|Methods and Results
A GIS base was created in order to draw up the requested environmental
hazard maps for slope instability and geo-archaeology. As the pixel
value increases, so too does the risk of slope instability and there
is a greater chance of geo-archaeological finds.
Four weighted factors were combined for the
slope instability mapping, selected on the basis of knowledge of
the field and supplementary literature. In this region a great many
factors (topography, lithology, climate, human influence, tectonics,
etc.) all play a part in the development of slope instability. The
most important factors that trigger mass movements are (1) slope
gradient, (2) lithology, (3) proximity of faults and lineaments
and (4), and proximity of thalwegs or wadis.
Slides causing large masses of material
to move down the slope
Faults due to (neo)tectonic activity
A fifth information layer consists of the
locations of earlier mass movements on the slopes. This information
is obtained from the geomorphological map drawn up on the basis
of TK-350 images.
The various factors are then weighted by
means of a statistical calculation based on the site of earlier
mass movements. The highest weight is given to the factor with statistically
the greatest influence on mass movements in the region.
Geomorphological map resulting from
mapping based on stereoscopic TK-350-images
The geomorphological mapping based on
TK-350 images serves a double purpose:
(1) A general mapping (1:100,000) of the whole canyon; and
(2) A detail mapping (1:50.000) of the dam site and immediate
canyon environment. The legend is based on the geomorphological
legend for semi-arid and arid regions developed at the University
of Saragossa (Spain).
The geomorphological map on a scale of
1:50,000 was checked in the field during a second survey.
At the same time, field research was carried out into faults
and other lineaments which provide evidence of neotectonic
activity in the region.
Danger of landslides!
The sum of these weighted layers results
in a map which shows the spatial variation for the relative risk
of mass movements or landslides. For greater clarity, the map is
classified into 4 risk categories (low risk, moderate risk, high
risk and very high risk) on the basis of the image histogram.
Slope instability risk map indicating the
of the planned dam (height of the dam: 10 to 50 m)
Block diagram of the digital terrain model
including the planned reservoir
The planned dam construction in the Wadi Mujib will create an upstream
reservoir whose volume will be dependent on the dam height. The
very unstable slopes already present a potential danger of further
landslides, the result being that the reservoir will progressively
lose its capacity due to sediment infill. In addition, by raising
the water table, the marly canyon walls will become moist (with
a higher pore pressure) making the present slope instability even
worse. As there is increased contact between the future lake and
the high risk zones, there is a considerably increased danger of
slope instability for dam walls of above 20 meters in height.
What is a good
A model similar to that for slope instability is drawn up
for the geo-archaeological mapping. In this case, however, there
is no statistical calculation and thus no weights are allocated.
The model consists of 8 information layers on the spatial variation
of possible locations of as yet undiscovered archaeological sites
or artefacts :
- The proximity of
the steep plateau rim: the location of most known sites indicates
the strategic importance of the plateau rim. From here there is
a good view of the surrounding area and protection from attacks
from the opposite side by the deep canyon.
- The proximity of
the important King's Highway: this trading route dating back several
thousand years has already yielded many archaeological finds.
- The proximity of
thalwegs or wadis: these are important for the supply of water.
- The gradient: the
flatter the terrain, the more likely it is to be attractive to people
to settle there.
- The lithology: the
presence of basalt (on which fertile earth develops) or fertile
alluvium is very likely to attract people.
- The proximity of
a water source, a seepage line, or of Pleistocene travertine indicates
the previous presence of water: this is important for the water
- The river terrace
remnants: the low gradient and presence of a fertile valley bottom
results in the formation of terraces which are an ideal place for
agriculture or settlement.
- The slope direction
: in hot climates people seek out shaded areas. Since the Neolithic
revolution the protecting vegetation has been removed, leaving overgrazed
and bare slopes. North-facing slopes are exposed to less sunlight,
certainly resulting in a lower evaporation level (= higher moisture
content of the soil) than on south-facing slopes. East and west-facing
slopes have a value between the two.
The TK-350 images
are a valuable aid in pinpointing these landscape elements.
The archaeological site of Lehun is located
on the northern edge of Wadi Mujib, overlooking the canyon which
is 700 m deep and 5 km wide
Travertine sediments (in situ) along the
Fluviatile terraces and current river bed
This is where to dig!
By combining these information
layers it is possible to produce a map which shows the spatial variation
of the likelihood of archaeological sites or artefacts being present.
The relevance of the resulting geo-archaeological map is tested
by comparing the location of already known archaeological excavations
with the probability map. For the test zone it appears that 52%
of the known sites lie in the zone of very high probability; the
remaining 48% in zones with average to high probability. No known
site is located in zones with a low to very low probability!
It can be assumed that building the
dam will result in the loss of a great deal of archaeological material
on the river terrace remnants and the alluvial deposits. Archaeologists
can use the geo-archaeological map of the dam site as a guide in
organising an effective emergency excavation campaign.
Map of probable archaeological sites
site of the planned dam (height of the dam: 10 to 50 m)
The combination of the geomophological interpretation
of the TK-350 images and field work in drawing up a geomorphological
map (and derived landslide map) provides the necessary qualitative
basic information with which to draw up a general slope instability
model. The TK-350 images also provide an important added value when
creating a geo-archaeological model for the location of terrace
remnants, alluvium, travertine deposits, the steep plateau rim,
etc. Even if time and/or money prevents a field survey, the TK-350
images provide sufficient qualitative information for the geomorphology
on a scale of £ 1:50,000.
It can be concluded that the use of stereoscopic
TK-350 images brings two major benefits, namely:
1. They can be effectively used for geomorphological
preliminary studies for environmental risk mapping.
2. For various reasons, they are more useful
than stereoscopic aerial photographs (£ 1:50,000): the lower
price (especially for large areas), shorter delivery period (just
1 month!) and image processing, and less complexity (advantage of
large coverage and metric indications on the images).