HomeMy WebLinkAbout53 Development Permit 343 Henton Subd Lot 3DRAINAGE COMPUTATIONS
/or
HENTON SUBDIVISION
College Station, Brazos County, Texas
Prepared for
J.V. Henton
HC 30, Box6M
Brownwood, Texas
Telephone: (915) 643-6486
Prepared by.·
Garrett Engineering
4444 Carter Creek Parkway -Suite 108
Bryan, Texas 77802
Telephone: (409) 846-2688
•
* Revised: August, 1996 *
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DRAINAGE COMPUTATIONS 0' ~{)~
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HENTON SUBDIVISION (~,~'// i,,
College Station, Braws County, Texas 1fJ°
Prepared for
J.V. Henton
HC 30, Box 6M
Brownwood, Texas
Telephone: (915) 643-6486
Prepared by:
Garrett Engineering
4444 Carter Creek Parkway -Suite 108
Bryan, Texas 77802 "
Telephone: (409) 846-2688
* Revised: August, 1996 *
·PROJECT
LOCATIO~
CC>LU:61: ST.ATl{)p..j
VICINITY MAP
-DOt to 8Cai~ -
VICINITY MAP
HENI'ON SUBDTVISJON • lot 3: College S101ion. BrtlZDs County , Tu.a.s
DRAINAGE COMPUTATIONS
for
HENTON SUBDIVISION ·Lot 3
College Station, Brazos County, Texas
*Revised: August, 1996 *
SUMMARY
Henton Subdivision is located off Lincoln Avenue across from the T-intersection at Munson Road in
College Station, Brazos Collllty, Texas. (See following Vicinity Map.) This site is to be divided into 14
lots -with a detention pond located within the commons area to the east of lot 6. These 14 lots are
established within the bounds of the 3.51 acre tract The project will include the construction of Alleys
"A" and "B" as shown at the end of this report. These concrete alleys are 28 foot (alley "A") and 24 foot
(alley "B") back-of-curb to back-of-curb in width of paved drive surface and provide access to all 14 lots
from Lincoln Avenue.
DRAINAGE CALCULATIONS
The total drainage area tributary to these calculations is 7.05 acres (due to the inclusion of the adjacent,
upstream 3.54 acre tract). The pre-development run-off coefficient was estimated to be 0.40. Taking into
account typical residential site improvements and existii;1g conditions on the adjacent tract, a total post-
development run-off coefficient of 0.61 was computed (for the Henton tract).
A detention facility is to be constructed in the southeast comer of the property as designated on the
attached plans. The detention pond is designed to facilitate up to and including the 100-year storm event,
and discharge at a rate less than the current pre-development peak discharge rate. The detention pond
will intercept, store and meter run-off collected from 1.42 acres. The run-off generated on 1.66 acres
(consisting of mostly lots 7-14) of the Henton tract will "free-flow" from the project site unmetered. The
run-off from the remaining 3.98 acres {which consists of 3.55 acres from Putz and 0.43 acres from the
Henton tract) will be collected by a portion of alley "A" and will flow down alley "B" to the creek along
the east side of the Henton tract The total run-off from the Putz tract was calculated to be 16.53 cfs, and
the total run-off from a part of alley "A", alley "B ", and the small area of green space along the property
line along the Putz property was calculated to be 4.45 cfs. This results in a total of 20.98 cfs that will
drain down alley "B". It also results in 16.28 cfs of free flow from the Henton tract alone (10.70 cfs from
lots 7-14, 4.45 cfs from alley "B" and part of alley" A", and 1.14 cfs from the 6" diameter outlet), which is
slightly less than the predevelopment discharge rate of 16.34 cfs.
From the computations on page 3 incorporating the limiting basin of 3.51 acres, the pre-development run-
off coefficient being assumed at 0.40, and a minimum time of concentration of 10 minutes, the relative
pre-development peak discharge late ("Q") was determined. The Rational Method was utilized to
compute the estimated peak pre-development discharge rates. The equation that represents the Rational
Method is as follows: Q = CIA. "Q" is the peak discharge rate in cubic feet per second, "C" is the run-off
coefficient, assuuied to be 0.40 in the pre-development condition, "I" is the storm intensity in inches per
hour, and" A" is the area of the Henton basin in acres (3.51).
From the computations on page 4, based on the tributary area of 3.51 acres, the post-development run-off
coefficient being assumed at 0.61, and a minimum time of concentration of 10 minutes, the relative pre-
development peak discharge rate ("Q") was determined assuming the same minimum time of
concentration of 10 minutes. The Rational Method was again utilized to compute the estimated peak post-
development discharge rates. The equation that represents the Rational Method is as follows: Q = CIA,
where; "Q" is the peak discharge rate in cubic feet per second. "C" is the run-off coefficient. in the ~t
development condition "C" is assumed to be 0.61, "I" is the storm intensity in inches per hour and "A" is
the area of the basin in acres.
The comparison between the peak pre-development discharge rate and ~t-development discharge rate is
included as an aid to the designer to establish a preliminary "target" for the ne.cessary volwne in the
proposed detention facility. This preliminary estimate is arrived at by simply determining the volume
generated from the difference of the pre-development and post-development hydrographs (See pages 11-
13 for pre-development and ~t-development hydrographs). Please note that the ~t-development
hydrograph shown on these same graphs assumes no detention at this time.
Page 9 provides computations that determine the ~t-development run-off that is not routed through the
detention facility ("free-flow"), and similarly, the ~t-development run-off that is collected and routed
through the detention facility is shown on page 10. These are tabulations of the pre-development and
~t-development hydrograph information based on the computations performed on page 5 and page 6.
The ~t-development hydrograph assumes no detention to facilitate direct comparison in the pre-and
post-development conditions .
These computations on page 6 determine the actual peak ~t-development "inflow" rate supplied to the
proposed detention facility . From this "inflow" rate and the time of concentration (10 minutes minimum),
a hydrograph as tabulated on page 10 can be derived. The hydrograph is triangular in shape and is based
on the standard scs unit hydrograph with time to peak set equal to the time of concentration and the total
time base set at 3.00 times the time of concentration. Please note that a 30 minute total storm event
duration was utilized exclusively throughout this report due to the relative scope of the project
Having derived the preliminary volwne requirements and "allowable peak" discharge rate, it is now
possible to design the detention facility and outlet control structure. Page 14 presents a depth versus
volwne graph of the detention facility in addition to a tabulation of critical points reflecting the pond
geometry and storage characteristics. The maximwn detention pond volwne ne.cessary to provide
sufficient storage for the.100-year storm condition was determined to be 7,255 ft3• However, to provide
the required 0.5 ft of "freeboard", the volwne size was increased to 10,993 ft3 with a maximwn capacity
surface elevation of 304.50.
Page 15 supplies a tabulation and rating curve for the proposed outlet control structure. A 6" diameter
pipe outlet control structure was chosen due to depth verses discharge ratio characteristics. Page 16
presents a tabulation of the relationship between discharge from the detention and the dimensionless
quantity 2S/t-O and Storage Indication Curve for the detention facility based on the aforementioned
physical characteristics of the detention pond, storage volume, inflow hydrograph, and rating curve for the
outlet structure. The storage indication curve as shown on page 14 is a graphical solution to the equation
presented in the "Draina£:e Po1icy And Desj2n SlaIJdards" as follows:
2s1
01 +Ii)+(----------01) =
dt
2s2
(----------+ Oi)
dt
Page 17 through Page 28 present simulations of the 2, 5, 10, 25, 50, and 100 year storm events. The first
page of each storm simulation represents the tabulated data for the storm event The maximum depth
achieved in the detention pond during each simulation is shown below as is the time period during which
this maximum depth occurs after the beginning of the storm event The calculated peak discharge rate
corresponding to this depth over same period is also shown below. Additionally, the maximum water
surface elevation achieved during each event and the maximum peak discharge during this event are listed
below . The second page of each storm simulation represents the pre-development hydrograph, the ~t-
development hydrograph (with no detention) and the post-development hydrograph as routed through the
detention pond as well as the post-development "free-flow" hydrograph for each storm event
Page 29 through page 34 provide total inflow/outflow hydrographs for the 2, 5, 10, 25 , 50 and 100-year
pre-development and post-development storm events. It is shown graphically from these same
hydrographs that the post-development peak discharge rates have actually been slightly reduced from the
peak pre-development discharge rates due to the incorporation of the detention facility in the post-
development condition. Therefore, total post-development peak discharge rates from the detention pond
outlet control structure and project site are ensured to be less than the pre-development peak discharge
rates associated with the 3.51 acre basin due to the incorporation of the proposed metering device.
. . ..
Tributary Area ("A"): 3.55 Acres
Pervious Area: 3.55 Acres
Impervious Area: 0.00 Acres
Run-Off Coefficient ("Cwt"): 0.40
Time Of Concentration ("Tic"):
Woodlands:
Pastures:
Pavements:
Hourly Intensity Rates ("I"):
Peak Discharge Rate ("Q"):
Low Elevation:
High Elevation:
Distance (Feet):
Slope(% Grade):
Velocity ("Vw"):
Time:
Low Elevation:
High Elevation:
Distance (Feet):
Slope(% Graqe):
Velocity ("Vp"):
Time:
Low Elevation:
High Elevation:
Distance (Feet):
·slope (%Grade):
Velocity ("Vpave"):
Time:
Total Travel Time:
2-Year: 6.33
5-Year: 7.69
10-Year: 8.63
25-Year: 9.86
50-Year: 11.15
100-Year: 11.64
2-Year: 8.98
5-Year: 10.92
10-Year: 12.26
25-Year: 14.00
50-Year: 15.83
100-Year: 16.53
•total flow from Putz
C: 0.40
c .. 0.90
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
Cubic Feet I Second
Cubic Feet I Second
Tributary Area ("A"): 0.43 Acres
Pervious Area : 0.01 Acres
Impervious Area : 0.42 Acres
Run-Off Coefficient ("Cwt"): 0.89
Time Of Concentration ("T/c "):
Woodlands: Low Elevation:
High Elevation:
Distance (Feet):
Slope (% Grade):
Velocity ("Vw"):
Time :
Pastures: Low Elevation :
High Elevation:
Distance (Feet):
Slope (% Grade):
Velocity ("Vp"):
Time :
Pavements: Low Elevation:
High Elevation:
Distance (Feet):
Slope (% Grade):
Velocity ("Vpave"):
Time :
Tota l Travel Time:
Hourly Intensity Rates ("I"): 2-Year: 6.33
5-Year: 7.69
10-Year: 8.63
25-Year: 9.86
50-Year: 11 .15
100-Year: 11 .64
Peak Discharge Rate ("Q "): 2-Year: 2.42
5-Year: 2 .94
10-Year: 3.30
25-Year: 3.77
50-Year: 4.26
100-Year: 4.45
*total flow from part of alley •A•,
alley "B", and some green
area along Putz line
C: 0.40
C= 0.90
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
Cubic Feet I Second
Cubic Feet I Second
Determine Total Pre-Development
Peak Storm Water Discharae Rates
Tributary Area ("A "): 3 .51 Acres
Pervious Area: 3 .51 Acres
Impervious Area : 0.00 Acres
Run-Off Coefficient ("Cwt"): 0 .40
Time Of Concentration ("T/c"):
Woodlands: Low Elevation :
High Elevation :
Distance (Feet):
Slope(% Grade):
Velocity ("Vw"):
Time:
Pastures: Low Elevation :
High Elevation :
Distance (Feet):
Slope (%Grade):
Velocity ("Vp"):
Time :
Pavements: Low Elevation :
High Elevation :
Distance (Feet):
Slope (%Grade):
Velocity ("Vpave"):
Time :
Total Travel Ti me:
Hourly Intensity Rates ("I "): 2-Year: 6 .33
5-Year: 7.69
10-Year: 8.63
25-Year: 9.86
50-Year: 11.15
100-Year: 11 .64
Peak Discharge Rate ("Q"): 2-Year: 8 .88
5-Year: 10.80
10-Year: 12.12
25-Year: 13.85
50-Year: 15.65
100-Year: 16.34
P age3
*detaining o nly lots 1-6
C = 0.40
C = 0 .90
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
Cubic Feet I Second
Cubic Feet I Second
Determine Post-development Peak
Storm Water Dischar e No Detention
Tributary Area ("A"): 3.51 Acres
Pervious Area : 2 .02 Acres
Impervious Area: 1.49 Acres
Run-Off Coefficient ("Cwt"): 0.61
Time Of Concentration ("T/c"): 10 Minutes (Min)
Hourly Intensity Rates ("I"): 2-Year: 6.33
5-Year: 7.69
10-Year: 8.63
25 -Year: 9.86
50-Year: 11 .15
100-Year: 11 .64
C= 0.40
C= 0 .90
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Inches I Hour
Peak Discharge Rate ("Q"): 2-Year: 13 .60 Cubic Feet I Second
5-Year: 16.53 Cubic Feet I Second
10-Year: 18.56 Cubic Feet I Second
25-Year: 21.19 Cubic Feet I Second
50-Year: 23 .96 Cubic Feet I Second
100-Year: 25 .01 Cubic Feet I Second
Comparison Of Predevelopment And
Post-development Peak Discharge Rates
Predevelopment Post-Development
2-Year: 8.88 Ft3/sec 13.60 Ft3/sec
5-Year: 10.80 Ft3/sec 16 .53 Ft3/sec
10-Year: 12.12 Ft3/sec 18 .56 Ft3/sec
25-Year: 13 .85 Ft3/sec 21 .19 Ft3/sec
50-Year: 15 .65 Ft3/sec 23.96 Ft3/sec
100-Year: 16.34 Ft3/sec 25 .01 Ft3/sec
Preliminary Determination Of
Detention Pond Volume
2-Year:
5-Year:
10-Year:
25-Year:
50-Year:
100-Year:
4 .71 Ft3/sec x '(30 Min . x 60 Sec . I 2 ) =
5.73 Ft3/sec x (30 Min . x 60 Sec. I 2 ) =
6.43 Ft3/sec x (30 Min . x 60 Sec . I 2 ) =
7 .35 Ft3/sec x (30 Min. x 60 Sec . I 2 ) =
8.30 Ft3/sec x (30 Min . x 60 Sec . I 2 ) =
8.67 Ft3/sec x (30 Min . x 60 Sec . I 2 ) =
P a ge4
Increase
4.71 Ft3/sec
5.73 Ft3/sec
6.43 Ft3/sec
7.35 Ft3/sec
8.30 Ft3/sec
8 .67 Ft3/sec
4,242 Cubic
5,158 Cubic
5 ,789 Cubic
6,612 Cubic
7,474 Cubic
7,804 Cubic
Feet
Feet
Feet
Feet
Feet
Feet
Determine Post-Development "Free-Flow"
Peak Storm Water Discharge
Tributary Area ("A "): 1.66 Acres
Pervious Area: 1.15 Acres
Impervious Area: 0 .51 Acres
Run -Off Coefficient ("Cwt"): 0 .55
Time Of Concentration ("T/c"): 10 M inutes (Min)
Hourly Intensity Rates ("I "): 2-Year: 6 .33
5-Year: 7 .69
10-Year: 8 .63
25-Year: 9.86
50-Year: 11 .15
100-Year: 11 .64
Peak D ischarge Rate ("Q "): 2-Year: 5 .81
5-Year: 7 .07
10-Year: 7 .94
25-Year: 9.06
50-Year: 10.24
100-Year: 10.70
P age 5
C = 0 .40
C = 0 .90
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
Determine Post-Development
Flow Into Detention Pond
Tributary Area ("A"): 1.42 Acres
Pervious Area : 0.93 Acres
Impervious Area : 0.49 Acres
Run-Off Coefficient ("Cwt"): 0.57
Time Of Concentration ("T/c"): 1 O Minutes (Min)
Hourly Intensity Rates ("I"): 2-Year: 6.33
5-Year: 7 .69
10-Year: 8.63
25-Year: 9 .86
50-Year: 11.15
100-Year: 11 .64
Peak Discharge Rate ("Q"): 2-Year: 5.14
5-Year: 6.25
10-Year: 7 .02
25-Year: 8.02
50-Year: 9.06
100-Year: 9.46
Page6
C= 0.40
C= 0.90
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
Pre-Development Inflow Hydrograph Ordinates (Cubic Feet per Second) 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 0.89 1.08 1.21 1.38 1.57 1.63 2 1.78 2.16 2.42 2.77 3.13 3.27 3 2.66 3.24 3.64 4.15 4.70 4.90 4 3.55 4.32 4.85 5.54 6.26 6.54 5 4.44 5.40 6.06 6.92 7.83 8.17 6 5.33 6.48 7.27 8.31 9.39 9.80 7 6.22 7.56 8.49 9.69 10.96 11.44 8 7.11 8.64 9.70 11.08 12.52 13.07 9 7.99 9.72 10.91 12.46 14.09 14.71 Peak=> ' 10 8.88 10.80 12.12 13.85 15.65 16.34 11 8.44 10.26 11.52 13.15 14.87 15.52 12 7.99 l 9.72 10.91 12.46 14.09 14.71 13 7.55 9.18 10.30 11.77 13.30 13.89 14 7.11 8.64 9.70 11.08 12.52 13.07 15 6.66 8.10 9.09 10.38 11.74 12.26 16 6.22 7.56 8.49 9.69 10.96 11.44 17 5.77 7.02 7.88 9.00 10.17 10.62 18 5.33 6.48 7.27 8.31 9.39 9.80 19 4.89 5.94 6.67 7.62 8.61 8.99 20 4.44 5.40 6.06 6.92 7.83 8.17 21 4.00 4.86 5.46 6.23 7.04 7.35 22 3.55 4.32 4.85 5.54 6.26 6.54 23 3.11 3.78 4.24 4.85 5.48 5.72 24 2.66 3.24 3.64 4.15 4.70 4.90 25 2.22 2.70 3.03 3.46 3.91 4.09 26 1.78 2.16 2.42 2.77 3.13 3.27 27 1.33 1.62 1.82 2.08 2.35 2.45 28 0.89 1.08 1.21 1.38 1.57 1.63 29 0.44 0.54 0.61 0.69 0.78 0.82 30 0.00 0.00 0.00 0.00 0.00 0.00 Page7
Post-Development
Inflow Hydrograph Ordinates
(Cubic Feet per Second)
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 1 .36 1 .65 1 .86 2 .12 2.40 2 .50
2 2 .72 3 .31 3 .71 4 .24 4 .79 5.00
3 4 .08 4 .96 5 .57 6 .36 7 .19 7 .50
4 5 .44 6 .61 7.42 8.48 9 .58 10.01
5 6.80 8.27 9 .28 10.60 11.98 12.51
6 8 .16 9.92 11 .13 12.72 14.37 15.01
7 9 .52 11 .57 12 .99 14.83 16.77 17.51
8 10.88 13.23 14.84 16.95 19.17 20.01
9 12.24 14.88 16.70 19.07 21 .56 22.51
Peak=> 10 13.60 16.53 18.56 21.19 23.96 25.01
11 12.92 15.71 17.63 20.13 22.76 23.76
12 12.24 14.88 16.70 19.07 21 .56 22 .51
13 11 .56 14.05 15.77 18.01 20.36 21 .26
14 10.88 13.23 14.84 16.95 19.17 20.01
15 10.20 12.40 13 .92 15 .89 17.97 18.76
16 9.52 11 .57 12.99 14.83 16.77 17.51
17 8 .84 10.75 12 .06 13.78 15.57 16.26
18 8 .16 9 .92 11 .13 12 .72 14.37 15.01
19 7 .4 8 9.09 10 .21 11.66 13.18 13.76
20 6 .80 8 .27 9.28 10.60 11 .98 12 .51
2 1 6 .12 7 .44 8 .35 9 .54 10 .78 11 .26
22 5.44 6 .61 7.42 8.48 9 .58 10.01
23 4 .76 5 .79 6.49 7 .42 8 .38 8 .75
24 4 .08 4 .96 5 .57 6 .36 7 .19 7 .50
25 3.40 4 .13 4 .64 5 .30 5.99 6.25
26 2.72 3 .31 3 .71 4 .24 4 .79 5 .00
27 2.04 2.48 2 .78 3 .18 3 .59 3 .75 .
28 1.36 1.65 1.86 2 .12 2.40 2 .50
29 0 .68 0 .83 0 .93 1.06 1.20 1.25
30 0 .00 0 .00 0 .00 0 .00 0 .00 0.00
P age 8
Post-Development "Free-Flow" Inflow Hydrograph Ordinates (Cubic Feet per Second) 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 0.58 0.71 0.79 0.91 1.02 1.07 2 1.16 1.41 1.59 1.81 2.05 2.14 3 1.74 2.12 2.38 2.72 3.07 3.21 4 2.33 2.83 3.17 3.63 4.10 4.28 5 2.91 3.53 3.97 4.53 5.12 5.35 6 3.49 4.24 4.76 5.44 6.15 6.42 7 4.07 4.95 5.55 6.34 7.17 7.49 8 4.65 5.66 6.35 7.25 8.20 8.56 9 5.23 6.36 7.14 8.16 9.22 9.63 Peak=> I 10 5.81 7.07 7.94_ 9.06 10.24 10.70 11 5.52 6.72 7.54 8.61 9.73 10.16 12 5.23 6.36 7.14 8.16 9.22 9.63 13 4.94 6.01 6.74 7.70 8.71 9.09 14 4.65 5.66 6.35 7.25 8.20 8.56 15 4.36 5.30 5.95 6.80 7.68 8.02 16 4.07 4.95 5.55 6.34 7.17 7.49 17 3.78 4.60 5.16 5.89 6.66 6.95 18 3.49 4.24 4.76 5.44 6.15 6.42 19 3.20 3.89 4.36 4.98 5.63 5.88 20 2.91 3.53 3.97 4.53 5.12 5.35 21 2.62 3.18 3.57 4.08 4.61 4.81 22 2.33 2.83 3.17 3.63 4.10 4.28 23 2.03 2.47 2.78 3.17 3.59 3.74 24 1.74 2.12 2.38 2.72 3.07 3.21 25 1.45 1.77 1.98 2.27 2.56 2.67 26 1.16 1.41 1.59 1.81 2.05 2.14 27 0.87 1.06 1.19 1.36 1.54 1.60 28 0.58 0.71 0.79 0.91 1.02 1.07 29 0.29 0.35 0.40 0.45 0.51 0.53 30 0.00 0.00 0.00 0.00 0.00 0.00 Page9
Post-Development Flow Routed Through Detention Pond Inflow Hydrograph Ordinates (Cubic Feet per Second) 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 0.51 0.63 0.70 0.80 0.91 0.95 2 1.03 1.25 1.40 1.60 1.81 1.89 3 1.54 1.88 2.11 2.41 2.72 2.84 4 2.06 2.50 2.81 3.21 3.63 3.79 5 2.57 3.13 3.51 4.01 4.53 4.73 6 3.09 3.75 4.21 4.81 5.44 5.68 7 3.60 4.38 4.91 5.61 6.34 6.62 8 4.11 5.00 5.62 6.41 7.25 7.57 9 4.63 5.63 6.32 7.22 8.16 8.52 Peak=> I 10 5.14 6.25 7.02 8.02 9.06 9.46 11 4.89 ' 5.94 6.67 7.62 8.61 8.99 12 4.63 5.63 6.32 7.22 8.16 8.52 13 4.37 5.32 5.97 6.81 7.70 8.04 14 4.11 5.00 5.62 6.41 7.25 7.57 15 3.86 4.69 5.26 6.01 6.80 7.10 16 3.60 4.38 4.91 5.61 6.34 6.62 17 3.34 4.07 4.56 5.21 5.89 6.15 18 3.09 3.75 4.21 4.81 5.44 5.68 19 2.83 3.44 3.86 4.41 4.98 5.20 20 2.57 3.13 3.51 4.01 4.53 4.73 21 2.31 2.81 3.16 3.61 4.08 4.26 22 2.06 2.50 2.81 3.21 3.63 3.79 23 1.80 2.19 2.46 2.81 3.17 3.31 24 1.54 1.88 ~.11 2.41 2.72 2.84 25 1.29 1.56 1.75 2.00 2.27 2.37 26 1.03 1.25 1.40 1.60 1.81 1.89 27 0.77 0.94 1.05 1.20 1.36 1.42 28 0.51 0.63 0.70 0.80 0.91 0.95 29 0.26 0.31 0.35 0.40 0.45 0.47 30 0.00 0.00 0.00 0.00 0.00 0.00 Page 10
14 .00
... 12 .00 G>
A--G> 10.00 G>
"'-
o-·-'O 8.00 ..c c:
:::J 0 0 0
-G> 6.00 G> Cl)
O> ...
0 4.00 .c
~
0 2.00
0.00
0
Total Pre-Development Vs. Total Post-Development (No
Detention) Peak Discharge Rates 2-Year Storm Event
5 10 15 20 25
Time (Minutes)
---Pre-Development Hydrograph ----Total Post-Development Hydrograph
18.00 ... 16 .00 G>
A-
1> 14.00
G> "'-12.00 o-·-'O 10.00 ..c c:
:::J 0
0 0 8.00 -G> G> Cl)
O> 6.00 ...
0 .c 4.00 0
Cl)
0 2.00
0.00
0
Total Pre-Development Vs. Total Post-Development (No
Detention) Peak Discharge Rates 5-Year Storm Event
5 10 15 20 25
Time (Minutes)
Pre-Development Hydrograph ----Post-Development Outflow
Hydrograph Without Detenti on
Page 11
30
30
20.00
... 18.00 Q)
A. 16 .00 -Q)
Q) 14 .00 u...
0 ........ 12 .00 ·-"O .0 c
::::J 0 10 .00 0 0
-Q) 8 .00 Q) (I)
O> ... 6 .00 0 .s::.
~ 4 .00
i:5 2 .00
0 .00
0
Total Pre-Development Vs. Total Post-Development (No
Detention) Peak Discharge Rates 10-Year Storm Event
5 10 15 20 25
Time (Minutes)
30
----Pre-Development Hydrograph ----Post-Development Outflow Hydrograph
Wit hout Detention
25.00
...
Q)
A. 20 .00 -Q)
Q) u...
0 ........ 15.00 ·-"O .0 c
::::J 0 0 0
-Q) 10 .00 Q) (I)
O> ...
0 .s::.
0 5 .00 I/)
i:5
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 (Minutes)
30
---Pre-Development Hydrograph ----Post-Development Outflow Hydrograph
Without Detention
Page 12
...
G>
Q.. -G>
G> u..
o-·-"O .0 c
j 0
0 0
-G> G> Cl)
O> ...
0 .c
~
i:3
Total Pre-Development Vs. Total Post-Development (No
r
Detention) Peak Discharge Rates 50-Year Storm Event
25 .00
20.00
15 .00
10.00
5.00
0.00
0 5 10 15 20 25 30
Time (Minutes)
Pre-Development Hydrograph Post-Development Outflow Hydrograph
Without Detentio n
0
Total Pre-Development Vs . Total Post-Development (No
Detention) Peak Discharge Rates 100-Year Storm Event
5 10 15 20 25
Time (Minutes)
Pre -Develo pment Hydrograph ----Post-Developm ent Outflow
Hydrograph Without Detention
Page 13
30
Detention Pond
De th Vs. Volume
Elevation Depth Volume
(Feet) (Ft3)
302 .00 0 .00 0
302 .50 0 .50 843
303.00 ·1.00 3 ,022
303 .50 1.50 5 ,679
304.00 2.00 8 ,336
304.50 2 .50 10,993
Detention Pond Depth Vs. Volume
1.50 -a;
G> ..... -= a.
G> a
l.00
0 2,000 4 ,000 6 ,000 8 ,000 10 ,000 12 ,000
Detention Pond Volume (Cubic Feet)
P age 14
Rating Curve For Outlet Control Structure Pipe Dia. (Inches): 6 0 .50
De th Vs. Volume Pipe Length 42 .00 Feet
Elevation Depth Discharge
(Feet) Ft3/Sec
302 .00 0 .00 0 .00
302 .50 0.50 0 .51
303 .00 1.00 0 .72
303 .50 1.50 0 .88
304 .00 2.00 1.02
304 .50 2 .50 1.14
Rating Curve For Outlet Control Structure Depth Vs.
Volume
1.50 --G>
G> u.. -.i::. -a.
G> a
l.00
0 .00 0 .20 0 .40 0.60 0 .80 l.00 1.20
Discharge (Cubic Feet Per Second)
P a ge 15
I Storage Indication Curve
Depth Storage Discharge 2s/t 2s/t+O
(Feet) (Ft3} (Ft 3/Sec) (Ft3/Sec) (Ft3/Sec)
0 .00 0 0 .00 0 .00 0 .00
0.50 843 0.51 28.10 28.61
1.00 3 ,022 0 .72 100.73 101.45
1.50 5 ,679 0 .88 189.30 190.18
2.00 8 ,336 1.02 277 .87 278 .88
2.50 10,993 1.14 366.43 367 .57
Storage Indication Curve
1.20
-"O c:
0 0 .80 ()
~ ...
G>
A. -G>
G> u.. 0 .60 ()
:0
:J u -G>
/
,,,,,,
,/
v ,
/
/
/
( "
1.00
O> ...
0 0 .40 ~
()
II)
0
0.20
0 .00
0 .00 50.00 l 00 .00 150 .00 200 .00 250 .00 300 .00 350 .00 400 .00
2s/t+O (Cubic Feet Per Second)
P age 16
Inflow I Outflow Simulation 2-Year Storm Event Time Inflow 11+ 12 2s/t-0 2s/t+O Outflow Depth Elevation (Minutes) (Ft3/Sec) (Ft3/Sec) (Feet) 0 0.00 0.00 0.00 0.00 0.00 0.00 302.00 1 0.51 0.51 0.50 0.51 0.01 0.00 302.00 2 1.03 1.54 1.97 2.04 0.04 0.00 302.00 3 1.54 2.57 4.38 4.54 0.08 0.01 302.01 4 2.06 3.60 7.69 7.98 0.14 0.04 302.04 5 2.57 4.63 11.89 12.32 0.22 0.09 302.09 6 3.09 5.66 16.92 17.54 0.31 0.19 302.19 7 3.60 6.69 22.77 23.61 0.42 0.34 302.34 8 4.11 7.72 29.46 30.48 0.51 0.51 302.51 9 4.63 8.74 37.13 38.20 0.54 0.56 302.56 10 5.14 9.77 45.78 46.90 0.56 0.61 302.61 11 4.89 10.03 54.64 55.81 0.59 0.67 302.67 12 4.63 9.52 62.93 64.15 0.61 0.72 302.72 13 4.37 9.00 70.67 71.93 0.63 0.78 302.78 14 4.11 8.49 77.85 79.16 0.65 0.83 302.83 15 3.86 7.97 84.47 85.82 0.67 0.88 302.88 16 3.60 7.46 90.55 91.93 0.69 0.92 302.92 17 3.34 6.94 96.08 97.49 0.71 0.97 302.97 18 3.09 6.43 101.07 102.51 0.72 1.01 303.01 19 2.83 5.91 105.53 106.99 0.73 1.03 303.03 20 2.57 5.40 109.46 110.93 0.74 1.05 303.05 21 2.31 4.89 112.86 114.35 0.74 1.07 303.07 22 2.06 4.37 115.74 1.17.23 0.75 1.08 303.08 23 1.80 3.86 118.10 119.60 0.75 1.09 303.09 24 1.54 3.34 119.93 121.44 0.75 1.10 303.10 25 1.29 2.83 121.24 122.76 0.76 1.11 303.11 26 1.03 2.31 122.04 123.56 0.76 1.12 303.12 27 0.77 1.80 122.33 123.84 0.76 1.12 303.12 28 0.51 1.29 122.09 123.61 0.76 1.12 303.12 29 0.26 0.77 121.35 122.87 0.76 1.11 303.11 30 0.00 0.26 120.10 121.61 0.75 1.10 303.10 --Page 17
Inflow/Outflow Simulation 2-Year Storm Event
14 .00
I '
I ...
' I
I ' 12.00 I ' I '
I ' I '
I '
10 .00 -' 'O I ' c I 0 ' 0 I ' Cl> I ' Cl) ... , ...
Cl> 8.00 ' 0... I -' Cl> I ' Cl> u..
0 ' :0 ' ::J 6 .00 0 -' ~ ' 0 / ' ' ~ ' ' / .
' 0 I ' ' 4 .00 ' I ' ' ' I ' ' ' '
' ' 2 .00
0 .00
0 5 10 15 20 25 30
Time (Minutes)
Pre -----Post -Post --• -• -· Post-
Developme Developme Developme Developme
nt nt Outflow nt Outflow nt 'Free-
Hydrograph Hydrograph With Flow '
Without Det ention
Detention
Page 18
Inflow I Outflow Simulation 5-Year Storm Event Time Inflow 11+ 12 2s/t-0 2s/t+O Outflow Depth Elevation (Minutes} (Ft3/Sec) (Ft3/Sec) (Feet) 0 0.00 0.00 0.00 0.00 0.00 0.00 302.00 1 0.63 0.63 0.60 0.63 0.01 0.00 302.00 2 1.25 1.88 2.39 2.48 0.04 0.00 302.00 3 1.88 3.13 5.32 5.52 0.10 0.02 302.02 4 2.50 4.38 9.36 9.70 0.17 0.06 302.06 5 3.13 5.63 14.45 14.99 0.27 0.14 302.14 6 3.75 6.88 20.58 21.33 0.38 0.28 302.28 7 4.38 8.13 27.69 28.71 0.51 0.50 302.50 8 5.00 9.38 36.01 37.07 0.53 0.55 302.55 9 5.63 10.63 45.52 46.64 0.56 0.61 302.61 10 6.25 11.88 56.22 57.40 0.59 0.68 302.68 11 5.94 12.20 67.17 ~)8.42 0.62 0.75 302.75 12 5.63 11.57 77.44 78.74 0.65 0.83 302.83 13 5.32 10.94 87.02 88.38 0.68 0.90 302.90 14 5.00 10.32 95.93 97.34 0.71 0.97 302.97 15 4.69 9.69 104.17 105.62 0.73 1.02 303.02 16 4.38 9.07 -111.76 113.24 0.74 1.06 303.06 17 4.07 8.44 118.70 120.20 0.75 1.10 303.10 18 3.75 7.82 124.99 126.52 0.76 1.13 303.13 19 3.44 7.19 130.63 132.18 0.77 1.16 303.16 20 3.13 6.57 135.63 137.20 0.78 1.19 303.19 21 2.81 5.94 139.99 141.57 0.79 1.21 303.21 22 2.50 5.32 143.71 145.31 0.80 1.23 303.23 23 2.19 4.69 146.80 148.40 0.80 1.25 303.25 24 1.88 4.07 149.24 150.86 0.81 1.27 303.27 25 1.56 3.44 151.06 152.68 0.81 1.28 303.28 26 1.25 2.81 152.25 153.88 0.81 1.28 303.28 27 0.94 2.19 152.81 154.44 0.81 1.29 303.29 28 0.63 1.56 152.74 1.54.37 0.81 1.29 303.29 > 29 0.31 0.94 152.06 153.68 0.81 1.28 303.28 30 0.00 0.31 150.75 152.37 0.81 1.27 303.27 Page 19 ...
Inflow/Outflow Simulation 5-Yeor Storm Event
18.00
16 .00 ' -' I ' I ' I '
14 .00 • ' ' I
' • ' -I ' "O 12 .00 c
0
I '
I ' 0
G>
Cl) ....
G>
A.. 10.00 -G>
G> .....
0 :.a 8 .00 :J u -~
0
=§ 6 .00
0
4.00
2.00
I '
I ' J ~ ' I ' . I ~ ' I ' I ' if ' . "
I I ~ ' ' I " ' . ' . . . ' ' . ' ' I . ' '
I I . . '
' "\ ... . . ' // . ' ~ ' ' . . .
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'•·,,, ~ ' .. .
I .• ' ' ' '\. "' I ' ' ' 1· .. ', 0: . ''... ' . ' . . -~
0.00
0 5 10 15 20 25 30
Time (Minutes)
Pre -----Post-Post---------Post -
Developme Developme Developme Developme
nt nt Outflow nt Outflow nt "Free-
Hydrograph Hydrograph With Flow "
Without Detention
Detention
Page 20
Inflow I Outflow Simulation
10-Year Storm Event
Time Inflow 11+ 12 2s/t-0 2s/t+O Outflow Depth Elevation
(Minutes) (Ft3/Sec) (Ft3/Sec) (Feet)
0 0 .00 0.00 0 .00 0 .00 0 .00 0.00 302.00
1 0 .70 0 .70 0.68 0.70 0.01 0.00 302.00
2 1.40 2 .11 2.68 2 .78 0 .05 0 .00 302.00
3 2.11 3 .51 5.97 6.19 0.11 0 .02 302.02
4 2.81 4.91 10.50 10.89 0 .19 0 .07 302.07
5 3 .51 6.32 16.22 16.82 0.30 0 .17 302 .17
6 4 .21 7 .72 23 .09 23.94 0.43 0 .35 302 .35
7 4 .91 9 .13 31 .18 32 .22 0.52 0.52 302.52
8 5 .62 10.53 40.62 41 .71 0.55 0.58 302 .58
9 6.32 11.93 51.40 52 .56 0.58 0.65 302 .65
10 7 .02 13.34 63 .52 64 .74 0.61 0 .73 302.73
11 6.67 13 .69 75.91 77 .20 0 .65 0.81 302 .81
12 6.32 12 .99 87 .53 88 .89 0.68 0.90 302 .90
13 5 .97 12 .28 98.39 99 .82 0 .71 0.99 302 .99
14 5 .62 11 .58 108.50 109 .97 0.73 1.04 303.04
15 5.26 10.88 117.88 119 .38 0 .75 1.09 303 .09
16 4 .91 10.18 126.53 128.06 0 .77 1.14 303 .14
17 4.56 9.48 134.44 136.01 0 .78 1.18 303 .18
18 4 .21 8.77 141.63 143.22 0 .79 1.22 303.22
19 3 .86 8.07 148.09 149.70 0.81 1.26 303 .26
20 3.51 7 .37 153.83 155.46 0 .82 1.29 303.29
21 3.16 6.67 158.85 160.50 0.83 1.32 303.32
22 2.81 5.97 163.15 164.81 0.83 1.35 303 .35
23 2 .46 5.26 166.73 168.41 0 .84 1.37 303.37
24 2.11 4 .56 169 .60 171.30 0 .85 1.39 303 .39
25 1.75 3.86 171 .77 173.47 0 .85 1.40 303.40
26 1.40 3 .16 173.22 174.93 0.85 1.41 303.41
27 1.05 2.46 173.97 175.68 0.85 1.41 303.41
28 0.70 1.75 174.02 175.73 0 .85 1.41 303.41
29 0 .35 1.05 173.37 175.07 0 .85 1.41 303 .41
30 0.00 0 .35 172.02 173.72 0 .85 1.40 303.40
Page 21
Inflow/Outflow Simulation 10-Year Storm Event
20.00
'' 18 .00 I ' ' I
' • ' 16 .00 I "'
I ' I '
I '
...... 14 .00
"O c
I ' ~
I
' 0 I ' 0
Cl>
Cl) 12 .00 -Cl>
Q.. -Cl>
Cl> 10.00 y,.
0
:0
:::J
0 ...... 8 .00 ~
0
~
0 6 .00
4 .00
I ' . ~
I I ~ '
I •
I ' .. . I "' ' r
'
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',,'',,, . . . / ..
' ' ', ~ ' .· ' ' . ' ""'-'',. ' I , ' . ' ' I ' ' l : ',,,~ . •, ' .
·-~
2 .00
0.00
0 5 10 15 20 25 30
Time (Minutes)
Pre-----Post -Post--.. -.... Post-
Developme Developme Developme Developme
nt nt Outflow nt O utflow nt "F ree-
Hydrograph Hydrograph With Flow "
Without Detention
Detention
Page 22
Inflow I Outflow Simulation
25-Year Storm Event
Time Inflow 11+ 12 2s/t-0 2slt+O Outflow Depth Elevation
(Minutes) (Ft3/Sec) (Ft3/Sec) (Feet)
0 0 .00 0 .00 0 .00 0 .00 0 .00 0 .00 302.00
1 0 .80 0 .80 0 .77 0 .80 0.01 0 .00 302 .00
2 1 .60 2.41 3.07 3 .18 0.06 0 .01 302.01
3 2.41 4 .01 6 .82 7 .07 0 .13 0 .03 302.03
4 3 .21 5 .61 11.99 12.44 0 .22 0.09 302.09
5 4 .01 7 .22 18.53 19.21 0.34 0 .23 302.23
6 4 .81 8 .82 26.38 27.35 0 .49 0.46 302.46
7 5.61 10.42 35.74 36 .80 0 .53 0 .55 302 .55
8 6.41 12.03 46.64 47.76 0 .56 0.61 302.61
9 7 .22 13.63 59 .07 60.26 0.60 0 .70 302 .70
10 8.02 15.23 73.02 74.30 0.64 0 .79 302 .79
11 7.62 15.63 87 .29 88.65 0.68 0 .90 302 .90
12 7 .22 14.83 100.69 102.12 0.72 1.00 303 .00
13 6.81 14.03 113.23 114.72 0.74 1.07 303 .07
14 6 .41 13.23 124.93 126.46 0 .76 1.13 303.13
15 6.01 12.43 135.79 137.36 0.78 1.19 303 .19
16 5.61 11.63 145.81 147.42 0 .80 1.25 303.25
17 5.21 10.82 155.00 156.64 0 .82 1.30 303 .30
18 4 .81 10.02 163.35 165.02 0 .83 1.35 303 .35
19 4.41 9.22 170.88 172.57 0 .85 1.39 303.39
20 4 .01 8.42 177.58 179 .30 0.86 1.43 303.43
21 3 .61 7 .62 183.45 185.19 0.87 1.47 303.47
22 3 .21 6 .81 188.51 190.27 0 .88 1.50 303.50
23 2 .81 6.01 192.75 194.52 0.89 1.52 303.52
24 2.41 5 .21 196.18 197.96 0 .89 1.54 303.54
25 2 .00 4.41 198.80 200.59 0.90 1.55 303.55
26 1.60 3 .61 200.61 202.40 0 .90 1.56 303 .56
27 1.20 2.81 201 .61 203.41 0 .90 1 .57 303 .57
28 0 .80 2 .00 201.82 203.62 0 .90 1.57 303.57
29 0.40 1.20 201 .22 203.02 0 .90 1.57 303 .57
30 0.00 0.40 199.83 201 .62 0.90 1.56 303.56
'i
P age 23
Inflow/Outflow Simulation 25-Year Storm Event
25 .00
I ' 20 .00
I '
I '
I ' ' -I ' 'O I ' c
0 I ' 0 I ' G>
Cl) 15.00 ... ' G> I
Q., ' -I ' G> I ' G> ..... I ' 0 I ' :0 ' :J I
(.) ' -10.00 ~ I ' 0 I ' = • ' ' ~
:J . ' ' ' 0 I . ' ' ' ...
I ' ' I
I ' ' ' ' .
5.00 '
' ...
' . . ' ' ' ' '
0.00
0 5 10 15 20 25 30
Time (Minutes)
Pre-----Post-Post-........ Post -
Developme Developme Developme Developme
nt nt Outflow nt Outflow nt "Free-
Hydrograph Hydrograph With Flow ·
Without De t ention
Detention
Page 24 .
Inflow I Outflow Simulation 50-Year Storm Event Time Inflow 11+ 12 2s/t-0 2s/t+O Outflow Depth Elevation (Minutes) (Ft3/Sec) (Ft3/Sec) (Feet) 0 0.00 0.00 0.00 0.00 0.00 0.00 302.00 1 0.91 0.91 0.87 0.91 0.02 0.00 302.00 2 1.81 2.72 3.47 3.59 0.06 0.01 302.01 3 2.72 4.53 7.71 8.00 0.14 0.04 302.04 4 3.63 6.34 13.56 14.06 0.25 0.12 302.12 5 4.53 8.16 20.94 21.71 0.39 0.29 302.29 6 5.44 9.97 29.88 30.91 0.51 0.51 302.51 7 6.34 11.78 40.58 41.67 0.55 0.58 302.58 8 7.25 13.59 53.01 54.17 0.58 0.66 302.66 9 8.16 15.41 67.17 68.41 0.62 0.75 302.75 10 9.06 17.22 83.05 84.39 0.67 0.87 302.87 11 8.61 17.67 99.29 100.72 0.72 0.99 302.99 12 8.16 16.77 114.57 116.06 0.74 1.08 303.08 13 7.70 15.86 128.89 130.43 0.77 1.15 303.15 14 7.25 14.95 142.25 1,~3.84 0.80 1.23 303.23 15 6.80 14.05 154.66 156.30 0.82 1.30 303.30 16 6.34 13.14 166.13 167.80 0.84 1.36 303.36 17 5.89 12.24 176.65 178.36 0.86 1.43 303.43 18 5.44 11.33 186.22 187.97 0.88 1.49 303.49 19 4.98 10.42 194.87 196.65 0.89 1.53 303.53 20 4.53 9.52 202.58 204.38 0.90 1.58 303.58 21 4.08 8.61 209.37 211.19 0.91 1.61 303.61 22 3.63 7.70 215.23 217.07 0.92 1.64 303.64 23 3.17 6.80 220.17 222.03 0.93 1.67 303.67 24 2.72 5.89 224.19 226.06 0.93 1.69 303.69 25 2.27 4.98 227.30 229.18 0.94 1.71 303.71 26 1.81 4.08 229.49 231.37 0.94 1.72 303.72 27 1.36 3.17 230.77 232.66 0.94 1.73 303.73 28 0.91 2.27 231.15 233.04 0.95 1.73 303.73 29 0.45 1.36 230.62 232.51 0.94 1.73 303.73 30 0.00 0.45 229.19 231.07 0.94 1.72 303.72 Page 25 -
Inflow/Outflow Simulation 50-Year Storm Event
25 .00
I ' ' I
' • ' I ' I ..
20 .00 I '
I
I '
I ' -"O I ' c I ' 0
0 I ' G> 15.00 ' Cl) ..... ' G> • ' Q. I -' G> I ' G>
LL. I ' 0
:0 I ' :::> ' u I -10 .00 ~ ' ' ..
0 I . ' ' . ' =§ I ' '
0 I ' • . ' " . ' ' I ' ' . ' ' I . . ' ' 5.00 '
' ' ' . .
' '
0.00
0 5 10 15 20 25 30
Time (Minutes)
Pre-----Post-Post-........ Post -
Developme Developme Developme Developme
nt nt Outfl ow nt Outflow nt 'Free-
Hydrograph Hydrograph With Flow "
Wit hout Detention
Detenti o n
P age 26
Inflow I Outflow Simulation 100-Year Storm Event Time Inflow 11+ 12 2s/t-O 2s/t+O Outflow Depth Elevation (Minutes) (Ft3/Sec) (Ft3/Sec) (Feet) 0 0.00 0.00 0.00 0.00 0.00 0.00 302.00 0.95 0.95 0.91 0.95 0.02 0.00 302.00 2 1.89 2.84 3.62 3.75 0.07 0.01 302.01 3 2.84 4.73 8.05 8.35 0.15 0.04 302.04 4 3.79 6.62 14.16 14.68 0.26 0.13 302.13 5 4.73 8.52 21.87 22.67 0.40 0.31 302.31 6 5.68 10.41 31.24 32.28 0.52 0.52 302.52 7 6.62 12.30 42.44 43.54 0.55 0.59 302.59 8 7.57 14.19 55.46 56.63 0.59 0.67 302.67 9 8.52 16.09 70.28 71.54 0.63 0.77 302.77 10 9.46 17.98 86.90 88.26 0.68 0.90 302.90 11 8.99 18.45 103.90 105.35 0.73 1.02 303.02 12 8.52 17.51 119.90 121.41 0.75 1.10 303.10 13 8.04 16.56 134.89 136.46 0.78 1.19 303.19 14 7.57 15.61 148.89 150.51 0.81 1.26 303.26 15 7.10 14.67 161.90 163.56 0.83 1.34 303.34 16 6.62 13.72 173.91 175.62 0.85 1.41 303.41 17 6.15 12.77 184.94 186.69 0.87 1.48 303.48 18 5.68 11.83 194.99 196.77 0.89 1.53 303.53 19 5.20 10.88 204.06 205.87 0.90 1.58 303.58 20 4.73 9.94 212.17 214.00 0.92 1.63 303.63 ' 21 4.26 8.99 219.30 221.16 0.93 1.67 303.67 22 3.79 8.04 225.47 227.34 0.94 1.70 303.70 23 3.31 7.10 230.68 232.57 0.94 1.73 303.73 24 2.84 6.15 234.93 236.83 0.95 1.75 303.75 25 2.37 5.20 238.22 240.13 0.96 1.77 303.77 26 1.89 4.26 240.56 242.48 0.96 1.79 303.79 27 1.42 3.31 241.95 243.87 0.96 1.79 303.79 28 0.95 2.37 242.39 244.31 0.96 1.80 303.80 29 0.47 1.42 241.88 243.81 0.96 1.79 303.79 30 0.00 0.47 240.44 242.36 0.96 1.79 303.79 Page 27 -
30 .00
25.00
-"O 5 20.00
0
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Cl)
a;
0..
$
LL. 15.00
0
:0
::::J
~
~
~ 10 .00
0
5.00
0.00
Inflow/Outflow Simulation 100-Year Storm Event
I
I
I
I
I
I
0 5
Pre-----
Developme
nt
Hydrograph
•
' I '
I
10
'
' '
'
'
' ...
15 20
Time (Minutes)
Post -Post-
Developme Developme
nt Outflow nt O utflow
Hydrograph With
Without Detention
Detention
Page 28
25 30
.... -. -. Post-
Developme
nt 'Free-
Flow'
9.00
8.00
7.00
~ 6.00
c
0
()
Cl>
Cl)
a;
0.. 5.00
Qi
Cl> .....
()
:0 8 4.00 -~
0
=§
0 3 .00
2.00
l.00
0.00
0
Inflow/Outflow Simulation 2-Year Storm Event
5 10 15 20 25 30
Time (Minutes)
---Pre -Development Hydrograph ----Total Post-Development Hydrograph
Page 29
Inflow/Outflow Simulation 5-Year Storm Event
12.00
10.00
-8 .00 'O c
0
0
Cl>
Cl) ....
Cl>
Q.. -Cl>
Cl> 6 .00 ~
0
:0
::::J
0 -~
0
=§
0 4 .00
2 .00
0.00
0 5 10 15 20 25 30
Time (Minutes)
----Pre-Development Hydrograph ----Total Post-Development Hydrograph
Page 30
Inflow/Outflow Simulation 10-Year Storm Event
14 .00
12 .00
10 .00 -"O c
0
0
G>
Cl) .. 8 .00 G>
0... -G>
G>
i&..
0
:0 ;::,
0 6 .00 -~
0
== ;::,
0
4 .00
2 .00
0.00
0 5 10 15 20 25 30
Time (Minutes)
----Pre-Devel o pment Hydrograph ----Total Post-Development Hydrograph
Page 3 1
-
Inflow/Outflow Simulation 25-Year Storm Event
14.00
12 .00
10.00 -"O c
0 g
Cl) ... 8 .00 Q)
0.. -Q)
Q)
LI.
(J
:0
:J
() 6.00 -~
0
=§
0
4.00
2.00
0.00
0 5 10 15 20 25 30
Time (Minutes)
Pre-Development Hydrograph ----Total Post-Development Hydrograph
Page 32
-
Inflow/Outflow Simulation 50-Year Storm Event
12 .00
-'O c
0 10.00 0
G>
Cl) -G>
iCI.. -G>
G> 8 .00 LL.
0
:0
:::7
0 -3';
0 6.00 =§
0
4.00
0 5 10 15 20 25 30
Time (Minutes)
Pre-Development Hydrograph ----Tot al Post-Development Hyd rograph
Page 33
Inflow/Outflow Simulation 100-Year Storm Event
18.00
16 .00
14 .00
-12.00 "O c
0
0
G>
Cl) ...
G> 10.00 Q. -G>
G>
LL.
0
:0 8 .00 :::J
(.) -~
0
=§
0 6 .00
4 .00
2 .00
0.00
0 5 10 15 20 25 30
Time (Minutes)
Pre-Development Hydrograph ----Total Post-Developm ent Hydrograph
Page 34
•
9.00
8 .00
7 .00
£1 6 .00 c
0
0
Cl)
Cl) ...
l. 5.00
1 .....
0
:0
:::J 4 .00 ~
~
0
=§
0 3.00
2.00
1.00
0.00
0
Inflow/Outflow Simulation 2-Year Storm Event
5 10 15 20 25 30
Time (Minutes)
----Pre-Development Hydrograph ----Total Post-Development Hydrogra ph
~oes not include the 0.43 acres that include alley "B ", alley "A", and a small section
of property along th e Putz line .
'See page 36 for Summary of Inflow/Outflow Simulations Page29 A
Inflow/Outflow Simulation 5-Year Storm Event
-8.00 ,,
c
0
0
G>
Cl) ...
G>
Q..
'i
G> 6 .00 u..
0 :a
:J
0 -~
0
~
0 4 .00
0 5 10 15 20 25 30
Time (Minutes)
Pre-Development Hydrograph ----Total Post-Development Hydrograph
*does not include the 0.43 acres that include alley "B", alley "A", and a small section
of property along the Putz line.
**See page 36 for Summary of Inflow/Outflow Simulations Page 30 A.
Inflow/Outflow Simulation 10-Year Storm Event
10.00 -"O c
0
0
G>
Cl) .. 8.00 G>
0.. -G>
G> u..
0
:0
:::J
() 6.00 -~
0
=§
0
4.00
0 5 10 15 20 25 30
Time (Minutes)
----Pre-Development Hydrograph ----Total Post-Development Hydrograph
snot include the 0.43 acres that include alley "B", alley "A", and a small section
f property along the Putz line.
ee page 36 for Summary of Inflow/Outflow Simulations Page31 ~
Inflow/Outflow Simulation 25-Year Storm Event
14 .00
12 .00
10.00
-,,
c
0
()
Cl>
Cl) ... 8 .00 Cl>
A. -Cl>
Cl> u..
()
:0 ;::,
0 6.00 -~
0
=§
0
4.00
2.00
0.00
0 5 10 15 20 25 30
Time (Minutes)
----Pre-Development Hydrograph Total Post-Development Hydrograph
*does not include the 0.43 acres that include alley "B ", alley "A", and a small section
of property along the Putz line.
**See page 36 for Summary of Inflow/Outflow Simulations Page 32 A.
Inflow/Outflow Simulation 50-Year Storm Event
12.00
-"O c
0 10.00 ~
Cl) ...
Cl>
A. -Cl>
Cl> 8.00 .....
0
:0
::::J
0 -~
0 6.00 =§
0
4.00
0 5 10 15 20 25 30
Time (Minutes)
Pre-Development Hydrograph Total Post-Development Hydrograph
tioes not include the 0.43 acres that include alley "B", alley" A", and a small section
of property along the Putz line.
See page 36 for Summary of Inflow/Outflow Simulations Page 33 A
•
Inflow/Outflow Simulation 100-Year Storm Event
-12.00 "O c
0
(,)
G>
U) ...
G> 10.00 0..
-;
G> u..
(,)
:0 8 .00 ;:,
0 -~
0 :s
0 6 .00
0 5 10 15 20 25 30
Time (Minutes)
Pre-Development Hydrograph ----Total Post-Development Hydrograph
*does not include the 0.43 acres that include alley "B ", alley "A", and a small section
of property along the Putz line .
*See page 36 for Summary of Inflow/Outflow Simulations
Page 34 A
-
Detention Pond Storage Volumes as Percent of Maximum
Volume
100%
90%
80%
70%
Q)
E
::::J
0 60% >
E
::::J
E
")( 50% 0
~
0 -40% c
G>
0 -G>
0..
30%
20%
10%
0%
2-year 5-year 10-year 25-year SO-year 100-year
Design Storm
Storm Simulation Synopsis
2-ear 5-ear 10-ear 25-ear
Storm Depth 1.12 1.29 1.41 1.57
Storm Elevation 303 .12 303 .29 303.41 303 .57 303.73 303.80
Storm Volume 3642 4544 5210 6057 6915 7255
Maximum Capacity 10993 10993 10993 10993 10993 10993
Percent of Capacity 33% 41% 47% 55% P age35 ~-63% 66%
Summary of Inflow/Outflow Simulations
(all values in cfs, unless otherwise noted)
Free Flow Detention Pond Flow Total fQst. Pre-devel,
0.43 ac. 1.66 ac, at IO minutes
2-yr. 2.42 5.81 0.56 8.79 8.88
5-yr. 2.94 7.07 0.59 10.60 10.80
10-yr. 3.30 7.94 0.61 11.85 12.12
25-yr. 3.77 9.06 0.64 13.47 13.85
50-yr. 4.26 10.24 0.67 15.17 15.65
100-yr. 4.45 10.70 0.68 15.83 16.34
Page 36 Pt
Engineeri:11g Report
For
Sanitary Sewer Improvements
&
\Vaterlmprovements
To
Lot 3 -Henton Subdivision
College Station, Brazos County, Texas
Prepared By:
* Revised -February, 1996 *
ENGINEERING REPORT
to accompany
Lot 3 -HENTON SUBDIVISION
COLLEGE STATION, BRAZOS COUNTY, TEXAS
Lot 3 is situated adjacent to Lincoln Avenue, on the north side of the
intersection of Lincoln Avenue and Munson Street. The Henton Subdivision
consists of 14 lots, established within the bounds of a 3 .51 acre tract.
Soils within the area have shown historically to be quite stable. No faults exist
within the project area, and no stability problems are expected, other than
those caused by the expansive clays typical of Brazos County.
SANITARY SEWER IMPROVEMENTS
Sarvtary sewer needs are met by a 6" diameter SDR 26-3034 PVC sanitary
sewer line. The grade varies from + 1. 70% from station 0+00 to station 0+50 to
+2.42% from station 0+50 to station 3+ 78. The pipe loading was calculated to
be 0.022 cubic feet per second (cfs). This loading was calculated by
considering the 14 lots, with 3 residents per lot, with each resident producing
100 gallons per day (gpd). The daily loading was factored by 3 to get the peak
loading, and 10% of the daily loading accounts for infiltration. The infiltration
was added to the peak loading to calculate the total loading.
Daily loading = 14 lots * 3 residents per lot * 100 gpd per resident
= 4200 gpd
Peak loading = Daily loading * 3
= 4200 gpd * 3
= 12600 gpd
Infiltration = Daily loading * 0.10
= 4200 gpd * 0.10
= 420 gpd
Total loading = Peak loading+ Infiltration
= 12600 gpd + 420 gpd
= 13020 gpd
Flow in cubic feet per second = flow in gallons per day * A * B
Where:
A= 0.13368 cubic feet per gallon
B = 0.00001157 days per second
Flow in cubic feet per second (cfs) = 13020 gpd * 0.13368 * 0 .0000 1157
= 0.020 cfs
According to Manning Pipe Theory, an 6" line graded at 2.42% can carry
0.8750 cfs, and a 6" line graded at 1 . 70% can carry 0. 7334 cfs.
where:
Q = (1. 49 I n) * A * R2 13 * S 0 ·5
Q =flow in cubic feet per second (cfs)
n =Manning's roughness coefficient
n = 0.013 for cast iron pipe
A = area of pipe in square feet
A= rcd 2 /4
A= 7t * (0.50)2 I 4 = 0.1963
R = hydraulic radius = A I P
where:
A = area of pipe in square feet
P =wetted perimeter= n *diameter
p = 7t * 0 .50 = 1.57
R = 0.1963 I 1.57 = 0.125
S =slope of pipe= 0.0242 and 0.0170
Q = (1.49 I 0.013) * 0.1963 * 0.1252 13 * 0.02420.5
Q = 0.8750 cfs
Since the ratio of loading to capacity is 0.020/0.8750 = 0.0229, the resulting
velocity (from design tables) will be 0.41 of full flow velocity. Full flow velocity
is calculated as follows :
Where:
V =QI A
fu ll
vfull = full flow velocity
Q = full flow capacity in cfs = 0.8750 cfs
A= area of pipe in square feet = 0.1963 ft2
Vru1i = 0.8750 I 0.1963 = 4.46 ft/ sec
The resulting velocity for a pipe flowing at 0.0251 full, being 0.41 of full flow
velocity, is thus :
v0.025Jfull = 0.41 * 4.46 ft/sec= 1.829 ft/sec
The 6" diameter pipe flows into an 8" diameter line, at an existing manhole
(station 0+00 on the plans). A proposed manhole is located at station 0+50,
and the final manhole is located at the end of the proposed 6" diameter
sanitary sewer line at station 3+ 78.
WATER IMPROVEMENTS
The proposed water needs are met by a 18 " diameter C900 Class 200 PVC water
line. The pipe loading was calculated to be 8. 75 gallons per minute (gpm).
This loading was calculated by considering the 14 lots, with 3 residents per lot,
with each resident using 100 gallons per day. The daily loading was factored
by 3 to get the peak loading.
Daily loading = 14 lots * 3 residents per lot * 100 gpd per resident
= 4200 gpd
Reference
Peak loading = Daily loading * 3
= 4200 gpd * 3
= 12600 gpd
Total loading = 12600 gpd = 8. 75 gpm
Design and Construction of Sanitary and Storm Sewers. Prepared by a joint
committee of the American Society of Civil Engineers (ASCE) and the Water
Pollution Control Federation (WPCF): 1969. AS'CE -Manuals and Reports on
Engineering Practice No . 37. WPCF -Manual of Practice No. 9 .
@. 0~ Cl~y of College Station ~ ~ POST OFFICE BOX 9960 1101 TEXAS AVENUE ~ ~ COLLEGE STATION, TEXAS 77840
Planning Division
CITY OF COl.LEGE STATION
January 5, 1996
Garrett Engineering
Attn: Mr. Donald Garrett, P .E.
Post Office Box 9960 1101 Texas Avenue
College Station, Texas 77842-9960
(409) 764-3500
4444 Carter Creek Parkway, Suite 108
Bryan, Texas 77802
RE: Construction Plans for Lot 3 -Henton Subdivision
Dear Mr. Garrett:
Upon review of the above referenced construction plans, it was detennined that sufficient
information was not submitted. For this reason, these plans will not accepted as a first review . I
did however red line the plans with the information I received. Again, when these plans are
resubmitted, it will constitute the first review.
If you have any questions , feel free to contact me at 764-3570.
Steve Homeyer
Graduate Engineer
cc: ~onica Morgan, Asst. City Engineer
Shirley Volk, Development Coordinator
Home of Texas A&M University
Field Notes
Access Easement
Being an access easement lying and being situated in College Station,
Brazos County, Texas and being a part of Lot 1 of1WO LINCOLN PLACE
according to the Plat recorded in Volume 2251 page 329, Official Records of
Brazos County, Texas and being described as follows:
COMMENCING; at a 1 /2" iron rod found at the south corner of said Lot 1;
same being in the northwest right-of-way line of Lincoln Avenue; also being the
east corner of the J. V. Henton tract;
THENCE: N 45°29'48" W -118.96 feet along the common line between said
Lot 1 and said Henton tract to a 1 /2" iron rod for the POINT OF BEGINNING;
THENCE: N 45°29'48" W -28.40 feet continuing along said common line to a
1/2" iron rod set for corner;
THENCE: 40.55 feet around a curve to the right with a central angle of
10°57'34", a radius of 212.00 feet and whose chord bears N 80°12'15" E -
40.49 feet to a 1/2" iron rod set at the end of said curve;
THENCE: N 85°41'02" E -10.05 feet to a 1/2" iron rod set at the beginning of
a curve to the left;
THENCE: 27.20 feet around a curve to the left with a central angle of
77°55'58", a radius of 20.00 feet and whose chord bears N 46°43'03" E -25.15
feet and corner;
THENCE: 6.67 feet around a curve to the left with a central angle of 1°08'04",
a radius of 336.66 feet and whose chord bears S 7°11 '02" W -6.67 feet at the
beginning of another curve to the left;
THENCE: 26.99 feet around said curve to the left with a central angle of
15°29'57", a radius of 99.77 feet and whose chord bears S 00°15'14" W -26.91
feet to another curve to the left;
THENCE: 22.86 feet around said curve to the left with a central angle of
6°07'04", a radius of 214.14 feet and whose chord bears S 12°18'41" E -22.85
feet to another curve to the left; .
THENCE: 27.56 feet around said curve to the left with a central angle of
78°56'46", a radius of 20.00 feet and whose chord bears N 54°50'35" w ·-25.43
feet to the end of said curve;
THENCE: S 85°41 '02" W -9.68 feet to the beginning of a curve to the left;
. ,.
THENCE: 21.65 feet around said curve to the left with a central angle of
6 °35'53", a radius of 188.00 feet and whose chord bears S 82°23'05" W -21.64
feet to the PLACE OF BEGINNING; according to a survey made on the ground
under the supervision of Donald D. Garrett, Registered Professional Land
Surveyor, No. 2972 on June 20, 1996.
..
Engineer's Estimate l D FOR ·
For 1 • .'J
Sanitary Sewer, Water, and Street and Drai age I
To <:t=tt:=.=
~~U.L..4 Lot 3 -Henton Subdivisio
College Station, Brazos County T
~~:.:.;:~;..:...--------
August, 1996
Wt I I t
~ rt:mi~ ~;~,~I· " ~ 111; '-~·
~:...--···-~ ~-· .. ' . ...n.a ... . . .. . -' ... ,, ) , .. ...It." ........... . .. ,. < •• t ••
1. 6" Diameter C900 Class 200 P.V.C. 41 L.F . $17.00 $ 697.00
Water Line (0-5 feet)
2. 6" Diameter C900 Class 200 P.V.C. 366 L.F . $18.00 $ 6,588.00
Water Line (5-7 feet)
3 . 6" Diameter C900 Class 200 P.V.C . 460 L.F . $ 19.00 $ 8 ,740 .00
Water Line (7-9 feet)
4. 6" Diameter C900 Class 200 P.V.C . 70 L.F . $ 20.00 $1,400.00
Water Line (9-11 feet)
5. 1 1/2" Diameter Type "K" Copper 156 L.F. $14.00 $ 2,184.00
Service Line
6. Trench Safety 1 L.S . $ 1,416.00 $1,416.00
7. 6" M.J . Pluq 1 Ea . $ 50.00 $ 50.00
8. 6" M.J. Gate Valve And Box 2 Ea . $ 475 .00 $ 950.00
9. 6"X6"X6" M.J. Tee 1 Ea . $ 570 .00 $ 570.00
10. 6" X 11 .25 Degree M.J . Bend 6 Ea . $ 250.00 $1,500.00
11 . 6" X 22 .5 Degree M.J . Bend 2 Ea. $ 250.00 $ 500.00
12. 6" X 45 Degree M.J. Bend 2 Ea . $ 250.00 $ 500.00
13. 6" X 90 Deqree M.J . Bend 1 Ea . $ 250.00 $ 250.00
14 . 6"X6" M.J. Tapping Saddle w/6" M.J. 1 Ea . $ 2 ,000.00 $ 2,000.00
Gate Valve And Box
Engineer's Estimate
For
Sanitary Sewer, Water, and Street and Drainage Improvements
To
Lot 3 -Henton Subdivision
College Station, Brazos County, Texas
August, 1996
(continued)
Water Im rovements I
[Gbiii ' j =n j titr:lr~~ ~ i -~!fi)ii i /il'
~L Ii'•~, ... ~ .J t :~)
15. 6"X1 1/2" Bronze Service Saddle w/ 4 Ea . $ 250.00
1 1/2" Corp. Stop , 1 1/2"X1 "X1" Wye
and 2-1" Stops
16 . Standard City Of College Station 1 Ea . $ 1,400.00
Fire Hydrant w/aoourtenances
17. 16"X6" M .J. Reducer 1 Ea. $ 250.00
JI
Total Estimated Cost For Water Improvements:
1.'
S n·t r Sewer I I v nt
irrJii11 · •m':ft t.rrt:l11 :.~~w·:~ .. ,jfjff:
' "' " 'Cf It
,, . ..,,
, .. ~~~~ .. (;(ta:: ~l .jl., "
1. 6" Diameter SOR 26-3034 San . Swr. 180 L.F. $17.00
(7 - 9 feet)
2. 6" Diameter SOR 26-3034 San . Swr. 198 L.F. $ 18.00
(9 -11 feet)
3. Standard Manhole (0 - 8 feet) 2 Ea . $1 ,000.00
4. Extra-depth Manhole (>8 feet) 6 V.F. $135.00
5. Trench Safety 1 L.S . $ 954 .00
6. Tie Into Existinq Manhole 1 .. Ea. $ 250.00
7. 6"X6 "X4 " Tee 3 Ea . $ 250.00
, :~iir·l 'rn ,
$1 ,000.00
$ 1,400.00
$ 250.00
$ 29 ,995.00
~
$ 3 ,060 .00
$ 3 ,564 .00
$ 2 ,000.00
$ 810.00
$ 954.00
$ 250 .00
$ 750.00
mate Engineer's Esti
For
Sanitary Sewer, Water, and Street an
To
d Drainage Improvements
division Lot 3 -Henton Sub
College Station, Brazos C ounty, Texas
August, 199 6
(continued)
s "t s I t
"~I; .11,1:
-~-.z.J:i.a
,,,.:'I....; _, .. lf' ... '(.,,..,. ' .•. !!\'tfll "°! 1 lff I\." ' ~~--~----------... ~ci~ .. :z_-~
8. 4" Diameter Schedule 40 San . Swr. 117 L.F. $14.00
Service Line
9. 4" Sin 6 Ea . $ 225.00
•;
Total Estimated Cost For Sanitary Sew er Improvements:
I
...
\'1'1 'r.J 1 li~ ·
--....
$1,638.00
$1,350.00
$14,376.00
•
Engineer's Estimate
For
Sanitary Sewer, Water, and Street and Drainage Improvements
To
Lot 3 -Henton Subdivision
College Station, Brazos County, Texas
August, 1996
(continued)
St t A d D . I I nt
rrrm'i'" ·! ' 1":t1l1;·,1.w;: 'i ~1 Ifill • , j1;
.Jli:i~ ~··i'' ~ L. .. L ... Htl< . ,, ~li8
1. 6" Thickness Reinforced Concrete 23200 S.F. $2.50
Pavement -3000 P.S.I., 28-day
Strength (reinforced with #4
bars at 18" O.C .)
2. 48" Reinforced Concrete Pipe 75 L.F . $ 48 .00
3. 6" P.V.C Pipe 42 L.F . $ 20 .00
4. Detention Pond Outlet Structure 1 Ea . $150.00
5. T .C.S. #018 Rock Rubble 100 S.F. $2 .50
Rip -Rap
6. Reinforced Concrete Sloped 285 S.F. $5.00
Headwalls (w/toewalls)
7. Inlet Structure 1 L.S. $ 650.00
8. 4" Concrete Reinforced Sidewalk 1760 S.F. $2.50
Total Estimated Cost For Street And Drainage Improvements:
Water Improvements :
Sanitary Sewer Improvements :
Street and Draina e Im rovements :
Total Estimated Cost:
''. \!';t~f1'i1 '
$ 58 ,000 .00
$ 3 ,600.00
$ 840 .00
$150.00
$ 250 .00
$1,425.00
$ 650.00
$ 4,400.00
$ 69 ,315.00
$ 29 ,995 .00
$14,376 .00
$ 69 ,315.00
$ 113,686 .00
E i'I GI i'I EE RI i'I G
CONSULTING ENGINEERING & LAND SURVEYING
4444 CARTER CREEK PKWY, SUITE 108 BRYAN, TEXAS 77802 1409J 846-26881409> 846-3094
March 15, 1996
Mr. Ken t Laza, P.E.
CITY ENGINEER
Department of Development Services
P.O. Box 9960
College Station, Texas 77842-9960
RE: DRAINAGE PLAN FOR HENTON SUBDIVISION -LOT 3
COLLEGE STATION, BRAZOS COUNTY, TEXAS
Dear Kent,
We are writing in rega rd to the abov e referenced project, the 3.51 acre tract off
Lincoln Avenue in College Station . Analysis of the property and surrounding area
found the appropriate drainage basin to include a total of 7.05 acres. The project site
itself (3.51 acre tract; owned by Mr. Henton) and the adjacent, upstream property to the
west (3.54 acre tract; owned by Mr. Putz; presently undeveloped) cornpri6e the existing
runoff area relevant to this project. Site and grading plans for the Henton project were
developed by our firm according to the existing 7 .05 acres contributing run-off area. An
appropriate detention pond and outlet control wPre designed and sized to ensure that
post-development conditions would not exceed pre-development (current) runoff flows.
Upon several submissions to the City of College Station, we receiv2d a comment
from Mr. Steve Homeyer regarding the adjacent property. His concern was the
possibility that, in the future, runoff from the adjarent site may be diverted from the
proposed detention facility. Considering the topography and roadway layout, the
chance of the runoff from the adjacent d evelopment not draining to the Henton pond is
unlikely. (This possibility is explored, however, so as not to restrict the grading options
and flexibility that may be pursued by the adjacent land owner.) We do recognize the
validity of the argument that if this situation did indeed arise, the 18 inch diameter pipe
specified would not be sufficient to properly detaii1 and meter runoff from the Henton
tract alone. Therefore, we conducted a second analysis to model the event that only the
Henton tract runoff entered the pond. Another outlet control structure was designed to
meter the flow according to the pre-development conditions of only the 3.51 acre tract.
This analysis is shown in the second portion of the drainage report (included with this
letter) and calls for a 5 inch weir to be placed ahead of the 18 inch pipe in the event that
the adjacent runoff is diverted .
The proposal wa s made by our firm that the second outlet control structure could
be added at the time of the adjacent development if needed. That is, if the adjacent
property was indeed graded in such a manner that runoff bypassed the Henton
detention area, the 5 inch weir could be placed ahead of the 18 inch diameter pipe to
further restrict discharge. This proposal was considered by our firm to be the best
soluti on for the following reasons:
(Pl ease Not e: Th e CihJ has ad vocat ed that th e outl et control structur e be size d
ac cordin g to th e 3.51 acre tra ct . A t th e sam e tim e how eve r, we hav e al so bee n
told that runoff infl ow, determinin g detention pond volum e and capacity, should
be com puted accordin g to th e 7.05 acres.)
a) It provided an effective, safe detention and metering system for the existing
stormwater runoff condition; as well as a second option for the potential
complication regarding the uncertainty in adj a cent land development.
b) It allowed progress on the Henton project to continue as quickly as possible.
Plans were all but complete and finalized -pending approval on this drainage
issue. This proposal enabled work to continue while still thoroughly addressing
and planning for the pos sibility of changes in drainage patterns.
c) As the cost to add the weir is small, this option was also the most cost-effective.
The engineer's estimate show s a construction cost for the weir structure and
concrete box of $200.00 or less. Re-work of plans, drainage reports, further
anal ysis and modeling calculations will incur greater expense to the project
owner -considerably greater than the up front investment toward this second
outlet control s tructure .
d) The priority remained on the current, existing condition.
The controlling desi g n parameters in this proposal were the 7.05 acre drainage
basin and corresponding outlet control structure and discharge restrictions. It
seems unwise to redesign the facility with the controlling metering device
selected for a drainage possibility which may or may not occur -a possibility
which will more than likely not occur. The situation remains at present that the
Henton project receives runoff flow accumulating from the entire 7.05 acres. In
the event that there is a change in runoff flow, an alternate design has already
been provided and may be easily added on an as-needed basis.
e) To design a detention facility for two possible runoff basins, particularly when
the basins vary significantly in size, is inherently inefficient.
The capacity for either of the runoff conditions alone is under half of that
required to handle both scenarios with one metering device. (That is, the
detention volume required for either the 7.05 acre or the 3.51 acre case -with
the appropriate outlet control structure -is under 14,000 cubic feet. The
additional storage capacity needed with one metering device restrictive enough
for the 3.51 acre condition is 30,000 cubic feet.)
Such a pond will be overdesigned in either event. So long as the 7.05 acre runoff
condition remains, the pond will detain twice as much water as is necessary. In
the event that drainage patterns do change to the 3.51 acre basin scenario, the
pond is grossly too large with less than 9,000 cubic feet required.
f) This option most closely achieved the intentions of the drainage ordinance
regulations as we understand them .
It is our understanding that, at this level, the primary purpose of drainage
assessment with new development is to account for changes made in drainage
behavior and as closely as possible return the effects felt downstream to that
which existed prior to development. (e.g., Increased, rapid runoff due to the
establishment of impermeable surfaces is usually collected in a detention facility
and released no faster than peak pre-development flows.)
In other words, the regulations at the basic level emphasize maintaining the
natural drainage patterns of the area . Only with a larger scale, more
comprehensive analysis and reference to the City's overall drainage goals would
there be a reason to alter the existing drainage conditions.
g) This proposal was also considered by our firm to be both fair and just in terms
of responsibility and reasonable expectations.
It enabled a considerably smaller detention facility to be constructed for the
Henton project, and still offered a solution to the potential basin change with
adjacent development. Addition of the weir structure would be provided for by
Mr. Henton -both financially and in terms of technical design.
Analysis and design for the outlet weir structure is included in the Drainage
Report for the Henton project. Feasibility, incorporation with the proposed
pond geometry and capacity, and integration with the 18 inch pipe outlet
structure have been verified. Capital to finance the weir construction would also
be provided by Mr. Henton at the onset of the project.
Following the suggestion made by the City, Mr. Henton will be required to
construct a detention pond large enough to store and meter the increased runoff
generated from his own development and all of the existing stormwater runoff
from the adjacent tract. In effect, downstream peak flow conditions are reduced
from pre-development flows. (e .g ., For the 100-year storm event, peak flows fall
from 34 cubic feet per second to 17 cubic feet per second.) Detention of this
additional runoff volume seems an unfair burden to place on Mr. Henton
without any justification given. If there is some other consideration or objective
that we are presently unaware of, please let us know that we may inform our
client.
In conclusion, we do not understand why our proposal was so quickly
dismissed. We received little explanation and were informed that the City will only
accept an outlet control structure sized according to the 3.51 acre runoff basin. We have
since redesigned the detention pond and drainage structures to meet with this
restriction. However, we were quite disappointed by the seemingly uncooperative
nature of the City in working towards a mutually beneficial solution on this project. We
would like to arrange a time to meet with you to discuss the project so that we might
better understand the City's point of view and the drainage requirements as they may
influence future projects . If you have any questions or comments concerning the
Henton project or this letter, please contact me at your earliest convenience (409) 846-
2688 .
d;:~d~
Lisa R. Hagerman
Garrett Engineering enclosure: PREVIOUS DRAINAGE REPORT
cc : MR. STEVE HOMEYER
MR. J .V . HENTON
I
E i'l G I i'l EE R. I i'l G
CONSULTING ENGINEERING & LAND SURVEYING
4444 CARTER CREEK PKWY, SUITE 108 BRYAN, TEXAS 77802 (409J 846-2688 (409) 846-3094
March 15 , 1996
Mr. Kent Laza, P.E .
CITY ENGINEER
Department of Development Services
P.O . Box 9960
College Station, Texas 77842-9960
RE: DRAINAGE PLAN FOR HENTON SUBDIVISION -LOT 3
COLLEGE STATION, BRAZOS COUNTY, TEXAS
Dear Kent,
We are writing in regard to the above referenced project, the 3.51 acre tract off
Lincoln Avenue in College Station. Analysis of the property and surrounding area
found the appropriate drainage basin to include a total of 7.05 acres. The project site
itself (3.51 acre tract; owned by Mr. Henton) and the adjacent, upstream property to the
west (3.54 acre tract; owned by Mr. Putz; presently undeveloped) comprise the existing
runoff area relevant to this project. Site and grading plans for the Henton project were
developed by our firm according to the existing 7.05 acres contributing run-off area. An
appropriate detention pond and outlet control were designed and sized to ensure that
post-development conditions would not exceed pre-development (current) runoff flows.
Upon several submissions to the City of ColJege Station, we received a comment
from Mr. Steve Homeyer regarding the adjacent property. His concern was the
possibility tha t, in the future, runoff from the adjacent site may be diverted from the
proposed detention facility. Considering the topography and roadway layout, the
chance of the runoff from the adjacent deve lopment not draining to the Henton pond is
unlikely. (This possibility is explored, however, so as not to restrict the grading options
and flexibility that may be pursued by the adjacent land owner.) We do recognize the
validity of the argument that if this situation did indeed arise, the 18 inch diameter pipe
specified would not be sufficient to properly detain and meter runoff from the Henton
tract alone. Therefore, we conducted a second analysis to model the event that only the
Henton tract runoff entered the pond. Another outlet control structure was designed to
meter the flow according to the pre-development conditions of only the 3.51 acre tract.
This analysis is shown in the second portion of the drainage report (included with this
letter) and calls for a 5 inch weir to be placed ahead of the 18 inch pipe in the event that
the adjacent runoff is diverted.
The proposal was made by our firm that the second outlet control structure could
be added at the time of the adjacent development if needed. That is, if the adjacent
property was indeed graded in such a manner that runoff bypassed the Henton
detention area, the 5 inch weir could be placed ahead of the 18 inch diameter pipe to
further restric t .clisch(lrge. This proposal was considered b y our firm to he the hes.t
solution for the following reas ons :
(Plea se Not e: Th e Citt; has advocated that th e outlet control structure be sized
accordin g to th e 3.51 acre tra ct. At the same time how ever, we hav e also been
told that runoff inflow, determining detention pond volume and capacity, should
be computed according to th e 7.05 acres.)
a) It provide d an effective, safe detention and metering system for the existing
stormwater runoff condition; as well as a second option for the potential
complication regarding the uncertainty in adjacent land development.
b) It allowed progress on the Henton project to continue as quickly as possible.
Plans were all but complete and finalized -pending approval on this drainage
issue . This proposal enabled work to continue while still thoroughly addressing
and planning for the poss ibilit y of changes in drainage patterns.
c) As the cost to add the weir is small, this option was a lso the most rn:,t-effe1::t°i ve .
The engineer's estimate shows a construction cost for the weir structure and
concrete box of $200 .00 or les s. Re -w ork of plans, drair..age reports, further
analysis and mode ling calculations will incur greater expense to the project
owner -considerably greater than the up front investment toward this second
outlet control structure.
d) The priority remained on the current, exis ting condition .
The controlling desi gn parameters in this proposal were the 7.05 acre drainage
basin and corresponding outlet control structure and discharge restrictions. It
G J.\R REI I E i'I GI i'I EE RI i'I G
seems unwise to redesign the facility with the controlling metering device
selected for a drainage possibility which may or may not occur -a possibility
which will more than likely not occur. The situation remains at present that the
Henton project receives runoff flow accumulating from the entire 7.05 acres. In
the event that there is a change in runoff flow, an alternate design has already
been provided and may be easily added on an as-needed basis.
e) To design a detention facility for two possible runoff basins, particularly when
the basins vary significantly in size, is inherently inefficient.
The capacity for either of the runoff conditions alone is under half of that
required to handie both scenarios with one metering device. (That is, the
detention volume required for either the 7.05 acre or the 3.51 acre case -with
the appropriate outlet control structure -is under 14,000 cubic feet. The
additional storage capacity needed with one metering device restrictive enough
for the 3.51 acre condition is 30,000 cubic feet.)
Such a pond will be overdesigned in either event. So long as the 7.05 acre runoff
condition remains, the pond will detain twice as much water as is necessary. In
the event that drainage patterns do change to the 3.51 acre basin scenario, the
pond is grossly too large with less than 9,000 cubic feet required.
f) This option most closely achieved the intentions of the drainage ordinance
regulations as we understand them.
It is our understanding that, at this level, the primary purpose of drainage
assessment with new development is to account for changes made in drainage
behavior and as closely as possible return the effects felt downstream to that
which existed prior to development. (e.g., Increased, rapid runoff due to the
establishment of impermeable surfaces is usually collected in a detention facility
and released no faster than peak pre-development flows.)
in other words, the regulations at the basic ievel emphasize maintain.ing the
natural drainage patterns of the area. Only with a larger scale, more
comprehensive analysis and reference to the City's overall drainage goals would
there be a reason to alter the existing drainage conditions.
g) This proposal was also considered by our firm to be both fair and just in terms
of responsibility and reasonable expectations.
It enabled a considerably smaller detention facility to be constructed for the
Henton project, and still offered a solution to the potential basin change with
adjacent development. Addition of the weir structure would be provided for by
Mr. Henton -both financially and in terms of technical design.
Analysis and design for the outlet weir structure is included in the Drainage
Report for the Henton project. Feasibility, incorporation with the proposed
pond geometry and capacity, and integration with the 18 inch pipe outlet
structure have been verified. Capital to finance the weir construction would also
be provided by Mr. Henton at the onset of the project.
Following the suggestion made by the City, Mr. Henton will be required to
construct a detention pond large enough to store and meter the increased runoff
generated from his own development and all of the existing stormwater runoff
from the adjacent trnct. fti effect, downstream pea.!-: fl e w conditions Me Teduced.
from pre-development flows. (e .g., For the 100-year storm event, peak flows fall
from 34 cubic feet per second to 17 cubic feet per second.) Detention of this
additional runoff volume seems an unfair burden to place on Mr. Henton
without any justification given . If there is some other consideration or objective
that we are presently unaware of, please let us know that we may inform our
client.
In conclusion, we do not understand why our proposal was so quickly
dismissed. We received little explanation and were informed that the City will only
accept an outlet control structure sized according to the 3.51 acre runoff basin. We have
since redesigned the detention pond and drainage structures to meet with this
restriction . However, we were quite disappointed by the seemingly uncooperative
nature of the City in working towards a mutually beneficial solution on this project. We
would like to arrange a time to meet with you to discuss the project so that we might
better understand the City's point of view and the drainage requirements as they may
influence future projects. If you have any questions or comments concerning the
Henton project or this letter, please contact me at your earliest convenience (409) 846-
2688 .
Respectfully Submitted,
~~~~ Li~~. Hagerman
Garrett Engineering enclosure: PREVIOUS DRAINAGE REPORT
cc: MR. STEVE HOMEYER
MR. J.V. HENTON