Most research on hillslope hydrology has been carried out in headwater basins where slopes directly adjoin the channel, with no intervening floodplain. Further downstream, a floodplain is commonly found (Fig. 1), though curiously there has been little field research in such locations. The important point is that the hydrology of many floodplains is closely controlled by hillslope inputs; indeed if floodplains are to function as effective buffer zones, the interaction of the floodplain with slope drainage water is of crucial concern. With this in mind, a brief review of hillslope hydrology will be provided before discussing the hydrology of the floodplain itself.
Hillslope hydrology has been the subject of much research over the last three decades; full reviews are contained in Kirkby (1978) and Anderson and Burt (1990). There are many different pathways by which hillslope runoff can reach a river; these are summarised in Fig. 2. It is usual to distinguish between water which reaches the stream channel quickly, quickflow or stormflow, causing floods, and that which moves more slowly, baseflow, maintaining flow in rain-free periods. In practice, however, the dividing line between stormflow and baseflow during flood recession is a purely arbitrary one since subsurface drainage of soil and bedrock dominates the latter stages of flood runoff generation as well as providing baseflow (Burt, 1996).
Figure 2. Hydrological pathways. 1, Infiltration-excess overland flow. 2, Saturation-excess overland flow; 2a, direct runoff; 2b, return flow. 3, Subsurface stormflow.
4, Groundwater flow. See text for
discussion of these various hydrological processes.
23 The hydrological role of floodplains
T.P. Burt
The route by which hillslope runoff reaches the stream channel is determined by a number of factors: soil, bedrock, vegetation cover and climate (Anderson and Burt, 1990). Hydrological pathways are particularly important with respect to the speed, volume and peak rate of runoff (Fig. 3) and, perhaps more crucially, in terms of the sediment and solute load carried by the water. Infiltration-excess overland flow occurs when rainfall intensity exceeds the infiltration capacity of the soil; once surface depression become full, excess water overflows downslope. This stormflow mechanism has become closely associated with arable land in recent years; indeed wash and rill erosion are predominantly affected by this flow mechanism. Bare soils or those with a low crop cover are most at risk; such soils may become easily crusted during heavy rainfall, greatly increasing the likelihood of runoff and erosion (Burt and Slattery, 1996). Depending on crop type, the ‘window of opportunity’ for erosion (Boardman, 1992) may open in any season. In the UK for example, erosion of autumn-sown wheat is common in the early winter, but an increasing problem is erosion of spring-sown maize by early summer thunderstorms (Boardmanet al., 1995).
Figure 3. Schematic diagram of the water balance of a floodplain. Abbreviations are defined in the following section of the text.
Overland flow may also be generated when the entire soil profile becomes saturated. Here, rainfall intensity may be well below the soil’s infiltration capacity: the soil becomes saturated either because of prolonged rainfall or because of inflow from further upslope; often both processes operate together. Return flow occurs where water exfiltrates the soil because upslope drainage exceeds the soil’s storage capacity. Where rain falls onto a saturated soil, no infiltration can take place and direct runoff happens. Together, return flow and direct runoff comprise saturation-excess overland flow. Source areas for saturation-excess overland flow are those parts of the drainage basin where soil moisture tends to accumulate: at the bottom of any hillslope, especially where soil water converges within a hillslope hollow or where the profile shape is concave, or where the soil profile becomes thinner limiting moisture storage. The erosion of ephemeral gullies along the floors of dry (zero-order) valleys may well be encouraged by generation of saturation-excess overland flow in the valley bottom; such gullies are often the major source of sediment leaving the catchment hillslopes (Slatteryet al., 1994).
Subsurface flow (sometimes termed interflow or throughflow) may also drain rapidly enough from slopes to contribute to stormflow. Subsurface stormflow will be produced in large quantities where permeable soils overlie impermeable bedrock and where steep slopes adjoin the stream channel. Until recently, hillslope hydrologists have emphasised the importance of flow through the micropores of the soil matrix, but rapid infiltration and drainage through macropores is now recognised as a significant process in many locations too, both in relation to flood production and pollutant transport. The subsurface stormflow response tends to be more attenuated than that of overland flow; though its peak runoff rate may be lower, volumetrically it tends to dominate the flood hydrograph, particularly
upland soil and regolith floodplain alluvium UOF USSQ FOF ET RF FSSQ
PERC
GW
OBIin the later stages, and may indeed provide a second flood peak a day or two after the main hydrograph (Anderson and Burt, 1978). Subsurface flow usually comprises long-residence soil water even where macropore flow is an important contributor, so that it invariably has a high total dissolved solids concentration (McDonnell, 1990). Continued drainage of soil and bedrock sustains streamflow during dry periods.