HomeMy WebLinkAboutFile Documents.1130 Black Birch Dr.0096.2018 (12).ARBKAspen City Offices i
Preliminary Drainage Report
Drainage Report
1130 BLACK BIRCH DRIVE
ASPEN, CO
March 22, 2018
Prepared by
Daniel Stewart, P.E.
Roaring Fork Engineering
592 Highway 133
Carbondale, CO
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Table of Contents
1.0 Existing Conditions ........................................................................................................................... 1
1.1 Site Description ................................................................................................................................... 1
1.2 Existing Drainage Patterns .................................................................................................................. 1
1.3 Existing Drainage Structures ............................................................ Error! Bookmark not defined.
1.4 Previous Drainage Studies ................................................................ Error! Bookmark not defined.
2.0 Proposed Improvements .................................................................................................................... 1
2.1 Proposed Site Improvements .............................................................................................................. 1
2.2 Drainage Basins .................................................................................................................................. 1
3.0 Hydraulics ......................................................................................................................................... 3
3.1 Storm Recurrence and Rainfall ......................................................... Error! Bookmark not defined.
3.2 Peak Runoff and Storage Volume Methodology ................................................................................ 3
3.3 Peak Discharge Calculations ............................................................................................................... 3
4.0 Conveyance Structures ...................................................................................................................... 4
4.1 Temporary Structures ........................................................................ Error! Bookmark not defined.
5.0 Low Impact Site Design .................................................................................................................... 5
5.1 Principles ............................................................................................................................................. 5
6.0 Treatment Facilities .......................................................................................................................... 6
6.1 Proposed Bio Swales ........................................................................................................................... 6
6.2 Proposed Detention Areas ................................................................. Error! Bookmark not defined.
7.0 Operation and Maintenance .............................................................................................................. 7
7.1 Bio Swales .......................................................................................................................................... 7
8.0 Appendices ...................................................................................... Error! Bookmark not defined.
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1.0 Existing Conditions
1.1 Site Description
The following report is an evaluation of 1130 Black Birch Drive Aspen, Colorado. The project is a
residential site located between Black Birch Drive to the east, the Roaring Fork River to the west and
between two residential homes to the north and south. An existing irrigation ditch separates the residence
from Black Birch Drive. The ditch flows under the driveway via two 18 inch pipes.-
2. The 0.465-acre lot consists of an existing multi-story house and a separated garage.
1.2 Existing Drainage Patterns
Drainage patterns on the site are split by a high point in the center of the lot. Drainage on the western half
of the property flows into an existing irrigation ditch, both from the residence and from Black Birch
Drive. The irrigation ditch flows from the south to the north across the site. The eastern half of the
property flows directly into the Roaring Fork River.
2.0 Proposed Improvements
2.1 Project Classification
This project is classified as a ‘Major Project’ as per Table 1.1 of the City of Aspen Urban Runoff
Management Plan (URMP). The proposed development is over 1,000 square feet and disturbs an area of
approximately 0.303 acres. The intent of this report is to demonstrate compliance with the requirements
of the City of Aspen URMP. The Low Impact Design (LID) Principles in the introduction of the manual
were used as a guide throughout the design process.
2.2 Proposed Site Improvements
A complete demolition of existing structures is proposed for the site. A new residence will be built in
place of the old house with all new patios, landscaping and driveway. Large mature trees onsite are being
preserved. The existing culverts under the driveway will be replaced with a concrete box culvert to
convey the flow of the irrigation ditch. The proposed culvert was sized to maintain the flow of the
existing culverts. Other drainage from roofs and the driveway will be conveyed through a new pipe
system to a series of ponds and level spreaders. Runoff from the impervious patios will sheet flow into
the surrounding landscaped areas but has been included in the water quality volume.
Due to the site’s proximity to the Roaring Fork River, only the water quality volume was accounted for
while designing the site’s storm system due to no downstream properties being impacted by site runoff to
the river.
2.3 Drainage Basins
The proposed construction has been split into four major basins. Basin 1 consists of 798 square feet of the
northwest corner of the roof. Basin 2 consists of 671 square feet of the northeast corner of the roof.
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Runoff from both Basin 1 and Basin 2 will be collected in downspouts and discharge on grade on the
north side of the structure. Runoff from these basins will flow into bioretention ponds. Basin 3 includes
the remaining 5,248 square feet of the roof not collected in Basin 1 or Basin 2. Runoff from Basin 1 is
collected through as series of downspouts and conveyed to the northeastern bioretention pond. The
proposed system includes runoff piped inside the house and pumped into the system. Basin 4 consists of
all uncovered imperious patio areas. Rainfall in these areas will sheet flow into the landscaped area
surrounding the proposed residence.
Basin areas are shown below, including the impervious areas and percentages.
Onsite Basin Data Entry
Basin # Total Area Impervious Area Impervious
(ft2)(ft2)%
1 798.00 798.00 100.00%
2 671.00 671.00 100.00%
3 5248.00 5248.00 100.00%
4 1247.00 1247.00 100.00%
TOTAL 7964.00 7964.00 100.00%
Onsite Sub‐Basin Data Entry
Basin # Total Area Impervious Area Impervious
(ft2)(ft2)%
1.1 494.00 494.00 100.00%
1.2 304.00 304.00 100.00%
2.1 207.00 207.00 100.00%
2.2 464.00 464.00 100.00%
3.1 257.00 257.00 100.00%
3.2 501.00 501.00 100.00%
3.3 170.00 170.00 100.00%
3.4 232.00 232.00 100.00%
3.5 279.00 279.00 100.00%
3.6 279.00 279.00 100.00%
3.7 1291.00 1291.00 100.00%
3.8 402.00 402.00 100.00%
3.9 300.00 300.00 100.00%
3.10 1537.00 1537.00 100.00%
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3.0 Hydraulics
3.1 Peak Runoff and Storage Volume Methodology
The drainage design is routing the 100-year event but will only detain the WQCV before discharging into
the Roaring Fork River. The WQCV was determined using Figure 8.16 from the City of Aspen URMP for
developed conditions. Each major basin was analyzed for the WQCV as well as 100-year flood routing.
The table below shows the required WQCV for each basin.
3.2 Peak Discharge Calculations
The peak flows were calculated for each Major Basin for the 100-year storm event. Rainfall intensity was
calculated using a Time of Concentration (Td) of 5 minutes. Actual Time of Concentration on the site is
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) is given in Table 2.2 as 1.23 inches for
the 100-year event. The intensity, in inches per hour, for different storm duration (Td), is calculated using
the Equation 2.1 from the Aspen URMP.
I = 88.8P1/ (10+Td )1.052
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.
Qp = CIA
Qp = Peak Discharge (cfs)
A = Area (Acres)
I = Rainfall intensity (inches per hour)
C = Runoff Coefficient
These peak flow values are used to calculate the size of the proposed conveyance structures, such as inlets
and piping. The tables below contain the peak flows for developed and undeveloped conditions for the
100-year storm 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)
1 798.00 798.00 100.00% 0.255 16.96 1.5 25.4 Pond 1
2 671.00 671.00 100.00% 0.255 14.26 1.5 21.4 Pond 2
3 5248.00 5248.00 100.00% 0.255 111.52 1.5 167.3 Pond 2
4 1247.00 1247.00 100.00% 0.255 26.50 1.5 39.7 Pond 2
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4.0 Conveyance Structures
Based on the above runoff calculations, inlets, area drains and pipes were sized to convey the 100-year
storm event with the pipe flowing eighty percent full. The table below shows flows through each pipe by
contributing basin as designed and shown on the design plans. Inlets are labeled to match numbering
system on the design plans.
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)
1 798.00 798.00 100.00% 0.950 5 6.33 0.11
2 671.00 671.00 100.00% 0.950 5 6.33 0.09
3 5248.00 5248.00 100.00% 0.950 5 6.33 0.72
4 1247.00 1247.00 100.00% 0.950 5 6.33 0.17
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)
1 798.00 0.00 0.00% 0.350 5 6.33 0.04
2 671.00 0.00 0.00% 0.350 5 6.33 0.03
3 5248.00 0.00 0.00% 0.350 5 6.33 0.27
4 1247.00 0.00 0.00% 0.350 5 6.33 0.06
Storm System Pipes
Pipe System Pipe Contibuting Sub‐Basins Design Flow Rate
Qdes
A A1‐A0 3.1‐3.10 0.72
A2‐A1 3.2‐3.10 0.69
A3‐A2 3.2‐3.10 0.69
A4‐A3 3.3‐3.10 0.62
A5‐A4 3.3‐3.10 0.62
A6‐A5 3.4‐3.10 0.60
A7‐A6 3.4‐3.10 0.60
A8‐A7 3.5‐3.10 0.56
A9‐A8 3.6‐3.10 0.53
A10‐A9 3.6‐3.9 0.31
A11‐A10 3.7‐3.9 0.27
A12‐A11 3.7‐3.9 0.27
A13‐A12 3.8, 3.9 0.10
A14‐A13 3.9 0.04
A15‐A14 3.9 0.04
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The Pipe Sizing table below shows required diameter and proposed pipe size based on slope, roughness
and flow rate. All pipes were conservatively sloped at one percent for design purposes. See the design
plans for actual pipe slopes.
5.0 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. By using bio-retention ponds and a screened
rock bed, storm water will be infiltrated to the maximum extent feasible.
5.1 Principles
Principle 1: Consider storm water quality needs early in the design process.
The grading and drainage was coordinated with the landscape architect and architect during the design
phase of the project.
Principle 2: Use the entire site when planning for storm water quality treatment.
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‐A0 0.72 1.00% 0.01 0.50 5.99 8.0
A2‐A1 0.69 1.00% 0.01 0.49 5.88 8.0
A3‐A2 0.69 1.00% 0.01 0.49 5.88 8.0
A4‐A3 0.62 1.00% 0.01 0.47 5.65 6.0
A5‐A4 0.62 1.00% 0.01 0.47 5.65 6.0
A6‐A5 0.60 1.00% 0.01 0.46 5.57 6.0
A7‐A6 0.60 1.00% 0.01 0.46 5.57 6.0
A8‐A7 0.56 1.00% 0.01 0.45 5.45 6.0
A9‐A8 0.53 1.00% 0.01 0.44 5.31 6.0
A10‐A9 0.31 1.00% 0.01 0.36 4.38 6.0
A11‐A10 0.27 1.00% 0.01 0.35 4.17 6.0
A12‐A11 0.27 1.00% 0.01 0.35 4.17 6.0
A13‐A12 0.10 1.00% 0.01 0.23 2.82 4.0
A14‐A13 0.04 1.00% 0.01 0.17 2.05 4.0
A15‐A14 0.04 1.00% 0.01 0.17 2.05 4.0
Pipe Sizing
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The site was designed with multiple bioretention ponds and a screened rock bed. Most area not occupied
by the proposed structure or river setback has been used for storm water treatment.
Principle 4: Reduce runoff rates and volumes to more closely match natural conditions.
Runoff from impervious surfaces is routed and partially infiltrated on-site in water quality systems. The
bio-retention ponds are sized to meet the water quality capture volume.
Principle 5: Integrate storm water quality management and flood control.
The bio-retention ponds will serve to improve storm water quality using a landscaped approach. The
ponds have been designed to allow a gentle flow to the Roaring Fork River via level spreaders during any
rain event larger than the WQCV.
Principle 6: Develop storm water quality facilities that enhance the site, the community and the
environment.
RFE is working closely with Design Workshop to create areas that enhance the visual impact of the
property and simultaneously treat storm water from the residence.
Principle 7: Use treatment train approach.
The bio-retention ponds have been tied together with a four-inch PVC pipe. The tie between the ponds
includes a screened rock bed that will further infiltrate and filter any runoff before overflowing into the
Roaring Fork River.
Principle 8: Design sustainable facilities that can be safely maintained.
The bio-retention ponds are at grade and easily accessible. Pipes and inlets have all been sized to allow
access and easy cleaning. All infrastructure in the storm system is just below grade for ease of access and
maintenance. Two inspection ports have been included in the screened rock for monitoring the infiltration
effectiveness.
Principle 9: Design and maintain facilities with public safety in mind.
No grading will be steeper than 3:1 onsite and proper grading of the driveway and patios will reduce the
chance of ice buildup. The Driveway and patios will be snow melted.
6.0 Treatment Facilities
6.1 Proposed Bio‐retention Ponds and Screened Rock Bed
There are two proposed bio-retention ponds located to the north of the proposed residence. The ponds will
be tied together with a 4” PVC pipe to allow more water to be treated before overflow occurs. The pipe
connecting the two ponds is perforated and surrounded in a screened rock bed. The ponds and screened
rock bed were designed to accommodate the WQCV for the entire site. Ponds were designed to have a
level overflow to reduce erosion.
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The tables below show the sizing of the screened rock bed and the total storage proved by the water
treatment facilities.
7.0 Operation and Maintenance
7.1 Bio‐retention Ponds
Sediment build-up may require periodic removal of sediments and plants when clogging reduces
infiltration capacity to unacceptable levels. Access to facility must be provided to enable maintenance
operation. Plant materials in areas prone to sediment build-up should be limited to grasses and
groundcovers tolerant of periodic wet-dry cycles.
Screened Rock Bed Storage
Storage System Basins Width Length Depth Void Ratio Total Capacity Required Capacity
(Name) (#) (ft) (ft) (ft) (ft
3)(ft3)
Screened Rock Bed 1, 2, 3, 4 2 45 2 0.3 54.0 49.8
Storage Summary
BMP Storage Type Volume Required Volume Provided
(WQCV, FAA or Full detention) ft
3 ft3
Pond 1 WQCV 25.00 90.40
Pond 2 WQCV 113.6 113.60
Screened Rock Bed WQCV 49.8 54.00
Total 253.8 258.00
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