HomeMy WebLinkAboutFile Documents.200 S Aspen St.0007.2017 (32).ACBK1
Drainage Report
200 S. ASPEN STREET
ASPEN, CO
November 21, 2016
Comment Responses: April 24, 2017
Comment Responses II: June 12, 2017
Prepared by
Richard Goulding, P.E.
Roaring Fork Engineering
592 Highway 133
Carbondale, CO 81623
City of Aspen
Received
6/27/17
Building Department
Reviewed by Engineering
07/05/2017 2:18:22 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.
2
Drainage Report
200 S. ASPEN STREET
ASPEN, CO
I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 200 S. ASPEN ST. WAS
PREPARED BY ME FOR THE OWNERS THEREOF IN ACCORDANCE WITH THE PROVISIONS OF THE
CITY OF ASPEN (COA) URBAN RUNOFF MANAGEMENT PLAN (URMP) 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.
City of Aspen
Received
6/27/17
Building Department
3
RICHARD GOULDING, P.E.
RFE Project # 2016-02
Contents
1.0 General .................................................................................................................................................... 4
1.1 Existing Site ........................................................................................................................................ 4
1.2 Proposed Conditions ........................................................................................................................... 4
1.3 Previous Drainage Studies .................................................................................................................. 5
1.4 Offsite Drainage & Constraints ........................................................................................................... 5
2.0 Drainage Basins ...................................................................................................................................... 5
2.1 Basins .................................................................................................................................................. 5
2.2 Peak Discharge Calculations ............................................................................................................... 5
3.0 Low Impact Site Design .......................................................................................................................... 6
3.1 Principles ............................................................................................................................................. 6
4.0 Hydrological Criteria .............................................................................................................................. 8
4.1 Storm Recurrence and Rainfall ........................................................................................................... 8
4.2 Storage Volumes Methodology .......................................................................................................... 8
5.0 Hydraulic Criteria ................................................................................................................................... 8
5.1 Piping .................................................................................................................................................. 8
5.2 Inlet Sizing ........................................................................................................................................ 10
6.0 Proposed Facilities ................................................................................................................................ 10
6.1 Proposed structures ........................................................................................................................... 10
7.0 Operation and Maintenance .................................................................................................................. 11
7.1 Bio-Planter/Swale with Screened Rock ............................................................................................ 11
8.0 Appendices ............................................................................................................................................ 12
Drawings 11x17 ...................................................................................................................................... 12
Weir Hydraulics ...................................................................................................................................... 12
City of Aspen
Received
6/27/17
Building Department
4
1.0 General
1.1 Existing Site
The property under review is located near the downtown core of Aspen at 200 S. Aspen St. This parcel is
located on the south east corner of the inter section of S. Aspen St. and Hopkins Ave. Currently Hotel
Lenado sits on the 8,970 square foot (sf) lot. The proposed structure is a multi-unit 22,850 sf lodge, and
will result in the demolition /remodeling of the existing structure. To the south of the property is an alley
while to the east is a City of Aspen park. The high point on the property is to the southeast at 7906’ while
the low point is to the northwest at 7895’. There are large mature cotton wood trees running along the
north side of the property that will be preserved. This property is within the Aspen Mountain Drainage
Basin delineated by the City of Aspen’s Master Drainage Plan.
HP Geotech performed a field exploration on February 23th, 2016. A sub-surface soils report was
produced on March 21st, 2016. The soil profile consists of 3 feet of granular fill overlaying dense, slightly
silty sandy gravel and cobbles with boulders down to the depth explored of 40 ft. No ground water was
encountered during the exploration. A percolation test was also conducted yielding an infiltration rate of
about 1/2 minute per inch. The bedrock is also known to be generally deep in this area.
1.2 Proposed Conditions
This project is classified as a ‘Major Project’ in Table 1.1 of the Urban Runoff Management Plan
(URMP), as the proposed development is over 1000 square feet and disturbs an area greater than 25% of
the site footprint. The intent of this report is to demonstrate compliance with the requirements of the City
of Aspen’s (COA) URMP. The Low Impact Design (LID) Principles in the introduction of the manual
were used as a guide throughout the design process.
The proposed development involves the demolition and renovation of the existing structure. Grading
around the new structure will also take place to accommodate new walkways, planters and the driving
surfaces for access to garages and underground parking. Site walls will also be utilized to retain grades to
make flatter areas for walkways and storm retention planters/swales. No changes to land use or soil types
are planned. Cut depths of up to 24’ feet are expected and site grading cuts and fills are very minor
compared to the foundation excavation.
The runoff from impervious surfaces will be collected in a system of roof drains, inlets, trench drains and
under drains. These drains will daylight into a series of two bio-swale planters to the west of the structure.
These bio-swales will have screened rock beneath them wrapped in 140N Mirafi filter fabric to prevent
them from clogging with fines. There will be enough storage within the voids of the washed/screened
rock for the Water Quality Capture Volume (WQCV). The screened rock has a PVC liner beneath it to
prevent infiltration near the structure. The PVC liner will be sloped to an orifice plate at the bottom of the
planter. This orifice will restrict the out flow form each planter. There are two planters in series for
capturing runoff. During large events overflow weirs will convey water into the next planter and into a
24” inlet leading to the COA storm system via a 15” RCP pipe.
City of Aspen
Received
6/27/17
Building Department
5
1.3 Previous Drainage Studies
The property is not within a Mudflow Area as defined by the COA Storm Drainage Master Plan. The
City of Aspen Master Drainage Plan shows a 30” CMP storm pipe running down S. Aspen St. This is part
of System 3 delineated by the Drainage Master Plan.
1.4 Offsite Drainage & Constraints
There is City park to the east the does have some grading sloping toward the 200 S. Aspen St. property.
Grading has been permitted by the City parks department to mend this issue. Fill will be placed along the
north side of the building to create a swale leading north to the curb and gutter.
2.0 Drainage Basins
The entire site is routed to one overall point of concentration, so the entire site was considered as a single
drainage basin. This was done to determine the Water Quality Capture Volume. This basin was then split
into sub-basins comprised of the roof area, driveways, walkways and patios. The sub-basins were then
used to calculate 100 year peak flow conditions for sizing individual inlets and piping.
2.1 Basins
Basin 1 is comprised of most of the lot, including some walkway and driveway areas along the property
line. This basin is 9,866 sf, with 7,852 sf impervious, resulting in the basin being 79.59% impervious.
2.2 Peak Discharge Calculations
Peak Flows were calculated for 10 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 0.77 inches for the
10-year event and 1.23 inches for the 100-year event. Equation 2.1 was referenced when solving for the
Rainfall Intensity.
I = 88.8P1/(10+Td )1.052
Runoff Coefficients (C), a function of the Soil Group (in this case 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 the Major Basin. Q allowable was calculated the same way, except the basin was treated as
undeveloped, or 100% pervious. The Peak Discharge (Qp) in cubic feet per second (cfs) is given by
equation 3.1.
Qp= CIA
Qp= Peak Discharge (cfs)
A= Area (Acres)
I= Rainfall intensity (inches per hour)
C= Runoff Coefficient (Unitless)
The tables below contain the peak flows for developed and undeveloped conditions for 10 and 100 year
storm events.
City of Aspen
Received
6/27/17
Building Department
6
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 with the architects early in the design. Comments from
owners were considered and analyzed. Multiple site visits ensured proper understanding of existing
conflicts. Little room was left for WQCV so creative measures were explored right away.
Principle 2: Use the entire site when planning for storm water quality treatment.
The entire site was analyzed when determining the WQCV. The screened rock beds beneath the bio-swale
planters are sized for the WQCV. No runoff from an impervious surface will be discharge without first
being collected and routed through these planters.
Principle 3: Avoid unnecessary impervious area.
10 Year Peak Discharge Developed Calculations
1 Hour(P 1)0.77
Return Period 10
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 9866.00 7852.00 79.59% 0.580 5 3.96 0.52
10 Year Peak Discharge Pre Development Calculations
1 Hour(P 1)0.77
Return Period 10
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 9866.00 0.00 0.00% 0.150 5 3.96 0.13
100 Year Peak Discharge Developed Calculations
1 Hour(P 1)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 9866.00 7852.00 79.59% 0.650 5 6.33 0.93
100 Year Peak Discharge Pre Development Calculations
1 Hour(P 1)1.23
Return Period 10
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 9866.00 0.00 0.00% 0.350 5 6.33 0.50
City of Aspen
Received
6/27/17
Building Department
7
The only surfaces that are to be impervious are imperative for walkways and driving surfaces. The rest are
roof area.
Principle 4: Reduce runoff rates and volumes to more closely match natural conditions.
All runoff from the site will be routed to the top of the bio-swale planters at the west of the property,
which has capacity for the WQCV. This will decrease the existing runoff rates from impervious areas on
the property. Over flow from these planters will be controlled via weirs sized to handle the 100 year peak
flow.
Principle 5: Integrate storm water quality management and flood control.
All inlets and piping are sized to accommodate the 100 year peak flows. Pipes were sized to ensure 100
year flows didn’t exceed 80% of the pipe’s capacity. Orifice plates are utilized at the bottom of each
water quality planter while overflow weirs are sized to accommodate the peak 100 yr flow. Overflows
will be captured and routed to the COA storm system.
Principle 6: Develop storm water quality facilities that enhance the site, the community and the
environment.
Runoff will be discharged at the top of a bio-swale running through a landscaped planter with specific
species of plants and medium to help filter runoff. Water running through the swale and planter will also
create a visual effect for anyone passing by.
Principle 7: Use treatment train approach.
There are two planters in series where all runoff will be routed. First the storm water will discharge on the
surface of the planter where a landscape swale with rip rap will slow the flows allowing them to permeate
into the soil of the planter and down into the screened rock reservoir. From here a orifice plate will restrict
the flow. If the first planter is to fill a weir will discharge into the next planter where a similar process will
take place.
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 reduces ice buildup. All facilities are easily accessible and
are installed with cleanouts when applicable. The planters are shallow and planting material along with
the screened rock wrapped in filter fabric can be easily accessible and maintained .
Principle 9: Design and maintain facilities with public safety in mind.
All slopes were reduced as much as possible to eliminate steep walking surfaces or uneven transitions.
Wall and planters meet zoning codes and walk ways meet ADA requirements where possible.
City of Aspen
Received
6/27/17
Building Department
8
4.0 Hydrological Criteria
4.1 Storm Recurrence and Rainfall
The property is in the commercial core and all storm water will discharge into the COA storm system.
Therefore this property classifies as a “Urban area served by public storm sewer”. This site is also within
the Aspen Mountain Drainage Basin so only the WQCV requires detaining.
The 1 hour Rainfall depth (P1) is given in Table 2.2 of the URMP as 0.77 inches for the 10-year event and
1.23 inches for the 100-year event. The intensity in inches per hour for different storm durations (Td) were
calculated using Equation 2.1 from the URMP.
4.2 Storage Volumes Methodology
Storage requirements were calculated using the impervious area of the site established in section 2.2. The
impervious percentage of 79.59% was used in conjunction with Figure 8.13 of the URMP to obtain the
rainfall depth. This value was then multiplied by the basin area to obtain the WQCV. Below is a table
documenting these values.
5.0 Hydraulic Criteria
5.1 Piping
Pipes used in all drainage systems will be SDR-35 PVC with a Manning’s coefficient of 0.01. The pipes
were sized to accommodate peak flows for a 100 year event at 80% full. If the water level in the pipe
exceeds the 80% full criteria, then the pipe is deemed inadequate. Sub-basins were delineated to isolate
specific peak flows. Below is a table of sub-basin peak flows.
Onsite Required Water Quality Capture Volume (WQCV)
Basin Total Area Impervious Area Impervious WQCV Table Value WQCV Storage
(#) (ft
2)(ft2) (%) (in) (ft
3)
Onsite basin 1 9866.00 7852.00 79.59% 0.161 132.37
Peak Discharge Developed Calculations
1 Hour(P 1)1.23
Return Period 100
Sub‐Basin ID Total Area Total Area Imp. Area Impervious % C Value Time of C Intensity Q Max
See(D1) acre ft2 ft2 Percentage From Table (Td) I=88.8P1/(10+Td)01.052 ft3/sec
1.1 0.102 4421.60 4421.60 100.00% 0.950 5 6.33 0.61
1.2 0.020 873.15 873.15 100.00% 0.950 5 6.33 0.12
1.3 0.023 991.80 991.80 100.00% 0.950 5 6.33 0.14
1.4 0.009 384.15 384.15 100.00% 0.950 5 6.33 0.05
1.5 0.007 296.85 296.85 100.00% 0.950 5 6.33 0.04
1.6 0.018 785.74 0.00 0.00% 0.350 5 6.33 0.04
City of Aspen
Received
6/27/17
Building Department
9
Below is a table showing which sub-basins contribute runoff to each pipe and the total cfs accumulated
during a 100 year peak flow event.
Calculated pipe sizes were tested for hydraulic capacity at 80%. Values for depth of flow for each pipe
were calculated. Design charts giving Q design / Q full were downloaded from FHWA and the equations in
Section 4.8.4 of the URMP were used as the basis for these calculations. Calculated pipe sizes and
internal depth of flow within the pipes is shown below.
Storm System Pipes
Pipe System Pipe Contibuting Sub‐Basins Peak Flows (CFS)
A 1 1.2, 1.3, 1.4, 1.5 0.35
2 1.2, 1.3, 1.4, 1.5 0.35
3 1.2, 1.3, 1.4, 1.5 0.35
4 1.6 0.04
5 1.6 0.04
6 1.6 0.04
RCP 1 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 1.00
RCP2 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 1.00
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
1 0.35 0.01 12.80% 0.24 2.831 4.0 3.200 Yes
2 0.35 0.01 2.00% 0.33 4.009 8.0 6.400 Yes
3 0.35 0.01 2.00% 0.33 4.009 8.0 6.400 Yes
4 0.04 0.01 14.39% 0.10 1.225 4.0 3.200 Yes
5 0.04 0.01 9.11% 0.11 1.335 4.0 3.200 Yes
6 0.04 0.01 9.18% 0.11 1.333 4.0 3.200 Yes
RCP 1 1.00 0.013 1.12% 0.61 7.305 15.0 12.000 Yes
RCP2 1.00 0.013 1.00% 0.62 7.462 15.0 12.000 Yes
Onsite Piping Capacity
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
1 0.35 0.01 4.0 0.087 12.80% 0.887 0.396 0.49 1.94
2 0.35 0.01 8.0 0.349 2.00% 2.226 0.158 0.30 2.40
3 0.35 0.01 8.0 0.349 2.00% 2.226 0.158 0.30 2.40
4 0.04 0.01 4.0 0.087 14.39% 0.941 0.042 0.16 0.62
5 0.04 0.01 4.0 0.087 9.11% 0.748 0.053 0.18 0.70
6 0.04 0.01 4.0 0.087 9.18% 0.751 0.053 0.18 0.70
RCP 1 1.00 0.013 15.0 1.227 1.12% 6.851 0.146 0.29 4.31
RCP2 1.00 0.013 15.0 1.227 1.00% 6.474 0.155 0.30 4.50
Depth Of Flow‐section 4.8.4 Sewer Sizing
City of Aspen
Received
6/27/17
Building Department
10
5.2 Inlet Sizing
Inlets were sized to accommodate peak flow conditions for each basin. Equations 4-17 through 4-20 were
used to size inlets, which incorporate a 50% clogging factor and a 40% opening in grates. A water depth
of 0.04’ was assumed for the rectangular inlets like the trench drain and slot drain while a 3” water depth
was used for the round inlets set in landscaped depressions. All the inlets were treated as sumps as they
will be set a minimum of 0.5 inches below the flow lines. Calculated inlet sizes and specifications for the
selected trench drain channel flows are shown below.
6.0 Proposed Facilities
6.1 Proposed structures
A series of two planters overflowing into one another with screened rock below are being utilized to meet
URMP requirements for the entire site. A series of pipes, inlets and roof down-spouts route runoff to this
structure. The analysis of the conveyance structures is contained in section 5.0 of the report. The screend
rock below the planting medium was sized for the WQCV calculated in section 4.2. The planters are lined
with PVC liner below the screened rock to prevent infiltration next to the structure. This PVC liner will
eventually convey the runoff into the COA storm system. Sheet C104 shows the dimensions and location
of these planters. Each planter is fitted with an over flow weir. The weirs are large enough to contain the
100 yr developed peak flow. The water elevation calculated during the 100 yr. event is shown in the
appendix. This water surface elevation was then used to size the orifice plates.
The planters have been designed to retain the WQCV above the finished grade or bio-planting medium
and below the weirs. The 3D surfaces within Auto CAD Civil 3D were utilized to calculate the storage of
the bio-planter/ponds. Water surfaces at the level of the weir were created to compare against the finished
grade surface. This was done to ensure the WQCV was retained before the lower weir is over toppled.
The Upper Planter has a storage capacity of 2.1 cubic yards or 56.7 cubic feet. The Lower Planter has a
capacity of 3.36 cubic yards or 90.72 cubic feet. Below is the cut fill summary. This results in a storage
volume of 147.42 cubic feet.
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) (ft
2)(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)
SLOT DRAIN 1.4 384.15 384.15 100.00% 0.950 5 6.33 0.053 .5" x 17.25' 0.5 1068 0.742 0.744 Yes
ALLEY TRENCH DRAIN 1.5 296.85 296.85 100.00% 0.950 5 6.33 0.041 4" x 11.25' 4 135 0.750 0.753 Yes
Sub Basin and Circular Inlet Calculations
1 Hour(P 1)1.23 m=40% Ys=.25 (Depress inlet by 3")
Return Period 100 Cg=50% Co=0.65
Inlet ID Basin ID Total Area Imp. Area Impervious C Value Concentration Intensity Q Max Inlet Type Diameter Area(EQ. 4‐20) Inlet Capacity (EQ 4‐19) Has Capacity
See(D1) (ft
2)(ft2)(%)From Table (Td) I=88.8P1/(10+Td)1.052 ft3/sec Wo (inches) Ae=(1‐Cg)mA Q=CoAe√2gYs (Yes/No)
24" STORM MANHOLE 1.1‐1.5 6967.55 6967.55 100.00% 0.950 5 6.33 0.961 24" Round 24 0.628 1.638 Yes
8" INLET 1.6 785.74 0.00 0.00% 0.350 5 6.33 0.040 8" Round 8 0.070 0.081 Yes
City of Aspen
Received
6/27/17
Building Department
11
At the bottom of each layer of screened rock is an orifice plate. This orifice plate restricts the flow down
to the 100yr historic peak flow when the water level reaches the 100yr developed elevation flow over the
weir. In other words the effective head elevation for sizing the orifice was derived from the water surface
elevation when the 100 peak flow is flowing over the weir. Below is a table showing the values used
along with section 5.8.1 of the URMP.
The WQCV= 132.37 cf. When the water level is at 5.87 feet above the weir is will take 4.41 minutes for
132.37 cubic feet of runoff to flow through the orifice.
7.0 Operation and Maintenance
7.1 Bio-Planter/Swale with Screened Rock
The screened rock bed beneath the bio-swale planters and piping must be maintained periodically to
ensure proper operation. A maintenance plan shall be submitted to the City 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:
‐ During the first year draw down should be checked for every event over 0.25” of precipitation
to ensure no significant backups are occurring.
‐ The maximum drain down time is 24 hours.
‐ The fabric surrounding the bed will be the first component to clog. If the fabric becomes
clogged replacement will be needed.
‐ Piping systems and sumps should be checked during and after storms routinely.
‐ Clean out and drainage basins provide access for hoses and vacuum equipment.
‐ After the first year the system should be cleaned out at least once a year and more if the first
year inspection proves more maintenance is required.
‐ More frequent cleaning reduces the amount of debris entering the system and reduces the
need for more intense maintenance.
‐ Remove debris from the gravel bed routinely. If the gravel has been contaminated by soil and
sand, clean or replace gravel as necessary. Gravel will possibly have to be replaced every 5-
10 years for proper perforation into the lawn.
‐ Clean the inside of the perforated pipe with a 6” or 12” pipe cleaner accessed through
cleanouts. This should be done yearly, or as necessary if the system is not infiltrating properly
or if the system has become contaminated.
‐ Ensure heat tape is functioning before colder months to prevent damage to piping.
‐ If the storm system is not maintained properly, replacement of parts or of the entire system
may be necessary.
Perforation Orifice Calc
Q=Flow Rate 0.50 cfs
Co=Discharge Coeff. (square) 0.6
A=Area of Orifice 0.043 ft 2
H= Effective Head 5.87 ft
g= Grav Const. 32.2 ft/sec2
Perforation Diameter 0.23 ft
2.81 in
City of Aspen
Received
6/27/17
Building Department
12
8.0 Appendices
Drawings 11x17
Weir Hydraulics
City of Aspen
Received
6/27/17
Building Department