HomeMy WebLinkAboutFile Documents.433 W Bleeker St.0170.2017 (10).ARBK1
Drainage Report
433 WEST BLEEKER
ASPEN, CO
June 29th, 2017
Revised: January 10th, 2018
Prepared by Richard Goulding, P.E.
Roaring Fork Engineering
592 Highway 82
Carbondale, CO 81623
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Drainage Report
433 WEST BLEEKER
ASPEN, CO
I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 433 WEST BLEEKER, ASPEN,
CO 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.
RICHARD GOULDING, P.E.
RFE Project # 2017-02
04/11/2018
Reviewed by Engineering
04/16/2018 1:36:01 PM
"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.
3
Table of Contents
1.0 General .................................................................................................................................................... 4
1.1 Existing Site ......................................................................................................................................... 4
1.2 Proposed Conditions ........................................................................................................................... 4
1.3 Previous Drainage Studies .................................................................................................................. 5
1.4 Offsite Drainage & Constraints ........................................................................................................... 5
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 ........................................................................................................................... 6
3.1 Principles ............................................................................................................................................. 7
4.0 Hydrological Criteria ............................................................................................................................... 8
4.1 Storm Recurrence and Rainfall ........................................................................................................... 8
4.2 Peak Runoff Methodology .................................................................................................................. 8
5.0 Hydraulic Criteria .................................................................................................................................... 8
5.1 Inlets .................................................................................................................................................... 9
5.2 Pipes .................................................................................................................................................... 9
6.0 Proposed Facilities ................................................................................................................................ 11
6.1 Drywell .............................................................................................................................................. 11
7.0 Operation and Maintenance ................................................................................................................. 12
7.1 Drywell .............................................................................................................................................. 12
7.2 Pervious Paver Area .......................................................................................................................... 12
8.0 Appendices ............................................................................................................................................ 13
Drawings 11x17 ....................................................................................................................................... 13
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1.0 General
1.1 Existing Site
The following report is an evaluation of an undeveloped site located to the south of West Bleeker Street
between North 3rd and North 4th Street in Aspen, Colorado. The property is addressed at 433 West Bleeker
Street, and is located between two residential homes. To the north is the West Bleeker Street Right-of-
Way, and to the south is an alley Right-of-Way. The lot consists of 6,000 square feet and consists of a
grassy field with a minor slope downhill to the northeast. Large mature trees are scattered along the
eastern half of the property, and a large mature tree is located north of the lot and the right of way. No
sidewalk is installed in front of the residence, but curb and gutter was recently installed. All utilities are
located in the alley, excluding the water line, which is located in Bleeker Street.
View of the site from West Bleeker Street
A Geotechnical Report was produced on March 13th, 2007 by CTL Thompson. Two exploratory pits were
dug 10.5 feet below grade to perform the analysis, resulting in 1 foot of sandy clay “topsoil” over clayey
to silty gravel with cobbles and boulders. No groundwater was encountered. A percolation test was
conducted on December 19th, 2017 by HP Kumar, and the site was determined to have an infiltration
rate of 6 inches per hour.
1.2 Proposed Conditions
This project is classified as a ‘Major Project’ as per Table 1.1 of the URMP. This is because the proposed
development is over 1,000 square feet (sf) and disturbs an area of approximately 7,400 sf., which is over
98% of the site. This has implications for the design. The intent of this report is to demonstrate
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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. Onsite storm
infrastructure has been sized for conveyance and full detention storage of a 100 year event.
The proposed residence is 6,210 square feet (sf) with a 2,500 sf footprint. Extensive landscaping will
extend to the property line surrounding the residence. Above patios and walkways will be located on
the east and north sides of the structure. A two car garage will be accessed from the alley with a
snowmelted concrete strip. Cuts of up to 12’ deep are expected for the sub-level excavation. For
drainage, one drywell will capture all runoff from roofs and hardscape and is sized to have capacity for
full detention.
Utility connections will be made to the existing infrastructure.
1.3 Previous Drainage Studies
The City of Aspen updated their URMP in 2001 and the property is within the boundaries of the study.
The study indicates that the property is not within a Mudflow Area.
1.4 Offsite Drainage & Constraints
No offsite basins effect the site, so no analysis was required.
2.0 Drainage Basins and Sub-basins
The site was divided into one major drainage basin, which was then subdivided into smaller sub-basins. A
Drainage Exhibit in the appendices illustrates the basin and sub-basin delineations. It lists Impervious
Areas, Runoff Coefficients, and Peak Flows.
The sub-basins were created to calculate the concentrated flow from each impervious area, including
patios, decks and roofs. These sub-basin peak flows were then used to size the proposed infrastructure.
2.1 Drainage Basins
Basin 1 is 3,647 square feet (sf), 87% impervious, and consists of roof drains, inlets, trench drains, and
the landscaping surrounding the residence. Runoff from this basin is collected and conveyed to the
drywell. This drywell has capacity for full detention for the entire basin.
2.2 Peak Discharge Calculations
The peak flows were calculated for each Major Basin for the 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 (P1), given in Table 2.2 as
0.64 inches for the 5-year event and 1.23 inches for the 100-year event. Equation 2.1 was referenced
when solving for the Rainfall Intensity.
I = 88.8P1/(10+Td )1.052
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Runoff Coefficients (C), a function of the Soil Group (in this case B) and the percentage of impervious
area within each 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. Q allowable was calculated the same way except each basin was treated as
undeveloped or 100% pervious. The Peak Discharge (Qp) is given by equation 3.1.
Qp= CIA
Qp= Peak Discharge (cfs)
C= Runoff Coefficient (Unitless)
I= Rainfall intensity (inches per hour)
A= Area (Acres)
Peak flow values were used to calculate the size of the proposed detention and conveyance structures,
such as drywells, inlets and piping. The tables below contain the peak flows for developed and
undeveloped conditions for 5 and 100 year storm events.
3.0 Low Impact Site Design
Low Impact Development (LID) aims to mimic the natural pre-development hydrologic pattern. The goal
is to manage storm water as close to its source as is possible. This entire developed site is approximately
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)I=88.8P1/(10+Td)1.052 (ft3/sec)
1 3646.81 3189.17 87.45%0.650 5 3.29 0.18
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)1.052 (ft3/sec)
1 3646.81 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)1.052 (ft3/sec)
1 3646.81 3189.17 87.45%0.750 5 6.33 0.40
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)1.052 (ft3/sec)
1 3646.81 0.00 0.00%0.350 5 6.33 0.19
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87% impervious. The treatment train approach is used on all runoff to increase water quality and
percolation.
3.1 Principles
Principle 1: Consider storm water quality needs early in the design process.
The Grading and Drainage design was coordinated with the architect during the design phase. Due to
the lack of space for the project, coordination in the design process was key.
Principle 2: Use the entire site when planning for storm water quality treatment.
Because of the size and limitations of the parcel, it was necessary in the design process to use the site
efficiently. Grading away from the residence for the project was difficult to achieve with the flat site.
Principle 3: Avoid unnecessary impervious area.
Green roof has reduced the impervious area, along with pervious paver systems for the walkways
surrounding the residence.
Principle 4: Reduce runoff rates and volumes to more closely match natural conditions.
The runoff will all be infiltrated into the ground, as the drywells are sized for full detention. There will be
no runoff leaving the site.
Principle 5: Integrate storm water quality management and flood control.
The Drywells are being used for water quality, which in itself increases flood control. The drywells will
eliminate the peak flow as there is no runoff leaving the site.
Principle 6: Develop storm water quality facilities that enhance the site, the community and the
environment.
The proposed design encourages replenishing groundwater, and does not introduce any runoff into the
city infrastructure. This reduces the flows being introduced to the Roaring Fork River.
Principle 7: Use treatment train approach.
Several downspouts disperse into the landscaping before entering the proposed inlets. The emergency
overflow from the drywell will release into a grass field, which will treat the water before entering the
city storm system.
Principle 8: Design sustainable facilities that can be safely maintained.
Screens will be placed over downspouts to provide a barrier against vermin and debris. Drainage
systems were simply designed so maintenance is minimized. Infrastructure will be just below grade
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providing little labor for maintenance. The Owner will sign a maintenance agreement as part of their
Certificate of Occupancy.
Principle 9: Design and maintain facilities with public safety in mind.
Proper drainage and grading of the driveway and walkways reduces ice buildup and dangerous icy
conditions. All grading was done with safety in mind.
4.0 Hydrological Criteria
4.1 Storm Recurrence and Rainfall
The property is not in the commercial core and is served by any city curb and gutter so this property
classifies as a “Sub-urban area served by public storm sewer”. However, due to limitations on the site,
the curb and gutter cannot be utilized, so the site cannot disperse into the curb and gutter. Due to this,
the 5 and 100 year events were analyzed.
4.2 Peak Runoff Methodology
This site could drain to city storm infrastructure, however due to site limitations and the inability to
disperse into the curb and gutter, full detention is necessary. To determine these capacities, the rainfall
from a 100 year storm that is collected on all impervious areas must be detained. No detention is required
for pervious areas. Below is a summary of the required storage.
5.0 Hydraulic Criteria
Sub-basins were delineated per the design points of concentrations created by roof drains and inlets.
Pipe networks were then created connecting the sub-basins and conveying the flows to the overall point
of concentration for the basin. The 100 year peak flow for each sub-basin was calculated.
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 3646.81 3189.17 87.45%1.23 1 327 DRYWELL
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5.1 Inlets
The 100 year peak flows were used in the sizing of inlets. Equations 4-17 to 4-20 from the URMP were
used in the analysis. They incorporate a 50% clogging factor and 40% opening in the grates. A water
depth of 0.04’ was assumed and all the inlets were treated as sumps as they will be set a minimum of
.04‘(½ Inch) below the flow lines. Below is a summary of each square inlet being tested for capacity
against their tributary basin, and below that is every circular inlet calculation.
5.2 Pipes
The pipes were analyzed by calculating the flow from the sub basins entering them. Below is table which
groups what sub basins are conveyed in each pipe. The TOC is below 5 minutes for all sub-basins, so a
reduction was not taken for the intensity. They were tested for hydraulic capacity at 80% of pipe
diameter. Depth of flow was also calculated in the spread sheets below. The pipes are all SDR 35 PVC
with a manning’s coefficient of .01.
Design Q design / Q full charts were downloaded from FHWA. The equations in Section 4.8.4 was used as
the basis for these calculations.
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)Ai (ft2)Ai/At (%)From Table (Td)I=88.8P1/(10+Td)01.052 Qsub (ft3/sec)
1.1 493.30 493.30 100.00%0.950 5 6.33 0.07
1.2 640.34 640.34 100.00%0.950 5 6.33 0.09
1.3 160.16 160.16 100.00%0.950 5 6.33 0.02
1.4 464.71 333.54 71.77%0.620 5 6.33 0.04
1.5 198.99 198.99 100.00%0.950 5 6.33 0.03
1.6 242.64 142.61 58.77%0.540 5 6.33 0.02
1.7 366.46 366.46 100.00%0.950 5 6.33 0.05
1.8 863.79 541.95 62.74%0.570 5 6.33 0.07
1.9 85.83 85.83 100.00%0.950 5 6.33 0.01
1.10 284.10 284.10 100.00%0.950 5 6.33 0.04
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)(ft2)(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)
B3-TRENCH DRAIN 1.9 85.83 85.83 100.00%0.950 5 6.33 0.012 4" x 25.50'4 306 1.700 1.706 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)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 ft3/sec Wo (inches)Ae=(1-Cg)mA Q=CoAe√2gYs (Yes/No)
B1-INLET 1.7 366.46 366.46 100.00%0.950 5 6.33 0.051 8" Round 8 0.070 0.081 Yes
B2-INLET 1.8 863.79 541.95 62.74%0.570 5 6.33 0.071 8" Round 8 0.070 0.081 Yes
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Storm System Pipes
Pipe System Pipe Contibuting Sub-Basins Peak Flows (CFS)
A A1 1.1 0.07
A2 1.1, 1.2 0.16
A3 1.1-1.3 0.18
A4 1.1-1.4 0.22
A5 1.1-1.5 0.25
A6 1.1-1.6 0.27
B B1 1.7 0.05
B2 1.7, 1.8 0.12
B3 1.7-1.9 0.13
B4 1.7-1.10 0.17
K=0.462
Pipe Design Flow
Rate
Proposed
Slope
Manning
Coefficient
Required Pipe Diameter
Equation 4-31
Required Pipe
Diameter
Proposed Pipe
Diameter
Qdes (ft3/sec) S (%)n d (ft) = {nQdes/K√S}3/8 Dreq (in) Dpro (in)
A1 0.07 1.50%0.01 0.19 2.29 4.0
A2 0.16 1.50%0.01 0.26 3.12 4.0
A3 0.18 2.00%0.01 0.26 3.11 4.0
A4 0.22 2.00%0.01 0.28 3.37 6.0
A5 0.25 1.50%0.01 0.31 3.71 6.0
A6 0.27 2.00%0.01 0.30 3.62 6.0
B1 0.05 1.50%0.01 0.17 2.05 4.0
B2 0.12 1.50%0.01 0.24 2.85 4.0
B3 0.13 1.50%0.01 0.25 2.95 4.0
B4 0.17 2.00%0.01 0.26 3.07 4.0
Pipe Sizing
Hydraulic Grade Line and Pipe Capacity
Pipe Design Flow
Rate
Proposed Pipe
Diameter Slope 80% of Proposed
Pipe Diameter
Manning
Coefficient
Full Pipe Cross
Sectional Area Full Pipe Flow Rate Q Design /
Q Full d/D Hydraulic Grade Line
(Depth of Flow)
Depth of Flow Less Than
80% of Pipe Diameter
Qdes (ft3/sec) Dpro(in)S (%)Dpro*.8 (in)n A (ft) = π (Dpro/2)2 Qfull (ft3/s) = A(1.49/n)((Dpro/48)2/3)S1/2 Qdes/Qfull (from Chart)d (in) = (d/D)*Dpro (Yes/No)
A1 0.07 4.0 1.50%3.2 0.01 0.087 0.304 0.22 0.35 1.40 Yes
A2 0.16 4.0 1.50%3.2 0.01 0.087 0.304 0.51 0.57 2.28 Yes
A3 0.18 4.0 2.00%3.2 0.01 0.087 0.351 0.51 0.57 2.28 Yes
A4 0.22 6.0 2.00%4.8 0.01 0.196 1.034 0.21 0.35 2.10 Yes
A5 0.25 6.0 1.50%4.8 0.01 0.196 0.895 0.28 0.41 2.43 Yes
A6 0.27 6.0 2.00%4.8 0.01 0.196 1.034 0.26 0.38 2.28 Yes
B1 0.05 4.0 1.50%3.2 0.01 0.087 0.304 0.17 0.31 1.25 Yes
B2 0.12 4.0 1.50%3.2 0.01 0.087 0.304 0.40 0.50 2.00 Yes
B3 0.13 4.0 1.50%3.2 0.01 0.087 0.304 0.44 0.52 2.06 Yes
B4 0.17 4.0 2.00%3.2 0.01 0.087 0.351 0.49 0.55 2.20 Yes
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6.0 Proposed Facilities
6.1 Drywell
The proposed drywell collects all runoff from the site and is designed to have capacity for full detention.
The drywell is 5’ in diameter and 17’ deep, with 13’ of storage capacity. It is 4.3’ away from the
foundation footing for the lightwell and almost 12’ away from the footing for the basement foundation.
A 30-mil PVC liner has been proposed between the drywell and foundation. The bottom of the drywell is
4 feet below the bottom of footing to prevent infiltration from occurring near the foundation.
Perforations have been proposed beginning at 7898 feet down to 7894 feet. In a 100-yr event the
drywell will fill to an elevation of 7904.50, and the system will have an additional 43 cubic feet of
storage beyond city requirements before reaching the lowest invert of the system at 7906.70.
Using the infiltration rate concluded in the report by HP Geotech on December 19th, 2017, an infiltration
rate 6 inches per hour was used to determine the drain down time of the drywell.
Pipe Design Flow
Rate
Proposed Pipe
Diameter Slope d/D Manning
Coefficient Rh/D Hydraulic Radius Exit Velocity
(ID)Qdes (ft3/sec) Dpro(in)(%)(from Chart)n (from Chart)Rh (ft) = (Rh/D) Dpro V (ft/sec) = [1.49/n] Rh2/3 √S
A1 0.068 4.0 1.50%0.35 0.01 0.19 0.19 6.11
A2 0.156 4.0 1.50%0.57 0.01 0.27 0.27 7.63
A3 0.178 4.0 2.00%0.57 0.01 0.27 0.27 8.81
A4 0.220 6.0 2.00%0.35 0.01 0.19 0.19 7.05
A5 0.248 6.0 1.50%0.41 0.01 0.21 0.21 6.53
A6 0.267 6.0 2.00%0.38 0.01 0.21 0.21 7.35
B1 0.051 4.0 1.50%0.31 0.01 0.18 0.18 5.72
B2 0.122 4.0 1.50%0.50 0.01 0.25 0.25 7.24
B3 0.134 4.0 1.50%0.52 0.01 0.25 0.25 7.30
B4 0.173 4.0 2.00%0.55 0.01 0.26 0.26 8.69
Exit Velocities
Drywell Storage
Drywell Basins Diameter Storage Depth Internal Volume External (18" of Screened Rock) Volume Total Capacity Required Capacity
(Name)(#)D (ft)H (ft)π*H*(D/2)2) (ft3)0.3*π*H*((D/2)+1.5)2 - (D/2)2) (ft3)(ft3)(ft3)
DRYWELL 1 5 13 255 119 375 327
Drywell Infiltration
Name Diameter Perforation Height Perforated Area Total Capacity Infiltration Rate Infiltration Time Volume Infiltrated in 24 Hours
(Name)D (ft)H (ft)A (ft2) = 3.14*D*H V (ft3)I (in/hr)T (hr) = V/(A*I/12)Vtotal (ft3) = A*I/12*24
DRYWELL 5 4 62.83 375.00 6 11.94 753.98
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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. A maintenance plan shall be submitted to the City in the Drainage Report 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:
• 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.
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8.0 Appendices
Drawings 11x17
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