1064S
SEDIMENT TRANSPORT AND EROSION
INTRODUCTIONThe literature on the subject of erosion and sediment transport
is vast and is treated in the publication of such disciplines as
civil engineering, soil science, agriculture, geography and geol-
ogy. This article provides a brief introduction to the subjects of
soil erosion, transport of detritus by streams and the response
of a stream channel to changes in its sediment characteristics.
The agriculturalist is concerned with the loss of fertile
land through erosion. Sheet, gully and other erosion mecha-
nisms result in the annual movement of about five billion
tons of sediment in the United States.^1 By this process, plant
nutrients and humus are washed away and conveyed to the
streams, reservoirs, and lakes.
The sediment characteristics of a stream also affect its
aquatic life. Changes in the character of the sediment load
will normally tend to change the balance of aquatic life. Fine
sediment, derived from sheet erosion, causes turbidity in the
waterways. This turbidity may interfere with photosynthesis
and with the feeding habits of certain fish, thus favoring the
less susceptible (often less desirable) varieties of fish. The
resulting mud deposits may have similar selective results on
spawning. The plant nutrients (phosphates and nitrates) that
accompany erosion from farmlands may contribute to the
eutrophocation of the receiving waters.
Turbidity also makes waters less desirable for municipal
and industrial use. Mud deposits may ruin sand beaches for
recreational use.
From the engineer’s point of view an understanding of sed-
iment transport processes is essential for proper design of most
hydraulic works. For example, the construction of dam on a
stream is almost always accompanied by, a reservoir siltation
or aggradation problem and a degradation problem. A reason-
able prediction of the rate of reservoir siltation is necessary in
order to establish the probable useful life and thus the econom-
ics of a proposed reservoir. The degradation or erosion of the
downstream channel and the consequent lowering of the river
level may, unless properly accounted for, endanger the dam
and other downstream structures (due to under-cutting). After
construction of the Hoover Dam the bed of the Colorado Riverdownstream from the dam started to degrade. In 12 years the
bed level dropped about 14 feet (Brown^1 ).
In addition the downstream channel may change its
regime (i.e. its dominant stable geometry). For example, a
wide braided channel or delta area may become a much nar-
rower and deeper meandering channel thus affecting the prior
uses of the stream. This appears to have happened as a result
of the Bennett Dam on the Peace River in British Columbia.^2CLASSIFICATION OF STREAMBORNE SEDIMENTSTerminologyThe materials transported by a stream may be grouped under
the following type of load:1) dissolved load,
2) bed load,
3) suspended load.The dissolved load, although a significant portion of total
stream load, is generally not considered in sediment trans-
port processes. According to Leopold, Wolman and Miller,^3
the dissolved load in US streams increases with increas-
ing annual runoff, reaching a maximum of about 125 tons
sq.mileyear for runoffs of 10 inchesyear or more.
Bed load consists of granular particles, derived from the
stream bed, which are transported by rolling, skipping or slid-
ing near the stream bed. Einstein 4,5 defines the bed load as the
sediment discharge within the bed layer which he assumes to
have an extent of two sediment grain diameters from the bed.
Suspended sediment load is that part of the sediment load
which is transported within the main body flow, i.e. above the
bed layer in Einstein’s terminology. Turbulent diffusion is the
primary mechanisms of maintaining the sediment particles in
suspension. The suspended load may be subdivided into:1) Wash load which consists of fine sediments mainly
derived from overland erosion and not found inC019_001_r03.indd 1064C019_001_r03.indd 1064 11/18/2005 11:05:59 AM11/18/2005 11:05:59 AM