# Hydraulic Analysis of a Passive Wedge Wire Water Intake Screen for Ichthyofauna Protection
## 1 Introduction
Sweeping to approach velocity ratios greater than 15 yields a strong hydraulic cleaning component, whereas \(V_{s}\)/\(V_{a}\) ratios lower than 5 result in high debris and fish impingement on the screen, and ratios between 5 and 10 result in a high percentage of the debris being carried or \"rolled\" along the screen [30]. According to U.S. recommendations, to achieve the highest possible \(V_{s}\)/\(V_{a}\) quotient, the angle between the screen surface and the direction of water flow in the channel should not exceed 45\({}^{\circ}\)[30]. US design guidelines vary the maximum allowable approach velocity \(V_{a}\) depending on the size and swimming ability of the individuals in question [31]. For fly less than 60 mm in length, the approach velocity at a distance of approximately 7.5 cm (originally in the guidelines 3 inches) from the screen surface should not exceed 0.06 m/s, and 0.12 m/s for longer individuals [32]. If the intake would be equipped with its own cleaning system, these velocities should not exceed 0.12 m/s and 0.24 m/s, respectively [33]. For UK guidelines, the maximum allowable approach velocities at a distance of 30 cm from the screen surface should not exceed 0.15 m/s [34]. According to New Zealand guidelines, the inlet velocity expressed in m/s should not be greater than 4 times the length of the smallest fish living near the intake expressed in mm [23]. U.S. guidelines also specify a value for the maximum allowable inlet velocity \(V_{in}\) at intake screen openings, and it is 0.15 m/s [30]. For Canadian guidelines for intakes with a capacity of less than 125 dm\({}^{3}\)/s, the value is 0.11 m/s when there are \"trout\" type fish moving with their hindquarters and 0.038 m/s when there are \"eel\" type fish moving with their whole body [35]. The mentioned values are summarized in Table 1.  
According to a study conducted at a cooling water intake on the Columbia River, USA, equipped with a T-screen cylindrical screen design with axial flow, the low number of \begin{table}
\begin{tabular}{c c c c c} \hline
**According to Guidelines** & **Allowable Approach** & **Allowable Inlet** & **Fish Category** & **Remarks** \\  & **Velocity \(V_{a}\) [m/s]** & **Velocity \(V_{in}\) [m/s]** & **Fish Category** & **Remarks** \\ \hline Canada & - & 0.11 & “trout” type fish moving & intakes with a capacity of less than 125 dm\({}^{2}\)/s \\  & - & 0.038 & “eel” type fish moving & L – fish length expressed in mm \\ New Zealand & 4\(\times\)\(L\) & - & - & in mm \\ United Kingdom & 0.15 & - & - & velocity at a distance of 30 cm \\  & 0.06 & & & from the screen surface \\ USA & (0.12—when intake equipped with cleaning system) & 0.15 & 60 mm & approach \\  & 0.12 & & & velocity measured at a distance of \(\sim\)7.5 cm from screen surface \\  & (0.24—when intake equipped with cleaning system) & & Length greater than 60 mm & screen surface \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of maximum allowable velocity, based on [16; 23; 31; 32; 33; 34; 35]. Figure 2: Components of the water stream flow velocity vector. entrained fish and larvae was influenced by the bow-wave-like hydraulics at the structure's nose cone as well as pressure and velocity changes upstream that helped the fish detect danger [29]. Wedge wire screens, in addition to capturing water for cooling and municipal purposes, are also used as a drain for washing sand filters in water and wastewater treatment plants or for collecting fish feces from the tank outlet water [36]. Wedge wire screen allows for inlet velocities of 15 cm/s or less when the inlet velocities across the surface are uniform or close to each other [37]. One way to achieve inlet velocities that are close to each other over the entire inlet surface of the screen is enabled by a specially shaped deflector. The deflector can also affect an increase in the ratio of sweeping velocity to inlet velocity, which also increases the chances of fish avoiding being drawn into the water intake.