Construction considerations

The following presents a summary of considerations when planning the construction of a low impact development project. More details can be found in the following reference:

• The site of the infiltration facility must remain outside the limit of disturbance and blocked from site traffic until construction of the facility begins, to prevent soil compaction by heavy equipment.
• This area must not be used as the site of sediment basins during construction, as the concentration of fines will reduce post-construction infiltration.
• This area must not be use as a staging area, for storing materials.
• To prevent sediment from clogging the surface, stormwater must be diverted away from the facility until the drainage area is fully stabilized.
• As many infiltration facilities are installed in the road right-of-way or tight urban spaces, considerations of traffic control and utility conflicts must be part of the plans and inspections.

Sequencing

The following is a typical construction sequence to properly install an infiltration practice:

• The area should be fully protected by silt fence or construction fencing to prevent compaction by construction traffic and equipment.
• Installation may only begin after entire contributing drainage area has been either stabilized or flows have been safely routed around the area. The designer should check the boundaries of the contributing drainage area to ensure it conforms to original design.
• The pretreatment part of the design should be excavated first and sealed until full construction is completed.
• Excavators or backhoes working adjacent to the proposed infiltration area should excavate to the appropriate design depth.
• The soil in the bottom of the excavation should be ripped to promote greater infiltration.
• Any accidental sediment accumulation from construction should be removed at this time.

1. Excavate subsurface water storage reservoir to base elevation,
2. Check base elevation and slope,
3. Fracture/rip bottom and roughen side of the excavation to remove smeared surfaces,
4. Install optional geotextiles (or liner for biofilter); overlapping according to design drawings,
5. Install coarse reservoir gravel, and any void forming structures (e.g. underdrains, infiltration chambers, or wells),
6. Check elevation and slope at top of reservoir,
7. Install choking layer and optional geotextile (typically only over the perforated pipe),
8. Check elevation and slope at top of choking layer,
9. Install filter media with additional 30 cm over finish grade of the filter bed,
10. Thoroughly saturate and allow to settle for at least one week. After this time, tamp manually to check settling is complete. Alternatively, installations made in the fall can be left to settle over the whole winter season at this point,
11. Install temporary erosion and sediment control practices,
12. Conduct all other site construction activities (buildings/servicing etc.)
13. Check condition of bioretention after settling period, remediate any deficiencies,
14. Install curbs and pavements and concrete pretreatment devices,
15. Check elevations of curb cuts and other inlets
16. Install erosion control to all inlets!!
17. Remove excess filter media along with any accumulated construction sediment,
18. Install any surface applied additives,
19. Conduct fine grading to surface of filter bed, checking elevations/slopes/compaction,
20. Apply stone or mulch cover for decorative systems, or turf reinforcement for grassed systems,
21. Install erosion control blankets or matting
22. Plant or lay sod,
23. Saturated system thoroughly to settle filer media particles around the roots of new plants,
24. Irrigated the system as required to establish healthy vegetation cover,
25. Inspect and remediate deficiencies after any significant rainfall within the next 3 months or remainder of the first growing season.

Facilities containing media

Bioretention

Sequencing depends on the design:

• Full infiltration:Pack 50 mm diameter clear stone to storage design depth, top with 100 mm of the choker course,
• Partial infiltration:Place design depth of 50 mm diameter clear stone for the infiltration volume on bed and then lay the perforated underdrain pipe over it. Pack more clear stone to 75 mm above the top of the underdrain, top with 100 mm of choker layer.

Stormwater planters

• Place an impermeable liner on the bed with 150 mm overlap on sides. Lay the perforated underdrain pipe, Pack 50 mm diameter clear stone to 75 mm above top of underdrain, top with 100 mm of choker layer;

Rain gardens

No storage or drainage is required, filter media or amended topsoil is laid onto native soils

Media installation

Media installed over the choker course in 0.3 m lifts until desired top elevation is achieved. Each lift must be thoroughly wetted and drained before adding the next. Wait three weeks to check for settling, and add additional media and regrade as needed.

• Prepare planting holes for any trees and shrubs, install vegetation, and water accordingly.
• Install any temporary irrigation.
• Plant landscaping materials as shown in the landscaping plan, and water them weekly in the first two months.
• Lay down surface cover in accordance with the design (mulch, riverstone, or turf).
• Conduct final construction inspection, checking inlet, pretreatment, bioretention cell and outlet elevations.
• Remove erosion and sediment controls, only when the entire drainage area is stabilized.

Checklists

• Bioretention construction checklist

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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.

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

Soil particles with a diameter less than 0.050 mm.

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

Penetration of water through the ground surface.

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.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

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.

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.

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.

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

Includes the protection of soil from dislocation by water, wind or other agents.

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.

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

A perforated pipe used to assist the draining of soils.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

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

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

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.

Credit Valley Conservation