General Principles of Erosion Protection
6.4 SURFACE DRAINAGE
Inadequate provision for surface drainage seldom results in complete failure of the
work, but it should not be neglected as it can be a major concern to adjacent property owners.
Inadequate design with respect to surface erosion gives the appearance of incompetent
design, affecting public perception of the success of the work. Mistakes occur easily, because
the designer's primary focus is usually on overall channel stabilization, and proper design for
overbank drainage flow outlets can be a tedious process, especially if rigorous design
procedures are to be followed.
Attention to surface drainage is even more important if the stream is degrading, and
flowline lowering is anticipated, since rills, gullies, and channels draining from the floodplain
will similarly degrade if not adequately protected.
The amount of design effort which is appropriate is determined by the:
project purpose;
susceptibility of a site to surface erosion, which depends on topography,
rainfall, vegetation, soil characteristics, and the type of bank stabilization to
be used;
engineering, environmental, and political consequences of erosion; and
feasibility of collecting sufficient data to permit a rigorous design.
The potential for surface erosion is best determined by identification and observation
of pre-existing problems. However, the construction of bank stabilization work can make the
problem worse, as well as more noticeable, since gullies leading into the stream will no longer
be periodically destroyed by streambank caving. Freshly graded banks are particularly
susceptible to surface erosion, and natural levees and existing drainage patterns and
vegetation may be disturbed by construction operations.
The basic steps in preventing erosion from surface drainage are to:
Protect all bare ground;
Collect the overland flow; and
Provide controlled outlets into the stream.
In the simplest situations, surface drainage occurs by sheet flow which is directed
away from the stream into a natural interior drainage channel. In this case, protection of bare
ground on unarmored bank slopes and in areas disturbed by construction activities is all that
is necessary. This is usually provided by vegetative treatments. Various types of chemical soil
stabilizers are also available and are often effective.
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