Geoscience Reference
In-Depth Information
Fig. 11.2 Illustration of the overland flow
(OF) mechanism as infiltration
excess. The precipitation rate
P
exceeds infiltration capacity, and
the water table is at the ground
surface.
P
OF
Infiltration
11. 2
STORM RUNOFF MECHANISMS IN RIPARIAN AREAS
11.2.1 Overland flow
Infiltration excess overland flow
This type of flow occurs when the rainfall rate is larger than the infiltration capacity, so that
there is an excess which runs off over the surface. Although this flow generation concept
is sometimes associated with the name of Horton (1933), it goes back much earlier.
It was already the basis of the well-known rational method, introduced 150 years ago
by Mulvany (1850), and of the various runoff routing procedures subsequently derived
from it by Hawken and Ross (1921) and others (see also Dooge, 1957; 1973). It is also
implicit in the unit hydrograph, as originally proposed by Sherman (1932a; b). In these
and other early studies concerned with maximal rates of runoff in problems of flooding
and erosion, it was assumed that the infiltration rate is smaller than the precipitation rate
over the entire catchment. In the rational method, the infiltration is taken as a fraction
of the precipitation, whereas in the unit hydrograph approach and in Horton's work,
the infiltration capacity or a related index is subtracted from the precipitation. Thus it
was assumed that the infiltrated water is “lost” and that virtually all stormflow results
from the overland flow of the precipitation excess (see Figure 11.2). In the prediction
of extreme flows for design purposes indisaster situations, this assumption of overland
flow was not unreasonable.
It is now understood that overland flow isnotauniversally occurring phenomenon,
that in many situations it may not occur at all, and that its prevalence depends on the
nature of the catchment and of the intensity of the precipitation. But it can be expected
to be the main mechanism in catchments with relatively impermeable surfaces, and with
only a thinsoil layer; such surfaces cover mostly urban environments, factory and farm
yards and other trampled soil areas, and rocky and stony areas with little or no soilor
vegetation, as seen inarid and desert environments. Thus it occurs most frequently in
areas where people live and work and in denuded aridregions. It can also occur on other
more permeable surfaces, provided the rainfall is sufficiently intense. For instance, ina
study of a 20 ha first-order agricultural catchment with steep slopes in semi-arid Shanxi
(China), Zhu
et al
. (1997) reported that most storms generate no overland flow. However,
