HomeMy WebLinkAboutFile Documents.546 McSkimming Rd.0190.2019 (5).ARBK
Drainage Report
546 MCSKIMMING ROAD
ASPEN, CO
81611
Prepared by
Richard Goulding, P.E.
Roaring Fork Engineering
592 Highway 133
Carbondale, CO 81623
08/08/2019
Drainage Report
546 MCSKIMMING ROAD
ASPEN, CO
81611
I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 546 MCSKIMMING ROAD
IS PREPARED BY ME FOR THE OWNERS THEREOF IN ACCORDANCE WITH THE PROVISIONS OF
CITY OF ASPEN AND APPROVED VARIANCES AND EXCEPTIONS LISTED THERETO. I
UNDERSTAND THAT IT IS THE POLICY OF THE CITY OF ASPEN THAT ASPEN DOES NOT AND
WILL NOT ASSUME LIABILITY FOR DRAINAGE FACILITIES DESIGNED BY OTHERS.
RICHARD GOULDING, P.E.
RFE Project # 2019-11
08/08/2019
Table of Contents
1.0 General ................................................................................................................................. 4
1.1 Existing Site ..................................................................................................................... 4
1.2 Proposed Site .................................................................................................................... 5
1.3 Previous Drainage Studies ............................................................................................... 5
1.4 Offsite Drainage ............................................................................................................... 5
2.0 Drainage Basins and Sub-basins .......................................................................................... 5
2.1 Drainage Basins ................................................................................................................ 5
2.2 Peak Discharge Calculations ............................................................................................ 6
3.0 Low Impact Site Design....................................................................................................... 8
3.1 Principles .......................................................................................................................... 8
4.0 Hydrological Criteria ........................................................................................................... 9
4.1 Storm Recurrence and Rainfall ........................................................................................ 9
4.2 Storage Volumes Methodology ........................................................................................ 9
5.0 Hydraulic Criteria .............................................................................................................. 12
5.1 Inlets ............................................................................................................................... 13
5.2 Pipes ............................................................................................................................... 13
6.0 Proposed Facilities ............................................................................................................. 16
6.1 Drywell ........................................................................................................................... 16
7.0 Operation and Maintenance ............................................................................................... 16
7.1 Drywell ........................................................................................................................... 16
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1.0 General
1.1 Existing Site
546 McSkimming Road, parcel number 273718102002, is located in the Aspen Grove
Subdivision at the east end of city limits, about two thousand feet north east from Highway 82.
The site contains a three-story residence with an approximate gross sq.ft. of 4,034. Vegetation on
the property includes large fir trees, shrubs, and landscape lawn.
The parcel is surrounded by heavy vegetation. The existing topography slope from northeast to
the southwest away from the footprint of the home. An aerial photograph is provided as Figure 1.
An existing conditions sheet provided by the surveyor is part of the building permit set.
A geotechnical report was developed by H-P Kumar on November 6, 2018, with a percolation
test performed on May 31, 2019. A copy of the geotechnical information is included in the
submittal package. The geotechnical investigation resulted in an observation slightly silty sandy
gravel with cobbles and possible boulders found below the driveway on the west side of the
property. No free water was encountered in the borings at the time of drilling and the subsoils
were slightly moist to moist. Sieve analysis and lab results of the subsurface conditions
encountered at the site are shown on Figure 1 & 2 of the geotechnical report.
Figure 1: Aerial map of existing site.
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1.2 Proposed Site
This project is classified as a βMajor Projectβ 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 14,000 square feet, roughly 72 percent 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 scope of work includes the implementation of a stormwater detention system for a
remodeled residence. It also included widening of the driveway access and auto court, as well as
earthwork and soil retention by wall to improve the safety of the three-story residence.
The topography of the parcel is sloped with an existing grade typically draining away from the
site towards the lower portion of McSkimming Road. The proposed drainage infrastructure
includes two onsite drywells buried southwest of the home. This drainage report will focus on
the drainage basins being captured and conveyed by the storm drainage system. The onsite basin
that is no longer following historical runoff flow paths has been analyzed for full detention of a
100-year storm event.
1.3 Previous Drainage Studies
The parcel of land belonging to 546 McSkimming Road is not a part of any drainage study or
master plan for the City of Aspen. Therefore, 100-year detention with historical release is being
implemented on the property and away from the road.
1.4 Offsite Drainage
Based on the survey, McSkimming Road does not drain onto the property. The steep slope
between the road and the residence will drain toward the property and into the proposed storm
system.
2.0 Drainage Basins and Subβbasins
The development on the parcel is proposed as two large onsite basins. These basins were then
subdivided into smaller sub-basins and analyzed to aid with design of the storm water
infrastructure. Basin and Sub-basin delineations are shown on sheets C2 and C3 of the civil
plans. These sheets list impervious areas, runoff coefficients, peak flows, and the required
volume of runoff to be detained.
2.1 Drainage Basins
Basin 1 is a major basin within the parcel and consists of the developed area of the residence,
including the asphalt driveway, a roof structure, and concrete auto-court. As well as the pervious
green ROW between the home and McSkimming Road. The basin has a total area of 8,252
square feet and is 61.79% impervious. Impervious sections of the basin include the northside of
the roof structure, the asphalt driveway, and concrete auto-court. The remainder of the basin is
made up of pervious landscaped areas that surround the residence. Runoff from the basin is
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collected by inlets and downspouts. The runoff is conveyed into a drywell buried under the
landscaped lawn. The drywell has been sized for detention of a 100-year storm event and water
quality capture volume with a historical release.
Sub-basin 1.1 & 1.2 are an asphalt driveway collected by two 4β trench drains at two separate
low points.
Sub-basin 1.3 is an impervious roof basin to be drained by a downspout.
Sub-basin 1.4 is an area inlet at the low point of the concrete auto court.
Sub-basin 1.5 is an impervious roof basin to be drained by a downspout.
Basin 2 is a major basin within the parcel and consists of the developed area of the residence,
including the hardscaped patios, a plunge pool, various pervious landscaped surfaces, and the
southwest portion of the impervious roof. The basin has a total area of 3,613 square feet and is
71.38% impervious. Impervious sections of the basin include the hardscaped patios, plunge pool,
and the southwest portion of the impervious roof. The remainder of the basin is made up of
pervious landscaped areas that surround the residence. Runoff from the basin is to be collected
by inlets and downspouts. The runoff is conveyed into a drywell buried under the landscaped
lawn. The drywell has been sized for detention of a 100-year storm event and water quality
capture volume with a historical release.
Sub-basin 2.1, 2.2, 2.3, 2.4 are impervious roof basins to be collected by downspout.
Sub-basin 2.5 is an impervious concrete surface that holds the equipment for the pool.
Sub-basin 2.6 is the plunge pool and associated landscaped areas around it. The basin is to be
collected by area inlet located at the base of an access ramp.
Sub-basin 2.7 is the largest hardscaped patio portion with a small landscaped lawn and low point
graded to collect drainage at an area inlet.
Sub-basin 2.8 is the portion of stone steps and landscaped lawn to be collected by an area inlet
at a low point.
Sub-basin 2.9 is the remaining impervious roof basin to be collected by downspout.
2.2 Peak Discharge Calculations
The peak flows were calculated for the Major Basin for 5 and 100-year storm events using the
Rational Method. The Rational Method is an acceptable method to calculate runoff for this basin
as the area is under 90 acres. Rainfall intensity was calculated using a Time of Concentration
(Td) of 5 minutes. The actual time of concentration for this 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 5 minutes or greater so the smallest valid time of
concentration value was used. The 1-hour Rainfall depths (P1) used for these calculations was
taken from Table 2.2 of the URMP and is equal to 0.64 inches for the 5-year event and 1.23
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inches for the 100-year event. Equation 2.1 was referenced when solving for the Rainfall
Intensity (I).
π°ΰ΅ ππ.π π·π
αΊππ ΰ΅ π»π α»π.πππ αΊπΈππ’ππ‘πππ 2.1α»
Runoff Coefficients (C), a function of the Soil Group (in this case B for the onsite basin) and the
percentage of impervious area within each sub basin were developed using Figure 3.3. The
Runoff Coefficient (C) was then multiplied by the Rainfall Intensity (I) and the area of the Major
Basin (A, in acres) to determine the peak discharge.
πΈπΰ΅πͺπ°π¨
πΰ―£ ΰ΅ ππππ π·ππ πβππππ αΊπππ α»
πΆ ΰ΅ π π’ππππ πΆππππππππππ‘
πΌ ΰ΅ π πππππππ πΌππ‘πππ ππ‘π¦ αΊπππβππ πππ βππ’πα»
π΄ΰ΅π΄πππ αΊπππππ α»
These peak flow values were used to calculate the size of the proposed detention and conveyance
structures, such as the drywell, inlets and pipes. The tables below contain the peak flows for
developed and undeveloped conditions for 5 and 100-year storm events for the major basin, and
the 100-year peak flow rate for the sub basins.
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)
1 8252.00 5099.00 61.79% 0.410 5 3.29 0.26
2 3613.00 2579.00 71.38% 0.490 5 3.29 0.13
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)
1 8252.00 0.00 0.00% 0.080 5 3.29 0.05
2 3613.00 0.00 0.00% 0.080 5 3.29 0.02
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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. This entire developed site
is approximately 64 percent impervious. The treatment train approach is used on all runoff to
increase water quality and infiltration.
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 and water quality requirements were
discussed early on. 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.
Principle 3: Avoid unnecessary impervious area.
The total impervious area on the site was kept to a minimum while meeting the architectural
design goals by incorporating pervious landscaped areas throughout the site.
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 BMP structures.
The infrastructure has been sized to capture the water quality capture volume, as well as detain
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 8252.00 5099.00 61.79% 0.570 5 6.33 0.68
2 3613.00 2579.00 71.38% 0.620 5 6.33 0.33
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 8252.00 0.00 0.00% 0.350 5 6.33 0.42
2 3613.00 0.00 0.00% 0.350 5 6.33 0.18
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the 100-year developed runoff and releasing at a historical pre-developed rate of flow. The
drywells are designed to infiltrate storage capacity into the surrounding earth.
Principle 5: Integrate storm water quality management and flood control.
Oversized trench drains are proposed to allow for easier access for removal of pollutants. The
drywells have been oversized to account for the water quality capture volume. The area inlets
provide a six-inch sump for separating sediment from water to improve overall water quality.
Principle 6: Develop storm water quality facilities that enhance the site, the community and the
environment.
The drywells are being implemented to improve the water quality of the already existing
structure and provide mitigation for large storm events.
Principle 7: Use treatment train approach.
The design implements sheetflow across landscaping, and sumps in the area drains to ensure
treatment throughout the system, and storage volume size for water quality capture volumes.
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. Cleanouts are located where necessary to ensure the lifetime
of the drainage infrastructure.
Principle 9: Design and maintain facilities with public safety in mind.
Proper drainage and grading of the driveway and walkways reduce ice buildup and dangerous icy
conditions. All grading was done with safety and accessibility in mind.
4.0 Hydrological Criteria
4.1 Storm Recurrence and Rainfall
The property is located outside of the commercial core and isnβt served by any municipal storm
system, so this property classifies as a βSub-urban area not served by public storm sewerβ.
Therefore, the storm system for the site was designed to meet detention requirements for the 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)
was calculated using Equation 2.1 from the City of Aspen URMP.
4.2 Storage Volumes Methodology
The storage requirements for this site were calculated using the total impervious area along with
the historic and developed peak runoff rates that were established in section 2.2. The proposed
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storm drainage system is designed for detention of a 100-year storm event with historical release.
No detention is required for pervious areas. The FAA procedure calculations were used to calculate
the storage requirements for the basins. Below is a summary of the required storage.
5 Year Storage Calculations β Basin 1
Rainfall Duration
(minutes)
Intensity (inches/Hour)
EQ 5β1 Volume In ft3 EQ 5β2 Volume Out ft3 EQ 5β3
Volume Difference
ft3 EQ 5β4 C 0.410
(Td) I=88.8P1/(10+Td)1.052 Vi=(1/720)*C*I*Td*A Vo=30(1+Tc/Td)*Qa*Td Vd=ViβVo A 8252.00 ft2
5 3.29 77.33 14.96 62.36 Tc 5 Minutes
8 2.72 102.13 19.45 82.68 Qa 0.050 ft3
11 2.31 119.40 23.94 95.46 P1(5)0.64
14 2.01 132.05 28.43 103.62
17 1.77 141.66 32.92 108.74
20 1.59 149.18 37.41 111.77
23 1.44 155.19 41.90 113.29
26 1.31 160.08 46.39 113.70
29 1.20 164.14 50.88 113.26
32 1.11 167.53 55.36 112.17
35 1.04 170.41 59.85 110.56
38 0.97 172.87 64.34 108.53
41 0.91 174.99 68.83 106.16
44 0.86 176.84 73.32 103.52
47 0.81 178.45 77.81 100.64
50 0.77 179.87 82.30 97.57
53 0.73 181.12 86.79 94.34
56 0.69 182.23 91.28 90.96
59 0.66 183.23 95.77 87.46
62 0.63 184.11 100.25 83.86
65 0.61 184.91 104.74 80.16
68 0.58 185.62 109.23 76.39
71 0.56 186.27 113.72 72.55
74 0.54 186.85 118.21 68.64
77 0.52 187.38 122.70 64.68
80 0.50 187.86 127.19 60.67
83 0.48 188.29 131.68 56.62
86 0.47 188.69 136.17 52.53
Maximum Difference 113.70
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100 Year Storage Calculationsβ Basin 1
Rainfall Duration
(minutes)
Intensity (inches/Hour)
EQ 5β1 Volume In ft3 EQ 5β2 Volume Out ft3 EQ 5β3
Volume Difference
ft3 EQ 5β4 C 0.570
(Td) I=88.8P1/(10+Td)1.052 Vi=(1/720)*C*I*Td*A Vo=30(1+Tc/Td)*Qa*Td Vd=ViβVo A 8252.00 ft2
5 6.33 206.61 125.81 80.79 Tc 5Minutes
7 5.54 253.56 150.98 102.59 Qa 0.419 ft3
9 4.93 290.01 176.14 113.87 P1(100)1.23
11 4.44 319.03 201.30 117.73
13 4.03 342.63 226.47 116.16
15 3.70 362.14 251.63 110.51
17 3.41 378.51 276.79 101.72
19 3.16 392.40 301.95 90.45
21 2.95 404.32 327.12 77.20
23 2.76 414.64 352.28 62.36
25 2.59 423.64 377.44 46.20
27 2.45 431.55 402.61 28.95
29 2.31 438.55 427.77 10.78
31 2.20 444.77 452.93 β8.17
33 2.09 450.32 478.10 β27.77
35 1.99 455.31 503.26 β47.95
Maximum Difference 117.73
5 Year Storage Calculations β Basin 2
Rainfall Duration
(minutes)
Intensity (inches/Hour)
EQ 5β1 Volume In ft3 EQ 5β2 Volume Out ft3 EQ 5β3
Volume Difference
ft3 EQ 5β4 C 0.490
(Td) I=88.8P1/(10+Td)1.052 Vi=(1/720)*C*I*Td*A Vo=30(1+Tc/Td)*Qa*Td Vd=ViβVo A 3613.00 ft2
5 3.29 40.46 6.55 33.91 Tc 5 Minutes
8 2.72 53.44 8.52 44.92 Qa 0.022 ft3
11 2.31 62.48 10.48 52.00 P1(5)0.64
14 2.01 69.10 12.45 56.65
17 1.77 74.13 14.41 59.71
20 1.59 78.06 16.38 61.68
23 1.44 81.20 18.34 62.86
26 1.31 83.77 20.31 63.46
29 1.20 85.89 22.27 63.61
32 1.11 87.66 24.24 63.42
35 1.04 89.17 26.21 62.96
38 0.97 90.46 28.17 62.29
41 0.91 91.57 30.14 61.43
44 0.86 92.53 32.10 60.43
47 0.81 93.38 34.07 59.31
50 0.77 94.12 36.03 58.09
53 0.73 94.78 38.00 56.78
56 0.69 95.36 39.96 55.39
59 0.66 95.88 41.93 53.95
62 0.63 96.34 43.89 52.44
65 0.61 96.75 45.86 50.89
68 0.58 97.13 47.83 49.30
71 0.56 97.47 49.79 47.68
74 0.54 97.77 51.76 46.02
77 0.52 98.05 53.72 44.33
80 0.50 98.30 55.69 42.61
83 0.48 98.53 57.65 40.88
86 0.47 98.74 59.62 39.12
Maximum Difference 63.61
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5.0 Hydraulic Criteria
This property is not connected to the COAβs storm water infrastructure. All hydraulics are sized
for onsite infrastructure. Below is a table that was used for an in-depth analysis of the flows
through the conveyance structures.
100 Year Storage Calculations β Basin 2
Rainfall Duration
(minutes)
Intensity (inches/Hour)
EQ 5β1 Volume In ft3 EQ 5β2 Volume Out ft3 EQ 5β3
Volume Difference
ft3 EQ 5β4 C 0.620
(Td) I=88.8P1/(10+Td)1.052 Vi=(1/720)*C*I*Td*A Vo=30(1+Tc/Td)*Qa*Td Vd=ViβVo A 3613.00 ft2
5 6.33 98.39 55.09 43.31 Tc 5Minutes
6 5.91 110.32 60.59 49.73 Qa 0.184 ft3
7 5.54 120.76 66.10 54.65 P1(100)1.23
8 5.22 129.95 71.61 58.34
9 4.93 138.11 77.12 60.99
10 4.67 145.40 82.63 62.77
11 4.44 151.94 88.14 63.80
12 4.23 157.83 93.65 64.19
13 4.03 163.17 99.15 64.02
14 3.86 168.03 104.66 63.37
15 3.70 172.47 110.17 62.29
16 3.55 176.53 115.68 60.85
17 3.41 180.26 121.19 59.07
18 3.28 183.70 126.70 57.00
19 3.16 186.88 132.21 54.67
20 3.05 189.82 137.71 52.11
Maximum Difference 64.19
FAA Storage
Required Calculated Volumes Required Total
SubβBasin Total Area Impervious Area Impervious WQCV Tbl. Val. WQCV Volume 5βyr 100βyr Volume Volume BMP
(ft2)(ft2) (%) (in) (ft
3)(ft3)(ft3)(ft3)(ft3)
1 8252.00 5099.00 61.79% 0.117 80.5 113.70 117.73 118.0 198.5 DRYWELL 1
2 3613.00 2579.00 71.38% 0.14 42.2 63.61 64.19 65.0 107.2 DRYWELL 2
WQCV Detention
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 1814.00 1297.00 71.50% 0.620 5 6.33 0.16
1.2 3943.00 2221.00 56.33% 0.540 5 6.33 0.31
1.3 778.00 778.00 100.00% 0.950 5 6.33 0.11
1.4 1327.00 413.00 31.12% 0.480 5 6.33 0.09
1.5 390.00 390.00 100.00% 0.950 5 6.33 0.05
2.1 227.00 227.00 100.00% 0.950 5 6.33 0.03
2.2 168.00 168.00 100.00% 0.950 5 6.33 0.02
2.3 460.00 460.00 100.00% 0.950 5 6.33 0.06
2.4 457.00 457.00 100.00% 0.950 5 6.33 0.06
2.5 69.00 69.00 100.00% 0.950 5 6.33 0.01
2.6 1202.00 880.00 73.21% 0.620 5 6.33 0.11
2.7 799.00 87.00 10.89% 0.400 5 6.33 0.05
2.8 231.00 231.00 100.00% 0.950 5 6.33 0.03
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5.1 Inlets
The peak flows for the 100-year event in each sub-basin were used to size the proposed inlets.
Equations 4.17 through 4.20 from the URMP were used in these calculations. The equations
incorporate a 50 percent clogging factor and assume a 40 percent opening in the grates. Water
depths used in these calculations are based on the grading around each inlet and safe ponding
levels above the inlets. The tables on the following page summarize the calculations for each
inlet as well as for the trench drains.
5.2 Pipes
The pipes were sized by using the calculated flow from the sub-basins they are connected to.
Below is a table which groups what sub-basins are conveyed to each pipe. The Time of
Concentration (TOC) is below 5 minutes for all sub-basins, so a reduction was not taken for the
intensity. Depth of flow was also calculated in the spreadsheets below. The pipes are all SDR 35
PVC with a manningβs 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.
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. 4β20) Inlet Capacity (EQ 4β19) Has Capacity
See(D1) (ft
2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 ft3/sec Wo (inches) Ae=(1βCg)mA Q=CoAeβ2gYs (Yes/No)
AREA INLET 1.4 1.4 1327 413 31.12% 0.48 5 6.33 0.092 8" Round 8 0.070 0.096 Yes
AREA INLET 2.5 2.5 69 69 100.00% 0.95 5 6.33 0.010 8" Round 8 0.070 0.081 Yes
AREA INLET 2.7 2.7 799.00 87.00 10.89% 0.40 5 6.33 0.046 8" Round 8 0.070 0.081 Yes
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. 4β20) Inlet Capacity (EQ 4β19) Has Capacity
See(D1) (ft
2)(ft2)(%)(From Table) (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) Rectangular Wo (inches) Lo (inches) Ae=(1βCg)mWoLo Q=CoAeβ2gYs (Yes/No)
TRENCH DRAIN 1.1 1.1 1814.00 1297.00 71.50% 0.62 5 6.33 0.163 4" x 13.5'4 162 0.900 0.903 Yes
TRENCH DRAIN 1.2 1.2 3943.00 2221.00 56.33% 0.54 5 6.33 0.309 4" x 24' 4 288 1.600 1.605 Yes
TRENCH DRAIN 2.6 2.6 1202.00 880.00 0.73 0.62 5 6.33 0.108 3" X 11' 4 132 0.733 0.736 Yes
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Pipe sizes were tested for hydraulic capacity at 80 percent of their full flowrate. 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.
Storm System Pipes
Pipe System Pipe Contibuting SubβBasins Design Flow Rate
Qdes
11.1 0.16
21.1 0.16
31.1 0.16
41.5 0.05
5 1.4β1.5 0.15
6 1.2, 1.4, 1.5 0.46
71.3 0.11
8 1.2β1.5 0.56
9 1.1β1.5 0.72
10 2.8 0.03
11 2.1 0.08
12 2.1,2.8β2.9 0.16
13 2.1,2.8β2.9 0.23
14 2.2 0.02
15 2.3 0.06
16 2.2β2.3 0.09
17 2.5 0.01
18 2.5 0.01
19 2.4β2.5 0.07
20 2.4β2.5 0.07
21 2.4β2.5 0.07
22 2.6 0.11
23 DRYWELL 1 0.42
24 DRYWELL 2 0.18
North
System
South
System
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15
K=0.462
Pipe Design Flow
Rate
Proposed
Slope
Manning
Coefficient
Required Pipe Diameter
Equation 4β31
Required Pipe
Diameter
Proposed Pipe
Diameter
Qdes (ft3/sec) S (%) n d (ft) = {nQdes/KβS}3/8 Dreq (in) Dpro (in)
1 0.16 3.00% 0.01 0.23 2.78 4.0
2 0.16 3.00% 0.01 0.23 2.78 4.0
3 0.16 3.00% 0.01 0.23 2.78 4.0
4 0.05 2.00% 0.01 0.17 1.98 4.0
5 0.15 2.00% 0.01 0.24 2.89 4.0
6 0.46 2.00% 0.01 0.37 4.42 6.0
7 0.11 2.00% 0.01 0.21 2.57 4.0
8 0.56 2.00% 0.01 0.40 4.78 6.0
9 0.72 30.00% 0.01 0.26 3.17 4.0
10 0.03 7.00% 0.01 0.11 1.29 4.0
11 0.08 2.00% 0.01 0.19 2.28 4.0
12 0.16 14.00% 0.01 0.17 2.06 4.0
13 0.23 2.00% 0.01 0.29 3.45 4.0
14 0.02 2.00% 0.01 0.12 1.45 4.0
15 0.06 2.00% 0.01 0.18 2.11 4.0
16 0.09 2.00% 0.01 0.20 2.37 4.0
17 0.01 2.00%0.01 0.09 1.04 4.0
18 0.01 2.00%0.01 0.09 1.04 4.0
19 0.07 2.00%0.01 0.18 2.22 4.0
20 0.07 2.00%0.01 0.18 2.22 4.0
21 0.07 2.00% 0.01 0.18 2.22 4.0
22 0.11 2.00% 0.01 0.21 2.58 4.0
23 0.42 2.90% 0.01 0.33 3.99 4.0
24 0.18 2.70% 0.01 0.25 2.97 3.0
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) = Ο (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)
1 0.16 4.0 3.00% 3.2 0.01 0.087 0.429 0.38 0.49 1.94 Yes
2 0.16 4.0 3.00% 3.2 0.01 0.087 0.429 0.38 0.49 1.94 Yes
3 0.16 4.0 3.00% 3.2 0.01 0.087 0.429 0.38 0.49 1.94 Yes
4 0.05 4.0 2.00% 3.2 0.01 0.087 0.351 0.15 0.30 1.20 Yes
5 0.15 4.0 2.00% 3.2 0.01 0.087 0.351 0.42 0.50 2.00 Yes
6 0.46 6.0 2.00% 4.8 0.01 0.196 1.034 0.44 0.52 3.09 Yes
8 0.56 6.0 2.00% 4.8 0.01 0.196 1.034 0.54 0.59 3.51 Yes
9 0.72 4.0 30.00% 3.2 0.01 0.087 1.358 0.53 0.59 2.34 Yes
10 0.03 4.0 7.00% 3.2 0.01 0.087 0.656 0.05 0.16 0.62 Yes
11 0.08 4.0 2.00% 3.2 0.01 0.087 0.351 0.22 0.35 1.40 Yes
12 0.16 4.0 14.00% 3.2 0.01 0.087 0.928 0.17 0.31 1.25 Yes
13 0.23 4.0 2.00% 3.2 0.01 0.087 0.351 0.67 0.66 2.64 Yes
14 0.02 4.0 2.00% 3.2 0.01 0.087 0.351 0.07 0.20 0.80 Yes
15 0.06 4.0 2.00% 3.2 0.01 0.087 0.351 0.18 0.33 1.30 Yes
16 0.09 4.0 2.00% 3.2 0.01 0.087 0.351 0.25 0.37 1.46 Yes
17 0.01 4.0 2.00% 3.2 0.01 0.087 0.351 0.03 0.12 0.48 Yes
18 0.01 4.0 2.00% 3.2 0.01 0.087 0.351 0.03 0.12 0.48 Yes
19 0.07 4.0 2.00% 3.2 0.01 0.087 0.351 0.21 0.35 1.40 Yes
20 0.07 4.0 2.00% 3.2 0.01 0.087 0.351 0.21 0.35 1.40 Yes
21 0.07 4.0 2.00% 3.2 0.01 0.087 0.351 0.21 0.35 1.40 Yes
22 0.11 4.0 2.00% 3.2 0.01 0.087 0.351 0.31 0.43 1.72 Yes
23 0.42 4.0 2.90% 3.2 0.01 0.087 0.422 0.99 0.88 3.52 N/A
24 0.18 3.0 2.70% 2.4 0.01 0.049 0.189 0.97 0.86 2.58 N/A
Hydraulic Grade Line and Pipe Capacity
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6.0 Proposed Facilities
This property is not connected to the COAβs storm water infrastructure, therefore, onsite
drywells are sized for 100-year detention with historical release, as clarified in section 2.2 of this
report. Below are the analyses for the individual detention structureβs capacity and infiltration.
6.1 Drywells
Below is a table that shows the proposed drywell meeting the capacity required for 100-year
detention with historical release for basins 1 and 2.
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:
ο· Inspect drywells at least four times a year and after every storm exceeding 0.5 inches.
ο· 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.
ο· 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.
Drywell Storage
Drywell Basins Diameter Storage Depth Perforated Depth Internal Volume External (18" of Screened Rock) Volume Total Capacity Required Capacity
(Name) (#) D (ft) H (ft) P (ft) Ο*H*(D/2)
2) (ft3) 0.3*Ο*P*((D/2)+1.5)
2 β (D/2)2) (ft3)(ft3)(ft3)
1166 5 170 53 223 198
2245.5 5 69 39 108 107
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