HomeMy WebLinkAboutMONTAVA - PHASE G & IRRIGATION POND - BDR210013 - MONTAVA SUBMITTAL DOCUMENTS - ROUND 4 - LEGAL DOCUMENTSJuly 1
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October 8, 2018
Max Moss
HF2M Colorado
430 N College Ave. Suite 410
Fort Collins, CO 80524
Re: Montava Poudre School District Site
Martin/Martin, Inc. Project No.: 17.0946
The purpose of this letter is to quantify the required size of a stormwater detention and water quality pond for
the Montava Poudre School District (PSD) site.
This pond sizing exercise was performed assuming the PSD site is 118 acres and has an imperviousness of 55%
per Urban Drainage Criteria. The EPA SWMM version 5.1 modelling program was used in accordance with the City of
Fort Collins Stormwater Criteria Manual. The program established the allowable release of the stormwater pond which
is limited to the runoff from the site under existing conditions assumed to be undeveloped site at 10% impervious
during the 2 year storm event. The program then established the proposed conditions assumed to be a developed site
at 55% impervious during the 100 year storm event. The required pond volume for detention is the volume required to
attenuate the 100 year developed flows from the site while restricting the release from the pond to the undeveloped 2
year flow. The detention volume of the pond was determined to be 17.2 acre feet and a summary is attached to this
letter as Exhibit A.
Per COFC stormwater criteria the water quality capture volume (WQCV) is to be added to the detention
volume. The WQCV for the PSD site was determined to be 2.2 acre feet and is attached to this letter as Exhibit B.
The total pond volume for the PSD site is 19.4 acre feet.
Martin/Martin performed a grading exercise to accommodate a pond of 19.4 acre feet on the PSD site and is
attached to this letter as Exhibit C. The total volume of the designed pond is calculated using the an excel spreadsheet
calculating the volume using the prismoidal method and is attached to this letter as Exhibit D. The volume provided in
the design is slightly higher than the required volume and accounts for approximately 5% construction tolerance. The
required size of the pond would be 640’ wide by 640’ long and would cover 10 acres in area.
Sincerely,
Peter S. Buckley, P.E.
Associate
Attachments: Exhibits A, B, C & D.
EPA SWMM Modeling Basin L (School Parcel)
Detention Pond Output
EXHIBIT A
PROJECT INFORMATION
PROJECT NAME:
PROJECT #:
POND NAME:
DATE:
Required Water Quality Volume:
Detention Sizing Method: WQCV
NRCS Hydrologic Soil Group: C & D
*Figure 3-1, UDFCD (V.3), Chapter 3, Page 3-5
*Equations 12-1, 12-2, 12-3, UDFCD (V.2), Chapter 12, Page 12-4
Where:
WQCV = Water Quality Capture Volume (Watershed Inches)
a = Constant Dependent on Drain Time (Typically a=1.0 40-Hr Drain Time)
i = Percent Imperviousness
i = 55.0%
WQCV = 0.220 (watershed inches)
*Equation 3-3, UDFCD (V.3), Chapter 3, Page 3-6
Where:
WQCV = Water Quality Capture Volume (Watershed Inches)
Area = Contributing Watershed Area (Acres)
Area = 118.00 (acres)
Required Storage = 2.1675 (ac-ft)
MONTAVA
17.0946
School Pond
10/4/2018
( )
´=
43.0
5.0 WQCVLEED WQCV
( )AREAEURV´
=
12
Storage Required
()iiiaWQCV78.019.191.0 23 +-´=
( )AREAWQCV´
=
12
Storage Required
08.1
/
08.1
28.1
20.1
36.1
68.1
iEURV
iEURV
iEURV
DC
B
A
=
=
=
10/8/2018 8:51 AM
WQCV
G:\SCHLAGETER\17.0946-Anheuser-Busch Mix-Used Development\ENG\DRAINAGE\School Site Pond WQCV.xlsx
Required pond = 17.2 ac-ft + 2.2 ac-ft = 19.4 ac-ft
EXHIBIT B
EXHIBIT C
REGIONAL POND
PSD SITE
PROJECT INFORMATION
PROJECT NAME:Montava
PROJECT #:17.0946
POND NAME:PSD Prelim Onsite Pond
DATE:10/08/18
Required Water Quality Volume = (ft³)
(ac-ft)
Water Quality Surface Elevation =
Required 5-Year Volume = (ft³)
(ac-ft)
5-Year Water Surface Elevation =
Required 10-Year Volume = (ft³)
(ac-ft)
10-Year Water Surface Elevation =
Required 100-Year Volume = (ft³)
(ac-ft)
100-Year Water Surface Elevation =
Total Pond Volume =894747 (ft³)
20.541 (ac-ft)
Incremental Volume=
Contour
Elevation
Contour Area
(ft²)Cumulative Volume (ft³)Cumulative Volume
(acre-ft)
4983.00 0.0 0 0.00
4984.00 59536.0 19845 0.46
4985.00 121104.0 108363 2.49
4986.00 204304.0 269264 6.18
4987.00 309136.0 524181 12.03
4988.00 435600.0 894747 20.54
160901
254917
370565
Volume
(ft³)
0
19845
88517
10/8/2018 5:32 PM
Pond_WSEL
\\mmcivil\civil\SCHLAGETER\17.0946-Anheuser-Busch Mix-Used Development\SENT\2018.10.04 School Pond\PSD Prelim Onsite Pond.xlsm
EXHIBIT D
June 20