HomeMy WebLinkAboutWELLS FARGO PARKING LOT AND ATMS - MJA190003 - DOCUMENT MARKUPS - ROUND 3 - DRAINAGE REPORTDRAINAGE REPORT
FOR
FINAL DRAINAGE REPORT
PARKING AT 112 WEST MAGNOLIA STREET
LOTS 1-2, BLOCK 113, FTC
112 WEST MAGNOLIA STREET
FORT COLLINS, CO 80524
Prepared For:
Wells Fargo Bank
RO Box 2609
Carlsbad, CA 92018
Prepared By:
Coffey Engineering & Surveying, LLC
4045 St. Cloud Drive, Suite 180
Loveland, CO 80538
970-622-2095
Project No. 2283.00
October 25, 2019
PG. 6
After treatment within the permeable pavers, stormwater will infiltrate through the bottom of
the paver sections and dry well. If the paver system overflows due to clogging, catch basins
at the low point of the pavers will direct runoff into the underdrain system.
When improvements are being added to an existing developed site, onsite detention is only
required if there is an increase in impervious area greater than 5,000 square feet in the Old
Town Basin. The effective impervious area for the existing site is 7,602 square feet and the
effective impervious area for the proposed site is 12,205 square feet, a difference of 4,603
square feet. Since the effective impervious area added to the site is under 5,000 square feet,
detention is not required.
Sub-basin P1 produces a 100-year flow of 3.48 cfs. The Modified FAA spreadsheet was
created to incorporate rainfall data from the Fort Collins Stormwater Criteria Manual, Table
3.4-1. This spreadsheet was used to calculate the inflows and outflows from the site. With
permeable pavers, the City of Fort Collins does not require a volume treatment based on
WQCV. The treatment method is filtration and infiltration. The pavers criteria is a maximum
run-on:pavers ratio of 3:1.
The sub-base under the pavers is designed per Fort Collins Detail D-54, Interlocking Pavers
Cross Section. The water quality volume is held in the 10” #2 rock layer.
V. CONCLUSIONS
A. Compliance with Standards
The drainage design for the Parking at 112 West Magnolia Street follows the requirements of
the Fort Collins Stormwater Criteria Manual as well as the City’s floodplain regulations. The
criteria and recommendations of the Urban Storm Drainage Criteria Manual are also
reflected in the design of the drainage systems.
B. Drainage Concept
The drainage design for the Parking at 112 West Magnolia Street will safely convey onsite
flows from the parking lot into the existing storm inlet, via the permeable paver detention
system. Permeable pavers and the preservation of an existing large tree help to minimize the
impact of the development to the surrounding properties. Water quality and LID treatment
will be achieved by the permeable paver system.