The four goals
Most stormwater designs balance four goals. Quantity control limits peak flows, often to pre-development levels, to reduce flooding downstream. Quality control removes pollutants such as suspended solids before runoff reaches a stream or lake. Erosion control holds back small, frequent storms so channels are not scoured. Water balance keeps some rainfall infiltrating to protect groundwater and stream health.
What the system looks like
Typical components include stormwater ponds and wetlands, underground storage, oil and grit separators, and low impact development (LID) features such as bioswales, permeable pavement and infiltration trenches. Outlet structures control how quickly water leaves a pond.
Minor and major systems
Drainage is usually designed as a dual system. The minor system, such as storm sewers, handles frequent storms. The major system is the overland route, such as roads and channels, that safely carries larger storms when pipes are full.
Who decides what is required
In Ontario, criteria are set by the municipality, the local Conservation Authority and the province, and they vary by watershed. Confirming the applicable criteria is one of the first steps on any project.
When to involve an engineer
As early as possible. Pond size and location, grading and drainage routes shape the site layout, so addressing stormwater at the concept stage avoids redesign later.