France: In the Netherlands, local authorities are looking to satellite observations as a promising option for dike monitoring and to protect against dike failures.
Cut off from the Wadden Sea by a causeway, the shallow IJsselmeer lake is lined by dikes that protect the surrounding land from rising water levels. In a recently completed Terrafirma study, the IJsselmeer dikes were checked using Envisat radar data for the period 2003–10.
Results show that a large stretch of the dike near the town of Medemblik is subsiding up to 5 mm per year.
These numbers are normal for dikes with recent reconstruction works. Other areas also show some subsidence, but overall the dikes appear stable.
The study was just the first step in a project that combines satellite data, information on subsurface structure and the knowledge of dike managers for the early detection of weak spots in dikes.
Dike management authorities Wetterskip Fryslân and Hoogheemraadschap Hollands Noorderkwartier are enthusiastic about the first results generated by the project.
“For us, the benefit of satellite monitoring is that it tracks down small anomalies that we can’t see with the naked eye,” said Fryslân water authority manager Harry Boon.
“This is a highly promising approach, and a valuable addition to our usual physical inspections.”
To track movement, scientists use Synthetic Aperture Radar Interferometry (InSAR) – a remote sensing technique where two or more radar images over the same area are combined to detect surface changes occurring between acquisitions. This provides deformation data at centimetre precision.
In a more recent way of processing satellite radar images called Persistent Scatterer Interferometry (PSI), movements over wide areas can be detected and monitored with even greater sensitivity.
PSI typically works best with hard structures such as buildings, roads, railways or dikes with a hard cover.
This technique is therefore particularly useful in monitoring stability and tracing weak spots in dikes with a hard cover, like stone or asphalt.
Roald van Gameren, a dike manager with Hoogheemraadschap Hollands Noorderkwartier, said that the technology has already proven its worth during a previous study looking at the Hondsbossche en Pettemer Sea Dike.
“The results show that satellite observations are very useful for designing dike upgrades, for testing dikes and for detecting instability. The results also show that the data density is still too low for practical purposes when it comes to grass dikes. We see this as an avenue for improvements to the technology,” he added.
In 2013, ESA plans to launch the first satellite of the Sentinel missions. Owing to the Sentinels’ high revisit rate over the Netherlands, more data will be available on the dikes.