Planning amendments for resilience

The municipality’s general plan is more than twenty-five years old. Its drainage regulations did not require separate sewer networks and, for stormwater, simply set an absorption rate of no less than 50 % in the calculation of flows, with a return period of at least 25 years. After the flood of October 2024, and alongside the planning of the riverside set out in the special plan, the studio has prepared a number of amendments to the general plan itself so that the town built from now on manages rainwater to current standards.

The piece already approved is a detailed amendment with five regulatory changes, one of which concerns rainwater. In new development units, drainage must be separate —foul water on one side, stormwater on the other— and stormwater from streets and green spaces is managed exclusively through sustainable drainage systems (SuDS). Buried stormwater pipes are reserved for draining buildings. The aim is to improve wastewater treatment, save water and energy and meet the legal requirement for separate networks in integrated developments.

The SuDS are sized to absorb 60 % of the rainfall over the whole area, with a return period of at least 25 years or with retention volumes set by zone and street type: from 8 mm in the urban extension and on narrow streets to 60 mm in parks and gardens. In this way, much of the rain stays where it falls instead of loading the network.