Surface Armor for Erosion Protection
7.1.4.4 Typical Applications
Longitudinal stone toe protection is especially suitable where the upper bank slope is
fairly stable (due to vegetation, cohesive material, or relatively low flow velocities), and
erosion can be arrested by placing a windrow along the toe of the bank. This avoids the
wasted effort of disturbing, then rearmoring, an existing stable slope. Small or ephemeral
streams are especially suited to this approach.
The longitudinal stone toe technique may be appropriate where the existing stream
channel is to be realigned, although for maximum effectiveness the top elevation of the stone
must be high enough that it is not overtopped frequently. In this application, it actually
functions as a retard.
7.1.4.5 Design Considerations
There are basically two variations of the longitudinal stone toe. These will be referred
to as longitudinal peaked stone toe protection, and longitudinal stone fill toe protection.
Design consideration for these two stabilization measures are discussed below.
Longitudinal Peaked Stone Toe Protection. An efficient design for a longitudinal
stone toe is to simply specify a weight or volume of stone to be placed per unit length of
streambank, rather than to specify a given finished elevation and cross-section dimensions.
This basically results in a triangular shaped section of stone placed along the toe of the
streambank. This type of protection is commonly referred to as a longitudinal peaked stone
toe protection (Figures 7.7 and 7.8). A primary attraction of this treatment is its simplicity.
Extensive surveys and analysis during design and construction would reduce that attraction.
Since the volume of stone required at each section is determined by the estimated scour
depth, simply specifying a volume or weight is all that is required. In the small streams of
north Mississippi, longitudinal peaked stone toe protection placed at a rate of 1 to 2 tons per
linear foot of streambank has proven to be one of the most successful bank stabilization
measures used in that area. This generally results in a height of stone between 3 and 5 feet
high above the streambed. A "typical" cross-section can be specified on the drawings, along
with a relatively smooth alignment to fit site conditions. During construction, the selected
alignment for the structure is flagged, and increments of length are measured as appropriate
for the size of delivery vehicles or placement buckets. Design, bidding, and supervision of
construction is, therefore, greatly simplified.
With longitudinal peaked stone toe protection, the establishment of vegetation
landward of the structure is a critical component for a successful project. Consequently, it is
important to maintain as much of the natural vegetation as possible. If at all possible, the
construction site should be approached and the construction work accomplished from the
riverward side of the bank to leave the existing upper bank vegetation undisturbed.
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