How Can We Identify Active, Former, and Potential Floodplains? Methods and Lessons Learned from the Danube River

Abstract

Floodplains are a fundamental source of multiple functions and services. Despite their various benefits, a dramatic reduction in floodplain areas has occurred in most large river systems over the last few centuries, and is still ongoing. Human modifications (such as river regulation, dam construction, and land use changes) due to economic growth, increasing population size, etc., were and still are drivers of major floodplain losses. Therefore, studies offering solutions for floodplain preservation and restoration are of great importance for sustainable floodplain management. This paper presents methods to identify active, former, and potential floodplains, and their application to the Danube River. We used hydraulic data, historical sources, and recent geospatial data to delineate the three floodplain types. Fifty hydraulically active floodplains larger than 500 ha were identified. According to our results, the extent of Danube floodplains has been reduced by around 79%. With the support of different representatives from the Danube countries, we identified 24 potential floodplains. However, the share of active and potential floodplains in relation to former floodplains ranges between 5% and 49%, demonstrating the huge potential for additional restoration sites. This analysis contributes to an understanding of the current and the past floodplain situation, increases awareness of the dramatic floodplain loss along the Danube, and serves as a basis for future floodplain management.

Full text

Citation: Eder, M.; Perosa, F.;
Hohensinner, S.; Tritthart, M.;
Scheuer, S.; Gelhaus, M.; Cyffka, B.;
Kiss, T.; Van Leeuwen, B.; Tobak, Z.;
et al. How Can We Identify Active,
Former, and Potential Floodplains?
Methods and Lessons Learned from
the Danube River. Water 2022,14,
2295. https://doi.org/10.3390/
w14152295
Academic Editor: Chang Huang
Received: 19 June 2022
Accepted: 14 July 2022
Published: 24 July 2022
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4.0/).
water
Article
How Can We Identify Active, Former, and Potential
Floodplains? Methods and Lessons Learned from the
Danube River
Markus Eder 1, *, Francesca Perosa 2, Severin Hohensinner 3, Michael Tritthart 1, Sabrina Scheuer 1,
Marion Gelhaus 4, Bernd Cyffka 4, Tímea Kiss 5, Boudewijn Van Leeuwen 5, Zalán Tobak 5, György Sipos 5,
Nándor Csikós6, Anna Smetanová7, Sabina Bokal 7, Andrea Samu 8, Tamas Gruber 8, Andreea-Cristina Gălie 9,
Marinela Moldoveanu 9, Petri¸sor Mazilu 10 and Helmut Habersack 1
1Institute of Hydraulic Engineering and River Research, Department of Water-Atmosphere-Environment,
University of Natural Resources and Life Sciences, Vienna, Muthgasse 107, 1190 Vienna, Austria;
michael.tritthart@boku.ac.at (M.T.); sabrina.scheuer@inode.at (S.S.); helmut.habersack@boku.ac.at (H.H.)
2Chair of Hydrology and River Basin Management, School of Engineering and Design,
Technical University of Munich, Arcisstrasse 21, 80333 Munich, Germany; francesca.perosa@tum.de
3Christian Doppler Laboratory for Meta Ecosystem Dynamics in Riverine Landscapes,
Institute of Hydrobiology and Aquatic Ecosystem Management, Department of
Water-Atmosphere-Environment, University of Natural Resources and Life Sciences, Vienna,
Gregor-Mendel-Straße 33, 1180 Vienna, Austria; severin.hohensinner@boku.ac.at
4Floodplain Institute Neuburg, Catholic University of Eichstaett-Ingolstadt, Schloss Grünau,
86633 Neuburg an der Donau, Germany; marion.gelhaus@ku.de (M.G.); bernd.cyffka@ku.de (B.C.)
5Department of Geoinformatics, Physical and Environmental Geography, University of Szeged,
Egyetem Str. 2-6, 6722 Szeged, Hungary; kisstimi@geo.u-szeged.hu (T.K.); leeuwen@geo.u-szeged.hu (B.V.L.);
tobak@geo.u-szeged.hu (Z.T.); gysipos@geo.u-szeged.hu (G.S.)
6
Centre for Agricultural Research, Institute for Soil Sciences, Department of Soil Mapping and Environmental
Informatics, H-1022, Hermann Otto 15., 1052 Budapest, Hungary; csntact@gmail.com
7Global Water Partnership Central and Eastern Europe, Jeseniova 17, 833 15 Bratislava, Slovakia;
anna.smetanova@gwpcee.org (A.S.); sabina.bokal@gwpcee.org (S.B.)
8Nature Conservation Department, WWF Hungary, Álmos vezérútja 69/a, 1141 Budapest, Hungary;
andrea.samu@wwf.hu (A.S.); tamas.gruber@wwf.hu (T.G.)
9National Institute of Hydrology and Water Management, Water Management Department,
Sos. Bucuresti-Ploiesti 97E, Sector 1, 013686 Bucuresti, Romania; andreea.galie@hidro.ro (A.-C.G.);
marinela.moldoveanu@hidro.ro (M.M.)
10 National Administration “Romanian Waters”, Emergency Situations Department, Str. Edgar Quinet nr. 6,
Sector 1, C.P. 010018 Bucure¸sti, Romania; petrisor.mazilu@rowater.ro
*Correspondence: markus.eder@boku.ac.at; Tel.: +43-1-47654-81931
Abstract:
Floodplains are a fundamental source of multiple functions and services. Despite their
various benefits, a dramatic reduction in floodplain areas has occurred in most large river systems
over the last few centuries, and is still ongoing. Human modifications (such as river regulation, dam
construction, and land use changes) due to economic growth, increasing population size, etc., were
and still are drivers of major floodplain losses. Therefore, studies offering solutions for floodplain
preservation and restoration are of great importance for sustainable floodplain management. This
paper presents methods to identify active, former, and potential floodplains, and their application
to the Danube River. We used hydraulic data, historical sources, and recent geospatial data to
delineate the three floodplain types. Fifty hydraulically active floodplains larger than 500 ha were
identified. According to our results, the extent of Danube floodplains has been reduced by around 79%.
With the support of different representatives from the Danube countries, we identified
24 potential
floodplains. However, the share of active and potential floodplains in relation to former floodplains
ranges between 5% and 49%, demonstrating the huge potential for additional restoration sites.
This analysis contributes to an understanding of the current and the past floodplain situation,
increases awareness of the dramatic floodplain loss along the Danube, and serves as a basis for future
floodplain management.
Water 2022,14, 2295. https://doi.org/10.3390/w14152295 https://www.mdpi.com/journal/water

Water 2022,14, 2295 2 of 31
Keywords:
floodplain management; historical floodplains; preservation; restoration potential; flood
risk management; hydrodynamic modeling
1. Introduction
1.1. Research Context
Floodplains are a fundamental source of multiple functions and services [
1
–
4
]. During
overbank flooding, water is temporally stored in the floodplain, leading to the deceleration
of the flood wave and reduction in the flood peak. Hence, floodplains contribute to mitigate
flood risk in riparian areas [
5
–
7
], a characteristic that will be even more important in the
future, as climate change is expected to increase flood and drought magnitude and fre-
quency of occurrence. During droughts, floodplains slowly release stored water, reducing
the impact of dry seasons [
8
]. Moreover, floodplains provide important ecological and
recreational services [
2
]. They are very prolific habitats, even more than their neighboring
river and uplands, due to their exchange with lateral and upstream sources (e.g., nutrient-
rich sediments) [
9
]. This leads to high biodiversity (floodplains are the habitats of many
Europewide endangered species), productivity (agriculture, fishery, pasture land), regu-
lation processes (climate, nutrients, flood events), recreational and aesthetic values, and
corresponding economic importance [
2
,
9
–
13
]. The sum of ecosystem service provisioning is
greater in natural systems [
1
,
14
]. Therefore, scientists, governments, and the public are cur-
rently increasing their interest in floodplain habitats and their restoration [
10
]. This interest
corresponds with the urgent need to maintain and restore floodplains and their vegetation,
hydrologic dynamics, and sediment transport capabilities [
9
]. Human modifications of
river morphologies, and our activities on floodplains, have led to a dramatic reduction in
the extent of formerly flooded areas [
9
,
15
,
16
]. The reasons for this are manifold: floodplains
are extremely attractive for agricultural cultivation and urban development due to their soil
fertility, low topographic slope, and water availability [
6
]. The technical achievements of
the industrial revolution allowed large-scale engineering work, such as river training and
drainage systems, leading to an intensified usage of the flood-prone areas for agriculture
and urban development [
17
]. In fact, Europe and North America have faced a loss of up to
90% of former floodplains [
9
,
16
]. Regarding the Danube area, previous studies report a loss
of around 70% along the Danube River, and up to 80% in the Danube Catchment [
18
,
19
].
The Middle and Lower Danube sections have lost 75% and 72% of their morphological
floodplains, respectively, excluding the Danube Delta [20].
Not only have floodplains been lost in the past, but their preservation is still threat-
ened by various factors. Only very few large rivers are free-flowing [
21
]. Social changes
(economic growth, increasing population, urbanization) connected to other anthropogenic
drivers (climate change, land use changes, overharvesting, pollution) [
9
,
22
,
23
] are pressur-
ing freshwater resources, and, in turn, are modifying the hydrological cycle, which is the
most predominant factor affecting floodplains [
9
]. Lately, the presence of invasive species,
the increased cultivation of crops for energy production, and the renovation of existing
dikes, have also proven to be potential floodplain threats [24].
Floodplain preservation and restoration are key components in environmental and
water policies in Europe [
25
–
27
], since a series of major floods revealed the vulnerability and
limits of structural flood protection measures [
7
,
28
–
30
]. Floodplain restoration measures
are considered a valid solution to reviving lost functions and services in terms of flood
protection and ecology [
11
,
14
,
31
]. Accordingly, historic river courses and their floodplains
should be taken into consideration for potential restoration. In the Danube River Basin,
520 km2
of floodplain was partly or totally reconnected to the main channel between 2009
and 2015, followed by about 100 km
2
during 2015–2021. Until 2027, 234 km
2
of floodplain
is planned to be partly or totally restored [32].
Despite these great achievements and ambitious plans, many obstacles still hinder
floodplain restoration measures. On the floodplain, we find settlements, forestry, and/or

Water 2022,14, 2295 3 of 31
agricultural land uses, and the river is used for navigation, hydropower, and/or as a
drinking water supply [
19
]. For transboundary rivers such as the Danube, the diversified
legal framework also plays an important role: small-scale, site-specific projects do exist,
but restoration efforts should aim towards a larger scale [19,24].
1.2. Background
In general, floodplains are the areas alongside a river course that are flooded when
the river discharge exceeds the capacity of the channel [
33
]. Riparian areas that are more
or less regularly flooded under natural conditions (without any dikes or levees) are called
morphological floodplains. If they are flooded at the time, they are called “recent flood-
plains” (or “current floodplains” or “levee foreshores”). The disconnected areas, which
have been cut off from the river’s flooding regime, are referred to as “old floodplains”
(“historic floodplains”, “levee backlands”, or “former floodplains”) [24].
According to Tockner and Stanford [
9
], a floodplain can be identified hydrologically
as an area inundated by a 100-year return period, geomorphologically as an area covered
by recent alluvial deposits, and ecologically as an area colonized by organisms adapted to
flooding. Nanson and Croke [
34
] also observe, on one hand, an engineering interpretation of
the term (called “hydraulic floodplain”), which focuses on the aspect of regular inundation,
separate from the presence of alluvial sediment; on the other hand, “genetic floodplains”
are referred to as “the largely horizontally-bedded alluvial landform adjacent to a channel,
separated from the channel by banks, and built of sediment transported by the present
flow-regime” [
34
]. Using the inundated areas of a 100-year return period to identify the
“active” or “recent” floodplains is the most common approach in the literature. Several
other studies [
18
,
20
,
24
] have used this approach. Our definitions of active, former, and
potential floodplains are provided in Section 2.3, as the steps of the methodology are
derived based on these definitions.
Several methods [
18
,
20
,
35
–
37
] can be found in the literature to identify former and
morphological floodplains. Schwarz [
18
] delineated morphological floodplains along
the Danube River using the most recent and accurate digital elevation model (DEM) at
that time, focusing on the definition of postglacial terraces. This author also used high-
resolution satellite data and physical riparian landscape features to define such terraces.
They concluded that additional data sources will be available in the next few years that will
help improve their raw delineation. In Germany, flood probability maps of less frequent
(extreme) events were used together with digital terrain models to identify morpholog-
ical floodplains [
24
]. Hauer et al. [
35
] used satellite data to delineate the morphological
floodplain at the Vjosa River in Albania. Guerrero et al. [
38
] used a functional landscape
approach to identify the spatial extent of potential locations for floodplain restoration. The
study was applied along the Lahn River in Hesse, Germany, where hydrologic, topographic,
and pedological information was combined to delineate potential restoration areas. This
approach is useful for areas with limited data availability, but does not take the historical
aspect of the floodplains into account. Moreover, the parameters used for the estimation
are only valid for the study area, as shown by Perosa et al. [39].
On smaller scales, various methods have been used in the past few years to investigate
former floodplains from disparate historical periods. Niller [
40
] explored the changes in the
prehistoric riverine landscape close to Regensburg (Germany). By studying soils, indicators
of the stability of a landscape, interrelationships can be found between anthropogenic
influences on the landscape and soil type. Based on an analysis of colluvial deposits,
flood loams, and soils, the author discovered anthropogenic interventions in the natural
landscape in the prehistoric era. However, ultimately, only absolute dating makes it possible
to establish reliable relationships for settlement archaeological features. To analyze the
Danube River and its floodplains in Roman times, Arauner [
41
] used HQ
1000
simulations,
DTM, geological maps, archaeological reference points, historical maps, and groundwater
simulations, and combined them with the performance data of reconstructed Roman
military vessels with the conditions of an ancient river system, to obtain realistically

Water 2022,14, 2295 4 of 31
quantifiable predictions for Roman inland navigation. This method is detailed, but its
tradeoff is that it requires archaeological data, which are often not available in other
river systems.
In the pursuit of understanding the status of landscapes in the past, Molnar et al. [
42
]
and the Bavarian State Library [
43
] digitalized historical maps by Heinrich von Schmitt and
Adrian von Riedl, respectively. However, the sole digitalization, i.e., georeferencing, of these
maps is not sufficient for the delineation of former floodplains. Skublics et al. [
44
,
45
] created
an historical DEM of a river section in Germany for a two-dimensional (2D) hydrodynamic
model using the historical maps of Adrian von Riedl to determine the river course, and
estimated the bed elevation from other historical sources (e.g., Kern [
46
]). With hydraulic
modeling, they investigated the effect of river training on flood retention [
44
,
45
]. Schober
et al. [
47
] calculated the impact of floodplain changes using the following Floodplain
Evaluation Matrix (FEM) [
5
,
48
] parameters: flood peak reduction, flood wave translation,
and water level change. They compared the current status with the historic status and
found that the floodplain losses and consequent land use changes significantly modified
flood characteristics and flow regimes over the last few decades, as also shown by Skublics
and Rutschmann [
36
]. In addition to the hydrological and hydraulic effects, the loss of
floodplains is also accompanied by ecological and socio-economic changes (e.g., valuable
habitat losses, land use change, etc.). Therefore, the FEM is a method that uses hydraulic,
hydrological, ecological, and socio-economic parameters to evaluate floodplains, and thus,
should contribute to their preservation. To investigate floodplains with the FEM, clear
start and end points of the floodplain need to be defined. It is possible to assess all three
floodplain types with this method, and determine their value using the set of parameters.
The active and potential floodplains identified in this paper were subsequently evaluated
using the FEM; the results of this evaluation can be found in Eder et al. [
49
]. In this way,
a basis for decision making is created, which could lead to the reconnection of identified
potential floodplains, and support a fundamental principle of flood risk management,
to providing more space for rivers [
50
,
51
]. Schwarz [
18
] and the DPRP [
20
] estimated
potential restoration areas along the Danube River based on land use, hydromorphological
properties (e.g., flooding dynamics), size, location, and the overlap with protected areas.
Experience has demonstrated that political and socio-economic factors (such as property
structure or the political agenda) are far more important than actual land use when it comes
to the realization of restoration projects [20].
1.3. Research Questions
Since accurate global assessments of wetland extent are elusive due to the complexity
and highly dynamic nature of wetlands themselves [
9
], focus should be on specific catch-
ments, to reduce uncertainty. The goals of the current authors are to present methods used
to identify active, former, and potential floodplains, and to compare the three floodplain
types for the Danube River case study. Therefore, the following research questions apply to
this work:
1.
What are the most suitable methods for identifying active, former, and potential floodplains?
2.
Where are the active, former, and potential floodplains along the Danube River, and
what are their land uses?
3.
What is the ratio between active, former, and potential floodplains along the Danube River?
Moreover, this work provides an upgrade on the inventory and status of the flood-
plains along the Danube River.
2. Material and Methods
2.1. Study Area—Danube River
The Danube River Basin covers more than 800,000 km
2
, which accounts for 10%
of continental Europe, and includes territories of 19 countries, with 80 million people
living within its perimeter, making it the most international river basin in the world [
52
].
The basin is heterogeneous, as one third is mountainous and has a mean altitude of

Water 2022,14, 2295 5 of 31
450 m [53]
. The Danube River flows from the Black Forest Mountains in Germany to the
Black Sea in Romania; it is 2857 km long, and has a mean annual discharge at the mouth of
6486 m3/s [54]. The Danube River connects 27 large, and over 300 smaller, tributaries [1].
The river is important for inland transportation, a use that is incompatible with the
conservation or restoration of ecologically sensitive river stretches and floodplains [
19
].
About 39% (or 1111 river km) of the Danube River itself is impounded by 83 dams [
52
]. The
river has been channelized, confined by levees, impounded, and polluted, and its flood-
plains have been converted into agricultural land, poplar plantations, and fisheries
[54,55]
.
Nonetheless, the river has relatively unaltered hydrology, with frequent “flood” and “flow”
pulses [
9
]. The riparian vegetation is adapted to the changes from dry to wet ecosys-
tems [
54
], although invasive species can also be found [
9
]. Among the many Danube
fauna species found on the IUCN Red List (https://www.iucnredlist.org/ accessed on
20 May 2022
), the ship sturgeon, the Danube salmon, and the white-tailed eagle are the best
known [
19
]. Simultaneously, invasive fauna species were found in the last Joint Danube
Survey [19].
Regarding water quality, between the 1950s and the 1970s, urban and industrial
pollution has been a relevant issue, although nowadays, the river exhibits relatively good
water quality (classes II to II–III) [54].
Concerning future trends in terms of expected climatic changes, the Danube River
Basin is going to be affected differently due to its huge area. In Northwest Europe, where
part of the Upper Danube is located, increasing autumn and winter rainfalls are expected to
enhance flood probability. In Southern and Eastern Europe, where the Middle and Lower
Danube are located, decreasing precipitation (up to 20% in Central Europe, as high as 45%
in Eastern Europe, as in Bulgaria, Hungary, Slovakia, and Romania) and increasing evapo-
ration are expected to decrease flood probability in medium and large catchments
[19,56]
.
The projected effects of climate change (low water during the growing season and higher
water temperatures) are likely to negatively affect disconnected floodplains [
9
,
19
]. In terms
of land use changes, population growth is expected to enhance demands for land for
settlement, agriculture, and forestry, which in the past have already resulted in large-scale
river regulation measures for flood protection, navigation, and hydropower. Due to these
past changes, the Danube River and its tributaries have been progressively constrained for
flood protection, navigation, and hydropower [53].
2.2. Data Sources
This paper builds on the EU-funded “Danube Floodplain (https://www.interreg-
danube.eu/approved-projects/danube-floodplain#! accessed on 25 March 2022)” project
that aimed at improving transnational water management and flood risk prevention,
focusing on floodplain management. Different representative entities (national water
agencies, universities, etc.) from the countries along the Danube River collaborated in this
project. In Table 1, we list the data sources that we used to identify and analyze the active,
former, and potential floodplains along the Danube River.
In terms of hydrological data, this study used the medium (HQ
100
) and extreme
(HQ
1000
) flood event outlines from the “Danube FLOODRISK” project to identify active,
former, and potential floodplains. In the scope of the “Danube FLOODRISK” project,
hazard and risk maps were prepared for the entire Danube River in 2012. Three different
flood scenarios (frequent event, HQ
30
; medium event, HQ
100
; extreme event, HQ
1000
) were
investigated using various hydrodynamic models (2D, quasi-2D, and 1D). In this study, for
some countries (e.g., Germany, Hungary) we used more recent maps of inundated areas to
use the most updated data set in terms of flood-prone areas. Table 2provides an overview
of the models that were used to calculate the inundation areas.

Water 2022,14, 2295 6 of 31
Table 1.
Overview of all data sources used for identifying the active, former, and potential flood-
plains along the Danube River. Country code: DE = Germany, BW = Baden-Württemberg, BY-
Bavaria,
AT = Austria
, SK = Slovakia, HU = Hungary, HR = Croatia, RS = Serbia, BG = Bulgaria,
RO = Romania.
Data Source Name Description Countries
Hydrological
HQ100 outlines
The project “Danube FLOODRISK
1
” and/or national flood hazard
maps provided the inundation areas of a flood with a 100-year
return period. The representatives from each country involved in
the “Danube Floodplain” project decided which sources (“Danube
FLOODRISK” or national flood hazard maps) were used for their
country. In the end, the HQ
100
outlines used are based on calibrated
1D, quasi-2D and 2D hydraulic models.
DE, AT, SK, HU, HR,
RS, BG, RO, MD, UA
HQ1000 outlines
The project “Danube FLOODRISK” provided the inundation areas
of a flood with a 1000-year return period. As with the HQ100
outlines, the representatives from each country selected the source
of the HQ
1000
outlines in their country, leading to the use of results
from calibrated 1D, quasi-2D and 2D hydraulic models.
AT, SK, HU, HR, RS,
BG, RO, MD, UA
HQextreme, BW outlines
The Baden-Württemberg State Institute for Environmental
Protection provided the inundation areas of an extreme flood. An
extreme event is statistically less likely than once in 100 years.
DE (State: Baden-
Württemberg, BW)
HQextreme, BY outlines
The Bavarian Environment Agency provided the inundation areas
of an extreme flood. An extreme event is statistically less likely than
once in 100 years.
DE (State: Bavaria BY)
Historical
Pasetti map
A cartographic inventory of the Danube River and its riparian areas
at the onset of the river regulation in the second half of the 19th
century (1857–1867).
AT, SK, HU, HR, RS
Franziscean cadastre 2The “Franziscean cadastre” is an historical map created between
1817 and 1861 to calculate land taxes, showing land cover/uses of
each plot for the Habsburg Monarchy.
AT, HU
I. 3and II. 4Military
Survey Maps
The military survey maps from 1763–1787 and 1806–1869 show
landscape structures, such as watercourses, and terrain topography.
AT, SK, HU, HR, RS,
BG, RO
Schmitt’sche Map 5In 1797, Heinrich von Schmitt started drawing a military map on
behalf of the German Emperor Franz II. DE
Historical flood
events map
The Bavarian Environment Agency provided data on flood events
that have been observed in the past. The areal expansion and the
flood line (outer boundary of the flood), as well as historical water
level marks, are recorded.
DE (State: Bavaria BY)
Written historical
sources
Local chronicles, newspapers, and books are used to verify the
delineated former floodplains; e.g., Sartori [57,58], LfU [59],
Peckarova and Miklanek [60], and Nagy [61]
DE, AT, HU, RS
Geospatial data
Digital Elevation Model
The EU Digital Elevation Model 6is found in the Copernicus
database, and is based on SRTM and ASTER GDEM data,
providing information in 25 m resolution.
DE, AT, SK, HU, HR,
RS, BG, RO
Land cover data set
The CORINE land cover data set 7from the Copernicus database
summarizes land cover into five categories: “Artificial surfaces”,
“Agricultural areas”, “Forest and semi-natural areas”, “Wetlands”,
and “Water bodies”.
DE, AT, SK, HU, HR,
RS, BG, RO
Satellite images Different sources (Google Earth, OpenStreetMap, World Imagery)
were used for high-resolution satellite images.
DE, AT, SK, HU, HR,
RS, BG, RO, MD, UA
Previous projects
Floodplains
Previous delineated morphological floodplains from Schwarz [18],
a WWF funded project in 2010, are used as a basic reference for
identifying former floodplains in the present study.
DE, AT, SK, HU, HR,
RS, BG, RO, MD, UA
BfN morphological
floodplains (Altauen)
The morphological floodplain map was delineated by the Federal
Agency for Nature Conservation [10], following the methodology
reported in Günther-Diringer et al. [62].
DE
Notes:
1
https://environmentalrisks.danube-region.eu/projects/danube-floodrisk/ accessed on 25 March
2022;
2
www.mapire.eu/en/map/cadastral/ accessed on 20 May 2022;
3
https://maps.arcanum.com/en/
browse/country/firstsurvey/ accessed on 20 May 2022;
4
https://maps.arcanum.com/en/browse/country/
secondsurvey/ accessed on 20 May 2022;
5
https://maps.arcanum.com/de/synchron/schmittschekarte/?layers=
35&bbox=705838.5570651566%2C4890580.581526183%2C2244363.0623891843%2C7177576.467818657 accessed on
20 May 2022;
6
https://land.copernicus.eu/imagery-in-situ/eu-dem/eu-dem-v1.1;
7
https://land.copernicus.
eu/paneuropean/corine-land-cover/clc2018 accessed on 20 May 2022.

Water 2022,14, 2295 7 of 31
Table 2.
Overview of the hydraulic models that were used to calculate the inundation areas for HQ
100
and HQ1000 in each country.
Country Model Used for HQ100 Outlines Model Used for HQ1000 Outlines
Germany 2D models used for latest national flood
hazard maps
2D models used for latest national
flood hazard maps
Austria
2D steady models used for latest national
flood hazard maps
2D steady models used in the
"Danube FLOODRISK" project
Slovakia 2D models used for latest national flood
hazard maps
2D models used for latest national
flood hazard maps
Hungary 2D models used for latest national flood
hazard maps
2D models used for latest national
flood hazard maps
Croatia 1D unsteady model used in the "Danube
FLOODRISK" project
1D unsteady model used in the
"Danube FLOODRISK" project
Serbia 1D unsteady model used in the "Danube
FLOODRISK" project
1D unsteady model used in the
"Danube FLOODRISK" project
Bulgaria 1D unsteady model used in the "Danube
FLOODRISK" project
1D steady model used in the "Danube
FLOODRISK" project
Romania 1D and quasi-2D unsteady models used
in the "Danube FLOODRISK" project
1D and quasi-2D unsteady models
used in the "Danube
FLOODRISK" project
An example of historical data is the “Pasetti map”, a cartographic inventory of the
Danube River and its riparian areas, created between 1857 and 1867 at a scale of 1:28,800;
based on the Franziscean land survey, this map represents the time before the river regu-
lation (1830). The “Pasetti map” represents an area from the German–Austrian border to
the Iron Gate, and provides information regarding the riverbanks, the hydraulic situation,
and roads in the riparian areas, features that are not included in the I. and II. military
survey maps. For example, the inundation areas of a large flood event in 1830 are shown
on the “Pasetti map”. More information about this map can be found in Zeilinger [
63
].
The “Franziscean cadastre” and the military survey maps (1763–1787; 1806–1869) are also
historical maps, produced by the later “k.k. Militärgeographische Institut”, and provide
valuable information about formerly flooded areas. Additionally, the “Schmitt map” (in
German “Schmitt’sche Karte”), another military map, was also available; this map cov-
ers the missing German part of the Danube River. We used written documents, such as
the reports of Satori [
57
,
58
], about the 1830 flood event, and reports on the disastrous
1876 floods collected by Nagy [
61
] to verify the inundated areas of the “Pasetti map”. In
general, we searched for local chronicles, newspapers, and other written documents for
additional information about historical flood events to improve the delineation of the
former floodplains.
In terms of geospatial data, the EU-DEM, part of the Copernicus database, is a single
consistent elevation data set for Europe. It is based on SRTM and ASTER GDEM data, and
provides information at 25 m resolution. Different sources (Google Earth, OpenStreetMap,
World Imagery) provided high-resolution satellite images.
The CORINE land cover data set from the Copernicus database was used to analyze
the land use of each active floodplain. The data set summarizes the land cover at the
first level into five categories: “Artificial surfaces”, “Agricultural areas”, “Forest and semi-
natural areas”, “Wetlands”, and “Water bodies”. Appendix Areports all the land use
categories and subcategories of the CORINE land cover data set.
Schwarz [
18
] delineated active morphological floodplains, and proposed areas for
restoration (potential floodplains). The delineation is mainly based on the most recent
and accurate DEM at that time, and focuses on the definition of postglacial terraces. High-
resolution satellite data and physical riparian landscape features were also used to define
such terraces. In the project’s report, Schwarz [
18
] stated that additional data sources were

Water 2022,14, 2295 8 of 31
going to be available in the upcoming years, with potential for improvement of the raw
delineation. In another recent work [
23
], German morphological floodplains were identified
and interpreted as natural inundation areas of a river, which would be occupied by more or
less regularly recurring floods under natural conditions (no flood protection measures, such
as dikes). Separated areas that are no longer flooded are called “old floodplains”. These
include polders without ecological flooding. Therefore, the floodplain areas delineated in
these previous projects were used as references for identifying former floodplains in the
present study.
2.3. Identification of Active, Former, and Potential Floodplains
In the following three subsections, we describe the methods used to delineate active,
former, and potential floodplains along the Danube River. These methods can be applied
to any river if the necessary data sets are available.
2.3.1. Active Floodplains
We define active floodplains as inundated areas (hydraulically active) during a flood
event with a return period of 100 years (HQ
100
). Several other studies, e.g., [
1
–
3
], used the
inundation areas of HQ
100
for delineating recent/active floodplains. Moreover, this return
period was chosen because the EU Floods Directive (2007/60/EC) requires the preparation
of flood hazard and risk maps for flood events with low, medium, and high frequency, in
which “medium” frequency corresponds to a 100-year return period. Hence, inundation
areas for a 100-year return period are widespread and available in Europe.
For our study, the flood hazard and risk maps for three frequencies (high, HQ
30
;
medium, HQ
100
; low, HQ
1000
) from the “Danube FLOODRISK” project were available
for most of the Danube River. If more recent national flood hazard and risk maps were
provided from individual countries, we used these maps as a data source.
Our developed method differs from previous methods, e.g., [
1
–
3
], as we introduce
two delineation criteria to identify the main active floodplains (with the highest retention
effects):
(i) ratio of widthFloodplain/widthChannel ; (ii) minimum size.
In a further step [
49
],
we applied the Floodplain Evaluation Matrix (FEM), which requires delineated floodplains
with a defined start- and endpoint. The two delineation criteria can vary and be adapted
depending on the research question and the characteristics of the investigated river (e.g.,
basin size, river type). Considering our research intent and the characteristics of the Danube
River, we set the width ratio to 1:1, meaning that an active floodplain starts if the width
of the floodplain (excluding the main channel) is at least as wide as the channel width. If
the width of the flooded area was smaller, then the area was defined as a riparian zone.
In general, the width of the main channel was used for the ratio factor. An exception
was the Eastern Wallachian Danube with the three largest islands (Balta Ialomita, Big
and Small Islands of Braila), where the width of the respective largest branch was used.
This was conducted because of the special characteristics of this section, where the side
channels were considered as part of the river for several kilometers. In Figure 1, the applied
delineation criteria for the Danube River are summarized, and an example of locating the
start- and endpoint of a floodplain is presented.
The second criterion is the minimum size, which we used to distinguish two groups
of floodplains that fulfilled the chosen width ratio of
WidthFloodplain/WidthChannel
. This
differentiation was required, since we focused on floodplains with the largest retention
effects in the second stage of our research [
49
]. We assigned all floodplains larger than
500 ha to the 1st group. We chose this size since all Danube River Basin Management
Plans (DRBMPs) [
32
,
52
,
64
] concern floodplains of basin-wide importance (area > 500 ha).
Nevertheless, we also delineated all floodplains smaller than 500 ha (2nd group), and all
riparian areas (3rd group) that neither fulfill the width ratio nor the minimum size, since
these areas may be relevant for ecological and/or socio-economic aspects (e.g., habitats,
agriculture, leisure time). We used a Geographic Information System (GIS, such as ArcGIS)

Water 2022,14, 2295 9 of 31
to delineate the active floodplains and to assign them to one of the three groups. The three
groups of this study are therefore defined as follows:
•1st group:
floodplains with delineated start- and endpoint and larger than the defined
minimum size;
•2nd group:
floodplains with delineated start- and endpoint but smaller than the
minimum size;
•3rd group: riparian zones, where the two delineation criteria are not fulfilled.
Water 2022, 14, x FOR PEER REVIEW 9 of 34
which “medium” frequency corresponds to a 100-year return period. Hence, inundation
areas for a 100-year return period are widespread and available in Europe.
For our study, the flood hazard and risk maps for three frequencies (high, HQ30; me-
dium, HQ100; low, HQ1000) from the “Danube FLOODRISK” project were available for most
of the Danube River. If more recent national flood hazard and risk maps were provided
from individual countries, we used these maps as a data source.
Our developed method differs from previous methods, e.g., [1–3], as we introduce
two delineation criteria to identify the main active floodplains (with the highest retention
effects): (i) ratio of widthFloodplain/widthChannel; (ii) minimum size. In a further step [49], we
applied the Floodplain Evaluation Matrix (FEM), which requires delineated floodplains
with a defined start- and endpoint. The two delineation criteria can vary and be adapted
depending on the research question and the characteristics of the investigated river (e.g.,
basin size, river type). Considering our research intent and the characteristics of the Dan-
ube River, we set the width ratio to 1:1, meaning that an active floodplain starts if the
width of the floodplain (excluding the main channel) is at least as wide as the channel
width. If the width of the flooded area was smaller, then the area was defined as a riparian
zone. In general, the width of the main channel was used for the ratio factor. An exception
was the Eastern Wallachian Danube with the three largest islands (Balta Ialomita, Big and
Small Islands of Braila), where the width of the respective largest branch was used. This
was conducted because of the special characteristics of this section, where the side chan-
nels were considered as part of the river for several kilometers. In Figure 1, the applied
delineation criteria for the Danube River are summarized, and an example of locating the
start- and endpoint of a floodplain is presented.
Figure 1. Left: Applied delineation criteria for identifying active floodplains along the Danube River.
Right: Identified floodplain HU_DU_AFP07 (HU = Hungary, DU = Danube River, AFP = active
floodplain) showing the concept of locating the start- and endpoint of a floodplain based on the
width ratio (widthFloodplain/widthChannel).
The second criterion is the minimum size, which we used to distinguish two groups
of floodplains that fulfilled the chosen width ratio of WidthFloodplain/WidthChannel. This dif-
ferentiation was required, since we focused on floodplains with the largest retention ef-
fects in the second stage of our research [49]. We assigned all floodplains larger than 500
ha to the 1st group. We chose this size since all Danube River Basin Management Plans
Figure 1.
Left: Applied delineation criteria for identifying active floodplains along the Danube
River. Right: Identified floodplain HU_DU_AFP07 (HU = Hungary, DU = Danube River,
AFP = active floodplain
) showing the concept of locating the start- and endpoint of a floodplain
based on the width ratio (widthFloodplain/widthChannel).
2.3.2. Former Floodplains
We define former floodplains as areas that have been cut off from the river due to
river training and regulation, but that would have potentially been flooded without these
human interventions. As in earlier literature [
18
,
24
], we use the term morphological
floodplain to describe the combination of active and former floodplains. Delineating former
floodplains that were hydraulically active during an extreme flood event is challenging
because hydraulic processes in the river and floodplains have changed significantly due
to strong human intervention. Using different sources ranging from historical documents
(e.g., reports, military survey maps) to current hazard maps (e.g., outlines of HQ
extreme
) and
DEM allows the estimation of the former hydraulically active floodplains. As an innovative
part of this work, we developed a schematic approach to identify former floodplains
along four consecutive steps (Figure 2). Each step includes mandatory tasks, and leads
to potentially different data sources, as each river is described through different data sets.
Therefore, the presented method is structured in a way that allows it to be applicable, even
when certain data (e.g., HQextreme) are not available.
Step I: Estimation of former floodplains with current HQextreme outlines

Water 2022,14, 2295 10 of 31
Water 2022, 14, x FOR PEER REVIEW 13 of 34
Figure 2. Schematic approach for the identification of former floodplains.
Figure 2. Schematic approach for the identification of former floodplains.
Very often, flood protection measures disconnect the former floodplains from the river
system. In Europe, the design discharge of these measures mostly corresponds to a 100-year
return period (medium frequency). Events with a low frequency (e.g.,
1/1000 probability
)
are called “extreme” (which corresponds to the discharge HQ
extreme
), and can lead to
overflow or failure of the existing flood defenses. This results in flooding of the areas
behind the protection facilities, thereby reactivating the former floodplains, or at least part
of them. Hence, current HQ
extreme
outlines can serve as a starting point for the extent of
former floodplains. This step depends on the data availability of the HQ
extreme
outlines,
and is therefore optional. If the data set is available, the outlines are imported into a GIS.
The extent of HQ
extreme
outlines is seen as an estimation of the former floodplains. Two
methods exist to verify whether the HQ
extreme
inundation areas reliably represent the former
floodplains: (i) using historical sources (e.g., documentations, maps) and
(ii) cross-checking
whether existing flood protection measures are overtopped or not. In fact, if the flood
defenses protect the hinterland during an HQ
extreme
event, the formerly inundated areas
identified with the HQ
extreme
outlines might yield an underestimation of the actual former
floodplains. Additionally, historical sources can deliver further information about the
hinterland, for example, the presence of flood-prone areas (see step III).

Water 2022,14, 2295 11 of 31
For the Danube River, we used the HQ
1000
outlines from the “Danube FLOODRISK”
project as a starting point for the extent of the formerly flooded areas. Except for Germany,
the HQ
1000
inundation areas were not available, so we used updated HQ
extreme
maps
outlined by the environmental agencies of the Federal States of Baden-Württemberg [
65
]
and Bavaria [
66
] in the German sections. In most cases, the current flood protection
facilities are overtopped along the Danube during extreme events, indicating areas that
were disconnected from the river by the flood defense.
In areas where measures were not overtopped, information from other sources is
needed to understand whether the hinterland behind the defense measures can be classified
as a former floodplain. In step II, sources of such information were collected. In step III, the
collected data were used to check and extend the estimated former floodplains.
Step II + III:
Search for data sources (historical, current) and delineation of
former floodplains
For simplicity, we present steps II and III together in this subsection. Step II focuses on
the data research. In step III, former floodplains are delineated based on the data acquired
in step II.
The goal was to delineate the maximum extent of former floodplains, and historical
sources (such as military survey or cadastre maps, local chronicles, newspapers, gauging
data, etc.) can contain crucial information about formerly flooded areas. Hence, step II
focuses on finding such sources.
First, historical flood events can be identified based on local chronicles, newspapers,
historical–topographical descriptions, and gauging data [
67
]. A compilation of past flood
events serves as a basis for an in-depth investigation of the extent such events, which could
subsequently lead to the former floodplains. For the Danube case study, we used previous
investigations [
59
,
60
,
67
] on historical floods along the Danube River to identify the major
flood events from the past.
The next step was to search for maps showing the flooded areas of historical events.
For the Danube River, we found the “Pasetti map”, a cartographic inventory of the Danube
River and its riparian areas at the onset of the river regulation in the second half of the 19th
century, showing the inundation areas of an extreme flood event in 1830. For Bavaria, the
Bavarian Environment Agency (LfU) provided an historical flood events map [
68
], which
indicated the greatest extent of a flood observed in the past. The maps showed that the
flood events exceeded, in some parts, the HQ
extreme
outlines identified in step I. In these
sections, we extended the preliminary former floodplains according to the inundation areas
from the “Pasetti map” and the LfU historical map.
On occasion, (military) cartographers mapped the flooded areas several years after
the flood event in a simplified and partly flawed manner. Historical sources from authors
more familiar with the local situation (e.g., flood damages, victims) can help to correct such
errors [
69
]. Hence, the inundation outlines must be verified with written documents and
geodata, such as digital elevation models, when used to delineate the former floodplains.
Since the “Pasetti map” was created about 30 years after the 1830 flood, and before the
1876 floods
, we verified the mapped inundation outlines with other historical sources
(reports on flood events, local chronicles, newspapers, etc.). Some of these written docu-
ments [
57
,
58
,
61
] indicated villages that were affected by the flood of 1830, which were not
shown in the map, or, in contrast, the map showed larger inundated areas than documented.
In these cases, the inundation outlines were corrected according to the written sources.
In the German section of the Danube, the extension of the HQ
extreme
outlines, the LfU
historical maps, and the floodplains drawn from the “Schmitt map” (see next paragraph)
were compared with the locations of the settlements, where floods took place according to
literature [
59
,
60
]. The comparison was performed by georeferencing the settlements using
GIS software (QGIS [
70
]), and none of the added points were located outside of the already
identified areas.
However, most maps do not show any information about previous flood events.
Instead, they show the typical coloring of wetlands, floodplain forests, moist pastures, and

Water 2022,14, 2295 12 of 31
meadows, as well as landscape structures, such as watercourses, and terrain topography,
which allow one to delineate former inundation areas.
Cadastral maps, which show the land use of each plot, and military maps, which
focus on the geographical situation and landscape structures, complement each other, and
can be used to identify formerly flooded areas. At the Danube River, the “Franziscean
cadastre”, the I. and II. military survey maps, and the “Schmitt map” were used in sections
that were not covered by the “Pasetti map” or by the LfU map (e.g., for the German section
in Baden-Württemberg). The maps were incorporated into QGIS using the Web Map Tile
Service (WMTS) of the company Arcanum (https://www.arcanum.com/en/ accessed on
25 March 2022). Again, the starting point was the HQ
1000
outline. If the cadastral map
and/or the military survey maps indicated former floodplain areas that the HQ
1000
did not
cover, then the polygon was adapted and extended.
Searching for previous projects or studies on former floodplains is also part of step
II. Keywords such as “morphological floodplains”, “former floodplains”, or “historical
floodplains” may be used to find further studies on floodplain reconstruction. We used
the previous studies of Schwarz [
18
] and Koenzen and Günther-Diringer [
10
], which we
previously described in Section 2.2, as basic references, and compared their delineated
floodplains to the newly identified former floodplains.
If none of the above data sets are available, recent DEMs and satellite images can be
used to identify postglacial lower terraces and breakthrough valleys, which often represent
the boundaries of the former floodplains. In theory, these data sets alone could be used to
identify former floodplains. However, delineating formerly flooded areas without historical
and hydraulic data is not recommended. If other data sets are available, DEMs and satellite
images are used to verify the delineated former floodplains (see step IV).
Step IV: Verification of former floodplains with DEM and satellite images
The verification of former floodplains with a current DEM and high-resolution satellite
images is very limited, since the terrain might have changed significantly. This step aims at
discovering and correcting coarse errors in the historical maps. For example, according to
the historical maps, the river’s course runs over an older/higher river terrace, valley slope,
or even a mountain. This apparent error can be easily detected with the DEM or satellite
images. We used the EU-DEM and current satellite images (Google Earth, OpenStreetMap,
World Imagery) to check the plausibility of the delineated former floodplains along the
Danube River.
2.3.3. Potential Floodplains
We define potential floodplains as areas that are currently not inundated by an HQ
100
event, but have the potential (from a hydraulic, ecological, and socio-economic perspective)
to be reconnected to the river system, leading to inundation during flood events with a
100-year return period. A step-by-step approach based on hydraulic and historical data
was developed to identify potential and “operational” potential floodplains. The method is
organized along five consecutive steps (Figure 3), with the option to exclude the last step V,
which leads to the “operational” potential floodplains that have already been discussed
in detail (regarding technical, political, and economic feasibility) among stakeholders,
decision makers, and experts. The first four steps are compulsory, and sufficient to identify
potential floodplains of a river. Depending on the individual project or research question,
it might be necessary to include and apply step V (Detailed Planning of Reconnection
Measures with Stakeholders and Decision Makers). To meet our objectives and answer our
research questions, it was sufficient to apply the first four steps of the method, which led to
the potential floodplains.
Step I: Estimation of Former Floodplains

Water 2022,14, 2295 13 of 31
Water 2022, 14, x FOR PEER REVIEW 14 of 34
2.3.3. Potential Floodplains
We define potential floodplains as areas that are currently not inundated by an HQ
100
event, but have the potential (from a hydraulic, ecological, and socio-economic perspec-
tive) to be reconnected to the river system, leading to inundation during flood events with
a 100-year return period. A step-by-step approach based on hydraulic and historical data
was developed to identify potential and “operational” potential floodplains. The method
is organized along five consecutive steps (Figure 3), with the option to exclude the last
step V, which leads to the “operational” potential floodplains that have already been dis-
cussed in detail (regarding technical, political, and economic feasibility) among stakehold-
ers, decision makers, and experts. The first four steps are compulsory, and sufficient to
identify potential floodplains of a river. Depending on the individual project or research
question, it might be necessary to include and apply step V (Detailed Planning of Recon-
nection Measures with Stakeholders and Decision Makers). To meet our objectives and
answer our research questions, it was sufficient to apply the first four steps of the method,
which led to the potential floodplains.
Figure 3. Sequence of the identification method for potential and “operational” potential flood-
plains.
Step I: Estimation of Former Floodplains
A detailed delineation of former floodplains is challenging and time-consuming. In
general, the method described in Section 2.3.2 or the application of hydraulic models is
recommended. The latter is the most time-consuming and cost-intensive approach, espe-
cially if the former floodplains must be identified for an entire river. Hence, using the
inundation areas of an HQ
extreme
event provides an opportunity to roughly estimate the
extent of the former floodplains, especially if the focus of the investigation is on potential
floodplains and not on formerly flooded areas. For the Danube River, we used the former
floodplains identified via the method described in Section 2.3.2.
Step II: Exclusion of Settlements and Infrastructure
Figure 3.
Sequence of the identification method for potential and “operational” potential floodplains.
A detailed delineation of former floodplains is challenging and time-consuming. In
general, the method described in Section 2.3.2 or the application of hydraulic models
is recommended. The latter is the most time-consuming and cost-intensive approach,
especially if the former floodplains must be identified for an entire river. Hence, using the
inundation areas of an HQ
extreme
event provides an opportunity to roughly estimate the
extent of the former floodplains, especially if the focus of the investigation is on potential
floodplains and not on formerly flooded areas. For the Danube River, we used the former
floodplains identified via the method described in Section 2.3.2.
Step II: Exclusion of Settlements and Infrastructure
The estimated former floodplains are the basis for identifying potential floodplains.
Nowadays, settlements, individual houses, and infrastructure are often located in formerly
flooded areas. If this is the case, the operator of the method must decide if complementary
local flood defense measures (e.g., dikes, protective walls, etc.) are feasible to protect the
human-made structures. With additional flood protection measures, the settlements and
infrastructure are excluded from the former floodplain.
Step III: Exclusion of Agricultural Land without Compensation Possibility
Many formerly flooded areas are used for agriculture due to their natural characteris-
tics (low slope, soil fertility, water availability). In general, such areas are not considered
“no go” areas for potential floodplains, but certain agricultural areas can be excluded from
the potential floodplains, if land ownership or the legal framework do not allow compensa-
tion, or if the compensation is too expensive. To identify only realistic potential floodplains,
the user can exclude some agricultural areas.
The potential floodplains presented in this study are one output of the EU project
“Danube Floodplain”, for which different representative entities (national water agencies,
universities, etc.) from countries along the Danube River collaborated with each other and

Water 2022,14, 2295 14 of 31
with external stakeholders in delineating potential floodplains. The entities identified the
potential floodplains in their own country, and excluded some agricultural areas where
they identified no realistic chance for restoration, i.e., due to political agenda, urban
development, or land ownership.
After excluding selected agricultural land and settlements/infrastructure from the
previous step, the former floodplains are reduced to the delineated potential floodplains.
Step IV: Definition of Restoration Measures
Restoration measures (e.g., relocation, slitting, or removal of dikes) must be defined
to ensure that the identified potential floodplains are inundated, at least during an HQ
100
flood, which corresponds in this study to a reconnection of the river system. Hydrodynamic
models are used to verify whether the defined/potential restoration measures would lead
to a reconnection of the delineated potential floodplain. If not, the potential measures must
be adapted. If reconnection is not feasible, then the defined area should not be selected as a
potential floodplain. We defined several restoration measures for the potential floodplains
at the Danube River, ranging from dike relocation to deepening lateral branches/oxbows
or removing bank stabilizations. The results of step IV can then be used as a discussion
basis in step V.
Step V:
Detailed Planning of Restoration Measures with Stakeholders and Decision Makers
The last step is optional, and gives, as a result, the “operational” potential floodplains,
upon which stakeholders, decision makers, and experts can agree regarding technical,
political, and economic feasibility. The potential floodplains and the defined restoration
measures/ideas from step IV are the basis for discussion among the interested parties.
The discussions should focus on the feasibility of reconnection of the identified areas
from a technical, political, and economic perspective; for example, land availability and
purchase, planning and implementation costs, future compensation in the case of flooding,
maintenance and monitoring costs, or political willingness.
In the Danube River case study, this last step was excluded for time and funding
reasons, since the first four steps were sufficient to achieve the “Danube Floodplain”
project’s goal of identifying potential floodplains along the Danube River.
2.3.4. Naming Convention
To assign each floodplain a unique, identifiable code, we developed a naming con-
vention. The code consists of four different parts. The first part is the country code with
two characters. The following country codes are used in this study: DE = Germany,
AT = Austria
, SK = Slovakia, HU = Hungary, HR = Croatia, RS = Serbia, BG = Bulgaria,
RO = Romania, MD = Moldova, UA = Ukraine. The second part is the river code with
two characters (e.g., DU = Danube River), the third part the floodplain type (AFP = active
floodplain, FFP = former floodplain, PFP = potential floodplain), and the fourth is the
consecutive number of the floodplain within the country in flow direction of the river.
If a floodplain is transboundary, all countries are listed in the code, beginning with the
upstream country, and the number starts at the first transboundary floodplain of these
countries. Here are two examples:
•
The 1st transboundary active floodplain at the Danube River between Austria and
Slovakia, AT_SK_DU_AFP_01;
•The 4th potential floodplain at the Danube River in Germany, DE_DU_PFP_04.
3. Results
In this section, we present the results of applying the presented methods along the
Danube River. The results build on the work of the “Danube Floodplain” project, where the
spatial database “Danube Floodplain GIS (http://www.geo.u-szeged.hu/dfgis/ accessed
on 20 May 2022)” was created to present the project’s outputs and make them available to
the public.

Water 2022,14, 2295 15 of 31
3.1. Active Floodplains along the Danube River
In total, 4711 km
2
(see Table 3) of hydraulically active floodplains were identified
(incl. main river channel and excl. the Delta), with Romania having the largest share of
30% (around 1435 km
2
). Hungary, Serbia, Germany, and Austria have a relatively large
share between 11–15%, and 6 and 7% of the active floodplains are located in Croatia and
Bulgaria, respectively. The share of Ukraine and Moldova is relatively low, especially since
the Danube Delta is not considered in this estimation (due to its unique characteristics).
However, with an area of 3394 km
2
, the Danube Delta is the largest wetland along the
Danube River, and its importance for ecology is immense.
Table 3.
Comparison of the active floodplains of the Danube River (without the Delta) in the riparian
states. In the 1st group, all active floodplains fulfill the two delineation criteria (ratio factor, minimum
size); 2nd group, the minimum size is not fulfilled; 3rd group, neither the ratio factor nor the
minimum size is fulfilled. Country codes: DE = Germany, AT = Austria, SK = Slovakia, HU = Hungary,
HR = Croatia
, RS = Serbia, BG = Bulgaria, RO = Romania, MD = Moldova, UA = Ukraine. The relative
share in the last column presents the proportion of the total floodplain area in a country in relation to
the total floodplain area along the Danube River.
Size of Active Floodplain Incl. Main Channel in km2without Delta
Country 1st Group 2nd Group 3rd Group Total Rel. Share [%]
DE 492 101 13 606 13
AT 415 21 65 500 11
SK 89 0 78 168 4
HU 595 29 65 688 15
HR 294 0 13 307 6
RS 331 42 267 640 14
BG 82 37 232 351 7
RO 728 146 561 1435 30
MD - - 0.22 0.22 <1
UA - - 15 15 <1
Danube River
without Delta 3027 375 1309 4711 100
[%] 64 8 28 100
Fifty hydraulically active floodplains (without the Danube Delta) meet the two delin-
eation criteria described in Section 2.3.1. We assigned these 50 floodplains to the 1st group
of floodplains, which are evaluated in more detail in Eder et al. [
49
]. These 50 floodplains
cover 3027 km
2
, which is about 64% of the total area (4711 km
2
) inundated during an
HQ
100
event. A total of 101 active floodplains are smaller than 500 ha, and were assigned
to the 2nd group of floodplains. Most of them (58) are found in Baden-Württemberg,
where the river width is relatively small, and therefore, the first criterion (ratio factor of
width
Floodplain
/width
Channel
> 1) was easy to fulfil. With 375 km
2
, the floodplains in the 2nd
group account for only 8% of the total inundated area. The 3rd group includes the riparian
zones, where the two delineation criteria (ratio factor, minimum size) are not fulfilled. The
riparian zones cover 1309 km
2
, which is about 28% of the total area inundated during an
HQ100 event. Romania has the largest share of the 3rd group, with 43% (561 km2).
In Figures 4and 5, all active (red), potential (orange), and former (green) floodplains
are presented for the three Danube sections (upper, middle, and lower).

Water 2022,14, 2295 16 of 31
Water 2022, 14, x FOR PEER REVIEW 18 of 34
Figure 4. Overview of the active, former, and potential floodplains at the Upper and Middle Dan-
ube.
Figure 4.
Overview of the active, former, and potential floodplains at the Upper and Middle Danube.

Water 2022,14, 2295 17 of 31
Water 2022, 14, x FOR PEER REVIEW 19 of 34
Figure 5. Overview of the active, former, and potential floodplains at the Lower Danube.
In Figure 6, all active floodplains larger than 500 ha (average value around 5800 ha)
are sorted from up- to downstream, and the total area of each floodplain is presented.
Each floodplain received a unique, identifiable code (see Section 2.3.4) that is plotted on
the abscissa. Out of the 50 floodplains, only five have an area larger than 150 km2: two of
them are located in the upper (DE, AT), two in the middle (HU, HR-RS), and one in the
lower (RO) Danube section. Thirty-two floodplains have an area below 50 km2. Germany
has ten hydraulically active floodplains larger than 500 ha, Austria five, and one is trans-
boundary. Slovakia has only six transboundary floodplains. Hungary has the most flood-
plains, eight only in Hungary and ten transboundary. Croatia shares one floodplain with
Hungary and five with Serbia. Serbia has ten floodplains (five transboundary and five in
Serbia only). Bulgaria shares six with Romania. Romania also has four floodplains on its
own territory.
Figure 5. Overview of the active, former, and potential floodplains at the Lower Danube.
In Figure 6, all active floodplains larger than 500 ha (average value around 5800 ha)
are sorted from up- to downstream, and the total area of each floodplain is presented. Each
floodplain received a unique, identifiable code (see Section 2.3.4) that is plotted on the
abscissa. Out of the 50 floodplains, only five have an area larger than 150 km
2
: two of them
are located in the upper (DE, AT), two in the middle (HU, HR-RS), and one in the lower
(RO) Danube section. Thirty-two floodplains have an area below 50 km
2
. Germany has ten
hydraulically active floodplains larger than 500 ha, Austria five, and one is transboundary.
Slovakia has only six transboundary floodplains. Hungary has the most floodplains, eight
only in Hungary and ten transboundary. Croatia shares one floodplain with Hungary and
five with Serbia. Serbia has ten floodplains (five transboundary and five in Serbia only).
Bulgaria shares six with Romania. Romania also has four floodplains on its own territory.
Figure 7illustrates the land uses for all active floodplains > 500 ha. The percentage of
agricultural land varies from 0.4% to 96%, with a mean of 25%. In the upper and middle
part of the Danube, the floodplains have, in general, a higher percentage of agricultural
areas and a lower percentage of forest and semi-natural areas, especially in Germany and
Austria. The share of the forest and semi-natural areas ranges from 0 to 95%, with a mean of
41%. At the end of the Middle Danube, and in the Lower Danube, forest and semi-natural
areas are dominant. Wetland cover is only present at 20 out of 50 active floodplains, mainly
located at the Lower Danube. The percentage of artificial surfaces is always under 7%.

Water 2022,14, 2295 18 of 31
Water 2022, 14, x FOR PEER REVIEW 20 of 34
Figure 6. Area distribution of active Danube floodplains (> 500 ha) from up- to downstream.
Figure 7 illustrates the land uses for all active floodplains > 500 ha. The percentage of
agricultural land varies from 0.4% to 96%, with a mean of 25%. In the upper and middle part
of the Danube, the floodplains have, in general, a higher percentage of agricultural areas and
a lower percentage of forest and semi-natural areas, especially in Germany and Austria. The
share of the forest and semi-natural areas ranges from 0 to 95%, with a mean of 41%. At the
end of the Middle Danube, and in the Lower Danube, forest and semi-natural areas are dom-
inant. Wetland cover is only present at 20 out of 50 active floodplains, mainly located at the
Lower Danube. The percentage of artificial surfaces is always under 7%.
3.2. Former Floodplains along the Danube River
The identified former floodplains amount to 22,853 km2 (incl. main river channel and
excl. the Delta), with Romania having the largest share of around 6459 km2 (28%), fol-
lowed by Hungary with 5338 km2 (23%), Serbia 3389 km2 (15%), and Slovakia 3161 km2
(14%). Around 6% are each located in Germany (1457 km2), Austria (1251 km2), and Bul-
garia (1435 km2). Croatia has the lowest share, with 645 km2 (3%). Figures 4 and 5 show in
green the delineated former floodplains. Table 4 compares the active with the former
floodplain area and provides the percent loss of floodplain for each country. About 60%
of the former floodplains were lost in Germany and Austria. With 95%, Slovakia shows
the highest floodplain loss of all countries. In Hungary, the former floodplains have been
reduced by 87%. Croatia shows the lowest reduction, with 52%. Serbia and Romania’s
current active floodplain areas are about 20% of the original area. Bulgaria shows a de-
crease of 70%. The total loss of former floodplains along the Danube River equals 79%
without the Danube Delta.
Figure 6. Area distribution of active Danube floodplains (>500 ha) from up- to downstream.
Water 2022, 14, x FOR PEER REVIEW 21 of 34
Figure 7. Distribution of land use classes, as percentages, for all active Danube floodplains from up-
to downstream.
Table 4. Comparison between active and former floodplain area for each country (incl. main river
channel and excl. Delta).
Country Active FP
(km
2
) Former FP (km
2
) Floodplain
Loss (%)
DE 606 1457 58%
AT 500 1251 60%
SK 168 3161 95%
HU 688 5338 87%
HR 307 645 52%
RS 640 3389 81%
BG 351 1152 70%
RO 1435 6459 78%
MD 0.2 -
UA 15 -
Danube without Delta 4711 22,853 79%
Figure 8 presents the current land uses of the delineated former floodplains in each
country. The figure shows that the former floodplain areas are mostly used as agricultural
Figure 7.
Distribution of land use classes, as percentages, for all active Danube floodplains from up-
to downstream.

Water 2022,14, 2295 19 of 31
3.2. Former Floodplains along the Danube River
The identified former floodplains amount to 22,853 km
2
(incl. main river channel and
excl. the Delta), with Romania having the largest share of around 6459 km
2
(28%), followed
by Hungary with 5338 km
2
(23%), Serbia 3389 km
2
(15%), and Slovakia 3161 km
2
(14%).
Around 6% are each located in Germany (1457 km
2
), Austria (1251 km
2
), and Bulgaria
(1435 km
2
). Croatia has the lowest share, with 645 km
2
(3%). Figures 4and 5show in green
the delineated former floodplains. Table 4compares the active with the former floodplain
area and provides the percent loss of floodplain for each country. About 60% of the former
floodplains were lost in Germany and Austria. With 95%, Slovakia shows the highest
floodplain loss of all countries. In Hungary, the former floodplains have been reduced
by 87%. Croatia shows the lowest reduction, with 52%. Serbia and Romania’s current
active floodplain areas are about 20% of the original area. Bulgaria shows a decrease of
70%. The total loss of former floodplains along the Danube River equals 79% without the
Danube Delta.
Table 4.
Comparison between active and former floodplain area for each country (incl. main river
channel and excl. Delta).
Country Active FP
(km2)Former FP (km2)Floodplain
Loss (%)
DE 606 1457 58%
AT 500 1251 60%
SK 168 3161 95%
HU 688 5338 87%
HR 307 645 52%
RS 640 3389 81%
BG 351 1152 70%
RO 1435 6459 78%
MD 0.2 -
UA 15 -
Danube without Delta 4711 22,853 79%
Figure 8presents the current land uses of the delineated former floodplains in each
country. The figure shows that the former floodplain areas are mostly used as agricultural
areas. In Slovakia, 80% of the former floodplains are used for agricultural purposes. In
Germany, Hungary, Serbia, and Romania, between 60–69% of the original area is cultivated.
The share of the former floodplains used for farming in Austria and Bulgaria is 47% and
55%, respectively. The former floodplain areas in Croatia are the most natural, with 37%
forest and semi-natural areas, 16% wetlands, and 11% water bodies. With 18%, the highest
share of artificial surfaces is found in Austria, followed by Germany with 13%. In all
other countries, the percentage of artificial surfaces is 10% or lower. Solely in Croatia, the
combined share of forest and semi-natural land use, wetlands, and water bodies exceeds
50%. In all other countries, the share of these combined land uses averages only at 28%.
3.3. Potential Floodplains along the Danube River
In the scope of the “Danube Floodplain” project, representative entities from the
countries Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, and Romania
applied the methodology presented in Section 2.3.3 to delineate potential floodplains
in their countries. In total, 24 potential floodplains covering an area of 1090 km
2
were
identified. In Figures 4and 5, all potential floodplains along the Danube River are shown
in yellow. Half of the delineated potential floodplains are extensions of active floodplains.
Some of these extensions are relatively small and increase the flooded area only by 2–10%
(median = 13%). Others double or quadruple the size of the active floodplains. The other
half of the delineated potential floodplains are additional areas that could be flooded in the
case of an HQ
100
event after some restoration measures. The newly inundated areas range
between 3 and 205 km
2
. The analysis of the current land use on the potential floodplains

Water 2022,14, 2295 20 of 31
shows that the share of agricultural areas ranges from 54% to 83% (Table 5). In the Upper
Danube, this share is the highest. Hungary and Serbia have the largest share of forest and
semi-natural areas (39% and 33%, respectively).
Water 2022, 14, x FOR PEER REVIEW 22 of 34
areas. In Slovakia, 80% of the former floodplains are used for agricultural purposes. In
Germany, Hungary, Serbia, and Romania, between 60–69% of the original area is culti-
vated. The share of the former floodplains used for farming in Austria and Bulgaria is 47%
and 55%, respectively. The former floodplain areas in Croatia are the most natural, with
37% forest and semi-natural areas, 16% wetlands, and 11% water bodies. With 18%, the
highest share of artificial surfaces is found in Austria, followed by Germany with 13%. In
all other countries, the percentage of artificial surfaces is 10% or lower. Solely in Croatia,
the combined share of forest and semi-natural land use, wetlands, and water bodies ex-
ceeds 50%. In all other countries, the share of these combined land uses averages only at
28%.
Figure 8. Current land uses in the former floodplains for each country.
3.3. Potential Floodplains along the Danube River
In the scope of the “Danube Floodplain” project, representative entities from the
countries Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, and Romania
applied the methodology presented in Section 2.3.3 to delineate potential floodplains in
their countries. In total, 24 potential floodplains covering an area of 1090 km
2
were iden-
tified. In Figures 4 and 5, all potential floodplains along the Danube River are shown in
yellow. Half of the delineated potential floodplains are extensions of active floodplains.
Some of these extensions are relatively small and increase the flooded area only by 2–10%
(median = 13%). Others double or quadruple the size of the active floodplains. The other
half of the delineated potential floodplains are additional areas that could be flooded in
the case of an HQ
100
event after some restoration measures. The newly inundated areas
range between 3 and 205 km
2
. The analysis of the current land use on the potential flood-
plains shows that the share of agricultural areas ranges from 54% to 83% (Table 5). In the
Upper Danube, this share is the highest. Hungary and Serbia have the largest share of
forest and semi-natural areas (39% and 33%, respectively).
Figure 8. Current land uses in the former floodplains for each country.
Table 5. Current land use of the identified potential floodplains.
Country Artificial
Surfaces
Agricultural
Areas
Forest and
Semi
Natural
Areas
Wetlands Water
Bodies
DE 0% 79% 15% 0% 7%
AT 0% 83% 17% 0% 0%
HU 0% 57% 39% 4% 0%
RS 0% 65% 33% 0% 2%
BG 0% 54% 18% 1% 28%
RO 0% 67% 13% 7% 13%
In Figure 9, the active, former, and potential floodplains in each country are compared.
To assess how much of the former floodplain is still a hydraulically active or a potential
floodplain, the percentage of the former floodplain that is active or potential, and the sum
of the two, is provided for each country. In Germany and Austria, the identified potential
floodplain area is between 39 and 45 km
2
, which would increase the active floodplain by
around 3–4% of the former floodplains. Combined, the active and potential floodplains
represent 45% (DE) and 44% (AT) of the former floodplains in these two countries. In the
present study, no potential floodplains were identified in Slovakia and Croatia, although
only 5% (168 km
2
) of the former Slovakian floodplains are still active. Forty-eight percent
of the former floodplains have been preserved in Croatia, but it must be considered that, at
645 km
2
, the former Croatian floodplains are by far the smallest. Additional 28 km
2
(1%
of the former floodplains) floodplain areas were delineated in Hungary. In total, active

Water 2022,14, 2295 21 of 31
and potential Hungarian floodplains cover an area of 716 km
2
, or 14% of the previously
inundated areas. Serbia and Bulgaria could increase their active floodplains by about
221 km
2
(7% and 19% of the former floodplains) with the potential ones, resulting in a
preservation of 861 km
2
(26%) and 573 km
2
(49%). The identified potential floodplains in
Romania equal 8% (536 km
2
) of the former floodplains. The sum of active and potential
floodplains shows that, in Romania, 30% (1971 km
2
without Danube Delta) of the former
floodplains could be preserved.
Water 2022, 14, x FOR PEER REVIEW 23 of 34
Table 5. Current land use of the identified potential floodplains.
Country Artificial
Surfaces
Agricultural
Areas
Forest and Semi
Natural Areas Wetlands Water Bodies
DE 0% 79% 15% 0% 7%
AT 0% 83% 17% 0% 0%
HU 0% 57% 39% 4% 0%
RS 0% 65% 33% 0% 2%
BG 0% 54% 18% 1% 28%
RO 0% 67% 13% 7% 13%
In Figure 9, the active, former, and potential floodplains in each country are com-
pared. To assess how much of the former floodplain is still a hydraulically active or a
potential floodplain, the percentage of the former floodplain that is active or potential,
and the sum of the two, is provided for each country. In Germany and Austria, the iden-
tified potential floodplain area is between 39 and 45 km2, which would increase the active
floodplain by around 3–4% of the former floodplains. Combined, the active and potential
floodplains represent 45% (DE) and 44% (AT) of the former floodplains in these two coun-
tries. In the present study, no potential floodplains were identified in Slovakia and Croa-
tia, although only 5% (168 km2) of the former Slovakian floodplains are still active. Forty-
eight percent of the former floodplains have been preserved in Croatia, but it must be
considered that, at 645 km2, the former Croatian floodplains are by far the smallest. Addi-
tional 28 km2 (1% of the former floodplains) floodplain areas were delineated in Hungary.
In total, active and potential Hungarian floodplains cover an area of 716 km2, or 14% of
the previously inundated areas. Serbia and Bulgaria could increase their active flood-
plains by about 221 km2 (7% and 19% of the former floodplains) with the potential ones,
resulting in a preservation of 861 km2 (26%) and 573 km2 (49%). The identified potential
floodplains in Romania equal 8% (536 km2) of the former floodplains. The sum of active
and potential floodplains shows that, in Romania, 30% (1971 km2 without Danube Delta)
of the former floodplains could be preserved.
Figure 9.
Area analysis of active, former, and potential floodplains along the Danube River. The
percentages show how much of the former floodplains are still hydraulically active, or a potential
floodplain, in each country.
4. Discussion
4.1. Methods for Identifying Active, Former, and Potential Floodplains
To identify the active floodplains, a hydraulic approach that uses the inundated areas
of a 100-year return period was chosen, in alignment with previous literature [
10
,
18
,
20
,
71
].
One major advantage of this approach is that the HQ
100
outlines are widespread and avail-
able for Europe. In contrast to previous studies [
18
,
20
], we classified the active floodplains
into three groups, allowing a detailed analysis of the largest active floodplains along the
Danube River. We focused on floodplains of basin-wide importance (area
> 500 ha
) since
all DRBMPs [
32
,
52
,
64
] also deal with these floodplains. Other methods that use pedological
and geological information [
38
] were not chosen due to the different availability and data
quality in the Danube countries. Moreover, we considered the hydraulic approach more
suitable, since, in a previous study, Eder et al. [
49
] used hydraulic models to calculate the
retention capacity of each identified floodplain.
Identifying former floodplains is a major challenge, since human interferences have
changed the river landscapes considerably, but as Hohensinner et al. [
72
] and Kiss et al. [
73
]
showed, reconstruction of the historical characteristics and its surroundings prior to these
interventions is possible. Various studies and methods [
11
,
13
–
15
,
17
,
18
] can be found in the
literature that delineate former floodplains along different rivers and river sections. Some
approaches use geospatial data (DEM, satellite imagery), hydraulic data (e.g., HQ
extreme
outlines), and/or historical sources, such as maps and local chronicles. Based on the

Water 2022,14, 2295 22 of 31
concepts and techniques presented in previous studies, we developed a systematic method
organized along four consecutive steps leading to the former floodplains along a river. The
goal was to develop a replicable method that can be applied to other large rivers along
their whole watercourse. Since the data basis for each river is individual, our method
uses different data sources (geospatial, hydraulic data, and historical sources), and is
structured such that it is also applicable if certain data (e.g., HQ
extreme
) are unavailable. The
application of the developed method shows that the different data sets complemented each
other very well. Hauer et al. [
35
] showed that an approximate delineation of the former
floodplains is possible only with a current DEM and satellite imagery. These data sets
allow for identifying postglacial low terraces and breakthrough valleys. Delineation using
only these data sets is not recommended, and should only be applied if no other data are
available. Another method to delineate the former floodplains more accurately is to use
2D hydrodynamic models. To delineate the formerly flooded areas, the hydraulic modeler
has to create an historical DEM that represents the historical characteristics of the river
and its surrounding landscape before human interference. Skublics and Rutschmann [
36
]
developed a 2D hydraulic model for a 270 km section along the German Danube, and
simulated the inundation areas prior to the implementation of any human modifications.
Since creating such a model requires a lot of time and resources for an entire river, we did
not consider this option.
After delineating the former floodplains, potential floodplains should be identified to
find possible restoration sites. Floodplains have a fundamental role in the river ecosystem,
and provide numerous ecosystem services [
1
–
3
,
74
]. However, the reconnection of former
floodplains is still a major challenge because different social needs (e.g., settlements, agricul-
ture, hydropower, nature protection, etc.) and legal frameworks must be considered [
75
,
76
].
Our goal was to identify potential floodplains that have the potential to be reconnected
in the future. Based on the former floodplains, areas for settlements and infrastructure,
and certain agricultural areas (where land ownership or the legal framework does not
allow compensation, or is too expensive) are excluded from the potential floodplains. Since
decision makers and stakeholders play key roles in restoration projects [
77
–
79
], they should
already be involved in the delineation process of the potential floodplains. In our case
study (see Section 2.3.3—step III), some stakeholders excluded certain agricultural areas,
where they considered restoration measures unfeasible. Hence, the potential floodplains
delineated in this study represent only a small part of all potential restoration sites at the
Danube River. Our aim was not to identify all potential restoration sites, but only the
more realistic ones, where decision makers and stakeholders can see opportunity for future
restoration projects.
All the applied methods and used data come with limitations and uncertainties,
starting with the HQ
100
outlines, which were calculated with a hydraulic model. Statistical
inference plays a role here, since extreme values were extrapolated from historical data sets
that are used in the model. Other uncertainties exist, for example, in the representation of
the terrain and the roughness parameterization in the model. We used the results from
calibrated and validated hydrodynamic models (used in the “Danube FLOODRISK” project
and from national flood hazard and risk maps) to reduce uncertainty. We relied only on
hydraulic data to define the active floodplains, since this approach has proven to be reliable
in the past, and because we used HQ
extreme
outlines to delineate former floodplains. Thus,
our approaches are based on comparable sets of data.
Moreover, we used historical data, which are also characterized by substantial un-
certainties. For examples, different cartographers were involved in creating the historical
maps used in our study, who certainly deployed different levels of accuracy in their work.
As mentioned above, information from historical sources should be verified with current
data sets or other historical sources: DEM or inundation areas based on hydraulic modeling
can help conduct a plausibility check. Nevertheless, the delineated active, former, and
potential floodplains and their absolute numbers presented here must be seen in light of
inherent uncertainties. The mentioned sources of uncertainties may lead to incorrect results

Water 2022,14, 2295 23 of 31
on a local scale, but the larger the study area, the better the accuracy, since they can level off.
Combining and cross-checking different data sources (hydraulic data, historical sources,
and current geospatial data) helps to minimize incorrect results and allows for the creation
of a floodplain inventory that serves as a basis for future floodplain management.
4.2. Active, Former, and Potential Floodplains at the Danube River
The report by the DPRP [
20
] was the first evaluation of active, former, and potential
floodplains along the Danube River. The study is mainly based on historical, topographical,
and thematical maps and satellite images. Schwarz [
18
] updated the evaluation of the
floodplains along the Danube River based on the most recent and accurate DEM at that time,
and focused on the definition of postglacial terraces. As recommended by Schwarz [
18
] (the
study by Hein et al. [
19
] is based on the results of his research), the present investigation
completely revised and refined the delineation of active floodplains based on the results
of hydraulic models, improving the delineation of earlier work. In total, we identified
347 km
2
fewer active floodplains than Schwarz [
18
]. Fifty hydraulically active floodplains
larger than 500 ha were evaluated in a next step with the Floodplain Evaluation Matrix
(FEM) [
5
,
48
] to assess the value of these areas for flood protection, ecology, and socio-
economy [49].
The comparison with previously delineated former floodplains of Schwarz [
18
] showed
that the delineation with the data at that time was matching in some parts. Still, in other
areas, the new sources led to changes in the extent of the former floodplains. In total, we
identified 2067 km
2
, in addition to the formerly flooded areas from the previous study [
18
].
Especially at the Middle Danube, the former floodplains are around 2180 km
2
larger than
in Schwarz [
18
]. In the upper and lower sections, the former floodplains are smaller by
97 km2and 16 km2, respectively.
This paper builds on the outputs of the “Danube Floodplain” project, where represen-
tatives ranging from national water administrations and universities to NGOs collaborated
to delineate the potential floodplains using the presented method. All the national partners
identified the potential restoration sites in their countries of responsibility. The Danube
River case study shows that, even though perhaps more realistic potential floodplains
were identified, with 24 potential sites and a total area of 1090 km
2
, the potential flood-
plains delineated in our work are much smaller than reported in an earlier assessment by
the DPRP [
20
], which delineated ten areas for future restoration covering 2774 km
2
. In
Schwarz [
18
], 196 potential restoration sites with a total size of 8102 km
2
(1797 km
2
on active
and 6305 km
2
on former floodplains) were reported. In both past studies, agricultural land
uses were not excluded from the potential floodplains. In our investigation, the political
and socio-economic conditions have led to large areas, such as Balta Ialomita, Big and
Small Islands of Braila, which are former floodplains, not being identified as potential areas,
although they are mainly used for agriculture. The reason for this is that the involved
representatives of these countries did not see these areas as realistic restoration areas. The
combined share of active and potential floodplains compared to the former floodplains
ranges between 5% and 49% in the individual countries, with a median of 37%, showing
that the potential in each country is still high. Hence, the potential floodplains delineated
in this study represent only a small part of all potential restoration sites at the Danube
River. One future goal should be to increase these percentages and identify even more
potential floodplains.
The total loss of former floodplains along the Danube River equates to 79%, without
the Danube Delta, which is comparable with other large rivers, such as the Rhine, with a
loss of 84% [
80
], or the Lower Mississippi, with a reduction of about 90% [
81
]. The results
of the ratio between active, potential, and former floodplains show that there are significant
differences between the three Danube sections. In the upper section, the share of active
floodplains on the former floodplains is about 41%, which is much higher compared to
the 14% coverage of the formerly flooded areas in the Middle Danube. One reason for
this is that the Upper Danube is an alpine river with many breakthrough valleys, where

Water 2022,14, 2295 24 of 31
a large expansion of floodplains is not possible due to the limitation of the mountains,
and thus only smaller areas could be disconnected there. In the Middle Danube, there
is the Pannonian lowland, where the flat terrain enabled the inundation of an average
30–35 km wide area. These large floodplain areas offered the perfect place for agricultural
activity, so they were disconnected by artificial levees. Moreover, the construction of flood
protection dikes began earlier in the middle section [
82
], which might have contributed
to the disconnection of larger areas from the former floodplains. After breaking through
the Carpathian Mountains, the Danube River passes through the Wallachian lowlands in
the lower part of the river, where the river had more space to expand than in the upper
section. At the Lower Danube, 23% of the formerly flooded areas are still inundated
during a 100-year return period. The current land use shows that most hydraulically
preserved floodplains in the upper section have lost their natural properties. Especially
in mountainous regions where space is limited, the valley bottom, where the rivers and
their floodplains are located, is used for agriculture. At the Lower Danube, hardly any
agricultural uses are found on the active floodplains. One reason for this might be that
the flood characteristics in the upper and lower sections are quite different. At the Lower
Danube, the floodplains can be inundated for several weeks during a flood event due
to lower flood dynamics [
83
,
84
], which would also substantially reduce the period for
agricultural use. However, on the former and potential floodplains, the land uses are
more or less similar for all countries, since these areas are mainly used for agriculture,
followed by forest and semi-natural areas. Fremling et al. [
81
] showed that most of the
former floodplains along the Lower Mississippi are also used for agricultural purposes.
4.3. Floodplain Management
Floodplain ecosystems are among the most diverse and productive on the planet [
85
].
Despite their numerous benefits, up to 90% of former floodplains are lost or impaired by
human activities in Europe and North America [
9
]. Economic growth, increasing population
size, and ongoing urbanization threaten the remaining active floodplains. Creating an
inventory of active, former, and potential floodplains along a river could support the
preservation and restoration of floodplains since, in this way, the losses and potential
restoration sites can be shown.
However, various stakeholders and decision makers have differing and sometimes
conflicting priorities [
19
]. In fact, floodplain restoration implies tradeoffs among multiple
aspects (ecology, ecosystem services, land uses, costs), thus, a compromise must be met
among all interested groups [
12
], and requires solving multiple organizational issues
(discussions with landowners, meeting deadlines, securing funding, supervising staff, and
cooperating with politicians), as well as scientific work [86].
How can we encourage stakeholders to see floodplain restoration as a positive mea-
sure? It should be made clearer that the restoration of potential floodplains does not
only indirectly benefit people by benefitting the environment, but also directly via better
economic possibilities. For example, the restoration in Babina and Cernovca (RO) in the
Danube Delta has brought back the traditional economic occupations of the local popu-
lation —fishing, hunting, reed harvesting [
87
]. Therefore, funders and planners should
ensure that the full range of values of restoration projects is assessed and measured to
facilitate decision making [4]. Multiple methods exist for evaluating restoration measures
of potential floodplains in all its facets, e.g., through ecosystem services assessments [
88
,
89
],
multicriteria decision-making methods [
90
], or cost-benefit analyses [
91
,
92
]. Another way
to facilitate floodplain restoration is to evaluate the identified potential floodplains with the
Floodplain Evaluation Matrix (FEM), which could demonstrate the value of the additional
areas for flood protection, ecology, and socio-economy, and also increase the willingness to
reconnect the identified areas. Eder et al. [
49
] assessed the potential floodplains delineated
in this study with the FEM.
A major class of stakeholders involved in the implementation of floodplain restoration
projects are landowners, who, in some cases, do not agree with trading their land. If land

Water 2022,14, 2295 25 of 31
acquisition is not possible, another solution would be to make a contract with private
partners, i.e., a public–private partnership [
86
]. Payments for ecosystem services, i.e.,
payments to landowners or managers to provide or protect ecosystem services [
88
], can
be effective elements with which landowners and farmers receive payments to manage
their land properly and avoid public costs related to unsustainable land use (e.g., water
contamination or soil degradation) [
93
], benefitting landowners as well as others [
94
]. This
voluntary scheme could facilitate the adoption of environmentally sound land use practices,
making economic factors include otherwise unvalued externalities in their floodplain
management [
95
]. For this, it should be made clear in advance how the restoration measures
might affect landowners, for example, it would be useful to estimate whether and by how
much farmers might experience losses in terms of agricultural production.
Learning from previously implemented projects could also help get ideas on the next
steps to follow after delineating active, former, and potential floodplains. For example,
the “Room for the River” program (completed in 2018 in the Netherlands [
96
]), included
over 30 projects along four major rivers, and was not only able to reduce the risk of
flooding, but also to provide valuable habitats and offer high potential for tourism and
recreation [
93
,
97
,
98
]. On the Danube, much work was already conducted by the Integrated
Danube Programme (IDP), which was adopted for the Upper Danube (from the source
rivers Brigach and Breg to the town of Ulm), and aimed to combine flood protection and
ecological restoration of floodplains [
12
], floodplain restoration along the Danube between
Neuburg and Ingolstadt, and along the Danube’s Green Corridor [99].
Some results from the “Danube Floodplain” project and this study contribute to
the latest version of the Danube River Basin [
32
] and Flood Risk Management Plan [
32
],
giving some strategic guidance and a restoration roadmap. However, a concrete large-
or basin-scale restoration program is still missing at the Danube level. This is due to
the high international level of the catchment [
19
], which leads to inconsistencies in river
management legislation, differences in socio-economic conditions, and high complex-
ity and heterogeneity of the environmental problems. Moreover, data availability also
plays a role [1].
Nevertheless, even on large rivers, restoration work can still be performed success-
fully [
12
]. To reach this goal, stakeholder involvement should be improved and long-term
planning should be undertaken [
86
]. On one hand, we need to show all the benefits
of restoration (flood risk, ecosystem improvement, etc.) to attract funding from differ-
ent sources [
100
]. On the other hand, we should not take ecological improvement for
granted, and standards for measuring the success of an ecological river restoration should
be used [
101
]. Within the “Danube Floodplain” project, a catalogue of floodplain restora-
tion measures (Deliverable 5.2.1 (https://www.interreg-danube.eu/approved-projects/
danube-floodplain/outputs accessed on 20 May 2022)) was created to promote win–win
measures for reaching multiple goals (e.g., flood risk reduction, ecological improvements,
etc.). After restoration is implemented, monitoring the floodplain will be fundamental, to
ensure that the system functions in a sustainable way or to understand whether additional
measures are needed for the system to operate in a self-regulatory manner [
12
]. The re-
sults of monitoring should then be communicated to funders, stakeholders, restoration
practitioners, scientists, and policy makers [101].
5. Conclusions
Methods used to identify active, former, and potential floodplains are presented and
applied for the case study of the Danube River. For identifying active floodplains, a
hydraulic approach based on the inundation areas of a 100-year return period was used, as
this data set is widespread and available for many large rivers. Hydraulic data, historical
sources, and current geospatial data were combined to delineate former floodplains. This
approach enables a more accurate delineation of formerly flooded areas in comparison
to using only one data set. The comparison with the active floodplains showed that 79%
of the former floodplains along the Danube River were lost due to human pressures,

Water 2022,14, 2295 26 of 31
such as land use change, river regulation, and dam construction. Based on the identified
former floodplains, potential floodplains were delineated with the support of different
representative institutions from the basin countries, ranging from national water agencies
and universities to NGOs. Twenty-four potential floodplains covering 1090 km
2
were
identified. However, the share of active and potential (arising from the former) floodplains
ranges between 5% and 49%, indicating that further work could be undertaken to identify
additional restoration sites. It was also shown that the involved institutions were acting
conservatively when establishing restoration areas so that large-scale restoration sites were
rarely delineated, although former floodplains are mainly used for cultivation purposes.
Agricultural usage is also the predominant land use on the active floodplains at the Upper
Danube. At the Lower Danube, the active floodplains have a more natural land use,
where the forest and semi-natural class is dominant. Additionally, potential floodplains
are mainly used for cultivation purposes, a factor that will have to be considered when
actually implementing the restoration measures.
This work, along with others before, is a wake-up call regarding the dramatic flood-
plain loss that has taken place so far, and could still happen in the future. A comprehensive
and systematic analysis of active, former, and potential floodplains along a river or in a
catchment contributes to an understanding of the current, as well as the past, floodplain
situation. Others can use this example as a basis for the analysis of floodplain loss and
restoration potential, and for future-oriented floodplain management. An application at
other large rivers (e.g., Nile, Mississippi) is desirable, and an inventory of active, former,
and potential floodplains would help floodplain managers in the future (or preferably
at present time) with the preservation and restoration of floodplains. Providing an on-
line inventory, such as the “Danube Floodplain GIS (http://www.geo.u-szeged.hu/dfgis/
accessed on 20 May 2022)”, which shows some of the results of this study, could raise
awareness of the immense floodplain loss, allowing the public to actually visualize the
former floodplains on a map and compare them with the identified active floodplains.
Evaluating active and potential floodplains can deliver arguments for preserving and
restoring these areas, and support more sustainable floodplain management.
Author Contributions:
Conceptualization, M.E., F.P., S.H., M.T. and H.H.; methodology, M.E., F.P.,
S.H., M.T., S.S., M.G., B.C., T.K., B.V.L., Z.T., N.C., G.S., A.S. (Anna Smetanová), S.B., A.S. (Andrea
Samu), T.G., A.-C.G., M.M., P.M. and H.H.; validation, M.E., F.P., S.H., T.K., B.V.L., Z.T., N.C.,
A.-C.G.
and M.M.; formal analysis, M.E., F.P., S.H., M.T. and H.H.; investigation, M.E., F.P., S.H.,
S.S., T.K., B.V.L., N.C., A.-C.G., M.M. and P.M.; resources, H.H.; data curation, B.V.L., Z.T. and G.S.;
writing—original
draft preparation, M.E. and F.P.; writing—review and editing, M.E., F.P., S.H., M.T.,
S.S., M.G., B.C., T.K., B.V.L., Z.T., G.S., N.C., A.S. (Anna Smetanová), S.B., A.S. (Andrea Samu), T.G.,
A.-C.G., M.M., P.M. and H.H.; visualization, M.E.; supervision, H.H.; project administration, M.E.,
F.P., S.S., M.G., B.C., T.K., B.V.L., Z.T., G.S., A.S. (Anna Smetanová), S.B., A.S. (Andrea Samu), T.G.,
A.-C.G., M.M., P.M. and H.H.; funding acquisition, M.E., B.C., B.V.L., Z.T., G.S., A.-C.G., M.M., P.M.
and H.H. All authors have read and agreed to the published version of the manuscript.
Funding:
This work was supported by the European Union’s Interreg Danube Transnational Coop-
eration Programme in the Danube Floodplain project—Reducing the flood risk through floodplain
restoration along the Danube River and tributaries (grant number DTP2-003-2.1, 2018). Moreover,
Markus Eder was supported by the Doctoral School “Human River Systems in the 21st Century
(HR21)” of the University of Natural Resources and Life Sciences, Vienna. Severin Hohensinner’s
contribution to the study was funded by the Austrian Federal Ministry for Digital and Economic
Affairs; the National Foundation for Research, Technology and Development; and the Christian
Doppler Research Association.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The identified active, former, and potential floodplains can be found
in the “Danube Floodplain GIS” (http://www.geo.u-szeged.hu/dfgis/ accessed on 20 May 2022),
which is a spatial database, created in the scope of the “Danube Floodplain” project.

Water 2022,14, 2295 27 of 31
Acknowledgments:
The authors would like to thank all the project partners involved in the Danube
Floodplain project for the excellent collaboration and their support for this paper. Further, the authors
would like to thank Ulrich Schwarz for his valuable input and shared data and experiences.
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
Appendix A
Water 2022, 14, x FOR PEER REVIEW 30 of 34
Appendix A
Figure A1. Overview of the three levels of land uses categories of the CORINE land cover data set.
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