Major differences in flow velocity and positive feedback effect explain large scour holes

Published: 5 June 2020

Major transverse differences in horizontal flow velocity can accelerate the development of large erosion pits, or 'scour holes', explains Yorick Broekema, a Deltares expert, in his PhD thesis. The focus of his study was the ongoing development of large scour holes adjacent to the bed protection near the Eastern Scheldt storm surge barrier.

Rijkswaterstaat monitors the formation of these holes intensively and, where necessary, deposits steel slag or rock to maintain the stability of the barrier. Broekema’s study represents an important step towards an enduring solution and the prevention of scour-hole formation.

Datasets and experimental studies show flow behaviour

It was found that flow velocities were stronger locally than had been assumed during the design of the barrier. Detailed investigations to determine the cause were necessary. Broekema went to work with the numerous datasets describing flow rates at different depths and distances around the barrier. He converted the essential data into an experimental set-up in a flow flume at Delft University of Technology.

Expertimental research in the flow flume by Yorick Broekema

Expertimental research in the flume by Yorick Broekema.

By combining the data and the experiments, he discovered that the large transverse differences in horizontal flow velocities led in some cases to relatively high flow velocities and high loads along the bed of the pit. This effect turned out to be partly dependent on the increase in flow depth at the scour hole, and it could therefore result in a positive feedback loop. ‘The data showed that there was a lot of variation in flow rates at the different locations’, commented Broekema, who was awarded his PhD at Delft University of Technology for this study last week. It turned out that these differences in flow velocity near a local increase in depth can cause the flow to contract horizontally. This contraction depends on the increase in depth and it can lead to the flow velocity remaining relatively high. In some cases, the flow can then also follow the contours of the bed on relatively steep slopes, resulting in relatively high loads along the bed and the holes getting larger quickly.  ‘The experiments showed that, even in the case of a very steep 1:2 slope, the flow continues to follow the bed contours. It has never been demonstrated before that flows can continue to follow the bed contours at such steep slopes and that this effect, which could be self-reinforcing in the case of scour hole development, occurs.’

Extension of morphodynamic knowledge base

The research represents a fundamental extension of the knowledge base in the area of morphodynamics and flow theory in general. The findings are not only relevant in terms of arriving at an enduring solution for scour-hole formation near the Eastern Scheldt storm surge barrier. They are also applicable to other infrastructure where strong horizontal differences in flow velocity occur, examples being the new design of the Afsluitdijk barrier dam or a passage in the Brouwersdam. In addition, this phenomenon could also be a feature in natural situations, for example where rivers meet large lakes or the sea. Or in the case of flows along coral reefs, as one of the examination committee members suggested during Broekema’s defence.

Feeding numerical models with results

The next step will be to input the research results into numerical models, rapid assessment tools and design guidelines so that they can easily be included in the calculation of new structures or be used to deploy adequate measures for the protection of existing structures.