HomeMy WebLinkAboutFile Documents.1419 Crystal Lake Rd.0009.2017 (5).ARBK1
Drainage Report
1419 CRYSTAL LAKE ROAD
ASPEN, CO 81623
February 13th, 2017
Updated May 19th, 2017
Prepared by
Richard Goulding, P.E.
Roaring Fork Engineering
592 Highway 133
Carbondale, CO 81623
Reviewed by Engineering
06/21/2017 9:04:37 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.
2
Drainage Report
1419 CRYSTAL LAKE ROAD
ASPEN, CO 81623
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.
RICHARD GOULDING, P.E.
RFE Project # 2016-35
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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 and Storage Volumes Methodology ............................................................................... 9
5.0 Hydraulic Criteria ................................................................................................................................... 9
5.1 Inlets .................................................................................................................................................... 9
5.2 Pipes .................................................................................................................................................. 10
6.0 Proposed Facilities ................................................................................................................................ 11
6.1 Proposed structures ........................................................................................................................... 11
6.2 Infiltration ......................................................................................................................................... 12
6.2.1 Drywell ...................................................................................................................................... 12
7.0 Operation and Maintenance .................................................................................................................. 12
7.1 Drywell ............................................................................................................................................. 12
7.2 Pervious Paver Area .......................................................................................................................... 12
8.0 Appendices ............................................................................................................................................ 13
Drawings 11x17 ...................................................................................................................................... 13
4
1.0 General
1.1 Existing Site
The residence under evaluation is located at 1419 Crystal Lake Road in Aspen, Colorado. This property is
a 1.03 acre parcel and is located within the Smuggler Hunter Drainage Basin. The existing structure
includes an 8,000 square foot two-story residence, a walkout basement, and a finished garage. The
residence is built directly above the top of the bank on a flat area. The southern property line runs along
the center of the Roaring Fork River, with neighboring houses located to the north and west of the site and
the Aspen Club Condominiums located to the east. Along the western property line is an asphalt trail on
an easement that provides access to the Aspen Club and Spa, which is south of the property across the
river. A large berm lined with trees is located along the north property line to provide privacy from
Crystal Lake Road. A majority of the site consists of a grass lawn with large aspen and pine trees. There
are a number of landscape beds leading up to the structure and the stone patio to the south of the structure.
1419 Crystal Lake Road
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A Geotechnical Report has been performed by H-P Kumar for this project as of permit submittal. All
calculations with percolation information included in this document currently use percolation test results
performed on January 17, 2017.
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 16, 500 sf., roughly 37% 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 a significant remodel of the interior of the existing structure, the
demolition of the patio and stairway on the south side of the residence, walkway and pond removal on the
west side of the residence, and regrading and resurfacing of existing snowmelted concrete driveway. The
existing building footprint will not change.
The runoff from impervious surfaces will be collected through a system of inlets, roof drains, and trench
drains and routed either to a drywell located at the northwest corner of the driveway or to a screened rock
detention bed located under the improved patios to the south of the house. Drainage from northern
sections of existing roof and the driveway will be routed to the drywell located at the northeast corner of
the driveway. Runoff from southern sections of existing roof and all of the southern patios will be routed
to a screened rock bed located under the new patio pavers. Refer to sheet C6 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.
2.0 Drainage Basins and Sub-basins
The site was divided into two drainage basins, which were then subdivided into smaller sub-basins. Basin
and Sub-basin delineations are shown on Sheets C4 and C5 of the permit drawings. 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.
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2.1 Drainage Basins
Basin 1 is the largest of the basins at 8560.7 square feet (sf) and is 99% impervious. Impervious sections
of this basin include the proposed driveway, the northern section of the roof structure, and the patio
located west of the structure. The remaining pervious sections of this basin include landscaping areas near
the front entry and west of the existing structure. There are five sub-basins in Basin 1. Two downspout
tie-ins collect runoff from the roof portions, two trench drains collect runoff from the patio west of the
existing structure and the proposed driveway, and Storm Systems A and B routes runoff from these sub-
basins to Drywells A and B. Each drywell is 6 ft. in diameter and 12 ft. deep, located on the northwest
corner of the proposed driveway.
Basin 2 consists of the southern portions of the roof and patios to the south and east of the structure.
Basin 2 is 3684.0 sf and 100% impervious. There are seven sub-basins in Basin 2. Four downspout tie-
ins collect runoff from the roof portions, one slot drain collects runoff from the east patio, and one center
drain collects runoff on the stairway. All of these inlets drain to Screened Rock Bed C. The 3 ft. deep
screened rock bed located below the patio will only extend to the 15 ft. top of slope setback. No treatment
will occur within the setback.
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 (P1), 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.
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.
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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.
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 8560.72 8508.72 99.39%0.896 5 3.29 0.58
2 3749.43 3749.43 100.00%0.896 5 3.29 0.25
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 8560.72 0.00 0.00%0.080 5 3.29 0.05
2 3749.43 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 8560.72 8508.72 99.39%0.950 5 6.33 1.18
2 3749.43 3749.43 100.00%0.950 5 6.33 0.52
100 Year Peak Discharge Pre Development Calculations
1 Hour(P1)1.23
Return Period 10
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 8560.72 0.00 0.00%0.350 5 6.33 0.44
2 3749.43 0.00 0.00%0.350 5 6.33 0.19
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Storm water quality was considered in the design of every part of the site that is being affected by the
proposed construction. Gutters and downspouts will be added to collect runoff from the entire roof and
routed to either to a drywell or a screened rock bed.
Principle 3: Avoid unnecessary impervious area.
All pervious patios are to be refinished with infiltration beds underneath. Although the size of the
driveway is increasing, full detention will be infiltrated through the drywell next to the drive.
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 either a drywell or a
screened rock bed. Because the drywells are 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 the drywells and screened rock bed 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. Both the drywell and screened rock
bed will be easily accessible.
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.
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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 (P1) 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 (Td) 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, Drywell A, Drywell B, and Screened Rock Bed C were sized to fully detain the runoff
from a 100 year storm event. The table below shows storage volume requirements for the proposed
detention system.
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 and 2 were 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.
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 8560.72 8508.72 99.39%1.23 1 872 Drywell A, Drywell B
2 3749.43 3749.43 100.00%1.23 1 384 Screened Rock Bed C
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)
TRENCH DRAIN-A1 1.1 787.08 787.08 100.00%0.950 5 6.33 0.109 4" x 19.7'4 236.4 1.313 1.318 Yes
TRENCH DRAIN-B3 1.4 3652.57 3600.57 98.58%0.880 5 6.33 0.467 4" x 39.0'4 468 2.600 2.609 Yes
SLOT DRAIN-C3 2.3 272.51 272.51 100.00%0.950 5 6.33 0.038 0.25" x 19.8'0.25 237.6 0.083 0.083 Yes
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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.
Calculated pipe sizes were tested for hydraulic capacity at 80% of their full flowrate. Calculations of
depth of flow for each pipe were calculated. Design charts giving Qdesign / 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.
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
Time of
Concentration Intensity Q Max Inlet Type Diameter
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 Wo (inches)Ae=(1-Cg)mA Q=CoAe√2gYs (Yes/No)
INLET-C4 2.4 198.57 198.57 100.00%0.950 5 6.33 0.027 6" Round 6 0.039 0.046 Yes
Storm System Pipes
Pipe System Pipe Contibuting Sub-Basins Peak Flows (CFS)
A A1 1.1 0.11
A2 1.1 0.11
A3 1.1, 1.2 0.18
B B1 1.3 0.28
B2 1.3 0.28
B3 1.4 0.47
C C1 2.1 0.11
C2 2.1, 2.2 0.22
C3 2.1-2.3 0.25
C4 2.1-2.4 0.28
C5 2.1-2.4 0.28
C9 2.6 0.11
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
A1 0.11 0.01 3.00%0.20 2.393 4.0 3.200 Yes
A2 0.11 0.01 2.00%0.22 2.582 4.0 3.200 Yes
A3 0.18 0.01 8.00%0.20 2.411 4.0 3.200 Yes
B1 0.28 0.01 2.00%0.31 3.699 6.0 4.800 Yes
B2 0.28 0.01 4.00%0.27 3.249 6.0 4.800 Yes
B3 0.47 0.01 2.00%0.37 4.461 6.0 4.800 Yes
C1 0.11 0.01 6.00%0.18 2.126 4.0 3.200 Yes
C2 0.22 0.01 20.00%0.18 2.169 4.0 3.200 Yes
C3 0.25 0.01 4.00%0.26 3.115 4.0 3.200 Yes
C4 0.28 0.01 5.00%0.26 3.104 6.0 4.800 Yes
C5 0.28 0.01 25.00%0.19 2.296 6.0 4.800 Yes
C9 0.11 0.01 2.00%0.21 2.553 4.0 3.200 Yes
Onsite Piping Capacity
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6.0 Proposed Facilities
6.1 Proposed structures
Drywells and a screened rock bed are 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.
Drywell A and Drywell B are located at the Northwest corner of the driveway and collects all runoff
from Basin 1. These drywells have a diameter of 6 ft., a storage depth of 12 ft. each with 18 in. of
screened rock surrounding the drywells, giving each drywell a storage capacity of 467 ft3, for a total
capacity of 934 ft3. The total capacity for the drywells exceeds the required detention volume of 872 ft3
for Basin 1.
Screened Rock Bed C is located under the snowmelted patio area along the south edge of the building
and collects runoff from Basin 2. This bed is 3 ft. deep with a footprint of 427 ft2, and a gravel void ratio
of 0.3 giving the bed a total storage volume of 386 ft3, which exceeds the required capacity of 384 ft3 for
Basin 2.
Pipe Combined
Design Flow
Manning
Coefficient
Design
Diameter X-section Slope Q -Full Q-Design/Q Full d/D Depth
(ID)Q (ft3/sec)n (inches) (ft2) (%) (ft3/sec)Q/Qfull (from Chart)d= (d/D)*D
A1 0.109 0.01 4.0 0.087 3.00%0.429 0.253 0.38 1.52
A2 0.109 0.01 4.0 0.087 2.00%0.351 0.310 0.43 1.72
A3 0.181 0.01 4.0 0.087 8.00%0.701 0.258 0.38 1.52
B1 0.283 0.01 6.0 0.196 2.00%1.034 0.274 0.38 2.28
B2 0.283 0.01 6.0 0.196 4.00%1.462 0.194 0.34 2.01
B3 0.467 0.01 6.0 0.196 2.00%1.034 0.451 0.53 3.18
C1 0.112 0.01 4.0 0.087 6.00%0.607 0.184 0.33 1.30
C2 0.216 0.01 4.0 0.087 20.00%1.109 0.195 0.34 1.34
C3 0.253 0.01 4.0 0.087 4.00%0.496 0.511 0.57 2.28
C4 0.281 0.01 6.0 0.196 5.00%1.635 0.172 0.31 1.88
C5 0.281 0.01 6.0 0.196 25.00%3.655 0.077 0.22 1.32
C9 0.105 3.01 4.0 0.087 2.00%0.001 90.505 0.90 3.60
Depth Of Flow-section 4.8.4 Storm Sewer Sizing
Drywell Storage
Drywell Basins Diameter Storage Depth Internal Volume External (18" of Screened Rock) Volume Total Capacity Required Capacity
(Name)(#)(ft)(ft)π*H*(D/2)2) (ft3)0.3*π*8*((D/2)+1.5)2 - (D/2)2) (ft3)(ft3)(ft3)
Drywell A 1 6 12.5 353 85 438 431.98
Drywell B 1 6 12.5 353 85 438 431.98
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6.2 Infiltration
6.2.1 Drywells and Screened Rock Bed
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 drywells and screened rock bed to drain within 24 hours using the
percolation rate determined on January 17, 2017 of 3 inches per hour for the entire site. Section 8.5.4.2
was referenced for these calculations.
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.
Screened Rock Bed Storage
Storage System Basins Area Depth Void Ratio Total Capacity Required Capacity
(Name)(#)(ft2)(ft)(ft3)(ft3)
Screened Rock Bed C 2 428.64 3 0.3 385.78 384
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
Drywell A 6 8 150.80 438.25 3 11.63 5094.68
Drywell B 6 8 150.80 438.25 3 11.63 5094.68
Screened Rock Bed Infiltration
BMP Max Volume Infiltration Area Infiltration Rate Time To Drain Volume Infiltrated in 24 Hours
(name)V (ft3)A (ft2)I (in/hr)(hr)Vtotal (ft3) = V*T
Screened Rock Bed C 384.32 204 3 7.54 22.61
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8.0 Appendices
Drawings 11x17