202
J. Ojeda et al.
The chapter summarizes some of the cartographic results of a study of the spit’s recent
evolution using various sources - maps, charts, air-photos, satellite images - treated
digitally in a GIS context. IDRISI was used for satellite images, Desktop Mapping
System for digital photogrammetry and stereoplotting, and ArcInfo - ArcView 3.0 for
general mapping output and spatial analysis.
Time Scales
Analysis of the spit’s recent evolution has been handled at different time
scales:i)
long term change
Evidence from old maps and charts make it quite clear that the change from a
system of barrier islands to an estuary-spit occurred at the end of the nineteenth
century.
ii)
Decadal time scale
Since its establishment the spit has been subject to rapid longitudinal
progradation. Historical cartography, current maps, and the earliest air photos
integrated geometrically and consistently into a GIS have formed the basis for
quantification. The temporal rates of longitudinal progradation have been very
significant (40-60m/yr.) throughout the period. Progradation has been by the accretion
of beach ridges at the spit’s distal end, and as interpreted from satellite images and air
photos these beach ridges reveal a clear evolution from an alignment perpendicular to
the coast towards a more parallel alignment. This progressive change in orientation
has been interpreted in terms of weakening tidal currents and also in terms of greater
and lesser availability of sediment supply.
Nautical charts with detailed point bathymetry have made it possible to
evaluate the sedimentary balance of the submarine beach (shoreface) between the
reference dates of 1943 and 1981. This was done by generating separate DEMs (TIN
Data Structure) and exporting them to a grid format for analysis of spatial and
volumetric changes (chart differencing).
The volumetric difference between the reference dates gives an annual rate of
which, given the theoretical rate of
of sediment
transport, suggests that the spit incorporates some 45% of the theoretical sediment
transport mobilized by littoral drift. Finally the three dimensional graphs and the
sedimentary balance image show how the volumetric changes reflect, from a spatial
perspective, the change from an estuary model with a single main tidal channel and a
clear associated submarine delta to two tidal channels model and a tidal delta which
extends parallel to the coast.
J. Ojeda et al.
The chapter summarizes some of the cartographic results of a study of the spit’s recent
evolution using various sources - maps, charts, air-photos, satellite images - treated
digitally in a GIS context. IDRISI was used for satellite images, Desktop Mapping
System for digital photogrammetry and stereoplotting, and ArcInfo - ArcView 3.0 for
general mapping output and spatial analysis.
Time Scales
Analysis of the spit’s recent evolution has been handled at different time
scales:i)
long term change
Evidence from old maps and charts make it quite clear that the change from a
system of barrier islands to an estuary-spit occurred at the end of the nineteenth
century.
ii)
Decadal time scale
Since its establishment the spit has been subject to rapid longitudinal
progradation. Historical cartography, current maps, and the earliest air photos
integrated geometrically and consistently into a GIS have formed the basis for
quantification. The temporal rates of longitudinal progradation have been very
significant (40-60m/yr.) throughout the period. Progradation has been by the accretion
of beach ridges at the spit’s distal end, and as interpreted from satellite images and air
photos these beach ridges reveal a clear evolution from an alignment perpendicular to
the coast towards a more parallel alignment. This progressive change in orientation
has been interpreted in terms of weakening tidal currents and also in terms of greater
and lesser availability of sediment supply.
Nautical charts with detailed point bathymetry have made it possible to
evaluate the sedimentary balance of the submarine beach (shoreface) between the
reference dates of 1943 and 1981. This was done by generating separate DEMs (TIN
Data Structure) and exporting them to a grid format for analysis of spatial and
volumetric changes (chart differencing).
The volumetric difference between the reference dates gives an annual rate of
which, given the theoretical rate of
of sediment
transport, suggests that the spit incorporates some 45% of the theoretical sediment
transport mobilized by littoral drift. Finally the three dimensional graphs and the
sedimentary balance image show how the volumetric changes reflect, from a spatial
perspective, the change from an estuary model with a single main tidal channel and a
clear associated submarine delta to two tidal channels model and a tidal delta which
extends parallel to the coast.