HomeMy WebLinkAboutFile Documents.320 W Bleeker St.0188.2017 (31).ARBK1
Drainage Report
320 W. BLEEKER STREET
ASPEN, CO
July 24, 2017
Revised: 11/8/2017
Revision 2: 4/17/2018
Prepared by
Richard Goulding, P.E.
Roaring Fork Engineering
592 Highway 133
Carbondale, CO 81623
Reviewed by Engineering
05/02/2018 6:46:29 AM
"It should be known that this review shall not
relieve the applicant of their responsibility to
comply with the requirements of the City of
Aspen. The review and approval by the City is
offered only to assist the applicant's
understanding of the applicable Engineering
requirements." The issuance of a permit based
on construction documents and other data shall
not prevent the City of Aspen from requiring the
correction of errors in the construction
documents and other data.
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Drainage Report
320 W. BLEEKER ST.
ASPEN, CO
I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 320 W. BLEEKER ST. WAS
PREPARED BY ME FOR THE OWNERS THEREOF IN ACCORDANCE WITH THE PROVISIONS OF THE
CITY OF ASPEN (COA) URBAN RUNOFF MANAGEMENT PLAN (URMP) AND APPROVED
VARIANCES AND 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.
RICHARD GOULDING, P.E.
RFE Project # 2017-09
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Contents
1.0 General .................................................................................................................................................... 4
1.1 Existing Site ........................................................................................................................................ 4
1.2 Proposed Conditions ........................................................................................................................... 4
1.3 Previous Drainage Studies .................................................................................................................. 4
1.4 Offsite Drainage & Constraints ........................................................................................................... 4
2.0 Drainage Basins ...................................................................................................................................... 5
2.1 Basins .................................................................................................................................................. 5
2.2 Peak Discharge Calculations ............................................................................................................... 5
3.0 Low Impact Site Design .......................................................................................................................... 6
3.1 Principles ............................................................................................................................................. 6
4.0 Hydrological Criteria .............................................................................................................................. 7
4.1 Storm Recurrence and Rainfall ........................................................................................................... 7
4.2 Storage Volumes Methodology .......................................................................................................... 7
5.0 Hydraulic Criteria ................................................................................................................................... 8
5.1 Piping .................................................................................................................................................. 8
5.2 Inlet Sizing ........................................................................................................................................ 10
6.0 Proposed Facilities ................................................................................................................................ 10
6.1 Proposed Structures .......................................................................................................................... 10
7.0 Operation and Maintenance .................................................................................................................. 11
7.1 Screened Rock Bed ........................................................................................................................... 11
8.0 Appendices ............................................................................................................................................ 11
Drawings 11x17 ...................................................................................................................................... 11
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1.0 General
1.1 Existing Site
The site is located at 320 West Bleeker Street in the West End of Aspen, Colorado. There is currently a
single-family residence on the property that will be demolished. The property is bordered by two other
lots to the east and west, Bleeker St. to the south and an alley to the north. There are mature trees and
landscaping surrounding the lot. The topography of the lot slopes from the northwest down to the south
east.
H-P Kumar performed a field exploration on March 1st, 2017. A sub-surface soils report was produced on
March 8, 2017. The soil profile consists of 6” of top soil or 4’ of silty clayey sand with gravel fill. Below
this is natural relatively dense, silty sandy gravel and cobbles with probable boulders down to the boring
depths of 7 and 13 feet. No free water was encountered at the time of drilling and the sub soils were
slightly moist to moist. A final percolation rate of 7.5 inches per hour, or 8 minutes per inch (mpi) was
observed.
1.2 Proposed Conditions
This project is classified as a ‘Major Project’ in Table 1.1 of the URMP, as the proposed development is
over 1000 square feet and disturbs an area greater than 25% of the site footprint. The intent of this report
is to demonstrate compliance with the requirements of the City of Aspen’s (COA) URMP. The Low
Impact Design (LID) Principles in the introduction of the manual were used as a guide throughout the
design process.
The proposed development involves the demolition of the existing structure and the construction of a new
single-family home. Grading around the new structure will also take place to accommodate new
walkways, lawn area and patios. No changes to land use or soil types are planned. Cut depths of up to 16
feet are expected and site grading cuts and fills are very minor compared to the foundation excavation.
The runoff from impervious surfaces will be collected in a system of roof drains, trench drains and inlets,
which will convey runoff to a screened rock bed under the southeast patio. The screened rock bed will be
sized for the Water Quality Capture Volume (WQCV), as the property is within the Aspen Mountain
Drainage Basin. A planter just south of the patio will allow overflows from the screened rock bed to be
evenly disperse into the front lawn area. The from lawn then slopes down to the curb and gutter. A
foundation drain system will collect ground water from around the building foundation and pipe it to an
internal drywell located under the mechanical room in the basement.
1.3 Previous Drainage Studies
The property is not within a mudflow area as defined by the COA Storm Drainage Master Plan. The
latest URMP does not show any infrastructure near the property. The curb and gutters are the primary
means of conveyance for the COA’s storm system in this area.
1.4 Offsite Drainage & Constraints
There appears to be no offsite drainage flowing onto the property. The alley to the north is paved with
proper grading as to not inundate private property. To the east, and west, pervious lawn areas and
landscaping border the property. W. Bleeker St. is lined with curb and gutter on the south side of the
property.
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2.0 Drainage Basins
The entire site is routed to one overall point of concentration, the screened rock bed, and the entire site
was modeled as a single drainage basin to determine the total WQCV required. This basin was then
divided into sub basins comprised of the roof area, driveways, walkways and patios. The sub basins were
then used to calculate 100-year peak flow conditions for sizing individual inlets and piping.
2.1 Basins
Basin 1 is comprised of the entire lot. This includes roof area, the driveway off of the alley, the patios,
lawn area and walkways. This basin is 6,000 square feet (sf), and is 65.04% impervious (3,902.6 square
feet).
2.2 Peak Discharge Calculations
Peak flows were calculated for 10- and 100-year storm events. Rainfall intensity was calculated using a
Time of Concentration (Td) of 5 minutes. Actual time of concentration on the site is significantly less
than 5 minutes, but according to the City of Aspen URMP, equations used to calculate rainfall intensity
are only valid for a Time of Concentration of greater than 5 minutes, so the smallest valid time of
concentration value was used. The 1-hour Rainfall depth (P1), given in Table 2.2 as 0.77 inches for the
10-year event and 1.23 inches for the 100-year event. Equation 2.1 was referenced when solving for the
rainfall intensity (I).
I = 88.8P1/(10+Td )1.052
Runoff coefficients (C), a function of the hydrologic soil group (in this case, B) and the percentage of
impervious area within each sub basin were developed using Figure 3.2. The runoff coefficient was then
multiplied by the rainfall intensity (I) and the acreage of each major basin (A) to determine the peak
discharge for the Major Basin. Q allowable was calculated the same way, except the basin was treated as
undeveloped, or 100% pervious. The Peak Discharge (Qp) in cubic feet per second (cfs) is given by
equation 3.1.
Qp= CIA
Qp= Peak Discharge (cfs)
A= Area (acres)
I= Rainfall intensity (inches per hour)
C= Runoff Coefficient (unitless)
The tables below contain the peak flows for developed and undeveloped conditions for 10- and 100-year
storm events.
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3.0 Low Impact Site Design
3.1 Principles
Principle 1: Consider storm water quality needs early in the design process.
The grading and drainage design was coordinated with the architects early in the design. Comments from
owners were considered and analyzed. Multiple site visits ensured proper understanding of existing
conflicts.
Principle 2: Use the entire site when planning for storm water quality treatment.
The entire site was analyzed when determining the WQCV. The screened rock bed is sized for the
required total WQCV. No runoff from an impervious surface will be discharge without being collected
and routed to the screened rock bed. Overflows will discharge to the south lawn.
Principle 3: Avoid unnecessary impervious area.
10 Year Peak Discharge Developed Calculations
1 Hour(P1)0.77
Return Period 10
Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max
See(D1) (ft
2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec)
1 6000.00 3902.60 65.04% 0.500 5 3.96 0.27
10 Year Peak Discharge Pre Development Calculations
1 Hour(P1)0.77
Return Period 10
Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max
See(D1) (ft
2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec)
1 6000.00 0.00 0.00% 0.150 5 3.96 0.08
100 Year Peak Discharge Developed Calculations
1 Hour(P1)1.23
Return Period 100
Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max
See(D1) (ft
2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec)
1 6000.00 3902.60 65.04% 0.590 5 6.33 0.51
100 Year Peak Discharge Pre Development Calculations
1 Hour(P1)1.23
Return Period 100
Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max
See(D1) (ft
2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec)
1 6000.00 0.00 0.00% 0.350 5 6.33 0.30
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Pervious pavers will be utilized on the south-east patio. Other impervious surfaces are for walkways and
driveways.
Principle 4: Reduce runoff rates and volumes to more closely to match natural conditions.
All runoff from impervious surfaces will be collected in the screened rock bed, which has capacity for the
WQCV. The percolation rates of the soil are very high so this area will infiltrate storm water effectively.
Principle 5: Integrate storm water quality management and flood control.
All inlets and piping are sized to accommodate the 100-year peak flows. Pipes were sized to ensure
100-year flows do not exceed 80% of the pipe capacity. Overflows will discharge toward the right of way
and the curb and gutter.
Principle 6: Develop storm water quality facilities that enhance the site, the community and the
environment.
An oil and sand separator is specified for the driving surface while all inlets have 6-inch sumps to help
trap sediment. Also, no large concrete storm structures are proposed.
Principle 7: Use treatment train approach.
After runoff has passed through the piping and inlets with numerous sumps it discharges into a screened
rock bed. If the bed were to overflow the front lawn area will provide more treatment before it reaches the
curb and gutter.
Principle 8: Design sustainable facilities that can be safely maintained.
Inlets and piping will be vacuumed or flushed periodically to maintain adequate flow. Proper grading
reduces dangerous slopes and proper drainage reduces ice buildup. All facilities are easily accessible and
are installed with cleanouts when applicable.
Principle 9: Design and maintain facilities with public safety in mind.
The proposed storm system adds gutters and drains to roofs. This reduces ice buildup and the damages
that follow. There are no drop-offs or steep grades proposed. Discharges are far from public right of way.
4.0 Hydrological Criteria
4.1 Storm Recurrence and Rainfall
The 10 and 100-year events were analyzed for this site. The site’s runoff overflows toward W. Bleeker St.
where the City’s infrastructure takes over. The property is within the Aspen Mountain Drainage Basin so
only the WQCV is detained.
4.2 Storage Volumes Methodology
The total Water Quality Capture Volume (WQCV) calculation is shown below.
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A factor of safety of 1.5 was used as well.
5.0 Hydraulic Criteria
5.1 Piping
Two separate systems were established to covey storm water down the east and west sides of the property
to the screened rock bed.
Pipes used in all drainage systems will be standard dimension ratio (SDR) 35 PVC with a Manning’s
coefficient (n) of 0.01. The pipes were sized to accommodate peak flows for a 100 year event at 80%
full. If the water level in the pipe exceeds the 80% full criteria, then the pipe is deemed inadequate. Sub-
basins where delineated to isolate specific peak flows. Below is a table of sub-basin peak flows.
Below is a table showing which sub basins contribute runoff to each pipe and the total flow (Q)
accumulated during a 100-year peak flow event.
Water Quality Capture Volume Storage
Basin Total Area Impervious Area Impervious WQCV Table Value WQCV Storage F.O.S. Required Storage BMP
(#) (ft
2)(ft2)(%) (in) (ft
3)(ft3)
1 6000.00 3902.60 65.04% 0.128 64.00 1.5 96.0 Screened Rock Bed
Peak Discharge Developed Calculations
1 Hour(P1)1.23
Return Period 100
Basin ID Total Area Imp. Area Impervious % C Value Time of C Intensity Q Max
See(D1)ft2 ft2 Percentage From Table (Td) I=88.8P1/(10+Td)01.052 ft3/sec
E1 261.00 261.00 100.00% 0.950 5 6.33 0.04
E2 493.29 493.29 100.00% 0.950 5 6.33 0.07
E3 299.00 299.00 100.00% 0.950 5 6.33 0.04
E4 296.00 296.00 100.00% 0.950 5 6.33 0.04
E5 35.00 35.00 100.00% 0.950 5 6.33 0.005
E6 343.00 343.00 100.00% 0.950 5 6.33 0.05
W1 1013.58 1013.58 100.00% 0.950 5 6.33 0.14
W2 71.00 71.00 100.00% 0.950 5 6.33 0.01
W3 114.60 114.60 100.00% 0.950 5 6.33 0.02
W4 193.36 193.36 100.00% 0.950 5 6.33 0.03
W5 385.83 385.83 100.00% 0.950 5 6.33 0.05
FP 284.40 0.00 0.00% 0.350 5 6.33 0.01
Total 3790.06 3505.66 92.50%0.50
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Calculated pipe sizes were tested for hydraulic capacity at 80%. Values for depth of flow for each pipe
were calculated. Design charts giving Q design / Q full were downloaded from Federal Highway
Administration (FHWA) and the equations in Section 4.8.4 of the URMP were used as the basis for these
calculations. Calculated pipe sizes and internal depth of flow within the pipes is shown below.
Storm System Pipes
Pipe System Pipe Contibuting Sub‐Basins Peak Flows (CFS)
EAST E1 E1 0.04
E2 E1, E2 0.10
E3 E1, E2, E3 0.15
E4 E1, E2, E3, E4 0.19
E5 E1, E2, E3, E4 0.19
E6 E1, E2, E3, E4, E5 0.20
E7 E1, E2, E3, E4, E5 0.20
E8 E1, E2, E3, E4, E5 0.20
E9 E1, E2, E3, E4, E5 0.20
E10 E1, E2, E3, E4, E5 0.20
E11 E1, E2, E3, E4, E5, E6 0.24
WEST W1 W1, W2 0.15
W2 W1, W2, W3 0.17
W3 W1, W2, W3, W4 0.19
W4 W1, W2, W3, W4, W5 0.25
W5 W1, W2, W3, W4, W5 0.25
Screened Rock
Bed Overflow Overflow
E1, E2, E3, E4, E5, E6, W1, W2,
W3, W4, W5. FP 0.50
K=0.462
Pipe Combined
Design Flow
Manning
Coefficient Flattest Slope Equation 4‐31
Required
Diameter
Design
Diameter
Design
Diameter‐80%
Has Capacity@
80% full
(ID)Q (ft3/sec)n (%)S0 d={nQ/K√So}3/8 (inches) (inches) (inches) Yes/No
E1 0.04 0.01 2.00% 0.14 1.707 4.0 3.200 Yes
E2 0.10 0.01 2.00% 0.21 2.541 4.0 3.200 Yes
E3 0.15 0.01 2.00% 0.24 2.880 4.0 3.200 Yes
E4 0.19 0.01 3.00% 0.24 2.929 4.0 3.200 Yes
E5 0.19 0.01 3.00% 0.25 2.957 4.0 3.200 Yes
E6 0.20 0.01 3.00% 0.25 2.985 4.0 3.200 Yes
E7 0.20 0.01 3.00% 0.25 2.985 4.0 3.200 Yes
E8 0.20 0.01 3.00% 0.25 2.985 4.0 3.200 Yes
E9 0.20 0.01 3.00% 0.25 2.985 4.0 3.200 Yes
E10 0.20 0.01 3.00% 0.25 2.985 4.0 3.200 Yes
E11 0.24 0.01 2.00% 0.29 3.493 6.0 4.800 Yes
W1 0.15 0.01 2.00% 0.24 2.911 4.0 3.200 Yes
W2 0.17 0.01 2.00% 0.25 3.023 6.0 4.800 Yes
W3 0.19 0.01 2.00% 0.27 3.198 6.0 4.800 Yes
W4 0.25 0.01 2.00% 0.29 3.505 6.0 4.800 Yes
W5 0.25 0.01 2.00% 0.29 3.505 6.0 4.800 Yes
Overflow 0.50 0.01 2.00% 0.38 4.572 6.0 4.800 Yes
Onsite Piping Capacity
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5.2 Inlet Sizing
Inlets were sized to accommodate peak flow conditions for each basin. Equations 4-17 through 4-20 were
used to size inlets, which incorporate a 50% clogging factor (Cg) and a 40% opening in grates (m). A
water depth of 0.05 feet was assumed and all the inlets were treated as sumps as they will be set a
minimum of 0.05 feet below the flow lines. The trench drain was treated a rectangular sump.
Inlets set in landscaping will be circular. Below are the sump calculations for circular inlets.
6.0 Proposed Facilities
6.1 Proposed Structures
A single screened rock bed is being utilized to meet the WQCV requirement for the entire site. Roof area
will be collected via roof drains that will then be piped into the system. Inlets will also be used to capture
runoff from roof overflow scuppers. The driveway will flow into a 4-inch wide trench drain piped to an
oil/sand separator. Other patios and impervious surface will be graded toward landscaping where inlets
will be placed at low points to collect runoff. Everything will be piped to the screened rock bed. Analysis
of the conveyance structures is contained in section 5.0 of this report. The screened rock bed was sized for
the total the WQCV calculated in section 4.2. The screened rock bed is 15 feet long, 6 feet wide and 4 feet
deep. With a 30% void space in the screened rock the bed provides 108 cubic feet of storage. The soil on
the site was determined to have a percolation rate of 7.5 inches per hour or 8 minutes per inch (mpi) as
stated in the Geotechnical Report. The infiltration area used to determine the drain time was the bottom 2
feet of the south vertical side of the screened rock bed. This is a 2 foot by 15-foot strip at the bottom of
the bed. The bottom of the screened rock bed is lined with a PVC liner sloping away from the structure at
10%. This ensures infiltration occurs 10’ away from the structure. A 10’ set back from the neighboring
property has also been established.
The drain time for the WQCV is 1.76 hours.
Sub Basin and Rectangular Inlet Calculations
1 Hour(P1)1.23 m=40% Ys=.04 (Depress inlet by 0.04')
Return Period 100 Cg=50% Co=0.65
Inlet ID Basin ID Total Area Imp. Area Impervious C Value Time of Concentration Intensity Q Max Inlet Type Inlet Width Inlet Length Effective Open Area (EQ. 4‐20) Inlet Capacity (EQ 4‐19) Has Capacity
See(D1) (ft
2)(ft2)(%)(From Table) (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) Rectangular Wo (inches) Lo (inches) Ae=(1‐Cg)mWoLo Q=CoAe√2gYs (Yes/No)
TRENCH DRAIN W1 261.00 261.00 100.00% 0.950 5 6.33 0.036 4" x 20' 4 240 1.333 1.338 Yes
Sub Basin and Circular Inlet Calculations
1 Hour(P1)1.23 m=40% Ys=.04 (Depress inlet by 0.04')
Return Period 100 Cg=50% Co=0.65
Inlet ID Basin ID Total Area Imp. Area Impervious C Value Concentration Intensity Q Max Inlet Type Diameter Area(EQ. 4‐20) Inlet Capacity (EQ 4‐19) Has Capacity
See(D1) (ft
2)(ft2)(%)From Table (Td) I=88.8P1/(10+Td)1.052 ft3/sec Wo (inches) Ae=(1‐Cg)mA Q=CoAe√2gYs (Yes/No)
CATCH BASIN‐E1 E2 493.29 493.29 100.00% 0.950 5 6.33 0.068 8" Round 8 0.070 0.081 Yes
CATCH BASIN‐E2 E3 299 299 100.00% 0.950 5 6.33 0.041 8" Round 8 0.070 0.081 Yes
CATCH BASIN‐E3 E4 296 296 100.00% 0.950 5 6.33 0.041 8" Round 8 0.070 0.081 Yes
CATCH BASIN‐E4 E5 35 35 100.00% 0.950 5 6.33 0.005 8" Round 8 0.070 0.081 Yes
CATCH BASIN‐E5 E6 343 343 100.00% 0.950 5 6.33 0.047 8" Round 8 0.070 0.081 Yes
CATCH BASIN‐W1 W2 71.00 71.00 100.00% 0.950 5 6.33 0.010 8" Round 8 0.070 0.081 Yes
CATCH BASIN‐W2 W3 114.60 114.60 100.00% 0.950 5 6.33 0.016 8" Round 8 0.070 0.081 Yes
CATCH BASIN‐W3 W4 193.36 193.36 100.00% 0.950 5 6.33 0.027 8" Round 8 0.070 0.081 Yes
CATCH BASIN‐W4 W5 385.83 385.83 100.00% 0.950 5 6.33 0.053 8" Round 8 0.070 0.081 Yes
Infiltration Claculation
BMP Max Volume Infiltration Area Infiltration Rate Time To Drain
(name) V (ft
3)A (ft2)I (in/hr) (hr)
Screened Rock Bed 99.00 30 7.5 5.28
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To the south of the previously paved patio a small planter will help retain the pavers and help with
grading to eliminate steep slope down to the existing grade at the property line. The screened rock bed
will be hydraulically connected to the planter. The edge of the planter will be graded level to act as a level
spreading mechanism for overflow. The south edge of the planter is the lowest point in the system where
over flows will first discharge. Overflows shall discharge evenly into the front lawn and eventually reach
the COA curb and gutter.
7.0 Operation and Maintenance
7.1 Screened Rock Bed
The screened rock bed beneath the patio and piping must be maintained periodically and inspected to
ensure proper operation. A maintenance plan shall be submitted to the City describing the maintenance
schedule that will be undertaken by the owners of the new residence or building. Minimum inspection
and maintenance requirements include the following:
‐ During the first year draw down should be checked for every event over 0.25” of precipitation
to ensure no significant backups are occurring.
‐ The maximum drain down time is 24 hours.
‐ The fabric surrounding the bed will be the first component to clog. If the fabric becomes
clogged replacement will be needed.
‐ Piping systems and sumps should be checked during and after storms routinely.
‐ Clean out and drainage basins and provide access for hoses and vacuum equipment.
‐ After the first year the system should be cleaned out at least once a year and more if the first-
year inspections prove more maintenance is required.
‐ More frequent cleaning reduces the amount of debris entering the system and reduces the
need for more intense maintenance.
‐ Remove debris from the gravel bed routinely. If the gravel has been contaminated by soil and
sand, clean or replace gravel as necessary. Gravel will possibly have to be replaced every 5-
10 years for proper perforation into the lawn.
‐ Clean the inside of the perforated pipe with a 6” or 12” pipe cleaner accessed through
cleanouts. This should be done yearly, or as necessary if the system is not infiltrating properly
or if the system has become contaminated.
‐ Ensure heat tape is functioning before colder months to prevent damage to piping.
‐ If the storm system is not maintained properly, replacement of parts or of the entire system
may be necessary.
‐ The south edge of the patio and screened rock bed must also be inspected to ensure the grade
is level and overflows are being dispersed evenly over the edge of the screened rock.
8.0 Appendices
Drawings 11x17