HomeMy WebLinkAboutTHE SAVOY - FDP230012 - DOCUMENT MARKUPS - ROUND 2 - DRAINAGE REPORT
FINAL DRAINAGE REPORT
THE SAVOY
FORT COLLINS, COLORADO
MARCH 29, 2023
NORTHERNENGINEERING.COM
970.221.4158
FORT COLLINS
GREELEY
This Drainage Report is consciously provided as a PDF. Please
consider the environment before printing this document in its entirety.
When a hard copy is necessary, we recommend double-sided printing.
NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: THE SAVOY
FORT COLLINS | GREELEY 6 | 11
D. HYDROLOGICAL CRITERIA
The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in Figure
3.4-1 of the FCSCM, serves as the source for all hydrologic computations associated with The
Savoy development. Tabulated data contained in Table 3.4-1 has been utilized for Rational
Method runoff calculations.
The Rational Method has been employed to compute stormwater runoff utilizing coefficients
contained in Tables RO-11 and RO-12 of the FCSCM.
The Rational Formula-based Federal Aviation Administration (FAA) procedure has not been
utilized for detention storage calculations since detention is provided off-site at the ODP Pond
or Detention Pond A – Brookfield.
Three separate design storms have been utilized to address distinct drainage scenarios. The
first event analyzed is the "Minor" or "Initial" Storm, with a 2-year recurrence interval. The
second event considered is the "Major Storm," which has a 100-year recurrence interval. The
final event analyzed was the 10-year recurrence interval for comparative analysis only.
E. HYDRAULIC CRITERIA
The drainage facilities proposed with The Savoy project are designed in accordance with
criteria outlined in the FCSCM and/or the Mile High Flood District's (UDFCD) Urban Storm
Drainage Criteria Manual.
As stated in Section I. C. 1. above, the subject property is not located within a FEMA or a City of
Fort Collins designated floodplain.
F. FLOODPLAIN REGULATIONS COMPLIANCE
As stated in Section I. C. 1. above, the subject property is not located within a FEMA or a City
of Fort Collins designated floodplain.
G. MODIFICATIONS OF CRITERIA
No formal modifications are requested at this time. However, as previously stated, The
Savoy will not provide detention on-site. Two regional detention ponds will provide
detention and extended water quality for a portion of the runoff.
H. CONFORMANCE WITH WATER QUALITY TREATMENT CRITERIA
City Code requires 100% of runoff from a project site to receive water quality treatment. This
project proposes to provide water quality treatment by using an underground infiltration
gallery (aka – underground chambers) located at the north end of the project and a rain
garden at the south end of the site. Stormtech isolator chambers are considered a LID
treatment method. Due to the physical constraints associated with an infill project of this
nature and the prohibition of providing water quality facilities within the public right-of-way,
some areas around the perimeter of the project cannot be captured and adequately routed
to either the rain garden or the Stormtech chambers. However, these areas have been
accounted for within the ODP Pond and Detention Pond A – Brookfield to receive water
quality.
Additionally, when the ODP Pond was installed, it did meet the current LID requirements set
forth by the FCSCM at the time of approvals. The ODP Pond has a sand filter along the entire
length of the bottom. However, the ODP Pond is also used for detention, and thus that is
why it no longer meets current LID standards.
Project Title Date:
Project Number Calcs By:
City
Basins
0.8
WQCV = Watershed inches of Runoff (inches)80%
a = Runoff Volume Reduction (constant)
i = Total imperviousness Ratio (i = Iwq/100)0.263 in
A =4.67 ac
V = 0.1227 ac-ft
V = Water Quality Design Volume (ac-ft)
WQCV = Water Quality Capture Volume (inches)
A = Watershed Area (acres)
5343 cu. ft.
Drain Time
a =
i =
WQCV =
Figure EDB-2 - Water Quality Capture Volume (WQCV), 80th Percentile Runoff Event
The Savoy March 20, 2023
514-003 B. Mathisen
Fort Collins
Stormtech LID Pond
0.231
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
00.10.20.30.40.50.60.70.80.91WQCV (watershed inches)Total Imperviousness Ratio (i = Iwq/100)
Water Quality Capture Volume
6 hr
12 hr
24 hr
40 hr
()iii78.019.10.91aWQCV 23 +-=
()iii78.019.10.91aWQCV 23 +-=
12 hr
1.20
WQCV
12 ∗