coastal
Juyana
djacent
maps.
sistant
meters
hanged
y years.
ies and
to estimate coastal changes in Guyana. The detail use of SAR
images for coastal zone mapping is described by Singhroy (1996).
The study area extends for 100 km of coastline between the
Demerara and Berbice rivers, which is one fifth of the entire
Guyanese coastline. This area is drained by the Demerara, Berbice,
Abary, Mahaica and Mahaicony rivers. Currently, drainage and
irrigation development projects have considerably reduced this
overbank flooding of the major rivers, which results from the
combined effect of the average annual rainfall of 200 cm and the
tidal influences.
The geology, geomorphology, and agricultural land use of coastal
Guyana are described in several publications (Bleackley 1957,
Daniel 1986, Singhroy and Bruce 1983, and others).
Topographically, the whole coastal zone has been mapped in detail
at 1:50,000 by the British Ordnance Survey. The study area, 100 by
50 km, is below sea level, with gradual rise of one to four meters in
areas covered by beach deposits. Approximately 30 km from the
coastline, the land rises to 10 m, above sea level, in the white sand
region.
The surficial sediments of the coastal plain include mud flats, fluvio-
littoral sand ridges and aprons, alluvial silt and clay, and pegasse
(organic) accumulations. These deposits are underlain by recent soft
marine fossiliferous clay. The original vegetation was once tropical
forest, most of which has been cleared for agriculture and settlement.
Mangrove forest fringes the tidal flats. The major commercial
agricultural crops include sugar, rice and coconuts. Secondary
agricultural products are fruits, market vegetables and livestock.
Based on the recommended guidelines on the use of RADARSAT
images for geological mapping, described by Singhroy and St-Jean
(1997), a Standard 7 RADARSAT with incidence from 45-49
degrees was used to estimate coastal changes. Because of permanent
cloud cover there were no optical images of the entire Guyana
coastline. The Standard beam mode 7 of RADARSAT with a low
incidence angle are particularly useful for mapping these subtle
geomorphic expressions.
2.1 Coastal Processes
Between 1990 and 1994, there were several cases where the dykes
and dams were eroded and broken by coastal storms resulting in
severe flooding. For instance, in 1990, there were 49 breaches to the
coastal structures. Assessing the risk and damage from coastal
flooding requires detailed geomorphological and land use
information, which does not exist in Guyana at this time. Coastal
erosion and shoreline recession has been recorded over the past 200
years at rates varying between five and 20 metres a year (Cambers et
al. 1994). This may be true where specific recordings were taken,
but regional estimates from remote sensing techniques show a larger
change. A comparison between the 1992 SAR images and the 1972
topographic maps shows that parts of the coast have been subjected
to severe erosion and accretion and some parts remain unchanged.
Our estimate shows that the shoreline has retreated to a maximum of
a half of a kilometre in pockets between the Demerara and Mahaica
rivers and between the Abary and Berbice rivers, over the past 20
years.
The eastern bank of the Berbice river is experiencing a considerable
deposition and regeneration of mangroves. Mudflat deposition and
mangrove growth have resulted in an increase of the shoreline
toward the sea, by an average of half of a kilometre (Figure 1).
Pastakia (1991) recommended that the planting of mangroves to
extend the existing mangal seaward, would further stabilise some of
the shoreline. Mangroves are the constituent plants of tropical
intertidal forest. These salt tolerant (halophytic) plants are
dominated by trees and shrubs (one to six metres), which help to
protect the coast from erosion by waves, and their root structures
promote accretion of silt and mud.
The entire coastline of Guyana was fringed with mangrove forest.
Now as shown from the RADARSAT images, all the areas of severe
coastal erosion have no mangrove forest. To reduce the cost of sea
defence maintenance, serious consideration is given to the
management of the mangrove forest, an as such the estimates of
coastal changes are essential.
3. COASTAL CHANGE IN NORTH EAST BRAZIL
Using a 1996 RADARSAT Standard mode image (Beam S7, 45
- 49 degrees incidence) and 1971 topographic maps, we estimate
that parts of shoreline north east Brazil have been eroded from
150 - 300 meters over the past 25 years. This rate of erosion is
increasing in recent years with serious damage to recreation and
coastal infrastructure. In these flat shorelines, the RADARSAT
S7 image is particularly useful to map areas of erosion, as well
as the adjacent affected coastal land use. Because of the
permanent cloud cover in these coastal areas, other optical data
is not available.
The study area extends for about 29 km to the south of Jodo
: Pessoa. Coastal erosion problems in Jodo Pessoa region, and
other areas in the Brazilian north east is affecting tourism and
urban development. In this region, the tropical forest was cleared
for plantation agricultural such as sugar cane and coconuts.
Rudimentary fishing is practised by the local population. The
coast is protected only by reefs. Recently, coastal erosion has
been intensified resulting in damage to property and
infrastructure
3.1 Coastal Processes
Coastal erosion and deposition processes are controlled by local
geology and geomorphology. Recently, the increasing frequency
and intensity of recent storm events, which occurs during high
tides, is accelerating the erosion process. The geomorphology of
the shoreline include eroded steep Tertiary-quaternary sandstone
cliffs; flat quaternary marine and alluvial deposits with
mangroves and spits. For instance, the areas of intense erosion,
such as the Gramame and Abiri estuaries (Figure 1) occurs on
fractured shorelines, with soft quaternary sediments.
RADARSAT images were used to identify simple and complex
lineaments can be correlated with Precambrian and post-
Cretaceous linear geological structures in the interior of the State
of Paraiba (CPRM, 1982). Coastal erosion in these areas
averaged about 300 meters. According to the local reports, the
Gramame river’s mouth continuously changes. During the high
tides, the ocean destroys the spit and flood local mangrove areas.
Comparison of the 1971 topographic maps with the 1996
RADARSAT images shows large changes at the mouth of the
Abiai river. (Figure 1). The old (1971) mouth of the river was in
the north of the image. It is now blocked by marine and alluvial
sediments, and now the river flows to the south.
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 175
