HomeMy WebLinkAboutFile Documents.1322 Snowbunny Ln.0071-2021-BCHO (19)
DRAINAGE REPORT
FOR
1320 SNOWBUNNY LANE
CITY OF ASPEN, COLORADO
PARCEL ID: 273501311004
PREPARED FOR:
CRAWFORD DESIGN BUILD
1001 Village Road, Unit LL2B
Carbondale, Colorado 81623
(970) 963-3833
PREPARED BY:
High Country Engineering, Inc.
1517 Blake Avenue, Suite 101
Glenwood Springs, CO 81601
(970) 945-8676
November 30, 2018
Revised: March 10, 2021
HCE JOB NUMBER: 2181041.00
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TABLE OF CONTENTS
SECTION PAGE
I. GENERAL LOCATION AND HISTORIC DESCRIPTION 4
II. DRAINAGE STUDIES 5
III. DRAINAGE DESIGN CRITERIA 5
IV. DRAINAGE FACILITY DESIGN 8
V. CONCLUSION 13
VI. REFERENCES 14
EXHIBITS:
1. Vicinity Map (8.5”x11”)
2. SCS Soils Map (8.5”x11”)
3. FEMA Map (11”x17”)
4. Historic Drainage Conditions (24”x36”)
5. Proposed Drainage Conditions (24”x36”)
6. Soil Reports (H-P/Kumar)
Appendices
Hydrologic Computations
Historic Conditions
Proposed Conditions
Hydraulic Computations
Trench Drain Calculations
Swale Calculations
Weir Calculations
Pond Calculations
Aspen Charts and Figures
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Engineers Certification
“I hereby affirm that this report and the accompanying plans for the construction of the
duplex, driveway, patios and general site improvements at 1320 Snowbunny Lane was
prepared by me (or under my direct supervision) for the owners thereof in accordance with the
provisions of the City of Aspen Urban Runoff Management Plan and approved variances are
exceptions listed thereto. I understand that it is the policy of the City of Aspen that the City of
Aspen does not and will not assume liability for drainage facilities designed by others.”
License No. 29975
Roger D. Neal, P.E.
Licensed Professional Engineer, State of Colorado
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I. GENERAL LOCATION AND DESCRIPTION
A. Location
The site is located at 1320 Snowbunny Lane within the City of Aspen, County of Pitkin, State
of Colorado, along the north side of Snowbunny Lane and south of Mountain View Drive.
Although within the City of Aspen, the site is not within the four major drainage basins shown
in the Urban Runoff Management Plan (URMP). The City of Aspen Drainage Basins Map can
be found in the appendices under Aspen Charts and Figures. The property is approximately
1,300 feet west of Castle Creek and 3,000 feet east of Maroon Creek. The Roaring Fork River
lies approximately 1,200 feet north of the property. A Vicinity Map has been included as
Exhibit #1.
B. Description of E xisting Property
The site is approximately 15,000 square feet (0.34 acres). The existing lot consists of a single
family residence, deck, gravel driveway, attached garage, landscaping and numerous trees.
The site is bordered by private properties along the north, east and west property lines. The
Snowbunny Lane right-of-way borders the south property line. The site shares the north
property line with two private properties, 1325 Mountain View Drive and 1315 Mountain
View Drive. The site drains north within two existing swales located near the east and west
property lines. The north lane of Snowbunny Lane drains onto the site. Existing grades range
from approximately 1-percent to 6-percent.
C. Soils Description
H-P/Kumar, Inc. on October 5, 2018, project number 18-7-575, completed a site-specific
geotechnical soil study. The geotechnical study describes the site as having 1 to 11/2 feet of
topsoil overlying dense, slightly silty, sandy gravel with cobbles and boulders. There was no
free water encountered in the boring at the time of excavation, and the subsoils were slightly
moist to moist. The report references the City of Aspen soil map to classify the hydrologic soil
group Type C having a moderate infiltration rate. The site percolation test determined an
average percolation rate of less than one inch per minute. The site is also well above the river
elevation, and groundwater was not encountered in the exploratory pits 5 to 6 feet deep. The
City of Aspen soils map locates this site in the Type “C” soils area; however, according to the
USDA Web Soil Survey, the property is within section 107 and the report states that it consists
of soil Type “B”. See USDA Web Soil Survey exhibit #2. The NRCS soil map in the City of
Aspen URMP was last revised in February, 2010 and the NRCS area data from the USDA
Web Soil Survey is from September, 2018. For this reason, the site has been analyzed with
Type “B” soils.
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II. DRAINAGE STUDIES
A. Major Drainage Way Planning and Influential Parameters
The site is located within FEMA’s major drainage study of the area on its Flood Insurance
Rate Map (FIRM) No. 08097C0203C which has an effective date of June 4, 1987. The area of
interest within the site is located in Zone X. This zone is described as areas determined to be
outside the 100-year and 500-year floodplains. Refer to Exhibit #3 for the FEMA map.
Mud flow was not analyzed for the site since the site is located outside of the Mud Flow Zone
as indicated in the Storm Drainage Master Plan for the City of Aspen, Colorado by WRC
Engineering, Inc. in November of 2001.
B. Previous Drainage Studies
The site was not within the study area boundary of the November 2001 study completed by
WRC Engineering, Inc. titled, “Storm Drainage Master Plan for the City of Aspen, Colorado”.
There are no previous site specific drainage studies for the site, and the site does not drain to
the storm sewer system for the City of Aspen.
C. Receiving System and Effects of Adjacent Drainage Issues
There are no major drainage issues with the adjacent properties that affect the site or that the
site affects. The existing site discharges runoff to the north property line, primarily where two
swales travel along the east and west property lines to the northeastern and northwestern
corners of the site, where runoff exits the property. From there, runoff flows between the
neighboring properties, 1325 Mountain View Drive and 1315 Mountain View Drive, and
continues north across private properties north of Mountain View Drive. Then stormwater
drains north along Sage Court before travelling northwest on Red Butte Drive. Finally the
runoff flows west and into the Roaring Fork River.
III. DRAINAGE DESIGN CRITERIA
A. Criteria
This drainage study was prepared in conformance with the City of Aspen, Colorado Urban
Runoff Management Plan (URMP), dated April of 2010 and the revised sections dated
thereafter. More than 1,000 square feet of area and 25-percent of the entire site will be
disturbed with the proposed construction; therefore, the site is viewed as a Major Project with
water quality required for the entire site per the URMP. Stormwater detention is also required
for the site. The existing site was analyzed in its historic condition (i.e. no improvements).
The offsite basin, consisting of a portion of Snowbunny Lane and R.O.W. landscaping, was
analyzed as existing (impervious area and open space) per the URMP. The onsite water
treatment systems were sized to pass the offsite flow.
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Water Quality Capture Volume (WQCV) will be determined for the entire site, per URMP
standards. The WQCV is defined as the treatment for up to the 80th percentile runoff event,
corresponding to between a 6-month to 1-year event. The WQCV was determined using the
equations and Figure 8.13 from Chapter 8 of the URMP. The WQCV equation is: Volume
(ft3) =WQCV (watershed-inches) x 1/12(ft./in) x area(acres) x 43,560 ft2/acre. The runoff for
the onsite basins will be routed through a series of swales receiving storm water from sheet
flow and downspouts to two bioretention areas/planters, where the runoff will be treated for
WQCV.
B. Hydrologic Criteria
The hydrologic methods for this study are outlined in the URMP from the City of Aspen,
Colorado (April, 2010) and the Microsoft Excel spreadsheet for the Rational Method.
The rainfall amounts for each basin were obtained using Figure 2.1 “IDF Curves for Aspen,
Colorado” in the URMP publication from the City of Aspen, Colorado. Using these curves, the
rainfall intensity corresponding to the 2-yr, 1-hr storm 10-yr, 1-hr storm, and 100-yr, 1-hr
storm event were determined based on the time of concentration for each basin.
Figure 3.3 from the URMP was used to determine the runoff coefficients for the 2-year, 10-
year and 100-year storm events since the soils were determined to be type ‘B’ soils.
For areas not capable of discharging runoff into the City’s system without impacting
neighboring properties, detention beyond WQCV is required. The site must meet both water
quality and detention requirements.
The bioretention area and drywell were sized to handle the WQCV, and to detain the 10-year
and 100-year runoff per the URMP. The rational method and the FAA procedure were utilized
to determine required 100-year detention using the historic release rate. Hydrologic
calculations can be found in the appendices.
All charts and figures mentioned from the URMP are located in the last section of the
appendices under the “Aspen Charts/Figures” section.
C. Hydraulic Criteria
The swales, trench drains, storm pipes and weir within the proposed drainage system have
been calculated utilizing the Hydraflow Express Extension for Autodesk AutoCAD Civil 3D.
All drainage features and structures have the ability to carry basin design flows anticipated in a
major rain event. Some design flowrates were determined by taking a percentage of the
tributary area within the basin. See basin descriptions below for explanation.
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D. Site Constraints
There are no utilities, streets or structures that cause major site constraints for the drainage
system design. Tree locations were considered during the design process that was adapted to
provide low-impact to site trees.
E. Easements and Irrigation Facilities
There are no major drainage ways, drainage easements or tracts located on the site. There are
also no irrigation facilities onsite that affect the overall proposed development. Existing utility
easements run parallel to the west (5’ offset), north (10’ offset) and east (5’ offset) property
lines.
F. Low Impact Site Design
A bioretention area and drywell will be implemented to allow for the capture of the required
WQCV and detention per the URMP code. Should the bioretention area and drywell exceed
the WQCV and required detention capacity, runoff will discharge from a weir and overflow
path at historic site release points and follow an existing flow path reaching the Roaring Fork
River.
G. 9 Principles
The 9 Principles for storm water quality management were followed during the design process
to create the best storm water design and water quality management. The following is a
summary of compliance with the Storm Drainage Principles outlined in the City of Aspen
Urban Runoff Management Plan:
1. Consider storm water quality needs early in the design process
Storm water quality needs were considered early in the design process, as
recommended.
2. Use the entire site when planning for storm water quality treatment.
With the use of a bioretention pond, drywell and vegetated swales, the entire site is
utilized for water quality treatment.
3. Avoid unnecessary impervious area
Efforts were made to avoid unnecessary impervious areas in drainage design.
Existing impervious areas will be redeveloped, but the site will have an overall
increase in impervious area. Impervious area in the Snowbunny R.O.W. has been
reduced by the replacing an existing driveway with two curb-cuts with a narrow
single curb-cut driveway. Permeable pavers capable of treating R.O.W. drainage
have been proposed.
4. Reduce runoff rates and volumes to more closely match natural conditions
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Runoff rates and volumes have been reduced, as recommended, by implementing a
bioretention area, drywell and multiple vegetated swales. All impervious areas will
drain to a bioretention area or drywell via swales, inlets and storm pipes. Weir in the
west bioretention pond will provide controlled release rate at historic release point.
5. Integrate storm water quality management and flood control
A proposed bioretention area and drywell capture runoff onsite, and have a weir to
control the discharge of overflow. Vegetated swales and storm pipes have been
sized to accommodate 10-year and 100-year peak flowrates for the entire basins.
6. Develop storm water quality facilities that enhance the site and environment.
The proposed water quality facilities enhance the site and the environment with a
bioretention area that will become part of the landscape.
7. Use a treatment train approach
The treatment train approach has been implemented by incorporating vegetated
swales, inlets and storm pipes convey all impervious runoff to bioretention area and
drywell.
8. Design sustainable facilities that can be safely maintained
The proposed storm water quality facilities have been designed to be easily
accessible and safely maintained, as recommended.
9. Design and maintain facilities with public safety in mind
The proposed storm water quality facilities have been designed with public safety in
mind, as requested.
IV. DRAINAGE FACILITY DESIGN
A. General Concept
The proposed construction calls for the redevelopment of an existing site. A proposed two-
story duplex, with a basement in the east unit and crawl space in the other, will be constructed
after the removal of the existing residence, attached garage and gravel driveway. The
impervious areas for the entire site will be treated for WQCV by a bioretention area and
drywell. Runoff will be routed through sheet flow, drainage swales, perforated pipe buried in
pervious rock, inlets and storm pipes receiving runoff from downspouts and hardscape. Offsite
stormwater enters the site from a portion of the Snowbunny Lane R.O.W. that is north of the
road centerline. Runoff greater than WQCV and the required detention will leave the site in
historical fashion at the northeast and northwest corners of the property.
B. Historic Drainage Basins Descriptions
The proposed site’s historic drainage pattern is generally from the south to the north and offsite
to adjoining private properties on Mountain View Drive. The existing site has been analyzed in
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its historic conditions.
The historic site has been broken into two on-site basins and one off-site. Refer to sheet
EXDR (exhibit #4) for a map of existing basin layouts. Two swales along the east and west
property lines and low points at the northeast and southeast corners of the lot delineate basins
EX-1 and EX-2 to encompass the entire lot. The basins have also been delineated to split the
lot in half to separately analyze detention requirements for each duplex unit. Drainage design
has been kept separate from the adjoining units for a potential lot split. Basin EXOS-1
encompasses an area south of the lot, as well as areas east and west of the lot where offsite
runoff enters the swales along the east and west property lines.
Historic Flow Path One:
Runoff from basin EX-1 sheet and channel flows north from the southwest corner of the site
before entering the north neighboring property. Design point one has been associated with the
western half of the lot. Basin EX-1 receives additional runoff from off-site basin EXOS-1.
Table 1 below is a summary of the existing basin information.
Historic Flow Path Two:
Runoff from basin EX-2, similarly to basin EX-1, sheet and channel flows from the southwest
corner of the basin to the northeast corner of the site. This basin discharges at the northeast
corner offsite into the north neighboring property of Mountain View Drive. Design point two
has been associated with the eastern half of the lot. Basin EX-2 receives additional runoff from
off-site basin EXOS-1. Table 1 below is a summary of the existing basin information.
There are no negative impacts from the runoff to the adjacent properties. By observing Pitkin
County GIS contours, the runoff appears to flow over permeable land between the neighboring
properties, 1325 Mountain View Drive and 1315 Mountain View Drive, with low discharge
rates. The nearest neighboring residence to the north is approximately 70 feet from the historic
discharge points. No runoff drains directly to any downstream structures. Refer to Exhibit #4
in the appendices for the existing basin delineation and information. Table 1, below, is a
summary of the existing basin information.
Table 1. Historic Basin Characteristics
BASIN AREA,
ACRES
C,
10YR I, 10YR Q10-YEAR,
CFS C, 100YR I, 100 YR Q100-YEAR,
CFS
EX-1 0.172 0.15 2.35 0.06 0.35 3.75 0.23
EX-2 0.172 0.15 2.23 0.06 0.35 3.56 0.21
EXOS-1 0.065 0.15 2.07 0.03 0.35 3.31 0.13
ONSITE
TOTAL 0.12 ONSITE
TOTAL 0.44
C. Proposed Basin Description
The proposed site has been separated into four proposed onsite drainage basins, and one offsite
basin.
Proposed basin PR-1 encompasses the western portion of the site and consists of the majority
of the west proposed duplex unit (unit 1322), the proposed driveway in front on the single stall
garage, concrete patio, fire pit and water feature. The runoff from basin PR-1 is captured by a
perforated pipe buried in pervious rock and discharges into a proposed bioretention area.
Downspouts from the proposed roof and a trench drain along the proposed driveway discharge
into the previously mentioned pipe. Runoff will also sheet flow into the perforated pipe.
Design point one is located at the north end of the basin, where the perforated pipe enters the
bioretention pond. Should the PR-1 bioretention exceed the required WQCV and detention,
runoff will overtop the bioretention’s spillway weir onto permeable landscape at the north
property line, before continuing flow north, as it did historically.
Proposed basin PR-2 encompasses the remaining portion of the western half of the site. The
basin includes the covered rear terrace, master bedroom and master bathroom, as well as
surrounding landscape. A drainage swale north of the rear terrace receives runoff as sheet flow
from surrounding landscape and discharging downspouts before conveying stormwater runoff
into the west bioretention pond. Design point 2 is associated with the discharge point for the
swale conveying runoff from basin PR-2.
Proposed basin PR-3 encompasses a small the northeastern half of the site. The basin includes
part of the two-car garage, the den, small terrace, spa, lightwell and concrete pavers, as well as
surrounding landscape. A drainage swale north of the spa receives runoff as sheet flow from
surrounding landscape and discharging downspouts before conveying stormwater runoff into a
drywell via inlets and storm pipes. Design point 3 is associated with the discharge point for the
swale conveying runoff from basin PR-3.
Proposed basin PR-4 encompasses the southeastern portion of the site and consists of
approximately half of the east proposed duplex unit (unit 1320), the proposed driveway in
front on the double stall garage, concrete pavers, patio, fire pit, bocci court and surrounding
landscape. Area inlets and trench drain capture the runoff from basin PR-4; drainage is then
discharged into a proposed drywell via storm pipes. Downspouts from the proposed roof and a
trench drain along the proposed driveway discharge into the previously mentioned storm
system. Runoff will also sheet flow into the inlets. Design point four is located at the north end
of the basin for the overflow. Should the drywell exceed the required WQCV and detention,
runoff will overtop the lowest inlet onto permeable landscape near the fire pit at the north
property line, before continuing flow north, as it did historically.
The offsite basin, PROS-1, consists of the northern portion of Snowbunny Lane, driveway
entrance and R.O.W. landscaping. Two permeable grassblock paver areas are proposed
between the edge of Snowbunny Lane and the south property line. These areas, most likely to
be used for parking, will provide water quality treatment for stormwater runoff from
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Snowbunny Lane. Both permeable paver areas have an impervious pavement to permeable
pavement ratio of 1.5, below the required 2.0 maximum ratio.
See Exhibit #5 for the proposed basin delineation. Table 2, below, is a summary of the
proposed (developed) basins.
Table 2. Proposed (Developed) Basin Characteristics
BASIN AREA,
ACRES
C,
10YR I, 10YR Q10-YEAR,
CFS C, 100YR I, 100 YR Q100-YEAR,
CFS
PR-1 0.149 0.41 3.96 0.24 0.52 6.33 0.49
PR-2 0.025 0.45 3.96 0.04 0.55 6.33 0.09
PR-3 0.048 0.58 3.49 0.10 0.66 5.57 0.18
PR-4 0.123 0.45 2.98 0.16 0.56 4.75 0.33
PROS-1 0.069 0.45 3.96 0.12 0.56 6.33 0.24
ONSITE
TOTAL 0.54 ONSITE
TOTAL 1.08
D. Downstream Impacts
The proposed onsite grading and detention facilities will have positive downstream impacts
during frequent storm events by capturing and treating the onsite WQCV. This will result in
less flow from the site during frequent storm events. There are no downstream facilities from
the site to be negatively impacted by the site’s redevelopment.
The onsite runoff will leave the site after treated in the water quality facilities thus preventing
the spread of pollutants downstream.
Table 3. Proposed WQCV Table
BASIN AREA (S.F.)
IMPERVIOUS
AREA (SF)
PERCENT
IMPERVIOUS (%)
WQCV (WATERSHED-
INCHES)
PR-1 6,480 3,252 50.2 0.100
PR-2 1,077 623 57.9 0.110
PR-3 2,091 1,553 74.3 0.155
PR-4 5,352 3,149 58.8 0.115
TOTAL 15,000 8,577 57.2 0.105
BASIN BIORETENTION AREA REQUIRED WQCV (CF) PROVIDED WQCV (CF)
PR-1 & PR-2 NORTHWEST 63.9 107.7
PR-3 & PR-4 NORTHEAST 78.3 86.6
If the bioretention and drywell facilities surpass the WQCV, the system will allow runoff to
overflow the pond weir and lowest inlet before discharging onto permeable landscape at
points along the north property line, and leave the site as it did historically. Calculations for
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the proposed drainage facilities are included in the appendices of this report under the
Facility Calculations section.
F. Site Detention
Runoff is not capable of discharging to a storm sewer system or directly into the river;
therefore, detention beyond WQCV is required for the site runoff. The bioretention pond and
drywell have been oversized to effectively provide detention for the site. Existing and
proposed grades do not allow a controlled outlet structure to discharge historic release rates for
small events, so runoff cannot discharge out of the bioretention ponds until the water surface
elevation reaches the weir crest. Due to this condition, required detention calculations have
been conservatively modified for the storage volume used in design. The amount of storage
required was determined by calculating full detention until the runoff hydrograph intersects the
outflow hydrograph. Rather than calculating the difference between inflow volume and
outflow volume, the minimum storage volume was determined to be the difference between
the total inflow volume and the inflow volume after the peak outflow of the predeveloped
hydrograph. Required detention volumes for Bioretention ‘A’ (west pond) and drywell were
determined to be 215.7 CF and 252.6 CF, respectively. By providing full detention until the
outflow hydrograph reaches the 100-year peak flowrate, the proposed bioretention pond and
drywell are capable of detaining 100-year storage volumes while keeping the weir discharge to
100-year historic peak flowrates or less. Using Microsoft Excel and AutoCAD Civil 3D
volume surfaces and manual check calculations, the design volumes were determined to be
219 CF for Bioretention ‘A’ (west pond) and 254.5 CF for drywell. Bioretention pond and
drywell WQCV and required detention calculations can be found in the appendices.
G. Operation and Maintenance
The proposed drainage facilities are to be constructed in conformance with the City of Aspen
Urban Runoff Management Plan, dated April 2010 and revised thereafter.
The bioretention basin will need to be inspected and maintained quarterly to make sure that the
reservoirs have not become clogged and that the reservoirs are functioning properly. Debris
and liter removal shall occur routinely.
Should clogging or standing water remain after debris and liter removal, the growing media
mix layer shall be removed and replaced with new growing media mix.
Accumulated material should be removed as a source control measure. Inspect and maintain all
surface areas for healthy grass growth, areas of dead grass, tire rutting, surface erosion,
accumulation of sediment and slow infiltration quarterly or after major storm events. Repair
worn out or damaged sod with sod grown in very sandy loam type soils. Remove and replace
the sod layer for majorly damaged or aged sod, to maintain a healthy vegetative cover or when
sod layer builds up significant amount of silt above the originally installed surface layer. All
manufacturer installation and maintenance procedures shall be followed.
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The drywell will need to be inspected and cleared of rubbish and debris quarterly to make sure
that the filter/riser pipe has not become clogged and is functioning properly. Routinely
evaluate the drain-down time of the drywell to ensure the max time of 24 hours is not being
exceeded. If drain-down times are exceeding the max, drain the drywell via pumping and
clean out the percolation area. The drywell grate must also be inspected to make sure it has
not clogged. Review of the overflow structure should occur at least every 6 months or after
large storm events.
The owners of the property will be responsible for the maintenance and upkeep of the drainage
facilities. The property owner shall dispose of sediment and any other waste material removed
from a reservoir at suitable disposal sites and in compliance with local, state, and federal waste
regulations.
This project includes “Low Impact Site Design” to mimic the natural pre-development
hydraulic pattern. Storm water runoff is to be in contact with soils and plants prior to reaching
the site outfall points. The plants and soil are to act as filters to remove pollutants. The
proposed plants and soils are present along the lengths of proposed graded swales and within
the proposed bioretention basins.
V. CONCLUSION
A. Compliance with Standards
This drainage report has been prepared in accordance with City of Aspen Regulations. The
proposed bioretention ponds will capture and treat the proposed WQCV for all impervious
areas on the entire site.
B. Drainage Concept
The proposed drainage design will be effective in controlling any adverse downstream impacts
on landowners or structures. Water quality issues will be minimal as the runoff will be
intercepted and routed to the proposed bioretention ponds.
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VI. REFERENCES
United States Department of Agriculture, Natural Resources Conservation Service: Soil
Survey of Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties.
Version 9, September 10, 2018.
City of Aspen, Colorado: Design and Construction Standards, June 2005.
City of Aspen, Colorado: Urban Runoff Management Plan. April 2010.
WRC Engineering, Inc. Storm Drainage Master Plan for the City of Aspen, Colorado.
November 2001.
HP Kumar, Inc. “Subsoil Study for Foundation Design, Proposed Duplex, Lot 4, Block 2,
Snowbunny Subdivision, 1320 Snowbunny Lane, Aspen, Colorado” dated October 5, 2018,
Project No. 18-7-575.
Urban Drainage and Flood Control District: Urban Storm Drainage Criteria Manual.
Volume III. August 2013. www.udfcd.org.
Autodesk, Inc. Hydraflow Express Extension for Autodesk AutoCAD Civil 3D. Version
10.5 www.autodesk.com/civil3d-stormwater.
EXHIBITS
Soil Map—Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
11/15/2018
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Map projection: Web Mercator Corner coordinates: WGS84
0 50 100 200 300Feet
0 20 40 80 120Meters
Map Scale: 1:1,500 if printed on A portrait (8.5" x 11") sheet.
Soil Map may not be valid at this scale.
EXHIBIT #2: NRCS SOILS MAP AND DESCRIPTION
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: Aspen-Gypsum Area, Colorado, Parts of
Eagle, Garfield, and Pitkin Counties
Survey Area Data: Version 9, Sep 10, 2018
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Data not available.
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—Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
11/15/2018
Page 2 of 3
Map Unit Legend
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
107 Uracca, moist-Mergel complex,
1 to 6 percent slopes,
extremely s
0.5 100.0%
Totals for Area of Interest 0.5 100.0%
Soil Map—Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
11/15/2018
Page 3 of 3
Aspen-Gypsum Area, Colorado, Parts of Eagle,
Garfield, and Pitkin Counties
107—Uracca, moist-Mergel complex, 1 to 6 percent slopes,
extremely s
Map Unit Setting
National map unit symbol: jq4g
Elevation: 6,800 to 8,400 feet
Mean annual precipitation: 16 to 19 inches
Mean annual air temperature: 40 to 43 degrees F
Frost-free period: 75 to 95 days
Farmland classification: Not prime farmland
Map Unit Composition
Uracca, moist, and similar soils: 50 percent
Mergel and similar soils: 40 percent
Minor components: 10 percent
Estimates are based on observations, descriptions, and transects of
the mapunit.
Description of Uracca, Moist
Setting
Landform: Alluvial fans, structural benches, valley sides
Down-slope shape: Linear
Across-slope shape: Linear
Parent material: Mixed alluvium derived from igneous and
metamorphic rock
Typical profile
H1 - 0 to 8 inches: cobbly sandy loam
H2 - 8 to 15 inches: very cobbly sandy clay loam
H3 - 15 to 60 inches: extremely cobbly loamy sand
Properties and qualities
Slope: 1 to 6 percent
Depth to restrictive feature: More than 80 inches
Natural drainage class: Well drained
Runoff class: Low
Capacity of the most limiting layer to transmit water (Ksat):
Moderately high to high (0.20 to 2.00 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Calcium carbonate, maximum in profile: 10 percent
Available water storage in profile: Very low (about 2.6 inches)
Interpretive groups
Land capability classification (irrigated): 6s
Land capability classification (nonirrigated): 6s
Hydrologic Soil Group: B
Map Unit Description: Uracca, moist-Mergel complex, 1 to 6 percent slopes, extremely s---
Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
11/15/2018
Page 1 of 2
Ecological site: Stony Loam (R048AY237CO)
Other vegetative classification: Stony Loam (null_82)
Hydric soil rating: No
Description of Mergel
Setting
Landform: Alluvial fans, structural benches, valley sides
Down-slope shape: Linear
Across-slope shape: Linear
Parent material: Glacial outwash
Typical profile
H1 - 0 to 8 inches: cobbly loam
H2 - 8 to 20 inches: very cobbly sandy loam
H3 - 20 to 60 inches: extremely stony sandy loam
Properties and qualities
Slope: 1 to 6 percent
Depth to restrictive feature: More than 80 inches
Natural drainage class: Well drained
Runoff class: Very low
Capacity of the most limiting layer to transmit water (Ksat):
Moderately high to high (0.60 to 6.00 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Calcium carbonate, maximum in profile: 10 percent
Available water storage in profile: Low (about 3.3 inches)
Interpretive groups
Land capability classification (irrigated): 4s
Land capability classification (nonirrigated): 4s
Hydrologic Soil Group: A
Ecological site: Stony Loam (R048AY237CO)
Other vegetative classification: Stony Loam (null_82)
Hydric soil rating: No
Minor Components
Other soils
Percent of map unit: 10 percent
Hydric soil rating: No
Data Source Information
Soil Survey Area: Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and
Pitkin Counties
Survey Area Data: Version 9, Sep 10, 2018
Map Unit Description: Uracca, moist-Mergel complex, 1 to 6 percent slopes, extremely s---
Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
11/15/2018
Page 2 of 2
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FEMA
2181041.00
APPENDICES
HYDROLOGIC
COMPUTATIONS
HISTORIC CONDITIONS
1320 & 1322 Snowbunny, Aspen CO
DRAINAGE REPORT
BY: BDB
CHECKED BY: RDN
EX-1 7,500.0 0.172 0.0 0.0 0.08 0.15 0.35
EX-2 7,500.0 0.172 0.0 0.0 0.08 0.15 0.35
EXOS-1 4,747.8 0.109 0.0 0.0 0.08 0.15 0.35
TOTAL ON-SITE 15,000.0 0.344 0.0 0.0 0.08 0.15 0.35
DATE: 11/16/18
Type B Soils
5 YR RUNOFF
COEFFICIENT
10 YR RUNOFF
COEFFICIENT
100 YR RUNOFF
COEFFICIENTPERCENT IMPERVIOUSBASIN AREA (S.F.) AREA (ACRE) IMPERVIOUS AREA (SF)
1320 & 1322 Snowbunny, Aspen CO
DRAINAGE REPORT
BY: BDB
CHECKED BY: RDN
11/16/2018
REACH
I P1-10yr P1-100yr Td
EX-1 2.35 0.77 14.6
EX-1 3.75 1.23 14.6
I P1-10yr P1-100yr Td
Tc EX-2 2.23 0.77 15.9
TC EX-2 3.56 1.23 15.9
Tc
10 YEAR I P1-10yr P1-100yr Td
100 YEAR
EXOS-1 2.07 0.77 17.8
To = [0.395 (1.1 - C5) SQRT(L)] / (S0.333) EQUATION 3-4 EXOS-1 3.31 1.23 17.8
C= 5 YR runoff coefficient from City of Aspen Urban Runoff Management Plan
Tc=To+Tt
INTENSITY I=29p/((10+T)^0.789) EQUATION 2-1
P TAKEN FROM TABLES 2.2 AND 2.3
AREA IDENTIFIER
3.31
EXOS-1
0.0000
1
2.07
0.0
3.75
17.8
TRAVEL TIME = L/(60V) (min.)
URBAN CHECK = 10+L/180
BASIN
0.0
10.8
(MIN)15.9
EX-1
2.35
5.0MINIMUM 5 MINUTES
14.6
RATIONAL COEFFICIENT. C (FIGURE 3.2 OF URMP) 0.08
FLOW LENGTH, L (ft.)
FLOW VELOCITY, V (FIGURE *RO-1 UDFCD) (fps.)
FLOW LENGTH, L (TOTAL <300 FT.) (ft.)
LAND SLOPE, S (ft./ft.)
SURFACE DESCRIPTION
To (MIN)
1
Rainfall Intenstity Chart EX-1
Rainfall Intenstity Chart EX-2
EXOS-1
0.08
17.8
0
0.0000
0.028 0.0180
0
OV
E
R
L
A
N
D
FL
O
W
TR
A
V
E
L
T
I
M
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FLOW SLOPE, S (ft./ft.) 0.0000
0
1
0.0
0.033
14.6 15.9
EX-1
139.06 137.62
EX-2
0.08
147.4
Rainfall Intenstity Chart EXOS-1
10.8
5.0
10.8
2.23
3.56
5.0
EX-2
CALCULATED BY: BDB STANDARD FORM SF-3
DATE:11/16/18 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
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(
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(
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(
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(
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Q
(
C
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(
M
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)
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(
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(
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(
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SL
O
P
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(
%
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CH
A
N
N
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L
F
L
O
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(C
F
S
)
DE
S
I
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N
F
L
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(C
F
S
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SL
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(
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(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
DESIGN POINT 1
DESIGN POINT 2
DESIGN POINT 33 EXOS-1 0.109 0.15
TOTAL RUNOFF
0.172 0.03
17.8 0.02 2.07 0.03
15.9 0.06
PROJECT: 1320 & 1322 Snowbunny
JOB NO. 2181041.00
PIPE
DESIGN STORM: EXISTING 10 YEAR
TRAVEL TIME
REMARKS
CHANNEL
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF
0.15 14.61 EX-1 0.062.35
0.172EX-2 0.152 0.03 2.23
CALCULATED BY: BDB STANDARD FORM SF-3
DATE: 11/16/18 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
E
A
(
A
C
)
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N
O
F
F
C
O
E
F
F
.
Tc
(
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(
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(
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(
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F
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(C
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(
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(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
DESIGN POINT 1
DESIGN POINT 2
DESIGN POINT 317.8 0.04 3.31 0.13
EX-2
0.23
3 EXOS-1 0.109 0.35
TRAVEL TIME
REMARKS
1 EX-1 0.172 0.35 14.6 0.06 3.75
JOB NO. 2181041.00
PROJECT: 1320 & 1322 Snowbunny
DESIGN STORM: EXISTING 100 YEAR
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF TOTAL RUNOFF CHANNEL PIPE
0.172 0.35 0.21215.9 0.06 3.56
PROPOSED CONDITIONS
1320 & 1322 Snowbunny, Aspen CO
DRAINAGE REPORT
BY: BDB
CHECKED BY: RDN
DATE: 02/12/2021
PR-1 6,480.1 0.149 3252.2 50.2 0.100 54.0 0.30 0.35 0.41 0.52
PR-2 1,077.2 0.025 623.2 57.9 0.110 9.9 0.35 0.40 0.45 0.55
PR-3 2,091.3 0.048 1553.3 74.3 0.155 27.0 0.52 0.54 0.58 0.66
PR-4 5,351.4 0.123 3149.1 58.8 0.115 51.3 0.35 0.40 0.45 0.56
PROS-1 3,010.4 0.069 1763.7 58.6 0.115 28.8 0.35 0.40 0.45 0.56
Onsite Only 15,000.0 0.344 8577.8 57.2 0.11 137.5 0.33 0.38 0.43 0.54
5 YR RUNOFF
COEFFICIENT
EFFECTIVE
IMPERVIOUS (%)
Type B Soils
BASIN AREA (S.F.) AREA (ACRE)
IMPERVIOUS AREA
(SF)
PERCENT
IMPERVIOUS
2 YR RUNOFF
COEFFICIENT
10 YR RUNOFF
COEFFICIENT
100 YR RUNOFF
COEFFICIENT
WQCV (Watershed
inches) WQCV (CF)
1320 & 1322 Snowbunny, Aspen CO
DRAINAGE REPORT
BY: BDB
CHECKED BY: RDN
DATE: 02/12/2021
REACH
Basin I P1-2yr P1-10yr P1-100yr Td
PR-1 2.42 0.47 5.0
PR-1 3.96 0.77 5.0
3149 0.12 PR-1 6.33 1.23 5.0
Basin I P1-2yr P1-10yr P1-100yr Td
Tc PR-2 2.42 0.47 5.0
TC PR-2 3.96 0.77 5.0
Tc PR-2 6.33 1.23 5.0
2 YEAR INTENSITY Basin I P1-2yr P1-10yr P1-100yr Td
10 YEAR INTENSITY
100 YEAR INTENSITY PR-3 2.13 0.47 6.9
PR-3 3.49 0.77 6.9
To = [0.395 (1.1 - C5) SQRT(L)] / (S0.333) EQUATION 3-4 PR-3 5.57 1.23 6.9
C= 5 YR runoff coefficient from City of Aspen Urban Runoff Management Plan
INTENSITY I=88.8p/((10+T)^1.052) EQUATION 2-1 Basin I P1-2yr P1-10yr P1-100yr Td
P TAKEN FROM TABLES 2.2 AND 2.3 WITHIN THE URMP
*INTENSITIES TAKEN FROM FIGURE 2.1 "IDF CURVES FOR ASPEN, COLORADO" FROM URMP PR-4 1.82 0.47 9.7
PR-4 2.98 0.77 9.7
PR-4 4.75 1.23 9.7
Basin I P1-2yr P1-10yr P1-100yr Td
OS-1 2.42 0.47 5.0
OS-1 3.96 0.77 5.0
OS-1 6.33 1.23 5.0
Rainfall Intenstity Chart OS-1
OS-1
2.42
3.96
6.33
0.04
1
0
4.6
10.2
5.0
OS-1
0.40
32
0.04
4.6
0
Rainfall Intenstity Chart PR-3
Rainfall Intenstity Chart PR-4
Rainfall Intenstity Chart PR-1
Rainfall Intenstity Chart PR-20.0486
0.0
6.9
10.4
3.31
5.0
PR-3
0.54
72.5
0.02
6.9
0.0
3.49
PR-3
2.42 2.13
5.576.33
PR-2
3.96
0.0
10.1
5.0
0.02
2.12
OV
E
R
L
A
N
D
FL
O
W
FLOW VELOCITY, V (FIGURE *RO-1 UDFCD) (fps.)
FLOW LENGTH, L (ft.)
TR
A
V
E
L
T
I
M
E
SURFACE DESCRIPTION
FLOW LENGTH, L (TOTAL <300 FT.) (ft.)
LAND SLOPE, S (ft./ft.)
BASIN
MINIMUM 5 MINUTES
6.33
PR-1
3.96
2.42
PR-1
31.7
RATIONAL COEFFICIENT. C (FIGURE 3.2 OF URMP) 0.35
AREA IDENTIFIER
FLOW SLOPE, S (ft./ft.)
(MIN)
TRAVEL TIME = L/(60V) (min.)
URBAN CHECK = 10+L/180
0.0
PR-2
0.017
10.2
0.40
12.0
To (MIN)
0.07
5.0
1.96
0.078
3.9
0.0
2.33.9
0.0
2.3
5.0
PR-4
0.40
110.5
0.027
9.7
0.0
PR-4
1.82
2.98
4.75
0.017
1.96
0.0
9.7
10.6
CALCULATED BY: BDB STANDARD FORM SF-3
DATE: 02/12/2021 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
E
A
(
A
C
)
RU
N
O
F
F
C
O
E
F
F
.
Tc
(
M
I
N
)
C
*
A
(
A
C
)
I
(
I
N
/
H
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)
Q
(
C
F
S
)
Tc
(
M
I
N
)
SU
M
(
C
*
A
)
(
A
C
)
I
(
I
N
/
H
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)
Q
(
C
F
S
)
SL
O
P
E
(
%
)
ST
R
E
E
T
F
L
O
W
(
C
F
S
)
DE
S
I
G
N
F
L
O
W
(
C
F
S
)
SL
O
P
E
(
%
)
PIP
E
S
I
Z
E
(
I
N
C
H
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S
)
LE
N
G
T
H
(
F
T
)
VE
L
O
C
I
T
Y
(
F
P
S
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Tt
(
M
I
N
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(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)
Design Point 3
Design Point 44 PR-4 0.123 0.35 9.68 0.043 1.82 0.08
0.025 2.13 0.053 PR-3 0.048 0.52 6.93
2.42
Design Point 1
Design Point 2PR-2
1
0.35
PR-1 5.000.149
0.025
PROJECT: 1320 & 1322 Snowbunny
JOB NO. 2181041.00
DIRECT RUNOFF
2
0.11
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
REMARKS
0.02
DESIGN STORM: PROPOSED 2 YEAR
0.30
5.00 0.009
0.04 2.42
TRAVEL TIMEPIPESTREETTOTAL RUNOFF
OS OS-1 0.069 0.35 5.00 Design Point OS0.024 2.42 0.06
CALCULATED BY: BDB STANDARD FORM SF-3
DATE: 02/12/2021 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
E
A
(
A
C
)
RU
N
O
F
F
C
O
E
F
F
.
Tc
(
M
I
N
)
C
*
A
(
A
C
)
I
(
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N
/
H
R
)
Q
(
C
F
S
)
Tc
(
M
I
N
)
SU
M
(
C
*
A
)
(
A
C
)
I
(
I
N
/
H
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)
Q
(
C
F
S
)
SL
O
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E
(
%
)
ST
R
E
E
T
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DE
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(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)
Design Point 40.055 2.98 0.164 PR-4 0.123 0.45 9.68
0.10 Design Point 3
Design PoInt 2
3 PR-3 0.048 0.58 6.93 0.028 3.49
0.011 3.96 0.042 PR-2 0.025 0.45 5.00
0.24 Design Point 1
TRAVEL TIME
REMARKS
1 PR-1 0.149 0.41 5.00 0.06 3.96
JOB NO. 2181041.00
PROJECT: 1320 & 1322 Snowbunny
DESIGN STORM: PROPOSED 10 YEAR
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF TOTAL RUNOFF STREET PIPE
OS OS-1 0.069 0.45 5.00 Design Point OS0.031 3.96 0.12
CALCULATED BY: BDB STANDARD FORM SF-3
DATE: 02/12/2021 STORM DRAINAGE SYSTEM DESIGN
CHECKED BY: RDN (RATIONAL METHOD PROCEDURE)
Contributing Area
AR
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A
(
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C
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C
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Tc
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Q
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Tc
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A
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)
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Q
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(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)
Design Point 40.069 4.75 0.334 PR-4 0.123 0.56 9.68
0.18 Design Point 3
Design Point 2
3 PR-3 0.048 0.66 6.93 0.032 5.57
0.014 6.33 0.092 PR-2 0.025 0.55 5.00
0.49 Design Point 1
TRAVEL TIME
REMARKS
1 PR-1 0.149 0.52 5.00 0.08 6.33
JOB NO. 2181041.00
PROJECT: 1320 & 1322 Snowbunny
DESIGN STORM: PROPOSED 100 YEAR
STRUCTURE NO.
DE
S
I
G
N
P
O
I
N
T
DIRECT RUNOFF TOTAL RUNOFF STREET PIPE
OS OS-1 0.069 0.56 5.00 Design Point OS0.039 6.33 0.24
HYDRAULIC
COMPUTATIONS
TRENCH/SLAB DRAIN & PIPE CALCULATIONS
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Monday, Feb 15 2021
East Slab Drain 10yr (PR-3 & 4)
Rectangular
Bottom Width (ft) = 0.67
Total Depth (ft) = 0.33
Invert Elev (ft) = 1.00
Slope (%) = 0.50
N-Value = 0.013
Calculations
Compute by: Known Q
Known Q (cfs) = 0.15
Highlighted
Depth (ft) = 0.14
Q (cfs) = 0.150
Area (sqft) = 0.09
Velocity (ft/s) = 1.60
Wetted Perim (ft) = 0.95
Crit Depth, Yc (ft) = 0.12
Top Width (ft) = 0.67
EGL (ft) = 0.18
0 .25 .5 .75 1 1.25
Elev (ft) Depth (ft)Section
0.75 -0.25
1.00 0.00
1.25 0.25
1.50 0.50
1.75 0.75
2.00 1.00
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Monday, Feb 15 2021
East Slab Drain 100yr (PR-3 & 4)
Rectangular
Bottom Width (ft) = 0.67
Total Depth (ft) = 0.33
Invert Elev (ft) = 1.00
Slope (%) = 0.50
N-Value = 0.013
Calculations
Compute by: Known Q
Known Q (cfs) = 0.31
Highlighted
Depth (ft) = 0.23
Q (cfs) = 0.310
Area (sqft) = 0.15
Velocity (ft/s) = 2.01
Wetted Perim (ft) = 1.13
Crit Depth, Yc (ft) = 0.19
Top Width (ft) = 0.67
EGL (ft) = 0.29
0 .25 .5 .75 1 1.25
Elev (ft) Depth (ft)Section
0.75 -0.25
1.00 0.00
1.25 0.25
1.50 0.50
1.75 0.75
2.00 1.00
Reach (ft)
Weir Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Monday, Nov 26 2018
WEST SLAB DRAIN OPEN END OUTLET @ 10YR PEAK FLOWRATE (21% PR-1 & 21% PR
Rectangular Weir
Crest = Sharp
Bottom Length (ft) = 0.67
Total Depth (ft) = 0.33
Calculations
Weir Coeff. Cw = 3.33
Compute by: Known Q
Known Q (cfs) = 0.08
Highlighted
Depth (ft) = 0.11
Q (cfs) = 0.080
Area (sqft) = 0.07
Velocity (ft/s) = 1.10
Top Width (ft) = 0.67
0 .25 .5 .75 1 1.25
Depth (ft)Depth (ft)WEST SLAB DRAIN OPEN END OUTLET @ 10YR PEAK FLOWRATE (21% PR-1 & 21% PROS-1)
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
Length (ft)Weir W.S.
Weir Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Monday, Nov 26 2018
WEST SLAB DRAIN OPEN END OUTLET @ 100YR PEAK FLOWRATE (21% PR-1 & 21% P
Rectangular Weir
Crest = Sharp
Bottom Length (ft) = 0.67
Total Depth (ft) = 0.33
Calculations
Weir Coeff. Cw = 3.33
Compute by: Known Q
Known Q (cfs) = 0.18
Highlighted
Depth (ft) = 0.19
Q (cfs) = 0.180
Area (sqft) = 0.12
Velocity (ft/s) = 1.44
Top Width (ft) = 0.67
0 .25 .5 .75 1 1.25
Depth (ft)Depth (ft)WEST SLAB DRAIN OPEN END OUTLET @ 100YR PEAK FLOWRATE (21% PR-1 & 21% PROS-1)
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
Length (ft)Weir W.S.
Inlet Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Tuesday, Nov 27 2018
Trench Drain Inlet Capacity at Maximum Depth
Drop Grate Inlet
Location = Sag
Curb Length (ft) = -0-
Throat Height (in) = -0-
Grate Area (sqft) = 5.97
Grate Width (ft) = 0.67
Grate Length (ft) = 25.00
Gutter
Slope, Sw (ft/ft) = 0.020
Slope, Sx (ft/ft) = 0.020
Local Depr (in) = -0-
Gutter Width (ft) = 0.67
Gutter Slope (%) = -0-
Gutter n-value = -0-
Calculations
Compute by: Known Q
Q (cfs) = 0.68
Highlighted
Q Total (cfs) = 0.68
Q Capt (cfs) = 0.68
Q Bypass (cfs) = -0-
Depth at Inlet (in) = 0.32
Efficiency (%) = 100
Gutter Spread (ft) = 3.36
Gutter Vel (ft/s) = -0-
Bypass Spread (ft) = -0-
Bypass Depth (in) = -0-
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Thursday, Mar 11 2021
4in Pipe 100yr
Circular
Diameter (ft) = 0.33
Invert Elev (ft) = 1.00
Slope (%) = 1.70
N-Value = 0.010
Calculations
Compute by: Known Q
Known Q (cfs) = 0.33
Highlighted
Depth (ft) = 0.29
Q (cfs) = 0.330
Area (sqft) = 0.08
Velocity (ft/s) = 4.14
Wetted Perim (ft) = 0.80
Crit Depth, Yc (ft) = 0.31
Top Width (ft) = 0.21
EGL (ft) = 0.56
0 1
Elev (ft)Section
0.75
1.00
1.25
1.50
1.75
2.00
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Tuesday, Nov 10 2020
6in Pipe 100yr PR-1 & 2
Circular
Diameter (ft) = 0.50
Invert Elev (ft) = 1.00
Slope (%) = 1.00
N-Value = 0.012
Calculations
Compute by: Known Q
Known Q (cfs) = 0.49
Highlighted
Depth (ft) = 0.35
Q (cfs) = 0.490
Area (sqft) = 0.15
Velocity (ft/s) = 3.33
Wetted Perim (ft) = 0.99
Crit Depth, Yc (ft) = 0.36
Top Width (ft) = 0.46
EGL (ft) = 0.52
0 1
Elev (ft)Section
0.75
1.00
1.25
1.50
1.75
2.00
Reach (ft)
SWALE CALCULATIONS
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Monday, Nov 26 2018
WEST SWALE @ 100 YR PEAK FLOWRATE (Design Point 1)
Triangular
Side Slopes (z:1) = 4.00, 5.00
Total Depth (ft) = 0.50
Invert Elev (ft) = 7844.10
Slope (%) = 1.40
N-Value = 0.030
Calculations
Compute by: Known Q
Known Q (cfs) = 0.61
Highlighted
Depth (ft) = 0.30
Q (cfs) = 0.610
Area (sqft) = 0.40
Velocity (ft/s) = 1.51
Wetted Perim (ft) = 2.77
Crit Depth, Yc (ft) = 0.26
Top Width (ft) = 2.70
EGL (ft) = 0.34
0 .5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
Elev (ft)Depth (ft)Section
7843.75 -0.35
7844.00 -0.10
7844.25 0.15
7844.50 0.40
7844.75 0.65
7845.00 0.90
Reach (ft)
WEIR CALCULATIONS
Weir Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Thursday, Nov 29 2018
West Weir @ Historic (EX-1) 10YR Release Rate
Rectangular Weir
Crest = Sharp
Bottom Length (ft) = 2.50
Total Depth (ft) = 0.10
Calculations
Weir Coeff. Cw = 3.33
Compute by: Known Q
Known Q (cfs) = 0.06
Highlighted
Depth (ft) = 0.04
Q (cfs) = 0.060
Area (sqft) = 0.09
Velocity (ft/s) = 0.64
Top Width (ft) = 2.50
0 .5 1 1.5 2 2.5 3 3.5
Depth (ft)Depth (ft)West Weir @ Historic (EX-1) 10YR Release Rate
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
Length (ft)Weir W.S.
Weir Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Thursday, Nov 29 2018
West Weir @ Historic (EX-1) 100YR Release Rate
Rectangular Weir
Crest = Sharp
Bottom Length (ft) = 2.50
Total Depth (ft) = 0.10
Calculations
Weir Coeff. Cw = 3.33
Compute by: Known Q
Known Q (cfs) = 0.23
Highlighted
Depth (ft) = 0.09
Q (cfs) = 0.230
Area (sqft) = 0.23
Velocity (ft/s) = 1.01
Top Width (ft) = 2.50
0 .5 1 1.5 2 2.5 3 3.5
Depth (ft)Depth (ft)West Weir @ Historic (EX-1) 100YR Release Rate
-0.50 -0.50
0.00 0.00
0.50 0.50
1.00 1.00
Length (ft)Weir W.S.
BIORETENTION & DRYWELL CALCULATIONS
Hydrology Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Wednesday, Nov 28 2018
Required 100yr Detention for West Pond (PR-1 & PR-2)
Hydrograph type = Mod Rational Peak discharge (cfs) = 0.421
Storm frequency (yrs) = 100 Time interval (min) = 1
Drainage area (ac) = 0.170 Runoff coeff. (C) = 0.53
Rainfall Inten (in/hr) = 4.672 Tc by FAA (min) = 10
IDF Curve = AspenIDFcurve.IDF Storm dur factor = 1.00
Hydrograph Volume = 253 (cuft); 0.006 (acft)
0 5 10 15 20
Q (cfs)
0.00 0.00
0.05 0.05
0.10 0.10
0.15 0.15
0.20 0.20
0.25 0.25
0.30 0.30
0.35 0.35
0.40 0.40
0.45 0.45
0.50 0.50
Q (cfs)
Time (min)
Runoff Hydrograph
100-yr frequency
Runoff Hyd - Qp = 0.42 (cfs)Outflow Hyd *Req. Stor = 118 (cuft) *
* Estimated
Required Detention:
215.7 CF
37.3 CF
Required Detention=(253-37.3)CF=215.7 CF
Detention Provided=219.5 CF
FAA Formula Tc Worksheet
Tc = 1.8(1.1 - C) x Flow length^0.5 / Watercourse slope^0.333 Hydraflow Express by Intelisolve
Mod Rational
Required 100yr Detention for West Pond (PR-1 & PR-2)
Description
Flow length (ft) = 154.10
Watercourse slope (%) = 2.30
Runoff coefficient (C) = 0.54
Time of Conc. (min) = 9
Hydrology Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Monday, Mar 8 2021
Required 100yr Detention for East (PR-3 & PR-4)
Hydrograph type = Mod Rational Peak discharge (cfs) = 0.469
Storm frequency (yrs) = 100 Time interval (min) = 1
Drainage area (ac) = 0.170 Runoff coeff. (C) = 0.59
Rainfall Inten (in/hr) = 4.672 Tc by User (min) = 10
IDF Curve = AspenIDFcurve.IDF Storm dur factor = 1.00
Hydrograph Volume = 281 (cuft); 0.006 (acft)
0 5 10 15 20
Q (cfs)
0.00 0.00
0.05 0.05
0.10 0.10
0.15 0.15
0.20 0.20
0.25 0.25
0.30 0.30
0.35 0.35
0.40 0.40
0.45 0.45
0.50 0.50
Q (cfs)
Time (min)
Runoff Hydrograph
100-yr frequency
Runoff Hyd - Qp = 0.47 (cfs)Outflow Hyd *Req. Stor = 159 (cuft) *
* Estimated
28.4 CF
Required Detention:
252.6 CF
Required Detention=(281-28.4)CF= 252.6 CF
Provided Detention=254.5 CF
FAA Formula Tc Worksheet
Tc = 1.8(1.1 - C) x Flow length^0.5 / Watercourse slope^0.333 Hydraflow Express by Intelisolve
Mod Rational
Required 100yr Detention for East Pond (PR-3 & PR-4)
Description
Flow length (ft) = 158.02
Watercourse slope (%) = 2.30
Runoff coefficient (C) = 0.56
Time of Conc. (min) = 9
POND WQCV DEPTH AND AREA CALCULATIONS
West Pond:
PR-1 WQCV (watershed-inches): 0.10
PR-2 WQCV (watershed-inches): 0.11
PR-1 WQCV ሺ݂ݐଷሻ: 0.10ሺݓܽݐ݁ݎݏ݄݁݀ െ ݄݅݊ܿ݁ݏሻ ൈ ቀ ଵ௧
ଵଶቁ ൈ 6,480݂ݐ
ଶ ൌ54 ݂ݐଷ
PR-2 WQCV ሺ݂ݐଷሻ: 0.11ሺݓܽݐ݁ݎݏ݄݁݀ െ ݄݅݊ܿ݁ݏሻ ൈ ቀ ଵ௧
ଵଶቁ ൈ 1,077 ݂ݐ
ଶ ൌ9.9 ݂ݐଷ
Pond:
WQCV Depth Capacity ൌ0.9 ݂ݐ
Flat Area Required ൌ ଷ.ଽ௧య
.ଽ௧ ൌ71 ݂ݐଶ
Flat Area Provided ൌ ૢ.ૠ ࢌ࢚
Volume to Width of Depth of gravel Dia of Percent Concrete Cylinder
Detain (cf) Gravel (ft) Below M.H. (ft) M.H. Voids (%) Thickness (in)
252.6 1.3 1 5 30 5
Area of M.H. (sqft) 19.6
Area of Gravel (sqft) 27.8
Area of Concrete Cylinder (sqft) 7.09
Volume of 6" PCC Slab (sqft) 9.8
Volume of Gravel Bottom 54.5
Depth Volume of Vol of Gravel Vol of Drywell Vol of Drywell Vol of Total Volume
Varied Gravel Voids Conic Section Cyl. Section Drywell Volume Met
(ft) (cf) (cf) (cf) (cf) (cf) (cf)
1 0.0 0.0 0.0 0.0 0.0 0.0 no
2 0.0 0.0 12.8 0.0 3.0 3.0 no
3 0.0 0.0 12.8 19.6 22.6 22.6 no
4 0.0 0.0 12.8 39.3 42.3 42.3 no
5 82.4 24.7 12.8 58.9 61.9 86.6 no WQCV 78.3
6 110.2 33.1 12.8 78.5 81.6 114.6 no
7 138.0 41.4 12.8 98.2 101.2 142.6 no
8 165.8 49.7 12.8 117.8 120.8 170.6 no
9 193.6 58.1 12.8 137.4 140.5 198.5 no
10 221.4 66.4 12.8 157.1 160.1 226.5 no
11 249.3 74.8 12.8 176.7 179.7 254.5 Meets Min.Provided Detention
12 277.1 83.1 12.8 196.3 199.4 282.5 Meets Min.
13 304.9 91.5 12.8 216.0 219.0 310.5 Meets Min.
14 332.7 99.8 12.8 235.6 238.6 338.4 Meets Min.
Transmissivity Calc.
Depth of Gravel (ft)= 6.5
K from NRCS National Engineering Handbook, figure 3-10, soil class GM
AP=(VR)/(K)(43,200)
AP=Total area of sides (sf)170.17
VR = runoff volume (cf)252.6
K= Hydraulic Conductivity (ft/s)0.000035
.
Area of Percolation Required (sf)167.06
1320 Snowbunny
DRAINAGE STUDY DRYWELL CALCULATIONS
March 11, 2021 HCE #: 2181041.00 J:/sdskproj/218/1041/excel/detention drywell.xls
Detention Drywell Calculations
Tele: (970) 945-8676 - Fax (970) 945-2555
Calculation of Drywell and Gravel
Variables
5' DIAMETER, 11' DEEP DRYWELL
1517 BLAKE AVENUE
Glenwood Springs, CO 81601
ASPEN CHARTS AND
FIGURES
City of Aspen Urban Runoff Management Plan
Chapter 1 – Policy and Permit Requirements 1-15 Rev 11/2014
Figure 1.2 City of Aspen Drainage Basins
City of Aspen Urban Runoff Management Plan
Chapter 3 - Runoff 3-2 Rev 2/2010
Figure 3.1 Natural Resource Conservation Service (NRCS) Soil Map for Aspen
City of Aspen Urban Runoff Management Plan
Chapter 3 - Runoff 3-6 Rev 10/2014
Figure 3.2 – Runoff Coefficients for NRCS Hydrologic Soil Group A
Figure 3.3 – Runoff Coefficients for NRCS Hydrologic Soil Group B
City of Aspen Urban Runoff Management Plan
Chapter 2 - Rainfall 2-2 Rev 9/2014
into thunderstorms. Autumn in Aspen is usually dry and warm and during September daytime temperatures
can reach 70°F, but night temperatures can drop to freezing. Aspen is renowned for its warm winter sun.
Winter daytime temperatures typically range from 20 to 40°F in the City and from 10 to 30°F on the
mountain. Once the sun goes down, the temperature drops dramatically. Table 2.1 presents monthly
statistics for temperature, precipitation, snowfall, and snow depth in the Aspen area.
Table 2.1 Monthly Statistics for Temperature and Precipitation in Aspen
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Average Max. Temperature (F) 35 39 45 52 63 72 78 76 69 58 43 35 55.5
Average Min. Temperature (F) 9.1 12 20 26 35 41 47 46 39 30 19 9.7 27.7
Average Total Precipitation (in.) 1.7 2.1 2.7 2.5 2.1 1.4 1.8 1.6 2.1 2 2.6 1.9 24.37
Average Total Snowfall (in.) 25 27 28 20 7.8 1 0 0 1 11 28 25 173.8
Average Snow Depth (in.) 21 28 27 12 1 0 0 0 0 1 6 14
(Source: Station 050372 at Aspen 1 SW, Colorado)
2.3 Rainfall Depth, Duration, Frequency, and Intensity
The rainfall intensity-duration-frequency (IDF) curve is a statistical formula to describe the relationship
among the local rainfall characteristics and return periods. The IDF curve is used in the Rational Method
for peak runoff predictions of basins smaller than 90 acres. Based on the NOAA Atlas Volume 3, the
IDF curve for the City of Aspen can be derived according to the locality and elevation. The City of Aspen is
located at approximately 39°11′32″N and 106°49′28″W, at an elevation of approximately 8,100 feet.
Based on depth and duration data (Appendix B, Table 1), rainfall intensities can be calculated for various
frequencies. Rainfall intensity data, which form the basis of the Intensity-Duration-Frequency (IDF) curves
in Figure 2.1 are provided in Table 2.2.
Table 2.2 Rainfall Intensity-Duration-Frequency in Aspen, Colorado
Return Rainfall Intensity in inch/hr for Various Periods of Duration
Period 5-min 10-min 15-min 30-min 1-hr (P1) 2-hr 3-hr 6-hr 24-hr
2‐yr 2.06 1.51 1.23 0.77 0.47 0.28 0.21 0.13 0.06
5-yr 2.98 2.17 1.77 1.09 0.64 0.36 0.26 0.16 0.07
10-yr 3.72 2.72 2.22 1.35 0.77 0.43 0.30 0.18 0.08
25‐yr 4.75 3.47 2.82 1.71 0.95 0.53 0.36 0.21 0.09
50‐yr 5.53 4.05 3.30 1.98 1.09 0.60 0.41 0.24 0.11
100-yr 6.32 4.63 3.76 2.24 1.23 0.67 0.45 0.26 0.12
Using the data in Table 2.2 (derived from NOAA Atlas 14 Volume 8), the following equation was derived
that can be used to determine intensities not shown in the IDF table or curve:
052.1
1
)10(
8.88
dT
PI (Equation 2-1)
Where, I = rainfall intensity (inch/hr),
P1 = 1-hr rainfall depth (inches), and
Td = duration or time of concentration (minutes).
City of Aspen Urban Runoff Management Plan
Chapter 2 - Rainfall 2-4 Rev 9/2014
Note: Accuracy is more reliable at 5 minute increments.
Figure 2.1 IDF Curves for Aspen, Colorado
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30 35 40 45 50 55 60
In
t
e
n
s
i
t
y
(i
n
c
h
/
h
r
)
Duration in Minutes
Rainfall IDF for Aspen, Colorado
2‐yr 5‐yr 10‐yr 25‐yr 50‐yr 100‐yr
City of Aspen Urban Runoff Management Plan
Chapter 8 – Water Quality 8-33 Rev 8/2009
Figure 8.13 Aspen Water Quality Capture Volume