An analytical model of crenulate shaped beaches

An analytical model for the development of crenulate shaped beaches is constructed. It is assumed that the shape of the crenulate shaped beach remains constant with time and expands with a rate according to a time function. Based upon a sediment balance, a simplified model of wave diffraction and refraction and a nearshore current model an expression for the shape function and the time function of crenulate shaped beaches is derived. It is shown that in the diffraction zone the time function should follow a t1/3 law, while for the refraction zone the well known t1/2 law is found. This implies that the evolution of a crenulate beach in the diffraction zone should initially be faster and on the long term slower than is found in refraction zones. The hypothesis is verified using available data. The resulting shape function of crenulate shaped beaches is expressed in terms of the diffracted wave field

Numerical modelling of erosion and sedimentation around offshore pipelines

In this paper a numerical model is presented for the description of the erosion and sedimentation near pipelines on the sea bottom. The model is based on the Navier-Stokes equations and the equation of motion and continuity of sediment.\ud \ud The results of the simulations have been compared with the results of tests in a large-scale facility. The agreement between the results of the simulations and the experimental results is good.\ud \ud The applicability of the method is twofold: firstly, the processes of erosion and sedimentation around bodies on the sea bottom can be simulated; secondly, the method can be used for the design of pipelines, including erosion stimulating elements, such as spoilers

Assessment of flood damages and benefits of remedial actions: "What are the weak links?"; with application to the Loire

Flood damage models are used to determine the impact of measures to reduce damage due to river flooding. Such models are characterized by uncertainty. This uncertainty may affect the decisions made on the basis of the model outcomes. To reduce uncertainty effectively, the most important sources of uncertainty must be found. Uncertainty analysis serves this purpose.\ud \ud By way of a questionnaire experts were asked about their judgment of the significance of uncertainty sources in flood damage assessment. The results of this questionnaire are compared to an uncertainty analysis by Monte Carlo Simulation, which Torterotot (1993) applied to the French model CIFLUPEDE.\ud \ud The paper concludes that the role of uncertainty in flood damage assessment is highly significant and cannot be neglected. Both the experts and the analysis on the flood damage assessment model indicate the hydrologic relations ‘frequence of occurrence — river discharge — river water level’ and the damage estimates as the most important uncertainty sources. For embanked rivers dike breach is the most significant uncertainty source.\ud \ud A question which appears is, taking into account these uncertainties, to what level of precision can flood damage assessment models predict the expected annual flood damage and the costs and revenues of flood alleviation measures? It is of importance to explore the boundaries of flood damage modeling and to try to find ways to move these boundaries. The uncertainty analysis presented in this paper can be seen as one more step on the way to this goal

Multiobjective decision-making in integrated water management

Traditionally, decision-making by water authorities in the Netherlands is largely based on intuition. Their tasks were, after all, relatively few and straight-forward. The growing number of tasks, together with the new integrated approach on water management issues, however, induces water authorities to rationalise their decision process. In order to choose the most effective water management measures, the external effects of these measures need to be taken into account. Therefore, methods have been developed to incorporate these effects in the decision-making phase. Using analytical evaluation methods, the effects of various measures on the water system (physical and chemical quality, ecology and quantity) can be taken into consideration. In this manner a more cognitive way of choosing between alternative measures can be obtained. This paper describes an application of such a decision method on a river basin scale. Main topics, in this paper, are the extent to which uncertainties (in technical information and deficiencies in the techniques applied) limit the usefulness of these methods, and also the question whether these techniques can really be used to select measures that give maximum environmental benefit for minimum cost. It is shown that the influence of these restrictions on the validity of the outcome of the decision methods can be profound. Using these results, improvement of the methods can be realised