MECHANISM OF BOULDER TRANSPORT DURING EXTREME WAVE EVENTS ALONG THE EASTERN FLANK OF THE ARABIAN SEA

Abstract

The study presents hydrodynamic mechanism of boulder transportation along the rocky coastal margins with a case study from the eastern flank of the Arabian Sea. Limestone blocks ranging in size from small (1.3, 0.95, 0.35) m to large (2.9, 1.68, 0.5)m scattered over a rocky platform were considered for the approximation of wave energy. The boulders with prominent imbrications indicative of flow direction were used to assess the minimum flow velocity to initiate boulder transport during extreme wave events. The irregular /polyhedral boulders, previously a part of sub-aerially exposed jointed limestone strata, were detached and transported from the rocky coastal platform under the impact of high energy waves. We calculated velocity and height for dynamic waves required to transport boulders onshore using boulder dimensions i.e., length (a-axis), width (b-axis), and height (c-axis). By applying hydrodynamic equations, it is deduced that the average wave velocity required to transport boulders of such dimensions onshore must be ≥7.28m/s and the corresponding average wave heights must be ≥5.51m and ≥1.38m in case of the storm and tsunami events respectively. Based on largest clast recorded, ≈7m high storm waves and ≈2m high tsunami waves are capable of transporting ≈2.5m3 tabular boulders along rocky coastal margins. Since this region has documented records of very-severe cyclonic storms affecting the coastal landscape in addition to the widespread impact of 1945 tsunami, we suggest that boulders deposition on the eastern flank had received inputs from storm as well as tsunami events for the last few decades.