HomeMy WebLinkAboutPOWERHOUSE 2 - PDP220006 - SUBMITTAL DOCUMENTS - ROUND 1 - STORMWATER-RELATED DOCUMENTS (3) MEMORANDUM
COPH201_NoRise_Memo.docx Page 1 of 12 April 12, 2022
DATE: April 12, 2022 ACE PROJECT NO.: COPH201
TO: Bryan Willson, Colorado State University Powerhouse
Will Welch, Wm. T. Welch Company
FROM: Greg Koch, Anderson Consulting Engineers, Inc.
Parker Maddocks, Anderson Consulting Engineers, Inc.
SUBJECT: Powerhouse 2 – No-Rise Floodplain Evaluation
This memo is intended to document a no-rise condition analysis for the proposed
Powerhouse 2 Project. This memo is provided as a pre-cursor to a full CLOMR
application and report that is being prepared by Anderson Consulting Engineers for
this project.
PROJECT DESCRIPTION
The Powerhouse 2 (PH2) site is located at the northeast corner of the College Avenue/Vine Drive
intersection in north-central Fort Collins, bounded on the north and east by the Lake Canal and Vine Drive,
respectively. PH2 is a proposed research facility intended to function as an extension of the Colorado
State University Powerhouse Energy Campus located a short distance south of the PH2 Site. Proposed
improvements for the site include a single elevated building (elevated to meet floodplain regulations)
fronting both College Avenue and Vine Drive. Parking lots and a stormwater detention pond would be
located on the eastern portion of the site, while an east-west driveway would run between the building
and the Lake Canal. Raised plazas would surround much of the building’s perimeter, with the plazas being
formed with a series of retaining walls. A vicinity map is included in Figure 1 attached to the end of this
memo.
The PH2 site is located in the north overbank 1% annual chance of occurrence (100-year) floodplain
resulting from flows from the Poudre River. More specifically, the site lies within the College Avenue Split
Flow Path (SFP). Flows which form this SFP become hydraulically separate from flows along the main
Poudre River corridor west of College Avenue. These flows initially run west to east passing over College
Avenue, largely between Vine Drive and the Lake Canal. East of College Avenue, flows along the College
Avenue SFP trend generally southeast, rejoining the Poudre River south of Vine Drive, west of the BNSFRR.
Although the PH2 Site is not located in the regulatory floodway, due to unique circumstances concerning
the definition of the floodway in this area (as described in the following section) and the site’s position in
the main flow corridor of the College Avenue SFP, City Stormwater Staff requires that proposed
improvements result in no-rise in 1% Annual Chance flood elevations. Per City of Fort Collins Code, prior
to the proposed building being approved for occupancy, a LOMR would need to be approved and adopted
by FEMA. Although the City is requiring the PH2 Project to demonstrate no-rise, since the project is not
located in a regulatory floodway, the City does not require a Conditional Letter of Map Revision (CLOMR)
in this case. However, the owner intends to submit a CLOMR to be approved prior to construction.
EFFECTIVE FLOOD STUDY
Effective floodplain modeling and mapping for the reach of the Poudre River adjacent to the PH2 Site is
the Poudre River Whitewater Park and Oxbow Levee Letter of Map Revision (LOMR, FEMA Case No. 20-
08-0643P) which became effective June 2021. The 2021 LOMR, encompassing an approximately 2-mile
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Figure 1. Vicinity Map for the Powerhouse 2 Site.
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reach from upstream of Mulberry Street to downstream of Shields Street, documented floodplain changes
due to several recent projects in that 2-mile reach, the most germane of which to the PH2 Project is the
Poudre River Whitewater Park (PWWP) located directedly south of Vine Drive and east of College Avenue.
The PWWP Project lowered both river channel and flood elevations along the river, including through the
College Avenue Bridge, such that the bridge could convey the regulatory 1% Annual Chance flow.
However, flow patterns and topography west of College Avenue allow a portion of the flows in the north
overbank to be become hydraulically separated from the main channel during larger flood events (2%
annual chance, aka 50-year, and greater). These flows give rise to the College Avenue SFP.
Since the College Avenue Bridge has the capacity to convey the 1% Annual Chance flow and the City
intends to implement upstream improvements to eliminate the College Avenue SFP sometime in the
future, for the 2011 LOMR it was decided by the City to handle the floodplain and floodway in this area in
the following manner: (a) the 1% annual chance floodplain along the river was defined using the full 1%
annual chance discharge, rather than reduce these flows by the flow in the College Avenue SFP; (b) the
actual split flow for the 1% annual chance event was used to define the floodplain along the College
Avenue SFP; (c) the floodway is defined along the river using the full 1% annual chance discharge; and (d)
a floodway is not defined along the College Avenue SFP.
TOPOGRAPHIC MAPPING
Pre-project condition topographic mapping for the PH2 Project started with the mapping used for the
PWWP LOMR. That is, the overall topography for the area was provided by the City of Fort Collins based
on 2013 LiDAR data produced by Ayres Associates with a vertical datum referenced to NAVD88, while
topography for the PWWP Project Site was developed in 2019 by Majestic Surveying using as-built survey
data collected by Majestic. For the PH2 study, the LOMR topographic data was supplemented with
detailed field survey conducted by Northern Engineering in 2021 for both the PH2 Site and the Jerome
Street Station Site located directed to north, along the north bank of the Lake Canal.
DUPLICATE EFFECTIVE ANALYSIS
The regulatory hydraulic model (HEC-RAS, Version 5.0.7) for the PWWP LOMR was obtained and used to
re-analyzed effective conditions. Resulting 1% annual chance water surface elevations (WSELs) from the
duplicate effective analysis matched the effective 1% annual chance WSELs at all cross sections along the
College Avenue SFP and along the Poudre River main channel in the vicinity of the SFP except two cross
sections, where the difference in 1% annual chance WSELs was ±0.01 feet (as shown in Table 1), thereby
indicating that the proper model had been obtained.
It is noted that for the remainder of this memo, only hydraulic modeling results along the College Avenue
SFP are reported due to regulatory hydraulic conditions along the mainstem of the Poudre River being
independent from any potential PH2 Site improvements. Since, in accordance with the PWWP LOMR, the
regulatory 1% annual chance flow along the Poudre River main channel in this area is set to the full 1%
annual chance discharge, improvements to the PH2 Site cannot change the regulatory discharge along the
river. In addition, changes to the PH2 Site cannot directly influence hydraulic conditions along the river
as the entire site is contained within area encompassed by the College Avenue SFP.
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Table 1. Comparison of Effective and Duplicate Effective 1% Annual Chance WSELs
Cross Section
ID
1% Annual Chance WSEL
(NAVD 1988) Change in
1% Annual
Chance WSEL
(ft) Effective
Condition
Duplicate
Effective
Condition
College Avenue Split Flow Path
150 4964.23 4964.23 0.00
282 4964.23 4964.23 0.00
583 4964.23 4964.23 0.00
645 4964.23 4964.23 0.00
723 4964.23 4964.23 0.00
930 4964.31 4964.31 0.00
1235 4966.01 4966.02 0.01
1341 4967.57 4967.56 – 0.01
1470 4968.74 4968.74 0.00
1556 4968.77 4968.77 0.00
1601 4968.76 4968.76 0.00
1676 4969.44 4969.44 0.00
1731 4970.26 4970.26 0.00
2520 4971.20 4971.20 0.00
Cache la Poudre River
230278 4963.25 4963.25 0.00
230388 4963.35 4963.35 0.00
230489 4963.91 4963.91 0.00
230572 4964.46 4964.46 0.00
230617 4964.43 4964.43 0.00
230679 4964.88 4964.88 0.00
230775 4964.95 4964.95 0.00
230891 4965.74 4965.74 0.00
231080 4966.15 4966.15 0.00
231119 4966.32 4966.32 0.00
231212 4967.63 4967.63 0.00
231628 4968.58 4968.58 0.00
232134 4968.92 4968.92 0.00
232362 4969.08 4969.08 0.00
232397 4969.16 4969.15 0.00
232771 4969.25 4969.25 0.00
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CORRECTED EFFECTIVE ANALYSIS AND FLOODPLAIN MAPPING
The duplicate effective model was modified to better reflect pre-project conditions by incorporating the
more detailed topographic data provided by Northern Engineering for the PH2 Site and the adjacent
Jerome Street Station Site (JSS). A base map for this evaluation was created by overlaying detailed pre-
project topography for the PH2 and JSS Sites on the topographic mapping utilized for the effective PWWP
LOMR. 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 slightly
to better reflect pre-project conditions. Specifically, ineffective flow areas were modified for Cross Section
1341.
In addition, based on a close review of existing conditions within the PH2 Site, a small adjustment was
made to one of the Manning’s n values at Cross Section 930 where the roughness coefficient for a short
portion of this cross section was changed from 0.045 to 0.04.
Manning’s n roughness coefficients were left unchanged from the effective hydraulic model. Consistent
with the PWWP LOMR and the current duplicate effective analysis, HEC-RAS 5.0.7 was utilized to analyze
the 1% Annual Chance event for the corrected effective condition. The resulting corrected effective
condition and duplicate effective 1% Annual Chance WSELs are summarized and in Table 2. The tabular
results indicate that utilizing the more detailed pre-project topography results in a slight reduction in 1%
Annual Chance WSELs at Cross Sections 930, 1235 and 1341. Sheet 1 (attached) shows both the effective
and corrected effective flood hazard mapping in the vicinity of the PH2 Site. It is noted that only the
floodplain along the College Avenue SFP is changed from the effective condition, and those floodplain
changes are shown to be relatively minimal.
Table 2. Comparison of Duplicate Effective and Corrected Effective 1% Annual Chance WSELs
Cross Section
ID
1% Annual Chance WSEL
(NAVD 1988) Change in
1% Annual
Chance WSEL
(ft)
Duplicate
Effective
Condition
Corrected
Effective
Condition
College Avenue Split Flow Path
723 4964.23 4964.23 0.00
930 4964.31 4964.30 – 0.02
1235 4966.02 4965.98 – 0.04
1341 4967.56 4967.56 – 0.01
1470 4968.74 4968.74 0.00
PROPOSED CONDITION ANALYSIS AND FLOODPLAIN MAPPING
Northern Engineering provided ACE with a revised grading and site plan for the PH2 site. ACE evaluated
the proposed condition hydraulics by modifying the corrected effective hydraulic geometry to reflect the
proposed grading and site improvements. As currently proposed, the PH2 building and related site
improvements would obstruct a significant portion of the middle of relatively broad, shallow floodplain
crossing the site. Due to the proposed grading lowering ground north of the building, flood levels would
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not increase but the hydraulic baseline for the College Avenue SFP would shift north around the building
between Cross Sections 723 and 1235. This lengthening of the hydraulic baseline alters the stationing of
Cross Section 930 and all upstream cross sections along the College SFP. The horizontal alignment of both
corrected effective and proposed condition hydraulic baselines are shown on Sheet 2 (attached). The
alignment of the northern portion of Cross Section 1004 (formerly Cross Section 930) was modified to
reflect the more west-east flow pattern around the north side of the proposed building; the new
alignment of this cross section is also shown on Sheet 2.
Proposed cross section geometry was defined based on the proposed grading within the site, with
ineffective flow areas defined based on the proposed building and associated retaining walls. Based on
the proposed site plan and the change in use over the site from generally a gravel surface to a paved and
landscaped surface, roughness coefficients were modified to reflect the proposed site condition. For the
proposed condition, Manning’s n values through the PH2 Site, from Cross Section 723 through 1370, range
from 0.020 for flow over ponded water to 0.050 for vegetated areas. The resulting proposed condition
1% annual chance WSELs, summarized in Table 3, indicate that no increase in 1% annual chance WSELs
would occur due to the proposed grading and site improvements when compared to the corrected
effective WSELs. Furthermore, proposed condition 1% annual chance WSELs would be equal to or lower
than effective 1% annual chance WSELs.
Table 3. Comparison of Corrected Effective and Proposed Condition 1% Annual Chance WSELs
Cross Section ID 1% Annual Chance WSEL
(NAVD 1988) Change in
1% Annual
Chance WSEL
(ft)
Corrected
Effective
Condition
Proposed
Condition
Corrected
Effective
Condition
Proposed
Condition
College Avenue Split Flow Path
723 723 4964.23 4964.23 0.00
930 1004 4964.30 4964.19 – 0.11
1235 1264 4965.98 4965.94 – 0.05
1341 1370 4967.56 4967.31 – 0.24
1470 1499 4968.74 4968.74 0.00
Given the current site grading plan and associated 1% annual chance WSELs along the College Avenue
SFP, the proposed condition 1% annual chance floodplain was delineated along the College Avenue SFP;
the proposed condition floodplain is shown on Sheet 2. Both the corrected effective and proposed
condition flood hazard mapping in the vicinity of the PH2 Site are shown on Sheet 2. It is again noted that
only the floodplain along the College Avenue SFP is changed from the corrected effective condition, and
those floodplain changes are shown to be relatively minimal. While the 1% Annual Chance floodplain
would still encroach on much of the property, the proposed building and adjacent plaza areas would be
elevated above the floodplain, along with a portion of the driveway north of the building. The proposed
finished floor elevation for the building is 4968.8 ft, NAVD, which 2.1 ft higher than the 1% annual chance
flood elevation at the west (upstream) face of the building of 4966.7 ft, NAVD. Consequently, the building
would have slightly more than 2.0 feet of freeboard, thereby meeting City requirements for the flood
protection elevation in the Poudre River floodplain.
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TWO-DIMENSIONAL MODELING
In addition to the one-dimensional (1D) modeling documented above, two-dimensional (2D) modeling of
proposed conditions was conducted for the project area using HEC-RAS 2D, Version 5.0.7. The 2D
modeling was conducted at the request of City Staff to inform and verify the results of the 1D model effort.
In addition, the results of the 2D modeling are being utilized to identify potential access points and
conditions for emergency responders during large flood events. Accordingly, the 2D modeling was
conducted for both the 2% and 1% annual chance events.
The 2D model grid was defined for a portion of the College Avenue SFP to encompass the entire PH2 Site,
extending from 1D model Cross Section 583 (along Vine Drive) to Cross Section 1731 (along the UPRR,
west of College Avenue). In order to closely represent the proposed grading and site plan, a 5-foot grid
was utilized for the 2D model. Manning’s n values were defined to correspond to the roughness
coefficients utilized in the 1D model, while an n values of 15 was used to represent the proposed building.
The primary outflow boundary condition was defined along the alignment of 1D Cross Section 583 using
the computed 1D WSEL at that cross section for both the 2% annual chance and 1% annual chance events.
Two other outflow boundaries were defined, one each east and west of College Avenue, where flows can
return from the SFP to the river corridor. These two boundary conditions were defined as rating curves
based on the 1D modeling results. Two upstream boundary conditions were defined along the UPRR
representing inflows to the 2D model based on the results of the 1D model.
The 2D model analysis was stepped up to the maximum inflow at each of the two inflows, and the
modeling extended until steady-state conditions were established. The results of the 2D modeling for the
1% annual chance event are shown in Figures 2 and 3. In both figures, the proposed condition 2D model
results are represented graphically using a graduated color scale based on flood depth; in addition, the
1D 1% annual chance floodplain boundary is shown for comparison.
In Figure 2, areas where 2D flood depths would be less than 0.05 feet have been filtered out to avoid
focusing on areas that may be subject to localized nuisance flooding but may not actually be impacted by
riverine flood flows. In Figure 3, areas where 2D flood depths would be less than 1 foot are not shown in
an attempt to identify all areas outside of the 1D floodplain that, based on the 2D model, would be subject
to more than 12 inches of flooding.
In evaluating Figure 2 it is apparent that flow patterns through the PH2 Site are similar for both the 1D
and 2D analyses. In addition, the dry zone along the north driveway indicated by the 1D floodplain is
confirmed. Reviewing Figure 3, it is apparent that based on the 2D model results, virtually no areas
outside of the 1D floodplain are subject to more than 12 inches of flooding.
The results of the 2D modeling for the 2% annual chance event are shown in Figures 4 and 5. As with the
1% annual chance figure, in both Figures 4 and 5, the proposed condition 2D model results are
represented graphically using a graduated color scale based on flood depth; in addition, the 1D 2% annual
chance floodplain boundary is shown for comparison.
Reviewing Figures 4 and 5, it is apparent that the 1D and 2D modeling of the 2% annual chance event
results in similar flow patterns through the PH2 Site and there are virtually no areas outside of the 2%
annual chance floodplain that would be subject to more than 12 inches of flooding.
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Figure 2. Comparison of the 1% Annual Chance 1D and 2D Modeling Results.
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Figure 3. Comparison of the 1% Annual Chance 1D and 2D Modeling Results (>1 ft only).
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Figure 4. Comparison of the 2% Annual Chance 1D and 2D Modeling Results.
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Figure 5. Comparison of the 2% Annual Chance 1D and 2D Modeling Results (>1 ft only).
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FLOODPLAIN MAPS
VINE DRIVE
COY DITCH
BNSF RR
CA
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LA POUDRE RIVER
REDWOOD STUNION
PAC
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MATCH LINE MATCH LINEC
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4963.44959.
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3.
14967.64963.24973.34973.14972.94972.94974.14974.24972.54972.14970.34968.74966.44964.24964.2
4964.2
4964.24964.24969.44961.
04965.04966.04968.04975.04978.02
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T Anderson Consulting Engineers, IncCivil ▪ Water Resources ▪ Environmental375 East Horsetooth Road, Building 5, Fort Collins, CO 80525Phone (970) 226-0120 / Fax (970) 226-0121www.acewater.com11EFFECTIVE ANDCORRECTED EFFECTIVEFLOOD HAZARD WORKMAPPOWERHOUSE 2 NO-RISEANALYSIS
VINE DRIVE
COY DITCH
BNSF RR
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LA POUDRE RIVER
REDWOOD STUNION
PAC
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T Anderson Consulting Engineers, IncCivil ▪ Water Resources ▪ Environmental375 East Horsetooth Road, Building 5, Fort Collins, CO 80525Phone (970) 226-0120 / Fax (970) 226-0121www.acewater.com22CORRECTED EFFECTIVEAND PROPOSED CONDITIONFLOOD HAZARD WORKMAPPOWERHOUSE 2 NO-RISEANALYSIS