Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 128

(a)

(b)
Fig. 3.6 (a) Sediment mining from the Lishui River in Hunan Province, China; (b) Channelization of the Charlooz
River in Iran causing channel incision, which endangered the banks and a nearby road

3.1.3 Evolution process of Incised River

3.1.3.1 Channel Incision
The incision of the river channel indicates a period of vertical instability or disequilibrium by
degradation. Lane (1955) suggested degradation occurs if

Qs kQ ! (3.1)
where Q is the channel-forming discharge (m 3 s -l ), s is the channel gradient, k is a coefficient, Q s is the
unit bed-material discharge (m 2 s -l ), and D 50 is the median grain size of the bed material (m). Figure 3.7
shows the factors affecting channel degradation or aggradation and their dynamic relations.
The bankfull discharge of non-incised, stable streams is generally accepted to be in the range of the
1-2 year recurrence interval flow (Wolman and Leopold, 1957; Williams, 1978). As degradation
progresses, discharge capacity increases and the channel is able to pass progressively larger volumes of
water within its enlarged cross section. The frequency of floodplain inundation is, therefore, reduced to
less than the 1 to 2-year recurrence interval and the floodplain becomes a terrace. This has important
implications for the geomorphic effectiveness of moderate and high recurrence-interval flows, which
would have previously spilled out over the floodplain, dissipating flow energy. In an incised system, such
flows are constrained within the deeper, narrower cross section and exert higher shear stresses, transporting

s 50