Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 315

et al., 1982). The Gediz River in Turkey shifted 6 times in the last 10,000 years, with a frequency of about
once per 1,600 years. The latest shift of the river occurred in 1980, and since then the river flows in the
present channel, the Kirdeniz River (Aksu and Piper, 1983). The frequency of avulsion of the Rhine-Meuse
River is very low because the river carries much less sediment load. Stouthamer (2001) studied the
avulsions in the Holocene Rhine-Meuse Delta. Five avulsions occurred in the Rhine-Meuse Delta from
about 6,500 yr BP to 1950 yr BP, when the Rhine-Meuse Delta experienced aggradation. The frequency
of avulsion is about 1/800 years.
At present, an avulsion is occurring on the Mississippi River. Since the 1930s, the river gradually
began shifting to the Atchafalaya River. Since 1950s the avulsion has been stopped by human hydro
structures, which control the flow into the Atchafalaya at 1/3 of the discharge with 2/3 of the discharge
remaining in the old Mississippi River channel to guarantee sufficient channel depth for navigation.
5.4 New Approaches
5.4.1 Rate of Bed Load Transport in Mountain Streams
Bed load motion in mountain streams is very complex and the bed load formulas available, including the
formulas proposed by Meyer-Peter-Mullër, Einstein, Bagnold, Engelund and Yalin, are not applicable in
many cases. Since the 1970s, researchers tested, analyzed and compared these formulas with measured
data from mountain streams. The difference between the calculated and the measure rate of bed load
transport were as large as on several orders of magnitude. It is because the disagreement of the bed load
formulas and data that research on bed load transport theories and computing methods have never been
stopped.
Barry et al. (1997, 2006) used bed load transport data measured from 24 gravel bed rivers in Idaho to
compare the accuracy of eight different formulas and the results of this analysis showed substantial
differences in performance of these formulas. Bathurst et al. (1987) tested the validity of bed load
formulas for mountain rivers, and found that the Shields approach (based on constant dimensionless
shear stress) failed for slopes S ı 0.01 and the ratio of water depth/median diameter İ 10.
The measured rate of bed load transport in mountain streams is sometimes much lower, or sometimes
much higher compared with the calculated values from the bed load formulas. Carson and Griffiths (1987)
evaluated the validity of bed load formulas using time-averaged transport measurements available for the
Waimakariri River and other gravel-bed rivers in New Zealand. In particular, they focused on the ability
of bed load equations, including the Bagnold formula, to estimate transport in braided rivers. They concluded
that bed load formulas often under-predict transport rates by several orders of magnitude.
Martin (2003) evaluated the original and revised versions of the Bagnold formula, the Meyer-Peter-
Mullër formula and a stream power correlation formula based on the data from Vedder River, a mountain
stream in British Columbia. From the evaluations Martin found the formulas under-predict gravel transport
rates by orders. Bagnold formula generally over-predicted bed load transport rates for braided rivers that
were in equilibrium or aggrading. The Vedder River is situated on an aggrading alluvial fan, yet the
original Bagnold formula under-predicted transport rates in the braided reaches.
Yu et al. (2009) measured the rate of bed load transport in the Diaoga River in Yunnan Province of
southwestern China with a double-box sampler. The outer box was buried under the stream bed with the
top edges of the box even with the local bed surface. Bed load particles were trapped, weighed and
analyzed with sieves. Figure 5.51 shows the measured rate of bed load transport per width as a function
of stream power, P , and the Shields dimensionless shear stress, Ĭ . For the same P or Ĭ , the rate of bed
load transport varied with a range of three orders of magnitude. It also clearly shows that the rate of bed
load transport before the first flood was 100 times higher compared with the rate after the flood.