Figure 6.12. Flow and scour patterns at a sloping sill (after Laursen and Flick 1983).
6.5 RIPRAP DESIGN AND PLACEMENT
6.5.1 Factors to Consider
When available in sufficient size, rock riprap is usually the most economical material for bank
protection. Rock riprap has many other advantages over other types of protection. A riprap
blanket is flexible and is neither impaired nor weakened by slight movement of the bank
resulting from settlement or other minor adjustments. Local damage or loss is easily repaired
by the placement of more rock. Construction is not complicated so for many applications
special equipment or construction practice is not necessary. Riprap is usually durable and
recoverable and may be stockpiled for future use. The cost-effectiveness of locally available
riprap provides a viable alternative to many other types of bank protection. Riprap stability
increases with increasing thickness as more material is available to move to damaged areas
and more energy is dissipated before it reaches the filter and streambank. Although riprap
must be placed to the proper level below the bed, there are no special foundation
requirements. The appearance of rock riprap is natural and after a period of time vegetation
will grow between the rocks. Wave runup on rock slopes is usually less than on other types of
bank protection.
The important factors to be considered in designing rock riprap bank protection are:
Durability of the rock
Density of the rock
Slope of the bed and bankline being protected
Angle of repose for the rock
Shape and angularity of the rock
What shape and weight of stones will be stable in the streamflow
What blanket thickness is required
Is a filter needed between the bank and the blanket to allow seepage but to prevent
erosion of bank soil through the blanket
6.20
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