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FINAL DRAINAGE.& EROSION CONTROL REPORT
for
Spring Creek Country Day School
1900 Remington Street
Fort Collins, Colorado $0525
Prepared by
ADVANCED PROFESSIONAL ENGINEERING INC.
P.O. Box 1391
Fort Collins, CO 80522
Phone & Fax 970-484-0533
Project # 69-7N-24-332
August 1, 1995
August 31, 1995
1
1 A ADVANCED PROFESSIONAL ENGINEERING INC.
REGISTERED PROFESSIONAL, CONSULTANT ENGINEERS AND LAND SURVEYORS
'P.O. BOX 1391, FORT COLLINS, COLORADO 80522-1391 USA
P PHONE & FAX (303) 484-0533
1 August 1, 1995
IE
Mr. Lee Fetters
Spring Creek Country Day School
' I 1900 Remington Street
Fort Collins, CO 80525
1 RE: Drainage Report for Spring Creek Country Day School.
1 Dear Lee,
This is the final drainage and erosion control report for the proposed addition to
1 Spring Creek Country Day School, 1900 Remington Street, Fort Collins,
Colorado. The location of the School is between Spring Park Drive, Mathews
and Remington Streets. The legal description of the site is Tract 2 of the Replat
1 of Block 2, and Lots 1 to 7 inclusive of Block 1 Saint Wain Subdivision, in the
City of Fort Collins, State of Colorado.
1 The hydrology data and the hydraulic analysis presented here meet the City of
Fort Collins "Storm Drainage Design Criteria," dated May 1984. The data and
the analysis are determined based on the historic, proposed, and developed
1 conditions and according to Spring Creek Master Drainage Way Plan, dated
March 1988.
If you have any further questions or would like additional information, please feel
free to call me at 970-484-0533.
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TABLE OF CONTENTS
Title
Introducation
Site Location and Conditions
Hydrology and Hydraulics Analysis
Proposed Construction
Summary
Discussion
Detention Pond Size
Erosion Control Report
Page
1
1
2
10
15
16
1.7
20
1 DRAINAGE REPORT
INTRODUCTION
This drainage and erosion control report provides the hydrology data using City of
' Fort Collins rainfall data and utilizes hydraulic analysis to determine historic,
existing, and proposed drainage conditions for the proposed addition to Spring
Creek Country Day School (SCCDS), located at 1900 Remington Street, Fort
t Collins, Colorado. Location of the School is between Spring Park Drive, Mathews
and Remington streets. The site was not developed until February 1991. The total
area of the site before 1991 was 1.338 acres. Another 0.249 acre was added since
then which made the total area equal to 1.587 acres. This drainage report is
prepared and compiled based on the following references:
' A. Storm Drainage Criteria, City of Fort Collins, dated May 1984.
B. Spring Creek Master Drainage Way Plan by EPI, dated March 1988.
' C. Drainage Report, and Plans for SCCDS by CDS Eng. , dated Feb. 7, 1991.
D. Site plan for addition to Spring Creek Country Day School prepared by
David E. Knox Architect, dated January 5, 1995.
SITE LOCATION AND CONDITIONS
' The site is bordered by Spring Park Drive at the north, by Mathews Street to the
east, and Remington Street to the west. The address of the site is 1900 Remington
Street, Fort Collins, Colorado. The legal description of the site is Tract 2 of the
' Replat of Block 2, and Lots 1 to 7 inclusive of Block 1 Saint Wain Subdivision, in
Northwest 1/4 of Section 24, Township 7 North, Range 69 West of the 6th P.M., City
of Fort Collins, State of Colorado. The existing condition of the property consists of
' an existing building used as a day school, and a parking lot. The rest of the site is
landscaped or used as a playground. The existing general topography of the site is
relatively flat with a slight overall slope of 1.7% from south to north and northeast.
According to CDS Eng.'s drainage report and plan dated February 1991, which is
approved by the City of Fort Collins, there was no detention pond planned.
' Along the north side of Spring Park Drive is Spring Creek that flows from west to
east. The Spring Creek is a major off -site conveyance channel that flooded
' significantly in the past. The existing 100-year flood computed water surface
elevation (CWSEL) is estimated in Spring Creek Master Drainage Way Plan at
4,977.79 feet above sea level, at cross section # 65 at the west end of the property.
' At the same cross section # 65 the floodway is at an elevation of 4,980.40. At cross
section # 64 which is in the middle of the site, the 100-year floodplain is estimated
at 4,978.61, and the floodway is estimated at 4,979.60 feet. These values have
' been confirmed with FEMA floodplain information.
0
The 100-year floodplain covers about 1/4 of the total area at the most northwest
corner of the site. The 100-year flood way covers less than the floodplain, which is
about 1/8 of the total area at the most northwest corner of the'site. The existing
building encroaches on the 100-year floodplain boundary slightly, but it is well
outside the 100-year floodway boundary. The first floor of the existing building had
been set at 4,980.88, where the first floor of the new proposed additional building is
set at 4,982.88. The 100-year floodway and floodplain are shown on the drainage
plan according to Spring Creek Master Drainage Way Plan.
All the streets have curbs, gutters, and sidewalks. There are two curb cuts and
driveways existing on Spring Park Drive and Remington Street. There is a parking
lot at the west end of the site. The site is landscaped using grass, trees, an' open
playground area, and covered playground area with concrete pads. The total area
of the site is 1.587 acres or 69,122.39 square feet.
HYDROLOGY DATA AND HYDRAULICS ANALYSIS
The site is part of Spring Creek Drainage Way Master Plan, dated March 1988.
This report will present and discuss the historic, existing, and proposed conditions.
Total area involved in this drainage study is 69,122.39 square foot or '1.587 acres
gross. Since the area is far less than 200 acres, Rational Method to calculate the
storm runoff quantity can be used. The site is zoned business (B-P). The site was
not developed until February 1991. Total area of the site before 1991 was 1.338
acres. Another 0.249 acre was added to the site since then, which made the total
area equal to 1.587 acres.
HISTORIC DRAINAGE CONDITIONS
The historic condition of the site is determined prior to any development which took
place in 1991. The whole site is considered as one basin "A" when determining the
runoff of historic conditions. The historic drainage condition over the undeveloped
area is a sheet flow over the entire site from the south to the north with an average
slope of 0.90%. The ground is covered with local hay, native grasses, and wild
weeds which grow all over the field. Before 1991, there were no trees on the
property. The length of the basin is 245.00 feet of overland sheet flow. The runoff
then overtopped the south curb along Spring Park Drive and drained along the
gutter into the storm drainage intake. The intake storm drainage box is 2 x 3 feet
and is connected to 15-inch reinforced concrete (RCP) storm sewer pipe which is
drained into Spring Creek to the north. Two -and ten-year initial historic storms are
used to calculate the historic runoff. Historic runoff is calculated for the site as
follows:
2
A. Historic drainage conditions for 2 and 10 year storm, based on design
storm frequencies for natural ground area:
1. The Rational Method runoff coefficient for 1.587 acres or 69,122.39
square feet under historic conditions for grassland with 0.90% slope is
equal to 0.20.
2. Time of concentration = T, minutes.
S = Slope of basin S% = 0.90%.
Cf = Frequency adjustment factor for 2 yr = 1.0.
Cf = Frequency adjustment factor for 10 yr = 1.0.
C = Rational Method Runoff Coefficient = 0.20.
L = Length of Basin in feet = 245.00 feet.
Tc = 1.87 x (1.1 - CxCf) x (L) 1/ = 27.2 minutes
S113 I .
From Figure 3-2, T, for 245 feet of overland flow = 25.50 minutes.
Use time of concentration of 25.5 minutes.
3. Precipitation in inches/hour using Rainfall Intensity Duration Curves
for 2 and 10-year frequency for City of Fort Collins, from figure 3-1.
Rainfall intensity = IZ = 1.60 & 110 = 2.85 inches/hour.
4. Runoff quantity for undeveloped conditions = Q = C(COIA where
Q = Flow quantity, cfs. A = Total area of basin = 1.587 acres.
C = Runoff coefficient = 0.20.
Cf = Frequency adjustment factor = 1.0.
1 = Rainfall intensity = IZ = 1.60 & 110 = 2.85 inches/hour
Then Q2 = 1.0 x 0.2 x 1.60 x 1.587 = 0.51 cfs
Qt0 = 1.0 x 0.20 x 2.85 x 1.587 = 0.91 cfs.
B. Historic drainage conditions for 100-year'storm, based on design storm
frequencies for natural ground area:
1. Rational Method runoff coefficient for 1.587 acres or 69,122.39 square
feet under historic conditions for grassland with 0.90% slope is equal
to 0.20.
2. Time of concentration of 245.00 feet of overland flow ,= Tc minutes
S = Slope of basin S% = 0.90%. Cf = Freq. adj. factor for 100 yr =
1.25.
C = Rational Method Runoff Coefficient = 0.20
3
L = Length of Basin in feet = 245.00 feet.
Tc = 1.87 x (1.1 - CxCf) x (L)'/Z = 25.8 minutes
S'/
From Figure 3-2, T, for 245.00 feet of overland flow = 25.50 minutes.
Use time of concentration = 25.50 minutes.
3. Precipitation in inches/hour using Rainfall Intensity Duration Curves
for 100 year frequency for City of Fort Collins, from figure 3-1, then
rainfall intensity = I100 = 4.55 inches/hour.
2. Runoff quantity for undeveloped historical conditions = Q = C(Cf IA,
where:
Q = Flow quantity, cfs. A Total area of basin = 1.587 acres.
C = Runoff coefficient = 0.20. Cf = Frequency adjustment factor =
1.25
Q100 = 1.25 x 0.20 x 4.55 x 1.587 = 1.81 cis
Runoff Summary for Historical Conditions
Total area =
69,122.39 square feet
or 1.587 acres
Slope (S) =
0.90%
Length of Basin (D) =
245.00 feet
Runoff Coefficient =
0.20
Frequency adj. factor for 2 yr =
1.0
Frequency adj. factor for 10 yr =
1.0
Frequency adj. factor for 100 yr =
1.25
Time of Concentration:
T, for historic 2 yr. =
27.2 minutes
T, for historic 10 yr. =
25.5 minutes
T, for historic 100 yr. =
25.8 minutes
Precipitation for 2 yr. =
1.60 In/hr..
Precipitation for .10 yr. =
2.85 in/hr.
Precipitation for 100 yr. =
4.55 in/hr.
Historic runoff Q2 =
0.51 cfs
Historic runoff Q,o =
0.91 cis
Historic runoff Q100 =
1.81 cfs
4
Existinq Drainage Conditions
A. Existing drainage conditions for 2- and 10-year historical storm, based
on design storm frequencies for existing conditions:
Total area of the site in 1991 (when CDS Eng. prepared their drainage report) was
1.338 acres. Another 0.249 acre was added to the site since then which made the
total area equal to 1.587 acres. The existing drainage is divided into three sub -
basins called A, B, and C as per CDS Eng. drainage report. The additional acreage
of 0.247 is added to sub -basin A and C. Since the site has been developed,
streets, curbs, gutters, and sidewalks have already been built around the lot along
Spring Park Drive, Remington Street, and Mathews Street. The site is landscaped
with grass, trees, open playground area, and a covered playground area with
concrete pads.
There are two curb cuts and drive approaches existing on Spring Creek Drive and
' Remington Street. The runoff from the parking lot and the building is directed to a
drainage concrete pan that carries the runoff to Spring Park Drive. At Spring Park
Drive there is an intake storm drainage box connected to al 5-inch reinforced
' concrete storm sewer pipe. Existing conditions consist of a 7,244 square feet one
story brick building, and approximately 9,073 square feet of pavement parking area.
Landscaped and grass area is equal to 43,310 square feet. Total area of existing
playground which consists of sand on the ground and few slides and swings is
equal to 8,968 square feet.
' The existing flow after development in 1991 is sheet flow at the parking lot and
around the building. Then the flow becomes concentrated along the concrete pan
across the parking lot. The flow in the streets is a gutter flow as shown on the
drainage plan of CDS Eng. The runoff along the south side of Spring Park Drive
drains along the gutter to the east and into a storm drainage intake then through an
' existing 15-inch RCP into Spring Creek. The following is the site data:
Site Area Gross = 69,122.39 S.F. (100%) (1.587 Acres)
' Net = 69,122.39 S.F. (100%) (1.587 Acres)
Building (roof) Area = 7,244.00 S.F. (10.5%) (0.166 Acres)
Parking, Pay. & Side Walks = 9,073.00 S.F. (13.1 %) (0.208 Acres)
Grass & Landscape Area = 43,310.00 S.F. (62.7%) (0.990 Acres)
BASIN DELINEATION: The site was divided by CDS Eng., into 3 basins. The
following are the three basins:
Basin "A" is the smallest basin for the site which includes the parking lot and the
driveway at the southwest of the lot. Basin "A" has been increased by 527 square
5
' feet or 0.012 acres. Total area of Basin "A" is equal to 3,450 square feet or 0.082
acres. The slope of this basin is 0.01 % toward the west. The length of the basin is
' equal to 80.00 feet. Basin "A" runoff drains into Remington Street through the
driveway which then runs into the intake storm drainage box at Spring Park Drive
which is connected to a 15-inch reinforced concrete (RCP) storm sewer pipe.
' Basin "B" is for the area of the parking lot and driveway on Spring Park Drive, also
the west, one-half (%) of the existing building. Total area of Basin "B" is 22,904
' square feet or 0.526 acres. All of Basin "B" drains through the curb opening and
then through the driveway into Spring Park Drive, then into the storm drainage
intake. The slope of this basin is 0.0065% toward the north. The length of the
' basin is equal to 250 feet. Basin "B" has not changed since 1991 for the existing
conditions.
' Basin "C" is that portion of the east one-half ('/) of the existing building and all the
northeast area that is part of the original parcel. Basin "C" has been increased by
' 10,320.39 square feet or 0.237 acres. Total area of Basin "C" is 42,767.35 square
feet or 0.982 acres. All of Basin "C" drains as sheet flow toward the northeast,
overtops the curb then flows along curb and gutter of the south side of Spring Park
' Drive into storm drainage intake. The slope of this basin is 0.7% toward the
northeast. The length of the basin is equal to 330 feet.
B. Existing drainage conditions for 2, 10, and 100 year storm, based on
design storm frequencies for natural ground area.
BASIN "A"
Total area contributing to Basin "A" runoff is equal to 3,450 square feet or 0.079
acres.
Land Cover
Area/sq.ft.
Area/Acr
% of area
Runoff/Coeff.
Parking & Drives
= 2,548
0.058
74%
0.95
Landscape Area
= 902
0.021
26%
0.20
Total Area
= 3,450
0.079
100%
0.76
Calculated composite runoff coefficient is equal to:
C = 2,548 x 0.95 + 902 x 0.20 = 0.75
3,450 .
M
2. Time of concentration in minutes:
Basin Length = 80 feet, sheet flow over the driveway with slope of 0.01 %
From Figure 3-2, T, = 18.50 minutes.
3. Precipitation in inches/hour using Rainfall Intensity Duration Curves for 100
year frequency for City of Fort Collins, from figure 3-1, then rainfall intensity:
IZ = 1.90 inches/hour, 110 = 3.40 inches/hour, 1100 = 5.40 inches/hour
4. Runoff quantity for existing development:
Q2=1.0x0.75x1.90x0.079=0.113cfs
Q10=1.0x0.75x3.40x0.079=0.202cfs
Q100= 1.25 x 0.75 x 5.40 x 0.079 = 0.400 cfs
BASIN "B"
Total area contributing to Basin "B" runoff is equal to 22,904 sq. ft. or 0.526 acres.
' Land Cover
Area/sq.ft.
Area/Acr
% of area
Runoff/Coeff.
Building
= 3,622
0.083
15%
0.95
'
Conc. & Pay.
= 11,370
0.261
50%
0.95
Landscape Area
= 7,912
0.182
35%
0.20
Total
= 22,904
0.526
100%
0.70
1. Calculated composite runoffcoefficient is equal to
C = (3,622 + 11,370) x 0.95 + 7,912 x 0.20 = 0.70
22,904
2. Basin length = 80 feet, with slope of 2.4% & sheet flow of 250 feet of pan and
gutter flow with slope of 0.4%.
3. Time of concentration in minutes: T, = 9.0 minutes for overland flow, Tc for
conc. Pan & gutter flow is equal to: Time = length/ velocity.
Velocity is determined by Manning equation for section H where;
V=1.486/n(R)"2/3(S)"'rz
25'
Section H
W-KYS1l
7
78.94 fl
Area = % x 0.46 x 25.0 = 5.75 sq.ft.
R = A/P = 5.75/25.46 = 0.23, S = 0.4%, L = 250 feet
Then V=1.486/0.016(0.23)*23(0.004)*'n= 2.2 ft/sec.
Time = 250 x /2.2x60 = 2.0 min, then Q = A x V = 5.75 x 2.2 = 12.65 cfs.
Then total time of concentration T, = 9.0 + 2.0 = 11.0 minutes.
4. Precipitation in inches/hour using Rainfall Intensity Duration Curves for 100
year frequency for City of Fort Collins, from figure 3-1, then rainfall intensity:
IZ = 2.40 inches/hour, 110 = 4.20 inches/hour, time of concentration 100 yr =
5.0 minutes, then total T, = 5.0 + 2.0 = 7.0 min, then I100 = 7.50 inches/hour.
5. Runoff quantity for existing development:
Q2= 1.0 x 0.70 x 2.40 x 0.526 =0.880cfs
Q,o = 1.0 x 0.70 x 4.20 x 0.526 = 1.546 cfs
Q100= 1.25 x 0.70 x 7.50 x 0.526 = 3.452 cfs
BASIN "C"
Total area contributing to Basin "C" runoff is equal to 42,767 square feet or 0.982
acre.
Land Cover
Area/sq.ft.
Area/Acr
% of area
Runoff/Coeff.
Building
= 3,622
0.083
8%
0.95
Concrete
= 3,841
0.088
9%
0.95
Playground (sand)
= 8,968
0.206
21%
0.10
Landscape Area
= 26.336
0.605
62%
0.20
Total Area
= 42,767
0.982
100%
0.30
2.
3.
Calculated composite runoff coefficient is equal to
C=(3.622+3.841)x0.95+8,968x0.10+26,336x0.2 = 0.30
42,767
Basin length = 330 feet, with slope of 0.6% & sheet flow of 115 feet of gutter
flow with slope of 0.58%.
Time of concentration in minutes:
Tr = 32.22 minutes for overland flow, T, for gutter flow is equal to:
Time = length/ velocity, velocity is determined by Manning.equation for
section H where ,
Area ='/ x 0.50 x 27.5 = 6.88 sq.ft.
R = A/P = 6.88/28.0 = 0.25, S = 0.58%, L = 115 feet.
27.50
Spring Park Dr. 77.48 fl
76.98 fl
V=1.486/n(R)'(S)'rz=1.486/0.017(0.25)73(0.0058)"= 2.64 ft/sec.
Time = 115/2.64 x 60 = 0.73 min, then Q=AxV=6.88x2.64=18.16cfs.
Then total time of concentration T, = 33.22 + 0.73 = 33.95 minutes.
4. Precipitation in inches/hour using Rainfall Intensity Duration Curves for 100
year frequency for City of Fort Collins, from figure 3-1, then rainfall intensity:
12 = 1.35 inches/hour, l,o = 2.40 inches/hour.
Time of concentration for 100 year = 29.5 minutes.
Then total Tc = 29.5 + 0.73 = 30.23 min. 1100 = 4.15 inches/hour
5. Runoff quantity for existing development:
Q2 = 1.0 x 0.30 x 1.35 x 0.982 = 0.400 cfs
Q10 = 1.0 x 0.30 x 2.40 x 0.982 =0.707cfs
Q100 = 1.25 x 0.30 x 4.15 x 0.982 = 1.528 cfs
Runoff Summary for Existing Conditions
Basin Area in sq.ft. _
Basin Area in Acres =
Slope (S) % _
Length of Basin (D)feet =
Runoff Coefficient =
Time of Concentration:
T. for existing 2 yr. min. _
Tc for existing 10 yr. _
T, for existing 100 yr. _
Precipitation for 2 yr. In/hr =
Precipitation for 10 yr. _
Precipitation for 100 yr.
Existing runoff Q2 cfs
Existing runoff Q10 cfs =
Existing runoff Q100 cfs =
Basin A
Basin B
Basin C
Total
3,450
22,904
42,767
69,122
0.079
0.526
0.982
1.587
0.01
2.4/0.4
0.6/0.58
80
80/250
330/115
0.75
0.71
0.30
18.5
11.0
33.95
18.5
11.0
33.95
18.5
7.0
30.23
1.90
2.4
1.35
3.40
4.2
2.40
5.40
7.5
4.15
0.113
0.880
0.400
1.405
0.202
1.546
0.707
2:471
0.400
3.452
1.528
5.380
F7
Proposed Development
The proposed construction consists of an additional building next to the existing one
story day school building and landscaped areas. Total area of the site is equal to
1.587 acres. The proposed drainage conditions is divided into three sub -basins
called A, B and C. The proposed addition will be landscaped with grass, trees, an
open play ground area and a covered play ground area with concrete pads. A new
sidewalk will be built along Mathews Street.
Developed Drainage Conditions
' There are two curb cuts and drive approaches existing on Spring Park Drive and
Remington Street. The runoff from the parking lot and the existing building is
directed to a concrete pan that carry the run off to Spring Park Drive. At Spring
' Park Drive there is an intake storm drainage box connected to a 15-inch reinforced
concrete storm sewer pipe. The proposed conditions consist of 12,737 square feet
one story brick building and approximately 19,148 square feet of concrete and
' pavement parking area. Landscaped and grass area is equal to 33,130 square feet.
t Total area of existing play ground which consist of sand on the ground and few
slides and swings is equal to 8,968 square feet. The runoff is a sheet flow at the
parking lot and around the building. Then the flow becomes concentrated along the
' concrete pan across the parking lot. The flow in the streets is in a gutter. The
runoff along the South side of Spring Park Drive drains along the gutter to the East
and into a storm drainage intake. Then through an existing 15-inch reinforced
' concrete pipe (R.C.P.) into Spring Creek. The following is the site data:
Site Area Gross = 69,122.39 S.F. (100%) (1.587 Acres)
Net = 69,122.39 S.F. (100%) (1.587 Acres)
' Building (roof) Area = 12,737 S.F. (18%) (0.292 Acres)
Parking, Pay. & Side Walks = 23,251 S.F. (34 %) (0.534 Acres)
' Grass & Landscape Area = 33,130. S.F. (48%) (0.761 Acres)
Basin Delineation The proposed development is divided into 3 basins. The
' following are the three basins:
Basin "A" is the smallest basin for the site which includes the parking lot and the
' driveway at the Southwest of the lot. Basin A has been increased by 527 square
feet or 0.012 acres. Total area of basin A is equal 3,450 square feet or 0.082
' Acres. The slope of this basin is 0.01 % toward the west. The length of the basin is
equal to 80.00 feet. Basin "A" runoff drains into Remington Street through the
Driveway, which then runs into the intake storm drainage box at Spring Park Drive
' which is connected to a 15-inch reinforced concrete (R.C.P.) storm sewer pipe.
10
Basin "B" is for the area of parking lot and driveway on Spring Park Drive, also the
west one-half (%) of the existing building. Total area of Basin B is 22,904 square
' feet or 0.526 acres. All of basin "B" drains through a curb opening and then through
the driveway into Spring Park Drive, then into the storm drainage intake. The slope
of this basin is 0.65% toward the north. The length of the basin is equal to 250.00
' feet. Basin B did not change from the existing conditions.
Basin "C' is that portion of the east one-half (%) of the existing building and all the
north area of the new addition. The new school building is added to basin D. Total
area of Basin C is 25,000.00 square feet or 0.574 Acres. Basin "C" consist of a
' grass area, sand and play ground which slopes toward the north and northeast.
Basin "C" drains as sheet flow toward the north and northeastThe slope of this
basin is 1.25% toward the northeast. The length of the basin is equal to 200 feet.
' Basin "D" is allthe new addition and area south. of the old and the new biulding
which was portion of Basin C at the existing conditions. The new school building is
' added to basin C. Total area of Basin C is 17,771.78 square feet or 0.408 Acres. A
new sidewalk is built along Mathews Street. All of basin "D" drains to the south to
detention pond B. Then along a 2 feet concrete pan, the flow drains trough a 6-
inch pipe toward the north along the east side of the new biulding, then through
sidewalk culvert as shown in the drainage plan to cross the sidewalk and the curb,
then into the gutter of the Mathews Street, then west into Sprink Park Drive, and into
' the storm drainage intake. Average slope of basin D is 0.75% toward north. Total
length of basin D is equal to 235.00 feet.
' C. Developed drainage conditions for 2-, 10- and 100-year storm, based on
design storm frequencies for natural ground area:
' BASIN "A" Area contributing to basin A is equal to 3,450 sq. ft. or 0.079 acres.
' Land Cover Area/sq.ft. Area/Acr % of area Runoff/Coeff.
Parking & Drives = 2,548 0.058 74% 0.95
Landscape Area = 902 0.021 26% 0.20
Total Area = 3,450 0.079 100% 0.75
1. Calculated composite runoff coefficient is equal to
' C = 2,548 x 0.95 + 902 x 0.20 = 0.75
3,450
2. Time of concentration in minutes; Basin Length = 80 feet sheet flow over
t driveway with slope of 0.01 %. From Fig. 3-2, Tc = 18.50 mina
3. Precipitation in inches/hour using Rainfall Intensity Duration Curves for 100
year frequency for City of Ft. Collins, from figure 3-1, then rainfall intensity: IZ
' = 1.90 inches/hour, 110 = 3.40 inches/hour, 1100 = 5.40 inches/hour
4. Runoff quantity for proposed development
' Q2=1.0x0.75x1.90x0.079=0.113cfs
Q10=1.0x0.75x3.40x0.079=0.202cfs
0100= 1.25 x 0.75 x 5.40 x 0.079 = 0.400 cfs;
BASIN "B"
tBasin B developed drainage conditions is the same as the existing conditions. Area
contributing to basin B runoff is equal to 22,904 square feet or 0.526 acres.
'. Land Cover Area/sq.ft. Area/Acr % of area Runoff/Coeff.
' Building = 3,622 0.083 15% 0.95
Conc. & pay. = 11,370 0.261 50% 0.95
Landscape Area = 7.912 0.182 35% 0.20
' Total = 22,904 0.526 100% 0.70
1. Calculated composite runoff coefficient is equal to
' C = (3.622 + 11.370) x 0.95 + 7,912 x 0.20 = 0.70
22,904
' 2. Basin length = 80 feet, with slope of 2.4% & sheet flow of 250 feet of pan and
gutter flow with slope of 0.4%.
' 3. Time of concentration in minutes: T, = 9.'0 minutes for overland flow, T, for
conc. Pan & gutter flow is equal to: Time = length/ velocity, velocity is
' determined by Manning equation for section H. Where: Area = % x 0.46 x
25.0 = 5.75 sq.ft. R = A/P = 5.75/25.46 = 0.23, S = 0.4%, L = 250 feet
V=1.486/n(R)w(S)12=1.486/0.016(0.23) "(0.004)"= 2.2 ft/sec.
' Time = 250 x /2.2 x 60 = 2.0 min, then Q = A x V = 5.75 x 2.2 = 12.65 cfs.
Then total time of concentration Tc = 9.0 + 2.0 = 11.0 minutes.
' 4. Precipitation in inches/hour using Rainfall Intensity Duration Curves for 100
year frequency for City of Ft. Collins, from figure 3-1, then rainfall intensity: 12
' = 2.40 inches/hour, Ito.= 4.20 inches/hour, Time of concentration 100 yr = 5.0
minutes, then total Tc = 5.0 + 2.0 = 7.0 min, then 1100 = 7.50 inches/hour.
' 5. Runoff quantity for existing development:
Q2= 1.0 x 0.70 x 2.40 x 0.526 =0.88cfs
Q10 = 1.0 x 0.70 x 4.20 x 0.526 =1.55cfs
' Q100 = 1.25 x 0.70 x 7.50 x 0.526 = 3.45 cfs
12
BASIN "C"
' The new addition of the school building is in basin C. The roof of the new building
slopes to the south. All the drainge from the roof of the new biulding draining
' toward the south, toward a new location of detention pond "B". The drainage from
the rest of basin "C" drains as sheet flow toward the north. Total length of basin C
is equal to 474.00 feet, with an average slope of 0.85% toward north. Total area
' contributing to basin C runoff is equal to 42,767 square feet or 0.982 acres.
Land Cover Area/sq.ft. Area/Acr % of area Runoff/Coeff.
' Building = 9,115 0.209 21% 0.95
Concrete = 5,352 0.123 12% 0.95
' Play ground (sand) = 8,968 0.206 21% 0.10
Landscape Area = 19.335 6.444 46% 0.20
Total Area = 42,767 0.982 100% 0.43
' 1. Calculated composite runoff coefficient is equal to
' C=(9.115+5.352)x0.95+8,968x0.10+19,335x0.2 = 0.43
42,767
2. Total basin length = 474 feet which is broken into segments as follows: 100
feet of overland flow with slope = 0.25%. Concrete pan along the north side
of new building for 35 feet with a slope of 0.5%. Then through a 4-inch PVC
pipe across a concrete side walk for 5 feet with a slope of 0.6%. Another
concrete pan for 35 feet across the grass landscaped area east of the new
biulding with a slope of 0.5%. Then through a metal sidewalk culvert for 4
feet with a slope of 0.6% across the sidewalk. Then into the gutter along
west side of Mathews Street for 175 feet with a 1.8% slope. Then into the
gutter along south side of Spring Park Drive for 120 feet with a slope of
0.58%.
3. Time of concentration in minutes for 2- and 10-year storms is equal to as
follows:
L = 100 feet of overland flow, S = 0.25%.
From Tc = 1.87 x (1.1 - CxCf) x (L)" = 20 min.,
S13
L = 35 feet of concrete pan with a slope of S = 0.5%. Velocity is determined
by Manning equation. Where: V=1.486/n(R)"7j3(S)*"2. Where n = 0.013, d =
0.13 feet, S = 0.005, R = A/P = 0.25/4.0.
then V = 1.486/0.013(0.25/4.0)"213(0.005)"'rz. = 1.27 ft/sec.
13
Time = 35/1.27 x 60 = 0.50 min, then Q = A x V = 0.25 x 1.27= 0.32 cfs,
Tc=1.0min.
L = 5 feet of 4" PVC pipe. S = 0.6%. Time = length/velocity.
Velocity is determined by Manning equation. Where:
V=1.486/n(d)"2J3(S)*'2 n = 0.01, d = 6.0", S = 0.006,
V = 1.486/0.01(6/12)*213(0.006)*'2=2.88 ft/sec.
Time = 5/2.88 x 60 = 1.83 min, then Q = A x V = 0.57 cfs, Tc = 1.83 min.
L = 35 feet of concrete pan with a slope of S = 0.5%. Velocity is determined
by Manning equation. Where: V=1.486/n(R)*23(S)*'rz. Where n = 0.013, d =
0.13 feet, S = 0.005, R = A/P = 0.25/4.0.
then V =1.486/0.013(0.25/4.0)*23(0.005)*'n =1.27 ft/sec.
Time = 35/1.27 x 60 = 0.50 min, then Q = A x V = 0.25 x 1.27= 0.32 cfs,
T, = 1.0 min.
' Time of concentration for gutter flow along Mathews Street is equal to: Time
= length/ velocity, velocity is determined by Manning equation for section H.
Where: Area = % x 0.50 x 25.0 = 6.25 sq.ft. R = A/P = 6.25/25.50 = 0.25, S =
' 1.8%, L = 175 feet.
V=1.486/n(R)m(S)'n=1.486/0.013(0.25)23 (0.018)'n= 6.0 ft/sec.
Time = 175 x /6.Ox60 = 1.0 min, then Q = A x V = 6.25 x 6.0 = 37.5 cfs.
Time of concentration for gutter flow along Spring Park Drive is equal to:
Time = length/ velocity, velocity is determined by Manning equation for
' section H. Where: Area ='/ x 0.50 x 30.0 = 7.5 sq.ft. R = A/P = 7.50/30.50 =
0.25, S = 0.58%, L = 120 feet.
V=1.486/n(R)73(S)"=1.486/0.013(0.25) 213 (0.0058)'rz= 3.5 ft/sec.
' rime =120x/3.5x60=1.0min, then Q=AxV=7.5x3.5=26.25cfs.
Total time of concentration Tr = 20 + 2.5 + 1.0 + 1.83 + 1.0 + 1.0 = 27.33 min.
4. Precipitation in inches/hour using Rainfall Intensity Duration Curves for 100-
' year frequency for City of Fort Collins, from figure 3-1, then rainfall
intensity:
' 12 = 1.40 inches/hour, 110 = 2.45 inches/hour. Time of concentration for 100-
year = 28.0 minutes. Then I100 = 4.30 inches/hour
' 5. Runoff quantity for existing development;
QZ = 1.0 x 0.43 x 1 A0 x 0.982 0.59 cfs
Q10=1.0x0.43x2:45x0.982=1.04cfs
Q100 = 1.25 x 0.43 x 4.30 x 0.982 = 2.27 cfs
14
BASIN "D"
' The new addition of the school building is in basin D. The roof of the new building
slopes to the south. All the drainge from the roof of the new biulding draining
toward the south, toward a new location of detention pond "B°. The drainage from
' the rest of basin "D" drains as sheet flow toward the north. Total length of basin C
is equal to 474.00 feet, with an average slope of 0.85% toward north. Total area
contributing to basin C runoff is equal to 42,767 square feet or 0.982 acres.
' Land Cover Area/sq.ft. Area/Acr % of area Runoff/Coeff.
Building = 9,115 0.209 21% 0.95
' Concrete = 5,352. 0.123 12% 0.95
Play ground (sand) = 8,968 0.206 21 % 0.10
' Landscape Area = 19.335 0.444 46% 0.20
Total Area = 42,767 0.982 100% 0.43
' 1. Calculated composite runoff coefficient is equal to
C=(9115+5352)x0.95+8,968x0.10+19,335x0.2 = 0.43
' 42,767
2. Total basin length = 474 feet which is broken into segments as follows: 100
feet of overland flow with slope = 0.25%. Concrete pan along the north side
of new building for 35 feet with a slope of 0.5%. Then through a 4-inch PVC
pipe across a concrete side walk for 5 feet with a slope of 0.6%. Another
' concrete pan for 35 feet across the grass landscaped area east of the new
biulding with a slope of 0.5%. Then through a metal sidewalk culvert for 4
feet with a slope of &6% across the sidewalk. Then into the gutter along
' west side of Mathews Street for 175 feet with a 1.8% slope. Then into the
gutter along south side of Spring Park Drive for 120 feet with a slope of
0.58%.
' 3. Time of concentration in minutes for 2- and 10-year storms is equal to as
follows:
' L = 100 feet of overland flow, S = 0.25%.
' From T, = 1.87 x (11.1 - CxCf) x (L)" = 20 min.,
SIM
L = 35 feet of concrete pan with a slope of S = 0.5%. Velocity is determined
by Manning equation. Where: V=1.486/n(R)*w(S)*"2. Where n = 0.013, d =
' 0.13 feet, S = 0.005, R = A/P = 0.25/4.0.
then V=1.486/0.013(0.25/4.0)*213(0.005)*'rz. = 1.27 ft/sec.
Time = 35/1.27 x 60 = 0.50 min, then Q = A x V = 0.25 x 1.27= 0.32 cfs,
15
' T, = 1.0 min.
' L = 5 feet of 4" PVC pipe. S = 0.6%. Time = length/velocity.
Velocity is determined by Manning equation. Where:
V=1.486/n(d)*2'3(S)w12 n = 0.01, d = 6.0", S = 0.006,
' V = 1.486/0.01(6/12)*23(0.006)*12 =2.88 ft/sec.
Time = 5/2.88 x 60 = 1.83 min, then Q = A x V = 0.57 cfs, Tc = 1.83 min.
L = 35 feet of concrete pan with a slope of S = 0.5%. Velocity is determined
by Manning equation. Where: V=1.486/n(R)*2J3(S)*12. Where n = 0.013, d =
0.13 feet, S = 0.005, R = A/P = 0.25/4.0.
then V = 1.486/0.013(0.25/4.0)*23(0.005)*12 =1.27 ft/sec.
Time = 35/1.27 x 60 = 0.50 min, then Q = A x V = 0.25 x 1.27= 0.32 cfs,
T, = 1.0 min.
Time of concentration for gutter flow along Mathews Street is equal to: Time
= length/ velocity, velocity is determined by Manning equation for section H.
Where: Area = % x 0.50 x 25.0 = 6.25 sq.ft. R = A/P = 6.25/25.50 = 0.25, S =
1.8%, L = 175 feet.
V=1.486/n(R)'(S)12=1.486/0.013(0.25)2r' (0.018)12= 6.0 ft/sec.
Time = 175 x /6.Ox60 = 1.0 min, then Q = A x V = 6.25 x 6.0 = 37.5 cfs.
Time of concentration for gutter flow along Spring Park Drive is equal to:
Time = length/ velocity, velocity is determined by Manning equation for
section H. Where: Area =1/ x 0.50 x 30.0 = 7.5 sq.ft. R = A/P = 7.50/30.50 =
0.25, S = 0.58%, L = 120 feet.
V=1.486/n(R)2`3(S)12=1.486/0.013(0.25) m (0.0058)12= 3.5 ft/sec.
Time = 120 x /3.5 x 60 = 1.0 min, then Q = A x V = 7.5 x 3.5 = 26.25 cfs.
Total time of concentration T, = 20 + 2.5 + 1.0 + 1.83 + 1.0 + 1.0 = 27.33 min.
4. Precipitation in inches/hour using Rainfall Intensity Duration Curves for 100-
year frequency for City of Fort Collins, from figure 3-1, then rainfall
intensity:
12 = 1.40 inches/hour, 110 = 2.45 inches/hour. Time of concentration for 100-
year = 28.0 minutes. Then 1100 = 4.30 inches/hour
5. Runoff quantity for existing development;
Q2 = 1.0 x 0.43 x 1.40 x 0.982 = 0.59 cfs
Q10 = 1.0 x 0.43 x 2.45 x 0.982 =1.04cfs
Q100 = 1.25 x 0.43 x 4.30 x 0.982 = 2.27 cfs
16
Runoff Summary for Developed Conditions
Total area = 69,122.39 square or 1.587 acres
Basin A Basin B Basin C Total
■ Basin Area in sq.ft. = 3,450 22,904 42,767 69,122
Basin Area in Acres = 0.079 0.526 0.982 1.587
' Slope (S) % = 0.01 2.4/0.4 0.5/0.58
Length of Basin (D)feet = 80 80/250 445/115
Runoff Coefficient = 0.75 0.71 0.43
' Time of Concentration:
T, for developed 2 yr. min. = 18.5 11.0 33.22
T. for developed 10 yr. = 18.5 11.0 33.22
' T. for developed 100 yr. = 18.5 7.0 28.50
Precipitation for 2 yr. In/hr = 1.90 2.4 1.4
' Precipitation for 10 yr. 3.40 4.2 2.45
Precipitation for 100 yr. 5.40 7.5 4.3
Devloped runoff QZ cfs = 0.11 0.90 0.59 1.60
Developed runoff Q,o cfs = 0.20 1.57 1.04 2.81
Developed runoff Q100 cfs = 0.40 3.50 2.27 6.17
Allowable release Q,o cfs = 0.20 1.57 1.05 2.81
' Dif. between Q100 - Q,o cfs = 0.20 1.93 1.24 3.37
Summary of Runoff
' The entire developed site is divided into three sub -basins "A", "B" and "C". The
allowable release rate for the 10-year rainstorm, for the entire site is Q10 = QA + QB
+ QC = 0.20 + 1.57 + 1.04 = 2.81 cfs. The total runoff for the 100-year major
' rainstorm for the proposed condition for the three sub -basins "A", "B", and "C" are =
Q,00 = QA + QB + QC = 0.40 + 3.5 + 2.27 = 6.17 cfs. The total difference between
' the major storm runoff (Q100) for the proposed conditions and the historic conditions
for 10 -year storm of the entire site is equal to = Q100 - Q10 = 6.17 - 2.81 = 3.36 cfs.
' According to the City of Fort Collins Drainage Design Criteria, a detention pond is
needed to detain the difference between the 100 year major flood storm and the 10
' year minor flood storm. The difference between 100 year storm and 10 year storm
which is 3.36 cfs will be required to be detained. Since the developed runoff from
basin A is minimal, at 0.40 cfs, the runoff from basin A will be allowed to drain
' directly to Remington Street, then along Spring Park into the storm drainage.
The discharge out of the detention pond will be limited to the allowable historical
' release flow of 3.36 - 0.40 = 2.96 cfs. A 6-inch PVC will be installed at the existing
' 17
curb cut at the north end of the parking lot to regulate the flow:
Velocity is determined by Manning equation. Where: V=1.486/n(R)"�`3(S)*'rz.
Where n = 0.009, d = 0.5 feet, S = 0.005, R = A/P.
then V = 1.486/0.03(1/4.12)*u'(0.005)*'rz = 2.11 ft/sec. L = 35 feet of concrete pan
with a slope of S = 0.5%, then Q = A x V = 2.11 cfs,
' Discussion
' According to the City of Fort Collins Drainage Design Criteria, a detention pond is
needed to detain the difference between the 100-year major flood storm and the 10-
year minor flood storm. The difference between the 100-year storm and 10-year
' storm, which is 3.37 cfs, will be required to be detained. Since the developed runoff
from basin A is minimal, at 0.40 cfs, the runoff from basin A will be allowed to drain
t directly to Remington Street, and then along Spring Park into the storm drainage
system.
' Along the north side of Spring Park Drive is Spring Creek, which flows from west to
east. The Spring Creek is a major off -site conveyance channel, which has flooded
significantly in the past. The existing 100-year flood computed water surface
t elevation (CWSEL) is estimated in Spring Creek Master Drainage Way Plan at
4,977.79 feet above sea level, at cross section # 65 at the west end of the property.
At the same cross section # 65, the floodway is at an elevation of 4,980.40. At
' cross section # 64, which is in the middle of the site, the 100-year floodplain is
estimated at 4,978.61 and the floodway is estimated at 4,979.60 feet. These values
have been confirmed with FEMA floodplain information.
1 The 100-year floodplain covers about one -quarter (1/4) of the total area at the most
northwest corner of the site. The 100-year flood way covers less than the
' floodplain, which is about tine -eighth (1/8) of the total area at the most northwest
comer of the site. The existing building encroaches on the 100-year floodplain
boundary slightly, but is well outside of the 100-year floodway boundary. The first
' floor of the existing building had been set at 4,980.88, where the first floor of the
proposed additional building is set at 4,982.88. The 100-year floodway and
' floodplain are shown on the drainage plan according to Spring Creek Master
Drainage Way Plan. The following assumptions have been considered in
determining the location, type and size of the detention ponds:
' 1. All off -site runoff is determined in the CDS drainage report and approved by
the City.
' 2. Since the slope of the site is toward the North and since the storm sewer inlet
box with 15-inch storm sewer pipe is along Spring Park Drive, then the
' location of the detention pond "B" is chosen at the North side of the parking
' 18
lot. The location of the detention pond is shown on the drainage plan.
3. All surface drainage from the site, except basin C, will be directed to the
North and into the detention pond, then through a 4-inch PVC pipe into the
gutter of Spring Park Drive, then into the 15-inch storm drainage sewer pipe.
4. The historical and existing runoff of the site is determined in this report within
the values of that in the CDS drainage report.
5. The off -site runoff flow around the streets surrounding the site is a gutter flow
as shown on the drainage plan. All off -site runoff quantities are based on
the CDS drainage report.
DETENTION POND SIZE
Mass Diagram:
BASIN A: The developed runoff from basin A is minimal, which is 0.40 cfs. The
runoff from basin A will be allowed to drain directly to Remington Street and then
along Spring Park into the storm drainage sewer pipe.
BASIN B:
Total Discharge
Historic Required
Time
I
CfCA
Q100
Volume
910
Volume
Storage
Min.
Sec.
In/hr
Acre
cfs
Fe
cfs
Fe
Fe
5
300
8.6
0.47
4.0
1,204
1.57
471
733
10
600
7.2
0.47
3.4
2,030
1.57
942
1.088*
20
1,200
5.2
0.47
2.4
2,933
1.57
1,884
1,049
30
1,800
4.2
0.47
2.0
3,600
1.57
2,826
774
40
2,400
3.5
0.47
1.7
3,948
1.57
3,768
180
60
3,600
2.6
0.47
1.2
4,399
1.57
5,652
XXXX
90
5,400
1.9
0.47
0.9
4,822
1.57
8,478
XXXX
120
7,200
1.4
0.47
0.7
4,738
1.57
111304
XXXX
180
10,800
1.2
0.47
0.6
6,091
1.57
16,956
XXXX
19
From the Mass diagram, the maximum difference between the 100- and 10-year
storm is 1,088 cubic feet, which is required to be detained. Discharge out of the
' detention pond will be limited to the allowable historical release flow of 1.57 cfs by
using a 6" PVC pipe.
' STORAGE VOLUME
The volume needed to be retained is 1,088 cubic feet. The volume in the detention
' pond area is the storage volume between contour line of 4,977.78 feet and contour
line of 4,978.50. The volume needed is determined using the following formula:
' Volume = d (A + B + (AB) %)
3
' Where D = Depth between contour line, ft
A = Surface area of top contour line,
B = Surface area of bottom contour line,
' Storage volume between contours 4,977.78 and 4,978.00 is:
A = 250.00 ftZ, Surface area at contour line 4,977.78.
B = 500.00 ftZ., Surface area at contour line 4,978.00
with a depth of d = 0.22 feet, Then V, = 80.00 cubic feet
Storage volume between contours 4,978.00 and 4,978.25 is:
A = 500.00 ft', Surface area at contour line 4,978.00.
B = 2,000.00 ft', Surface area at contour line 4,978.25.
' with a depth of d = 0.25 feet, Then VZ = 300.00 cubic feet.
Storage volume between contours 4,978.25 and 4,978.50 is:
' A = 2,000.00 ft', Surface area at contour line 4,978.25..
B = 4,000.00 ft', Surface area at contour line 4,978.50.
with a depth of d = 0.25 feet, Then V3 = 740.00 cubic feet.
' Total storage volume provided by detention pond is equal to 1,120 ft, which
' exceeds the needed storage volume of 1,088 ft3. High water line is at 4,978.50 ft.
20
BASIN C:
Total
Discharge
Historic Required
' Time I
CfCA
Q100
Volume
f310
Volume Storage
Min. Sec. In/hr
Acre
cfs
Fe
cfs
f Ft3
5 300 8.6
0.53
4.6
1,367
1.04
312 1,055
t10 600 7.2
0.53
3.8
2,290
1.04
.624 1,666
20 1,200 5.2
0.53
2.8
3,307
1.04.
1,248 2,059
' 30 1,800 4.2
0.53
2.2
4,007
1.04
1,872 2,135•
40 2,400 0.5
0.53
1.9
4,452
1.04
2,496 1,956
' 60 3,600 2.6
0.53
1.4
4,961
1.04
3,744 1,217
90 5,400 1.9
0.53
1.0
5,438
1.04
.5,616 XXXX
t 120 7,200 1.4
0.53
0.7
5,342
1.04
7,488 XXXX
180 10,800 1.2
0.53
0.6
6,869
1.04
11,232 XXXX,
From the Mass diagram the maximum difference between
100- and 10-year storm is
2,135 cubic feet, which
is required to be detained.
Discharge
out of the detention
'
pond will be limited to the allowable historical release flow
of 1.04 cfs by using a 6"
PVC pipe.
' Storage Volume
' The volume needed to be retained is 2,135 cubic feet. The volume in the detention
pond area is the storage volume between contour line of 4,978.88 feet and contour
' line of 4,980.00. The volume needed is determined using the following formula:
Volume = d (A + B + (AB)72)
3
Where d =Depth between contour line, ft.
A = Surface area of top contour line, ft.
' B = Surface area of bottom contour line, ft.
Storage volume between contours 4,978.88 and 4,979.00 is:
' A = 25.00 ft2, Surface area at contour line 4,978.88.
B = 1,500.00 ft2, Surface area at contour line 4,979.00
with a depth of d = 0.12 feet, Then V, = 70.00 cubic feet
t 21
Storage volume between contours 4,979.00 and 4,979.50 is:
A = 1,500.00 ftZ, Surface area at contour line 4,979.00.
B = 2,500.00 ftZ, Surface area at contour line 4,979.50.
with a depth of d = 0.50 feet, Then V, = 990.00 cubic feet.
Storage volume between contours 4,979.50 and 4,980.00 is:
A = 2,500.00 ftZ, Surface area at contour line 4,979.50.
B = 3,500.00 ftZ, Surface area at contour line 4,980.00.
with a depth of d = 0.50 feet, Then V, = 1,380.00 cubic feet.
Total storage volume provided by detention pond is equal to 2,440 ft3, which
exceeds the needed storage volume of 2,135 W. The high water line is at 4,980.00.
II EROSION CONTROL REPORT
INTRODUCTION
This erosion control report provides the wind and rainfall erosion control plan using
the City of Fort Collins wind and rainfall data, for the addition to Spring Creek
Country Day School, 1900 Remington Street, Fort Collins, Colorado. The total area
of the site before 1991 was 1.338 acres. Another 0.249 acres was added since
then, which made the total area equal to 1.587 acres. The site is landscaped with
grass, trees, an open play ground area and a covered play ground area with
concrete pads.
■ Existing drainage is divided into three sub -basins called A, B, and C as per the CDS
Engeering drainage report. An additional acreage of 0.247 is added to subbasin A
' and C. Since the site had been developed, streets, curbs, gutters and sidewalks
have already been built around the lot along Spring Park Drive, Remington and
' Mathews Streets. There are two curb cuts and drive approaches existing on Spring
Park Drive and Remington Street. Runoff from the parking lot and the building is
directed to a drainage concrete pan that carry the run off to Spring Park Drive. At
' Spring Park Drive there is an intake storm drainage box connected to a 15-inch
reinforced concrete storm sewer pipe.
■ Wind and Rainfall Erosion Control
The erosion control plan presents methods to control wind and rainfall erosions. An
erosion control installation schedule is presented here for erosion control measures
that are taking place as soon as construction starts. Since wind erosion control
occurs as a result of providing control for. rainfall erosion, evaluation of the latter is
completed first. The entire site is in a moderate wind and rainfall erodibility zone.
22
Rainfall Erosion Control Plan
The existing conditions of the site are covered with irrigated grass and trees. Since
the inlet drainage box and storm sewer pipe are existing on -site along the south
side of Spring Park Drive, then an area inlet filter will be installed around the
existing drainage inlet box. The filter will consist of straw bales to be placed around
drainage inlet the (as shown in the drainage plan) to prevent erosion and sediment
from entering the storm sewer system. A silt fence is also proposed around the
construction site as a silt and sediment trap which will reduce any additional erosion
effect.
Rainfall Performance Standard
Basin A: Area (Asb,) = 0.079 acres. Average Slope (Ssb,) = 0.01%.
Flow path (Lsbj) = 80 feet
Basin B: Area (Asb,) = 0.526 acres. Average Slope (Ssbl) = 2.4%.
Flow path (Lsb,) = 250 feet
Basin C: Area (Asb,) = 0.982 acres. Average Slope (Ssb,) = 0.50%.
Flow path (Lsb,) = 474 feet
Basin A
Basin B
Basin C
All basins
RAINFALL
100% moderate
100% moderate
100% moderate
WIND
100% moderate .
100% moderate
100% moderate
Lb = sum (Lsb x A sb)/sum Asb
= (80 x 0.079 + 250 x 0.526 + 474 x 0.982)/1.587 = 380 feet
Sb = sum (Lsb x A sb) / sum Asb
_ (0.01 x 0.079 + 2.4 x 0.526 + 0.50 x 0.982)/1.587 = 1.11 %
Performance Standard = 77.0% ( from table 8-A)
Rainfall Erosion Control Effectiveness
Site Area Gross = 69,122.39 SY
Net = 69,122.39 SY
Building (roof) Area = 12,737 S.F.
Parking, Pay. & Side Walks = 23,251 S.F.
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(100%)
(1.587 Acres)
(100%)
(1.587 Acres)
(18%)
(0.292 Acres)
(34%)
(0.534 Acres)
Grass & Landscape Area =
24,162. S.F.
(35%)
(0.555 Acres)
Play ground (sand) =
8,968
(13%)
(0.206 Acres)
EFF = (1 - C x P) x 100. where:
EFF = Effectiveness (%),
C = C-Factor (cover factor)
P = P-Factor (Practice factor)
Erosion Control
C-Factor
P-Factor
Method
Value
Value
Comment
Established Grass
0.08
1.00
Basin A, B & C
Roads/Walks
0.01
1.00
Basin A, B & C
Asphalt/Conc.
0.01
1.00
Basin A, B & C
Bare Soil
1.00
1.00
Basin C
Gravel Filters
1.00
0.80
Basin C
Silt Fence
1.00
0.50
Basin C
Basin "A"
Land Cover
Area/so.ft.
Area/Acr
% of area
Runoff/Coeff.
Parking & Drives
= 2,548
0.058
74%
0.95
Landscape Area
= 902
0.021
26%
0.20
Total Area
= 3,450
0.079
100%
0.75
Grass area = 26% x 0.079 ac. = 0.021 acres
Roadstwalks = 74% x 0.079 ac. = 0.058 acres
Net C-Factor = (0.021 x 0.08 + 0.058 x 0.01)/0.079 = 0.029
(grass) (Roads/walks)
Net P-Factor = (0.021 x 1.0 + 0.058 x 1.00)/0.079 = 1.00
(grass) (Roads/walks)
EFF,bj=(1-CxP)x100=(1-(0.029x1.00))x100=97%
Basin "B"
Land Cover Area/sct.ft. Area/Acr % of area Runoff/Coeff.
Building = 3,622 0.083 15% 0.95
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Conc. & pay. = 11,370
0.261 50% 0.95
Landscape Area = 7.912
0.182 35% 0.20
Total = 22,904
0.526 100% 0.71
Grass area
= 35% x 0.526 ac. = 0.182 ac.
Roads/walks/Asphalt/Conc..
= 65% x 0.526 ac. = 0.344 ac.
Net C-Factor = (0.182 x 0.08 + 0.344 x 0.01)/0.526 = 0.034
(grass)
(Roads)
Net P-Factor = (0.182 x 1.00 + 0.344 x 1.00)/0.526 = 1.00
(grass)
(Roads)
EFF,bj=(1-CxP)x100
= (1 - (0.034 x 1.00) x 100 = 97%
Basin "C'
Land Cover
Area/sq.ft. Area/Acr % of area Runoff/Coeff.
Building
= 9,115 0.209 21% 0.95
Concrete
= 5,352 0.123 12% 0.95
Play ground (sand)
= 8,968 0.206 21 % 0.10
Landscape Area
= 19.335 0.444 46% 0.20
Total Area
= 42,767 0.982 100% 0.43
Grass area
= 46% x 0.982 ac. = 0.444 ac.
Conc./Bldg.
= 33% x 0.982 ac. = 0.322 ac.
Bare Soil (sand)
= 21 % x 0.982 ac. = 0.206 ac.
Net C-Factor = (0.444 x 0.08 + 0.322 x 0.01 + 0.206 x 1.00 )/0.982 = 0.25
(grass) (Conc..) (Bare Soil)
Net P-Factor = (0.444 x 1.00 + 0.322 x 1.00 + 0.206 x 0.90 )/0.982 = 0.97
(grass) (Conc..) (Bare Soil)
EFF,bj
= (1 -C x P) x 100
=(1- (0.25 x 0.97) x 100=76%
The net EFF value for the entire 1.587 acres basin is calculated as follow;
EFF„d
= sum (EffsbxAsb) /sum A sb
97% x 0.079 + 97% x 0.526 + 76% x 0.982)/1.587 = 84%
Since the net EFF is greater than PS (84% > 77%), the proposed erosion control
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methods will meet the City of Fort Collins requirements.
Wind Erosion Control Plan
A strip of 18' x 18' x 6" of gravel or crushed rocks are to be laid on the bare ground
surface at both driveways during the construction period to prevent the dropping of
sediment, silt and deposits from the work site to the streets. Sediment traps will be
installed at the storm drainage box inlet and can be constructed of hay bales placed
around drainage outlet (as shown in the drainage plan) to prevent erosion and
sediment from entering the irrigation ditches. The landscaped area will be covered
with irrigated lawn, shrubs, and trees.
Cost Estimate for Erosion Control Plan
The erosion control plan requires the following:
A. Silt fence is proposed around the site.
250 linear feet of silt fence x 4 feet high of 6 mill at $20.00/100 feet long.
$20.00 x 250/100 = $50.00.
B. 10 pieces of concrete blocks and 3/4" gravel to be placed around the
drainage inlet. Ten (10) pieces of concrete blocks x $1.0 = $10.00 + $10.00
gravel = $20.00.
C. A strip of 18' x 18' x 6" of gravel or crushed rocks are to be laid on the bare
ground surface at both driveways during construction period.
The cost = 18 x 18 x 0.5 x 2 = 324 cubic feet = 12.00 cubic yard x $20.
The cost = $ 250.00.
D. 5,000 square feet of. landscaped grass at the end of project
cost = 5,000 x $0.25 = $1,250.00
Total cost = $50.00 + $20.00 + $250.00 + $1,250 = $1,570.00
Then 150% of the cost = 1.5 x 1,570.00 = $ 2,355.00, which is greater than the $
1,000.00 minimum.
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