HomeMy WebLinkAboutDrainage ReportDrainage Report
for
Castlegate Subdivision -Section 13
College Station, Texas
May 2003
Developer:
Greens Prairie Investors, Ltd.
By Greens Prairie Associates, LLC
4490 Castlegate Drive
College Station, Texas 77845
(979) 690-7250
Prepared By:
TEXCON General Contractors
1 707 Graham Road
College Station, Texas 77845
(979) 690-7711
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COi..-.. -""'.... v G ENG\NEEf;i I
CERTIFICATION
I, Joseph P. Schultz, Licensed Professional Engineer No. 65889, State of Texas, certify that this
report for the drainage design for the Castlegate Subdivision -Section 13 infrastructure, was
prepared by me in accordance with the provisions of the City of College Station Drainage
Policy and Design Standards for the owners hereof.
TABLE OF CONTENTS
DRAINAGE REPORT
CASTLEGATE SUBDIVISION -SECTION 13
CERTIFICATION ................................................................................................................................................................. 1
TABLE OF CONTENTS ....................................................................................................................................................... 2
LIST OF TABLES .................................................................................................................................................................. 2
INTRODUCTION .................................................................................................................................................................. 3
GENERAL LOCATION AND DESCRIPTION ................................................................................................................. 3
FLOOD HAZARD INFORMATION ................................................................................................................................... 3
DEVELOPMENT DRAINAGE PATTERNS ...................................................................................................................... 3
DRAINAGE DESIGN CRITERIA ....................................................................................................................................... 3
STORM WATER RUNOFF DETERMINATION .............................................................................................................. 4
DETENTION FACILITY DESIGN ..................................................................................................................................... 5
STORM SEWER APRON & FLUME DESIGN ................................................................................................................. 5
CONCLUSIONS ..................................................................................................................................................................... 6
APPENDIX A ......................................................................................................................................................................... 7
Storm Sewer Apron & Flume Design Calculatio11s
EXHIBIT A ........................................................................................................................................................................... 11
Post-Development Drainage Area Map
LIST OFT ABLES
TABLE 1 -Rainfall Intensity Calculations .............................................................................................. 4
TABLE 2 -Post-Development Runoff Information ................................................................................ 5
2
DRAINAGE REPORT
CASTLEGATE SUBDIVISION -SECTION 13
INTRODUCTION
The purpose of this report is to provide the hydrological effects of the construction of the
infrastructure for the Castlegate Subdivision, Section 13, and to verify that the proposed storm
drainage system meets the requirements set forth by the City of College Station Drainage
Policy and Design Standards.
GENERAL LOCATION AND DESCRIPTION
The project is located on a portion of a 111.46 acre tract located west of State Highway 6 along
the north side of Greens Prairie Road in College Station, Texas. This report addresses Section
13 of this subdivision, which is made up of 6.238 acres. The site is pastureland with
approximately 30% of the area wooded with oak trees and yaupons. The existing ground
elevations range from elevation 300 to elevation 310. The general location of the project site is
shown on the vicinity map in Exhibit A.
FLOOD HAZARD INFORMATION
The project site is located in the Spring Creek branch of the Lick Creek Drainage Basin. Most
of the proposed developed area of the site is located in a Zone X Area according to the Flood
Insurance Rate Map prepared by the Federal Emergency Management Agency (FEMA) for
Brazos County, Texas and incorporated areas dated February 9, 2000, panel number
48041 C0205-D. Some of the proposed residential lots are partially in Special Flood Hazard
Area, Zone AE, as shown on the panel. This area is shown on Exhibit A as the 100-year
floodplain limit. Also shown on this exhibit are the floodplain limits as determined by the
Castlegate Floodplain Analysis Report which was previously submitted. The rear portions of
some of the residential lots in Section 13 are within the 100-year floodplain limits. However,
the floodplain limit determined by the Castlegate Floodplain Analysis results in less of the lot
area being in the Flood Hazard Area. The majority of the Flood Hazard Area has been
designated as Common Area, which will have no development and it will be left in its current
condition.
DEVELOPMENT DRAINAGE PATTERNS
Prior to development, the storm water runoff flows easterly until it enters the floodplain and
then into Tributary D of Spring Creek. Ultimately, the runoff flows in Spring Creek and then
north to the regional detention facility. Refer to the vicinity map on Exhibit A for the location
of this facility.
DRAINAGE DESIGN CRITERIA
The design parameters for the storm sewer are as follows:
• The Rational Method is utilized to determine peak storm water runoff rates for the storm
sewer apron & flume design.
3
• Design Storm Frequency
Storm Sewer system 10 and 100-year storm events
• Runoff Coefficients
Pre-development
Post-development (single family residential)
c = 0.30
c = 0.55
• Rainfall Intensity values for Brazos County for a minimum time of concentration of 10
minutes can be found in Table 1. Where a longer time of concentration was necessary, it is
noted in the respective table, and the intensities are calculated with the higher values where
required.
• Time of Concentration, tc -Due to the small sizes of the drainage areas, the calculated
times of concentration, tc, are less than 10 minutes. Therefore, a minimum tc of 10 minutes
is used in most cases to determine the rainfall intensity values. Where a longer time of
concentration was necessary, it is noted and used accordingly.
STORM WATER RUNOFF DETERMINATION
The peak runoff values were determined in accordance with the criteria presented in the
previous section for the 5, 10, 25, 50, and 100-year storm events. The runoff coefficients are
based on the future development of this tract. Exhibit B shows the post-development drainage
areas for the storm sewer apron and flume designs. Post-development runoff conditions are
summarized in Table 2.
TABLE 1 -Rainfall Intensity Calculations
Rainfall Intensity Values (in/hr)
Storm t, =
Event 10 min
Is 7.693
110 8.635
125 9.861
lso 11 .148
1100 11 .639
Brazos County:
5 }".ear storm 10 }".ear storm
b = 76 b = 80
d = 8.5 d = 8.5
e = 0.785 e = 0.763
I = b I (tc+d)e
I = Ra infall Intensity (in/hr)
t, = U(V*60)
t, =Time of concentration (m in)
L = Length (ft)
V =Velocity (ft/sec)
25 }".ear storm 50 }".ear storm 100 }".ear storm
b = 89 b = 98 b = 96
d = 8.5 d = 8.5 d = 8.0
e = 0.754 e = 0.745 e = 0.730
(Data taken from State Department of Highwa}".s and Public Transportation H}".drau/ic Manual , page 2-16)
4
TABLE 2 -Post-Development Runoff Information
Area# Area c
(acres)
13-1 0.60 0.55
13-2 0.62 0.55
13-3 0.53 0.55
13-4 0.62 0.55
The Rational Method:
Q=CIA
Q = Flow (cfs)
A= Area (acres)
C = Runoff Coeff.
tc
(min)
10
10
10
10
I = Rainfall Intensity (in/hr)
5 year storm 10 year storm
Is Os 110 010
(in/hr) (cfs) (in/hr) (cfs)
7.693 2.54 8.635 2.85
7.693 2.62 8.635 2.94
7.693 2.24 8.635 2.52
7.693 2.62 8.635 2.94
I = b I (tc+d)e
tc =Time of co ncentration (min)
DETENTION FACILITY DESIGN
25 year storm 50 year storm
l2s
(in/hr)
9.861
9.861
9.861
9.861
02s lso
(cfs) (in/hr)
3.25 11.148
3.36 11 .148
2.87 11.148
3.36 11 .148
tc = L/(V*60)
L = Length (ft
Oso
(cfs)
3.68
3.80
3.25
3.80
V =Velocity (fVsec)
100 year storm
1100 0100
(in/hr) (cfs)
11.639 3.84
11 .639 3.97
11.639 3.39
11.639 3.97
The detention facility handling the runoff from this site is a regional facility designed by LJA
Engineering & Surveying, Inc. Also, a detention pond was constructed upstream of Castlegate
Drive to reduce the peak flow resulting from the Castlegate development. In Exhibit A, the
location of the regional detention facility is shown on the vicinity map, and the detention pond
is shown upstream of Castlegate Drive. The runoff from this project flows into existing
drainages and then into Spring Creek. The regional detention facility is located adjacent to
Spring Creek prior to Spring Creek entering the State Highway 6 right-of-way.
STORM SEWER APRON & FLUME DESIGN
There wi ll not be any storm sewer piping for this project. However, there will be a concrete
apron to drain the gutter flow across the street where it will enter a concrete flume which will
take the runoff to the 100-year floodplain and then into Tributary D of Spring Creek.
Dissipater blocks will be constructed at the downstream end of the flume to reduce the velocity
of the water prior to entering the existing drainage.
The concrete apron will have a 5' bottom width with a slope of 1 % and side slopes of 2%. The
concrete flume will also have a bottom width of 5 ', but it will have a slope of 1.8% with
vertical side slopes. The flume will be 8" in depth. Appendix A presents a summary of the
storm sewer apron and flume design parameters and calculations. These two features were
designed based on a 10-year design storm, and they pass the 100-year storm event as well.
The streets were designed to maintain a gutter flow depth of 5" or less. This design depth will
prevent the spread of water from reaching the crown of the road for the 10-year storm event.
The maximum depth of flow in the street gutter will be 2.61". The depth of flow in the
concrete apron will be 2.04". However, the flow depth in the flume is 3.99", which results in
the depth of water being 3.24" in the apron. All of these values are for the 10-year storm event.
A summary of these flow depths are also shown in Appendix A.
5
CONCLUSIONS
The construction of this project will increase the storm water runoff from this site. The
proposed apron and flume should adequately control the runoff and release it into existing
drainages. As shown in the Castlegate Floodplain Analysis, the Castlegate Subdivision does
not have a significant effect on the 100-year floodplain water surface elevations or the
floodplain limits. Also, the regional detention facility should adequately control the peak post-
development runoff so that it will not have any impact on the properties downstream of the
Crowley Tract.
6
APPENDIX A
Storm Sewer Apron & Flume Design Calculations
7
Castlegate Subdivision -Section 13
Depth of Flow -Street Gutter 10-year storm 100-year storm
Station Gutter Area# A c 0 10 Y 10-actual 0 100 Y100
Location Location No. (acres) (cfs) (ft) (in) (cfs) (ft) (in)
Sta. 3+67.13 Apron -Upper Right 13-2 0.62 0.55 2.94 0.218 2.61 3.97 0.244 2.92
Sta. 3+42.13 Apron -Upper Left 13-3 0.53 0.55 2.52 0.205 2.47 3.39 0.230 2.76
Sta. 3+67.13 Apron -Lower Right 13-1 0.60 0.55 2.85 0.215 2.58 3.84 0.241 2.89
Sta. 3+42.13 Apron -Lower Left 13-4 0.62 0.55 2.94 0.218 2.61 3.97 0.244 2.92
Transverse (Crown) slope (ft/ft) = 0.0330
Straight Crown Flow (Solved to find actual depth of flow in gutter, y):
Q = 0.56 * (z/n) * S112 * Ys13 ¢ y ={QI [0.56 * (z/n) * S112]}31s
n = Roughness Coefficient= 0.018
z = Reciprocal of crown slope = 30
S = StreeUGutter Slope (ft/ft)
y = Depth of flow at inlet (ft)
Depth of Flow -Apron & Flume 10-year storm 100-year storm
Location Area# Total A c 0 10 Y10-actual 0 100 Y100
Nos. (acres) (cfs) (ft) (in) (cfs) (ft) (in)
Apron 13-1 1.22 0.55 5.79 0.170 2.04 7.81 0.194 2.33 13-4
13-1
Flume 13-2 2.37 0.55 11 .26 0.333 3.99 15.17 0.402 4.82 13-3
13-4
Concrete Apron -10 Year Storm
Channel Calculator
Given Input Data:
Shape .......................... .
Solving for .................... .
Flowrate ....................... .
Slope .......................... .
Manning's n .................... .
Height ......................... .
Bottom width ................... .
Left slope ..................... .
Right slope .................... .
Computed Results:
Depth .......................... .
Velocity ....................... .
Full Flowrate .................. .
Flow area ...................... .
Flow perimeter ................. .
Hydraulic radius ............... .
Top width ...................... .
Area ......................... ·. ·
Perimeter ...................... .
Percent full ................... .
Trapezoidal
Depth of Flow
5.7900 cfs
0.0100 ft/ft
0.0130
6.0000 in
60.0000 in
0.0200 ft/ft (V/H)
0.0200 ft/ft (V/H)
2.0364 in
2.5302 fps
72.0996 cfs
2.2883 ft2
263.6764 in
1.2497 in
263 .6357 in
15.0000 ft2
660.1200 in
33.9393 %
Concrete Apron -100 Year Storm
Channel Calculator
Given Input Data:
Shape .......................... .
Solving for .................... .
Flowrate ....................... .
Slope .......................... .
Manning's n .................... .
Height ......................... .
Bottom width ................... .
Left slope ..................... .
Right slope .................... .
Computed Results:
Depth .......................... .
Velocity ....................... .
Full Flowrate .................. .
Flow area ...................... .
Flow perimeter ................. .
Hydraulic radius ............... .
Top width ...................... .
Area ........................... .
Perimeter ...................... .
Percent full ................... .
Castlegate Subdivi sion -Section 13
College Station, Texas
Trapezoidal
Depth of Flow
7.8100 cfs
0.0100 ft/ft
0.0130
6.0000 in
60.0000 in
0.0200 ft /ft (V/H)
0.0200 ft/ft (V/H)
2.3304 in
2.7339 fps
72.0996 cfs
2.8568 ft2
293.0903 in
1.4036 in
293.0437 in
15.0000 ft2
660.1200 in
38.8406 %
Concrete Flume -10 Year Storm
Channel Calculator
Giv en Input Data:
Shape .......................... .
Solving for .................... .
Flowrate ....................... .
Slope .......................... .
Manning' s n .................... .
Height ......................... .
Bottom width ................... .
Computed Results:
Depth .......................... .
Velocity ....................... .
Full Flowrate .................. .
Flow area ...................... .
Flow perimeter ................. .
Hydraulic radius ............... .
Top width ...................... .
Area .......................... ·.
Perimeter ...................... .
Percent full ................... .
Rectangular
Depth of Flow
11.2600 cfs
0.0180 ft/ft
0.0130
8.0000 in
60 .0000 in
3.9901 in
6.7727 fps
33.3238 cfs
1.6626 ft2
67.9803 in
3.5217 in
60.0000 in
3.3333 ft2
76.0000 in
49.8767 %
Concrete Flume -100 Year Storm
Channel Calculator
Given Input Data:
Shape .......................... .
Solving for .................... .
Flowrate ....................... .
Slope .......................... .
Manning' s n .................... .
Height ......................... .
Bottom width ................... .
Computed Results:
Depth .......................... .
Velocity ....................... .
Full Flowrate .................. .
Flow area ...................... .
Flow perimeter ................. .
Hydraulic radius ............... .
Top width ...................... .
Area ........................ · · · ·
Perimeter ...................... .
Percent full ................... .
Castlegate Subdivision -Section 13
College Station, Texas
Rectangular
Depth of Flow
15.1700 cfs
0.0180 ft /ft
0. 0130
8.0000 in
60.0000 in
4.8176 in
7.5572 fps
33.3238 cfs
2 .0074 ft2
69.6353 in
4.1510 in
60.0000 in
3 .3333 ft2
76.0000 in
60.2206 %
EXHIBIT A
Post-Development Drainage Area Map
11