Inlet Capacity of a Trench Drain

Trench Drains

• Trench, or French, drains keep low-lying areas from flooding during heavy rain and are commonly installed along roadways and parking areas. Heavy grills laid over trench drains match the type of traffic expected in the area. Parking area drains need grills strong enough to support vehicles, with small openings that won't trip pedestrians or trap bicycle tires. As the strength of the grill increases, inlet capacity drops. Larger drain channels compensate for the reduced intake, but other factors including the degree of slope, the amount of acreage served, and the slope of the drain itself also affect the drain's maximum capacity.

Outside the Home

• Simple trench drains can reduce the severity of problems such as seasonal flooding of basements and crawl spaces. Where a sloping landscape directs groundwater towards the home, digging a trench drain across the water's drainage path diverts the flow away from the house. Even perforated drainage pipe, laid in a ditch back-filled with gravel, helps. Accurately figuring flow and inlet capacity might not be possible. The gravel acts as the trench drain grill, and the space available for water decreases as plant roots invade the ditch and natural erosion clogs the drain with soil. A gravel-topped trench drain can improve problems caused by slow groundwater movement.

Inside the Home

• Small trench drains installed along the entryways of curb-less shower stalls provide unrestricted access for elderly or disabled residents. Correctly installed, the trench drain channels shower water into an ordinary drain pipe without splashing water onto floors outside the shower stall. If kept clean, the inlet capacity of the trench drain exceeds that of the round drain inlet in an ordinary shower, and uses a drain channel of about the same size as a shower drain pipe. Waterproof installations can fit trench drains to sloping shower stall floors in either wood frame or masonry construction, according to North Carolina State University Extension.

Design Factors

• Calculating the size and type of trench drain for any large project requires professional expertise. How much water an outside trench drain must handle depends on factors including the maximum rainfall, the surface area of the land above the drain, and the type of surfacing on the site, according to TrenchDrain.net. Movement through the drain depends on the drain's cross-sectional area, the drain's resistance to flow and the drain slope. Inlet capacity depends on the drain's surface area, the grill design, and the percentage of inlet area that the grill obstructs. Keep existing trench drains clear of debris, because a cover of fallen leaves can reduce inlet capacity almost to zero.