On a watershed scale, factors contributing to flooding include the total volume of stormwaterSurface runoff from at-grade surfaces, resulting from rain or snowmelt events., the peak flow and the duration of peak flow. All low impact developmentA 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. facilities can help to prevent flooding by providing watershed distributed stormwater storage, and most feature some form of peak flow control device at the outflow or overflow. The limiting factors in the contribution that a system can make to flood control include the total storage and the rate at which the system can be emptied between events. Flow is controlled by:
- the properties of the filter mediaThe 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.. See flow through media,
- the exfiltrationThe 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). of water from the practice through an underdrain. See flow through perforated pipe,
- one or more orifices,
- weirs, or
- proprietary low flow devices.
Where either device may be installed, a weir is recommended over an orifice, as they are less susceptible to blockage under freezing conditions.
The smallest diameter orifice accepted by most municipalities to ensure that clogging does not occur in a stormwater system is 75 mm. The preferred minimum orifice size is 100 mm where the effects of freezing are a concern. It is recommended that this latter size be maintained for exposed outlet designs (i.e., reverse sloped pipes).
In instances where a perforated riser outlet is designed, the orifice is protected by the smaller perforations in the riser and a minimum orifice size of 50 mm is acceptable. Where small orifices are required, consideration should be given to providing an overflow outlet which would operate in the event that blockage of the primary orifice occurs.Flow through an orifice
Low flow devices
Vortex valves have a unique head/discharge relationship. Flow is directed tangentially into the volute to form a vortex that reduces the design peak dischargeThe greatest volume of stream flow occurring during a storm event. flow rate from the vortex valve far below an equivalent diameter simple orifice.
- Under low flow conditions, water passes through the valve with negligible pressure drop.
- Under high flow conditions, a vortex flow pattern develops within the device creating an air filled core. This phenomenon restricts and throttles flow through the device, creating a back pressure immediately upstream of its discharge. During high flow rates, a vortex valve with a relatively large outlet opening performs similarly to a conventional orifice with a much smaller outlet opening. However, debris that might clog a smaller orifice is able to pass through the vortex valve because of the relatively larger flow path opening. The relatively large opening also makes vortex valves less prone to complete clogging from ice formation compared to an orifice of similar specification. 
In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
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- Ontario Ministry of Environment. (2003). Stormwater Management Planning and Design Manual. Retrieved January 15, 2017, from https://www.ontario.ca/document/stormwater-management-planning-and-design-manual/stormwater-management-plan-and-swmp-design
- Contech Engineered Solutions. (2018). Vortex Valves. Retrieved March 15, 2018, from http://www.conteches.com/products/stormwater-management/stormwater-flow-controls/vortex-valves#tools