Key Design and Plan Review Considerations

Fill from Section 4.0 of the 2016 Guide!!

draw on LID I&M Guide section 4 content (see training presentation for ideas for images/photos) and include content on Inspection and Maintenance page under section title of the same name. Add links to the new page to relevant BMP landing pages under “Design” section titles too.

Overview

Designing LID BMPs with ease of inspection and maintenance in mind is critical to the affordability of municipal stormwater infrastructure asset management programs and must be considered early on in the overall activity. Generally this occurs during the design and an early stage in the plan review and approval process. The following sections provide tips on tailoring the design of LID BMPs to help reduce the frequency of structural repairs and make inspection and maintenance tasks easier and cheaper to perform by either municipal staff or hired external consultants.

Provide Runoff Pretreatment

Pretreatment refers to techniques or devices used to retain coarse materials suspended in stormwater runoff, either through filtration or settling, before it enters the BMP rendering it inefficient over time. Proper pretreatment extends the operating phase of the BMP’s life cycle by reducing the rate of accumulation of coarse sediment, thus delaying the need for at times expensive structural repairs like unclogging filter beds, pipes and orifices. Common pretreatment devices include vegetated filter strips, grass swales, geotextile-lined inlet filters/liners, check dams, forebays, eavestrough screens or filters, Oil and grit separators (i.e., hydrodynamic separators) and manholes containing baffles, filters and sumps. One important consideration for pretreatment is that these devices require frequent (i.e., at minimal, annual) sediment and trash removal maintenance and should be easy to access by inspection personnel.

Design Low Maintenance Conveyance Systems

The design of conveyance systems that carry stormwater into the BMP should anticipate potential maintenance issues and include features to minimize or avoid them.

For example, during large storm events, rapidly flowing water into or out of the BMP often causes erosion in vegetated practices. Inlet and outlet designs should consider protective features that prevent erosion. The size of inlets to BMPs and their slope also needs careful consideration as small, gently sloping openings are easily clogged with coarse debris and sediment which could cause stormwater flows to by-pass or not enter the BMP, increasing maintenance needs.

• Curb cuts should curve into the BMP so that flowing stormwater does not have to turn sharply to enter and inlets should be sloped at between 5 and 10%.
• Inlets should also be easily accessible and unobstructed by permanent covers to make trash, sediment and debris removal maintenance easy to perform.
• When designing stormwater infiltration BMPs, consideration should be given to where the majority of trash, sediment and debris will accumulate in the BMP and where snow storage will occur (a significant source area for sediment and debris).
• Infiltration BMPs should include pretreatment devices, inlet designs or forebays that allow accumulation to occur without blocking inflow (e.g., 5 cm change in grade between pavement surface and BMP surface) and that isolate sedimentation areas from the main portions of the filter bed so that the area disturbed through routine sediment removal maintenance is minimized.

Inspection & Maintenance Features

During planning and design phases of an LID management plan, consideration for how they will be maintained (what equipment is needed, who is responsible for conducting maintenance, how often should said maintenance occur for a given LID BMP? etc.) and what features are needed to perform necessary inspection and maintenance tasks are all required questions to be answered by the municipality.

The following table provides some examples of inspection and maintenance features that should be considered in the BMP process:

I&M Features to be Considered in BMP Design Phase

LID BMP Type	Description
Infiltration BMPs
	For infiltration BMPs, monitoring wells that extend to the bottom and standpipes or manholes connected to sub-drains that allow access for drainage performance verification through water level measurements and routine flushing of sediment from pipes.
	For infiltration BMPs, sub-drain pipes should be 200 mm in diameter and be connected to manholes, maintenance hatches or standpipes (for standpipes, it is best to use two (2) 45 degree couplings) to allow for inspection by closed circuit camera (i.e., push camera) and sediment removal by jet-vac equipment.
Vegetated BMPs
	For vegetated BMPs, consideration should be given to what source of water will be drawn upon for irrigation during the establishment/warranty period, how it will be delivered to the BMP (e.g., is equipment with sufficient volume and pressure available?), and in some cases an irrigation system should be part of the BMP design (e.g., green roofs).
	For vegetated BMPs in high pedestrian traffic areas, consider the need for walkways or stepping stones and barriers to help limit foot traffic to designated portions of the BMP, or discourage it altogether if it is causing vegetation maintenance issues.
Underground Pretreatment or BMPs
	For underground pretreatment devices or BMPs, the associated manholes or maintenance hatches should be close to a drivable surface/path that can support the heavy vehicles needed for sediment removal maintenance (e.g., hydrovac trucks) and consider installing a staff gauge or graduated measuring tape to allow sediment depth to be assessed visually from the ground surface, without having to enter the confined space.
Green Roofs and Blue Roofs
	Safe and efficient means of accessing and exiting a green roof or blue roof site for installation, inspection and maintenance is a primary consideration as it will influence the time and effort required to transport tools, equipment and materials to and from the site.
	For green roofs and blue roofs, tie-off points for ladders and personal fall protection safety equipment should be incorporated in the roof design for use by individuals performing inspection and maintenance work.
General Considerations
	Lockable caps on monitoring wells and sub-drain clean-out standpipes to prevent unauthorized access, tampering, or vandalism.
	Features for taking the BMP off-line or draining stored water by gravity to improve ease of inspection and sediment removal maintenance tasks.
	Inlets should be readily accessible from surface conveyances, catchbasins, manholes or access hatches to avoid the need for specialized equipment for inspection (e.g., closed circuit or remote controlled cameras).

Inclusion of Planting Plans

All vegetated BMP designs should include planting plans that specify species that can tolerate both wet and dry conditions and, for BMPs that will receive de-icing salt laden runoff during winter, species that are salt tolerant. Further consideration should be given to species that are both pollution and soil compaction tolerant given the location of many BMPs along roadways and near public spaces. Where possible, planting should done during the wettest seasons/months of the year (e.g., early spring and mid-to-late fall) to help minimize the need for irrigation (and staff time and equipment) during the establishment/warranty period. Use of drought-tolerant and native species will help minimize or eliminate the need for irrigation during the operating phase of the BMP life cycle. Good vegetation cover on the surface and side slopes of BMPs helps to maintain infiltration function, contributes to runoff volume reduction function through evapotranspiration, and helps prevent erosion of soil and/or mulch cover from flowing overland water.

Both common and botanical (i.e., species Latin name) names should be used on planting plans so that inspectors and maintainers of the vegetation are better able to recognize or develop the means of recognizing the plants in the field and distinguishing them from pervasive weeds. Planting plans should also specify the planting method (e.g., seed vs. sod) and plant or container size (e.g., saplings vs. caliper tree; plugs vs. pots; bare root vs. root ball). Any deviations from the planting plan or species substitutions should be noted on as-built drawings/planting plans.

All construction contracts that include vegetation should specify a minimum two (2) year warranty period (i.e., establishment period) for the plants, which begins after planting is completed, and ends when the BMP is assumed by the owner. Over the warranty/establishment period the contractor is responsible for routine maintenance tasks (e.g., watering, weeding, and sediment and trash removal).

Consideration should be given to specifying a phased approach to planting in construction contracts, in which planting occurs in two stages (e.g., fall and the following spring; spring and the following fall) to help ensure the full palette of plants specified in the plan are available. Thereby, any failed plantings from the first stage of planting are sure to be replaced in the second. If many or all plantings of a certain species do not survive the first phase, they can be substituted with another more tolerant or suitable species in the second (A list of suitable shrubs, trees, graminoids, etc. can be found on our Plant lists page).

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.

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 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.

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 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.

The technique of removing pollutants from runoff as it infiltrates through the soil.

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.

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.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

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

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.

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

The quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.

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