HomeMy WebLinkAboutDrainage MemoSWR GUEST CABIN 2 DRAINAGE MEMORANDUM
To: Chris Bendon
From: Jesse K. Swann, PE
Date: August 29, 2025
Re: Drainage Memo -Sweetwater Ranch Guest Cabin 2
Eric[: Appendix
Sopris Engineering LLC (SE) has prepared this drainage memo in support of the Grading & Building permit application for a cabin
located in Garfield County. The proposed residence is situated on Tract GA which encompasses 165.2 acres within the SB35
Subdivision Exemption Plat. The SB35 Subdivision is a recently recorded subdivision consisting of several parcels, each exceeding
35 acres, all located within an active 2,659+/- acre working ranch under common ownership. This memo includes a description of
the existing site conditions, proposed improvements, and the general approach to stormwater routing, considering the open
expansiveness of the ranch and surrounding area.
Section A-ExistingSite Overview:
The existing site is located approximately 1 mile west of Sweetwater Road. Access to this portion of the ranch is currently available
via a private gravel road that extends from Sweetwater Road through the ranch. The SB35 Subdivision is a working ranch that will
continue operations involving cattle raising and hay production. Improvements to the existing gravel access road, including an
extension to serve the proposed residence and other structures have been reviewed and approved by the U.S. Army Corps of
Engineers, Garfield County, and Eagle County. Additionally, design of the road has been coordinated and approved by the Gypsum
Fire District.
The surrounding topography consists of relatively hilly terrain, with slopes ranging from 10% to 20%, generally trending east to
northeast. Vegetation in the area includes pinyon pine, Gambel oak, and sagebrush, along with cultivated pastures featuring several
lateral irrigation ditches used for flood irrigation.
Sweetwater Creek, located approximately 0.8 miles east of the proposed residence site (as the crow flies), is the ultimate receiving
waterbody. Although Sweetwater Creek is not included in FEMA Flood Insurance Rate Maps, floodplain studies conducted in
anticipation of future bridge crossings indicate that the 100-year floodway and floodplain remain confined near the ordinary high-
water mark, all of which are well below the proposed residential site. Surface runoff from the building site follows existing drainage
patterns, which are conveyed over and through irrigated pastures and areas of partially vegetated ground cover typical of high -
elevation, and environments. Figure 1 is provided to illustrate the proposed location of the residence and surrounding area.
Figure 1-Aerial Image of Site
SOPRiS ENGINEERING LLC
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
pg. 1
Section B-Proiect Overview:
The proposed project involves the construction of a cabin, detached carport, gravel driveway, covered patio, utility improvements,
and landscaping enhancements. The landscaping will be designed to complement the surrounding vegetation and will be irrigated
using water supplied from Beaver Reservoir, located approximately 0.5 miles west of the site on the SB35 Subdivision property.
Grading activities will be carefully planned to minimize disturbance to existing topography and vegetation. Cut and fill operations
will be conducted in accordance with best management practices (BMPs) to ensure erosion control and sediment containment
during construction. Temporary sediment control measures, such as sediment control logs and silt fences, will be implemented
around the site to prevent sediment migration from the development area and to protect existing waterways and drainage courses.
Additionally, erosion and sediment control BMPs will be maintained throughout construction, with inspections conducted in
compliance with the State issued Storm Water Management Plan permit, regularly to ensure their effectiveness. Final grading will
be completed to promote positive drainage away from the residence, reducing the potential for ponding or surface runoff issues.
After construction is complete, disturbed areas will be stabilized through landscaping and revegetation to prevent erosion and
establish permanent ground cover.
Section C-Post Development Drainage Analysis:
To properly size the proposed storm water mitigation infrastructure for the project, the post development site was divided into
several drainage basins and sub -basins. The post development drainage basins and sub -basins are described in detail below.
Design Point 1— is the collection point for all flows collected by Swale 1 flowing to west of the building. There are two post
development basins tributary to DP1.
Basin 1: A small narrow and basin that is tributary to Swale 1.
Roof 1: A small section of sloped roof designed to shed water uninterrupted from the roof to Swale 1.
Desiqn Point 2 — This design point is situated between the proposed cabin and carport and aligns with the outfall of the proposed
culvert (Culvert 1) that passes beneath the access walk from the carport to the cabin. Culvert 1 is located within a natural
drainage channel that was previously used to flood irrigate pastures. However, the proposed water transmission system will
result in abandoning the ditch for flood irrigation purposes and therefore the existing drainage channel will only route storm water
runoff. The drainage basins tributary to Design Point 2 includes the following:
Basin 2: A large offsite basin that conveys flows to the existing drainage channel that will direct runoff towards the proposed
culvert located under the access path from the driveway to the cabin.
Sub -Basin 2.1: Included within Basin 2, Sub Basin 2.1 is delineated to size Swale 2 which will direct runoff west to east
around the cabin before blending into the existing drainage channel.
Roof 2: This basin includes a portion of the cabin's shed roof that will direct runoff towards the south side of the cabin and
towards Swale 2.
Design Point 3 — This design point corresponds with the outfall of proposed Culvert 2 located on the furthest northeast side of
the development and aids in the design of culvert 2 routed under the proposed driveway. The drainage basins tributary to Design
Point 3 include:
Basin 3: A small basin to the east of the covered carport and bounded by the driveway and the separately permitted access
road.
Roof 3: This roof line is designed to shed water to the south and allow water to flow uninterrupted off the roof into Basin 3,
runoff will sheet flow towards Culvert 2.
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
pg. 2
Section D-H drola is Methods and Assumptions:
Post -development drainage areas were analyzed using the Rational Method (Equation 1) since the cumulative total of tributary
areas are less than 90 acres.
Equation 1: Q = C * I * A
Q = Runoff Flow Rate (cfs); C = Runoff Coefficient
I = Rainfall Intensity (in/hr); A= Area of Basin (acres)
The runoff coefficient (C) represents the ratio of runoff volume to rainfall volume during a storm event. Its determination is influenced
by several factors, including soil type, the percentage of impervious area within the watershed, and the frequency of storm events.
Each drainage basin was analyzed to quantify its percentage of impervious area. The effective impervious area for each basin was
then used to derive a weighted runoff coefficient. A spreadsheet tool developed by the Mile High Flood District (MHFD) of Denver,
CO was used to calculate site -specific runoff coefficients. This tool allows for the calculation of site -specific C values based on a
Type C hydrologic soil classification.
The MHFD spreadsheet, which is included in Appendix, also computes the time of concentration (Tc), the time it takes for runoff to
travel from the furthest point upstream in a basin to the designated design point, accounting for both overland and channelized
flow. A minimum time of concentration of 5 minutes was adopted for the smaller developed tributary areas. These Tc values were
then used to estimate the corresponding 100-year rainfall intensities based on NOAA rainfall data for the surrounding area.
Table 1, provided below, presents the areas, C values, Tc, intensities and resultant 100-year peak runoff rates for the post -
development drainage basins.
Table 1 - Post Development Peak 100-Year Runoff Rates
Table 1: Post Develo ment Drainage Basin: Rational Method Summary
Drainage Basin ID
(ac)
Rainfall
Runoff Peak Flow, QArea Percent Selected Intensity
Impervious Coefficient, C Tc (min) inlhr) (cfs)
100- r 100- r 100- r
POST DEVELOPMENT BASINS
BASIN 1
0.21
2.%
0.49
1 13.79
1 4.49
0.46
BASIN2
33.16
2%
0.49
1 88.32
1 1.02
16.64
BASIN 3
0.25
30%
0.61
1 10.00
5.18
0.79
POST DEVELOPMENT SUB BASINS
SUB BASIN 2.1 0.46 2% 0.49 13.85 4.44 1.00
ROOF BASINS
ROOF-1
0.01
100%
0.89
5.00
7.08
0.06 _
ROOF-2
0.06
100%
0.89
5.00
7.08
0.40
ROOF-3
0.03
100%
0.89
5.00
7.08
0.18
Section E-Hydraulic Methods and Assumptions:
The 100-year post -development peak runoff rates summarized in Table 1 were used to size the proposed stormwater conveyance
system, which includes two culverts and two swales.
Culvert Sizing: Hydraflow Express Extension was used to size the proposed culverts. Hydraflow Express employs an energy -
based Standard Step methodology to estimate culvert flow capacities. For maintenance and efficiency purposes, a minimum culvert
diameter of 18 inches was adopted. The proposed culverts will be constructed from ADS N-12 smooth interior pipe, utilizing a
Manning's Roughness Coefficient of 0.013, which reflects the flow roughness characteristics of the pipe material. A description of
each culvert is summarized below.
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
pg. 3
Culvert 1: Located under a proposed concrete walking path that connects the driveway to the proposed cabin, Culvert 1 is a
24" culvert designed to maintain existing flows of Basin 2 as well as post development flows associated with portions of the
cabin's shed roof
Culvert 2: Is an 18" ADS N-12 pipe located at the eastern end of the driveway and is designed to convey water from Basin 3
and Roof 3.
To accurately estimate culvert capacity, flow increments of 0.25 cubic feet per second (cfs) were evaluated iteratively until just prior
to overtopping. These flow estimates were cross-referenced with the total 100-year peak runoff rate tributary to the culvert. Table
2 summarizes the tributary peak runoff rates, the corresponding culvert capacities based on size, depth, material, and slope.
Supporting calculations are provided in the Appendix.
Table 2-Culvert Sizing Summary_
CULVERT DESIGN SUMMARY
Culvert ID
Tributary Drainage
Basin ID
Qwo
cfs
Size
(in)
Capacity
(cfs)
CULVERT 1
BASIN 2, ROOF 2
17.04
24
20.5
CULVERT 2
BASIN 3, ROOF 3 0.97
18
12.0
Swale Sizing: Swales are proposed within the development to convey surface runoff around the proposed development. Manning
Equation (Equation 2) was used to estimate the dimensions of the proposed drainage swa.les, employing the 100-year peak runoff
rates associated with tributary basins in conjunction with the proposed longitudinal slope for each swale. Roughness coefficients
were determined based on the intended treatment for each swale. Swale dimensions were inputted into Hydraflow software
accounting for maximum and minimum longitudinal slopes. A brief description of the proposed swales is provided below:
2
49 Equation 2: Q = "n ( ,)3 A * S2
Q = Channel Capacity (cfs)
n = manning's runoff coefficient (native: n = 0.027)
A = Area of flow (sf)
Pw = Wetted perimeter of channel (ft)
S = Channel longitudinal slope (ft/ft)
Swale 1: Tributary to Basin 1 and Roof 1, this swale is engineered to intercept both historic offsite runoff and post -development
flows. It conveys these flows along the western perimeter of the main building where the concentrated flow will travel —100ft
before discharging into an existing drainage ditch.
Swale 2: Tributary to Sub -Basin 2.1 and Roof 2, this swale is engineered to intercept both historic offsite runoff and post -
development flows. It conveys these flows along the eastern perimeter of the main building, ultimately discharging into an
existing drainage ditch.
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
pg. 4
A summary of the resultant swale analysis as performed with Hydraflow is provided in Table 3.
Table 3-Swale Sizing Summary_
SWALE DESIGN
SUMMARY
CONTRIBUTING
SLOPE
Q100
VELOCITY
FLOW DEPTH
MIN. DEPTH
FULL FLOW
SWALE ID
BASINS
(%)
(CFS)
n
(ft/sec)
(ft)
PROVIDED (ft)
. CAPACITY (CFS)
SWALE #1:
BASIN 1, ROOF 1
2.0%
0.52
0.03
1.80
0.31
1.0
12.8
SWALE #2:
BASIN SUB 2.1,
3.00%
1.40
0.03
2.78
0.41
1.0
15.7
ROOF 2
Section F-Erosion Control:
Erosion control measures are essential to mitigate drainage issues and prevent soil erosion during construction activities. While
the responsibility for implementing these measures lies with the Contractor under the State -issued Stormwater Management Plan,
the following erosion control practices are minimum recommendations to help reduce sediment transport and soil degradation.
Pre -Construction Measures: Before any clearing, grubbing, lot grading, or construction work begins, the contractor
shall establish temporary sediment control logs and/or embedded silt fencing around the anticipated limits of
disturbance.
Culvert Protection: Hay bales and sediment control logs should be placed at the inlets and outlets of all culverts to
prevent sediment from contaminating the culverts prior to the establishment of vegetation.
e Topsoil Management: Topsoil designated for removal and reuse shall be stockpiled with sediment control logs or silt
fencing around their perimeters. If stockpiles are to remain for more than 15 days, temporary seed should be applied
to prevent erosion and weed growth.
a Ditch Control: Install sediment control logs within the flowline of ditches at appropriate intervals to reduce flow
velocities and capture sediment.
Site Inspections: The site must be inspected and recorded in accordance with the State issued SWMP. Silt deposits
behind silt fencing and sediment control logs should be regularly cleared to ensure the effectiveness of the erosion
control system. These inspections and maintenance activities must be documented in a logbook readily available for
inspection.
• Vegetation Establishment: Seed and mulch shall be applied over disturbed cut and fill slopes, with watering as
necessary, to establish permanent vegetative ground cover.
Slope Stabilization: Erosion control blankets and/or hydromulching shall be applied to all cut and fill slopes that exceed
a 3:1 slope ratio.
e Vehicle Tracking Control: Vehicle tracking control devises shall be installed at the entrance to Sweetwater Road to
prevent tracking onto the public roadways.
Temporary erosion control measures installed during construction should remain in place and be maintained until new vegetation
is established at a 70% growth level. Once soil stabilization is satisfactory, temporary erosion control structures may be removed.
Given the dynamic nature of construction sites, the final location and selection of BMPs shall be at the contractor's discretion. All
necessary permits must be acquired prior to the commencement of construction, and the criteria outlined in these permits must be
adhered to until the associated permits are closed.
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
pg. 5
Section G-Conclusions:
The drainage analysis indicates that the proposed improvements will not adversely impact the subject property or surrounding
areas. Offsite and onsite runoff will be appropriately managed via the proposed drainage system which has been sized to safely
convey the 100-year storm event associated with the proposed development. Lastly, erosion control BMPs will be enforced prior
throughout construction in compliance with State issued SWMP permit.
For further questions or additional information, please feel free to contact me.
Prepared by: Jesse K. Swann, PE
42787
�o . Gad-29-ZO25 ; 1. �4o
SS70NAl- EN��
Encl:
Exhibit A: Post Development Drainage Basin Delineation Map
NOAA Rainfall Depth Chart
Hydraflow Culvert Hydraulic Calcs
Hydraflow Swale Capacity Calcs
Mile High Flood District Spreadsheet
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
•• •
OFF SITE BASINS'
ONSITE BASINS
use
Precipitatign Frequency Data Server
https://hdsc.nws.noaa.gov/pfds/pfds_printpage.html?lat=39.755 I &Ion=...
NOAA Atlas 14, Volume 8, Version 2
Location name: Gypsum, Colorado, USA*
Latitude: 39.7551°, Longitude:-107.1036'
Elevation: 6997 ft-
'source: ESRI Maps
source: USGS
POINT PRECIPITATION FREQUENCY ESTIMATES
Sanja Perica, Deborah Martin, Sandra Pavlovic, Ishani Roy, Michael St. Laurent, Carl Trypaluk, Dale
Unruh, Michael Yekta, Geoffery Bonnin
NOAA, National Weather Service, Silver Spring, Maryland
PF tabular I PF_graphical 1 Maps_&_aerials
PF tabular
PDS-based point precipitation frequency estimates with 90% confidence intervals (in inches/hourp `
Averege recurrence interval (years) !!
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1 Precipitation frequency (PF) estimates in this table are based on frequency analysis of partial duration series (PDS).
Numbers in parenthesis are PF estimates at lower and upper bounds of the 90% confidence interval. The probability that precipitation frequency estimates
(for a given duration and average recurrence interval) will be greater than the upper bound (or less than the lower bound) is 5%. Estimates at upper bounds
are not checked against probable maximum precipitation (PMP) estimates and may be higher than currently valid PMP values.
Please refer to NOAA Atlas 14 document for more information.
Back to Top
PF graphical
1 of 4 10/17/2024, 10:15 AM
Culvert Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.
Monday, Aug 25 2025
GC2 CULVERT 1
Invert Elev Dn (ft)
= 7719.60
Calculations
Pipe Length (ft)
= 18.00
Qmin (cfs)
= 20.00
Slope (%)
= 3.00
Qmax (cfs)
= 21.00
Invert Elev Up (ft)
= 7720,14
Tailwater Elev (ft)
= (dc+D)/2
Rise (in)
= 24.0
Shape
= Circular
Highlighted
Span (in)
= 24.0
Qtotal (cfs)
= 20.50
No. Barrels
= 1
Qpipe (cfs)
= 20.50
n-Value
= 0.013
Qovertop (cfs)
= 0.00
Culvert Type
= Circular Concrete
Veloc Dn (ft/s)
= 6.85
Culvert Entrance
= Square edge w/headwall (C)
Veloc Up (ft/s)
= 7.51
Coeff. K,M,c,Y,k
= 0.0098, 2, 0.0398, 0.67, 0.5
HGL Dn (ft)
= 7721.41
HGL Up (ft)
= 7721.76
Embankment
Hw Elev (ft)
= 7723.15
Top Elevation (ft)
= 7723.17
Hw/D (ft)
= 1.50
Top Width (ft)
= 10.00
Flow Regime
= Inlet Control
Crest Width (ft)
= 5.00
8m ml GQ CULVEHr 1
G—C— MGL L 1.
uwo"M trti
in
IN
1N
am
A1<
4.1.
—CM
Culvert Report
Hydraflow Express Extension for Autodesk® Civil 3D0 by Autodesk, Inc. Tuesday, Aug 19 2025
GC2 CULVERT 2
Invert Elev Dn (ft)
= 7711.58
Calculations
Pipe Length (ft)
= 40.80
Qmin (cfs)
= 11.00
Slope (%)
= 3.23
Qmax (cfs)
= 13.00
Invert Elev Up (ft)
= 7712.90
Tailwater Elev (ft)
= (dc+D)/2
Rise (in)
= 18.0
Shape
= Circular
Highlighted
Span (in)
= 18.0
QtotaI (cfs)
= 12.00
No. Barrels
= 1
Qpipe (cfs)
= 12.00
n-Value
= 0.013
Qovertop (cfs)
= 0.00
Culvert Type
= Circular Concrete
Veloc Dn (ft/s)
= 6.97
Culvert Entrance
= Square edge w/headwall (C)
Veloc Up (ft/s)
= 7.32
Coeff. K,M,c,Y,k
= 0.0098, 2, 0.0398, 0.67, 0.5
HGL Dn (ft)
= 7712.99
HGL Up (ft)
= 7714.21
Embankment
Hw Elev (ft)
= 7715.72
Top Elevation (ft)
= 7715.73
Hw/D (ft)
= 1.88
Top Width (ft)
= 30.00
Flow Regime
= Inlet Control
Crest Width (ft)
= 5.00
S. m M gRVEar2 MwoepM ins
Crt 1:— MIL T
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.
GC2 SWALE 1
Triangular
Side Slopes (z:1)
= 3.00, 3.00
Total Depth (ft)
= 1.00
Invert Elev (ft)
= 1.00
Slope (%)
= 2.00
N-Value
= 0.030
Calculations
Compute by:
Known Q
Known Q (cfs)
= 0.52
Elev (ft)
3.00
2.50
2.00
1.50
1.00
0.50
0 1
Section
Tuesday, Aug 19 2025
Highlighted
Depth (ft)
= 0.31
Q (cfs)
= 0.520
Area (sqft)
= 0.29
Velocity (ft/s)
= 1.80
Wetted Perim (ft)
= 1.96
Crit Depth, Yc (ft)
= 0.29
Top Width (ft)
= 1.86
EGL (ft)
= 0.36
2 3 4 5 6 7
Reach (ft)
Depth (ft)
2.00
1.50
1.00
0.50
1
-0.50
8
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.
SWALE #2
Triangular
Side Slopes (z:1)
= 3.00, 3.00
Total Depth (ft)
= 1.00
Invert Elev (ft)
= 1.00
Slope (%)
= 3.00
N-Value
= 0.030
Calculations
Compute by: Known Q
Known Q (cfs) = 1.40
Elev (ft)
3.00
2.50
2.00
1.50
1.00
1 2 3
Section
4
Reach (ft)
Wednesday, Aug 20 2025
Highlighted
Depth (ft)
= 0.41
Q (cfs)
= 1.400
Area (sqft)
= 0.50
Velocity (fVs)
= 2.78
Wetted Perim (ft)
= 2.59
Crit Depth, Yc (ft)
= 0.43
Top Width (ft)
= 2.46
EGL (ft)
= 0.53
5 6 7
Depth (ft)
— 2.00
1.50
1.00
0.50
WIN
-0.50
8
n Calculation of Peak Runoff using Rational Method
Company: Sopris Ineerin
Date: Bl2W.025 t_i=(0.395(1.1-C_5 BtB minimum=5(urban)
S_iM33 CComputed t minimum=10 non -urban ued t�_c=max{t_mum , min(Compu[ed t c
Project: SWEET WATER RANCH ))/( ) B �- ( )
Location: GARFiELO CO, t tat t f(GOK4(S t BRegional tB c=(26-17i)+ L t/(G Q(cfs)=CIA
L_t/(60V_t) ,/(S t))
Runoff
Coefficient,
Overland (Initial) Flow Time
Channelized (Travel) Flow Time
Time of Concentration
NRCS
Percent
C
Overlap
Overland
Overland
Channelized
Channelized
NRCS
Channelized
Channelized
Computed
Regional
Selected
Subcatchmunt Name
Hydrologic
Soil Group
Imperviousness
100-yr
d Flow
Flow Slope
Flow Time
Flow Length
Flow Slope
Conveyance
Flow Velocity
Flow Time
t,(min)
t,(min)
t.(min)
Length
Si (ftlft)
t, (min)
Li M)
St (ftlft)
Factor K
V, (ftisec)
t, (min)
LI (ft)
BASIN
C
2.0
0a9
365.00
0.220
13.04
50.00
0.05
5
1.12
0.75
13.79
26.06
379
BASIN 2
C
2.0
049
600.00
0.116
18.86
3584.00
0.12
2.5
0 66
69.47
8832
44.37
44 37
BASIN 3
C
30.3
0.61
186.00
0.100
RA2
1.00
0.10
5
1.58
0.01
9A3
20.85
304M
SUB BASIN 2 1
C
2.0
0.49
36000
0.210
13.15
65.00
0.050
6
1 12
0 97
14A2
26.18
14.12
ROOF 1
C
100.0
0.64
y,pp
ROOF 2
C
100.0
0 89
5,OR
ROOF 3
C
100.0
0.89
S W