Estimating erosion rates of tropical shorelines from RADARSAT-1
Vern Singhroy
Canada Centre for Remote Sensing
588 Booth St, Ottawa, K1A 0Y7
email: vern.singhroy@cers.nrcan.gc.ca
Marx Barbosa
Federal University of Paraiba
Paraiba, Brazil
email: Marx@Imrs.br
Commission VII Working Group 2
KEY WORDS : RADARSAT, coastal hazards, erosion rates, sea defence, mangrove forest, tropical shorelines.
ABSTRACT
On a global scale, shorelines are eroding at an increasing rate. The local shoreline geomorphology and regional differences in coastal
processes are fundamental in estimating erosion rates. Our results show that in the permanently cloudy tropical coastal areas of Guyana
and north east Brazil, RADARSAT S7 image is particularly useful to map areas of erosion, and deposition, as well as the adjacent
affected coastal land use. Coastal changes were estimated by comparing recent RADARSAT images with previous topographic maps.
In north east Brazil erosion rates on fractured quaternary sediments vary from 0 to 12 meters a year, and on the more resistant
sandstone erosion rates vary fram 0 to 6 meters a year. In Guyana, low flat areas below sea level, erosion rates vary from 0 to 25 meters
a year. In El Nifio year, these estimates are expected to increase.
1. INTRODUCTION
On a global scale, shorelines are eroding at an increasing rate.
The local shoreline geomorphology and regional differences and
coastal processes are fundamental in estimating erosion rates. In
the United States, for example, the Coastal Erosion Information
System (CEIS) uses historical topographic maps and aerial
photographs to calculate erosion rates, which are used for
engineering planning purposes and to verify insurance claims.
Erosion rates range from 0 to 10 meters a year in some areas.
However, in most tropical areas historical airphotos and accurate
topographic maps are scarce. In these areas, the shorelines are
cloudy, and clear satellite optical images are not available. A
cheaper and more accurate monitoring technique for land use and
coastal changes is needed.
This study uses RADARSAT images to estimate the erosion
rates of two cloudy shoreline in Guyana and north east Brazil,
where the geomorphological and near shore processes are
different. The results of these two case studies are used to assist in
current sea defence and coastal land development projects, as well as
providing a regional baseline of erosion estimates for regional
monitoring of these sensitive coastlines.
In addition, the need for a sustainable approach to sea defence and
coastal land development projects has been recognised by many
developing nations in the tropics and a number of international
donor agencies, namely the Caribbean Development Bank, the Inter-
American Development Bank, the World Bank, and the European
Economic Community (World Bank, 1995). In the case of Guyana,
the multilateral financial agencies have allotted 44 million dollars
(US) for sea defence projects. Similar coastal development projects
exist in Northeast Brazil.
2. COASTAL CHANGE IN GUYANA
This study shows that parts of the Guyana coastline have changed
from a few meters to half of a kilometre over the past twenty years.
This has serious implications for sea defence, coastal fisheries and
commercial agriculture.
The coastal zone of Guyana is approximately 425 km in length and
varies from 12 km to 77 km in width, along the Atlantic Ocean.
Ninety per cent of the population (820 000) live on this narrow
coastal plain. Elevation varies from 0.5 m below to 2.4 m above
mean sea level. This coastal plain, which occupies only 7.5 per cent
of the total land area, is the main agricultural region of the country.
Sugar and rice are the principal agricultural exports. These crops
require extensive drainage and irrigation to maintain their
productivity. Sea defence, the construction and maintenance of
drainage, irrigation canals and other land development projects are
therefore vital to the economic growth of Guyana. Sixty per cent of
the coastline is protected with some form of sea defence (earthen
dams, concrete dikes, and boulder slopes). About 70 km is protected
by masonry sea walls, 170 km by earthen dams, and the rest by
beach ridges. Recently, the increasing frequency and intensity of
coastal storms has caused considerable damage to the sea defence
structures, resulting in serious flood damage to residential areas and
crops. Over the past 20 years, there has been little maintenance or
restoration of the sea defences. The sea walls have exceeded their
design life time and are on the verge of collapse due to material
deterioration and constant coastal erosion (Llosa, 1993). In addition,
any predicted rise in mean sea level from global warming, coupled
with the increase in coastal processes in El Nifio years, will severely
affect economic activity in the coastal areas. For these reasons, it is
necessary to provide a baseline of coastal changes in sensitive areas,
so as to predict the impacts of sea level rise and anomalous El Niño
events. This case study reports on the use of RADARSAT images,
174 International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998
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