FLOOD00950

• County: Statewide
• Title: Rock Riprap Design for Protection of Stream Channels near Highway Structures volume 2
• Date: 1/1/1986
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

An example of progressive rip rap failure and increased height of the scarp
caused by the particle erosion is given in table 3. Data for failures at three
sites on the Sacramento River at E-10 near Chico were collected in May 1983.
The larger failure was first observed in January 1983, after the initial damage
occurred during a flood in December 1982. This failure subsequently increased
in size, and the other two failures occurred during floods that overtopped the
bank in January and March 1983. All of these failures exhibit the geometric
shape of a horseshoe, shown in figure 8, during the beginning phase of riprap
failure. The riprap failure on Pinole Creek at Pinole represents the advanced
stage of particle erosion; in which the individual areas of failure are com-
bined. A significant characteristic of the Pinole Creek failure is that the new
side slope is flatter than the original side slope of 2.4:1, thus leaving an
exposed face (scarp) that is. easily eroded.

The probable causes of particle erosion are:

o Median size (Dso) stone not large enough to resist the shear stress of the
stream.

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o

Abrasion or removal by impact of individual stones.
previous situation, individual stones are removed, and
lative effect results in failure of the riprap.

For this and the
in time, the cumu-

o Side slope of the bank so steep that the angle of repose of the riprap is
easily exceeded, causing instability of the individual stones.

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o Gradation of riprap may be too uniform (all stones near the median size).
Without enough smaller diameter stones that tend to fill the voids and
provide lateral support for larger material, failure may occur even
though the median size is adequate and the bank side slope is not too
steep.

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Table 3. Geometry of progressive rip rap failures related to particle
erosion on the Sacramento River at Site E-IO near Chico, California
(surveyed May 2, 1983).
New side slope Length (feet) at stated Height
Fail- in area of Maximum intervals (in percent) of
ure erosion width1 of width2 scarp3
site (feet/feet) (feet) 0 25 50 75 90 (feet)
Ii 1 0.54 5.5 12 12 ll.5 9 7 1.6
! 2 .62 10.5 25 24 22 19.5 16 1.9
"
i 3 .59 11.0 44 43 40 30 14 2.4

1Width is maximum slope distance, measured perpendicular to shoreline.
2Length is the distance acroSs failure, measured parallel to shoreline.
The _z.ero.Jlercent length is at the downslope end of the failure. The
100 percent length is at the face of the scarp, as shown in figure 8.
3Maximum height of scarp. The height decreases to zero at the
o percent width interval.

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