Inspection and Maintenance: Permeable Pavement

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Inspection & Maintenance Guidance of a built vstormwater best management practices that treats the stormwater that falls on them and can be designed to also receive runoff from adjacent impermeable surfaces (e.g., pavements and roofs) as either sheet flow or from a pipe (e.g., roof downspout) connected to the washed gravel base, where captured water is temporarily stored in the base where it later soaks into underlying native soil or sent to a municipal stormsewer/other BMP via underdrains (TRCA, 2016)[1]

Overview[edit]

Permeable Pavement Permeable pavements contain many small openings (i.e., joints or pores) that allow rainfall and snowmelt (i.e., stormwater) to drain through them instead of running off the surface as it does on impervious pavements like conventional asphalt and concrete. An overflow outlet is needed to safely convey flows during flood events. Depending on the permeability of the underlying soil and other constraints, the pavement may be designed with no sub-drain for full infiltration, with a sub-drain for partial infiltration, or with an impermeable liner and sub-drain for a no infiltration practice. The sub-drain pipe may feature a flow restrictor (e.g., orifice cap or valve) for gradually releasing detained water and optimizing the amount drained by infiltration into the underlying soil.

Associated Practices[edit]

  • Permeable Interlocking Pavers (i.e., Block Pavers) – Precast modular units made of concrete, pervious concrete or rubber/plastic composite designed to create open joints between pavers that are filled with fine, washed aggregate and installed on an open graded aggregate (i.e., clear stone) base and sub-base.
  • Permeable Interlocking Grid Systems (i.e., grid pavers) – Precast concrete or manufactured plastic grids with open cells that can be filled with aggregate or a mixture of sand, gravel and topsoil and planted with grass or low-growing ground covers and are installed on an open-graded aggregate base.
  • Pervious Concrete – a rigid pavement installed on an open-graded aggregate base that uses a cementitious binder to adhere aggregate together, similar to conventional concrete, except that the fine aggregate component is minimized or eliminated which results in the formation of connected pores throughout.
  • Porous Asphalt – a flexible pavement installed on an open-graded aggregate base that uses a bituminous binder to adhere aggregate together, similar to conventional asphalt, except that the fine aggregate component is minimized or eliminated which results in the formation of connected pores throughout.

Inspection and Testing Framework[edit]

Permeable articulated block system at the LSRCA headquarters in Newmarket, ON is designed for heavy loads. This example shows an appropriate pavement surface condition when conducting any of, construction, assumption, routine or verification inspections.
Visual Indicators Framework - Permeable Pavements

Component

Indicators

Construction Inspection

Assumption Inspection

Routine Operation Inspection

Verification Inspection
Contributing Drainage Area
CDA condition x x x x
Perimeter
BMP dimensions x x x
Filter Bed
Standing water x x x
Trash x x
Vegetation
Vegetation cover x x x x
Vegetation condition x x
Vegetation composition x x x
Underdrain & Monitoring Well
Monitoring well condition x x x x
Sub-drain/Perforated pipe obstruction x x
Outlets Overflow outlet obstruction x x x x
Pavement Surface Pavement surface condition x x
Pavement surface sediment accumulation x x x x
Control structure condition x x x x
Control structure sediment accumulation x x x x



Permeable pavement in need of maintenance as excess sediment and unwanted vegetation growth has begun to occur near the perimeter of the installation. Source: (TRCA, 2016).[2].
Testing Indicators Framework - Permeable Pavements

Component

Indicators

Construction Inspection

Assumption Inspection

Routine Operation Inspection

Verification Inspection
Testing Indicators
Surface infiltration rate testing x (x)
Natural or simulated storm event testing x (x)
Continuous monitoring x (x)
Note: (x) denotes indicators to be used for Performance Verification inspections only (i.e., not for Maintenance Verification inspections)

Construction Inspection Tasks[edit]

A Vacuum Truck used for permeable pavement maintenance, which sucks up excess sediment and fines trapped between the pores of block pavers. Conducting this maintenance regularly helps maintain the practice's optimal infiltration rate during large rain events. (Source: STEP).

Construction inspections take place during several points in the construction sequence, specific to the type of LID BMP, but at a minimum should be done weekly and include the following:

  1. During site preparation, prior to BMP excavation and grading to ensure the CDA is stabilized and/or flow diversion devices are in place and confirm that construction materials meet design specifications
  2. At completion of excavation and grading, prior to backfilling and installation of pipes to ensure depths, slopes and elevations are acceptable
  3. At completion of installation of pipes, prior to completion of backfilling to ensure slopes and elevations are acceptable
  4. After final grading, prior to surface course installation to ensure depths, slopes and elevations are acceptable
  5. Prior to hand-off points in the construction sequence when the contractor responsible for the work changes (i.e., hand-offs between the storm sewer servicing, paving, building and landscaping contractors)
  6. After every large storm event (e.g., 15 mm rainfall depth or greater) to ensure flow diversion devices are functioning and adequately maintained. View the table below, which describes critical points during the construction sequence when inspections should be performed prior to proceeding further. You can also download and print the table here
Bioretention/Swales: Construction Inspections

Construction Sequence Step & Timing

Inspection Item

Observations*
Site Preparation - after site clearing and grading, prior to BMP excavation and grading Natural heritage system and tree protection areas remain fenced off
ESCs protecting BMP layout area are installed properly
CDA is stabilized or runoff is diverted around BMP layout area
BMP layout area has been cleared and is staked/delineated
Benchmark elevation(s) are established nearby
Construction materials have been confirmed to meet design specifications
BMP Excavation and Grading - prior to backfilling and installation of pipes/catchbasins Excavated soil is stockpiled outside the CDA
Excavation location, footprint, depth and slope are acceptable
BMP Installation – after installation of pipes/catchbasins, prior to completion of backfilling Structural components (e.g., pavement base, curbs) installation is acceptable
Impermeable liner installed correctly, if applicable
Installations of sub-drain pipes (e.g., locations, elevations, slopes), standpipes/monitoring wells are acceptable
Sub-drain trench dams installed correctly (location, elevation)
Surface coarse installation (elevation, slope, monitoring wells) is acceptable

Routine Maintenance - Key Components and I&M Tasks[edit]

Regular inspections (twice annually, at a minimum) done as part of routine maintenance tasks over the operating phase of the BMP life cycle to determine if maintenance task frequencies are adequate and determine when rehabilitation or further investigations into BMP function are warranted.

Table below describes routine maintenance tasks for permeable pavements, organized by BMP component, along with recommended minimum frequencies. It also suggests higher frequencies for certain tasks that may be warranted for BMPs located in highly visible locations or those receiving flow from high traffic (vehicle or pedestrian) areas or those designed with higher than recommended impermeable drainage area to permeable BMP footprint area ratios (I:P ratios). Tasks involving removal of trash, debris and sediment and weeding/trimming of vegetation for BMPs in such contexts may need to be done more frequently (i.e., higher standards may be warranted).

Individuals conducting vegetation maintenance and in particular, weeding (i.e., removal of undesirable vegetation), should be familiar with the species of plants specified in the planting plan and experienced in plant identification and methods of removing/controlling noxious weeds. Key resources on these topics are provided below at the links provided:

Permeable Pavements: Key Components, Descriptions and Routine I&M Requirements
Comnponent Description Inspection & Maintenance Tasks (Pass) Photo Example (Fail) Photo Example
Contributing Drainage Area (CDA)

Area(s) from which runoff directed to the BMP originates; includes the pavement itself and any adjacent impermeable pavement or roofs that drain to it.

  • Remove trash, debris and sediment from pavements (biannually to quarterly) and eavestroughs (annually);
  • Replant or seed bare soil areas as needed.
CDA has not changed in size or land cover. Sediment, trash or debris is not accumulating and point sources of contaminants are not visible.
Size of the CDA has changed from design assumptions (i.e. large asphalt area drains to a small portion of the permeable pavement). Evidence of surface ponding is visible.
Pretreatment

Devices or features that retain trash, debris and sediment; help to extend the operating life cycle; examples are eavestrough screens, catch basin inserts and sumps, oil and grit separators, geotextile-lined inlets, gravel trenches, grass filter strips and forebays.

  • Remove trash, debris and sediment annually to biannually or when the device sump is half full;
  • Measure sediment depth or volume during each cleaning, or annually to estimate accumulation rate and optimize frequency of maintenance.
Although permeable pavers are generally considered pretreatment for other BMPs in a treatment train system, using eavestrough screens can act as pretreatment as they don't add to sediment build up and accumulation on the paver surface. (Photo Source: Guertin, 2010)[3]
Build up of leads, sediment and leaf detritus in an eavestrough downspout disconnection leading to a permeable pavement surface. If not cleaned regularly this can lead to clogged pores between the pavers reducing the infiltration rate where the downspout deposits water onto the feature. (Photo Source: My Gutter Pro, 2021)[4]
Inlets

Structures that deliver water to the BMP (e.g., impermeable pavement edges, pipes from roof downspouts or catchbasins).

  • Keep free of obstructions;
  • Remove trash, debris and sediment biannually to quarterly;
  • Make note of sediment depth or volume during each cleaning or annually to estimate accumulation rate and optimize frequency of maintenance
Impermeable pavement edges along the edge of the pervious concrete and impermeable asphalt graded so excess sheet flow is infiltrated down through the pores of the infiltration BMP. (Photo Source: Fairfax County, 2014)[5]
Accumulated sediment, poor grading and vegetation is preventing stormwater from entering the swale. Sediment on the curb cut surface behind of the inlet indicates ponding is also occurring and is depositing water towards the permeable paver surface.
Pavement Surface

The surface of the pavement, including pavers/pavement, joints and edge restraints (e.g., curbs, edging); should not allow water to pond on the surface so any observation or evidence of surface ponding (e.g., sediment caking on the pavement) indicates a drainage problem.

  • Check for sediment, surface ponding, deformation, sink holes, damage and loss of joint fill;
  • Remove trash regularly (biannually-quarterly);
  • Grid systems with soil and grass fill are maintained like lawns;
  • Sweep and vacuum interlocking and rigid pavements regularly (annually-biannually) with regenerative air/pure vacuum sweeper;
  • Plow snow as needed and spread deicers sparingly during winter;
  • Repair ruts or local sinking of 15 mm or greater over a 3 metre length, adjacent pavers or cracks in pervious concrete or porous asphalt are vertically offset by 5 mm or greater and paver joint fill is missing or below 15 mm from the paver surface.
No damage, displacement or sinking of the permeable surface is visible and there are no weeds growing between paver joints.
The pavement surface has sunk in local areas, creating a trip hazard and the potential for further damage from snow plowing and parked vehicles nearby.
Vegetation

Applies to grid paver systems only; a mixture of deep rooting perennial grasses or low growing ground covers, tolerant to both wet and dry conditions and salt (if receiving impermeable pavement runoff); roots uptake water and return it to the atmosphere, provide habitat for soil organisms that break down pollutants trapped in the soil and help maintain soil structure and permeability.

  • Grid systems with soil and grass fill are maintained like lawns;
  • In the first 2 months water frequently (biweekly in the absence or rain) and as needed (e.g., bimonthly) over the remainder of the first growing season;
  • Remove weeds and undesirable plants biannually to quarterly;
  • Replace/reseed dead plantings annually to achieve 80% cover by the third growing season;
  • Do not apply chemical fertilizers.
The vegetation cover on the permeable pavement is turf grass as specified in the final design and contains very few weeds. (Photo source: WEF).
The turf grass cover on the permeable walkway is not thriving in some areas and needs cutting in others. It is also impairing the aesthetic value of the BMP.
Overflow Outlet

Structures (e.g., catchbasin, curb-cut, swale) that convey flow that exceeds the storage capacity of the BMP to another drainage system (e.g., municipal storm sewer or other BMP).

  • Keep free of obstructions;
  • Remove trash, debris and sediment biannually to quarterly.
The overflow outlet is free of damage and obstruction and functions as designed to safely convey excess water from the BMP.
The overflow outlet is obstructed with sediment which impairs its function to convey excess water from the BMP.
Sub-drain

Optional component; perforated pipe(s) surrounded by gravel and may be wrapped in geotextile filter fabric; installed in the base or sub-base gravel layer to collect and convey treated water to an adjacent drainage system or other BMP; may also include a flow restrictor.

  • Keep pipe and flow restrictor free of obstructions by flushing annually;
  • Inspect flow restrictor regularly (e.g., biannually to quarterly).
The solid section of the sub-drain pipe is not obstructed by sediment, debris or roots and shows no signs of damage.
A section of the sub-drain pipe has been crushed which substantially reduces its conveyance capacity
Monitoring well

Perforated standpipe that extends from the bottom of the excavation to just below the pavement surface and contains perforations or slots to allow measurement of subsurface water level; used to track drainage performance over the operating life cycle of the BMP.

  • Standpipes should be securely capped on both ends and protected from damage by vehicular traffic by a casing.
The well is undamaged and accessible and the cap is in place and secured to prevent unauthorized access
The well cap is missing and the casing is clogged by sediment, preventing access for monitoring and allowing sediment to flow into the sub-drain system.
Control Structure

Manhole or catchbasin to which the subdrain outlets that provides access to the underdrain and flow restrictor. Inspect for accessibility, damage and sediment.

  • Remove trash, debris and sediment regularly (biannually/quarterly)..
TThere is some sediment accumulated in the manhole but it is not impairing the flow of stormwater into or out of the BMP. (Photo source: SWC Canada)
The manhole sump is full of sediment and debris and it is beginning to impair flow of stormwater into a perforated pipe of the exfiltration storm sewer system.

Tips to Preserve Basic BMP Function[edit]

  • Never use sealants on porous asphalt nor pervious concrete.
  • Prohibit access by construction vehicles to prevent tracking of sediment on to the surface.
  • Prohibit storage of soil, compost, sand, salt or unwashed gravel on permeable pavements to prevent clogging of joints or pores, or protect the pavement surface with tarps or geotextile during temporary storage of such materials.
  • Landscaped areas adjacent to permeable pavements should be covered with vegetation and not drain to the pavement where possible to prevent eroding soil from reaching the surface.
  • Use a mulching mower to mow permeable interlocking grid systems with grass cover.
  • Permeable pavements can be plowed for snow removal like conventional pavements. To reduce the risk of dislodging pavers or grids and minimize displacement of joint/cell fill material, the plow blade should be slightly raised off the pavement surface (e.g., 0.6 cm or 1/4”) with a shoe attachment.
  • Plowed snow piles should not be stored on permeable pavements to reduce the risk of clogging from sediment accumulation upon melting.
  • Do not spread sand on permeable pavements as part of winter maintenance as it will quickly clog the joints or pores and impair drainage function. On permeable interlocking pavers and grid systems filled with gravel, if application of an anti-skid material is desirable, spread the same fine washed gravel material used to fill the paver joints or grid cells; and
  • De-icers should be used sparingly, as needed during winter. Due to their freely draining design, ice will not form on permeable pavements as readily as it does on conventional impermeable pavements during winter thaw-freeze cycles.
  1. TRCA. 2016. Fact Sheet - Inspection and Maintenance of Stormwater Best Management Practices: Permeable Pavement. https://sustainabletechnologies.ca/app/uploads/2018/02/Permeable-Pavement-Fact-Sheet.pdf
  2. STEP. 2018. Fact Sheet - Inspection and Maintenance of Stormwater Best Management Practices: Permeable Pavements. https://sustainabletechnologies.ca/app/uploads/2018/02/Permeable-Pavement-Fact-Sheet.pdf
  3. Guertin, M. 2010. Simple Screen Gutter Guards Better Than Pro-Installed Systems (and way cheaper). Fine Homebuilding - The Daily Fix. Accessed July 17 2022. https://www.finehomebuilding.com/2010/08/17/simple-screen-gutter-guards-better-than-pro-installed-systems-and-way-cheaper
  4. My Gutter Pro. 2021. Clogged Downspout : Causes and Solutions. 10 April 2021. Accessed July 19 2022. https://mygutterpro.com/downspout-clog/
  5. Fairfax County. 2014. Protecting our Environment, one Stormwater Practice at a Time - Permeable Pavements. April 2014. Accessed: July 20 2022. https://www.fairfaxcounty.gov/publicworks/sites/publicworks/files/assets/documents/pdf/factsheets/permeable-pavement.pdf