HomeMy WebLinkAboutVILLAGES AT HARMONY WEST PUD CHURCH OF JESUS CHRIST OF LATTER DAY SAINTS AMENDED FINAL - 3 90C - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTI
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THE CHURCH OF JESUS CHRIST
OF LATTER-DAY SAINTS
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may 19%CTEC
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III. DRAINAGE DESIGN
A. Scope of Design
' The site (lot 2) is proposed for expansion of the existing church parking lot. Because
we will be changing the current use of the site to an asphalt parking lot, the
' permeability of the site is altered. The site has been planned for areas of landscaping,
breaking up the "sea of asphalt" which will reduce the impermeability of the site. We
have considered a composite "C" factor when calculating the developed major storm
' runoff and have included this calculation in the technical appendix.. The proposed
parking lots will collect and convey runoff from the parking lot itself as well as the
unimproved portion of lot 2 to inflow concrete curb and gutter. Concrete channels will
carry runoff from the parking lots to the regional detention pond. Riprap has been
provided to protect against erosion at the entrance to the pond.
B. Gutter, Channel Sizing, and Location
' We have calculated the major storm developed runoff for the site as well as the
contributory flow from the existing parking lot and building. This runoff volume was
used to determine if the existing rectangular channel was large enough to carry the
combined flows. We also determined the major storm flows from lot 2 to determine if
the 6-inch vertical curb and gutter section, with a cross -slope of two percent, is
' adequate for this use. Please reference the Appendix of this report for these
calculations.
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ISUMMARY
1 The combined use of parking lots, curb and gutter, and concrete channels ensures a safe, economical, and
sensible means to direct storm runoff in both the minor and major storm events.
1 In the event clogging may occur in the channels, this would not pose a serious problem as runoff would top
the gutters and flow towards the detention pond, therefore, not endangering the safety of the adjacent
1 property.
1 If the recommended construction, as illustrated on Exhibit A and outlined in this report, is implemented, we
feel the site will adequately handle the runoff for the minor as well as the major storm.
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1 IV. TECHNICAL APPENDIX
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TECII IICAL APPENDIX
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DESIGN CALCULATIONS:
' Basin Areas
' To determine the amount of runoff to a concentrated release point, we need to determine the amount of
contributory drainage basin area. Referencing Exhibit "A", the basin boundaries are highlighted. Runoff.
from Lot 1 (existing Church site) and Lot 2 (expansion area) will join at the existing rectangular concrete
' channel on the north end of the property. The basin area which will contribute runoff from the site at this
release point is as follows:
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Contributory Area Description
Plammeter Reading
Conversion Factor
(20 ft. x 20 ft.)
Area
(sq. feet)
Area
(acres)
Impervious areas contributing to runoff to
51.84
400
20,736
0.48
regional detention pond from lot 2
Impervious areas contributing to runoff to
83.80
400
33,520
0.77
regional detention pond from lot 1, existing
Church bldg. and parking lot.
Total Impervious Areas
1.25
Pervious areas contributing to runoff to
68.86
400
27,544
0.63
regional detention pond from lot 2
Pervious areas contributing to runoff to
13.62
400
5,448
0.13
regional detention pond from lot 1
Total Pervious Areas
0.76
Therefore, the total impervious and pervious areaS contributing to storm runoff to release point "A" are
2.01 acres.
Composite Runoff Factor "C"
Because the site does not fit the categories for developments as described by the City of Fort Collins
Drainage Design Manual, we will determine a composite runoff factor based on the impervious and
pervious areas for use in determining the amount of runoff to release point "A".
From the City of Fort Collins, Stormwater Design Manual, Table 3-3, Rational Method Runoff
Coefficients for Composite Analysis, we have the following runoff factors:
Description Runoff Factor
Streets, drives, parking lots (asphalt) 0.95
Lawns, average slope, 2 - 7%, sandy soil 0.15
The composite runoff factor is as follows:
Impervious Area = 1.25 acres
Pervious Area = 0.76 acres
Composite "C" factor = 1 25(0 95 + 0.76(0.15) / 2.01 = 0.65
' Time of Concentration:
' The time of concentration is needed to determine the time runoff will take from the most remote location in
the contributory drainage basin to the release point of the basin. We determined by trial, the time of
' concentration shall be determined (as illustrated on Exhibit "A') from the grassed area to the west of the
proposed parking lot expansion, to the top back of the curb of the proposed parking lot, then sheet flow to
the inflow gutter, then north in the gutter to release point "A". The time of concentration in our case can be
' broken into two (2) distinct travel patterns.
' The first leg of the time of concentration will be over a grassed area whose time of concentration can be
determined using Figure 3-2, or the following equation:
' T, = (1.87 (1.1 - CCf) Dos) / S oas in minutes,
where C = rational method runoff coefficient
' Cf = rational method frequency adjustment factor
D = basin length (ft.)
S = basin slope (%)
' The second and final leg of the time of concentration will be sheet flow and gutter flow whose time of
concentration can be determined using Figure 3-1, or the following equation:
Tj=L/60 V
' where L = Basin length (ft)
V = gutter velocity, from figure 3-1
1 The combined times of concentration equal the total time of concentration. See the attached results and
calculations for the overall time of concentration from the most remote location in the basin to the release
' point.
Basin
Back
Back
Runoff
Cf
Ti(min.)
Paved
Paved
Velocity
Tt
Total
Lot
Lot
Flow
Area
(min.)
Length
Slope
Coeff.
Factor
Length
Slope
Fig 3-1
(IJ60'V
(ft.)
%
TbL 3-1
100-
(ft.)
%
(fps.)
year
A
100
1.2
0.15
1.25
16.1
257
1.10
2.0
2.1
18.21
Intensi
We know our time of concentration for the combined basin equals approximately 18.2 minutes.
From the City of Fort Collins, Rainfall Intensity Curve, the intensity for the 100-year storm equals
approximately 5.4 inches per hour.
Discharge to Release Point "A":
Per City of Fort Collins, design manual for drainage basins of 200 acres or less, the runoff may be
calculated by the Rational Method, which is essentially the following equation:
Q = CCJA
where: Q = flow (cfs)
A = total area of basin (acres)
Cf= storm frequency adjust factor
C = composite runoff factor
I = rainfall intensity (inches per hour)
Our runoff rate (cfs) at release point "A" is the following:
Q=CCfIA = 0.65x1.25x5.4x2.01=8.81cfs
' Capacity of Existing Rectangular Channel
' Release point "A" is at the beginning of the existing rectangular concrete channel, which carries stormwater
to the regional detention pond. We determined in the calculations for Phase 1, (existing building and
' parking lot) that the channel was of sufficient capacity to carry storm runoff during the major storm from
Phase 1.
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We need to determine the capacity of the channel to carry 8.8 cfs, which is the 100-year storm runoff
volume from Phase 1 and Phase 2.
Following are our calculations to determine the capacity of the existing rectangular channel:
The channel measures three feet (3') wide (opening) by eight- inches (8') deep, has a slope of 0.9 percent,
and a runoff coefficient of 0.13. Following are the calculations to determine the capacity of the channel:
Width
Depth
Depth
Area
WP
H R
Manning
Slope
Q
(ft.)
(ft.)
(inches)
(sq. ft.)
(ft.)
(ft.)
Coeff.
(ft. / ft.)
(cfs)
3
0.667
8.00
2.00
4.33
0.46
0.013
0.009
13.0
The channel can carry 13.0 cfs. The runoff volume for the major storm = 8.8 cfs, channel is adequate to
carry the major storm flows.
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Allowable Gruffer Capacity
Determine the capacity of the gutter to carry major storm flows. Our worst case flow during the major
storm would be 8.8 cfs, which is concentrated at release point 'W'. If the gutter section can carry this flow
or more, the gutter section would be adequate.
The section we will investigate will be the curb and gutter section along the east limit of the proposed
parking lot at the northernmost reach.
Using Figure 4-1, a nomograph for determining the capacity in triangular channels, we have the following
calculations:
where: gutter slope = 1.1 percent
cross slope of parking lot = 1.4 percent
Z = 1 / cross slope = 71.4
n = 0.013
Y = depth of flow in gutter = 5 inches , = 0.42 ft.
Z/n=71.4/0.013=5493
From figure 4-1, Qth,o,,�,= 30 cfs
From figure 4-2, the reduction factor = 0.80
Therefore, Qeu,,bi� = 0.80 x 30 = 24 cfs, > 8.8 cfs, okay!
FINAL DRAINAGE REPORT
for
THE CHURCH OF JESUS CHRIST
OF LATTER-DAY SAINTS
PARKING LOT EXPANSION
PREPARED BY
TEC, THE ENGINEERING CO.
2310 EAST PROSPECT, SUITE B
FORT COLLINS, CO 80525
RICK R PICKARD, P.E.
May 1996
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.ND CONNECT OISC... At 101 RITM = Ga
Z
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DEPTH III THESE TWO LINES MUST 0 02
AQ
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INTERSECT AT TURNING LINE FDA U)
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COMPLETE SOLUTION Q .01
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OLTM a ' r
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OISC-AMOC D� IN
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POWTIGN Of CHANNEL =J
HAVIMG WIDTH [:
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30
ot*uMI+E DEPTH > VON TOTAL DISCM•Rc[ IN
.002
.03
ENTIRE SECTION a TMEN USE NOMOORAPH TO
D[T[MMINE Ow IN SECTION IF FOR DEPT.
Q
20
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A TO DETERMINE DISCHARGE
in COMPOSITE SECTION;- tix�
CL
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FOLLOW IMaTRMCTIGN t
TO OOTAIM DISCHARGE IM E•>
•0O1IO
SECTION a AT ASSUMED
DEPTH > ; OOTAIM O 'ON
From BPR SLOPE RATIO to ANY DEPTH > THEN O, a.. 0.
.01
Figure 4-1
NONOGRAPH FOR FLOW IN TRIANGULAR GUTTERS
(From U.S. Dept. of Commerce, Bureau of Public
Roads, 1965)
MAY 1984 4-3 DESIGN CRITERIA
No Text
Storm Drainage Design Criteria
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r-- 2 (
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Z
Lu
0-
0
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w
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CU 2
w
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MERRIMAN
1 .2 .3 .5 1 2 3 5 10 20
VELOCITY IN FEET PER SECOND
FIGURE 5-1
ESTIMATE OF AVERAGE FLOW VELOCITY FOR USE WITH RATIONAL FORMULA
Most frequently occurring 'undeveloped' land surfaces in the region.
(Adapted from: 'Urban Hydrology for Small Watersheds'. Technical Release No. 55, USDA, SCS, Jan, 1975 )
EROSION CONTROL
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Wind Erosion Protection
Using City of Fort Collins standards as a guideline, we will discuss measures for wind erosion control.
The staging of construction for the expansion of the existing parking lot will include the clearing and
grubbing (minimal) of the area to be improved. This will not include the vacant land to the west of the
proposed parking lot. This area will be used to spoil the topsoil removed from the clearing operation after
the parking lot is near completion. Landscaping will not occur to the islands in the parking lot until after
the area is paved. At this time, any topsoil remaining will be spread over the vacant area west of the
proposed parking lot and will be seeded. Therefore, our calculations of the area which will be disturbed
does not include the vacant area. Our calculations of the area to be disturbed that warrants consideration
for wind erosion is limited to approximately 0.5 acres. Because the area is less than 1 acre, we will not
discuss a means to control wind erosion.
Sediment Loss and Erosion Control
Currently, Lot 1 (building and west half of the parking lot) drains to the northwest, where flows exit the site
through the rectangular channel, eventually emptying into the detention pond. Construction of the parking
lot will require the demolition of a section of curb and gutter which directs runoff from the existing site to
the concrete channel. This section of curb and gutter will eventually be replaced by a concrete valley pan.
For a short period of time, the contractor will use sand bags to insure storm runoff is allowed to effectively
exit the site. The sand bags will be used to prevent runoff from entering the construction area and direct
runoff to the rectangular channel. Once the valley pan is completed, the sand bags can be removed; and
runoff will again flow to the rectangular channel.
Construction of the proposed parking lot will require the cutting to subgrade of the paved areas. This
cutting will create a "pond" affect, thus trapping any rainfall onsite. Runoff will be prohibited from
exiting the site as long as the base course is not in place. Once the base coarse is in place, the contractor
will need to protect against sediment loss.
Once the base course material is in place, the parking lot will have been graded to direct runoff to the
rectangular charnel. Now, we have a means to direct flows to a concentrated area (release point "X)
where we can protect against sediment loss. We propose the use of hay bale barriers at the entrance to the
rectangular charnel prior to the placement of asphalt paving. These hay bales need to be placed such that
sediment will be collected from the construction site but not cause a blockage of runoff from the existing
parking lot.
The use of sand bags and hay bale barriers should provide an effective means to control sediment loss from
the construction site while not interfering with the normal flow of runoff from the existing parking lot.
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TABLE OF CONTENTS
INTRODUCTION
Scope of Work
Executive Summary
HYDROLOGY AND HYDRAULICS
A. Introduction
B. Design Objectives and Approach
C. Hydraulic Analysis
DRAINAGE DESIGN
A. Scope of Design
B. Concrete Channel Sizing, Location
IV. TECHNICAL APPENDIX
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I. INTRODUCTION
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tI. INTRODUCTION
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The Church of Latter -Day Saints (LDS) recently completed (1992) the construction of a new
church facility located on the northwest corner of Harmony Road and Regency Drive. The site
consisted of 3.02 acres. The property is bordered by Harmony Road to the south, Regency
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Drive to the west, the City of Fort Collins regional storm water detention pond to the north,
and Pineview PUD, Phase II, (undeveloped) to the east. The property owned by the Church
included lot 1, where the Church was constructed, lots 2 and 3, to the west and northwest of
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the Church which was left vacant. The Church proposes the expansion of the existing parking
lot to the west of the parking lot adjacent to the Church building. This report and
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investigation, will include the expansion of the existing parking lot as well as the impact on the
existing drainage features constructed with the Church facility.
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A. Scope of Work
The areas of emphasis, included as part of this investigation, are:
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1. A hydrologic analysis
2. A hydraulic analysis
3. Analysis of detention considerations (City's Regional Detention Pond)
B. Executive Summary
This report was prepared for the LDS Church site in Fort Collins, Colorado to
evaluate drainage improvements required as a result of the work proposed for the
expansion of the existing parking facilities.
' Drainage improvements to the site will include the use of parking lots to direct runoff
to the City of Fort Collin's regional detention pond to the north of the property. Due
to the proximity of the site, no on -site detention will be required. Some minor runoff
' will be directed to Regency Drive as indicated on the drainage exhibit attached to this
report.
' An integral part of the improvements to the LDS church property was the construction
of a thirty-six inch (36") diameter storm sewer along the west limit of lot 2. This
storm sewer carries runoff from developments to the west of Regency Drive, then
' under Regency Drive, then north along the west limit of lot 2, then northeast to the
regional detention pond. Prior to the construction of this storm sewer, a grass -lined
swale was used to transport storm runoff through lot 2. This Swale was a detriment to
the future development of lot 2, due to the size and location of the swale. The storm
sewer was constructed to allow the removal of the swale. The grading plan developed
1 for the expansion of parking onto lot 2, takes into account the location and depth of the
storm sewer.
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[TE]C
II. HYDROLOGY AND HYDRAULICS
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II. HYDROLOGY AND HYDRAULICS
' A. Introduction
' The hydrologic techniques and analytic tools utilized in the design flow hydrology for the
improvements to lot 2 are described in this chapter. The facilities constructed as a result of
this analysis will convey the runoff resulting from storm events with the design frequencies of
2-year and 100-year recurrence intervals. Because there will be no detention, the facilities
constructed will be sized to carry the major storm to the regional detention pond.
B. Design Objectives and Approach
' The objective of the hydrologic analysis for the LDS property concentrates on two principal
issues:
t 1. To investigate the impact of the proposed development upon the changes in the
magnitude of storm runoff.
' 2. Designing for the conveyance of storm runoff by means of appropriate
strategies as specified by the applicable drainage guidelines of the City of Fort
Collins.
The Rational Method, in widespread use in the Colorado area, will be utilized for the
determination of the runoff magnitude from the site in the developed condition and to size the
appropriate means of conveying this runoff.
The limit of application of the Rational Method is approximately 160 acres. Because the site is
2.01 acres, more or less, the Rational Method will be used for this analysis.
' Accomplishing the modeling objectives has been attained through a sequence of model
simulation runs for the two contributing basins. (please reference Exhibit "A"). The 100-year
storm output in the developed condition has been used for the design of curb and gutter
' sections as well as the rectangular concrete channel. We have not figured the minor storm as it
will be easily conveyed in the facilities proposed to handle the major storm (100-year
developed).
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' C. Hydraulic Analysis
' Historic Conditions
The site historically drains from west to east, or from Regency Drive to the existing
' parking lot. Lot 2 was graded to drain in this manner as part of the work done for the
construction of the existing parking lot. Given the constraints of the site, with the
existing parking lot to the east and the regional detention pond to the north, the
' grading scheme was designed to allow sheet flows in the parking lot and concentrated
flows in the curb and gutter to flow to the north. Due to the proximity of the site to
this detention pond, no on -site detention was required. Flows, after being collected by
' the parking lots, drain to the north exiting the site through an existing concrete channel
and dumping into the regional detention pond. The present use of the site is a vacant
lot that is maintained only to keep the weeds down.
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III. DRAINAGE DESIGN
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