Stormwater Drainage

Trench Drain Sizing Calculator

Channel & Outlet Capacity Analysis

Comprehensive analysis tool for sizing trench drains, considering both channel capacity and outlet configurations.

Trench Properties

Drainage Area Properties

Total width draining to both sides (e.g., 20ft = 10ft each side)

Outlet Configuration

adequate

Capacity Assessment

Channel Utilization

0.8%

Outlet Utilization

3.2%

How to read this result

The outlet system is using 3.2% of its calculated capacity. Both the open channel and outlet system are at or below the calculator's 80% screening limit, preserving at least a 20% capacity allowance.

Flow Rates

Trench Capacity

0.53cfs

Required Capacity

0.00cfs

Velocity

0.97 m/s

Flow Regime

critical
Check open-channel flow detailsEstimate runoff separately

Drainage Area Summary

200 sq ft

(0.005 acres)

10 ft × 20 ft

Design Considerations

  • Trench capacity should exceed expected runoff by at least 20%
  • Flow velocity should prevent sediment accumulation
  • Consider safety factor for intense rainfall events
  • Account for future changes in drainage patterns

Worked example

Parking-deck ramp trench

Screen a 12-inch-wide, 6-inch-deep HDPE trench at 1% slope serving a 100-by-100-foot paved area. Use a 0.95 runoff coefficient, 3.5 in/hr rainfall, and two 8-inch outlets.

Trench capacity

3.28 cfs

Required flow

0.76 cfs

Assessment

Adequate

The channel is about 23.3% utilized and the two outlets are about 31.6% utilized, so the outlets control while both components remain below the 80% screening limit. Confirm local design-storm criteria, inlet capture, grate open area, waterproofing, and allowable discharge before final design.