HomeMy WebLinkAboutWATERS' EDGE - PDP - PDP160006 - REPORTS - DRAINAGE RELATED DOCUMENT (3)October 19, 2016
Mr. Wes Lamarque
City of Fort Collins
Stormwater Department
700 Wood Street
Fort Collins, CO 80525
RE: Drainage Memorandum – Waters Edge P.U.D.
Dear Wes:
The purpose of this drainage memorandum is to amend the Final Drainage and Erosion Control
Report for Water’s Edge dated April 9, 2010, by Aspen Engineering. A summary of the drainage
design modifications proposed with the updated plans for Water Edge P.U.D is presented below.
The 107 acre project consists of single family and multi-family dwelling units, and was designed and
approved for construction July 2010. The project was not constructed and expired in 2014. The
development is being resubmitted with slight modifications to specific areas of the site, as required
for new standard details and to update porous paver treatment for the site. The existing site remains
as fallow grassland, as it was under the approved drainage report. Therefore, the original, approved
Final Drainage Report still applies to the proposed development, while the addendum provides the
additional details required for LID improvements and additional drainage features implemented for
Pond 110.
The proposed development entails resizing some of the lots and moving alley locations in the
northeast corner of the site from that previously shown. However, the overall imperviousness of the
site remains nearly the same and in general, the modifications are not significant enough to justify
changing the rational calculations. A “streetlet” will be constructed south of Morningstar Way for
access to a row of lots at the south east corner of the site (north of Pond 110).
The grade down from the adjacent homes (Lots 1-7) will encroach on detention Pond 110 and
retaining walls along the north and west side of Pond 110 will be separated into two, terraced walls
to comply with the current City of Fort Collins design criteria. These changes result in a net
decrease in available volume of approximately 1.25 acre-feet from the original 26.16 acre-feet, of
the SWMM Model.
To compensate for the loss in volume the release rate will be increased from 1.5 cfs to 3.2 cfs. This
new release rate will be negotiated by the Owner with the Ditch Company. The additional flow
entering the existing storm sewer system will be checked to ensure adequate capacity is available.
This approach was discussed with and agreed to with Wes Lamarque and Shane Boyle. Updated
SWMM modeling for the Water’s Edge West project was run using the revised release rate to
calculate the new pond volume required for Pond 110.
It was agreed that the original report should remain intact with an addendum speaking to the
LID/porous paver treatment for the multi-family and pool areas, as well as documenting the 1.25
acre-feet of volume reduction in Pond 110, and presenting the updated release rate and updated
SWMM results. It was also agreed to that the Mod SWMM may continue to be used for this project.
Please refer to the following details below and the attachments for specific information.
Specific Details
The encroachment of Lots 1 through 7 along the north boundary of Pond 110 and the addition of a
second retaining wall to terrace the walls along the north and west sides of Pond 110, will result in a
loss of approximately 1.25acre-feet of volume from Pond 110, as determined using the updated
rating curve for Pond 110 versus the original, approved site SWMM model.
As discussed with the City of Fort Collins Storm Water Staff, the release rate may be increased if
found to be acceptable with the Ditch Company and the Mod SWMM revised to recalculate pond
volume required. As an alternative, the City would also accept remodeling of the entire site using
EPA SWMM which may result in a smaller pond volume requirement and possibly allowing the
same release rate or lessen the increase in release rate. Due to the level of completion on this project
and the effort necessary to update the drainage model for the whole site, it was decided to continue
using the current MOD SWMM model for this revision. The updated MOD SWMM modeling
produced an increase in the required release rate from 1.5 cfs to 3.2 cfs. The required pond volume
is 24.86 acre-feet and the available volume is 24.91 acre-feet. The water quality control volume
(WQCV), 2.26 acre-feet will be provided at an elevation 5052.20. The revised MOD SWMM
results, WQCV, rating curve, and orifice control sizing for Pond 110 are provided as an attachment
to this memorandum.
Flow from Pond 110 will discharge through the existing pond outlet pipe with modified pond outlet
structure. The water quality control outlet structure is designed for the proposed detention volume
and water quality control volume. The structure elevations, 100-year detention orifice plate, water
quality control elevation and water quality control orifice plate will be constructed to control runoff
discharge to the existing storm sewer in Turnberry Road. The project ultimately discharge to the No.
8 Ditch, as historically occurs. Calculations for the site 100-year orifice plate, Extended Detention
Basin (EDB) volume and orifice calculations are provided as an attachment.
Slight changes in the sidewalk alignment north of Pond 501 also resulted in modification of two
storm culverts. Storm M has been removed, while Storm L has been re-positioned under a new
sidewalk crossing. A new 4-foot wide concrete sidewalk culvert has been added upstream of
proposed Storm L to convey runoff south and under the new sidewalk location. Calculations for
sizing of the storm sewers, sidewalk culvert, and riprap have been provided as attachments to this
memorandum.
Additional water quality measures and LID measures will be implemented on the site to treat runoff
from the single family lots, public pool area and multi-family lots proposed with this development.
Approximately 5,000 square feet of porous pavers will be constructed in the parking areas to treat
approximately 0.46 acres or 26% of the new pavement area (from multi-family and pool parking
areas).
Water quality measures and LID measures for treatment of the single family lots will include the use
of bioswales within detention ponds, a micro-pool and grass buffers. Bio-swales will be constructed
in the bottom of detention ponds 101, 201 and 301. Low flows will percolate through sand media
and enter a 4” perforated, underdrain and discharge to proposed storm sewer. Near the north end of
pond 501, a micro-pool designed to intercept first flush flow, will pond to a depth of 1 foot before
overtopping and spilling to Pond 501. An underdrain constructed in a sand media will filter flow
before entering a 4-inch underdrain which discharges to pond 501. The pavers, micro-pool and pond
bioswales will treat approximately 11.4 acres of the impervious area which is 25% of the site
impervious area.
An additional 7.5 acres is impervious area is treated with grass buffers and will drain through grass
swales. This area treats approximately 16% of the site impervious area. Considering all the
treatment methods, the combined impervious area of 19.4 acres is treated with bioswales, porous
pavers and grass buffer swales resulting in 40.4% of the 50% required. It should be noted that the
remaining site area not considered in these treatment methods will include 23.1 acres of impervious
area treated by grass buffer within the detention ponds 110 and 300. Although ponds 110 and 300
will not incorporate a bioswale/underdrain, they will be grass lined and provide filtering of sediment
as flow is routed to the pond and through the pond area. Considering these addition grass buffers in
the pond 110 and 300, a total of 89%.of the impervious area is treated with the LID measures
proposed for construction.
A summary table for the LID measures is provided on the Drainage Basin Exhibit.
Should you have any questions or concerns, please contact me.
Sincerely,
Anthony Willkomm, P.E.
Aspen Engineering
Attachments: Drainage Basin Exhibit
Drainage Calculations