HomeMy WebLinkAboutPARADIGM PROPERTIES - ODP - 29-01A - SUBMITTAL DOCUMENTS - ROUND 4 - DRAINAGE REPORTI
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Paradigm Properties O.D.P. Northern Engineering Services, Inc.
Conceptual Drainage Report
August 8, 2003
developed site (Basin 1) has been estimated as 0.44 acre feet. Please refer to Attachment
1 for water quality capture volume calculations.
We trust that this report provides you with the information you need concerning
the conceptual drainage plan for Paradigm Properties O.D.P. If you have any questions
or concerns please feel free to contact me a 221-4158 ext. 106.
Sincerely,
NORTHERN ENGINEERING SERVICES, INC.
Aaron Cvar, P.E. O REGWS
Senior Engineer
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iAugust 8, 2003
Basil Harridan
City of Fort Collins
I Stormwater Utility
700 Wood Street
Fort Collins, CO 80521
I
Re: Paradigm Properties O.D.P.
Conceptual Drainge Report
Dear Basil,
The intent of this conceptual drainage report is to summarize storm drainage
I concepts for the proposed Paradigm Properties O.D.P., located at the southeast corner of
the intersection of Interstate 25 and East Prospect Road (see vicinity map).
Existing Site Description
' Currently the site has a retail motorcycle business at the southwest corner; the
remainder of the site is undeveloped. Existing topography directs runoff to the southeast
corner of the property at slopes of roughly 0.3 to 0.5 percent. All runoff from the
existing site, including the retail motorcycle business, is directed to the frontage road
ditch at the southeast comer of the property. The Rational method has been used in
conjunction with current City of Fort Collins IDF curves to estimate historic runoff
quantities. Historic flows have been estimated as 3.7 cfs and 16.7 cfs in the 2- and 100-
II year events, respectively. Please see the historic basin exhibit and historic runoff
calculations provided in Attachment 1.
Boxelder Creek Floodplain
The site is located in the Boxelder Creek Floodplain, as shown in the Boxelder
Creek/Cooper Slough Basin Master Plan by Anderson Consulting Engineers, January
2002. The floodplain is currently a City of Fort Collins regulated floodplain with no
floodway. Based on discussion with the City, a floodway may be regulated for this reach
of Boxelder Creek in the future.
As shown on the conceptual drainage plan, the site plan has been designed to
allow for a large flood channel to be created along the east boundary of the property.
This channel will be designed at Final to safely convey 100-year Boxelder Creek flows
through the site, and to create no rise in the existing 100-year flood elevation at all
property boundaries.
= Floodplain modeling of a conceptual flood channel has been done in HEC-RAS
1 version 3.0.1. The model has been created using the effective floodplain model for
�i Boxelder Creek from the Boxelder Creek/Cooper Sough Master Plan by Anderson
Consulting as a base model. A copy of the conceptual flood model is provided on the
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(� PARADIGM PROPERTIES O.D.P.
VICWITY MAP
N.T.S.
Conceptual Drainage Study
For
Paradigm Properties O.D.P.
Fort Collins, Colorado
August 8, 2003
Prepared For:
Paradigm Properties
P.O.Box 3236
Ventura, California 93006-3236
Prepared By:
NENORTHERN
ENGINEERING
SERVICES, INC.
420 S. Howes, Suite 202 Fort Collins, Colorado 80521
Phone: (970) 2214158 Fax: (970) 221-4159
Project Number: PPI: 02-155.00
' Paradigm Properties O.D.P. Northern Engineering Services, Inc.
Conceptual Drainage Report
' August 8, 2003
' attached floppy disk; summary output tables and an exhibit cross-section locations are
provided in Attachment 2. A formal floodplain modeling report will be completed
according to City guidelines at Final design for this project.
A meeting was held with City Stormwater Staff, the Developer and his
consultants to discuss overtopping of the access road for the site. It was concluded that
due to site constraints and economic viability of the project, a variance could be made for
I overtopping of the access road. It was noted in the meeting that currently both Prospect
Road and the frontage road experience significant overtopping exceeding allowable
overtopping criteria (2.1 feet at Prospect Road and 2.8 feet at the frontage road), thus
' access to the site would be limited in any case.
The current proposed design results in a maximum of 4.3 feet of overtopping at
the access road. Several iterations were done, and this is the lowest amount of
overtopping that can be achieved without excessive encroachment upon the developable
area of the site. The current flood channel design requires 7.2 acres of the site to be used
for flood conveyance, which is 42% of the total site.
Developed Drainage
I The proposed site is zoned for commercial development; the current site layout is
shown in the conceptual drainage plan. As shown on this plan, storm water detention can
be provided primarily in parking lots. Water quality/detention ponds can be incorporated
into landscaped areas. A network of storm lines can connect parking lot detention ponds
to water quality/detention ponds in landscaped areas; which would ultimately discharge
to a roadside ditch along the frontage road. Culverts will likely be provided as shown on
the conceptual drainage plan, and will convey flows to the southeast corner of the
property, which is the historic'point of discharge for the site. From this point of discharge
existing flows are currently conveyed by a roadside ditch south along the frontage road a
few hundred feet to an existing culvert flowing west under I-25. This culvert discharges
into City of Fort Collins property where it is conveyed west to Boxelder Creek. A
drainage easement through this property is currently being sought, and it is anticipated
that there will not be any problems to obtain it prior to final approval of the project.
Easements along the frontage road and I-25 are not necessary since drainage is contained
within public right of way.
Detention ponds will be designed to release the historic 2-year peak discharge rate
in the 100-year storm. An allowable release rate of 1.36 cfs has been determined by
taking the historic 2-year discharge rate calculated for the entire site and dividing by the
acreage of the site to arrive at a cfs per acre discharge rate. Please see calculations
provided in Attachment 1.
Using the allowable discharge rate of 1.36 cfs, total required detention volumes
for the developed site (Basin 1) is 2.74 acre-feet for the100-year storm event. The FAA
method has been used to determine this volume due to the relatively small size of Basin 1
(9.74 acre). Please see Attachment 1 for calculations.
`l Water quality ponds will receive discharge from detention ponds and will be
11 designed according to criteria for 40-hour extended detention as outlined in Volume 3 of
the Urban Storm Drainage Criteria Manual. Water quality capture volume for the
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