Manning's Drain Sizing
Ref: MSMA 2nd Edition (2012), Chapters 14 (Drains & Swales) and 16 (Engineered Channel); Manning n from Table 2.3
Q = (1/n) × A × R^(2/3) × S^(1/2)
Ref: MSMA 2nd Edition (2012), Chapters 14 (Drains & Swales) and 16 (Engineered Channel); Manning n from Table 2.3
A = b×y | P = b + 2y
Rectangular channel
A = (b + z·y)·y | P = b + 2y·√(1 + z²)
Trapezoidal channel
Channel Shape
Channel Properties
MSMA Table 2.3: Concrete smooth 0.015, Brick 0.020, Grassed (short) 0.035, uPVC 0.011
Bottom width of drain channel (min 0.5 m for lined open drain)
Depth of water in drain (max 0.6 m without cover, 1.2 m with cover)
Typical 0.002–0.05; ensure 0.6–2.0 m/s velocity
Flow Area (A)
—m²
Engineering aid only — final design must be endorsed by a registered PE under BEM.
Wetted Perimeter (P)
—m
Engineering aid only — final design must be endorsed by a registered PE under BEM.
Hydraulic Radius (R)
—m
Engineering aid only — final design must be endorsed by a registered PE under BEM.
Flow Velocity
—m/s
Engineering aid only — final design must be endorsed by a registered PE under BEM.
Discharge Capacity
—m³/s
Engineering aid only — final design must be endorsed by a registered PE under BEM.