SEVERELY ERODING ERESSOS BEACH (LESVOS ISLAND) AND

Proceedings of the 10th International Conference on Environmental Science and Technology
Kos island, Greece, 5 – 7 September 2007
SEVERELY ERODING ERESSOS BEACH (LESVOS ISLAND) AND
POSSIBLE DAM IMPLICATIONS
M.I. VOUSDOUKAS1, O. ANDREADIS1, G. ADAMAKIS1, A.F. VELEGRAKIS1,
E. PASAKALIDOU1 and G. KOKOLATOS2
1
Department of Marine Sciences, Environmental School, University of the Aegean,
Mitilene, 81100, Greece
2
Department of Geography, University of the Aegean, Mitilene, 81100, Greece
e-mail: vousdoukas@marine.aegean.gr
EXTENDED ABSTRACT
Small water storage dams are nowadays regarded as the ideal solution to waterhungry Aegean Sea islands. Many of such dams have been constructed and more
are planned for the immediate future. However, these dams can also create
problems to coastal areas, particularly to the downstream beaches found at the lower
reaches of the dammed rivers. The present contribution reports the results of a study
undertaken on the effects of such a dam located at Eressos basin, S. Lesvos.
Eressos beach in recent years, shows evidence of erosion, which may be related to
the reduction of river sediment load, due to the construction of the upstream dam.
Estimation of the basin sediment yields and loads on the basis of the basin
topography, sedimentology and land use and generalized soil erosion equations
have shown that the dam stops more than half of the sediment load. Although other
factors can also be responsible for the observed beach erosion (e.g. coastal defence
works and changes of the annual wind climate), the dam’s contribution is regarded to
be significant, particularly over the long term.
Keywords: river discharge, beach erosion, dams, sediment transport, drainage
basins
1. INTRODUCTION
Coastal areas comprise highly dynamic zones, affected by atmospheric, marine and
terrestrial processes, as well as their interaction [e.g. see 3]. Moreover, the already
important human interference is steadily increasing, due to the high socio-economical
importance of the coastal zone, with often unexpected implications [e.g. see 2].
Hydrological cycle and in particular sediment balance in the drainage basin and the
coast is one of the most important components of the total system; rain water
interaction with the land leads to soil erosion and transport mechanisms. The
generated sediment load is finally deposited to the coast, a process initially
responsible for the generation of beaches, and thus to a great extent vital for their
conservation.
On the other hand water demand is constantly increasing and in many cases dam
construction appears as the only choice. 3800 km3 of fresh water is withdrawn
annually from the world’s lakes, rivers and aquifers, twice the volume extracted 50
years ago. Furthermore world population has passed 6 billion and still hasn’t reached
the peak point. As a result more than 45000 large dams have been built during the
past decades and more are scheduled for construction [10].
Apart from their undisputable merits, dams have significant impacts since they affect
downstream hydrology, morphology and ecology [e.g. 8]. They can trigger
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morphological changes through change of sediment supply, affect water properties
and quality downstream and threaten certain species [e.g. 4; 6]. Studies have shown
that 30% of riverine water is retained in dams resulting in significant impacts on
carbon balance, nutrients supply and biological production, thermo-saline balance
and water circulation of basins and coastal morphodynamics [10]. Especially in small
islandic areas, where sediment balance is very fragile, they pose a serious threat to
beaches found downstream, erosion of which is a serious blow to such tourismbased economies [9].
This study is focusing on an islandic area (Eressos, Lesvos Island, Greece)
consisting of a drainage basin where a small dam was constructed and the
downstream beach, which is under a severely eroding state. Sediment transport
processes in the watershed are studied numerically, while field topographic and
sedimentological data are analyzed and discussed.
2.
STUDY AREA
The study area is located in the north western part of Lesvos island (see Figure 1),
100 km (55 in a straight line) away from the capital city, Mytilene. It is a catchment
area of 57.6 km2 which includes two main inhabited areas the villages of Eressos and
Skala Eressos (at the coast) with population of 1247 and 306 people respectively.
The morphology of the area under consideration is semi-mountainous with ground
slopes varying from less than 100 in the lower flood plain close to the coast areas, to
steep rocky relieves on the northern part of the watershed. The main soils in the
area, similar to the soils of most north western part of Lesvos, consist of mainly of
volcanic rocks, with some amount of limestone in the lower coastal alluvial plains.
Newer fluvial sediments are also present in these lower parts of the river basin as
well. The watershed is crossed by a number of seasonal streams of short length and
several secondary branches eventually ending up to the beach of Skala Eressos, via
a couple of main streams the most important, in terms of size (14 km) and discharge
is Chalandra. The dam is constructed along Chalandra, 5 km upstream of the outfall
to the sea. The main type of ground cover appearing on the area is low shrubby type
with scant tree canopy, restricted around the lower inhabited areas and especially
Skala Eressos where olive trees are cultivated, along with stock-raising plants. The
main human activities are tourist business, land cultivation and animal farming. The
dam itself is constructed along Chalandra and it corresponds to 45% (26 km2) of the
total Eressos catchment area. It is designed to store more than 2.500.000 m3 of
water in the reservoir upstream for the cultivation and general water needs of the
municipality. It was constructed in 1999 using 765.000 m3 of soil materials and has a
height of 29 m and length of 340 m [1].
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Figure 1. Study area. (a) Aerial photo of the Eressos dam and (b) satellite image
showing the location of the dam, Eressos village and the main streams of the
drainage basin (Chalandra, Karasaris, Eleousas and Lifonakas).
3. METHODS
3.1. Drainage basin sediment transport model (UCLE equation)
To calculate the sediment yield within the study area the RUSLE equation [7] was
used. The acronym stands for Revised Universal soil Loss Equation and refers to an
algorithm for calculating the amount of soil produced (A) by overland erosion in tons
per hectare (tons/ha). The initial equation, USLE, was first developed in the United
States to estimate the amount of soil loss caused by rainfall in tilled low gradient
areas [11]. Thereafter it has evolved, modified and revised in order to be applicable
in a variety of conditions around the world giving accurate results and hence
receiving a universal character. Each parameter in the equation plays a significant
role with respect to the physical phenomenon of soil erosion and their correct
determination is crucial to the end result:
(A)=(R)*(K)*(LS)*(C)*(P)
Where, R - Rain erosivity is an energy factor that depends on the energy of the
precipitation over a period of time and shows the potential of the rain to cause
erosion. It is calculated by formulas developed through site specific experimentation
and needs measurement of certain values like I30, which is the maximum rain
intensity in a 30 minute recording period [e.g. 5].
K – Soil erodibility factor that counts for the sensitivity of the soil to the rainfall’s
erosive action. It largely depends on its composition, texture and organic content.
Experimental values of K represent the percentage of silt, clay and sand in the soil
mixture of the surface sediment.
LS – Relates to the ground slope and the length that it goes over. It is clear that
higher slope values and long gradient lengths cause higher erosion levels and hence
soil loss.
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C – Land cover/use factor is a parameter that comes into the algorithm as a
percentage in order to account for the effect of the various land covers and land
uses, to soil erosion.
P – Another parameter affecting the final amount of soil loss as it counts for the
protection practices against soil erosion that take place by humans (terraces etc) in a
similar manner as land cover.
The above algorithm was run in ESRI Arc Map environment using the Raster
Calculation function. A Digital Elevation Model (30m) was employed in order to
extract some of the above parameters (LS) whereas others where calculated using
tables produced for similar cases (K, C) and on site measurements (R).
Figure 2. Soil erosion estimation flow chart
3.2. Other field and meteorological data
Beach profile impressions were obtained at five stations along Eressos beach during
September 2004, January 2005 and June 2006. Typical topographical survey
equipment was used and the upper dry part of the profile was impressed until depths
that not exceeded 1.5 m.
Sediment samples were collected from several points along the beach, Chalandra
stream’s bed, as well as from the dam’s bed. Mineralogic composition was studied
using a X-Ray diffraction device (PHILIPS PW1820/00), with a PW1710/00
microprocessor, Cu tube and Νi filter for CuKa radiation, while the scanning area of
2θ angle was 3-63˚ and scanning speed 1.2 ˚/min. PC-APD (1994) software was
used for automated input/processing of the digital scanned data. Prior to samples
analysis, validation and sensitivity tests were carried out, and semi-quantitative
identification of phasies took place on the grounds of counts certain diffractions, as
well as density and Mass Absorption Coefficient CuKa values.
Daily precipitation data (height, duration) of the period 2000-2006 were collected
from the meteorological station of Andissa.
4.
RESULTS-DISCUSSION
A difficulty in sediment yield estimation, is to ascribe the proper values on the
several terms of the USLE equation, and in particular the soil erosibility K, land use C
and soil protection P terms. Most of the existing applications of the equation refer to
United States areas and it is very likely that terms’ values found on the literature are
not applicable for the case of Eressos. For that reason calculations for different cases
were carried out. For a sand terrain case the dammed area sediment yield was found
to be 6.1 t/km2/yr, almost 46% of the total (13.3 t/km2/yr) (see
Figure 3a), while for the silt terrain case (
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Figure 3b) the estimated total production was 791 t/km2/yr, of which 367 is retained
by the dam, a proportion similar to the sand case.
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Figure 3. Drainage basin model results for two studied cases regarding soil
characteristics. Both sand (a) and silt (b) cases show that 40-45% of the total
produced sediment yield is retained from dam.
The topographic data (Figure 4) show that the beach suffered from erosion during the
monitoring period (2004-2006), showing a significant shoreline retreat of ≈ 10 m or
even higher at some stations. The observed accretion on the westerly station also
suggests the presence of longshore transport to that direction, but the amount of
sediment deposited to the west, doesn’t correspond to the one lost form the rest of
the beach, showing that cross-shore seaward transport is prominent.
X-ray sediments analysis (Figure 5) showed significant similarities between the
beach and stream bed samples, in contrast to the ones from the dam’s bed. The
former suggests that Eressos beach is supplied by sediments found downstream of
the dam, while produced suspended sediments yields upstream are retained on the
artificial lake’s bed.
All the above suggest that there may be a link between the observed erosion on
Eressos beach and the dam construction. After several cases studied on the
drainage basin model the overall conclusion is that more than 40 % of the available
sediment is retained by the dam, a proportion high enough to disturb the area’s
sediment balance. On the other hand, a recent dramatic change in the hydrodynamic
regime cannot be excluded from the possible reasons for the observed erosion
problem (not easily examined since relevant wave or wind data are not available),
and further study of the area is required in order to reach to solid conclusions.
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Figure 4. Beach profile impressions showing the ≈10 m shoreline retreat during the
last two years. Beach accretion was observed only on the western station (e). The
stations arrangement on the figure is similar to the one on the inset photo.
Figure 5. X ray diffraction analysis results of samples from the beach (a, b),
Chalandra stream bed (c) and the dam bed. The similarities on the first three figures
suggest that sediments are characterized by similar compositions, in contrast to the
dam’s sediments. The latter implies that the dam is trapping all the upstream
sediments.
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5.
CONCLUSIONS
Dam construction is nowadays considered a common practice solution to respond to
the constantly increasing water demand; on the other hand, the negative impacts of
dams have been clearly outlined, after extensive practice during the past decades. A
small dam was constructed in the Eressos drainage basin for irrigation purposes and
shortly after (5-years period) the beach downstream is suffering from major erosion
problems. Eventhough it is not possible to exclude other factors as possible causes
(mainly a dramatic change in the hydrodynamic regime), model results show that
more than 40% of the total produced sediment yield of the area is retained by the
dam. The statement above is also supported by the fact that materials on the dam’s
bed have different characteristics compared to those form the rest of the area. All the
above suggest that dam construction should be carefully practiced, always
considering the possible consequences that may often be dramatic. The possible
‘loss’ of a beach on an area with tourism-dependent economy, like in the situation
studied, is unfortunately, a good such example.
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