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MEMORANDUM
COJSS01_FloodplainEval-PDP_Memo-July18 2022 Page 1 of 3 July 18, 2022
DATE: March 18, 2022 ACE PROJECT NO.: COJSS01
TO: Shane Ritchie, Northern Engineering
FROM: Greg Koch, Anderson Consulting Engineers, Inc.
Matt Clark, Anderson Consulting Engineers, Inc.
SUBJECT: Jerome Street Station Preliminary Floodplain Evaluation – Revised Grading, Fort Collins,
Colorado
The Jerome Street Station (JSS) property is partially located in the northern fringe of the Poudre River 1%
annual chance of occurrence (100-year) floodplain that has recently been revised by a Letter of Map
Revision (LOMR) for the Poudre River Whitewater Park; this LOMR that became effective in June 2021.
More specifically, the Jerome Street Station property is located along the College Ave Split Flow Path (SPF)
which originates at the Poudre River west of College Avenue and returns to the river south of the Jerome
Street/Vine Drive intersection. The entire property is located outside of the effective ½-foot rise floodway.
The proposed development on the Jerome Street Station site includes residential land use and is located
in the 100-year Poudre River floodplain. As such, per City of Fort Collins code, the portions of the property
on which residential buildings are planned would need to be removed from the regulatory floodplain,
through FEMA’s LOMR process, prior to construction. Normally, a LOMR application to FEMA needs to be
proceeded by an approved Conditional Letter of Map Revision (CLOMR) by FEMA. However, in some
unique instances, a no-rise application through the City can be processed in lieu of the CLOMR.
A preliminary hydraulic evaluation was conducted by Anderson Consulting Engineers, Inc. (ACE) in May of
2021 to determine potential impacts to 100-year flood elevations and the 100-year floodplain boundary
due to proposed grading within the Jerome Street Station site. The 2021 evaluation determined that the
initial grading for the JSS project may increase 100-year flood elevations and parking areas and driveways
may be inundated by 100-year water surface elevations. Pursuant to the 2021 evaluation, detailed pre-
project topography was collected for the JSS site, as well as for the property south of the site (bounded
by College Avenue, Vine Drive, and Jerome Street). In addition, the proposed site and grading plans for
the JSS site were revised.
ACE utilized the detailed, pre-project topography to better evaluate the pre-project (corrected effective)
hydraulic conditions near the JSS site. A base map for this evaluation was created by overlaying detailed
pre-project topography for the JSS property and adjacent areas on topography provided by the City of
Fort Collins, which is based on 2013 Lidar data. The corrected effective condition hydraulic model was
created by altering the effective geometry to reflect the detailed topographic information. Ineffective
flow areas were reviewed and altered as necessary to better reflect pre-project conditions. Manning’s n
roughness coefficients were left unchanged from the effective hydraulic model. Consistent with the
Poudre WW LOMR, HEC-RAS 5.0.7 was utilized to analyze the 100-year event for the corrected effective
condition. Sheet 1 (attached) shows the effective floodplain information at and in the vicinity of the JSS
site. The resulting corrected effective condition and effective 100-year water surface elevations (WSELs)
are summarized and in Table 1. The tabular results indicate that utilizing the more detailed pre-project
topography results in a slight reduction in 100-year WSELs at Cross Sections 1235 and 1341, and a slight
increase in the 100-year WSEL at Cross Section 930.
MEMORANDUM
COJSS01_FloodplainEval-PDP_Memo-July18 2022 Page 2 of 3 July 18, 2022
Table 1. Comparison of Effective and Corrected Effective 100-Year Water Surface Elevations
Cross Section
100-Year WSEL (NAVD 1988)
Change in 100-
Year WSEL (ft) Effective
Condition
Corrected
Effective
Condition
723 4964.24 4964.24 0.00
930 4964.31 4964.33 0.02
1235 4965.98 4965.97 -0.01
1341 4967.57 4967.54 -0.03
1470 4968.74 4968.74 0.00
Northern Engineers provided ACE with a revised grading plan for the JSS site in July of 2022. ACE evaluated
the proposed condition hydraulics by modifying the corrected effective hydraulic geometry to reflect the
proposed grading. Proposed cross sections were defined in HEC-RAS based on the proposed grading
within the site. Comparative cross section plots from HEC-RAS are attached. The resulting proposed
condition 100-year water surface elevations, summarized in Table 2, indicate that no rises would occur
due to the proposed grading when compared to the corrected effective water surface elevations. Based
on these results we anticipate producing a no-rise report and Floodplain Use Permit application rather
than a complete FEMA CLOMR.
Table 2. Comparison of Corrected Effective and Proposed 100-Year Water Surface Elevations
Cross Section
100-Year WSEL (NAVD 1988)
Change in 100-
Year WSEL (ft) Corrected
Effective
Condition
Proposed
Condition
723 4964.24 4964.24 0.00
930 4964.33 4964.33 0.00
1235 4965.97 4965.97 0.00
1341 4967.54 4967.54 0.00
1470 4968.74 4968.74 0.00
Given the current site grading plan and adjacent 100-year water surface elevations, a proposed condition
100-year floodplain in the vicinity of the Jerome Street Station was delineated; this proposed condition
floodplain is shown on Sheet 1. While the 100-year floodplain would still encroach minimally on the
property, all driveways, parking lots, and residential structures, would be outside the proposed condition
MEMORANDUM
COJSS01_FloodplainEval-PDP_Memo-July18 2022 Page 3 of 3 July 18, 2022
100-year floodplain. The residential structures would be elevated above the effective 100-year floodplain
with a minimum of 2 feet of freeboard provided.
930123513411
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0 723Anderson Consulting Engineers, Inc
Civil ▪ Water Resources ▪ Environmental
375 East Horsetooth Road, Building 5, Fort Collins, CO 80525
Phone (970) 226-0120 / Fax (970) 226-0121
www.acewater.com
JEROME STREET STATION
FLOODPLAIN EVALUATION
PRELIMINARY
100-YR FLOODPLAIN1
1
02004006008001000120014001600496049624964496649684970497249744976PWWP-OxBow LOMR Plan: 1) RC FP 2) JSS CE FP Lake Canal capped at 4963 because this is the elvation water wilStation (ft)Elevation (ft)LegendWS 1% - RC FPWS 1% - JSS CE FP- JSS CE FP- JSS CE FPGround - JSS CE FPIneff - JSS CE FPBank Sta - JSS CE FP- RC FP- RC FPGround - RC FPIneff - RC FPBank Sta - RC FPEFFECTIVE VS. PRE-PROJECT XS1341EFFECTIVE GROUNDPRE-PROJECTGROUND
0200400600800100012001400160049584960496249644966496849704972PWWP-OxBow LOMR Plan: 1) RC FP 2) JSS CE FP D/S College AvenueStation (ft)Elevation (ft)LegendWS 1% - RC FPWS 1% - JSS CE FP- JSS CE FPGround - JSS CE FPIneff - JSS CE FPBank Sta - JSS CE FP- RC FPGround - RC FPIneff - RC FPBank Sta - RC FPEFFECTIVE VS. PRE-PROJECT XS 1235EFFECTIVE GROUNDPRE-PROJECTGROUND
020040060080010001200495849604962496449664968PWWP-OxBow LOMR Plan: 1) RC FP 2) JSS CE FP Lake Canal capped at 4963 because this is the elvation water wilStation (ft)Elevation (ft)LegendWS 1% - JSS CE FPWS 1% - RC FP- JSS CE FPGround - JSS CE FPIneff - JSS CE FPBank Sta - JSS CE FP- RC FPGround - RC FPIneff - RC FPBank Sta - RC FPEFFECTIVE VS. PRE-PROJECT XS 930EFFECTIVE GROUNDPRE-PROJECTGROUND
02004006008001000120049584960496249644966PWWP-OxBow LOMR Plan: 1) RC FP 2) JSS CE FP Lake Canal capped at 4963 because this is the elvation water wilStation (ft)Elevation (ft)LegendWS 1% - JSS CE FPWS 1% - RC FP- JSS CE FP- JSS CE FPGround - JSS CE FPIneff - JSS CE FPBank Sta - JSS CE FP- RC FP- RC FPGround - RC FPIneff - RC FPBank Sta - RC FPEFFECTIVE VS. PRE-PROJECT XS 723EFFECTIVE GROUNDPRE-PROJECTGROUND
02004006008001000120014001600496049624964496649684970497249744976Jerome Street Station Hydraulic Analysis Plan: 1) JSS PC FP 2) JSS CE FP Lake Canal capped at 4963 because this is the elvation water wilStation (ft)Elevation (ft)LegendWS 1% - JSS CE FPWS 1% - JSS PC FP- JSS CE FP- JSS CE FPGround - JSS CE FPIneff - JSS CE FPBank Sta - JSS CE FP- JSS PC FP- JSS PC FPGround - JSS PC FPIneff - JSS PC FPBank Sta - JSS PC FPPre-Project GroundProposed Ground
020040060080010001200495849604962496449664968Jerome Street Station Hydraulic Analysis Plan: 1) JSS PC FP 2) JSS CE FP Lake Canal capped at 4963 because this is the elvation water wilStation (ft)Elevation (ft)LegendWS 1% - JSS CE FPWS 1% - JSS PC FP- JSS CE FPGround - JSS CE FPIneff - JSS CE FPBank Sta - JSS CE FP- JSS PC FP- JSS PC FPGround - JSS PC FPIneff - JSS PC FPBank Sta - JSS PC FPPre-Project GroundProposed Ground