D-3 Storm Drainage Systems Introduction

The following terms will be used throughout this section. A complete list of terms for this section can be found in the Glossary.

Please also see the separate list of NPC Definitions for DWV Systems in Section D-3.1.

• 15-minute rain event: A standard time frame used to express storm drainage load calculations, based on the volume of rainfall (in litres) expected over a 15-minute period. This metric helps determine appropriate pipe sizing. (Section D-3.3)
• berm: A raised area of soil that is typically built along the edge of a trench or drain to hold back water, directing it into the drain for proper collection and drainage. (Section D-3.7)
• catch basin: A ground-level stormwater collection point, often found in paved areas, that traps debris like leaves before directing rainwater or stormwater into pipes for proper drainage. It has a grate on top to catch debris and prevent blockages, helping direct the water into storm sewers for discharge into natural water bodies. (Section D-3.2; Section D-3.6)
• clamping collar: A metal ring used to hold parts together, like a roof drain and the waterproof membrane, to make sure everything stays tightly in place and doesn’t let water leak through. (Section D-3.5)
• combined drainage piping system: A system that carries both stormwater and wastewater in the same pipe. These systems can overflow during heavy precipitation, leading to direct discharge of untreated water into natural water bodies. (Section D-3.2)
• corrugated steel galvanized pipe: A type of durable, ribbed metal piping commonly used in stormwater systems such as culverts or detention systems. It must be joined with couplings that align pipes, resist separation, and prevent root or debris infiltration. (Section D-3.3)
• curtain drain: A French drain installed across a slope to intercept and drain water away from sensitive areas, such as septic disposal fields, reducing water accumulation and lowering the water table. (Section D-3.7)
• deck drain: A flat-strainer drain used in areas like walkways or patios, similar to a roof drain but intended for horizontal surfaces other than roofs. (Section D-3.2)
• drainage trough: A narrow channel or trench that collects and moves water away from areas like driveways or parking lots. Made of concrete or plastic, it has a grate on top for water to flow into, helping keep the area dry and safe. (Figure 4, Section D-3.6)
• drain tile: A type of pipe used in drainage systems, often made of clay, that helps collect and move water from the soil, typically used around foundations. (Section D-3.7)
• duplex system: Two pumps in a sump, each with its own pipes and valves. Float switches inside the sump detect water levels, and a control panel automatically turns the pumps on or off. This setup ensures the system works even if one pump fails. (Section D-3.6)
• fixture unit (FU): A unit of measure used in sanitary drainage systems to represent the load-producing effect of a plumbing fixture. In combined systems, 1 FU = 9.1 L/15 min when total load exceeds 260 FUs. (Section D-3.3)
• float switch: A device that controls water levels in tanks or pumps. It has a floating part that moves with the water level, and when the water gets too high or low, it sends a signal to turn the pump on or off to keep the water at the right level. (Section D-3.6)
• flow-control roof drain: A roof drain designed to limit the rate at which stormwater is released into the drainage system, reducing pipe size and minimizing strain on municipal sewers during peak rainfall. Design requirements include placement limits and overflow protection. (Section D-3.2)
• French drain: A drainage system consisting of a trench filled with perforated pipe and clean rock, designed to direct surface water away from an area. (Section D-3.7)
• green roof: A rooftop covered with vegetation that absorbs rainwater, reducing runoff and providing benefits like energy efficiency and improved air quality. (Figure 5, Section D-3.8)
• greywater: Water that comes from sinks, showers, and washing machines in your home. It’s not dirty like sewage water, but it’s not clean enough to drink either. Greywater can be reused for things like watering plants or flushing toilets to help save fresh water. (Section D-3.8)
• hydrology: The scientific study of the movement, distribution, and management of water, including the water cycle and water resources. (Section D-3.7)
• litres per 15 minutes (L/15 min): The unit used to express drainage loads in storm and combined systems, aligning volume with a 15-minute rainfall time frame. Helps convert area and rainfall depth into actionable flow data. (Section D-3.3)
• local rainfall intensity: The amount of rain, measured in millimetres, expected to fall over a 15-minute period in a specific geographic location. This data is obtained from the National Building Code and is critical to calculating drainage loads. (Section D-3.3)
• minimum slope: The smallest allowable incline for drainage pipes, typically 1:50 as per regulations like the NPC. In some cases, the slope may be reduced to 1:100 for building drains at least 100 mm (4 in.) in size to match other connected drainage systems. (Section D-3.5)
• moling: A process in agriculture where a torpedo-shaped mandrel creates tunnels or voids in the soil without leaving a visible trench, often used to improve drainage in waterlogged fields. (Section D-3.7)
• rainwater leader (RWL): A vertical pipe or tubing that conveys rainwater from roof drains or gutters to the ground or stormwater disposal system. Can be installed inside or outside a building. (Section D-3.2)
• Roof scupper (or scupper): An exterior, box-like device or opening in a parapet or wall that collects and drains stormwater from flat roofs. Scuppers serve as emergency overflow systems to prevent water buildup when primary roof drains are blocked. They are often used in combination with parapet walls and flow-control drains. Scuppers must be located no more than 30 m (100 ft) apart and be capable of handling up to 200% of local rainfall intensity when used with flow-control systems. (Section D-3.3)
• sheet metal leader: A vertical exterior pipe made of sheet metal that conveys rainwater from gutters to the ground. It is permitted for use only above ground and outside a building. (Section D-3.3)
• siphonic roof drain: A high-capacity roof drain that uses siphonic action to drain water more quickly than traditional gravity drains, reducing the number of required drains. (Section D-3.2)
• sub-drainage system: (subsoil drainage system); A network of pipes or drains placed underground to collect and carry away excess water from the soil. It helps prevent flooding or water buildup by directing water to safe areas, keeping the ground dry and stable. (Section D-3.6)
• trenchless plow: A type of equipment used in moling that creates a narrow void or tunnel in the soil without disturbing the surface or creating a visible trench. (Section D-3.7)
• vitreous clay pipe: A type of pipe made from baked clay that has a smooth, hard surface, like glass. It’s used in plumbing and drainage systems because it’s strong, durable, and resists wear and tear from water and chemicals. (Section D-3.7)