HomeMy WebLinkAboutDrainage Reports - 02/16/2012December 15, 2011
Mr. Wes Lamarque
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, CO 80522
Re: Ram Crossing K2 — Storm Drainage Report
Dear Wes,
JgF;1
INTERWEST I CONSULTING G R O U P
City of Ft. Collin ppro d Plans
Approved By—
Date -I -
We are pleased to submit for your review and approval this drainage letter showing the drainage
impacts from the proposed expansion and improvements to the Ram Crossing K2. City
comments dated November 29, 2011 have been addressed in this letter.
The project is located at 914 West Lake Street and is a part of Lots 12 and 13 of College Heights
Addition. The site currently has an existing apartment complex and large asphalt parking lot on
the site. The project will expand the building, reconfigure and significantly reduce the parking
area, and provide for stormwater quality improvements. The area of the site that will be
improved is approximately one acre.
The site is surrounded by Colorado State University campus and is bounded on the south by
Lake Street. There are no offsite flows entering the site. Today, the entire site sheet flows north
and east to the northeast corner of the site and then overland through CSU to Pitkin Street and
eventually outfalls into Spring Creek. After conversations with CSU there is no outfall adjacent
to the site other than the existing location. All of the CSU storm sewer and ponds near the site
are at capacity. Please refer to the attached Existing Conditions SWMM Schematic for reference
to the outfall. With the building addition and reconfiguration, all water will still flow to the
north and east, but around the new building in swales and in a small storm pipe system
(lengthening several of the existing flow paths). Please refer to the attached erosion control plan.
The site is required to provide water quality treatment, but there is no adjacent storm sewer
available near the site and it must flow overland in the northeast corner. Because of the lack of
an adjacent, below grade outfall, it is not feasible to install a standard water quality pond.
Today, there is no treatment as stormwater leaves the asphalt parking and the site (the asphalt is
on the existing property lines). In an effort to improve the stormwater quality, swales have been
added on the west and north sides of the site. The swale on the west is at slightly more than 2%
and the swale on the north is consistently at 1 %. We propose that neither swale have a trickle
pan in an effort to encourage infiltration. The swale on the north is easily maintained and over
1218 WEST ASH, SUITE C WINDSOR, COLORADO 80550
TEL. 970.674.3300 • FAX. 970.674.3303
Mr. Wes Lamarque
December 15, 2011
Page 2 of 2
20 feet from the proposed building, so we are not concerned about encouraging infiltration here.
At the northeast corner of the site, we propose a modified "sand filter" with a 10'x16' area being
lowered approximately 6" below the existing grade with cobble and granular material. A grade
beam placed across the filter will force the water through the cobble and sand layers. Please refer
to the attached detail. Again, there is not an opportunity to install an underdrain, but smaller
storms will be slowed and allowed to infiltrate in this area. Once the filter area is overwhelmed,
the stormwater will overtop the sump and flow to the northeast as it does today.
The existing and proposed imperviousness of the lot was compared as a part of this study.
Exhibits are attached which show that the existing imperviousness is 78% while the proposed
improvements will lower the imperviousness to 75%. This is mostly due to the required 5' offset
on the north and east from the property line to the proposed parking. Because the impervious
area will decrease with this project, detention is not required. Attached is a summary of the
flows for the existing and the proposed site. With the reduction of impervious area by 3%, the
100-year flow off the site is reduced by 8%.
The proposed 12" PVC storm system is the only hydraulic system on the project. Our
calculations show a 100 year flow to this system of 4.8 cfs. The system is a series of small
sumps and overtops to the east and west prior to reaching the existing or proposed building
finished floors. We have verified that the system can convey the 100-year flow, with the
safeguard the overtopping occurs below the finished floors.
This area has previously been studied in a master drainage plan for the CSU campus by Ayers
Associates and is included in the Braiden Hall Basin. All possible measures have been taken to
provide water quality for the site through the proposed grass swale and sand filter and the
lengthening of flow paths from existing conditions. The proposed expansions and improvements
have been designed with provisions for safe and efficient control of stormwater runoff in a
manner that will not adversely affect the downstream detention system and will improve water
quality for the Ram Crossing K2 project. Overall, it is evident that the proposed revisions will
have a positive impact on the proposed stormwater management system.
We appreciate your time and consideration in reviewing this submittal. If you have any
questions or need additional information please feel free to call at any time (970) 674-3300 Ext.
102.
Sincerely,
Michael Oberlander, P.E.
Colorado Professional Engineer 34288
Interwest Consulting Group
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RAM CROSSING K2
PROPOSED IMPERVIOUS AREA
63,000 SF = 75 f
25' 0 50'
SCALE: 1"= 50'
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Calculation Results Summary
Scenario: Base
»» Info: Subsurface Network Rooted by: 0-1
»» Info: Subsurface Analysis iterations: 1
»» Info: Convergence was achieved.
CALCULATION SUMMARY FOR SURFACE NETWORKS
I Label
I Inlet
I Inlet I
Total
Type
I I
Intercepted
I
I I
Flow
I
I
I I
(cfs)
I -------I---------------
1 I-3
1 Generic Inlet
I--------------------- -I-------------
I Generic Default 100% 1
0.00
1 I-2
1 Generic Inlet
I Generic Default 100% 1
0.00
I I-1
--------------------------------------------------------------
I Generic Inlet
I Generic Default 200% 1
0.00
CALCULATION SUMMARY FOR SUBSURFACE NETWORK WITH ROOT: 0-1
Total
I Capture
I Gutter
I Gutter I
Bypassed
I Efficiency
I Spread
I Depth I
Flow
1 M
I (ft)
I (ft) I
(cfs)
I I
I
I --------
I
I --------I
----------
0.00
1------------
I 100.0
1 0.00
1 0.00 1
0.00
I 100.0 I
0.00 1
0.00 1
0.00
•------------------------------------------
1 100.0 I
0.00 1
0.00 1
I Label
I Number
I Section
I Section
I Length
I Total I
Average
I Hydraulic
I Hydraulic I
of
I Size
I Shape I
(ft)
I System I
Velocity
I Grade
I Grade I
I
I Sections
I
I
I
I Flow I
(ft/s)
I Upstream
I Downstream I
I
I
I
I
I
I (cfs) I
I (ft)
-----------
I (ft) 1
I ------------I
I-------I----------I---------I----------I--------I--------I----------
1 P-1
i 1
1 12 inch
I Circular 1
7.50
1 4.80 1
I
6.11 1
20.57
1 20.44 1
P-2
I 1
1 12 inch
I Circular 1
22.10
1 4.80 1
6.11
1 21.10
1 20.86 1
I P-3
I 1
1 12 inch
I Circular 1
38.70
1 4.80 1
6.11 1
21.80
1 21.39 1
P-4 I
1
1 12 inch I
Circular 1
42.50
1 3.20 1
4.07 1
22.30
1 22.09 1
I P-5 I
-----------------------------------------------------------------------------------------------
1
1 12 inch I
Circular 1
42.50 1
1.60 1
2.04 1
22.48
1 22.43 1
I Label I
Total I
Ground
I Hydraulic
I Hydraulic I
I
System I
Elevation
I Grade
I Grade I
I I
Flow I
(ft)
I Line In
I Line Out I
I I
(cfs) I
I (ft)
I (ft) I
I - ------I--------
1 0-1 I
I
4.80 I
-----------I-----------I-----------I
23.30
I 19.53
I 19.53 I
I J-1 I
4.80 I
23.20
1 20.86
1 20.57 1
1 1-2 1
4.80 I
23.00 1
21.39
1 21.10 1
I I-1 1
4.80 I
22.70
1 22.09
1 21.80 1
I-2 I
3.20 1
22.70 1
22.43
1 22.30 1
1 I-3 I
------------------------------------------------------
1.60 1
22.70 1
22.51 1
22.48 1
Completed: 11/02/2011 12:28:29 PM
Title: RAM K2 - Minor Storm Sewer Project Engineer. Interwest Consulting Group
x:l..\drainage\designVam k2 storrn.stm Interwest Consulting Group StormCAD v5.5 (5.50051
11/02/11 12:29:01 PM 0 Haestad Methods, Inc. 37 Brookside Road Waterbury, CT 06708 USA +1-203-755-1666 Page I of 1