A culvert is a drainage structure that allows water to pass through or beneath a canal embankment, Writes Ralph Freeman.
It serving as a critical component in managing water flow and pressure. When these structures become defective, they can trigger a catastrophic chain of events leading to complete embankment failure, as evidenced by recent incidents on the Bridgewater Canal and Llangollen Canal in England.
The failure mechanism
The collapse process typically begins with a compromised culvert that develops leaks through cracks, corrosion, or structural deterioration. Water escaping through these defects begins eroding the surrounding soil and fill material that supports both the culvert and the embankment above it. This erosion creates voids in the ground as water washes away the culvert's ground support, causing the culvert structure itself to collapse and fracture.
Once the culvert loses integrity, water flowing through it drains into the surrounding ground rather than being properly conveyed away. This uncontrolled water flow accelerates erosion of the supporting material, creating progressively larger voids beneath and around the embankment. The embankment, which may have successfully held water for centuries, loses its foundational support.
Common culvert defects
Several factors contribute to culvert deterioration over time. Abrasion of the invert—the bottom portion of the culvert—occurs when water carries sand, soil, and other abrasive materials through the structure, gradually wearing away protective surfaces and exposing the soil below. Poor initial design or construction can result in inadequate hydraulic capacity or structural weaknesses that manifest years later. Inadequate maintenance allows culverts to become clogged with debris, vegetation, and sediment, which reduces water-carrying capacity and increases internal pressures.
Progressive collapse
As erosion continues, the embankment experiences what engineers call a 'rotational landslip', where cracks may first appear at the top of the affected area. The loss of support causes the embankment to fail catastrophically, with the canal bed eroding and creating trench-like structures as water rushes into adjacent fields. This rapid drainage can compromise stretches measuring 150 to 180 feet in length with depths of approximately 15 feet.
Serious consequences
A collapsed culvert and embankment poses significant risks beyond the immediate structural failure. The sudden release of canal water can flood adjacent land, damage infrastructure, and create hazardous conditions. Recent incidents have required emergency responses including the installation of floodgates to prevent further breaches that could affect local residents. Repairs are complex and costly, requiring excavation of damaged towpaths, embankments, and canal beds, followed by reconstruction using hundreds of tonnes of clay to reline and waterproof the canal. The repair process can take months and cost hundreds of thousands of pounds.
The risk of culvert-related embankment failure increases with extreme weather events, as unseasonably heavy rainfall can overwhelm compromised structures that might otherwise continue functioning. This makes regular inspection and maintenance of culverts essential for preventing catastrophic failures in aging canal infrastructure.