Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
Estimation of the torrential environment
(potential) of isle Zakynthos (Greece) using
GIS
P. stefanidisl & I. ~alinderis'
I
Institute of Mountainous Water Management and Control, Aristotle
University of Thessaloniki, Greece.
Abstract
The torrential conditions of an area are determined by four basic factors:
Climate, geologic support, relief and vegetation.
The combination of the factors, mentioned above, in a particular basin or in a
wider area, brings about an energy system, called "torrent potential". The
climate is a factor attacking the geological support, while the vegetation,
wherever it exists, acts as a protective cover. The relief has a regulative effect:
precipitous of flat relief aggravates or moderates accordingly the torrential
phenomena.
Therefore the torrent potentials of an area determine the process and the way the
overland flow, moves.
Mediterranean climate demonstrates certain particularities, with the long-drawnout dry and hot period during summer but also the intense rainfalls during
autumn and winter. These particularities cause respectively severe problems of
draughts and floods.
In this paper study, the torrential environment (potential) of isle Zakynthos in
Greece is determined. Initially the area was mapped (scale 1:50000). The
torrent basins were delimited and the basic factors (Climate, geologic support,
relief and vegetation) were estimated using G.I.S. Finally, the torrential
environment (potential) was determined and the proper watershed management
system was suggested in order to prevent the problems of water deficiency and
of flooding.
Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
120 River Ba~itlMarragernctlr I1
1 Introduction
Greece is a mountainous land, whch receives a great amount of intense rainfall.
Its geologic support consists of rather highly erodible rocks. That fact in
combination with the lack of plant cover, which is a highly protective factor
against erosion, and also with the immense man-made interference, causes the
development of many disastrous torrents on Greek land.
Torrents introduce a certain characteristic, which is described by the removal, the
transport and the deposit of debris from highlands to lowlands. As a result the
torrent basins on highlands are being degraded while on the lowlands debris
causes serious problems by alluviating streambeds and lakes.
The torrential conditions of an area are determined by four main torrent factors:
Climate, geologic support, relief and vegetation.
The combination of those factors in a particular basin or in a wider area brings
about an energy system, called "torrent potential". It's the torrent potential,
which appoints the way of the outflow of the torrents (Kotoulas, 1972). The
climate is a factor attacking the geologic support, while vegetation wherever it
exists, acts as a protective cover. Relief has a regulative effect: precipitous or
flat relief aggravates or moderates accordingly the torrential phenomena.
The rational confrontation of the intense torrential problems in Greece
necessitate the research of torrent conditions and the classification of torrents in
certain types. Therefore the proper watershed management system must be
determined.
The multiprocessor system that G.I.S. offer combined with the ability of
processing spatial data and attributes, makes their use irreplaceable.
In this paper study, the torrential environment (potential) of isle Zakynthos in
Greece is determined, with the use of G.I.S.
2 Bibliography review
Many researchers have tried to classify mountainous basins of torrents. In 1826
Duile was the fist who attempted to classify mountainous watersheds. Since
then, the most important researches were made by, Sure11 [10], Demonzey [3],
Stiny [9], Margaropoulos [8], Kotoulas (1972). The classification Kotoulas
introduced, was the only classification, which defined specific torrent types.
Besides those, there is also Aulitzky's [l] classification of depositional
formations and debris cones, which relates mainly to the hazardous activity of
torrents in Austria.
The main features of Kotoulas [5] classification, which was used also in the
specific paper, are the following. Three out of four basic torrent factors (Relief,
Climate and Vegetation) are depended on the maximum elevation (Maximum
torrential elevation). The sea water level, the limit of the full forest cover
impact, the natural forest cover limit as well as the limit of everlasting snow
determine the torrent environment in which watersheds are developed (Fig. 1).
1.
Torrent environment I. 0 - 1 0 0 0 ~
Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
2.
3.
Torrent environment 11. l OOlm - 2 0 0 0 ~
Torrent environment 111. 2001 - 3000m (Alpine zone)
Torrent environment IV. > 3001m Zone of everlasting snow
4.
In Greece only the first three torrent environments are formed.
As far as the geologic support is concerned, in order to associate torrential
phenomena to predominating rocks in each basin, rocks were classified in
torrential petrographical formations according to their liability on extreme
weather conditions.
The results of this classification are the following:
M = Crystalline, igneous or magmatic rocks
S = Sedimentary rocks
G = Schist (slate) rocks
K = limestone rocks
F = Flysch rocks
Basins composed by only one torrential petrographical formation, or composed
by more than one, but their percentage is lower than 15%, are described as
unmixed, while others are described as mixed ones.
A basin described as an M I type, means that it's an unmixed watershed, which
its geologic support consists of crystalline rocks, and belongs to torrent
environment I (0 - 1000m). A basin described as an M,G - I1 type, means that
it's a mixed watershed, which its geologic support consists of crystalline rocks
and Schist (slate) rocks, and belongs to torrent environment I1 (1001 - 2000m).
-
Torrent Enxuvnment I1
Toruent Enrilvnment I
Figure 1: Torrent environments according to the Kotoulas [5] classification
3 Research area
Our research was conducted in Zakynthos island in Ionian Sea (Fig. 2). It is one
of the rainiest areas in Greece. Zakynthos has an extent of 406.0. Kid and its
population exceed 39.015 people. During summer it attracts a great number of
tourists.
Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
122 River Ba~itlMarragernctlr I1
4 Research method
For the accomplishment of the goals of this research, the following methods
were applied. The research area was mapped (scale 1:50000). The watersheds
were defined and the whole area was digitised. The contours were also digitised
(contour interval 100m).
The watersheds were counted and encoded.
Using Arc/Info 3.5.1 for pc and ArcIView 3.2, the morphological features were
determined. The total area drainage basin (F), Perimeter (U), degree of
Hmed),maximum relief (H,) and basin's mean
roundness (B), altitude (H-, H,,
slope (J).
After that the basic torrent factors were analysed and the torrent potential of each
torrent was defined.
In order to complete the above mentioned research
The meteorological dates of the existing weather stations were
collected, interpreted and revises
A vegetation (plant cover) map was conducted (scale 1:50000), using
satellite data, thematic maps and aerial photographs
The kinds and the extension of the torrential petrographical formations
in the research area were defined, based on the geologic maps of Greece
(scale 1:50000), which were digitized and processed by G.I.S.
The contour lines were digitized and processed by G.I.S.
On completion of the above-mentioned work and after the determination of the
torrent environment, torrents were classified in torrential types.
5 Results
5.1 Torrent basins, morphological features
It is generally known that the most important morphological features that affect
torrent attributes are the total basin area (F), the perimeter (U) the degree of
roundness (B), elevation (Hfin, H-, Hmed),Maximum Relief (H,) and basin's
mean slope (J). These morphological features were estimated and used in our
research.
Table 1 lists the values of the morphological features described for every torrent
basin in Zakynthos. Figure 2 shows the research area, the total drainage system
and the watersheds. 78 watersheds drain Zakynthos. The total basin area varies
from 0.27 ~ m (No
' 5) to 29.40 ICrn2 (No 26). According to the elevation of
<
Zakynthos, all 78 torrents are classified in Torrent environment I (H,
l OOOm).
The basin's mean slopes vary from 11.69% to 55.24%.
Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
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River Ba~itlMarragernctlr I1
5.2 Relief
In Figure 3 is shown the distribution of contours in Zakynthos Island. It's clearly
evident that 35% of Zakynthos area is under 1OOm altitude and 65% varies Erom
0 - 700m altitude.
Moreover, torrent's relief range from fair to harsh, a fact that acts in favor of the
emergence of extreme torrential phenomena such as floods.
5.3 Vegetation - plant cover
Vegetation (plant cover) and especially forest cover is the torrent basic factor
that prevents torrent action. In order to define the types of vegetation in
Zakynthos, the following method was used. Information on plant cover was
extracted from Corine - Landcover, a European project that defines vegetation
using satellite data (Landsat - MMS, pixel resolution 25m), thematic maps (scale
1:100000) and aerial photographs.
In table 2 is shown the different types of land use that exists in Zakynthos. In
figure 5 is presented the map with the different types of land use.
Table 2: Land use in Zakynthos island
Types of Vegetation
Forests
Agricultural areas
Evermeen broadleaved species
Partially forested area
Grazing land
Area (~rn')
9.1 1
97.32
100.20
33.69
26.33
%
3.39
36.26
37.33
12.55
9.81
According to table 2, forests whose existence prevents torrential phenomena
occupy only 3.39%.
5.4 Geologic support
In figure 6 is presented the area distribution of rock formations in Zakynthos
Island. According to our research, Zakynthos island consists mostly of limestone
rocks (74.42%, see Table 3). The rest of the area consists of sedimentary rocks
(25.59%). Both these rock formations act in favour of the emergence of
torrential phenomena and especially in the production of sediment loads.
Table 3: Torrential petrographic formations in Zakynthos island
-
Torrential Petrographic Formations
Limestone rocks (K)
Sedimentary rocks (S)
Area ( ~ m ' )
199.81
68.72
%
74.41
25.59
Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
5.5 Climate
Islands in Ionian Sea accept a great amount of rainfalls (Mean yearly height of
rainfall 995.31-1114. The distribution of rainfall throughout the year is not normal.
During autumn and winter intense rainfalls cause severe problems, while in
summer rainfalls are minimized.
Table 4: Mean annual rainfall in Zakynthos island
Months
Rainfall (mm)
Months
Rainfall (mm)
J
162.8
J
F
114.5
A
4.2
8.3
M
86.2
S
39.1
A
44.0
0
131.5
M
18.8
N
153.0
J
11.4
D
221.5
Annual
995.3
It is important that in addition to the abnormal distribution of rainfall, intense
maximum daily rainfall depth aggravates the torrential phenomena (Maximum
daily rainfall depth: 182.0 d 2 4 h ) .
5.6 Torrential environment, torrential types
The main factor that has a strong impact on the kind of torrential phenomena that
will occur is the geologic support of the watershed (Kotoulas, 1972).
Based on that fact and after a spot test, an effort was made in order to classify the
different torrential environments, found in Zakynthos island, in specific torrential
types. In order to accomplish this, the Kotoulas method, mentioned earlier, was
used.
According to our research, 51 watersheds are classified in K - I type, 20
watersheds are classified in S I type and just 7 watersheds are classified in the
mixed K,S - I type.
The torrential phenomena that torrential types produce are given below.
Unmixed type K - I
Torrential environment: Rock Formation: Limestone (Limestones, dolornites,
hornstone, marbles or crystalline limestones)
Elevation 0 - 1000m.
Torrent phenomena: Intensive sheet erosions, weatherings, gully erosions and
landslides.
Unmixed type S - I
Torrential environment: Rock formation: Sedimentary (sands, clay,
conglomerate, breccius, sandstones, mollasic sediments)
Elevation 0 - l OOOm
Torrent phenomena: Intensive sheet and rill erosions, gully erosions, landslides,
lateral erosions.
The mixed type K,S I produces torrential phenomena similar to those produced
by the unmixed ones, depending on the extent of the petrographic formations.
-
-
Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
126 River Ba~itlMarragernctlr I1
Figure 2: The Drainage network system of Zakynthos island
6 Torrent control - watersheds management system
The outcome of our research showed that the features that best describe torrents
of Zakynthos Island are the following. Torrents belong to the hilly, semi mountainous areas with an extent of 0 . 2 7 ~ mto~ 29.40 Km2. The torrential
environment is such that produces moderate as well as intense torrential
phenomena.
The limited forested area as well as the intense human interference in the
streambeds of torrents result in the occurrence of many floods. The extreme
torrential phenomena that appear in Zakynthos island necessitate the application
of Forestry techniques and torrent control works system. The application of the
system will lead to the prevention of the existing problems as well as in the
enrichment of underground water.
Moreover the utilization of planting techniques in combination with technical
works will stabilize soil and will help in the untroubled abduction of water.
Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
Figure 3: Relief of Zakynthos island (contour interval 100m)
Figure 4: Land use in Zakynthos island
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128 River Ba~itlMarragernctlr I1
Figure 5: Area distribution in Petrographic Formations
I
I
Figure 6: Area distribution in torrential types
Transactions on Ecology and the Environment vol 60, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541
7 Conclusions - results
In our study the torrential environment and torrential types in Zakynthos island
were determined and described, using G.I.S.
The research was based on bibliography available on torrential environments,
torrential types.
All the data collected concerning the four basic torrential factors (Relief, climate,
geologic support and vegetation) were processed by G.I.S.
The results derived by this procession lead us to the determination of the
torrential types that exist in Zakynthos island. The knowledge of torrential types
helps in the determination of the appropriate management system that should be
applied.
G.I.S. offer great help in the procession of spatial variation of torrential factors
and is a great tool in the research of mountainous water management systems.
References
[ l ] Aulitzky H., Preliminary two - fold classification of torrents. Mittelungen
der forstlichen bundewersuchanstalt, Wien, p 144, 1982.
[2] Chow V. T., Handbook of applied Hydrology, USA 1964.
[ 3 ] Demonzey P,, L ' extension des torrents en France par le reboisement.
Paris, 1878.
[4] Jones C, Geographical Information Systems and computer cartography.
Logman Scientific and Technical, London 1998.
[5] Kotoulas D., Die Wildbache Suddeutschlands und Griechenlands, Teil 1.
Bericht Nr. 25. Munchen, 1972.
[6] Kotoulas D., Die Wildbache Suddeutschlands und Griechenlands, Teil 2.
Bericht Nr. 3 1. Munchen, 1972
[7] Magure J, David and al, Geographical Information Systems principles and
applications. (Logrnan Scientific and Technical), USA 1991
[S] Margaropoulos P., Rapport sur la classification des bassins torrentiels
FAO/EFC/TORlU64/2, 1964.
[9] Stiny J., Die geologischen Grundlagen der verbauund der Geschiebeherde,
Wien 1931.
[l 01 Surell, Etude sur les torrents des Hautes - Alpes, Paris, 1842.
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130 River B a i t 1 M a r r a g e r n ~ t ~I!r