HomeMy WebLinkAboutWHITHAM PROPERTY - ODP - 24-02B - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTFrom our preliminary calculations, we show that the 2-year historic release rate for the 80
acres contributory portion of the site is approximately 2.72 cfs. We assume that this rate
will need to be verified when the SWMM model is updated. This rate was calculated
using a runoff coefficient of 0.20 for the undeveloped historic site.
Based on a developed runoff coefficient of 0.60, and a 0.0170 cfs/acre release rate (2.72
cfs over 160 acres), we calculated the rough detention volumes that would be required for
the site;
Basin W-1
4.75 ac.ft.
0.45 cfs
Basin W-2
7.44 ac.ft.
0.70 cfs
Basins W-4,W5
12.00 ac.ft.
1.13 cfs
Basin W-6
3.68 ac.ft.
0.35 cfs
As mentioned above, these volumes do not include water quality capture volumes that
will be constructed in addition to the volumes shown above. Also, these volumes along
with the maximum allowable release rates will be verified with the SWMM model update
with the next design stage.
Please call if you have any questions regarding this study.
Sincerely,
Roger A Curtiss, P.E.
Project Engineer
There are two design alternatives for the area west of the tracks (Subbasin W4, W5, and
W6). One would be to detain developed flows, and release runoff to the Cooper Slough at
a reduced rate, based on a total release to the Cooper Slough discussed above). The other
alternative would be to obtain easements as necessary to conveyed runoff to the Lake
Canal (if there could be an agreement reached between the developer and the ditch
company), or to the Poudre River. We are currently still examining the second option, but
for the purposes of this ODP submittal, we have assumed that the basis of our drainage
plan for the site will be consistent with the first alternative discussed. Therefore, the
entire 160 acres in the Whitham ODP, will provide detention onsite, and will release
flows at the 2-year historic rate of the 80-acres that currently directs flows to the Cooper
Slough.
As design progresses on this site, we have also assumed that we will be required to
update the existing SWMM model for the Cooper Slough. It is our hope that through the
SWAM model for the site, that we demonstrate that the peak runoff from this site will be
sooner than the peak runoff from the entire basin, and that we can increase our allowable
release rate from the site. Again, until this model is updated, we will assume for the
purposes of this study, that our total allowable release rate will be limited to the 2-year
historic release rate of the 80-acre contributory basin.
Subbasin W-1 (which coincides with Parcel B) will have an onsite detention pond located
in the southeastern corner of Parcel B, between the tracks and the proposed Arterial road.
Release from this pond will be conveyed either via pipe or an open channel to the pond
located in Subbasin W-2. Subbasin W-2 (which coincides with Parcel C, will also have
an onsite detention pond, located in the southeast comer of the basin, between the tracks
and the 100-year flood zone. This pond will release flows to a proposed culvert under the
railroad tracks, to the detention pond located in Subbasin W6. Subbasin W-3 is that
portion of the site currently located in the 100-year flood limits, and it is our intent that
no development occur in this area.
Runoff from Subbasin W-4 (which coincides with Parcel A) will be conveyed via pipe or
open channel to the regional detention pond located in Subbasin W-5. Runoff from
Subbasin W-5 (which coincides with Parcel E, and the Neighborhood Park) will also be
conveyed to the regional detention pond, located at the southeast comer of W-5. Release
from this pond will be conveyed to the detention pond located in Subbasin W-6. Runoff
from Subbasin W-6 will be conveyed to a detention pond located at the southeast corner
of the subbasin. The total release from the pond in basin W-6, will be limited to the 2-
year historic runoff rate for the 80-acre contributory area discussed above. This release
will be directed to the Cooper Slough in an offsite easement, by an open channel.
All of the detention ponds discussed above, will have a staged outlet system, and will
also be designed to provide water quality capture volume.
NORTHERN
NESERVICES,ENGINEERING
INC. IR" I 3 I[,
April 28, 2003
Mr. Basil Hamdan
Stormwater Utilities
City of Fort Collins
700 Wood Street
Fort Collins, Colorado 80521
RE: Whitham ODP
Conceptual Drainage Study
Basil,
This study is in regards to the Whitham property. They have submitted plans for an ODP
for the site. This study will present our preliminary concepts for stormwater mitigation
for the site due to development.
The site is located directly south of Vine Drive, and west of the Cooper Slough. There are
approximately 160 acres in size. A Burlington Northern Railroad spur cuts across the
center of the site from the northwest corner to the southeast corner. The site is currently
undeveloped, and slopes are generally from the north to the south-southeast at 1 %. The
site is also located in the Cooper Slough Major Drainage Basin. This site can also be
described as being a portion of the northwest quarter of Section 9, Township 7 North,
Range 68 West of the 6"' P.M. The intent of the developer is to develop this site into
Low/Medium Density Residential.
As mentioned, there is an existing railroad spur that bisects the site. This railroad spur
also generally serves as a major basin boundary for the site, with the area east of the
tracks (approximately 80 acres) where historic runoff drains east to the Cooper Slough,
and the area west of the tracks (approximately 80 acres) where historic runoff drains '
south towards Lake Canal, and ultimately to the Poudre River. I have further divided the
site into 6 smaller drainage basins (see attached exhibit). I have also assumed, for the
purposes of this study, that there will be no offsite contributions to or conveyed through
this site (It is my understanding that the site to the west of this site, currently in the
design/review process plans to convey their runoff directly south to the Lake Canal, by
special agreement with the Ditch Company).
In general, for the Cooper Slough Major Drainage basin, developments are required to
detain the difference between runoff from the 2-year historic storm event, and runoff
from the 100-year developed storm events. Therefore, any runoff from this site directly to
the Cooper Slough will be limited to the 2-year historic runoff from the existing 80 acres
that currently contribute flows to the Cooper Slough.
420 SOUTH HOWES, SUITE 202 / FORT COLLINS, COLORADO 80521 / 970.221.4158 / FAX 970.221.4159