The locks are very closely spaced in this stretch of the canal. When a lock empties it causes a small wave to travel down the canal, hit the door of the next lock and travels back up.

If a ship exits a lock exactly when this wave comes back disaster can strike as the wave can lift the ship and crush the wheelhouse underneath the bridge. Many wheelhouses have found a premature end of their lifetime this way.

Prudent lockkeepers therefore open the lift bridge, if only for a few inches to avert this danger.