Foreword
Scour of rock downsteam of overtopping dams has become a dam safety concern in recent years. As dams age, the estimates of their associated design floods continues to increase as more hydrologic data becomes available and hydrologic prediction techniques improve. It is not uncommon for the magnitude of a design flood to increase manifold, at times increasing by a factor of two or more. In such cases, it is often required to either increase the dam's spillway capacity, or to allow overtopping in the case of concrete dams. In either case, but particularly when a decision is made to allow dam overtopping, it is necessary to assess the erodibility of the earth material downstream of the dam. When allowing a concrete dam to overtop, it is usually found that the foundation downstream of the dam consists of intact rock formations. In such cases it is necessary to show that the rock foundation will not scour excessively during flood conditions, thus endangering the dam. In cases where excessive scour is predicted, it is necessary to design mitigation measures.
Until recently, i.e., prior to about 1995, no methods existed to reliably investigate the erodibility of rock formations subject to flowing water, nor were any methods available to predict how deep it may scour, should it erode. The first methods that became available to make such assessments used the Erodibility Index, which is based on a geo-mechanical index originally developed to assess the excavatability of rock formations. The method related stream power, a proxie representing the relative magnitude of the erosive capacity of water, to the Erodibility Index, which represents the relative ability of rock to resist the erosive capacity of water. This method can be used to quantify the erodibility of any earth material, including rock formations, and to quantify the anticipated extent of scour.
Towards the end of the last century and the early part of this century detailed research was conducted that led to in-depth understanding of rock scour mechanisms. This research was a major step forward because it now allows engineers to not only predict the maximum extent of scour of rock, but to also predict the rate of such scour. The methodology uses principles of fracture mechanics and basic principles of physics to identify scour mechanisms as either brittle fracture, fatigue failure or block removal.
Experience has shown that implementation of both types of methods provide a check on predictions as well as more detailed understanding of site-specific scour potential. In the case of plunging jets, it is now possible to predict the maximum extent of scour using the Erodibility Index Method and then confirm that prediction using the methodology based on fracture mechanics and basic physics. This approach to rock scour analysis and prediction is recommended as it provides confirmation of predictions and creates deeper understanding of scour potential, rock scour mechanisms and scour extent. The understanding thus developed allows development of mitigation measures with increased confidence.
This White Paper summarizes the current state of the art of rock scour. Ongoing research, conducted internationally, continues to improve our understanding and abilities to predict scour of rock. This field of specialization is rapidly developing.
George Annandale, DIng, PE, USSD Committee on Foundations
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