Page 92 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 75

Precipitation

x 0

Overland flow profile

Critical distance
of overland flow Suspended m aterial
x C
Depth of overland flow
Belt of no Depression
erosion storage
Stream
Flow or lake
direction Active
erosion

Deposition
<r =r >r of sediment
Eroding stress
r Shearing resistance of soil surface
Figure 3.12 Horton’s model of overland flow production.
Source: Adapted from Horton (1945)

development in humid regions is very likely to occur deposits, channel margin deposits, overbank flood
where subsurface pipes are present. Pipe networks can plain deposits, and valley margin deposits (Table 3.4).
help initiate channel development, either through roof When studying stream deposition, it is useful to take
collapse or by the concentration of runoff and erosion the broad perspective of erosion and deposition within
downslope of pipe outlets. Piping can also be impor- drainage basins. Stream erosion and deposition take
tant in semi-arid regions. Channel initiation may also place during flood events. As discharge increases dur-
take place where slope wash and similar mass movements ing a flood, so erosion rates rise and the stream bed
dominate soil creep and creep-like processes (e.g. Smith is scoured. As the flood abates, sediment is redeposited
and Bretherton 1972; Tarboton et al. 1992). over days or weeks. Nothing much then happens until
the next flood. Such scour-and-fill cycles shift sedi-
ment along the streambed. Scour-and-fill and channel
deposits are found in most streams. Some streams actively
Fluvial deposition
accumulate sediment along much of their courses,
Rivers may deposit material anywhere along their course, and many streams deposit material in broad expanses
but they mainly deposit material in valley bottoms in the lower reaches but not in their upper reaches.
where gradients are low, at places where gradients change Alluviation is large-scale deposition affecting much of
suddenly, or where channelled flow diverges, with a a stream system. It results from fill preponderating scour
reduction in depth and velocity. The Hjulstrøm diagram for long periods of time. As a general rule, scour and
(p. 74) defines the approximate conditions under which erosion dominate upstream channels, and fill and deposi-
solid-load particles are deposited upon the stream bed. tion dominate downstream channels. This pattern arises
Four types of fluvial deposit are recognized: channel from steeper stream gradients, smaller hydraulic radii,