difference between the sediment load entering and exiting the outer-bank zone. Since
secondary currents transport sediment supplied, in large part, from outer bank erosion
toward the inner bank of a bend, hardening of the outer bank by longitudinal bank protection
structures may cause the channel cross section to narrow and deepen by preventing the
recruitment of eroded outer bank sediments.
Experience is usually the most reliable means of estimating scour depth when designing a
bank protection project for a particular stream. Lacking experience on a particular stream,
scour depths may be estimated using physically based analytical models or empirical
methods. Although scour-depth can be estimated analytically or empirically, empirical
methods were generally found to provide better agreement with observed data.
Maynord (1996) provides an empirical method for determining scour depths on a typical
bendway bank protection project. Although his studies are restricted to sand bed streams,
the Maynord method agrees reasonably well with the limited number of gravel-bed data
points obtained by Thorne and Abt (1993). Nonetheless, the techniques presented by
Maynord are restricted to meandering channels having naturally developed widths and
depths, and cannot be applied to channels that have been confined to widths significantly
less than a natural system.
HEC-23 provides application guidelines for Maynord's approach to estimating toe protection
requirements on stabilized bendways. HEC-23 also contains guidance for estimating scour
at vertical wall structures (e.g., retards and bulkheads) from flow parallel to and impinging on
the wall.
6.4 FLOW CONTROL STRUCTURES
A flow control structure is defined here as a structure, either within or outside a channel that
acts as a countermeasure by controlling the direction, velocity, or depth of flowing water.
Structures within this category are sometimes called "river training works". Among the most
important properties of a flow control structure is its degree of permeability. An impermeable
structure may deflect a current entirely, whereas a permeable structure may serve mainly to
reduce water velocity. As used here, the term "permeable" means that a structure has definite
openings through which water is intended to pass, such as openings between adjacent boards
or pilings, or the meshes of wire. Structures made of riprap, or filled with riprap, have some
degree of permeability, but these are classed as impermeable because they act essentially as
impermeable barriers to a rapidly moving current of water.
Types of flow control structures are distinguished on Figure 6.2.
6.4.1 Spurs
A spur is a structure or embankment projected into a stream from the bank at some angle and
for a short distance to deflect flowing water away from critical zones, to prevent erosion of the
bank, and to establish a more desirable channel alignment or width. By deflecting the current
from the bank and causing sediment deposits behind them, a spur or a series of spurs may
protect the stream bank more effectively and at less cost than revetment riprap applied directly
on the bank. Also, by moving the location of any scour away from the bank, failure of the
riprap on the spur can often be repaired before damage is done to structures along and across
the river. Conversely, failure of riprap on the bank may immediately endanger adjacent
structures.
6.10
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