Why Are Weirs So Dangerous?

Even though the water around weirs can often appear relatively calm, they can be extremely dangerous places to boat, swim, or wade, as the circulation patterns on the downstream side—typically called a hydraulic jump—can submerge a person indefinitely.

Why weirs are dangerous?

The greatest hazard is often the strong re-circulating flow (or towback) immediately downstream of a weir. This can trap a person, animal or object, leading to exhaustion and drowning. The flow is often highly aerated with reduced buoyancy, making it hard to stay afloat.

Is it safe to swim in a weir?

Weirs: Weirs are generally to be avoided – some types in particular (box weirs) are fatal to trapped swimmers and kayakers. In some places you will see people swimming happily upstream in the pools formed by weirs (their purpose is to deepen the water), that's probably safe in low flows.