Numerical simulation of floodplain hydrology

Abstract

Abstract. Despite the extensive research into hillslope and channel interactions in headwater catchments, surprisingly little attention has been paid to such processes in lowland rivers. In particular, previous studies have not addressed 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, floodplain, and channel interactions for a lowland river. Piezometric, precipitation, and river stage data were used to parameterize and test a new twodimensional finite element model of saturated-unsaturated flow applied to two vertically aligned cross sections through a lowland floodplain. Data for two major out-of-bank flood events were simulated which appeared to show the presence of a significant unsaturated zone extending up to 5 rn below the surface. The model simulated reasonably well the pressure head field that was recorded at a number of piezometers located internal to the computational domain on each transect, and we conclude that floodplain hydrology is predominately a two-dimensional (lateral) process. Three-dimensional (down reach) flow effects would seem to become more significant at the beginning and end of each event.The simulations also showed that the unsaturated zone remained close to saturation at all times and that it was not significant in terms of the floodplain hydrology. Examination of velocity vector patterns showed the formation of a strong groundwater ridge within the floodplain. This led to the development of strong velocities directed toward hillslope areas as the inundation front approached the hillslope/floodplain junction. This suggests that surface water may move into hillslope areas adjoining the floodplain during major floods. Thus the extent of the hyporheic zone may be larger than previously thought. Numerical Model DevelopmentA finite element model, ESTEL, was developed to simulate hillslope and channel fluxes to the floodplain alluvium during overbank flood events. Like WaTab2D [Whiting and Pomeranets, 1997] this model is also based on an exact formulation for time-dependent unconfined groundwater flow and hence provides an advance on models such as MODFLOW, which are based on the Dupuit-Forcheimer assumption. However, unlike WaTab2D, ESTEL applies to both saturated and unsaturated flow conditions, as this may be important in particular floodplain situations. The model was programmed using an object-oriented numerical library previously developed to solve the shallow water equations. For a complete description of the model and its development the reader is referred to Desitter et al. [1998, 2000], and here we provide only a brief summary of its major attributes.We solve the so-called "mixed" where t is the time (dimension T), 0 is the volumetric moisture content (L 3 L-3), h is the pressure head (L), K is the hy-

Hydrological impacts of floodplain restoration: a case study of the River Cherwell, UK

“…Overall, the comparisons between observed and predicted flows are similar to those presented elsewhere (e.g. Bates et al, 2000). Given that the objective of the study was to consider the general impacts of floodplain restoration and not to design any specific river engineering infrastructure, the calibration shown in Fig.…”

Influence of stream‐groundwater interactions in the streambed sediments on NO3− flux to a low‐relief coastal stream

“…The hydraulic gradient toward the stream is diminished during the passage of a flood wave (Figure 8), however, and flow rates through and residence times within the streambed sediments are affected. Unlike the extensive bank storage (with reversed flow) that occurs in floodplain areas [Bates et al, 2000;Squillace, 1996], the response to floods may be limited in hillslope areas without floodplains or riparian wetlands because of a much steeper hydraulic gradient toward the stream. In the present study, the effect of passage of a flood wave is to retard the flow of hillslope water to the stream during periods of high stream stage.…”

Modelling water flow and seasonal soil moisture dynamics in analluvial groundwater-fed wetland

“…The part of the left boundary extending higher than the maximum river water level is a no flux-boundary, whereas at the bottom the specified head distribution is extended to the bottom of the flow domain, lower than the river floor. This approach is also taken by Bates et al (2000) and is argued to be physically more realistic than a no-flux boundary from the bottom of the river channel downwards, since the latter can lead to the development of spurious flow features.…”

“…Groundwater flow models have been used to simulate water level variation in wetlands and estimate fluxes to and from the river with good results (Bradley, 1996;2002;Burek and Nestman, 2002;Mohrlok and Jirka, 2002), although lacking a description of processes such as capillary rise. A more complex model describing two-dimensional saturatedunsaturated flow was developed by Bates et al (2000) to investigate hillslope, floodplain and channel interactions for a lowland river during over-bank floods. This model was further tested for in-bank floods (Burt et al, 2002).…”