The purpose of these regulations is to set out the specific measures required within the urban community’s territory regarding the control of surface runoff, the treatment of rainwater, and its discharge into public underground or open drainage systems. To this end, it sets out the general legislative framework.
Extract from the regulations
7- General rules
a) It is forbidden to connect rainwater to the sewerage system or to an independent sewerage system.
b) Any increase in runoff caused by new waterproofing (creation or extension of existing buildings or infrastructures) must be compensated for by the installation of rainwater storage systems on the plot (infiltration or retention basins) or other alternative techniques*.
New waterproofing must be compensated for to the extent of 130 litres/m². (minimum) with a maximum leakage rate of 50 l/s per hectare of waterproofing.
If infiltration is not possible (or if the infiltration rate is insufficient) and there is no outlet, diffuse spreading may be considered at a regulated rate of 5 l/s/waterproofed hectare, subject to acceptance by the municipality.
The retention volume to be used in this case is : 170 l/m² of waterproofing.
c) The volume stored must be emptied by infiltration as a priority and not connected to the public network.
d) For building permits involving the demolition of existing buildings, the sizing of the works must take into account all the waterproofed surfaces of the land unit, regardless of the previous degree of waterproofing.
e) In the case of grouped urban development projects (housing estates, ZACs, etc.), storage facilities must be shared by the whole project in order to avoid storage on each plot. Storage facilities created as part of a subdivision permit must be sized for the roadway and for the total waterproofed surfaces likely to be created on each lot.
* Alternative techniques complement or replace conventional sewerage systems. Their main function is to limit peak flows downstream in order to prevent the concentration of water in saturated networks:
- by infiltration when the soil is suitable and outside the catchment protection area,
- by temporarily storing rainwater before releasing it at a controlled flow rate into the downstream network (collectors, gutters, ditches, etc.) if infiltration is not possible or is prohibited,
- by combining temporary storage and infiltration.
A non-exhaustive list of on-site storage techniques is set out in the annex to the regulations: swales, ditches, drainage trenches, rainwater-harvesting roofs, porous surfaces, reservoir pavements, etc.