## 1 Introduction  
A spillway is a flood discharge structure that has been applied in hydropower and irrigation projects for decades. Particularly, in the area with narrow river width, a stepped spillway is an alternative to a large energy dissipater [1]. The steps act as roughness elements and heighten the bottom friction, thanks to which more kinetic energy is dissipated in comparison with the smooth layout. As a result, the required extent of the downstream energy dissipation arrangement is reduced, leading to lower total engineering costs [2]. In addition, owing to the advances in roller-compacted concrete (RCC) techniques, stepped chutes have, over the past decades, found wide applications in handling extreme floods [3].  
Despite the advantages, traditional stepped spillways are, in certain situations, insufficient to effectively dissipate energy and prevent cavitation risk. In countries including Sweden, the criteria for design-flood determinations for dams are re-evaluated; new flood guidelines have come into force [4]. For many existing dams, this leads to higher flood magnitudes, which requires that the spillway discharge capacity should be enlarged. Another contributing factor is the climate changes. Improved spillway design is necessary to meet the new flood standards.  
[PERSON] and [PERSON] examined the air-water properties on a stepped spillway with modified steps and cavities [5]. It was concluded that partial cavity blockages have little effect on air entrainment, but were linked to an increased presence of large-scale structures in the overflow. [PERSON] et al. performed numerical simulations on a spillway with trapezoidal labyrinth-shaped steps [6]. Relative to the conventional layout, a lower residual head and a larger friction factor were found in the labyrinth one. [PERSON] and [PERSON] proposed a stepped chute formed with gabions [7]. Compared with a layout with smooth surface, it led to less air entrainment and lower turbulence intensity. [PERSON] and [PERSON] conducted model tests on a pooled stepped spillway and examined its two-phase air-water flow properties [8]. [PERSON] and [PERSON] performed experiments of a large stepped chute equipped with triangular vanes and longitudinal ribs [9]. In comparison with the conventional one, larger flow resistance and more energy dissipation are evidenced. In a chute with upward inclined steps, [PERSON] and [PERSON] looked into its flow regimes and proposed an empirical formula for energy loss prediction [10].  
The focus of the previous investigations is mainly on examinations of energy loss in stepped chutes. Though the studies are fewer in number, they contribute to the understanding of the importance of step layouts. Despite this, insights into the step effects and cavitation behaviors are limited. Based on the geometrical layout of a stepped chute spillway examined elsewhere, CFD modelling in 3D is made, with emphasis on the step angling of the horizontal surfaces. With several typical layouts included, the study examines the characteristics of flow pressure and velocity and illustrate their differences in cavitation potential. The purpose is to identify their pros and cons, evaluate their suitability and provide guidance for improved step design in engineering applications. The upward layout is found an effective alternative to improve energy dissipation and mitigate cavitation risks.