HomeMy WebLinkAbout42 Drainage Report Sandstone Psychiatric CenterDR.h I NA.GE R.:EPO~T
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WAI.T ON AND J,~)SIJCl.4TES I
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MAY 1 1 1987
CONSTRUCTION
WALTON 0 i\00CI:Ii\TE0
Consullin8 En8ineers. Inc.
DRAINAGE REPORT
FOR
SANDSTONE PSYCHIATRIC CENTER
APRIL 1987
by
WALTON AND ASSOCIATES/
CONSULTING ENGINEERS
Introduction
DRAINAGE REPORT
FOR
SANDSTONE PSYCHIATRIC CENTER
The Sandstone Psychiatric Center will be located on the 15 .00
acre tract called Lot 1 of the Barron Park Subdivision located in the
Robert Stevenson League, A-54, within the city limits of the City of
College Station, Brazos County, Texas. The project consists of an
approximately 45,000 square foot single storied structure, attendant
drives, parking facilities, and landscaping. The site is near but not
included in the floodplain for Lick Creek as shown on the Final
Subdivision Plat. There will be no dedicated street or drainage
facilities associated with the development, however water and sewer
improvements will be dedicated to and maintained by the City of
College Station.
Method of Analysis
The analysis presented herein is broken down into two basic areas
of consideration, Off-Site, and On-Site. The rational method using
Texas State Department of Highways and Public Transportation (SDHPT)
Hydraulic Manual procedures has been employed for the intensity,
duration, frequency relationships and other analysis parameters.
Figure 1 shows the general area including basic contour patterns on
and adjacent to the project site.
Off-Site Considerations
The project site rests at the divide between Lick Creek on the
north and Spring Creek on the south, however the major drainage
pathway serving the site leads to Lick Creek via a small tributary.
Figure 1 shows the position of the project site with respect to the
adjacent streams and drainage divides. The effects of off-site
drainage on the project site are negligible and have been neglected in
this analysis.
On-Site Considerations
Two major drainage pathways are utilized to convey stormwater
runoff from the site. The first drainage pathway serves the entire
building site and conveys all stormwater generated to a detention
facility in accordance with the requirements of the PRC drainage
statement. This area is considered in Section 1 of the following
discussion. The entry drive provides the second of these pathways and
runs coincident with the ridgeline dividing the previously mentioned
creeks. This area is considered in Section 2 of the following
discussion. As the ensuing discussion will show, the effects of the
entry drive construction are relatively insignificant.
Section 1
The first, and major, drainage pathway includes the building and
park i ng areas, which comprise the majority of the hard surfaced areas
being added to the site, and can be modeled using three points of
analysis which are: 1) Junction Box 1, 2) Junction Box 2, and 3) the
Outlet Control Box; each of which are shown on the construction
drawings and on the attached Drainage Plan. At points 1 and 2
flowrates and capacities are calculated, and at point 3 flowrate and
detention calculations are presented. Table 1 shows the compilation
of drainage areas as defined on the Drainage Plan and details acreage,
post development runoff coefficient, net acreage (gross acres *
coefficient), and a brief description of the area. These areas are
summed for each of the analysis points defined above . These
contributing areas are used in the calculations to compute f lowrates
based on the 5, 25, and 100 year rainfall intensities (SDHPT).
Following the flowrate calculations are capacity calculations which
indicate that the design represented on the construction drawings will
adequately handle the calculated flowrate. At analysis points 1 and 2
capacity is provided utilizing slotted corrugated metal drain pipe,
and a grate inlet into a reinforced concrete junction box. The design
flows are then released into the detention area via a short section of
reinforced concrete pipe and a headwall which opens into a small
channel. The channels are concrete lined with a low flow section to
provide for easy maintenance and good appearance.
Staged detention has been provided to attempt to simulate pre -
development peak flowrates. Analysis of the 5, 25, and 100 year
events are provided . Figures 2, 3, and 4 present the pre-development
and post-development hydrographs for analysis point 3 for the 5, 25,
and 100 year events, respectively. Figure 5 shows the elevation ,
storage relationship for this particular detention basin, and Figure 6
shows the discharge, storage relationship for this particular outlet
control structure. Table 2 summarizes the data used to construct
these figures. Using the above information a routing table and
outflow hydrograph was developed for each of the rainfall events.
Tables 3, 4, and 5, and Figures 7, 8, and 9 show these routing tables
and hydrographs for the 5, 25, and 100 year events, respectively.
It can be concluded from the Summary of Results presented in
Table 6 that the peak f lowrates closely resemble those of the pre-
development condition. The detention basin shown on the construction
drawings adequately provides the quantity of storage indicated and
maintains approximately 9 inches of freeboard in the 100 year event .
It can also be seen that this basin maintains some flexibility for
additional development of the project site in the future, however, an
updated analysis would be required at that time to insure the proper
performance.
Table 7 summarizes
analysis points 1 and 2
appended to this report .
those predicted for the 25
propos ed construction.
predicted flowrates and capacities for
which are developed in the calculations
It can be seen that f lowrates approaching
year rainfall event could be handled by the
S ection 2
The entry drive construction consists of a boulevard roadway
section with two 20 fo o t wide travel lanes , and a 16 foot wide median
section . Two median openings are provided which approximately bisect
the entry drive along its length. As was previously mentioned this
area rests atop a ridge which divides Lick Creek on the north, and
Spring Creek on the south as shown on Figure 1. The paving amounts
to approximately 26,000 square feet of hard surfaced area which would
have a runoff coefficient of about 0.90 and would produce estimated
post construction flowrates of 4.13, 5.30 , and 6.71 cfs for the 5, 25 ,
and 100 year rainfall events, respectively, if the flows were
concentrated at one point. Pre construction flowrates should be
approximately 1.38, 1.76, and 2.24 cfs for the 5, 25, and 100 year
rainfall events, respectively. The net effect of this construction is
to increase the total runoff rate in this area by approximately 2.75
to 4.5 cfs. It should be noted that this increase is total overall
increas if the flow was concentrated at one location . As the drainage
plan and construction drawings show this flow will be diffused by
taking it off of the roadway at 6 locations via a curb cut and flume .
this should closely resemble the existing drainage pattern of diffuse
surface flow. It can be concluded therefore, that the increased
runoff will have a negligible effect on adjacent areas and can be
disregarded with respect to additional outlet control measures .
\
SANDSTONE
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TABLE 1
DRAINAGE AREA SUMMARY
AREAS CONTRIBUTING TO ANALYS M PO~ 1 (JUNCTI~ B~ 1) A C. C Y..A C. ~ xtJ,
AREA AREA PRE ~v p POS ;ti)Ev T ~DESCRIPTION
NAME ACRES COE FF ACRES COEFF ACRES
Al 1.221 0.30 0.366 0.90 1.099 ROOF
A2 0.150 0.30 0.045 0.90 0 .135 PAVEMENT
A3 0.211 0.30 0.063 0.40 0.085 LANDSCAPE
A4 0.432 0.30 0.130 0.85 0.367 PVHT W/ISLANDS
A5 0.406 0.30 0.122 0.40 0.162 LNDSCP W/SW
A6 0 .434 0.30 0.130 0.85 0.369 PVHT W/ISLANDS
A9 0.461 0.30 0.138 0.40 0.184 LNDSCP W/SW
AlO 0.647 0.30 0.194 0.35 0.226 LANDSCAPE
Al3 0.151 0.30 0.045 0. 90 0.136 PAVEMENT
SUBTOTAL 1 4.112 0.30 1.234 0.67 2.763 NET AREA
~ ~ c4~~ £ c.x A. !(t)!~~ ~ c x t.') fft'f
A
AREAS CONTRIBUTING TO ANALYSIS POINT 2 (JUNCTION BOX 2)
AREA AREA PRE DEV NET PRE POST DEV NET POST DESCRIPTION
NAME ACRES COEFF ACRES COEFF ACRES
A7 0.385 0.30 0 .115 0.85 0.327 PVMT W/ISLANDS
AB 1.000 0.30 0.300 0.35 0.350 LANDSCAPE
All 0.497 0.30 0.149 0.35 0 .174 LANDSCAPE
Al2 0.463 0.30 0.139 0.85 0.394 PVMT W/ISLANDS
Al4 0.095 0 .30 0 .029 0.90 0.086 PAVEMENT
A16 0.468 0.30 0.140 0 .35 0.164 LANDSCAPE
A17 0 .332 0.30 0 .100 0.85 0.282 PVHT W/ISLANDS
SUBTOTAL 2 3.241 0.30 0.972 0.55 1.777 NET AREA
AREAS CONTRIBUTING TO ANALYSIS POINT 3 (OUTLET CONTROL BOX)
AREA AREA PRE DEV NET PRE POST DEV NET POST DESCRIPTION
NAME ACRES COE FF ACRES COEFF ACRES
A15 1. 738 0. 30 0.521 0.35 0.608 LANDSCAPE
SUBTOTAL 1 4.112 0.30 1.234 0.67 2.763 NET AREA
SUBTOTAL 2 3.241 0.30 0.972 0.55 1.777 NET AREA
TOTAL 9.091 0 .30 2 .727 0.57 5.148 SUM OF ALL AREAS
CALCULATIONS
PEAK FLOWRATE CALCULATION -GENERAL
Q=CIA Q=Flowrate, CFS
C=Runoff Coefficient
I=Rainfall Intensity , In/Hr
A=Drainage Area, Acres
I=b/(Tc+d)~e (SDHPT) Parameters for Brazos County
5 year frequency
10 year frequency
100 year frequency
b=76, d=8.5, e=.785
b=80, d=8.5, e=.763
unspecified by SDHPT
Tc=Time of Concentration
Tc=lO min (minimum used for small areas)
I (5 yr) -7 .69 In/Hr -
I (25 yr) = 9.86 In/Hr
I (100 yr)= 12.5 In/Hr
PEAK FLOWRATES AT THE ANALYSIS POINTS
ANALYSIS POINT 1 (JUNCTION BOX 1)
Q (5 yr) = 0.67 * 7.69 In/Hr * 4.112 Ac = 21. 25 Cf s
Q (25 yr) = 0.67 * 9.86 In/Hr * 4.112 Ac = 27.16 Cf s
Q (100 yr)= 0.67 * 12.50 In/Hr * 4.112 Ac = 34.44 Cf s
ANALYSIS POINT 2 (JUNCTION BOX 2)
Q (5 yr) = 0.55 * 7 .69 In/Hr * 3.241 Ac = 13 .71 Cf s
Q (25 yr) = 0.55 * 9.86 In/Hr * 3.241 Ac = 17.76 Cf s
Q (100 yr)= 0.55 * 12.50 In/Hr * 3.241 Ac = 22.28 Cf s
ANALYSIS POINT 3 (OUTLET CONTROL BOX
PRE DEVELOPMENT CONDITION
Q (5 yr) = 0.30 * 7.69 In/Hr * 9.091 Ac -20.97 Cf s -
Q (25 yr) = 0.30 * 9.86 In/Hr * 9.091 Ac = 26.89 Cf s
Q (100 yr)= 0.30 * 12.50 In/Hr * 9.091 Ac -34.09 Cf s -
POST DEVELOPMENT CONDITION
Q (5 yr) = 0.57 * 7.69 In/Hr * 9.091 Ac -39.87 Cf s -
Q (25 yr) -0.57 * 9.86 In/Hr * 9.091 Ac = 51. 09 Cf s -
Q (100 yr)= 0.57 * 12 .50 In/Hr * 9.091 Ac = 64.82 Cf s
CALCULATIONS (cont .)
CAPACITY AT THE ANALYSIS POINTS
ANALYSIS POINT 1 (JUNCTION BOX 1)
Capacity Qc = capacity of Grate + capacity of Slotted Drain Pipe
Capacity of Grate (Qg) at 6" depth = 7.5 Cfs (nomograph Appendix A)
Capacity of Slotted Drain (Qg) = L * d·5
1. 401
Qg -60 ft . * .5·5 I 1.401
Qg = 30.28 Cfs
Qc -7.5 Cfs + 30.28 Cfs = 37.78 Cfs
ANALYSIS POINT 2 (JUNCTION BOX 2)
L = length of pipe
d = depth of water
Capacity Qc = capacity of Grate + capacity of Slotted Drain Pipe
Capacity of Grate (Qg) at
Capacity of Slotted Drain
Qg = 20 ft . * .5 .5 I 1 .401
Qg = 10.09 Cf s
Qc = 7.5 Cf s + 10.09 Cfs =
6" depth = 7.5
(Qg) -L * d·5 -
1.401
17 .Q9 Cf~
Cf s (nomograph Appendix A)
L = length of pipe
d = depth of water
TABLE 2
GRATE 30" RCP TOTAL
ELEVATION CAPACITY CAPACITY CAPACITY STORAGE
(CFS) (CFS) (CFS) (CF)
284 0 0 0 0
284.5 13 34.09 13 4000
285 19 34.09 19 7880
285.5 24 34.09 24 16000
286 26 34.09 26 23301
286.5 30 35.22 30 33500
287 34 36.50 34 44226
FIGURE 2
HYDROGRAPH AT OUTLET CONTROL BOX
40 / ~ -a P :AK = 39.87 cfs
"' I ~ 35
30
r-.....
(/)
LL 25 0 ......_,,,
0
w 20 r-
<(
0::::
3 15 0
_J
LL
10
I ~
I """ ~
I " I IQ F :AK = 20 .9 ~ cfs ~ / ~ v v ~ ~ ~ ~ , / I\.
I v ~~ ~ / ~ ~ .......
v/ v ~ ~ ""' I ~ ~ //v ............ ~ '/Tc = 10 I hin k
u ....
5
0
0 4 8 1 2 1 6 20 24 28
TIME (MIN)
o 5 YR POST DEV + 5 YR PRE DEV
FIGURE 3
HYDROGRAPH AT OUTLET CONTROL BOX
60
50
,,-...__ 40 (f)
LL u
~
0
w 30
I-
<{
er::
3:
0
_J 20 LL
10
l~Q I ~EAK = 51.0S cfs
v ~
~' J
v ~
~
v / <:: t:AK = 26 .8~ cfs ~
~ ~ / ~
v v ............ ~ ~ ~ v ~ ~ ...._
_} v ~ ~ ~ /Tc= 10 mi1
~ 0
0 4 8 1 2 1 6 20 24 28
o 25 YR POST DEV
TIME (MIN)
+ 25 YR PRE DEV
FIGURE 4
HYDROGRAPH AT OUTLET CONTROL BOX
70
50
,,.-.._
(f) u..
() ....._,,,, 40
0
w
I-
<! n:: 30
3
0
_j u..
20
) Ka PEAK ~ 64.E 2 cfs
v " ~
) ' I ~ ~
I ,OF EAK = 34.0~ • cfs '~
/ ~ ..... "' / v ~ ~
I v ~ ~ ~
I// v ~ ~ ~ ~ / "" .f v VTc 1= 10 min ~ ~ ~"
u
60
10
0
0 4 8 12 1 6 20 24 28
o 100 YR POST DEV
TIME (MIN)
+ 100 YR PRE DEV
287
286 .8
286.6
286.4
286.2
286
z 285 .8
0 285.6 ~
GJ 285.4
_J
w 285.2
285
284.8 /
284 .6
284.4
284.2
284
/
/
./ ...
0
/
/
_/
v
/
10
FIGURE 5
ELEVATION VS. STORAGE
/
_/ /'
/
20
(Thousands)
./ /
./"
30
o STORAGE IN CUBIC FT
.......
~
~
/'
.,,..
40
45
40
35
30
,---. u_,,,..-.._
u CJ)
'-._./ -0 25 c
WO
('.) CJ)
<( ::J
O:'.: 0 20 o..r:::
r-f-......__,,
({)
1 5
10
5
--------~
Lµ 0
0
FIGURE 6
STORAGE VS. DISCHARGE
j
/
~ -------~
10 20
DISCHARGE (CFS)
I
I
) {
I
I
30
TABLE 3
ROUTE 5 YR EVENT Tc= 10
POSTQ= 39.87 239.22 0 PREQ= 20.97 125.82
TIME QI VI STORAGE QD VO QPRE
0 0 0 0 0 0
2 7.974 478.44 478.44 1. 7 102 4.194
4 15.948 1913.76 1811.76 6 564 8.388
6 23.922 4305.96 3741. 96 12.5 1674 12.582
8 31.896 7655.04 5981.04 16.3 3402 16.776
10 39.87 11961 8559 19.2 5532 20.97
12 35.883 16506.18 10974.18 21 7944 18.873
14 31.896 20572.92 12628.92 22 10524 16.776
16 27.909 24161.22 13637.22 22.5 13194 14.679
18 23.922 27271.08 14077.08 22.7 15906 12.582
20 19.935 29902.5 13996.5 22.7 18630 10.485
22 15.948 32055.48 13425.48 22.5 21342 8.388
24 11. 961 33730.02 12388.02 22 24012 6.291
26 7.974 34926.12 10914.12 21 26592 4.194
28 3.987 35643.78 9051.78 19.7 29034 2.097
30 0 35883 6849 17.5 31266 0
32 35883 4617 14 33156
34 35883 2727 8.8 34524
36 35883 1359 4.5 35322
38 35883 561 1.9 35706
40 35883 177 0.8 35868
42 35883 15 0.3 35934
--_______ ____...
30
" en
LL 25 u ......_,,
0
w 20 0
ct::
<{
I u en 1 5
-0
10
0
D
10
ROUTED +
FIGURE 7
5 YEAR HYDROGRAPH
20
TIME (MIN)
POST D~V
30 40
PRE DEV
TABLE 4
ROUTE 25 YR EVENT Tc= 10
POSTQ = 51.09 306.54 0 PREQ = 26.89 161.34
TIME QI VI STORAGE QD VO QPRE
0 0 0 0 0 0
2 10 .218 613.08 613 .08 2 120 5.378
4 20.436 2452 .32 2332.32 8 720 10.756
6 30.654 5517.72 4797.72 14 2040 16.134
8 40 .872 9809.28 7769.28 19 4020 21.512
10 51.09 15327 11307 21.2 6432 26.89
12 45 .981 21151.26 14719 .26 23.5 9114 24.201
14 40.872 26362.44 17248.44 24.5 11994 21.512
16 35 .763 30960.54 18966 .54 25 14964 18.823
18 30 .654 34945.56 19981. 56 25.1 17970 16.134
20 25.545 38317.5 20347.5 25 .2 20988 13.445
22 20.436 41076.36 20088 .36 25.2 24012 10 .756
24 15.327 43222 .14 19210 .14 25 27024 8.067
26 10 .218 44754.84 17730 .84 24 .7 30006 5 .378
28 5.109 45674.46 15668.46 24 32928 2 .689
30 0 45981 13053 22 35688 0
32 45981 10293 20 38208
34 45981 7773 19 40548
36 45981 5433 15.5 42618
38 45981 3363 11 44208
40 45981 1773 6 45228
42 45981 753 2 45708
44 45981 273 1 45888
46 45981 93 0.4 45972
48 45981 9 0 .2 46008
,,...........
40 ([)
LL u
'-"
0
w 30 (.'.)
0::::
<t
I
u
([) -20 0
0 10
o ROUTED
FIGURE 8
25 YEAR HYDROGRAPH
+
20
TIME (MIN)
POST DEV
30 40
PRE DEV
TABLE 5
ROUTE 100 YR EVENT Tc = 10
POSTQ = 64.82 388.92 0 PREQ = 34.09 204.54
TIME QI VI STORAGE QD VO QPRE
0 0 0 0 0 0
2 12.964 777.84 777 .84 2.5 150 6.818
4 25.928 3111. 36 2961.36 10.5 930 13.636
6 38.892 7000.56 6070.56 16 2520 20.454
8 51.856 12445.44 9925.44 20.2 4692 27.272
10 64.82 19446 14754 23.2 7296 34.09
12 58.338 26835.48 19539.48 25 10188 30.681
14 51.856 33447.12 23259.12 26 13248 27 .272
16 45.374 39280.92 26032.92 27 16428 23 .863
18 38.892 44336.88 27908.88 28 19728 20.454
20 32 .41 48615 28887 28.5 23118 17.045
22 25.928 52115.28 28997.28 28.5 26538 13.636
24 19.446 54837.72 28299.72 28.2 29940 10.227
26 12.964 56782.32 26842.32 27 33252 6 .818
28 6.482 57949.08 24697.08 26.2 36444 3.409
30 0 58338 21894 25.6 39552 0
32 58338 18786 24.8 42576
34 58338 15762 24 45504
36 58338 12834 22 48264
38 58338 10074 20 . 3 50802
40 58338 7536 18.8 53148
42 58338 5190 15.2 55188
44 58338 3150 10.6 56736
46 58338 1602 6 57732
48 58338 606 2 58212
50 58338 126 0 .4 58356
52 58338 -18 0.5 58410
50
,,--...
Cf)
LL u
"--/
0
40
w
(.'.)
O'.'.:
<t 30
I u
Cf)
0
20
0
o ROUTED
FIGURE 9
100 YEAR HYDROGRAPH
20
+
TIME (MIN)
POST DEV
40 60
PRE DEV
TABLE 6
SUMMARY OF RESULTS
RETURN PRE DEV POST DEV ROUTED Q PEAK MAX WS
FREQUENCY PEAK Q PEAK Q IN PEAK OUT STORAGE ELEVATION
(CFS) (CFS) (CFS) (CF)
5 YEAR 20.97 39.87 22 .7 14077 285.4
25 YEAR 26.89 51.09 25.2 20348 285.8
100 YEAR 34.09 64.82 28.5 28997 286.3
TABLE 7
CAPACITY AND FLOWRATE SUMMARY
5 YEAR 25 YEAR 100 YEAR
ANALYSIS CAPACITY FLOWRATE FLOWRATE FLOWRATE
POINT (CFS) (CFS) (CFS) (CFS)
1 37.78 21.25 27 .16 34.44
2 17.59 13.71 17.76 22.28
APPENDIX A
IV NEENAH~ 9--1 FOUNDRY COM~ -, . :J Discharge vs Dept-h On Grate R-4882-A GRATE AT ELEV = 284.0 AT OUTLET CONTROL BOX . 6 J 100 5-I 90 80 70 10 60 9 <4 8 w 7 a:: < 6 .z 5 . ~3 .._ LL 0 4 I ....: LL I " ........ ... ~ ............. --....__ -----I w 3 ..... I 287 0 ~ (,/) °' I 286 I C> ~2 2 °' I 286 < w a:: ........_ > C> ............... 10 0 I 285 LL ........_ 9 I 0 ........_ 8 .._ :x: 0.. < 1 7 ~ lw I 285 w 6 :::::> ........_ .9 0 a:: < 0 .............. .8 ~ 5 a:: :x: ........__ .7< ..... 4-1 w ..........___ .6 ~ ~ I 284 1 -I 3~~ .5 .9 -f t--.4 u V) .8--1 210 t·3 "1-~ .7 1 1 t ~ .6 .5 (0 1961 Neenah Foundry Company
IV . NEENAH~ 9--1 FOUNDRY COM~ 8--I Discharge vs Dept·h On Grate 7-1 R-4880-C AT JUNCTION BOXES 1 AND 2 6 J 100 5--1 90 80 70 10 60 9 f J . 8 50 7 . u 40 w 6 Cl) .z °' ~5~ ~3 .. 30 w .. Q.. 0 4 I t-....: u.. u.. • w . 20 ::> 3 t-0 u < Cl) °' I C> ~2 2 °' < w a.:: > C> 0 u.. ......... :J: 0 ~ Q.. < lw w .9 c °' < .8 ~ .7~ 4--1 w ~;~ 1~ <!> 3~~ .9 ~ ~.4 Cl) -.8--1 2--r t·3 1-~ .7 1 1 t .~ .6 .5 (f'I 1981 Neenah Found!)' Company