Inspection and Maintenance: Underground Infiltration Systems

Inspection & Maintenance Guidance of underground infiltration systems' best management practices that refers to LIDs that capture and temporarily store rainwater and snowmelt from hard surfaces (e.g., roofs and pavements) below ground in geotextile filter fabric lined excavations filled with washed gravel or other void space forming structures (TRCA, 2018)

Overview

Underground infiltration systems is a general term that refers to best management practices (BMPs) that capture and temporarily store stormwater from hard surfaces. These systems treat stormwater by detaining it to allow suspended sediments to settle out and soaking it into the ground where it is filtered and cleansed by interaction with soil. Runoff water is delivered to the practice through pipes connected to catchbasins, hydrodynamic (i.e., oil and grit) separators, filters, manholes, sub-drains of other features or roof downspouts. They are installed below the local maximum frost penetration depth to ensure they continue to drain year-round. Water that is in excess of the storage capacity overflows to an adjacent drainage system (e.g., municipal storm sewer or other BMP), typically via pipe or manhole containing a control structure (e.g., weir wall), to safely convey flows during flood events. Depending on the permeability of the underlying soil, such practices may be designed without a sub-drain for full infiltration or with a sub-drain for partial infiltration. The sub-drain pipe may feature a flow restrictor (e.g., orifice cap, valve) for gradually releasing detained water and optimizing the amount drained by infiltration.

General plan and profile (cross-section) views showcasing key components and sections of a typical infiltration trench.

Key components of Underground Infiltration Systems to pay close attention to are the:

• Inlets
• Sub-Drains
• Monitoring Wells
• Overflow outlets

Trash, debris and sediment builds up at these locations and can prevent water from flowing into or out of the practice.

Associated Practices

• Soakaways: Typically service individual lots and receive only roof and walkway runoff but can also be designed to receive overflows from other BMPs (e.g., rain barrels or cisterns, rain gardens, green roofs. Also known as infiltration galleries, French drains, dry wells or soakaway pits.
• Infiltration trenches: Linear oriented soakaways designed to fit into narrow strips of land between structures or properties, or along road rights-of-way; can also receive road runoff with adequate pretreatment devices upstream of inlets.
• Infiltration chambers: Include a range of proprietary modular structures installed underground that create large void spaces for temporary storage of stormwater while providing sufficient load bearing capacity to allow construction of structures on top of them. Applications are similar to infiltration trenches. Also known as infiltration tanks or vaults.
• Perforated pipe storm sewer systems or Exfiltration trenches: Linear-oriented infiltration trenches installed parallel with conventional storm sewer pipes and catchbasins that receive stormwater from them. May include manholes with perforated risers. Also known as exfiltration storm sewer, percolation drainage, or clean water collector systems.

Inspection and Testing Framework

An inlet to an infiltration trench during the fall in need of maintenance (TRCA, 2018).

Visual Indicators Framework - Underground Infiltration Systems

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Contributing Drainage Area
	CDA condition	x	x	x	x
Inlet
	Inlet/Flow Spreader Structural Integrity		x	x	x
	Inlet/Flow Spreader Structural Integrity	x	x	x	x
Perimeter
	BMP dimensions	x	x		x
Filter Bed
	Filter bed sediment accumulation		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
Control Structure
	Control structure condition	x	x	x	x
	Control structure sediment accumulation	x	x	x	x

An example of a soakaway under construction with a newly installed monitoring well. To ensure performance assessment and infiltration testing can be conducted efficiently over the life cycle of this LID feature, the well condition must stay in good condition and be repaired immediately upon notice of significant damage (TRCA, 2018).

Testing Indicators Framework - Underground Infiltration Systems

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Testing Indicators
	Sediment accumulation testing	x	x	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

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 geotextile/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 ESCs and pretreatment or 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
   

A pretreatment device (catchbasin insert) upstream of an underground infiltration system inlet in need of cleaning out. These devices are designed to limit the amount of trash, debris, sediment, oil and grease entering the infiltration system, which helps to prevent clogging and maintain its drainage performance (Source: TRCA, 2018).

Underground Infiltration Systems: Construction Inspections

Construction Sequence Step & Timing	Inspection Item	Observations*
Site Preparation - after site clearing and grading, prior to BMP excavation 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 geotextile/pipes	Excavated soil is stockpiled outside the CDA
	Excavation location, footprint, depth and slope are acceptable
	Compaction of subsoil where load-bearing portions of the system will be installed is acceptable
	Excavation bottom and sides roughened to reduce smearing and compaction
BMP Installation – after installation of geotextile/ pipes/structures, prior to completion of backfilling	Installation of structural components (e.g., control manhole, maintenance hatches) is acceptable
	Sub-drain trench dams installed correctly (location, elevation)
	Installations of sub-drain pipes (e.g., locations, elevations, slopes) & maintenance access hatches are acceptable

Routine Maintenance - Key Components and I&M Tasks

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:

• Agriculture and Agri-food Canada’s Weed Info database
• Ontario Ministry of Agriculture, Food and Rural Affairs’ Ontario Weed Gallery
• Ontario Ministry of Agriculture, Food and Rural Affairs’ Noxious Weeds In Ontario list
• Ontario Invasive Plant Council’s Quick Reference Guide to Invasive Plant Species
• Oregon State University Stormwater Solutions, 2013, Field Guide: Maintaining Rain Gardens, Swales and Stormwater Planters, Corvallis, OR.
• Plants of Southern Ontario (book), 2014, by Richard Dickinson and France Royer, Lone Pine Publishing, 528 pgs.
• Weeds of North America (book), 2014, by Richard Dickinson and France Royer, University of Chicago Press, 656 pgs.

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)	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)
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)	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.

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.

Toronto and Region Conservation Authority

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

An underground water storage reservoir into which stormwater is directed and allowed to percolate into the underlying native subsoil.

An excavated area lined with geotextile filter cloth and filled with clean granular stone or other void forming material, that receives runoff and allow it to infiltrate into the native soil; can also be referred to as infiltration galleries, French drains, dry wells or soakaway pits.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

The downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.

Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed).

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

Structure to control the volume or rate of flow of water through or over it.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

A body of water smaller than a lake, often artificially formed.

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.