What is the importance of sedimentation?

Sedimentation is another important feature of spate irrigation and spate irrigation is as much about managing water as it is about managing sedimentatin. The spate waters are usually laden with sediment. Scour and siltation are part and parcel of spate irrigation. Rivers in spate lift and deposit huge quantities of sediment. As a result there is constant change in bed levels, resulting in changes in bed levels and water distribution. The impact of these processes differs between the various systems. It depends on the amount and composition of the sediment load that a river carries, which depends on the rainfall pattern and the characteristics of the catchment area; its geology, morphology and vegetation cover. Farmers are usually able to identify the origin of a flood by the type of sediment that is transported by it. The degree of siltation and scour also depends on the local topography and the type of material. In spate irrigated areas with low gradients, as are found on the plains, a river is always in danger of choking itself with its own silt deposits and finding another way. Moreover, in the fine sandy deposits of the plains, the scouring of the riverbed is a larger danger than it is in the armuoured river beds of the highlands. As a result, the lowland flood irrigation systems are particularly dynamic.

Farmers, however, are not passive actors in these scour and siltation processes. They actively manipulate the scour and sedimetnation processes. They may deepen the headreach of a flood channel, in order to attract a larger flood that will further scour out the channel. If a flood river breaks its banks, farmers may close the breaches, if it deflects water away from their land or on other occasions, they will leave the breaches intact, so that these will act as escapes, creaming off the peaks of the very high floods and maintaining the flow at their own system at a manageable level. In other cases farmers will manipulate the siltation process to force the river bed to purposely silt up. The latter is in practice where the river has become uncontrollable, because its bed may has become to deep or to steep. The remedy is to built a strong permanent bund across the river and force the river to deposit its sediment load upstream of the bund.

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