Smart coastal management takes sandbar positions into account

Published: 14 June 2016

On 17 June 2016, Deltares researcher Dirk-Jan Walstra will be awarded a doctorate for his research into the anatomy of (submerged) sandbars off the coast. Walstra’s study, entitled ‘On the anatomy of nearshore sandbars’, offers insight into sandbanks’ behaviour and how we can reinforce our coastlines through the targeted positioning of underwater sand deposits.

In the Netherlands, we already have many years of experience using sand replenishment to preserve our coastline. One way to do this is by depositing sand on the beach itself, but since the early 1990s, we are increasingly depositing sand off the coast, under water. This is cheaper than beach nourishment, meaning that we can deposit a larger volume of sand with the same budget. Another advantage of this approach is that the beach is no longer disrupted by the work.

Underwater sandbanks wander away from the coast

However, over time, submerged sandbanks start to ‘wander away’ from the coast. Existing sandbanks in the littoral zone have a net lifespan of 3 to 15 years. Incoming waves and longshore drift also have a number of effects on sandbanks’ behaviour. Strong, wave-driven longshore currents result in a net growth of the sandbanks, while in deeper waters, the absence of such littoral drifts causes sandbanks to erode. Walstra has analysed the behaviour of the submerged sandbanks with the aid of model calculations. Since underwater sand replenishment and sandbanks influence one another, this newly-obtained knowledge can be used to improve the design of the deposits.

This image shows how the profile develops over a period of 10 years. Looking from left to right, one can clearly see the gradual seaward movement of the sandbanks. The sandbanks first form near the beach, then increase in size in the littoral zone and finally gradually flatten out in deeper waters just beyond the littoral zone.

The next step

Walstra: “For my dissertation, I studied the behaviour of the sandbanks. The next step for me will be to take a targeted look at the designs of the sand deposits. I intend to apply this model to a variety of deposit designs, in order to identify opportunities for increasing efficiency. In the Netherlands, most sand replenishment is handled by Rijkswaterstaat. Looking abroad, I believe this research could be put to good use in Florida, for example, since most of the coastal management there is based on offshore sand deposits too.”