Numerical simulation of floodplain hydrology

Despite the extensive research into hillslope and channel interactions in h eadwater catchments, surprisingly little attention has been paid to such pr ocesses in lowland rivers. In particular, previous studies have not address ed the influence of hillslope contributions and have concentrated solely on in-bank floods rather than more complex out-of-bank cases. Accordingly, we combine field monitoring and numerical modeling to study hillslope, floodp lain, and channel interactions for a lowland river. Piezometric, precipitat ion, and river stage data were used to parameterize and test a new two-dime nsional finite element model of saturated-unsaturated flow applied to two v ertically aligned cross sections through a lowland floodplain. Data for two major out-of-bank flood events were simulated which appeared to show the p resence of a significant unsaturated zone extending up to 5 m below the sur face. The model simulated reasonably well the pressure head field that was recorded at a number of piezometers located internal to the computational d omain on each transect, and we conclude that floodplain hydrology is predom inately a two-dimensional (lateral) process. Three-dimensional (down reach) flow effects would seem to become more significant at the beginning and en d of each event. The simulations also showed that the unsaturated zone rema ined close to saturation at all times and that it was not significant in te rms of the floodplain hydrology. Examination of velocity vector patterns sh owed the formation of a strong groundwater ridge within the floodplain. Thi s led to the development of strong velocities directed toward hillslope are as as the inundation front approached the hillslope/floodplain junction. Th is suggests that surface water may move into hillslope areas adjoining the floodplain during major floods. Thus the extent of the hyporheic zone may b e larger than previously thought.