FLUVIAL PROCESSES AND MORPHOLOGICAL THRESHOLDS IN INCISED CHANNEL RESTORATION1

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Abstract
Incised channels are those in which an imbalance between sediment transport capacity and sediment supply has led to degradation of their beds. This is a frequent response to stream channelization, changes in land use, or lowering of base level. If the degradation causes a critical bank‐height threshold to be exceeded, which is dependent on the geotechnical properties of the bank materials, then bank failure and channel widening follow. Interdependent adjustments of channel slope and cross‐sectional area occur until a new state of dynamic equilibrium with the imposed discharge and sediment load is attained. These geomorphic adjustments can be described and quantified by using location‐for‐time substitution and a model of channel evolution can be formulated. Three approaches to rehabilitation of the degraded channels are possible; geomorphic, engineering and rational. The rational approach, which integrates elements of both the engineering and geomorphic approaches, is based on the channel evolution model, and it generally involves control of grade, control of discharge, or a combination of both.