Floods play an important role in deltas, particularly in their geomorphological and ecological processes. Current deltas are exposed to strong anthropogenic influences. In the Neretva River Delta (Croatia), which is located in the Mediterranean, numerous hydrotechnical facilities have been built for flood protection and flow regulation. The level of flood protection has significantly increased compared to natural conditions, because the main cause of flooding in this delta is fluvial flow, but there are still certain risks for material assets and the population. The hydrological regime of the Neretva River is strongly influenced by reservoirs and hydropower plants built upstream of the basin. This study presents a simulation of floods in the Neretva River Delta for various return periods (from 5 to 1000 years) to assess damages and risks. The extent of flooding is classified into three statistical levels: high, medium, and low probability. Floods in this area can cover approximately 110 km² with a low probability of flooding and material damage of approximately 250 million Euros. The average number of potential human victims would be 0.67 per year, and the potential average annual damage would be 11.63 million Euros, in the Donja Neretva according to the conditions of 2015. Climate change would increase the mentioned risks of floods, the potential number of victims and damage by 31 and 41%, respectively. Based on a multi-criteria analysis, protective measures including the construction of embankments and self-regulating gate were proposed. Their implementation would reduce the average annual potential number of victims and the average material damage by 70% and 83%, respectively. The results of this study should be used to identify critical areas and make decisions to improve the flood protection system in the delta area, as well as to improve water management in the entire basin, which is located in the territory of two countries Croatia and Bosnia and Herzegovina.

Delta, Croatia, flood, Neretva, risk

Estuaries and deltas are considered particularly vulnerable to floods. These areas are exposed to water action in two directions. On the one hand, the sea acts on the region (maritime influence), on the other hand, there are meteorological and hydrological processes that occur in the entire basin and affect the natural processes of the estuary and/or delta on the upstream side. Among the mentioned influences, there are several factors (causes) of floods, natural and human, which often act simultaneously (Bonacci 1987; Kranen 2010; Ljubenkov and Papić 2015; Poulos et al. 2022; Ljubenkov et al. 2024).

Estuaries and deltas are found worldwide in various climatic regions, maritime and continental influences are different and vary over in time (Ward et al. 2018). In general, flood risks are particularly important in low-lying and densely populated estuaries and deltas, where floods can cause human casualties and large material and social damage, as reported in numerous studies (Kranen 2010; Schwarz 2024). In addition to settlements and industrial and commercial facilities, agriculture is often present in these areas. Because of fertile land, large areas along estuaries and deltas were often meliorated, and agricultural production was established in former wetlands globally.

Schwarz (2024) specified the 250 largest deltas in the Mediterranean and provided their basic characteristics, including those of the Neretva River. Most deltas in the Mediterranean are exposed to greater or lesser anthropogenic influences. Hydrotechnical interventions, such as the regulation of watercourses and canals, the construction of infrastructure, or the reduction of flooded areas, lead to changes in space and stop the natural formation of deltas. Schwarz suggested that the protection and restoration of deltas in the Mediterranean should be initiated because these are important ecological systems. Therefore, it is important to analyze extreme situations, such as floods and droughts, because they have a significant impact on the delta region, well as on the natural and social processes that occur there (Meslard et al. 2022).

The Neretva Delta is one of the most valuable wetlands on the eastern coast of the Adriatic Sea and one of the few remaining wetlands in the Mediterranean region of Europe. Although a large part of the former extensive wetlands of the Neretva Delta has been converted into agricultural land, the remaining wetland habitats are representative and important at the international level. Therefore, this delta has been declared a wetland of international importance (Ramsar site) according to the Ramsar Convention of 1992.

In professional literature, estuaries and deltas are often considered from the perspective of small waters. A significant problem affecting deltas and estuaries is the increased intrusion of salt water, which is particularly pronounced during dry conditions (Ljubenkov and Vranješ 2012; Vranješ 2019; Lovrinović et al. 2023; Ljubenkov and Haddout 2024). This leads to an increase in salt content in aquifers and surface waters, which has a direct impact on water supply, agriculture, and other social activities (Romić et al. 2008; Racetin et al. 2020). The processes of salinization during the year are not constant, they vary and depend on hydrological and maritime conditions. The interaction between fresh and saltwater in surface streams and underground is a dynamic process that changes throughout the year. During periods of high water, including floods, the interaction between fresh and saltwater moves downstream and is suppressed by the stronger action of freshwater. Floods and droughts can cause extensive material and social damage and have a strong impact on the ecology. The lack of fresh water in estuaries has been described in numerous studies (Romić et al. 2008; Bellafiore et al. 2021; Kvesić et al. 2024; Ljubenkov and Haddout 2024).

This study focused on fluvial flooding in delta areas. The delta of the Neretva River (Croatia), which flows into the Adriatic Sea in the Mediterranean region, was analyzed in detail. There are eight operational hydropower plants in the Neretva Basin that directly affect the hydrological regime of the river. However, occasional floods still occur in the downstream of the Neretva River.

Batalla et al. (2004) stated that the magnitude of large waters decreased by 30% on average after the construction of hydropower plants in the Ebro River basin (Spain) compared to the period before their construction. The influence of regulatory and hydrotechnical structures on the hydrological regimes of estuaries has been reported in numerous studies (Bonacci 1987; Kresic 2013; Ljubenkov 2014; Kamidis et al. 2021; Meslard et al. 2022; Poulos et al. 2022). Van Alphen (2016) described a delta flood protection program in the Netherlands and proposed measures to improve it. One method for flood risk assessment is the application of appropriate hydrodynamic models. Suarez and Rodriguez (2007) used the soil and water assessment tool (SWAT) and HEC RAS models to simulate flood risks in the Aguan River Delta area (Honduras). Pramanik et al. (2009) used MIKE FLOOD model to simulate flood inundation extent in the Mahanadi River Delta (India).

To date, many studies and models have been developed to assess the extent of floods in deltas in the world (Pramanik et al. 2009; Belica et al. 2013; Leauthaud et al. 2013; Dinh et al. 2019; Poulos et al. 2022; Khemiri et al. 2024). A few studies refer to the impact of sea level rise on deltas in the Mediterranean (Alvarado-Aguilar et al. 2012; Sayol and Marcos 2018; Falciano et al. 2023). However, there is lack of studies that assess the fluvial flood events, associated damages and risks in deltas, particularly in the Mediterranean. Therefore, this study makes a contribution to the research and provides new perspectives on delta fluvial flooding. The effort becomes more demanding considering the transboundary character of both the Neretva River delta and its catchment, that makes management and mitigation of flooding a challenging task.

The goal of this study was to determine the causes of floods and to quantify large water bodies and their distribution in space using the example of a delta from Croatia, which improves our understanding of these processes. In addition, this study provides a methodology for collecting and analyzing input data to make the best possible assessment of flood risks. Hydrodynamic models MIKE 11 and MIKE FLOOD (DHI) were used to simulate the floods. The EU Directive on the assessment and management of flood risks (2007/60/EC) obliges the EU member states to develop flood risk maps. However, in order to obtain reliable flood risk maps, it is important to select proper type of inundation model as well as properly apply the models. Therefore, this study contributes to this task and proposes practical measures to mitigate floods in the Neretva River delta. Floods are important for numerous aspects of social activity and are important considerations for spatial planning. Therefore, this methodology can be applied to similar systems worldwide.

The flood risk in the Neretva delta was determined in the form of the average annual potential number of victims and damage for three scenarios: a) existing conditions (year 2015), b) the impact of climate change, and c) new conditions with the implementation of recommended protection measures.

The main drawback of this study is that the results of the 2015 research were used (Kvesić et al. 2016). From a hydrological point of view, it would be useful to consider more recent data, i.e. to extend the input series after 2013. Since it is a large and complex system, such modelling would require considerable resources and time. Therefore, the extension of the input hydrologic data could be considered as part of future activities. In certain areas, only a digital model of the terrain was used instead of geodetic measurements, which gives a certain inaccuracy to the topography of the terrain. Regardless of the stated limitations of the input data, the conducted research determined the critical areas in the Neretva River delta, and the potential damages and risks were quantified.

The Neretva River rises southeast of the Zelengora Mountain (peak 2032 m above sea level) in Bosnia and Herzegovina (BiH), at a height of 1,095 m above sea level. The total length of this river is approximately 220 km, of which 22 km is in the Republic of Croatia (Fig. 1). The basin of this river, with numerous hydrotechnical structures, is located on the territory of two countries. The management of this system is particularly demanding because it consists of two independent states with separate protocols and professional services. Although the relationships and obligations of both the states regarding water management issues are defined, such systems have certain limitations.

In its upper and middle reaches (upstream from the Počitelj settlement; approximately 35 km from the mouth), the river flows through narrow valleys and canyons, representing a typical mountain river with occasional steep slopes (Fig. 1). After leaving the canyon, the Neretva passes through the valley and has a lowland course with gentle falls.

The total surface area of the Neretva topographic catchment is approximately 10,975 km² (Fig. 1) and is mostly spread over the Dinaric Mountain massif in BiH (Bonacci 1987; Bonacci and Jelin 1988; Kresic 2013; Vranješ 2019). Only a small part of the basin (692 km²) is located in the Republic of Croatia. It is also the most downstream part of the basin, where a complex hydrographic network has developed over an alluvial area of approximately 190 km², forming a delta of the Neretva River with numerous branches (Fig. 2) (Ljubenkov and Vranješ 2012; Vranješ 2019; Ljubenkov and Haddout 2024).

The Neretva Delta formed during the last ice age, when the sea level rose and flooded the karst areas along the river to Hutovo Blato. Gravel and sand brought by the river from the upstream part of the basin have been deposited in the valley, thereby increasing the ground level. The delta existed naturally until the end of the 19^th century, when the first regulatory work began. This first focused on the maintenance of the waterway from the sea to Metković, for commercial transport. Then, during the 20^th century, numerous embankments for flood protection, dams, and other hydrotechnical facilities were built and wetland reclamation began, significantly changing the natural features of the delta. In its current state, downstream from Metković, the river is directed into the main riverbed. In addition to the mainstream, traces of the former delta are visible, including numerous abandoned meanders and backwaters.

In the second half of the 20^th century, a number of hydropower facilities with reservoirs were built in the upstream basin of the Neretva, greatly influencing the water regime: hydroelectric power plant (HEPP) Jablanica (1954), HEPP Rama (1969), RHEPP Čapljina (reversible, 1979), HEPP Grabovica (1981), HEPP Salakovac (1981), and HEPP Mostar (1985) (Fig. 1, Table 1). At the beginning of the 21^st century, the HEPP Peć Mlini was built on Tihaljina (2004), the right tributary of the Neretva, which is called Trebižat in the downstream, and the HEPP Mostarsko Blato (2010). In the upper reaches of the Neretva, the HEPP Konjic was planned, and the construction of the HEPP Ulog began (Fig. 1). In addition to the inflow of the Neretva River, which provides the majority of the water, there are numerous occasional or permanent sources along the edge of the valley (Donja Neretva, Fig. 1) that feed the Neretva and Mala Neretva rivers, particularly during the wet season (Ljubenkov and Vranješ 2012; Ljubenkov and Haddout 2024).

The Neretva and Trebišnjica river basins (Fig. 1) are interconnected through deep and underexplored underground karst. Therefore, it is currently impossible to reliably define their mutual borders, that is, to define their hydrological and hydrogeological relationship. The entire area of the Neretva and Trebišnjica basins is represented by developed Dinaric karst, which is characterized by complex surface and underground flow processes (Bonacci et al. 2006; Kresic 2013).

Figure 1. 

Neretva River basin with marked locations of hydroelectric power plants (HEPP).

Figure 2. 

Donja Neretva area with marked locations of meteorological and hydrological stations.

The climatological characteristics of the Neretva Basin are characterized by a transition between a moderate continental and maritime regime, given that the largest part of the basin is located in the area of the Dinaric Mountain massif, which separates the maritime and continental zones. In most of the basin, the most significant precipitation occurs in winter, and the minimum occurs in summer. The average annual precipitation ranges from 1,000 mm on the coast to 1,900 mm in the mountainous parts of the basin. The maximum annual precipitation in the elevated parts of the basin is over 2,900 mm. There are two climatological stations in the Donja Neretva area Metković and Opuzen (Fig. 2, Table 2). Fig. 3 shows the characteristic values of monthly precipitation at Metković station for 2014–2023. On average, the rainiest quarter of the year is November–January, both in the coastal and continental parts of the watershed therefore, it is the period when higher waters in watercourses are most frequent. In this quarter, the highest recorded monthly precipitation was 400 mm (November 2021).

The long-term average air temperature in Donja Neretva ranges from 14 to 16 °C. The highest average monthly temperature occurred in July and the lowest in February. Average daily air temperatures in winter reach -6 °C. The highest mean daily temperatures exceeded 30 °C.

In the Donja Neretva Basin, there are no particularly strong winds, and the dominant winds are jugo (southeast wind) and bura (northeast wind), which are most common in the spring and November.

Figure 3. 

Characteristic values of monthly precipitation (Metković, 2014–2023).

In Donja Neretva, the most widespread strata are permeable carbonate deposits, mostly limestone. The basic characteristic is that most of the precipitation water immediately seeps underground, and occasional surface watercourses are formed only during heavy and long-lasting rains (Bonacci 1987; Bonacci and Jelin 1988; Kresic 2013; Vranješ 2019).

The Quaternary sediments deposited in the valley of the Donja Neretva are fluvial Pleistocene-Holocene sediments composed of porous sandy and clay gravels, which are most often covered with dusty clays whose base is formed by fluvioglacial deposits, because of the high level of underground water. Part of the surface is covered by muddy swampy, and occasionally flooded terrains.

The Croatian Donja Neretva area has twelve operational hydrological stations (Fig. 2). Basic data on the hydrological stations are listed in Table 3. The Metković and Opuzen stations are located on the Neretva River. Water levels are continuously measured at each station. It was not possible to determine the flows unambiguously because of the influence of the sea. Three stations were located on the Mala Neretva watercourse. However, because the inflow into the Mala Neretva is controlled by floodgate, which is lowered during periods of high water, the Mala Neretva accepts only water from its basin. The three hydrological stations are located on the right tributary of the Neretva River, the Norin River. Of these, two are located relatively close to the Neretva (Fig. 2), whereas one is at its source, Prud. Other hydrological stations are located on smaller watercourses of the left tributaries of the Neretva. Two stations are on the Koševo-Vrbovci Lateral canal, one on the Bijeli Vir spring (Mislina River), and one on Lake Kuti.

Floods in the Donja Neretva area may have several causes. Floods are primarily caused by the inflow of large waters through the Neretva River from the upstream part of the basin (BiH) and to a lesser extent, by the inflow of water from springs on the right edge of the valley and the rise in sea level due to tides. Floods on the left bank of the Neretva and Mala Neretva rivers are primarily influenced by sources in the Metković-Kuti area (the most significant source is the Bijeli Vir), which are part of the Trebišnjica River basin, the left tributary of the Neretva and the largest European sinkhole river. The smallest impact on the flooding on Neretva was the precipitation that fell directly on the Donja Neretva area. The impact of local precipitation was significant only for floods on the left banks of the Neretva and Mala Neretva rivers. The basin of the Neretva River belongs to the Dinaric Karst. Therefore, most of the precipitation quickly sinks, accumulating large amounts of water in the underground system, which reappears on the surface via numerous springs, particularly along the edges of the valley, and flows into the Neretva, Mala Neretva, and Norin watercourses. “Stretched” runoff hydrographs are formed under such complex runoff conditions and the interaction of surface and underground water in the Donja Neretva area.

The constructed system of HEPPs, primarily with large storage lakes, such as HEPP Jablanica (Table 1), has a dominant influence on the frequency, extent, and duration of floods in Donja Neretva. In addition, its effect on the flow regime, the construction of reservoirs in the upstream Neretva has a long-term impact on the reduction of sediment delivery to the valley, deepening the Neretva bed. Combined with the reduction in inflow in certain periods, this worsens the problems of water and soil salinity in Donja Neretva.

Water structures constructed on the Trebišnjica River (Table 1) have a dominant influence on the size and duration of floods on the left banks of the Neretva and Mala Neretva rivers. Several of these structures are under construction, and other currently planned, the so-called upper and lower horizons (HEPP Nevesinje, Davar and Bileća; Fig. 1) with the large accumulation lake, Bileća. It will have an area of 33 km² and volume of 1,277 mil. m³, which will further change the hydrological regime of the Trebišnjica River and its influence on Donja Neretva (Vranješ 2019). Floods on the left banks of Neretva and Mala Neretva do not coincide with flood events in Neretva.

The hydrological station furthest upstream on the Neretva River in the Republic of Croatia is Metković, located approximately 21 km from the mouth of the river and 1 km from the state border. This station also represents the upstream boundary conditions of the hydrodynamic model. Fig. 4 depicts a series of maximum and average annual water levels from 1935–2022 (N = 88), with breaks in four years. The highest water levels in Metković (> 430 cm) were recorded in the 1950s before the construction of HEPP Jablanica and HEPP Rama (Fig. 4). After 1961, the highest recorded water level was 414 cm (December 2010) (Table 4). The highest water levels in 2010 caused flooding in a large part of the valley.

Figure 4. 

Series of maximum and average annual water levels in Metković (1935–2022).

The Neretva River is the largest river on the eastern coast of the Adriatic Sea with its inflow and basin. The river delta formed in the geological past over an area of approximately 190 km². Natural geomorphological and hydrological processes occurred mostly undistributed until the end of the 19^th century when the first regulatory work in the region began. The current hydrological regime is strongly influenced by reservoirs and hydroelectric power plants built in the middle and upper upstream areas of the Neretva River, in the territories of Bosnia and Herzegovina. The plan is to build new hydropower plants in the Neretva and Trebišnjica basins, further impacting the hydrological regime.

Flood situations in the Neretva Delta are primarily caused by the inflow of large amounts of water from the upstream part of the basin (BiH). A smaller contribution to large waters was provided by numerous springs on the edge of the valley and the action of the sea. Precipitation that falls directly into the Donja Neretva area has the least impact on floods.

To quantify of flood events, a hydrological analysis of the available measurements and flood simulations were conducted. Owing to the influence of the sea, hydrological measurements in the delta area are focused on water level measurements, where there is no unambiguous relationship between water level and flow. Synthetic water waves were determined for return periods of 5–1,000 years. The MIKE FLOOD hydrodynamic model determined the coverage of floods for the specified return period, as well as the relevant flood parameters that were used to calculate damage and flood risk.

For the Donja Neretva area (Republic of Croatia), the potential material damage of approximately 250 million Euros and the possible number of human victims were determined for the existing conditions and different return periods. The estimated possible damage in the Neretva River delta amounts to 1% of the total flood damage for the Republic of Croatia. Flood risk was classified into three probability categories: high (return period of 25 years), medium (100-year flood), and low (1,000-year flood), with an average annual damage of 11.63 million Euros.

The present flood defense system of Donja Neretva includes numerous hydrotechnical structures (protective dikes, levees, canal networks, pumping stations, etc.). The level of flood protection has significantly increased in the last 50 years. Floods are natural phenomena that cannot be completely prevented; however, with the constant development of flood defense systems and the construction of protective and regulatory water structures, as well as the implementation of flood defense measures and other appropriate measures, flood risks can be reduced to an acceptable level. With the implementation of the proposed measures the average annual potential number of victims and the average material damage would be reduced by 70% and 83%, respectively.

Part of the conducted research was related to modelling the impact of climate change, which included sea level rise and increased water inflow from the upstream part of the basin. The estimated average annual number of victims was 0.88, and the potential annual damage was 16.5 million Euros, for the end of the 21^st century, which represents an increase of 31 and 41%, respectively.

The flood simulation presented in this study was as part of the 2015 project (Kvesić et al. 2016). Hydrological series of up to 53 members were used, with the last year being 2013. As part of further activities, the simulations could be repeated with extended series. In this way, climatic and hydrological variations from the last decade would also be considered.

In a large and complex system such as the Neretva delta, there is always a certain geometric (topographic) inaccuracy. Also, there is an influence of temporary and permanent springs along the edge of the valley without defined associated hydrographs. It is certainly reflected in the results of the simulations. However, such influences are partially compensated by the calibration process. In future activities, appropriate hydrological analyzes could be carried out with the aim of determining the hydrographs of marginal sources.

The analysis of flood events, the calculation of potential damages and risks, and the proposal of optimal measures for additional risk reduction represent a quality basis for the further development of flood protection and should help experts in making decisions related to flood defense. The aforementioned methodology is universal and can be applied to similar estuary and delta systems worldwide with their own specificities. Similarly, the occurrence of floods is important for numerous natural and ecological processes that occur in delta areas (estuaries). Therefore, the application and results of hydrodynamic modelling can also be used for ecological studies, as they quantify numerous hydrological parameters (e.g., water depth, flow rate, flood duration, and salinity). This is particularly important in the context of recommendations to preserve natural wetlands in the Mediterranean region, which includes this delta.