Technical Note Gabion Stepped Spillway: Interactions between Free-Surface, Cavity, and Seepage Flows Gangfu Zhang 1 and Hubert Chanson 2 Abstract: On a gabion stepped chute, the steps contribute to the dissipation of turbulent kinetic energy, free-surface aeration may be intense, and there are complex interactions between the free-surface flow and seepage motion. Detailed measurements were conducted in a relatively large gabion stepped spillway model. Using a combination of high-speed movies and phase-detection probe measurements, the air–water flow properties in the step cavities and in the gabions were documented. Strong air–water exchanges between seepage and stepped cavity flows were observed. The data showed a complex bubbly seepage motion in the gabions associated with a high level of interactions between seepage and free-surface flows, leading to a modification of the step cavity recirculation and lesser flow resistance. DOI: 10.1061/(ASCE) HY.1943-7900.0001120. © 2016 American Society of Civil Engineers. Author keywords: Air–water flow; Gabion stepped spillways; Seepage flow; Overflow-seepage flow interactions; Physical modeling. Introduction Stepped spillways have been used as flood release facilities for several centuries (Chanson 2001). The steps contribute to some dissipation of the turbulent kinetic energy, thereby reducing or eliminating the need for a downstream stilling structure. Highly tur- bulent flows are experienced down a stepped chute (Rajaratnam 1990). For low- to medium-head stepped chutes, gabions may be a suitable construction material. Their advantages include sta- bility, low cost, flexibility, porosity, and noise abatement (Agostini et al. 1987; Chanson 2001). Peyras et al. (1992) investigated the flow patterns and energy dissipation performances of gabion stepped weirs with a 0.2 m step height. Wüthrich and Chanson (2014) compared the energy dissipation performances of flat imper- vious and gabion stepped chutes. Their findings showed lesser rates of energy dissipation on the gabion chutes, which motivated the present investigation. Herein the turbulent flow properties above a gabion stepped chute were investigated experimentally. The air–water flow was studied in the mainstream flow and step cavities and within the ga- bions using a combination of dual-tip phase-detection probe and high-speed video observations. It is the aim of this study to detail the interactions between air–water seepage and cavity flows and to characterize the flow through gabions. Experimental Setup and Instrumentation Experiments were conducted at the University of Queensland in a gabion stepped chute, previously used by Wüthrich and Chanson (2014). The test section was 3 m long and 0.52 m wide, consisting of an impervious broad crest followed by ten 0.1 m high, 0.2 m long gabion steps: the chute slope was θ ¼ 26.6°(1V∶2H). Each gabion was 0.3 m long, 0.1 m high, and 0.52 m wide, filled with 14 mm sieved gravel, and the gabions overlapped (Fig. 1). The bulk density of the dry gravel was 1.6 t=m 3 with a porosity of approximately 0.35–0.4. The hydraulic conductivity of the gabions ranged from 1.1 × 10 −1 to 2.3 × 10 −1 m=s. The discharge was recorded from the upstream head above the crest. The air–water flow measure- ments were performed with a dual-tip phase-detection probe (O = ¼ 0.25 mm) in the free-surface flow and step cavities, with the probe sensors sampled at 20 kHz for 45 s. The translation of the probe in the normal direction was controlled by a fine adjustment traverse. Visual observations were made with high-speed video re- cordings at speeds of up to 1,000 fps (fps = frames per second). The flow investigations were conducted for discharges per unit width up to 0.28 m 2 =s, with a focus on the aerated transition and skimming flows, i.e., d c =h > 0.6, where d c is the critical flow depth [d c ¼ðq 2 =gÞ 1=3 ], q is the discharge per unit width, g is the gravity acceleration, and h is the vertical step height. Phase- detection probe measurements were performed in the overflow including in the step cavities. Bubble trajectories in the gabions were tracked with the high-speed video camera. The tracking was performed using a manual frame-by-frame analysis to guaran- tee the maximum reliability of the data. Flow Patterns On the gabion stepped spillway, the water seeped through the ga- bions at very low discharges (d c =h < 0.2), and no overflow was observed at the step edges. For d c =h > 0.2, some overflow was observed above all the gabion steps. A nappe flow regime occurred for 0.2 < d c =h < 0.5 − 0.6. A transition flow was seen for 0.6 < d c =h < 0.9, characterized by large hydrodynamic instabilities and intense splashes in the overflow. For d c =h > 0, a skimming flow was seen. The upstream flow was nonaerated. The inception of free-surface aeration was clearly marked. Downstream, the air- entrainment process was highly three-dimensional and complex. The gabions were fully saturated for d c =h > 0.2. Downstream of the inception point, a strong bubbly motion was observed inside all gabions, as sketched in Fig. 1. A large amount of air entered the gabions through the downstream half of the horizontal step face. The entrapped air flowed through the gabions as individual bubbles 1 Ph.D. Research Student, School of Civil Engineering, Univ. of Queensland, Brisbane, QLD 4072, Australia. 2 Professor in Hydraulic Engineering, School of Civil Engineering, Univ. of Queensland, Brisbane, QLD 4072, Australia (corresponding author). E-mail: h.chanson@uq.edu.au Note. This manuscript was submitted on April 15, 2015; approved on October 20, 2015; published online on January 8, 2016. Discussion period open until June 8, 2016; separate discussions must be submitted for indi- vidual papers. This technical note is part of the Journal of Hydraulic En- gineering, © ASCE, ISSN 0733-9429. © ASCE 06016002-1 J. Hydraul. Eng. J. Hydraul. Eng., 2016, 142(5): 06016002