HomeMy WebLinkAboutRIVER SONG WALDORF SCHOOL - PDP - 11-98 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTPROJECT: Waldorf School
SEQUENCE FOR 1998-99 ONLY
COMPLETED BY: Chris Carlson
DATE: 2/9/98
Indicate by use of a bar line or symbols when erosion control measures will be installed. Major modifications
to an approved schedule may require submitting a new schedule for approval by the City Engineer.
May
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Jan.
OVERLOT GRADING
WIND EROSION CONTROL
Soil Roughing
Perimeter Barrier
Vegetative Methods
Mulching / Sealant
RAINFALL EROSION
CONTROL
STRUCTURAL:
Detention Pond
Pond Outlet Filters
Straw Barriers
Silt Fence Barriers
1111111111111110
Sand Bags
Bare Soil Preparation
Contour Furrows
Asphalt / Concrete Paving
VEGETATIVE:
Permanent Seed Planting
Mulching / Sealant
Temporary Seed Planting
Sod'. Installatiorif
Nettings / Mats / Blankets
BUILDING CONSTRUCTION:
STRUCTURES INSTALLED BY: Contractor
VEGETATION/MULCHING CONTRACTOR: Contractor
DATE SUBMITTED: 219/98
MAINTAINED BY: Contractor
APPROVED BY CITY OF FORT
COLLINS ON:
Figure 5. Construction schedule.
11 Ayres Associates
During overlot and final grading, the soil will be roughened and furrowed perpendicular to
the prevailing winds (northwest to southeast). All areas not covered by asphalt, concrete, or
foundations and not seeded within two weeks of final grading will have 2 tons/acre of straw
mulch either crimped 4 or more inches into the soil or sprayed with a tackifier. The inlet
filter and silt fences will remain in place until construction is complete. They should be
checked after each storm event to determine whether repairs or alterations are needed.
With proper precautions, damage to off -site areas from a greater than historic sediment load
' can be eliminated and the loss of sediment from the site minimized. Calculations for
performance standards and effectiveness will be submitted with the final drainage and
erosion control plan. The proposed construction sequence is shown in Figure 5.
f
10 Ayres Associates
Table 5. Detention Pond 179 - Outlet Rating Curve.
WSEL
Volume
(ac-ft)
Discharge
(cfs)
5103.50
0.000
0.00
5104.00
0.035
0.24
5104.50
0.206
0.37
5105.00
1 0.377
1 0.47
100-year WSEL = 5104.34
100-year Volume = 0.15 ac-ft
100-year Peak Discharge = 0.49 cfs
2-year WSEL = 5104.04
2-year Volume = 0.049 ac-ft
1 2-year Peak Discharge = 0.25 cfs
NOTE: UDSWMM output is to the nearest cfs and tenth of an ac-ft. The
volumes and discharges shown are the highest possible derived
from that output.
Runoff basin 80 will flow through a swale directly into the drainageway. The swale will have
4:1 side slopes and be lined with grass. Since the swale slope is less than 2.0 percent, a
perforated underdrain will be placed 6 inches below the surface and along the swale
centerline. The underdrain pipe will improve drainage of low flows and standing water.
The drainageway's left bank will be improved with side slopes ranging between 8:1 and 4:1.
With the exception of approximately the first 0.5 foot of channel depth, most of the right
bank will remain unimproved. The existing retention pond will be relocated slightly farther to
the west and the area cleaned up. The retention area will be placed as part of the
drainageway. The existing drainageway makes a 90 degree left bend, runs for
approximately 85 feet then makes another 90 degree right bend. These bends have
historically created flooding of the adjacent residential lots in Mountaire Subdivision. The
proposed drainageway will follow the same course, but will be improved to contain runoff
within the channel banks and to eliminate flooding of these lots. Along the parking lot, the
channel will be contained within in (2) 2.5 foot high retaining walls.
EROSION CONTROL PLAN
Erosion from the site can be classified into erosion caused by wind and rainfall. Since
construction will occur during the winter and spring months, effective wind erosion plans
must be established. On moderate erodibility soil, significant erosion could occur without
steps taken to protect the soil. Those portions of the site not covered with pavement,
concrete, or buildings and not seeded within a month of final grading will have 2 tons/acre of
straw mulch applied for water erosion control, which will also prevent wind erosion. All
mulches will be properly anchored.
The effects of rainfall erosion will be eliminated by the use of several erosion control
techniques to prevent sediment from leaving the site. A silt fence, which will prevent
sediment from flowing off the property will be placed along the drainageway. A silt fence will
also be placed along the west property line. A straw bale inlet filter will be placed at the
detention pond outlet until construction is complete.
9 Ayres Associates
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Ayres Associates
Table 1. Subbasin Constants.
Parameter
Value
Manning's, n for impervious surfaces
0.016
Manning's n for pervious surfaces
0.25
Surface retention storage for impervious surfaces
0.1 in.
Surface retention storage for pervious surfaces
0.3 in.
Maximum infiltration rate
0.51 in/hr
Minimum infiltration rate
0.50 in/hr
Decay rate
0.0018
Storm duration
3 hours
Time step
1 minute
Table 2. Basin Parameters.
Subbasin
Connected
To:
Width
(ft)
Area
(ac)
Percent
Impervious
Slope
I(ft/ft)
80
180
157
0.573
26.0
0.010
81
179
136
0.782
64.0
0.005
82
179
121
0.836
21.0
0.020
0
7 Ayres Associates
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6 Ayres Associates
During the 2-year storm event, the peak runoff in the drainageway leaving the site is 5 cfs
with the Scenic Views development and 12 cfs without the Scenic Views development
(flows shown for element 61). SWMM output is shown in the Appendix.
PROPOSED DRAINAGE CONDITIONS
A detailed site, grading, and drainage plan (enclosed in pocket) shows the building layout
and storm drainage facilities that will be used for the Waldorf School site. The map uses a
1-foot contour interval. A schematic of the proposed conditions SWMM network is shown in
Figure 4.
' The developed site will consist of 3 subbasins that contribute drainage to a detention
pond and the drainageway. SWMM, which was used for determining the existing condition
runoff, was used to determine the proposed condition 2- and 100-year peak discharges and
' detention pond volumes. Subbasin constants are shown in Table 1, basin parameters in
Table 2, and conveyance elements and runoff in Table 3.
I Subbasins 81 and 82 contribute drainage to a detention area labeled 179. Curb and gutter
and overland flow will convey the runoff to the pond outlet structure. The pond will release
runoff into a 15-inch diameter ADS pipe which will daylight in the drainageway.
Subbasin 80 will contribute undetained runoff directly into the drainageway. Although
improvements will be made to the drainageway itself, offsite conditions will not change. A
I comparison of SWMM results between the Master Plan fully developed model, the existing
conditions model, and the proposed conditions model is shown in Table 4.
STORM DRAINAGE FACILITIES
A major portion of runoff flows off of the parking lot through a curb cut and into the detention
pond. Other areas drain directly to the detention pond. The detention pond, element 179,
was sized to contain the 100-year runoff from subbasins 81 and 82.
The site's storm drainage facilities consist of concrete pans, a detention pond, Runoff will be
released at a controlled rate so that the total runoff leaving the site in the drainageway is
equal to or below both the historical flows and the Master Plan fully developed flows.
The detention pond's outlet structure will consist of a 15-inch diameter ADS pipe with a 4- '
inch diameter orifice. In the event the orifice becomes clogged, water will flow over an
emergency spillway into the "drainageway. The spillway will be designed as part of final
design. For a 3-hour, 100-year storm, the detention pond must detain 0.15 acre-foot (ac-ft)
of water. The 100-year water surface elevation (WSEL) is 5104.34, corresponding to a
maximum depth at the outlet of 0.84 feet. For a 3-hour, 2-year storm, the detention pond
must detain 0.05 ac-ft of water. The 2-year WSEL is 5104.04, corresponding to a maximum
depth at the outlet of 0.54 feet. The detention pond's outlet rating curve is shown in Table
5.
5 Ayres Associates
CSU EQUESTRIAN
CENTER
N
PLEASANT
VALLEY AND
LAKE CANAL
12
1
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PROPOSED
,DE TENTIO >,1
',POND ��
'PROPOSED-`
CENIC VIEWS
SUBDIVISION
STREET
SUBCATCHMENT BASIN BOUNDARY
-------------
CHANNEL OR GUTTER
11
BASIN LABEL
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CONVEYANCE ELEMENT
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DETENTION POND
CONCENTRATION POINT
RIVER SONG
WALDORF SCHOOL
- PROPOSED SITE
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Figure 3. Overall West Plum Street Basin.
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Ayres Associates
VICINITY MAP
Figure 1. Vicinity map.
2 Ayres Associates
I INTRODUCTION
This report summarizes the storm drainage investigations performed for the proposed River
Song Waldorf School. A hydrologic analysis of the proposed development was completed
to determine the magnitude and location of storm runoff, allowable release rates into an
existing drainageway, develop a detention pond, and to determine downstream effects of
the proposed development. This study and drainage plan was prepared to meet the City of
Fort Collins Storm Drainage Design Criteria for the development.
The River Song Waldorf School property is located along the south side of Orchard Place
adjacent to the intersection with Orchard Place and Louise Lane as shown in Figure 1. The
' site is a 2.4 acre lot created by subdividing Happy Heart Farm and occupies part of the
northwest quarter of Section 16, Township 7 North, Range 69 West, of the 6th Principal
Meridian.
PREVIOUS STUDIES
Previous drainage studies of the area include (1) West Plum Street Basin Master Drainage
Plan (RCI 1990), and (2) Update of Hydrologic Analysis for West Plum Street Basin (Ayres
Associates 1995). These were reviewed to assist with the preparation of this report.
EXISTING DRAINAGE CONDITIONS
The River Song Waldorf School lies within the West Plum Street Basin. Orchard Place and
Louise Lane lie to the north, Happy Heart Farm to the south, Mountaire Subdivision to the
east, and unplatted natural land to the west. A major. drainageway runs -eastward across
the property from the unplatted land to the Mountaire Subdivision. The drainageway
collects all existing runoff from the area and is unimproved. There is an existing retention
pond near the southeast corner of the property. Although the pond becomes submerged
and overtopped during the 100-year storm, it is not of sufficient size to be included as a
detention area. J
Existing runoff calculations were performed to determine the 2- and 100-year runoffs for the
site using the Storm Water Management Model (SWMM, UDFCD 1985). A schematic
showing the linkages of the subbasins and conveyance elements for the existing conditions
is shown in Figure 2. The overall basin is shown in Figure 3. Both figures contain
information from the West Plum Street Master Drainage Plan SWMM model. The
hydrologic analysis used the Master Plan SWMM model with those subbasins representing
the Waldorf School area altered to reflect existing conditions.
Runoff leaves the site only through the drainageway. Although the Master Plan contains a
large detention pond as part of the proposed Scenic Views development, it is our
understanding that the pond has not been approved or constructed at this time. Existing
condition SWMM models were developed with and without the Scenic Views development
and detention pond in order to determine the highest flow condition through the site. For
the 100-year storm, results indicate 103 cfs with the Scenic Views development and 89 cfs
without the Scenic Views development (flows shown for element 61).
Ayres Associates
TABLE OF CONTENTS
INTRODUCTION..................................................................................................................1
PREVIOUSSTUDIES...........................................................................................................1
EXISTING DRAINAGE CONDITIONS..................................................................................1
PROPOSED DRAINAGE CONDITIONS...............................................................................5
STORM DRAINAGE FACILITIES.........................................................................................5
EROSIONCONTROL PLAN.................................................................................................9
LIST OF FIGURES
Figure1. Vicinity map.....................................................................................:....................2
Figure 2. Existing conditions and master plan SWMM network............................................3
Figure 3. Overall West Plum Street Basin............................................................................4
Figure 4. Proposed conditions SWMM network...................................................................6
Figure 5. Construction schedule.........................................................................................11
LIST OF TABLES
Table1. Subbasin Constants...............................................................................................7
Table2. Basin Parameters...................................................................................................7
Table 3. Conveyance Elements and Peak Discharges.........................................................8
Table 4. SWMM Results Comparison..................................................................................8
Table 5. Detention Pond 179 - Outlet Rating Curve.............................................................9
i Ayres Associates
/SIRES
ASSOCIATES
February 10, 1998
Mr. Basil Hamdan
City of Fort Collins
Stormwater Utility
235 Mathews Street
Fort Collins, Colorado 80524
Re: River Song. Waldorf School
Dear Basil,
Enclosed please find our "Preliminary Drainage and Erosion Control Report For River Song
Waldorf School." This report was prepared in accordance with the City of Fort Collins Storm
Drainage Criteria as revised in January 1997.
We appreciate your time and consideration in reviewing this submittal. Please call if you have
questions.
Sincerely,
Owen Ayres & Associates, Inc.
�� CI
Chris Carlson, E.I.T.
Civil Engineer
r
Lyle W. Zevenbergen, Ph.D., P.E.
Hydraulic Engineer
CC/LWZ/sp
Enclosure
u
C Owen Ayres & Associates, Inc.
Engineers/Scientists/Surveyors HAMD-2A.LTR
3665 JFK Parkway, Building 2, Suite 300, P.O. Box 270460, Fort Collins, CO 80527 34-0615.01
(970) 223-5556, Denver Metro (303) 572-1806, FAX (970) 223.5578
Printed on recycled paper
PRELIMINARY DRAINAGE AND EROSION CONTROL REPORT
FOR
RIVER SONG WALDORF SCHOOL
Prepared for
River Song Waldorf School
Fort Collins, Colorado
AV ■ as
ASSOCIATES
P.O. Box 270460 -
Fort Collins, Colorado 80527
(970) 223-5556, FAX (970) 223-5578
Ayres Project No. 34-0615.01
WALDORF2.TXT
February 1998