HomeMy WebLinkAboutFile Documents.121 E Hyman Ave.0149.2018 (4).ARBK02/28/2019
Reviewed by Engineering
03/18/2019 1:48:24 PM
"It should be known that this review shall not
relieve the applicant of their responsibility to
comply with the requirements of the City of
Aspen. The review and approval by the City is
offered only to assist the applicant's
understanding of the applicable Engineering
requirements." The issuance of a permit based
on construction documents and other data shall
not prevent the City of Aspen from requiring the
correction of errors in the construction
documents and other data.
DRAINAGE REPORT
For
CAYTON RESIDENCE REMODEL
121 E. HYMAN AVE.
Parcel Number 2735-12471-002
Menendez Architects
715 W. Main Street, Suite 104
Aspen, Colorado 81611
Prepared By:
Hans E. Brucker, PE
Pinnacle Design Consulting Group, Inc.
0805 Buckpoint Road
Carbondale, Colorado 81623
Revised
January 28, 2019
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CAYTON RESIDENCE REMODEL Drainage Report
121 E. HYMAN AVE.January 28, 2019
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TABLE OF CONTENTS
Cover Sheet……………………………………………………………………...…1
Table of Contents……………………………………………………………...…...2
Introduction / General Description………………………………………………...3
Project Description………………………………………………………………... 3
Existing Drainage Conditions……………………………………………………...3-4
Proposed Drainage System Design………………………………………………...4
URMP Design Principles…………………………………………………………..4
Water Quality…….………………………………………………………………...5
Peak Flow Rates On-Site - Rational Method………………………………............5-7
Pipe Capacities …………………………………………………………………….7-8
Inlet Capacities…………………………………………………………….…..…...8
Detention………………………………………………………….……………..…8-9
Outlet Works………………………………………………………………...……..9
Erosion Control………………………………………………………………….....9
Maintenance………………………………………………………………..………10
Nearest Flood Zone……………………………………………………...…….….. 10
Mudflow Area…………………………………………………………….…….….10
Conclusions……………………………………………………………….………..10-11
Recommendations ……………………………..………………………...…11
References……………………………………………………………………….....11
Engineering Certificate………………………………………………………...…..11
Appendix:
Reduced Civil Drawings (Sheets C1, through C10)
Computer Printouts
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121 E. HYMAN AVE.January 28, 2019
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Introduction / General Description
This drainage report has been prepared for the redevelopment of a detached duplex unit located at 121 E.
Hyman Avenue in Aspen, Colorado. The residence lies just south of Hyman Avenue, and is bordered by
an alley to the south and two existing residences on both the east and west sides. As part of the remodel,
all of the existing structure will be disturbed, including the basement slab. In addition, most of the
hardscape will be replaced to accommodate a new snow melt system.
A site visit was performed as part of this evaluation to verify, to the extent possible, the existing flow
regimes and to provide the necessary input data required to model storm water runoff quantities, which in
this instance, will be routed to a detention/water quality tank under the existing basement.
Project Description
The drainage areas analyzed in this report consist of the entire Lot area, including the proposed
driveway and tributary alley area, located on the south side of the duplex. The aforementioned
impervious areas, analyzed for water quality and detention, consist of approximately 3,099
square feet.
To complete the drainage analysis, we have analyzed the developed conditions on the site as
primarily (13) thirteen separate drainage Sub-Basins. Based on an analysis of the Lot and a site
visit, we have determined that little, if any of the storm water from the Lot flows offsite. This is
largely due to the site lying in a bowled area with the localized topography low lying in nature.
See Sheet C4 and C5 for the analyzed Basin and Sub-Basin areas.
As part of the drainage design, and per our discussions with City of Aspen Engineering staff, we
have concluded that redevelopment of the site, due to its low-lying nature and minimal green
space area, requires 100% detention. Furthermore, in this particular case, there is not sufficient
area on the site, except for under the basement slab, to satisfy the detention volume requirement.
As part of our final design calculations, we will a provide a detention tank which will
accommodate both the 100-year storm, 1-hour precipitation rate volume as well as provide for
the required water quality control volume for the site tributary area.
Existing Drainage Conditions
The Lot is considered flat with grades generally between zero (0) and three (3) percent. Steeper manmade
slopes occur along the west portion of the Lot where landscape rock walls exist. The Lot slopes from
southeast to northwest and storm water is ultimately tributary to the Roaring Fork River Basin via
infiltration.
The gross Lot area is 3,060 square feet or .0702 acres. The total disturbance area proposed, in order to
complete the proposed project improvements, is approximately 2,800 square feet. This includes all re-
graded areas, newly remodeled areas, and the hardscape improvements.
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Vegetation on the site consists of aspen trees, spruce trees and native grasses.
There are no known major defined drainage basins on the Lot or immediately up-gradient of the Lot
which pose a flooding concern in the study area.
A soils report was not provided at the time of this report, however, based an observation of an excavation
in 1999, the underlying soils consist of mostly sands and coarse gravels. Our experience suggests that
larger cobbles and boulders may also be present.
Based on a percolation test that we conducted near the Lot (see Sheet C1 for perc test location) we found
that the soils will percolate an acceptable rate in order to use a detention/water quality tank. In fact, in the
percolation test that we conducted, we observed a stabilized percolation rate of between (2) two and (4)
four minutes per inch (See Appendix for the results of the percolation test we conducted).
According to the City of Aspen Urban Runoff Management Plan (URMP) the project is classified as a
Major since more than 1,000 square feet of impervious area will be added or disturbed and more than
50% of the existing structure will be disturbed. To the best of our knowledge, a comprehensive drainage
study has not been previously performed for Lot.
Proposed Drainage System Design:
Per the City of Aspen URMP,latest addition, mitigation of on-site storm water requires providing water
quality treatment and detention for all of the impervious areas. As part of the drainage analysis, we have
provided calculations for the Major peak flow rates, water quality control volumes, and detention volume
requirements. In addition, we have utilized the 100-year storm precipitation in order to size the required
100-year detention tank.
The proposed design contemplates capturing the runoff from the residential impervious areas and routing
it to a single centralized detention tank and providing centralized detention.
To the best of our knowledge, items such as streets, utilities, and existing structures will not be adversely
impacted by the proposed drainage design, except for items that have been specifically noted on the
proposed Civil Plans for the Project.
URMP Design Principles:
The project, to the extent possible considering all of the input parameters, has been designed in concert
with the principles of the URMP. We considered water quality and detention early in the design process
and have used the only feasible location on the site for incorporating detention and water quality
mitigation measures for the project. The impervious areas have been minimized, to the extent possible,
given the ultimate design goals by the design team and the owner. We have maintained, to the extent
possible, the flat area on the site for water quality benefits.
The drainage system has been designed to allow for maintenance via the use of manhole access lids, pipe
cleanouts, and the incorporation of an accessible sand/grit interceptor.
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Water Quality:
As part of the site drainage calculations using the WQCV methodology, we have analyzed the site
primarily as a single, fully developed drainage basin. This drainage basin is shown on sheet C5. There
are also a few miscellaneous impervious areas that we are proposing to treat with the adjacent lawn area
on a 1:1 basis, as provided under the URMP water quality provisions. One location is a lower level which
includes access steps near an adjacent lawn on the east side of the residence. The second area that will be
treated on a 1:1 basis is a small section of roof located on the west side of the residence.
The Water Quality Control Volume (WQCV) calculations for the main drainage basin are based on
effective values determined by applying Level 0 MDCIA. The analysis includes the application of
Figures 8.14 and 8.16. Below is a summary table of the required water quality control volume for the
overall drainage basin, as well as, application of the following formula:
The WQCV (ft3) = (watershed in)( Basin Area ft2)(1 ft / 12 in)
121 E. Hyman Ave - Lot "A"
Water Quality Control Volume Requirement
Level 0
Basin Total %MDCIA
or Area Impervious Effective (Watershed-in)WQCV
Sub-Basin sf %in cf
Imperviousness
"C-1"3,099 100%100 0.255 65.9
Peak Flow Rates On-Site – Rational Method:
Based on the URMP, we will calculate the peak flow rates for each Sub-Basin and return period by using
the Rational Method as follows:
Rational Method:
Given:
Q = CIA
Where:
Q = Peak Discharge, cfs for the return period
C = Runoff Coefficient
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I = Rainfall Intensity, inches/hour
A = Drainage Area, acres
Based on the time-intensity-frequency curve for the Aspen area and for a 60-minute duration storm, the
intensity for a 5-minute time-of-concentration (the minimum rate in the URMP and recommended for
small City of Aspen Lots) is approximately as follows:
100-year intensity:I100 = 6.32 inches per hour
The historic 100-year runoff coefficient is as follows for NRCS Hydrologic Soils Type “B”:
C100 hist = 0.35
The developed 100-year runoff coefficients, assuming 100% impervious area, are as follows for NRCS
Hydrologic Soils Type “B”:
C100 dev = 0.96
Since we are not flood routing in this case with the 100-year and 5-year storms, we only calculated the
historic and developed 100-year peak flow rates for each Basin or Sub-Basin respectively.
The historic 100-year Sub-Basin Peak Runoff Rates are as follows:
121 E. Hyman - Lot "A"
Historic 100-Year Peak Flow Rate
Major
Basin Runoff %Total Area 100-year
Tc= 5-
min.100-year
Classification Impervious Area acres Runoff 100-year Peak Flow
sf Coefficient Intensity cfs
in/hr
"A"Pervious 0 3,060 0.070 0.35 6.32 0.16
The developed 100-year Sub-Basin Peak Runoff Rates are as follows:
121 E. Hyman Ave - Lot "A"
Developed 100-year Peak Flow Rates
Basin %Area Area 100-year Tc= 5-min.Major
or Impervious sf Acres Runoff 100-year 100-year
Sub-
Basin Coefficient Intensity Peak Flow
in/hr cfs
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"B1"91%1360 0.0312 0.82 6.32 0.162
"B2"100%293 0.0067 0.96 6.32 0.041
"B3"100%312 0.0072 0.96 6.32 0.043
"B4"100%9 0.0002 0.96 6.32 0.001
"B5"100%31 0.0007 0.96 6.32 0.004
"B6"31%119 0.0027 0.47 6.32 0.008
"B7"100%99 0.0023 0.96 6.32 0.014
"B8"100%284 0.0065 0.96 6.32 0.040
"B9"100%196 0.0045 0.96 6.32 0.027
"B10"69%144 0.0033 0.61 6.32 0.013
"B11"65%348 0.0080 0.59 6.32 0.030
"B12"76%196 0.0045 0.67 6.32 0.019
"B13"100%8 0.0002 0.96 6.32 0.001
Pipe Capacities:
The following table shows the maximum pipe flow capacities and the anticipated maximum flow rates
that will be generated to each exterior pipe based on the current drainage plan design:
Calculated Pipe Flows and Corresponding Maximum Pipe Capacities
Is Pipe Capacity
Pipe Peak Flow sufficient
Description in Pipe per
cfs Channel Reports
4" PVC Drain 0.215 Yes
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CAYTON RESIDENCE REMODEL Drainage Report
121 E. HYMAN AVE.January 28, 2019
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at -1.71% grade d/Dfull = 0.19/0.33 = 0.57
57% full
4" PVC 0.062 Yes
at -1.76% grade d/Dfull = 0.10/0.33 = 0.30
30% full
4" PVC 0.277 Yes
at -2.0% grade d/Dfull = 0.22/0.33 = 0.67
67% full
6" PVC 0.277 Yes
at -1.0% grade d/Dfull = 0.21/0.50 = 0.42
42% full
Inlet Capacities:
The 12” NDS Catch Basins with a cast iron grate will handle approximately 0.07 cfs with a 50% clogging
factor. The maximum required overflow from to a 12” inlet will be no greater than about 0.03 cfs based
on Sub-Basin “B11”.
Detention:
In order to provide detention for the proposed improvements, the only feasible solution is a cast in place,
concrete detention tank. Based on a 100-year precipitation rate of 1.23 inches and a total impervious
tributary area of 3,099 ft2, the total detention requirement is (3,099 ft3 x 1.23 in/(12 in/ft) = 318 ft3).
The total tank volume required, assuming we add in the water quality, is (318 ft3 + 66 ft3 = 384 ft3).
We have proposed a tank with the following usable inside dimensions:
W = 5.5 ft
L = 13 ft
H = 6.93 ft
The total usable volume is therefore (5.5 ft x 13 ft x 6.93 ft = 495.5 ft3).
The tank, therefore, has been designed with sufficient capacity.
We have located the detention tank in the only feasible position on the Lot, in order to maximize the
setback distances from the property line, and adjacent foundation. The tank will be cast in place concrete
and has been designed in concert with the structural plans for the new remodel.
The percolation area within the perimeter of the detention tank must also be verified by the following,
based on the drywell side wall area:
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If we use a percolation rate of 4 min/in, the hydraulic conductivity of the soil will be 30 ft/day or 3.47 x
10-4 ft/sec.
The minimum area can be calculated based on the URMP using the following equation:
AP = (Vr)/(K)(43,200)
Where:
AP = Total area of the sides of the percolation area, square feet
Vr = Runoff volume for 100-year event and Water Quality Control Volume, cubic feet
K = Hydraulic conductivity of soil, feet/second.
AP = (384 ft3)/[(3.47 x 10-4)(43,200)] = 25.6 ft2
The bottom 48” of the detention tank area is far greater then 25.6 ft2, by inspection, so this requirement is
satisfied.
The drain time of the detention tank is a function of the total storage height. For calculation purposes we
will assume that the total height is 7 feet. Therefore, if we use a conservative percolation rate of
5 min./inch, the total drain time will be as follows:
7 ft x 12 in/ft x 5 min./in) x (1 hr/60 min) = 7 hours
Therefore, a conservative estimate of the drain time will be 7 hours.
Outlet Works:
The outlet works for the project will be facilitated via infiltration from the detention/water quality tank.
We have also provided a secondary duplex pump, backup system which will pump to an exterior area
drain (release location) adjacent to the Hyman Ave. R.O.W. Given the side of the detention/water quality
tank, coupled with the screened rock under the basement slab, we do not anticipate that this system will
ever initiate pumping.
Erosion Control:
Given the size of the project, disturbance to the site will be minimal. However, we are recommending
that silt fence be installed along the down-gradient portion of the site during construction, if feasible.
There is not sufficient area on the site for a vehicle tracking pad, however, the site will likely be hand
excavated, so mud tracking will not be an issue. All excavations should be planned to minimize, to the
extent possible, damage to surrounding vegetation.
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CAYTON RESIDENCE REMODEL Drainage Report
121 E. HYMAN AVE.January 28, 2019
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Maintenance:
Area drains, and inlets shown on the plan should be checked for debris buildup on an annual basis. Any
encountered debris should be removed as required to ensure that facilities are working properly. The
drainage collection system should also be checked after construction to ensure that they are free of debris
and silt. If debris and silt are encountered, it should be removed via pumping or other mechanical means.
Per the City of Aspen URMP, drywells must be inspected and maintained yearly to remove sediment and debris that
may be encountered. Maintenance should be in accordance with a submitted maintenance plan to the City
describing the maintenance schedule that will be facilitated by the owners of the residence.
Minimum inspection and maintenance for the drywell should include the following:
- Inspect drywell annually and after every storm that exceeds 0.5 inches of precipitation.
- Remove sediment, debris and trash or other waste material from the drywell and dispose of at suitable
disposal site per local, state and federal waste regulations.
- Evaluate the drain-down time of the drywell to ensure that the maximum 24 hour maximum is not being
exceeded. If the drain-down time is being exceeded then the drywell percolation area should be further
cleaned via pumping or jetting. Additional percolation perforations in the sidewalls could also be pursued
by additional drilling methods.
The following checklist should be administered for the pump vault biannually:
- Check the pump vault twice annually. Before the Spring snowmelt is preferable.
- Refer to manufacturer’s instructions for specific pump maintenance.
- Check pump electrical circuit and Ground Fault Circuit Interrupter (GFCI) to assure proper function.
- Remove the pump vault cover and check for silt or debris. – Remove as may be required.
- Check that the float level controls including the alarm float are functioning properly.
- Check that the weep hole in the discharge pipe is free of obstructions.
- Exercise pumps and any valves on the system piping.
- Check for any leaks around discharge pipe.
- Make sure that the pump body is submersed in water so that the seals do not dry out.
- Check that the exterior discharge location is free of obstructions and that it will drain
back to the pump vault accordingly.
-Verify that alarm (activated when the tank is full) is functioning.
Nearest Flood Zone:
The subject property is not located within a major FIRM area.
Mudflow Area:
Based on URMP Figure 7.1c, the Lot is not in a mudflow Zone.
Conclusions:
The proposed design utilizes a treatment train approach, whereby the runoff will be collected once it has
passed through lawn grass and drain pipe with disconnected impervious areas (used as much as possible
in this case) via the use of area drains and roof drains. The drainage will then be conveyed to the
detention tank/water quality tank from where it will infiltrate into the ground.
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APPENDIX
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Channel Report
Hydraflow Express Extension for AutoCAD® Civil 3D® 2013 by Autodesk, Inc.Friday, May 18 2018
Pipe A - 4 inch PVC flowing 0.215 cfs at -1.71% Grade
Circular
Diameter (ft)= 0.33
Invert Elev (ft)= 100.00
Slope (%)= 1.71
N-Value = 0.009
Calculations
Compute by:Known Q
Known Q (cfs)= 0.22
Highlighted
Depth (ft)= 0.19
Q (cfs)= 0.220
Area (sqft)= 0.05
Velocity (ft/s)= 4.31
Wetted Perim (ft)= 0.57
Crit Depth, Yc (ft)= 0.27
Top Width (ft)= 0.33
EGL (ft)= 0.48
0 1
Elev (ft)Section
99.75
100.00
100.25
100.50
100.75
101.00
Reach (ft)
02/28/2019
Channel Report
Hydraflow Express Extension for AutoCAD® Civil 3D® 2013 by Autodesk, Inc.Friday, May 18 2018
Pipe B - 4 inch PVC flowing 0.062 cfs at -1.76% Grade
Circular
Diameter (ft)= 0.33
Invert Elev (ft)= 100.00
Slope (%)= 1.76
N-Value = 0.009
Calculations
Compute by:Known Q
Known Q (cfs)= 0.06
Highlighted
Depth (ft)= 0.10
Q (cfs)= 0.060
Area (sqft)= 0.02
Velocity (ft/s)= 2.72
Wetted Perim (ft)= 0.39
Crit Depth, Yc (ft)= 0.14
Top Width (ft)= 0.30
EGL (ft)= 0.21
0 1
Elev (ft)Section
99.75
100.00
100.25
100.50
100.75
101.00
Reach (ft)
02/28/2019
Channel Report
Hydraflow Express Extension for AutoCAD® Civil 3D® 2013 by Autodesk, Inc.Friday, May 18 2018
Pipe C - 4 inch PVC flowing 2.77 cfs at -2.00% Grade
Circular
Diameter (ft)= 0.33
Invert Elev (ft)= 100.00
Slope (%)= 2.00
N-Value = 0.009
Calculations
Compute by:Known Q
Known Q (cfs)= 0.28
Highlighted
Depth (ft)= 0.22
Q (cfs)= 0.280
Area (sqft)= 0.06
Velocity (ft/s)= 4.61
Wetted Perim (ft)= 0.63
Crit Depth, Yc (ft)= 0.30
Top Width (ft)= 0.31
EGL (ft)= 0.55
0 1
Elev (ft)Section
99.75
100.00
100.25
100.50
100.75
101.00
Reach (ft)
02/28/2019
Channel Report
Hydraflow Express Extension for AutoCAD® Civil 3D® 2013 by Autodesk, Inc.Friday, May 18 2018
6 inch PVC flowing 0.277 cfs at -1.00% Grade
Circular
Diameter (ft)= 0.50
Invert Elev (ft)= 100.00
Slope (%)= 1.00
N-Value = 0.009
Calculations
Compute by:Known Q
Known Q (cfs)= 0.28
Highlighted
Depth (ft)= 0.21
Q (cfs)= 0.280
Area (sqft)= 0.08
Velocity (ft/s)= 3.55
Wetted Perim (ft)= 0.71
Crit Depth, Yc (ft)= 0.27
Top Width (ft)= 0.49
EGL (ft)= 0.41
0 1
Elev (ft)Section
99.75
100.00
100.25
100.50
100.75
101.00
Reach (ft)
02/28/2019
Note: The 12” (1212) grate will flow 66.35 gpm or 0.14 cfs (0.07 with a 50% clog factor).
02/28/2019
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