Monitoring Rockflow in Zevenaar (NL)

Under the car park of a Jumbo supermarket, the system of stone wool blocks captures rainwater and then allows it to gradually infiltrate into the soil. The package of 12 x 28.8 x 0.5m (173m³) is designed for a maximum rainfall of 35.7mm per hour. This package is used to disconnect the roof and the car park of the supermarket, as well as part of the street, from the sewer system.

The total disconnected surface area is 4500m². Due to the high absorption capacity of the subsoil, the system is empty within a day and available for the next rainfall. This is in accordance with the water management regulation.

At the Rockflow system near the Jumbo supermarket in Zevenaar, the groundwater often reaches just below or even partially into the buffer. Nevertheless, the system can infiltrate the collected water within a few hours. This was concluded by the Land Engineering Bureau after analysing several years of measurement data on groundwater, the water level in the Rockflow system, and the amount of precipitation. Below are some findings from the measurement report.

To continuously monitor the filling and emptying behaviour of the system, the following measurements are taken:

• Water level in the buffer using two-level measurements in the stone wool package (B01, B02)
• Groundwater level below the package using a piezometer (G01)
• Groundwater level next to the package using a piezometer (G02)
• Water level, overflow frequency, and overflow height in an existing well behind the Rockflow stone wool package (Overflow Well)
• Amount of precipitation using a rain gauge on the roof of the Jumbo supermarket

The amount of incoming water is estimated from a combination of the precipitation measurement and the catchment area of the system. The average measured permeability coefficient (k-value) is 4.7m/day, despite the presence of a clay layer. The relatively high permeability coefficient (k-value) is probably due to cracks in the clay layer and the presence of sand.

The key conclusions that can be drawn from the measurement results are:

1. the system functions properly; also years after installation;
2. the infiltrating action of the buffer works very well, as seen by the spiky behavior of the groundwater level under the buffer and no level rise in the buffer;
3. good lateral distribution of infiltrating water over the groundwater surface when the groundwater level is in the buffer.

Long before the construction began, the project attracted significant interest from local residents who actively participated in the planning process. Over 200 residents were present at the open consultation round. 

Jan Heemskerk from Jumbo supermarket is enthusiastic about the design and implementation. The parking lot has been expanded and has become more attractive for customers as well as for the local fish shop and snack bar. Less cleaning in the store due to fewer wet feet is a nice bonus for the businesses.

Rockflow offers great design freedom combined with high absorption and infiltration capacity. However, it is nice not to be alone during the design phase. We are not just a material supplier but also a partner and service provider: our advisors think along with you. Thus, the permeability of the ground is included in the design, and your buffer gets the right shape and dimensions for optimal inflow speed, drainage, and maintenance ease. This way, the design will always meet the requirements.