HomeMy WebLinkAboutDrainage Reports - 03/21/2011City of Ft. Collins proved Plans
App ved Bye/iacc_
February 17, 2010
Mr. Wes Lamarque
City of Fort Collins Stormwater Department
700 Wood Street
Fort Collins, CO 80521
RE: Registry Ridge Park
Registry Ridge P.U.D. Fifth Filing - Drainage Addendum
ICG Project No.1096-012-27
Dear Wes,
We are pleased to submit to you for your review and approval this letter and attachments as an
addendum to the Final Drainage Report for Registry Ridge Fifth Filing dated October 25, 2007
(an addendum to the original Registry Ridge First Filing Report dated June 14, 1999). This
addendum is for the proposed Registry Ridge Park, Tract H of Registry Ridge P.U.D. First
Filing.
In the First Filing Report, the proposed park area was included in sub -basin 403H. It was one of
five sub -basins contributing to design point 20. The Fifth Filing Report broke up sub -basin
403H and 4031 into multiple basins for the development of their property. The Fifth Filing
renamed the proposed park area sub -basin OS-2. For the purpose of this project, the drainage
basin map includes the new sub -basins proposed by the Fifth Filing Report. The park has been
divided into four sub -basins, all of which follow the original drainage concept and flows to
design point 20. Please refer to the attached Drainage Basin Map and the section below ,for these
sub -basin descriptions.
The First Filing Report created a composite runoff coefficient for all basins draining to design
point 20. That composite runoff coefficient was C=0.49 (C100=0.63). The 100-year flow to
design point 20 was originally 70.4 cfs. Approximately the same area is still draining toward
design point 20 following the development of the park and the Fifth Filing. Per the Fifth Filing
Report, "the capacity needed for inlets at historic Design Point 10 and 18 will be decreased due
to 7% of historic Basin 402D and 31 % of historic Basin 403G begin redirected to the proposed
site by form of overland and gutter flow". A new composite runoff coefficient has been
calculated using the new sub -basin information. The proposed composite runoff coefficient is
C=0.46 (C1004.58). The 100-year flow to design point 20 is proportionally less than the original
because of the reduced runoff coefficient.
Since the new composite runoff coefficient is lower than the original from the First Filing
Report, all existing conveyance systems and detention facilities are adequate. There are two
proposed culverts that were part of the Fifth Filing design (not yet constructed) that need to be
constructed with the park to allow for conveyance. These culverts are labeled "Storm Line 2"
and "Storm Line 3" and follow the original design of the Fifth Filing. (Note that "Storm Line 2"
will need to be extended and connected to future inlets with the development of the Fifth Filing.
The outlet invert and slope are per the Fifth Filing design).
As previously mentioned, all proposed drainage basins will be collected at design point 20. As
the drainage leaves the site, water quality will be achieved in the existing channels. The Fifth
Filing Report states: "Through a network of established swales, water quality will be achieved
through 931 feet of an existing grassed swale. The first water quality treatment will occur after
exiting the proposed site into a small pond lined with maintained landscape. The remainder of
the water quality shall occur through an 805 foot long channel heavily vegetated with high
grasses. " The addition of the park does not change the intent or effectiveness of the approved
water quality method from the Fifth Filing.
Sub -Basin Descriptions:
Sub -Basin A consists of mostly turf area and a small portion of concrete sidewalk. Runoff will
flow to the north where it will be intercepted by the existing curb and gutter in Ranger Drive. It
will then follow the curb and gutter to existing inlet 18, where it will be discharged into swale 18
and ultimately conveyed to design point 20.
Sub -Basin B consists of the majority of the park improvements including the restrooms, shelter,
patio space, a tennis court and basketball court, playground area, soccer field, and concrete
sidewalk. All runoff from the playground area will enter an underdrain system conveyed by
Storm Line 1 that discharges into swale 18. All other runoff will flow overland to the south until
it is intercepted by swale 18 which ultimately conveys all flow to design point 20.
Sub -Basin C is a small area of turf that slopes toward Ranger Drive. Runoff will be intercepted
by the existing curb and gutter and follow the same drainage pattern as Sub -Basin A.
Sub -Basin D consists of mostly turf area and a small portion of concrete sidewalk. Runoff will
flow northeast toward Leahy Drive, a future road to be constructed with the Fifth Filing. Until
the Fifth Filing is developed, this runoff will be conveyed by a temporary swale to Storm Line 2
and ultimately swale 20 which is conveyed to design point 20 as well.
Off -site 2 (which originally included the entire park site in the Fifth Filling) has been reduced to
the back lots form the First Filing development just south of the park site. It is assumed that all
this area is pervious. Runoff flows to the north where it is intercepted by swale 18 which
ultimately conveys all flow to design point 20.
Erosion Control:
There will be multiple erosion control devices implemented within the site. Wattles will be
implemented every 100 feet along Swale 18 and 20 to prevent sediment accumulation at all
existing culverts and Design Point 20. Silt fence will be installed and maintained off the back of
existing walk of Ranger Drive to prevent sediment from going into the existing street. A
construction entrance and concrete washout will also be installed to prevent sediment from
entering the existing streets. The contractor will be required to obtain a CDPHE Construction
permit and create and maintain the permit and Stormwater Management Plan.
In conclusion, the proposed park improvements fall within the design assumptions used in the
First Filing report and follow the same drainage patterns to Design Point 20. The site is
ultimately routed into Pond 603 through the use of existing drainage facilities. The proposed
composite runoff coefficients are reduced from the original design, water quality is addressed by
the existing drainage facilities, and proper erosion control devices shall be installed and
maintained.
We appreciate your time and consideration in reviewing this submittal. Please call if you have
any questions.
Sincerely,
Sk lar Brower, EIT
Reviewed By
Michael Oberlander, PE
Colorado Professional Engineer No. 34288
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RUNOFF COEFFICIENTS AND COMPOSITE COEFFICIENT
LOCATION: Registry Ridge Park
PROJECT NO: 1096-012-27
COMPUTATIONS BY: SMB
DATE: 2/22/2010
FIRST FILING COMPOSITE RUNOFF
COEFFICIENT
Basin
C
C100
Area (AC)
403A
0.6
0.75
4.213
403G
0.74
0.93
1.116
403H
0.45
0.56
11.612
4031
0.45
0.56
3.966
403 AB
0.25
0.31
0.563
Total
21.47
Comp. C
0.49
0.61
PROPOSED COMPOSITE RUNOFF
COEFFICIENT INCLUDING C VALUES FROM
THE FIFTH FILING
Basin
C
C100
Area (AC)
403A
0.6
0.75
4.213
403 AB
0.25
0.31
0.563
H
0.74
0.93
0.77
A
0.31
0.39
0.13
B
0.33
0.41
5.19
C
0.25
0.31
0.2
D
0.3
0.37
0.4
5A
0.34
0.43
1.23
56
0.72
0.9
1.59
5C
0.53
0.66
4.36
5D
0.39
0.49
0.67
OS1
0.25
0.31
0.98
OS2
0.25
0.31
1.43
Total
21.73
Comp. C
0.46
0.57
Note: Per the Fifth Filing Report, 7% of basin 402D (0.25 AC) is now part of sub -basin 5C. This accounts for
the total area descrepency.
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