628 COASTAL ENGINEERING
large number of periodicities in the tidal motion. The time periods of these
periodic constituents are known. Further, tidal flow is strongly influenced
by the Coriolis acceleration, by the sea bed topography and features of
coastlines. It may be amplified by the local resonance effects in bays and
estuaries. Accurate prediction of tidal behaviour at a particular site by
theoretical means is thus not possible. In practice, the measured data at a
site are expressed as a summation of periodic components, the number of
which depends on the duration of the record. Once the periodic com-
ponents have been computed, they are then used in the summation to
predict the water level at the site in the future. In the USA, tide predic-
tions are made by the National Oceanic and Atmospheric Administration;
in the UK, the Admiralty Tide Tables provide information about astro-
nomical tides around the coasts.
Seawater levels are also raised significantly by strong onshore winds
and by suction due to low atmospheric pressure. Apart from generating
waves the wind causes the water surface to assume a slope due to wind
shear, thus forming a storm surge which is further affected by the topogra-
phy of the coastline. The Shore Protection Manual (US Army, 1984)
presents a method of determining the storm surge based on wind shear.
Water level at a particular site depends on tidal and wind conditions.
The design of structures must take into account the tide levels to which
should be added the depth due to storm surge. The combined effects of
high water level and large waves are the cause of coastal flooding or
damage to coastal structures. Observations made along the coast over a
period of time could form a useful basis for statistical analysis to deter-
mine the individual probabilities of wave heights and water levels (Will,
Willis and Smith, 1985). However, in recent times the joint probability of a
given water level with a wave of given height occurring at the same time,
forms the basis for specifying the design conditions for coastal structures.
Seiches are oscillations of water levels in a body of water such as
lakes and reservoirs; the disturbance may be initially caused by sudden
gusts of wind (see also Section 4.4). Tsunamis are caused by submarine
earthquakes. Seiches, harbour oscillations and tsunamis are long period
waves and hence are treated as shallow water waves. Tsunamis generated
in deep sea can grow to a considerable height as they approach the shore
due to shoaling effects (equation (14.34)) and they can cause extensive
flooding, costly damage to property and heavy loss of life along coastal
regions.
Coastal currents may be generated by the density differences in the
seas, wind stresses, tidal flow, rivers entering the sea and waves. Currents
produced by breaking waves are important as far as sediment transport
along the beaches is concerned. River flows discharging into the sea can
cause near shore currents and circulation. Currents caused by the flood and
ebb of the tide and by wind stresses, particularly in shallow water, can lead
to vortex shedding in the wake of immersed structures. As with offshore
