Enter Details

Open Channel diagram
    Mannings 'n' (Typical Values)
  • Mowed Grass - 0.025
  • Concrete   - 0.015
  • Asphalt     - 0.016
  • Rocks    - 0.035

Channel Size

This free online calculator is especially applicable for building sites where it is desirable to design an overland flow path to carry water away from buildings. Especially when the storm is greater than the design storm for the pipes. Or as a fail safe system should a blockage occur in the pipes.

However it is becoming increasingly popular (even mandatory sometimes) to provide some sort to storm water treatment facility before discharging into the Authorities storm water system, or into the local river.

This usually involves a gross pollutant trap and nutrient removal.
However, with the right design, a grass swale (open Channel) can achieve a similar result.

If the velocity is slow enough, and the channel long enough, sediment will settle out, and nutrients can be taken up by the vegetation. If the channel is also a filter system, eventually discharging to an agi drain (slotted pipe) underneath, this has an added benefit of leaving all gross pollutants on the surface where they can be cleaned up.

For tenanted building sites this can be important, as usually the tenant does not know of the existence of any underground pollutant traps. Hence such things won't get cleaned out, until it all goes horribly wrong.

So, grass swales, overland flow paths, and open channels are becoming increasingly popular, for both the dispersal and treatment of storm water runoff.

The program uses Mannings formula (for metric):-

  Q = A/n * R^0.66 * S^0.5

To use imperial units, use the converter below.
Convert both the input and the answers.
Conversions (Rounded)
mm  ft3/s ft/sec 
inch Litres/s m/s

Q = Flow
A = Cross sectional area of the water flow
R = Hydraulic radius = A/P
P = Wetted Perimeter
S = bed slope

Enter a known flow (runoff) in Litres/sec. Or calculate from scratch.
Runoff (L/s) = Coefficient of runoff * Storm Intensity(mm/hr) * Catchment Area (sqm) / 3600.
Or, get from Here for a single catchment or Here for multiple catchments.

Side Slopes
Side slopes are of the form vertical : horizontal, so for example a 1 in 2 slope means 1 unit vertical to every 2 units horizontal. (about 26.6 degrees) Enter 2 in the program. A simple conversion calculator for converting degrees, or percent slope, to the 1:? format can be found here.

Entering zero in the S1 or S2 box will give a vertical side to the open channel. When entering two vertical faces a base width is required. This will give a rectangular open channel.

Bed slope
The bed slope is the longitudinal slope of the channel. Refer to 'Side Slopes' above for the format.

Bed Width
A bed width of zero will give a triangular channel, if there is slope greator than zero on at least one side.
If both sides are vertical (ie S1=0 and S2=0) then a base width of zero won't give you an open channel at all.

Mannings 'n'
Mannings 'n' is a roughness coefficient devised by Manning that takes into account the surface roughness of the open channel. The higher the value the rougher the channel.

Copyright Ken Sutherland 2015
If you find that the programs useful, please feel free to buy me a few beers.