Surface Armor for Erosion Protection
The velocity and characteristics of the stream dictate the size of stone used to form
the windrow revetment. The stone must be large enough to resist being transported by the
stream. Results obtained from windrow revetments constructed on the Missouri River
indicated that small gradation stone (200-pound top size with D50 of 7 to 8 inches) was more
effective than large gradation stone (500-pound top size with D50 of 9 to 10 inches) because
the smaller gradation forms a more dense, closely chinked protective blanket layer than the
larger gradation. A well graded stone is important to ensure that the revetment does not fail
from leaching of the underlying bank material.
7.1.4 LONGITUDINAL STONE TOE
Longitudinal stone toe is another form of a windrow revetment, with the stone placed
along the existing streambed rather than on top bank. The longitudinal stone toe is placed
with the crown well below top bank, and either against the eroding bankline or a distance
riverward of the high bank. Typical crown elevations may vary but are commonly between
1/3 and 2/3 of the height to top bank.
The success of longitudinal stone toe protection is based on the premise that as the
toe of the bank is stabilized, upper bank failure will continue until a stable slope is attained
and the bank is stabilized. This stability is usually assisted by the establishment of vegetation
along the bank.
A longitudinal stone toe has the same advantages as a trenchfill and windrow. It also
allows for the preservation of much of the existing vegetation on the bank slope, and
encourages the growth of additional vegetation as the bank slope stabilizes. An additional
advantage is that the treatment is amenable to the planting of additional vegetation behind it.
A longitudinal stone toe also has the same disadvantages as trenchfill. By definition,
longitudinal stone toe protection only provides toe protection and does not directly protect
mid and upper bank areas. Some erosion of these mid and upper bank areas should be
anticipated during long-duration, high energy flows, especially before these areas stabilize
and become vegetated.