GEOLOGICA CARPATHICA, JUNE 2008, 59, 3, 261—268
www.geologicacarpathica.sk
Application of grain-size trend analysis and spatio-temporal
changes of sedimentation, as a tool for lagoon management.
Case study: the Kotychi lagoon (western Greece)
PAVLOS AVRAMIDIS
1*
, DIONISIOS BOUZOS
1
, VASILIOS ANTONIOU
2
and
NIKOLAOS KONTOPOULOS
1
1
University of Patras, Department of Geology, 26500 Patras, Greece; p.avramidis@upatras.gr
*Present address: Technological Educational Institute of Mesolonghi, Department of Aquaculture and Fisheries Management, 30200
Mesolonghi, Greece; pavramid@teimes.gr
2
Agricultural University of Athens, Iera Odos 75, Athens, Greece
(Manuscript received June 15, 2007; accepted in revised form December 13, 2007)
Abstract: The Kotychi lagoon is located on the northwest Peloponnese Greece, along a wave dominated and microtidal
coast. The lagoon is one of the most important ecological areas in Greece, with international significance as it is protected
by the Ramsar International Convention and is listing in the Natura (E.C.) Catalogue. For the present study 59 surficial
samples were collected from the bottom of the lagoon, on a grid basis (approximately 300 m spacing) using a grab and
were analysed for their grain size distribution as well as grain size parameters, mean, variance and skewness. Using the
moment measures, grain-size trend analysis was made and the definition of trend vectors were estimated. Grain-size trend
analysis indicates that the sediments of the lagoon are mainly sandy mud. Application of trend analysis indicates that the
sediment pathways along the lagoon can be related to a) the sediments source area of the lagoon (small river supplies), b)
wind related water circulation and c) the NW and SW wind directions. Moreover, comparing 1990 and 2000 satellite
images we have detected three major areas where the shallow waters show a significant increase: area (a) in the northeast
part of the lagoon, area (b) in the middle-west part and area (c) in the southern part. All these changes are related to river
discharges and to the winnowing effect. Detection of changes in the coastline of the lagoon, showed a total reduction of
areas covered by water by up to 13.8 %. The biggest change was detected in the northeast and in the southeast area of the
lagoon and is related to the sediment supply from the periphery of the lagoon and to a minor degree to the bottom sediment
pathway and winnowing effect. The rate of change of the area covered by water shows a rapid filling up of the lagoon and
that additional measures have to applied for the protection of the lagoon ecosystem.
Key words: Greece, Kotychi lagoon, sedimentology, coastline changes, grain-size trend, remote sensing.
Introduction
Sediments are typically described by particle size distribution
based on the log of the grain size. Use of the moment mea-
sures (mean, variance and skewness) highlights important fea-
tures which can help in the interpretation of depositional
environments and the mean of sediment transport. The use of
change of the moment measures demonstrate the transport
processes, selective entrainment transport and deposition.
Several methods using grain-size parameters of bottom sedi-
ments have been proposed (McCave 1978; McLaren 1981;
McLaren & Bowles 1985; Gao & Collins 1992; Le Roux
1994; Le Roux et al. 2002). All methods are based on the
same premise that sedimentary processes produce changes in
the grain-size parameters. McLaren (1981) used the spatial
changes in moment measure to identify the transport direc-
tions in a modern environment. This author concluded that
successive deposits along the transport path must become bet-
ter sorted and more skewed, although they can be finer or
coarser. Later McLaren & Bowels (1985) modified the previ-
ous conclusion which relates two cases of grain-size trends to
net transport paths. Although this approach is widely accept-
ed, Gao & Collins (1991) reexamined the model and support
the view that other cases can also occur along the transport
path. Later Gao & Collins (1992) developed a model for the
analysis of grain-size trends, which defines transport vectors
for a grid of sampling sites and based on this model the signif-
icance of the exported trends are statistically tested on the ba-
sis of the length of the characteristic vector. This model is
supported by Gao (1996) with a FORTRAN program which
defines the sediment transport pathways.
All the above methods have been applied mainly in har-
bours where deposition of silt clay and sand is a problem, and
the pattern of sediment movement is the main parameter
which has to be evaluated. Moreover, the estimation of sedi-
ment transport paths is a tool for the protection and the evolu-
tion of wetlands. The pathways in lagoons and lakes and
specially in environmentally protected areas, can help in pro-
tection and planning administrative strategies for these eco-
systems. In the present paper we apply the Gao & Collins
(1992) method to the Kotychi lagoon, which is located in
western Greece and is one of the main environmentally pro-
tected areas in Greece. Remote sensing satellite data were
used, as they provide a unique tool for research and monitor-
ing coastal areas and deltaic environments (Ciavola et al.
1999). Additionally, mapping of the coastline using multi-
262
AVRAMIDIS, BOUZOS, ANTONIOU and KONTOPOULOS
temporal satellite data has been proved to be an important
tool, as existing maps are often not accurate (in the first
place), not updated and so sometimes unable to follow rapid
coastline changes.
The objectives of the present paper are the calculation of the
surfare sediments distribution pattern, the estimation of sedi-
ment transport pathways along the lagoon and the relationship
between hydraulic conditions and lagoon siltation. Moreover,
the topographical alteration of the lagoon, based on satellite
data, during the last ten years is presented and compared with
the sediment pathways and their source areas. Modeling the
sediment transport pathways and changes around the Kotychi
lagoon could be a managerial tool for researchers to propose
solutions in these kinds of ecological problems.
Geological setting
Greece has 24 lagoons, with a total surface of 24,500 ha.
10 % of this surface is natural, 85 % partially natural and 5 %
man-made. There are also marshes with a total surface of
70,900 ha (Greek Coastal Zone Management Report, 2006).
Wetlands are among the most threatened environmental ele-
ments. A lot of detrimental impacts have been observed as a
result of intensive agriculture, aquaculture, industry, overex-
ploitation of the water resources, pollution due to human ac-
tivities and urbanization, intense pasturing as well as
over-fishing.
The study area is the Kotychi lagoon (Fig. 1a,b), located in
the western Peloponnese (Fig. 1a), Greece. It is one of the
Fig. 1. a – Location of the study area and b – Geological map of the area around the Kotychi lagoon and lithological type distribution of
the bottom lagoon sediments based on Folk’s (1974) diagram.
263
GRAIN-SIZE TREND ANALYSIS AND SPATIOTEMPORAL CHANGES (KOTYCHI LAGOON, GREECE)
most important ecological areas in Greece, with international
significance as it is protected by the Ramsar International
Convention and is listing in the Natura 2000 (E.C.) Catalogue
with code GR2330006 as a Special Protected Area.
The lagoon is located along a wave dominated and mi-
crotidal coast. On the west side it is separated from the open
sea by a low relief barrier island, and has limited communica-
tion with the open sea, with a stable, short and narrow inlet
(Fig. 1a). On the landward lagoonal margins to the east, small
scale deltas are prograded into the lagoon. Intertidal and su-
pratidal mud flats are developed among deltas, covered with
plants, e.g. Salicornía (Fig. 1b). Depths in lagoon decrease
gradually with distance from the landward side of the barrier
island to the inner lagoonal margins. Although the maximum
depth is 2.5 m in front of the inlet, the average depth is only
0.5 m (Fig. 2a). Four artificial, very shallow channel-like fea-
tures run at right angles to the barrier coast and one more par-
allel to the coast. The average surface water temperature
ranged from 10 °C during the winter to 27 °C in the summer.
The lagoon is affected by semidiurnal tides. The tidal range is
of the order of 10—15 cm. The maximum combined tide and
meteorological tide amplitude is of the order of 25—30 cm.
The tidal current speed is 10—30 cm/sec at the inlet and ap-
proximately 0.5—1 cm/sec in the lagoon and is subjected by
NW and SW winds (Bouzos & Kontopoulos 1998) (Fig. 2b).
Kalivas et al. (2003), using as a reference lagoon area in the
year 1945, showed that the water area of the lagoon was per-
manent decreased between the years 1945 and 1987 (from
Fig. 2. a – Bathymetric map with the location of surface sediment samples; b – the development of current trends inside the lagoon un-
der the influence of the main prevailing winds, NW (white arrow) and SW (black arrow) (based on nearest meteorological station of An-
dravida airport; current trends inside the lagoon were measured using buoy drifter) (Bouzos & Kontopoulos 1998) and c – representative
hydrodynamic model, simulating water transport, based on a two dimensional model of finite differences (Fakiris 2005).
264
AVRAMIDIS, BOUZOS, ANTONIOU and KONTOPOULOS
iii) Topographical maps scale 1 : 50,000, from the Hellenic
Army Geographical Service.
The following steps describe the various image processing
methods and techniques:
Initially, a digital elevation model (DEM) of the study area
was extracted from the 1 : 50,000 scale topographic maps pub-
lished by the Hellenic Army Geographical Service. The con-
tour lines were digitized with an interval of 20 meters.
Additionally, surface-specific point elevations (using bathy-
metric data from 1995), including high and low points, were
digitized in order to improve the final digital product. A linear
interpolation method was applied, based on the Bongefors dis-
tance method (ILWIS User’s Guide, 2003) to transform the
contour data into a DEM, with a spatial resolution of 25
meters. With the intention of verifying its fidelity, the digital
elevation model was plotted against the 1 : 50,000 scale con-
tour map, by interpolating the elevation values and overlapped
onto the original topographical map. It showed a very good
correspondence of the contour lines.
In order to create a False Colour Composite image we have
to combine the adequate spectral bands and display them in
the RGB colour system. There are 120 possible colour combi-
nations of the data for a large number of applications. Theory
and experience, however, show that a small number of colour
combinations are suitable for most applications. The optimum
band combination is determined by the terrain, climate and na-
ture of the interpretation project (Sabins 1997). In the present
study the selected bands are 1, 2, 3 forming the false colour
composite image 3, 2, 1 (RGB) for detecting shallow areas of
the lagoon, and 4, 5, 7 forming the false colour composite im-
age 7, 5, 4 (RGB). After constructing the 4 composite images
(2 for each scene), the ERDAS Imagine software program was
used to detect any changes in the shallow waters and coastline.
The final products of the above procedure were two images
pointing out the total changes between the two scenes.
Results
Grain size distribution – Trend vectors
The main physical geographical characteristics of the area
around the Kotychi lagoon are presented in Fig. 1b as well as
the geological map of the area and the distribution of litholog-
ical types of the lagoon surface sediments. The lithological
types were estimated according to Folk’s (1954) classifica-
tion. The main lithological type of the lagoon sediments is
sandy mud, with exceptions in the eastern edge of the lagoon
and some locations around the lagoon where thinner material
of silt-clay has been deposited. The sand class is almost absent
from the lagoon sediments and is restricted only to a narrow
zone on the sea side. The mean grain size of the Kotychi la-
goon sediments ranges from 5.15
Φ to 9.9 Φ with an average
of 7.38 (Table 1). The distribution of mean size shows three
small regions that are covered by finer sediments (Fig. 3a).
These are: 1) the area close to the marsh environment, 2) the
zone in front of the Gouvos river mouth, and 3) the part of the
lagoon just behind the northeastern part of the barrier island
(Fig. 1). The rest of the lagoon is characterized by coarser ma-
97.8 to 40.3 % of the lagoon reference area). Moreover, in
1987, the area of the lagoon covered by land or swampy areas
reached its maximum (59.7 % of the reference area) (Kalivas
et al. 2003). These dramatic developments are due to irrigation
works, which began in the early 70’s and rapidly changed the
ecological and environmental situation of the lagoon (Karan-
tounias et al. 2005). The irrigation trenches which directly in-
fluence the lagoon are established in the southern part of the
Kotychi watershed. The major source area for the lagoonal
sediments is river supply. The main rivers which supply the
lagoon with water are the Vergas, Klimatsidi, Kapeleteikos,
Pepa, Gouvos, Sykios and Trykokias (Fig. 1b). The biggest
flood discharges are from the Vergas (278 m
3
/sec) and
Trykokias rivers (108 m
3
/sec). A minor contribution is shell
fragments from autochthonous biogenic production. Aeolian
sands, suspended sands eroded on tidal flats and lagoonal mar-
gins, or sand washed into the lagoon by washover fans are
negligible.
Methods
Sedimentological data
In 1995, 59 surface samples were collected from the bottom
of the lagoon (Fig. 2a), on a grid basis (approximately 300 m
spacing) using a grab. The grab sampler enables the top 10 cm
of sediment to be sampled. Positioning was achieved with a
hand held GPS (accuracy 5 m). All the samples were analysed
for their grain size distribution. Material coarser than 4
Φ was
dry sieved, while fine-grained material > 4
Φ was analysed us-
ing the pipette method, and grain size distribution was calcu-
lated. Grain size parameters (mean, variance and skewness)
were calculated. In each sample location, the lagoon depth
were measured and a bathymetric map of the lagoon was pro-
duced on the basis of 1995 data (Fig. 2a).
The grain-size trend analysis was done, based on Gao &
Collins (1992) model, using Type 1 (
σ
2
<
σ
1
,
µ
2
>
µ
1
and
Sk
2
< Sk
1
) and Type 2 (
σ
2
<
σ
1
,
µ
2
<
µ
1
and Sk
2
> Sk
1
) trends.
The above models apply to inner continental shelf and coastal
environments, and seem to be the most appropriate as their
patterns have been used in investigations with artificial tracer
experiments using hydrodynamic data (Delisle 1994; Turner
1994). The calculations were made with a FORTRAN pro-
gram developed by Gao (1996) and the definition of trend
vectors were estimated.
Satellite data set
In order to estimate the spatio-temporal changes, based on
satellite images of the lagoon, the following data sets were used:
i) A Landsat 5 Enhanced Thematic Mapper (ETM + ) image
(7 spectral bands) was obtained covering the study area (path
184, row 034), acquisition date: 28 August 1990 (5 % cloud
cover).
ii) A Landsat 7 Enhanced Thematic Mapper (ETM + ) image
(7 spectral bands) was obtained covering the study area (path
184, row 034), acquisition date: 15 August 2000 (5 % cloud
cover).
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GRAIN-SIZE TREND ANALYSIS AND SPATIOTEMPORAL CHANGES (KOTYCHI LAGOON, GREECE)
terials (Fig. 3a). This pattern of the distribution of mean size
does not relate to the water depth; thus wave energy cannot
significantly distribute sediment according to grain size.
The sorting fluctuates between 1.81
Φ and 5.30 Φ with an
average of 3.57
Φ (Table 1). The sediments are very poorly
sorted to extremely poorly sorted (Fig. 3b). This indicates that
Table 1: Particle size analysis results and the moment measures for the analysed samples.
the lagoon sediments were not transported for a considerable
time or distance. The skewness varies from —0.64 to 0.17 with
an average of —0.26 (Table 1). The major part of the lagoon is
covered with coarse to very coarse skewed sediments
(Fig. 3c). The near symmetrical and fine skewed sediments
constitute only 27 % of the lagoon and occur mostly in the
266
AVRAMIDIS, BOUZOS, ANTONIOU and KONTOPOULOS
Fig. 3. Distributions of: a – graphic mean size (Mz), b – graphic standard deviation (
σ), c – skewness (Sk) and d – kurtosis (Kg).
first two mentioned regions (the area close to the marsh envi-
ronment and the zone in front of the Gouvos river mouth),
(Fig. 3c). The dominance of the negatively skewed sediments
suggests that they are unaffected by the current action. Further-
more the negative sign of skewness shows the operation of the
winnowing action and the absence of deposition (Duane 1964).
On the contrary, the two regions (the area close to the marsh en-
vironment and the zone in front of the Gouvos river mouth) in-
dicate deposition or a state of flux (Fig. 3) (Duane 1964).
The kurtosis varies from 0.72 to 3.13 with an average of
1.27 (Table 1). The major part of the lagoon is covered with
mesokurtic to very leptokurtic sediments (Fig. 3d). The
platykurtic sediments constitute only 15 % of the lagoon. The
mesokurtic to leptokurtic nature of sediments refers to the
continuous addition of finer or coarser materials after the win-
nowing action and retention of their original characters during
deposition.
Interpreting the results of the grain-size trend analysis
(Fig. 4) we cannot recognize a dominated unidirectional trend.
We observed two probable trends, a) NW and b) NE that
could be related to the main wind directions and to the river
supplies. The small depth of the lagoon can explain the close
relation between the dominant winds and the sediment trans-
port paths. As coarse material seems to be absent, the source
material of the river seems to be fine-grained and does not dif-
ferentiate the grain-size distribution of the lagoon sediments.
Furthermore, the pattern of the transport paths illustrates a
subtle cyclic circulation (Fig. 4). It is probable that the wind
plays a dominant role in this circulation of the lagoon as a re-
sult of the fact that the lagoon has a small size and is very shal-
low. On the basis of data from the nearest meteorological
station of Andravida airport and using a buoy drifter, Bouzos
& Kontopoulos (1998) suggest that the NW winds drive the
water to the SE and introduce a counter-clockwise circulation
while the SW winds cause an anti-counter-clockwise
(Fig. 2b). In order to simulate the water transport and set up a
reliable hydrodynamic model, using the two dimensional
model in finite differences program, Mike21 (DHI), Fakiris
267
GRAIN-SIZE TREND ANALYSIS AND SPATIOTEMPORAL CHANGES (KOTYCHI LAGOON, GREECE)
Fig. 4. The identified residual grain-size trend pattern, consisting of
transport vectors, for Kotychi lagoon, obtained from the Gao (1995)
FORTRAN program. After exporting the vector data from the FOR-
TRAN program, the plot was produced by SURFER.
(2005) also suggests that the wind currents produce a two cell
of cyclonic circulation with an opposite direction to each other
(Fig. 2c). For the above interpretation he took into account the
bathymetry of the lagoon, the bottom friction coefficient and
the prevailing winds. So, this circulation should be considered
the main mechanism for the winnowing effect.
Spatio-temporal changes
After constructing the 4 composite images (2 for each
scene) (Fig. 5), we detected three major areas where the shal-
low waters show a significant increase from 1990 to 2000:
area (a) in the northeast part of the lagoon, area (b) in the mid-
dle-west part and area (c) in the southern part (Fig. 6a).
We have also detected two areas with a small decrease of
shallow waters: one in the middle-east area of the lagoon (d)
and another one in the northwest (e) (Fig. 6a).
Detection of changes in the coastline of the lagoon showed
that we have a total reduction of areas covered by water esti-
mated at 13.8 % (Fig. 6b). The total water surface in 1990 was
up to 5,800,000 m
2
while in 2000 the water surface had de-
creased up to 800,000 m
2
. The greatest changes were detected
in the northeast and in the southeast area of the lagoon. The
above results are similar to those of Kalivas et al. (2003). Kali-
Fig. 5. The four false colour scenes of 1990 and 2000, we used for change detection. a – Bands 3, 2, 1, of 1990; b – Bands 7, 5, 4, of
1990; c – Bands 3, 2, 1, of 2000, and d – Bands 7, 5, 4, of 2000.
Fig. 6. Change detection a – of shallow areas
and b – of coastline in the lagoon. Bright co-
lours show an increase of the property, dark
colours a decrease and black shows that no
changes were detected.
268
AVRAMIDIS, BOUZOS, ANTONIOU and KONTOPOULOS
vas et al. (2003) using as a reference area the lagoon surface in
the year 1945, showed that the water surface of the lagoon was
permanent decreased between the years 1945 and 1987 (from
97.8 to 40.3 % of the lagoon reference area). This dramatic de-
velopment of the lagoon seems to be constant up to 2000 and
is a result of all man’s activities as deforestation and agricul-
ture, rapidly changing the ecological and environmental situa-
tion of the lagoon.
Conclusion
On the basis of the grain-size distribution, of the bottom la-
goon sediments, and the sedimentological environments dis-
tribution around the Kotychi lagoon, we can infer that the
main source area of the lagoon sediments is the river supplies
and a minor contribution is shell fragments from autochtho-
nous biogenic production. Aeolian sands, suspended sands
eroded on tidal flats or sand washed into the lagoon by wash-
over fans are negligible. Interpreting the results of the grain-
size trend analysis it seems that the sediment transport
pathways are related to the contemporary effect of prevailing
waves – winds and river discharges. The spatio-temporal
changes of the lagoon water depth from 1990 to 2000 suggest
three main sites of deposition/siltation and a reduction of ar-
eas covered by water up to 13 % (800,000 m
2
). Moreover the
lagoon area has the greatest reduction in the northeast and
southeast parts of the landward margin. All these changes ap-
pear to be regulated of the winnowing action of the water cir-
culation and are related to the river discharges.
The same regulating factors also operate for the distribution
of sediments. The water circulation is the product of the action
of the NW and NE winds and probably has a cyclonic charac-
ter. In order to avoid the silting up of the lagoon, disposal of
debris must be prohibited, particularly in the northeast and
southeast parts of the landward margin. If not, the landward
margin of the lagoon will be moved seawards from these two
parts. Moreover, all human activities such as deforestation and
agriculture, influenced the sediment accumulation pattern.
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