and design procedures are summarized in charts and examples. Design guidance is also
presented for:
Wire-enclosed rock (gabions)
Precast concrete blocks
Concrete paved linings
The following section (6.4) summarizes design concepts for selected flow control (river training)
structures. Section 6.5 deals with riprap design and placement, and Section 6.6 describes
design for revetment types other than riprap. General filter design concepts are discussed in
Section 6.7 and issues related to overtopping flows on embankments are summarized in
Section 6.8. Environmental considerations for streambank protection are presented in Section
6.9.
6.3.5 Protection of Training Works
Granular or geosynthetic filters are essential to the performance of hydraulic counter-
measures, especially armoring countermeasures such as bankline revetment or armoring
used as protection for river training works such as spurs and guide banks. Filters prevent
soil erosion beneath the armoring material, prevent migration of fine soil particles through
voids in the armoring material, distribute the weight of the armor units to provide a more
uniform settlement, and permit relief of hydrostatic pressure within the soils. Experience has
indicated that the proper design of filters is critical to the stability of revetments. If openings
in the filter material are too large, excessive piping through the filter can result in erosion of
the subgrade beneath the armor. Conversely, if openings in the filter are too small,
hydrostatic pressures can build up in the underlying soil and result in failure of the
countermeasure. Guidelines for the selection, design, and specifications of filter material
can be found in HEC-11 (Brown and Clyde 1989), HEC-23 (Lagasse et al. 2001), and
Section 6.7.
Undermining of the edges of armoring countermeasures is another of the primary
mechanisms of failure. The edges of the armoring material (head, toe, and flanks) should
be designed so that undermining will not occur. For revetment slope protection, this is
achieved by trenching the toe of the revetment below the channel bed to a depth which
extends below the combined expected contraction scour and long-term degradation depth.
When excavation to the contraction scour and degradation depth is impractical, a launching
apron can be used to provide enough volume of rock to launch into the channel while
maintaining sufficient protection of the exposed portion of the bank. Continuous systems,
such as articulating concrete block systems and grout filled mattresses applied on side
slopes, should be designed with an apron or toe trench so that the system provides
protection below the combined expected contraction scour and long-term degradation depth.
Tension anchors may be used to increase stability at the edges of these continuous
systems. Additional guidelines on edge treatment for armoring countermeasures can be
found in HEC-11 and HEC-23.
Variations in bed elevation during flow events or after bank hardening can result in the
undermining of bank protection structures including longitudinal structures. Deep sections at
the toe of the outer bank of a bendway are the result of scour. As discussed in Section
5.4.3, high velocity along the outer bank is caused by secondary currents and greater outer-
bank depths, and together with the resultant shear stress, produce scour and cause a
6.9
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