Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 260

5 Sediment Movement in Alluvial Rivers
Abstract
Sediment is transported by river flow in the forms of suspended load and bed load. The fluvial process is
a result of sediment deposition and erosion. Flood, avulsion and sediment transportation and deposition
are the natural processes in alluvial rivers and water diversion, channelization, and navigation are human
disturbances to rivers. The fluvial process is the macroscopic view and long-term consequence of sediment
movement. This chapter introduces the basic knowledge of sediment movement and fluvial processes,
paying attention to the fall velocity of sediment particles, flow resistance, bed forms, and the rate of
sediment transportation, hyperconcentrated floods, river patterns, and unsteady sediment transportation.
This knowledge is useful for alluvial river management.
Key words
Hydraulics, Sediment load, Fluvial processes, Hyperconcentrated flows, Meandering river, Braided river
5.1 Hydraulics
5.1.1 Hydrograph
Streams flow out of mountainous areas and pour into flat-land sections of a river, which extend on the
plain between the estuary and the mountains. The flat-land sections of large rivers, such as the lower
reaches of the Yellow River and the middle and lower reaches of the Yangtze River, are alluvial rivers.
An alluvial river is defined as a river with its boundary composed of the sediment previously deposited
in the valley, or a river with erodible boundaries flowing in self-formed channels. Over time the stream
builds its channel with sediment it carries and continuously reshapes its cross section to obtain depths of
flow and channel slopes that generate the sediment-transport capacity needed to maintain the stream channel.
Alluvial rivers are mostly perennial streams and the channel bed is composed mainly of sand and silt.
Alluvial rivers are lowland rivers flowing through areas that often are densely populated. Many alluvial
rivers are confined within a channel defined by human constructed or artificially reinforced levees.
Flows in a river may range from no flow to flood flows in a variety of time scales. On a broad scale,
historical climate records reveal occasional persistent periods of wet and dry years. Many rivers in the U.S.,
for example, experienced a decline in flows during the “dust bowl” decade in the 1930s. Unfortunately,
the length of record regarding wet and dry years is short (in geologic time), making it difficult to predict
broad-scale persistence of wet or dry years. Seasonal variations of stream flow are more predictable,
though somewhat complicated by persistence factors. Because design work requires using historical
information (period of record) as a basis for designing for the future, flow information is usually
presented in a probabilistic format. Two formats are especially useful for planning and designing stream
corridor restoration: (1) Flow duration —the percentage of time over which a given flow discharge was
equaled or exceeded; (2) Flow frequency —the probability a given flow discharge will be exceeded (or
not exceeded) in a year.
Figure 5.1 presents an example of a flow frequency expressed as a series of probability curves. The
graph displays months on the x -axis and a range of mean monthly discharges on the y -axis. The curves
indicate the probability that the mean monthly discharge will be smaller than the value indicated by the
curve. For example, in January, there is a 90% chance that the discharge will be smaller than 9,000 m 3 /s.
Stage is the water surface elevation recorded relative to some horizontal elevation datum, usually sea
level. Stage records are valuable for the definition of high and low water levels. The record of stage is