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January 18, 2002
JIM SELL DESIGN
LanAoap:.L,Gilruure.
Mr. Don Carroll
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Weld County Public Works Department
1111 H Street Box 758
Greeley, Colorado 80632-0758
RE: Nelson Pipeline Facility Site Plan Drainage and Erosion Control Report
Dear Mr. Carroll:
Please refer to our enclosed "Drainage and Erosion Control Report for Nelson Pipeline Facility."
This report was prepared in accordance with the Urban Storm Drainage Criteria Manual, with
reference to the Comprehensive Drainage Study for the City of Evans, Volume 2, for regional
hydrologic information.
Please feel free to contact our office at (970) 484-1921 at your convenience if you have any
questions regarding this report. We thank you for your time and consideration in reviewing this
drainage report submittal.
Sincerely,
JIM SELL DESIGN, INC.
/OadC31444ilbtrj
_ Kent Bruxvoort, P.E.
Senior Engineer
Enclosure
2002-0991
E'NROJECT FILES LAND IE 170MOCS SI]OD DRAMA F RFPORT DOC
EXHIBIT
JIM SELL DESIGN,INC. 153 WEST MOUNTAIN AVENUE FORT COLLINS,COLORADO 80524 970 484.1921 FAX:97C I
-- FINAL DRAINAGE AND EROSION CONTROL REPORT FOR:
NELSON PIPELINE FACILITY
WELD COUNTY, COLORADO
Prepared for:
DAVID NELSON
NELSON PIPELINE CONSTRUCTORS, INC.
6215 COLORADO BOULEVARD
COMMERCE CITY, COLORADO 80022
(303) 289-5975
— Fax: (303) 289-5975
Project#2170
January 18, 2002
Prepared by:
•
JIM SELL DESIGN
153 West Mountain Avenue
Fort Collins,Colorado 80524
i-. FAX 970.484.2443
970.484.1921
ENGINEER'S CERTIFICATION BLOCK
I hereby certify that this report for the final drainage design for the Nelson Pipeline Facility was
prepared by me (or under my direct supervision) in accordance with the provisions of the Urban
Drainage and Flood Control District Drainage Criteria Manual for the owners thereof.
�Cro �1SRO4:<;<C„
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Reg' ered Prof ssional Engineer
State of Colorado No. 26325 *-p
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Drainage and Erosion Control Report For Nelson Pipeline Facililty
EPROIECr FILIS LAND pg01ACO @I]OD DRAINAGE RUIVRT DOC January 18,2002-Page 2
Two historic basins (H1 and H2) have been delineated within the proposed site. Historic basins
HI and H2 drain to the north-northeast via sheet flow through the proposed Nelson Pipeline
Facility. Runoff coefficients for both historic basins are 0.01, 0.05, and 0.20 for the 5, 10, and
100-year events, respectively. In accordance with the Weld County Code, detained storm runoff
from the site has been designed to release at the historic 5-year rate.
A private irrigation lateral, approximately 2 feet wide and 2 feet deep, lies on the south edge of
the site. The ditch conveys irrigation water from west to east to flood irrigate the existing site.
The ditch was assumed to be flowing at bank full capacity during the 5, 10, and 100-year events,
and therefore would not intercept storm water from offsite basins O1 and O2. Consequently,
offsite basin O1 flows directly into historic basin HI. Similarly, offsite basin O2 flows directly
into historic basin H2, as illustrated in Figure 2. Table 1 presents historic discharges for basins
Hl, H2, O1, and O2 for the 5, 10, and 100-year events.
TABLE 1 —HISTORIC AND OFFSITE BASIN DISCHARGES
Historic Predicted Discharge Q (cfs)
Basin ID Historic Release Minor Event(10 yr.) Major Event (100 yr.)
Event(5 yr.)
H1 0.10 0.60 4.74
H2 0.10 0.50 3.48
O1 6.63 7.84 18.60
O2 6.33 7.49 18.65
STORM DRAINAGE CRITERIA
This drainage report was prepared to conform to the Urban Storm Drainage Criteria Manual
(USDCM), prepared by the Urban Drainage and Flood Control District (March 1969). The
Comprehensive Drainage Study for the City of Evans, Volume II (March 1997) was also
referenced in the preparation of this report. Drainage calculations are presented in Appendix A.
Swale calculations are presented in Appendix B, detention pond calculations are presented in
Appendix C, and referenced information is presented in Appendix D.
Storm runoff for the proposed Nelson Pipeline Facility was analyzed using the Rational Method.
The Rational Method, widely accepted for design of sub-basins less than 160 acres, is
appropriate for the site, as the total site area is approximately 64 acres, with approximately 11.8
acres proposed to be developed. In accordance with Weld County Code, basin peak discharges
have been calculated for the major event (100-year return period) and the minor event (10-year
return period). Additionally, basin discharges were determined for the 5-year event for
comparison with historic discharges. Detention facilities have been designed to release at the
historic 5-year rate.
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Drainage and Erosion Control Report For Nelson Pipeline Facililty
E NROILCI IIIESLANO121t0U111CC51'I I0V DRAINAGE REPORT DOC January 18,2002-Page 5
FIGURE 2-HISTORIC BASIN DELINEATION
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DRAINAGE FACILITY DESIGN
The proposed development will increase peak flow rates and runoff volumes. As a result, a
detention pond has been designed to detain storm runoff on site. No storm sewers or inlets are
designed as part of the storm drainage system for the site, but a culvert will be placed beneath the
proposed access to the facility to convey flow along roadside swales. A storm pipe is also
required at the detention pond outlet.
Table A.1, in Appendix A, illustrates composite runoff coefficients for proposed conditions.
Runoff coefficients ranged from 0.20 to 1.00 for the developed condition. Basin 6 has the
highest runoff coefficient (1.00); it surrounds the detention pond, and therefore has the maximum
runoff coefficient. The runoff coefficients presented in Table A.1 are composite runoff
_ coefficients based on proportions of area within the basin with differing degrees of
imperviousness. These areas include pavement, gravel, roof, lawn and undeveloped land use.
Historic coefficients are 0.01 and 0.2 for all basins for the 5-year event and 100-year event,
respectively. The coefficients were selected from the recommended runoff coefficient for
"lawns, sandy soil" as published in the USDCM.
_ As depicted, there are six proposed drainage basins. Storm water will be conveyed overland
across the site, into roadside swales, and into the detention pond. The storm water will then be
released from the pond at the historic 5-year rate, and will discharge into the agricultural field
— north of the disturbed portion of the site. Grading downstream of the pond outlet has been
designed to encourage the point-source discharge to return to a sheet-flow condition. The
discharge will continue to flow north, and ultimately into Little Dry Creek.
Figure 3 illustrates the developed basin delineation.
The time of concentration calculations are presented in Table A.2 of Appendix A for the
proposed condition. Historic time of concentration calculations are presented in Table A.3.
Table A.4 presents rainfall intensity-duration data as referenced from Table 3.2.1 of the
Comprehensive Drainage Study for the City of Evans, Volume II. Rainfall intensity-duration
data was linearly interpolated from the data presented in Table 3.2.1 assuming that the time of
concentration is equal to the storm duration (a standard assumption, as discussed in the Urban
Drainage and Flood Control District Urban Storm Drainage Criteria Manual). Intensity-duration
calculations for design point flows and routed flows are also presented in Table A.4.
Rational Method calculations for the 5, 10, and 100-year events are presented in Tables A.5,
A.6, and A.7, respectively. These tables include routed discharges and discharges for each
drainage sub-basin. The "comments" column on each of the Rational Method tables indicates
which basins the storm runoff was routed from. Discharges from the developed basins are
summarized in Table 2, following Figure 3. Table A.8 in Appendix A presents Rational
Method calculations for both historic basins.
_ Drainage and Erosion Control Report For Nelson Pipeline Facililry
EIPROILCr EILrs ANDUIIODWocs\+1700 DRAINAGE REPORT IXX January 18,2002-Page 7
FIGURE 3 -DEVELOPED BASIN DELINEATION
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_ Drainage and Erosion Control Report Fm'Netmn Pipeline En,dill
C.VTROll CI bll.l:LLANue flon\IxM Sm)au DU NNULi ln'olrr.uo0 Jelin/My IS.2002-Page S
TABLE 2-DEVELOPED BASIN DISCHARGES
Developed Predicted Discharge Q (cfs)
Basin ID Historic Release Minor Event (10 yr.) Major Event (100 yr.)
_ Event(5 yr.)
1 5.43 6.41 11.88
2 3.49 4.12 8.47
3 0.92 1.09 2.28
4 0.65 0.77 1.86
5 0.50 0.59 1.19
6 6.89 _ 8.13 12.84
SWALE DESIGN
Five swales were designed to convey the peak 100-year discharge. Near the high points of the
drainage basins, the swales were designed to carry discharges less than the calculated peak for
that basin. Swale slopes will vary along between the high point of each respective basin and the
low point; however, the swale slope will generally follow the grade of the adjacent roadway.
Design for all five swales meets the minimum side slope standard of 3:1 as stated in the Weld
County Code.
Swale 1 will direct runoff from basins O2 and 2 around the south and southwest sides of the
proposed building. Swale 1 has been divided into segments Swale la, Swale lb, and Swale lc.
South of the proposed building, the swale will primarily collect runoff from offsite basin O2.
The 100-year discharge for Swale 1 a is 18.65 cfs. Swale la has been designed with 4:1 side
_ slopes because of width constraints on the west side of the proposed building. The required
design depth and depth with freeboard for Swale l a are 1.47 feet and 1.63 feet, respectively.
Freeboard depth for each swale was determined by adding 33 percent of the design discharge to
the channel calculation.
North of the proposed building, Swale la joins Swale lb. Swale lb will curve to the east at the
confluence with discharge from basin 1,just southeast of a crossing with a field access road.
Swale lb has been designed for a routed 100-year discharge for basins O2 and 2 of 23.04 cfs and
will have 5:1 side slopes. Swale 1 c is defined as the swale downstream of the confluence with
_ basin 1. Swale lc flows over a field access road, with 15:1 side slopes to allow agricultural
vehicles to cross. The swale is designed for a routed 100-year discharge of 31.27 cfs for basins
1, 2, and O2. The design depth of Swale lc over the road is 1.27 feet including freeboard.aDrainage and Erosion Control Report For Nelson Pipeline Facililty
E.Cxwect el£WANN'DC170D\DOCsmIDD DRAINAGE REPonr.DOC January 18,2002-Page 9
Swale 2 will drain the southern portion of Basin 3 on the east side of the access road off of
Highway 52. The contributing area for Swale 2 is essentially confined to the east half of the
access road and the swale itself; however, the entire 100-year discharge for Basin 3 was used in
the analysis of the swale. The 100-year design discharge for Swale 2 is 2.28 cfs. Swale 2 will
flow into Swale 3 at the confluence with runoff from Basins O1,4, and 5. The 100-year design
discharge from Swale 3 is 19.55 cfs. Discharge from basins O1,4, and 5 enter Swale 3 through
Culvert 2 (Culvert 1 is the proposed outlet from the detention pond). Culvert 2 will be a 24-inch
_ RCP culvert designed to convey the 100-year combined discharge from basins O1, 4, and 5
underneath the access road. Swale 3 will route storm runoff to the southeast corner of the
detention pond.
_ Swale 4 will route the 100-year discharge from Basin 4 to Culvert 2. The 100-year design
discharge for Swale 4 is 1.86 cfs. The design depth for Swale 4 including freeboard is 0.8 feet.
_ Swale 5 has been designed to route a 100-year discharge of 1.19 cfs from Basin 5 to Culvert 2.
The design depth for Swale 5 including freeboard is 0.56 feet.
DETENTION POND
A detention pond design was required for the site to release 100-year discharge during the
developed condition at the historic 5-year rate. The pond will collect runoff from all 6 on-site
basins and both offsite basins. An additional water quality capture volume (WQCV) was added
to the design of the pond to aid with sediment control. Urban Storm Drainage Criteria Manual
guidelines were used to generate the WQCV utilizing a 40-hour drain time. A drain time of 40
_ hours provides adequate time for suspend sediments to settle out before being transported
downstream. Appendix C presents the detention basin design details.
_ The detention pond will release discharge at the combined historic 5-year rate for historic basins
H1 and H2 and offsite basins O1 and O2. The detention pond release discharge is equal to the
summation of historic 5-year discharges for basins H1, H2, O1, and O2. The resultant release
rate is 13.2 cfs. The detention pond release discharge calculation is presented in Table C.2 in
Appendix C.
The outlet structure for the detention pond will consist of a cast-in-place concrete vault. The
vault will include a 5/8" thick steel plate with orifice holes for the release of the WQCV
discharge. Calculations for the sizing of the orifice holes are presented in Table C.4 in Appendix
C. Storm discharge will be released through a 9-inch wide opening in the concrete wall above
the steel plate. The opening will allow for weir flow at a maximum release rate of 13.2 cfs at the
100-year water surface elevation of 4955.71 feet. Detail drawings of the outlet structure are
presented on Sheet 5, of the plan set. The outlet structure will drain into a 24-inch RCP culvert,
identified as Culvert 1. The culvert will daylight into the field to the north through a standard
24-inch precast flared-end section.
_ Drainage and Erosion Control Report For Nelson Pipeline Facililry
r.wRorzcrFILES LANDQ170MDOcsaI7on DRAINAGE REPORrtoc January 18,2002-Page 10
The detention pond has been designed with a total depth of 5 feet and 7:1 side slopes to minimize
visual impact. Freeboard for the detention pond has been designed to provide an additional 10
percent of the required storage volume (including WQCV volume) for a total of 3.22 acre-feet.
The additional volume results in a freeboard depth of 0.3 feet, therefore requiring a variance (see
below).
EROSION CONTROL PLAN
Riprap basins were designed downstream of both culverts to protect the downstream swales from
erosion. Type L riprap is required for both culverts. Riprap basins 19 feet and 6.7 feet in length
are required for Culverts 1 and 2, respectively. According to the Urban Storm Drainage Criteria
Manual, Type L riprap should be buried with native topsoil and revegetated to protect from
vandalism. Riprap calculations are presented in Appendix B.
In addition to the erosion protection provided by the water quality pond, a temporary erosion
control plan is to be implemented for the site during construction. During construction, straw
bale barriers will be placed at regular intervals (every 2 feet of drop) in the swales to dissipate
energy, and ultimately intercept and detain sediment from being transported from the site. In
addition, silt fences are to be constructed in accordance to the UDFCD Manual (V. 3) around the
proposed access road to limit the off-site conveyance of sediment. Silt fences are also proposed
on the north side of Swale 1 and the detention pond. Grading will be limited to the access road,
and building envelopes to maintain much of the surface vegetation.
Areas disturbed by grading will be protected from further erosion during the construction
process. Surface roughening, mulching, and temporary or permanent revegetation may be
implemented on the site. Which method to be used in different areas will be determined by
construction scheduling, as different methods protect the site for a given duration of time. Also,
as outlined in the UDFCD Manual (V. 3), all temporary erosion control measures will be
removed after the temporary measures are deemed unnecessary.
VARIANCE REQUESTED
A variance to the design standard for detention pond freeboard is requested. Many
municipalities along the Northern Colorado Front Range suggest a freeboard depth of 1 foot for a
detention pond. The detention pond was designed to minimize visual impact by maintaining 7:1
side slopes, and limiting pond depth. The resulting design allowed 0.3 feet for freeboard,
providing an additional 10 percent of the required storage volume. We believe that the
freeboard provided in the design will not be detrimental to public health, safety or welfare, will
not reduce the longevity of the pond, and will not have a negative impact on capital or
maintenance cost requirements for the County and/or the property owner.
Drainage and Erosion Control Report For Nelson Pipeline Fertility
E.\PROJECT FILES\LAND 170D\DOCSLI]OD DRAINAGE REroRr.DOC January 1 R,2002-Page 11
CONCLUSIONS
The Nelson Pipeline Facility drainage report has been designed to comply with The Urban Storm
Drainage Criteria Manual and the Weld County Code, with reference to the Comprehensive
Drainage Study for the City of Evans, Volume 2, for hydrologic information. A variance to
allow an additional 10 percent of storage volume in place of 1-foot for freeboard in the detention
pond is requested as part of this submittal. The drainage plan presented in this report is intended
to effectively and safely convey storm runoff through the site.
REFERENCES
Comprehensive Drainage Study for The City of Evans, Volume 2, Rocky Mountain Consultants,
Inc., March 1997.
Larimer County Storm Water Management Manual,Resource Consultants, Inc., May, 1979.
Urban Storm Drainage Criteria Manual, Denver Regional Council of Governments,March 1969,
Volumes 1, 2, & 3
The Weld County Code, Colorado Code Publishing Company, Fort Collins, CO, July 2001
Drainage and Erosion Control Report For Nelson Pipeline Facililry
PROJECT egLesvv nrmmroomocsmron DRAINAGE REPORT OOC January 18,2002-Page 12
GENERAL DESCRIPTION AND LOCATION
The proposed Nelson Pipeline Facility is located on the north side of Colorado State Highway 52
between County Road 21s and 23 in Weld County. The project site is approximately 64 acres
with an area affected by site plan development of approximately 11.8 acres (See Figure 1). The
property lies in the west half of the northeast quarter of Section 2, Township 1 North, Range 67
West in Weld County, Colorado. The proposed development consists of a storage yard and
office/shop facilities to be used as the headquarters for Nelson Pipeline Constructors, Inc.
Neighboring land use is rural residential and agricultural.
EXISTING CONDITIONS
Currently, the site is primarily used for agricultural purposes. Areas not used for agricultural
purposes predominantly contain grasses. There is a gas well existing within the area affected by
site plan development. The access for the gas well will be relocated to the eastern border of the
site. An existing irrigation lateral is also present on the site. Some of this water will be re-routed
west around the perimeter of the site to provide continued irrigation of the land directly north of
the site. Soils on the site are primarily sandy loam. According to the Soil Conservation Service
Soils Report for Weld County, soils contained on the site are Olney Fine Sandy Loam and Nunn
Clay Loam. There are no existing wetlands on the site.
DRAINAGE BASINS AND SUB BASINS
As of the printing of this report, the proposed Nelson Pipeline Facility does not lie within a
defined regional drainage basin. A regional basin containing the Nelson Pipeline Facility has
been delineated for this report to determine historical flows through the site (see Historical
Drainage Plan, Figure 2). Colorado State Highway 52 traverses the basin just upstream (south)
of the site. A swale on the south side of County Road 52 collects runoff from the road and
ultimately drains eastward to Brighton Ditch. Therefore, the offsite basins' upstream boundary
has been delineated along the north bank of the ditch on the north side of Highway 52. This
regional basin slopes to the north at an average of approximately 1.8 percent with slopes ranging
from roughly 1 percent to 2 percent. The basin ultimately drains into the South Platte River.
Historical use of the basin was primarily agricultural, however some residential land use exists in
the basin.
Offsite flows from the south are generated from an area of 17.8 acres. The entire site drains in a
north-northeast direction. Using runoff coefficients from the Urban Storm Drainage Criteria
Manual and Intensity-Frequency Tables from the City of Evans, the offsite flows were computed
as 7.8 cfs and 7.5 cfs for the 10-year event from offsite basins O1 and O2, respectively. The
100-year offsite flows were computed as 18.6 cfs and 18.7 cfs for offsite basins O1 and O2,
_ respectively. Basins O1 and O2 include two existing residences. No significant off-site flows
enter the proposed site from the north, west or east. Flow exits the site as sheet flow along the
northeast boundary of the site, ultimately flowing northward to Little Dry Creek.
Drainage and Erosion Control Report For Nelson Pipeline Faciiily
e.waoJEC FlLLN.AND➢70o\COCS\2170D DRANAGF.RI-PORT DOC January 18,2002-Page 3
FIGURE 1 -VICINITY MAP
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_ Drainage and Erosion Control Report Far Nelson Pipeline Facililty
cu ROJFCI mEt ANucroomocsc 1700 uR INAGL Iu Iroar.uc c January 18,2002-Page 4
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