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Swift and Associates
Town Planning U Urban Design • Civil Engineering
PRELIMINARY DRAINAGE REPORT
LYONS 66 PACIFIC COMMERCE PARK
WELD COUNTY, COLORADO
I hereby certify that this drainage report(plan)for the preliminary drainage design of Lyons 66
Pacific Commerce Park, Weld County, Colorado, was prepared by me(or under my direct supervision) in
accordance with the provisions of the Storm Drainage Design and Technical Criteria Regulations in Section
10.13 of the Weld County Subdivision Ordinance for the owners thereof.
Peter Swift, P.E., No. 26511, Colorado
PREPARED FOR: Lyons 66 Pacific, LLC
1210 Commerce Court,Unit#36
Lafayette, Colorado 80026
October 9, 2000
421 21", Suite 212, Longmont, Co, 80501 (303)772-7052; FAX 651-7226 EXHIBIT
2003-1518
Introduction:
The subject property is located in the NW V. of the NW %of Section 26. Township 3 North, Range 68
West of the 6th P.M. The site contains a total of 35.01 acres, more or less. The property is currently going
through the Change of Zone process in the county to subdivide the property into 15 commercial/light
industrial lots varying in size from 1 acre to 3 acres more or less.
The property presently undeveloped and bound by 1-25 to the west, Highway 66 to the north, a John Deere
Dealership to the east and the Rademacher farm to the south. The existing site is not currently being
utilized. The improvements on the property consist of a private irrigation lateral and a storm water
conveyance from 1-25. It is understood that each of these facilities needs to be realigned when the property
develops.
The property is approximately 1'/:miles north of the St Vrain Creek and Mulligan Reservoir and Foster
Reservoir are each about '/mile to the west. The drainage from this property eventually reaches the St.
Vrain Creek to the south. The irrigation lateral that bounds the west and part of the northern boundary
characterizes the property. This ditch is constructed as a flat concrete lined ditch but separates the highway
drainage from the property drainage. The site slopes from the northwest corner down to the southeast
corner at 1.2%. And the ground slopes up from the northwest corner to both the southwest and northeast
corners. The 1-25 drainage ditch is the low point in the property and traverses from the west to east
property lines. This concrete lined ditch is fed from a 30 inch RCP from under the frontage road and the
adjacent ROW. It appears that the basin to this pipe includes all of the 1-25 ROW for a '/:mile to the north
and approximately 3/e mile to the south as well as the north and south ROW of State Highway 66 from 1-25
to the eastern boundary of the subject property.
The site is not in the 100 year flood zone.
The proposed development consists of 15 commercial/light industrial lots and the associated access roads
and utilities. Because of the size and location of the property, the rational method has been used to
determine historic and developed runoff rates.
Drainage Design:
The property is best analyzed as an onsite basin and an offsite basin. The onsite basin has been modeled
according to the rational method as described in the Urban Drainage and Flood Control Drainage Criteria
manual Volume 1. A time of concentration of 68 minutes was derived and the Intensity Frequency
Duration curves prescribed by Boulder county for Region 1 were used to determine the rainfall intensity.
The curves published by Boulder County were created using the method described by Urban Drainage and
Flood Control and this property is in close enough proximity the curves are deemed adequate. A copy of
these curves is included in Appendix A. This analysis reveals a 10-year historic flow rate of 9.73 cfs and a
100-year flow rate of 30.8 cfs. Because CDOT could not provide any information regarding the highway
drainage in this area, the I-25 ROW drainage was determined by the 30 inch pipe being the limiting factor.
It would be possible for water to pond in the ROW area above the inlet to a depth of 5.9 feet. Utilizing the
orifice equation and Manning's full flow pipe equation it was determined that approximately 64.85 cfs
could reach the property and need to be conveyed. The 10 year flow was determined by the fact that
generally in a developed state the 10 year flow is approximately 60%of the 100 year flow. These values
can not be more reasonably calculated without a great deal of extra survey work complicated routing and
the help of CDOT. Much of the I-25 ROW drainage infrastructure was in disrepair with pipes being silted
full and crushed in some instances. These values should be very conservative as compared to the actual
flows.
To determine the developed flows associated with the new construction, the property was broken into six
subbasins. Basins A through D include some lots and a portion of the access loop. Basin E is just the
development within the lot lines of lot 11. Basin F includes lots 8, 9 and 10 as well as the I-25 frontage
road and the detention pond. These basins were chosen to analyze certain design points for conveyances.
421 21", Suite 212, Longmont, Co, 80501 (303)772-7052; FAX 651-7226
The initial intent is to have the roadways carry the water to a point between Lot 9 and 10 where an earthen
channel will convey the water to the detention pond. The pond will be bermed to a height of
approximately 4.5 feet above the flowline of the proposed pans with a 36 inch RCP for the 10-yr outlet and
weir over the top of the pipe to control the 100-yr flows. The pond berming will be built with 1 foot of
freeboard and the emergency outlet would be through the weir. The outfall of the weir will be ripraped to
UDFCD standards and the water will be channeled to the existing concrete ditch that has carried the water
historically.
This report respectfully requests a waiver of item 10.13.4.3 of the Weld County Subdivision Ordinance
173-E. required the 100-year release rate to be equal to the 5-year undeveloped flow. This report requests
that the 100-year developed flows be detained and released at the historic 100-year flow and the 10-year
developed flow be detained and released at the historic 10-year flow.
All of the calculations as well as site drawings are included in appendix A.
Conclusion:
The information and design provided herewith comply with the standards as outlined in the Storm Drainage
Design and Technical Critena Regulations in Section 10.13 of the Weld County Subdivision Ordinance and
the Urban Drainage and Flood Control District Design Manual. The proposed development of this site
poses no adverse impact on down stream development or drainage ways.
421 21",Suite 212, Longmont, Co, 80501 (303)772-7052; FAX 651-7226
•
References:
1. Urban Storm Drainage Criteria Manual, Urban Drainage and Flood Control District, Denver,
Colorado, Sixth Printing September 1978.
2. City of Longmont Storm Drainage Criteria Manual, City of Longmont Public Works, Longmont,
Colorado, 80501, Adopted September 1987.
421 21', Suite 212, Longmont, Co, 80501 (303)772-7052; FAX 651-7226
APPENDIX A
• 421 21', Suite 212, Longmont, Co,80501 (303)772-7052; FAX 651-7226
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BOULDER COUNTY
_ STORM DRAINAGE CRITERIA MANUAL FIGURE 501
_ RAINFALL ZONES
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WRC ENG. REFERENCE:
• Base map from USGS map for State of Colorado, Rev 1980
-5119_
BOULDER COUNTY
STORM DRAINAGE CRITERIA MANUAL FIGURE 502
TIME-INTENSITY-FREQUENCY CURVES ZONE I
•
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-510-
DRAINAGE CRITERIA MANUAL RUNOFF
TABLE 3-1 (42)
RECOMMENDED •RUNOFF COEFFICIENTS AND PERCENT IMPERVIOUS
LAND USE OR PERCENT FREQUENCY
- SURFACE CHARACTERISTICS IMPERVIOUS 2 5 10 100
Business:
Commercial Areas 95 .87 .87 .88 .89
Neighborhood Areas 70 . 60 .65 . 70 .80
_ Residential :
Single-Family * .40 .45 .50 .60
Multi-Unit (detached) 50 .45 .50 .60 .70
Multi-Unit (attached) 70 .60 .65 .70 .80
1/2 Acre Lot or Larger * .30 .35 .40 .60
— Apartments 70 .65 .70 .70 .80
Industrial :
Light Areas 80 .71 .72 .76 .82
Heavy Acres 90 .80 .80 .85 .90
Parks , Cemeteries: 7 . 10 . 18 .25 .45
Playgrounds: 13 . 15 .20 .30 .50
Schools: 50 .45 .50 .60 .70
- Railroad Yard Areas 20 .20 .25 .35 .45
Undeveloped Areas:
- Historic Flow Analysis- 2 (See "Lawns")
Greenbelts , Agricultural
-
Offsite Flow Analysis 45 .43 .47 .55 .65
(when land use not defined)
Streets:
Paved 100 .87 .88 .90 .93
Gravel (Packed) 40 .40 .45 .50 .60
Drive and Walks: 96 .87 .87 .88 .89
Roofs : 90 .80 .85 .90 .90
- Lawns, Sandy Soil 0 .00 .01 .05 .20
Lawns , Clayey Soil 0 .05 . 15 .25 .50
— NOTE: These Rational Formula coefficients may not be valid for large basins.
*See Figure 2-1 for percent impervious.
11-1-90
— URBAN DRAINAGE AND FLOOD CONTROL DISTRICT
DRAINAGE CRITERIA MANUAL RUNOFF
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30
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VELOCITY IN FEET PER • SECOND
FIGURE 3- 2. ESTIMATE OF AVERAGE FLOW VELOCITY FOR
USE WITH THE RATIONAL FORMULA.
* MOST FREQUENTLY OCCURRING "UNDEVELOPED"
- LAND SURFACES IN THE DENVER REGION.
REFERENCE: "Urban Hydrology For Small Watersheds" Technical
Release No. 55, USDA, SCS Jan. 1975.
5 -1-84
• URBAN DRAINAGE & FLOOD CONTROL DISTRICT
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HISTORIC RUNOFF CALCULATIONS
TOTAL Lot size 1525079 SF 35.011 AC
Runoff Coefficients C 10 = 0.20 Table601
C ioo = 0.40 Agricultural
• NBA
Historic Basin A
Site
Basin Area 1525079 SF 35.011 AC
Time of Concentration
tc=ti+ ft L 1 = 815.00 ft Channel Flow
S = 0.10 %
V = 1.00 ft/s Fig.602
t t = 13.58 min if L<500' then Tt•0
t = 1.8(1.1-05)'sgrt(L)/ (s)^1/3
C 5 = 0.10 Overland Flow
length of travle L 2 = 917.00 ft
Overall % slope s = 1.00 %
t I = 54.51 min
t = 68.09 min
Rainfall Intensities
Time Intensity Frequency
10 year storm 110 = 1.39 in/hr awe
100 year storm 1100 = 2.20 in/hr
Historic Runoff Rates
Q = CIA
aWliMMEMINFONDlOCEIN
Off site contribution -
—
Highway ROW- 30" RCP at approximately 2.5% slope with a head of 7.15 feet
Q 10 = 38.91 cfs
Q 1ao = 64.85 cfs
Historic Runoff 10/10/00 Page 1
Lyons 66 Pacific Commerce Park
Drainage Design!
Developed Drainage Basin A
Lots 1, 2, 3, and 4
AREA: 1 278277, SF 6.388 AC
Runoff Coefficients C10 C100 Table 3-1 (42)
Area Roof 01831 sf 0.33 0.90 0.9
Area Gravel 0 sf o 0.35 0,65
Area Landscp • 41742 sf o,s 0.25 0.5
Area Parking lot • 444704 sf 052 0.90 0.93
Total area: 278277 sf
Combination Runoff C 10 = 0.80
Coefficient C loo = 0.86
Time of Concentration for urbanized lot
t = (L/180)+10
L = 881,00 ft
t = 14.89 min
Rainfall Intensities
10 year storm I ,o = 3.99 in/hr Fig 502 LSDCM
— 100 year storm I ,00 = 6.26 in/hr
Developed Runoff Rates
Q = CIA
Developed Drainage Basin A Page 2 10/10/00
Lytr s 66 P cific o mm rc�e Perk
gags Desa
DEVELOPED DRAINAGE BASIN B
Lots 5, 6, and 7
AREA: 225528! SF 5.177 AC
Table 3-1
Runoff Coefficients C10 C100 (42)
Area Roof 58382 sf 025 0.90 0.9
Area Gravel 0 sf o 0.35 0.65
Area Landscp 33829 sf 015 0.25 0.5
Area Parking lot 135317' sf 06 0.90 0.93
Total area: 225528 sf
Combination Runoff C 10 = 0.80
Coefficient C 100 = 0.86
Time of Concentration for urbanized lot
t = (L/180)+10
L = 623.i1Q ft
t = 13.46 min
Rainfall Intensities
_ 10 year storm 110 = 4.16 in/hr Fig 502 LSDCM
100 year storm 1100 = 6.54 in/hr
Developed Runoff Rates
Q = CIA �yy
iletwENIISOKSANNIM
Developed Drainage Basin C Page 3 10/10/00
ytr s f 6'P*dfle Commerce Park
D atnage Design
DEVELOPED DRAINAGE BASIN C
Lots 12 and 13
AREA: 133957 SF 3.190 AC
Table 3-1
Runoff Coefficients C10 C100 (42)
Area Roof 3473$' sf 025 0.90 0.9
Area Gravel C sf o 0.35 0.65
Area Landscp 20844 sf 015 0.25 0.5
Area Parking lot 83374! sf 06 0.90 0.93
Total area: 138957' sf
Combination Runoff C 10 = 0.80
Coefficient C 100 = 0.86
Time of Concentration for urbanized lot
t < = (L/180)+10
L = 84$.00;ft
t < = 13.59 min >1=5.0 min
Rainfall Intensities
— — 10 year storm 110 = 4.15 in/hr Fig 502 LSDCM
100 year storm 1 100 = 6.51 in/hr
_ Developed Runoff Rates
Q = CIA 1.4,12#1111111
//gyyp .. :.k..kt':: .j�:a�.per, .. ....11•
Developed Drainage Basin C Page 4 10/10/00
_ ^ L�66 Pacific Commerce Park
ll7ratna a ties€gn
DEVELOPED DRAINAGE BASIN D
Lots 14 and 15
AREA: 141521 SF 3.249 AC
Table 3-1
Runoff Coefficients C10 C100 (42)
Area Roof • 35380 sf oze 0.90 0.9
Area Gravel C sf o 0.35 0.65
Area Landscp • 21228 sf a15 0.25 0.5
Area Parking lot 84913 sf oe '0.90 0.93
Total area: 141521 sf
Combination Runoff C 10 = 0.80
Coefficient C 100 = 0.86
Time of Concentration for urbanized lot
t = (L/180)+10
L u = 597.00+ft
t = 13.32 min
Rainfall Intensities
— 10 year storm 110 = 4.18 in/hr Fig 502 LSDCM
100 year storm 1100 = 6.56 in/hr
_ Developed Runoff Rates
Q = CIA
t 'Air'IMM . 18 >
Developed Drainage Basin C Page 5 10/10/00
_ Lyons 66 Pacific Com merce Park
Poklaiii5 Design;
DEVELOPED DRAINAGE BASIN E
Lot 11
AREA: 125205 SF 2.874 AC
Table 3-1
Runoff Coefficients C10 C100 (42)
Area Roof • 31301 sf 025 0.90 0.9
Area Gravel • 0 sf o 0.35 0.65
Area Landscp 18781 sf 01s 0.25 0.5
Area Parking lot 75123 sf 06 0.90 0.93
Total area- 125205 sf
Combination Runoff C 10 = 0.80
Coefficient C 100 = 0.86
Time of Concentration for urbanized lot
t 0 = (L/180)+10
L = 387,00 ft
t t = 12.15 min
Rainfall Intensities
— -- 10 year storm 110 = 4.34 in/hr Fig 502 LSDCM
100 year storm 1100 = 6.81 in/hr
Developed Runoff Rates
Q = CIA
to *_`r 4t ..........._.::1101•.::; 1C .............:..:
•
Developed Drainage Basin C Page 6 10/10/00
_ Lyons 66 Pacific Commerce Park
Drainage Design
DEVELOPED DRAINAGE BASIN F
Lots 8, 9, 10 and frontage road
AREA: 630241 SF 14.468 AC
•
Table 3-1
Runoff Coefficients C10 C100 (42)
Area Roof 220594; sf 035 0.90 0.9
Area Gravel 0 sf o 0.35 0.65
Area Landscp 63024 sf 01 0.25 0.5
Area Parking lot 409657' sf 0ss 0.90 0.93
Total area: 093265.1! sf
- Combination Runoff C 10 = 0.84
Coefficient C 100 = 0.88
Time of Concentration for urbanized lot
t = (L/180)+10
L u = 140200!ft
.........................
t = 17.79 min
Rainfall Intensities
— ^ 10 year storm 110 = 3.69 in/hr Fig 502 LSDCM
100 year storm 1100 = 5.79 in/hr
- Developed Runoff Rates
Q = CIA
-
Developed Drainage Basin C Page 7 10/10/00
Lyons 66 Pacific Commerce Park
Dr gle Design
DEVELOPED DRAINAGE
Developed Runoff Rates
Q = CIA
DETENTION POND SIZING
Using the asymmetrical triangular hydrograph
100 year storm:
V 100 = 1.335(tc)(60)(Qd-Qh)
Q d = 254.87 cfs
Q h = 95.65 cfs
10 year storm: V 10 = 1.335(tc)(60)(Qd-Qh)
Q d = 153.07 cfs
Q = 48.64 cfs
Yr.j0 'i4;0.f: :
Ponding Area Required
100 yr pond depth h 100 = 4.80 ft
100 yr vol. v 100 227079.94 cf
10 yr pond depth h 10 = 3.30 ft
10 yr vol. v 10 148965.70 cf
— Orifice Design
10 Year outlet Q = C*A*squr(2gh)
Coeffecient C d 0.650
_ gravitational constant g = 32.200 fUsA2
Head from pipe center h = 1.815 ft total h 3.30 ft
Area A = 6.924 sf
Opening Diam. d = SL630 in
.......................................................
Allowable flow Q = 48.664 cfs
- Diam for 1/2 blockage d 2 = 50.39 in
100 Year outlet
— Weir flow
Q=CLHA(3/2)
C=3.27+0.40*H/P For sharp crested rectangular wiers
H = 1.5
P=10yr d = 3.3
C=3.27+0.40*H/P = 3.5
Q 100h- Q10h = 47.01 cfs
L = 7.413445 ft
Developed Drainage Basin C Page 8 10/10/00
POND STOR
DETENTION POND 1
STAGE/STORAGE
Pond bottom area I 40500.00 sf
side slope 2.001ft h for 1ft v
- pond geom. triangular
base 150.00ift
height 540.00 ft
10 yr depth i ft
100 yr depth ft
- depth increment 1 0.10 ft
!depth (ft) Ivolume(cfs)
0.00 0.00
0.101 4050.00
_ 0.20 8155.26
0.301 12274.42
0.40; 16421.31
0.50'', 20596.00
0.601 24798.53
0.70i 29028.94
0.80' 33287.30
- 0.901 37573.63
1.00; 41888.00
1.101 46230.45
1.20! 50601.02
1.30 54999.78
1.40: 59426.75
1.50 63882.00
1.60, 68365.57
•
1.70 72877.50
1.80; 77417.86
1.90 81986.67
2.00' 86584.00
2.10 91209.89
2.20 95864.38
2.30 100547.54
2.40 105259.39
2.50 110000.00
2.60 114769.41
_ 2.70' 119567.66
_ 2.80 124394.82
2.90 129250.91
3.00 134136.00
3.10 139050.13
3.20 143993.34
3.30 148965/0
3.401 153967.23
Page 9
POND STOR
3.50' 158998.00
3.601 164058.05
3.70 169147.42
1 3.80 174266.18
3.901, 179414.35
4.00 184592.00
4.101 189799.17
4.20 195035.90
4.301 200302.26
4.40' 205598.27
4.50, 210924.00
4.601 216279.49
4.7Q 221664.78 •
4.80 227079.94
4.901 232524.99
5.00' 238000.00
5.10 243505.01
5.20r 249040.06
5.301 254605.22
5.40r 260200.51
5.5011 265826.00
5.60r 271481.73
5.701 277167.74
5.801 282884.10
5.90,! 288630.83
_ 6.00 294408.00
6.10, 300215.65
6.20 306053.82
6.30' 311922.58
6.40 317821.95
6.50 323752.00
6.60 329712.77
6.70 335704.30
6.80 341726.66
6.90] 347779.87
7.00 f 353864.00
Page 10
MANNINGS EQUATION
Circular Pipe - Full Flow
Pipe Diameter d = 30 000
Slope s = 0.025 ft/ft
Hydralic Radius R = 0.625
Mannings Number n = 0,013!,
Velocity V = 13.212 fUs
Flow Q = 64.854 cfs
Orifice Design
Q = C*A•squr(2gh)
Coeffecient C d 0.650
gravitational constant g = 32.200 ft/s^2
Head from pipe center h = 7.150 ft total h = 8.4 ft
Area A = 4.909 sf FROM FL
Pipe Diam, d = 30,000 in
Allowable flow Q = 68.467 cfs
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