including redeployment of its wastewater systems from now to 2050.

The cliffs in Ault are regularly retreating, under repeated attack from the sea wearing them away from the bottom, and rainwater infiltration eating away at them from the top, plus the combined effects of freezing and thawing. According to estimates, in some places, they are losing 30 to 50m every 100 years.

Faced with this danger, the town was added to the Picardy cliffs risk prevention plan (prefectural order of 6 June 2013), which addresses the risk of coastal erosion and predicts changes to the coastline, as well as making the most of the area’s development potential. This plan has two aims: to protect people and property in affected areas and avoid increasing risks in the area over the next 100 years.

The redeployment of the wastewater systems in Ault, carried out in conjunction with the Syndicat Mixte Baie de Somme-Grand Littoral Picard, is part of this overall local development strategy.

Against this backdrop, SADE (contractor) was selected to secure wastewater runoff to prevent any risk of discharge into the sea if the cliffs collapse. So teams from the Abbeville branch moved into the town centre to work on the existing wastewater facilities and systems:

• disconnection of the main pumping station,
• modification of the direction of flow from the sea to the land,
• creation of 5 backflow stations,
• laying of 950m of return piping and replacement of a 500m section of gravity sewer main, either using traditional methods or close-fit lining,
• reworking of a hundred or so service pipe connections,

Important: due to the risk of collapse, technical specifications were drawn up to minimise the tonnage of on-site vehicles down by the seafront and limit vibrations by focusing on self-compacting material backfilling techniques.

A complex 5-month project with high environmental stakes over the next century.