Malcolm G. Anderson

The importance of spatial resolution in hydraulic models for floodplain environments

For many numerical modelling applications the problem of specifying an optimum mesh resolution remains unbounded and for mesh construction objective a priori rules do not exist. By contrast, the problem of specifying model parameter surfaces is largely bounded within known physical error distributions. In this paper we thus investigate the impact of varying mesh resolution on a typical non-linear finite numerical solver. Specifically, a two-dimensional finite element code which solves the Shallow Water equations was used to simulate unsteady flows in a meandering compound channel. A range of different mesh resolutions and parameter surfaces were simulated to determine relative dominance and, unlike previous studies, the effect on both bulk flow and distributed outputs were analysed. The results showed a wide variation in performance for mesh discretizations which fulfilled traditional length scale-based construction. Mesh resolution effects were at least as important as a typical calibration parameter and model response was shown to be highly complex.

Automatic monitoring of soil moisture conditions in a hillslope spur and hollow

Abstract An automatically-recording tensiometer system was designed and installed to record the spatial variation of soil moisture in a hillslope spur and hollow. Results are presented which demonstrate the significant control of topography on soil moisture conditions and the resulting stream-flow response. From the storm event examined and the month-long recession which followed, the hillslope hollow is shown to be significant in both generating a through flow peak in stream discharge and in maintaining the subsequent base flow.

Water table fluctuations within the floodplain of the River Severn, England

Abstract In contrast to extensive research on hydrological processes operating in headwater basins, there has been relatively little attention paid to the hydrological processes that occur on the floodplains of lowland rivers. This dearth of information is all the more surprising given current interest in the use of floodplains as buffer zones between farmland and the riverine environment. In the previous paper (Water Resour. Res. 36 (2000) 2517), Bates et al., reported preliminary results from a field site on the floodplain of the River Severn in Shropshire, UK, a large lowland river by British standards. Piezometric data suggested that during out-of-bank conditions a reverse groundwater ridge develops in the floodplain subsurface and results in strong groundwater flux velocities directed towards the base of hillslopes adjoining the floodplain. Bates et al. showed that the impact of such ridges was to switch off hillslope inputs to the riparian zone; they hypothesised that this occurred when surface inundation approached the back of the floodplain. In this paper, we provide a more detailed analysis of the events considered by Bates et al. and extend the analysis to the more common in-bank flood condition. In total, five events are considered (two out-of-bank and three in-bank); these were chosen to represent a wide range of event magnitudes, antecedent conditions and local patterns of rainfall and runoff. The analysis demonstrates that the reverse groundwater ridging process identified by Bates et al. also occurs during in-bank events. Hillslope inputs to the floodplain are also ‘switched off’ in these events if the flood stage is high. In smaller floods, water continues to move from slope to floodplain, although coupling between slope and channel is only re-established later in the recession. We conclude that, contrary to the conclusions of Bates et al., this process switching is not necessarily dependent on surface inundation approaching the back of the floodplain. Whilst the paper broadly confirms the operation of the simple reverse groundwater ridging process described by Bates et al., it shows that antecedent conditions, local rainfall and runoff, and flood stage all act to complicate this basic pattern. Lastly, we consider the implications for catchment water quality of the newly identified processes.

Topography and hillslope soil water relationships in a catchment of low relief.

Soil water potentials are mapped on a 6° hillslope during summer and winter conditions in a 0.8-km2 catchment. It is shown that the dynamic hillslope contributing area is maintained at the slope base for long periods after precipitation has ceased. Additionally, the focus of the hillslope soil water convergence is not always at the hollow base, but is seen to migrate in the downstream direction and to accord with a hillslope spur. Microtopographic features are shown thereby to influence, but not to be the sole dominant control upon the spatial disposition of hillslope contributing areas in low-angled topography.

Sensitivity analysis, calibration and predictive uncertainty of the Institute of Hydrology Distributed Model

The physically based Institute of Hydrology Distributed Model is briefly described and its sensitivity to the parameters of the surface and subsurface flow components is examined for the Tanllwyth catchment, Powys, Wales. Model predictions are most sensitive to a surface flow roughness parameter and the saturated hydraulic conductivity of the soil. Uncertainty in the effective values of these parameters is assessed by comparing observed and predicted discharges for a number of storms. The effect of uncertainty in the effective parameter values on the predicted discharges for two additional storms is assessed using the Rosenblueth method.

USING A COMBINED SLOPE HYDROLOGY/STABILITY MODEL TO IDENTIFY SUITABLE CONDITIONS FOR LANDSLIDE PREVENTION BY VEGETATION IN THE HUMID TROPICS

The susceptibility of cut slopes to landsliding can be reduced in certain circumstances by the establishment of a vegetation cover. However, the hydrological implications of allowing a cover to develop may offset the mechanical benefits of soil reinforcement by roots. The balance between hydrological and mechanical effects is critical on slopes which are susceptible to the development of an infiltration-induced transitory perched water table, a common cause of landslides in deep, tropical residual soils. This balance is likely to change both between slopes of different types as well as temporally on any given slope. The net effect of a vegetation cover must be predicted either before natural vegetation covers are allowed to encroach on bare slopes, or if engineers are considering the use of trees as a protective measure. This paper presents a method of calculating the impact of a vegetation cover on slope stability. Simulations carried out on a wide range of slope types suggest that where failure is most likely to be triggered by infiltration rather than ground water rise, large-scale vegetation covers may contribute to instability. Whether vegetation had a positive or negative impact on slope stability was controlled by the permeability of the soil matrix, whilst the magnitude of impact was controlled by the soil strength and the slope height.

The influence of low-angled topography on hillslope soil-water convergence and stream discharge

Abstract Results from an instrumented 6° hillslope are presented to show that significant shifts in the focus of soil-water convergence can occur in relatively minor hollow-and-spur topography. It is suggested that such shifts can significantly change the location of contributing partial areas for stream hydrograph generation. Inflow unit hydrographs from four subcatchments are generally supportive of this assertion. Further investigations must seek to define more accurate topographic indices for the identification of hillslope partial area zones in shallow topography.

Interpretation of recession flow

Abstract Various attempts at interpreting recession flow using graphical techniques are reviewed. Data derived from a laboratory slope drainage experiment and from an instrumented catchment are plotted using the same graphical presentations. These data are also interpreted using predictions based upon Darcys law. It is shown that graphical techniques may falsely interpret the factors controlling recession and the need for field prediction of recession flow is stressed.

A Two-Dimensional finite element model for river flow inundation

Current solutions to the problem of simulating river flood flows over reach lengths of 10-30 km are examined in terms of their ability to represent particular significant aspects of the flow physics. Inadequacies with such approaches are highlighted and an alternative method based on a two dimensional finite-element solution is suggested. This scheme is outlined and a discussion made of the analytical developments necessary that enable functional solutions to be obtained at the above scale. Numerical results are presented that establish the viability of this alternative modelling strategy.

Modelling suspended sediment deposition on a fluvial floodplain using a two-dimensional dynamic finite element model

Abstract In this paper we outline a new numerical model for predicting floodplain sediment deposition resulting from out-of-bank flow in reach-scale natural compound channels. Simulation of this problem requires models capable of dealing with the hydraulic and sediment transport effects of a dynamically moving inundation front, as well as a complex set of flow processes including momentum exchange between main channel and floodplain, spillage of water across meander loops and the impact of complex topography. Whilst the treatment of dynamic moving boundary problems is difficult, but attainable in finite element hydraulic codes, the necessity to include dry areas within the model generates a number of problems that numerical solvers for fluvial sediment transport have, to date, failed to overcome. Accordingly, we develop a two-dimensional finite element approach that specifically accounts for sediment transport in domains undergoing wetting and drying. The hypothesis that at the reach-scale a two-dimensional depth averaged representation of flow and suspended sediment is able to reproduce observed deposition patterns is then tested against average annual rates determined using 137 caesium analysis of floodplain sediments. Using reasonable parameterisation and no calibration of the sediment transport component, the developed model is able to replicate an encouraging amount of the observed spatial variability in this data set. Whilst further testing of both the hydraulic and sediment transport components of the model is undoubtedly required, the results provide an initial assessment of reach scale process dominance for floodplain systems.