Infiltration chambers

Overview

As their name suggests infiltration galleries work exclusively to infiltrate stormwater. They are an underground facility and are often used beneath parking lots or playing fields to treat flow routed from other areas.

Infiltration galleries are an ideal technology for:

• Installing below any type of surface or landscape
• Receiving and infiltrating large volumes of water

The fundamental components of an infiltration gallery are:

• Structurally reinforced chambers
• Layers of coarse aggregate to bed the chambers and redistribute water.
• Perforated pipe

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  Schematic diagram of an underground infiltration gallery

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Design

Sizing

This is an image map; clicking on components will load the appropriate article.

• To calculate the required depth of an infiltration facility in a specified footprint area...
• To calculate the required footprint area of an infiltration facility with a known depth constraint....
• To calculate the drainage time of ponded water on the surface of a facility footprint...
• To calculate the drainage time of an underground infiltration facility...

The sizing calculations require that most of the following parameters be known or estimated. The exceptions are the depth (d) and the permeable footprint area of the practice (Ap), as only one of these is required to find the other. Note that some of these parameters can be limited by site conditions and factors influencing constructability:

1. The maximum total depth will be limited by construction practices i.e. usually ≤ 2 m to avoid the need for benching and shoring open cut excavations.
2. The maximum total depth may be limited by the conditions underground (e.g. water table or underlying geology/infrastructure).
3. The maximum total depth may be limited by the desire to install the practice below the maximum frost penetration depth in the proposed location.
4. The maximum total depth may be limited by the desire to support vegetation cover over it (e.g. at least 30 cm of planting soil backfill over the BMP to support grasses)
5. Infiltration trenches, chambers and bioretention have a maximum recommended I/P ratio of 20.

Inputs

Symbol	Units	Parameter
D	h	Duration of design storm
i	m/h	Intensity of design storm
f'	m/h	Design infiltration rate of the underlying native soil, calculated from measured infiltration rate and applied safety factor
n	-	Porosity of the aggregate or other void-forming fill material(s) in the storage reservoir of the practice. *Note: For systems that have significant storage in open chambers surrounded by clear stone aggregate, an effective porosity value (n') may be estimated for the whole installation and used in the calculations below. Effective porosity will vary according to the geometry of the storage chambers, so advice should be sought from product manufacturers. Permit applications should include the basis for n' estimates.
Ai	m2	Catchment impervious area
dr	m	Water storage reservoir depth. For practices without an underdrain (i.e. full infiltration design), dr is the total depth of the practice (i.e. includes surface ponding and filter media depths). For practices with an underdrain (i.e. partial infiltration design), dr is the depth below the invert of the underdrain perforated pipe outlet.
Ap	m2	Permeable footprint area of the practice. For practices where runoff is directed to a surface ponding area, Ap is the surface ponding area. For practices where runoff is directed to a subsurface water storage reservoir, Ap is the footprint area of the storage reservoir, Ar
x	m	Perimeter of the practice
Kf	m/h	Minimum acceptable saturated hydraulic conductivity of the filter media or planting soil used in the practice, when compacted to 85% maximum dry density

This spreadsheet tool has been set up to perform all of the infiltration practice sizing calculations shown below
Download the infiltration practice sizing tool

To calculate the required storage reservoir footprint area where the depth is fixed or constrained (1D drainage)

To ensure that the water storage capacity of the facility is available at the onset of a storm event, it is recommended to size the storage reservoir despth, d_r, based on the depth of water that will drain via infiltration between storm events. So d_r can be calculated as:
$d_{r}={\frac {f'\times t}{n}}$ Where
f' = design infiltration rate of the native soil (m/h)
t = drainage time, based on local criteria or long-term average inter-event period for the location (e.g. 72 hr in southern Ontario).
n = Porosity of the stone bed aggregate material (typically 0.4 for 50 mm dia. clear stone)
In many locations there may be a limited depth of soil available above the seasonally high water table or top of bedrock elevation into which stormwater may be infiltrated. In such cases the required storage needs to be distributed more widely across the landscape.
Where the storage reservoir depth is fixed or constrained the footprint area of the water storage reservoir, A_r can be calculated: $A_{r}={\frac {i\times D\times Ai}{(n\times d_{r})+f'D}}$

To calculate the required storage reservoir depth where the area is fixed or constrained (1D drainage)

On densely developed sites, the surface area available for the practice may be constrained. In such cases the required storage reservoir depth, d_r of the bioretention cell or infiltration trench can be calculated based on available surface area, A_p:

$d_{r}={\frac {D\left[\left({\frac {Ai}{Ap}}\right)i-f'\right]}{n}}$

Note that in most cases the results of this calculation will be very similar to those from the equation below assuming three-dimensional drainage.

To calculate the required storage reservoir depth, where the area is fixed or constrained (3D drainage)

On densely developed sites, the surface area available for the facility may be constrained. In such cases the required water storage reservoir depth of the bioretention cell or infiltration trench, d, can be calculated assuming three-dimensional drainage as follows

$d_{r}=a[e^{\left(-bD\right)}-1]$

Where

$a={\frac {A_{r}}{x}}-{\frac {i\times A_{i}}{x\times f'}}$

and

$b={\frac {x\times f'}{n\times A_{r}}}$

(The rearrangement to calculate the required footprint area of the facility for a given depth assuming three-dimensional drainage is not available at this time. Elegant submissions are invited.)

Time required to drain surface ponded water (1D drainage)

The following equation assumes one dimensional drainage over the surface ponding area. It is best applied to calculate the maximum duration of ponding on the surface of bioretention cells, and upstream of the check dams of bioswales and enhanced grass swales to ensure all surface ponding drains within 48 hours. To calculate the time (t) to fully drain surface ponded water through the filter media or planting soil: $t={\frac {d_{p}'}{K_{f}}}$ Where
d_p' is the effective or mean surface ponding depth (mm).
K_f is the minimum acceptable saturated hydraulic conductivity of the filter media or planting soil when compacted to 85% maximum dry density (mm/h).

Time to drain the storage reservoir

The target drainage time for the active storage reservoir depth of an infiltration facility is typically between 48 and 72 hours or based on the average inter-event period for the location. Contact the local municipality or conservation authority for criteria. See Drainage time for more information about how inter-event periods vary across Ontario and to help select what is suitable for the site.

Try the Darcy drainage time calculator tool for estimating the time required to fully drain the water storage reservoir of the facility assuming either one or three-dimensional drainage:
Download the Darcy drainage time calculator

Three footprint areas of 9 m².
From left to right x = 12 m, x = 20 m

For some geometries, particularly deep or linear facilities, it desirable to account for lateral drainage, out the sides of the storage reservoir. The following equation makes use of the hydraulic radius (A_r/x), where x is the perimeter (m) of the facility.
Maximizing the perimeter of the water storage reservoir of the facility will enhance drainage performance and directs designers towards longer, linear shapes such as infiltration trenches and bioswales. See illustration for an example.

To calculate the time (t) to fully drain the facility assuming three-dimensional drainage: $t={\frac {n\times A_{r}}{f'\times x}}ln\left[{\frac {\left(d_{r}+{\frac {A_{r}}{x}}\right)}{\left({\frac {A_{r}}{x}}\right)}}\right]$ Where "ln" means natural logarithm of the term in square brackets
Adapted from CIRIA, The SUDS Manual C753 (2015).

Materials

Construction

CVC staff conducting a construction inspection at Kenollie Public School, Mississauga, Ontario. (Photo source: CVC, 2015)

Overview

LID techniques and technologies are new to many municipalities, consulting engineers, and contractors. STEP's construction guidance aims to give practical advice, specific to LID construction, to enable practitioners to successfully construct LID practices.

Common reasons LID projects fail at the construction stage are:

• lack of detail in designs and construction documents
  • Contractors can struggle to build LID facilities properly without enough detail in the contract drawings and without guidance and inspection throughout the construction process.
• lack of knowledge
  • Designers often do not understand the complexities of the construction process, and contractors often don't understand the purpose of LID practices or the technologies they employ.
• lack of effective erosion and sediment control during construction
  • LID practices are most vulnerable to sedimentation and clogging during their own construction or construction of adjacent lands.
• lack of planning and communication
  • Poor communication protocols and the pace and extent of construction may preclude proper inspections and certifications.

Published research corroborates STEP’s experiences in the field (e.g., (DelGrosso et al., 2019 ); LSRCA, 2011); (CWP, 2009).; see the inspection and maintenance page’s overview for a more detailed discussion). Del Grosso et al., 2019 provide a helpful summary of positive considerations for LID construction: “[LID] requires more considerations during construction compared to traditional stormwater management facilities. … The proper construction of [LID] is centered around thoughtful construction sequencing, ensuring all parties involved know their responsibilities, protecting soils and media from compaction and clogging, property installing filter media and aggregate, and ensuring facilities are kept off-line until the entire drainage area is stabilized. More generally, it is important for contractors to be aware of the sensitivity of GSI and for municipalities to ensure that facilities are being inspected by qualified inspectors at critical points (Del Grosso et al., 2019, pg. 8).

Regular inspections throughout the construction process of LID practices prevent end products that are not built to the design specifications. Specifically, they ensure that:

• the LID practice has the proper layout, location, footprint, and volume;
• materials meet design specifications;
• any material substitutions and field changes to the design are verified and documented;
• the contributing drainage area is stabilized before the LID practice becomes operational;
• environmentally sensitive areas and the LID practices themselves are protected during construction; and
• inlet, outlet, pretreatment, and piped elements have the correct elevations and inverts.

Furthermore, keeping records of these inspections helps to certify the works after construction and makes for a smooth assumption process.

Construction stages and LID types

The five-step construction process for ground-level LIDs.

STEP divides the construction process for LID practices into five over-arching stages:

1. Pre-construction
2. Excavation and grading
3. Sub-surface components
4. Finishing grades and surface layer installation: vegetated LIDs and finishing grades and surface layer installation: permeable pavements, and
5. Post-construction

Most LID practices work at the sub-surface and ground-level by routing stormwater flows from impervious surfaces into excavated or natural depressions or by allowing stormwater to pass through a pervious surface, as is the case with permeable pavements. These depressions are designed and constructed to the meet goals of the LID practice, which may be quality control, quantity control, or water balance restoration. Bioretention gardens, stormwater planters, bioswales, rain gardens, enhanced swales, exfiltration trenches, permeable pavements, infiltration systems (chambers, trenches, and soakaways), and stormwater tree trenches fall into this category.

For this reason, Stages 1-2 and 5 of the LID construction process are fundamentally similar for all sub-surface and ground-level LID types. To illustrate, STEP's recommended processes for excavation do not differ between LID practice types. Excavation procedures are the same, whether for a bioretention garden or a permeable pavement parking lot. On the other hand, stage 4 sub-tasks will vary depending on whether the LID practice's surface is vegetated or permeable pavement. Some sub-tasks in stage 3 will also vary depending on the LID type. For example, permeable pavements often require compaction of sub-surface storage layers. The following sections give a brief description of each over-arching stage, a list of sub-tasks for each stage, and links to the page dedicated to each main LID construction stage.

STEP has developed practice-specific construction inspection checklists for bioretention and dry swales, permeable pavement, enhanced swales, vegetated filter strips, underground infiltration systems (exfiltration systems, trenches, chambers, soakaways), green roofs, and rainwater harvesting.

Green roofs and rainwater harvesting systems have specific construction sequences and requirements that differ from the main sequence described above. STEP has developed guidance for green roof construction; guidance on installing rainwater harvesting practices is forthcoming. When installing these BMP types, always consult the product manufacturer's guidance.

Pre-construction

Pre-construction activities set the stage for the successful construction of an LID practice. The pre-construction page gives guidance on:

• design verification and site walk-through
• LID construction notes
• tendering and contract
• communication, inspection plan, and utilities coordination
• erosion and sediment control measures
• mobilization, access, staging, and perimeter controls

Excavation and grading

Excavation of a rain garden at Belfountain Public School, Caledon, Ontario. (Photo source: CVC, 2021)

Excavation and grading are necessary for installing LID practices with sub-surface components, re-grading land to hold more water, and re-routing overland flow routes into an LID practice. The excavation and grading page gives guidance on:

• clearing and grubbing
• excavation and rough grade
• sub-grade
• final excavated grade and verification

Sub-surface components

Most LID practices use a combination of sub-surface features such as gravel storage reservoirs, liners, underdrains, monitoring wells, and other components to meet their design objectives. The construction process for sub-surface components works from the ground up. While some LID practices include all the sub-surface components listed below, most designs will not include one or more of these layers or components. Permeable pavements can have different construction requirements at this stage, mostly regarding compaction of sub-surface layers. Installing infiltration chambers also requires guidance specific to that LID type. Stormwater Tree Trenches, which consist of subsurface trenches filled with modular structures and growing medium, or structurally engineered soil medium to supporting an overlying pavements, also require advice specific to them.

The sub-surface components page gives guidance on:

• geotextile
• underdrain
• impermeable liner
• overflow
• monitoring wells
• storage reservoir
  • sub-base reservoir (permeable pavements)
  • base course (permeable pavements)
  • infiltration chambers
• choker layer
• curbing and curb cuts
• pretreatment and inlets

Finishing grades and surface layer installation

CVC and Toronto Zoo staff planting the surface layer of a rain garden at Janet I McDougald Public School in Mississauga, Ontario. (Photo source: CVC, 2021)

This construction stage differs between LID practice type. The finishing grades and surface layer installation: vegetated LIDs page has guidance for vegetated LIDs, and the finishing grades and surface layer installation: permeable pavements page has guidance for and non-vegetated LIDs. In many cases the surface of infiltration systems will be traditional asphalt, concrete, or pavers; STEP does not provide guidance on installing non-permeable surfaces.

Stage 4 guidance for vegetated LIDs and permeable pavements

LID practices with vegetated surfaces	Permeable pavements
soil media installation and soil amendments	bedding layer
finish grading	placement and finishing
large stone and riprap	paver installation
plant verification and installation	tamping
mulch placement	joint cutting
stabilizing contributing drainage area and planting adjacent vegetation	joint aggregate
--	curbing
--	stabilizing contributing drainage area

Post-construction

Post-construction tasks ensure that the LID practice was built to specs and that any outstanding issues with it are resolved before assumption. The post-construction page give guidance on:

• Addressing deficiencies
• Final certification

References

1. REDIRECT Special:ArticleFeedbackv5

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

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 ratio of the impervious catchment (drainage) area to the pervious (footprint) area of the receiving BMP.

The natural ground material characteristic of or existing by virtue of geographic origin.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

A perforated pipe used to assist the draining of soils.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

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 parameter that describes the capability of a medium to transmit water.

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

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Credit Valley Conservation

Sustainable Technologies Evaluation Program

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.

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

Lake Simcoe Region Conservation Authority

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.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

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 practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

Open space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System.

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

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

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.

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

Ground depression acting as a flow control and water treatment structure, that is normally dry.