HomeMy WebLinkAboutGUARDIAN SELF STORAGE - FDP210018 - DOCUMENT MARKUPS - ROUND 1 - DRAINAGE REPORTGuardian Storage Fort Collins
Final Drainage Report
8
DRAINAGE DESIGN CRITERIA
REGULATIONS
This report was prepared to meet or exceed the City of Fort Collins stormwater criteria. The City of
Fort Collins Stormwater Criteria Manual (dated 2018)(FCSCM) and the Urban Drainage Flood Control
District’s (UDFCD) Drainage Criteria Manual (USDCM) Volumes 1, 2 and 3 were referenced as
guidelines for this design.
MINIMIZATION OF THE DCIA
UDFCD recommends the Four Step Process to protect receiving waters. The four step process
incorporates reducing runoff volume, treating water quality capture volume, stabilizing streams, and
implementing long-term source controls. Volume reduction is an important part of the Four Step
Process and is fundamental to effective stormwater management. Per City criteria, a minimum of 75
percent of new impervious surface area must be treated by a Low-Impact Development (LID) best
management practice (BMP). The site incorporates a sand filter designed to capture and treat 120% of
the sites water quality capture volume. The sand filter media needs to be properly cleaned and media
maintained to allow for long term protection of the receiving waters. The proposed LID BMPs will have
the effect of slowing runoff through the site lot and increasing infiltration and rainfall interception by
encouraging infiltration and careful selection of vegetative cover. The improvements will decrease the
runoff coefficient from the site and are expected to have no adverse impact on the timing, quantity, or
quality of stormwater runoff. The majority of new impervious area is treated by the proposed sand filter
in the southeast corner of the site as illustrated in the LID/Surface Map is provided in Appendix G.
Runoff generated on the proposed Snead Drive will not be treated by sand filter. In order to
compensate for portions of Snead drive not being treated via LID, untreated offsite runoff that
historically entered the site from the north west will be routed to the sand filter and treated. See the
LID Map & Table 3 for a summary of the areas to be treated.
DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS
The master drainage report for the Fossil Creek Drainage Basin was prepared by ICON Engineering,
Inc. April 9th, 2002. Guardian Storage falls within element 30 in the SWMM file associated with the
master drainage report. The development of the Guardian Storage site must not have a negative impact
Please retitle to "Four Step Process to
Protect Receiving Waters"
City of Fort Collins
requires the..
Fort Collins Water Utilities Comments
Matt Simpson
2/15/2021
JR Response: Addressed
Guardian Storage Fort Collins
Final Drainage Report
9
on downstream discharges, volumes and water surface elevations. The elements directly downstream
from the Guardian Storage site are elements 47 and 29 from the master SWMM file.
HYDROLOGIC CRITERIA
The rational method was performed to calculate the peak runoff rates for each basin. Weighted percent
imperviousness and weighted runoff coefficients were calculated for each basin using the Fort Collins
Stormwater Criteria Manual Table 3.2-2 and Table 4.1-2. The time of concentration was calculated using
FCSCM Equation 5-3 and the intensity was interpolated from Table 3.4-1 in the FCSCM. The City of
Fort Collins area has 2-year, 1-hour rainfall depth of 0.82 inches and a 100-year, 1-hour rainfall depth of
2.86 inches. These depths do account for the 1997 adjusted rainfall depths.
HYDRAULIC CRITERIA
The ultimate Guardian Self Storage storm drainage system will be designed to convey the minor and
major storm events through the property with the proposed inlets, storm sewer pipes, and swales for
the flows being calculated by this report. Per the requirements provided by the City of Fort Collins
Storm Drainage Design Criteria and Construction Manual, all inlets and storm pipes will be designed to
convey the 100-year storm flows. During the major storm event, the developed condition 100-year
storm, the accumulated water depth in the sand filter will be held to a maximum level of one-foot below
all building finished floor elevations.
It is important to note that the site’s designed hydraulic system is controlled by the tailwater in Fossil
Creek at the outfall of the storm system. When the tailwater in Fossil creek was set at the crown of the
discharge pipe, the Guardian drainage system has more than enough capacity as seen in the “100 YR
Profile-Tailwater @ Crown” profile in Appendix C. When the tailwater is set to the 10-Year WSEL in
Fossil Creek, the drainage system will pass approximately 74 cfs through the outfall pipe while the
remaining 67 cfs leaves the site via surface flow to the East. In this scenario, the 67 cfs leaving the site is
less than the ditch overflow leaving the site in the existing condition. The 74 cfs passing through the
outfall pipes was determined by iterating the flow in the system until the hgl at the sump in proposed
Snead Drive was at the overflow elevation in the curb and gutter as shown in the “100 Yr Profile-HGL
Set to Overflow @ Sump” StormCAD Profile in Appendix C.
to convey the full
100-yr flow rate (or
xx-cfs)
JR Response: Addressed