Real Risk

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OST GEOLOGICAL HAZARDS

in Cameroon are linked to

the existence of the Cameroon Volcanic Line (CVL), a

1,600km long chain of Cenozoic-Present volcanoes

that traverses the country in a SW-NE direction from the Gulf

of Guinea island of Annobon across Cameroon to eastern

Nigeria. Such hazards include toxic gas emissions from volcanic

crater lakes, landslides, floods and hazards from volcanic erup-

tions.

In the twentieth century and early twenty-first century, the

country recorded a series of natural disasters located within and

around the corridors of the CVL. These include:

• The Lake Manoun toxic gas disaster (1984) in which 37

people were killed

• The Lake Nyos toxic gas disaster (1986) which claimed

approximately 1,700 to 2,000 lives

• The Bafaka Balue landslide (1995) with 3 lives lost

• The Mount Cameroon volcanic eruption (1999) where lava

consumed an enormous amount of forest (about 800

hectares) with its rich bio-diversity and cut off 83m of tarred

road, disrupting economic activities.

Limbe subdivision is located between latitude 3

90’ and 4

05’N and longitude 9

29’ and 9

06’E in the southwestern part

of Cameroon. This is a coastal region with an approximate 50.5

km of coastline. The subdivision consists of more than 25

villages with the city of Limbe, the capital.

The project

Over the years, this region has experienced persistent landslides

and floods with the worst being in June 2001, taking some 30

lives, rendering over 2,000 people homeless, and destroying

properties and social amenities such as roads and telephone

lines worth millions of CFA francs. The occurrence of disasters

in this region varies periodically, resulting from oceanographic

processes, heavy rains and seismic activity from MC, particu-

larly lava flows leading to fire outbreaks from volcanoes.

Delineating the potential risk zone is an attempt to provide deci-

sion makers and planners with geospatial information on areas

where they need to concentrate concerted effort in order to

develop mitigation plans.

Geospatial tools were employed in the execution of this

project for data collection and analysis. Extensive use was made

of existing feature datasets considering their source, quality, and

date of publication. Landsat 7 ETM+ satellite imagery was also

used extensively for categorizing, interpreting and digitizing

features. Unfortunately, only the April 2003 image was cloud-

free and therefore was put to full use for this project. Images

were orthorectified and georeferenced by EarthSat to its

GeoCover dataset, a commonly used and available system.

Image interpretation

The ETM+ scanner measures the sun’s reflectance in seven

spectral channels known as bands. These include the visible

spectrum (bands 1-3), near infrared (bands 4 and 5), thermal

(band 6) and medium infrared (band 7). These bands were

selectively combined into colour-composite images to enhance

visibility. Different band combinations were used to maximize

the visibility of landslide scars and different land uses under a

range of conditions. To improve the detection of narrow land-

slides, scars and less prominent features, the panchromatic band

(14.5m resolution) was merged with the other bands (28.5m

resolution) to create a ‘pan-sharpened’ image. This greatly

improved the visibility of older landslide scars before catego-

rization and digitization.

Topographic map update

A topographic map update was performed by image analysis

and manipulation. Pixel transformation was carried out by

computing the normalized difference vegetative index (NDVI) to

improved visual interpretation of enhanced false colour compos-

ite enabling discrimination between different vegetations and

non-vegetative surfaces. This visual enhancement of the image

permitted easy digitization.

The topographic map of the subdivision was updated with

focus on new roads and land use/land cover patterns. Based on

an accumulated pattern of NDVI, four main groups were cate-

gorized:

1) Non-vegetative areas (settlement, bare ground or outcrop,

road)

2) Natural vegetative area (evergreen forest, deciduous forest)

3) Agricultural area (CDC plantations, farmland etc.)

4) Water surfaces.

The Cameroon Development Corporation (CDC) has colossal

palm and rubber plantations in the subdivision, accounting for

over 30 per cent of total land use. The landscape has undulat-

ing relief with heights varying from 2m to 362m above sea level.

Most human settlements are around the coast, with a few

villages situated above 200m. Low-lying areas adjacent to the

sea include Mbojo, Mutowoh, Wovia, Ngeme, Man’O War Bay,

Sonara Bota-Land and a host of other villages on the west coast

of the subdivision. Areas around the city of Limbe with remark-

able hills include Towel, Miletwo, Coconut Island, Mabetal New

Layout, Ambas Bay, and Mount Etindi. Over the years the city

Delineation of potential risk zones,

Limbe subdivision, Cameroon

Buh Wung Gaston, GIS and Remote Sensing Department, Limbe Botanic Garden, Cameroon