6.5.5 Riprap Gradation and Thickness
The concept of a representative grain size for riprap is simple. A uniformly graded riprap with a
median size D50 scours to a greater depth than a well-graded mixture with the same median
size. The uniformly distributed riprap scours to a depth at which the velocity is less than that
required for the transportation of D50 size rock. The well-graded riprap, on the other hand,
develops an armor plate. That is, some of the finer materials, including sizes up to D50 and
larger, are transported by the high velocities, leaving a layer of large rock sizes which cannot
be transported under the given flow conditions. Thus, the size of rock representative of the
stability of the riprap is determined by the larger sizes of rock. The representative grain size
Dm for riprap is larger than the median rock size D50.
The recommended gradation for riprap is illustrated in Figure 6.15 in terms of D50. The
Figure 6.15. Suggested gradation for riprap.
Table 6.2. Data for Suggested Gradation.