River classification has always been a controversical filed of study. There are mainly two aspects in river classification, the in-channel processes and the extra-channel processes. Commonly, river are classified according to innerprocesses, as can be seen in Fig. 3 Rosgen classification (1994). The main parameters for this type of classification, are slope and sinuosity.
However, as Makaske1 suggests, this type of classification gives us a biased knowledge about rivers, as the extra-channel processes are as important as the inner ones.
1 Bart Makaske, Anastomosing rivers: a review of their classification, origin and sedimentary products. 2000.
Fig. 4 Avulsion Process
Avulsiosn are primarly driven by aggradation of the channel belt and/or loss of channel capacity by in-channel deposition. Both processed are favoured by low floodplain gradient. An avulsion is the abandonment of a part or the whole of a channel belt by a stream in favour of a new course, (Fig. 4).
In the same way the avulsions can take different forms along the course, according to the way they bifurcate and flow downstream. (Fig. 6) If the at the same point of the river, several avulsions occur it will be a nodal avulsion.
As it can bee seen in the Local Avulsion case, it suggests a different pattern of water management and organisation for irrigated lands. This pattern could generate a new set of Micro Flooding Units1 along the riparian lands of rivers in floodplains. S. Ribot, Technical Essay.
Random avulsions will happen if they start at different points along the river belt.
Avulsion can happen in different ways, as can be seen in Fig. 5, it can be a full avulsion provoking the river to change channel, a partial avulsion in which the river bifurcates and starts flowing in two channels and finally a failed avulsion, in which the new channel is not consolidated.
Nodal Avulsion
Fig. 5 Type of Avulsions
Local avulsions, bifurcate at one point and flows separately through a specific length to come back to the river.
Random Avulsion
1 The term Micro Flooding Unit will be explained in further chapters
Local Avulsion
Fig. 6 Type of Avulsions along course.
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