Lock gates are built using the same technique as bridges. "In fact, the bridges on their side. " Big difference is that the bridge stiffness is indicative of the design. This is motivated by the fact that people do not like a wobbly bridge when they walk or drive.

Due to this focus on stiffness, our composite bridges are much stronger than necessary.
At the lock gates, comfort requirement plays a smaller role. The gates can thus be designed much thinner, making the initial costs turn out lower than for a door of steel or wood. In addition there is the benefit in maintenance. Because the gates are expected to last decades longer, without maintenance.

Another big advantage is that the doors can be built weight neutral. This produces far less load on the suspension and movement work. These can then be made considerably lighter as well.

Brochure sluisdeuren in pdf (in Dutch)