F.3.2 Riprap Filter Systems

Riprap should be underlain by a sand and gravel filter (or filter fabric) to keep the fine materials in the underlying channel bed from being washed through the voids in the riprap. Likewise, the filter material shall be selected so that it is not washed through the voids in the riprap. Adequate filters can usually be provided by a reasonably well graded sand and gravel material where:

D₁₅ < 5 * d₈₅

The variable d₈₅ refers to the sieve opening through which 85 percent of the material being protected will pass, and D₁₅ has the same interpretation for the filter material. A filter material with a D₅₀ of 0.5 mm will protect any finer material including clay. Where very large riprap is used, it is sometimes necessary to use two filter layers between the material being protected and the riprap.

Example

Problem:

What embedded riprap design should be used to protect a streambank at a level culvert outfall where the outfall velocities in the vicinity of the downstream toe are expected to be about 8 fps?

Solution:

From Figure F.17, W₅₀ = 6.5 lbs, but since the downstream area below the outfall will be subjected to severe turbulence, increase W₅₀ by 400 percent so that:

W₅₀ = 26 lbs, D₅₀ = 8.0 inches

The gradation of the riprap is shown in Figure F.16, and the minimum thickness would be 1 foot (from Table II-4.5); however, 16 inches to 24 inches of riprap thickness would provide some additional insurance that the riprap will function properly in this highly turbulent area.

Figure F.18 shows that the gradation curve for ASTM C33, size number 57 coarse aggregate (used in concrete mixes), would meet the filter criteria. Applying the filter criteria to the coarse aggregate demonstrates that any underlying material whose gradation was coarser than that of a concrete sand would be protected.

Figure F.18. Riprap/Filter Example Gradation Curve.