Tracing the Recent Evolution of a Littoral Spit
203
iii)
Annual temporal scale
Analysis has concentrated on the years 1984 - 1996 in an attempt to trace the evolution
of the coastline (backshore/foreshore) in the emerged spit and its associated beaches
(using air photographs and digital steroplotting in Desktop Mapping System 4.0) and
in the sandy intertidal sediments, in effect tidal deltas, (using satellite images and
digital image processing in IDRISI).
The results are very interesting in that the evolution of the photo-interpreted
coastline shows a change in behaviour. The emerged spit undergoes minimal
morphological change and practically ceases its longitudinal progradation whilst the
adjacent beaches, which previously moved in step with the spit’s prolongation continue
their eastward migration (90 m/yr.), although this is no longer directly related to the
progradation of the emerged spit.
The westward migration of the beaches is now explained in terms of the
intertidal sediments. The chapter covers the evolution of these intertidal sand bodies
from 1989-1996 in a set of enhanced TM Landsat images that present two conclusions.
i) the intertidal sediments have continued their eastward progradation at rates
similar to the adjacent beaches providing shelter from the dominant southwesterly wave action and augmenting the beach accumulation processes
ii) there is a clear cyclicity in progress
* In the dry years (1989-1994) with low storm evidence and low river discharges, the
intertidal sediments move eastward, progressively deflecting the main channels until
they are parallel to the coast and only one main tidal channel operates in the mouth.
* During the wet phase 1994-1996 with high storm incidence and high river
discharges the intertidal sediments are broken up and a second tidal channel is
established which becomes hydraulically more efficient than the old channel which is
gradually reduced. In this way, part of the intertidal sediment is passed beyond the
estuary mouth and continues its eastward migration driven by the littoral drift.
Key points for further investigation
what happened at the end of the 19th century?
what is the relationship between the evolution of the spit and the changing
orientation of the coast at its current distal point?
203
iii)
Annual temporal scale
Analysis has concentrated on the years 1984 - 1996 in an attempt to trace the evolution
of the coastline (backshore/foreshore) in the emerged spit and its associated beaches
(using air photographs and digital steroplotting in Desktop Mapping System 4.0) and
in the sandy intertidal sediments, in effect tidal deltas, (using satellite images and
digital image processing in IDRISI).
The results are very interesting in that the evolution of the photo-interpreted
coastline shows a change in behaviour. The emerged spit undergoes minimal
morphological change and practically ceases its longitudinal progradation whilst the
adjacent beaches, which previously moved in step with the spit’s prolongation continue
their eastward migration (90 m/yr.), although this is no longer directly related to the
progradation of the emerged spit.
The westward migration of the beaches is now explained in terms of the
intertidal sediments. The chapter covers the evolution of these intertidal sand bodies
from 1989-1996 in a set of enhanced TM Landsat images that present two conclusions.
i) the intertidal sediments have continued their eastward progradation at rates
similar to the adjacent beaches providing shelter from the dominant southwesterly wave action and augmenting the beach accumulation processes
ii) there is a clear cyclicity in progress
* In the dry years (1989-1994) with low storm evidence and low river discharges, the
intertidal sediments move eastward, progressively deflecting the main channels until
they are parallel to the coast and only one main tidal channel operates in the mouth.
* During the wet phase 1994-1996 with high storm incidence and high river
discharges the intertidal sediments are broken up and a second tidal channel is
established which becomes hydraulically more efficient than the old channel which is
gradually reduced. In this way, part of the intertidal sediment is passed beyond the
estuary mouth and continues its eastward migration driven by the littoral drift.
Key points for further investigation
what happened at the end of the 19th century?
what is the relationship between the evolution of the spit and the changing
orientation of the coast at its current distal point?