Arjan Tabak

Smart prioritization of public space management

Roël de Leede – Senior Policy Officer for Water and Sewerage at the Municipality of Vlaardingen.

Vlaardingen is a South Holland municipality, which has about 72,000 inhabitants. The municipality is responsible for the management and maintenance of public space, and needs insight into the quality of its properties in order to perform this task properly. Because of its location on largely soft soil, the municipality is experiencing structural subsidence. This has a major impact on the rate of degradation of the public space, resulting in frequent embankments.

1. Underground work

Roël de Leede is employed by the municipality of Vlaardingen as Senior Policy Officer for Water and Sewerage. In this capacity, he is partly responsible for prioritizing the maintenance of sewers and roads. He is also concerned with water management and the consequences of climate change, such as the effects that dry periods have on the municipal environment. His goal is to carry out behind-the-scenes management in such a way as to minimize disruption to the city’s residents.

Maintenance work regularly causes inconvenience to local residents.

2. Insufficient information from subsurface maps

Subsidence plays an important role in the uplift strategy of the municipality of Vlaardingen. Previously, the municipality only had subsurface maps and models available to measure subsidence, fed by low-frequency level measurements. Because of the strong spatial variation in subsidence, however, these maps are not an adequate basis for a data-driven policy. As a result, the municipality was often in the dark and assumptions had to be made in maintenance planning about the amount of subsidence in the past and toward the future. This resulted in high maintenance costs and a greater number of complaints from residents. Amongst other things, settlement can cause broken pipes at home connections or a decline in the quality of the main sewer due to breaks, cracks, or open joints and connections.

“Replacing a meter of sewer in Vlaardingen soon costs €1,000,” Roël said, “so you only want to do that if it’s really necessary.” In addition, the maps offer only an indirect indication of subsidence; they tell nothing, for example, about the effects that water levels have on subsidence.

An old sewer pipe that needed to be replaced.

“Ideally, we replace sewers the day before they collapse.”

3. InSAR data for the municipality

So there is great demand from the municipality for data on the actual subsidence that has occurred. Because, as Roël says, “Ideally, we would replace the sewer the day before it collapses.” With that in mind, Vlaardingen turned to Sensar. With our InSAR satellite measurements, Vlaardingen gets a detailed picture of the subsidence that has occurred since 2015 for the entire municipality and is enabled to make object-level decisions about future maintenance prioritization. In addition, the data provides input for the choice of fill material to be used.

So the Property Department at Vlaardingen also uses the data. Topics here are the value assessment of properties and the identification of houses with foundation problems. Vlaardingen also wants to use this data in the future to focus more on providing information to citizens, as described in the new Environment Act.

4. Drought stress test

To gain better insight into the effects of drought on the subsidence of the municipal area, Vlaardingen, Sensar and consulting firm Wareco joined forces for a drought stress test. This looked specifically at the extremely dry summer of 2018 and the differences in soil subsidence from other summers. This showed that the ground level dropped as much as 5mm extra due to the drought! This effect was not only limited to the sharply declining neighborhoods, but could also be seen in the normally relatively stable parts of the city. In some cases, even the water withdrawal effect of trees could be linked to a higher ground level drop.

Land subsidence in the municipality of Vlaardingen, shown by neighborhood.

“With smarter prioritization of some sewer strands, I’ve already got the cost out of it.”

5. Quick savings

All in all, the use of satellite-based soil subsidence data saves Vlaardingen a lot of money, and also provides better and more proactive services to residents. In particular, the benefit lies in better estimating the replacement timing and early identification of bottlenecks. “With smarter prioritization of a number of sewer lines, I already got the costs out of it,” says Roël. By using our data, the municipality of Vlaardingen now has a data-driven and smart prioritization of public space maintenance.