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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 (ft.)
Rainfall intensity in inches per hour

Once catchment run-off is calculated, other inflows, e.g., down spouts, can be added.

Link to the US government rainfall frequency atlas

Other factors that affect how much liquid reaches the trench and how quickly;

 Ground fall percentage
Pavement material - some materials absorb liquids, e.g., brick pavers
Position and size of outlet pipe
Surface roughness of trench material. Manning’s coefficient of roughness figures are available
Angle of approach to trench - this can affect grate hydraulics (steep slopes may cause bypass)

Determining trench size

Once run-off is calculated, it is used with the other factors above to determine the correct size of trench. The trench size
is dictated 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., 4”, 8”, etc.
Overall grate and outside trench dimensions are misleading. Some trench
drain companies promote these sizes to mislead specifiers and installers, e.g., overall trench dimension of 6”. Usually
clear opening will be approximately 4”
- this is the dimension available for hydraulic flow, not 6”.

 
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