Boulder transport by tsunamis: what happens if we add some sand?

This work presents an overview of two sets of experiments, carried out at the Hydrodynamics Laboratory of the University of Oslo in a small 3 m long and 10 cm wide wave tank, filled with 5 cm of water. The purpose of the experiments was to examine how tsunami-driven transport of boulders is influenced by the presence of sand or any other smaller sediments on the bottom.

The “tsunami” wave input for all three experiments was kept the same and was presented by breaking solitary waves with an amplitude normalized by water depth a/ℎ = 0.5. Generated solitary waves propagated towards a 1:10 beach, which was also kept the same for all experiments.

The boulders were represented by concrete blocks of different shape and size. They were placed alternately (one at a time) at different locations on the beach slope with respect to the wave breaking point.

Experiment 1 was conducted in two set-ups: (i) empty Polymethyl methacrylate (PMMA) bottom of the flume, (ii) the slope covered by a thin layer of 65 μm sand. Transport of boulders and their dynamics was studied with respect to boulder characteristics (size, orientation), their initial position regarding the wave breaking point and inclusion of sediment. It was shown that presence of sediment enhanced boulder transport. In particular, in this set-up, the presence of sediment increased the boulder transport in 2–5 times. The maximum displacement increase was observed for boulders with the smallest length and height and the largest width initially located at the breaking position.

Another result regarded the type of boulder motion. The boulders experienced either sliding or turning over. Boulders whose height was at least twice as large as their length exhibited turning-over. This held for boulders placed both on an empty PMMA slope and on a sedimentary slope. However, the largest boulder displacement on an empty PMMA slope occurred due to turning over, while on a sedimentary slope it occurred due to sliding.

Experiment 2 examined the influence of the thickness and the size of the sediments. In this set of experiments the slope was covered with a thicker layer of sediment (2 cm) compared to the one used in Experiment I, and in addition to the 65 μm sand, coarser sand with a grain size of 250 μm was used. For this 60 cm long, 10 cm wide, and 2 cm deep section was carved out of the slope and filled with either 65 μm sand or 250 μm sand, forming a sandy beach. The results showed that boulders traveled farther over the coarser sand due to reduced friction. Furthermore, the sandy slopes caused the boulders to rotate or turn over.