Hollows (Elemen t HO}
Cowan (1991) proposed that these hollows formed by a process of deep scouring at points of channel
conve rgence. Scour depths of up to six times the
mean channel depth have been documented in rivers
and laboratory flumes by Mosley (1976), Mosley and
Schumm (1976), Best and Brayshaw (1985), Best
(1987, 1988), Best et al. (1989), and Salter (1993).
Bristow et al. (1993) proposed a facies model for the
scour and fill processes at channel confluences, and
Ashmore (1993) modeled their development in
gravel-bed rivers. These studies suggest that a
mechanism of deep scouring may be a significant
process at channel junctions (Fig. 6.45) and below
mid-channel macroforms in braided streams, where
stream junctions and bar forms abound (Best 1987,
p. 34; Best et al. 1989). Salter (1993) documented
scour where flow is deflected against banks or islands. Best (1987) showed that avalanche faces can
develop on the upstream end of these scours (Fig.
6.45). This allows the scours to be filled laterally,
167
obliquely, or vertically by an avalanche deposit in a
short period of time during channel switching or a
flood event. The scours are, therefore, envisaged to
form as clusters or isolated features, depending on
the density and spacing of the channels within a
braided channel belt.
Cant (1976, p. 125) interpreted deep scours,
reaching nearly three times the mean braidMchannel
depth, to have formed upstream of a large emergent
bar. This process is analogous to scouring on the
upstream margin of an obstacle clast in a flow (cf.
Best and Brayshaw 1985), with the emergent bar
acting as an obstacle within the channel.
Because these scours result in the deposition of
sediment below mean channel depth, they have a
high preservation potential. In the deposits studied
by Cowan (1991), they are common, whereas other
types of macroforms, such as DA and LA units, are
preserved only in fragmentary form.
Cowan (1991) proposed that these hollows formed by a process of deep scouring at points of channel
conve rgence. Scour depths of up to six times the
mean channel depth have been documented in rivers
and laboratory flumes by Mosley (1976), Mosley and
Schumm (1976), Best and Brayshaw (1985), Best
(1987, 1988), Best et al. (1989), and Salter (1993).
Bristow et al. (1993) proposed a facies model for the
scour and fill processes at channel confluences, and
Ashmore (1993) modeled their development in
gravel-bed rivers. These studies suggest that a
mechanism of deep scouring may be a significant
process at channel junctions (Fig. 6.45) and below
mid-channel macroforms in braided streams, where
stream junctions and bar forms abound (Best 1987,
p. 34; Best et al. 1989). Salter (1993) documented
scour where flow is deflected against banks or islands. Best (1987) showed that avalanche faces can
develop on the upstream end of these scours (Fig.
6.45). This allows the scours to be filled laterally,
167
obliquely, or vertically by an avalanche deposit in a
short period of time during channel switching or a
flood event. The scours are, therefore, envisaged to
form as clusters or isolated features, depending on
the density and spacing of the channels within a
braided channel belt.
Cant (1976, p. 125) interpreted deep scours,
reaching nearly three times the mean braidMchannel
depth, to have formed upstream of a large emergent
bar. This process is analogous to scouring on the
upstream margin of an obstacle clast in a flow (cf.
Best and Brayshaw 1985), with the emergent bar
acting as an obstacle within the channel.
Because these scours result in the deposition of
sediment below mean channel depth, they have a
high preservation potential. In the deposits studied
by Cowan (1991), they are common, whereas other
types of macroforms, such as DA and LA units, are
preserved only in fragmentary form.