In many applications, river flood modelling is performed by a one-dimensional full hydrody-namic modelling system. To model the floodplains with such a system, a quasi two-dimensional approach can be used. In this approach, the floodplains are modelled as a network of fictitious river branches and spills with the rivers. The river branches represent the topographical depressions and the spills correspond with the river embankments. Overflow structures are used to model barriers (roads, railways,...) within the floodplains, possibly in combination with culverts. When a digital elevation model (DEM) is available, a GIS system can be used to extract geometric data for the river branches and spills/overflows from the DEM. It can also be used to demarcate the potential flooding areas (areas to be modelled in a quasi two-dimensional way) and to visualise the spatial extent of the modelled floods. In this way, the GIS system is applied both as a pre-processing and post-processing tool. The communication between the hydrodynamic model and the GIS system acts in two directions. An evaluation of this approach has been made by the practical application of the river modelling package MIKE11 (Danish Hydraulic Institute, Denmark), in combination with ArcView/MIKE-GIS. It has been implemented for the Flemish basin of the river Dender in Belgium (708 km~2). To identify the potential flood risk zones, representative hydrographs (so called "composite hydrographs") have been simulated in the model for return periods in the range from 1 to 100 years. These composite hydrographs were derived on the basis of an extreme value analysis. They have the important feature that river states with the same safety level at all locations along the river can be derived by one single short-term simulation. In this way, flooding maps can be created for different return periods in an accurate and easy way.
展开▼
