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:

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.