ROCK MOVEMENT IN LARGE-SCALE TESTS OF RIPRAP STABILITY UNDER WAVE ACTION

Thorndike Saville

Abstract


There have been several instances in the past four or five years of damage to the riprap protection of some earth dams and embankments in major reservoirs in the middle western portion of the United States. In particular, in small sections of the embankment of the Snake Creek sub-impoundment in the Garrison Reservoir in North Dakota, some riprap was removed by a severe storm in 1964. It was recognized at the time of construction that the riprap protection to be placed on this embankment was considerably lighter than desirable, and was knowingly placed as an experiment to see if lighter graded material might still provide sufficient protection in a reservoir where the water surface elevation changed periodically. High waves can develop over the 32-mile fetch in this area of frequent high wind velocity. Loss of some riprap in this area has led to an investigation of various schemes of upgrading the riprap. As a part of this investigation, tests have been made at the Coastal Engineering Research Center in Washington, D.C. of various types of riprap exposed to wave action. Tests have been made at both small and large scale. As with most wave tank tests of breakwater or embankment structures, the tests must be run as a short series of bursts of waves, followed by periods of calm. This is necessary because when the wave approaches the structure and breaks upon it, a small portion of the wave energy is not absorbed in the breaking process, but is reflected back along the wave tank. This reflected wave, upon reaching the wave generator, is re-reflected and travels again down the tank toward the structure. If mechanical generation of waves continues after the wave is re-reflected from the generator, this re-reflected wave adds to the mechanically generated wave, and gives a higher wave at the structure than is desired for tests. Accordingly, the wave generator must be stopped at the time the wave reflected from the structure has travelled to and reaches the generator. Thus the tests are run in a short burst of perhaps 10 or 1$ waves, followed by a period of time sufficient to allow the tank water level to calm, and then again a burst of 10 or 1'j waves.

Keywords


riprap stability; riprap movement; large-scale test

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