Erosion pits in the Old Meuse and Dordtse Kil rivers

Published: 13 February 2015

It is too easy to conclude that the closure of the Haringvliet inlet is the only factor explaining the erosion pits in the Old Meuse and Dordtse Kil rivers, explains Kees Sloff, a river morphologist with Deltares, at the 'Deltawater now and later' conference.

‘It would have happened anyway, albeit slightly later. The bed of this river consists of clay and sand. Where the clay gets worn away, the sand is exposed. The sand layer erodes faster and, as a result, large pits form quite suddenly. It is true that the flow velocity has increased since the inlet was closed off but the structure of the bed is the main cause of the problem,’ argues Kees.

Predicting new pits on the basis of bed data

The consequences of closing the Haringvliet inlet are becoming increasingly apparent. The question is how we should respond. Deltares has studied bed erosion in the Dutch rivers Old Meuse and Dordste Kil. The first step, which involved mapping out the structure of the bed and erosion as precisely as possible, has now been completed. On that basis, we can make an estimate of where new erosion pits may possibly be formed, when, and what levels of erosion we can expect. And whether the erosion could potentially result in a dike collapse.

Erosion pit in the Spui river

Erosion pit in the Spui river

Management plan under construction

In addition, Deltares supplies the knowledge and instruments needed for the management plan to control erosion and to fill in the pits where necessary or record the locations. Bed monitoring is an important component of the plan because it is not possible to predict exactly where the pits will be formed. However, when the pits are formed, there is enough time to intervene. This has been demonstrated by the study of existing pits conducted by Deltares. In this way, we can get to grips better with how the rivers behave and limit the risk of dike collapse.