HomeMy WebLinkAboutCOLORADO EARLY COLLEGES - SITE PLAN ADVISORY REVIEW - SPA160003 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE RELATED DOCUMENTDecember 9, 2016
Mr. Shane Boyle
City of Fort Collins
Storm Water Utility
700 Wood Street
Fort Collins, CO 80522
Re: Colorado Early College – 4424 Innovation Drive
ICG Project No. 1292-028-00
Dear Shane,
Please accept the following memo on behalf of Colorado Early College to demonstrate the site’s
compliance with City Code.
EXISTING CONDITIONS
Colorado Early College Charter School is a remodel of an existing commercial building located
at 4424 Innovation Drive. The site is approximately 6.0 acres and includes an existing 92,600
square foot building, parking lot with associated drive areas, sidewalks, and landscape areas.
The existing site is 66% impervious.
PROPOSED IMPROVEMENTS
The proposed improvements for the site primarily consist of interior building modifications. As
part of Poudre Fire Authority’s requirements to bring the building up to code, a new 20 foot wide
concrete fire lane will be constructed on the east side of the building. A single parking space
will be removed in the west parking lot near the entrance of the site to accommodate the fire
truck turning movements. These two improvements are the only proposed site improvements.
The concrete fire lane will add 4,522 square feet of impervious area to the site. However, the
concrete will be replacing an existing gravel road (assumed to be 50% impervious) so the net
increase in impervious area is 2,261 square feet. The elimination of the single parking stall on
the west side of the building and associated rebuilding of the curb is considered negligible, but it
is a decrease in impervious area. The addition of the fire lane increases the sites total impervious
area from 66% to 67%. City code states that if the increase in imperviousness is greater than
1,000 square feet then drainage improvements – detention or water quality – will be required.
However, this increase in impervious area is very small compared to the size of the overall site.
The additional runoff produced can be considered negligible and will not have a detrimental
impact to any downstream drainage facilities.
All historic drainage patterns will be maintained with the construction of the fire lane. Runoff
from the fire lane will sheet flow to the east until it is intercepted by an existing drainage channel
that runs along the eastern property boundary.
Mr. Shane Boyle
December 9, 2016
Page 2 of 2
LOW IMPACT DEVELOPMENT (LID)
An alternative compliance/variance application has been submitted for this project. As
previously mentioned, runoff from the fire lane will follow existing drainage patterns and be
intercepted by the existing drainage channel. The drainage channel cab be classified as a grass
swale (UDCM Vol. III, Chapter 4, Section 2.0:T-2) where water quality treatment will occur
prior to exiting the site. This is the proposed alternative compliance to be implemented in place
of pervious pavement in the fire lane or a rain garden downstream of the fire lane.
If you have any questions or comments please contact me at (970) 460-1091.
Sincerely,
Skylar Brower, P.E.
Colorado Professional Engineer 44248
Interwest Consulting Group
1218 West Ash, Suite C
Windsor, Colorado
LOCATION: Colorado Early College
PROJECT NO:
COMPUTATIONS BY: smb
DATE: 12/9/2016
Recommended Runoff Coefficients from Table 3-3 of City of Fort Collins Design Criteria
Recommended % Impervious from Urban Storm Drainage Criteria Manual
Runoff %
coefficient Impervious
C
Streets, parking lots (asphalt): 0.95 100
Concrete: 0.95 96
Roofs: 0.95 90
Gravel: 0.50 40
Lawns (flat <2%, sandy soil): 0.10 0
SUBBASIN TOTAL TOTAL PAVED CONCRETE ROOF GRAVEL LANDSCAPE RUNOFF %
DESIGNATION AREA AREA AREA AREA AREA AREA AREA COEFF. Impervious REMARKS
(ac.) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (C)
1 6.02 262,030 75,157 12,457 92,604 7,780 74,032 0.70 66 Existing impervious areas
1 6.02 262,030 75,157 16,979 92,604 3,258 74,032 0.70 67 Proposed impervious areas
Equations
- Calculated C coefficients & % Impervious are area weighted
C = Σ (Ci Ai) / At
Ci = runoff coefficient for specific area, Ai
Ai = areas of surface with runoff coefficient of Ci
n = number of different surfaces to consider
At = total area over which C is applicable; the sum of all Ai's
RUNOFF COEFFICIENTS & % IMPERVIOUS
CEC-Impervious Area Calcs(2016-12-02).xls Existing Impervious Areas