HomeMy WebLinkAboutWATERFIELD PUD - PRELIMINARY - 7-95B - CORRESPONDENCE - CITY STAFF65.00
2.47
10.86
0.25
10.61
70.00
2.35
11.10
0.27
10.84
75.00
2.22
11.26
028
10.98
80.00
2.10
11.34
0.30
11.03
85.00
2.00
11.47
0.32
11.15
90.00
1.90
11.54
0.34
I1.20
95.00
1.80
11.54
0.36
11.18
100.00
1.70
11.47
0.38
11.09
105.00
1.65
11.69
0.40
11.29
110.00
1.60
11.88
0.42
11.46
115.00
1.55
12.03
0.44
11.59
120.00
1.50
12.15
0.45
11.69
125.00
1.45
12.23
0.47
11.76
130.00
1.40
12.28
0.49
11.79
135.00
1.35
12.30
0.51
11.79
140.00
1.30
12.28
0.53
11.75
145.00
1.25
12.23
0.55
11.68
150.00
1.20
12.15
0.57
11.58
155.00
1.17
12.20
0.59
11.62
160.00
1.13
12.24
0.61
11.63
THE REQUIRED POND SIZE = 11.78894 ACRE -FT
THE RAINFALL DURATION FOR THE ABOVE POND STORAGE= 130 MINUTES
75.00
2.22
10.70
0.47
10.23
80.00
2.10
10.77
0.50
10.27
85.00
2.00
10.90
0.53
10.37
90.00
1.90
10.96
0.56
10.40
95.00
1.80
10.96
0.59
10.37
100.00
1.70
10.90
0.62
10.27
105.00
1.65
11.11
0.66
10.45
110.00
1.60
11.28
0.69
10.60
115.00
1.55
11.43
0.72
10.71
120.00
1.50
11.54
0.75
10.79
125.00
1.45
11.62
0.78
10.84
130.00
1.40
11.67
0.81
10.86
135.00
1.35
11.68
0.84
10.84
140.00
1.30
11.67
0.87
10.79
145.00
1.25
11.62
0.90
10.71
150.00
1.20
11.54
0.94
10.60
155.00
1.17
11.59
0.97
10.63
160.00
1.13
11.62
1.00
10.63
THE REQUIRED POND SIZE = 10.85633 ACRE -FT
THE RAINFALL
DURATION
FOR THE ABOVE POND STORAGE= 130 MINUTES
DETENTION POND SIZING BY FAA METHOD
Developed by
Dr. James Guo, Civil Eng. Dept., U. of Colorado
Supported by Denver Metro Cities/Counties Pool Fund Study
Denver Urban Drainage and Flood Control District, Colorado
EXECUTED ON 04-08-1997 AT TIME l 1:28:04
PROJECT TITLE: TYPE IN THE PROJECT TITLE
**** DRAINAGE BASIN DESCRIPTION
BASIN ID NUMBER = 2.00
BASIN AREA (acre)= 65.01
RUNOFF COEF = 0.71
***** DESIGN RAINFALL STATISTICS
DESIGN RETURN PERIOD (YEARS) = 100.00
INTENSITY(IN/RR)-DURATION(MIN) TABLE IS GIVEN
DURATION 5 10 20 30 40 50 60 80 100 120 150 190
INTENSITY 9.0 7.3 5.2 4.2 3.5 3.0 2.6 2.1 1.7 1.5 1.2 1.0
***** POND OUTFLOW CHARACTERISTICS:
MAXIMUM ALLOWABLE RELEASE RATE = 4.53 CFS
OUTFLOW ADJUSTMENT FACTOR = 1
AVERAGE RELEASE RATE = 4.53 CFS
AVERAGE RELEASE RATE = MAXIMUM RELEASE RATE * ADJUSTMENT FACTOR.
***** COMPUTATION OF POND SIZE
RAINFALL RAINFALL INFLOW OUTFLOW REQUIRED
DURATION INTENSITY VOLUME VOLUME STORAGE
MINUTE INCH/HR ACRE -FT ACRE -FT ACRE -FT
0.00
0.00
0.00
0.00
0.00
5.00
9.00
2.88
0.03
2.85
10.00
7.30
4.68
0.06
4.62
15.00
6.25
6.01
0.09
5.92
20.00
5.20
6.67
0.12
6.54
25.00
4.70
7.53
0.16
7.38
30.00
4.20
8.08
0,19
7.89
35.00
3.85
8.64
0.22
8.42
40.00
3.50
8.97
0.25
8.73
45.00
3.25
9.38
0.28
9.09
50.00
3.00
9.62
0.31
9.30
55.00
2.80
9.87
0.34
9.53
60.00
2.60
10.00
0.37
9.63
65.00
2.47
10.31
0.41
9.91
70.00
2.35
10.55
0.44
10.11
DETENTION POND SIZING BY FAA METHOD
Developed by
Dr. James Guo, Civil Eng. Dept., U. of Colorado
Supported by Denver Metro Cities/Counties Pool Fund Study
Denver Urban Drainage and Flood Control District, Colorado
.............
EXECUTED ON 04-09-1997 AT TIME 07:52:26
PROJECT TITLE: WATERFIELD
**** DRAINAGE BASIN DESCRIPTION
BASIN ID NUMBER = 1.00
BASIN AREA (acre)= 77.12
RUNOFF COEF = 0.63
***** DESIGN RAINFALL STATISTICS
DESIGN RETURN PERIOD (YEARS) = 100.00
INTENSITY(IN/HR)-DURATION(MIN) TABLE IS GIVEN
DURATION 5 10 20 30 40 50 60 80 100 120 150 180
INTENSITY 9.0 7.3 5.2 4.2 3.5 3.0 2.6 2.1 1.7 1.5 1.2 1.0
***** POND OUTFLOW CHARACTERISTICS:
MAXIMUM ALLOWABLE RELEASE RATE = 2.75 CFS
OUTFLOW ADJUSTMENT FACTOR = 1
AVERAGE RELEASE RATE = 2.75 CFS
AVERAGE RELEASE RATE = MAXIMUM RELEASE RATE * ADJUSTMENT FACTOR.
***** COMPUTATION OF POND SIZE
RAINFALL RAINFALL INFLOW OUTFLOW REQUIRED
DURATION INTENSITY VOLUME VOLUME STORAGE
MINUTE INCH/HR ACRE -FT ACRE -FT ACRE -FT
0.00
0.00
0.00
0.00
0.00
5.00
9.00
3.04
0.02
3.02
10.00
7.30
4.93
0.04
4.89
15.00
6.25
6.33
0.06
6.27
20.00
5.20
7.02
0.08
6.94
25.00
4.70
7.93
0.09
7.83
30.00
4.20
8.50
0.11
8.39
35.00
3.85
9.09
0.13
8.96
40.00
3.50
9.45
0.15
9.30
45.00
3.25
9.87
0.17
9.70
50.00
3.00
10.12
0.19
9.93
55.00
2.80
10.39
0.21
10.18
60.00
2.60
10.53
0.23
10.30
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We have also reviewed comments contained in the report and on the preliminary Utility plans and
can address these comments at time of final design.
Thank you for your time and consideration. Please feel free to call if you have any questions
regarding this letter.
Sincerely,
Roger Curtiss P.E. - Northern Engineering Services, Inc.
cc: Jim McCory - Colorado Land Source
Mike Ludwig - City of Fort Collins
could provide approximately 11.2 acre-feet of storage, based on an outlet at elevation of
approximately 4940, and a 100 year high water surface elevation of 4945.
In our original design the wetlands is being utilized for some detention storage. In our original
calculations, we assumed a high water surface elevation of 4952, with a pond outlet of 4950. This
resulted in storage volume of approximately 16.58 acre-feet. We used a release rate of 12.75 cfs,
and calculated a required storage volume of 7.62 acre-feet. As we discussed, we could utilize
more of the available storage in this wetlands area by reducing the outflow. Attached are revised
pond calculations showing a reduced outflow of 2.75 cfs, resulting in a required detention storage
needed to 11.79 cfs, still well below the existing capacity. Since the runoff from this pond is not
directly released into the Lake Canal, but into detention pond 2, this should not directly reduce
the amount of permissible release from the site, based on the 2 year historic rate of Basin H4.
At time of final design, a SWMM model will be developed for this site to account for routing, and
differences in detention volumes. The excess volumes shown in both ponds should account for
any small additional requirements that a SWMM model might reveal.
Item No. 4 - Historic values were recalculated for Basin H-4 using a runoff coefficient of 0.20.
Item No. 5 - There is only one outlet to the Lake Canal, and it is being sized for the historic rate.
There is a small amount of developed runoff from Vine Drive which due to grade cannot be
routed through detention pond 2. At time of final design the peaks from both the pond and
undetained portion of Vine Drive will be analyzed to insure that the peak discharge from the site
does not exceed the agreed to historic rates.
Item No. 6 - We are working with the Ditch Company, and will get a letter of intent as soon as
possible.
Item No. 7 - Again we have made contacts with the affected parties, and will endeavor to obtain
letters of intent as soon as possible.
Item No. 8 - These will be provided at time of final design.
Item No. 9 - The original runoff calculations have the frequency adjustment factor already
included. The attached detention calculations have been modified to include the adjustment factor.
Item No. 10 - We will coordinate design of drainage conveyance with the landscape consultant.
Item No. 11 - OK
April 9, 1997
Mr. Basil Harridan
City of Fort Collins
Utility Services Stormwater
235 Mathews
Fort Collins, Colorado 80522
RE: Waterfield PUD Preliminary
Stormwater comments dated January 22, 1997
Dear Basil,
This letter is in response to Stormwater comments regarding project #7-95B Waterfield PUD
Preliminary dated January 22, 1997. Our response to your comments is also based on our meeting
of April 3, 1997.
Item No. I & 2 - Our original design included routing 100 cfs from upstream through the
Waterfield site. This was based on the Cooper Slough Master plan. We will now design to
accommodate the historic 100 year flows through our site. Attached are preliminary calculations
for this upstream basin, and a copy of the USGS map which was used to determine the
approximate boundaries for this basin. We found the historic flows to be approximately 27.5 cfs.
Since our original design was for 100 cfs, this should not be a problem.
The preliminary intent is to design an interception swale just to the south of the Irrigation ditch to
intercept any overtopping that may occur. The upstream overflow can then be routed through the
Waterfield site. The purpose of the interception swale is that it may be difficult to isolate where
overtopping may occur.
Item No. 3 - Generally, it is assumed that the Wetland area would eventually overtop. At that
point, then basins H1-H4 would be historically drain to Lake canal. We are in the process of
preparing an analysis of how this wetlands is fed, and it's relationship to the water surface
elevation of the Larimer and Weld Canal. From our initial findings, the water surface elevation in
the wetlands is directly related to the water surface in the ditch. Therefore, if the Larimer and
Weld Canal was assumed full, it is highly unlikely that the wetlands would completely retain
runoff from basins H1-H3. This will be investigated further at time of final design.
For the purposes of this preliminary however, we assumed that basins H 1-H3 are completely
retained in the wetlands area. Attached are calculations showing what the 2 year historic rate is
for Basin H4 only. This also uses a historic runoff coefficient of 0.20. The historic rate for basin
H-4 was found to be approximately 4.54 cfs. This rate was then used to recalculate the required
on -site detention. The resulting detention requirement was found to be approximately 10.86 acre-
feet (Calculations for this is all attached). Attached also is a preliminary design of a pond which
420 SOUTH HOWES, SUITE 202, FORT (OLLINS, (OLORADO 80521, (970) 221-4158, FAX (970) 22 1.4 159