In lowland rivers, bed morphological processes at spatial scales much larger than the water depth are generally slower than hydrodynamic processes. The morphological changes on river reaches of tens of kilometers or more typically develop over a period of years to centuries, which is here referred to as the engineering timescale. The engineering timescale is relevant from a perspective of planning river interventions and operational river management, which is often focused on navigability, flood prevention and nature conservation. In the long term, the riverbed develops toward a (quasi) equilibrium situation. De Vries (1975) introduced a morphological timescale for the development of longitudinal riverbed profiles. He and others (e.g., Church & Ferguson, 2015;Dade & Friend, 1998) showed that larger lowland rivers may take 10 3 -10 5 years to adapt to permanent changes, for
Many scientists feel that scientific outcomes are not sufficiently taken into account in policy-making. The research reported in this paper shows what happens with scientific information during such a process. In 2001 the Dutch Ministry of Transport, Public Works and Water Management commissioned their regional office in Limburg to assess how flood management objectives can be achieved in future in the Dutch Meuse valley, assuming climate change will increase peak discharges. To ensure political support, regional discussion rounds were to help assess the measures previously identified. This paper discusses the ways in which hydrological and hydraulic expertise was input, understood and used in this assessment process. Project participants as a group had no trouble contesting assumptions and outcomes. Nevertheless, water expertise was generally accepted as providing facts, once basic choices such as starting situation had been discussed and agreed. The technical constraints determined that politically unacceptable measures would have to be selected to achieve the legally binding flood management objective. As a result, no additional space will be set aside for future flood management beyond the already reserved floodplain. In this case, political arguments clearly prevail over policy objectives, with hydraulic expertise providing decisive arbitration between the two.
Many scientists feel that scientific outcomes are not sufficiently taken into account in policy-making. The research reported in this paper shows what happens with scientific information during such a process. In 2001 the Dutch Ministry of Transport, Public Works and Water Management commissioned their regional office in Limburg to assess how flood management objectives can be achieved in future in the Dutch Meuse valley, assuming climate change will increase peak discharges. To ensure political support, regional discussion rounds were to help assess the measures previously identified. This paper discusses the ways in which hydrological and hydraulic expertise was input, understood and used in this assessment process. Project participants as a group had no trouble contesting assumptions and outcomes. Nevertheless, water expertise was generally accepted as providing facts, once basic choices such as starting situation had been discussed and agreed. The technical constraints determined that politically unacceptable measures would have to be selected to achieve the legally binding flood management objective. As a result, no additional space will be set aside for future flood management beyond the already reserved floodplain. In this case, political arguments clearly prevail over policy objectives, with hydraulic expertise providing decisive arbitration between the two.
For restoration of natural habitats in fluvial ecosystems secondary channels have been proposed. However, the other functions of the river should not be affected too much. In this paper the possibilities and limitations of re-opening these secondary channels are assessed. A methodology is outlined for the design of secondary channels and for the assessment of their effects on the other functions of the river. From the results of several complementary research projects we conclude that secondary channels in the flood plains can be designed to be quasi-stable. When applied only locally, additional measures in the main river are necessary to avoid deterioration of shipping conditions during low flows.
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