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NORTHERNENGINEERING.COM | 970.221.4158 Graham Property Sewer Impact Study Technical Memorandum
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GRAHAM PROPERTY SEWER IMPACT STUDY TECHNICAL MEMORANDUM
February 25, 2022
Heidi Hansen - Water Utilities Development Review Manager
City of Fort Collins Utilities
215 N. Mason Street
Fort Collins, Colorado 80525
RE: Graham Property Sewer Impact Study Technical Memorandum
NE Project No. 1893-001
Dear Ms. Hansen,
This memorandum is intended as an amendment to the previous sewer impact study submitted by Jeff
Christopherson in July 2021. While adhering to previously vetted parameters defined in the 2021 Christopherson
technical memorandum, a new capacity model was developed rather than attempting to obtain and reuse any
models created by others. Results of the newly created model were calibrated with previous model results to
validate the output.
Summary of Findings
Based on the results of this analysis, the following conclusions can be drawn.
With only the Northfield development (peak flows totaling 306 gpm), (Jacobs, 2018), the pipe
profile as provided in the Utility Plans for Northfield (Highland Development Services, 2020) has
eight (8) pipe segments that do not meet the City’s d/D = 0.5 criterion within the study area. These
results are consistent with the previous analysis.
The addition of the proposed Enclave and Graham developments under flow condition #2 add
peak flows of 205 gpm (Harris Kocher Smith, 2021) and 186 gpm, respectively. Thus, the total peak
flow under this scenario is 698 gpm. The new model indicates there are eight (8) pipe segments
that do not meet the City’s d/D = 0.5 criterion within the study area.
Modeling results indicate that there are numerous pipe segments not meeting the City’s minimum
2 feet per second velocity criterion under all modeled conditions for peak wet weather flow.
Based on the results of the conflicting utilities assessment, the NECCO A-4 storm line is the
prohibiting factor from connection to the existing sanitary sewer system from the proposed
Graham Property.
Caveats to these model results are as follows.
Conditions of this model assume all peak flows from each proposed development and offsite
flows meet in the Lemay sewer main at the exact same instance, which is highly unlikely to occur
in the reality of this system.
The baseline scenario in flow condition #1 carries forward flows defined by a previous analysis
(Christopherson, 2021). These flows seem to have high inflow and infiltration (I/I) flows of over 15
gpm that are added throughout the pipe segments in the study area. These values could be
overstated, especially since I/I flows from each development are already accounted for in the
peak flow calculations.
The flow generation of 120 gallons per capita per day (gpd/capita) is extremely conservative and
unlikely to occur in new construction. Criteria in neighboring communities and utility districts
suggest values of 80 – 100 gpd/capita for multifamily residential.
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GRAHAM PROPERTY SEWER IMPACT STUDY TECHNICAL MEMORANDUM
Project Background
The section of sanitary sewer main in question for this analysis runs along North Lemay Avenue in Fort Collins,
Colorado. The focus of this analysis is an impact study of this sewer main between Conifer Street and East Vine
Drive. The current and future development areas included in this analysis are Northfield, Enclave, and Graham
properties. A map of the project area is provided in Exhibit 1. Previous sewer impact studies on this system have
included the Northfield and Enclave properties, but have not included the proposed Graham property located on
the eastside of N. Lemay Ave. Thus, the intent of this report is to evaluate what impacts the addition of this
property may have on the existing sewer system section. In addition, this study will also provide a preliminary
investigation of the feasibility to construct a gravity driven sewer lateral that can serve the Graham property.
The Northfield development is currently in the final stages of sitework construction, while both the Enclave and
Graham properties are still early in the planning stages. As such, initial flow estimates for the properties still in
the planning stages are not definitive and are subject to further refinement.
Design Assumptions and Proposed Flow Demands
General operational design checks of this analysis compare modeled values with City of Fort Collins Utilities
Water and Wastewater Design Criteria Manual. However, this criteria manual does not have any defined design
criteria for estimating sanitary sewer flows. Thus, the sanitary sewer design standards of the City and County of
Denver Department of Public Works were applied for the sewer flow demand estimations for the Graham
property. These Denver standards are the same used in previous technical memorandums and utility reports for
estimating sewer demands for the Enclave property. A summary of design standards and sources utilized for this
analysis is provided in Table 1 below.
Table 1: Sanitary Sewer Design Assumptions
Parameter Value Source
Maximum Allowable d/D 50% Fort Collins Utilities Design Standards
Minimum Sewer Line Slopes
For 8-inch = 0.40%
For 10-inch = 0.28%
For 12-inch = 0.22%
For 15-inch = 0.15%
Fort Collins Utilities Design Standards
Manning’s “n” value for Sewer Pipes 0.013 Fort Collins Utilities Design Standards
Multi-Family Residential (MFR)
Occupancy
Low Density: 2.8 capita/unit
Medium/High Density: 2.1 capita/unit
Denver Design Standards/ Enclave
Utility Report
Gallons per Capita per Day 120 gpd/capita Denver Design Standards/ Enclave
Utility Report
Inflow and Infiltration (I/I) Rate 500 gpd/tributary acre Denver Design Standards/ Enclave
Utility Report
The Graham property has anticipated a total of 280 multi-family units that will be built on approximately 12-
acres. Based on the Denver criteria, this can be categorized as a High Density MFR land use. Thus, peak sewer
design flows have been determined to be 186 gpm. The complete rationale behind this calculation is provided in
Table 2 below.
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GRAHAM PROPERTY SEWER IMPACT STUDY TECHNICAL MEMORANDUM
Table 2: Graham Property Sewer Demands Calculations
Parameter Value
No. of MFR High Density Units 280 Units
MFR High Density Occupancy 2.1 persons/unit
Average Residential Daily Usage 120 gal/cap/day
Total Residential Population 588 persons
Average Daily Wastewater Flow 49.0 gpm
Peaking Factor 3.71
I/I Rate per Unit Area 500 gal/day/acre
Gross Site Area 12 ac
Total I/I Flow 4.7 gpm
Total Peak Design Flow 186.3 gpm
Model Assumptions and Flow Conditions
This analysis was performed using EPA-SWMM 5.1 software. The sewer model built in this software was
developed using critical elevations, pipe lengths, and pipe slopes gathered from the approved utility construction
plans for the Northfield development (Highland Development Services, 2020). The analysis begins at the existing
sewer manhole WW16606 and extends downstream to the existing manhole WW16603. Minor losses were also
applied to the model links using the same methodology described in the previous technical memorandum. Table
4 provides the minor loss coefficients that were assumed at the manholes in the modeling analysis. These
headloss coefficients were applied as exit losses on the corresponding downstream pipes.
Table 3: Head Loss Coefficients used for Modeling Analysis of the Existing and New Interceptors
Angle through Manhole between
Entrance Pipe and Exit Pipe Headloss Coefficients*
180o 0.15
157.5o 0.45
135o 0.75
120o 0.85
90o 1.0
*Extracted from Table 7-5 of HEC-22, FHWA, 2009
Since this study’s is intended as an update to prior analyses done by others, sewer demand flows determined
previously for Northfield and Enclave developments will be utilized. In addition, contributing offsite flows and
additional I/I determined in the previous technical memorandum will be used to make an alike comparison of
results. In particular, the Northfield only flow comparison will be used as a model calibration.
Flow Condition #1 (Northfield + Existing Offsite and I/I Flows):
The flows from this condition were determined from previous analysis performed on this section of the sewer to
calibrate the newly created model and to compare results using identical sewer flows. A detailed summary of
modeled flow inputs at their respective nodes is provided in Table 4 below.
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Table 4: Model Node Input Flows for Flow Condition #1
Key
Location Node ID
Estimated POC Flows (gpm)
Estimated
Offsite
Flows (gpm)
Estimated
I/I Flows
(gpm)
Total Node Input
Flow (gpm) Northfield Enclave Graham
Start of
Analysis WW16606 567 0 567
POC-1 SSMH_38 120.97 1.03 122
WW16607 1 1
POC-4 SSMH_33 35.18 1.82 37
POC-3 SSMH_22 32.33 0.67 33
WW16608 0
POC-2 SSMH_08 69.26 0.74 70
WW16609 1 1
WW16597 2 2
SSMH_52 1 1
SSMH_51 1 1
SSMH_50 1 1
SSMH_49 1 1
POC-5 WW16595 48.7 9.3 0 58
WW16610 2 2
WW16602 7.0 0 7
End of
Analysis WW16604 1 1
Flow Condition #2 (Northfield + Enclave + Graham + Existing Offsite and I/I Flows):
This flow condition is similar to that of flow condition #1 with the addition of the anticipated peak flows
associated with the proposed Enclave and Graham developments. All previous Northfield, offsite and I/I flows
from condition #1 remain the same in this scenario. Proposed flows from the Enclave development are
anticipated to connect at the existing Lemay sewer system at model node SSMH_38 (POC-1) while proposed flows
from the Graham property will connect to the Lemay system at model node SSMH_08 (POC-2). A detailed
summary of modeled flow inputs at their respective nodes is provided in Table 5 below.
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Table 5: Model Node Input Flows for Flow Condition #2
Key
Location Node ID
Estimated POC Flows (gpm)
Estimated
Offsite
Flows (gpm)
Estimated
I/I Flows
(gpm)
Total Node Input
Flow (gpm) Northfield Enclave Graham
Start of
Analysis WW16606 567 0 567
POC-1 SSMH_38 120.97 205.4 1.03 327.4
WW16607 1 1
POC-4 SSMH_33 35.18 1.82 37
POC-3 SSMH_22 32.33 0.67 33
WW16608 0
POC-2 SSMH_08 69.26 186.3 0.74 256.3
WW16609 1 1
WW16597 2 2
SSMH_52 1 1
SSMH_51 1 1
SSMH_50 1 1
SSMH_49 1 1
POC-5 WW16595 48.7 9.3 0 58
WW16610 2 2
WW16602 7.0 0 7
End of
Analysis WW16604 1 1
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Model Results for Flow Condition #1
This section provides the modeled results for the flow condition #1 scenario, which includes upstream demands
north of manhole WW16606 along with previously projected peak flows from the Northfield development. Model
results provided in Table 6 and Figure 1 indicate there are seven (7) pipe segments that do not meet the City’s d/D
= 0.5 criterion within the study area. The non-conforming pipe segments are highlighted in red in Table 6.
Table 6: Output Model Results for Flow Condition #1
Key Location Pipe
ID
Upstream
Node ID
Downstream
Node ID
Pipe
Diameter
(in.)
Pipe
Slope
(%)
Maximum
Flow (gpm)
Maximum
Velocity
(fps)
Maximum
d/D
Start of Analysis P101 WW16606 SSMH_38 15 0.19 567 2.22 0.47
POC-1 P102 SSMH_38 WW16607 15 0.18 689 2.31 0.53
P103 WW16607 SSMH_33 15 0.15 690 2.12 0.57
POC-4 P104 SSMH_33 SSMH_22 15 0.14 727 2.12 0.60
POC-3 P105 SSMH_22 WW16608 15 0.14 760 2.22 0.60
P106 WW16608 SSMH_08 15 0.16 760 2.11 0.62
POC-2 P107 SSMH_08 WW16609 18 0.11 830 1.99 0.52
P108 WW16609 WW16597 18 0.11 831 1.99 0.52
P109 WW16597 SSMH_52 24 0.08 833 1.75 0.37
P110 SSMH_52 SSMH_51 24 0.08 834 1.62 0.39
P111 SSMH_51 SSMH_50 24 0.08 835 1.73 0.37
P112 SSMH_50 SSMH_49 24 0.08 836 1.73 0.38
P113 SSMH_49 WW16595 24 0.08 837 1.65 0.39
POC-5 P114 WW16595 WW16610 24 0.08 895 1.71 0.40
P115 WW16610 WW16602 24 0.08 897 1.85 0.38
P116 WW16602 WW16604 24 0.08 904 1.55 0.43
End of Analysis P117 WW16604 WW16603 24 0.08 905 1.81 0.38
In comparing these results to those of the previous technical memorandum, there are a few minor discrepancies
related to maximum pipe velocities and d/D ratios. However, the values consistent enough and within an
acceptable tolerance to raise any concerns. Besides the difference in modeling software, other likely causes of
variation are:
Several pipe slopes used in the previous analysis were slightly off in comparing the actual slopes
shown in the Northfield Utility Plans.
Existing manhole WW16608 was not included in the previous analysis.
The previous memo did not specify what minor loss values were assigned to each segment, so it is
possible different minor loss values were applied.
Manhole inlet/outlet offsets (drops) were applied to this model, but not the previous study.
However, despite these minor discrepancies, results from the new model still show the same segments of pipes
not meeting City defined criteria for d/D ratios and minimum pipe velocities.
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Figure 1: Output Model Hydraulic Profile Results for Flow Condition #1
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Model Results for Flow Condition #2
This section provides the modeled results for the flow condition #2 scenario, which includes previous peak flows
from flow condition #1 plus the addition of projected peak flows from the Enclave and Graham properties. Model
results provided in Table 7 and Figure 2 indicate that eight (8) pipe segments do not meet the City’s d/D = 0.5
criterion within the study area.
Table 7: Output Model Results for Flow Condition #2
Key Location Pipe
ID
Upstream
Node ID
Downstream
Node ID
Pipe
Diameter
(in.)
Pipe
Slope
(%)
Maximum
Flow (gpm)
Maximum
Velocity
(fps)
Maximum
d/D
Start of Analysis P101 WW16606 SSMH_38 15 0.19 567 2.22 0.47
POC-1 P102 SSMH_38 WW16607 15 0.18 894 2.45 0.63
P103 WW16607 SSMH_33 15 0.15 895 2.24 0.68
POC-4 P104 SSMH_33 SSMH_22 15 0.14 932 2.23 0.71
POC-3 P105 SSMH_22 WW16608 15 0.17 965 2.32 0.71
P106 WW16608 SSMH_08 15 0.16 965 2.2 0.74
POC-2 P107 SSMH_08 WW16609 18 0.11 1222 2.16 0.67
P108 WW16609 WW16597 18 0.11 1223 2.17 0.67
P109 WW16597 SSMH_52 24 0.08 1225 1.94 0.46
P110 SSMH_52 SSMH_51 24 0.07 1226 1.79 0.49
P111 SSMH_51 SSMH_50 24 0.08 1227 1.92 0.46
P112 SSMH_50 SSMH_49 24 0.08 1229 1.82 0.48
P113 SSMH_49 WW16595 24 0.07 1287 1.88 0.49
POC-5 P114 WW16595 WW16610 24 0.08 1289 2.04 0.46
P115 WW16610 WW16602 24 0.08 1296 1.7 0.53
P116 WW16602 WW16604 24 0.05 1297 1.99 0.47
End of Analysis P117 WW16604 WW16603 24 0.08 1228 1.92 0.46
Based on these results presented with the addition of the Enclave and Graham properties, all non-conforming
pipe segments from flow condition #1 remain as such under flow condition #2. There is an additional
downstream pipe segment between existing manholes WW16602 and WW16604 that is barely exceeds the City’s
d/D threshold under this flow scenario as well.
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Figure 2: Output Model Hydraulic Profile Results for Flow Condition #1
Sewer Constructability for Graham Property
The ±12-acre Graham property is located north of Suniga Road between “old” North Lemay Avenue (aka,
Lindenmeier Road) and realigned Lemay Avenue (see Exhibit #1). To connect the Graham Property to the existing
SSMH_08, a new sewer line must cross two active waterlines, an existing storm sewer line, and existing buried
cable and electric utilities within the Lindenmeier Road right-of-way. A test hole evaluation was performed by
Kinetic in January 2022 to determine the depths of these various utilities. A summary of the test hole results is
provided in Table 8.
Table 8: Utility Crossing Test Hole Results
TH
#
Test Hole
Date Utility Owner Utility Type Utility
Size
Utility
Material
Depth from
Ground to
Top of Pipe
Surface
Elevation
Top of
Pipe
Elevation
1 01/07/2022 City of Fort Collins Electric 1" PVC 2’-3” 4952.72 4950.47’
2 01/07/2022 Comcast CATV 1" PVC 2’-3” 4952.73 4950.48’
3 01/12/2022 ELCO Water District Water 12" PVC 4’-11” 4953.05 4948.13’
4 01/07/2022 City of Fort Collins Water 24" STEEL 5’-0” 4952.57 4947.57’
5 01/07/2022 City of Fort Collins Storm Sewer 48" RCP 4’-3” 4951.48 4947.23’
Based on these results, a plan and profile exhibit was developed to assess the utility crossings for conflicts
(Exhibit #2). The results of this assessment indicate at least three conflicts requiring mitigation. The proposed
remedies for the conflicting utilities are described below.
The 12” ELCO waterline should be lowered to provide acceptable vertical clearance from the
proposed sewer line. ELCO has indicated that an air release valve will be required on the south
side of the 12” lowering.
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GRAHAM PROPERTY SEWER IMPACT STUDY TECHNICAL MEMORANDUM
The 24” City of Fort Collins waterline should also be lowered to provide acceptable clearance from
the proposed sewer line. The need for an air release valve should be evaluated early in the design
process.
A design variance is recommended to allow 12” of vertical separation between the 48” City of Fort
Collins NECCO A-4 storm line and the proposed 8” sewer line to serve the Graham property. The
need for casing, insulation board or other special installation requirements should be evaluated
early in the design process. The variance is not expected to pose public health or safety concerns,
nor make future maintenance of either utility more onerous.
The existing ground depicted on Exhibit #2 is drawn from Fort Collins LiDAR topographic data. Based on the
information currently available, it appears possible to gravity serve the Graham property. However, fill
placement and/or insulation board will be required to satisfy City design standards and construction
specifications. Fortunately, the property is zoned MMN and development is expected to be multifamily
dwelling units without basements. This will help limit the amount of fill required for the property to develop.
The shallow sanitary sewer depths will require close attention during future site planning and design of the
Graham property.
Conclusion
While the modeling results of both flow scenarios indicate that many pipe segments do not meet current City of
Fort Collins criteria, that does not necessarily mean there are system capacity concerns. Multiple layers of
(over)conservative parameters are currently built into the model, such as concurrent peak flows, heavy I/I, and
excessive per capita flow generation. Even with these inflated values, the segments not meeting City criteria for
d/D and minimum velocity are very close to the City’s thresholds in most instances. The sections with the highest
d/D occur upstream of POC-2, and therefore are not attributed to the proposed Graham connection. Additionally,
it is not uncommon to see d/D ratios of 0.6 – 0.8 allowed during peak flow conditions. If flows from Enclave are
reduced from those currently modeled, the d/D results only improve.
For these reasons, it is reasonably probable that the North Lemay Sanitary sewer system has capacity to serve the
Graham property.
Northern Engineering appreciates the opportunity to provide this initial sewer capacity and constructability
analysis. If you have any questions as you review this memo, please feel free to contact us.
Sincerely,
NORTHERN ENGINEERING SERVICES, INC.
Michael Castillo, EIT
Project Engineer
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GRAHAM PROPERTY SEWER IMPACT STUDY TECHNICAL MEMORANDUM
References
1. City of Fort Collins. 2017. Fort Collins Utilities Water and Wastewater Design Criteria Manual.
2. City and County of Denver Department of Public Works. 2008. Sanitary Sewer Design Technical Criteria
Manual.
3. Harris Kocher Smith, 2021. Wastewater Utility Report for Enclave at Redwood. Prepared for DHI
Communities.
4. Highland Development Services. 2020. Utility Plans for Northfield. February 2020 plans for Sanitary
Sewers; Sheets 3 – 6, 28 – 36.
5. Jacobs Engineering Group Inc. 2018. City of Fort Collins Sanitary Sewer System – Development Review
(Piping Alternatives for Northfield). Prepared for City of Fort Collins.
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8" SS10" SS10" SS10" SS8" SS8" SS8" SSPROPOSED FUTUREGRAHAM PROPERTYPROPOSED FUTUREENCLAVE PROPERTYNORTHFIELDPROPERTYPOC-1POC-4POC-3POC-2POC-524" SS 24" SS
24" SS18" SS 15" SS 15" SS
WW16606SSMH_38WW16607SSMH_33SSMH_22WW16608SSMH_08WW16609WW16597SSMH_52SSMH_51SSMH_50SSMH_49WW16595WW16610WW16602WW16604E VINE DRN LEMAY AVE
ALTA VISTA ST
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MAIN STEND OFANALYSISSTART OFANALYSISEXHIBIT #1: SANITARY SEWER CAPACITY ANALYSISNE PROJECT NO.: 1893-001FORT COLLINS, COGRAHAM PROPERTYENGINEERNGIEHTRONRN01.21.2022P:\1893-001\DWG\EXHIBITS\1893-001_SEWER_EXHIBIT.DWG( IN FEET )01 INCH = 300 FEET300300
LINDENMEIER RD EXIST. BURIEDELECTRIC & CATVEXIST. SSMH_08EXIST. 48" CITY OFFC STORM SEWERFUTURE 8"SANITARY SEWER12" W 12" W
ABANDONED10" ELCOWATERLINEEXIST. 24" CITY OFFC WATERLINE12" ELCOWATERLINETHTHTHTHTH#1#2#3#4#5494049454950495549404945495049550+751+002+003+003+5012"CLEARANCESTA. 1+11.16CATV
ABANDONED
10" WATER
STA. 1+60.97
24" WATER
STA. 1+94.85
48" STORM
APPROX.EXISTINGGROUNDSTA. 1+10.44ELECTRICSTA. 1+00.00SSMH_08
STA. 1+33.87
12" WATER
FUTURE 8" SS MAINSTA.=1+00.00EX. INV. OUT = 4947.351POTENTIALINSULATIONBOARDFILL AS NECESSARYTO PROVIDE COVEREXISTING WATERMAINS TO BELOWEREDEXHIBIT #2: UTILITY CONFLICTSNE PROJECT NO.: 1893-001FORT COLLINS, COENGINEERNGIEHTRONRN02.25.2022P:\1893-001\DWG\EXHIBITS\1893-001_SSWR_CONFLICTS_EXBT_8.5X11.DWGGRAHAM PROPERTYPROFILE SCALE:( IN FEET )01 INCH = 30 FEET3030