HomeMy WebLinkAboutChris Hale Stormwater Memo Update COAFrom:Chris@mountaincross-eng.com
To:Philip Berry
Subject:RE: stormwater plan - eagle springs organic large solar
Date:Tuesday, August 15, 2023 1:56:06 PM
Philip:
I reviewed the document and I had no comments. Let me know if you need anything else. Thanks.
Sincerely,
Mountain Cross
Engineering, Inc.
Chris Hale, P.E.
826 1/2 Grand Avenue
Glenwood Springs, CO 81601
Ph: 970.945.5544
Fx: 970.945.5558
From: Philip Berry <pberry@garfield-county.com>
Sent: Monday, July 31, 2023 11:08 AM
To: Chris@mountaincross-eng.com
Subject: stormwater plan - eagle springs organic large solar
Attached is an updated version of their stormwater plan for application (MPPA-04-23-8956). I’m
finalizing my staff report for the BOCC hearing that will be next Monday. This item is a
recommended Condition of Approval that they are trying to fulfill as soon as possible. No giant rush
on our end for review though. Please call if you have any questions or concerns.
Thanks
Philip Berry, AICP
Planner Garfield County – Community Development Department
108 8th. St., Suite 401
Glenwood Springs, CO 81601
Pberry@garfield-county.com
Office: 970-945-1377 (1580)
Cell: 970-456-4404
neiengineering.com + 303-431-7895
12600 W Colfax Avenue, Suite C500, Lakewood, CO 80215
To: Joshua Mayer
AES Clean Energy
The AES Corporation
282 Century Place, Suite 2000
Louisville, CO 80027
From: NEI Electric Power Engineering, Inc.
12600 W Colfax Avenue, Suite C500
Lakewood, CO 80215
Date: July 12, 2023
Project Name: Eagle Springs Solar
Project No.: 4084.001
Subject: Stormwater Memo (30% Review)
Eagle Springs Solar is a proposed solar facility located near County Road 315 and 346, Garfield
County, approximately 1 mile east of the Rifle Garfield County Airport. Disturbed area of this
project will be limited to approximately 68.0 acres of the 301-acre lot. The project is expected to
change approximately 64,843 sq-ft (1.49 ac; 2.19% lot coverage) of pervious area into
impervious area. The impervious area will consist of four (4) concrete equipment pads, two (2)
gravel access roads, and foundations for solar arrays. Onsite drainage patterns will be
maintained with exception to minor grading related to proposed gravel roads; there a Drainage
Report is not required per county standards.
This memo is intended to provide Garfield County with drainage and land disturbance
information with respect to this project.
Existing Conditions
The project site is mostly developed agricultural fields throughout the property. The
undeveloped land cover consists of agriculture fallow, native vegetation, sparse weeds and
grasses. The topography consists of rolling hills, plains and mesas. The existing soils on site are
primarily type C. The site generally drains from southeast to northwest atop a mesa with
approximately 160 ft of topographic relief. Stormwater runoff sheet flows northwest and
AES Clean Energy Eagle Springs Solar
STORMWATER MANAGEMENT PLAN p. ii
collects in dispersed ephemeral ditches. These ditches convey flow northwest into an
approximately 1.8 ac. detention basin. This basin discharges to the Last Chance Ditch, thence
into Mamm Creek, thence into the Colorado River approximately 1.0 mile northwest of the
project site.
Proposed Conditions
Construction activities associated with the proposed solar facility will affect approximately 68.0
acres onsite. However, much of the disturbed area will involve construction of support structures
for solar panel arrays, leaving the surface beneath the panels undeveloped and pervious. This
undeveloped area will be seeded with a grass mix to further reduce peak runoff. These solar
arrays are not expected to result in a significant creation of impervious area. The project also
proposes four (4) concrete equipment pads, an integrated switch gear pad, and gravel access
roads. In total, the project will result in the generation of approximately 64,843 sf of impervious
area which covers 2.19% of the project site. Grading activities will be localized around the
gravel access road and existing drainage patterns will be maintained.
Impervious Area Calculations
Quantity Area (ft2) Impervious Factor Impervious Area (ft 2)
Equipment Pads 3 7,920 1.0 23,760
Equipment Pad with
Switchgear 1 11,991 1.0 11,991
Array Piles 3,705 ~0.037 1.0 139
Gravel Access Road 1 72,383 0.4 28,953
Total Area (ft2) 64,843
Total LOD (ft2) 2,961,082
Coverage Ratio 2.19%
Table 1 – Impervious Area Calculations
A gravel access road will cross existing stormwater conveyances; therefore, the project also
proposes three (3) culverts, which will be discussed below.
Stormwater Management Strategy
Permanent stormwater management is not required for the proposed development. Project area
drainage areas are to be maintained and runoff will decrease to due changed land use. The
land use is being changed from agricultural row crops to grass cover. Using a grass seed mix to
restore all areas of disturbance, stormwater quality will be improved. The seed mix will also
decrease the quantity of peak stormwater runoff previously experienced on site.
AES Clean Energy Eagle Springs Solar
STORMWATER MANAGEMENT PLAN p. iii
The pre and post development runoff volumes have been modeled using the TR-55 method in
Autodesk’s Storm and Sanitary Analysis. Calculation details can be found in the attachments of
this memo. The runoff volumes are as follows:
2-Year Runoff
(CFS)
25-Year Runoff
(CFS)
Existing 16.24 66.15
Proposed 5.90 42.67
Table 2 – Pre & Post Development Basin Runoff Results
Flood Zone
The project site would be reported on Flood Insurance Rate Map (FIRM) community panel
0802051360B. Currently the panel is not printed, and flood hazard areas are unidentified.
However, the project site is approximately 160 and 130 ft above the intermittent creeks to the
north and west, respectively. Therefore, this project is not expected to be negatively impacted,
nor negatively impact these creeks.
Culvert Design
This project proposes an access road which intersects existing drainage routes. This crossing
requires culverts to maintain drainage through the site.
The culverts are designed to convey the 25-yr peak flow from their contributing drainage basin.
The 25-yr storm event (4% annual exceedance probability) is the typical design storm for
stormwater conveyances per the County’s Land Use & Development Code (2013).
The UDFCD’s modified rational method, an allowable reference per CDOT’s Drainage Design
Manual (DDM, 2019), was used to quantify the 25-yr peak flow for each culvert’s contributing
drainage basin. The following equation is referenced:
Q25 =C25 ∙ I25 ∙ A
Where Q25 is the 25-yr peak flow (cfs), A is the contributing drainage area (acres), I 25 is the
rainfall intensity in inches per hour for a duration equal to the time of concentration, and C 25 is
the runoff coefficient from equations based on NRCS soil groups and storm return period. For
soil groups C/D, the runoff coefficient was determined from the equation:
CC/D = 0.56i + 0.319 (i = % Imperviousness expressed as a decimal)
The runoff coefficient, C, used was 0.33 and the rainfall intensity, I, used was 0.502 inches per
Hour with a basin Tc of 2 hours. Rainfall depth was derived from “NOAA ATLAS 14 POINT
PRECIPITATION FREQUENCY ESTIMATES”.
AES Clean Energy Eagle Springs Solar
STORMWATER MANAGEMENT PLAN p. iv
Existing culverts are constructed of CMP, proposed culverts are to be RCP, which both have a
roughness coefficient (n) of 0.012 per the DDM. The results of the hydraulics calculations are
shown in Table 3 below.
Table 3 – Culvert Sizing Results
Summary
The project proposes 1.49 ac of impervious area, which consists of concrete equipment pads,
gravel access roads, and foundations for solar arrays. This impervious area constitutes 2.19%
of the solar development area. The project will not alter existing drainage patterns, thus does
not require a drainage report.
The proposed access roads will require the installation of three (3) culverts. Initial sizing has
been provided and will be refined as the site design develops.
Attachments
• Hydrology Basin Exhibit
• Pre & Post Development SSA ASCII Report
W
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562
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5632
DRAINAGE
BASIN 1
(33.08 AC.)
DRAINAGE
BASIN 2
(0.69 AC.)
DRAINAGE
BASIN 3
(19.06 AC.)
DRAINAGE
BASIN 4
(18.36 AC.)
DATE
PROJECT TITLE:
DESCRIPTION
APV:
PROJECT LOCATION:
NO.
KEY PLAN:
PE STAMP:
REVISIONS:
SHEET NO:REV:
SCALE AT 24" x 36":
CHK:
DWN:
DES:
DATE:
PROJ
NUM:
SHEET TITLE & DESCRIPTION:
2180 South 1300 East, Suite 600
Salt Lake City, UT 84106-2749
(801) 679 - 3500
ELECTRIC POWER ENGINEERING, INC.
12600 W. COLFAX AVE, STE. C500
LAKEWOOD, CO 80215
(303) 431-7895 www.neieng.com
0 100'200'300'400'
1" = 200'
A 3/29/23 HYDROLOGY_BASIN_EXHIBIT
B 7/12/23 HYDROLOGY_BASIN_EXHIBIT
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JPD
07/12/2023
EX-1.0 A
4084.001
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9
PRELIMINARY
NOT FOR CONSTRUCTION
FOR REVIEW & APPROVAL ONLY
200'100'0 200'400'600'
1"=200'
N
LEGEND:
MODULE BUILDABLE AREA
PARCEL BOUNDARY
FENCE SETBACK LINE
LoD LIMIT OF DISTURBANCE
IRRIGATION LINES
FENCE SETBACK LINE
W
OVERHEAD POWER LINESOHE
DRAINAGE BASIN
HYDROLOGY BASIN
EXHIBIT
SILT, CO
HYDROLOGY BASIN
EXHIBIT
CIVIL
Autodesk® Storm and Sanitary Analysis 2016 - Version 13.5.302 (Build 0)
-----------------------------------------------------------------------------------------
*******************
Project Description
*******************
File Name ................. Eagle Springs Hydrology.SPF
****************
Analysis Options
****************
Flow Units ................ cfs
Subbasin Hydrograph Method. SCS TR-55
Time of Concentration...... SCS TR-55
Storage Node Exfiltration.. None
Starting Date ............. JUN-28-2023 00:00:00
Ending Date ............... JUN-29-2023 00:00:00
Report Time Step .......... 00:05:00
*************
Element Count
*************
Number of rain gages ...... 3
Number of subbasins ....... 1
Number of nodes ........... 1
Number of links ........... 0
****************
Subbasin Summary
****************
Subbasin Total Peak Rate
Area Factor
ID acres
------------------------------------------
Sub-01 128.81 484.00
************
Node Summary
************
Node Element Invert Maximum Ponded External
ID Type Elevation Elev. Area Inflow
ft ft ft²
------------------------------------------------------------------------------
Out-01 OUTFALL 0.00 0.00 0.00
************************** Volume Depth
Runoff Quantity Continuity acre-ft inches
************************** --------- -------
Total Precipitation ...... 13.041 1.215
Surface Runoff ........... 0.289 0.027
Continuity Error (%) ..... -0.000
************************** Volume Volume
Flow Routing Continuity acre-ft Mgallons
************************** --------- ---------
External Inflow .......... 0.000 0.000
External Outflow ......... 2.883 0.939
Initial Stored Volume .... 0.000 0.000
Final Stored Volume ...... 0.000 0.000
Continuity Error (%) ..... 0.000
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
******************************************
Composite Curve Number Computations Report
******************************************
------------------
Subbasin Sub-01
------------------
Area Soil
Soil/Surface Description (acres) Group CN
----------------------------------------------------------------------------------------
Row crops, straight row, Good 128.81 C 85.00
Composite Area & Weighted CN 128.81 85.00
***************************************************
SCS TR-55 Time of Concentration Computations Report
***************************************************
Sheet Flow Equation
-------------------
Tc = (0.007 * ((n * Lf)^0.8)) / ((P^0.5) * (Sf^0.4))
Where:
Tc = Time of Concentration (hrs)
n = Manning's Roughness
Lf = Flow Length (ft)
P = 2 yr, 24 hr Rainfall (inches)
Sf = Slope (ft/ft)
Shallow Concentrated Flow Equation
----------------------------------
V = 16.1345 * (Sf^0.5) (unpaved surface)
V = 20.3282 * (Sf^0.5) (paved surface)
V = 15.0 * (Sf^0.5) (grassed waterway surface)
V = 10.0 * (Sf^0.5) (nearly bare & untilled surface)
V = 9.0 * (Sf^0.5) (cultivated straight rows surface)
V = 7.0 * (Sf^0.5) (short grass pasture surface)
V = 5.0 * (Sf^0.5) (woodland surface)
V = 2.5 * (Sf^0.5) (forest w/heavy litter surface)
Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hrs)
Lf = Flow Length (ft)
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
Channel Flow Equation
---------------------
V = (1.49 * (R^(2/3)) * (Sf^0.5)) / n
R = Aq / Wp
Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hrs)
Lf = Flow Length (ft)
R = Hydraulic Radius (ft)
Aq = Flow Area (ft²)
Wp = Wetted Perimeter (ft)
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
n = Manning's Roughness
------------------
Subbasin Sub-01
------------------
Sheet Flow Computations
-----------------------
Subarea A Subarea B Subarea
C
Manning's Roughness: 0.20 0.00
0.00
Flow Length (ft): 100.00 0.00
0.00
Slope (%): 2.50 0.00
0.00
2 yr, 24 hr Rainfall (in): 1.20 1.20
1.20
Velocity (ft/sec): 0.09 0.00
0.00
Computed Flow Time (minutes): 18.42 0.00
0.00
Shallow Concentrated Flow Computations
--------------------------------------
Subarea A Subarea B Subarea
C
Flow Length (ft): 3464.00 0.00
0.00
Slope (%): 2.50 0.00
0.00
Surface Type: Straight rows Unpaved
Unpaved
Velocity (ft/sec): 1.42 0.00
0.00
Computed Flow Time (minutes): 40.66 0.00
0.00
================================================================================================
Total TOC (minutes): 59.08
================================================================================================
***********************
Subbasin Runoff Summary
***********************
--------------------------------------------------------------------------
Subbasin Total Total Peak Weighted Time of
ID Precip Runoff Runoff Curve Concentration
in in cfs Number days hh:mm:ss
--------------------------------------------------------------------------
Sub-01 1.20 0.28 16.24 85.000 0 00:59:04
--------------------------------------------------------------------------
Analysis began on: Wed Jun 28 15:15:41 2023
Analysis ended on: Wed Jun 28 15:15:42 2023
Total elapsed time: 00:00:01
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
Autodesk® Storm and Sanitary Analysis 2016 - Version 13.5.302 (Build 0)
-----------------------------------------------------------------------------------------
*******************
Project Description
*******************
File Name ................. Eagle Springs Hydrology.SPF
****************
Analysis Options
****************
Flow Units ................ cfs
Subbasin Hydrograph Method. SCS TR-55
Time of Concentration...... SCS TR-55
Storage Node Exfiltration.. None
Starting Date ............. JUN-28-2023 00:00:00
Ending Date ............... JUN-29-2023 00:00:00
Report Time Step .......... 00:05:00
*************
Element Count
*************
Number of rain gages ...... 3
Number of subbasins ....... 1
Number of nodes ........... 1
Number of links ........... 0
****************
Subbasin Summary
****************
Subbasin Total Peak Rate
Area Factor
ID acres
------------------------------------------
Sub-01 128.81 484.00
************
Node Summary
************
Node Element Invert Maximum Ponded External
ID Type Elevation Elev. Area Inflow
ft ft ft²
------------------------------------------------------------------------------
Out-01 OUTFALL 0.00 0.00 0.00
************************** Volume Depth
Runoff Quantity Continuity acre-ft inches
************************** --------- -------
Total Precipitation ...... 23.908 2.227
Surface Runoff ........... 0.998 0.093
Continuity Error (%) ..... -0.000
************************** Volume Volume
Flow Routing Continuity acre-ft Mgallons
************************** --------- ---------
External Inflow .......... 0.000 0.000
External Outflow ......... 9.969 3.249
Initial Stored Volume .... 0.000 0.000
Final Stored Volume ...... 0.000 0.000
Continuity Error (%) ..... 0.000
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
******************************************
Composite Curve Number Computations Report
******************************************
------------------
Subbasin Sub-01
------------------
Area Soil
Soil/Surface Description (acres) Group CN
----------------------------------------------------------------------------------------
Row crops, straight row, Good 128.81 C 85.00
Composite Area & Weighted CN 128.81 85.00
***************************************************
SCS TR-55 Time of Concentration Computations Report
***************************************************
Sheet Flow Equation
-------------------
Tc = (0.007 * ((n * Lf)^0.8)) / ((P^0.5) * (Sf^0.4))
Where:
Tc = Time of Concentration (hrs)
n = Manning's Roughness
Lf = Flow Length (ft)
P = 2 yr, 24 hr Rainfall (inches)
Sf = Slope (ft/ft)
Shallow Concentrated Flow Equation
----------------------------------
V = 16.1345 * (Sf^0.5) (unpaved surface)
V = 20.3282 * (Sf^0.5) (paved surface)
V = 15.0 * (Sf^0.5) (grassed waterway surface)
V = 10.0 * (Sf^0.5) (nearly bare & untilled surface)
V = 9.0 * (Sf^0.5) (cultivated straight rows surface)
V = 7.0 * (Sf^0.5) (short grass pasture surface)
V = 5.0 * (Sf^0.5) (woodland surface)
V = 2.5 * (Sf^0.5) (forest w/heavy litter surface)
Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hrs)
Lf = Flow Length (ft)
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
Channel Flow Equation
---------------------
V = (1.49 * (R^(2/3)) * (Sf^0.5)) / n
R = Aq / Wp
Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hrs)
Lf = Flow Length (ft)
R = Hydraulic Radius (ft)
Aq = Flow Area (ft²)
Wp = Wetted Perimeter (ft)
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
n = Manning's Roughness
------------------
Subbasin Sub-01
------------------
Sheet Flow Computations
-----------------------
Subarea A Subarea B Subarea
C
Manning's Roughness: 0.20 0.00
0.00
Flow Length (ft): 100.00 0.00
0.00
Slope (%): 2.50 0.00
0.00
2 yr, 24 hr Rainfall (in): 1.20 1.20
1.20
Velocity (ft/sec): 0.09 0.00
0.00
Computed Flow Time (minutes): 18.42 0.00
0.00
Shallow Concentrated Flow Computations
--------------------------------------
Subarea A Subarea B Subarea
C
Flow Length (ft): 3464.00 0.00
0.00
Slope (%): 2.50 0.00
0.00
Surface Type: Straight rows Unpaved
Unpaved
Velocity (ft/sec): 1.42 0.00
0.00
Computed Flow Time (minutes): 40.66 0.00
0.00
================================================================================================
Total TOC (minutes): 59.08
================================================================================================
***********************
Subbasin Runoff Summary
***********************
--------------------------------------------------------------------------
Subbasin Total Total Peak Weighted Time of
ID Precip Runoff Runoff Curve Concentration
in in cfs Number days hh:mm:ss
--------------------------------------------------------------------------
Sub-01 2.20 0.94 66.15 85.000 0 00:59:04
--------------------------------------------------------------------------
Analysis began on: Wed Jun 28 15:16:14 2023
Analysis ended on: Wed Jun 28 15:16:15 2023
Total elapsed time: 00:00:01
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
Autodesk® Storm and Sanitary Analysis 2016 - Version 13.5.302 (Build 0)
-----------------------------------------------------------------------------------------
*******************
Project Description
*******************
File Name ................. Eagle Springs Hydrology - Proposed.SPF
****************
Analysis Options
****************
Flow Units ................ cfs
Subbasin Hydrograph Method. SCS TR-55
Time of Concentration...... SCS TR-55
Storage Node Exfiltration.. None
Starting Date ............. JUN-28-2023 00:00:00
Ending Date ............... JUN-29-2023 00:00:00
Report Time Step .......... 00:05:00
*************
Element Count
*************
Number of rain gages ...... 3
Number of subbasins ....... 1
Number of nodes ........... 1
Number of links ........... 0
****************
Subbasin Summary
****************
Subbasin Total Peak Rate
Area Factor
ID acres
------------------------------------------
Sub-01 128.81 484.00
************
Node Summary
************
Node Element Invert Maximum Ponded External
ID Type Elevation Elev. Area Inflow
ft ft ft²
------------------------------------------------------------------------------
Out-01 OUTFALL 0.00 0.00 0.00
************************** Volume Depth
Runoff Quantity Continuity acre-ft inches
************************** --------- -------
Total Precipitation ...... 13.041 1.215
Surface Runoff ........... 0.146 0.014
Continuity Error (%) ..... -0.000
************************** Volume Volume
Flow Routing Continuity acre-ft Mgallons
************************** --------- ---------
External Inflow .......... 0.000 0.000
External Outflow ......... 1.461 0.476
Initial Stored Volume .... 0.000 0.000
Final Stored Volume ...... 0.000 0.000
Continuity Error (%) ..... 0.000
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
******************************************
Composite Curve Number Computations Report
******************************************
------------------
Subbasin Sub-01
------------------
Area Soil
Soil/Surface Description (acres) Group CN
----------------------------------------------------------------------------------------
Row crops, straight row, Good 59.52 C 85.00
Imperv-areas 1.34 - 98.00
> 75% grass cover, Good 67.95 C 74.00
Composite Area & Weighted CN 128.81 79.33
***************************************************
SCS TR-55 Time of Concentration Computations Report
***************************************************
Sheet Flow Equation
-------------------
Tc = (0.007 * ((n * Lf)^0.8)) / ((P^0.5) * (Sf^0.4))
Where:
Tc = Time of Concentration (hrs)
n = Manning's Roughness
Lf = Flow Length (ft)
P = 2 yr, 24 hr Rainfall (inches)
Sf = Slope (ft/ft)
Shallow Concentrated Flow Equation
----------------------------------
V = 16.1345 * (Sf^0.5) (unpaved surface)
V = 20.3282 * (Sf^0.5) (paved surface)
V = 15.0 * (Sf^0.5) (grassed waterway surface)
V = 10.0 * (Sf^0.5) (nearly bare & untilled surface)
V = 9.0 * (Sf^0.5) (cultivated straight rows surface)
V = 7.0 * (Sf^0.5) (short grass pasture surface)
V = 5.0 * (Sf^0.5) (woodland surface)
V = 2.5 * (Sf^0.5) (forest w/heavy litter surface)
Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hrs)
Lf = Flow Length (ft)
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
Channel Flow Equation
---------------------
V = (1.49 * (R^(2/3)) * (Sf^0.5)) / n
R = Aq / Wp
Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hrs)
Lf = Flow Length (ft)
R = Hydraulic Radius (ft)
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
Aq = Flow Area (ft²)
Wp = Wetted Perimeter (ft)
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
n = Manning's Roughness
------------------
Subbasin Sub-01
------------------
Sheet Flow Computations
-----------------------
Subarea A Subarea B Subarea
C
Manning's Roughness: 0.20 0.00
0.00
Flow Length (ft): 100.00 0.00
0.00
Slope (%): 2.50 0.00
0.00
2 yr, 24 hr Rainfall (in): 1.20 1.20
1.20
Velocity (ft/sec): 0.09 0.00
0.00
Computed Flow Time (minutes): 18.42 0.00
0.00
Shallow Concentrated Flow Computations
--------------------------------------
Subarea A Subarea B Subarea
C
Flow Length (ft): 3464.00 0.00
0.00
Slope (%): 2.50 0.00
0.00
Surface Type: Straight rows Unpaved
Unpaved
Velocity (ft/sec): 1.42 0.00
0.00
Computed Flow Time (minutes): 40.66 0.00
0.00
================================================================================================
Total TOC (minutes): 59.08
================================================================================================
***********************
Subbasin Runoff Summary
***********************
--------------------------------------------------------------------------
Subbasin Total Total Peak Weighted Time of
ID Precip Runoff Runoff Curve Concentration
in in cfs Number days hh:mm:ss
--------------------------------------------------------------------------
Sub-01 1.20 0.14 5.90 79.330 0 00:59:04
--------------------------------------------------------------------------
Analysis began on: Wed Jun 28 15:12:53 2023
Analysis ended on: Wed Jun 28 15:12:54 2023
Total elapsed time: 00:00:01
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
Autodesk® Storm and Sanitary Analysis 2016 - Version 13.5.302 (Build 0)
-----------------------------------------------------------------------------------------
*******************
Project Description
*******************
File Name ................. Eagle Springs Hydrology - Proposed.SPF
****************
Analysis Options
****************
Flow Units ................ cfs
Subbasin Hydrograph Method. SCS TR-55
Time of Concentration...... SCS TR-55
Storage Node Exfiltration.. None
Starting Date ............. JUN-28-2023 00:00:00
Ending Date ............... JUN-29-2023 00:00:00
Report Time Step .......... 00:05:00
*************
Element Count
*************
Number of rain gages ...... 3
Number of subbasins ....... 1
Number of nodes ........... 1
Number of links ........... 0
****************
Subbasin Summary
****************
Subbasin Total Peak Rate
Area Factor
ID acres
------------------------------------------
Sub-01 128.81 484.00
************
Node Summary
************
Node Element Invert Maximum Ponded External
ID Type Elevation Elev. Area Inflow
ft ft ft²
------------------------------------------------------------------------------
Out-01 OUTFALL 0.00 0.00 0.00
************************** Volume Depth
Runoff Quantity Continuity acre-ft inches
************************** --------- -------
Total Precipitation ...... 23.908 2.227
Surface Runoff ........... 0.693 0.065
Continuity Error (%) ..... -0.000
************************** Volume Volume
Flow Routing Continuity acre-ft Mgallons
************************** --------- ---------
External Inflow .......... 0.000 0.000
External Outflow ......... 6.921 2.255
Initial Stored Volume .... 0.000 0.000
Final Stored Volume ...... 0.000 0.000
Continuity Error (%) ..... 0.000
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
******************************************
Composite Curve Number Computations Report
******************************************
------------------
Subbasin Sub-01
------------------
Area Soil
Soil/Surface Description (acres) Group CN
----------------------------------------------------------------------------------------
Row crops, straight row, Good 59.52 C 85.00
Imperv-areas 1.34 - 98.00
> 75% grass cover, Good 67.95 C 74.00
Composite Area & Weighted CN 128.81 79.33
***************************************************
SCS TR-55 Time of Concentration Computations Report
***************************************************
Sheet Flow Equation
-------------------
Tc = (0.007 * ((n * Lf)^0.8)) / ((P^0.5) * (Sf^0.4))
Where:
Tc = Time of Concentration (hrs)
n = Manning's Roughness
Lf = Flow Length (ft)
P = 2 yr, 24 hr Rainfall (inches)
Sf = Slope (ft/ft)
Shallow Concentrated Flow Equation
----------------------------------
V = 16.1345 * (Sf^0.5) (unpaved surface)
V = 20.3282 * (Sf^0.5) (paved surface)
V = 15.0 * (Sf^0.5) (grassed waterway surface)
V = 10.0 * (Sf^0.5) (nearly bare & untilled surface)
V = 9.0 * (Sf^0.5) (cultivated straight rows surface)
V = 7.0 * (Sf^0.5) (short grass pasture surface)
V = 5.0 * (Sf^0.5) (woodland surface)
V = 2.5 * (Sf^0.5) (forest w/heavy litter surface)
Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hrs)
Lf = Flow Length (ft)
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
Channel Flow Equation
---------------------
V = (1.49 * (R^(2/3)) * (Sf^0.5)) / n
R = Aq / Wp
Tc = (Lf / V) / (3600 sec/hr)
Where:
Tc = Time of Concentration (hrs)
Lf = Flow Length (ft)
R = Hydraulic Radius (ft)
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
Aq = Flow Area (ft²)
Wp = Wetted Perimeter (ft)
V = Velocity (ft/sec)
Sf = Slope (ft/ft)
n = Manning's Roughness
------------------
Subbasin Sub-01
------------------
Sheet Flow Computations
-----------------------
Subarea A Subarea B Subarea
C
Manning's Roughness: 0.20 0.00
0.00
Flow Length (ft): 100.00 0.00
0.00
Slope (%): 2.50 0.00
0.00
2 yr, 24 hr Rainfall (in): 1.20 1.20
1.20
Velocity (ft/sec): 0.09 0.00
0.00
Computed Flow Time (minutes): 18.42 0.00
0.00
Shallow Concentrated Flow Computations
--------------------------------------
Subarea A Subarea B Subarea
C
Flow Length (ft): 3464.00 0.00
0.00
Slope (%): 2.50 0.00
0.00
Surface Type: Straight rows Unpaved
Unpaved
Velocity (ft/sec): 1.42 0.00
0.00
Computed Flow Time (minutes): 40.66 0.00
0.00
================================================================================================
Total TOC (minutes): 59.08
================================================================================================
***********************
Subbasin Runoff Summary
***********************
--------------------------------------------------------------------------
Subbasin Total Total Peak Weighted Time of
ID Precip Runoff Runoff Curve Concentration
in in cfs Number days hh:mm:ss
--------------------------------------------------------------------------
Sub-01 2.20 0.66 42.67 79.330 0 00:59:04
--------------------------------------------------------------------------
Analysis began on: Wed Jun 28 15:13:20 2023
Analysis ended on: Wed Jun 28 15:13:21 2023
Total elapsed time: 00:00:01
Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis
From:Chris@mountaincross-eng.com
To:Philip Berry
Subject:RE: stormwater plan - eagle springs organic large solar
Date:Tuesday, August 15, 2023 1:58:44 PM
Philip:
No additional comments on these, either. Thanks again.
Sincerely,
Mountain Cross
Engineering, Inc.
Chris Hale, P.E.
826 1/2 Grand Avenue
Glenwood Springs, CO 81601
Ph: 970.945.5544
Fx: 970.945.5558
From: Philip Berry <pberry@garfield-county.com>
Sent: Monday, July 31, 2023 3:50 PM
To: Chris@mountaincross-eng.com
Subject: RE: stormwater plan - eagle springs organic large solar
I apologize, I also have these updated documents. I don’t think I forwarded them all to you. Sorry
about that.
Philip Berry, AICP
Planner Garfield County – Community Development Department
108 8th. St., Suite 401
Glenwood Springs, CO 81601
Pberry@garfield-county.com
Office: 970-945-1377 (1580)
Cell: 970-456-4404
From: Philip Berry
Sent: Monday, July 31, 2023 11:08 AM
To: Chris@mountaincross-eng.com
Subject: stormwater plan - eagle springs organic large solar
Attached is an updated version of their stormwater plan for application (MPPA-04-23-8956). I’m
finalizing my staff report for the BOCC hearing that will be next Monday. This item is a
recommended Condition of Approval that they are trying to fulfill as soon as possible. No giant rush
on our end for review though. Please call if you have any questions or concerns.
Thanks
Philip Berry, AICP
Planner Garfield County – Community Development Department
108 8th. St., Suite 401
Glenwood Springs, CO 81601
Pberry@garfield-county.com
Office: 970-945-1377 (1580)
Cell: 970-456-4404