HomeMy WebLinkAbout27 Development Permit 316 Ponderosa Place II F2 F3. '
·.
SITE DRAINAGE REPORT
PONDEROSA PLACE, SECTION TWO
TRACTS "F-2" AND "F-3"
JUNE 27, 1995
PREPARED FOR :
Rock Pra irie , LTD .
PREPARED BY :
M .L. Hammons, P .E .
1700 Kyle South , Suite 240
College Station , TX 77840
( 409)696-1444
..
PROJECT DESCRIPTION
Construction of retail facilities, including an auto oil & lube service, a dry cleaning servic e
and a self-serve car wash .
The proposed facility is to be situated in Tract "F" of the Ponderosa Place Subdivis ion ,
Section Two and is located in the northwest quadrant of the intersection of Rock Pra irie
Road and State Highway 6 , also identified as Texas Avenue South (See Appendix #1 ,
"Area Map"). Proposed improvements will occupy ·tracts "F-2" and "F-3". The
development is to be a common driveway/common parking retail trade area among the
three separate businesses and in conjunction with the Exxon facil it y located on the
adjacent tract "F-4".
The facilities proposed in this project will include buildings with combined "footprint" are as
of approximately 4,600 square feet. Stormwater detention facilities are presently located
on tract "F-2" to provide stormwater run-off control requirements for tracts "F-1", "F-2",
"F-3" and "F-4". This proposal includes modification of the detention facility , but the
drainage control function will be continuously operative during the modification .
STORMWATER CONTROL
The development site (tracts "F-2" and "F-3"), addressed herein , is an in tegral part of the
storm drainage system that was designed and implemented in 1993 by Victoria Bank and
Trust. There is no proposal, herein, to change or modify the watershed run -off patt e rn s
or to change the degree of run-off attenuation or the discharge routing . Th i s propos al
provides strict adherence to the original drainage plan , even though the stormwater
drainage and detention facilities will be modified to enlarge the land area available for
retail facilities development.
Changes to accomplish land area usage improvements are to be accomplished by :
1. Deeper detention pond (5 .0' vs 3.0')
2. Vertical concrete pond walls -vs -sloped earth walls
The original plan, as followed herein, was prepared and submitted by B.J . Kling. A partial
reproduction of the report is included herein as Attachment I. Further, the plan was
updated in February, 1995 to include the Exxon development of tract "F-4". A p artial
reproduction of the updated report is included herein as Attachment 11.
According to Attachment I, page 2 , first paragraph , "the detention pond has been
designed to attenuate enough peak run-off for the full development of the site , incl ud ing
the two tracts to be sold at a later time ." Since the land to be used in this development
1
.
; .
I '
''
is a part of the "tracts to be sold at a later time", no additional stormwater ma na ge m K .t
facilities are required . Therefore , the objective of th is report is to show adhe rence t o the
conditions of the original analysis , thus maintain ing the performan c e inte grit y of the
system as it was originally designed and approved .
Essential factors for adherence include :
1. Acceptance and discharge of sto r mw~ter run -off fro m adja ce nt t ra cts.
2 . Proper drainage routing (direct d ischarge -vs -detention ) fro m tr acts
"F -2" and "F-3".
3 . Detention volume capacity of at least 17 , 9 3 8 ft ( 100 year sto r m ev ent).
4. Discharge outflows to closely match the original sto r m ev en t de si g n .
DRAINAGE AREAS
Neither this development site , nor any of the upstream drainage areas are w it h i n the 100
year flood plain . (See Appendix #2 , "FIRM flood p lain map").
As reported in the tract "F-4" update , a small portion of the "F-4" tract (E xxo n ) d rai n s
directly to tract "F-3" (this site). As shown by grading plan (See App en d ix #3 , "G rad in g
Plan") all incoming surface flow will flow i nto the detention pond .
Also , all tract "F-1" (Victoria Bank & Trust) stormwater run -off, w ith det en t io n pond
destination, flows onto the paved access easement along the west side of t ra ct s "F -4 ",
"F-3" and "F-2" as originally planned . As provided by this development pla n , All of t h at
inflow then flows into the detention pond via surface flow or via the proposed un d erg ro u '.1d
collection and transfer system . Therefore , acceptance and r un -off from ad j a cen t tr act s
is not modified and is in compliance with the original plan .
All Rock Prairie, Ltd. site generated stormwater run-off eithe r flows d i rectly t o t h e street
(or paved private access) or into the detention pond (See Appendix #4 , "S ite r un -off flow
plan"). As the plan shows , a 0 .176 acre perimeter area drains directly to street flow an d
the 0 .908 acre mid section of the site drains to the detention pond. Therefore , d r ai n a ge
from tracts "F-2" and "F-3" a re properly included in the dete nt ion po nd colle cti o n sy stem
and comply with the original storm drainage plan .
As shown by the grad ing plan , (Append ix #3) a detention pond is to be c o nst r u ct ed on
tract "F-2". The original drainage plan in cludes a detention pond on this t r a c t , b ut is
larger in area . According to Attachment I, page 3 , 1st pa ragraph , the orig i nal p o nd
provides volumes of 14 , 131 ft 3 for the 2 5 year storm and 17 , 9 38 ft 3 for th e 100 year
2
storm. This proposal is to increase the depth and reduce the surface area of th e pond
while maintaining or exceeding the originally planned storage volumes . The de pth a nd
surface areas of the redesigned pond are tabulated thus:
ELEVATION AREA
301.8 0
302.0 2150 sf
302 .3 3680 sf
303.0 3800 sf
304 .0 4170 sf
305.0 4580 sf
306 .0 5430 sf
306 .3 5780 sf
306 .8 6330 sf
VOLUME
0
140 ft 3
940 ft 3
3550 ft 3
7530 ft3
11 , 910 ft 3
16, 910 ft 3
18,590 ft 3
21 ,620 ft 3
(Discharge i nvert)
(100 year volume )
(Ultimate volume )
Therefore, originally planned detention volume requirements have been met .
Detention pond discharge is presently controlled by a discharge structure located as
shown by Appendix #3 and as detailed by Append ix #5 (Detention disc harge str uct u r2)
Discharge structure modifications to accomplish a deeper storage facility in clude :
1. Raising the overflow level from 306.0 to 306.3 , by extending the top of the
concrete structure with brick and epoxy grout mix .
2 . Lowering the f low control orifice from elevation 303 .0 to 301 .8 . The size a nd/or
shape of the orifice will be adjusted to prov ide flows properly metered
(according to original plan) at the 10 , 25 and 100 year storm eve nts .
Discharge flows as shown by Attachment I, page 3, (and the target flows fo r redesi gn)
are :
25 year ..... .
100 year .... .
4 .00 cfs
4 .82 cfs
Flow control integrity will thereby be attained .
DRAINAGE VOLUME
In order to remain in full compliance with the original stormwater control pl an , the actual
run-off for each contributing area must not exceed the originally calcu lated run -off . Since
the area and the intensity of the rainfall event used in the Rational Method of cal cul ati o n
is identical, the only variable is the C-factor . The o riginal run -off calc u lations w ere b as ed
on a "C-factor of 0 .78". (See Appendix #6 , "C -factor Confir mation")
3
'.
The composite "C-factor" for tracts "F-2" and "F -3" is 0 .777 ; thus in full compliance .
C factor has been calculated thus :
CONCLUSIONS
Impervious Areas C = 0 .90
Buildings
Paving
Walkways
Etc.
Other Areas C = 0 .35
Landscaping
Lawn
Run-off area F2DA - 1
Run-off area F2DA - 2
5 ,310 sf@ 0.90
2 ,335 sf@ 0 .35
31 ,313 sf@ 0 .90
8 ,262 sf@ 0 .35
47 ,220 sf@ C = 0 .777
Since tracts "F-2" and "F-3" have been included in the original ordinance comliant stor m
drainage plan for the tracts that were then (1993) Victoria Bank and Trust land , prope r
storm drainage for tracts "F-2" and "F-3" must comply with the design perameters of the
originally designed and approved plan . All drainage inflows from adjacent tracts , an d all
site generated drainage flows have been accommodated . Detention storage vo lume s
have been maintained or exceeded and all metered discharge rates have been
maintained. Accordingly, as provided in the original plan , "no impact is expect ed to
downstream landowners from this development".
4
CERTIFICATION
I hereby certify that this drainage analysis and the fac ilities des ign result ing the refro m was
prepared by me (or under my supervision) and that the plan has been devised for
compliance with the City of College Station Drainage Policy and Design Stand ard s and
for adherence to the original V ictoria Bank and Trust area dra inage plan of Septem ber
1993.
M . L. Hammons
Registered Professional Eng ineer
State of Texas #25308
5
.. ,
,,.
'·
,.
APPENDICES
#1 Area Map
#2 FIRM Flood Plain Map (Copy)
#3 Site Grading Plan
#4 Site Run-Off Flow Pattern Sketch
#5 Detention Discharge Structure Drawing
#6 C-Factor Confirmation
ATTACHMENTS
Original Victoria Bank and Trust Drainage Analysis and Plan (Copy)
II 1995 Updated Plan (Copy)
VICINITY MAP
I Z
f--D
Ck'.~
D --i
Z I
NTS
1
If"
I
I
I
i
3 /
A
Bra zos Co unt y
U nin c orp o ra t ed Areas
4 8 1195
ZONE X
~ . /)
· /
Ci t y of Co ll ege St ati o n
4 800 83
C0
C it y of College Station
480083
·.
\
ON
WATER LINE
AND DEPTH
J VERIF IED
~·
/ /
Ji 1
I
I
I
I
·~1
~ N j
!
/
. . d
. J..
.d , -l , .. --..
.d
GRATE INLET-NEENAH R-1879-A9G OR EQUAL
Ei CIRCULAR OPE NIN G
IN FA CE OF BOX
FL IN ELEY 303 .00
--_ 14
·--..
--4 .
--·-t· ..
TOP ELEY .306.00
#3 REINFORCING 8.A.~S AT
l 5° O.C.8.W., 7 CLEAR
15" RCP @ 0. 757.
D£T£/\ITJON POND OlJTLD WORKS DETAJL
NOT TO SCALE
A ppi:::: N 0 I x $ 5
---,--------r--1 310
•
06 /19 /95 18:05 '0'409 846 8252 KLHiG EN GR
KLING ENGINEERING & SURVEYING
TELECOPIER COVER SHEET
(Ef:faxcover)
Please deliver the following pages to:
Name: J<~ \k""'~'-o&-
Firm: _______________________ _
Fax Number: ~Cl_o -~S \
From:----...S~~~-1~ .......... ~~~T ________ _
Kling Engineering & Surveying
4103 South Texas Ave .• Suite 212
Bryan, Texas 77902
Phone: (4091846-6212
Fax: (4091846 -8252
We are transmitting page(s), including this cover letter from our
fax number listed above. If any pages are missing or incomplete, please
contact sender.
Regarding: ~a..-''cl\ ~ t..WRJ ~..\----l~"-'>-U b-.O-lQ
[41001
O"'\ ~Le~ t= -~ -Q.St-\-\ A.t_fu.' · .~f\h ~~O~I\ ~"lr,
~@-\~:a <,C-81\~i"'"' ~~~~[1,.J)m~,\:
']) /\ \ _.,_ .1-·l5>1-~~ l~ tD~\.t'1 ~Let ui. \--\ \ ·~ UcDk~~
'f-1.. D·~L\:}~ ~T~~ o.<4l kr °'"'& \J'd-Q..~ ~
Co~l?~o~ ,, C \ 7-. Q ,!'( l ~ •
1.
I
I
I
I
I
I
I
I
I
I
I
il
I ,
I
.I
I
I
Drainage Report
for
Victoria Bank and Trust -College Station Branch
Robert Stevenson Survey, A-54
College Station, Brazos County, Texas
September 8, 1993
GENERAL LOCATION & DESCRIPTION
The proposed site location is Tract F of Ponderosa Place, Section Two, 3.81 Acres,
which is located at the intersection of Rock Prairie Road and State Highway No. 6 . The tract
is bounded by Tract A and Tract E of the beforementioned subdivision to the north, the frontage
road for Highway 6 to the east, Rock Prairie Road to the south and Longmire Drive to the west.
The tract is currently vacant and is covered entirely by grass. Proposed improvements consist
of subdividing the tract into three separate tracts, the largest of which will consist of an
approximately 5700 square foot bank with 65 parking spaces and a 9 lane drive-thru facility.
The remaining two tracts are to be developed after being sold and platted at a later date.
DRAINAGE BASINS & SUB-BASINS
Two primary studies have been performed on Carter's Creek and its tributaries. The first
is a study performed by the Federal Emergency Management Agency in July, 1992, titled Flood
Insurance Study -Brazos County. Texas and Incorporated Areas. The purpose of this report was
to investigate the existence and severity of flood hazards for Brazos County and to aid in the
administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection
Act of 1973. The second study is the Stormwater Management Plan. Phase III, performed by
Walton and Associates -Consulting Engineers, Inc. contracted with the City of College Station
in 1987. The purpose of this study was to gain knowledge of the existing and future drainage
conditions in the City and to identify flood problems and flood prone areas.
The site is located on a ridge line dividing the Bee Creek drainage basin, Lick Creek
drainage basin and the Foxfire Tributary to Carter's Creek drainage basin. The site itself mostly
drains into the Foxfire Tributary's basin, thus the tributary is the primary drainageway.
Secondary drainageways include a series of culverts and open ditches along Highway 6 until the
flow reaches the tributary. (See Appendix 1: "General Location Map"). The site is not within
the 100 year flood plain according to the Flood Insurance Rate Map for College Station, Texas.
(See Appendix 2: "Flood Plain Map").
Almost all offsite runoff is intercepted by the curb and gutter systems along Longmire
and Rock Prairie Road which leads into two curb inlets that discharge into the right-of-way of
Highway 6. The runoff then proceeds down an open ditch until it reaches Lick Creek. Runoff
from Tract A drains over the 5 foot retaining wall and down the property line, more or less, into
the curb and gutter parking lot and drives of Tract F. From here, runoff enters the frontage
road and proceeds into an open ditch and ultimately into the Foxfire Tributary.
Victoria Bank and Trust -Drainage Report Page 2
DRAINAGE DESIGN CRITERIA/FACILITY DESIGN
Because of the increase in peak runoff expected from this development detention will be
provided for development of the entire site . That is, the detention pond has been designed to
attenuate enough peak runoff for the full development of the site, including the two tracts to be
sold at a later time. The pond will be located at the north corner of the tract and will discharge
into the right-of-way of Highway 6. Currently runoff drains onto the frontage road as sheet
flow. After development, it will become concentrated flow, therefore, the pond outlet will be
discharged across the frontage road through a 15" R.C.P culvert to be constructed. Due to
topography and the flowline elevation of the curb inlet at the Rock Prairie Road intersection, the
existing storm sewer system cannot be utilized .
The design storm used is the 25 year event, though the 10 year and 100 year storms were
investigated. The Rational Method was used to calculate peak runoffs and the Modified Rational
Method was used to estimate detention storage volume requirements. A runoff coefficient of
0 .35 was used for pre-development conditions ·while a runoff coefficient of 0.90 was used for
post-development impervious areas and 0 .50 for post-development landscaped areas. This
yielded a pre-development peak runoff of Q25 = 13.18 cfs and a post-development peak runoff
of Q25 = 29 .38 cfs and thus a required attenuation of 16.20 cfs.
Because of topography, Reserve Tract "A" will not be required to drain into the detention
pond. All i nflow into the pond will be from the bank's tract and Reserve Tract "B". Therefore,
the tract can be divided into two drainage areas: Drainage Area #1 is captured by the detention
pond and Drainage Area #2 bypasses the pond. DA #1 contributes Qi 5 = 20.15 cfs and DA #2
contributes Q25 = 9.23 cfs . (See Appendix 3: "Drainage Area Maps"). This requires the 16 .20
cfs attenuation be taken from DA #1 and thus the maximum outflow of the pond to be 3 .95 cfs.
Most drainage will be sheet flow, curb and gutter, and open channel flow, although one
portion of the drive and parking lot will be storm sewered with three grate inlets and a 15"
R.C.P. The parking lot will have two curb breaks located at the south end of the tract with 6: 1
side sloped swales leading to a main interceptor channel. The 15" storm sewer will daylight
near the middle of this channel while the drive-thru lanes will drain to a curb break with a 6: 1
side sloped swale leading to the north end of the channel. The interceptor channel itself will
have 6: 1 side slopes , 0. 98 % channel slope, maximum water depth of 1. 0 ft, and a velocity of
3.6 fps.
The interceptor channel then leads into the detention pond, which will have a maximum
volume of approximately 23,000 ft3 and varies in elevation 303.0 to 306 .5 . The outlet works
will be an 8" orifice in a concrete box with the orifice flow line at elevation 303.0 and a grate
inlet at the top of the box with an elevation of 306.0. The box then leads to a 15" R .C.P. on
a 0. 75 % grade that will be bored beneath the frontage road and daylight in an area between the
southbound exit ramp and the turnaround exit ramp for Highway 6 . The turnaround exit ramp
currently has an 18" R.C.P. culvert beneath it and has sufficient capacity for the pond's
discharge. The ditch section between the proposed 15" and existing 18" pipe will require
regrading with a minimum slope of 0.8 % . It is anticipated that at such time as the frontage road
is converted to one-way traffic, the turnaround exit ramp will be removed and the ditch section
can then be i mproved to obtain more positive drainage.
!
i.
i
I
f
I
t
I
I
I
I
.I
·I.
Victoria Bank and Trust -Drainage Report Page 3
This design provides for a water surface elevation in the pond of 305. 78, peak outflow
of 4.00 c f s, and a storage volume of 14,131 ft3 for the 25 year storm. The 100 year storm will
have a water surface elevation of 306.10, peak outflow of 4.82 cfs , and a storage volume of
17,938 ft3 . ..
CONCLUSIONS
Peak runoff after development increases by 16.2 cfs for the 25 year event and 19.12 cfs
for the 100 year event. The Detention Pond will bring post-development, 25 year peak runoffs
down to their pre-development conditions and will bring post-development, 100 year peak
runoffs down from 34 .68 cfs to 15. 71 cfs, an increase of 0.15 cfs from pre-development
conditions. Since the site currently drains into the Highway 6 right-of-way and ultimately into
the same ditch section that the detention pond discharges into, and peak runoffs have been
attenuated , no impact is expected to downstream landowners from this development.
"I hereby certify that this report (plan) for the drainage design of Victoria Bank & Trust -
College Station Branch was prepared by me (or under my supervision) in accordance with the
provisions of the City of College Station Drainage Policy and Design Standards for the owners
thereof." -
I
I
I .
I
I
I
V1 '-f o rt <A L >'"'" L ·~ in-'--·
A r-ec... ... ? I rt/l AcrO:>
~10-.. 8.~3 71,.,
Q -= J l . i?Y c 1 ~ 10 f _,
/. o.c.? ~ 15. J)b d-::.
,
Vo>+-yp,,,efop~
11: >.ct7..Ac.-
~,o = B.b:>
~l(K),; /I, b ~
Q,0 -=-t6.'7l J-s
Oz.:;-: z.q, "'Jct; r
Q,Ct? ~ 3~ • b~ ~r~
£/(;v
~
3o t/
305"
'50b
so 7
30&
3()0
~
0
1 0 5
'21 ~r;
'-1 2.:-I S
f:7V)J0
LJ3J(
Cul f e..· :.-{""v:: ,·crv
1
[ • A ~ I. 'LO
~7...
Q,o~l\o .... "" '3,'i/
Qz.;; ::;. 3,c,5
O.o-= )1, ~"{
Qis1 Z0.15
C:,(() = '2.3 .'7°l
Q,0-;. 0.0 7
Qi>~ <1,'2..'3
QIOC>-;. ) 01'0~
I.
!
'
DRAINAGE REPORT
FOR
EXXON EXPRESS
HWY 6 & ROCK PRAIRIE ROAD
Rqbert Stevenson Survey, A-54
College Station, Texas
February 22, 1995
DEVELOPED BY:
Exxon Express -Hwy 6
c/o Ray Hansen
730 N. Rosemary
Bryan, TX 77802
( 409) 846-7861
PREPARED BY:
Kling Engineering & Surveying
4103 Texas Avenue, Suite 212
Bryan, TX 77802
(409) 846-6212
Drainage Report
for
Exxon Express -Hwy 6 & Rock Prairie Road
Robert Stevenson Survey, A-54
College Station, Brazos County, TeJ1,as
February 20, 1995
GENERAL LOCATION & DESCRIPTION
The proposed site location is Tract "F-4", Resubdivision of Ponderosa Place, Section Two,
"Tract F", (hereafter referred to as Tract "F-4 ") located at the intersection of Rock Prairie Road
and State Highway No. 6. The tract is bounded by Tract "F-3" of the beforementioned
subdivision to the northwest , the frontage road for Highway 6 to the northeast, Rock Prairie Road
to the southeast and Tract "F-1" of the beforementioned subdivision to the southwest (See
Appendix 3: "Resubdivision plat of Ponderosa Place Section II Tract F). The tract is currently
vacant and is covered entirely by grass. Proposed improvements consist of constructing a 3869
square foot Exxon Express Convenience Store, 8 covered gas pumping islands, 38 parking spaces,
and installing 2 -12,000 gallon and 1 -8000 gallon underground storage tanks.
DRAINAGE BASINS & SUB-BASINS
Two primary studies have been performed on Carter's Creek and its tributaries . The first
is a study performed by the Federal Emergency Management Agency in July, 1992, titled Eloo.d
Insurance Study -Brazos County, Texas and Incorporated Areas. The purpose of this report was
to investigate the existence and severity of flood hazards for Brazos County and to aid in the
administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act
of 1973 . The second study is the Stormwater Management Plan, Phase III, performed by Walton
and Associates -Consulting Engineers, Inc. contracted with the City of College Station in 1987.
The purpose of this study was to gain knowledge of the existing and future drainage conditions
in the City and to identify flood problems and flood prone areas .
The subject tract was formerly a portion of the 3.81 acre Tract "F" of Ponderosa Place
Section Two (hereafter referred to .as Tract "F"), which has since been subdivided into Tracts Fl, /
F2, F3, & F4. A drainage analysis was performed and a drainage report prepared for Tract "F" V
in regards to the construction of the Victoria Bank & Trust, College Station Branch which is now
built and situated on Tract "F-1". The Tract "F" study provided a detention pond that was
designed to attenuate enough peak runoff for the full development of Tract F (see Victoria Bank
& Trust Drainage Report Dated September 13 , 1993). Subsequently, runoff for the resubdivision
Tracts F-1, F-2, F-3 & F-4 has been attenuated as well. Therefore, no detention is needed for
Tract F-4.
' ' ' I
I
I
I
I
I
I
I
I
I
I
I
I.
I.
I
Exxon Express -Hwy 6 & Rock Prairie Road
Drainage Report Page 2
.. -. -. ............. · .. ;..; .... ~11.J
The site is located on a ridge line dividing the Bee Creek drainage basin, Lick Creek
drainage basin and the Foxfire Tributary to Carter's Creek drainage basin. The site itself mostly
drains into the Foxfire Tributary's basin , thus the tributary · is the primary drainageway .
Secondary drainageways include a series of culverts and open ditches along Highway 6 until the
flow reaches the Foxfire Tributary. (See Appendix 1: "General Location Map"). The site is not
within the 100 year flood plain according to the Flood Insurance Rate Map for College Station,
Texas. (See Appendix 2: "Flood Plain Map").
Almost all off-site runoff is intercepted by two fackties. The first being the curb and
·gutter system along Rock Prairie Road which leads into two curb inlets that discharge into the
right-of-way of Highway 6, runoff then proceeds down an open ditch until it reaches Lick Creek.
The second is an open ditch that runs parallel to the east line of Tract "F-1" and discharges into
the detention pond on Tract "F-2" which is metered out into the Highway 6 right-of-way and
proceeds to the Foxfire Tributary.
Currently, the majority of the runoff from the subject tract sheet flows into the gutters
along Highway 6 frontage road, proceeds to an open ditch, and then ultimately into the Foxfire
Tributary. A portion of the tract sheet flows into the curb and gutter systems along Rock Prairie
Road, then follows the discharge pattern that flows into Lick Creek as mentioned earlier (See
Appendix 5: "Hydrologic Calculations -Rational Method).
·DRAINAGE DESIGN CRITERIA/FACILITY DESIGN
Because of the detention requirements for Tract "F" have been met, no detention for Tract
"F-4" is required. In the previous study performed by Kling Engineering, Tract "F" was divided
into Drainage Area #1 and Drainage Area #2. DA #1 is captured by the detention pond and DA
lfl, which contains Tract "F-4", bypasses the pond . (Reference Appendix 3 of the Victoria Bank
& Trust Drainage Report Dated September 13 , 1993).
To show that the detention requirements for Tract "F" have been met, the design storm for
the 25 year return period will be used, even though the 10 year and 100 year storm were also
investigated . The Rational Method was used to calculate peak runoff. A runoff coefficient of
0.35 was used for pre-development conditions while a runoff coefficient of 0.90 was used for
post-development and impervious areas and 0.35 for post-development landscaped areas. This
yielded a pre-development runoff of Q25 = 13.18 cfs. The detention pond outlet peak flow is 4.00
cfs for the 25 year storm, leaving 13.18 cfs -4.00 cfs = 9.18 cfs of allowable direct runoff from
the site. The development site for Tract "F-4" can be divided into five drainage areas: Drainage
Area #1 flows into the detention pond while Drainage Area lfl, #3, #4, and #5 contribute to direct
runoff (See Appendix 4: "Drainage Area Map"). The collective runoff of DA #2, DA #3, DA
#4, and DA #5 for the 25 year storm is 6.31 cfs which is substantially less than the 9.18 cfs
allowable. Therefore, the detention requirements are in compliance with city ordinances . The
results for all the storm events are tabulated below:
I
' '
Ill
• • • • '. • •
Exxon Express -Hwy 6 & Rock Prairie Road
Drainage Report Page 3
Storm
return period
10
25
100
Pre-Dev. Q
Tract F (cfs)
11.54 /
13 .18 ./
15.56 /
Det Pond
outfl<ry (cfs)
3.84
4.00 /
4.82 /
Allowable
direct runoff
7.70 /
9.18
10.74 /
Post-Dev
Tract 11 F-4 11 (cfs)
(sum~ #2 ... DA #4)
5.53.
6.31 /
7.44 /
Most drainage will be sheet flow, curb and gutter, and open channel flow. The site will have three
concrete flumes, two of which discharge at the right-of-way of Highway 6 and one that discharges into
the .gutter of Rock Prairie Road .
DA #1 has a Q 25 of 3.64 cfs that discharges through the proposed driveway into the detention
pond. DA #2 has a Q25 of 2.13 cfs that discharges through the proposed Highway 6 entrance into the
Highway 6 frontage road curb and gutter system and ultimately into the Foxfire Tributary. DA #3 has
a Q25 of 1.29 cfs that discharges through a concrete flume onto Highway 6 frontage and ultimately into
the Foxfire Tributary. DA #4 has a Q25 of 2.38 cfs that discharges through a concrete flume into the
curb and gutter system on Rock Prairie Road and ultimately into Lick Creek. DA #5 has a Q25 of 0.51
cfs that discharges through a concrete flume onto the Highway 6 frontage road and ultimately into the
Foxfire Tributary .
CONCLUSIONS
A detention pond for Tract 11 F 11 of Ponderosa Place Sectio Two has been designed to attenuate
runoff for the full development of the tract. Tract 11 F-4" Resubdivisio f Ponderosa Place Section Two
is a subsidiary of Tract 11 F 11 mentioned above. The sum of the peak out w of the detention pond and
the direct runoff from Tract 11 F-4 11 is substantially less than the pre-developed runoff for Tract 11 F 11
•
Therefore detention is not required for Tract 11 F-4 II and no impact to upstream or downstream landowners
is expected.
11 1 hereby certify that this report (plan) for the drainage design of Exxon Express -Hwy 6 & Rock
Prairie Road was prepared by me (or under my supervision) in accordance with the provisions of the City
of College Station Drainage Policy and Design Standards for the owners thereof. 11
I
I
I
I
B
i
I
..
II
II
Ill ,
•
I
Exxon Express -Hwy 6 & Rock Prairi e Road
Drainage Report Page 4
REFERENCES
Stormwater Management Plan, Phase Il: Drainage Policy and Design Standards, City of College
Station , Ordinance No. 1728 , prepared by : Walton & Associates , October, 1987 .
Stormwater Management Plan, Phase III: Drainage System Analysis, City of College Station ,
prepared by: Walton & Associates, August , 1986 .
APPENDICES
1)
2)
3)
4)
5)
General Location Map
Flood Plain Map -Commun ity No. 480083, Panel No. 0182 Effective Date : July
2 , 1992
Resubdivision Plat of Ponderosa Place Section II, Tract F
Drainage Area Map I Grading Plan -Prepared by Kling Engineering & Surveying
Hydrologic Calculations -Rational Method
• lr.cs95-0lb :\exxon .rpt
II
J ............-: ..-e: ' . s 60". •Stf' w .. 21~.52 .
~ . 11,,_7''1.? . tinUTY fAS£M£NT
--..,-_.,...,,..,.., Arr.rlT-...., .. ti 6 45.}B VOL 1 ~-4 'PC. 217
_mo~ Of 28 ' TRAC: '. T . ~-·,,,. F-2'" . --.:
I ACCESS ESl.IT . . . '
t" •• ::·
. 0 .543. ACRES.
DETENTION F AGILITY ., ;.--: .
_:-:.. ..
, F~ 1"·
:RES · ·-
., .
.....
-:"' '
·. '\: . -
; ~· ' .
. ··~.
iRIE . ROAD rv 80'• R. 0. W.
.-· . .-, .. , < '', ...... · Ct./RVE. . DEL TA
•. ··' ... ·.··:_,;,, ........ ., •. -_:1·_6~46'!8." · .. :;;_::.'jp:/":~· -· .•-.; _·,:~:7·;~i . so·oo ·od'. ·
RADIUS
599.26
«2 5 .00 .
I ..
I t:
'··I 8
·l ~-::~ -.. ' . . J ,.._ .. l
.. -~· !;:·.·
•I ;
"·
i /2" 1 ~~f'No
"-'(; u:;
N'-fT22 ;!.
2.J:ss ·
1 /'t' t.R.. ·ffiD .
' ·,_ ~-f.. ,.
... ~-
:~
. _·::
·25.00 ~ .. ~>~::" ...
. ·:-·
. . . . .. ~
(/) (/)
WW a: a: :3 :3 a: 00~
• (/) (/) _J
<Ill LO U::
I I I
I (/) (/) (/)
·1-1-1-
IWWW
IWWW
:III
I (/) (/) (/)
:888 ..-f".\IN
I I I
) <O ,... ,...
)(')(')(')
l ~ C\I .... ) y 9 9
) <? (") <?
I
~
I
.... w
-~
u
l
r
I
I I
;J,-,,r, :;.. {!'. '/! :tv
( ir::i ,. , 1. 1 r d , X ___ o_z.._.;' _"'_3_. &,_il-_c._~_J _)( a,o. ,
C,-::.o .ro
;4q:.. C .e>ifac ),.,..!'~ ['./?,,-.'-,
c; .. c .3:."(o.o +) + o.cio(c.13) -o. o.77
0 .17
c D • 7 ;, ') { ~ I 1...) +-D . 9 C ( 0 . 'l. 't l
D . ""'·· '/·
C.71
!/. -c .c '> """r -
0 . V5 ( C . 1.' -z.. ) -+ D 9 C> ( 0 · 0 S ) c., '.:-·-----.. -· . --c . 7.j.
O c ]
Q 10 -=-5. 5?.; c ~s <..:. A !loc..Vc.....b I e..
Gz,-:..{o.)f cH .C:: Allcw .... lol~
Oi c<>:.. {.y4 cf'S < A llow4 Lie
(!,_I'= o .'J o
'>j? frJ • ~ 0 I -f"u
m'.:Y/I''./ 1J,.hf/ Ho y1 ''7r
(_s-_f_J _"l_s_'5_1_""' -~0_(/_) { rf 'N f I · ·"0 l
I I
II _-I I/ _J J f./d