# Hydraulic Analysis of a Passive Wedge Wire Water Intake Screen for Ichthyofauna Protection
## 1 Introduction
This implies the need to ensure the safe migration of fish in watercourses to find spawning grounds, search for feeding grounds, cooler waters, a place to overwinter, or return after being carried away by flood waters, but also the safe movement of ichthyofauna near water intakes [13; 14; 18; 19]. In many countries, such as the United States [20], Canada [21], the United Kingdom [22], and New Zealand [23], regulations already exist to limit the adverse effects of water intake on ichthyofauna. In particular, these regulations apply to water intakes for cooling purposes, characterized by very high yields. Large hydropower plant intakes are also dangerous for fish, as on their way downstream, they are exposed to various related negative effects, ranging from a delay in downstream movement to being injured or killed by rotating blades [24]. Physical and behavioral barriers and systems that guide fish and fry away from areas that pose a risk to them to areas that are safe for them are used to protect them from entering the water intake screen. There are different design solutions for physical barriers widely used. They can be vertical, horizontal, or inclined and have different bar shapes and bar spacings [25]. The examples of the barriers are shown in Figure 1.  
The purpose of behavioral barriers (bubble curtains, strobe lights, sound, electricity, magnetic fields, or changed direction and velocity of the water flow) is to deter fish from entering zones that may pose a threat to them. Compared to physical barriers, behavioral barriers are based on the natural behaviors of fish [27] and require their activity; therefore, the effectiveness of the barriers largely depends on the swimming ability of the fish. The efficiency of physical barriers especially with narrow bar spacing in avoiding fish passage through and impingement risks at the rack is confirmed [25]. However, narrow bar spacing causes higher head losses and contributes to clogging of inlet surfaces by attracting floating debris such as leaves and, therefore, adversely affecting the exploitation of the intake [25]. Of the existing methods, screening technologies are considered the best available [20], among which traveling screens and wedge wire passive water intake screens stand out in particular. Wedge wire passive screens for water intakes are considered the most effective type of screens for minimizing impingement and entrainment, protecting larval-sized juvenile fish. They have installation flexibility and prevent contaminants from entering the intake system, but do not remove contaminants from the water body. They have low operating costs due to the lack of moving parts and thus no need for power [28]. An important piece of equipment of modern wedge wire screens is the use of automatic systems that clean their surface from contaminants that are deposited during operation [13]. The frequency of cleaning is determined by equipping the screens with measuring systems that control the pressure difference on the inside and outside of the screen. Cleaning can be carried out using appropriate brushes that are mounted inside or outside the screen or compressed air. The most effective method is counter-current cleaning with compressed air. In order to protect ichthyofauna and minimize the pollutants drawn in by the intake, it is necessary to design the intake properly. One of the parameters to be considered in such a design is the value of the channel velocity vector \(V_{c}\), which can be decomposed into two component vectors: parallel to the screen surface sweeping velocity vs. and perpendicular approach velocity \(V_{a}\) (Figure 2). Figure 1: Examples of barriers: (**a**) horizontal bar rack [25] (**b**) drum screen in a hydroelectric power plant [26], (**c**) flat screen mounted horizontally [23], (**d**) inclined rack [25]. While \(V_{a}\) causes debris and fish impingement on the screen, vs. makes it easier to pull them away from its surface [29]. Generally, higher ratios of \(V_{s}\)/\(V_{a}\) shed debris and fish better than low ratios.