The calculations were reached through timing of water infiltration of soil (water does pool after down pours) and 1 in100 year event of flooding.
There is no road drainage or sewers nearby as we are out in the sticks. However, there is an open ditch that runs along two sides of the property (just outside the plot) and onto the lane and this could act as a storm flow if connected. I suppose what I really need to know - do planning check whether I install all 200 crates or not!?