Author(s)

O. Hoes & F. Nelen

Abstract

Risk analysis will become more important in formulating cost-effective solutions to flood problems. For this purpose, we need reliable estimates of flood frequencies in terms of inundation depths, as well as the possible damage that may result from the flooding. This article deals with the first aspect. Hydrologists are using many different modelling techniques for estimating the flood frequency. These methods can be divided into two categories: event based design and continuous simulation approaches. In the first approach, the probability of extreme water levels is derived directly by simulating a large number of storm events, for which the probability has been determined in advance. Each event is characterized by a combination of stochastic variables that may contribute to flooding, such as rainfall volume, antecedent conditions, rainfall pattern, event duration, etc. An important feature of this approach is that it is assumed that the probability of the simulated storm event is equal to the probability of the calculated maximum water level. In the second approach, the water level probabilities are derived with standard flood frequency analyses of a (measured or calculated) time series of water levels. In this paper, both approaches are discussed. Their results are compared for two catchments in the Netherlands. It is shown that the probability of floods is different when using continuous simulations and a stochastic event-based approach. It is concluded that for this type of problem a continuous simulation is to be preferred. Keywords: probability of floods, continuous simulation, stochastic events. 1 Introduction Many studies have been carried out to estimate the consequences of expected climate change on the behaviour of water systems (IPCC [1], Parker [2], Monirul

Keywords

probability of floods, continuous simulation, stochastic events.