Living with floods: a change of strategy in The Netherlands
Frans Klijn and Jos Dijkman of WL/Delft Hydraulics describe the Dutch government's 'room for rivers' policy which aims to prevent flooding by 'building with nature.'
Figure 1: Interventions in the river and floodplain currently under consideration
Decades-old plans for dike reinforcements were reviewed and revised. All river dikes must now provide flood safety at a level of a 1 in 1250 per year probability, and improvement projects are currently taking place along 600km of dikes in order to reach this goal by the year 2000. The dike reinforcement projects now give significant attention to landscape and ecological considerations, and circumvent demolition of buildings as much as possible by the application of special dike reinforcement methods like sheet piling, removable flood walls, etc.
The Dutch have learned that however high the dikes are, there still is a limit to the safety that they can guarantee. Experience at home and abroad shows that land use quickly changes in response to enhanced safety levels. Investments in housing, industry and transportation infrastructure increase, leading to the situation where overall risks (probability of occurrence multiplied with potential flood damage) also increase. Thus, lowering the probability of flooding is soon counteracted by the larger potential damage of flooding. This phenomenon of mutual adaptation of land-use and improvements in flood prevention has led to much degradation of the scenic and natural landscape values along the main waterways of The Netherlands. The societal acceptance of further heightening and strengthening dikes is now very low despite attempts to reduce the impacts on the landscape to a minimum.
Consequently, great efforts are being put into the design of alternative strategies and measures for minimising flood risk. In the 4th National Policy Document on Water Management (1997) the government emphasised the need for a sustainable water management strategy. An important element of this strategy is to provide 'room for the rivers'. This policy translates into very strict regulations for further developments in the riverbed and floodplain (other than for agriculture and recreation). It also provides a watertight definition (with maps) of the riverbed and floodplain. The maps are of special importance for identifying the floodplain regions not bounded by dikes.
The 'room for the rivers' policy also implies providing room for natural processes and trying to 'build with nature' to the benefit of river management. Here, the tendency of a river to achieve dynamic equilibria in discharge, sediment transport, width-depth ratios etc. must be exploited in such a way that more reliable and sustainable solutions can be expected. In this context, a wide variety of river interventions is available, as depicted in Figure 1. Possible measures currently under scrutiny range from dredging the river, lowering the floodplain (in combination with nature development) even to setting back dikes, thereby enlarging the floodplain. Also, the removal of existing weirs on one of the Rhine branches that is not intensively used for navigation is being considered.
For the Meuse River, which mainly flows through a natural terraced valley with elevated land on both sides, WL/Delft Hydraulics has recommended a combined strategy for lowering flood risks. This consists of deepening the riverbed and lowering the present floodplains, requiring the excavation of over 100M m3 of sand and gravel. This plan is now being further examined and turned into a concrete plan by the National Water Authorities. The measures are to a large extent paid for by the revenues of the gravel and sand mining, as the excavation can be tuned in such a way as to be economically beneficial.
While most of the Meuse River has to meet navigation requirements and will therefore be kept under strict flow control, in one unregulated stretch of about 60km, the Grensmaas, a far-reaching nature development plan will be carried out. The plan along the Grensmaas is in essence a large nature rehabilitation project in the context of the so-called Ecological Main (infra)Structure, aiming at the establishment of a vast and connected network of natural areas in a country which is sometimes addressed as a desert of agricultural land. The nature rehabilitation project relies on the restoration of the fluvial processes such as erosion and sedimentation, and the subsequent establishment of a riverine environment where vegetation and fauna can thrive. At the same time, the plan will significantly reduce flood stages, and thereby provide an important reduction in flooding risks.
In the long run 'room for the rivers' may require setting aside large tracts of former floodplain which were diked centuries ago. Under changing climatic circumstances the present floodplain area may prove to be too narrow, especially if there is a change in the discharge character of the rivers in the next century. This change is expected to increase the frequency of high discharges as well as yield discharges substantially higher than can presently be coped with. Especially in combination with a sea level rise of as much as 0.8m or more, this may require drastic measures in the river delta region.
WL/Delft Hydraulics is presently engaged in a policy analysis study commissioned by the national authorities to investigate the effectiveness of short to medium-term measures (10-20 years) for minimising flood risks. These include: lowering the floodplains of the Rhine branches (which will most probably involve the excavation of hundreds of millions of cubic metres of floodplain material), removing river obstructions (bridge ramps, ferry landings etc.), digging side channels, managing vegetation in order to prevent forest development by eg. grazing cattle etc. Many of these measures can be regarded as no-regret, but may be only temporarily effective, unless intensive maintenance is carried out to mitigate effects of accelerated sedimentation of silt.
For the long term, WL/Delft Hydraulics is exploring several additional possibilities such as: relocating the dike system, changing the distribution of the discharge over the various branches of the Rhine, creating by-passes with spill-ways around bottlenecks resulting from unfortunate locations of large cities etc., all in order to approach the natural behaviour of a large river in a deltaic environment. These possibilities logically result from a thorough conceptual rethinking of river management and engineering in a context of global change. Only by adopting a strategy of resilience instead of resistance can sustainable solutions be designed. This requires an adaptation of society, i.e. land use and land use planning, to the natural and ever-changing dynamics that are related to river systems. The study reveals that flood protection, river management and physical planning must soon become more closely integrated, a notion which has also been formulated by the responsible ministry in the most recent National Document on Water Management Document.