HomeMy WebLinkAboutSCOTT PLAZA - FDP - FDP140004 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTJanuary 29, 2014
FINAL DRAINAGE AND
EROSION CONTROL REPORT FOR
SCOTT PLAZA
Fort Collins, Colorado
Prepared for:
Taylor Fitzpatrick Capital, LLC
7825 E. Gelding St., Suite 102
Scottsdale, AZ 85260
Prepared by:
200 South College Avenue, Suite 10
Fort Collins, Colorado 80524
Phone: 970.221.4158 Fax: 970.221.4159
www.northernengineering.com
Project Number: 948-001
This Drainage Report is consciously provided as a PDF.
Please consider the environment before printing this document in its entirety.
When a hard copy is absolutely necessary, we recommend double-sided printing.
January 29, 2013
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Final Drainage and Erosion Control Report for
SCOTT PLAZA
Dear Staff:
Northern Engineering is pleased to submit this Final Drainage and Erosion Control Report for your
review. This report accompanies the Project Development Plan submittal for the proposed Scott
Plaza development.
This report has been prepared in accordance to Fort Collins Stormwater Criteria Manual (FCSCM),
and serves to document the stormwater impacts associated with the proposed project. We
understand that review by the City is to assure general compliance with standardized criteria
contained in the FCSCM.
If you should have any questions as you review this report, please feel free to contact us.
Sincerely,
NORTHERN ENGINEERING SERVICES, INC.
Aaron Cvar, PE
Project Engineer
Scott Plaza
Preliminary Drainage Report
TABLE OF CONTENTS
I. GENERAL LOCATION AND DESCRIPTION ................................................................... 1
A. Location ............................................................................................................................................. 1
B. Description of Property ..................................................................................................................... 2
C. Floodplain.......................................................................................................................................... 4
II. DRAINAGE BASINS AND SUB-BASINS ....................................................................... 5
A. Major Basin Description .................................................................................................................... 5
III. DRAINAGE DESIGN CRITERIA ................................................................................... 5
A. Regulations........................................................................................................................................ 5
B. Four Step Process .............................................................................................................................. 5
C. Development Criteria Reference and Constraints ............................................................................ 6
D. Hydrological Criteria ......................................................................................................................... 6
E. Hydraulic Criteria .............................................................................................................................. 6
G. Modifications of Criteria ................................................................................................................... 7
IV. DRAINAGE FACILITY DESIGN .................................................................................... 7
A. General Concept ............................................................................................................................... 7
B. Specific Details .................................................................................................................................. 8
V. CONCLUSIONS ........................................................................................................ 9
A. Compliance with Standards .............................................................................................................. 9
B. Drainage Concept .............................................................................................................................. 9
References ....................................................................................................................... 10
APPENDICES:
APPENDIX A–Hydrologic Computations
APPENDIX B–Detention Calculations
APPENDIX C–Water Quality/LID Supporting Documentation
APPENDIX D –Erosion Control Report
Scott Plaza
Preliminary Drainage Report
LIST OF FIGURES:
Figure 1 – Aerial Photograph ................................................................................................ 2
Figure 2– Proposed Site Plan ................................................................................................ 3
Figure 3 – Existing Floodplains ............................................................................................. 4
MAP POCKET:
Proposed Drainage Exhibit
Scott Plaza
Preliminary Drainage Report 1
I. GENERAL LOCATION AND DESCRIPTION
A. Location
1. Vicinity Map
2. The project site is located in the northeast quarter of Section 15, Township 7 North,
Range 69 West of the 6th Principal Meridian, City of Fort Collins, County of Larimer,
State of Colorado The project site is located on the south side of West Plum Street,
and just west of Shields Street.
The project site lies within the Old Town Master Drainage Basin. Per the Old Town
Master Drainage Plan, onsite detention is required. Onsite detention is required for the
runoff volume difference between the 100 year developed inflow rate and the 2 year
historic release rate. It is acknowledged that this is the standard requirement, but
since the existing outfall into Plum already floods a home the release rate could be
more restrictive. The proposed preliminary design addresses this with a reduced
release rate.
3. The proposed project is surrounded by existing and new development. Apartment
developments exist to the east and west of the proposed project. Commercial
development exists to the south of the proposed project. A new apartment
development is currently under construction to the north of the proposed project,
Scott Plaza
Preliminary Drainage Report 2
which was referred to as “The Retreat at 1200 Plum” (Ref. 6).
4. No offsite flows of significance enter the site.
B. Description of Property
1. The development area is roughly 1.2 net acres.
Figure 1 – Aerial Photograph
2. The subject property currently four multi-family buildings and a gravel drive/parking
area along the east side of the development site. Existing ground cover generally
consists of residential landscaping and gravel drive. Existing ground slopes are
generally mild (i.e., 1 to 5±%) through the interior of the property. A portion of the
site topography slopes south to north. The remainder of the existing site slopes to the
south, directing existing runoff into a concrete pan located along the south property
line.
3. According to the United States Department of Agriculture (USDA) Natural Resources
Conservation Service (NRCS) Soil Survey website:
http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx,
the site consists of Nunn clay loam, which falls into Hydrologic Soil Group C.
Scott Plaza
Preliminary Drainage Report 3
Figure 2– Proposed Site Plan
Scott Plaza
Preliminary Drainage Report 4
4. The proposed project will consist of the construction of an apartment complex.
Parking areas and associated utilities will be constructed. LID features are provided
throughout the project site, with permeable pavers being proposed as the main LID
feature. It is acknowledged that the current City draft standards require that 50% of
the new impervious area be treated by an LID method and 25% of new parking lots must be
pervious. The proposed plan exceeds this requirement.
5. No major irrigation ditches or related facilities are in the vicinity of the project site.
6. The proposed land use is an apartment complex.
C. Floodplain
1. The project site is not encroached by any City or FEMA floodplain.
Figure 3 –Area Floodplain Mapping
2. No offsite improvements are proposed with the project.
PROJECT
SITE
Scott Plaza
Preliminary Drainage Report 5
II. DRAINAGE BASINS AND SUB-BASINS
A. Major Basin Description
3. The project site is located within the Old Town Master Drainage Basin. Onsite
detention is required for the runoff volume difference between the 100 year developed
inflow rate and the 2 year historic release rate. It is acknowledged that this is the
standard requirement, but since the existing outfall into Plum already floods a home
the release rate could be more restrictive. The proposed preliminary design addresses
this with a reduced release rate.
B. Sub-Basin Description
4. A portion of the subject property historically drains overland north into the adjacent
Plum Street. The remainder of the site historically drains overland south into an
existing concrete pan located along the south property line. A more detailed
description of the project drainage patterns follows in Section IV.A.4., below.
III. DRAINAGE DESIGN CRITERIA
A. Regulations
There are no optional provisions outside of the FCSCM proposed with the proposed
project.
B. Four Step Process
The overall stormwater management strategy employed with the proposed project utilizes
the “Four Step Process” to minimize adverse impacts of urbanization on receiving waters.
The following is a description of how the proposed development has incorporated each
step.
Step 1 – Employ Runoff Reduction Practices
Several techniques have been utilized with the proposed development to facilitate the
reduction of runoff peaks, volumes, and pollutant loads as the site is developed from the
current use by implementing multiple Low Impact Development (LID) strategies including:
Conserving existing amenities in the site including the existing vegetated areas.
Providing vegetated open areas throughout the site to reduce the overall impervious
area and to minimize directly connected impervious areas (MDCIA).
Routing flows, to the extent feasible, through vegetated swales to increase time of
concentration, promote infiltration and provide initial water quality.
Step 2 – Implement BMPs That Provide a Water Quality Capture Volume (WQCV) with
Slow Release
The efforts taken in Step 1 will facilitate the reduction of runoff; however, urban
development of this intensity will still generate stormwater runoff that will require
additional BMPs and water quality. The majority of stormwater runoff from the site will
ultimately be intercepted and treated using extended detention methods prior to exiting the
site.
Scott Plaza
Preliminary Drainage Report 6
Step 3 – Stabilize Drainageways
There are no major drainageways within the subject property. While this step may not
seem applicable to proposed development, the project indirectly helps achieve stabilized
drainageways nonetheless. By providing water quality where none previously existed,
sediment with erosion potential is removed from the downstream drainageway systems.
Furthermore, this project will pay one-time stormwater development fees, as well as
ongoing monthly stormwater utility fees, both of which help achieve City-wide drainageway
stability.
Step 4 – Implement Site Specific and Other Source Control BMPs.
The proposed project will improve upon site specific source controls compared to historic
conditions:
Trash, waste products, etc. that were previously left exposed with the historic trailer
park will no longer be allowed to exposure to runoff and transport to receiving
drainageways. The proposed development will eliminate these sources of potential
pollution.
C. Development Criteria Reference and Constraints
The subject property is tied currently developed properties adjacent to the site. Thus,
several constraints have been identified during the course of this analysis that will
impact the proposed drainage system including:
Existing elevations along the property lines will generally be maintained.
As previously mentioned, overall drainage patterns of the existing site will be
maintained.
Elevations of existing downstream facilities that the subject property will release to
will be maintained.
D. Hydrological Criteria
1. The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in
Figure RA-16 of the FCSCM, serve as the source for all hydrologic computations
associated with the proposed development. Tabulated data contained in Table RA-7
has been utilized for Rational Method runoff calculations.
2. The Rational Method has been employed to compute stormwater runoff utilizing
coefficients contained in Tables RO-11 and RO-12 of the FCSCM.
3. Three separate design storms have been utilized to address distinct drainage
scenarios. A fourth design storm has also been computed for comparison purposes.
The first design storm considered is the 80th percentile rain event, which has been
employed to design the project’s water quality features. The second event analyzed is
the “Minor,” or “Initial” Storm, which has a 2-year recurrence interval. The third
event considered is the “Major Storm,” which has a 100-year recurrence interval.
The fourth storm computed, for comparison purposes only, is the 10-year event.
4. No other assumptions or calculation methods have been used with this development
that are not referenced by current City of Fort Collins criteria.
E. Hydraulic Criteria
1. As previously noted, the subject property maintains historic drainage patterns.
2. All drainage facilities proposed with the project are designed in accordance with
Scott Plaza
Preliminary Drainage Report 7
criteria outlined in the FCSCM and/or the Urban Drainage and Flood Control District
(UDFCD) Urban Storm Drainage Criteria Manual.
3. As stated above, no part of the subject property is located in a City or FEMA
regulatory floodplain.
4. The proposed project does not propose to modify any natural drainageways.
F. Modifications of Criteria
1. The proposed development is not requesting any modifications to criteria at this time.
IV. DRAINAGE FACILITY DESIGN
A. General Concept
1. The main objectives of the project drainage design are to maintain existing drainage
patterns, and to ensure no adverse impacts to any adjacent properties.
2. A list of tables and figures used within this report can be found in the Table of
Contents at the front of the document. The tables and figures are located within the
sections to which the content best applies.
3. The drainage patterns anticipated for proposed drainage basins are described below.
Basin 1
Basin 1 consists of the apartment building; the basin area is composed primarily of
rooftop. This basin will generally drain via parking and drive curb and gutter to the
permeable paver system within Basin 3, where water quality and detention will be
provided for this basin.
Basin 2
Basin 2 consists of a small area (0.03 Ac.) of frontage along Plum Street. This basin
will sheet flow into Plum Street undetained. Compensation for this undetained release
is discussed below.
Basin 3
Basin 3 consists of of drive area which will be captured and detained within the
permeable paver system designed within this basin. The permeable paver system has
been designed per USDCM Volume 3 (Reference 5) methodology. Runoff from Basin
1 is also designed to be conveyed to this permeable paver system and detained within
the subgrade of the paver system.
Basin 4
Basin 4 consists of a small area (0.02 Ac.) of landscaped area along the back and
side of the proposed building. This basin will sheet flow into existing offsite
development.
Basin 5
Basin 5 consists of drive, parking, and landscaped area. This basin will sheet flow
into into an existing concrete pan located along the south property line. Historically,
0.55 Acres of existing development drained to design point 5. Historic development
consisted of rooftop and gravel. With the proposed 0.23 Acres now draining to this
Scott Plaza
Preliminary Drainage Report 8
design point, consisting of permeable pavers and landscaping, there will be a
reduction in runoff. Proposed runoff to design point 5 is 2.3 cfs, versus an estimated
historic 4.1 cfs in a 100-year storm event.
A full-size copy of the Drainage Exhibit can be found in the Map Pocket at the end of
this report.
B. Specific Details
A required detention volume of 0.12 acre-feet has been calculated for the subgrade
detention provided below the permeable paver system within Basin 3. Basins 1,3,
and 4 are directed into the paver system and detained within. Please see detention
calculations provided in Appendix B. The release rate determined for the proposed
detention facility has been preliminarily set at 0.60 cfs. This release rate has been
determined based on the methodology utilized for the approved Final Drainage Report
for “The Retreat at 1200 Plum” (Ref. 6). The methodology accounts for impervious
area that is allowed to be “grandfathered”. There is 0.24 acres of impervious area
within the development site which drains to Plum Street. A 100-year discharge from
this impervious area of 2.27 cfs has been calculated. There is 0.37 acres of pervious
area within the development site which also drains to Plum Street. A 2-year
discharge of 0.74 cfs has been calculated. The sum of “grandfathered” impervious
area discharge into Plum Street combined with 2-year pervious area discharge is
3.01, which is considered as the allowable peak release rate for the site. We have
subtracted the 100-year discharge computed from Basin 2, and 4 (0.30 cfs total) for
an allowable release rate of 2.71 cfs. We then reduced this release rate to 1.35 cfs,
in order to take into consideration the downstream flooding issues that have been
identified in the March 2013 City Conceptual Review for this project.
LID features include the aforementioned sand filter, and permeable pavement systems
within Basins 3 and 5. The total area draining to all LID features combined is 1.13
acres, which comprises 97.8% of the site. This exceeds the requirement stating that
50% of the site must pass through an LID feature. The site also exceeds the
requirement that 25% of parking and drive area must be composed of permeable
pavement, with the majority of parking and drive area outside of the parking structure
proposed as a permeable pavement.
In summary, a total detention volume of 0.12 acre-feet (5086 cubic feet) will be
provided within the subgrade of the permeable paver system.
V. CONCLUSIONS
A. Compliance with Standards
1. The drainage design proposed with the proposed project complies with the City of Fort
Collins’ Stormwater Criteria Manual.
2. The drainage design proposed with this project complies with all applicable City of
Fort Collins Master Drainage Plans.
3. The drainage plan and stormwater management measures proposed with the
proposed development are compliant with all applicable State and Federal regulations
governing stormwater discharge.
Scott Plaza
Preliminary Drainage Report 9
B. Drainage Concept
1. The drainage design proposed with this project will effectively limit any potential
damage associated with its stormwater runoff as all runoff is being captured and
routed to offsite drainage facilities which have either been previously approved by the
City of Fort Collins or are in the review and approval process with the City of Fort
Collins.
2. The drainage concept for the proposed development is consistent with all applicable
City of Fort Collins Master Drainage Plans.
Scott Plaza
Preliminary Drainage Report 10
References
1. City of Fort Collins Landscape Design Guidelines for Stormwater and Detention Facilities,
November 5, 2009, BHA Design, Inc. with City of Fort Collins Utility Services.
2. Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, as adopted by Ordinance No.
174, 2011, and referenced in Section 26-500 (c) of the City of Fort Collins Municipal Code.
3. Larimer County Urban Area Street Standards, Adopted January 2, 2001, Repealed and
Reenacted, Effective October 1, 2002, Repealed and Reenacted, Effective April 1, 2007.
4. Soils Resource Report for Larimer County Area, Colorado, Natural Resources Conservation
Service, United States Department of Agriculture.
5. Urban Storm Drainage Criteria Manual, Volumes 1-3, Urban Drainage and Flood Control
District, Wright-McLaughlin Engineers, Denver, Colorado, Revised April 2008.
6. Final Drainage and Erosion Control Report for The Retreat at 1200 Plum, Northern
Engineering, April 22, 2009.
APPENDIX A
HYDROLOGIC COMPUTATIONS
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: 948-001
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: ATC
Asphalt ……....……………...……….....…...……………….………………………………… 0.95 100% Date:
Concrete …….......……………….….……….………………..….…………………………… 0.95 90%
Gravel ……….…………………….….…………………………..…………………………… 0.50 40%
Roofs …….…….………………..……………….……………………………………………. 0.95 90%
Pavers…………………………...………………..……………………………………………. 0.40 22%
Lawns and Landscaping
Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0%
Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0% 2-year Cf
= 1.00 100-year Cf = 1.25
Basin ID
Basin Area
(s.f.)
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(ac)
Area of
Lawns and
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Composite
% Imperv.
1 28892 0.66 Note: Composite Runoff Coefficients are 0.95 0.95 1.00 90%
2 1389 0.03 based on Table RO-10. 0.95 0.95 1.00 90%
3 10454 0.24 Composite % Imperviousness has been estimated based 0.95 0.95 1.00 90%
4 897 0.02 on UDFCD USDCM, Vol. I, Figures RO-3 and RO-4. 0.95 0.95 1.00 90%
5 10018 0.23 0.95 0.95 1.00 90%
DEVELOPED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I.
10-year Cf = 1.00
January 15, 2014
Overland Flow, Time of Concentration:
Project: 948-001
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt = L / 60V
Tc = Ti + Tt (Equation RO-2)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
NOTE: C-value for overland flows over grassy surfaces; C = 0.25
Is Length
>500' ?
C*Cf
(2-yr
Cf=1.00)
C*Cf
(10-yr
Cf=1.00)
C*Cf
(100-yr
Cf=1.25)
Length,
L
(ft)
Slope,
S
(%)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
Rational Method Equation: Project: 948-001
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
1 1 0.66 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 1.8 3.1 6.6
2 2 0.03 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 0.1 0.1 0.3
3 3 0.24 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 0.6 1.1 2.4
4 4 0.02 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 0.1 0.1 0.2
5 5 0.23 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 0.6 1.1 2.3
Intensity,
i10
(in/hr)
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
C10
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
DEVELOPED RUNOFF COMPUTATIONS
C100
Design
Point
Flow,
Q100
(cfs)
Flow,
Q2
(cfs)
10-yr
Tc
(min)
2-yr
Tc
(min)
C2
Flow,
Q10
(cfs)
Intensity,
i100
(in/hr)
Basin(s)
ATC
January 15, 2014
Q C f C i A
APPENDIX B
D DETENTION CALCULATIONS
ETED
ATC
Pond No : 1
1
100-yr
1.00
Area (A)= 0.92 acres 5055 ft
3
Max Release Rate = 1.35 cfs 0.12 ac-ft
Time Time
100-yr
Intensity
Q100
Inflow
(Runoff)
Volume
Outflow
(Release)
Volume
Storage
Detention
Volume
(mins) (secs) (in/hr) (cfs) (ft
3
) (ft
3
) (ft
3
)
5 300 9.950 9.15 2746 405.0 2341.2
10 600 7.720 7.10 4261 810.0 3451.4
15 900 6.520 6.00 5399 1215.0 4183.6
20 1200 5.600 5.15 6182 1620.0 4562.4
25 1500 4.980 4.58 6872 2025.0 4847.4
30 1800 4.520 4.16 7485 2430.0 5055.1
35 2100 4.080 3.75 7883 2835.0 5047.6
40 2400 3.740 3.44 8258 3240.0 5017.9
45 2700 3.460 3.18 8595 3645.0 4949.6
50 3000 3.230 2.97 8915 4050.0 4864.8
55 3300 3.030 2.79 9199 4455.0 4744.1
60 3600 2.860 2.63 9472 4860.0 4612.3
65 3900 2.720 2.50 9759 5265.0 4494.4
70 4200 2.590 2.38 10008 5670.0 4337.8
75 4500 2.480 2.28 10267 6075.0 4192.2
80 4800 2.380 2.19 10510 6480.0 4030.1
85 5100 2.290 2.11 10745 6885.0 3859.7
90 5400 2.210 2.03 10979 7290.0 3689.3
95 5700 2.130 1.96 11170 7695.0 3474.7
100 6000 2.060 1.90 11371 8100.0 3271.2
105 6300 2.000 1.84 11592 8505.0 3087.0
110 6600 1.940 1.78 11780 8910.0 2869.7
115 6900 1.890 1.74 11998 9315.0 2682.7
120 7200 1.840 1.69 12188 9720.0 2468.2
DETENTION POND CALCULATION; FAA METHOD
Project Number : 948-001
Date : 1/15/14
1.35
Design Point
Design Storm Required Detention Volume
Developed "C" =
Project Location : Fort Collins
OUTLET RATING CURVE
Subgrade Detention Orifice
Project: 948-001
Date: 1/15/2014
By: ATC
100-YR ORIFICE RATING
Orifice Dia (in) 5.25
Orifice Area (sf) 0.1503
Orifice invert (ft) 0.00
Orifice Coefficient 0.65
Outlet
Orifice Area Stage release
(SF) (FT) (CFS)
0.1503 0.00 0.00
0.1503 0.25 0.39
0.1503 0.50 0.55
0.1503 0.75 0.68
0.1503 1.00 0.78
0.1503 1.25 0.88
0.1503 1.50 0.96
0.1503 1.75 1.04
0.1503 2.00 1.11
0.1503 2.25 1.18
0.1503 2.50 1.24
0.1503 2.75 1.30
0.1503 3.00 1.36
APPENDIX C
WATER QUALITY/LID SUPPORTING DOCUMENTATION
ETED
WATER QUALITY DESIGN CALCULATIONS
Permeable Paver System - Subgrade Volume
Project: 948-001
By: ATC
Date: 1.15.14
REQUIRED STORAGE & OUTLET WORKS:
BASIN AREA = 0.920 <-- INPUT from impervious calcs
BASIN IMPERVIOUSNESS PERCENT = 90.00% <-- INPUT from impervious calcs
BASIN IMPERVIOUSNESS RATIO = 0.9 <-- CALCULATED
WQCV (watershed inches) = 0.321 <-- CALCULATED from Figure EDB-2
WQCV (ac-ft) = 0.025 <-- CALCULATED from UDFCD DCM V.3 Section 6.5
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-1
Urban Storm Drainage Criteria Manual Volume 3
Photograph PPS-1. The reservoir layer of a permeable pavement
provides storage volume for the WQCV. Photo courtesy of Muller
Engineering and Jefferson County Open Space.
Description
The term Permeable Pavement System, as
used in this manual, is a general term to
describe any one of several pavements that
allow movement of water into the layers
below the pavement surface. Depending
on the design, permeable pavements can
be used to promote volume reduction,
provide treatment and slow release of the
water quality capture volume (WQCV),
and reduce effective imperviousness. Use
of permeable pavements is a common Low
Impact Development (LID) practice and is
often used in combination with other
BMPs to provide full treatment and slow
release of the WQCV. A number of
installations within the UDFCD
boundary have also been designed with an increased depth of
aggregate material in order to provide storage for storm events in
excess of the water quality (80th percentile) storm event. This
requires some additional design considerations, which are
discussed within this BMP Fact Sheet.
Site Selection
This infiltrating BMP requires consultation with a geotechnical
engineer when proposed near a structure. In addition to providing
the pavement design, a geotechnical engineer can assist with
evaluating the suitability of soils, identifying potential impacts,
and establishing minimum distances between the BMP and
structures.
Permeable pavement systems provide an alternative to
conventional pavement in pedestrian areas and lower-speed
vehicle areas. They are not appropriate where sediment-laden
runoff could clog the system (e.g., near loose material storage
areas).
This BMP is not appropriate when erosive conditions such as
steep slopes and/or sparse vegetation drain to the permeable
pavement. The sequence of construction is also important to
preserve pavement infiltration. Construction of the pavement
should take place only after construction in the watershed is
complete.
For sites where land uses or activities can cause infiltrating
stormwater to contaminate groundwater, special design
requirements are required to ensure no-infiltration from the
pavement section.
Permeable Pavement
Functions
LID/Volume Red. Yes
WQCV Yes
WQCV+Flood Control Yes
Fact Sheet Includes
EURV Guidance No
Typical Effectiveness for Targeted
Pollutants3
Sediment/Solids Very Good1
T-10 Permeable Pavement Systems
PPS-2 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Benefits
Permeable pavement systems
provide water quality treatment
in an area that serves more than
one purpose. The depth of the
pavement system can also be
increased to provide flood
control.
Permeable pavements can be
used to reduce effective
imperviousness or alleviate
nuisance drainage problems.
Permeable pavements benefit tree
health by providing additional air
and water to nearby roots.
Permeable pavements are less
likely to form ice on the surface
than conventional pavements.
Some permeable pavements can
be used to achieve LEED credits.
Limitations
Additional design and
construction steps are required
for placement of any ponding or
infiltration area near or
upgradient from a building
foundation, particularly when
potentially expansive soils exist.
This is discussed in the design
procedure section.
In developing or otherwise
erosive watersheds, high
sediment loads can clog the
facility.
Permeable pavements and other BMPs used for infiltration
that are located adjacent to buildings, hardscape or
conventional pavement areas can adversely impact those
structures if protection measures are not provided. Wetting of
subgrade soil underlying those structures can cause the
structures to settle or result in other moisture-related
problems. Wetting of potentially expansive soils or bedrock
can cause those materials to swell, resulting in structure
movements. In general, a geotechnical engineer should
evaluate the potential impact of the BMP on adjacent
structures based on an evaluation of the subgrade soil,
groundwater, and bedrock conditions at the site. In addition,
the following minimum requirements should be met:
In locations where subgrade soils do not allow infiltration,
the growing medium should be underlain by an
underdrain system.
Where infiltration can adversely impact adjacent
structures, the filter layer should be underlain by an
underdrain system designed to divert water away from the
structure.
In locations where potentially expansive soils or bedrock
exist, placement of permeable pavement adjacent to
structures and conventional pavement should only be
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-3
Urban Storm Drainage Criteria Manual Volume 3
Example Construction Drawing Notes
Excavation of subgrade shall not commence until after the pre-construction meeting.
Subgrade shall be excavated using low ground pressure (LGP) track equipment to
minimize over compaction of the subgrade. 1
Grading and compaction equipment used in the area of the permeable pavement should be
approved by the engineer prior to use.
Loose materials shall not be stored on the permeable pavement area.
The contractor shall, at all times during and after system installation, prevent sediment,
debris, and dirt from any source from entering the permeable pavement system.
Placement of the wearing course shall be performed after fine grading and landscaping in
adjacent areas is complete. If the wearing course becomes clogged due to construction
activities, clean the surface with a vacuum machine to restore the infiltration rate after
construction is complete.
1 For partial and full infiltration sections only.
Call for construction fence on the plans around pervious areas where infiltration rates need to be
preserved and could be reduced by compaction from construction traffic or storage of materials.
Design Procedure and Criteria
Note: This manual includes a variety of specific pavements, which are discussed and distinguished in
supplemental BMP Fact Sheets T-10.1, T-10.2, etc. This BMP Fact Sheet outlines the design procedure
and other design components and considerations that are common to all of the systems. Review of the
supplemental Fact Sheets is recommended to determine the appropriate pavement for a specific site or
use.
1. Subsurface Exploration and Determination of a No-Infiltration, Partial Infiltration, or Full
Infiltration Section: Permeable pavements can be designed with three basic types of sections. The
appropriate section will depend on land use and activities, proximity to adjacent structures and soil
characteristics. Sections of each installation type are shown in Figure PPS-1.
No-Infiltration Section: This section includes an underdrain and an impermeable liner that
prevents infiltration of stormwater into the subgrade soils. Consider using this section when any
of the following conditions exist:
o Land use or activities could contaminate groundwater if stormwater is allowed to infiltrate.
o Permeable pavement is located over potentially expansive soils or bedrock that could swell
due to infiltration and potentially damage the permeable pavement system or adjacent
structures (e.g., building foundation or conventional pavement).
T-10 Permeable Pavement Systems
PPS-4 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Partial Infiltration Section: This section does not include an impermeable liner, and allows
some infiltration. Stormwater that does not infiltrate is collected and removed by an underdrain
system.
Full Infiltration Section: This section is designed to infiltrate the water stored in the voids of
the pavement into the subgrade below. UDFCD recommends a minimum infiltration rate of 2
times the rate needed to drain the WQCV over 12 hours.
Subsurface Exploration and Testing for all Sections: A geotechnical engineer should scope and
perform a subsurface study. Typical geotechnical investigation needed to select and design the
pavement system for handling anticipated traffic loads includes:
Prior to exploration review geologic and geotechnical information to assess near-surface soil,
bedrock and groundwater conditions that may be encountered and anticipated ranges of
infiltration rate for those materials. For example, if the site is located in a general area of known
shallow, potentially expansive bedrock, a no-infiltration section will likely be required. It is also
possible that this BMP may be infeasible, even with a liner, if there is a significant potential for
damage to the pavement system or adjacent structures (e.g., areas of dipping bedrock).
Drill exploratory borings or exploratory pits to characterize subsurface conditions beneath the
subgrade and develop requirements for subgrade preparation. Drill at least one boring or pit for
every 40,000 ft2, and at least two borings or pits for sites between 10,000 ft2 and 40,000 ft2. The
boring or pit should extend at least 5 feet below the bottom of the base, and at least 20 feet in
areas where there is a potential of encountering potentially expansive soils or bedrock. More
borings or pits at various depths may be required by the geotechnical engineer in areas where soil
types may change, in low-lying areas where subsurface drainage may collect, or where the water
table is likely within 8 feet below the planned bottom of the base or top of subgrade. Installation
of temporary monitoring wells in selected borings or pits for monitoring groundwater levels over
time should be considered where shallow groundwater that could impact the pavement system
area is encountered.
Perform laboratory tests on samples obtained from the borings or pits to initially characterize the
subgrade, evaluate the possible section type, and to assess subgrade conditions for supporting
traffic loads. Consider the following tests: moisture content (ASTM D 2216); dry density
(ASTM D 2936); Atterberg limits (ASTM D 4318); gradation (ASTM D 6913); swell-
consolidation (ASTM D 4546); subgrade support testing (R-value, CBR or unconfined
compressive strength); and hydraulic conductivity. A geotechnical engineer should determine the
appropriate test method based on the soil type.
For sites where a full infiltration section may be feasible, perform on-site infiltration tests using a
double-ring infiltrometer (ASTM D 3385). Perform at least one test for every 160,000 ft2 and at
least two tests for sites between 40,000 ft2 and 160,000 ft2. The tests should be located near
completed borings or pits so the test results and subsurface conditions encountered in the borings
can be compared, and at least one test should be located near the boring or pit showing the most
unfavorable infiltration condition. The test should be performed at the planned top of subgrade
underlying the permeable pavement system, and that subgrade should be prepared similar to that
required for support of the permeable pavement system.
Be aware that actual infiltration rates are highly variable dependent on soil type, density and
moisture content and degree of compaction as well as other environmental and construction
influences. Actual rates can differ an order of magnitude or more from those indicated by
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-5
Urban Storm Drainage Criteria Manual Volume 3
infiltration or permeability testing. Selection of the section type should be based on careful
assessment of the subsurface exploration and testing data.
2. Required Storage Volume: Provide the WQCV based on a 12-hour drain time.
Find the required WQCV (watershed inches of runoff). Using the effective impervious area of
the watershed area, use Figure 3-2 located in Chapter 3 to determine the WQCV based on a 12-
hour drain time. The maximum recommended ratio for tributary impervious area to permeable
pavement area is 2.0. Higher loading is not recommended, as it may increase the required
maintenance interval.
Calculate the design volume as follows:
𝑉 = �
WQCV
12
� 𝐴 Equation PPS-1
Where:
A = watershed area tributary to the permeable pavement (ft2)
V = design volume (ft3)
Add flood control volume if desired. When designing for flood control volumes, provide an
overflow that will convey runoff in excess of the WQCV directly into the reservoir. A gravel
strip or inlet that is connected to the reservoir can provide this overflow.
T-10 Permeable Pavement Systems
PPS-6 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Figure PPS-1. Permeable Pavement Sections
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-7
Urban Storm Drainage Criteria Manual Volume 3
3. Depth of Reservoir: The minimum recommended depth of AASHTO No. 57 or No. 67 coarse
aggregate is 6 inches. Additional depth may be required to support anticipated loads or to provide
additional storage, (i.e., for flood control). This material should have all fractured faces. UDFCD
recommends that void storage be calculated only for the reservoir, assuming the aggregate filter layer
is saturated. With the exception of porous gravel pavement, use a porosity of 40% or less for both
No. 57 and No. 67 coarse aggregate. For porous gravel pavement use a porosity of 30% or less to
account for reduced volume due to sediment. Porous gravel pavements typically allow greater
sediment volumes to enter the pavement. See Figures PPS-2 and PPS-3 for alternative pavement
profiles. Calculate available storage using equation PPS-2 for a flat subgrade installation, and PPS-3
for a sloped subgrade installation. These equations allow for one inch of freeboard. Flat installations
are preferred as the design spreads infiltration evenly over the subgrade. For sloped subgrade
installations, the increased storage depth located upstream of the lateral barrier (see step 7) can
increase lateral movement (parallel to the flow barrier) of water into areas adjacent to the pavement
section.
When used for vehicular traffic, a pavement design should be performed by a qualified engineer
experienced in the design of permeable pavements and conventional asphalt and concrete pavements.
The permeable pavement should be adequately supported by a properly prepared subgrade, properly
compacted filter material and reservoir material.
Reservoir aggregate should have all fractured faces. Place the aggregate in 6-inch (maximum) lifts,
compacting each lift by using a 10-ton, or heavier, vibrating steel drum roller. Make at least four
passes with the roller, with the initial passes made while vibrating the roller and the final one to two
passes without vibration.
For flat or stepped installations (0% slope at the reservoir/subgrade interface):
𝑉 = 𝑃 �
𝐷 − 1
12
� 𝐴
Equation PPS-2
Where:
V = volume available in the reservoir (ft3)
P = porosity, ≤0.30 for porous gravel, ≤0.4 for all other pavements
using AASHTO No. 57 or No. 67 coarse aggregate in the reservoir
D = depth of reservoir (in)
A = area of the permeable pavement (ft2)
T-10 Permeable Pavement Systems
PPS-8 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Figure PPS-2. Permeable Pavement Profile, Stepped Installation
For sloped installations (slope of the reservoir/subgrade interface > 0%):
𝑉 = 𝑃 �
𝐷 − 6𝑠𝐿 − 1
12
� 𝐴
Equation PPS-3a
While:
𝐿 <
2 WQCV
𝑠𝐴𝑃
Equation PPS-3b
Where:
V = volume available in the reservoir (ft3)
P = porosity, ≤0.30 for porous gravel, ≤0.4 for all other pavements using AASHTO
No. 57 or No. 67 coarse aggregate in the reservoir
s = slope of the reservoir/subgrade interface (ft/ft)
D = depth of the reservoir (in)
L = length between lateral flow barriers (see step 4) (ft)
A = area of the permeable pavement (ft2)
WQCV = water quality capture volume (ft3)
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-9
Urban Storm Drainage Criteria Manual Volume 3
Figure PPS-3. Permeable Pavement Profile, Sloped Installation.
4. Lateral Flow Barriers: Construct lateral flow cutoff barriers using concrete walls or a 30 mil
(minimum) PVC geomembrane. Lateral flow barriers should be placed parallel to contours (normal
to flow). This will preserve the volume available for storage and ensure that stormwater will not
resurface, washing out infill material. See Figure PPS-6 and Table PPS-4 when using a PVC
geomembrane for this purpose. Also include a separator fabric, per Table PPS-3, between the
geomembrane and all aggregate materials. Lateral flow barriers should be installed in all permeable
pavement installations that have a reservoir/subgrade interface greater than 0%. Lateral flow barriers
should be spaced, as necessary, to satisfy equations PPS-3a and PPS-3b. One exception is reinforced
grass pavement. Infill washout is not a concern with reinforced grass pavement.
5. Perimeter Barrier: For all no-infiltration sections, provide a reinforced concrete barrier on all sides
of the pavement system. Perimeter barriers may also be recommended for other permeable pavement
installations depending on the type or use of the pavement. For PICP and concrete grid pavement, a
barrier is required to restrain movement of the pavers or grids. Precast, cast-in-place concrete or cut
stone barriers are required for commercial vehicular areas. For residential use and commercial
pedestrian use, a metal or plastic edge spiked with 3/8-inch-diameter, 10-inch-long nails provides a
less expensive alternative for edge restraint.
For all pavements, consider the section beyond the permeable pavement when evaluating the
perimeter design. The perimeter barrier helps force water into the underdrain and reduces lateral flow
of water. Lateral flow can negatively impact the adjacent conventional pavement section, structure,
or embankment (especially when the subgrade is sloped). Also consider material separation.
Consider construction of the interface between the permeable pavement and the adjacent materials
and how the design will prevent adjacent materials from entering the permeable pavement section.
Depending on the soils, depth of pavement, and other factors, this may be achieved with fabric or
may require a more formalized barrier.
When a permeable pavement section is adjacent to conventional pavement, a vertical liner may be
required to separate the reservoir of the permeable pavement system from dense-graded aggregates
and soils within the conventional pavement. An impermeable linear can be used to provide this
vertical barrier and separate these two pavement systems.
No-Infiltration Section: For this type of section, the perimeter barrier also serves to attach the
impermeable membrane. The membrane should extend up to the top of the filter layer and be firmly
T-10 Permeable Pavement Systems
PPS-10 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Design Opportunity
Pollutant removal occurs in the filter material layer of the section. The basic permeable pavement
section may be considered with other wearing courses to provide water quality as long as:
the filter layer is included in the section,
the wearing course provides adequate permeability, and
the new section does not introduce new pollutants to the runoff.
attached to the concrete perimeter barrier using batten bars to provide a leak-proof seal. A nitrile-
based vinyl adhesive can be used when the need for an impermeable liner is less critical. See Figures
PPS-4 and PPS-5 for installation details. For ease of construction, including the placement of
geotextiles, it is suggested that the barrier extend to the bottom of the filter layer.
Partial and Full Infiltration Section:
6. Filter Material and Underdrain System: An aggregate filter layer and underdrain are required for
all partial and no-infiltration sections. Without this filter layer, the section will not provide adequate
pollutant removal. This is based on research performed by UDFCD monitoring sites with and
without this component. A filter or separator fabric may also be necessary under the reservoir in a
full infiltration section if the subgrade is not filter compatible with the reservoir material such that
finer subgrade soils could enter into the voids of the reservoir.
The perimeter barrier for these sections also restricts lateral flow
to adjacent areas of conventional pavement or other structures where excessive moisture and/or
hydrostatic pressure can cause damage. When this is of particular concern, the perimeter barrier
should be extended to a depth 12 inches or more below the underdrain. Otherwise, extend the barrier
to the bottom of the filter layer.
In previous versions of the USDCM, UDFCD recommended that the underdrain be placed in an
aggregate drainage layer and that a geotextile separator fabric be placed between this drainage and the
filter layer. This version of the USDCM replaces that fabric, which could more easily plug or be
damaged during construction, with aggregate filter material that is filter-compatible with the
reservoir, and a drainpipe with perforations that are filter-compatible with the filter material. This
eliminates the need for a separator fabric between the reservoir and the underdrain layer. The filter
material provided below should only be used with the underdrain pipe specified within this section.
The underdrain should be placed below a 6-inch-thick layer of CDOT Class C filter material meeting
the gradation in Table PPS-1. Extend the filter material around and below the underdrain as shown in
Figure PPS-1.
Provide clean-outs to allow inspection (by camera) of the drainpipe system during and after
construction to ensure that the pipe was not crushed or disconnected during construction and to allow
for maintenance of the underdrain.
Use of Class C Filter material with a slotted PVC pipe that meets the slot dimensions provided in
Table PPS-2 will eliminate the need for an aggregate layer wrapped geotextile fabric.
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-11
Urban Storm Drainage Criteria Manual Volume 3
Table PPS-1. Gradation Specifications for Class C Filter Material (Source: CDOT Table 703-7)
Sieve Size
Mass Percent Passing
Square Mesh Sieves
19.0 mm (3/4") 100
4.75 mm (No. 4) 60 – 100
300 µm (No. 50) 10 – 30
150 µm (No. 100) 0 – 10
75 µm (No. 200) 0 - 3
Table PPS-2. Dimensions for Slotted Pipe
Pipe Diameter Slot
Length1
Maximum Slot
Width
Slot
Centers1
Open Area1
(per foot)
4" 1-1/16" 0.032" 0.413" 1.90 in2
6" 1-3/8" 0.032" 0.516" 1.98 in2
1 Some variation in these values is acceptable and is expected from various pipe
manufacturers. Be aware that both increased slot length and decreased slot centers
will be beneficial to hydraulics but detrimental to the structure of the pipe.
Compact the filter layer using a vibratory drum roller or plate. The top of each layer below the
leveling course must be uniform and should not deviate more than a ½ inch when a 10-foot straight
edge is laid on its surface. The top of the leveling course should not deviate more than 3/8 inch in 10
feet.
7. Impermeable Geomembrane Liner and Geotextile Separator Fabric: For no-infiltration sections,
install a 30 mil (minimum) PVC geomembrane liner, per Table PPS-4, on the bottom and sides of the
basin, extending up at least to the top of the filter layer. Provide at least 9 inches (12 inches if
possible) of cover over the membrane where it is attached to the wall to protect the membrane from
UV deterioration. The geomembrane should be field-seamed using a dual track welder, which allows
for non-destructive testing of almost all field seams. A small amount of single track and/or adhesive
seaming should be allowed in limited areas to seam around pipe perforations, to patch seams removed
for destructive seam testing, and for limited repairs. The liner should be installed with slack to
prevent tearing due to backfill, compaction, and settling. Place CDOT Class B geotextile separator
fabric, per Table PPS-3, above the geomembrane to protect it from being punctured during the
placement of the filter material above the liner. If the subgrade contains angular rocks or other
material that could puncture the geomembrane, smooth-roll the surface to create a suitable surface. If
smooth-rolling the surface does not provide a suitable surface, also place the separator fabric between
the geomembrane and the underlying subgrade. This should only be done when necessary because
fabric placed under the geomembrane can increases seepage losses through pinholes or other
geomembrane defects. Connect the geomembrane to perimeter concrete walls around the basin
perimeter, creating a watertight seal between the geomembrane and the walls using a continuous
batten bar and anchor connection (see Figure PPS-5). Where the need for the impermeable
T-10 Permeable Pavement Systems
PPS-12 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
membrane is not as critical, the membrane can be attached with a nitrile-based vinyl adhesive. Use
watertight PVC boots for underdrain pipe penetrations through the liner (see Figure PPS-4).
Table PPS-3. Physical Requirements for Separator Fabric1
Table PPS-4. Physical Requirements for Geomembrane
Property
Thickness
0.76 mm
(30 mil)
Test Method
Thickness, % Tolerance ±5 ASTM D 1593
Tensile Strength, kN/m (lbs/in) width 12.25 (70) ASTM D 882, Method B
Modulus at 100% Elongation, kN/m (lbs/in) 5.25 (30) ASTM D 882, Method B
Ultimate Elongation, % 350 ASTM D 882, Method A
Tear Resistance, N (lbs) 38 (8.5) ASTM D 1004
Low Temperature Impact, °C (°F) -29 (-20) ASTM D 1790
Volatile loss, % max. 0.7 ASTM D 1203, Method A
Pinholes, No. Per 8 m2 (No. per 10 sq. yds.) max. 1 N/A
Bonded Seam Strength, % of tensile strength 80 N/A
8. Outlet: The portion of the WQCV in each cell should be slowly released to drain in approximately
12 hours. An orifice at the outlet of the underdrain can be used for each cell to provide detention and
slow release of the WQCV to offset hydromodification. Use a minimum orifice size of 3/8 inch to
avoid clogging. If lateral walls are required, each cell should be considered a separate system and be
Property
Class B
Elongation Test Method
< 50%2
Elongation
> 50%2
Grab Strength, N (lbs) 800 (180) 510 (115) ASTM D 4632
Puncture Resistance, N (lbs) 310 (70) 180 (40) ASTM D 4833
Trapezoidal Tear Strength, N (lbs) 310 (70) 180 (40) ASTM D 4533
Apparent Opening Size, mm
(US Sieve Size)
AOS < 0.3mm (US Sieve Size No. 50) ASTM D 4751
Permittivity, sec-1 0.02 default value,
must also be greater than that of soil
ASTM D 4491
Permeability, cm/sec k fabric > k soil for all classes ASTM D 4491
Ultraviolet Degradation at 500
hours
50% strength retained for all classes ASTM D 4355
1 Strength values are in the weaker principle direction
2 As measured in accordance with ASTM D 4632
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-13
Urban Storm Drainage Criteria Manual Volume 3
controlled independently. See Figure PPS-6 for underdrain system layout and outlet details showing
a multi-cell configuration. Equations PPS-4 and PPS-5 can be used to determine the depth of the
WQCV within the pavement section (based either on the stepped/flat installation shown in Figure
PPS-2 or the sloped installation shown in Figure PPS-3) and Equation PPS-6 can be used to size the
WQCV orifice. If the design includes multiple cells, these calculations should be performed for each
cell substituting WQCV and VTotal with the volumes provided in each cell. The UD-BMP workbook
available at www.udfcd.org can be used when multiple cells are similar in area. The workbook
assumes that the WQCV is distributed evenly between each cell.
For calculating depth of the WQCV using a flat/stepped installation, see Figure PPS-2:
𝑑 =
12 WQCV
𝑃𝐴
Equation PPS-4
Where:
d = depth of WQCV storage in the reservoir (in)
P = porosity, ≤0.30 for porous gravel, ≤0.4 for all other pavements using AASHTO No. 57
or No. 67 coarse aggregate in the reservoir
A = area of permeable pavement system (ft2)
WQCV = water quality capture volume (ft3)
For calculating depth of the WQCV using a sloped installation, see Figure PPS-3:
𝑑 = 6 �
2 WQCV
𝑃A
� + sL Equation PPS-5
Where:
d = depth of WQCV storage in the reservoir (in)
A = area of permeable pavement system (ft2)
s = slope of the reservoir/subgrade interface (ft/ft)
L = length between lateral flow barriers (see step 4) (ft)
T-10 Permeable Pavement Systems
PPS-14 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
For calculating the diameter of the orifice for a 12-hour drain time (Use a minimum orifice size of 3/8
inch to avoid clogging.):
𝐷12 hour drain time = �
𝑉
1414 𝑦0.41 Equation PPS-6
Where:
D = diameter of the orifice to drain a volume in 12 hours (in)
Y = distance from the lowest elevation of the storage volume (i.e. the bottom of the reservoir) to
the center of the orifice (ft)
V = volume (WQCV or the portion of the WQCV in the cell) to drain in 12 hours (ft3)
Additional Design Considerations
Subgrade Preparation
Partial Infiltration and Full Infiltration Installations
For sites, or portions thereof, requiring excavation to the final subgrade level, compaction of the
subgrade may not be needed, provided that loose materials are removed from the excavation, and a
firm subgrade is provided for the support of the pavement system. A geotechnical engineer should
observe the prepared subgrade. Local soft areas should be excavated and replaced with properly
compacted fill. As an alternative to excavating and replacing material, stabilization consisting of
geogrid and compacted granular fill material can be used to bridge over the soft area. Fill material
should be free draining and have a hydraulic conductivity significantly higher than the subgrade soil.
Fill is typically compacted to a level equivalent to 95% Standard Proctor compaction (ASTM D 698).
The designer should specify the level of compaction required to support the pavement system.
: The subgrade should be stripped of topsoil or other
organics and either excavated or filled to the final subgrade level. Unnecessary compaction or over-
compaction will reduce the subgrade infiltration rate. However, a soft or loosely compacted subgrade
will settle, adversely impacting the performance of the entire permeable pavement system. The following
recommendations for subgrade preparation are intended to strike a balance between those competing
objectives:
For sites (or portions thereof), requiring placement of fill above the existing subgrade to reach the
final subgrade level, the fill should be properly compacted. Specify the hydraulic conductivity for the
material that is to be placed. This should be at least one order of magnitude higher than the native
material. If the type or level of compaction of fill material available for construction is different than
that considered in design, additional testing should be performed to substantiate that the design
infiltration rate can be met. However, additional infiltrometer testing may not be necessary, provided
that it can be demonstrated by other means that the compacted fill material is more permeable than
that considered for design.
Low ground pressure (LGP) track equipment should be used within the pavement area to limit over-
compacting the subgrade. Wheel loads should not be allowed.
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-15
Urban Storm Drainage Criteria Manual Volume 3
No-Infiltration Sections
Filter and Reservoir Layer Compaction
: Unless otherwise indicated by the geotechnical engineer, the subgrade for this
section should be scarified and properly compacted to support the liner and pavement system. A level of
compaction equivalent to 95% of the Standard Proctor density (ASTM D 698) is typically used. The
designer should specify the level of compaction. No-infiltration sections should be smooth rolled with a
roller compactor, and the prepared subgrade surface should be free of sharp objects that could puncture
the liner. Both the designer and the liner installer should inspect the subgrade for acceptance prior to liner
placement.
Filter material placed above the prepared subgrade should be compacted to a relative density between
70% and 75% (ASTM D4253 and ASTM D4254) using a walk-behind vibratory roller, vibratory plate
compactor or other light compaction equipment. Do not over-compact; this will limit unnecessary
infiltration into the underlying subgrade. The reservoir layer may not be testable for compaction using a
method based on specified density (e.g., nuclear density testing). The designer should consider a method
specification (e.g., number of passes of a specified vibratory compactor) for those materials. The number
of passes appropriate is dependent on the type of equipment and depth of the layer.
Figure PPS-4. Geomembrane Liner/Underdrain Penetration Detail
T-10 Permeable Pavement Systems
PPS-16 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Figure PPS-5. Geomembrane Liner/Concrete Connection Detail
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-17
Urban Storm Drainage Criteria Manual Volume 3
Figure PPS-6. Lateral Barrier Installation
T-10 Permeable Pavement Systems
PPS-18 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Figure PPS-7. Underdrain System Layout and Outlet Details
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-19
Urban Storm Drainage Criteria Manual Volume 3
Figure PPS-8. Observation Well
T-10 Permeable Pavement Systems
PPS-20 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Construction Considerations
Proper construction of permeable pavement systems requires measures to preserve natural infiltration
rates (for full and partial infiltration sections) prior to placement of the pavement, as well as measures to
protect the system from the time that pavement construction is complete to the end of site construction.
Supplemental Fact Sheets on the specific pavements provide additional construction considerations. The
following recommendations apply to all permeable pavement systems:
When using an impermeable liner, ensure enough slack in the liner to allow for backfill, compaction,
and settling without tearing the liner.
Provide necessary quality assurance and quality control (QA/QC) when constructing an impermeable
geomembrane liner system, including, but not limited to fabrication testing, destructive and non-
destructive testing of field seams, observation of geomembrane material for tears or other defects, and
air lace testing for leaks in all field seams and penetrations. QA/QC should be overseen by a
professional engineer. Consider requiring field reports or other documentation from the engineer.
Keep mud and sediment-laden runoff away from the pavement area.
Temporarily divert runoff or install sediment control measures as necessary to reduce the amount of
sediment run-on to the pavement.
Cover surfaces with a heavy impermeable membrane when construction activities threaten to deposit
sediment onto the pavement area.
Design Example
The UD-BMP workbook, designed as a tool for both designer and reviewing agency is available at
www.udfcd.org. This section provides a completed design form from this workbook as an example.
Permeable Pavement Systems T-10
August 2011 Urban Drainage and Flood Control District PPS-21
Urban Storm Drainage Criteria Manual Volume 3
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Type of Permeable Pavement Section
A) What type of section of permeable pavement is used?
(Based on the land use and activities, proximity to adjacent
structures and soil characteristics.)
B) What type of wearing course?
2. Required Storage Volume
A) Effective Imperviousness of Area Tributary to Permeable Pavement, Ia Ia = 65.0 %
B) Tributary Area's Imperviousness Ratio (I = Ia / 100) i = 0.650
C) Tributary Watershed Area ATotal = 55,000 sq ft
(including area of permeable pavement system)
D) Area of Permeable Pavement System APPS = 15,000 sq ft
(Minimum recommended permeable pavement area = 13491 sq ft)
E) Impervious Tributary Ratio RT = 1.7
(Contributing Imperviuos Area / Permeable Pavement Ratio)
F) Water Quality Capture Volume (WQCV) Based on 12-hour Drain Time WQCV = 932 cu ft
(WQCV = (0.8 * (0.91 * i3 - 1.19 * i2 + 0.78 * i) / 12) * Area)
G) Is flood control volume being added?
Provide overflow to carry runoff directly
into the reservoir layer to ensure use
of flood control volume regardless
H) Total Volume Needed VTotal = 6,340 cu ft of infiltration rates.
3. Depth of Reservoir
A) Minimum Depth of Reservoir Dmin = 18.0 inches
(Minimum recommended depth is 6 inches)
B) Is the slope of the reservoir/subgrade interface equal to 0%?
C) Porosity (Porous Gravel Pavement < 0.3, Others < 0.40) P = 0.40
D) Slope of the Base Course/Subgrade Interface S = ft / ft
E) Length Between Lateral Flow Barriers L = ft
F) Volume Provided Based on Depth of Base Course V = 8,500 cu ft
Flat or Stepped: V = P * ((Dmin-1)/12) * Area
Sloped: V = P * [(Dmin - (Dmin - 6*SL-1)) / 12] * Area
4. Lateral Flow Barriers
A) Type of Lateral Flow Barriers
B) Number of Permeable Pavement Cells Cells = 1
5. Perimeter Barrier
A) Is a perimeter barrier provided on all sides of the
pavement system?
(Recommeded for PICP, concrete grid pavement, or for any
no-infiltration section.)
Shops at 56th Ave.
SE corner of 56th Ave. and 83rd St.
Design Procedure Form: Permeable Pavement Systems (PPS)
G. Frazer
BMP Inc.
November 29, 2010
Choose One
No Infiltration
Partial Infiltration Section
Full Infiltration Section
Choose One
YES
T-10 Permeable Pavement Systems
PPS-22 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3
Sheet 2 of 2
Designer:
Company:
Date:
Project:
Location:
6. Filter Material and Underdrain System
A) Is the underdrain placed below a 6-inch thick layer of
CDOT Class C filter material?
B) Diameter of Slotted Pipe (slot dimensions per Table PPs-2)
C) Distance from the Lowest Elevation of the Storage Volume y = 3.8 ft
(i.e. the bottom of the base course to the center of the orifice)
7. Impermeable Geomembrane Liner and Geotextile Separator Fabric
A) Is there a minimum 30 mil thick impermeable PVC geomembrane
liner on the bottom and sides of the basin, extending up to the top
of the base course?
B) CDOT Class B Separator Fabric
8. Outlet
(Assumes each cell has similar area, subgrade slope, and length
between lateral barriers (unless subgrade is flat). Calculate cells
individually where this varies.)
A) Depth of WQCV in the Reservoir DWQCV
= 1.86 inches
(Elevation of the Flood Control Outlet)
B) Diameter of Orifice for 12-hour Drain Time DOrifice
= 0.62 inches
(Use a minimum orifice diameter of 3/8-inches)
Notes:
Shops at 56th Ave.
Design Procedure Form: Permeable Pavement Systems (PPS)
G. Frazer
BMP Inc.
November 29, 2010
SE corner of 56th Ave. and 83rd St.
Choose One
YES
NO
Choose One
4-inch
6-inch
Choose One
Choose One
YES
NO
Placed above the liner
Placed above and below the liner
N/A
APPENDIX D
EROSION CONTROL REPORT
Scott Plaza
EROSION CONTROL REPORT
A comprehensive Erosion and Sediment Control Plan (along with associated details) HAS BEEN
PROVIDED BY SEPARATE DOCUMENT. It should be noted, however, that any such Erosion and
Sediment Control Plan serves only as a general guide to the Contractor. Staging and/or phasing of
the BMPs depicted, and additional or different BMPs from those included may be necessary during
construction, or as required by the authorities having jurisdiction.
It shall be the responsibility of the Contractor to ensure erosion control measures are properly
maintained and followed. The Erosion and Sediment Control Plan is intended to be a living
document, constantly adapting to site conditions and needs. The Contractor shall update the
location of BMPs as they are installed, removed or modified in conjunction with construction
activities. It is imperative to appropriately reflect the current site conditions at all times.
The Erosion and Sediment Control Plan shall address both temporary measures to be implemented
during construction, as well as permanent erosion control protection. Best Management Practices
from the Volume 3, Chapter 7 – Construction BMPs will be utilized. Measures may include, but are
not limited to, silt fencing along the disturbed perimeter, gutter protection in the adjacent roadways
and inlet protection at existing and proposed storm inlets. Vehicle tracking control pads, spill
containment and clean-up procedures, designated concrete washout areas, dumpsters, and job site
restrooms shall also be provided by the Contractor.
Grading and Erosion Control Notes can be found on the Utility Plans. The Final Plans contain a
full-size Erosion Control sheet as well as a separate sheet dedicated to Erosion Control Details. In
addition to this report and the referenced plan sheets, the Contractor shall be aware of, and adhere
to, the applicable requirements outlined in the Development Agreement for the development. Also,
the Site Contractor for this project will be required to secure a Stormwater Construction General
Permit from the Colorado Department of Public Health and Environment (CDPHE), Water Quality
Control Division – Stormwater Program, prior to any earth disturbance activities. Prior to securing
said permit, the Site Contractor shall develop a comprehensive StormWater Management Plan
(SWMP) pursuant to CDPHE requirements and guidelines. The SWMP will further describe and
document the ongoing activities, inspections, and maintenance of construction BMPs.
MAP POCKET
DRAINAGE EXHIBITS
S
UD
UD
UD
UD
UD
UD
UD UD UD UD
UD
UD UD
UD
X
X
X
X X X
X X X X
X
X
X
X
X
X
X
X
E E
X
X X X
X X
X
X X
G
X
X X X
X X
X X X
X X
X
X X
EXISTING
BUILDING
EXISTING
BUILDING
EXISTING
BUILDING
CAMBRIDGE
HOUSE LLC
1113 W PLUM ST
FORT COLLINS,
CO 80521
TOWNSQUARE CONDOS
1209 W PLUM ST
FORT COLLINS,
CO 80521
EXISTING
BUILDING
EXISTING
WALL
EXISTING
RETAINING
WALL
EXISTING
RETAINING WALL
EXISTING
WALL
EXISTING
RETAINING WALL
COLLINS CAMPUS WEST LLC
1220 W ELIZABETH ST
FORT COLLINS,
CO 80521
PROPOSED
STORM OUTFALL
EXISTING
CONCRETE PAN
EXISTING ASPHALT
PARKING LOT
PROPOSED STORM
DRAIN (BY OTHERS)
PERMEABLE PAVER DETENTION
DETENTION VOLUME=5086 CU.FT.
(40% VOID RATIO)
FORMER SCOTT AVE.
(VACATED 25' ROW)
PLUM STREET
(ROW VARIES)
TRASH
ROOF OUTLINE
(ABOVE)
RESIDENTIAL
ROOF OUTLINE
(ABOVE)
PROPOSED
UNDERDRAIN
PROPOSED
UNDERDRAIN
ROOF OUTLINE
(ABOVE)
PROPOSED
DRY WELL
1
4
2
3
5
5
3
1
2
4
PROPOSED
UNDERDRAIN
These drawings are
instruments of service
provided by Northern
Engineering Services, Inc.
and are not to be used for
any type of construction
unless signed and sealed by
a Professional Engineer in
the employ of Northern
Engineering Services, Inc.
NOT FOR CONSTRUCTION
200 South College Avenue, Suite 010
Fort Collins, Colorado 80524
N O R T H E RN
PHONE: 970.221.4158 FAX: 970.221.4159
www.northernengineering.com
C600
CALL 2 BUSINESS DAYS IN ADVANCE BEFORE YOU
DIG, GRADE, OR EXCAVATE FOR THE MARKING OF
UNDERGROUND MEMBER UTILITIES.
CALL UTILITY NOTIFICATION CENTER OF
COLORADO
R
NORTH
LEGEND:
A
BENCHMARK
BENCHMARK #1:
City of Fort Collins Benchmark 19-97
At the southeast corner of West Elizabeth and Shields Street, on a
concrete traffic signal base.
Elevation=5022.56
BENCHMARK #2:
City of Fort Collins Benchmark 20-97
At the southwest corner of West Elizabeth and Constitution Avenue, on
a concrete traffic signal base.
Elevation=5046.97
NOTES:
FOR DRAINAGE REVIEW ONLY
NOT FOR CONSTRUCTION
A
Basin
Area
(acres)
Treatment
Type LID ? Percent
1 0.66 Permeable Pavers yes 56%
2 0.03 none yes 3%
3 0.24 Permeable Pavers yes 20%
4 0.02 Permeable Pavers yes 2%
5 0.23 Permeable
Pavers/Grass Buffer
yes 19%
Totals 1.18 - - 100%
UD
LID SUMMARY TABLES:
Pavement1
Area2
(sq.ft.) LID ? Percent
Traditional
Concrete/Asphalt 5,233 no 34%
Permeable Pavers 9,959 yes 66%
Totals 15,192 - 100%
1. Represents "new/additional" pavement beyond the existing condition.
2. Only pavement areas exposed to surface rainwater are included.
NO
Choose One
YES- Flat or Stepped Installation
NO- Sloped Installation
Choose One
Concrete Walls
PVC geomembrane installed normal to flow
N/A- Flat installation
Other (Describe):
Choose One
YES
NO
Choose One
PICP
Concrete Grid Pavement
Pervious Concrete
Porous Gravel
considered if the BMP includes an underdrain designed to
divert water away from the structure and is lined with an
essentially impermeable geomembrane liner designed to
restrict seepage.
Designing for Maintenance
Recommended ongoing maintenance practices for all BMPs
are provided in the BMP Maintenance chapter of this manual.
During design and construction, the following should be
considered to ensure ease of maintenance over the long-term:
Hold a pre-construction meeting to ensure that the
contactor has an understanding of how the pavement is
intended to function. Discuss the contractor’s proposed
sequence of construction and look for activities that may
require protection of the permeable pavement system.
Ensure that the permeable pavement is protected from construction activities following pavement
construction (e.g., landscaping operations). This could include covering areas of the pavement,
providing alternative construction vehicle access, and providing education to all parties working on-
site.
Include an observation well to monitor the drain time of the pavement system over time. This will
assist with determining the required maintenance needs. See Figure PPS-8.
Nutrients Good
Total Metals Good
Bacteria Unknown
Other Considerations
Life-cycle Costs4 High2
1 Not recommended for watersheds with
high sediment yields (unless pretreatment is
provided).
2 Does not consider the life cycle cost of the
conventional pavement that it replaces.
3
Based primarily on data from the
International Stormwater BMP Database
(www.bmpdatabase.org).
4
Based primarily on BMP-REALCOST
available at www.udfcd.org. Analysis
based on a single installation (not based on
the maximum recommended watershed
tributary to each BMP).
Input Variables Results
Calculations By:
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
11No0.95 0.95 1.00 40 2.00% 1.4 1.4 0.9 215 0.50% 1.41 2.5 N/A N/A 5 5 5
22No0.95 0.95 1.00 30 2.00% 1.2 1.2 0.8 50 0.50% 1.41 0.6 N/A N/A 5 5 5
33No0.95 0.95 1.00 40 2.00% 1.4 1.4 0.9 120 0.50% 1.41 1.4 N/A N/A 5 5 5
44No0.95 0.95 1.00 8 2.00% 0.6 0.6 0.4 50 0.50% 1.41 0.6 N/A N/A 5 5 5
55No0.95 0.95 1.00 40 2.00% 1.4 1.4 0.9 235 0.50% 1.41 2.8 N/A N/A 5 5 5
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow
ATC
January 15, 2014
Time of Concentration
(Equation RO-4)
3
1
1 . 87 1 . 1 *
S
Ti C Cf L