The Netherlands increasingly have to deal with larger variations in river discharges due to climate change. At the same time, the river bed is eroding and the floodplains are silting up as a result of historic human interventions. These developments contribute to the desiccation of floodplains and consequently threaten the characteristic Dutch riverine nature. This article describes the impact of these developments on the condition of Dutch terrestrial floodplain nature and the potential design and management measures that could be taken to counteract the negative effects.
Various developments are imposing pressures on our river system. Climate change causes river discharges to become both higher and (prolonged) lower. Embankments and normalisations in recent centuries have resulted in the river to be narrower and this reduced the extent of the floodplains. This process reduced discharge capacities significantly. Another effect is that a much narrower river relates to more gentle river bed slopes to which the river gradually adapts. Due to embankments on both sides of the river, the river can only adapt by eroding the river bed and by releasing sediments on the floodplain during high water. This is a self-reinforcing process with ever-increasing consequences for water safety, fresh water supply, shipping, nature and many other functions that depend on (ground) water levels.
River bed erosion results in lower water levels on sections without weirs, lower ground water levels in floodplains, less frequent inundation of floodplains and a reduction of dynamics that are vital for nature. Floodplains desiccate as a consequence and this causes a loss in diversity of floodplain nature. This river bed erosion in combination with climate change, which is related to lower water levels during summer and higher water levels in spring and fall, can have significant consequences for the valuable riverine nature in The Netherlands.
This exploratory study shows which groundwater conditions are needed for the different types of nature in floodplains, what the current and future conditions are, and with which design and management measures these conditions can be realised. For the assessment of floodplain nature this study used ecotopes: relatively homogeneous, spatial vegetation types that have roughly the same environmental demands.
Based on literature study and a session with ecologists and vegetation experts, ground water conditions required for the ecological functioning of terrestrial floodplain nature were identified. Subsequently, the conditions were compared with the current and future hydrological conditions in the floodplains. These were simulated with the National Water Model, which also includes a ground water model. Climate effects were investigated using the future scenarios of the Deltaprogramma Zoetwater (English: Delta Program Freshwater). The extent of river bed erosion was based on annual bed measurements and extrapolated for future scenarios. This shows an erosional trend of approximately one to two centimetres per year (the extent differs greatly along the river; both per river branch and per river).
An ecotope map (5th edition) was used to map floodplain nature. The following five terrestrial ecotope types, relevant for the study area, were distinguished: hardwood forests and shrubs, softwood forests and shrubs, wet grasslands, dry grasslands, and reeds and marshes. These ecotope types can roughly be categorised as wet (wet grasslands and reeds/marshes) and dry nature (dry grasslands and hardwood forests/shrubs), with softwood forests and shrubs in an intermediate position. The (future) suitability of the locations for the ecotope types was assessed using the calculated average spring ground water levels (GVG) in the current situation and the year 2050 (based on the Steam scenario of the Deltaprogramma Zoetwater). The GVG was chosen because of the importance of the spring ground water level for the development of vegetation and its reliability as a boundary condition for each ecotope type. Two scenarios have been taken into account for 2050: a situation without river bed management (in other words: ongoing erosion) and a situation with management in which the river bed is kept on its current level. The assessments are categorised as: too wet, wet, good, dry and too dry.
The Programmatische Aanpak Grote Wateren (PAGW, English: Programmatic Approach Large Waters) has investigated what is needed to realise a robust and futureproof river system. The outcome of this program is part of the program Integraal Rivier Management (IRM, English: Integral River Management). The PAGW study resulted in four hotspots (Biesbosch, IJssel-Vechtdelta, Grensmaas and Gelderse Poort) that form the base for ecological improvement of the river area (Van der Sluis et al., 2020). The methods for the assessment of hydrological conditions of ecotopes was applied to the Gelderse Poort as river bed erosion is most evident in this area: the eroding stretches of the Waalbochten, the Pannerdens Kanaal and the Boven-Ijssel are adjacent to the Gelderse Poort. The area of the Gelderse Poort further includes the Oude Rijn and Groenlanden because of the closely related ecology and the potential to achieve sustainable populations of species in low-dynamic ecotopes.
Current condition of floodplain nature
In the past centuries, the river bed in the Gelderse Poort has eroded with one to two metres. For ecotopes that prefer wet conditions, this erosion has already led to a severe degradation (Fig. 1, current situation). For the ecotope reeds and marshes the spring ground water levels are too low and the situation is too dry. The hydrological conditions are more suitable for the dry ecotope types and in some locations even (too) wet. Softwood forests and shrubs thrive in a wide range of hydrological conditions: for this ecotope type the conditions on the present locations are on the dry side, but still acceptable. This analysis is based on an assessment of the average spring ground water levels only and does not consider other aspects in relation to the location of ecotopes, such as soil type, inundation frequency and the level of continuity. These aspects also determine whether or not a location is suitable for an ecotope (e.g. hardwood forests and shrubs require a higher location with a low flood frequency of less than once every 10 years). Our conclusions thus only focus on the ground water level.
Future condition of floodplain nature
The effect of climate change on spring river discharge is relatively small in the climate scenarios considered (Fig. 2). This means that the effect of climate change on mean spring ground water levels is also limited. As a result, there are hardly any differences between the assessment of the hydrological conditions in the current situation and the future situation in 2050 (with only the climate effect, right column in Fig. 1). Due to the ongoing river bed erosion, conditions become drier in floodplain. This desiccation causes the conditions for dry ecotopes to improve, while the conditions for wet ecotopes deteriorate (fig. 1). This threatens the characteristic wet river nature and will lead to a decrease in the variety of floodplain nature: homogeneity is therefore imminent.
Two types of measures are possible to ensure that the floodplain will regain its needed wet conditions. The first one is to bring the river water level and the floodplain level closer together. Maintaining the present level of the river bed is crucial and should be combined with raising the bed level where possible. Additionally, the floodplain level can be lowered when redesigning floodplains. This way additional relief and variation can be regained in the floodplain such that the higher and dryer habitats remain suitable for the dry ecotopes. The second type of measures focuses on retaining water for a longer period of time in the floodplain in spring and early summer. By managing inlets differently, water can be let in and out in a controlled manner, which could lengthen the inundation period and keeping the ground water levels higher. This is beneficial for the low-dynamic reeds and marshes situated in these areas. Both types of measures will increase hydrodynamics. It will be necessary to combine several interventions to achieve an optimal ecotope distribution.
In the present situation, wetland ecotopes have already dried out considerably and may disappear from the Dutch river landscape in the future. Climate change in combination with river bed erosion increases the desiccation of floodplains which threatens the characteristic wet river nature. This may lead to a decrease in the variety of floodplain nature: homogeneity is imminent. The concern of Rijkswaterstaat is that targets set up for nature (described in the Natura 2000 management plans and executed in the PAGW) will be under pressure.
This exploratory study shows that it is of great importance to stop river bed erosion. This can be done by active river bed management. Other measures to limit desiccation of floodplains are lowering the floodplain level or retaining water for a longer period of time. It is crucial to elaborate these measures. The method described in this article can be used to assess the effectiveness of the measures and also perform an assessment for other PAGW hotspots and floodplain areas along the Rhine and the Meuse.
River bed erosion has already led to deterioration of characteristic floodplain nature. Without intervention, this ongoing erosion in combination with lower discharges in summer due to climate change, will lead to further deterioration, especially of wet ecotope types. Managing the river bed at the present bed level will limit further damage. Other types of design and management measures are needed to improve the hydrological conditions in floodplains and make floodplains suitable again for both wet and dry ecotopes.
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