Stepped spillway model pressures characteristics, Susu Dam Malaysia

Susu Dam is an RCC Dam under construction in Peninsula Malaysia. The dam design has been carried out by SMEC International in its offices in Kuala Lumpur. The dam is some 90 m tall. The overfall stepped spillway provides for discharges to enter a hydraulic jump stilling basin for energy dissipation, prior to passing through a culvert under a roadway. The spillway was subject to hydraulic model testing at the Utah Water Research Laboratory of Utah State University (USA), at a length scale of 1:30. The hydraulic performance of the spillway design was evaluated up to the 4,700 m3/s Probable Maximum Flood (PMF) discharge (almost 1,000 L/s model scale). The spillway was tested through a number of configurations, prior to the development of the final arrangement. The purpose of the paper is to describe the modelling detail, and then focus on the flow behaviour at the stair-stepped spillway chute with 2.4 m high steps, in particular the piezometric pressures and transient pressures on the tread and riser of the steps. The spillway chute converged from ~100 m at the crest to 78 m at the bottom at entrance to the stilling basin; the unit discharge entering the stilling basin was ~ 60 m2/s for the PMF. Of particular interest was the occurrence of negative pressures on the steps, and the paper will describe the transients for several discharges from the AEP 1 in 1,000 up to the PMF; the results indicating very low pressures into a cavitation region. The design provides for an aerator across the spillway in order to counter the effects of possible cavitation. Results will be presented with and without the aerator operating. The paper provides useful design information for the hydraulic design of stepped spillways

Spillway Rock Scour Analysis - Composite of Physical & Numerical Modelling, Paradise Dam, Australia

A number of problems and solutions of rock scour downstream of spillways have been evaluated using a composite approach, based on the gathering of detailed data from a physical model and utilising those data in a sequence of calibration and application of numerical modelling of the scour. Our paper will describe the application of the Computational Scour Model (CSM - Bollaert, 2002 and subsequent) as the numerical procedure that makes the composite approach a proven methodology for such problems. The paper will focus on a case study application of the procedure based on the experiences of flooding and scour at the Paradise Dam, Queensland, Australia. Flooding in 2013 caused substantial scour downstream of the primary spillway. The occurrence led to a series of studies for the evaluation of the geology, and the evident hydraulics behaviour using a well-instrumented physical model to capture pressure and velocity transients, all as part of a process to determine the scour mechanism, and to determine the response of the spillway and areas downstream to future floods of larger magnitude. Utilising the transient data from approximately 60 pressure transducers, ADV measurements for transient velocities, together with the detailed geologic assessment, the comprehensive scour modelling procedures developed by Bollaert were applied for calibration of the numerical model and its application for possible discharge scenarios. The paper will discuss the design and construction of the physical model and instrumentation as a key part of securing adequate data for the composite procedure, and go on to illustrate the outcomes of the CSM procedures

Rock scour in Australia: some latest Queensland experiences

From 2010, a succession of floods in eastern Australia, and particularly in Queensland, brought about spillway operation at high head dams with return periods in the region of Annual Exceedance Probabilities (AEP) of up to 1 in 2,000 years. As such, a number of spillways experienced extensive scour of rock downstream – including Boondooma Dam and Paradise Dam – the subject of the present paper. For both dams, part of the scour assessment process has been to utilise a large-scale physical model to obtain transient data which, together with the detailed geologic assessment, have been incorporated into the numerical scour modelling procedures developed by Dr Erik Bollaert. This paper will first of all describe the features of the 2011 and 2013 flood events at both dams, as well as the resulting rock scour and damage on both spillways and the geology of the rock area below. The paper will then go on to describe the computational scour modelling procedures of calibration and application, used in conjunction with a large-scale physical model of both dam and spillway, demonstrating a “system” approach to spillway scour analysis for plunge pools and similar situations with energy dissipation on natural materials