HomeMy WebLinkAboutFile Documents.1112 Waters Ave.0089.2017 (22).ARBK Drainage Report
1112 WATERS AVENUE
ASPEN, CO 81611
May 5,2017
Prepared by
Richard Goulding,P.E.
Roaring Fork Engineering
592 Highway 133
Carbondale, CO 81623
Atlw ROARING FORK
ENGINEERING
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Drainage Report
1112 WATERS AVENUE
ASPEN, CO 81611
I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 1419 CRYSTAL LAKE ROAD WAS
PREPARED BY ME FOR THE OWNERS THEREOF IN ACCORDANCE WITH THE PROVISIONS OF THE CITY
OF ASPEN URBAN RUNOFF MANAGEMENT PLAN 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.
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RICHARD GOULDING,P.E. :Q r, •
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RFE Project#2017-03 �'Fs+"••"••.•.••`,�.. .
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Table of 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 and Sub-basins 5
2.1 Drainage Basins 5
2.2 Peak Discharge Calculations 5
3.0 Low Impact Site Design 7
3.1 Principles 7
4.0 Hydrological Criteria 8
4.1 Storm Recurrence and Rainfall 8
4.2 Peak Runoff and Storage Volumes Methodology 8
5.0 Hydraulic Criteria 8
5.1 Inlets 8
5.2 Pipes 9
6.0 Proposed Facilities 10
6.1 Proposed structures 10
6.2 Infiltration 10
6.2.1 Drywell 10
7.0 Operation and Maintenance 11
7.1 Drywell 11
7.2 Pervious Paver Area 11
8.0 Appendices 12
Drawings 11x17 12
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1.0 General
1.1 Existing Site
The residence under evaluation is located at 1112 Waters Avenue in Aspen, Colorado. This property is a
0.185 acre(8,050 square feet)parcel and is located within the Aspen Mountain Drainage Basin. The
existing structure includes a one-story 1,825 square foot multi residential property with a basement. The
site is bounded by Waters Avenue cul-de-sac to the west, 1110 Waters Avenue to the northeast, 1114
Waters avenue to the southwest, and 100 Ute Place to the east. The site also has the Wheeler Ditch
running through along the southern property line. The existing vegetation on the site includes aspen trees
of various sizes and irrigated grasses.
CTL Thompson completed a"Geotechnical Consultation"for this lot on March 29, 2017 as part of the
permit submittal. Per the consultation,the subsurface conditions for the site consists mainly of silty gravel
with large cobbles and boulders at depths of at least 60 feet below grade. This information was gathered
using prior investigations from adjacent/nearby sites to 1112 Waters Avenue. A recommendation on the
infiltration rate of the soils was completed on April 10,2017. All percolation provided in the infiltration
rate letter is incorporated into the drainage design calculations for this project.
1.2 Proposed Conditions
This project is classified as a `Major Project' as per Table 1.1 of the URMP. The proposed development is
over 1000 square feet(sf)and disturbs an area of approximately 5,450 sf.,roughly 68%of the site. The
intent of this report is to demonstrate compliance with the requirements of the URMP. The Low Impact
Design(LID)Principles in the introduction of the manual were used as a guide throughout the design
process.
The proposed project consists of the demolition of the existing residence and all associated flatwork,
patios, driveway, and small planters. The proposed two-story, single family residence has multiple patios
with a hot tub and a partially snowmelted driveway and walkways. There are a number of plantings and
landscaped areas associated with this project as well.
The runoff from impervious surfaces will be collected through a system of inlets,roof drains,and a slot
drain that is routed to a drywell located on the northwest corner of the property. Refer to sheet C-102 of
the civil set for proposed grading and drainage of the site.
1.3 Previous Drainage Studies
The City completed a Drainage Master Plan in November 2001 using the consultant WRC,however this
project is outside the area of that study.
Figure 7.1 of the URMP defines the mudflow zones for the City of Aspen. This property is not impacted
by mudflow.
1.4 Offsite Drainage & Constraints
There is no offsite drainage that impacts the property. The proposed development will not impact any
irrigation ditches or waterways, as grading limits do not exceed the top of the Wheeler Ditch running
along the southern property line of the site which inhibits any onsite flow from entering.
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2.0 Drainage Basins and Sub-basins
The site was able to be designed as one large basin,which was then subdivided into smaller sub-basins.
Basin and Sub-basin delineations are shown on sheet C-101 of the civil drawings for permit. These
sheets list impervious areas,runoff coefficients,peak flows, and the required volume of runoff to be
detained. Sub-basins were created to calculate the maximum flow entering each inlet of the proposed
storm systems. The sub-basin peak flows were then used to size pipes and inlet capacities. The table
below shows each sub-basin's peak flow rate for a 100 year storm.
100 Year Sub Basin Peak Discharge Developed Calculations
1 Hour(P1) 1.23
Return Period 100
Sub Basin Total Area Imp.Area Impervious C Value Time of C Intensity Sub Basin Flow Rate
(Name) At(ft2) A (ft2) A;/At(%) From Table (Td) I=88.8P1/(10+Td)o1.o52 C4ub(ft3/sec)
1.1 208.77 208.77 100.00% 0.950 5 6.33 0.03
1.2 1137.67 1137.67 100.00% 0.950 5 6.33 0.16
1.3 959.93 959.93 100.00% 0.950 5 6.33 0.13
1.4 405.15 192.67 47.56% 0.510 5 6.33 0.03
2.1 Drainage Basins
Basin 1 is the only basin on site at 2711.52 square feet(sf)and is 92%impervious. Impervious sections
of the basin include the roof structure,patios, and walkways. The remaining pervious areas of the basin
include the permeable paved driveway and the landscaping areas by the front entry. There are 4 sub-
basins in Basin 1. Two roof drain tie-ins collect runoff from the roof structure,one inlet collects runoff
onto the southeastern patio and the hot tub area,and one slot drain collects runoff on the snowmelted
entry to the residence. All runoff from each sub-basin is then directly routed to the proposed drywell on
the northwest corner of the site. The proposed drywell is 5 ft. in diameter and 10' deep.
2.2 Peak Discharge Calculations
The peak flows were calculated for each Major Basin for 5 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(Pi), given in Table 2.2
as 0.64 inches for a 5-year event and 1.23 inches for a 100-year event. Equation 2.1 was referenced when
solving for the Rainfall Intensity(I).
I=88.8P1/(10+Td)1.052 (Equation 2.1)
Runoff Coefficients(C),a function of the 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(C)was then multiplied by
the Rainfall Intensity(I)and the acreage of each Major Basin(A)to determine the peak discharge for each
Major Basin.
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Qp =CIA
Qp =Peak Discharge (cfs)
A =Area(Acres)
I =Rainfall intensity(inches per hour)
C =Runoff Coefficient
These peak flow values were used to calculate the size of the proposed detention and conveyance
structures, such as drywells, inlets and pipes. The tables below contain the peak flows for developed and
undeveloped conditions for 5 and 100 year storm events.
5 Year Peak Discharge Developed Calculations
1 Hour(P1) 0.64
Return Period 5
Basin ID Total Area Imp.Area Impervious C Value Time of C Intensity Q Max
See(D1) (ft2) (ft2) (%) From Table (Td) 1=88.8P1/(10+Td)1.o52 (ft3/sec)
1 2711.52 , 2499.04 92.16% 0.720 5 3.29 0.15
5 Year Peak Discharge Pre Development Calculations
1 Hour(P1) 0.64
Return Period 5
Basin ID Total Area Imp.Area Impervious C Value Time of C Intensity Q Max
See(D1) (ft2) (ft2) (%) From Table (Td) I=88.8P1/(10+Td)1.°52 (ft3/sec)
1 2711.52 0.00 0.00% 0.080 5 3.29 0.02
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) (ft2) (ft2) (%) From Table (Td) I=88.8P1/(10+Td)1.°52 (ft3/sec)
1 2711.52 2499.04 92.16% 0.820 5 6.33 0.32
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) (ft2) (ft2) (%) From Table (Td) I=88.8P1/(10+Td)1.°52 (ft3/sec)
1 2711.52 0.00 0.00% 0.350 5 6.33 0.14
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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 between the architect, landscape architect and civil
engineering teams throughout the design process. Multiple site visits ensured proper understanding of
existing conflicts and opportunities to improve existing drainage patterns.
Principle 2: Use the entire site when planning for storm water quality treatment.
Storm water quality was considered in the design of every part of the site that is being affected by the
proposed construction. Gutters and roof drains will be added to collect runoff from the entire roof and
routed to either to a drywell.
Principle 3: Avoid unnecessary impervious area.
All pervious patios are to be refinished with infiltration beds underneath.
Principle 4: Reduce runoff rates and volumes to more closely match natural conditions.
All runoff from impervious surfaces on the property is collected and routed to a drywell. Because the
drywell is sized to fully detain a 100 year storm event,runoff rates from the property will be greatly
reduced.
Principle 5: Integrate storm water quality management and flood control.
The use of a drywells to collect runoff from all impervious surfaces simultaneously aids in storm water
quality and flood control on the site.
Principle 6: Develop storm water quality facilities that enhance the site,the community and the
environment.
The use of the two-chambered drywell for water quality reduces the runoff of sediment and contaminants
from the site. This reduces the site's effect on the Roaring Fork River and the community.
Principle 7: Use treatment train approach.
Because storm water drywells are sized for full detention,the treatment train approach is not applicable in
this project.
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 to reduce ice buildup. The drywell will be easily
accessible for maintenance.
Principle 9: Design and maintain facilities with public safety in mind.
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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.
4.0 Hydrological Criteria
4.1 Storm Recurrence and Rainfall
The property is located outside of the commercial core and isn't served by any storm drains so this
property classifies as a"Sub-urban area not served by public storm sewer". The total site shall meet
detention requirements for 5 and 100 year historical storm events.
The 1 hour Rainfall depth(Pi)is given in Table 2.2 as 0.64 inches for the 5-year event and 1.23 inches for
the 100-year event. The Intensity in inches per hour for different storm duration(TO is calculated using
the Equation 2.1 from the Aspen URMP.
4.2 Peak Runoff and Storage Volumes Methodology
Using the peak flows for each Major Basin established in section 2.2, storage requirements were
calculated for a 100 year storm event. Because there is no storm infrastructure available for detention
system overflow,the proposed Drywell was sized to fully detain the runoff from a 100 year storm event.
The table below shows storage volume requirements for the proposed detention system.
Full Detention Storage
Basin Total Area Impervious Area Impervious Full Detention Depth Factor of Safety Required Storage BMP
(ft2) (ft2) (%) (in) F.O.S. (ft3)
1 2711.52 2499.04 92.16% 1.23 1 256 Drywell
5.0 Hydraulic Criteria
This property is not connected to the COA's storm water infrastructure. All hydraulics are sized for onsite
infrastructure.
5.1 Inlets
Basins 1 was divided into sub-basins according to which inlet they discharged into. The peak flows for
the 100 year event in each sub-basin were used as the flowrate to size the proposed inlet drains,trench
drains, and slot drains. Equations 4.17 through 4.20 from the URMP were used in these calculations. The
equations incorporate a 50%clogging factor and assume a 40% opening in the grates. A water depth of
0.5 in. was assumed and all the inlets were treated as sumps, as they will be set a minimum of 0.5 in.
below flow lines.
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Sub Basin and Rectangular Inlet Calculations
1 HourlPrl 1.23 m=40% Vy.04(Depress inlet by 0.04'l
Return Period 100 Cg 50% C.0.65
Inlet ID Basin ID Total Area Imp.Area Impervious CValue Time of Concentration Intensity (Max Inlet Type Inlet Width Inlet Length Effective Open Area(Eta.4-20) Inlet Capacity lE84-19) Has Capacity
See(Ds) (ft) (ft) (%) (From Table) CO 1=88.8P,/(10FT1°5-- (ft'/sec) Rectangular Walinches) 4,linches) Ap(1-CdinW.L, Q=C.M2gY, (Yes/No)
SLOT DRAIN-12 1.4 405.15 192.67 47.56% 0.510 5 6.33 0.030 0.25"x 26.7' 0.25 354 0.123 0.123 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.:165
Inlet ID Basin ID Total Area Imp.Area Impervious C Value Concentration Intensity Q Max Inlet Type Diameter Area(EQ,4-20) Inlet Capacity (EQ4.19) Has Capacity
See(D1) (ft'l (ft') (%) From Table (Td) 1=88.8P1/(109-T4l115° fta/sec W0(inches) A,(1-C,)mA Q=CnA,V2gY, (Yes/No)
INLET-Al 1.1 208.77 208.77 100.00% 0.950 5 6.33 0.029 8"Round 8 0.070 0.081 Yes
5.2 Pipes
Pipes used will be PVC SDR-35 with a Manning's coefficient(n) of 0.01. The pipes were sized to
accommodate peak flows for a 100 year event from all contributing sub-basins. A table delineating which
sub-basins contribute to which pipes is shown below.
Storm System Pipes
Pipe System Pipe Contibuting Sub-Basins Design Flow Rate
Qdes
Al 1.1 0.03
A2 1.1 0.03
A A3 1.1 0.03
A4 1.2 _ 0.16
A5 1.1,1.2 0.19
B B1 1.3 0.13
B2 1.3,1.4 0.16
Pipe sizes were tested for hydraulic capacity at 80%of their full flowrate. Design charts giving Qdes,g„/Q
full were downloaded from FHWA and the equations in Section 4.8.4 were used as the basis for these
calculations. Calculated pipe sizes and depth of flow for onsite pipes are shown below.
Pipe Sizing
K=0.462
Design Flow Proposed Manning Required Pipe Diameter Required Pipe Proposed Pipe
Pipe
Rate Slope Coefficient Equation 4-31 Diameter Diameter
Qdes(ft3/sec) S(%) n d(ft)={nodes/KVS}3/8 Dreg(in) Dpr.(in)
Al 0.03 2.00% 0.01 0.13 1.57 4.0
A2 0.03 2.00% 0.01 0.13 1.57 4.0
A3 0.03 2.00% 0.01 0.13 1.57 4.0
A4 0.16 2.00% 0.01 0.25 2.96 4.0
A5 0.19 2.00% I 0.01 0.26 3.16 4.0
B1 0.13 2.00% 0.01 0.23 2.78 4.0
B2 0.16 5.00% 0.01 0.21 2.53 4.0
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Hydraulic Grade Line and Pipe Capacity
Design Flow Proposed 80%of o M Cross
Full Pipe Flow Rate d/D Q Design/ Hydraulic Grade Line Depth of Flow Less Than
Pipe Slope
Rate DiameterPipe Pipe DiamPropetersed Coefficientanning SectionalFullPipe Area Q Full (Depth of Flow) 80%of Pipe Diameter
Q. (ft'/sec) Dpr,(in) S(%) Dp„*.8(in) n A(ft)=n(Dp„/2)' Qra0(ft'/s)=A(1.49/n)((Dr448)u')S'n Qa„/Qrm (from Chart) d(in)=(d/D)*Dp„ (Yes/No)
Al 0.03 4.0 2.00% 3.2 0.01 0.087 0.351 0.08 0.22 0.88 Yes
A2 0.03 4.0 2.00% 3.2 0.01 0.087 0.351 0.08 0.22 0.88 Yes
A3 0.03 4.0 2.00% 3.2 0.01 0.087 0.351 0.08 0.22 0.88 Yes
A4 0.16 4.0 2.00% 3.2 0.01 0.087 0.351 0.45 0.52 2.06 Yes
A5 0.19 4.0 2.00% 3.2 0.01 0.087 0.351 0.53 0.59 2.34 Yes
B1 0.13 4.0 2.00% 3.2 0.01 0.087 0.351 0.38 0.49 1.94 Yes
B2 0.16 4.0 5.00% 3.2 0.01 0.087 0.554 0.29 0.41 1.62 Yes
6.0 Proposed Facilities
6.1 Proposed structures
A Drywell is being utilized to meet URMP requirements for storm water management. Detention
volumes were calculated using equations 5-1 through 5-4 from the City of Aspen URMP.
The proposed Drywell is located at the Northwest corner of the property and collects all runoff from
Basin 1. These drywells have a diameter of 5 ft.,a storage depth of 10 ft. each with 18 in. of screened
rock surrounding the drywells,giving the drywell a storage capacity of 288 ft3. The total capacity for the
drywells exceeds the required detention volume of 259 ft3 for Basin 1.
Drywell Storage
Drywell Basins Diameter Storage Depth Internal Volume External(18"of Screened Rock)Volume Total Capacity Required Capacity
(Name) (#) D(ft) H(ft) n*H*(D/2)Z)(ft3) 0.3*n*H*((D/2)+1.5)2-(D/2)2)(ft3) (ft3) (ft)
Drywell 1 5 10 196 92 288 259
6.2 Infiltration
6.2.1 Drywell
Part of the Analysis is to ensure that the drainage structures can completely drain within 24 hours. The
minimum depth of perforation a drywell must have is 4 ft. Below is a calculation showing that there is
enough perforation area for the drywell to drain within 24 hours using the percolation rate determined
from the"Infiltration Rate Letter"on April 10,2017 as part of CTL Thompson's Geotechnical
Consultation for the project. The determined percolation rate of 3 inches per hour for the entire site.
Section 8.5.4.2 was referenced for these calculations.
Drywell Infiltration I
Name Diameter Perforation Height Perforated Area Total Capacity Infiltration Rate Infiltration Time Volume Infiltrated in 24 Hours
(Name) D(ft) H(ft) A(ft')=3.14*D*H V(ft) I(in/hr) T(hr)=V/(A*I/12) Vturei(ft3)=V*T
Drywell 5 4 62.83 288.24 3 18.35 5289.22
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7.0 Operation and Maintenance
7.1 Drywell
Drywells must be inspected and maintained quarterly to remove sediment and debris that has washed into
them. Minimum inspection and maintenance requirements include the following:
• Inspect drywells at least four times a year and after every storm exceeding 0.5 inches.
• Dispose of sediment, debris/trash, and any other waste material removed from a drywell at suitable
disposal sites and in compliance with local, State, and Federal waste regulations.
• Routinely evaluate the drain-down time of the drywell to ensure the maximum time of 24 hours is
not being exceeded.If drain-down times are exceeding the maximum,drain the drywell via pumping
and clean out the percolation area (the percolation barrel may be jetted to remove sediment
accumulated in perforations. If slow drainage persists,the system may need to be replaced.
7.2 Pervious Paver Area
As per section 8.5.3.1 of the URMP,the following schedule will be undertaken by the owners of the
property to achieve long term performance of the BMP's.
Table 8.8 Maintenance Recommendations for Modular Block Pervious Pavement
Required Action Maintenance Objective Frequency of Action
and Action
Debris and litter Accumulated material should be Routine—As needed.
removal removed as a source control
measure.
Sod maintenance If sandy loam turf is used, provide Routine—As dictated by inspection.
lawn care, irrigation system,and
inlay depth maintenance as needed.
Inspection Inspect representative areas of Routine and during a storm event to
surface filter sand or sandy loam turf ensure that water is not bypassing these
for accumulation of sediment or poor surfaces on frequent basis by not
infiltration. infiltrating into the pavement.
Rehabilitating sand To remove fine sediment from the Routine—Sweep the surface annually
infill surface top of the sand and restore its and, if need be,replace lost sand infill to
infiltrating capacity. bring its surface to be Y.below the
adjacent blocks.
Replacement of Remove,dispose,and replace Non-routine—When it becomes evident
Surface Filter Layer surface filter media by pulling out turf that runoff does not rapidly infiltrate into
plugs or vacuuming out sand media the surface. May be as often as every two
from the blocks. Replace with fresh year or as little as every 5 to 10 years.
ASTM C-33 sand or sandy loam turf
plugs,as appropriate.
Replace modular Restore the pavement surface. Non-routine—When it becomes evident
block pavement Remove and replace the modular that the modular blocks have deteriorated
pavement blocks,the sand leveling significantly. Expect replacement every 10
course under the blocks and the infill to 15 years dependent on use and traffic.
media when the pavement Surface
shows significant deterioration.
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8.0 Appendices
Drawings 11x17
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