HomeMy WebLinkAboutFile Documents.183 Red Mountain Rd.0038.2018 (53).ARBK183 Red Mountain Road-Drainage Report August 29, 2018
183 Red Mountain Road
Development Project
Drainage Report
Aspen, Colorado
Prepared for:
Aspen Starwood LLC
623 East Hopkins Avenue
Aspen, CO 81611
Prepared by:
Sopris Engineering, LLC
502 Main Street Suite A3
Carbondale, Colorado 81623
SE Project Number: 14237.05
January 2, 2018
Revised August 29, 2018
12/1/2018
Reviewed by Engineering
04/01/2019 9:55:16 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.
183 Red Mountain Road-Drainage Report August 29, 2018
Table of Contents
I. General ..............................................................................................................................................................2
A. Description of Existing Site ...........................................................................................................................2
B. Description of the Proposed Improvements .................................................................................................2
C. Purpose of Report .........................................................................................................................................2
D. Previous Drainage Studies: ..........................................................................................................................2
E. Flood Hazard Delineation: ............................................................................................................................2
II. Onsite and Offsite Drainage Basin Descriptions..............................................................................................3
A. Historic Drainage Basins ...............................................................................................................................3
B. Post Development Drainage Basin Conditions ............................................................................................3
III. Drainage Analysis Methods ..............................................................................................................................5
A. Hydrologic Design Criteria and Assumptions:..............................................................................................5
B. Hydraulic Design Criteria and Assumptions:................................................................................................6
IV. Proposed Water Quality Treatment Mitigation .................................................................................................7
V. Stormwater Detention .......................................................................................................................................8
VI. Low Impact Design ............................................................................................................................................9
VII. Maintenance Plan ........................................................................................................................................... 10
VIII. Sediment and Erosion Control/Construction BMPs ...................................................................................... 12
IX. Conclusion ...................................................................................................................................................... 12
X. Engineer’s Statement of Design Compliance ............................................................................................... 13
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I. General
A. Description of Existing Site
The site is located at 183 Red Mountain Road. The drainage study area contains 0.92 acres of land located
south of Hunter Creek and west of Red Mountain Road in Aspen, Colorado. Area topography generally
consists of varying slopes towards the northwest. The development portion of the property contains 0.331
acres and consists of existing buildings, established ground cover, trees, and sparsely grown shrubs with an
overall estimated impervious ground cover of 3,000 sf. The subject property is bounded by Hunter Creek on
the north, Red Mountain Road on the east, private land on the south and Rio Grande Trail Corridor on the west
sides and is within a fully developed semi urban neighborhood. Drainage patterns are overland flow with
several local depressions, and stormwater runoff is intercepted by Hunter Creek located north of the site.
B. Description of the Proposed Improvements
The proposed improvements for this property include construction of a new building within the established
disturbance envelope with a resultant effective impervious ground cover area of approximately 1,600 sf which
is almost half of what currently exists onsite. The reduction of impervious area is a direct result of integrating
green roofs and permeable pavers into the design.
Drainage patterns will be collected and conveyed through onsite water quality treatments, primarily consisting
of green roof, a bio-retention cell, and conveyance through grassed swales to promote onsite runoff routing
and to achieve maximum possible practice of “Minimizing Directly Connected Impervious Area” (MDCIA) as
defined in the City’s URMP 8.5.1.1..
C. Purpose of Report
Based on the location of the subject property and the proposed uses for redevelopment, the purposes of this
Drainage Report are to:
• Comply with the City of Aspen’s URMP for a “Major Design” project;
• Estimate flow rates for the improved areas and to size stormwater mitigation infrastructure;
• Estimate required water quality capture volumes and design facilities to provide the required treatment
of stormwater runoff;
• Promote the stormwater quality management principles outlined within COA URMP to include
promoting Low Impact Design strategies within the drainage mitigation system;
D. Previous Drainage Studies:
Given the age of the existing building and vacant land, we could not find a site drainage study or grading plans
associated with the subject property.
E. Flood Hazard Delineation:
The majority of site is located in Zone X which is outside the 500-Year Flood Plain according to FIRM Panel
203 of 325 (map ID #08097C0203 C) with effective date of June 4, 1987. A portion of the disturbance
envelope is within Zone AE where the 100-year base flood elevations are determined. The proposed
development will stay out of that area and will be left undisturbed.
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II. Onsite and Offsite Drainage Basin Descriptions
A. Historic Drainage Basins
1. Historic Onsite Basins: Two primary onsite drainage basins have been delineated to evaluate,
estimate and compare historic peak runoff rates against proposed conditions. The delineated basins
are consist with the anticipated limits of disturbance associated with the proposed improvements and
also correspond to an existing point of concentration where flows enter Hunter Creek. This location is
identified on Exhibit D-1 as Design Point 1 (DP 1) and correlates to the primary discharge location
under proposed conditions. The two onsite historic basins are further described below:
a. Historic Onsite Basin 1: This basin includes the majority of the anticipated limits of
disturbance and includes the majority of area that contributes runoff towards DP 1.
b. Historic Onsite Basin 2: This basin includes a localized depression located along the southern
edge of the subject property. Flows that exceed the available volume within this natural
depression overtop and continue to flow generally towards DP 1 and ultimately to Hunter
Creek.
Each of these basins were analyzed under historic conditions; ie no impervious area, to estimate peak
runoff rates for the 10 and 100-year storm events. The results are summarized within Table 1 and an
historic drainage basin delineation map is provided as an attachment to this report; re. to Exhibit D-1.
2. Offsite Drainage Basins: A study of the surrounding offsite topography and tributary areas were
performed through survey topography, GIS topography and site visits. The results were the
delineation of three (3) offsite basins as described below and as illustrated on attached Exhibit D-2.
a. Offsite Basin 1: This basin consists of runoff generated from the west side of Red Mountain
Road and areas south of the property. In summary, the curb/gutter on the west side of Red
Mountain Rd directs runoff down Shady Lane which is a private road that passes through the
property south of 183 Red Mountain Rd. It appears that the majority of this runoff is likely
contained within the corridor of Shady Lane however there is the potential for some of the
runoff to overtop the roadside berm and be conveyed onto the neighboring property to the
south and captured within an existing 3-ft deep depression that extends onto the subject
property. Flows captured within this existing depression are either evaporated or infiltrated
into the underlying soils.
b. Offsite Basin 2: This basin lies along the east and southeast side of the site and consists of
natural landscaping areas. Runoff from this area is directed onto the subject property at the
southeast corner and will be collected within a proposed drainage swale and routed through
the site to include the proposed bioretention area.
c. Offsite Basin 3: This basin lies along the west side of the site and consists of the Rio Grande
Trail and landscape berm area. Runoff from this area is directed onto the northwest corner of
the subject property before being conveyed to Hunter Creek. This basin falls outside the limits
of the proposed development and therefore will have no impact on the proposed stormwater
mitigation infrastructure.
B. Post Development Drainage Basin Conditions
To mimic historic drainage conditions, the proposed drainage concept routes site runoff through the site
towards Design Point 1 (DP 1) and ultimately towards Hunter Creek. In order to achieve this strategy
stormwater infrastructure is required to include inlets, swales and storm pipes. In addition, water quality
treatment mitigation will be provided throughout the project. For these reasons several post development
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drainage basins were created to design and size the proposed stormwater mitigation infrastructure as further
described below and as illustrated on Exhibit D-3:
Post-1: Consists of the proposed landscaping areas located along the south and east sides of the proposed
residence. It also includes a portion of the green roof and impervious portion of the driveway. Runoff from this
basin will be routed within a swale (Swale A) towards a shallow bioretention cell. Larger storm events will
overtop the overflow weir and convey runoff towards Hunter Creek via Swale B.
Post-2: This basin includes the lawn area located at the northwest corner of the residence. Runoff from this
basin will be routed to a couple of inlets and then conveyed towards an infiltration chamber located at the end
of Swale B. Flows that exceed the infiltration chamber will overtop the grated lid at low velocities and continue
towards Hunter Creek.
Post-3: Encompasses the landscaping buffer located along the northern edge of the residence. This
landscape buffer will include a geomembrane liner with an underdrain as well as a series of inlets which will
capture excess runoff and route flows to the proposed infiltration chamber located within Swale B. Flows that
exceed the infiltration chamber will overtop the grated lid at low velocities and continue towards Hunter Creek.
Post-4: Encompasses the permeable paver patio area and lawn area north of the pool. Several inlets will be
installed to collect excess runoff which will then be routed to the infiltration chamber located within Swale B.
Post-5: Encompasses the permeable paver patio area along the west, south and east sides of the pool.
Several inlets will be installed to collect excess runoff which will then be routed to infiltration chamber located
within Swale B.
Post-6: This basin includes the majority of the roof area which is made up primarily of green roof. The
downspout for this portion of the roof will discharge at the east side of the residence. A sediment catchment
basin is proposed at the outfall to provide additional water quality treatment. Discharged flows will be routed
towards Swale B and ultimately to Hunter Creek.
Post-7: Includes the proposed pool area. An impervious pool cover will likely be included with the pool
design. Water quality for the impervious cover will be provided via the permeable pavers that make up the
pool’s perimeter patio.
Post-8: This basin includes the proposed landscaping along the north side of the proposed site
improvements. No impervious areas are contemplated within this basin. Runoff will be directed towards DP-1
via surface conveyance.
Post-9: This basin consists of the existing depression that falls on the subject property and is also identified
as Hist. Onsite Basin 2. Runoff from this basin will follow existing drainage patterns. Flows that exceed the
infiltration capacity of the underlying soils will overtop the proposed bypass weir and be directed towards
Hunter Creek via Swale B.
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III. Drainage Analysis Methods
A. Hydrologic Design Criteria and Assumptions:
The drainage criteria used for this study was based on the COA’s URMP dated December 2014. The
improvements associated with this project classify it as a “Major Design” which requires an analysis of the 10-
and 100-year storm events. This section describes the hydrological assumptions and methods used to
estimate peak flow rates for each of the historic and post development drainage basins.
Peak Runoff rates for the 10- and 100-year storm events were calculated using the Rational Hydrologic
Method (Eq. 1) since the cumulative total of basin areas was less than 90 acres.
Equation. 1: Q = C* I * A
Q = Runoff Flow Rate (cfs)
C = Runoff Coefficient
I = Rainfall Intensity (in/hr)
A= Area of Basin (acres)
The runoff coefficient (C) is a variable that represents the ratio of runoff to rainfall volumes during a storm
event. The determination of C mainly depends on the soil type, watershed impervious and storm event
frequency. Each drainage basin was studied to determine the percent of impervious area and then the latest
version of the UD-Rational Spreadsheet was used to determine the runoff coefficient for each of the delineated
drainage basins.
The design rainfall duration used in the Rational Method is referred to as the time of concentration. The time
of concentration is the cumulative travel time, including overland flow and channelized flow, for runoff to get
from the furthest point upstream of a basin to a designated design point. Per COA URMP, 5 minutes was
used as the absolute minimum time of concentration. This minimum value was adopted for the smaller basins.
The resultant times of concentration and corresponding rainfall intensities were used to estimate the peak
runoff rates for each of the offsite and onsite drainage basins. A summary of the results are illustrated within
Table 1 & Table 2.
Table 1: Historic Runoff Summary
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Table 2: Post Development Runoff Summary
The peak runoff rates identified within Table 2 were used to size the proposed stormwater mitigation
infrastructure as further discussed within Section B.
B. Hydraulic Design Criteria and Assumptions:
As mentioned above, several post improvement sub-basins have been established to assist in the sizing of
stormwater infrastructure improvements. The items that were studied included vegetative swales, inlets and
subsurface stormwater collection pipes.
Vegetative Swales are proposed in various locations however the primary drainage swales are identified
as Swale A and Swale B which lie along the south and west sides of the proposed residence. These
swales will be conveying runoff from several sub-basins and have been sized accordingly. The proposed
dimensions and sizes of all swales were confirmed using Manning’s Equation (Eq. 2) and were based on
the sizing requirements for Swale B which will be receiving the most flows.
Equation 2: Q = 1.49/n * (A/Pw)2/3 * A * S0.5
Q = Channel Capacity (cfs)
n = manning’s runoff coefficient (native: n = 0.027)
A = Area of flow (sf)
Pw = Wetted perimeter of channel (ft)
S = Channel longitudinal slope (ft/ft)
The vegetative swale design considered 100-year peak runoff rates from tributary areas to Swale B, a
roughness coefficient of 0.027, maximum side slopes of 3:1 with a 2% min. longitudinal slope. The results
indicate that a swale with a 6” bottom width and 8” min. depth can accommodate a peak flow rate of
approximately 6.5 cfs which exceeds the total estimated 100-year peak runoff rates associated with Swale
B and all other proposed drainage swales. Supporting calculations are provided within the appendix of
this report.
Storm Sewer Hydraflow Software was used to size the various stormwater drain pipes. Hydraflow utilizes
Manning’s equation (Eq. 2) to compute the flow rate in open channels and partially full closed conduits. In
summary, a 6” PVC drain pipe installed at 1% has an 80% flow capacity of 0.55 cfs. The anticipated 100-
year cumulative peak runoff being conveyed through the proposed 6” storm drain was estimated at 0.22
cfs and therefore the 6” PVC at 1% has more than adequate capacity for the intended use.
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Inlets The Orifice Equation (Eq 3) was used to size each of the proposed inlets. Table 3 below
summarizes the results along with the corresponding recommended inlet. Alternative inlets may be
substituted provided the minimum area and available head outlined within Table 3 are provided.
Eq. 3: Q = 0.5* [Cd*Amin * (2gh)0.5]
Q =Design Flow Rate (cfs)
Cd = Coefficient of Discharge (0.6)
Amin = Minimum allowable area (sf)
h = Available head (ft);
Table 3: Inlet Sizing Summary
Supporting inlet calculations are provided within the Appendix of this report.
IV. Proposed Water Quality Treatment Mitigation
Water quality treatment is required for all projects that disturb more than 200 square feet. The overall goal of
the water quality treatment requirements is to protect receiving waters including the Roaring Fork River,
Maroon Creek, Castle Creek and tributaries to these water ways. The treatment is provided by strategically
incorporating stormwater Best Management Practices into the project’s stormwater infrastructure that are
capable of providing full water quality treatment for up to the 80th percentile runoff event which corresponds to
the volume of runoff generated from a storm event with a magnitude falling between a 6-month and 1-year.
The water quality capture volume associated with these more common storm events is directly correlated to
the amount of impervious area within a contributing drainage basin. The integration of the proposed water
quality treatment BMPs are discussed below and illustrated on the attached Building Permit plans.
Bio-retention Cells (BRC) are depressed landscaping areas designed to capture and filter or infiltrate the
water quality capture volume. The BRC for this project (BRC-A) is proposed along the southern edge of the
proposed residence and will receive stormwater runoff generated from Offsite Basin 2 and post development
Basin Post-1. The water quality capture volume for the respective BRC was calculated based on the criteria
outlined within chapter 8 of the COA URMP. Drain times were also evaluated under two separate methods.
The first utilized minimum area equation per Section 8.5.4.2 of the URMP while the other method simply used
the minimum infiltration rate of 2 in/hr for bioretention grow media and the available area to determine the time
to drain the captured water quality volume. Both methods incorporated a 50% clogging factor. The results are
summarized within Table 4 below and supporting water quality calculations are provided within the appendix of
this report.
Table 4: Bioretention Cell Summary
Permeable Pavers have been integrated into the design primarily to reduce overall impervious areas and are
located within a few areas surrounding the proposed pool on the north side of the residence. The permeable
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pavers will not accept any runoff from adjacent impervious areas and will include an underdrain system given
the close proximity to structures; therefore the effective impervious areas were 40% of the total area for sizing
of stormwater conveyance infrastructure. These paver areas will also consist of snowmelt which will require
removal of some of the underlying insulation. A detail has been provided within the civil permit set of
drawings.
Green Roofs are structural roof components that filter, absorb and retain/detain stormwater runoff. The water
quality benefits of green roofs include: biological uptake of stormwater runoff, evapotranspiration, moderates
stormwater runoff temperatures, and reduces peak runoff rates and volumes by decreasing the amount of
impervious area typically associated with traditional roof systems. Currently this project is proposing
approximately 2,500 +/- square feet of extensive green roof areas which makes up over 65% of the total roof
area. The extensive green system will be 6” in total depth and will include 3” of “LiteTop” media, a hydrodrain
matt, insulation and underlying substrate. An aluminum or galvalume flashing with a 2-ft wide gravel band will
be installed around the perimeter which will all be pitched inward towards the green roof. The green roof
“LiteTop” extensive grow media has a maximum media water retention of 39%. Applying this to the overall
available square footage it is determined that the green roof has an available retention volume of
approximately 245 cf which is the volume required to treat approximately 11,500 sf of impervious area which
far exceeds the square footage associated with the impervious perimeter roof banding and impervious
mechanical equipment located on the roof. The criteria within the City’s URMP estimates volume within a
green roof as the available volume within the sand/grow media area with a 30% void ratio. The “LiteTop”
consists of 75% sand and therefore the available volume per URMP standards was determined to be 144 cf
which also exceeds the required volume. This available volume will also provide stormwater detention that will
assist with the overall reduction of site generated runoff and volumes of runoff.
An Infiltration Chamber has also been incorporated into the drainage mitigation design for this project. An
infiltration retention chamber is a BMP that incorporates manhole structures with perforated to help infiltrate
incoming stormwater as well as reduce velocities exiting from a storm pipe. Washed screened rock is installed
around the exterior of the perforated sections. Stormwater will be captured within this structure will be stored
and allowed to infiltrate into the underlying soils while flows that exceed the capacity will simply stage up and
discharge out of the grated cover at a much reduce velocity and be carried to Hunter Creek within proposed
Swale B. When sub-soils are capable of moderate to high infiltration rates, infiltration chambers are
considered to be a viable BMP. They reduce the increased runoff and volume of stormwater generated from
surrounding impervious areas and promote infiltration, provide additional water quality treatment and reduce
runoff velocities.
Grass Lined Swales are proposed throughout the project to minimize directly connected impervious areas.
Grass lined swales are an integral part of the Low Impact Development (LID) concept and they provide
additional water quality treatment by increasing the travel time of surface runoff which helps remove pollutants
as well as reduces overall peak runoff rates and volumes.
V. Stormwater Detention
Typically the URMP would require Major Design projects to analyze and provide stormwater detention since
the site is not directly connected to the City storm sewer system. However common engineering practice
suggests that properties directly adjacent or in close proximity to receiving water bodies should release storms
above the water quality storm directly to the receiving body. Reasoning behind this concept is to prevent
runoff from the lower basin from lagging and combining with the peak runoff from the upper reaches of the
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design basin, which would increase the peak runoff for the design basin. This concept attempts to maintain the
runoff hydrograph for the design basin and reduces the potential impacts to downstream properties. This
concept has been utilized by City Staff on multiple projects.
VI. Low Impact Design
Low Impact Design (LID) is a stormwater management strategy that aims to control stormwater at the source
by promoting infiltration, evaporation, filtering and detain runoff close to its source. The LID techniques that
have been incorporated into this project’s stormwater mitigation plan include: disconnecting impervious areas
where practical, reducing impervious areas, reducing peak runoff rates and volumes, and incorporating water
quality treatment facilities. Below is a list of the 9 Principles outlined within the URMP as well as the ways this
project has attempted to implement these principles.
• Principle #1-“Consider stormwater quality needs early in the design process”: SE was consulted
continually through the design process to ensure stormwater mitigation was considered during the
initial design phases. The results were the implementation of several BMPs that will improve the the
quality of stormwater exiting the site.
• Principle #2-“Use the entire site when planning for stormwater quality treatment”: The stormwater
mitigation approach outlined within this report integrates numerous BMPs for managing stormwater
runoff. These elements are spread throughout the site which results in a post developed site that
closely mimics the historic drainage rates and patterns. Grass swales are utilized whenever possible
to minimize directly connected impervious areas and to create a treatment train approach to water
quality.
• Principle #3- “Avoid unnecessary impervious areas”: Impervious areas associated with this project
have been minimized by implementing permeable pavers and green roofs.
• Principle #4- “Reduce runoff rates and volumes to more closely match natural conditions”: Post
development runoff rates and volumes have been reduced to closely match historic conditions. This
was achieved by reducing the overall impervious area of the developed site by utilizing bio-retention
cells, permeable pavers and green roofs which will promote infiltration and reduce surface runoff while
increasing the time of concentration of stormwater runoff which results in a decrease of runoff rates.
• Principle #5- “Integrate stormwater quality management and flood control”: The proposed bio-
retention cells, permeable pavers and green roofs will provide storage detention volumes as well as
stormwater treatment. The results are post development peak runoff rates that closely match historic
levels.
• Principle #6- “Develop stormwater quality facilities that enhance the site, the community and the
environment”: Implementation of green roofs and patio landscaping offer stormwater treatment as well
as aesthetic benefits.
• Principle #7- “Use a treatment train approach”: A treatment train approach is provided by utilizing
grass swales, green roofs, bioretention cells and permeable pavers to direct and control stormwater
runoff prior to flows entering Hunter Creek.
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• Principle #8- “Design sustainable facilities that can be safely maintained”: There are no risks
associated with maintaining the proposed BMPs. A full maintenance plan has been provided within
Section VII of this report.
• Principle #9- “Design and maintain facilities with public safety in mind”: The current drainage design
poses no risks to public safety.
VII. Maintenance Plan
This section describes the stormwater management systems proposed for the project as well as the
associated maintenance anticipated with these improvements. All of the stormwater mitigation improvements
will be owned and maintained by the property owner and the following maintenance program should be
followed to ensure proper functioning of the proposed improvements.
Bio-Retention Cell: Sediment build-up may require periodic removal of sediments and plants when clogging
reduces infiltration capacity to unacceptable levels. Plant materials in areas prone to sediment build-up should
be limited to grass and groundcovers tolerant of periodic wet-dry cycles.
Inspect detention area to determine if the sandy growth media is allowing acceptable infiltration. This
should be performed annually.
Occasional removal of weeds and unwanted vegetation will be required.
Remove debris and litter from detention area to minimize clogging of filter grow media.
The grow media will clog in time as materials accumulate on it. This layer will need to be removed and
replaced to rehabilitate infiltration rates, along with all turf and other vegetation growing on the surface.
This will be required every 5- 15 years, depending on infiltration rates needed to drain the WQCV in
12-hours or less.
Permeable Pavers: After the installation of permeable paver, maintenance is relatively minimal but absolutely
critical to ensure the long lifetime of the system. The key maintenance objective for any permeable pavement
system is to know when runoff is no longer rapidly infiltrating into the surface, which is typically due to void
spaces becoming clogged and requiring sediment removal. This section identifies key maintenance
considerations for the Permeable Pavers.
Inspect pavement condition and observe infiltration at least once a year to ensure water infiltrates into
the surface. This can be done during a rain event or with a garden house. Video, photographs or
notes should be taken to help assess the infiltration degradation over time.
Debris should be removed, routinely, as a source control measure. Use a vacuum or regenerative air
sweeper to help maintain or restore infiltration as required. This should be done on a warm dry day for
best results. Do not use water with the sweeper. The frequency of sweeping is site specific and it
may be determined that biannual vacuuming is not necessary. After vacuuming pavers, replace infill
aggregate as needed.
In general pervious pavers do not form ice to the same extent as conventional pavements; therefore
sanding of these areas should not be needed. In fact, placing sand on the pervious pavers is not
recommended as it can reduce the infiltration capacity of the area. Snow shovels in lieu of mechanical
plows will be utilized to clear snow from the permeable paver area to avoid damaging the system.
When properly installed the system should not require much for repair/replacement. If a repair is
required, it is frequently due to poor placement of the paver blocks. Follow industry guidelines for
installation and replacement after underground repairs. If surface is completely clogged and rendering
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a minimal surface infiltration rate, restoration of surface infiltration can be achieved by removing the
first ½ to 1 inch of soiled aggregate infill material with a vacuum sweeper. After cleaning, the voids
between the pavers will need to be refilled with clean aggregate infill material. Replacement of the
infill is best accomplished with push brooms.
Storm sewers and Inlets: Inspections of storm sewer piping and inlets should be performed by a person who
is familiar with the operation and configuration of the system.
Inspection of all storm pipes and associated inlets should occur at least quarterly for the first two years
of operation and then at least twice a year thereafter, if a reduced inspection schedule is warranted
based on initial two years of inspection. Strong odors may be a good indication that the facility is not
draining properly.
Inspection of inlets and grates should occur at a minimum every spring melt, early and late fall and
after any significant rainfall event. Any debris on or around the grated inlets should be removed.
Vegetation around the inlet should be trimmed and or cut from the perimeter of the grate. Remove
any weeds or invasive root material around all inlets. Inspect drain inlet riser pipe to the invert and
remove any debris and securely replace grate.
Open Channels/Swales: At least once a year and more often if required the following inspections should be
performed on all temporary and permanent drainage swales:
Inspect swales for erosion. Any obvious damage to grass or to the swale bottom soil bed should be repaired
immediately.
Remove trash and other debris from all parts of the swale.
Maintain a healthy dense grass in channel and side slopes. Grass re-seeding and mulching may be
required to promote healthy growth.
Remove sediment build up as necessary
Green Roofs: If installed properly the maintenance of a green roof should be minimal. All manufacturer’s
recommendations for maintenance shall be adhered to and proper documentation shall be provided to the
individuals responsible for maintaining the facility. Below are just a few items that need to be considered as
part of the green roof’s maintenance schedule:
Initial water and fertilizing is anticipated until the plants have fully established. This will require
supplemental irrigation source which should be checked for leeks and operating at the scheduled
times.
Weeding, especially in during the establishment stage will be necessary.
Inspection of the roof drainage system is crucial. If drainage routes become blocked, green roofs can
cause some flat roofs to leak due to continuous contact with the water or wet soil; therefore all
drainage inlets and conveyance gutters should be checked and collected debris removed. This should
occur whenever any type of maintenance inspection is taking place.
Infiltration Chamber: Inspection of this structure should occur annually to remove sediment and debris that is
washed into them. Other items that should be inspected include:
Inspect the filter fabric on the bottom of the infiltrating surface at least twice a year; preferably after
snow melt in the spring and late fall.
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Verify that the structures are infiltrating properly. This can be confirmed by inspecting the chamber 24
hours after a rainfall event. If standing water is encountered clogging should be further investigated
and remedied.
Remove sediment, debris, trash and any other waste accumulated within the structures. Dispose of
this material at a suitable disposal site and in compliance with local, state and federal waste
regulations.
Replace the geo-fabric at the bottom of the structure when it appears saturated with sediment or
infiltration time is slow. Replacement fabric should be attached to the concrete wall of the perforated
portion of the structure.
Annually, after a large rain event or with a water hose, evaluate the drain-down time of the structure to
ensure the maximum drain time of 24 hours is not being exceeded. If drain-down times are exceeding
the maximum, drain the structure via pumping and clean out the percolation areas.
VIII. Sediment and Erosion Control/Construction BMPs
Current practice standards provide parameters for mitigation of drainage and soil erosion activities relative to
site development. These parameters are referred to as best management practices (BMP’s). These BMP’s
are primarily grouped for two stages of the development, the construction phase and the post-development
phase, with the main emphasis on soil erosion and sediment transport controls.
During the construction phase for the proposed improvements the contractor will have to prepare and provide
a Construction Management Plan (CMP) that will address site erosions, dust control and disturbed ground
stability.
Final construction stages of work must follow a complete landscaping and ground covering task to
permanently re-vegetate and cover bear grounds that will remain open space to avoid long-term soil erosion.
This effort will reduce the risk of unnecessary degradation of the City’s drainage system. Temporary erosion
control structures installed during construction shall be left in place as necessary and maintained until new
vegetation has been re-established at a 70% level. Upon reaching a satisfactory level of soil stabilization from
the new vegetation, all erosion control structures shall be removed.
IX. Conclusion
The proposed improvements incorporate stormwater and drainage mitigation strategies consistent with the
requirements of the current URMP. Low Impact Design concepts to include bio-retention cells, vegetated
swales, reduced basin imperviousness, and the integration of green roofs will result in a much improved
drainage condition than what exists today. Additionally, a maintenance plan outlining the recommended
maintenance requirements for each of the proposed stormwater mitigation measures has been provided. Best
Management Practices (BMPs) have been identified and will be implemented during the construction of the
improvements. Finally, the results from this drainage study suggest that no long term, adverse impacts of
drainage are anticipated with the improvements associated with this project.
12/1/2018
183 Red Mountain Road Drainage Report August 29, 2018
13 | P a g e
X. Engineer’s Statement of Design Compliance
I hereby affirm that this report and the accompanying plans for the site drainage mitigation of the property
located at 183 Red Mountain Road was prepared under my direct supervision for the owners thereof in
accordance with the provisions of 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 by others.
________________________
Jesse K. Swann, PE
License No. 42787
12/1/2018
183 Red Mountain Rd, Aspen, CO Major Design Drainage Study SE Project No. 14237.05
S OPRIS E NGINEERING • LLC civil consultants
502 Main Street Suite A3 Carbondale Colorado 81623 (970)704-0311 Fax:(970)704-0313
APPENDIX
• Historic & Post Development UD-Rational Spreadsheets
• Swale Sizing Calculations
• Storm Sewer Sizing Calculations
• BRC Design & Sizing Calculations
• Exhibit D-1.0: Historic Onsite Drainage Basin Delineations
• Exhibit D-2.0: Offsite Drainage Basin Delineations
• Exhibit D-3.0: Onsite Post Development Drainage Basin Delineations
12/1/2018
Designer:
Company:10-yr
Date:1-hour rainfall depth, P1 (in) =0.77
Project:c
Location:Rainfall Intensity Equation Coefficients =0.983
10-yr Selected
tc (min)10-yr 10-yr
0.13
5.00 3.72 0.19
0.13
5.00 3.72 0.03
0.50
5.00 3.72 1.06
0.13
5.00 3.72 0.03
0.27
5.00 3.72 0.20
Rainfall Intensity, I (in/hr) Peak Flow, Q (cfs)
Calculation of Peak Runoff using Rational Method
Time of ConcentrationRunoff Coefficient, C
Subcatchment
Name
Area
(ac)
NRCS
Hydrologic
Soil Group
Percent
Imperviousness
HIST OS 1 0.377 C 0.0
Select UDFCD location for NOAA Atlas 14 Rainfall Depths from the pulldown list OR enter your own depths obtained from the NOAA website (click this link)
Cells of this color are for required user-input
Cells of this color are for optional override values
Cells of this color are for calculated results based on overrides
11/9/2018
183 Red Mountain Road - 10 yr Historic
Aspen, Colorado
Version 2.00 released May 2017
HIST OS 2 0.056 C 0.0
OFFSITE 2 C 0.0
OFFSITE 1 C 50.0
0.062
0.57
OFFSITE 3 C 19.00.194
183 Red Mountain Road
10-Year Historic
12/1/2018
Designer:
Company:10-yr
Date:1-hour rainfall depth, P1 (in) =0.77
Project:c
Location:Rainfall Intensity Equation Coefficients =0.983
10-yr Selected
tc (min)10-yr 10-yr
0.19
5.00 3.72 0.11
0.13
5.00 3.72 0.02
0.22
5.00 3.72 0.01
0.43
5.00 3.72 0.02
0.43
5.00 3.72 0.02
0.35
5.00 3.72 0.09
0.42
5.00 3.72 0.02
0.47
5.00 3.72 0.13
0.20
5.00 3.72 0.04
0.13
5.00 3.72 0.005
0.13
5.00 3.72 0.003
0.50
5.00 3.72 0.01
0.50
5.00 3.72 0.01
0.87
5.00 3.72 0.03
39.8C0.01POST-7
C 0.0
Rainfall Intensity, I (in/hr)
POST-3 0.02 C 12.3
Peak Flow, Q (cfs)
Calculation of Peak Runoff using Rational Method
Time of ConcentrationRunoff Coefficient, C
Subcatchment
Name
Area
(ac)
NRCS
Hydrologic
Soil Group
Percent
Imperviousness
POST-1 0.15 C 7.6
Select UDFCD location for NOAA Atlas 14 Rainfall Depths from the pulldown list OR enter your own depths obtained from the NOAA website (click this link)
Cells of this color are for required user-input
Cells of this color are for optional override values
Cells of this color are for calculated results based on overrides
11/9/2018
183 Red Mountain Road - 10 yr Developed
Aspen, Colorado
Version 2.00 released May 2017
POST-2 0.03
POST-5 0.01 C 40.0
POST-4 0.01 C 40.0
POST-6 0.07 C 29.7
POST-9 0.056 C 9.9
POST-8 0.07 C 45.6
PDSA4 0.01 C 0.0
PDSA6 0.01 C 50.0
PDSA5 0.01 C 0.0
PDSA8 0.01 C 100.0
PDSA7 0.00 C 50.0
183 Red Mountain Road
10-Year Developed
12/1/2018
Designer:
Company:100-yr
Date:1-hour rainfall depth, P1 (in) =1.23
Project:
Location:Rainfall Intensity Equation Coefficients =
100-yr Selected
tc (min)100-yr 100-yr
0.48
5.00 6.32 1.15
0.48
5.00 6.32 0.17
0.69
5.00 6.32 2.49
0.48
5.00 6.32 0.19
0.56
5.00 6.32 0.69
Rainfall Intensity, I (in/hr) Peak Flow, Q (cfs)
Calculation of Peak Runoff using Rational Method
Time of ConcentrationRunoff Coefficient, C
Subcatchment
Name
Area
(ac)
NRCS
Hydrologic
Soil Group
Percent
Imperviousness
HIST OS 1 0.38 C 0.0
Select UDFCD location for NOAA Atlas 14 Rainfall Depths from the pulldown list OR enter your own depths obtained from the NOAA website (click this link)
Cells of this color are for required user-input
Cells of this color are for optional override values
Cells of this color are for calculated results based on overrides
11/9/2018
183 Red Mountain Road - 100 yr Historic
Aspen, Colorado
HIST OS 2 0.06 C 0.0
OFFSITE 2 C 0.0
OFFSITE 1 C 50.0
0.06
0.57
OFFSITE 3 C 19.00.19
183 Red Mountain Road
100-Year Historic
12/1/2018
Designer:
Company:100-yr
Date:1-hour rainfall depth, P1 (in) =1.23
Project:
Location:Rainfall Intensity Equation Coefficients =
100-yr Selected
tc (min)100-yr 100-yr
0.52
5.00 6.32 0.49
0.48
5.00 6.32 0.10
0.53
5.00 6.32 0.06
0.65
5.00 6.32 0.05
0.65
5.00 6.32 0.04
0.56
5.00 6.32 0.25
0.89
5.00 6.32 0.06
0.48
5.00 6.32 0.22
0.48
5.00 6.32 0.17
POST-6 0.07 C 17.6
POST-9 0.056 C 0.0
POST-8 0.07 C 0.0
POST-7 0.01 C 100.0
POST-2 0.03
POST-5 0.01 C 40.0
POST-4 0.01 C 40.0
Cells of this color are for calculated results based on overrides
11/9/2018
183 Red Mountain Road - 100 yr Developed
Aspen, Colorado
POST-3 0.02 C 12.3
C 0.0
Rainfall Intensity, I (in/hr) Peak Flow, Q (cfs)
Calculation of Peak Runoff using Rational Method
Time of ConcentrationRunoff Coefficient, C
Subcatchment
Name
Area
(ac)
NRCS
Hydrologic
Soil Group
Percent
Imperviousness
POST-1 0.15 C 7.6
Select UDFCD location for NOAA Atlas 14 Rainfall Depths from the pulldown list OR enter your own depths obtained from the NOAA website (click this link)
Cells of this color are for required user-input
Cells of this color are for optional override values
183 Red Mountain Road
100-Year Developed
12/1/2018
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Friday, Nov 9 2018
SWALE B CAPACITY
Trapezoidal
Bottom Width (ft) = 0.50
Side Slopes (z:1) = 3.00, 3.00
Total Depth (ft) = 0.67
Invert Elev (ft) = 1.00
Slope (%) = 2.00
N-Value = 0.027
Calculations
Compute by: Known Depth
Known Depth (ft) = 0.67
Highlighted
Depth (ft) = 0.67
Q (cfs) = 6.560
Area (sqft) = 1.68
Velocity (ft/s) = 3.90
Wetted Perim (ft) = 4.74
Crit Depth, Yc (ft) = 0.67
Top Width (ft) = 4.52
EGL (ft) = 0.91
0 1 2 3 4 5 6 7
Elev (ft)Depth (ft)Section
0.75 -0.25
1.00 0.00
1.25 0.25
1.50 0.50
1.75 0.75
2.00 1.00
Reach (ft)
12/1/2018
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Friday, Aug 10 2018
6-IN PIPE AT 80 PERCENT
Circular
Diameter (ft)= 0.50
Invert Elev (ft)= 1.00
Slope (%)= 1.00
N-Value = 0.013
Calculations
Compute by:Known Depth
Known Depth (ft)= 0.40
Highlighted
Depth (ft)= 0.40
Q (cfs)= 0.548
Area (sqft)= 0.17
Velocity (ft/s)= 3.26
Wetted Perim (ft)= 1.11
Crit Depth, Yc (ft)= 0.38
Top Width (ft)= 0.40
EGL (ft)= 0.56
0 1
Elev (ft)Section
0.75
1.00
1.25
1.50
1.75
2.00
Reach (ft)
12/1/2018
9329
11.7
12/1/2018
HIST. ONSITE BASIN 1
(HIST OS-1)RE
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PR
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(
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EX:7851.80'±
RE
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XGAS
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XGAS
XGAS
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HUNTER CREEK
Top of Bank
Top of
B
a
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Top of
B
a
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PROPERTY LINE (TYP.)
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1
6
0
HIST. ONSITE BASIN 2
(HIST OS-2)
DP
1
OFFSITE BASIN 2
OFFSITE BASIN 1
HP:7839.37'
HP:7839.44'
HP:7839.45'
HP:7839.27'
1 inch = ft.
( IN FEET )
GRAPHIC SCALE
0
10
OPRIS ES NGINEERING, LLC.
CIVIL CONSULTANTS
14237.05JOB NO.
DATE:02-21-18
502 MAIN STREET
CARBONDALE, CO 81623
(970) 704-0311
FAX: (970)-704-0313
PRE-DEVLOPMENT
DRAINAGE PLAN
DESIGNED BY
DRAWN BY
CHECKED BY
JPS 06/20/17
JPS 06/20/17
DATE REVISION
02/21/18 UPDATE GRADING
D-1
DRAWING NO.
CA
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H
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PRE DEVELOPMENT DRAINAGE BASIN
EXISTING CONTOUR
EXISTING CONTOUR INTERVAL7900
LEGEND
ZONE AE 100 YEAR FLOOD
DP
#DESIGN POINT
HISTORIC ONSITE DRAINAGE BASIN PEAK RUNOFF SUMMARY
JKS 08/15/18
SCALE REDUCE FOR REPORT EXHIBIT
12/1/2018
>
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PROPERTY LINE (TYP.)
CITY OF ASPEN BOUND
A
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CITY
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7
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78
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78
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787
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786
9
786
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5786
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OFFSITE BASIN 1
OFFSITE BASIN 3:
PROJECT SITE
183 RED MOUNTAIN ROAD
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OFFSITE BASIN 2
Know what's below.
before you dig.Call
R
OPRIS ES NGINEERING, LLC.
CIVIL CONSULTANTS
14237.05JOB NO.
DATE:02-21-18
502 MAIN STREET
CARBONDALE, CO 81623
(970) 704-0311
FAX: (970)-704-0313
OFFSITE DRAINAGE
BASINS
DESIGNED BY
DRAWN BY
CHECKED BY
NA 01/02/18
JPS 01/02/18
DATE REVISION
08-15-18 COA COMMENTS
D-2
DRAWING NO.
TITLE
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( IN FEET )
GRAPHIC SCALE
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OFFSITE DRAINAGE BASIN
EXISTING CONTOUR
EXISTING CONTOUR INTERVAL7900
LEGEND
DP
#DESIGN POINT
HISTORIC OFFSITE BASIN PEAK RUNOFF SUMMARY TABLE
JKS 08/15/18
G:\2014\14237\CIVIL\Civil DWGs\X-REFS\JKS STAMP 8-15-2018tiff_Page1.tiff
REDUCED FOR ATTACHMENT TO REPORT
12/1/2018
SWALE APOST-1
S
W
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17 SHADY LANE
ASPEN, CO 81612
PARCEL ID: 273707300010
(SPA)
7837
7837
DAYLIGHT
STRUCTURE\LEVEL
SPREADER. SEE DETAIL
SHEET 7.0
INLET #2a
WQCV BIO-RETENTION
CELL A (BRC A)
7837
7839
7839
7838
7837
7838
7
8
3
7
78
3
7
7838
7841
7
8
4
1
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>>>>>>>>>>>>>>>>>>>>>
784
0
78
3
9
TOW:78
3
7
.
7
5
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TOW:78
3
7
.
7
5
'
TO
W
:
7
8
3
7
.
7
5
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TOW
:
7
8
3
7
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:
7
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3
7
.
7
5
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HP:
7
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3
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78
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ROOF LEADER
INTERCEPTOR/SEDIMENT
TRAP. SEE DETAIL
SHEET C7.0
ROOF LEADER
INTERCEPTOR/SEDIMENT
TRAP. SEE DETAIL
SHEET C7.0
>>>>>>>>
>>>>>>>>>>>>>>>
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HUNTER CREEK
Top of Bank
Top of
B
a
n
k
Top of
B
a
n
k
x x x x x x x x x x x x x x x x x
SITE BENCHMARK
Elevation = 7839.34'
Fnd. Rebar w/Cap L.S. 2376
PROPERTY LINE (TYP.)
PROPERTY LINE (TYP.)
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Lov=135'
7851'
7837.75
7837.75
>>>>>>>>>>>>>>>>Lch=62'
7845'
>
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EXISTING ONSITE
DRAINAGE WAY
7837.5
7837
ROOF DRAIN
INTERCEPTOR(TYP.)
WQCV BIO-RETENTION CELL
CONTACT SURFACE AREA: 65 S.F.
STORAGE VOLUME: 16.3 C.F.
INLET #2b
INLET #4aINLET #4c
INLET #5c
INLET #5b
INLET #5a
INLET #3f
INLET #3e
INLET #3d
INLET #3c INLET #3b INLET #3a
6" DRAIN PIPE (TYP)
4" PERF. PAVER
UNDERDRAIN (TYP)
4" SOLID PAVER
UNDERDRAIN (TYP)
INLET #3g
INLET #5d
INLET #5e
INLET #4b
POST-6
POST-2
POST-8
POST-3
POST-5
POST-4
POST-7
OFFSITE BASIN 2
OFFSITE BASIN 1
HP:7839.37'
HP:7839.44'
HP:7839.45'
HP:7839.27'
DP
1
POST-9
1 inch = ft.
( IN FEET )
GRAPHIC SCALE
0
10
OPRIS ES NGINEERING, LLC.
CIVIL CONSULTANTS
14237.05JOB NO.
DATE:02-21-18
502 MAIN STREET
CARBONDALE, CO 81623
(970) 704-0311
FAX: (970)-704-0313
POST-DEVLOPMENT
DRAINAGE PLAN
DESIGNED BY
DRAWN BY
CHECKED BY
JPS 06/20/17
JPS 06/20/17
DATE REVISION
02/21/18 UPDATE GRADING
D-3.0
DRAWING NO.
CA
N
O
P
Y
H
O
U
S
E
18
3
R
E
D
M
O
U
N
T
A
I
N
R
O
A
D
CI
T
Y
O
F
A
S
P
E
N
,
C
O
L
O
R
A
D
O
BU
I
L
D
I
N
G
P
E
R
M
I
T
G:
\
2
0
1
4
\
1
4
2
3
7
\
C
I
V
I
L
\
C
I
V
I
L
D
W
G
S
\
P
L
O
T
\
1
4
2
3
7
.
0
5
-
P
R
E
-
P
O
S
T
D
R
G
.
D
W
G
-
N
o
v
0
9
,
2
0
1
8
-
2
:
0
2
p
m
PRE DEVELOPMENT DRAINAGE BASIN
EXISTING CONTOUR
EXISTING CONTOUR INTERVAL7900
LEGEND
ZONE AE 100 YEAR FLOOD
PROPOSED CONTOUR
PROPOSED CONTOUR INTERVAL7900
PDSA-X POST-DEVELOPED SUB-AREA
DESIGNATION
ONSITE POST DEVELOPMENT DRAINAGE BASIN
PEAK RUNOFF SUMMARY TABLE JKS 08/15/18
SCALE REDUCE FOR REPORT EXHIBIT
08/15/18 COA COMMENTS
INLET SUMMARY TABLE
BRC SIZING AND DESIGN SUMMARY TABLE
1
3
ON-SITE SWALE
8"
VEGETATED GRASS
AND BUSHES SEE
LANDSCAPE PLAN FOR
INFORMATION
6" MIN.
SEED AND PLACE COCONUT
FIBER EROSION CONTROL
BLANKET
6" min. SAND, COMPOST,
TOPSOIL MIX W/ PROPOER
COMPACTION OR SOD
MATERIAL
12/1/2018
12/1/2018
12/1/2018
12/1/2018
12/1/2018
12/1/2018
12/1/2018
12/1/2018
12/1/2018
12/1/2018