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Natural processes that operate (and may
interact) in the geographic environment.
Geographic environment = Tasman Bay coastline between Separation Point
and Cable bay.
Natural Process – Wave action
The Tasman Bay coast is dominated by the action of waves. Beaches are like
coastal defence mechanisms. When wave energies are low the beaches of
Tasman Bay generally build up, become steeper, and have a well defined
berm. Constructive waves (fig. 1) are dominant (longer wave period and not
as steep). Because of this longer wave period the backward movement of the
water (backwash) does not interfere with the water movement up the beach
which is called swash. This means that swash is greater than backwash and
more sediment is carried up the beach than back to sea.
Fig. 1
When wave energies are high during storms destructive wave (fig. 2) action
dominates. The wave period is much shorter and so backwash meets swash
before it has a chance to carry sediment landward resulting in a seaward
migration of sediment which is often stored in off shore bars. Little or no berm
is present and waves can travel up the beach at high tides eroding the
foredune. In Tasman bay these destructive waves come from the northerly
quarter where the fetch is more open (fig. 3).This allows the wind greater time
to travel over the water creating bigger and more frequent swells which in turn
break into waves in the shallow waters of Tasman Bay.
Fig. 2
Fig.3
Destructive Waves causing beach erosion Tahunanui Beach – 1st October
2000
Most erosion of Tasman Bay beaches occur when a storm surge and high
tides occur at the same time (fig. 4) A storm surge occurs when there is an
intense low pressure system out to sea. The low pressure means that air is
rising very rapidly which lifts the level of the sea. The incoming air associated
with the low pressure systems also generates large swell waves. When this
storm surge meets the coast at high tide destructive waves are able to travel
well inland eroding coastlines previously above high water.
Fig. 4
Tahunanui Beach Storm Surge 19th March 1957 – 4.82m tide