HomeMy WebLinkAboutFile Documents.1300 Red Butte Dr.0272.2018 (2).ARBK1
Drainage Report
1300 RED BUTTE
ASPEN, CO
October 18, 2018
Comment Responses: January 3, 2019
Prepared by
Danny Stewart, P.E.
Roaring Fork Engineering
592 Highway 82
Carbondale, CO 81623
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Drainage Report
1300 RED BUTTE
ASPEN, CO
I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 1300 RED BUTTE DR.
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.
DANNY STEWART, P.E.
RFE Project # 2018-52
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Table of Contents
1.0 General .................................................................................................................................................... 4
1.1 Existing Site ......................................................................................................................................... 4
1.2 Proposed Conditions ........................................................................................................................... 5
1.3 Previous Drainage Studies .................................................................................................................. 5
1.4 Offsite Drainage & Constraints ........................................................................................................... 5
2.0 Drainage Basins and Sub‐basins .............................................................................................................. 5
2.1 Drainage Basins ................................................................................................................................... 6
2.2 Peak Discharge Calculations ................................................................................................................ 6
3.0 Low Impact Site Design ........................................................................................................................... 7
3.1 Principles ............................................................................................................................................. 7
4.0 Hydrological Criteria ............................................................................................................................... 9
4.1 Storm Recurrence and Rainfall ........................................................................................................... 9
4.2 Peak Runoff Methodology .................................................................................................................. 9
5.0 Hydraulic Criteria .................................................................................................................................... 9
5.1 Inlets .................................................................................................................................................. 10
5.2 Pipes .................................................................................................................................................. 10
6.0 Proposed Facilities ................................................................................................................................ 11
6.1 Drywell .............................................................................................................................................. 11
7.0 Operation and Maintenance ................................................................................................................. 12
7.1 Drywell .............................................................................................................................................. 12
7.2 Gutters and downspouts ................................................................................................................... 12
8.0 Appendices ............................................................................................................................................ 12
Drawings 11x17 ....................................................................................................................................... 12
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1.0 General
1.1 Existing Site
The following report is an evaluation of 1300 Red Butte Dr. It is located along the west bank of the Roaring
Fork River on the north side of Red Butte Dr. The lot is approximately 0.85 acres and consists of landscaped
areas, well vegetated lawn, mature trees, patios, driveway, auto court and the existing residence. Steep
slopes extend north of the house downhill to the Roaring Fork River. No disturbance along the river bank
is permitted or proposed for this project. The 15‐foot top of bank set back will be the do not cross line for
construction. No sidewalk or curb and gutter are present in front of the residence, however there is a
small drainage ditch running parallel along the road. All utilities are located in or near Red Butte Dr. and
are currently connected to the residence. No alterations to the existing utilities are proposed.
View of the site from West Bleeker Street
Roof drainage currently is captured by gutters and downspouts. On the south side of the structure,
outside of the top of bank set back, the down spouts discharge or daylight onto the lawn and
landscaping. The down spouts are piped underground on the north side of the house beyond the top of
bank. They are then routed around the east side of the home and then south toward the front of the
building. Roto Rooter was contracted to do an investigation on the existing system to determine the
routing and depth of the existing storm lines.
A Geotechnical Report was produced on March 5th, 2018 by HP Kumar. Two exploratory borings were
drilled 31 and 26 feet below grade to perform the analysis, resulting in silty sands and gravels with
cobbles over medium dense silty sands with gravels. Groundwater was encountered between 20‐26
feet below grade. A percolation test was conducted on February 16th, 2018 by HP Kumar, and the site
was determined to have an infiltration rate of 60 inches per hour or 1 minute per inch.
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1.2 Proposed Conditions
The proposed development is limited to the south of the 15‐foot top of bank set back and includes
construction of a new auto court, driveway, landscape and walkways. Two drywells will be implemented
to mitigate for the impervious area runoff.
This project is classified as a ‘Major Project’ per Table 1.1 of the URMP due to the proposed
development being over 1,000 square feet (sf) and disturbing more than 25% of the site. As a Major
Project, the area of disturbance has implications for the design. 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. Onsite storm
infrastructure has been sized for conveyance of the 100‐year storm event and to retain the full
detention storage for the 100‐year rainfall depth of 1.23 inches for all off the impervious area. Full
detention is proposed due to the inability to overflow to the north over the top of bank and down to the
Roaring Fork River. There is also no City storm infrastructure to connect to.
The existing roof drainage that is being collected by gutters and downspouts on the north side of the
house is currently being piped around the east side of the structure to the front lawn area. This under
ground piping will be tied into the new proposed system just beyond the top of bank set back as to not
disturbed the top of bank area. The invert for this tie in has been determined by a field investigation by
Roto Rooter with a TV/Cam line. The location and depths of the pipes were determined. A schematic
was provided to RFE. This tie in point is just beyond the top of back set back where disturbance is
allowed. The depth to the invert was determined to be 4.5’ down from existing grade. This will be the
starting point for the east storm system. All pipes have been sized to accommodate this additional
drainage from the existing roof drains.
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. This property is not served by City
storm sewer infrastructure.
1.4 Offsite Drainage & Constraints
Red butte Dr. has its own drainage that keeps runoff from flowing onto the property via roadside swales
and ditches that collect runoff from the road. No other offsite drainage flows onto the property.
2.0 Drainage Basins and Sub‐basins
The impervious area on the site was turned into one major drainage basin, which was then subdivided
into smaller sub‐basins. A drainage exhibit in the permit set illustrates the basin and sub‐basin
delineations. The exhibit 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.
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No sub‐basins that drain to the north side of the house were included in the analysis because this runoff
will not be collected. This is because these areas are beyond the top of bank set back and no disturbance
is permitted here. These areas are still considered when calculating the total detention.
2.1 Drainage Basins
The overall impervious area is 9435 square feet (sf), and consists of roof area, patios, the proposed auto
court, driveway and walkways. Run off that can be captured and routed to the south without disturbing
north of the 15‐foot top of bank set back is conveyed to the three proposed drywells. The drywells have
capacity for full detention for the entire basin. The total 100% impervious area of the site is used due to
the total detention method of storm retention.
2.2 Peak Discharge Calculations
Peak flows were calculated the Basin 1 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, therefore 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 within the
URMP was referenced when solving for the Rainfall Intensity.
I = 88.8P1/(10+Td )1.052
Runoff Coefficients (C), a function of the Soil Group (in this case C) and the percentage of impervious
area within each 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
Basin 1. The allowable discharge flow rate or 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
within the URMP.
Qp = CIA
Where:
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.
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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. 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 landscape architect during the design phase.
Due to the existing trees and vegetation, coordination in the design process was key to ensure any
disturbance was strategically planned.
Principle 2: Use the entire site when planning for storm water quality treatment.
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) (ft
2)(ft2)(%)From Table (Td) I=88.8P1/(10+Td)1.052 (ft3/sec)
OVERALL IMPERVIOUS AREA 9435.43 9435.43 100.00% 0.896 5 3.29 0.64
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) (ft
2)(ft2)(%)From Table (Td) I=88.8P1/(10+Td)1.052 (ft3/sec)
OVERALL IMPERVIOUS AREA 9435.43 0.00 0.00% 0.080 5 3.29 0.06
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)
OVERALL IMPERVIOUS AREA 9435.43 9435.43 100.00% 0.950 5 6.33 1.30
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)
OVERALL IMPERVIOUS AREA 9435.43 0.00 0.00% 0.350 5 6.33 0.48
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Because of the size of the parcel, the design was able to allow the proposed stormwater detention
system to be far from the residence. Disturbance was also limited to south of the 15‐foot top of bank set
back so infrastructure is limited to one side of the residence.
Principle 3: Avoid unnecessary impervious area.
All proposed impervious area is for walkways and driving surfaces.
Principle 4: Reduce runoff rates and volumes to more closely match natural conditions.
To the extent possible, runoff will be infiltrated into the ground. The runoff over impervious areas will
be directed to the two drywells that are sized for full detention. The Roaring Fork River is also just
downhill of the property so there are no downstream properties that will be impacted.
Principle 5: Integrate storm water quality management and flood control.
The proposed drywells are sized for total detention which is greater than the water quality capture
volume and the flood detention volume required by the City.
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 direct discharged flows being introduced to the Roaring Fork River.
Principle 7: Use treatment train approach.
The proposed inlets for the storm drain system are located away from the residence, allowing the storm
water from the downspouts to disperse into the landscaping before entering the proposed inlets. Inlets
are also equipped with sumps to trap sediment and debris that can easily be removed.
Principle 8: Design sustainable facilities that can be safely maintained.
Grates and screens will be placed over inlets and downspouts to provide a barrier against vermin and
debris. Drainage systems were simply designed so maintenance is minimized. Infrastructure will be just
below grade 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.
There are no steep slopes or large drops proposed. Walkways are raised a maximum of 6 inches from
finished grade. Walkways will be snowmelted to prevent dangerous conditions for pedestrians.
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4.0 Hydrological Criteria
4.1 Storm Recurrence and Rainfall
The property is not in the commercial core and is not served by any City curb and gutter or storm sewer
so this property classifies as a “Sub‐urban area not served by public storm sewer”. Due to this, the 5 and
100‐year events were analyzed.
4.2 Peak Runoff Methodology
No proposed surface runoff will be directed to the banks of the Roaring Fork River as the north side of the
residence will remain undisturbed from its current condition. Therefore, due to the lack of City storm
infrastructure, full detention is necessary. The 100‐year storm rainfall depth for this region of 1.23 inches
was used.
5.0 Hydraulic Criteria
Sub‐basins were delineated per the design points of concentrations created by existing downspouts,
proposed inlets, piping and drywells. The 100‐year peak flows for each sub‐basin was calculated. These
sub‐basins are the only basin that the design is able to mitigate for based on the allowable area of
disturbance delineated by 15‐foot the top of bank set back. The roof areas that are beyond the set back
or drain to downspouts beyond the setback will be connected into the proposed storm system. This
connection will occur south of the 15‐foot top of bank set back. No trenching or grading will occur north
of the setback. These impervious areas have been accounted for in the sizing of the drywells.
Full Detention Storage
Basin Total Area Impervious Area Impervious Full Detention Depth Factor of Safety Required Storage BMP
(ft2)(ft2) (%) (in) F.O.S. (ft
3)
OVERALL
IMPERVIOUS
AREA
9435.43 9435.43 100.00% 1.23 1 967
DRYWELLS
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 1310 1310 100.00% 0.950 5 6.33 0.18
1.2 170.37 170.37 100.00% 0.950 5 6.33 0.02
1.3 572.42 572.42 100.00% 0.950 5 6.33 0.08
1.4 1147.72 1147.72 100.00% 0.950 5 6.33 0.16
1.5 244.41 244.41 100.00% 0.950 5 6.33 0.03
1.6 575.9 575.9 100.00% 0.950 5 6.33 0.08
1.7 150.0 150.0 100.00% 0.950 5 6.33 0.02
1.8 3200.4 3200.4 100.00% 0.950 5 6.33 0.44
1.9 334.0 334.0 100.00% 0.950 5 6.33 0.05
1.10 368.2 368.2 100.00% 0.950 5 6.33 0.05
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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 0.25’ was assumed because all of the inlets will be installed within depressions or low points
created by the grading and landscaping. Below is a summary of each circular inlet being tested for
capacity against their tributary basin.
5.2 Pipes
The pipes were analyzed by calculating and summing the flow from the sub‐basins entering them. Below
is a table showing what sub‐basin peak flows 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% full. Depth of flow was also calculated in the spread sheets below. The pipes are all PVC with a
Manning’s coefficient of 0.010.
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.
Sub Basin and Circular Inlet Calculations
1 Hour(P1)1.23 m=40% Ys=.25 (Depress inlet by 0.25')
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)
INLET‐E1 1.1 1310.00 1310.00 100.00% 0.950 5 6.33 0.181 8" Round 8 0.070 0.182 Yes
INLET‐E2 1.1 1310.00 1310.00 100.00% 0.950 5 6.33 0.181 8" Round 8 0.070 0.182 Yes
INLET‐E3 1.2, 1.3 742.79 742.79 100.00% 0.950 5 6.33 0.102 8" Round 8 0.0700.182Yes
INLET‐E4 1.4, 1.5 1392.13 1392.13 100.00% 0.950 5 6.33 0.192 10" Round 10 0.109 0.284 Yes
INLET‐W1 1.6,1.7 725.80 725.80 100.00% 0.950 5 6.33 0.100 8" Round 8 0.070 0.182 Yes
INLET‐W2 1.8 3200.40 3200.40 100.00% 0.950 5 6.33 0.441 15" Round 15 0.245 0.640 Yes
INLET‐W4 1.9,1.10 702.18 702.18 100.00% 0.950 5 6.33 0.097 8" Round 8 0.0700.182Yes
Storm System Pipes
Pipe Contibuting Sub‐
Basins
Design Flow Rate
(Qdes)
E1 1.1 0.18
E2 1.1 0.18
E3 1.1‐1.3 0.28
E4 1.1 ‐1.5 0.48
W1 1.6, 1.7 0.10
W2 1.8 0.44
W3 1.6‐1.8 0.54
W4 1.6‐1.10 0.64
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6.0 Proposed Facilities
6.1 Drywell
Three proposed drywells are designed to have capacity for full detention. Two drywells are 6 feet in
diameter and one drywell is 4 feet in diameter. All the drywells have a 10‐foot storage depth.
Perforations have been proposed for the bottom 4 feet of each drywell. Below are the storage capacities
for the drywells.
Using the infiltration rate stated in the geotechnical report by HP Geotech on March 5th, 2018 of 1 inch
per minute (60 in/hr), the drain down time of the drywells were determined. The 24‐hour infiltration
volume was also calculated to ensure the drywells drain within 24‐hours. These drywells will be
hydraulically connected to act as a single point of concentration and storage BMP.
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)
E1 0.18 1.00%0.01 0.30 3.56 4.0
E2 0.18 1.00% 0.01 0.30 3.56 4.0
E3 0.28 1.00% 0.01 0.35 4.21 6.0
E4 0.48 1.00% 0.01 0.43 5.11 6.0
W1 0.10 1.00% 0.01 0.24 2.85 4.0
W2 0.44 1.00% 0.01 0.41 4.97 6.0
W3 0.54 1.00% 0.01 0.45 5.37 6.0
W4 0.64 1.00% 0.01 0.48 5.71 6.0
Pipe Sizing
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)
E1 0.18
41.00%3.2 0.01 0.087 0.248 0.73 0.70 2.81 Yes
E2 0.18
41.00%3.2 0.01 0.087 0.248 0.73 0.70 2.81 Yes
E3 0.28 61.00%4.8 0.01 0.196 0.731 0.39 0.49 2.91 Yes
E4 0.48 61.00%4.8 0.01 0.196 0.731 0.65 0.66 3.96 Yes
W1 0.10 41.00%3.2 0.01 0.087 0.248 0.40 0.50 2.00 Yes
W2 0.44 61.00%4.8 0.01 0.196 0.731 0.60 0.63 3.78 Yes
W3 0.54 61.00%4.8 0.01 0.196 0.731 0.74 0.70 4.22 Yes
Hydraulic Grade Line and Pipe Capacity
Drywell Basins Diameter Storage Depth Internal Volume External (18" of Screened Rock) Volume Total Capacity Total Provided Capacity Total 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)(ft3)
1 6 10 283 106 388.8
2 6 10 283 106 388.8
3 4 10 126 78 203.4
981.0 967OVERALL IMPERVIOUS AREA
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) = (V/T)*24
1 6 4 75.40 505.40 60 1.34 9047.79
2 6 4 75.40 505.40 60 1.34 9047.79
3 4 4 50.27 505.40 61 1.98 6132.39
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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 Gutters and downspouts
The existing gutter and downspouts should be clean and inspected before they are connected to the
new system. No debris from the existing gutters or piping should be allowed to enter the new system.
The entire piping system should be inspected and flushed periodically to avoid clogging and flooding.
8.0 Appendices
Drawings 11x17
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