Thickness and grain-size distribution of the 2004 Indian Ocean tsunami deposits in Periya Kalapuwa Lagoon, eastern Sri Lanka

Abstract

In this paper we describe the sedimentary characteristics of the 2004 Indian Ocean tsunami deposits in and around Periya Kalapuwa Lagoon, Sri Lanka. Periya Kalapuwa is a coastal lagoon of about 13 km2 area and has an average depth of about 1 m. It is separated from the Indian Ocean by coastal barrier sand dunes of up to 9- m elevation through which two inlet channels open the lagoon to the ocean. This region was hit by three waves during the 2004 Indian Ocean tsunami. The second wave was largest (4–6 m) and entered the lagoon not only via the two inlet channels, but also by flowing over the sand dunes. Erosive scars were found on the sand dunes adjacent to the two inlets. Twenty-seven core samples, along with trenching and hand-auger data, show that the tsunami deposits are 9 cm thick on average (up to 35 cm in the lagoon and up to 66 cm on the shore) and are composed mainly of medium sand (mean grain size 1.06 φ) with low mud content (0.61 wt.%), which is similar to the composition of sand from near the erosive scars in the sand dunes (mean grain size 0.94 φ), but different from the lagoon deposits (mean grain size 1.68 φ; mud content 4.7 wt.%). The distribution of the tsunami deposits was limited to within about 1 km from each inlet. The tsunami deposits become thinner and finer grained with increasing distance from the inlets. Most of the tsunami deposits are massive, but some show sedimentary structures: single or multiple-graded bedding structures, parallel laminations defined by layers of heavy minerals, and muddy laminations. Our observations and analyses suggest that the tsunami deposits were formed mainly from sand eroded from sand dunes near the two inlets. We estimated the total volume of tsunami sediments to be 83 000 m3 . By assuming that the sediments of the tsunami deposits were supplied only by erosion of sand dunes from near the two inlets, this is equivalent to erosion of 83 m3 of sand per meter of sand dune traversed by the tsunami wave.