HomeMy WebLinkAboutPOWERHOUSE 2 - PDP220006 - SUBMITTAL DOCUMENTS - ROUND 2 - STORMWATER-RELATED DOCUMENTS (4) MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 1 of 13 June 28, 2022
DATE: June 28, 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. The information provided in this memo
has been revised from the previous memo dated April 25, 2022 in response
to comments provided by the City on May 22, 2022. 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. Figure 1 provides a vicinity map of the proposed PH2 Site. 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. Two of the areas
adjacent to the building, above the retaining walls, will be used as storage yards for materials used for
experimentation processes in the building. These materials will consist of tanks containing various gases
which result in the proposed building being considered a critical facility. The critical facility component of
this project is discussed in a variance request that is being prepared by ACE for this project.
The PH2 site is located in the north overbank of the 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 separated 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. 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). However, the owner intends to submit a
CLOMR to be approved prior to construction.
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 2 of 13 June 28, 2022
Figure 1. Vicinity Map for the Powerhouse 2 Site.
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 3 of 13 June 28, 2022
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
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 2021 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 directly 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%
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 4 of 13 June 28, 2022
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
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 5 of 13 June 28, 2022
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 the area encompassed by the College Avenue SFP.
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. This adjustment was made to reflect the presence of a small, mapped 1% annual chance island
along Cross Section 1235 directly north of Vine Drive that is in close proximity to Cross Section 1341,
resulting in a small portion of Cross Section 1341 being ineffective.
With only one exception, Manning’s n roughness coefficients were left unchanged from the duplicate
effective hydraulic model. Based on a review of existing conditions within the PH2 Site, an adjustment
was made to one of the Manning’s n values at Cross Section 930 where the roughness coefficient for most
of this cross section was changed from 0.045 to 0.04. This change was made to better reflect the average
roughness condition within the center portion of the site that is characterized by Cross Section 930. This
area consists primarily of a gravel surface with a few small, fixed objects which would cause minor, local
obstructions, along with a relatively narrrow band of vegetation along the banks of the Lake Canal.
Where base topography remained unaltered from the effective condition, bank stations remained
unchanged from the effective model. For cross sections where more detailed base topography was
available, bank station locations were set to closely align with effective bank stations.
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 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.
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 6 of 13 June 28, 2022
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
(HEC-RAS Plan:
DE FP Flow
Determination)
Corrected
Effective
Condition
(HEC-RAS Plan:
CE FP Flow
Determination)
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
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.
Both the channel and left overbank reach lengths were modified for Cross Section 1004 (CE XS 930) and
Cross Section 1264 (CE XS 1235) to reflect both the change to the hydraulic baseline and the re-orientation
of the left portion of Cross Section 1004.
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. This
primarily included a change to the ineffective flow area on Cross Section 1370 (CE XS 1341) to reflect the
proposed building and related improvements, while removing the effect of the previous island of high
ground which will be eliminated due to site grading.
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 7 of 13 June 28, 2022
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.
For the proposed condition model, bank station locations were modified at a number of cross sections
within the site to better encompass the bulk of the main conveyance corridor along the hydraulic baseline,
which was realigned to the north side of the proposed building.
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
(ft, NAVD 1988) Change in
1% Annual
Chance WSEL
(ft)
Corrected
Effective
Condition
(HEC-RAS Plan:
CE FP Flow
Determination)
Proposed
Condition
(HEC-RAS Plan:
PC FP Flow
Determination)
Corrected
Effective
Condition
(HEC-RAS Plan:
CE FP Flow
Determination)
Proposed
Condition
(HEC-RAS Plan:
PC FP Flow
Determination)
College Avenue Split Flow Path
723 723 4964.23 4964.23 0.00
930 (917.5) 4964.30 (4964.20) – 0.10
(973.7) 1004 (4964.54) 4964.18 – 0.36
1235 1264 4965.98 4965.85 – 0.13
1341 1370 4967.56 4967.40 – 0.16
1470 1499 4968.74 4968.74 0.00
Note: (###.#) represents interpolated station and WSEL
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.
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 8 of 13 June 28, 2022
FREEBOARD EVALUATION
Freeboard was evaluated for effective conditions to demonstrate that the required flood protection
elevation would be met for the proposed building and adjacent storage yards. The 1% annual chance
flood elevation at the west (upstream) face of the proposed building is 4966.61 feet, NAVD. This implies
a flood protection elevation of 4968.61 feet, NAVD. The proposed finished floor elevation of the PH2
Building is 4969.00 feet, NAVD, while the lowest slab elevation for the two storage yards along the north
side of the building is 4968.73 feet, NAVD. Consequently, the building would be elevated 0.39 feet above
the flood protection elevation, while the storage yards would be elevated a minimum of 0.12 feet above
the flood protection elevation.
Freeboard was also evaluated relative to the effective 0.2% annual chance (500-year) water surface
elevation to show that the building and storage yards would be substantially above the 0.2% annual
chance flood level. At the west (upstream) end of the proposed building, the 0.2% annual chance flood
elevation would be 4967.35 feet, NAVD. This indicates that the proposed building would be elevated 1.65
feet above the 0.2% annual chance flood level, and the storage yards would be elevated a minimum of
1.38 feet above the 0.2% annual chance flood level.
Since base flood elevations for the proposed condition would be equal or lower than those for the
effective condition, the building and storage yards would have slightly greater freeboard in the proposed
configuration. Freeboard calculations for the 1% and 0.2% annual chance events are attached at the end
of this memo.
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 value of 15 was used to represent the proposed building.
Since the 2D model roughness coefficients were set to generally be consistent with the 1D values, rather
than incorporating a slight lowering of values that can often be justified, the resulting 2D water surface
elevations and floodplain extents may be slightly conservative.
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.
Three other outflow boundaries were defined, one west and two east of College Avenue, where flows can
return from the SFP to the river corridor. These three 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.
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 9 of 13 June 28, 2022
In order to promote model stability, the inflow hydrographs were stepped up at an appropriate rate to
reach steady-state conditions, an appropriate grid size was selected given the detail of the site plan and
topography, and an adjustable time step was utilized based on the courant number.
The 2D model analysis was stepped up to the maximum inflow at each of the two inflow locations, and
the modeling extended until steady-state conditions were established. The 2D model was utilized to
analyze the study area using both momentum and diffusion wave methodologies. It appeared that the
diffusion wave technique better represented the shallow flooding area north of the Lake Canal;
consequently, in all cases the diffusion waves results are reflected in the figures presented in this memo.
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 be materially 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.
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 10 of 13 June 28, 2022
Figure 2. Comparison of the 1% Annual Chance 1D and 2D Diffusion Wave Modeling Results.
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 11 of 13 June 28, 2022
Figure 3. Comparison of the 1% Annual Chance 1D and 2D Diffusion Wave Modeling Results (>1 ft only).
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 12 of 13 June 28, 2022
Figure 4. Comparison of the 2% Annual Chance 1D and 2D Diffusion Wave Modeling Results.
MEMORANDUM
COPH201_NoRise_Memo_2022-06-28.docx Page 13 of 13 June 28, 2022
Figure 5. Comparison of the 2% Annual Chance 1D and 2D Diffusion Wave Modeling Results (>1 ft only).
MEMORANDUM
FLOODPLAIN MAPS
VINE DRIVE
COY DITCH
BNSF RR
CA
C
H
E
LA POUDRE RIVER
REDWOOD STUNION
PAC
IF
IC
RA
ILROAD
L
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M
A
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T
I
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SF
P N COLLEGE AVEHICKORY ST
MATCH LINE MATCH LINEC
O
L
L
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AVE.
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F
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LAKE
CANAL4977.1496
1.
6
4963.44959.
74963.74964.44966.64964.
3
4965.94964.54969.1
4968.94969.4
4969.44969.
6
4970.34969.
5
4970.34971.24969.54971.44965.1496
3.
14967.64963.24973.34973.14972.94972.94974.14974.24972.54972.14970.34968.74966.44964.24964.2
4964.2
4964.24969.44961.
04965.04966.04968.04975.04978.02
3
1
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8
0231119231212 229756230617 230186
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7
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228915230388230572230489150282583645930
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4.
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4967.64964.24968.74968.84970.34971.24969.44968.84968.84968.8LIMIT
UPST
RE
A
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DOW
N
S
T
R
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A
M
STU
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4964.2
PROPOSED
PH2 BLDG 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
CA
C
H
E
LA POUDRE RIVER
REDWOOD STUNION
PAC
IF
IC
RA
ILROAD
L
E
E
M
A
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T
I
N
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SF
P N COLLEGE AVEHICKORY ST
MATCH LINE MATCH LINEC
O
L
L
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AVE.
S
F
P
LAKE
CANAL
2
3
1
0
8
0231119231212 229756230617 230186
230
8
9
1
230
7
7
5
2306
7
9
2
3
2
3
6
2
228915230388230572230489150282583645930
1235
147
016
7
6
13417232302
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4.
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4964.24964.2
4964.2
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2
4964.24964.2 4967.34964.2100417602549LIMIT
UPST
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DOW
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STU
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4964.2
PROPOSED
PH2 BLDG 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
MEMORANDUM
FREEBOARD CALCULATIONS
T
D
CONTROL
IRR
CONTROL
IRR
D FE
S
TFTF
UD
UD UD UD UD UD UD UD UD UD UD UD UD
UD
UD
UD
UD
IR
EAST VINE DRIVENORTH COLLEGE AVENUE(63.13)(6
7
.
0
0
)
(66.59)(66.75)(6
7
.
0
2
)
EXISTING 30' UTILITY &
DRAINAGE EASEMENT
LAKE CANAL EASEMENT
PROPOSED RAIN GARDEN PROPOSED RAIN GARDEN
PROPOSED STORM LINE
PROPOSED
STORM LINE
PROPOSED STORM LINE
PROPOSED 10'
CONCRETE PAN
AREA INLET (TYP.)
PROPOSED STORM LINE
PROPOSED RAIN GARDEN
PROPOSED STORM LINE
PROPOSED STORM LINE
PROPOSED STORM LINE
PROPOSED RAIN GARDEN
100 YR FLOOD PLAIN
(CURRENT EFFECTIVE)500 YR FLOOD PLAIN
(CURRENT EFFECTIVE)
30' CANAL EASEMENT
PER BRUSH MOBILE
HOME P.U.D.
15' UTILITY EASEMENT15' UTILITY EASEMENTPROPOSED OUTDOOR
LANDSCAPED AMPHITHEATER
24' EMERGENCY
ACCESS EASEMENT
24' EMERGENCY
ACCESS EASEMENT
PROPOSED BUILDING
FFE=4969.0068.8268.8268.8268.8267.6064.0863.7263.7663.5068.73 68.
8
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2.0%
1.5%
2.
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64.74
64.18
63.99
63.01
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61.79
61.40
61.61
61.08
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60.64
60.54
60.86
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62.6562.2162.5861.3961.2561.1361.00NATURAL HABITAT
BUFFER ZONE
SheetPOWERHOUSE 2These drawings areinstruments of serviceprovided by NorthernEngineering Services, Inc.and are not to be used forany type of constructionunless signed and sealed bya Professional Engineer inthe employ of NorthernEngineering Services, Inc.NOT FOR CONSTRUCTIONREVIEW SETof 16
G1 GRADING PLAN10
NORTH
( IN FEET )
1 inch = ft.
Feet02020
20
40 60
PROPOSED CONTOUR
EXISTING STORM SEWER
PROPOSED STORM SEWER
PROPOSED SWALE FLOW LINE
EXISTING CONTOUR
PROPOSED CURB & GUTTER
PROPERTY BOUNDARY
PROPOSED SPOT ELEVATION
PROPOSED SLOPES
1.THE SIZE, TYPE AND LOCATION OF ALL KNOWN UNDERGROUND UTILITIES ARE
APPROXIMATE WHEN SHOWN ON THESE DRAWINGS. IT SHALL BE THE
RESPONSIBILITY OF THE CONTRACTOR TO VERIFY THE EXISTENCE OF ALL
UNDERGROUND UTILITIES IN THE AREA OF THE WORK. BEFORE COMMENCING
NEW CONSTRUCTION, THE CONTRACTOR SHALL BE RESPONSIBLE FOR LOCATING
ALL UNDERGROUND UTILITIES AND SHALL BE RESPONSIBLE FOR FOR ALL
UNKNOWN UNDERGROUND UTILITIES.
2.REFER TO THE PLAT FOR LOT AREAS, TRACT SIZES, EASEMENTS, LOT DIMENSIONS,
UTILITY EASEMENTS, OTHER EASEMENTS, AND OTHER SURVEY INFORMATION.
3.ALL PROJECT DATA IS ON VERTICAL DATUM; NAVD88. SEE COVER SHEET FOR
BENCHMARK REFERENCES.
4.ALL CURB SPOTS SHOWN ARE FLOWLINE ELEVATIONS. ALL OTHER SPOTS ARE
FINISHED GRADE ELEVATIONS.
5.LOT GRADING IS DESIGNED FOR TYPICAL RECTANGULAR FOOTPRINT WITH
DIMENSIONS AS SHOWN HEREON. SHOULD BUILDING FOOTPRINT CHANGE,
GRADING SHALL BE ADJUSTED BY PLOT PLAN FOR THE SPECIFIC FOOTPRINT. ALL
MINIMUM OPENINGS SHALL BE A MINIMUM OF 18" ABOVE FRONT LOT ELEVATIONS
AND PLOT PLAN DESIGN SHALL MEET FHA GRADING GUIDELINES AND BUILDING
CODE REQUIREMENTS.
6.THE NATURAL HABITAT BUFFER ZONE IS INTENDED TO BE MAINTAINED IN A NATIVE
LANDSCAPE. PLEASE SEE SECTION 3.4.1 OF THE LAND USE CODE FOR ALLOWABLE
USES WITHIN THE NATURAL HABITAT BUFFER ZONE.
NOTES:
PROPOSED STORM INLET
2.0%
(47.45
)
EXISTING SPOT ELEVATION
EXISTING LOT LINE
PROPOSED CONCRETE
CROSS PAN (TYP.)
33.43
EXISTING RIGHT OF WAY
PROPOSED LOT LINE
PROPOSED RIGHT OF WAY
LEGEND:
PROPOSED GRADE BREAK
PROPOSED UNDERDRAIN UD
KEYMAP
T
GAS
G1 G2
CALL 2 BUSINESS DAYS IN ADVANCE BEFORE YOU
DIG, GRADE, OR EXCAVATE FOR THE MARKING OF
UNDERGROUND MEMBER UTILITIES.
CALL UTILITY NOTIFICATION CENTER OF
COLORADO
Know what'sbelow.
before you dig.Call
R
MEMORANDUM
INTERNAL QA/QC NOTES
MEMORANDUM
RESPONSES TO COMMENTS
Page 1 of 3
Utilities
electric • stormwater • wastewater • water
700 Wood St.
PO Box 580
Fort Collins, CO 80522
970.221.6700
970.221.6619 fax • 970.224.6003 TDD
utilities@fcgov.com • fcgov.com/utilities
RESPONSES TO COMMENTS
JUNE 28, 2022
May 22nd, 2022
Gregory J. Koch, P.E.
Anderson Consulting Engineers
375 E. Horsetooth Road
Fort Collins, CO 80525
RE: Review Comments for Powerhouse 2 No-Rise Floodplain Evaluation at the Poudre River
Dear Greg,
The City of Fort Collins Master Planning & Floodplain Management Division (City) has reviewed the report titled,
Powerhouse 2 – No-Rise Floodplain Evaluation, herein referred to as the Report.
Based on our review of the first submission to the City, we offer the following comments:
General Comments
1. Please provide the Anderson Consulting Engineers internal QA/QC documentation for these hydraulic
models.
Our internal QA/QC notes are now included in the memo attachments.
2. Please review the comments in the body of the PDF Report.
The comments in the report have been reviewed and addressed with changes to the report.
3. Please state in the Report from which model plan the WSE values are derived.
a. Example: Plan .p11, “PC FP”
Plan references have been added in the report tables.
4. Please check effective WSE at XS 1235 for linear Interpolation for freeboard. It is noted that this is a
small difference and will have little impact on the final freeboard values.
a. Hand calcs show WSE = 4966.01 ft @ XS 1235
b. Model states WSE = 4965.98 ft @ XS 1235
Effective WSELs, which were taken from the PWWP LOMR, are shown in Table 1 of the memo
which reflects the 4966.01 value. It appears the 4965.98 elevation is from the corrected
effective analysis. The effective WSELs continue to be used for the freeboard calculation, but
the calculation has changed slightly as architectural elements have been added to the building
which effectively moves the west face of the building 5 feet upstream. The flood protection
elevation is slightly higher and the building and storage yards have been elevated accordingly.
Page 2 of 3
Mapping Comments
5. Please incorporate any changes to the modeling into the mapping products.
Due to the nominal changes to proposed condition WSELs between the initial submittal and the current
submittal, based on a review of local topography and modeled floodplain limits it was determined that
floodplain mapping changes were not warranted.
Hydraulic Model Comments
6. Please explain the reason for the addition of the IFA on the right side of XS 1341 in the corrected
effective model. This IFA is similar to the IFA used for the proposed building but shifted by ~50 feet and
lower in elevation.
There is a small mapped 100-year island along XS 1235 for the effective condition. This island is in close
proximity to XS 1341 resulting in a small portion of XS 1341 being ineffective. Defining this ineffective
area was overlooked in the LOMR model so it was added to the corrected effective model. This has
been noted in the report.
7. Please explain the reason behind the change in roughness value at XS 903 from 0.045 to 0.040 in the
corrected effective model?
The majority of the center portion of the existing site, which is represented by XS 930, consists primarily
of a gravel surface with a few small, fixed objects which would cause local obstructions; there is also a
local band of vegetation along the banks of the Lake Canal. This has been noted in the report.
8. The change in model station line between cross-section 1235 and 723 and the change in alignment for
cross-section 930 both look appropriate given the new grading and the proposed PH2 building. Thank
you for going to this level of detail to refine this model.
Acknowledged.
9. Please consider updating the downstream reach lengths (L, R, and Ch) to reflect the change in the cross-
section alignment and model station line. Please inspect cross-sections 1235 – 930 to ensure the
distance the flood wave travels overland between cross-sections is representative in the model.
Both the channel and left overbank downstream reach lengths were modified in the proposed condition
model for Cross Sections 1004 (old 930) and 1264 (old 1235).
10. Bank stations appear to be similar to the effective condition in some places and altered in others. How
were these bank stations determined? Bank stations do not follow similar elevations or features for
placement. Please explain reasons and methodology for bank station changes and placement.
Without a defined channel along the College Avenue SPF, locating bank stations can be somewhat
subjective. Bank stations in the corrected effective model have been adjusted to more closely match
the effective bank stations; at some cross sections, the locations are not exactly the same due to
changes in base topography. For the proposed condition model, bank station locations were modified at
selected cross sections to better encompass the bulk of the main conveyance corridor along the
hydraulic base line which was realigned to the north side of the proposed building. This has been noted
in the report.
11. After updating the downstream reach lengths, please make a comparison of the changes in water
surface elevation at a coincident location. Please use the effective cross-section location and
Page 3 of 3
interpolate a new proposed conditions WSE based on the bounding cross-sections data. Please update
Table 3 of the Report with these new data.
Since both the hydraulic baseline and cross section orientation changed between corrected effective
and proposed condition, two interpolations were completed, as shown in the new Table 3.
12. A detailed review of the 2D model was not done for this no-rise evaluation. We offer these basic
comments after a high-level review of the 2D model:
a. Thank you for taking the additional time to create a more robust analysis and evaluating both
the diffusive wave and full momentum equations.
i. Please state in the report which model results are being used in the figures and
compared to the 1D model. There are some differences in extents and depths between
the two equations.
It is now mentioned in the memo and noted on the figures that the diffusion wave
results are being presented. It was felt that this methodology better reflected the
shallow flooding shown in the northern portion of the model.
b. Near “Outflow LS 1004” at the southern edge of the mesh, flow appears to be “stacked up”
against the boundary of the mesh without an external boundary condition to allow the flow to
leave the study area.
An extra outflow boundary has been added just downstream of the LS 1004 boundary to allow
flow to exit at this location. It will still appear as though flow is stacking directly downstream of
this new boundary. This is intentional as the 1D model indicates that 100-year WSELs along the
river are nearly identical to those along the SPF at this location and, as such, water transfer
would be negligible.
c. What steps were taken to ensure a stable model run?
i. No velocity spikes were observed in this high-level review and the Courant Numbers
look reasonable for both equations analyzed.
In order to promote model stability, the inflow hydrographs were stepped up at an
appropriate rate to reach steady-state conditions, an appropriate grid size was selected
given the detail of the site plan and topography, and an adjustable time step was
utilized based on the courant number. This is now mentioned in the memo.
If there are any questions or concerns, please don’t hesitate to contact us.
Respectfully,
Theodore Bender, PE, CFM
Hydraulic Engineer
970-221-6503
tbender@fcgov.com
Marsha Hilmes-Robinson, CFM
Floodplain Administrator
970-224-6036
mhilmesrobinson@fcgov.co