Scientists who went to Sri Lanka immediately after the 26 December 2004 tsunami say coastal development removed natural protection and led to greater damage.
Publishing their observations today (10 June) in Science, the researchers also highlight the importance of education to limit deaths from natural disasters.
The researchers, from the United States and New Zealand, worked with four Sri Lankan scientists to survey the island's east and south-west coasts.
By recording debris and watermarks, and also eyewitness accounts, the team built up a picture of how high the tsunami waves were and how far inland they reached.
At Sri Lanka's southern tip of, near Galle, the first wave — just one metre high — arrived at about 9.10 a.m. It was followed, ten minutes later, by a wave about ten metres high.
Eyewitnesses reported a third wave on the western coast, north of Galle, several hours later. The researchers suggest this might have been a wave that reached India or the Maldives and 'bounced back'.
On the east coast of Sri Lanka, the first wave was also about one metre high. The sea then drew back by several hundred metres, before the second, larger wave hit.
The researchers illustrate the importance of education with the example of a villager who had previously witnessed a tsunami in Chile.
Recognising the withdrawal of the sea, he urged fellow residents to run to higher ground, and most did so.
"In this village of a few hundred, only one died," write the researchers.
They also give examples of places where coastal development had led to greater damage.
For instance, in an area where substantial coral mining linked to tourism development had occurred, the tsunami rose up to eight metres above sea level.
In the town of Yala, one tourist resort had removed part of a sand dune in front of the resort, to have better sea views. The hotel was destroyed by the waves, which were higher and caused greater damage there than in the neighbouring areas sheltered by the dunes.
A second paper, also published in Science today (10 June), describes the waves that hit Banda Aceh and neighbouring Lhoknga, in Indonesia.
Using similar methods to the Sri Lankan team, Jose Borrero, of the University of Southern California, United States, found that land was submerged beneath nine metres of water in Banda Aceh. The water reached three to four kilometres inland.
In Lhoknga, the water was more than 15 metres deep at the shoreline.
The tsunami moved Banda Aceh's shoreline inland by close to 1.5 kilometres, writes Borrero.
Read more about tsunamis in SciDev.Net's tsunami update.
References: Science 308, 1595 (2005), Science 308, 1596 (2005)