FLOOD06100

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Design of Stable Channels with Flexible Linings
• Date: 10/1/1975
• Prepared By: Hydraulics Branch, Federal Highway Adminstration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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This concept is verified by the MSU tests (3), the Louisiana
Department of Highway tests (4), and the empirical equations
from NCHRP Report 108 (6). For most lining materials, the
erodibility of the underlying soil is a design parameter.
For rock riprap channel linings, the underlying soil is not
a consideration because a properly designed filter blanket
should be used, as necessary, to prevent leaching of the
underlying soil through the riprap. Design of the filter
blanket will be discussed in a latter section of this circular.
The scs results on maximum permissible velocity for vegetative
linings can be transformed into permissible depth curves by
the method explained in Appendix C.

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The erodibility of cohesive soils has thus far eluded quan-
titative definition. It is suggested that the erodibility of
specific soils be based on the designer's experience rather than
any quantitative analysis based, for example, on the plasticity
index. The difficulties involved in defining the erodibility of
cohesive soils is well described by Partheniades (9). Based on
the MSU work, which covered ten soils of different characteristics,
soils with a gravel, sand and clay mixture are erosion resistant;
fine-grained sands or silts are erodible; and plastic and semi-
plastic soils are in the intermediate range. The soil erodibility
index (K) for the Universal Soil-Loss Equation, developed by the
Agricultural Research Service, could also be used as a guide to
soil erodibility. For example, in Maryland, all soils have been
assigned K values of from 0.17 to 0.49 (10). A soil with a K
value of 0.17 would be considered erosion resistant, while a soil
with a K value of 0.49 would tend toward the erodible classificatio~.
These K determinations by the Soil Conservation Service are
subjective for the most part, but they give the designer some
basis for his appraisal of a particular soil.

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Another source of information on the erodibility of the soils
in a particular area are the county soil reports published in
many areas of the country by the Soil Conservation Service.
These reports often describe the erodibility of the various
soils in a particular location with enough detail to make an
estimate for the site under consideration.

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