WSP06041

• File Number: 8210.470
• Description: Pacific Southwest Interagency Committee
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/1968
• Author: Unknown
• Title: Report of the Water Management Technical Subcommittee - Factors Affecting Sediment Yield and Measures for the Reduction of Erosion and Sediment Yield - October 1968
• Water Supply Pro - Doc Type: Report/Study

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force which signifies the hydraulic stress exerted by the flow on the bed
of the stream. This average bed-shear stress is obtained as the product
of the specific weight of the fluid, hydraulic radius, and energy gradient
slope. Thus, greater depth results in a greater bed shear and a greater
potential for moving sediment. By the same reasoning, steep slopes (the
energy slope and bed slope are assumed to be equivalent) also result in high
bed-shear stress.

The boundary between sediment yield classifications in much of the Pacific
Southwest may be at the mountain front, with the highest yield designation
on the alluvial plain if there is extensive channel erosion. In contrast,
many mountain streams emerge from canyon reaches and then spread over fans
or valley flats. Here water depths can decrease from many feet to only a
few inches in short distances with a resultant loss of the capacity to trans-
port sediment. Sediment yield of the highest classification can thus drop to
the lowest in such a transition from a confined channel to one that has no
definition.

Channel bank and bed composition may greatly influence the sediment yield of
a watershed. In many areas within the Pacific Southwest, the channels in
valleys dissect unconsolidated material which may contribute significantly
to the stream sediment load. Bank sloughing during periods of flol', as I'ell
as during dry periods, piping, and bank scour generally add greatly to the
sediment load of the stream and often change upl'ard the sediment yield class-
ification of the watershed. Field examination for areas of head cutting,
aggradation or degradation, and bank cutting are generally necessary prior
to classification of the transport expectancy of a stream. Geology plays a
significant role in such an evaluation. Geologic controls in channels can
greatly affect the stream regimen by limiting degradation and headcuts.
Thus, the transport capacity may be present, but the supply of sediment from
this source is limited.

Man-made structures can also greatly affect the transport characteristics of
the stream. For example, channel straightening can temporarily upset the
channel equilibrium and cause an increase in channel gradient and an increase
in the stream velocity and the shear stress. Thus, the sediment transport
capacity of the stream may be temporarily increased. Structures such as
debris dams, lined channels, drop spillways, and detention dams may drasti,-
cally reduce the sediment transport.