Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 83

Storm tide refers to the level of inundation resulting from the storm surge
and tide combined. This is not a simple addition of the tide height plus the
surge height to give this level of inundation as the two do not interact or com-
bine linearly because of the nature of bottom ocean currents. However, the tide
together with the surge produce a greater level of inundation than the surge
alone. Most eyewitness observations of marine inundation during a cyclone are
observations of storm tides not surges, plus any wave set-up and possibly wave
run-up. Wave set-up is the addition to the water column from broken waves and
wave run-up is the uprush of those waves against an object or a sloping surface
such as a beach. Wave set-up is normally regarded to be approximately 10% of the
significant wave height ( H s ) which is the average height of the highest one-third
of waves. Wave run-up can vary considerably depending upon the gradient and
composition of the slope surface that is overwashed. Wave run-up is an impor-
tant component of marine inundation and is discussed in more detail later in
this chapter.
Storm surges and storm tides can be measured directly by tide gauges if
located sufficiently close to the zone of marine inundation. Eyewitness accounts
can also be valuable as a guide to the level of inundation. However, people can
often be under some stress during an intense cyclone and will usually be seeking
shelter -- hence eyewitness observations are not always reliable. Post-marine inun-
dation surveys are valuable in those locations where tide gauges are absent to
determine the height of the inundation and also to measure the impact upon
the coast. Such surveys have been undertaken in Hawaii following Hurricane
Iniki in 1992 (Fletcher et al ., 1995)and also along the Western Australian coast
following several of the category 5 cyclones that occurred between 1998 and
2002 (Nott and Hubbert, 2005). These surveys involve topographically surveying
theheight of debris left by the inundation above the tide level at the time of
theevent at several locations along the coast. The results of one such survey
following Tropical Cyclone Chris (915 hPa central pressure) in January 2002 can
be seen in Table 4.2.
Nott and Hubbert's (2005)survey results highlight two very important aspects.
First, erosion of the sandy coast including sand dunes occurred to the level of the
maximum inundation level not just the level of the surge or storm tide. Second,
it is possible to estimate, through numerical modelling of the surge heights
during the event, the extent of wave run-up as noted in the difference in height
column in Table 4.2.Both of these aspects are important for assessing risk to
coastal communities from storm tides because many emergency strategies along
with building guidelines only take into account the level of a surge or storm
tide during a particular intensity cyclone. So for example the minimum building
level above sea-level will be determined from the estimated height of a storm
tide during various cyclone scenarios. Wave run-up is often not included in