7. Stormwater – Elements for calculating rainfall

7.1 Base data: ten-year rainfall

Since the primary characteristic of atmospheric precipitation is irregularity, the quantity of rainwater to be evacuated must be determined more or less empirically. Collectors must be able to evacuate rainwater under certain self-cleaning and filling conditions.

The flow rates to be taken into account when calculating cross-sections are based on a ten-year rainfall, corresponding to the highest rainfall recorded in the last ten years. In the event of a higher rainfall, it is assumed that the collector will flow backwards and be unable to absorb the excess inflow. This basic data (ten-year rainfall) is weighted by coefficients corresponding to the nature and morphology of the basin. This results in two practical values for...