How do I choose the channel width?

Back-calculate it. First get design flow from Q = psi x q x A, then check the section with Manning v = (1/n) x R^(2/3) x i^(1/2) and require capacity above the design flow with 30-60% margin. Example: for 18 L/s, a 150x100 mm section passes only 15.3 L/s and fails; 200x120 mm passes 28.7 L/s and works.

D400 or C250 load class?

Specify the worst case, not the everyday case. A15 pedestrian only, B125 footway edge and light parking, C250 estate roads and car parks, D400 city roads and fire-truck routes, E600 forklift aisles and ports, F900 airports and container terminals. Fire routes need at least D400 even when rarely used.

How wide should a slot drain opening be?

Keep it at 10-12 mm or less so heels, wheelchair casters and ankles cannot catch, but remember a narrow slot limits inlet capacity - check inlet flow per metre on long runs and add slots or switch to a grated top if needed. When safety and capacity conflict, safety wins and you compensate with length or section.

Drainage · Sizing

How to Size a Linear Drainage Channel: Flow Rate First, Load Class Second

Linear and slot drains are everywhere now - plazas, car parks, factory yards, curtain-wall bases - and they are also among the most frequently mis-specified products. The usual mistake is picking a channel width that looks right and hoping it drains. The correct order is the reverse: calculate the water first, size the channel second, then match the load class.

Step 1: Design flow Q

Q = psi x q x A

Q - design flow (L/s)
psi - runoff coefficient: 0.85-0.95 for roof/concrete/asphalt, 0.15-0.30 for green area
q - rain intensity (L/(s*ha)) from the local intensity-duration-frequency formula
A - catchment area (ha, 1 ha = 10,000 m2)

Rain intensity formulas are issued locally, typically q = A(1 + B*log P)/(t + C)^D, with parameters that differ by city. Always use the formula published for the project location. Return period: P = 3 years for ordinary yards, P = 10-50 years for sunken plazas and critical areas.

Worked example

Catchment A = 500 m2 = 0.05 ha, concrete surface (psi = 0.90), q = 400 L/(s*ha):

Q = 0.90 x 400 x 0.05 = 18 L/s

The channel must safely pass 18 L/s.

Step 2: Check capacity with Manning

v = (1/n) x R^(2/3) x i^(1/2)　　Q_ch = A_w x v

n - roughness: 0.011-0.013 stainless, 0.013-0.015 polymer concrete
R - hydraulic radius (m) = flow area / wetted perimeter
i - longitudinal slope (0.01 = 1%)

First attempt - too small

b = 150 mm, h = 100 mm, i = 1%, n = 0.012:

A_w = 0.15 x 0.10 = 0.0150 m2
wetted perimeter = 0.15 + 2 x 0.10 = 0.35 m
R = 0.0429 m, R^(2/3) = 0.1225
i^(1/2) = 0.100
v = (1/0.012) x 0.1225 x 0.100 = 1.02 m/s
Q_ch = 0.0150 x 1.02 = 15.3 L/s

15.3 < 18 - fails. On site this shows up as ponding and water overtopping the kerb.

Upsize and re-check

b = 200 mm, h = 120 mm:

A_w = 0.0240 m2; wetted perimeter = 0.44 m
R = 0.0545 m, R^(2/3) = 0.1436
v = 83.33 x 0.1436 x 0.100 = 1.20 m/s
Q_ch = 0.0240 x 1.20 = 28.7 L/s

28.7 > 18 - passes with roughly 60% margin for silt and debris.

Keep velocity between 0.6 and 3.0 m/s: below 0.6 silt settles, above 3.0 scour and noise increase. Design water depth is normally 70-80% of channel depth, not full bore.

Step 3: EN 1433 load class

Class	Test load	Typical use
A15	15 kN	Pedestrian, landscape
B125	125 kN	Footway edge, car parks, courtyards
C250	250 kN	Kerbside, slow lanes, parking areas
D400	400 kN	City roads, heavy vehicle lanes
E600	600 kN	Ports, industrial yards, forklift routes
F900	900 kN	Airport aprons, container terminals

Specify for the worst case: fire-truck routes need at least D400 even if rarely used; forklift aisles E600. Under-specifying rarely crushes the grate outright - it deforms it permanently until the slot opens, catching wheels and tripping pedestrians.

Material, slot and installation

304 stainless - default for outdoor use; smooth wall gives lower n and better capacity
316/316L - mandatory for coastal, poolside, de-icing salt or chemical exposure (chloride pitting)
Polymer concrete - high compressive strength, freeze-thaw resistant, heavier traffic
Slot width - keep openings at 10-12 mm or less so heels, wheelchair casters and ankles cannot catch
Slope - maintain at least 0.5%, never below 0.3%, or self-cleansing velocity is lost
Bedding - concrete haunch both sides to spread wheel load into the sub-base
Galvanic isolation - never bed stainless channel directly against carbon steel embedment
Maintenance - provide silt boxes or inspection openings every 20-30 m

Five common mistakes

Sizing from pipe diameter instead of open-channel hydraulics
Losing the longitudinal slope during site grading
Choosing load class from everyday traffic, ignoring fire trucks and forklifts
Mixing stainless with carbon steel without isolation
No cleaning access - the line silts up and has to be dug out

Need a sizing calculation?

Send us the catchment area, surface type and location - we return a sizing sheet with flow calculation and load class recommendation.

Foshan source factory · manufacturing since 1982 · Stainless · Liquid-cooling manifolds · Architectural metalwork