unit Hyd. Shape Factor = 483.432 (37.46% under rising limb)
K = 483.43/645.333, K = .7491 (also, K = 2 /(1 +(Tr /Tp))
Receding /Rising, Tr /Tp = 1.6698 (solved from K = .7491)
Unit peak,
Unit peak time
Unit receding limb,
Total unit time,
cgs = 221.62 cfs
Tp = 3.28392 hrs
Tr = 13.13570 hrs
Tb = 16.41962 hrs
SIN: HOMOL0902508 Kling Engineering & Surveying
Pond Pack Ver: 8 -01 -98 (61) Compute Time: 09:16:52 Date: 02 -10 -2005