Transport of large boulders by waves in the Brazilian coast: challenge for the engineering, reality for the geomorphology

Abstract

Abstract: Were built approximately 300 km of breakwater in Brazil to protect the littoral from the storm waves, according to the measurements made in this work.  On the exposed and semi-exposed coast, breakwaters and groynes are going through metamorphosis due to the waves` impact on the structures, and some of them were destroyed. It was proved because some of those structures present cuspids shaped by the storm waves transporting large boulders. The majority of the breakwaters don’t have cuspid, however, the rock fragments transportation is intense, and   

the processes to change the boulders' shape from angular to rounded are weakening the structurers. Some are changing into boulder beaches. In the breakwaters of Marina da Gloria and Santos Dumont Airport, situated in the Guanabara Bay, waves over 2 m in height can occasionally transport boulders as heavy as 3650 kg in a distance of 10 m to the top of the breakwater, 4 m above the sea level.  Large boulders close to 1000 kg were carried for 60 m. On the exposed coast, the transportation rate is higher because the waves are bigger, and their frequency is higher as well. In the period 2016-2023 there were produced 96 storms that generated waves (Hm) from 4 to 8 m in height, registered by buoys of the SiMCoast system. In the Saquarema breakwater, boulders weighing up to 5400 kg were deposited 3 m high and 23 m from the shore by waves from 4 to 7 m in height. Boulders with barnacle and oyster encrustation lied above the maximum high tide level were used as methodology to measure the transportation. The rock fragments' movements were also monitored with photos taken from the same spot at the same angles at different times. The results can be used to improve coastal engineering projects, and to better understand the dynamics of boulder beach on the Brazilian coast.