HomeMy WebLinkAboutIMPALA REDEVELOPMENT - PDP220005 - DOCUMENT MARKUPS - ROUND 2 - DRAINAGE REPORTNORTHERNENGINEERING.COM | 970.221.4158
FORT COLLINS | GREELEY
PRELIMINARY DRAINAGE REPORT: THE FLATS AT HANSEN FARM
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(Hydrologic Soil Group C).
Figure 2 - Aerial Photograph
6. The proposed development will consist of four (4) multi-family residential buildings containing 56
units with onsite and street parking, and a clubhouse.
7. The proposed land use is multi-family. This is a permitted use in the Low Density Mixed-Use
Neighborhood District (LMN).
C. FLOODPLAIN
1. No portions of the site are located in a FEMA regulatory floodplain.
Figure 3 - Existing Floodplains
NORTHERNENGINEERING.COM | 970.221.4158
FORT COLLINS | GREELEY
PRELIMINARY DRAINAGE REPORT: THE FLATS AT HANSEN FARM
6 | 11
2. The City of Fort Collins High-Risk Canal Importation floodplain extends into the SE corner of the
project site.
3. There are no special floodplain considerations required regarding finished floor elevations of
building footprints.
4. No floodplain use permit will be required for this project.
II. DRAINAGE BASINS AND SUB-BASINS
A. Major Basin Description
The project area of the Impala Redevelopment is located within the City of Fort Collins Canal
Importation Drainage Basin. Detention requirements for this basin are to detain the difference
between the 100-yr developed inflow rate and the historic 2-year release rate. However, outflow
from this property is limited by previously established release rates and existing storm
infrastructure in W Mulberry Street.
B. Sub-Basin Description
1. The outfall for the project site is at the south end of the project site to existing storm infrastructure
in W Mulberry Street.
2. The existing subject site can be defined with 9 distinct drainage basins (see DR1 in the provided
map pocket).
3. The existing site runoff generally drains from Northwest to Southeast towards W Mulberry Street.
4. The project area receives offsite runoff from the north. This was accounted for in the previous
drainage report for the project and will continue to be accounted for with the proposed design.
III. DRAINAGE DESIGN CRITERIA
A. OPTIONAL PROVISIONS
There are no optional provisions outside of the FCSCM proposed with Impala Redevelopment.
B. STORMWATER MANAGEMENT STRATEGY
The overall stormwater management strategy employed with Impala Redevelopment utilizes the
“Four Step Process” to minimize adverse impacts of urbanization on receiving waters. The following
is a description of how the proposed development has incorporated each step.
Step 1 – Employ Runoff Reduction Practices. The first consideration taken in trying to reduce the
stormwater impacts of this development is the site selection itself. By choosing an already
developed site with public storm sewer currently in place, the burden is significantly less than
developing a vacant parcel absent of any infrastructure.
Impala Redevelopment aims to reduce runoff peaks, volumes and pollutant loads from frequently
occurring storm events (i.e., water quality (i.e., 80th percentile) and 2-year storm events) by
implementing Low Impact Development (LID) strategies. Wherever practical, runoff will be routed
across landscaped areas or through a rain garden or water quality pond. These LID practices reduce
the overall amount of impervious area, while at the same time Minimizing Directly Connected
Impervious Areas (MDCIA). The combined LID/MDCIA techniques will be implemented, where
practical, throughout the development, thereby slowing runoff and increasing opportunities for
infiltration.
Step 2 – Implement BMPs that Provide a Water Quality Capture Volume (WQCV) with Slow
Release. The efforts taken in Step 1 will help to minimize excess runoff from frequently occurring
storm events; however, urban development of this intensity will still have stormwater runoff
leaving the site. The primary water quality treatment will occur between several rain gardens
between major parking areas of the property and the existing detention ponds installed for Impala