Villa Park Police Station Urban BMP Demonstration Project%3A A Project to Demonstrate Integration of Green Roof, Permeable Paving, and Bioswale Urban Runoff Systems

              Villa Park Police Station Urban
BMP Demonstration Project
A Project to Demonstrate Integration of Green
Roof, Permeable Paving, and Bioswale Urban
Runoff Systems
Prepared by:
Conservation Design Forum
375 W. First Street
Elmhurst, IL 60126
Prepared For:
Village of Villa Park
20 S. Ardmore Avenue
Villa Park, IL 60181
Villa Park Police Station Urban
BMP Demonstration Project
A Project to Demonstrate Integration of Green
Roof, Permeable Paving, and Bioswale Urban
Runoff Systems
July 19, 2005
Financial Assistance Number 3190310
Village of Villa Park
Engineering Division
20 S. Ardmore Avenue
Villa Park, Illinois 60181
Conservation Design Forum
375 W. First Street
Elmhurst, Illinois 60126
www.cdfinc.com
Illinois Environmental Protection Agency
Bureau of Water – Watershed Management Section
1021 Grand Avenue East, P.O. Box 19276
Springfield, Illinois 62794-19276
This project was funded, in part, by U.S. Environmental Protection Agency
funds under Section 319 of the Clean Water Act distributed and administered
through the Illinois Environmental Protection Agency. The findings and
recommendations contained herein are not necessarily those of the funding
agencies.
Contents
Introduction .....................................................................................................................................1
Project Location and Pre-Project Conditions...........................................................................2
Project Purpose and Goals...........................................................................................................2
Project Description.........................................................................................................................2
BMP System Overview ......................................................................................................3
BMP System Components................................................................................................3
BMP System Hydrologic Analysis..................................................................................................6
Runoff Analysis ....................................................................................................................6
Detention Analysis..............................................................................................................6
BMP System Performance.............................................................................................................8
Runoff Rates and Volumes ..............................................................................................8
Water Quality......................................................................................................................8
Runoff Monitoring System..............................................................................................................9
Maintenance and Management...............................................................................................9
Summary ........................................................................................................................................11
References.....................................................................................................................................12
Tables
Table 1: Runoff Parameters...........................................................................................................6
Table 2: Estimate Flow Rates and Runoff Volumes..................................................................8
Table 3: Estimated Pollutant Removal Rates ............................................................................9
Figures
Figure 1: Project Location............................................................................................................13
Figure 2: Site Plan...........................................................................................................................14
Figure 3: Flow Monitoring Locations..........................................................................................15
Project Photos ...............................................................................................................................16
Appendices
Appendix 1: Villa Park Police Station Site Construction Drawings
Appendix 2: Stormwater Practice Maintenance Requirements
1
Introduction
The Village of Villa Park constructed a new police station east of the previous station
and south of Village Hall in the heart of Villa Park and on the site of a former asphalt
parking lot. The site is 0.72 acres in size and is covered mostly by the building footprint
and parking lot. Despite this small size, runoff volumes and rates are expected to be
reduced to undeveloped conditions using green roofs, permeable paving, and
bioretention. The project is also expected to reduce pollutant loads by more than
90%.
Villa Park is in the Salt Creek watershed (WSID ILGL09) along segment GL09. This
segment of the Creek is rated as “partial support” for overall use and aquatic life
and “nonsupport” for swimming according to the 2000 Illinois Water Quality Report.
The impairments are reported to be caused by priority organics, PCBs, excess
nutrients including phosphorus, nitrogen and nitrates, salinity/TDS/chlorides, flow
alteration, and excessive algal growth/chlorophyll a. The sources of pollutants are
listed as municipal point sources including combined sewer overflows, collection
system failures, urban runoff/storm-sewers, hydrologic/habitat modifications,
upstream impoundment, flow regulation and modification, and contaminated
sediments.
Many of the causes and sources listed above are related to urban runoff, which is
largely unregulated at this time. Thus, implementing and providing examples of best
management practices to cleanse and reduce volumes of urban stormwater runoff
is greatly needed in this largely built out watershed.
Also, the DuPage County Department of Environmental Concerns encourages the use
of best management practices to address storm water management water quality
issues. However, detention is virtually the only BMP implemented under DEC’s stormwater
ordinance and detention basins are difficult to implement on small sites and do little to
control the volume of runoff reaching area waterways.
One of the obstacles to greater implementation of green roofs, permeable paving,
bioswales, and similar strategies is lack of understanding by local regulators (municipal
and county) on the runoff rate and volume reduction performance of these BMPs in
relation to stormwater release rate standards. Thus, runoff rates are being measured as
part of this project to provide hydrologic performance data.
This project addresses pollutant loadings as well as storm runoff induced water level
fluctuations within Salt Creek. The green roof on the new police facility will reduce
runoff and pollutant loading and provide a prototype green roof that we can easily
display to communities and businesses in the Salt Creek and other watersheds. The
site outside the building demonstrates innovative, infiltration based storm water
management approaches that can be used throughout the watershed to help
address nonpoint source pollution.
2
Project Location and Pre-Project Conditions
The project is located as shown in Figure 1 at the intersection of Ardmore Avenue
and Home Avenue, south of Village Hall. The site is a block south of St. Charles Road
and just north of the Prairie Path.
The site was previously an asphalt paved parking lot with no detention. The runoff
from the parking lot drained offsite via storm sewers and surface runoff. (See photos
at the end of the document for pre-project condition.)
Project Purpose and Goals
The purpose of this project was to address urban runoff impacts to Salt Creek, to
provide an example of a “green building” for the Village, County, and the general
public, and to demonstrate innovative, retention-based stormwater management
approaches for highly impervious areas. To achieve these purposes, the following
BMP design and performance objectives were used to guide design of the system.
• Improve water quality: Salt Creek is an impaired water body and urban runoff
is a significant contribution to that impairment.
• Reduce Runoff Volumes: Most BMPs do little to control runoff volumes and
therefore urban streams continue to be impacted despite improvements in
water quality. Control of runoff volumes is necessary to prevent increases in
streambank erosion and other impacts resulting from the “flashiness” of urban
streams.
• Provide Demonstration of Advanced Urban Runoff BMPs on Tight Urban Sites:
As many urban areas are experiencing a resurgence in development interest,
significant opportunity exists to retrofit our urban areas as redevelopment
occurs. The Village of Villa Park and Illinois EPA were interested in providing
demonstrations on how these small urban sites can be retrofit to address
runoff rates, volumes, and water quality.
• Meet County Stormwater Release Rate Standards: Although the County
ordinance has a threshold of 1 acre, the Village wished to meet the standard
for this 0.72 acre site.
• Ensure Long Term Performance of BMP Systems: Many BMPs receive very little
maintenance. Thus, to ensure long term performance, the system needed to
be designed for minimal maintenance.
Project Description
The Villa Park Police Station project includes a green roof, two bioswales with
underground storage, and a permeable paving parking lot. These features are
shown on the Site Plan in Figure 2 and the construction drawings in Appendix 1.
Virtually all runoff from the site is filtered by the various features to improve water
quality and the entire parking and bioswale area remains available for infiltration.
The green roof and bioswale soils also serve to retain water to allow for slow
evaporation between events.
The general operation of the system is described below, followed by detailed
descriptions of each of the BMPs used on the project.
3
Green Roof Section
Growing media
Drainage layer
BMP System Overview (See Figure 2)
• Excess rainfall that is not retained by the green roof system and runoff from
the non-vegetated roof area is discharged to the surface of the eastern
bioswale.
• Surface runoff from the permeable paving parking area that cannot
percolate through the openings in the pavement, drains to the surface of the
southern bioswale.
• Subsurface runoff from the permeable paving parking area (water that
percolates into the aggregate base under the paver parking surface) can
either infiltrate into the subgrade below the parking lot or drain laterally to the
underground storage trenches beneath the surface bioswales.
• Runoff to the surface of the bioswales percolates through an amended
topsoil down to the storage trench below. A portion of the runoff within the
bioswale infiltrates into the subgrade below the bioswale and a portion is
temporarily detained by a restrictor sized to meet the DuPage County and
Villa Park detention release rate standards.
BMP System Components
• Green Roof – The green roof of the Police
station is a Hydrotech system. Over most of
the roof, the system is composed of a 3-
inch rooting matrix and a gravel filled
plastic drain board. In the area of the roof
devoted to mechanicals (heating, cooling,
etc), the gravel is overlain by concrete
pavers that allow runoff to pass through to
the drainage media. (This area is denoted
as “non-vegetated roof” in Figure 2). The
growing media is a lightweight,
engineered soil composed of expanded
clay or slate mixed with organic matter.
The drainage layer is also expanded clay
or slate with a larger particle size. The exact mix of the materials is proprietary.
The expanded clay or slate material is designed to be light weight and have
high permeability as well as high water holding capacity. The approximate
properties of the growing media are as indicated below. (Information
provided by Midwest Trading Company – one provider of green roof soil and
drainage media.)
o Dry Unit Weight - approximately 30 - 45 lbs per cubic foot (water is 62
lbs per cubic foot).
o Saturated Unit weight – approximately 75 – 80 lbs per cubic foot.
o Porosity - 70% to 80% (conventional angular gravel is approximately
40%).
o Water holding capacity – approximately 25 - 35%.
o Permeability – approximately 30 inches per hour.
The runoff coefficient or runoff curve number of green roofs varies with green
roof thickness and drainage layer type. Information from Germany (where
green roofs have been in place for decades) report runoff coefficients as low
4
as 0.1 to as high as 0.8. However, it appears that event-based coefficients
are mixed with annual runoff-based coefficients, making the reliability of
these numbers for stormwater design uncertain. Recent paired rainfall and
runoff data from University of North Carolina suggests runoff coefficients of
0.55 to 0.60 with an initial abstraction of approximately 0.3 inches for small 3-
to 4-inch thick green roofs. Runoff monitoring data from this project will
provide additional data for determining runoff characteristics. However, for
purposes of design for this project, a runoff Curve Number (CN) of 74 (similar
to turf on C soils) was used. The discharge from the green roof drains to the
surface of the bioswales. Because both the vegetated and unvegetated
portions of the roof are covered with green roof media, the entire roof was
represented as one surface when estimating runoff.
• Permeable Paving – There are a range of
permeable paving strategies suitable for year
round, continuous use. These include porous
asphalt, porous concrete and interlocking
permeable pavers. Unlike rigid concrete
pavement (poured concrete), porous asphalt
and interlocking pavers are flexible pavement
systems and therefore are tolerant of modest
amounts of frost heave. Asphalt is a relatively
short-lived pavement that requires resurfacing
from time to time. Also, seal coating that is often part of maintenance would
render porous asphalt impermeable. Finally, interlocking paver systems are
extremely long lasting yet flexible, combining the benefits of asphalt and
poured concrete without the negatives. Partially due to this and other IEPA
funded projects, interlocking permeable paving systems are becoming
increasingly popular. As a result, installation costs are dropping, making
interlocking paver system installation costs more competitive.
The selected Unilock Ecoloc permeable paver system has an approximately
10% open area that is filled with a medium gravel material. Because of the
chamfers designed into the pavers, all runoff is routed to the open areas.
Thus, all runoff has the potential to infiltrate through the openings. At
installation, the net infiltration capacity is very high but tends to decrease over
time, leveling off after a period of approximately 5 years. German studies
have demonstrated initial infiltration rates of 9 in/hr and higher and two four
year old installations indicated rates of 3 in/hr. In another study of two and
five year old parking lot installations, infiltration rates were found to be
approximately 6 and 5 in/hr, respectively. These rates are sufficiently high to
capture the runoff from all but the most intense rainfalls. If the pavement
surface clogs, the infiltration can be restored by removing and replacing the
top 1 to 2 inches of crevice fill material. (See Maintenance Section.)
Because of the high surface infiltration capacity and due to the relatively low
soil infiltration capacities in northeastern Illinois, the infiltration of the subgrade
and the drainage characteristics of the base material control the runoff
5
Bioretention swale adjacent to permeable paving
characteristics rather than the surface. Thus, a permeable paving system
behaves hydraulically like a detention system with the detention located in
the gravel base under the paving surface. The release from the “detention”
can be controlled either by the hydraulic conductivity of the gravel or by the
capacity of perforated drains within the gravel base. In some cases, a
restrictor can be installed in the drain to meet stormwater release rate
requirements. The porosity of the open graded gravel base of the pavement
systems is approximately 40% (IDOT CA1 and CA7 aggregate porosities were
both measured to be 42% at one local quarry). Thus, the available detention
volume is equal to 40% of the gravel volume under the pavement surface.
For this project no restrictor was used to control the release of the pavement
and instead the aggregate hydraulic conductivity controls. The drainage
from the 12-inch gravel base discharges to the gravel beneath the bioswale
soil. In addition to lateral release from the aggregate, water is released via
infiltration into the subgrade. The measured infiltration capacity of the
subgrade soils under the previous parking lot was 0.75 in/hour. Due to
additional compaction that likely occurred during construction, a
conservatively low infiltration capacity of 0.1 in/hour for the permeable paver
subgrade was assumed. Virtually all the rainfall falling onto the permeable
pavers drains through to the gravel base below. Thus, a CN of 98 was used,
consistent with typical paving surfaces. The runoff is assumed to immediately
enter the gravel base.
The runoff into the gravel base was routed through the base using
conventional storage routing techniques. The discharge rate from the base
was computed based on Darcy’s law with a hydraulic conductivity of 3500
in/hr and a subgrade slope of 1%. The discharge from the gravel base is to
the gravel under the bioswales. See the following section on hydrologic
analysis for further discussion on storage routing of the permeable paving.
• Bioretention Swales – Bioswales
are composed of a surface topsoil
underlain by a gravel
drainage/storage layer. The
topsoil was amended with sand
and organic matter to achieve a
design infiltration capacity of 2 to
5 inches per hour. The design mix
of the eight inch soil layer was as
listed below:
o 70% - 80% coarse sand
o 8% - 10% organic content
o 10% maximum clay
content.
Because of the water holding capacity of the bioswale topsoil, a CN of 74
was used, consistent with turf on hydrologic soil group C soils.
6
As with the permeable paving, runoff was also storage routed through the
bioswales. The above ground and underground storage both function as
detention. The release from the system is controlled by a restrictor to meet
the county release rate of 0.1 cfs/acre. The depth of gravel drainage/soil
layer is 3 feet.
BMP System Hydrologic Analysis
The Villa Park system was evaluated using standard NRCS runoff curve number and
routing methods contained within “HydroCAD”.
Runoff Analysis
Runoff from permeable paving, green roofs and the other site surfaces (walks,
landscaping, etc.) were modeled independently to produce runoff hydrographs
and the sum of the individual hydrographs were routed through the detention
(bioswale) system. The green roof and permeable pavement descriptions above
discussed the runoff characteristics of these surfaces and Table 1 below
summarizes the runoff parameters.
Table 1: Runoff Parameters
Location
Area
(ac) CN
Time of
Concen.
(hours) Notes
Green Roof 0.0874 74 0.1 Runoff assumed similar to lawn on “C” soils
Permeable
Paving
0.215 98 0.1 Permeable paving runoff also routed through
gravel storage beneath pavement.
Discharge controlled by hydraulic
conductivity of gravel. Infiltration through
bottom at 0.1 inches per hour
Impervious
Paving
0.111 95 0.1
Landscape 0.138 65 0.1 Native landscaping beds
Bioswale 0.134 74 0.1 Turf over top of gravel infiltration bed
Total Area 0.72
Detention Analysis
Detention occurs in both the surface of the bioswales and the underground
coarse aggregate material of the bioswales and permeable paving. All runoff is
either filtered through the surface soils of the bioswale or through the permeable
paving before being discharged to the underground storage. This is done to
improve water quality and to prevent clogging of the permeable paving and
bioswale subgrade soils. The bottom six inches of the bioswale underground
storage is reserved for infiltration storage. Once the stage exceeds six inches,
runoff begins to discharge through the restrictor. The following paragraphs
describe analysis of the various components.
• Permeable Paving Underground Storage – Detention storage is provided
within the 12-inch base of the permeable paving. The base material is IDOT
CA-7 coarse aggregate with a size range from 0.2- to 1-inch. The total
volume of storage in the gravel layer is approximately 3750 ft3 (0.086 ac-ft).
The discharge from the base is controlled by the hydraulic conductivity of the
7
CA-7 aggregate and the slope of the subgrade. Based on information in the
drainage design guidelines prepared for the Uni-Group (Texas A&M, 1998),
the hydraulic conductivity of the aggregate was assumed to be 0.08 feet per
second. The slope of the subgrade is 1%. The drainage from the 12-inch
gravel base discharges to the gravel beneath the bioswale soil. In addition
to lateral release from the aggregate, water is released via infiltration into the
subgrade. The measured infiltration capacity of the subgrade soils under the
previous parking lot was 0.75 in/hour. Due to additional compaction that
likely occurred during construction, a conservatively low infiltration capacity
of 0.1 in/hour for the permeable paver subgrade was assumed. All the rainfall
falling onto the permeable pavers was assumed to drain through to the
gravel base below. However, if surface runoff occurs on the permeable
pavers, the runoff is directed to the surface of the bioswales where it can be
treated and detained.
• Bioswale Underground Storage - The underground storage is created with a 3
foot layer of IDOT CA-7 coarse aggregate below the bioswale topsoil. The
total volume of the subsurface storage is approximately 4370 ft3 (0.100 ac-ft).
The discharge from the underground storage is controlled by an orifice
restrictor as described under “Bioswale Release Rate” below.
• Bioswale Surface Storage – The surface storage is separated from the
underground storage with 8-inches of amended topsoil. Surface runoff from
all areas discharges to the surface of the bioswale where it filters through the
amended topsoil to the underground storage layer. If the depth in the
surface storage exceeds 6 inches, excess runoff can discharge down to the
underground storage through a surface structure. The underground storage
can also bubble up into the surface storage as needed. The total volume of
the surface storage is approximately 4520 ft3 (0.104 ac-ft) at the overflow
elevation of 93.4 feet. Overflow from the bioswale (for events exceeding 100-
year event) is to the street (Ardmore).
• Bioswale Release Rate – For the site area of approximately 0.72 acres, the
allowable release rate is 0.072 cfs (0.10 cfs/acre). The restrictor for the site is
located in a manhole and the controlling elevation is 6 inches above the
bottom of the underground aggregate storage. Runoff access to the
restrictor is via an 8-inch perforated pipe that runs the length of the bioswales.
The inflow capacity of the 124-foot perforated pipe is more than 30 cfs when
the detention area is full. Thus, the perforated pipe does not control the
discharge of the basin and the only control is the restrictor. Because the site is
small, the restrictor size required to achieve the allowable release rate is 1-
inch. Although the restrictor size is small, clogging should not be a concern
since runoff must discharge through the perforated pipe for most events prior
to reaching the orifice.
The assumed infiltration capacity of 0.3 in/hr of the bioswale subsoils was
based on a percolation test indicating a capacity of 0.75 inches/hour. This
infiltration rate over the 3640 ft2 of infiltration bed translates into a discharge
rate of 0.025 cfs out the bottom of the bioswale.
8
BMP System Performance
Runoff Rates and Volumes
Using the model parameters of Table 1and the methods described above, the
proposed system was modeled to estimate runoff volumes and flow rates for a
range of events. The system was modeled using HydroCAD with the NRCS Type II
rainfall distribution and 24-hour duration events.
Table 2: Estimated Flow Rates and Runoff Volumes
Event 24-hour
rainfall
Existing
Conditions
(24-hr runoff &
15-minute flow)
Proposed
Conditions
Notes
1-year discharge
(cfs)
- 1.7 cfs 0.02 cfs
2-year discharge
(cfs)
- 2.2 cfs 0.02 cfs
100-year
discharge (cfs)
- 5.8 cfs 0.07 cfs Allowable rate =
0.072
2-inch runoff 2.00 inches 1.48 inches 0.00 inches Threshold of zero
runoff
1-year runoff
(inches)
2.51 inches 1.97 inches 0.18 inches*
2-year runoff
(inches)
3.04 inches 2.49 inches 0.41 inches*
100-year runoff
(inches)
7.58 inches 6.98 inches 3.13 inches*
* Comparing the proposed conditions runoff volumes to the rainfall, the equivalent curve
number for the site is approximately 61.
Table 2 shows that the system not only meets the runoff rate requirements of
DuPage County but should also significantly reduce runoff volumes. The 100-year
runoff volume is reduced by more than 50% and the 1-year runoff volume is
reduced by more than 90%. No runoff is produced for approximately 2-inch and
small events.
Water Quality
Water quality is improved in two ways. First, all runoff is filtered by green roof
media, by permeable paving aggregate, and/or by bioswale amended topsoil.
For purposes of estimating pollutant reductions, IEPA pollutant removal rates for
sand filters were used to represent the expected pollutant removal rates for the
various media of this project. The filtered runoff then has an opportunity to be
infiltrated through the subgrade of the permeable pavement and bioswales. For
estimating the total load reduction, it was assumed that the infiltrated runoff
releases no constituents. To estimate the average annual runoff volume
reduction, NIPC’s chart showing the distribution of storm rainfall volumes was used
(Northeastern Illinois Planning Commission, 1998). The chart indicates that 2-inch
and smaller rainfall events (the threshold of zero runoff for the Villa Park BMP
system) represents 85% of the average annual rainfall volume.
9
Table 3: Estimate Average Annual Pollutant Removal Rates
Table 3 shows that average annual pollutant loads should be reduced 90% to
97%, depending on constituent. These removal rates are significantly higher than
virtually any single BMP system, largely due to the volume of runoff infiltrated.
Runoff Monitoring System
Rainfall and runoff rates are being monitored to verify the assumptions within the
modeling of the system and to provide data for municipal and county stormwater
agencies that must approve these systems to meet local stormwater ordinances.
This is intended to address one obstacle to implementation of green roof and
permeable paving systems. The monitoring locations are shown in Figure 3.
Rainfall is measured on the roof using a tipping bucker rainfall gage. Discharge rates
from the upper green roof are measured using an ultrasonic sensor within the internal
drain of the roof. Discharge leaving the site is measured in the pipe downstream of
the detention restrictor using a compound weir and depth sensor placed in the pipe.
There is a small amount of offsite runoff draining to the site and this runoff is measured
using the same equipment as the runoff leaving the site. The difference between
the offsite runoff entering the site and the runoff leaving the site is the runoff
generated by the site.
Monitoring data is not yet available but will be provided over the next year as data is
collected.
Maintenance and Management
The following paragraphs describe maintenance requirements for the various
systems used in the Villa Park Police Station project. Inspection and maintenance
activities are described in detail in Appendix 2.
Permeable Paving
Two levels of maintenance are required for permeable paving – routine
maintenance and remedial maintenance.
• Routine Maintenance: Routine maintenance involves normal street
sweeping, similar to that used on standard asphalt and concrete paving.
While high efficiency vacuum sweepers are more effective at capturing and
removing fine sediment than mechanical broom sweepers, mechanical
sweeper equipment is sufficient to dislodge surface encrusted sediment.
Permeable paving surfaces should be dry-swept (water should be turned off)
in dry weather to remove encrusted sediment that appears as small curled
TSS TN TP COD
1 Filter removal rate 80% 35% 50% 55%
Removal rate based on IEPA load reduction worksheet assuming removal
rate similar to sand filter
2 Remaining load after filter 20% 65% 50% 45% One minus the removal rate
3 % of infiltrated water 85% 85% 85% 85%
Percent of annual runoff volume infiltrated based on NIPC "Storm Rainfall
Volume Distribution" Chart (Figure 3.1)
4 Remaining runoff after infiltration 15% 15% 15% 15% One minus the infiltrated percent
5 Remaining load after infiltration 3% 10% 8% 7% Load from the runoff that is not infiltrated (Row 4 * Row 2)
6 Total Percent Removal 97% 90% 93% 93% One minus the remaining load
Constituent
Notes
10
“potato chips”. When vacuum equipment is used, vacuum settings should
be adjusted to prevent uptake of aggregate from the pavement openings
and joints.
• Snow Plowing: As with any pavement, snow plowing is required after snow
storms. Use of rubber or nylon rather than steel is recommended for the
replaceable snowplow tip. However, steel tips are used for plowing many
installations, particularly where plowing speeds are low. Manufacturers of
interlocking pavement blocks state that the chamfered top edges minimize
chipping and allows for normal plowing procedures. Shoes at the edges of
the blade and rubber or nylon tipped blades are recommended for all
pavements to protect the equipment and driver from impact at manholes,
pavement joints, etc that are common to asphalt and poured concrete
roads.
Due to the short flow distance from the paver surface to a joint opening, the
opportunity for ice formation is greatly reduced. Also, researchers state that
the underlying stone bed tends to absorb and retain heat, further reducing
ice formation. For these reasons, regular deicing should not be necessary
and is not recommended for water quality protection reasons. However, for
occasions when icing does occur, a mild application of deicing salt may be
used. If abrasives are necessary, stone chips should be used rather than sand.
If sand is used, it will tend to clog the openings and lead to the premature
need for remedial maintenance.
• Remedial Maintenance: As discussed previously, there is a high initial
infiltration capacity (9+ inches/hour net infiltration capacity over the area of
the pavement surface) with a decrease and leveling off over time. The
decrease in infiltration capacity over time is due to deposition of fine material
such as silt and vegetation in the joint aggregate. In a study of two four-year
old parking lots, infiltration capacities were found to be approximately 3 in/hr.
In another study of two- and five-year old parking lots, the infiltration rates
were found to be 6 in/hr and 5 in/hr, respectively (Borgwardt, 1994). Clogging
has been found to be limited to a depth of 0.5 to 1 inch, well confined within
the joint depth of 3 inches.
The need for remedial maintenance can be determined by visual inspection.
Areas that pond water in the openings between pavers will require remedial
maintenance. Remediation can be achieved using a vacuum sweeper with
water jets, sweeper, and vacuum bar attachment to evacuate clogged joint
material. The evacuated joint material can either be washed and replaced
or new joint material can be used to refill the joints. Private contractors are
often used to provide remedial maintenance and a Chicago area
contractor has indicated that remedial maintenance costs have come in at
about $0.25 per square foot of permeable pavement. This cost included both
sweeping and recharge of the joints with aggregate. The frequency of
required remedial maintenance depends on the degree of sediment and
debris loading as well as the level of routine maintenance. However, there
11
are many five to ten year and older installations that have never been
maintained and continue to function well.
Bioswales
Maintenance of the bioswales includes inspections to ensure that the amended
topsoil is not clogged, inspection of the outlet control, and vegetative
management. Specific maintenance activities are described in Appendix 2.
Inspections should occur twice annually and after large rainfall events as
indicated. Each inspection of the entire system should not require more than
approximately four man-hours.
Clogging of outlet structures is not expected to occur since stormwater entering
the system must pass through bioswale amended topsoil or through permeable
paving for most all events. However, should clogging occur, the orifices may be
cleared by simply sweeping away the obstruction.
Clogging of the amended topsoil within the rain gardens and bioswales is not
expected to occur provided the vegetation is maintained in a vigorous
condition. Should clogging occur, it is unlikely that the entire bioswale will clog
but more likely that isolated locations will clog. The cost to remove and replace
clogged amended topsoil should not exceed $1 per square foot for small areas.
Replacement for large areas (i.e., the entire bioswale) should not exceed $0.25
to $0.50 per square foot. These costs do not include replacement of vegetation.
Green Roofs
Very little maintenance is required for green roofs except vegetative
maintenance to control weeds. The sedums used to vegetate the green roof are
very drought tolerant and will go dormant under extreme dry conditions.
However, the roof can be irrigated if desired to prevent dormancy and improve
aesthetics during dry periods.
Summary
The Villa Park police station Urban BMP demonstration project utilizes advanced
runoff management techniques that minimize generation of runoff and associated
urban pollutants using green roof and permeable paving technology. Runoff that
does occur is directed to biofiltration swales that filter the runoff and provide
temporary storage for infiltration and runoff rate control.
The system was designed to restore pre-development hydrologic conditions as well
as water quality while also meeting local stormwater flood control standards. The
system has been estimated to remove 90% to 97% of urban runoff pollutants and
reduce runoff volumes to less than produced by a typical park area (the expected
equivalent curve number for the site is approximately 61), despite 65% coverage by
surfaces normally considered to be impervious (roofs, parking, and walks). Runoff
volumes are being monitored to verify the hydrologic analysis and to assist local
designers and regulators in designing and evaluating these systems.
12
References:
• Borgwardt, S., “Tests of Uni Eco-Stone Installations of Different Ages: Report of the
Uni-International. Institute for Planning Green Spaces and for Landscape
Architecture. University of Hanover. 1994.
• HydroCAD Software Solutions, LLC. HydroCAD Version 7.11. Chocorua, NH.
Copyright 2005.
• Moran, Amy Christine, “A North Carolina Field Study to Evaluate Greenroof Runoff
Quantity, Runoff Quality, and Plant Growth”, A thesis submitted to the Graduate
Faculty of North Carolina State University in partial fulfillment of the requirements
of the Degree of Master of Science. North Carolina State Biological and
Agricultural Engineering Department. Raleigh, North Carolina. 2004.
• Northeastern Illinois Planning Commission, “Urban Stormwater Best Management
Practices for Northeastern Illinois – Course Notebook”. May 1998.
• Texas A&M University, Department of Civil Engineering, “Drainage Design and
Performance Guidelines for Uni Eco-Stone Permeable Pavement”. Sponsored by
Uni-Group USA. Palm Beach Gardens, Florida. 1998.
13
Figure 1: Project Location
1
Green Roof
Permeable
Paving
Bioretention Swale
Bioswale
Figure 2: Site Plan
Non-
Vegetated
Roof
N
1
Bioswale
Rain Gage
Flow Meter
Flow Meter
Flow
Meter
Figure 3: Flow Monitoring Locations
Parking Lot Prior to Construction of Police Station
Installation of South Bioswale
South Bioswale curbs and curb cuts
Completed South Bioswale with Initial Cover Crop Growth
Compaction of Permeable Paving Base
Completed Permeable Paving
Upper Green Roof Prior to Planting
Lower Green Roof Beginning of First Growing Season