When is it safe to use rip-rap?

Posted on July 2, 2014

Layers of riprap (loose rocky material) are commonly used to protect river banks against erosion. However, if the size of riprap stones is too small, they are at risk of being dislodged by stream currents. Therefore, it’s important to have a reliable way of assessing the safety of existing riprap and calculating the minimum rock diameter that should be used in new designs. An innovative method of riprap sizing has been developed by utilizing the fact that loose stones tend to be dislodged more easily by rolling than by sliding. This means that a good index of riprap stability can be based on an analysis of stone overturning – specifically, the ratio of the moments that resist overturning to the moments that promote overturning. If the overturning ratio is greater than one, a riprap particle should be stable. Data from 38 studies of riprap performance at 26 different stream sites in the USA were used to test the approach. The overturning ratio varied from 0.6 to 1.3. Rip-rap failed in 72% of cases (13 out of 18) when the index was less than 1, but only 5% of cases (1 out of 19) when the index exceeded 1. The new approach was markedly better at predicting riprap damage than two alternative methods.

Reference: Froehlich, D.C. 2013. Sizing loose rock riprap to protect stream banks. River Research & Applications 29, 219–235. http://onlinelibrary.wiley.com/doi/10.1002/rra.1587/pdf