Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 153

in Chapter 5. The river bed inertia can be calculated with the formula:

- gg

b * bin

= J ( 1 - p ) (3.6)

I b

s

S r

where p is the porosity, J s (1 -p ) is the dry weight of the bed material, S r is the scour rate of the channel
bed in weight per area per time, g b* is the bed load carrying capacity of the flow, or the rate of bed load
transport by the flow when in equilibrium, g bin is the rate of bed load carried by the flow at the entrance
of the studied reach. For a given composition of bed material, the bed inertia I b is regarded as a constant.
Small bed inertia implies that the bed deforms quickly with the change of flow and has low stability. If
the bed inertia is large, the bed responds mimimally to the flow changes . The riverbed inertia depends on
the development degree of the bed structure and composition of the bed material. If there is no structure,
the river bed inertia increases with the sorting coefficient of bed material, D 90 /D 10 .
The development of a step-pool system enhances the river bed inertia. Figure 3.35 shows the channel
bed inertia of the Jiangjia Ravine and Fork Gully in comparison with the experimental results as a
function of , D 90 /D 10 , where the subscript indicates the percentage of the particles smaller than this
diameter For the Jiangjia Ravine, the bed load carrying capacity is equal to 0.759 kg/s.m for a flow with
hydraulic radius of 0.08 m. A field experiment was done by closing a branch of the stream and cutting
bed load transportation from upstream, thus, creating a clear water flow of a hydraulic radius of 0.08 m
in a single channel section. All sediment collected in the downstream end of the section was scoured
from the section. The measured scour rate of bed sediment from the section was on average 0.01 kg/sm 2 .
The riverbed inertia calculated with Eq. (3.6) equals to 141 t/m 2 . The water flowing in Fork Gully is from
fountain springs. The scour rate was measured at 6.6 u 10 -5 kg/s . m in a flow with hydraulic radius of
0.055 m. The river bed inertia calculated equals 17,000 t/m 2 . In Shengou and Jiuzhai creeks, the water
depth is large but the flow cannot scour sediment from the bed because the step-pool system protects the
bed sediment from erosion. In other words, the scour rate is zero for almost all flow conditions. For
instance, the step-pool system has developed in the middle and lower reaches of Shengou Creek in 2001.
The creek has not eroded down in the period 2001-2007. Only the sediment load from upper reaches may
have partly been transported through the middle and lower reaches in the period. For most cases the
scour rate is zero, therefore, the riverbed inertia is infinite.

Fig. 3.35 Riverbed inertia, I b as a function of the sorting coefficient D 90 /D 10 for channel beds without a
step-pool system and with a developing step-pool system