HomeMy WebLinkAboutMaster Permit.1015 Waters Ave.0204.2019 (59).ARBK c.--'_r-- , MOUNTAIN CROSS
ENGINEERING, INC.
/ Civil and Environmental Consulting and Design
April 4, 2018
Ms. Gretchen Greenwood
Gretchen Greenwood&Associates, Inc.
210 South Galena Street, Suite 30
Aspen, CO 81611
RE: Urban Runoff Management Plan— 1015 Waters Avenue
Dear Ms. Greenwood:
The purpose of this correspondence is to evaluate the construction of a residence at 1015 Waters
Avenue from a storm water perspective and provide design recommendations pertaining to urban
runoff management for incorporation into the site design. These recommendations were generated
from the site plans that were submitted to our office and a site visit performed on March 27, 2018.
Attached are plan sheets that show the design recommendations and details. Also attached are the
drainage calculations that were generated.
The project is at an elevation of approximately 7,960 feet above sea level. An existing residence
sits on the lot and will be razed prior to construction of the proposed residence. Ultimately,runoff
from the site will travel to the Roaring Fork River through a series of roadside conveyances and
storm drain appurtenances.
Runoff from snowmelt is typically very large in volume but because it happens over a much longer
time period, the peak flows are usually less than a rainfall event. Also, the Urban Runoff
Management Plan states that they are not necessary at this time. Therefore storm water is defined
in this study to be surface water that is a direct result from a rainfall event.
There is an irrigation ditch that cuts across the south west corner of the lot. The project is located
greater than 100' from any mapped floodplain boundaries. No other drainage studies are known
to have been performed specific to this lot.
Storm water from the site is evaluated in accordance with the standards of applicable sections of
the City of Aspen, Urban Runoff Management Plan (Code). All flows are listed in units of cubic
feet per second(cfs)unless otherwise noted.
Methodology
The design of a drainage plan begins with a calculation of the flowrate of water that will be
produced from a rainfall event. Since the watershed basins are small (less than 90 acres), per the
Code, the Rational method was used for estimating the amount of runoff that will occur. This
method calculates runoff(Q) in cubic feet per second (cfs) from basin area (A) in acres, runoff
coefficient(C), and rainfall intensity (I) in inches per hour:
RECEIVED
04/06/2018
8261/2 Grand Avenue, Glenwood Springs,CO 81601
P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN
BUILDING DEPARTMENT
1015 Waters Ave
April, 2018
Page 2 of 5
Q = C * I * A
When acres and inches per hour are used as the units, the conversion into cubic feet per second is
1.008 but is usually ignored and it has been here. The runoff coefficient is a dimensionless
coefficient.
Basin Area
Drainage basins have the characteristic that any precipitation falling within that area will
drain to the same point of discharge. The project basin was delineated from project
topography,project site plan, and building architecture.
Runoff Coefficient
A runoff coefficient is assigned to each basin that gives a relationship between the amount
of precipitation that becomes surface water and the amount of water that is lost to
infiltration, evaporation, or transpiration. The runoff coefficient is a function of drainage
basin soil types, surface area, and/or land-use. Because the land-use and the surface cover
often vary through the project, a composite coefficient is often assigned to each drainage
basin, based on the weight of the areas and their respective coefficients.
Rainfall Intensity
Rainfall intensity is determined from intensity duration frequency curves, or IDF curves.
IDF curves are graphs of, more or less, parallel frequency curves that yield rainfall
intensities based on storm durations.
Frequency: The return frequency of a rainfall storm is the statistical probability
that a given storm event will occur on average in a given period.For instance a 100-
year storm has the statistical probability of occurring once in a 100 year span or it
has a 1% chance of occurring in any given year. It is important to emphasize that
it is based on probability statistics and therefore does not reflect actual storm
frequency. Storms of a 100-year magnitude can occur in sequential years, even in
the same year. The return frequency of design is chosen and then referred to as the
design storm.
Duration: The duration of a storm is chosen to coincide with the time of
concentration. Per the Code,the FAA Overland flow equation was used to estimate
the time of concentration. The parameters needed to determine the flow elements
include length, slope, and the Rational runoff coefficient. The theory states that if
the duration is equal to the time of concentration,the length of time will be adequate
for the entire basin to contribute flow.
Analysis
Project basins were delineated based on a review of existing site conditions, aerial photography,
roof lines, site grading, and project topography. The lot drains from south to north int r E IVE D
Mountain Cross Engineering, Inc. �(�
Civil and Environmental Consulting and Design 0 4/0 6/2 018
8261/2 Grand Avenue, Glenwood Springs,CO 81601
P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN
BUILDING DEPARTMENT
1015 Waters Ave
April, 2018
Page 3 of 5
Avenue. The south boundary is the existing irrigation ditch. There is a site wall on the neighboring
property line that acts as the eastern boundary. The existing conditions may have some trespass
of runoff from the subject property onto the neighboring property to the west; this was difficult to
verify due to the still significant snow coverage. If this is the case, the proposed plan will remedy
this.
Other than maybe trespass to the west, the proposed drainage pattern is intended to remain. All
proposed storm water is captured and conveyed by swales and/or piping to the northwest corner
where a bio-retention pond provides the required water quality capture volume. A
scupper/opening in the proposed site wall drains water above the capture volume into Waters
Avenue. Since the project is within the Aspen Mountain Drainage Basin,detention is not required.
Runoff coefficients were determined based on site soils and percentage imperviousness, per the
appropriate figures in the Code. The hydrologic soil group was determined from Figure 3.1 and
based on the project location is, `B". Based on the corresponding Figure 3.3 the applicable
coefficients were determined and are shown in the attached calculations. The parameters for
calculating the time of concentration were determined from the site, slope, length, and land uses.
The time of concentration for the site basin is less than 5 minutes so 5 minutes was used. The
calculations are attached. The rainfall intensities were determined by using the appropriate storm
curve, duration to match the time of concentration, and the IDF curve provided in the Code.
The basin flow rates were calculated based on the Rational Method described in the Code for the
existing (pre-project flows) and also for the proposed conditions. The calculations are attached.
Basin Area Coefficient Flow
Description (acres) (cfs)
Existing - 100 yr 0.150 0.35 0.33
Proposed- 100 yr 0.150 0.52 0.49
Inlets and swales capture flows and direct water into 8" PVC storm pipes at a minimum of 2%
slope. 100 year flows would fill the 8"pipe halfway. Calculations are attached in the Appendix.
Water Quality
Water quality capture volume was calculated to be 51.7 cubic feet. Calculations are attached. The
plans call for a bio-retention pond in the northwest corner. Since the project has type B soils,
infiltration is proposed. Flows above the 51.7 cubic feet will be released through the wall with an
8"PVC pipe into Waters Avenue.
Temporary structures are intended to be used during construction activities. Permanent structures
are intended to be used continually after the construction activities have been completed.
Temporary erosion control measures that are to be employed during construction have been
designed to contain sediment on the site and to mitigate erosion from construction activities. Silt
fencing will be placed around the downhill limits of disturbance. Permanent erosion q rr E IVE D
Mountain Cross Engineering, Inc. KKtt
Civil and Environmental Consulting and Design 0 4/0 6/2 018
8261/2 Grand Avenue, Glenwood Springs,CO 81601
P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN
BUILDING DEPARTMENT
1015 Waters Ave
April, 2018
Page 4 of 5
measures are revegetation of disturbed areas and design of conveyances to prevent erosion. The
water quality structure will capture any transported sediment.
The site will disturb less than one acre and therefore will not require a permit from CDPHE.
Maintenance
Maintenance will be required periodically for the drainage system. At a minimum the following
should be done bi-annually in the spring and fall: swales and the water quality structure should be
cleaned and cleared of mud and debris. Temporarily, the general notes require the maintenance
and frequent inspection of the silt fence. Attached in the Appendix is a Maintenance Plan that
should be used by the Owner.
Low Impact Development
Principle 1: At the outset of the drainage design, WQCV was anticipated. The site grading was
designed to convey runoff to a WQCV.
Principle 2: The design used the entire site by minimizing and eliminating impervious areas.
Permeable pavers are used instead of impervious pavement. Where possible, flows are in
permeable swales.
Principle 3: Permeable pavers are used. The impervious areas have been minimized as much as
practical and still meet the requirements of the Owner.
Principle 4: Similar to Principal 2, permeable pavers are used and the impervious areas are
minimized. Swales are natural lined conveyances to allow for infiltration.
Principle 5: Only WQCV is proposed since the project is in the Aspen Mountain Drainage Basin.
Principle 6: There is a proposed WQCV facility, treating water to enhance the community.
Principle 7: The site has a treatment train approach in that drainage is conveyed in natural swales
to the WQCV where it is treated prior to leaving the site.
Principle 8: All features are accessible to make maintenance as easy as possible.
Principle 9: N/A. None of the facilities are located in areas accessible to the general public.
Results
The proposed drainage pattern is intended to remain. All proposed storm water is captured and
conveyed by swales and pipes into the bio-retention pond. An 8" PVC pipe will release water
through a wall into Waters Avenue. The proposed bio-retention provides the required WQC
volume of 51.7 cubic feet. Detention is not required.
There is a certain amount of uncertainty in hydrologic calculations. However, when constructed
in accordance with this report and the drainage plan, it is our opinion that the design will safely
convey the runoff flows and volume of the 10-year design storm event and the WQCV required
RECEIVED
Mountain Cross Engineering, Inc.
Civil and Environmental Consulting and Design 0 4/0 6/2 018
8261/2 Grand Avenue, Glenwood Springs,CO 81601
P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN
BUILDING DEPARTMENT
1015 Waters Ave
April,2018
Page 5 of 5
for this site and will not cause flooding damage to this or adjacent sites per the requirements of the
City of Aspen.
Thank you for the opportunity to provide this report. Feel free to call if you have any questions,
concerns,or comments.
Sincerely,
Mount in Cross ineering, Inc.
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Mountain Cross Engineering, Inc. RECEIVED
Civil and Environmental Consulting and Design 0 4/0 6/2 018
826 1/2 Grand Avenue,Glenwood Springs, CO 81601
P:970.945.5544 F:970.945.5558 www.mountaincross-eng.com ASPEN
BUILDING DEPARTMENT
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ENGINEERChris Hale 1015 Waters Avenue 'i" ENGI1NEERING, •
DWG FILE - / Civil and Environmental Consultii0li10A4Q 18
WatersDesign / 826 1/2 Grand Avenue Glenwood Springs,CO 81601
JOB Gretchen Greenwood & Assc. - /// '
ph 970.945.5544 fx 970.945.5558 www.mountaincr
556-014
BUILDING DEPARTMENT
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Figure 3.2— Runoff Coefficients for NRCS Hydrologic Soil Group B
04/06/2018
ASPEN
BUILDING DEPARTMENT
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Figure 3.2- Runoff Coefficients for NRCS Hydrologic Soil Group B
• 1 11 r rq
04/06/2018
ASPEN
BUILDING DEPARTMENT
DRAINAGE CALCULATIONS for
MOUNTAIN CROSS
1015 Waters Avenue
ENGINEERING,
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Drainage Area : Existing/Historic
Surface description: Unimproved
PROPERTY GENERAL SURFACE DATA
Total Drainage Area: 6,550 sq. ft. 0.150 ac.
RUNOFF COEFFICIENT
imperviousness = 0%
HSG = B from Fig 3.2
C100 = 0.35
C10 = 0.15
C5 = 0.08
C2 = 0.03
100% 0.150
TOTAL BASIN ACERAGE 0.150
TIME OF CONCENTRATION
Overland Flow Time Length Slope C5 Tc
Upstream Elevation 7971.00 30 0.333 0.08 3.17
Dnstream Elevation 7961.00
Channel Flow Time none Length Slope K Tt
Upstream Elevation 7961.00 130.0 0.023 7.000 2.04
Dnstream Elevation 7958.00
Total 5.21 min
RUNOFF
Existing/Historic
Intensity F l overate
(in/hr) (cfs)
2-yr Storm 2.0 0.01
10-yr Storm 3.6 0.08
100-yr Storm 6.2 0.33
NOTES:
- Soils are hydrologic soil group B from Figure 3.1
- K= 7.0 based on lawn
RECEIVED
04/06/ 018
ASPEN
BUILDING DEPARTMENT
DRAINAGE CALCULATIONS for
MOIINTfAIN CROSS
1015 Waters Avenue
ENGINEERING, INC 4r4f2018
Civil and Envlrgnmenfal Consulting and Deslgn ,,,
826112 Grand Avenue Glenwood Spdngs,GG 81601 ,,,;i
ph920.915.55661x 9T0.9655558 wwuv.movnialnciaeeen¢ ""✓
Drainage Area : Proposed
Surface description: Turf, landscaping, roof area, and hardscape
PROPERTY GENERAL SURFACE DATA
Total Drainage Area: 6,550 sq. ft. 0.150 ac.
RUNOFF COEFFICIENT sq.ft. or
Land Use est. % acres imperviousness = 48%
Building Roof 2610 0.060 HSG = B from Fig 3.2
Ponds 0 0.000 C100 = 0.52
Hardscape and Paving 565.5 0.013 C10 = 0.40
Landscaped turf and planters 3,375 0.077 C5 = 0.35
Pine and Aspen 0% 0.000 C2 = 0.28
Sage and Grasses 0% 0.000
TOTAL BASIN ACERAGE 0.150
TIME OF CONCENTRATION
Overland Flow Time Length Slope C5 Tc
Upstream Elevation 7970.50 20 0.488 0.35 1.68
Dnstream Elevation 7960.75
Channel Flow Time none Length Slope K It
Upstream Elevation 7960,75 75.0 0.020 7.000 1.26
Dnstream Elevation 7959.25
Total 2.94 min
use 5 min Tc
RUNOFF
Proposed
Intensity Flowrate
(in/hr) (cfs)
2-yr Storm 2.1 0.088
10-yr Storm 3.7 0.223
100-yr Storm 6.3 0.493
NOTES:
- Soils are hydrologic soil group B from Figure 3.1
- K= 7 based on gravel channel
04/06/2018
ASPEN
BUILDING DEPARTMENT
Rainfall IDF for Aspen, Colorado
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0
0 5 10 15 20 25 30 35 40 45 50 55 60
Duration in Minutes
t2-yr --0-5-yr -41- 10-yr —0-25-yr —t-50-yr —0--100-yr
RECEIVED
04/06/2018
ASPEN
BUILDING DEPARTMENT
14,_ WQC Volume
Level 1: Over turf to capture
w ,f Tot Imp = 48 %
19' --� from Fig. 8.14 yields
Letl3
TMa"y Eff Imp = 47 %
4 �
Pram Ike ' from Fig. 8.13 yields
SC
WQCV= 0.095 watershed-in
Area = 0.15 acres
Req'd Volume = 51.7 cubic feet
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Figure 8 10 Imperviousness Adjustments for Laval 1 and 2 NOC A(UDFCO 1919)
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to 0.15
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0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Effective Imperviousness of Tributary Area to BMP(percent)
Figure a.13 Aspen Water Quality Capful,Volume
RECEIVEID
04/06/2018
ASPEN
BUILDING DEPARTMENT
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk,Inc. Monday,Apr 2 2018
100- yr, 8 inch
Circular Highlighted
Diameter (ft) = 0.67 Depth (ft) = 0.30
Q (cfs) = 0.490
Area (sqft) = 0.15
Invert Elev (ft) = 1.00 Velocity (ft/s) = 3.19
Slope (%) = 1.00 Wetted Perim (ft) = 0.98
N-Value = 0.013 Crit Depth, Yc (ft) = 0.33
Top Width (ft) = 0.67
Calculations EGL (ft) = 0.46
Compute by: Known Q
Known Q (cfs) = 0.49
Elev (ft) Section
2.00
1.75
1.50
1.25
1.00
0.75 0 1 RECEIVED
04/06/2018
Reach (ft)
ASPEN
BUILDING DEPARTMENT
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk,Inc. Monday,Apr 2 2018
Swale
Triangular Highlighted
Side Slopes (z:1) = 2.00, 2.00 Depth (ft) = 0.33
Total Depth (ft) = 0.50 Q (cfs) = 0.490
Area (sqft) = 0.22
Invert Elev (ft) = 1.00 Velocity (ft/s) = 2.25
Slope (%) = 2.00 Wetted Perim (ft) = 1.48
N-Value = 0.025 Crit Depth, Yc (ft) = 0.33
Top Width (ft) = 1.32
Calculations EGL (ft) = 0.41
Compute by: Known Q
Known Q (cfs) = 0.49
Elev (ft) Section Depth (ft)
2.00 1.00
1.75 0.75
1.50 0.50
1.25 0.25
1.00 0.00
0.75 RECEIVED
.5 1 1.5 2 2.5
04/06/2018
Reach (ft)
ASPEN
BUILDING DEPARTMENT
(---rik
_ 1 MOUNTAIN CROSS
ENGINEERING, INC.
Civil and Environmental Consulting and Design
Maintenance Plan
The following is a checklist to aid the Property Owner of ongoing drainage system maintenance for
1015 Waters Avenue in Aspen CO. The site will require more frequent cleaning of sediment
initially. As the site matures, maintenance should be performed after large magnitude rainfall
events and at the changing of the seasons. Damaged, cracked, or missing items should be repaired
and replaced as discovered. This is not intended to be an all-inclusive list:
Drainage Swales:
• Verify positive drainage and grade any ponding areas to drain
• Check for exposed and/or damage to PVC liner and repair
• Clean out swales from deposited sediment and debris
Concrete:
• Clean off deposited sediment and debris
• Check for and fill any cracks
• Repair and/or replace broken and damaged items
Pipes and Inlets:
• Clear inlet grates of debris
• Clear piping of any obstructions
• Clean sumps and piping of deposited mud and debris
• Check offsite end is clear
• Check exposed PVC piping for UV degradation and cracking
Heat Tape and Snowmelt:
• Check for correct operation in pipes, roof gutters, and downspouts and repair as necessary
• Check for correct operation in trench drains and drywell and repair as necessary
• Check for correct operation in snowmelt area and repair as necessary
• Turn on heat tape for cold seasons and turn off for warm seasons
Storm Water Quality Structure
• Remove sediment, debris, and litter from structure
• Clear mud and debris from inlets and outlets
Overall:
• Check site for differential settlement creating any areas of ponding and grade to drain
• Check site for locations where runoff may breach and bypass the drainage system and correct
• Keep roof gutters and downspouts clear of debris
RECEIVED
826'A Grand Avenue,Glenwood Springs,CO 81601 04 0 6 2 018
P: 970.945.5544 F:970.945.5558 www.mountaincross-eng.com
ASPEN
BUILDING DEPARTMENT