Land Use Changes. Urbanization, agriculture, strip mining, and unregulated logging are
other activities that cause gradation problems. Natural vegetation is extremely important in
maintaining channel stability. The lateral stability of most streams in the United States,
particularly in regions where agriculture or lumbering is practiced, has very probably been
affected by the clearing of natural vegetation. Because this clearing has occurred more or less
gradually over the past hundred years, the magnitude of the effect at a particular crossing site
is sometimes difficult to assess.
The response to deforestation and agricultural activities is generally toward increased peak
flows and increased sediment yield. Channel widening and reduced sinuosity are common.
Grazing along the streambanks may have significant effects on bank stability.
Urbanization normally causes significant increase in the magnitude of runoff events while
reducing their duration. Urban areas are also low sediment producers because of the large
percentage of land covered by impervious surfaces. The combination of increased peak runoff
rates and reduced sediment loads results in channel degradation, channel widening, and a
reduction in channel sinuosity.
Improper construction activities, on the other hand, are known to increase both discharge and
sediment load. The removal of the vegetative cover accelerates the erosion process. The
response of the system to the increased discharge is to increase channel width and reduce the
radius of curvature. In response to increased sediment load, the stream will increase its
tendency for bank erosion.
Mining in an upland area may cause aggradation of channels, which are then subject to
degradation after the mining ceases.
Streambed Mining/Excavation. If sand or gravel is removed from an alluvial channel in
quantities that represent a substantial percentage of the annual bedload in transport, the
channel will probably degrade. In addition, removal of gravel from pits or trenches in or along
the stream may result in a change in flow alignment at a bridge.
Downstream mining can also produce headcutting through a bridge waterway, undermining the
structure. Mining operations upstream of the bridge can also produce degradation at the
bridge site and endanger the structure. Equation 5.28 provides a subjective tool for analysis of
gradation changes.
Highway engineers should, as a minimum, consider up- and downstream factors that might
cause gradation problems during scheduled bi-annual inspections of bridges.
Dams And Reservoirs. The effects of dams and reservoirs on a stream are complex. The
consequences include clear-water releases; high sustained, regulated flows; backwater; low
sustained, regulated flows; dam breach or removal; and high, controlled irrigation canal
releases.
Downstream from a reservoir, channel degradation is to be expected because of removal of
sediment. This effect has been documented for many streams (see for example, Williams and
Wolman 1984 and Lagasse 1994). The total amount of degradation is difficult to predict; if a
sand-bed channel becomes armored with gravel, the amount may be small. On gravel-bed
streams, aggradation may occur downstream from the dam because the flow releases are
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