HomeMy WebLinkAboutMORNINGSTAR ASSISTED LIVING & MEMORY CARE - PDP - PDP130024 - CORRESPONDENCE - DRAINAGE RELATED DOCUMENTproposed that the existing concrete pan shall be protected from sedimentation throughout the course
of construction that might limit the capacity of the channel, and upon stabilizing the channel side
slope, the existing concrete shall be cleaned of debris or sediment.
The proposed development will be divided into 2 sub -basins. Sub -basin A will consist of
approximately 1.2 ac and site runoff will be collected in a small extended detention basin located
southwest of the proposed building before releasing into the existing drainage channel to the west.
Sub -basin B consists of approximately 3.2 ac and drains to the northwest corner of the property
where runoff will be collected in a second extended detention basin before releasing into the existing
concrete drainage channel. Once the run-off enters the existing ditch, it will flow northeast
eventually reaching McClelland's Channel.
In addition to the extended detention ponds, there are a series of Low Impact Development (LID)
features to help slow and treat the quality of the runoff from the property. These include a Rain
Garden, Porous Landscape Swale and Pervious Pavement.
In accordance with City stormwater design criteria, the post -development 100-year storm event
runoff release rate must be at the same rate or less than the 2-year "historic", or existing site
condition rate. The two extended detention ponds are designed to initially treat the water quality
from the site by slowly releasing the initial storm flows over a 40 hour period and allowing sediment
to deposit in the pond bottoms. Heavier flows from larger storms are then directed to a control plate
system that is designed to release the 100-year storm event site runoff (calculated to be 6.7 cfs) at the
2-year historic runoff rate (calculated to be 1.5 cfs).
All computations to date have been completed in conformance with the City of Fort Collins (COFC)
Storm Water Criteria Manual, COFC Master Drainage Plans, and the Urban Drainage and Flood
Control District Criteria Manual. Therefore, it is my professional opinion that the proposed storm
drainage system will not adversely impact adjacent or downstream properties as the channel will be
graded to maintain a consistent cross slope and minimum flow depth of 2 feet, and will receive a
lower flow rate than under existing conditions in all storm events.
Please let me know if you have any questions regarding the drainage design for the project.
Sincerely,
&4
Robert Almirall, P.E.
Sr. Project Manager
1218 W. ASH, STE. C, WINDSOR, COLORADO 80550
TEL. 970.674.3300 - FAX 970.674.3303
INTRRwCfT C O N SULTING GROUP
November 6, 2013
Mr. Matt Turner
Chief Financial & Development Officer
Morningstar Senior Living
7555 East Hampden Ave., Suite 501
Denver CO 80231-4835
Re: MVG-Morningstar (Fort Collins)
Dear Mr. Turner,
Pursuant to your request, I am writing in regard to the storm drainage design associated with the
proposed MVG-Momingstar site located at the northwest corner of E. Horsetooth Road and
Lochwood Drive in Fort Collins, Colorado.
The property consists of 4.42 acres of land and the existing land currently drains from southeast to
northwest at approximately a 2% grade. The land is currently vacant and covered with natural dry -
land vegetation across the entire site. The only offsite flow entering the site originates at the
southwest comer of the property from pipe(s) under E. Horsethooth Road which convey offsite
runoff to the northeast through an existing drainage channel with a 4-foot wide concrete pan
centered on the channel. There is an existing 25 foot drainage easement extending from the ditch
centerline to the west. An additional 25 foot drainage easement is proposed, extending from the
ditch centerline 25 feet to the east, making the entire Drainage Easement 50 feet wide up to the
wider Tract A -Utility & Drainage Easement on the Collindale 3`d Filing.
The drainage channel was initially designed by James Stewart and Associates in 1978. The design
called for a 50 foot wide channel with a maximum flow depth of 2 feet. The resulting side slopes of
the channel are; therefore, at an approximately 8 percent grade. This channel was certified at the
time to convey the 100-year storm event without causing damage to the adjacent buildings.
Interwest Consulting Group has similarly confirmed that a consistent channel of 2 feet deep and 50
feet wide can adequately convey the runoff from a 100-year storm event.
It is the intent of the MVG-Morningstar project to provide a consistent open channel along the
western property line with side slopes of 8 percent and a minimum flow depth of 2 feet. It is also
1218 W. ASH, STE. C, WINDSOR, COLORADO 80550
TEL. 970.674.3300 - FAX 970.674.3303