2008 – Application of procedures for estimation of rock erosion for overflow of dams – illustrated by application to Julius Dam, Queensland

Eric Lesleighter, Erik Bollaert

Abstract: Many of the dams in Australia, and other countries, are potentially unprotected from the occurrence of extreme floods which would discharge either over the dam and/or the abutments due to inadequate spillway capacity.

The paper commences with the presentation of the evaluation procedure that has been applied by the first author for a number of dams in Queensland, initially with a detailed description of dams, the original hydraulics studies, the current hydrology, and the geological information. The procedure then comprises analysis of the extreme flood hydraulics and an erodibility assessment which leads to an estimate of the possible erosion of the rock for a range of flood magnitudes. A description of the components of the final evaluation and the typical conclusions is presented with reference to the Julius Dam in Queensland.

The paper includes a description of an alternative or complementary procedure that comes from the work of the second author. Comparison is made with the Comprehensive Scour Model (CSM) developed by Bollaert (2002, 2004). This model is physically based and allows estimating scour formation in rock or concrete as a function of time duration of discharge. The model compares the resistance against fracturing of concrete or rock layers with the hydrodynamic pressure fluctuations exerted by a turbulent aerated jet impacting in the plunge area. Once the fracture network is formed, dynamic uplift of single rock or concrete blocks in computed. Based on fracture mechanics and air-water hydrodynamics, a detailed time-evolution of scour formation is obtained. The model is generally used for projects involving plunge pool floors, ski-jump spillways, fractured rock and so on, where sufficient data are available on both rock quality and duration and intensity of spill from the dam. In the present paper, the CSM has been used to model scour of non-overflow abutment sections when there is overtopping flows.

Keywords: dams, spillways, extreme floods, rock scour, erosion, dam safety