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<br />A discussion of (1) geomorphic factors and the classification of stream <br />properties, (2) assessment of stability as related to sediment transport and <br />hydraulics, and (3) effectiveness of countermeasures for hydraulic problems at <br />bridges is given by Brice and Blodgett (1978). An overview of streambank stabi- <br />lization measures is given by Brown (1985). <br /> <br />Classification of Failures <br /> <br />During this study, the authors identified four basic types of riprap <br />failure along streambanks: Particle erosion, translational slide, modified <br />slump, and slump. The cause o.f each type of failure is different and certain <br />riprap design procedures will be needed that consider each type of failure. A <br />sketch of each type of failure is shown in figure 4. <br /> <br />Particle Erosion <br /> <br /> <br />Particle erosion is the transport of rip rap stones to the channel bed near <br />the installation or to a point downstream. Particle erosion is considered the <br />most common type of failure, and the mechanics of impending movement are docu- <br />mented in the literature. A mathematical analysis of particle erosion is <br />presented by Simons and Senturk (1977). The sketch in figure 4 shows an <br />advanced stage of failUre caused by particle erosion. Displaced rip rap usually <br />Comes to rest on the bed near the eroded areas and for some distance downstream. <br />A mound of displaced riprap on the channel bed indicates that the transport <br />capability of the stream is insufficient to move all of the eroded riprap from <br />the site. This situation occurred on Pinole Creek at Pinole, California (see <br />fig. 1). A detrimental effect of the mound is the tendency to confine flows of <br />high velocity between the mound and the toe of the embankment, causing addi- <br />tional bank and bed erosion. <br /> <br />STONES TOO LARGE <br />FOR TRANSPORT <br /> <br /> <br />ROCK RIPRAP <br /> <br />CHANNEL BED <br /> <br />PA R TlCLE ER OSION <br /> <br />Mound of displaced rlprap. Particle erosion results <br />Ifllow shear stress or velocities are excessive. <br />I! displaced stones are not transported from the <br />eroded area, the channel bed will show a mound. <br /> <br />FIGURE 4. Classification of principal types of riprap failures. <br /> <br />10 <br /> <br />