Three-stage water safety inspection of drainage windmills

Visual inspection, 3D scanning and crack detection along the Netherlands' most beautiful riversWater safety, experience-related value, nature and innovation all come together in the area around the Vlist and Rotte rivers. This is where RPS was commissioned by the Schieland and Krimpenerwaard district water board to drain three watercourses. In collaboration with Asset Hub, RPS selected various inspection techniques from its extensive toolbox to assess and inspect water safety, the masonry and the flood defence components in this area.

An iconic picture that is typical of the Netherlands past and present. The three drainage windmills standing proudly on the banks of the Vlist and Rotte rivers in this water-rich area managed by the Schieland and Krimpenerwaard district water board. The Vlist in particular is one of the prettiest rivers in the Netherlands: an archetype of a typical Dutch landscape with its busy riverside quays and luscious green banks.

The drainage windmills here date back to the 18th century and were originally built to keep lower areas of reclaimed land dry by ‘scooping up’ the water from those polders. That water is then expelled from the mill at a higher level into a masonry watercourse that runs to a basin. The water is finally discharged into the river from the basin. This task is now carried out by mechanical pumping stations, which pump up the excess water from the lower area of reclaimed land.

Statutory assessment

Although the windmills no longer play an active role in displacing the water, the watercourse is still used. It is an important drainage channel for transporting water away from the polder and intersects with the water defence structure. The Schieland and Krimpenerwaard district water board is responsible for the water defence barrier and thus also for water safety. They asked RPS to survey the condition of the water defence-related sections of all three windmills. This was part of the assessment activities for reviewing the civil engineering structures in the regional water defence barrier.

“Drainage windmills are quite unusual and difficult to accommodate in the flowchart for the statutory assessments of the civil engineering structures in regional water defences. A further snag is that the water board does not have detailed drawings of the watercourses. So the only realistic approach is to close the watercourses and pump them dry,” explains Michel de Vré, the RPS project manager responsible for this project. “After cleaning the walls and floors with a high-pressure washer, we inspected the watercourses for signs of damage such as deformation, subsidence and structural cracks.”


Point cloud

RPS' surveying team created drawings of the watercourses using a 3D scanning technique. 3D scanning is a measurement method that is gaining in popularity and increasingly used across a broad spectrum of applications. Because excellent accuracy and high resolution are among the advantages of using a scanner. Project leader Remko Gulickx explains: “A scanner was placed at various points in the watercourse. We used 5 different set-up locations. The scanner emits pulses of laser light that we track to record millions of points in a 360 degree pattern until we have mapped the entire visible surface.”

A computational program combines all these points to create a point cloud. “We generated the drawings from that point cloud. Because we also took pictures from the set-up locations, adding that information in colour leads to a better visualisation of the measurement”, Gulickx adds.

Objective baseline measurement

As the watercourses had been completely drained, this was also an opportunity to inspect the condition of the masonry in the channel. This area had already been visually assessed by the inspectors of course, but an innovative technique from Asset Hub also made it possible to perform a second, objective baseline assessment - i.e. without the involvement of the inspector - using photographs. The ideal starting point for detecting changes in the crack patterns in future years.

Asset Hub is a partner of RPS and has developed an artificial intelligence-based technique that recognises and measures cracks down to a width as small as 1 mm in masonry. An SLR camera or drone-mounted camera is used to take the photos.

Crack detection

Jeroen Kruithof, project leader at Asset Hub, which is a TNO spin-off, explains the procedure: “The RPS inspectors photographed the walls of the watercourse in line with Asset Hub's instructions. This means that they took a photo every 20 centimetres. Each side of the watercourse required 150 photos. So a total of 900 photos were taken for the three watercourses.”

The photos were automatically merged to create a single high-resolution orthophoto. This is a 2D orthogonal projection of an object to scale, which has been adjusted to eliminate lens distortion, topographic relief and camera tilt. The ideal technique for generating informative views of the masonry surfaces. The analysis of this orthophoto using Asset Hub's deep learning model revealed crack formation. This visual information is accurate to 1 mm.

Kruithof: “The baseline orthophoto allows us to track how crack patterns develop. If a crack is registered but does not change, you can assume that the structure is stable. However, if that crack grows bigger over time, the implications are obviously very different. Based on that information, RPS can provide more targeted advice and the water board can take action more effectively.”


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