Like any other wave tsunamis start to loose energy when they start to reach shore. Part of the wave energy is reflected offshore. Even though tsunamis loose their energy they still have enough to destroy what they can get to.
Tide gauges measure the night of the Sea surface and are used for tide levels. Many of tide gauges are operated by the Bureau of Meteorology's National Tidal Centre are SEAFRAME stations also known as (Sea level fine Resolution Acoustic measuring equipment).
Over 200,000 people have lost their lives during a tsunami in the Indian Ocean on the 26th of December 2004. The tsunami was caused by an underwater earthquake.
The waves devastated the shores of parts of Indonesia, Sri Lanka, India, Thailand and other countries with waves huge waves that reach up to 15 m high, reaching as far as Somalia near the east coast of Africa, 4500 km west of the epicentre. The amount of waves meant that the impact of the tsunami was noticed around the world and sea-level mo stations that were monitoring places such as Brazil and Queensland that also felt the effect of the tsunami.