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How far inland?
A Peradeniya University survey maps the extent of tsunami inundation
By Dr. Janaka J. Wijetunge
Nature has just reminded us rather forcefully that the waterline at the beach is not at all static, but very much a dynamic boundary between the land and the ocean; it moves back and forth with the tide, occasionally with the sea-storms, and extremely rarely with what we have just witnessed, the tsunamis. As we all painfully remember, on December 26, 2004, the waterline at the beach travelled several hundred metres inland as a wall of water with great destructive power inundating the low-lying coastal areas and causing considerable loss of life and property.

The enormity of the tragedy has awakened us all to the potential vulnerability of the coastal belt of our country to the forces of nature, especially to what goes on in the huge expanse of the Indian Ocean around us.

All of these tell us that we should not probably live far too close to the land-sea boundary, and that a little buffer zone is perhaps necessary, wherever possible. One question then is, how wide should the buffer zone be; the other is, how do we identify low-hazard site areas for rebuilding in the aftermath of the tsunami?

Therefore, a detailed assessment of the onshore distance within which significant damage has been caused by the recent tsunami may provide some helpful guidance to identify, among other things, high-hazard as well as low-hazard site areas. Obviously, such field data should be supplemented with further inundation information from mathematical simulation of other possible scenarios of tsunami threat. In other words, we ought to prepare inundation maps, indicating the extent of the coastal strip that would be significantly affected by tsunami attack, preferably for different recurrence intervals. With this long-term objective in mind, the Civil Engineering Department of the Peradeniya University recently carried out a tsunami inundation and run-up height survey at nearly 25 locations around the island and recorded eyewitness accounts of the tsunami overland flow.

The study covered the east coast from Mavadiodai through Batticaloa, Kalmunai and Akkarapattu down to Pottuvil; Jaffna Peninsula from Thondamannar to Manalkadu; and the west and the south coasts from Kalutara to Hambantota. The measurements at the selected locations of the damaged coastline around the island included tsunami height at the beach as well as the horizontal inundation distance, that is, how far the wall of water had travelled inland causing significant damage.

As it is not possible to make such inundation measurements at each and every kilometre along the coastline, the primary objective of the field survey was to verify the reliability of the tsunami inundation data that could be obtained from satellite images, for example, ESRI vector data. Fortunately, the inundation measurements at almost all the selected locations were found to match well with those derived from the ESRI data. Therefore, it appears that the inundation and damage shown on ESRI satellite images are sufficiently reliable to prepare a preliminary tsunami inundation map for the country.

The sketch shows the probable tsunami inundation distances obtained in this way for the south and the south-west coasts as well as for the north and the east coasts. Each of the four plots gives the probable horizontal inundation distance in metres from the shoreline against the latitude or the longitude of that part of the coastline. The proposed buffer zones of 100-metre for the south and the west coasts and 200-metre for the north and the east coasts are also marked on each plot as there has been considerable discussion and debate about the same.

What we see first is that, the tsunami inundation had been greater for the east coast than the south and the west coasts, barring a few locations like the area around Hambantota. This was because: (a) the earthquake that created the tsunami occurred just about 1000 kilometres east to south-east of Sri Lanka along the Andaman-Nicobar-Northern Sumatra line, so the east coast had the tsunami waves rolling in almost head-on compared to most parts of the south and the west coasts which only had relatively low energy tsunami waves spilling or leaking into the shadow zone, and, (b) the north and the east coasts generally consist of long, low-lying stretches of flat coastal lands compared to the rest of the country's coastal belt.

North and East coasts
On the north and the east coasts, tsunami inundation had been quite extensive at several locations along the coastline from Vakarai down to Batticaloa-Kalmunai, and along an eight-kilometre stretch south of Point Pedro from Tumpalai, Vallipuram down to Kudattanai, as well as around Mullaitivu. In the Jaffna Peninsula, we see particularly greater inundation, reaching a maximum of nearly 1800 m onshore, in the vicinity of the lagoon entrance at Tondamannar.

South coast
The deepest tsunami wave penetration in the south coast is at Hambantota, over two kilometres near the salt-pans. Significant inundation can also be seen further east of Hambantota in the Kirinda-Usangoda-Palatupana stretch, particularly around the smaller lagoons and bays. Hungama-Tangalle beach to the west of Hambantota too had recorded notable inundation, especially where tsunami surge waves had been conveyed inland through such water bodies opening to the sea as lagoons and lakes.

The inundation plot for the south coast shows a stretch of the shoreline without significant tsunami damage between the longitudes 80.550 to 80.750, to the west of Dickwella. This was not because the tsunami wave heights were low, but because the coastal lands there are at a comparatively higher elevation with steep beach slopes.

South-West coast
The coastline from Matara to Galle too suffered badly with particularly deep inundation occurring along the coastal belt of Ahangama-Koggala-Talpe, besides the tragic loss of life and destruction in the densely populated coastal cities of Galle and Matara.

On the south-west coast, we see a lessening of tsunami inundation, albeit with intermittent peaks, as we go from Galle to Kalutara and further up. Predominant inundation peaks on this stretch of the coast appear near Paraliya-Telwatte, Akurala, and Ahungalla points.

Post-tsunami land use
Finally, it must be added that it is neither economically feasible to neutralise the potential damage due to tsunami totally through engineering interventions such as building of giant seawalls, nor is it socially acceptable to exclude people and buildings entirely from the damage prone areas.

Therefore, a key attribute of any tsunami mitigation plan should obviously be a balanced management approach that can achieve a compromise between the above two competing factors after giving due consideration to hazard risks and likely damages, as well as the severity and the likely frequency of recurrence of such events.

(The writer is Senior Lecturer, Faculty of Engineering, University of Peradeniya)

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