Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 314

is blocked by a ice jam, the water stage rises, and the flooding water has to find a new channel. The new
channel extends downstream and finally meet the original channel at a downstream point. Thus, a net of
anastomosing channels develop. Ice jam floods occur very often in northeast China. This theory may
explain why there are many anastomosing rivers in northeast China but very few in other areas in China.
5.3.6 Avulsion
Avulsion is a kind of non-continuous channel motion. Avulsion was defined by Allen (1965) as the
abandonment of a part or the whole of a channel belt by a stream in favor of a new course. Avulsion is an
inevitable result of river aggradation and is, therefore, closely related to the sediment load the stream
carries. Avulsions are classified into nodal and random avulsions (Leeder, 1978). If, over time, more than
2 avulsions occur at approximately the same location this is called nodal avulsion. Random avulsion can
occur from any point along the active channel belt. Field (2001) studied the channel avulsion on alluvial
fans in southern Arizona. Channel avulsion invariably occurs where bank heights are low and often at
channel bends. The action of aggradation during floods is critical in the avulsion process since the greatest
amount of overland flow is generated where bank heights are lowest.
Avulsion is perhaps a final aspect of river behavior and concerns the large-scale movement of the river
course. The process occurs in meandering, braided, and wandering rivers and is recorded by abandoned
channel belts preserved on floodplains. The periodicity of avulsion appears to be on the order of
100-1,000 years. The diversion is actually gradual but can be considered instantaneous compared with
the recurrence time of avulsions. Avulsions are common only where the streams are aggrading relative to
their floodplain. In heavy sediment-laden rivers avulsion becomes the dominant mechanism of channel
shifting on alluvial fans and river deltas.
Sediment-laden rivers undergo periodic shifts. Avulsion occurred in the Mississippi River Delta along
the coast of Louisiana as successive channels searched for gradient advantages over their precursors
(Leeder, 1983). The same story has occurred for the Kosi River in India. From 1730-1960, the Kosi River
combed the Kosi River fan from east to west at a frequency of one avulsion per 23 years with the nodal
apex around Jogbani (Gole and Chitale, 1966). Major avulsions or changes in channel direction and form
occur regularly, particularly in semiarid areas and even in humid regions, during catastrophic, rare floods.
A single rainstorm of 3 days duration in California in 1938 produced as much as 189 m 3 /km 2 of sediment,
primarily from cultivated land, and initiated about 700 new channels on an area of 162 km 2 (Leopold et
al., 1964).
Slingerland and Smith (1998) studied the necessary conditions for a meandering river avulsion. They
presented a 1-D model and showed that whether a crevasse heals, runs away to an avulsion, or reaches a
steady state depends upon the ratio of crevasse to main-channel bed slopes, the height of the crevasse
bottom above the bed of the main channel, and the bed grain size. For fine to medium sand, crevasse
slopes greater than about eight times the main channel slope are predicted to capture all the main flow.
The Yellow River carries sediment from the Loess Plateau in central China to the delta and caused the
delta to expand by 2,000-3,000 ha per year in the period from 1855 to 1985. The extension of the river
channel reduces the gradient and capacity of the channel resulting in avulsions. The length of the new
channel is about 1/3-1/2 of the previous one and the gradient is 2-3 times higher.
As a comparison, the Mississippi River, transporting 240 million tons of sediment annually, extends
into the Mexico Gulf at a speed of 150 m/year, and shifts its course once per 1,000 years (Fisk, 1944).
The Danube River, carrying a much smaller sediment load, is quite stable with a frequency of channel
shift once per 2,300 years (Panin et al., 1983). The Fraser River in Canada shifted its course in 1827, 1864,
1892, 1896, 1900, and 1912, with a frequency of about once per 17 years (Clague et al., 1983). The Po
River in Italy shifted its delta channel 6 times in the past 3,000 years, about once per 500 years (Gandolfi