WATER BOARD THE STICHTSE RIJNLANDEN

Data-driven preliminary design for dike reinforcement Strong Lekdijk

a) HSDR

• Water board in central Netherlands
• Responsible for water level management, surface water quality and dike safety

Hoogheemraadschap De Stichtse Rijnlanden (HDSR) is using Sensar’s DikeScan to gain insight into the historical settlement rate per dike section. This allows the preliminary design to be determined smarter and at a higher level of detail for the dike reinforcement of the Sterke Lekdijk, part of the HWBP (High Water Protection Program).

2. Coarse picture from subsurface models

Soil conditions on and around a levee are of great importance in a reinforcement project. If future settlement can be accurately estimated, it can be taken into account in the preliminary design phase. In this way, it can be ensured that the dike will remain at a sufficient height and provide protection against high-water scenarios, even in the distant future. For this estimate of future settlement, HDSR previously worked with the subsidence map of the Department of Public Works; a relatively rough map based on publicly available data. This does not allow a distinction to be made between dikes and the hinterland, while there is a great deal of variation in the structure of the subsurface around the rivers.

Rijkswaterstaat’s subsidence map, as previously used by HDSR.

• Reinforce 55km of primary flood barrier between Amerongen and Schoonhoven
• Protects a large part of the Randstad against flooding from the Lek and Nederrijn rivers.
• Expected work: 2021-2029

Therefore, this map is often not accurate enough. Moreover, this interpolated map provides only an indirect indication of settlement based on soil type, leaving out other possible causes. As a result, the Water Authority had to use a conservative settlement of 0.5 to 1 cm/year over the entire section, even though large sections of the dike were constructed on historic sand ridges. A refinement could save a lot of money and prevent unnecessary inconvenience to the surrounding area.

“We can now determine the best approach for each dike segment. That saves a lot of money.”

c) HWBP

• National approach to reinforcing 1300km of primary flood defenses before 2050
• Goal: comply with recalibrated safety standards in the renewed Water Act

3. Insight per dike segment

For this reason, HDSR turned to Sensar. Our DikeScan is based on InSAR satellite measurements and provides the Water Authority with detailed and accurate information about the historical and current settlement rate of the Lekdijk and immediate surroundings. If necessary, it can even look back to 1992. The DijkScan provides direct input for estimating the future subsidence of the dike and thus the parameters to be taken into account in the preliminary design. “With this method we can determine the best approach per dike segment and that makes a big difference in costs,” says Waldo Molendijk, contract manager Sterke Lekdijk.

Historical settlements on part of the Lekdijk, visualized in DikeScan per dike segment as 25th percentile value.

4. Less settlement than expected

A section of the Lek Dike of Hoogheemraadschap De Stichtse Rijnlanden.

Whereas the district water authority initially took into account 1 cm/year of settlement along the entire length of the dike, the DikeScan showed that this settlement had not actually been achieved in the past for any of the dike segments. As a result, the requirements could be eased on many sections. Thus, the dike did not have to be raised nearly everywhere, resulting in major cost savings. In addition, HDSR used our dataset to map the reference situation of the surrounding properties. This allows any deformations of properties during construction to be put into perspective.

5. Significant savings

HDSR estimates that the use of satellite data has resulted in total savings of more than one million euros. This is due in part to a smarter definition of the approach per dike segment and better risk hedging toward the future. In addition to the monetary returns, this also provides environmental savings. Through less use of space, less use of materials and therefore less construction activity, CO2 emissions are reduced on the project. Thus, satellite data contributes to both economics and sustainability.

“Fellow water boards, take advantage of it!” says Waldo Molendijk (contract manager Sterke Lekdijk).