Current status and restoration options for floodplains along the Danube River

Science of the Total Environment 543 (2016) 778–790

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Current status and restoration options for floodplains along the Danube River Thomas Hein a,b,⁎, Ulrich Schwarz c, Helmut Habersack d, Iulian Nichersu e, Stefan Preiner a,b, Nigel Willby f, Gabriele Weigelhofer a,b a

WasserCluster Lunz GmbH, Dr. Carl Kupelwieser Promenade 5, 3293 Lunz am See, Austria University of Natural Resources and Life Sciences, Institute of Hydrobiology and Aquatic Ecosystem Management, Max-Emanuel Straße 17, 1180 Vienna, Austria FLUVIUS Floodplain Ecology and River Basin Management, Hetzgasse 22/7, 1030 Vienna, Austria d University of Natural Resources and Life Sciences, Institute of Water Management, Hydrology and Hydraulic Engineering, Muthgasse 107, 1190 Vienna, Austria e Danube Delta National Institute for Research and Development, 165 Babadag street, Tulcea 820112, Romania f Biological and Environmental Science, University of Stirling, Stirling FK9 4LA, Scotland, United Kingdom b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Floodplain areas in large river systems have decreased immensely during the last century. • The potential Danube floodplain area useful for restoration remains high (8102 km2) • Restoration is limited by stakeholder needs, acceptance, and resource availability. • Combining different EU targets can increase the effectiveness of restoration projects.

a r t i c l e

i n f o

Article history: Received 27 March 2015 Received in revised form 4 August 2015 Accepted 14 September 2015 Available online 23 October 2015 Editor: D. Barcelo Keywords: River restoration Water framework directive

a b s t r a c t Floodplains are key ecosystems of riverine landscapes and provide a multitude of ecosystem services. In most of the large river systems worldwide, a tremendous reduction of floodplain area has occurred in the last 100 years and this loss continues due to pressures such as land use change, river regulation, and dam construction. In the Danube River Basin, the extent of floodplains has been reduced by 68% compared to their pre-regulation area, with the highest losses occurring in the Upper Danube and the lowest in the Danube Delta. In this paper, we illustrate the restoration potential of floodplains along the Danube and its major tributaries. Via two case studies in the Upper and Lower Danube, we demonstrate the effects of restoration measures on the river ecosystem, addressing different drivers, pressures, and opportunities in these regions. The potential area for floodplain restoration based on land use and hydromorphological characteristics amounts to 8102 km2 for the whole Danube River, of which estimated 75% have a high restoration potential. A comparison of floodplain status and options

⁎ Corresponding author. E-mail addresses: [email protected] (T. Hein), ulrich.schwarz@fluvius.com (U. Schwarz), [email protected] (H. Habersack), [email protected] (I. Nichersu), [email protected] (S. Preiner), [email protected] (N. Willby), [email protected] (G. Weigelhofer).

http://dx.doi.org/10.1016/j.scitotenv.2015.09.073 0048-9697/© 2015 Elsevier B.V. All rights reserved.

T. Hein et al. / Science of the Total Environment 543 (2016) 778–790 Floodplains Danube river Flood protection

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for restoration in the Upper and Lower Danube shows clear differences in drivers and pressures, but certain common options apply in both sections if the local context of stakeholders and societal needs are considered. New approaches to flood protection using natural water retention measures offer increased opportunities for floodplain restoration, but conflicting societal needs and legal frameworks may restrict implementation. Emerging issues such as climate change and invasive non-native species will need careful consideration in future restoration planning to minimize unintended effects and to increase the resilience of floodplains to these and other pressures. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Floodplains are crucial components of riverine landscapes (Tockner and Stanford, 2002). They are important areas for biodiversity and contribute to a broad variety of ecosystem functions by controlling the regional water cycle and the retention and transformation of nutrients in river systems (Weigelhofer et al., 2015; Sanon et al., 2012; Schindler et al., 2014). Their importance in fluvial landscapes is related to the high spatial heterogeneity in concert with a high and flood-driven temporal variability (Tockner et al., 2000). These dynamic fluvial processes are responsible for habitat succession and rejuvenation (Hohensinner and Drescher, 2008), leading to a high biodiversity along aquatic – terrestrial boundaries (Bunn and Arthington, 2002). Lateral hydrological connectivity, defined as the water exchange between floodplain and river, is the key determinant of water-related ecosystem processes such as nutrient turnover, vegetation development, and geomorphic change (Amoros and Bornette, 2002; Hein et al., 2004; Schoenbrunner et al., 2012; Welti et al., 2012). Although there is a wide appreciation of the ecosystem services provided by floodplains and their vital role in riverine landscapes (e.g. Posthumus et al., 2010; Sanon et al., 2012; Schindler et al., 2014), there have been dramatic losses of floodplain habitat due to land reclamation and channel engineering, resulting in a functional degradation of these systems worldwide (Tockner et al., 2010). The reduction and degradation of floodplain systems has diminished their capacity for water retention, thus enhancing flood risks (Habersack et al., 2015), while other key floodplain functions and services, such as groundwater replenishment, nutrient storage (Hein et al., 2004), and water purification have also declined in effectiveness. The dramatic loss of riparian ecosystems threatens the conservation of key species and habitats, such as pioneer plants and soft- and hardwood tree species of alluvial forests (Scholz et al., 2012) and the aquatic vegetation of backwater systems (Keruzore et al., 2013). These problems are particularly acute in the case of the Danube. With a length of approximately 2800 km, the Danube crosses 15 countries from its origins in southern Germany to its confluence with the Black Sea (Romania). It drains a catchment of approximately 801,100 km2 linking different natural and cultural landscapes and shaping the history of the region like no other European river (Tockner et al., 2009). Human culture and development have greatly affected the Danube River and the surrounding landscapes. Over the last century, the floodplains of the Danube and its tributaries have been subject to major human interventions causing significant changes in the hydromorphology of the river-floodplain ecosystem (Hohensinner and Drescher, 2008). The increasing demands for land for settlement and agriculture have resulted in large-scale river regulation measures for flood protection, forcing the river into a single, spatially restricted channel between extended dams. About 39% or 1111 river km of the entire Danube are impounded by a total of 78 dams (ICPDR, 2009). As a result, more than 68% of the active floodplains of the Danube River, which are in frequent exchange with the main river channel, have already been lost (e.g. DPRP, 1999; Schwarz, 2010; Tockner et al., 2009). The consequences of these changes are predictable, and include an increase in serious flooding in different regions of the catchment, an increasing pollution load due to continuing emissions and reduced retention capacities, the loss of physical habitat diversity, and a correspondingly high percentage of endangered riverine species (ICPDR, 2009). Taken together, the Danube River Basin is therefore

among the most pressurized large river catchments in the world (Tockner et al., 2009). An improvement of the current status of residual floodplains through the basin-wide application of restoration measures is one of the key water management issues identified in the Danube River Basin Management Plan (DRBMP; ICPDR, 2009). The political changes in Central and Eastern Europe and respective EU policies (EC Water Framework Directive, EC Floods Directive, EC Habitats and Birds Directives; Council of the European Communities, 1992, 2000, 2007) are fostering efforts to protect the remaining Danube floodplains and restore the former hydrological dynamics by re-connecting floodplains with the main river. In addition, the 1975 Ramsar Convention on Wetlands (www.ramsar.org) supports the further conservation and restoration of floodplains and was instrumental in securing early protection for some examples of Danube floodplain wetland. This paper presents the current status and the development perspectives of floodplains along the Danube River and its major tributaries. Based on the assessment of the overall restoration potential of Danube floodplains, we (i) present case studies from the Upper and Lower Danube to compare different approaches, (ii) assess the potential of various restoration measures to improve floodplain conditions, considering local drivers, obstacles, and opportunities, and (iii) address societal aspects concerned within the Danube River Basin. 2. Current state and restoration potential of floodplains in the Danube River Basin (DRB) Assessing the restoration potential of Danube floodplains with respect to their current state and various pressures in the different reaches of the DRB is one of the pre-requisites to support and stimulate restoration projects. The first comprehensive and systematic evaluation of existing floodplains along the Danube and in the lowland parts of its major tributaries was commissioned by the UNDP/GEF as part of the Danube River Pollution Reduction Programme (DPRP, 1999; Zöckler, 2000). Initiated and funded by the WWF International Danube-Carpathian Programme, a revised floodplain inventory in the DRB was performed in 2010, including additional main tributaries of the Danube with larger floodplain areas (including the Mura, Drava, Tisza, Sava, Prut, and Mures; Schwarz, 2010). Floodplain areas were classified into a) the morphological floodplain delineated by post-glacial lower terraces and b) the active floodplain located within the existing flood protection dikes. Active floodplains were further distinguished according to the nature of flooding and the lateral hydrologic connectivity, being subdivided into near-natural floodplains (Type 1), elevated floodplains, impacted by strong aggradation and often used for agriculture (Type 2), and laterally disconnected floodplains along impounded reaches (Type 3). The main reason for type 3 floodplains is river bed incision, which reduces flooding, lowers the groundwater table, and promotes drying out of floodplain forests. According to the revised 2010 assessment, the morphological floodplain area totals 26,524 km2, equivalent to 3.3% of the DRB (Table 1). The current active floodplain area of the Danube amounts to 8452 km2 (out of which the Danube itself covers a water surface of approx. 1724 km2), thus showing a reduction by 68% of the original area (Table 1). If the main tributaries are included, the total loss equates to 80% of the original floodplain areas in the DRB (Schwarz, 2010). While in the upper and middle reaches most floodplains were assigned to types 2 and 3 (Schwarz, 2010), the

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Table 1 Comparison of floodplain areas and potential restoration areas for the DRB. Modified after Schwarz, 2010.

Upper Danube 950 km (DE, AT)b Middle Danube 900 km (SK, HU, HR, RS, RO) Lower Danube 850 km (RO, BG, MD, UA) Danube Delta 100 km (RO, UA) Danube total 2800

Main pressures

Dominant types

Impoundments, altered hydrological regime; construction of flood protection dikes between 1870 and 1950

Types 3 and partly 2; almost 92% loss of type 1c Type 2 (90% loss of type 1)

many free-flowing sections, but channel incision; disconnection of floodplains for drainage, agriculture, and flood protection between 1890 and 1970

Area of morphological floodplain [km2]a

Area of active floodplain, incl. Main channel [km]

Floodplain Potential Examples of significant floodplains loss [%] areas for restoration [km2]

2831

707

75

532

10,369

2143

79

1562

Gemenc National Park, Kopacki Rit

Lower Danube Islands (Belene), Islands of Braila

Systematic disconnection after 1960, but no large-scale aggradation yet; relatively good status

Types 1 and 2

8033

2208

73

5038

river modification for navigation and in agriculture after 1970; relatively good status

Types 1 and 2

5291

3394

36

970

26,524

8452

68

8102

Confluence Danube/Isar, Donau-Auen National Park

Sfântu Gheorge branch, Kiliya Channel

km a

Morphological floodplain = delineated by post-glacial terraces; active floodplain = area within flood protection dikes. b DE = Germany, AT = Austria, SK = Slovakia, HU = Hungary, HR = Croatia, RS = Serbia, RO = Romania, BG = Bulgaria, MD = Moldavia, UA = Ukraine. c Floodplain Type 1 = active floodplain with typical habitat conditions (near-natural), side-channels with pioneer stands, floodplain forests and pastures, wetlands and oxbows, Type 2 = active elevated floodplain, strongly altered due to substantial aggradation (sedimentation) and mostly used for agriculture; but still potentially flooded during major flood events, Type 3 = active floodplains along impounded reaches/backwaters (often disconnected laterally from the main channel) still flooded regularly by tributary confluences and during major flood events.

remaining floodplains of the lower reaches and the Danube Delta are less impacted by channel incision, showing a high percentage of morphologically intact reaches (natural islands which are often used for poplar and willow plantations). The estimation of potential restoration areas along the Danube was based on the criteria of land use, hydromorphological features (e.g. flooding dynamics), size and location, and the overlay with protected areas (Schwarz, 2010; DPRP, 1999). According to this assessment, potential restoration areas along the Danube amount to 8102 km2 in total, with 1797 km2 located in the active floodplain and 6305 km2 in the former floodplain (Table 1). About 8% of these areas lie in “near-natural” floodplains (including the free-flowing section of the Danube between Vienna and Bratislava, among others). The potential for restoration increases from the Upper Danube to the Lower Danube where large-scale restoration of the former floodplains is still considered feasible. In the Delta, about 75% of the potential restoration areas are agricultural polders, which can be reconnected at relatively low cost using existing simple engineering techniques, like breaching of embankments and blocking of man-made channels. Depending on the floodplain size, hydromorphology, intactness, and feasibility for restoration, about 19% of the potential restoration areas of the Danube were assigned “very high” restoration potential, 56% with “high” and the remaining 25% with “moderate” restoration potential (Schwarz, 2010). Fig. 1a gives an overview of restoration projects across the entire Danube River Basin. It comprises already completed or ongoing projects in the Upper Danube basin and Lower Danube/delta, officially planned restoration projects, namely in Romania (although Lower Danube restoration projects have

latterly been limited to a few sites and polder solutions are now preferred) and other potential restoration areas along the Danube and main tributaries. As an example, Fig. 1billustrates the restoration potential of the Middle Danube, which is part of the analysis of restoration potential for the Mura-Drava–Danube Transboundary Biosphere reserve (Schwarz, 2013). Especially in the Lower Danube and the Danube Delta, where extensive areas of mostly intact floodplain habitats can still be found, conservation rather than restoration is an issue. Floodplain conservation means the protection of the river system by changing the way the river is managed (Boon, 2005). Thus, priorities for sustainable river management in the Lower Danube and the Danube Delta are to minimize the risks and damage to existing floodplain habitats necessary for flood retention and to protect endangered species, such as the sturgeon (Bloesch, 2003). 3. Restoration effects in selected case studies in the Upper and Lower Danube 3.1. Austrian Danube floodplains between Vienna and Bratislava The 45 km river stretch between Vienna and Bratislava is located within one of the last free-flowing sections of the Upper Danube River (DPRP, 1999; Reckendorfer et al., 2005). Large-scale river regulation measures in the 19th century, designed to improve navigation and flood protection, have resulted in the isolation of the once extensive and dynamic floodplains from the main river channel and their degradation to a chain of decoupled floodplain remnants with prevailing

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Fig. 1. a. Overview of implemented, planned, and proposed restoration measures in the Danube River Basin (Schwarz, 2010). b. Potential restoration areas along the middle Danube (Schwarz, 2013).

lentic conditions and ongoing terrestrialization processes (Hein et al., 2006). As the largest remnant of alluvial landscapes in Central Europe, providing habitat for a diverse fauna and flora, this stretch was declared a National Park in 1996 (Schiemer et al., 2007). However, river bed incision, ongoing terrestrialization, and an increasing loss of aquatic habitat threaten the existence of these floodplains and require restoration concepts which re-integrate the remaining floodplain areas with the fluvial dynamics of the main channel (Reckendorfer et al., 2005). Applying the above mentioned assessment of the restoration potential (section 2), the area of the former floodplain amounts to 303 km2 between Vienna and Bratislava, with 98 km2 (32%) located in the active floodplain. The estimated potential restoration area is 60 km2.

The main drivers for floodplain restoration in this stretch of the Austrian Danube are the National Park regulations and the associated management plan (NP Donau-Auen Managementplan, 2009). This seeks to (i) rehabilitate rare aquatic and semi-aquatic habitats (e.g. oligotrophic to mesotrophic standing waters with Littorelletea and IsoëtoNanojuncetea communities; muddy river banks with Chenopodion rubri p.p. and Bidention p.p. vegetation communities), (ii) restore the ecological functioning of the Danube floodplains, (iii) protect the existing fauna, flora, and designated habitats based on European regulations (e.g. EC Habitats Directive), and (iv) to implement an integrated flood protection management scheme based on the EC Floods Directive (Fig. 2). The use of floodplains for water purification is of less importance due to the

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Fig. 2. Drivers, pressures, and management options: comparison of Upper Danube and Lower Danube (including the Danube Delta).

relatively moderate pollution in this part of the Danube (ICPDR, 2014). Challenges for floodplain restoration are the ongoing river bed incision, originating from river channelization and impoundments in the upstream reaches (Reckendorfer et al., 2005), and climate change effects such as water temperature increase and discharge fluctuations (Zweimüller et al., 2008). Floodplain restoration is restricted by various human demands. For example, navigation requires the maintenance of a homogenous and stable shipping channel, while settlements restrict the available space for restoration. In the currently active “Integrated River Engineering Project east of Vienna (River-km 1921–1872)”, managers aim to both maintain adequate fairway parameters for international navigation, and to improve the ecological state of the Danube and its floodplains by preventing further riverbed incision, removing bank protection, and re-connecting side-arms (Reckendorfer et al., 2005). Concerns of NGO's in the initial phase of this project were addressed in a second phase through an improved participatory approach based on the PLATINA guidelines (ICPDR, 2010). Besides, inconsistencies in different stakeholders' interests may also restrict the potential for floodplain restoration (Sanon et al., 2012). For example, restoration towards a hydrologically dynamic reference state may threaten the existence of protected limnophilic plant species (e.g. Stratiotes aloides, Nuphar lutea, Nymphea alba) that have been able to spread in the floodplain only after river regulation. The stretch of the Danube between Vienna and Bratislava comprises various floodplain sections which differ in their potential reversibility towards pre-regulation conditions, the existing lateral connectivity, the technical potential for improvement, socio-economic constraints, and nature protection values (Schiemer et al., 2007). These segments can be considered as discrete units for which individual restoration schemes have to be developed on a step-by-step basis (Reckendorfer et al., 2005). Starting in 1996, various sections of the floodplain were re-connected to the Danube by re-opening side-arms and lowering check-dams and culverts within the side-arm systems (Fig. 3, Table 2).

The different projects were funded by the EU (e.g. EU Life projects) and from various national sources (Reckendorfer et al., 2005). These projects have sought to (i) increase the lateral connectivity of the side-arms with the main channel, (ii) improve water supply to the floodplains, (iii) establish lotic habitats, (iv) mobilize or relocate sediment so as to rejuvenate habitats and reduce autochthonous terrestrialization and (iv) stimulate recolonization of floodplain sidearms by native rheophilic Danube fish species such as Chondrostoma nasus, Zingel streber, Rutilus pigus virgo, and Aspius aspius (Table 2). In general, the projects achieved a change in the structural features of the side-arms, but failed in their functional objectives, such as providing suitable habitat for Danube fish or notable rheophilic macrozoobenthos species, such as the odonate Ophiogomphus cecilia (Table 2). The hydrological connectivity of the floodplains increased from b30 days to N180 days per year in the re-connected sites, increasing also their discharge and reducing the water retention time (Hein et al., 2004; Schiemer and Reckendorfer, 2004). Re-connection resulted in the establishment of patches with increased water velocity in narrow parts of the side-arms, while large standing waters and small backwaters were not influenced to the same extent. Sediment relocation now occurs, but is restricted to fine sediment transport. This greatly limits the desired rejuvenation of floodplain surfaces (Reckendorfer et al., 2009), an intrinsic feature of dynamic floodplain systems (Gilvear and Willby, 2006). Erosion and sedimentation are only balanced when the duration and magnitude of the connection is sufficient for the size of the side-arms. In Regelsbrunn (site F, Table 2, Fig. 3), for example, sedimentation by fine particles prevails, especially in wider parts of the main side-arm, while in the Orth floodplain, sediment transport is more balanced, but is controlled by lateral bank erosion. Despite an increase in species diversity, rheophilic organisms from the Danube main channel have not yet recolonized any of the re-connected floodplain side-arms. While re-connection has, for example, supported the migration of eurytopic fish species into the floodplains (e.g. Alburnus

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Fig. 3. Floodplains along the Danube between Vienna and Bratislava showing realized or planned restoration projects. Site names are given in Table 2. A, C, D, E, G, I = restored side-arms, description see Table 2. B = re-connection project currently under discussion, description see Table 2. F and H mark the locations of the two isolated, back-flooded side-arms shown in Fig. 4.

islands, Borcea/Ialomița and Brăila (Silistra-Brăila, river km 195), have formed. The subsequent downstream section from Brăila to Ceatal Izmail (river km 90) is already tidally influenced. The Danube Fens (“Bălţile Dunării”) represent a floodplain region generated by alluvial deposits from two or more Danube arms (Badea, 1969). The floodplain and the terraces of the Lower Danube were and still are subject to continuous geomorphic transformations caused by river planform dynamics, erosion, deposition, and wind action, as well as by anthropogenic activities at regional and local scales. During the 20th century, the floodplains of the Lower Danube were embanked on a vast scale to enhance navigation (Buijse et al., 2002), leading to the alteration and conversion of the floodplains. Large parts of the wetland were converted into agricultural polders and ponds for fish production (Staras, 2001). In addition, channels were constructed

alburnus, Leuciscus cephalus or Rutilus rutilus), the percentage of rheophilic species remains less than 10% in the side-arms compared to almost 30% in the Danube (Zweimüller, 2004; Zauner et al., 2005; Fig. 4). Due to insufficient discharge and lack of gravel substrates for spawning, re-connected side-arms offer unsuitable habitat for endangered Danube species such as Zingel streber (Zauner et al., 2005). 3.2. Lower Danube and Danube Delta The Lower Danube downstream of the Iron Gates dams is characterized by extensive floodplain formation along a stretch of about 1000 km (Fig. 5). Between the Iron Gate at river km 943 and Silistra (Bulgaria) at river km 345, a sector of floodplains, fens, ponds, marshes, and shallow lakes has developed. Where the river splits into branches, two large

Table 2 Implemented floodplain restoration projects along the Danube between Vienna and Bratislava, showing success or failure regarding structural and functional aims. Capital letters refer to the map in Fig. 3. LW = Low water level, MW = Mean water level. ↑ = positive effects, ↔ = no effects, n. e. = not evaluated (from Kum, 2004; Reckendorfer and Steel, 2004; Zauner et al., 2005). Upper Lobau A

Schönau C

Orth D, E

Regelsbrunn G

Johler Arm I

Time of restoration Hydrological Connection with Danube conditions Mean discharge

2005

1998–2004

1999–2003

1996/97

2012–14

NLW 0.25 m3 s−1

NMW 15–30 m3 s−1

NLW + 0.5 m 50–70 m3 s−1

NMW 40–60 m3 s−1

NLW 100–125

Structural aims

↑ N120 days per year ↑

↑ N130 days per year

↑ N300 days per year

↑ N160 days per year

↑

↑

↑

↑/↔ Only at narrow passages ↔ No sediment relocation ↑

↑ Whole side-arm

↑ Whole side-arm

↑ Whole side-arm

↑ Whole side arm

↔ Fine sediment relocation, sedimentation prevails n. e.

↔ Fine sediment relocation, balance between erosion and sedimentation n. e.

n. e.

↔

↔

↔

↔ Fine sediment relocation, sedimentation prevails ↑ Local reductions of macrophytes by 10–25% (vegetation cover) ↔

Increase lateral connectivity Increase surface and subsurface water levels Increase percentage of lotic habitats

Functional aims

Rejuvenation by sediment relocation Reduce terrestrialization

Recolonization by rheophilic species from Danube

m3 s−1 ↑ N300 days per year ↑

n. e.

n. e.

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Fig. 4. Composition of fish fauna in re-connected and isolated floodplain side arms along the Danube between Vienna and Bratislava. Capital letters refer to the map in Fig. 3. Description of restoration projects, see Table 2 (Zweimüller, 2004).

to connect the various floodplain lakes, leading to an increased input of eutrophic water and sediments from the Danube (Buijse et al., 2002). While the fluvial dynamics are no longer sufficient to generate new habitats, the embanked sectors of the floodplain show clear signs of ongoing terrestrialization and a reduction of the former dynamic habitat mosaic, both features of other floodplains (Naiman, 1988; Townsend, 1989). This spatial reorganization and modification of the habitat mosaic has impacted the aquatic habitat structure and the fundamental ecological processes. The transformation of the Lower Danube floodplains into terrestrial ecosystems has restricted the original broad range of ecological, economic, recreational, esthetic, and educational functions to predominately economic uses (Nichersu, 2006). Nowadays, about 5300 km2 of the floodplain area along the Romanian Danube (6250 km2 in total) are embanked and about 4428 km2 are affected by desiccation and drainage (LUCAS, 2012; CLC, 2006). Areas in more pristine hydromorphological conditions, which can still be used for flood water retention, amount to only 839 km2 (13%) of the former floodplain, distributed along the mouth of the Danube tributaries and the Natural Park Small Island of Brăila (Staras, 2001).

The consequences of converting up to 80% of the Lower Danube floodplain to agricultural land through embanking have been an increase of river water levels at peak flow by 0.6–0.8 m (Bondar, 1996) and the collapse of commercial fisheries (Staras, 2001). Despite these negative developments, the Danube Delta is still a region with an extremely high biodiversity, harboring about 1800 plant species and about 3500 animal species (Nichersu, 2006). Due to political reforms and the severe physical changes already imposed, the Danube Delta was declared a Biosphere Reserve in 1991 and the conservation of natural values and recovery of wetlands functions have become priority objectives (Staras, 2001). The first restoration measures were implemented in the Danube Delta between 1994 and 1998. Because of the low economic efficiency of the agricultural polders and fish ponds, some were restored by opening the surrounding dams, for example at the polders Babina islet and Cernovca islet, located in the northern Danube Delta near the Ukraine border. This restoration aimed to restore the pristine hydrological regime in the polders with both dry and flooding phases depending on the water level of the Danube (Navodaru et al., 2005; Tudor, 2008). In addition, basic ecological functions should be re-established and previously damaged or destroyed habitats reinstated. The most important function was considered to be the recycling and storage of nutrients (Table 3). Nutrients and suspended material are now filtered and retained more efficiently in the bio-accumulative horizon of the polder (Schneider, 2002; Suciu et al., 2002). Nowadays, 20 years after the introduction of reconstruction measures, the pre-existing ecosystems have been gradually restored. The polders, for example, are natural spawning habitats for different fish species like Cyprinus carpio, Carassius carassius or Tinca tinca. The proportion of predatory fish (15.2%), like Silurus glanis, Esox lucius or Perca fluviatilis, was also typical of the Danube Delta within just six months of reopening connections (Navodaru et al., 2005). Popina is a large embanked floodplain located in the northeastern Danube Delta. An abandoned fish pond in the southern part of the area (Popina II; area 3600 ha) was designated an ecological reconstruction site in 2000. The main objectives were to maintain and increase the ecological value of this site (e.g. nutrient retention) and improve benefits for the local people by reconnecting the site to the main channel (Tudor, 2008). However, while the retention rates for suspended material and nitrogen were high, no effective retention was observed for phosphorus (Ooosterberg et al., 2000). As long as the phosphorus concentration of the Lower Danube River is high (0.12–0.15 mg P L− 1), algal blooming due to eutrophication is still a problem and can only be prevented by short residence times (Tudor 2008). Nevertheless,

Fig. 5. Floodplain areas along the Lower Danube. Partly restored according to the EC WFD. Floodplains reverted to wetlands (green), floodplains with restored water storage function (blue).

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Table 3 Implemented floodplain restoration projects in the Danube Delta, showing success or failure regarding structural and functional aims. Capital letters refer to the map in Fig. 3. LW = Low water level, MW = Mean water level. ↑ = positive effects, ↔ = no effects, n. e. = not evaluated (Tudor, 2008).

Time of restoration Hydrological conditions

Connection with Danube Mean discharge

Structural aims

Increase surface and subsurface water levels

Increase lateral connectivity

Increase percentage of lotic habitats Rejuvenation by sediment relocation Functional aims

Reduce terrestrialization Recolonization by rheophilic species from Danube

Babina and Cernovca islets

Popina fish pond

1994–1998 NLW 60–80 m3 s−1 ↑ N180 days per year ↑ ↑ Whole side-arm ↔ Fine sediment relocation, sedimentation prevails ↑

1998–2000 NLW 10–15 m3 s−1 ↑ N110 days per year ↑ ↑ Whole side-arm ↔ Fine sediment relocation, sedimentation prevails ↑

↑

↑

• Production of a revised flood risk map based on a digital terrain model (DTM) • Restoration of selected polders to create wetlands with high conservation value (Special Areas of Conservation (SAC) and Special Protection Areas (SPA) according to EC Habitats Directive (Council of the European Community, 1992)) • Evaluation of economic activities in polders to integrate these into a mixed polder concept (agricultural polders for cultivation and water retention polders for flood control)

the millennium (Willi and Eberli, 2006; Barredo, 2007; Habersack et al., 2014), existing flood protection concepts were re-evaluated and the preservation and/or restoration of river floodplains were officially recognized as efficient tools for natural flood management and an effective alternative to classic engineering approaches (JRC, 2012). The adapted goal of the EC Floods Directive (Council of the European Communities, 2007) is to avoid a further deterioration of the ecological status of rivers due to dam construction, in parallel with the reduction of flood risk by increasing “room for the river” instead (Pahl-Wostl, 2006). Thus, this new flood protection approach meets the aims of both the EC Water Framework Directive (Council of the European Communities, 2000) and the EC Floods Directive by targeting integrated river basin management (Habersack et al., 2015). The restoration of river floodplains can significantly reduce flood risk, as shown by Schober et al. (2015) for the Austrian Danube. In reaches with extended, re-connected floodplains, flood peak reduction per kilometer river length amounted to −110 m3 s−1 km−1 and was, thus, 18 times higher than in the rest of the Austrian Danube. Depending on the inundated floodplain area, reductions of peak discharge of more than 1000 m3 s−1 could be achieved during a 100 years flood, equivalent to about 10% of the maximum flood discharge (Fig. 6). While Machland [III] and especially Tullnerfeld [VI] show both flood peak reduction of up to 1000 m3 s−1 and flood wave translation, the National Park Donau-Auen is characterized mainly by flood wave translation. The efficiency of floodplains concerning both processes depends on the geomorphology and roughness of the floodplain, flood wave form and flood magnitude, but mainly on the relation between the time the floodplain is inundated and the time of the flood peak. In all cases studied, significant positive effects of the floodplains in reducing downstream flood risks were found. Thus, in contrast to classical engineering methods, this integrated flood protection approach not only protects the immediate catchment, but also improves the situation downstream. Furthermore, it is conducive to supporting EU biodiversity targets by preserving or regenerating important riverine habitats.

4. Challenges and opportunities for floodplain restoration in the Danube River Basin

4.2. Multiple conflicting socio-economic demands and administrative pitfalls

4.1. New approaches in flood protection

The role of floodplains as key components of river ecosystems and important providers of numerous ecosystem services has been well recognized since the late 1980s (e.g. Henry and Amoros, 1996; Lorenz et al., 1997; Meyerhoff and Dehnhardt, 2007). Nevertheless, restoration is often faced with conflicting socio-economic demands regarding the use of both the river channel and its floodplains. Settlements, agriculture, forestry, hydropower generation, navigation, the economic development of the catchment, but also nature protection are among the

wetland restoration was considered to be a valid ecological and economic alternative to the management of unprofitable polders in the Danube Delta (Staras, 2001) because of the high potential for rapid recovery of natural floodplain functions after reconnection (Zöckler, 2000) and the scope to significantly enlarge the delta wetland area (Buijse et al., 2002). A program considering these challenges, the Ecological and Economical Resizing of Lower Danube Floodplain (REELD, Covasnianu et al. 2010), was established to assist the authorities in fulfilling the goals of the EC Habitats Directive and the EC Water Framework Directive and to support effective measures for prevention, protection and mitigation of flooding effects, as stipulated in the EC Floods Directive. The main objective of the REELD program is to provide the basis for implementing an integrated management concept for sustainable development and also to elaborate priorities and criteria for rehabilitation of the Lower Danube floodplain in accordance with these EC Directives. The program is based on a recent assessment during the Joint Danube Survey 3 (ICPDR, 2015), which shows the continued high hydromorphological quality of the Lower Danube section, even though the current draft of the DRBMP classifies the Lower Danube section as a heavily modified waterbody according to the EU WFD definitions (ICPDR, 2009). The program is structured following three main activities (identification, evaluation, and prioritization) and establishes three priorities (Covasnianu et al. 2010):

River engineering to increase flood protection is one of the main reasons for the dramatic loss of floodplain areas in Europe (Hohensinner and Drescher, 2008). Until the end of the 20th century, flood protection consisted of the construction of dams and levees to retain flood water within the channel and thus prevent flooding of terrestrial surroundings. However, after the severe floods in Europe at the beginning of

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Fig. 6. Flow characteristics of three different floodplains; top: Machland [III]; middle: Tullnerfeld [VI]; bottom: National Park Donau-Auen [VII]. Left side: hydrographs showing flood wave reduction and translation of a HQ100 (flood wave type 1); Right side: Flow characteristics for HQ5 (6620 m3 s−1) and HQ100 (11200 m3 s−1). Gray shaded area displays flooded area; arrows indicate main flow directions (after Schober et al., 2015).

interests that commonly oppose floodplains being allowed to regain their natural flooding dynamics (Moss, 2008; Buijse et al., 2002). As a consequence, project managers have to deal with various stakeholders and institutions of different legal status, originating in different sectors and, thus, often with differing and sometimes conflicting priorities

(Hein et al., 2006). Tackling these problems may not only delay, but more often restrict or even prevent large-scale restoration measures in floodplains (Moss, 2008). Regarding navigation, for example, the Danube has been identified as one of the most important inland waterways and priority axis of the EU's Trans-European Network for

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Transportation (TEN-T; http://ec.europa.eu/transport/themes/ infrastructure/index_en.htm). The aim of this initiative is to optimize transport routes, including the removal of “bottlenecks” that are mostly situated in Natura 2000 areas. However, traditional river engineering measures for providing adequate fairway parameters, such as dredging, diking, or damming, are incompatible with the conservation or restoration of ecologically sensitive river stretches and floodplains. They also contradict the WFD non-deterioration principle.Thus, new integrative and ecology-oriented approaches are needed which also consider ecological demands within waterway projects, as formulated in guidelines such as the EU PLATINA manual (ICPDR, 2010). In the case of the Danube, the involvement of different nations with different legal frameworks further increases the complexity of the task to implement large-scale and basin-wide floodplain rehabilitation measures in the DRB. As a consequence, many restoration projects are often limited to site-specific, small-scale measures (Moss, 2008) when fundamental change on much larger scales is truly required. Efforts to establish transboundary co-operations across the DRB are a necessary prerequisite to solving this problem. The Danube Carpathian Programme (DCP) of the Word Wide Fund for Nature, for example, which was implemented in 1992, marked an important first step towards basinwide concerted actions for the restoration of Danube floodplains (WWF, 2000). In co-operation with governmental and nongovernmental groups, several initiatives have begun to protect and restore floodplains along the Danube (e.g. several EU LIFE projects along the Austrian Danube, see ec.europa.eu/environment/life/project/Projects), and subsequently also along major tributaries (e.g. in the recently established Mura–Drava–Danube Transboundary Biosphere Reserve). The ICPDR (International Commission for the Protection of the Danube River) is an international organization consisting of 14 cooperating states and the European Union (www.icpdr.org). Since its foundation in 1998, the ICPDR has grown into one of the largest and most active international bodies of river basin management expertise in Europe, if not the world. The main aims of this organization are to implement the Danube River Protection Convention (DRPC) across the entire DRB and to coordinate the implementation of the EC Water Framework Directive and the EC Floods Directive. With the development of the DRBMP (ICPDR, 2009), the ICPDR set an important milestone for the basinwide implementation of the EC Water Framework Directive. Based on an analysis of pressures and deficits across the entire Danube Basin, the DRBMP identifies a joint program of measures for the various surface and subsurface water bodies in the catchment, also addressing issues like flood risk management and climate change. Regarding the issue of navigation, for example, the “Joint Statement on Guiding Principles for the Development of Inland Navigation and Environmental Protection in the Danube River Basin” was endorsed by the ICPDR, the Danube Commission and the International Sava Commission (ISRBC) in 2007 to provide guidance for the planning and implementation of waterway projects (www.icpdr.org, 26.06.2015; ICPDR, 2008). Besides the establishment of transboundary networks and initiatives, the basin-wide implementation of floodplain restoration projects needs the support of a legal framework (Bogdanovic, 2005). With the creation of the EC Water Framework Directive in 2000, the European Union has provided politicians and water managers with both a tool to institutionalize river basin management across entire river basins and a legal justification for the implementation of restoration projects (Moss, 2008). Although floodplains are not explicitly mentioned in the WFD (Meyerhoff and Dehnhardt, 2007), the Directive makes explicit reference to river continuity and the structure of the riparian zone, albeit mainly as support for the biological elements. The WFD is also supported by guidance from the Common Implementation Strategy on the role of wetlands in the WFD that makes extensive reference to floodplain systems (European Commission, 2003). This obligation for member states to improve the ecological state of river systems supports decision makers and managers in their efforts to re-establish the lateral connectivity of Danube floodplains. Other important EU policies

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facilitating the realization of floodplain restoration projects are the EC Floods Directive, the EC guidance document on green infrastructures and the potential of natural water retention measures (European Commission, 2013), and the EC Habitats Directive. All members of the ICPDR agreed to implement the EC legislation concerning sustainable water management and use of water resources. However, the economic situation in some Danube countries, as well as a lack of political willingness, can impede efficient implementation of these Directives, thus, leading to inconsistencies in restoration efforts between Member States. Depending on the respective environmental conditions, the opening of dams in uninhabited areas can be a cost-efficient and highly effective measure to increase fluvial dynamics, as is shown in the case study of the Lower Danube. A further important factor in the implementation of restoration projects is their support by society. The inclusion of ecological sustainability in modern education concepts, at both European and national levels (e.g. European Commission, 2003; WRG, 2003; GZÜV, 2006) raises the ecological awareness necessary for public support. Co-operations between scientists and non-scientists, like education-research partnerships or citizen science projects, and public events like the Danube Day, organized by the ICPDR since 2004, increase public awareness of the need to improve the integrity of floodplains, even at the expense of other societal demands (Sommerwerk et al., 2010). The significance of the active involvement of the public in sustainable water management is also recognized in the Danube River Protection Convention. To date, 23 organizations, including environmental NGOs like the International Association for Danube Research (IAD), the World Wildlife Fund (WWF), and the Danube Environmental Forum (DEF), private industry, and intergovernmental organizations work as observers for the ICPDR. Besides, new decision support tools help to both resolve ecological and economic demands and find compromise solutions (Zsuffa 2001; Hein et al. 2006). 4.3. Unsolved problems and future challenges: species invasion and climate change The Joint Danube Survey 3 (JDS3) revealed the increasing incidence of invasive or non-native species in the Danube, such as zebra mussel Dreissena polymorpha, the shrimp Dikerogammarus villosus, and Asian clam Corbicula fluminea (Paunović et al., 2015). The Danube is part of the Southern Invasive Corridor which links the Black Sea with the North Sea via the Rhine-Main-Danube Canal (Sommerwerk et al., 2010). The increased volume of continental and intercontinental shipping and the construction of shipping channels connecting the Danube with other river systems, such as the Main and the Rhine, have facilitated the rapid spread of species from the Ponto-Caspian area, Asia, Australia, and North America, with potentially significant effects on the native biota (Gollasch and Nehring, 2006). River regulation seems to favor the colonization of the Danube by non-native species (Birk et al., 2012). Thus, the combination of heavily modified hydromorphology and frequent biological invasions has effectively turned large rivers such as the Danube into novel ecosystems. During JDS3, 25 neophytes, mostly from bankside and ruderal habitats, and 9 non-native fish species were recorded (Paunović et al., 2015). In the heavily regulated reaches of the Upper and Middle Danube, about 40 to 50% of all benthic invertebrate taxa, found during JDS3, were nonnative. In contrast, the proportion of invasive invertebrate taxa was less than 10% in the slightly regulated reaches of the Lower Danube, where species of Ponto-Caspian origin are considered to be native. Climate change may amplify the problem further as invasive species from warmer regions are favored by rising water temperatures (Rahel and Olden, 2008). The restoration of the hydromorphology of the Danube is expected to exert some control on the establishment of invasive species (Sommerwerk et al., 2010), although prospects for eliminating these species are negligible. However, regarding floodplain re-connection, invasive species may threaten the status of native

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floodplain biota if currently isolated side-arms are re-connected to the Danube and insufficient flood pulsing is achieved to re-initiate geomorphic dynamics. In the Danube floodplains near Vienna, for example, numbers of the gobies Neogobius kessleri and Neogobius melanostomus have increased in the floodplain side arms following their reconnection (Zweimüller, 2004). The management of invasive species is, thus, a major concern for both the Danube and its floodplains and needs consideration in all management actions to prevent unintended effects on floodplain habitats. Climate change is also seen as one of the major future challenges for river basin management (ICPDR, 2013). In general, the upper and middle basins of the Danube are assumed to be less affected by changes in water temperatures and flow patterns than the Lower Danube. Predictions for air temperature increases until 2050, for example, range from + 0.5 °C in the Upper Danube Basin to + 4 °C in the lower parts (Mauser et al., 2012). Besides, summer precipitation is expected to decrease by up to 20% in Central Europe, while reductions may be as high as 45% in Eastern Europe (Bulgaria, Hungary, Slovakia, and Romania). Various studies on long-term trends of water temperatures in the upper and middle reaches of the Danube support these predictions. Average increases in summer temperatures of the Danube during the last 50 to 60 years ranged from + 0.1 to + 0.2 °C per decade in Austria (Webb and Nobilis, 2007; Zweimüller et al., 2008), + 0.2 to +0.3 °C per decade in Slovakia (Pekarova et al., 2008), rising to + 0.4 °C per decade in Hungary (Lovász, 2012). All studies report a significant increase in this trend after 1970. In addition, decreases in summer discharge were observed (e.g. Pekarova et al., 2008; Lovász, 2012). Increased water temperatures and decreased discharge enhance other current pressures on riverine ecosystems and related water resources (ICPDR, 2013). Specifically, disconnected floodplains are more susceptible to climate change as decreasing water levels further decouple floodplain habitats from the parent channel, thereby amplifying warming effects on the biota. Therefore, increased efforts are needed to conserve surviving fragments of floodplains and their biodiversity. Re-connection with the main river can also lead to the cooling of naturally isolated floodplain water bodies. On the other hand, larger water surface areas may lead to enhanced evaporation and, thus, affect the overall water balance. Future alterations of the hydrological and thermal regime of river systems due to changing climatic conditions will have distinct impacts on floodplain restoration measures and needs early recognition in the planning of restoration. One package of measures, addressed in the “Climate adaption strategy for the Danube River Basin” of the ICPDR (2013), consists of ecosystem-based measures, which enhance the capacity of ecosystems to adapt to climate change. Explicitly mentioned are the “protection, restoration, and expansion of water conservation and retention areas”, such as natural floodplains. Floodplain re-connection can buffer the impacts of extreme hydrological events, improve the overall water availability in the region, and provide refuge areas for riverine species during flooding (Schiemer and Waidbacher, 1992). In return, an increased hydrological connectivity may reduce the impacts of global warming on floodplain biota and habitats. 5. Conclusion While the assessment of restoration potential clearly demonstrates the existence of large areas with a high potential for restoration, critical gaps remain in the implementation of restoration projects, highlighting the limitations posed by stakeholder needs, public acceptance, and resource availability. Including these factors alongside assessment at smaller spatial scales might improve the understanding of which factors are crucial for floodplain restoration under different environmental and societal scenarios. The case study on the Upper Danube demonstrates that clear aims framed around pre-regulation conditions (National Park) and a stepwise approach following an overall restoration concept can support

the implementation efficiently. In this scheme, a minimum areal share of different habitats based on the principle that active floodplain processes will maintain a dynamic habitat mosaic forms the template for restoration. In the Lower Danube case study, the much longer river stretch as well as economic and political changes offered the possibility to initiate a coordinated restoration program (REELD). While serving as a good example of how several goals of EU biodiversity targets, EC Water Framework Directive, and the Floods Directive can be addressed simultaneously via floodplain restoration, governmental structures and budget limitations have significantly constrained implementation. Knowledge gaps concerning floodplain restoration have been identified where targeted research is crucial. Climate change and the invasion of non-native species can significantly affect the current status of Danube floodplains as well as their restoration. In addition, research on floodplain conservation and restoration needs to consider potential interactions among the various local restoration measures being undertaken along the Danube and to include indicators that capture the efficiency of restoration in addressing both environmental and societal objectives. Last but not least, further improvements in success control of restoration projects is needed as present measures often lack an evaluation using quantitative and comparable data based on adequate methodological approaches.

Acknowledgments The DANCERS project received funding from the European Union's Seventh Programme for research, technological development and demonstration under grant agreement no. 603805. The project GWVLOBAU was funded by the European Agricultural Fund for Rural Development LE 07-13 under the grant agreement 323A/2010/043.

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