River and floodplain hydrodynamics

In a study for AMINAL - Division Water, the modelling systems for surface waters MIKE11 (Danish Hydraulic Institute) and ISIS (HR Wallingford) have been compared on the basis of a practical application for the Flemish river Dijle basin (Belgium). For this basin, river flood modelling has been performed by the quasi two-dimensional approach. The influence on the simulation results of the numerical scheme, the spill units with the flood plains and the hydraulic structures has been studied separately to identify the main sources of uncertainty in hydrodynamic surface water modelling. It has been found that the differences in the accuracy of the numerical scheme and the system features can be neglected when compared with the uncertainties associated with the model parameters of the hydraulic structures (bridges, culverts and weirs) and the model input. Therefore, experience and model validation against field measurements are considered of great importance in surface water modelling.

A detailed hydrodynamic river model is constructed for the river Dender using the MIKE11 software from Danish Hydraulic Institute. All hydraulic structures are implemented and its implementation validated based on water level measurements up- and downstream of the structures. A detailed cross-section survey was made available by the Flemish Administration AWZ, with cross-section measurements approx. every 50 m. The floodplains are modelled using the quasi 2D flood modelling approach. The model is used in combination with lumped conceptual models for the 12 hydrographic subcatchments in the river basin, for the simulation of both historical events and synthetic flood events. The model is currently in use at the Flemish Administration AWZ, to support the flood management in the river basin and to construct flood maps and flood risk maps.

In many applications of river flood modelling, a one-dimensional full hydrodynamic modelling system is used. To model the floodplains with such a system, a quasi two-dimensional approach can be applied. In this approach, the flooded areas are modelled as separate 1D river branches that are connected to the main river by means of spills or overflow structures (see Figure). These structures represent the dikes or embankment elevations between the river bed and the floodplains. For line elements such as railways and roads, which can stop the water in the floodplains, also spill units can be implemented. In this way, the flooded areas are modelled by a network of fictitious river branches and hydraulic structures. The direction of the flood branches is taken equal to the preferential flow direction. The cross-sections perpendicular to this direction are derived from a digital elevation model (DEM). In this way, the volumes along the flooded areas can be described accurately. By making appropriate assumptions for the roughness coefficients along these areas, the model is also able to describe the water surface profile along the flooded areas. When there is no preferential flow along these areas, the flood branches can be replaced by reservoir-type storage units (the so-called 'flood cells'). A GIS system is used to draw geometrical data for the river branches and spills/overflows from the DEM.

The quasi 2D approach has the advantage that, compared with the 2D approach, the computational load is limited. In this way, the model is able to describe the embanked river and the flooding conditions over its full length. By means of the spill units, both flooding by overtoppping, and flooding by dambreak can be considered.

An evaluation of this approach of quasi 2D floodplain modelling has been made by the practical application of the river modelling package MIKE11 (Danish Hydraulic Institute), in combination with ArcView / MIKE-GIS. It has been implemented for the Flemish basin of the river Dender in Belgium (708 km2), using data from and with support of the Flemish Water Authority AWZ.

Publications:

Willems P., K. Christiaens, G. Vaes, D. Popa, L. Timbe, J. Berlamont & J. Feyen (2001), 'Methodology for river flood modelling by the quasi two-dimensional approach', World Water and Environmental Resources (EWRI) Congress, Orlando, 20-24 mei 2001

Willems P., G. Vaes, D. Popa, L. Timbe & J. Berlamont (2002), 'Quasi 2D river flood modelling', In: River Flow 2002, D. Bousmar and Y. Zech (ed.), Swets & Zeitlinger, Lisse, Volume 2, 1253-1259