The recent tsunami affected up to two thirds of the entire coastline of Sri Lanka, changing it and its bathymetry. There is now an absolute and high-priority necessity for assessment of the amount of debris washed out to sea, and any changes in seabed topography which may have a definitely harmful affect upon coastal marine environment and safety of navigation. The NHO was at the time the tsunami struck already conducting a national nautical charting programme to produce nautical charts and bathymetric data for user-oriented format. This programme is now paralysed.

Two Challenges
Challenges facing the NHO are twofold. Firstly, there is the urgent need to re-establish NHO capability to conduct hydrographic surveys in near-shore and offshore areas. Germany has already provided assistance through the Federal Maritime Hydrographic Agency (BSH) in terms of equipment, thereby allowing replacement of lost equipment. The importance of lending assistance to the NHO of Sri Lanka was identified at the North Indian Ocean Hydrographic Commission on 3rd February 2005 and at the special tsunami session held in Monaco at the 3rd Extra Ordinary Hydrographic Commission on 26th April 2005. During this session, member states Germany, UK, Norway, India, Pakistan and France agreed to offer Sri Lanka various forms of assistance in hydrography.

Secondly, there is an equally urgent need for immediate survey of tsunami-affected areas. These immediate surveys are necessary both for safety of navigation and to gain insight into the post-tsunami coastal marine environment. Accurate coastal bathymetry may enable local authorities to forecast height and wave length of tsunami waves. In the absence of coastal bathymetry it is difficult to obtain useable output for any proposed and effective global warning mechanism. Coastal bathymetry is also an essential component in zoning decisions and in equitable use of coastal space. Both onshore and offshore mapping is necessary to determine energy dissipation at the coast and for new development of the relevant land areas. Such coastal surveys are therefore important for the success of both warning systems and rehabilitation programmes.

Choice of Method
There are two options that may be adopted for collection of bathymetric data: ship-borne bathymetric survey and airborne survey. Single-beam echo sounder and multi-beam echo sounder can be utilised in ship-borne bathymetric survey but compared to air-borne Lidar survey this method has disadvantages. It is time consuming and does not achieve 100% coverage of the seabed; the advantages include high reliability in achieving required accuracy, and comparatively low cost. The air-borne Lidar-system method has disadvantages in terms of high cost of operation and processing and lower reliability in respect of accuracy. The turbulent weather conditions of the waters of Sri Lanka may prove unfavourable for achieving required accuracy. The most advantageous factor in air-borne survey is far less time-consuming field operation.

By compromising all factors relating to advantages and disadvantages of both methods the NHO feels that their combined operation will provide maximum results from surveys. The NHO thus intends to carry out ship-borne bathymetric survey for main harbours and other areas requiring more detailed bathymetric information, and air-borne survey to cover other areas on land and offshore.