HomeMy WebLinkAboutDrainage Report & Revised Drainage ReportDrainage Report
for
SourceN et Solutions, Inc. Office Building
College Station Business Center
College Station, Texas
January 2002
Developer:
Universal Computer Services, Inc.
200 Quality Circle
Coilege Station, Texas 77845
£rg12_r1r:_ed By:
TEXCON General Contractors
1707 Graham Road
College Station, Texas 77845
(979) 690-7711
CERTIFICATION
I, Joseph P. Schultz, Licensed Professional Engineer No . 65889, State of Texas, certify that this report
for the drainage design for the SourceNet Solutions, lnc. Office Building in the College Station
Business Center, College Station, Texas, was prepared by me in accordance with the provisions of the
City of College Station Drainage Policy and Design Standards for the owners hereof, with the
exception that storm water runoff detention is not being required for this project since the site
discharges directly into an existing drainage and then almost immediately into the 100-year floodplain
limits.
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TABLE OF CONTENTS
SOURCENET SOLUTIONS, INC. OFFICE BUILDING
COLLEGE ST A TI ON BUSINESS CENTER
CERTIFICATION ......................................................................................................................................................... 1
TABLE OF CONTENTS ............................................................................................................................................... 2
LIST OFT ABLES .......................................................................................................................................................... 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
STORM SEWER SYSTEM DESIGN .......................................................................................................................... 6
STORM RUNOFF DESIGN CALCULATIONS ......................................................................................................... ?
CONCLUSIONS ........................................................................................................................................................... 12
EXHIBIT A ................................................................................................................................................................... 13
Pre-Development Drainage Area Map
EXHIBIT B ................................................................................................................................................................... 15
Post-Development Drainage Area Map
APPENDIX A ............................................................................................................................................................... 17
Calculations
LIST OF TABLES
TABLE 1 -Rainfall Intensity & Time of Concentration Calculations ............................................. 5
TABLE 2 -Post-Development Runoff Information ......................................................................... 6
TABLE 3 -Storm Sewer Pipe Data .................................................................................................. 6
TABLE 4 -Curb Opening Data ........................................................................................................ 7
2
DRAINAGE REPORT
SOURCENET SOLUTIONS, INC. OFFICE BUILDING
COLLEGE ST A TION BUSINESS CENTER
INTRODUCTION
The purpose of this report is to provide the hydrological effects of the construction of a two-
story office building and parking area, and to show that the storm water runoff will be
controlled in such a manner so as to have minimal offsite or downstream impact.
GENERAL LOCATION AND DESCRIPTION
The project is located on a 16.68-acre tract located in the College Station Business Center in
College Station, Texas. Over half of the site is open land with grass, and the remainder of the
site is wooded with yaupons and oak and elm trees. The existing ground elevations range from
elevation 258 to elevation 288. 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 Alum Creek Drainage Basin. The site is located in a Zone X
Area according to the Flood Insurance Rate Map (FIRM) prepared by the Federal Emergency
Management Agency for Brazos County, Texas and incorporated areas dated July 2, 1992,
panel number 48041C0205-C. Zone X Areas are determined to be outside of the 500-yr
floodplain. The site is not within the 100-year floodplain. The 100-year floodplain along
Stream AC-1 is located approximately 300 feet downstream of this property on Lot 1, Block 4
of the Business Center. This information is from the Final Plat of Lots 1 & 3, Block 4, The
Business Center at College Station, Phase 3.
DEVELOPMENT DRAINAGE PATTERNS
The storm water runoff from the site flows east into a tributary of Alum Creek designated as
Stream AC-1 on the FIRM. The pre-and post development drainage area boundaries are
shown on Exhibits A and B, respectively. An existing storm sewer system runs through the
property, which carries the runoff from the Pebble Creek Subdivision, Section 5, and the park
through this site. A drainage swale was constructed above the storm sewer to carry runoff in
excess of the capacity of the storm sewer pipes. The runoff from a portion of the park area
originally ran across this site and into Quality Circle. A drainage ditch was constructed during
the park construction, which diverts the runoff from the park into the drainage swale, which
then follows the storm sewer system. Area inlets are located on the SourceNet Solutions site to
collect runoff from the park and this site. The drainage report for Pebble Creek, Section 5,
shows that the storm sewer pipes and the drainage swale through the SourceNet site were
designed with excess capacity to handle the runoff from the park and this site.
DRAINAGE DESIGN CRITERIA
The design parameters for the storm sewer are as follows :
J
• The Rational Method is utilized to determine peak storm water runoff rates for the storm
sewer design.
• Design Storm Frequency
Storm Sewer system & curb openings
• Runoff Coefficients
Pre-and post-development
(grass and wooded areas)
Impervious surfaces
10 and 100-year storm events
c = 0.30
c = 0.90
• 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. Refer to Table 1 for
calculations.
STORM WATER RUNOFF DETERMINATION
The peak runoff values were determined in accordance with the criteria presented in the
previous section for the 10 and 100-year storm events. The runoff coefficients are based on the
future development of this tract. The drainage areas for post-development are shown in Exhibit
B. Post-development runoff conditions are summarized in Table 2.
4
TABLE 1 -Rainfall Intensity & Time of Concentration Calculations
The Rational Method: I = b I (tc+d)"
Q= CIA I = Rainfall Intensity (in/hr)
tc = U(V*60)
Q = Flow (cfs)
A= Area (acres)
C = Runoff Coeff.
I = Rainfall Intensity (in/hr)
le = Time of concentration (min)
L = Length (ft)
V = Velocity (ft/sec)
Storm
Event
110
1100
Brazos County:
10 year storm
b = 80
d = 8.5
e = 0.763
Rainfall Intensity Values (in/hr)
le=
10 min
8.64
11 .64
100 year storm
b = 96
d = 8.0
e = 0.730
t = c
16.9 min
6.78
9.18
(Data taken from State Department of Highways and Public
Transportation Hydraulic Manual , page 2-16)
For le= 16.9 minutes, the following data was used:
le=
19.0 min
6.38
8.66
300' Overland Flow at 2% slope with velocity = 1.0 fps => travel time = 300'/1 .0 fps = 300 seconds
1000' Gully Flow at 0.5% slope with velocity = 1.4 fps=> travel time= 1000'/1.4 fps= 714 seconds
Total travel time= 1014 seconds/60sec/min = 16.9 minutes
For le= 19.0 minutes, the following data was used:
Distance along swale = 378' with velocity = 3.0 fps => travel time = 378'/3.0 fps = 126 seconds
126 seconds/60sec/min = 2.1 minutes
Add this to the time of concentration for the park (16.9 minutes) to get 19.0 minutes
5
TABLE 2 -Post-Development Runoff Information
Area c 1 O year storm 100 year storm
Area# tc (acres) 110 010 1100 0100 C1 C2 Crotal (min) (in/hr) (cfs) (in/hr) (cfs) Al Ai Total
1 0.14 0 0.14 0.9 0.3 0.90 10 8.63 1.09 11 .64 1.47
2 0.21 0 0.21 0.9 0.3 0.90 10 8.63 1.63 11 .64 2.20
3 0.21 0 0.21 0.9 0.3 0.90 10 8.63 1.63 11 .64 2.20
4 0.23 0 0.23 0.9 0.3 0.90 10 8.63 1.79 11.64 2.41
5 0.21 0 0.21 0.9 0.3 0.90 10 8.63 1.63 11 .64 2.20
6 0.17 0 0.17 0.9 0.3 0.90 10 8.63 1.32 11 .64 1.78
7 0.04 0 0.04 0.9 0.3 0.90 10 8.63 0.31 11 .64 0.42
8 0.025 0.19 0.22 0.9 0.3 0.37 10 8.63 0.69 11 .64 0.93
9 0.31 0.03 0.34 0.9 0.3 0.85 10 8.63 2.49 11 .64 3.35
10 0.17 0.09 0.26 0.9 0.3 0.69 10 8.63 1.55 11 .64 2.10
11 0.12 0.03 0.15 0.9 0.3 0.78 10 8.63 1.01 11 .64 1.36
12 0.045 0.05 0.10 0.9 0.3 0.58 10 8.63 0.48 11 .64 0.65
13 0.06 0.063 0.12 0.9 0.3 0.59 10 8.63 0.63 11 .64 0.85
14 0.21 0.06 0.27 0.9 0.3 0.77 10 8.63 1.79 11 .64 2.41
15 0.12 0.025 0.15 0.9 0.3 0.80 10 8.63 1.00 11 .64 1.34
16 0.20 0.056 0.26 0.9 0.3 0.77 10 8.63 1.70 11 .64 2.29
17 0.086 0.023 0.11 0.9 0.3 0.77 10 8.63 0.73 11 .64 0.98
18 0.10 0.007 0.11 0.9 0.3 0.86 10 8.63 0.80 11 .64 1.07
19 0.22 0 0.22 0.9 0.3 0.90 10 8.63 1.71 11 .64 2.30
20 0.597 0.093 0.690 0.9 0.3 0.82 10 8.63 4.88 11 .64 6.58
20A 0.215 0.015 0.230 0.9 0.3 0.86 10 8.63 1.71 11 .64 2.30
21 0.229 0.01 0.24 0.9 0.3 0.87 10 8.63 1.81 11.64 2.43
22 0.03 0.37 0.40 0.9 0.3 0.35 10 8.63 1.19 11.64 1.61
23 0.27 0.51 0.78 0.9 0.3 0.51 10 8.63 3.42 11.64 4.61
23A 0.20 0.01 0.21 0.9 0.3 0.87 10 8.63 1.58 11 .64 2.13
24 0.10 0 0.10 0.9 0.3 0.90 10 8.63 0.78 11.64 1.05
25 0.021 0.028 0.05 0.9 0.3 0.56 10 8.63 0.24 11 .64 0.32
26 0.06 2.01 2.07 0.9 0.3 0.32 10 8.63 5.67 11 .64 7.65
27 0.29 1.44 1.73 0.9 0.3 0.40 10 8.63 5.98 11 .64 8.07
28 0 9.98 9.98 0.9 0.3 0.30 16.9 6.78 20.30 9.18 27.50
STORM SEWER SYSTEM DESIGN
Storm sewer piping is proposed to collect roof drainage and discharge it into the existing storm
sewer system. Runoff will also be collected by an area drain inlet at the front of the building,
which will is trapped by the sidewalk leading to the building entrance. Table 3 summarizes the
storm sewer pipe data for the SourceNet Solutions site. All pipes pass both the 10-and 100-
year storm events.
TABLE 3 -Storm Sewer Pipe Data
Inlet Invert Outlet Invert 10 Year Storm 100 Year Storm
Pipe# Size Length Slope Elev Elev Design Flow V 10 %Full Design Flow V100 %Full
(in) (ft) (%) (ft) (ft) (cfs) (fps) (cfs) (fps)
1 12 60.3 1.00 281 .02 280.42 1.09 4.2 36 1.47 4.6 43
2 12 60.5 1.25 280.42 279.55 2.72 5.8 58 3.67 6.2 71
3 15 60.3 1.00 279.55 278.95 4.35 6.0 57 5.87 6.4 70
4 15 58.2 1.50 279.95 278.09 6.14 7.6 63 8.28 8.0 79
5 12 30.2 0.80 275.09 274.85 1.63 4.3 48 2.20 4.7 58
6 8 83.2 0.80 276.03 275.36 0.31 2.8 35 0.42 3.1 41
7 24 15.4 0.60 274.34 274.25 9.4 5.4 55 12.68 5.7 66
8 12 208.4 0.60 281.30 280.05 0.69 3.1 33 0.93 3.4 38
All pipes are HOPE, except Pipe 7, which is RCP; n = 0.012 for HOPE and n = 0.014 for RCP
6
STORM RUNOFF DESIGN CALCULATIONS
As previously stated, the storm water runoff from this site currently either flows into Quality
Circle, or flows through the existing storm sewer system or drainage swale where it then
discharges into an existing tributary of Alum Creek.
The runoff from this site will increase due to the building and parking lot construction. The
following calculations wi ll show that the post-development runoff into Quality Circle will not
exceed the capacity of the street to handle the runoff. Also, the calculations will show that the
post-development runoff that is captured and enters the existing storm sewer system is less than
or equal to the pre-development runoff that entered the system. The flow in the drainage swale
will be contained by construction of a new swale along the fire lane behind the building or in
the proposed driveway.
Table 4 summarizes the curb openings for the site, which allow the runoff to exit the parking
lot.
TABLE 4 -Curb Opening Data
10 Year Storm 100 Year Storm
Curb
Opening Drainage 010 Required Curb Opening 0100 Required Curb Opening
# Areas Width for 6'" depth Width for 6'" depth
(cfs) (ft) (cfs) (ft)
A 18 0.80 0.75 1.07 1.01
B 17, 18 1.52 1.44 2.05 1.94
c 16, 17, 18 3.22 3.04 4.34 4.10
D 15 1.00 0.94 1.34 1.27
E 14, 19 3.50 3.30 4.71 4.44
F 19 1.71 1.61 2.30 2.17
G 13 0.63 0.59 0.85 0.80
H 12 0.48 0.45 0.65 0.61
I 11 1.01 0.95 1.36 1.28
J 10, 11, 12 3.04 2.87 4.10 3.87
K 9 2.49 2.34 3.35 3.16
L 21 1.81 1.70 2.43 2.29
M . 16.59 . 26.65 .
N . 18.40 . 29.08 .
0 . 23.38 . 35.78 .
p . 23.37 . 37.27 .
• Refer to subsequent sections of the report for information on these openings
The Rational Method:
Q=CIA
Q = Flow (cfs)
A = Area (acres)
C = Runoff Coeff.
I = Rainfall Intensity (in/hr)
7
Actual Curb
Opening Width
(ft)
2
2
4
2
4
2
2
2
2
4
3
4
12
8
20
20
Quality Circle Drive -Post-Development Conditions
Straight Crown Flow (Solved to find actual depth of flow, y):
Q = 0.56 * (z/n) * S 112 * y8'3 => y = {QI [0.56 * (z/n) * S 112 ]} 318
(Equation from City of College Station Design Stand ards, page 30; Design Procedures for Straight Crowns)
Transverse (Crown) slope (ft/ft)= 0.02
n =Roughness coefficient= 0.018
z = Reciprocal of crown slope= 50
S = Street/gutter slope (ft/ft)
y =Depth of flow (ft)
Evaluating depth of flow in street -Quality Circle:
Street details:
48' Back to Back width (23 .5 ' lane width), 2% cross slope, 1.8% street grade
Receives flow from Areas 9 thru 19 and 27 -Q10 = 22.02 cfs
(Refer to Table 2) Q100 = 29.67 cfs
Solving Straight Crown Flow Equation above gives a depth, y10 = 5.16 inches and y100 = 5.77
inches. The depth of water for the 10-year storm event will not spread to the crown of the road,
and the depth for the 100-year storm event is less than the curb height, so the runoff is
contained within the street.
Runoff Through Site
Pre-Development Conditions:
Park Area
Drainage area= 9.98 acres
Time of concentration = 16.9 minutes (See Appendix A for calculations)
Existing Site (excluding park)
Drainage area= 2.06 acres
Time of concentration= 2.1 minutes (See Appendix A for calculations)
Total Area= 12.04 acres
Total Time of Concentration= 19.0 minutes
C=0.3
(Refer to Table 1 for Intensity values)
Using Rational Equation, Q10 = 23.04 cfs and Q 100 = 31.28 cfs
This runoff either enters the existing area Inlets 4 & 5 on the SourceNet site or it flows into the
drainage swale above the storm sewer. A 10' curb inlet (Inlet 3) will be constructed on the
existing storm sewer pipe to reduce the amount of runoff which enters the proposed parking lot
driveway.
Inlet in Sump, Weir Flow Equation:
L = Q I (3 * y12)
Where: L = length of inlet opening, feet
Q = flow at inlet, cfs
y =total depth of flow on inlet, feet
8
Calculating the amount of water captured by Inlet 3:
Using the above equation with y = 7 inches, and L = 10 feet , then Q = 13.35 cfs. This is the
amount of flow captured by Inlet 3. To account for 10% clogging, this Q is multiplied by 0.9 to
get Q = 12.02 cfs.
Calculating the Overflow from Inlet 3:
First determine total flow for Q10 and Q100 going to inlet. The inlet receives flow from Areas
28 and 20. (Refer to Table 2 for Area flow calculations)
Q10 = 20.30 cfs (area 28) + 6.60 cfs (area 20) = 26.90 cfs
Q100 = 27.48 cfs (area 28) + 8.89 cfs (area 20) = 36.37 cfs
Subtracting the flow captured by the inlet (12.02 cfs) gives
Q10-overnow = 14.88 cfs
Q100-overnow = 24.35 cfs
Checking sum of Inlets 3 & 4 to verify pre-development conditions are not exceeded:
Q1o = 12.02 cfs (Inlet 3) + 1.21 cfs (Inlet 4) = 13 .23 cfs < 23.04 cfs =>OK
Qioo = 12.02 cfs (Inlet 3) + 1.63 cfs (Inlet 4) = 13.65 cfs < 31.28 cfs =>OK
(Refer to Appendix A for Inlet 4 flow calculations)
Note: Depth of flow for Inlet 4 for Q100 is 1.72 inches.
If the existing storm sewer piping becomes full, the excess runoff will be controlled by the
proposed drainage swale and driveway. Also, a portion of the existing drainage where it
parallels the property line along Pebble Creek, Section 6, will be left intact to carry excess
runoff
Calculating flow for Inlet 5:
This inlet is being plugged, so there is no surface flow captured, however, Areas l thru 7 are
being piped directly into the system at this point, so 100% of this piped flow is "captured."
This runoff is primarily from the bui !ding roof drainage system. (Refer to Table 2 for Area
flow calculations)
Area _.Q.!Q_ _.QIOO_
1 1.12 cfs 1.52 cfs
2 1.61 cfs 2.16cfs
3 1.61 cfs 2.16cfs
4 1.77 cfs 2.38 cfs
5 1.61 cfs 2.16 cfs
6 1.29 cfs 1.73 cfs
7 0.33 cfs 0.45 cfs
QIO = 9.34 cfs Q1 00 = 12.56 cfs
Checking sum of Inlets 3, 4 & 5 to verify pre-development conditions are not exceeded:
Q1o = 13.23 cfs (Inlets 3&4) + 9.34 cfs (Inlet 5) = 22.57 cfs < 23.04 cfs =>OK
Q100 = 13.65 cfs (Inlets 3&4) + 12.56 cfs (Inlet 5) = 26.21 cfs < 31 .28 cfs =>OK
The parking lot and driveways will have curb openings to allow the runoff to exit the pavement
area. A maximum depth of 10" for the 100-year storm event was selected to size the openings.
Capacity of curb openings solving Weir Flow Equation:
Q = 3* L * y3'2
9
Where: Q = flow at inlet, cfs
L =length of inlet opening, feet
y = total depth of flow on inlet, feet
For 1-4' opening flowing -6" deep (y = 0.500'), Q = 4.24 cfs
7" deep (y = 0.583'), Q = 5.34 cfs
8" deep (y = 0.667'), Q = 6.54 cfs
9" deep (y = 0.750'), Q = 7.79 cfs
10" deep (y = 0.833'), Q = 9.12 cfs
12" deep (y = 1.00'), Q = 12.00 cfs
For 1-2' divider with water flowing over it's top -
l" deep (y = 0.083'), Q = 0.14 cfs
2" deep (y = 0.167'), Q = 0.41 cfs
3" deep (y = 0.250'), Q = 0.75 cfs
4" deep (y = 0.333 '), Q = 1.15 cfs
6" deep (y = 0.500'), Q = 2.12 cfs
For Curb Opening M: [3-4' openings with 2-2' dividers]
Receives overflow from Inlet 3 -Qio-overnow = 14.88 cfs
Plus flow from Area 20A -
(Refer to Table 2)
Total flow going thru Curb Opening M -
Qioo-overnow = 24.35 cfs
Qio = 1.71 cfs
Qioo = 2.30 cfs
Q10 = 16.59 cfs
Q100 = 26.65 cfs
Determine depth of flow for Q10 by trial and error:
For 6" deep: (3 openings* 4.24 cfs/opening) + (2 dividers* 0 cfs) = 12.72 cfs
12 .72 < 16.59 =>flows deeper than 6"
For 7" deep: (3 * 5.34 cfs) + (2 * 0.14 cfs) = 16.30 cfs
16.30 < 16.59 =>flows deeper than 7"
For 8" deep: (3 * 6.54 cfs) + (2 * 0.41cfs)=20.44 cfs
20.44 > 16.59 =>flows less than 8"
For Q10, the flow thru Curb Opening M is between 7" and 8" deep.
Similarly, for Q 100, the flow thru Curb Opening Mis between 9" and 10" deep.
The actual depth of water will be less than these values because when the water reaches a depth
of 5", the runoff will also flow to Curb Opening L, which is sized to accept additional flow.
Evaluating Drainage Swale:
Receives flow from Curb Opening M -
Plus flow from Area 21 -
(Refer to Table 2)
Total flow going thru Drainage Swale -
Swale details:
Qio = 16.59 cfs
Qioo = 26.65 cfs
Q 1 o = l . 81 c fs
Qioo = 2.43 cfs
Q10 = 18.40 cfs
Q100 = 29.08 cfs
2' bottom width, 1 :5 side slopes, Manning 's n = 0. 03. Slope = 0. 9%
10
Using the Channel Calculator with Q100 = 29.08 cfs first ("worst case") gives
Velocity= 3.5 fps & Depth of Flow = 13.4 inches
For Q10 = 18.40 cfs, Velocity = 3.1 fps & Depth of Flow = 10 .9 inches.
From this data, it is determined that using a swale with a minimum depth of 15 inches
will carry the 100-year flow. The swale will be seeded to establish grass cover to
prevent erosion.
For Curb Opening N: [2-4' openings with 2-2' dividers]
Total flow received from Drainage Swale -Q10 = 18.40 cfs
Q100 = 29.08 cfs
Determine depth of flow for Q100 by trial and error:
For 10" deep: (2 * 9.12 cfs) + (2 * 1.15 cfs) = 20.54 cfs
20.54 < 29.08 =>flows deeper than 10" for 100 year storm
add area from corner due to spreading:
20.54 cfs + 0.82 cfs (2" deep, 4' width) = 21 .36 cfs
21.36 < 18.40 => flows less than 1 O" for 10 year storm
For 12" deep: (2 * 12.00 cfs) + (2 * 2.12 cfs) = 28.24 cfs
28.24 < 29 .08 => flows deeper than 12" for 100 year storm
A berm will be constructed adjacent to the swale and Curb Opening N to force the runoff to
enter the driveway.
Evaluating street/driveway
Receives flow from Curb Opening N -
Plus flow from Area 23 -
(Refer to Table 2)
Total flow in street/driveway -
Street details:
Qio = 18.40 cfs
Qioo = 29.08 cfs
Qio = 3.39 cfs
Qioo = 4.56 cfs
Q10 = 21.79 cfs
Q100 = 33.64 cfs
25 ' Back to Back width (24 ' wide channel), 1% cross slope, 1% street grade
Solving Manning's Equation (n = 0.014) gives a depth of flow, y10 = 4.26" and a
velocity, v10 = 3.94 fps . For Q,00, y,00 = 5.1 O" and V1 00 = 4.68 fps.
For Curb Opening 0: [5-4' openings with 4-2' dividers]
Receives flow from street -Q 10 = 21.79 cfs
Plus flow from Area 23A -
(Refer to Table 2)
Total flow going thru Curb Opening 0 -
Evaluate using Inlets On Grade Equation -
Q100 = 33 .64 cfs
Qio = 1.59 cfs
Qioo = 2.14 cfs
Q10 = 23.38 cfs
Q100 = 35.78 cfs
(Refer to Appendix A for formula & calculations)
Using street grade= 1.3% and cross slope = 1.0% the following results are found :
Y10 = 4.33 inches
11
Y100 = 5.08 inches
Capacity of Curb Opening 0 -Q10 = 7.41 cfs (5 openings* 1.48 cfs/opening)
with Q10-bypass = 15 .97 cfs
Q 100 = 8.471 cfs (5 openings* 1.69 cfs/opening)
with Q100-bypass = 27.31 cfs
Evaluate street between Curb Openings 0 & P:
Receives bypass from Curb Opening 0 -Q10 = 15.97 cfs
Plus flow from Area 24 -
(Refer to Table 2)
Total flow in street -
Street details:
Q100 = 27.31 cfs
Q10 = 0.81 cfs
Qioo = 1.09 cfs
Q10 = 16.78 cfs
Q100 = 28.40 cfs
25 ' Back to Back width (24 ' wide channel), 1% cross slope, 1.3% street grade
Solving Manning's Equation (n = 0.014) gives a depth of flow, Y10 = 3.7" and a
velocity, V10 = 3.82 fps. For Q 100, y1oo = 4.5" and V100 = 4.74 fps .
For Curb Opening P: (5-4' openings with 4-2' dividers]
Receives bypass from Curb Opening 0 -Q10 = 15.97 cfs
Q100 = 27.31 cfs
Plus flow from Areas 8 & 24-26 -
(Refer to Table 2)
Area _Q!Q_
8 0.70 cfs
24 0.81 cfs
25 0.24 cfs
26 5.65 cfs
Q10 = 7.40 cfs
_Q,oo_
0.94 cfs
1.09 cfs
0.32 cfs
7.61 cfs
Q100 = 9.96 cfs
Total flow going thru Curb Opening P -Q10 = 23.37 cfs
Q100 = 37.27 cfs
Determine depth of flow for Q10 by trial and error:
For 6" deep: (5 * 4.24 cfs) + (4 * 0 cfs) = 21.20 cfs
21.20 < 23.37 =>flows deeper than 6"
For 7" deep: (5 * 5.34 cfs) + (4 * 0.14 cfs) = 27.26 cfs
27.26 > 23.37 =>flows less than 7"
For Q10, the flow thru Curb Opening P is between 6" and 7" deep.
Similarly, for Q 100, the flow thru Curb Opening P is between 8" and 9" deep.
CONCLUSIONS
The construction of this project will significantly increase the storm water runoff from this site.
However, the runoff will be routed to an existing drainage, which will carry the runoff to the
100-year floodplain. The increased flow in this tributary should not have a significant impact
on the surrounding property. No flood damage to downstream or adjacent landowners is
expected as a result of this development.
12
EXHIBIT A
Pre-Development Drainage Area Map
13
EXHIBIT B
Post-Development Drainage Area Map
15
APPENDIX A
Calculations
17
SourceNet Solutions
Inlets on Grade
The Rational Method:
Q =CIA
Q = Flow (cfs)
C = Runoff Coeff.
I = Rainfall Intensity (in/hr)
A = Area (acres)
Capacity of Inlets on grade:
Oc = 0.7 * (1/(H1 -H2)] * [H1
512-H2
512]
Oc = Flow capacity of inlet (cfs)
H1 =a+ y
Transverse (Crown) slope (fUft) = 0.010
Straight Crown Flow (Solved to find actual depth of flow, y):
Q = 0.56 * (z/n) * S 112 * y'13 c:> y = {Q I (0.56 * (z/n) * S 1121}318
(Eqn from C. of C.S. Design Standards. page 30; Design Procedures for Straight Crowns)
n =Roughness Coefficient= 0.018
z = Reciprocal of crown slope = 100
S = StreeUGutter Slope (fUft)
y = Depth of flow at inlet (ft)
H2 = a = gutter depression (2" Standard; 4" Recessed)
y = Depth of flow in approach gutter (ft)
L = 5•4', a= 2" = 0.167'
Inlet a,. a, .. s Y10 Y100 Oc10 Oc1oeyp1111 Oc100 Oc1ooeypass
# (cfs) {cfs) (ft/ft) (ft) I (in) (ft) I (in) (cfs) (cfs) (cfs) (cfs)
Curb Opening 0 23.380 35.780 0.0130 o.361 I 4.33 0.423 I 5.08 7.41 15.97 8.47 27.31
SourceNet Solutions
Inlet Length Calculations
Inlet#
3
4
Inlets In Sump
Length & Type Flow from A
Area# (acres)
1 o· Standard 28 9.98
20 0.92
10' Standard 22 0.4
Inlets in sumps, Weir Flow:
L = QI (3 * y312)¢ y = (QI 3L)213
L = Length of inlet opening (ft)
Q = Flow at inlet (cfs)
y = total depth of flow on inlet (ft)
max y for inlet in sump = 7" = 0.583'
Grate Inlet Design Calculations:
0 = 4.82 *(Ag)* y112
Inlet 1: Pipe 6 -010 = 0.3 cfs, 0 100 = 0.5 cfs
c a,.
(els)
0.3 20.30 ---0.83 6.60
0.35 1.21
Ag= 0.3/4.82 • (0.5)112 = 0.088 sq ft= 12.7 sq in
1 O year storm
Ccarryover Crotal QTotal-+10% L10-Req'd . L10-11etual
(els) from Inlet I (els) (els) (ft) (ft)
20.30 22.33 22.15 10 >----6.60 7.26
1.21 1.33 1.00 10
·usilg y_ = 1·"" 0.583'
=> 8" Round Grate OK (open area of grate = 20.3 sq inches; with 10% clogging)
Inlet 2: 0 10 = 1.2 cfs, 0 100 = 1.6 cfs
Ag = 1.2/4.82•(0.5)112 = 0.35 sq ft= 51 sq in • 2(for clogging) = 102 sq inches
=>Use V-5718 (with 50% clogging)
100 year storm
a, .. Ocarryover Crotal Orotal .. 10%
(els) (els) rrom In~• (els) (els)
27.48 27.48 30.23
8.89 8.89 9.78
1.63 1.63 1.79
CERTIFICATION
I, Joseph P. Schultz, Licensed Professional Engineer No. 65889, State of Texas, certify that this report
for the drainage design for the SourceNet Solutions, fnc . Office Building in the College Station
Business Center, College Station, Texas, was prepared by me in accordance with the provisions of the
City of College Station Drainage Policy and Design Standards for the owners hereof, with the
exception that storm water runoff detention is not being required for this project since the site
discharges directly into an existing drainage and then almost immediately into the I 00-year floodplain
limits.
u..,,.,
_,,,,,,,
--"is OF f" ,,
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; 0 •• ••• ····~-<f'ui'• '* .. ·. ., I*: ··*I. f···jas·E·········· .......... : .. ~ .. ~ ~~·:····f.~ .. ~~ .. ~~.~.Y.~!~ .... J
1-:P\ 65889 :$J lto..(\ ·~~G' ~<:>··· q.; "' .,~;· •• ISTE'?-.·· ~ .,:' \'"'.s't ......... 'liA0.
\\'.t>.C!NAL €JI"-.,,~.,
4 ... 3,..oz_
RE~~~~fJJ>
MAY 0 S 2001
LLEGE ST TION CO ENGINEERING
TABLE OF CONTENTS
SOURCENET SOLUTIONS, INC. OFFICE BUILDING
COLLEGE STATION BUSINESS CENTER
CERTIFICATIO N ......................................................................................................................................................... 1
TABLE OF CONTENTS ............................................................................................................................................... 2
LIST OF TABLES .......................................................................................................................................................... 2
INTRODUCTION .......................................................................................................................................................... 3
GENERAL LOCATION AND DESCRJPTION ......................................................................................................... 3
FLOOD HAZARD INFORMATION ........................................................................................................................... 3
DEVELOPMENT DRAINAGE PATTERNS .............................................................................................................. 3
DRAINAGE DESIGN CRITERJA ............................................................................................................................... 3
STORM WATER RUNOFF DETERMINATION ..................................................................................................... .4
STORM SEWER SYSTEM DESIGN .......................................................................................................................... 6
STORM RUNOFF DESIGN CALCULATIONS ......................................................................................................... 7
CONCLUSIONS ........................................................................................................................................................... 12
EXHIBIT A ................................................................................................................................................................... 13
Pre-Developme11t Drainage Area Map
EXHIBIT B ................................................................................................................................................................... 15
Post-Developme11t Drainage Area Map
APPENDIX A ............................................................................................................................................................... 17
Calculations
LIST OF TABLES
TABLE 1 -Rainfall Intensity & Time of Concentration Calculations ............................................. 5
TABLE 2 -Post-Development Runoff [nformation ......................................................................... 6
TABLE 3 -Storm Sewer Pipe Data .................................................................................................. 6
TABLE 4 -Curb Opening Data ........................................................................................................ 7
2
DRAINAGE REPORT
SOURCENET SOLUTIONS, INC. OFFICE BUILDING
COLLEGE STATION BUSINESS CENTER
INTRODUCTION
The purpose of this report is to provide the hydrological effects of the construction of a two-
story office building and parking area, and to show that the storm water runoff will be
controlled in such a manner so as to have minimal offsite or downstream impact.
GENERAL LOCATION ANO DESCRIPTION
The project is located on a 16.68-acre tract located in the College Station Business Center in
College Station, Texas. Over half of the site is open land with grass, and the remainder of the
site is wooded with yaupons and oak and elm trees. The existing ground elevations range from
elevation 258 to elevation 288. 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 Alum Creek Drainage Basin. The site is located in a Zone X
Area according to the Flood Insurance Rate Map (FIRM) prepared by the Federal Emergency
Management Agency for Brazos County, Texas and incorporated areas dated July 2, 1992,
panel number 48041 C0205-C. Zone X Areas are determined to be outside of the 500-yr
floodplain. The site is not within the 100-year floodplain. The 100-year floodplain along
Stream AC-1 is located approximately 300 feet downstream of this property on Lot 1, Block 4
of the Business Center. This information is from the Final Plat of Lots 1 & 3, Block 4, The
Business Center at College Station, Phase 3.
DEVELOPMENT DRAINAGE PATTERNS
The storm water runoff from the site flows east into a tributary of Alum Creek designated as
Stream AC-1 on the FIRM. The pre-and post development drainage area boundaries are
shown on Exhibits A and B, respectively. An existing storm sewer system runs through the
property, which carries the runoff from the Pebble Creek Subdivision, Section 5, and the park
through this site. A drainage swale was constructed above the storm sewer to carry runoff in
excess of the capacity of the storm sewer pipes. The runoff from a portion of the park area
originally ran across this site and into Quality Circle. A drainage ditch was constructed during
the park construction, wh ich diverts the runoff from the park into the drainage swale, which
then follows the storm sewer system. Area inlets are located on the SourceNet Solutions site to
collect runoff from the park and this site. The drainage report for Pebble Creek, Section 5,
shows that the storm sewer pipes and the drainage swale through the SourceNel site were
desi gned with excess capacity to handle the runoff from the park and this site.
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 stom1
sewer design.
• Design Storm Frequency
Storm Sewer system & curb openings
• Runoff Coefficients
Pre-and post-development
(grass and wooded areas)
lmpervious surfaces
10 and 100-year storm events
c = 0.30
c = 0.90
• Rainfall lntensity values for Brazos County for a minimum time of concentration of 10
minutes can be found in Table l. 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 l 0 minutes. Therefore, a minimum tc of l 0 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. Refer to Table l for
calculations.
STORM WATER RUNOFF DETERMINATION
The peak runoff values were detem1ined in accordance with the criteria presented in the
previous section for the 10 and 100-year storm events. The runoff coefficients are based on the
future development of this tract. The drainage areas for post-development are shown in Exhibit
B. Post-development runoff conditions are summarized in Table 2.
TABLE 1 -Rainfall Intensity & Time of Concentration Calculations
The Rational Method: I= b I (tc+dt
Q=CIA I = Rainfall Intensity (in/hr)
tc = U(V*60)
Q = Flow (cfs)
A= Area (acres)
C = Runoff Coeff.
I = Rainfall Intensity (in/hr)
le = Time of concentration (min)
L = Length (ft)
V = Velocity (fUsec)
Storm
Event
110
1100
Brazos County:
10 year storm
b = 80
d = 8.5
e = 0.763
Rainfall Intensity Values (in/hr)
t.,=
10min
8.64
11 .64
100 year storm
b = 96
d = 8.0
e = 0.730
t.,=
16.9 min
6.78
9.18
(Data taken from State Department of Highways and Public
Transportation Hydraulic Manual , page 2-16)
For tc = 16.9 minutes, the following data was used:
le=
19.0 min
6.38
8.66
300' Overland Flow at 2% slope with velocity = 1.0 fps => travel time = 300'/1 .0 fps = 300 seconds
1000' Gully Flow at 0.5% slope with velocity= 1.4 fps =>travel time = 1000'/1.4 fps = 714 seconds
Total travel time= 1014 seconds/60sec/min = 16.9 minutes
For tc = 19.0 minutes, the following data was used:
Distance along swale = 378' with velocity = 3.0 fps => travel time = 378'/3.0 fps = 126 seconds
126 seconds/60sec/min = 2.1 minutes
Add this to the time of concentration for the park (16.9 minutes) to get 19.0 minutes
TABLE 2 -Post-Development Runoff Information
Area c 10 year storm 100 year storm
Area# (acres) tc
110 0 10 1100 0100 C1 C2 Cro1a1 A1 Az Total (min) (in/hr) (cfs) (in/hr) (cfs)
1 0.14 0 0.14 0.9 0.3 0.90 10 8.63 1.09 11.64 1.47
2 0.21 0 0.21 0.9 0.3 0.90 10 8.63 1.63 11.64 2.20 >------
3 0.21 0 0.21 0.9 0.3 0.90 10 8.63 1.63 11.64 2.20
4 0.23 0 0.23 0.9 0.3 0.90 10 8.63 1.79 11.64 2.41
5 0.21 0 0.21 0.9 0.3 0.90 10 8.63 1.63 11.64 2.20
6 0.17 0 0.17 0.9 0.3 0.90 10 8.63 1.32 11.64 1.78
7 0.04 0 0.04 0.9 0.3 0.90 10 8.63 0.31 11.64 0.42
8 0.025 0.19 0.22 0.9 0.3 0.37 10 8.63 0.69 11.64 0.93
9 0.32 0.03 0.35 0.9 0.3 0.84 10 8.63 2.56 11.64 3.44
9A 0.045 0.003 0.047 0.9 0.3 0.87 10 8.63 0.35 11.64 0.48
10 0.18 0.09 0.27 0.9 0.3 0.70 10 8.63 1.65 11.64 2.23
11 0.13 0.03 0.16 0.9 0.3 0.79 10 8.63 1.06 11.64 1.43
12 0.07 0.008 0.073 0.9 0.3 0.84 10 8.63 0.53 11.64 0.71
13 0.08 0.04 0.12 0.9 0.3 0.69 10 8.63 0.71 11.64 0.96
13A 0.11 0.04 0.15 0.9 0.3 0.74 10 8.63 0.98 11 .64 1.32
14 0.23 0.06 0.29 0.9 0.3 0.77 10 8.63 1.91 11.64 2.57
15 0.12 0.027 0.14 0.9 0.3 0.79 10 8.63 0.97 11.64 1.31
16 0.21 0.046 0.25 0.9 0.3 0.79 10 8.63 1.74 11.64 2.34
17 0.086 0.023 0.11 0.9 0.3 0.78 10 8.63 0.73 11.64 0.98
18 0.10 0.007 0.11 0.9 0.3 0.86 10 8.63 0.79 11.64 1.07
19 0.26 0 0.26 0.9 0.3 0.90 10 8.63 2.02 11.64 2.72
20 0.595 0.077 0.67 0.9 0.3 0.83 10 8.63 4.82 11 .64 6.50
21 0.43 0.40 0.83 0.9 0.3 0.61 10 8.63 4.39 11.64 5.92
22 0.21 0.006 0.21 0.9 0.3 0.88 10 8.63 1.63 11.64 2.20
23 0.24 0.33 0.57 0.9 0.3 0.55 10 8.63 2.70 11.64 3.64
24 0.39 0.01 0.40 0.9 0.3 0.88 10 8.63 3.08 11.64 4.16
25 0.07 0.58 0.65 0.9 0.3 0.36 10 8.63 2.04 11.64 2.74
26 0.07 1.53 1.60 0.9 0.3 0.33 10 8.63 4.53 11.64 6.10
27 0.37 1.11 1.48 0.9 0.3 0.45 10 8.63 5.75 11.64 7.75
28 0 10.13 10.13 0.9 0.3 0.30 16.9 6.78 20.60 9.18 27.91
STORM SEWER SYSTEM DESIGN
Storm sewer piping is proposed to collect roof drainage and discharge it into the existing storm
sewer system. Runoff will also be collected by an area drain inlet at the front of the building,
which will is trapped by the sidewalk leading to the building entrance. Table 3 summarizes the
storm sewer pipe data for the SourceNet Solutions site. All pipes pass both the l 0-and l 00-
year storm events.
TABLE 3 -Storm Sewer Pipe Data
Inlet Invert Outlet Invert 10 Year Storm 100 Year Storm
Pipe# Size Length Slope Elev Elev Design Flow V10 % Full Design Flow V100 % Full
(in) (ft) (%) (ft) (ft) (cfs) (fps) (cfs) (fps)
1 12 60.3 1.00 281 .02 280.42 1.09 4.2 36 1.47 4.6 43
2 12 60.5 1.25 280.42 279.55 2.72 5.8 58 3.67 6.2 71 --~---
3 15 60.3 1.00 279.55 278.95 4.35 6.0 57 5.87 6.4 70
-
4 15 58.2 1.50 279.95 278.09 6.14 7.6 63 8.28 8.0 79
!-·------
5 12 30.2 0.80 275.09 274.85 1.63 4.3 48 2.20 4.7 58
1----------·--6 8 83.2 0.80 276.03 275.36 0.31 2.8 35 0.42 3.1 41 -------------I-----------------------7 24 15.4 0.60 274.34 274.25 9.4 5.4 55 12.68 5.7 66 ----------· ----------------------
8 12 208.4 0.60 281 .30 280.05 0.69 3.1 33 0.93 3.4 38
All pipes are HOPE, except Pipe 7, which is RCP; n = 0.012 for HOPE and n = 0.014 for RCP
6
STORM RUNOFF DESIGN CALCULATIONS
As previously stated, the storm water runoff from this site currently either flows into Quality
Circle, or flows through the existing storm sewer system or drainage swale where it then
discharges into an existing tributary of Alum Creek.
The runoff from this site will increase due to the building and parking lot construction. The
following calculations will show that the post-development runoff into Quality Circle will not _
exceed the capacity of the street to handle the runoff. Also, the calculations wi II show that the
post-development runoff that is captured and enters the existing storm sewer system is less than
or equal to the pre-development runoff that entered the system. The flow in the drainage swale
will be contained by construction of a new swale along the fire lane behind the building or in
the proposed driveway.
Table 4 summarizes the curb openings for the site, which allow the runoff to exit the parking
lot.
TABLE 4 -Curb Opening Data
10 Year Storm 100 Year Storm
Curb Actual Curb
Opening Drainage
010
Required Curb Opening 0100 Required Curb Opening Opening Width
# Areas Width for 6" depth Width for 600 depth
(cfs) (ft) (cfs) (ft) (ft)
A 18 0.79 0.75 1.07 1.00 2
B 17, 18 1.52 1.43 2.05 1.93 2
c 16, 17, 18 ).26 3.07 4.39 4.14 4
D 15 0.97 0.92 1.31 1.24 2
E 14, 19 3.93 3.70 5.30 4.99 4
F 19 2.02 1.90 2.72 2.57 2
G 13 0.71 0.67 0.96 0.90 2
H 12 0.53 0.50 0.71 0.67 2
I 11 1.06 1.00 1.43 1.35 2
J 10, 11, 12 3.24 3.06 4.37 4.12 4
K 9 2.56 2.41 3.44 3.25 3
L 22 1.63 1.54 2.20 2.07 4
M . 17.79 . 28.31 . 12
N . 19.42 . 30.51 . 8
0 . 27.24 . 41 .05 . 20
p . 5.22 . 7.03 . 8
Q 9A 0.35 0.33 0.48 0.45 2
R 13, 13A 1.69 1.59 2.28 2.15 2
• Refer to subsequent sections of the report for information on these openings
7
Quality Circle Drive -Post-Development Conditions
Straight Crown Flow (Solved to find actual depth of flow, y):
Q = 0.56 * (z/n) * S 112 * y813 => y = {Q I [0.56 * (z/n) * S 112 ] } 318
(Equation from City of College Station Design Standards, page 30; Design Procedures for Strai ght Crowns)
Transverse (Crown) slope (ft/ft)= 0.02
n = Roughness coefficient = 0.0 l 8
z = Reciprocal of crown slope = 50
S =Street/gutter slope (ft/ft)
y =Depth of flow (ft)
Evaluating depth of flow in street -Quality Circle:
Street details:
48 ' Back to Back width (23.5 ' lane width), 2% cross slope, 1.8% street grade
Receives flow from Areas 9 thru l 9 and 27 -
(lncluding Areas 9A & l3A)(Refer to Table 2)
Qio = 21.75 cfs
Qioo = 29.3 l cfs
Solving Straight Crown Flow Equation above gives a depth, y10 = 5.14 inches and Y100 = 5. 75
inches. The depth of water for the 10-year storm event will not spread to the crown of the road,
and the depth for the l 00-year stom1 event is less than the curb. height, so the runoff is
contained within the street.
Runoff Through Site
Pre-Development Conditions:
Park Area
Drainage area = 9.98 acres
Time of concentration= 16.9 minutes (See Table l for calculations)
Existing Site (excluding park)
Drainage area = 2.06 acres
Time of concentration = 2. l minutes (See Table l for calculations)
Total Area= 12.04 acres
Total Time of Concentration= 19.0 minutes c =0.3
(Refer to Table l for Intensity values)
Using Rational Equation, Q 10 = 23.04 cfs and Q 100 = 31.28 cfs
This runoff either enters the existing area lnlet 5 on the SourceNet site or it flows into the
drainage swale above the storm sewer. A l O' curb inlet (Inlet 3) will be constructed on the
existing storm sewer pipe to reduce the amount of runoff which enters the proposed parking lot
driveway.
Inlet in Sump, Weir Flow Equation:
L = Q I (3 * /12)
Where: L = length of inlet opening, feet
Q = flo w at inlet, cfs
y =total depth of flow on inlet, feet
8
Calculating the amount of water captured by lnlet 3:
Using the above equation with y = 7 inches, and L = 10 feet, then O = 13.35 cfs. This is the
amount of flow captured by lnlet 3. To account for 10% clogging, this O is multiplied by 0.9 to
get Q = 12.02 cfs.
Calculating the Overflow from inlet 3:
First determine total flow for 0 10 and Q100 going to inlet. The inlet receives flow from Areas
28 and 20. (Refer to Table 2 for Area flow calculations)
OIO = 20.60 cfs (area 28) + 4.82 cfs (area 20) = 25.42 cfs
0100 = 27.91 cfs (area 28) + 6.50 cfs (area 20) = 34.41 cfs
Subtracting the flow captured by the inlet (12.02 cfs) gives
Q10-overnow = 13.40 cfs
Q100-overnow = 22.39 cfs
Checking lnlet 3 to verify pre-development conditions are not exceeded:
Qio = Qioo = 12.02 cfs < 23 .33 cfs => OK
Calculating flow for Existing lnlet 4:
This inlet will be plugged for this development.
If the existing storm sewer piping becomes full, the excess runoff will be controlled by the
proposed drainage swale and driveway. Also, a portion of the existing drainage where it
parallels the property line along Pebble Creek, Section 6, will be left intact to carry excess
runoff.
Calculating flow for Existing lnlet 5:
This inlet is being plugged, so there is no surface flow captured, however, Areas 1 thru 7 are
being piped directly into the system at this point, so 100% of this piped flow is "captured."
This runoff is primarily from the building roof drainage system. (Refer to Table 2 for Area
flow calculations)
Area _Q!Q_ _QIOO _
I 1.09 cfs 1.47 cfs
2 1.63 cfs 2.20 cfs
3 1.63 cfs 2.20 cfs
4 1.79 cfs 2.4 1 cfs
5 1.63 cfs 2.20 cfs
6 1.32 cfs 1.78 cfs
7 0.31 cfs 0.42 cfs
0 10 = 9.40 cfs 0 100 = 12.68cfs
Checking sum of lnlets 3 & 5 to verify pre-development conditions are not exceeded:
0 10 = 12.02 cfs (Inlet 3) + 9.40 cfs (In let 5) = 21.42 cfs < 23 .04 cfs =>OK
0 100 = 12.02 cfs (Inlet 3) + 12.68 cfs (Inlet 5) = 24.70 cfs < 31.28 cfs =>OK
The parking lot and driveways will have curb openings to allow the runoff to exit the pavement
area. A maximum depth of IO" for the I 00-year storm event was selected to size the openings.
Capacity of curb openings solving Weir Flow Equation:
0 = 3* L * /12
<)
Where: Q = flow at inlet, cfs
L =length of inlet opening, feet
y =total depth of flow on inlet, feet
For 1-4 ·opening flowing -4" deep (y=0.333 '), Q=2.3 l cfs
5" deep (y=0.417'), Q=3.23 cfs
6" deep (y = 0.500'), Q = 4.24 cfs
7" deep (y = 0.583 '), Q = 5.34 cfs
8" deep (y = 0.667'), Q = 6.54 cfs
9" deep (y = 0.750'), Q = 7.79 cfs
1 O" deep (y = 0.833 '), Q = 9.12 cfs
12" deep (y = 1.00'), Q = 12 .00 cfs
For 1-2' divider with water flowing over it's top -
l " deep (y = 0.083 '), Q = 0 .14 cfs
2" deep (y = 0.167'), Q = 0.41 cfs
3" deep (y = 0.250'), Q = 0.75 cfs
4" deep (y = 0.333 '), Q = 1.15 cfs
6" deep (y = 0.500'), Q = 2.12 cfs
For Curb Opening M: [3-4' openings with 2-2' dividers]
Receives overflow from Inlet 3 -Qi o-overtlow = 13.40 cfs
Plus flow from Area 21 -
(Refer to Table 2)
Total flow going thru Curb Opening M -
Q100-ovcrtlow = 22 .39 cfs
Qio = 4.39 cfs
Q100 = 5.92 cfs
Q10 = 17.79 cfs
Q100 = 28.31 cfs
Determine depth of flow for Q 10 by trial and error:
For 6" deep: (3 openings * 4.24 cfs/opening) + (2 dividers * 0 cfs) = 12. 72 cfs
12 .72 < 17 .74 =>flows deeper than 6"
For 7" deep: (3 * 5.34 cfs) + (2 * 0.14 cfs) = 16.30 cfs
16.30 < 17.74 =>flows deeper than 7"
For 8" deep: (3 * 6.54 cfs) + (2 * 0.41 cfs) = 20.44 cfs
20.44 > 17 .74 =>flows less than 8"
For Q10, the flow thru Curb Opening M is between 7" and 8" deep.
Similarly, for Q 100, the flow thru Curb Opening Mis between 9" and 10" deep.
The actual depth of water will be less than these values because when the water reaches a depth
of 6'', the runoff will also flow to Curb Opening L, which is sized to accept additional flow .
Evaluating Drainage Swale:
Receives flow from Curb Opening M -
Plus flow from Area 22 -
(Re fer to Table 2)
Total flow going thru Drainage Swale -
10
Q1 0 = 17.74 cfs
Qi oo = 28.31 cfs
Q1 0 = 1.63 cfs
Qioo = 2.20 cfs
Q10 = 19.42 cfs
Q100 = 30.51 cfs
Swale details:
2' bottom width, 1 :5 side slopes, Manning's n = 0. 03, Slope = 0. 9%
Using the Channel Calculator with Q 100 = 30.51 cfs first ("worst case") gives
Velocity = 3.5 fps & Depth of Flow = 15 .6 inches
For Q10 = 19.42 cfs, Velocity = 3.1 fps & Depth of Flow = 13.2 inches.
From this data, it is determined that using a swale with a minimum depth of 18 inches
will carry the 100-year flow. The swale will be seeded to establish grass cover to
prevent erosion.
For Curb Opening N: [2-4' openings with 1-2' divider]
Total flow received from Drainage Swale -Q 1o = 19.42 cfs
Q100 = 30.51 cfs
Determine depth of flow for Q100 by trial and error:
ForlO"deep: (2*9.12cfs)+(l * l.15cfs)=l9.39cfs
19.39 < 30.51 => flows deeper than 1 O" for 100 year storm
add area from corner due to spreading:
19.39 cfs + 0.82 cfs (2" deep, 4' width)= 20.21 cfs
20.21 < 19.42 => flows less than 1 O" for 10 year storm
For 12" deep: (2 * 12.00 cfs) + (1 * 2.12 cfs) = 26.12 cfs
26.12 < 30.51 => flows deeper than 12" for 100 year storm
A berm will be constructed adjacent to the swale and Curb Opening N to force the runoff to
enter the driveway.
Evaluating street/driveway
Receives flow from Curb Opening N -
Plus flow from Area 23 -
(Refer to Table 2)
Total flow in street/driveway -
Street details:
Qio = 19.42 cfs
Qioo = 30.51 cfs
Q10 = 2. 70 cfs
Qioo = 3.64 cfs
Q10 = 22.12 cfs
Q100 = 34.15 cfs
25' Back to Back width (24 ' wide channel), 1% cross slope, 1% street grade
Solving Manning's Equation (n = 0.014) gives a depth of flow, y10 = 4.30" and a
velocity, v,0 = 3.96 fps . For Qioo, Y100 = 5.13" and V100 = 4. 70 fps .
For Curb Opening 0 : [5-4' openings with 4-2' dividers]
Receives flow from street -Q10 = 22.12 cfs
Plus flow from Areas 24 & 25 -
(Refer to Table 2)
Total flow going thru Curb Opening 0 -
11
Qioo = 34.15 cfs
Qi o = 5.12 cfs
Q1 00 = 6.90 cfs
Q10 = 27.24 cfs
Q IOO = 41.05 cf S
Determine depth of flo w for Q10 by tri al and error:
Fo r 6" deep: (5 * 4.24 cfs) + (4 * 0 cfs) == 2 1.20 cfs
21.20 < 27.24 ==> fl ows deeper than 6"
Fo r 7" deep: (5 * 5.3 4 cfs) + ( 4 * 0.14 cfs) == 27.26 cfs
27.26 > 27.24 ==> fl ows just less than 7"
For Q10, the flow thru Curb Opening 0 is between 6" and 7" deep.
Similarly, for Q 100, the flow thru Curb Opening 0 is between 8" and 9" deep.
For Curb Opening P: [2-4' openings with 1-2 ' di vider]
Receives flow from Areas 8 & 26 -
(Refer to Table 2)
Area _QlQ_ _QIOO _
8 0.69 cfs 0.93 cfs
26 4.53 cfs 6.10 cfs
Q10 = 5.22 cfs Q100 = 7.03 cfs
Total flo w going thru Curb Opening P -Q10 == 5.22 cfs
Q100 == 7.03 cfs
Determine depth of flo w for Q10 by trial and error:
Fo r 4" deep: (2 * 2.31 cfs) + (1 * 0 cfs) == 4.62 cfs
4.62 < 5.22 ==> flo ws deeper than 4"
For 5" deep: (2 * 3.23 cfs) + (1 * 0 cfs) == 6.46 cfs
6.46 > 5.22 ==> flo ws less than 5"
For Q10, the flow thru Curb Opening P is between 4" and 5" deep.
Similarly, for Q100, the flow thru Curb Opening P is between 5" and 6" deep.
CONCLUSIONS
The construction of this project will increase the storm water runoff from this site. However,
the runoff will be routed to an existing drainage, which will carry the runoff to the 100-year
floodplain. The increased flow in this tributary should not have a significant impact on the
surrounding property. No flood damage to downstream or adjacent landowners is expected as a
result of this development. '
12
EXHlBIT A
Pre-Development Drainage Area Map
I , -'
EXHIBIT B
Post-Development Drainage Area Map
15
APPENDIX A
Calculations
17
SourceNet Solutions
Inlet Length Calculations -Revised
Inlet#
3
Inlets In Sump
Length & Type Flow from A
Area# (acres)
10' Standard 28 10.13
20 0.67
Inlets in sumps, Weir Flow:
L = QI (3 * y312)¢ y = (Q I 3L)213
L = Length of inlet opening (ft)
Q =Flow at inlet (cfs)
y = total depth of flow on inlet (ft)
max y for inlet in sump = 7" = 0.583'
Grate Inlet Design Calculations:
0 = 4.82 *(Ag)* y112
Inlet 1: Pipe 6 -0 10 = 0.3 cfs, 0 100 = 0.5 cfs
c
0.3
0.83
O,o
(els)
20.60
4.80
Ag= 0.3/4.82 • (0.5)112 = 0.088 sq ft = 12.7 sq in
10 year storm
O carry over Oro1a1 Orotal•10% L10-Req'd
(els) from inlet I (els) (els) (ft)
20.60 22.66 20.93
4.80 5.29
=> 8" Round Grate OK (open area of grate = 20.3 sq inches; with 10% clogging)
Inlet 2: 0 10 = 1.2 cfs, 0 100 = 1.6 cfs
Ag = 1.2/4.82 •(0.5)112 = 0.35 sq ft = 51 sq in • 2(for clogging) = 102 sq inches
=> Use V-5718 (with 50% clogging)
100 year storm
L10-actuat 0100 Qcarryover Orotal O rotal•10%
(ft) (els) (els) from inlet• (els) (els)
tO 27.90 27.90 30.69
6.47 6.47 7.12