Catchment Hydraulics
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Catchment Hydraulics - calculating run-off

To calculate the correct size of trench drain, catchment run-off must be calculated.

Catchment area - length x
width of pavement
Rainfall intensity in mm per hour  
Run-off coefficient - ie. pavement
material, some surfaces absorb
liquids e.g. block pavers
Refer to 'Australian Rainfall and Run-off' or AS/NZS 3500.3 - 'Plumbing and Drainage Code' for rainfall intensities.
Other factors that affect the transfer of liquids include:
Ground fall along trench %
Position and size of outlet pipe
Surface roughness of trench material. Manning's coefficient of roughness
Pavement angle of approach to trench - this can affect grate hydraulics (steep slopes may cause bypass)

Determining trench size

The calculated run-off , is combined with the other factors above to determine the correct size of trench. The trench size is specified by the clear opening and invert depth - changing either or both of these will create a smaller or larger ‘flow area’.
Clear opening of trench is the normal specified dimension, e.g., 100mm, 200mm, etc.
Overall grate and outside trench dimensions are misleading.


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