There are two occasions when the normal hillslope-to-floodplain flow direction may reverse:
1. As noted above, in the upper reaches of a catchment, subsurface contributions to streamflow aid in the build-up of a flood wave. In the lower reaches during a flood the river stage may be higher than the floodplain water table and river water will then flow into the floodplain sediments; this is often referred to as bank storage (Freeze and Cherry, 1979). The impact of bank storage in moderating the flood hydrograph is shown on figure 7. As the flood wave arrives, flow is induced into the bank; as the stage declines, the flow is reversed and the stored water flows back out into the channel. The effect is to attenuate the flood wave, reducing and delaying the peak discharge. A more extreme version of this effect occurs when the river bursts its banks and inundates the floodplain. This in itself causes large quantities of floodwater to be stored. Further storage takes place because some floodwater then infiltrates the floodplain soil. Current research at Bristol and Durham Universities aims to investigate these processes: the two-dimensional modelling schemes of Bates and Anderson (1993) will be extended by linking a one-dimensional finite difference model of subsurface hydrology to the two-dimensional finite element model of floodplain/channel flows. In addition, water fluxes to and from the hillslope will be simulated by integrating this scheme with a three- dimensional finite difference hillslope hydrology model. Hitherto, all current hydraulic models have had closed impermeable boundaries except at the inflow and outflow points to the modelled reach; in effect no flux of water occurs between the channel/floodplain flow and floodplain soils or adjacent hillslopes. As well as its impact on the flood hydrograph, overbank inundation may provide an additional pathway by which a floodplain may act as a sink for sediment-bound contaminants and nutrients. Current work by Price (in prep.) aims to couple a denitrification routine to the one-dimensional infiltration model mentioned earlier.
29 The hydrological role of floodplains
T. P. Burt
2. Under low-flow conditions, groundwater recharge by influent seepage from a river channel is a common occurrence. In many cases, flow from the hillslope across the floodplain to the channel takes place only in the winter months. In summer, the hydraulic gradient is reversed and water moves from channel to floodplain. Haycock (1991) has monitored this seasonal reversal of the hydraulic gradient for a small floodplain near Oxford (Fig. 8). In summer, the water table in the local limestone aquifer falls sufficiently so that inflow from the river takes place; once the aquifer has been recharged in the autumn, the aquifer then drains through the floodplain sediments to the channel. The period of hillslope-to-floodplain flow thus coincides with the time of maximum discharge and peak nitrate concentrations in subsurface flow and the floodplain is able to function as a nitrate buffer zone.
Figure 7. Flood wave modification due to bank storage effects (based on Freeze and Cherry, 1979): (a) change in flood stage during the flood; (b) attenuation of flood hydrograph
because of bank storage; (c) flow into and out of the channel bank during the flood; (d) volume of flood water stored in the channel bank; (e) the impact of flow reversal on groundwater discharge to the flood hydrograph.
Figure 8. Seasonal reversal of the hydraulic gradient across the floodplain of the River Leach near Oxford (reproduced from Haycock, 1991, by kind permission of the author). The graph shows the difference in water table elevation potential between row 4 and row 6 (distance = 11 m) at the grass floodplain site described by Haycock (1991; see also Haycock and Burt, 1993). In summer there is effluent seepage from the channel into the floodplain; in winter, the flow direction is reversed and groundwater discharges into the channel.
In both instances, river-to-floodplain flows assume more importance in the lower reaches of the drainage basin (Fig. 1) so that the floodplain becomes more important as a sink than a source of water, sediment and nutrients. Many buffer zone processes operate most effectively in headwater basins where hillslope-to-channel flows dominate and where most of the channel length (and by implication, most of the riparian land) is located. However, though the opportunity for buffer zone functions may be less in the downstream reaches of the basin, the importance of floodplain and channel storage in that zone should not be underestimated.
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31 The hydrological role of floodplains
-.8 -.6 -.4 -.2 0 .2 .4 .6 .8
Nov.'89 Dec.'89 Jan.'90 Feb.'90 March '90 April '90
Elevation
difference
(m)
300 320 340 360 380 400 420 440 460 480 500
Days since 1.1.89
Flow from hillslope into the floodplain Flow from floodplain into the hillslope
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