Record-breaking floods

Around noon, following very heavy rainfall, the dam on a canal ruptured along the right bank over about 20 metres. Roughly 800,000 m³ of water flowed into the already flooded Loing River. Between 3 p.m. and 7 p.m., its level increased by a few centimetres in the urban area located 12.5 km downstream. The exceptionally high flood waters, higher than the 100-year flood level, reached its peak the next day at 4 a.m. at a much higher level than that measured at 7 p.m. The damage from the overall event was extensive, but cannot be attributed to the rupture of the dam.

Rainfall accumulation over 4 days was equivalent to 3 months of precipitation for the entire region. An accidental overfill of the channel from the Loing River occurred just a few minutes before the rupture. The breach happened at a transition point between the original structure and the structural repair that was undertaken in 2003. The repair was performed immediately after a breach in 2002.

Combination of factors causing the failure

The expert evaluation conducted on this event suggests that a potentially more complex scenario than the simple overflow may have initiated the breach. Although it is not possible to establish the exact scenario with certainty, the most likely hypothesis is a combination of 2 triggering factors: overflow and caving-in of traffic between the slopes on the left and right banks. A local resident had noticed a jet of water at the top of the structure just before it broke. This may have been due to the ripping away of a sealing material anchor in the dam’s structure. This circulation of water would have damaged the ridge and initiated the overflow. The rupture would then have occurred rapidly as a result of external erosion.

The expert report recommended that the breach be repaired with materials similar to those used in the 2003 repair operation (compacted clay gravel). In order to prevent the risk of a new accident occurring in the repaired area, the report also recommended that the operator:

• equips the channel with a safety spillway to prevent overflow;
• continues draining the bottom of the canal over as long a stretch as possible, then backfills the drainage trench with suitable materials and restores the waterproofing.