FLOOD04263

• County: Statewide
• Basin: Statewide
• Title: Design of Open Channels
• Date: 10/1/1977
• Prepared By: USDA Soil Conservation Service
• Floodplain - Doc Type: Educational/Technical/Reference Information

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3. Log soils within stratigraphic units and classify and sample for eval-
uation or determination of erodibility characteristics.

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Geomorphology of Deposits

Si~ificance of the Stratigraphic Unit

There are a number of factors which influence the erodibility or stability
of soil materials. Erodibility and stability are explained in this context
as responses of the soils to the environment of channel boundaries. Erosion
results from the flow of water against the boundary as expressed by mean
veloCity, tractive force, or a combination of these parameters such as
tractive power, the product of mean velocity and tractive force. Instability
of stream banks often results from internal seepage forces. The soil's
ability to resist erosion is affected by the kinds, amount, and character
(dispersive or aggregated) of clay, the amount and size distribution of
coarse particles, and the nature and amount of cementing agents. The term
coherent is used to describe soils that resist erosion as a mass because
of bonding action of clay, cementing agents or other causes. The climate
and age of the deposit since its accumulation have a strong bearing on
these erodibility characteristics. There is an almost infinite variation
that can occur in each of these factors and in their influence on the
others. Deposits change with geologic .time. Each change produces an
expression of the dominant factor and interfactor relationships persisting
at that time. Thus, a combination of forces acting on a deposit produce a
layer of material identifiable as a stratigraphic unit. Relative uniformity e
of location in an alluvial profile, uniformity of appearance, and erodibility
are a result of a similarity in source of materials, the history of
deposition, and the weathering characteristias associated with the areas.
In time, with certain exceptions, weathering, consolidation, cementation,
accumulation of humus or other influences increases the significance of
structure on erodibility. By the same token the significance of local
variations in texture is diminished.

The delineation of stratigraphic units simplifies the field investigations
in that the units may extend for distances that can include the entire
route of channels to be improved. Their thickness and depth below the
surface may be relatively uniform and consistent, providing a means for
correlation and representative sampling. At the other extreme, alluvial
fan deposition can result in a heterogeneous mixture of sand, silt and
gravel where correlation of individual beds may be not only impossible
but not particularly useful.

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