9 th International Congress on Civil Engineering, May 8-10, 2012 Isfahan University of Technology (IUT), Isfahan, Iran 1 Transitional Steps Zone in Steeply Sloping Stepped Spillways Jalal Attari 1 and Mohammad Sarfaraz 2 1- Assistant Professor, Power and Water University of Technology, Iran 2- Graduate Student, Department of Civil Engineering, Sharif University of Technology, Iran attari@pwut.ac.ir Abstract Construction of large dams, using roller compacted concrete (RCC) technique, allows for steeply stepped spillways to form part of the dam body which leads into cost and time saving. If the ogee spillway crest and stepped chute profile are joined without a transitional stepped zone, the flow behavior would be unfavorable for low discharges. The flow falls on the bottom of first steps, springs out like a jet and impinges on some steps further downstream. This process of flow separation from the stepped bottom and successive impingement may cause significant damage on the steps. This paper presents the importance and need for the transition steps to join the ogee crest profile and the stepped chute. For this purpose, physical as well as numerical modeling was employed on a case study. The original design was modified by introducing a transitional stepped zone, in accordance with the CEDEX profile, and results showed a favorable flow pattern. Furthermore, the good agreement of the numerical results with physical modeling observations highlights the advantage of hybrid modeling for hydraulic structures. Keywords: Stepped Spillways, RCC dams, Transitional Steps Zone, Physical modeling, Numerical Modeling 1. INTRODUCTION Stepped spillways are playing a new role following developments in the field of construction technology and in view of the need to improve the visual impact of hydraulic structures used for dissipating energy of flood release from dams, effectively reducing the size of the energy dissipation basin required downstream and, hence cut costs [1]. Construction of large dams using roller compacted concrete (RCC) technique, allows for the stepped spillways to form part of the dam body, which leads into cost and time saving. This procedure results a steeply sloping stepped spillway with the slope of more than 50 degrees [2, 3]. A typical stepped spillway is divided into two distinct regions: the more or less conventional crest profile and the stepped chute. Until now, standard design procedures in the literature do not involve how to join these two regions [2]. So the designers attach the stepped chute directly to the point of tangency in the crest profile (see figure1). Figure1. Configuration of conventional design of stepped spillways [2]