631
eir gratitudes
n Geologic Sur-
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and mosaics,
and staff of
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orld oil, Circ-
d, p.99-112.
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A.A.P.G. Bull.
Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Monitoring geomorphological processes in desert marginal
environments using multitemporal satellite imagery
A.C.Millington & A.R.Jones
University of Reading, UK
N.Quarmby & J.R.G.Townshend
NERC Unit for Thematic Information Services, Reading, UK
ABSTRACT: Methods for geomorphological process monitoring and change detection using digitally processed
multitemporal Landsat TM & MSS imagery are evaluated in south-central Tunisia. Three categories of changes
are detected - sub-sampling unit, seasonal and long-term changes. Hydrological and geomorphological changes
in Tunisian playas are examined within these categories. Both surface water and groundwater are important
in determining salt and sediment budgets on these playas
RESUME: Les méthodes geomorphologiques des processus de mesurement et de detection des changements en
tilisant le traitement des images multitemporales Landsat TM and MSS sont évaluées au centre-sud de la
Tunisie. Trois changements de categories sont detectes - unite sous-echantillon, saisoniere et changements
a long terme Les changements hydrologiques et geomorphologiques dans les playas tunisiennes sont examines
dans les memes categories. Les eaux de surface ainsi que celles sous terre sont importantes pour determiner
la balance entre sel et sediment des playas
1 INTRODUCTION
The monitoring of sediment transfer processes in
arid and semi-arid environments has many important
applications for engineering problems (e.g. Cooke
et al., 1982; Doornkamp et al., 1980) and land
evaluation procedures (Mitchell, 1982; Purdie 1984) .
A closer examination of these problems focusses
attention on the difficulty of monitoring sediment
transfer processes to obtain information for resource
assessment and environmental planning.
Geomorphological processes on the desert margin
are characterised by:-
1. high-magnitude and low-frequency events,
2. a strong seasonality,
3. occurrences as spatially discrete and, often,
uncorrelated events.
These characteristics make semi-arid geomorpho
logical processes difficult to monitor using
conventional ground-based instrumentation because
there is a very low probability of measuring any
geomorphological event in a specific locality using
site-specific instrumentation. However the probab
ility is increased if either the time or spatial
dimensions are increased. The time dimension is,
for all practical monitoring purposes, inflexible.
However events can be monitored in a relatively short
time period if a suitably large area is examined
which will increase the number of sites where
processes are active. The synoptic capability of
satellite data provides large enough areas for this
and makes the monitoring of sediment transfer
processes, utilising change detection algorithms on
multidate imagery, a distinct possibility.
This methodology is currently being evaluated in
south-central Tunisia using Landsat MSS and TM data
acquired between 1981 and 1985. Attention is being
focussed on geomorphological change in three major
process-domains - alluvial fans, braided river
systems and playas. This paper presents the first
observations from this project and concentrates on
playa environments.
2 BACKGROUND TO THE STUDY AREA
2.1. Geomorphological Processes
Water plays a central role in the transfer of
sedimentary materials in south-central Tunisia although
wind becomes an increasingly important agent further
south. The main components of the fluvial system
on the desert margin are: i) Catchment slopes and
pediments ii) River channels iii) Alluvial fans
iv) Enclosed depressions containing playas (known
locally as Chotts).
Over time materials are transferred from mountain
slopes by slope erosion, river channels and alluvial
fans. Finer textured material is deposited in the
playas, whilst the coarse material is left in the
channels and fans and on the slope as lag deposits.
It is clear that this transfer of sediment is a far
from continuous process and episodic erosion and
deposition prevail often with long periods during
which materials are stored in the components of the
fluvial system.
Aeolian activity is important in the dry season
when gypsiferous sands are deflated from the playa
surfaces, and redistributed on the surrounding
landforms. The wind-blown material deposited in
channels is then flushed through the system during
the next spate of fluvial activity. Sandy materials
are also blown into the area from the south in the
late winter and early spring.
2.2. Geology
The study area is dominated by Cretaceous and
Tertiary sediments which have been subjected to
Alpine folding, (Burollet, 1967; Coque and Jauzien
1967). The area is dominated by a series of eroded
anticlines and synclines which give rise to many
resistant dolomite and limestone cuestas with
intervening valleys formed by the differential
weathering and erosion of marls and clays which are
now filled with late Tertiary and Quaternary
sediment.
