HomeMy WebLinkAboutTHE ELLIE AT OLD TOWN NORTH - PDP240002 - SUBMITTAL DOCUMENTS - ROUND 1 - Drainage Related DocumentPDP DRAINAGE REPORT
The Ellie at Old Town North
Old Town North 5th Filing Tract A
for:
Van Horn Development Management, LLC
14143 Denver West Parkway #100
Golden, CO 80401
engineer:
TJC Limited
8751 E Hampden Ave, Suite B10, Denver, CO 80231
James W. Allen, PE
303.489.3346
www.tjcivil.com
April 24, 2024
Preliminary Drainage Report
Page 2 of 8
TABLE OF CONTENTS
COVER LETTER .......................................................................... Error! Bookmark not defined.
GENERAL LOCATION AND EXISTING SITE INFORMATION ............................................. 4
MASTER DRAINAGE BASIN INFORMATION ........................................................................ 4
FLOODPLAIN INFORMATION .................................................................................................. 5
PROPOSED DRAINAGE FACILITIES ....................................... Error! Bookmark not defined.
DRAINAGE DESIGN CRITERIA................................................................................................. 6
VARIANCE REQUESTS ............................................................................................................... 7
EROSION CONTROL ................................................................................................................... 7
CONCLUSION ............................................................................................................................... 7
REFERENCES ............................................................................................................................... 7
APPENDICES ................................................................................................................................ 8
Preliminary Drainage Report
Page 3 of 8
I hereby attest that this report for the [preliminary or final] drainage design for the Ellie at Old
Town was prepared by me or under my direct supervision, in accordance with the provisions of
the Fort Collins Stormwater Criteria Manual. I understand that the City of Fort Collins does not
and will not assume liability for drainage facilities designed by others.
James W. Allen Licensed Professional Engineer State of Colorado No. 42256
Preliminary Drainage Report
Page 4 of 8
GENERAL LOCATION AND EXISTING SITE INFORMATION
a) Site Location Description
i. The site is located in Section 1, Township 7 North, Range 69 West of the 6th Principal
Meridian, County of Larimer, State of Colorado. Also known Old Town North 5th Filing,
Tract A.
ii. A vicinity map is attached in appendix A.
iii. The property is adjacent to Sungia Rd, Blondel St, and Alleys.
iv. Adjacent land uses are residential Old Town North filings.
v. The project is within North East College Corridor Outfall (NECCO) Basin 810. The
project is with Old Town North Filing 3 Basin 7.
vi. The existing site generally slopes west to east. There are two existing 18” RCP culverts
that outfall the site under Blondel Street. These culverts were constructed for interim
flows during the development of the area. These flows were eliminated when the NECCO
improvements were completed.
vii. There are no irrigation facilities on-site.
viii. The land is currently vacant.
ix. The property is covered with natural grass. Slopes range from 2% to 7%.
x. Soils consist of Nunn clay loan with Hydrologic Soil Group C. A NRCS Soils
Classification Map is attached as appendix B.
MASTER DRAINAGE BASIN INFORMATION
a) The site is located in the Lower Dry Creek Basin. The site was situated in the Dry Creek
floodplain when the original Old Town North plat was recorded in 2004 and Old Town North
was developed. The Dry Creek floodway has since been redefined to remove a majority of
the site from the Dry Creek floodplain. The immediate area adjacent to the property is fully
developed with residential use.
i. The subject property is within the Old Town North Third Filing and 2009 NECCO
study. The project runoff will conveyed through existing stormwater facilities to the
NECCO Detention pond north of Sungia Road.
ii. NECCO assumed a 90% imperviousness (developed condition).
b) Site Drainage Basins
i. The site is composed of one historic sub-basin and no offsite sub-basins:
Sub-basin H1 – consists of grass covered vacant land. Runoff sheet flows east to the
existing 18” RCP culverts under Blondel St.
Sub-Basin Design
Point
Area
(acres)
5-Year
Runoff
(cfs)
100-Year
Runoff
(cfs)
Preliminary Drainage Report
Page 5 of 8
H1 H1 0.81 0.26 1.08
c) There are no irrigation facilities on or adjacent to the property.
FLOODPLAIN INFORMATION
a) The site is within a Zone X according to FEMA Flood Insurance Rate Map No.
08013C0411K (appendix C).
b) The site was situated in the Dry Creek floodplain when the original Old Town North plat was
recorded in 2004 and Old Town North was developed. The Dry Creek floodway has since
been redefined to remove a majority of the site from the Dry Creek floodplain.
PROJECT DESCRIPTION
a) The project proposed two apartment buildings with tuck under garages. Surface parking is
being provided on the west end of the site. The existing alley will be widened. A rain garden
in proposed to meet Ft Collins LID requirements.
b) The site is approximately 0.81 acres.
PROPOSED DRAINAGE FACILITIES
a) The project site is composed of four sub-basins and no offsite sub-basins:
Sub-basin A – consists of roof, pavement, and landscaping. Runoff flows off-site
undetained to the western ally and conveyed via an existing gutter pan to the south
towards Emmaus Ln south of the site at Design Point 1.
Sub-basin B – consists of roof, pavement and a rain gardensidewalks. Runoff flows down
the private drive to a curb cut and into the rain garden. The rain garden outfalls into one
of the existing 18” RCP culverts under Blondel St at Design Point 2.
Sub-basin C – consists of sidewalks and landscaping. Runoff sheet flows into Sungia Rd
and into an existing storm inlet at the corner of Sungia Rd and Blondel St at Design Point
3.
Sub-basin D – consists of sidewalks and landscaping. Runoff sheet flows into Blondel St
at Design Point 4.
Sub-Basin Design
Point
Area
(acres)
5-Year
Runoff
(cfs)
100-Year
Runoff
(cfs)
A 1 0.16 0.24 0.99
B 2 0.47 0.91 3.82
C 3 0.11 0.08 0.33
D 4 0.06 0.06 0.23
Preliminary Drainage Report
Page 6 of 8
b) There is no on-site detention required for this project.
c) A rain garden is proposed within Basin B to treat a minimum of 75% of the new impervious
area. Site topography and the existing 18” RCP outfall limits the location of the rain garden
bottom requiring the use walls to obtain sufficient storage volume and provide 6 inches of
freeboard.
Total Impervious Area Added 26,054 sft
Min Area Treated (75% for RG) 19,541 sft
Actual Area Treated 19,550 sft
The rain garden has walls on all sides varying from 1.0-2.0 ft tall. Approximately 1.0 ft of the
wall will extrude from the ground.
d) LID techniques utilized in this design include minimizing directly connection impervious
areas by discharging roof downspouts onto the surface. No stormwater will be piped except
for the stormwater discharging from the rain garden.
e) Maintenance of the rain garden can be performed from the private drive or Blondel Street (20
ft east of the facility).
f) A drainage easement will be provided over the rain garden.
DRAINAGE DESIGN CRITERIA
a) The subject property is within the Old Town North Third Filing and 2009 NECCO study.
The project runoff will be conveyed through existing stormwater facilities to the NECCO
detention pond north of Sungia Road. NECCO assumed a 90% imperviousness (developed
condition). The previous studies have no impact to the proposed design.
b) Four-step process
a. Where possible, the runoff sheet flows across landscaping before discharging into
Sungia Rd and Blondel St.
b. A rain garden is proposed to treat a minimum of 75% of the new impervious
pavement and roof areas.
c. There are no streams within the project area to strengthen.
d. A construction erosion control plan will be included with this project.
c) Rainfall data is provided in table 3.4.1 IDF Table for Rational Method from the Fort Collins
Stormwater Criteria Manual.
d) Design frequencies used in this design:
Minor: 2-year
Major: 100-year
e) The Rational method was used to calculate hydrological data following the Fort Collins
Stormwater Criteria Manual chapter 5.0The peak runoff from minor and major storm events
are calculated using the Rational Method formula as detailed below:
Q=CIA
Where:
Preliminary Drainage Report
Page 7 of 8
Q= Storm runoff in cubic feet per second (cfs)
C= Runoff Coefficient
I = Runoff intensity in inches per hour
A= Drainage area in acres
Runoff coefficients were obtained from Table 3.2-2.
Times of concentration were calculated based on the initial time or overland flow time (for
urbanized undeveloped basins < 300’ overland flow), with a minimum time of concentration of 5
minutes.
t i = 1.87(1.1 - C 5 )√L / S0.33 MANUAL Eqn. (3.3-2)
V = 1.49/n R2/3S1/2 MANUAL Eqn. (5-4)
t t = L /(60V) MANUAL Eqn. (5-5)
t c = t i + t t MANUAL Eqn. (5-3)
MAX. t c = L/180 + 10 MANUAL Eqn. (3.3-5)
f) No on-site detention is required as part of this project.
g) No public streets requiring capacity calculations are required as part of this project.
h) The rain garden outlet structure is sized using weir flow calculations.
i) No storm sewer pipe networks are included with this project.
j) No swale or channels are included with this project.
k) No emergency spillways are included with this project.
VARIANCE REQUESTS
a) No variances are requested as part of this project.
EROSION CONTROL
a) This report is in compliance with all erosion control materials within the final Erosion
Control Report and Plans.
CONCLUSION
a) Compliance with requirements – this design complies with the requirements of the city of
Fort Collins and master drainage plans referenced in this report.
b) Drainage plan effectiveness – this design effectively routes developed flows throughout the
site and provides water quality for at least 75% of the new impervious areas. This design
does not impact adjacent properties.
REFERENCES
1. City of Fort Collins Stormwater Criteria Manuel, December 2018.
2. Urban Storm Drainage Criteria Manual, Volumes 1, 2, and 3, Revised March 2024
3. North East College Corridor Outfall (NECCO), August 2009.
Preliminary Drainage Report
Page 8 of 8
4. Old Town North Third Filing, Drainage, Sediment/Erosion Control, and Stormwater
Quality Report, June 2017
APPENDICES
A. Vicinity Map
B. FEMA Map
C. Soil Information
D. Old Town North Third Filing Excerpts
E. Drainage Calculations
F. Drainage Plans
Appendix A
National Flood Hazard Layer FIRMette
0 500 1,000 1,500 2,000250
Feet
Ü
SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT
SPECIAL FLOOD
HAZARD AREAS
Without Base Flood Elevation (BFE)
Zone A, V, A99
With BFE or DepthZone AE, AO, AH, VE, AR
Regulatory Floodway
0.2% Annual Chance Flood Hazard, Areas
of 1% annual chance flood with average
depth less than one foot or with drainage
areas of less than one square mileZone X
Future Conditions 1% Annual
Chance Flood HazardZone X
Area with Reduced Flood Risk due to
Levee. See Notes.Zone X
Area with Flood Risk due to LeveeZone D
NO SCREEN Area of Minimal Flood Hazard Zone X
Area of Undetermined Flood HazardZone D
Channel, Culvert, or Storm Sewer
Levee, Dike, or Floodwall
Cross Sections with 1% Annual Chance
17.5 Water Surface Elevation
Coastal Transect
Coastal Transect Baseline
Profile Baseline
Hydrographic Feature
Base Flood Elevation Line (BFE)
Effective LOMRs
Limit of Study
Jurisdiction Boundary
Digital Data Available
No Digital Data Available
Unmapped
This map complies with FEMA's standards for the use of
digital flood maps if it is not void as described below.
The basemap shown complies with FEMA's basemap
accuracy standards
The flood hazard information is derived directly from the
authoritative NFHL web services provided by FEMA. This map
was exported on 4/19/2024 at 2:17 PM and does not
reflect changes or amendments subsequent to this date and
time. The NFHL and effective information may change or
become superseded by new data over time.
This map image is void if the one or more of the following map
elements do not appear: basemap imagery, flood zone labels,
legend, scale bar, map creation date, community identifiers,
FIRM panel number, and FIRM effective date. Map images for
unmapped and unmodernized areas cannot be used for
regulatory purposes.
Legend
OTHER AREAS OF
FLOOD HAZARD
OTHER AREAS
GENERAL
STRUCTURES
OTHER
FEATURES
MAP PANELS
8
B 20.2
The pin displayed on the map is an approximate
point selected by the user and does not represent
an authoritative property location.
1:6,000
105°4'40"W 40°36'12"N
105°4'3"W 40°35'45"N
Basemap Imagery Source: USGS National Map 2023
Appendix B
Soil Map—Larimer County Area, Colorado
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2024
Page 1 of 3
44
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7
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493800 493810 493820 493830 493840 493850 493860 493870 493880 493890 493900 493910 493920
493800 493810 493820 493830 493840 493850 493860 493870 493880 493890 493900 493910 493920
40° 35' 59'' N
10
5
°
4
'
2
4
'
'
W
40° 35' 59'' N
10
5
°
4
'
1
8
'
'
W
40° 35' 57'' N
10
5
°
4
'
2
4
'
'
W
40° 35' 57'' N
10
5
°
4
'
1
8
'
'
W
N
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 30 60 120 180
Feet
0 5 10 20 30
Meters
Map Scale: 1:627 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
Appendix C
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Map Unit Polygons
Soil Map Unit Lines
Soil Map Unit Points
Special Point Features
Blowout
Borrow Pit
Clay Spot
Closed Depression
Gravel Pit
Gravelly Spot
Landfill
Lava Flow
Marsh or swamp
Mine or Quarry
Miscellaneous Water
Perennial Water
Rock Outcrop
Saline Spot
Sandy Spot
Severely Eroded Spot
Sinkhole
Slide or Slip
Sodic Spot
Spoil Area
Stony Spot
Very Stony Spot
Wet Spot
Other
Special Line Features
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Larimer County Area, Colorado
Survey Area Data: Version 18, Aug 24, 2023
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Jul 2, 2021—Aug 25,
2021
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Soil Map—Larimer County Area, Colorado
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2024
Page 2 of 3
Appendix C
Map Unit Legend
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
73 Nunn clay loam, 0 to 1 percent
slopes
1.6 100.0%
Totals for Area of Interest 1.6 100.0%
Soil Map—Larimer County Area, Colorado
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2024
Page 3 of 3
Appendix C
Larimer County Area, Colorado
73—Nunn clay loam, 0 to 1 percent slopes
Map Unit Setting
National map unit symbol: 2tlng
Elevation: 4,100 to 5,700 feet
Mean annual precipitation: 14 to 15 inches
Mean annual air temperature: 48 to 52 degrees F
Frost-free period: 135 to 152 days
Farmland classification: Prime farmland if irrigated
Map Unit Composition
Nunn and similar soils:85 percent
Minor components:15 percent
Estimates are based on observations, descriptions, and transects of
the mapunit.
Description of Nunn
Setting
Landform:Terraces
Landform position (three-dimensional):Tread
Down-slope shape:Linear
Across-slope shape:Linear
Parent material:Pleistocene aged alluvium and/or eolian deposits
Typical profile
Ap - 0 to 6 inches: clay loam
Bt1 - 6 to 10 inches: clay loam
Bt2 - 10 to 26 inches: clay loam
Btk - 26 to 31 inches: clay loam
Bk1 - 31 to 47 inches: loam
Bk2 - 47 to 80 inches: loam
Properties and qualities
Slope:0 to 1 percent
Depth to restrictive feature:More than 80 inches
Drainage class:Well drained
Runoff class: Medium
Capacity of the most limiting layer to transmit water
(Ksat):Moderately low to moderately high (0.06 to 0.20 in/hr)
Depth to water table:More than 80 inches
Frequency of flooding:None
Frequency of ponding:None
Calcium carbonate, maximum content:7 percent
Maximum salinity:Nonsaline (0.1 to 1.0 mmhos/cm)
Sodium adsorption ratio, maximum:0.5
Available water supply, 0 to 60 inches: High (about 9.1 inches)
Interpretive groups
Land capability classification (irrigated): 3e
Map Unit Description: Nunn clay loam, 0 to 1 percent slopes---Larimer County Area, Colorado
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2024
Page 1 of 2
Appendix C
Land capability classification (nonirrigated): 4e
Hydrologic Soil Group: C
Ecological site: R067BY042CO - Clayey Plains
Hydric soil rating: No
Minor Components
Heldt
Percent of map unit:10 percent
Landform:Terraces
Landform position (three-dimensional):Tread
Down-slope shape:Linear
Across-slope shape:Linear
Ecological site:R067BY042CO - Clayey Plains
Hydric soil rating: No
Wages
Percent of map unit:5 percent
Landform:Terraces
Landform position (three-dimensional):Tread
Down-slope shape:Linear
Across-slope shape:Linear
Ecological site:R067BY002CO - Loamy Plains
Hydric soil rating: No
Data Source Information
Soil Survey Area: Larimer County Area, Colorado
Survey Area Data: Version 18, Aug 24, 2023
Map Unit Description: Nunn clay loam, 0 to 1 percent slopes---Larimer County Area, Colorado
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2024
Page 2 of 2
Appendix C
Page 4
Drainage, Sediment/Erosion Control, and Stormwater Quality Report
Old Town North Third Filing; Fort Collins, Colorado
a. Runoff from the front lots of the single family lots fronting Osiander Street
and included in Sub -basin A is directed to the Old Town North detention
Pond. Refer to original Old Town North Drainage Plan in Appendix IV.
3. A majority of the Old Town North Third Filing property is included in the 2009
North East College Corridor Outfall (NECCO) study. Old Town North Third
Filing runoff will be conveyed to the NECCO detention Pond north of Suniga
Road via storm sewer.
a. A majority of the Old Town North Third Filing site is included in NECCO
Basin 810 and was modeled as a developed basin.
b. NECCO implemented an area of 8.2 acres for Basin 810 at 90%
imperviousness (developed condition) in the SWMM model.
4. A portion of the North East College Corridor Outfall (NECCO) improvements
were constructed adjacent to Old Town North Third Filing on the south side of
Suniga Road with the Aspen Heights Subdivision project to the north.
a. The section of storm sewer constructed in Suniga Road to convey runoff
from the south side of Suniga Road to the north side of Suniga Road is a
36" diameter RCP. The storm sewer profile included with the Aspen
Heights Subdivision Utility Plans is included with this report for immediate
reference. Refer to Appendix IV.
i. Reference Sheet 32 of 43 (record drawing) of the "Utility Plans for
Suniga Road to Blue Spruce Drive to Redwood Street and Redwood
Street from Lupine Drive to Cajetan Street" by Owen Consulting Group,
Inc.; Owen Project Number: 155-101FC; Plans dated April 29, 2015;
City Engineer approval date: 05/18/2015; Plan revision date:
10/28/2015 (As -Built storm).
b. The existing NECCO 36" RCP storm sewer is currently bulkheaded at both
ends with no outfall. Suniga Street existing inlets accept storm runoff and
snowmelt runoff to this section of storm sewer with no outfall. Therefore,
the NECCO 36" RCP storm sewer acts as a temporary storage facility.
There is approximately 960 LF of existing NECCO 36" RCP storm sewer
according to Sheet 32 of 43 (record drawing) of the "Utility Plans for Suniga
Road to Blue Spruce Drive to Redwood Street and Redwood Street from
Lupine Drive to Cajetan Street".
C. The NECCO Regional Detention Pond on the north side of Suniga Road is
not complete. It is currently built to accommodate Aspen Heights
Subdivision in an interim condition. Our understanding is that the NECCO
regional Pond and outfall will be completed in 2 to 4 years.
Apendix D
Project is part
of NECCO
Basin 810
Page 6
Drainage, Sediment/Erosion Control, and Stormwater Quality Report
Old Town North Third Filing; Fort Collins, Colorado
b. Refer to Aspen Heights Offsite Streets Sheet 30 of 43: Swale Plan and
Profile included in Appendix W.
C. Refer to Aspen Heights Offsite Streets Sheet 31 of 43: Swale Plan and
Profile included in Appendix IV.
3. Two (2) 14"x23" elliptical RCP culverts were placed under Blondel Street and a
channel was constructed on the Old Town north property with Aspen Heights
construction. Our understanding is that the culverts and channel were constructed
to convey interim historic flows from the west to Dry Creek and the Lake Canal.
Our understanding is that these flows are eliminated when the NECCO
improvements are completed.
a. The two (2) 14"x23" elliptical RCP culverts convey a 10-year storm flow
of 20.8 cfs.
b. The 50-year storm flow to this point is 35.8 cfs.
C. The 100-year storm peak flow is 46.8 cfs.
d. Refer to Appendix for April 27, 2015 e-mail from Larry Owen to Michael
Palizzi provided by Mr. Larry Owen discussing the Blondel Street culvert
capacity and interim peak flows.
4. After development of Aspen Heights Subdivision,construction of Suniga Road
and Redwood Street, and the construction of the interim channel on the Old Town
North property with the Aspen Heights Subdivision for conveyance of historic
flows from the west, runoff from a majority of the Old Town North Third Filing
site has no outfall. The channel berm was built up higher than existing grades on
the east and dammed the historic outfall point for this property.
a. Refer to Topographical Survey for Block 6, Tracts JJ and KK, Old Town
North by Intermill Land Surveying.
IV. DRAINAGE DESIGN CRITERIA
A. Regulations
Stormwater management improvements are designed in compliance with the
requirements of the City of Fort Collins Stormwater Criteria Manual adopted
December 2011. (Manual). The Manual incorporates most of the Urban Storm
Drainage Criteria Manual (USDCM) with amendments that are unique to Fort
Collins.
2. The latest versions of the USDCM software were used for runoff calculations.
Apendix D
Culverts no longer
required.
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CHECKED BY: WATERA A
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Apendix D
Project
Location
Anne mue, oftilRe.
a'[i(IP}I](J SCALE—
OI{ —0 EXISTING OVERNFAO METRIC TRANSMISSION UNE k PONE
FO— EXISTING FIBER OPTIC UNE \\
Imn - eon.
y_ EXISTING SANITARY SEWER W/MH \ 1 --
W
1
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3-15ItS— EXISTING IS' SANITARY SEWER
p4
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e — EXISTING OAS UNE \
It\>
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gia,
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PROPOSED UNITARY SEWER W/MH P'6q n
V
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9 PROPOSED CAS UNE /\_ _ _. -.
a. PROPOSED ELECTRIC UNE
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to ABU IN ACRES ?ECnd"SIIeHydrology ecCnoCOMPOSITERUNDWCOEFFICIENT (
IN YR. EVENT) y 1 "iJ1111'IdFI j NOfE9, yypWD t
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Apendix D
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.2 Runoff Coefficients
Page 5
Table 3.2-2. Surface Type - Runoff Coefficients
Surface Type Runoff Coefficients
Hardscape or Hard Surface
Asphalt, Concrete 0.95
Rooftop 0.95
Recycled Asphalt 0.80
Gravel 0.50
Pavers 0.50
Landscape or Pervious Surface
Lawns, Sandy Soil, Flat Slope < 2% 0.10
Lawns, Sandy Soil, Avg Slope 2-7% 0.15
Lawns, Sandy Soil, Steep Slope >7% 0.20
Lawns, Clayey Soil, Flat Slope < 2% 0.20
Lawns, Clayey Soil, Avg Slope 2-7% 0.25
Lawns, Clayey Soil, Steep Slope >7% 0.35
3.2.1 Composite Runoff Coefficients
Drainage sub-basins are frequently composed of land that has multiple surface types or zoning
classifications. In such cases a composite runoff coefficient must be calculated for any given drainage
sub-basin.
The composite runoff coefficient is obtained using the following formula:
( )
t
n
i
ii
A
xAC
C
∑
==1 Equation 5-2
Where: C = Composite Runoff Coefficient
Ci = Runoff Coefficient for Specific Area (Ai), dimensionless
Ai = Area of Surface with Runoff Coefficient of Ci, acres or square feet
n = Number of different surfaces to be considered
At = Total Area over which C is applicable, acres or square feet
3.2.2 Runoff Coefficient Frequency Adjustment Factor
The runoff coefficients provided in Table 3.2-1 and Table 3.2-2 are appropriate for use with the 2-year
storm event. For any analysis of storms with higher intensities, an adjustment of the runoff coefficient is
required due to the lessening amount of infiltration, depression retention, evapotranspiration and other
losses that have a proportionally smaller effect on high-intensity storm runoff. This adjustment is
Appendix E
TJC
Project Ellie
Date 4/8/2024
PAVEMENT 0.95
ROOFS 0.95
GRAVEL 0.50
LANDSCAPE/UNDEVELOPED 0.15 2-7%
BASIN OVERALL
(ACRE)OVERALL (SF)PAVEMENT
(SF)
GRAVEL
(SF)ROOFS (SF) LANDSCAPE (SF) 2 YR C 100 YR C
H1 0.81 35,197 - - - 35,197 0.15 0.18
Site Total 0.81 35,197 - - - 35,197 0.15 0.18
BASIN OVERALL
(ACRE)OVERALL (SF)PAVEMENT
(SF)
GRAVEL
(SF)ROOFS (SF) LANDSCAPE (SF) 2 YR C 100 YR C
A 0.16 7,130 3,445 - 1,054 2,631 0.65 0.79
B 0.47 20,348 6,146 - 13,404 798 0.92 1.10
C 0.11 4,932 1,105 - - 3,827 0.33 0.40
D 0.06 2,787 900 - - 1,887 0.41 0.49
Site Total 0.81 35,197 11,596 - 14,458 9,143 0.74 0.89
Total Imperviouss Area Added 26,054 SFT
Min Area Treated (75% for RG)19,541 SFT
Actual Area Treated 19,550 SFT
OKAY!
"C" CALCULATIONS
T:\Projects\Ellie\03_Project Reports\Drainage\C Calcx - C-CALCS 4/19/2024
Appendix E
TJC
Project Ellie
Date: 4/19/2024
ti = 1.87(1.1 - C5)√L / S0.33 MANUAL Eqn. (3.3-2)
V = 1.49/n R
2/3S1/2 MANUAL Eqn. (5-4)
tt =L /(60V )MANUAL Eqn. (5-5)
tc = ti + tt MANUAL Eqn. (5-3)
MAX. tc =L/180 + 10 MANUAL Eqn. (3.3-5)
COMP FINAL REMARKS
AVG. SLOPE Tc Tc
Design Point BASIN(S) AREA C LENGTH AVG. SLOPE Ti LENGTH S n R VEL Tt TOTAL Tc=(L/180)+10
(Acres) (ft/ft) (ft/ft) (fps) (min) LENGTH (ft) (min) (min)
H1 H1 0.81 0.15 300 0.02 118.83 0.0 0.0 118.83 300 11.7 11.7
1A0.16 0.65 45 0.04 16.16 76 0.01 0.12 0.50 0.8 1.6 17.78 121 10.7 10.7
2B0.47 0.92 45 0.02 8.27 214 0.008 0.12 1.00 1.1 3.2 11.48 259 11.4 11.4
3C0.11 0.33 15 0.02 20.30 200 0.01 0.12 0.50 0.8 4.3 24.56 215 11.2 11.2
4D0.06 0.41 12 0.02 16.29 200 0.01 0.12 0.50 0.8 4.3 20.55 212 11.2 11.2
(Urbanized Basins)
TIME OF CONCENTRATION - DEVELOPED
SUB-BASIN DATA INITIAL / OVERLAND TRAVEL TIME (Tt)Tc CHECK
T:\Projects\Ellie\03_Project Reports\Drainage\C Calcx - CONC. TIME 4/19/2024
Appendix E
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.4 Intensity-Duration-Frequency Curves for Rational Method
Page 8
Table 3.4-1. IDF Table for Rational Method
Duration
(min)
Intensity
2-year
(in/hr)
Intensity
10-year
(in/hr)
Intensity
100-year
(in/hr)
Duration
(min)
Intensity
2-year
(in/hr)
Intensity
10-year
(in/hr)
Intensity
100-year
(in/hr)
5 2.85 4.87 9.95
39 1.09 1.86 3.8
6 2.67 4.56 9.31
40 1.07 1.83 3.74
7 2.52 4.31 8.80
41 1.05 1.80 3.68
8 2.40 4.10 8.38
42 1.04 1.77 3.62
9 2.30 3.93 8.03
43 1.02 1.74 3.56
10 2.21 3.78 7.72
44 1.01 1.72 3.51
11 2.13 3.63 7.42
45 0.99 1.69 3.46
12 2.05 3.50 7.16
46 0.98 1.67 3.41
13 1.98 3.39 6.92
47 0.96 1.64 3.36
14 1.92 3.29 6.71
48 0.95 1.62 3.31
15 1.87 3.19 6.52
49 0.94 1.6 3.27
16 1.81 3.08 6.30
50 0.92 1.58 3.23
17 1.75 2.99 6.10
51 0.91 1.56 3.18
18 1.70 2.90 5.92
52 0.9 1.54 3.14
19 1.65 2.82 5.75
53 0.89 1.52 3.10
20 1.61 2.74 5.60
54 0.88 1.50 3.07
21 1.56 2.67 5.46
55 0.87 1.48 3.03
22 1.53 2.61 5.32
56 0.86 1.47 2.99
23 1.49 2.55 5.20
57 0.85 1.45 2.96
24 1.46 2.49 5.09
58 0.84 1.43 2.92
25 1.43 2.44 4.98
59 0.83 1.42 2.89
26 1.4 2.39 4.87
60 0.82 1.4 2.86
27 1.37 2.34 4.78
65 0.78 1.32 2.71
28 1.34 2.29 4.69
70 0.73 1.25 2.59
29 1.32 2.25 4.60
75 0.70 1.19 2.48
30 1.30 2.21 4.52
80 0.66 1.14 2.38
31 1.27 2.16 4.42
85 0.64 1.09 2.29
32 1.24 2.12 4.33
90 0.61 1.05 2.21
33 1.22 2.08 4.24
95 0.58 1.01 2.13
34 1.19 2.04 4.16
100 0.56 0.97 2.06
35 1.17 2.00 4.08
105 0.54 0.94 2.00
36 1.15 1.96 4.01
110 0.52 0.91 1.94
37 1.16 1.93 3.93
115 0.51 0.88 1.88
38 1.11 1.89 3.87
120 0.49 0.86 1.84
Appendix E
TJC
Project Ellie
Date: 4/19/2024 FALSE
DESIGN BASIN(S) AREA RUNOFF tc INTENSITY RUNOFF tc SUM INTENSITY RUNOFF DESIGN SLOPE PIPE LENGTH VEL Tt Tc
POINT (ACRES) COEFF (MIN) C*A (IN/HR) (CFS) (MIN) C*A (IN/HR) (CFS) Q (%) SIZE (FT) (FPS) (MIN) (MIN)
H1 H1 0.81 0.15 11.7 0.12 2.13 0.26
1 A 0.16 0.65 10.7 0.11 2.21 0.24
2 B 0.47 0.92 11.4 0.43 2.13 0.91
3 C 0.11 0.33 11.2 0.04 2.13 0.08
4 D 0.06 0.41 11.2 0.03 2.13 0.06
2 YR DESIGN STORM
2-YEAR DIRECT RUNOFF TOTAL RUNOFF TRAVEL TIME REMARKS
T:\Projects\Ellie\03_Project Reports\Drainage\C Calcx - 2-Yr 4/19/2024
Appendix E
TJC
Project Ellie
Date: 4/19/2024 FALSE
DESIGN BASIN(S) AREA RUNOFF tc INTENSITY RUNOFF tc SUM INTENSITY RUNOFF DESIGN SLOPE PIPE LENGTH VEL Tt Tc
POINT (ACRES) COEFF (MIN) C*A (IN/HR) (CFS) (MIN) C*A (IN/HR) (CFS) Q (%) SIZE (FT) (FPS) (MIN) (MIN)
H1 H1 0.81 0.18 11.7 0.15 7.42 1.08
1 A 0.16 0.79 10.7 0.13 7.72 0.99
2 B 0.47 1.10 11.4 0.51 7.42 3.82
3 C 0.11 0.40 11.2 0.04 7.42 0.33
4 D 0.06 0.49 11.2 0.03 7.42 0.23
100 YR DESIGN STORM
100-YEAR DIRECT RUNOFF TOTAL RUNOFF TRAVEL TIME REMARKS
T:\Projects\Ellie\03_Project Reports\Drainage\C Calcx - 100-Yr 4/19/2024
Appendix E
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia =100.0 %
(100% if all paved and roofed areas upstream of rain garden)
B) Tributary Area's Imperviousness Ratio (i = Ia/100)i = 1.000
C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.40 watershed inches
(WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including rain garden area)Area = 19,541 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV =651 cu ft
Vol = (WQCV / 12) * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 = in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER =cu ft
Water Quality Capture Volume (WQCV) Design Volume
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER =cu ft
(Only if a different WQCV Design Volume is desired)
2. Basin Geometry
A) WQCV Depth (12-inch maximum)DWQCV =12 in
B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 4.00 ft / ft
(Use "0" if rain garden has vertical walls)
C) Mimimum Flat Surface Area AMin =391 sq ft
D) Actual Flat Surface Area AActual =706 sq ft
E) Area at Design Depth (Top Surface Area)ATop =706 sq ft
F) Rain Garden Total Volume VT=706 cu ft
(VT= ((ATop + AActual) / 2) * Depth)
3. Growing Media
4. Underdrain System
A) Are underdrains provided?1
B) Underdrain system orifice diameter for 12 hour drain time
i) Distance From Lowest Elevation of the Storage y =2.5 ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 =651 cu ft
iii) Orifice Diameter, 3/8" Minimum DO =9/16 in
Design Procedure Form: Rain Garden (RG)
Jim Allen
TJC
April 19, 2024
Ellie
E Sungia & Blondel
UD-BMP (Version 3.07, March 2018)
Choose One
Choose One
18" Rain Garden Growing Media
Other (Explain):
YES
NO
Ellie RG Size, RG 4/19/2024, 12:11 PM
Appendix E
Sheet 2 of 2
Designer:
Company:
Date:
Project:
Location:
5. Impermeable Geomembrane Liner and Geotextile Separator Fabric
A)Is an impermeable liner provided due to proximity
of structures or groundwater contamination?
PROVIDE A 30 MIL (MIN) PVC LINER WITH CDOT CLASS B
GEOTEXTILE ABOVE IT. USE THE SAME GEOTEXTILE BELOW
THE LINER IF THE SUBGRADE IS ANGULAR
6. Inlet / Outlet Control
A)Inlet Control
7. Vegetation
8. Irrigation
A) Will the rain garden be irrigated?
Notes:
Design Procedure Form: Rain Garden (RG)
Jim Allen
TJC
April 19, 2024
Ellie
E Sungia & Blondel
Choose One
Choose One
Choose One
Sheet Flow- No Energy Dissipation Required
Concentrated Flow- Energy Dissipation Provided
Plantings
Seed (Plan for frequent weed control)
Sand Grown or Other High Infiltration Sod
Choose One
YES
NO
YES
NO
Ellie RG Size, RG 4/19/2024, 12:11 PM
Appendix E