Erosion and sedimentation processes in the Bojana River Delta at the Adriatic Sea

Abstract

Even though the Bojana River is, after the Po River, the second largest tributary of the Adriatic Sea, its hydrological characteristics are little known. The erosion and sedimentation processes in the Bojana River channel have not been monitored in detail or corroborated by measurement. However, historical records of changes in the basins of the Skadar Lake and the Drim River can be used to shed some light on morphodynamics of the Bojana River. The emergence of an extremely attractive tourist island – Ada Bojana -- is of special interest, focusing on its current status and projection of its future viability.

Full text

1 23
Journal of Coastal Conservation
Planning and Management
ISSN 1400-0350
J Coast Conserv
DOI 10.1007/s11852-018-0634-9
Erosion and sedimentation processes in the
Bojana River Delta at the Adriatic Sea
Sava Petković & Goran Sekulić

1 23
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Erosion and sedimentation processes in the Bojana River Delta
at the Adriatic Sea
Sava Petković
1
&Goran Sekulić
2
Received: 4 June 2015 /Revised: 25 April 2018 /Accepted: 6 July 2018
#Springer Nature B.V. 2018
Abstract
Even though the Bojana River is, after the Po River, the second largest tributary of the Adriatic Sea, its hydrological
characteristics are little known. The erosion and sedimentation processes in the Bojana River channel have not been
monitored in detail or corroborated by measurement. However, historical records of changes in the basins of the
Skadar Lake and the Drim River can be used to shed some light on morphodynamics of the Bojana River. The
emergence of an extremely attractive tourist island –Ada Bojana –is of special interest, focusing on its current
status and projection of its future viability.
Keywords The Bojana River .Sedimentation .Erosion .Hydrology
Introduction
Despite the fact that the Bojana River is relatively short
(40 km), its hydrologic regime is highly complex, de-
pending on a number of natural and anthropogenic fac-
tors. The main natural factors are linked to the hydro-
logic regime of the Skadar Lake and its tributaries, par-
ticularly the Drim River (Fig. 1). The anthropogenic
factors include the impact of three large reservoirs on
the Drim River, as will be discussed later.
Up to the middle of the nineteenth century, the hy-
drologic regime of the Bojana River, depended solely
on the regime of the Skadar Lake out of which it flows.
An ancient map from 1688 depicts the Bojana River
and the Drim River as two separate rivers, running par-
allel to each other, and discharging into the Adriatic
Sea, some 25 km apart (Fig. 2). In 1859, a disastrous
flood caused the channel of the Drim River to change
radically its course, and to connect with the Bojana
River channel (Fig. 1). Thus, the hydrologic regime of
the Bojana River was completely altered. As no
measurements were taken at the time, no quantitative assess-
ment of the new flow regime could be made, except an obvi-
ous conclusion that water levels of both the Bojana River and
the Skadar Lake were permanently increased.
As the Drim River brought large amounts of sedi-
ment, the Bojana River channel underwent a significant
aggradation and reduction of cross-sectional area. The
rising riverbed hindered the outflow from the Skadar
Lake and consequently caused a permanent increase of
its water levels. After the Drim River cut a new chan-
nel, floods in the Bojana River valley became more
frequent and severe.
The study of the hydrologic and morphologic regimes of
the Bojana River is increasingly becoming important over
time, as this river has a dominant effect on the coastal area.
This is particularly true for the Delta and the so called Ada
Bojana - an island formed by deposition of sediment that the
Bojana River has been delivering to the Adriatic Sea for cen-
turies (Fig. 1). The sustainability of this island, which has
become one of the best known tourist attractions in
Montenegro, is directly dependent on the sediment supply
from the Bojana River. Ecological issues are important as
well. The Bojana River Delta is an unique ecosystem in
Europe, and the BDesignation Act of the Bojana River Delta
as a Protected Area^classifies it asthe most important wetland
in the Eastern Mediterranean (Dömpke et al. 2008).
Conservation of its biodiversity, and habitats of some endan-
gered species, emerges as a task of the highest priority.
*Goran Sekulić
sgoran2000@gmail.com
1
Faculty of Agriculture, University of Belgrade, Belgrade, Serbia
2
Faculty of Civil Engineering, University of Podgorica,
Podgorica, Montenegro
Journal of Coastal Conservation
https://doi.org/10.1007/s11852-018-0634-9
Author's personal copy

Hydrology of the Bojana River
Available data
The first survey of the Bojana River channel was undertaken
in 1928. Later data collected in 1965 showed the aggradation
of the riverbed due to important sediment inflow from the
Drim River. It was estimated that about 2.5 million m
3
of
sediment has been deposited between 1928 and 1965
(Jovanovic & Djordjevic 1974). The recorded water level
and discharge data up to 1965 showed that the average annual
discharge of the Bojana River as it outflows the Skadar Lake
was around 320 m
3
/s. At the same timeframe, the mean annual
flow rate of the Drim River at its junction with the Bojana
River was 280 m
3
/s. Subsequent investigations showed that
the mean annual discharge of the Drim River increased to
360 m
3
/s, such that the mean annual discharge of the Bojana
River at its delta was 680 m
3
/s.Thisisinaccordancewiththe
Albanian sources, stating that the mean annual discharge of
the Bojana River at its delta is 682 m
3
/s (Drin River
Hydropower Plants 2006). Distribution of the mean monthly
flow rates of the Bojana and Drim Rivers is given in Table 1.
Data on flood stages of the Bojana River are highly unre-
liable. The rating curve could not be established because the
riverbed was constantly rising. However, according to some
estimates, the 100 year return period discharge is of the order
of 8500 m
3
/s (Schneider-Jacoby et al. 2006). The same return
period discharge of the Drim River is of the order of 6530 m
3
/s
(Schneider-Jacoby et al. 2006). This information is question-
able, because the probability of flood wave coincidence was
not analyzed. It seems that a more reliable estimate of the high
flows of the Bojana River can be based solely on the flood
stages of the Drim River, as, by creating strong backwater
effects, this river controls the outflow from the Skadar Lake.
Thus, it can be assumed that high flows of the Bojana River
equal those of the Drim River. Furthermore, it has been noted
that during extremely high floods a part of the Drim River
flows upstream and into Lake Skadar.
Flood stage data on the Drim River vary over a broad range.
It was estimated that during the enormous 1896 flood, the peak
discharge of the Drim River was about 7000 m
3
/s (Euronatur
2009). Albanian sources state that, according to their hydrologic
analyses, the extreme peak discharges of the Drim River can
reach 9000 m
3
/s (Drin River Hydropower Plants 2006).
Impact of the reservoirs
In the late sixties, Albania began to build three large reservoirs
on the Drim River: Vau I Dejes, Komani and Fierze (Fig. 3).
The one closest to the Skadar Lake and the junction of the
Bojana River and the Drim River –reservoir Vau I Dejes –
was formed by construction of three dams –Qyrsad, Zadeja
Fig. 1 Satellite image of the Bojana River between the Skadar Lake and the Adriatic Sea (Source: Google Earth©). The Bojana River flows out of the
Skadar Lake, and after 4.5 km joins the Drim River
S. Petković,G.Sekulić
Author's personal copy

and Ragam –in the early seventies, with a total storage vol-
ume of 0.623·10
9
m
3
. The spillway capacity of the three dams
was designed according to the estimated peak discharge of the
Drim River of 9000 m
3
/s. The Qyrsaq and Zadeja dams were
provided with spillways, the total capacity of these being 3 ×
1400+ 2 × 1200 = 6600 m
3
/s. When the flow through turbines
(565 m
3
/s) is taken into account, the maximum discharge from
the Vau I Dejes reservoir (which can be expected in the Drim
River, at times of extreme flood waves, and consequently in
the Bojana River) amounts to 7165 m
3
/s. It is interesting to
note that plans for a spillway on the third - Ragam Dam - were
abandoned when a decision was made to build the upstream
Fierze reservoir, with a storage volume of 2.7 ·10
9
m
3
. With
this reservoir, completed in 1979, and the Komeni reservoir
with a storage volume of 0.43 ·10
9
m
3
, completed in 1988, the
total storage volume of all three reservoirs on the Drim River
Fig. 2 The 1688 map of the Skadar Lake, showing separate channels of the Bojana River and the Drim River, and their confluences at the Adriatic Sea
(Source:https://commons.wikimedia.org/wiki/File:1690_Coronelli_Map_of_Montenegro)
Table 1 The mean monthly and annual discharges (m
3
/s) of the Drim and Bojana Rivers (2006)
River Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average
Drim 493 459 446 507 490 293 155 104 141 228 396 501 351
Bojana 1067 899 850 879 861 588 322 195 210 382 770 1030 682
Erosion and sedimentation processes in the Bojana River Delta at the Adriatic Sea
Author's personal copy

amounts to 3.76 ·10
9
m
3
, which surpasses the average storage
volume of the Skadar Lake, 2.6 ·10
9
m
3
.
Floods
The Skadar Lake, the Bojana River and the Drim River consti-
tute a complex hydrologic system with about 20,000 km
2
of
drainage area. The fact that all the water from such a large basin
reaches the Adriatic Sea only via the Bojana River represents a
major problem at times of heavy rainfall, because of the limited
bank-full flow capacity of this river, which is estimated by the
Hydrometeorological Service of Montenegro to be about
5000 m
3
/s.
Two major floods in the catchments of the Skadar Lake and
the Drim and Bojana rivers, caused by heavy rainfall in
Albania and Montenegro in late December 2009 and early
January 2010, and recurring in December 2010 and January
2011, provided some information about the flood regime, and
the flood management in the considered region. Although
water stages of the Skadar Lake and the Bojana River were
continually recorded by the Hydrological Service, no reliable
rating curve could be defined, due to a highaggradation rate of
the river channel. Thus, no reliable flow rate could be
established corresponding to the recorded stages. However,
the peak flow certainly surpassed 5000 m
3
/s as the floodplains
were inundated. This is explained by the fact that the water
which the Drim River delivered to the Bojana River signifi-
cantly hindered outflow from the Skadar Lake. As a certain
portion of the Drim River’s discharge went to the Skadar
Lake, and high discharges occurred on all of the Skadar
Lake tributaries, flooding was inevitable.
A question arises about the flood management offered by
the three reservoirs on the Drim River. Why were these par-
ticular flood waves not attenuated by the reservoirs? The an-
swer involves issues of planning and design, and the real-time
operation in practice. When the decision was made to build
the Fierze reservoir, the most upstream of the three, (during
construction of the VauI Dejes reservoir), hydrologic analyses
predicted that the maximum discharges of the Drim River
downstream from the Vau I Dejes reservoir would not exceed
2900 m
3
/s, thus no flooding was to be expected along the
Bojana River, and in the vicinity of the Skadar Lake. After
construction of the last reservoir, Komani, the cascade of all
three reservoirs was expected to reduce the peak flows of the
Fig. 3 Vau I Dejes, Komani and Fierze reservoirs on the Drim River (Source: World Bank 2008)
S. Petković,G.Sekulić
Author's personal copy

Drim River below 2000 m
3
/s, provided the reservoirs were
emptied ahead of the flood arrival. However, this is not what
actually happened. During the 2009–2011 floods, all three
reservoirs were full, and the flood waves were merely trans-
mitted over spillways downstream, causing huge damage.
Unfortunately, at the time, there was no communication be-
tween the water directorates of Montenegro and Albania, and
no early warning actions and emergency measures were un-
dertaken to reduce the risk to population and property.
However, it is not too late to draw a lesson from these flood
events. In fact, some collected data can be used to hind cast (or
back-calculate) the Drim River discharges. From daily logs of
the Vau I Dejes hydropower plant, the maximum reservoir
water levels could be determined, as well as the discharges
over the spillway and through the turbines. The total discharge
hydrograph would be indicative of the unsteady flow behavior
of the Drim River. This information would be valuable for
future operational flood management on the Drim River, and
subsequently on the Bojana River.
Sediment transport in the morphodynamics
of the Bojana River
Ada Bojana is an island which was created as a result of the
deposition of sediment carried by the Bojana River to the
Adriatic Sea after the Drim River altered course to flow into
the Bojana River. This island, with about 5 km
2
of surface
area, is situated in the Bojana River Delta (Fig. 4), and now
is one of the most important tourist resources of Montenegro
(IK Konsalting 2010).
It is well known that as an alluvial watercourse empties into
a sea, its longitudinal slope or grade diminishes over time, and
its sediment transport capacity becomes less. The result of this
process is the formation of islands and deltas. The
morphodynamics of the Bojana River delta can be tracked
from historical records –ancient maps of the Skadar Lake
region and the coastline of the Adriatic Sea. The oldest such
map dates back to 1570, and depicts two sandbars at the
mouth of the Bojana River, but no large island (Fig. 5).
Other maps from the 17th and 18th centuries show neither
sandbars nor an island. A map in Fig. 6shows no presence of
an island in the first half of the 19th centry. (An error is un-
likely, because this map shows two islands along the course of
the Bojana River, thus if there had been an island at the mouth,
the map would have certainly depicted it.).
The two maps thus presented are helpful in understanding
the origin of the Ada Bojana island. They show the Bojana
River and the Drim River as separate watercourses with
mouths at the Adriatic Sea, 25 km apart. While, at that time,
the Drim River’sDeltawaswellformed(Fig.5), the Bojana
River had then no apparent delta (Figs. 5and 6). This can be
explained by the fact that the Skadar Lake trapped sediments
from all its tributaries, thus the quantity of sediment the
Bojana River carried to the Adriatic Sea was relatively small
and insufficient for creation of a major delta. This situation
began to change after 1859, when a disastrous flood struck the
region; the new channel of the Drim River was formed, which
joined the Bojana River some 4.5 km downstream from the
Skadar Lake.
1
This event altered completely the hydrology,
hydraulics, and sedimentology of the two rivers. Water from
the Drim River was then introduced into the Bojana River
channel, while the old Drim River channel gradually silted
up and is now a recipient only for surface runoff. Large
amounts of sediment carried by the Drim River led to aggra-
dation of the Bojana River riverbed, and formation of sedi-
ment deposits at the junction with the Adriatic Sea. Thus be-
gan the formation of the Ada Bojana island (Fig. 7).
A qualitative assessment of the Adriatic Sea coastline evo-
lution, including Ada Bojana, can be made from maps and
aerial images in Fig. 7a-c. Even without proper
georeferencing and scaling, it is obvious that the surface area
of Ada Bojana and the western part of the Big Beach in Ulcinj
increased between 1900 and 1970. During that period, prior to
the construction of dams on the Drim River, large amounts of
sediment reached the Adriatic Sea via the Bojana River.
After the dams on the Drim River were completed in the
seventies and eighties, causing a significant decrease in sedi-
ment transport downstream from the Vau I Dejes reservoir, the
sediment supply to the Adriatic Sea dramatically diminished.
The disturbance of the natural balance between the amount of
sediment the river delivered to the sea and the amount of
Fig. 4 Ada Bojana, an island formed by the bifurcation of the Bojana
River at the Adriatic Sea. (Source: Google Earth © 2013)
1
Some believe that the Ada Bojana island is the result of sinking of the ship
BMerito^at the mouth of the Bojana River in 1858. This hypothesis seems
unlikely, for two reasons: (i) the obstacle was too small to initiate a significant
sediment buildup, and (ii) the amount of the incoming river sediment was
insufficient to form a large island.
Erosion and sedimentation processes in the Bojana River Delta at the Adriatic Sea
Author's personal copy

Fig. 5 Map of the Skadar Lake and the Bojana River in 1570 (Source: The French National Library)
Fig. 6 Mouth of the Bojana in the first half of the nineteenth century (Source:https://sq.wikipedia.org/wiki/Historia_e_Shkodres)
S. Petković,G.Sekulić
Author's personal copy

Big Ulcinj beach
Fig. 7 Evolution of the Adriatic Sea coastline, including the Bojana River Delta: (a) 1900, (b) 1970, (c) 2002 (Schneider-Jacoby et al. 2006)
Erosion and sedimentation processes in the Bojana River Delta at the Adriatic Sea
Author's personal copy

sediment washed away by the sea, has resulted in erosion of the
beaches around the mouth of the Bojana River, as well as its
right branch at the Ada Bojana island. Historically, in 1900 the
surface area of the Ada Bojana island was 36.4% of its area in
1970, while in 2002, the area was only 7% less than in 1970.
This indicates that over the past several decades, Ada Bojana
has been in a state of dynamic equilibrium, i.e. the sediment
supply from the Bojana River to the Adriatic Sea is sufficient
to compensate the erosion of the coastline due to sea waves and
currents. The evolution of the Bojana and Drim Rivers and the
Delta is schematically shown in Fig. 8.
Future trends
Instability of coastlines and beaches is a widespread global phe-
nomenon. A comprehensive elaboration of this topic can be
found, for example in (Komar 1976;O’Neill 1985). In addition
to the unrestricted natural physical action of sea currents and
waves on coastline erosion, the twentieth century has witnessed
the undesirable environmental impact of these natural actions as
may be exacerbated by human intervention –such as the intensive
development of tourist facilities, building reservoirs on rivers that
convey sediments to the sea, uncontrolled dredging of sediments
etc. Although the climate change impact on coastline erosion can-
not be predicted, due to many uncertainties, it can be expected that
an increase in global temperatures and the rise of seas and oceans
will generally worsen the present situation. Beach erosion in the
Mediterranean is very pronounced. Some 40% of beaches in the
European part of the Mediterranean are exposed to erosion pro-
cesses. For instance, about 1500 km of the coastline and 27% of
Italian beaches are threatened (Directorate General Environment,
European Commission 2004). In Albania, it is estimated that the
coastline in the vicinity of the old mouth of the Drim River is
receding toward the mainland, on the average, by 2.5 m each year,
and about 400 m of the coast has been lost in this region since
1936 (Regional Environmental Center (REC) 2006). An even
higher level of erosion has been observed at the mouth of the left
branch of the Bojana River, where the sea has washed away some
500 m of the coast since 1936. Further decrease of sediment
supply, which can be expected if Albania decides to construct
two more dams downstream of the Vau I Dejes reservoir, will
intensify the coastal erosion problem in the Bojana River Delta
(Euronatur 2009). One possible scenario of future coastal erosion
and disappearance of the Ada Bojana is shown in Fig. 8,No.5.
Activities of the European Council indicate the level of
attention devoted to erosion in the Mediterranean countries.
At the end of 2003, the European Parliament discussed a com-
prehensive report on coastal erosion in the Mediterranean and
its impact on tourism (Rigoni 2003).
How to save the Bojana River Delta?
The catastrophic floods of 2009–2011 induced a significant
morphological change of the bifurcation of the Bojana River,
Fig. 8 Historical development of the Bojana and Drim Rivers and the Delta in the last 150 years (Author’s synthesis)
S. Petković,G.Sekulić
Author's personal copy

atAdaBojanaisland(Fig.4). A barrier of amassed tree trunks
and sand bars was formed, obstructing flow in the right branch
of the river. A dramatic aggradation of the entire right branch
has been noted in the past ten years, such that there is justifi-
able fear that this branch will eventually disappear and merge
with the Big Beach at Ulcinj. The process is temporarily re-
tarded by large-scale dredging along the most critical reaches,
in order to preserve the Ada Bojana island.
The question is what can be done at this time to put a stop
to the highly unfavorable natural processes that threaten the
Bojana River Delta? The engineering approach calls for main-
taining the flow in the Bojana River as the first and utmost
priority, and to revitalize the banks of the Ada Bojana island,
as well as the shoreline of the Big Beach. However, environ-
mentalists strongly oppose any intervention in the channel of
the Bojana River (including the removal of deposited sedi-
ment and tree trunks), claiming that this would disturb the
unique ecosystem in the Delta. It seems that a trade-off is
needed to reconcile environmental, hydraulic engineering
and tourism perspectives. This would urgently require very
serious hydraulic, sedimentation, and environmental studies
to be carried out in order to reach a realistic, feasible scenario,
which could be implemented to the benefit of all stakeholders.
Conclusions
&Up to the middle of the nineteenth century, the hydrolog-
ical regime of the Bojana River, an outflow of the Skadar
Lake, depended solely on the hydrological regime of the
lake. After a disastrous flood in 1859, the Drim River,
which, until then was flowing directly to the Adriatic
Sea, changed its course, and became a tributary of the
Bojana River. After this, the hydrological, sedimentolog-
ical and morphological regimes of both rivers significantly
changed, as depicted on ancient maps and shown by his-
torical data.
&By a qualitative study of the morphodynamics of the
Bojana River Delta, it is shown that the Ada Bojana island
was formed by excessive sediment supply from the Drim
River, once it became a tributary of the Bojana River. The
surface area of the island has varied over time, reaching its
maximum surface area of about 5 km
2
.
&Further morphological changes of the Delta occurred with
the construction of three large reservoirs on the Drim
River in the seventies and eighties; by this action, the
sediment supply from the Drim River was thereby signif-
icantly diminished, thus causing erosion in the Delta and
its vicinity along the Adriatic Sea coast. The building of
two more reservoirs on the Drim River in Albania would
threaten the disappearance of the Ada Bojana island, and
is likely to intensify the coastal erosion problem in the
Bojana River Delta.
&Two disastrous floods in 2009–2011 have significantly
altered the bifurcation of the Bojana River, by forming a
barrier of tree trunks and sediment bars in the right river
reach. The civil engineering solution for restoring the flow
capacity, based on dredging sand bars, barrier removal,
and bank consolidation, is opposed by environmentalists
on the ground that these works are detrimental to the
unique ecosystem.
&This conflict of interest could be resolved by participatory
planning, which would evaluate all interactions, and pro-
pose the implementation of an integrated and balanced
solution to the benefit of all stakeholders.
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