HomeMy WebLinkAboutDrainage Reports - 06/13/2025Hoffman Mill Drainage Memo.docx 1/3
To:
Bill Welborn, City of Fort Collins
CC:
File
AECOM
7595 Technology Way
Denver, CO 80237
T: +1 (303) 694 2770
F: +1 (303) 694 3946
aecom.com
Project name:
Hoffman Mill Minor Amendment
Weigh Station Paving
From:
David Seyboldt
Date:
January 21st, 2022
Technical Memo
Subject: Drainage Calculations
1.Purpose and Objectives
This memo presents the results of the drainage analysis of the Hoffman Mill Weigh Station site and
assesses the compatibility of the Weigh Station site improvements with the existing drainage
infrastructure located on the Fueling Site. The existing infrastructure includes a detention pond, outlet
pipe, and overflow spillway and is located immediately downstream of the Weigh Station site. The
constructed Weight Station site improvements include a paved driveway and weigh scales. Proposed
improvements include a shallow swale graded into the parking lot to direct runoff toward the detention
pond instead of into the Fueling Site building.
A Final Storm Drainage Letter was prepared by Interwest Consulting Group (Interwest) for the building
adjacent to the Fueling Site in October 2018. The hydrologic assumptions and calculations made in
that letter are maintained here where possible, as discussed further below.
2.Calculations
The Weigh Station site improvements consist of a 30-foot-wide paved driveway extending from
Hoffman Mill Road north to an existing building, a distance of approximately 415 feet. Adjacent to the
driveway is a truck scale. These improvements have already been constructed. There was no grading
performed on the site with these improvements, so the existing drainage patterns have not changed.
The drainage basins delineated with the Interwest Drainage Letter remain applicable to this analysis.
There were two primary objectives for the drainage calculations. First, to calculate the additional runoff
generated on the site due to the added pavement. Second, to determine if the existing pond is
adequate to capture and attenuate this flow.
The pre-development condition was assumed to include the entire area tributary to the location of the
Fueling Site pond prior to the construction of Fueling Site improvements. This was used to determine
the allowable release rate from the pond. The drainage basins OS1 and H1 were combined into a
single basin named EX1. For the post-development condition, the additional Weigh Station pavement
area was added, the Fueling Site improvements were accounted for, and the basin was named PR1.
The runoff coefficient C values for individual land uses and the times of concentration for the tributary
areas were taken from the Interwest letter. The weighted C value for basin PR1 was updated to
account for the new impervious area of the driveway. To obtain the total time for the new basins, the
City of Fort Collins Approved Plans
Approved by:
Date:
Wes Lamarque
06/13/2025
Technical Memo
AECOM
Hoffman Mill Drainage Memo.docx 2/3
time of concentration for basin OS1 was added to the travel time from basin H1 for pre-development
conditions and D1 for post-development conditions. Finally, the 100-yr peak flow rates were computed.
It was assumed that the new driveway did not significantly change the time of concentration for the
basin.
To analyze the existing pond capacity a storage routing table was developed to determine the
maximum storage required, and then compared to the existing storage volume indicated on the
Interwest letter. The maximum inflow to the pond is the 100-yr peak flow rate for the post-development
condition. The maximum outflow is the 100-yr peak flow rate for the pre-developed condition. The
maximum required storage was calculated to be 0.066 ac-ft; the existing pond volume is 0.073 ac-ft.
A stage storage table for the existing pond was created using the pond contours from the Fueling Site
construction plans. A storage volume of 0.066 ac-ft corresponds to a water surface elevation of
4947.24. Next, a calculation was performed to analyze the existing restrictor plate on the 18” pond
outlet pipe. The calculation shows that the pipe can pass the pre-developed peak flow rate at a water
surface elevation of 4947.40, which is below the spillway elevation of 4947.70.
Since the outlet pipe will not convey its maximum flow until a certain headwater is reached, it is likely
the pond will fill to the 4947.40 elevation and discharge at a constant rate until the water surface
drops. But the storage routing calculation indicates that the volume at this elevation is greater than
necessary to attenuate the peak inflow.
3. Drainage Improvements
The users of the Fueling Site have indicated that during rainfall events the runoff flows across the site
from west to east and instead of discharging to the existing ditch and detention pond a substantial
amount flows down the driveway into the building adjacent to the Fueling Site and causes flooding. To
mitigate this, the site will be graded to create a minor swale which runs from north to south. The swale
will intercept runoff flowing from west to east and direct the runoff into the riprap rundown which leads
into the ditch and detention pond. Since the entire site is utilized for truck operations, the swale is
designed to have no impact to truck movements. It will have 2% side slopes and be approximately 4
inches deep; the longitudinal slope will be between 2.8% and 4%. The Bentley Flowmaster
calculations show that the swale has capacity to convey the 100-yr event runoff from the site. In the
area of the proposed swale, the site is currently compacted dirt / gravel; no changes are proposed to
the surface materials.
A new prefabricated 10’x12’ building will be installed on an existing concrete pad in front of the Weigh
Station building. Since the concrete pad is existing it was included in the original impervious area
calculations and will not affect these drainage calculations.
4. Conclusion
The calculations performed for this site maintain the calculations included with the approved Interwest
Drainage Letter with regards to land use coefficients and times of concentration. The weighted runoff
coefficient of the drainage basin was recalculated for this memo based on the new paved driveway
and weigh station. All the calculations referenced above are attached to this memo. The results
indicate that the existing pond on the Fueling Site has the capacity to attenuate the increased runoff
due to the pavement constructed on the Weigh Station Site. No changes to the existing pond, spillway,
or outlet pipe are required due to the Weigh Station Site improvements.
Technical Memo
ECOM
Hoffman Mill Drainage Memo.docx 3/3
Attachments:
Rational Calculations
Pond Routing Calculations
Orifice Calculation
Flowmaster Report
Fueling Site Calculations
Basin Map
Runoff Coefficients
Project: Hoffman Mill Calculated by DS Date: 9/28/2021
AECOM Project Number.: 60667912 Checked by RM Date: 10/1/2021
C2 C100 C Area (acres)C Area (acres)
EX1 3.65 0.35 0.44 0.95 0.55 0.25 3.10 Pre Development Tributary Area entire site
PR2 3.65 0.44 0.56 0.95 1.01 0.25 2.63 Post Development Tributary Area entire site
Basin ID
Hardscape:
Asphalt,Concrete, Rooftops
Lawns, Sandy Soil
>7% or Heavy Soil <2%Composite C
Total Area
(ac)
Fort Collins Rational Calc.xlsx Page 1
Standard Form SF-1 . Time of Concentration
Basin ID C2, C5 C100 Area (ac)
From
Interwest
Calcs
EX1 0.35 0.44 0.35 21.72 Tc from basin OS1 plus travel time in basin H1
PR2 0.44 0.56 0.44 21.22 Tc from basin OS1 plus travel time in basin D1
Fort Collins Rational Calc.xlsx Page 1
Standard Form SF-2 . Storm Drainage System Design (Rational Method Procedure)
Project: Hoffman Mill Computed: DS Date: 9/28/2021
Checked: RM Date: 10/1/2021
Design Storm: 100-yr
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EX1 3.65 0.44 21.72 1.62 5.36 8.67 Pre Development Tributary Area entire site
PR2 3.65 0.56 21.22 2.03 5.43 11.00 Post Development Tributary Area entire site
PROJECT STREET
DE
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DIRECT RUNOFF TOTAL RUNOFF
Existing Offsite Flows
REMARKS
Fort Collins Rational Calc.xlsx Page 1
EXISTING DET POND 100 YR
C(100)0.56 Tc (min) = 21.22
Area =3.65 acres
Qout =8.67 cfs = Historic Release Rate
Available Pond Vol=0.073 ac-ft
Time Intensity Area Qin Cum. Vin Qout pipe Cum. Vout Storage Qout Overflow Vout Overflow
(min) (in/hr) (AC) (cfs) (ac-ft) (cfs)(ac-ft)(2)(ac-ft)(1)(cfs) (cfs)
0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
5 9.95 0.86 4.75 0.03 4.75 0.03 0.00 0.00 0.00
6 9.31 1.03 5.33 0.04 5.33 0.04 0.00 0.00 0.00
7 8.80 1.20 5.88 0.06 5.88 0.06 0.00 0.00 0.00
8 8.38 1.38 6.40 0.07 6.40 0.07 0.00 0.00 0.00
9 8.03 1.55 6.90 0.09 6.90 0.09 0.00 0.00 0.00
10 7.72 1.72 7.37 0.10 7.37 0.10 0.00 0.00 0.00
11 7.42 1.89 7.79 0.12 7.79 0.12 0.00 0.00 0.00
12 7.16 2.06 8.20 0.14 8.20 0.14 0.00 0.00 0.00
13 6.92 2.23 8.59 0.15 8.59 0.15 0.00 0.00 0.00
14 6.71 2.41 8.97 0.17 8.67 0.17 0.01 0.00 0.00
15 6.52 2.58 9.34 0.19 8.67 0.18 0.01 0.00 0.00
16 6.30 2.75 9.62 0.21 8.67 0.19 0.02 0.00 0.00
17 6.10 2.92 9.90 0.23 8.67 0.20 0.03 0.00 0.00
18 5.92 3.09 10.17 0.25 8.67 0.21 0.04 0.00 0.00
19 5.75 3.27 10.43 0.27 8.67 0.23 0.05 0.00 0.00
20 5.60 3.44 10.69 0.29 8.67 0.24 0.06 0.00 0.00
21 5.46 3.61 10.95 0.32 8.67 0.25 0.066 0.00 0.00
22 5.32 3.65 10.78 0.33 8.67 0.26 0.06 0.00 0.00
23 5.20 3.65 10.53 0.33 8.67 0.27 0.06 0.00 0.00
24 5.09 3.65 10.31 0.34 8.67 0.29 0.05 0.00 0.00
25 4.98 3.65 10.09 0.35 8.67 0.30 0.05 0.00 0.00
26 4.87 3.65 9.87 0.35 8.67 0.31 0.04 0.00 0.00
27 4.78 3.65 9.68 0.36 8.67 0.32 0.04 0.00 0.00
28 4.69 3.65 9.50 0.37 8.67 0.33 0.03 0.00 0.00
29 4.60 3.65 9.32 0.37 8.67 0.35 0.03 0.00 0.00
30 4.52 3.65 9.16 0.38 8.67 0.36 0.02 0.00 0.00
31 4.42 3.65 8.95 0.38 8.67 0.37 0.01 0.00 0.00
32 4.33 3.65 8.77 0.39 8.67 0.38 0.00 0.00 0.00
33 4.24 3.65 8.59 0.39 8.67 0.39 0.00 0.00 0.00
Project:
Basin ID:
X
1
#1 Vertical #2 Vertical
Sizing the Restrictor Plate for Circular Vertical Orifices or Pipes (Input)Orifice Orifice
Water Surface Elevation at Design Depth Elev: WS = 4,947.40 feet
Pipe/Vertical Orifice Entrance Invert Elevation Elev: Invert = 4,944.56 feet
Required Peak Flow through Orifice at Design Depth Q = 8.67 cfs
Pipe/Vertical Orifice Diameter (inches) Dia = 14.4 inches
Orifice Coefficient Co =0.65
Full-flow Capacity (Calculated)
Full-flow area Af = 1.13 sq ft
Half Central Angle in Radians Theta = 3.14 rad
Full-flow capacity Qf = 8.8 cfs
Percent of Design Flow = 102%
Calculation of Orifice Flow Condition
Half Central Angle (0<Theta<3.1416) Theta = 2.64 rad
Flow area Ao =1.10 sq ft
Top width of Orifice (inches)To =6.97 inches
Height from Invert of Orifice to Bottom of Plate (feet)Yo =1.12 feet
Elevation of Bottom of Plate Elev Plate Bottom Edge = 4,945.68 feet
Resultant Peak Flow Through Orifice at Design Depth Qo =8.7 cfs
Width of Equivalent Rectangular Vertical Orifice Equivalent Width = 0.98 feet
Existing Orifice Plate with new design flow
RESTRICTOR PLATE SIZING FOR CIRCULAR VERTICAL ORIFICES
Hoffman Orifice Plate.xls, Restrictor Plate 10/13/2021, 11:53 AM
This is the WSEL required to
pass the peak flow through the
existing orifice
This calculation shows that the existing orifice can pass the historic flow of 8.67
cfs before the WSEL exceeds the spillway invert of 4947.70
Parking Lot Swale Grading
Project Description
Manning
FormulaFriction Method
DischargeSolve For
Input Data
0.022Roughness Coefficient
ft/ft0.028Channel Slope
in4.0Normal Depth
H:V50.000Left Side Slope
H:V50.000Right Side Slope
Results
cfs19.01Discharge
ft²5.6Flow Area
ft33.3Wetted Perimeter
in2.0Hydraulic Radius
ft33.33Top Width
in4.7Critical Depth
ft/ft0.012Critical Slope
ft/s3.42Velocity
ft0.18Velocity Head
ft0.52Specific Energy
1.478Froude Number
SupercriticalFlow Type
GVF Input Data
in0.0Downstream Depth
ft0.0Length
0Number Of Steps
GVF Output Data
in0.0Upstream Depth
N/AProfile Description
ft0.00Profile Headloss
ft/sInfinityDownstream Velocity
ft/sInfinityUpstream Velocity
in4.0Normal Depth
in4.7Critical Depth
ft/ft0.028Channel Slope
ft/ft0.012Critical Slope
Messages
Page 1 of 227 Siemon Company Drive Suite 200 W
Watertown, CT 06795 USA +1-203-755-1666
10/11/2021
FlowMaster
[10.02.00.01]
Bentley Systems, Inc. Haestad Methods Solution
CenterHoffman.fm8
Parking Lot Swale Grading
Messages
This
calculation
shows the
capacity of
the swale
which will
be graded
into the
existing
parking lot.
Side slopes
are 2%
and the
minimum
longitudinal
slope is
2.8%.
Notes
Page 2 of 227 Siemon Company Drive Suite 200 W
Watertown, CT 06795 USA +1-203-755-1666
10/11/2021
FlowMaster
[10.02.00.01]
Bentley Systems, Inc. Haestad Methods Solution
CenterHoffman.fm8
Design Engineer:
Design Firm:
Project Number:
Date:
%
Impervious
Runoff
Coefficient C
Return
Period
Frequency
Adjustment
Factor (Cf)
100% 0.95 2-year to
10-year 1.00
90% 0.95 100-year 1.25
90% 0.95
40% 0.50
0% 0.25
C2 to C10 C100
OS1 133,035 3.054 0 8,330 9,935 0 114,770 12.4% 0.35 0.43
H1 25,833 0.593 0 2,480 3,038 0 20,315 19.2% 0.40 0.50
Apavers
(sq feet)
Alawn
(sq feet)
Weighted %
Impervious
COMPOSITE
Hoffman Mill Fuel Site
EXISTING IMPERVIOUS AREA CALCULATION
Urban Storm Drainage Criteria Manual by Urban Drainage and Flood Control District, June 2001 (Revised January 2016)
DESIGN CRITERIA:
BASINS:
% Impervious values from Table RO-3 in the Urban Storm Drainage Criteria Manual
Sub-basin
Designation
Atotal
(sq feet)
Atotal
(acres)
Apaved
(sq feet)
Aroof
(sq feet)
Awalk/RAP
(sq feet)
Lawns
(Heavy, 2-7% Slope)
J.Claeys
Interwest Consulting Group
1344-095-00
June 6, 2018
Runoff Coefficients and Frequency Adjustment Factors for City of Fort Collins - Storm Water Criteria Manual
Land Use
Paved
Roof
Walks/RAP
Gravel/Pavers
1344-095-00 Rational Calcs (FC).xlsx Page 1 of 4 Interwest Consulting Group
The following pages are an excerpt from the Hoffman Mill Fueling Site Final Drainage Letter prepared by
Interwest Consulting Group, October 31, 2018.
Values highlighted in yellow were used unchanged for these hydrology and hydraulics calculations to
analyze the existing pond and its capacity to attenuate the Weigh Station Site runoff.
Design Engineer:
Design Firm:
Project Number:
Date:
EQUATIONS:
-Equation 6-2 -Equation 6-4 -Urbanized Check Equation 6-5
- CoFC Overland Flow
CONSTRAINTS:
300 ft - Overland flow shall not exceed for developed condition
500 ft - Overland flow shall not exceed for undeveloped condition
Final t c = minimum of t i + t t and urbanized basin check
recommended minimum t c = 5 min for urbanized basins
Time of Concentration (100-yr)
Type of Travel
Surface Cv
OS1 OS1 12.4% 0.43 3.054 300 0.0157 18.61 234 0.0120 Paved Areas 20 2.19 1.78 20.39 0.0141 21.16 20.39
H1 H1 19.2% 0.50 0.593 88 0.0291 7.40 154 0.0093 Paved Areas 20 1.93 1.33 8.73 0.0165 17.01 8.73
June 6, 2018
Hoffman Mill Fuel Site
HISTORIC TIME OF CONCENTRATION
J.Claeys
Interwest Consulting Group
1344-095-00
LENGTH
(ft)
DESIGN CRITERIA:
Urban Storm Drainage Criteria Manual by Urban Drainage and Flood Control District, June 2001 (Revised January 2016)
Overland Flow Equations for City of Fort Collins - Storm Water Criteria Manual
SUB-BASIN DATA INITIAL/OVERLAND TIME
(ti)TRAVEL TIME (tt)
tc=ti+tt
(min)
Urban Check Final
tc
(min)DESIGN
POINT Sub-basin %
Impervious C100
AREA
(acres)
tt
(min)
OVERALL SLOPE
(ft/ft)
tc
(min)
SLOPE
(ft/ft)
ti
(min)
LENGTH
(ft)
SLOPE
(ft/ft)
Table RO-2 VELOCITY
(ft/s)
ticttt+=5.0
wvSCV=
V
Ltt60
=
=18 − 15 +
60 24+12
=
1.87(1.1 − )
1344-095-00 Rational Calcs (FC).xlsx Page 3 of 4 Interwest Consulting Group
Design Engineer:
Design Firm:
Project Number:
Date:
%
Impervious
Runoff
Coefficient C
Return
Period
Frequency
Adjustment
Factor (Cf)
100% 0.95 2-year to
10-year 1.00
90% 0.95 100-year 1.25
90% 0.95
40% 0.50
0% 0.25
C2 to C10 C100
D1 25,836 0.59 0 2,481 10,384 0 12,971 44.8% 0.60 0.75
Hoffman Mill Fuel Site
DEVELOPED IMPERVIOUS AREA CALCULATION
DESIGN CRITERIA:
Urban Storm Drainage Criteria Manual by Urban Drainage and Flood Control District, June 2001 (Revised January 2016)
BASINS:
% Impervious values from Table RO-3 in the Urban Storm Drainage Criteria Manual
J.Claeys
Interwest Consulting Group
1344-095-00
June 6, 2018
Land Use
Paved
Roof
Walks
Runoff Coefficients and Frequency Adjustment Factors for City of Fort Collins - Storm Water Criteria Manual
Gravel/Pavers
Lawns
(Heavy, 2-7% Slope)
Sub-basin
Designation
Atotal
(sq feet)
Atotal
(acres)
Weighted %
Impervious
COMPOSITEApaved
(sq feet)
Aroof
(sq feet)
Awalk
(sq feet)
Agravel/pavers
(sq feet)
Alawn
(sq feet)
1344-095-00 Rational Calcs (FC).xlsx Page 1 of 4 Interwest Consulting Group
Design Engineer:
Design Firm:
Project Number:
Date:
EQUATIONS:
-Equation 6-2 -Equation 6-4 -Urbanized Check Equation 6-5
- CoFC Overland Flow
CONSTRAINTS:
300 ft - Overland flow shall not exceed for developed condition
500 ft - Overland flow shall not exceed for undeveloped condition
Final t c = minimum of t i + t t and urbanized basin check
recommended minimum t c = 5 min for urbanized basins
Time of Concentration (2-yr to 10-yr)
Type of Travel
Surface Cv
D1 D1 44.8% 0.60 0.593 156 0.0072 13.08 150 0.0229 Paved Areas 20 3.03 0.83 13.90 0.0149 13.11 13.11
June 6, 2018
Hoffman Mill Fuel Site
DEVELOPED TIME OF CONCENTRATION
J.Claeys
Interwest Consulting Group
1344-095-00
LENGTH
(ft)
DESIGN CRITERIA:
Urban Storm Drainage Criteria Manual by Urban Drainage and Flood Control District, June 2001 (Revised January 2016)
Overland Flow Equations for City of Fort Collins - Storm Water Criteria Manual
SUB-BASIN DATA INITIAL/OVERLAND TIME
(ti)TRAVEL TIME (tt)
tc=ti+tt
(min)
Urban Check Final
tc
(min)DESIGN
POINT Sub-basin %
Impervious C2-10
AREA
(acres)
tt
(min)
OVERALL SLOPE
(ft/ft)
tc
(min)
SLOPE
(ft/ft)
ti
(min)
LENGTH
(ft)
SLOPE
(ft/ft)
Table RO-2 VELOCITY
(ft/s)
ticttt+=5.0
wvSCV=
V
Ltt60
=
=18 − 15 +
60 24+12
=
1.87(1.1 − )
1344-095-00 Rational Calcs (FC).xlsx Page 2 of 4 Interwest Consulting Group
Hoffman Mill Fueling Site
Critical Pond Elevations
Design Engineer:
Design Firm:
Project Number:
Date:
DESIGN CRITERIA
Urban Storm Drainage Criteria Manual, Urban Drainage and Flood Control District, June 2001 (Revised April 2008)
Volume obtained utilizing 2018 AutoCAD Civil3D
Storage Summary
4947.00 ft
283 cu-ft
290 cu-ft
4947.70 ft
2,788 cu-ft
3,071 cu-ft
4944.56 ft
3,164 cu-ft
4946.90 ft
Require 100-yr Detention Volume
Total Detention Volume Provided
100-yr WSE
J.Claeys
Interwest Consulting Group
1344-095-00
September 25, 2018
Pond Invert Elevation
Sand Filter Invert Elevation
Required Water Qualtiy Capture Volume (WQCV)
WQCV Provided
WQCV Water Surface Elevation (WSE)
Combined Required 100-yr Detention & WQCV
1344-095-00 PondCalcs.xls - Pond Stage Storage Page 1 of 3 Interwest Consulting Group
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1344-095-00
1 of 1
BASIN COEFFICIENT (100-YR)
BASIN DESIGNATION
BASIN AREA
DESIGN POINT
DRAINAGE FLOW ARROW
DRAINAGE BASIN
BOUNDARY
DRAINAGE BASIN
FLOW PATH
LEGEND
SCALE 1" =
0
60'
6030