This study aims to investigate flow and turbulence structure of a diagonal brush fish passage which has been constructed at the existing Small Hydropower Plant (SHP) on İyidere River on the East Black Sea Coast of Turkey. The flow and turbulence characteristics of diagonal brush fish pass were investigated in a 1.1 m wide rectangular flume with a bed slope of 10% for Reynolds and Froude number ranging between 3.45x104-2.4x105 and 0.15-0.16, respectively. By taking into account dynamic upstream water levels (H=101.20 m to H=102.05 m) of fish pass structure throughout the year, the vertical velocity profiles of different flow regions have been obtained by using Nivus instrument; whereas the turbulence quantities were gathered by measuring the three-dimensional instantaneous velocity fields using Micro acoustic Doppler velocimeter. The flow data were grouped for 4 different relative submergence of bristles. Thus, the analysis is done based on a physical basis and it will contain the most important hydraulic parameters of velocity field, turbulence, and flow depth for fish passage design. The main findings of this study can be summarized as follows: (i) a significant proportion of energy dissipation (\u3e50%) takes place in the brush plates with the vibration and bending of bristles, (ii) turbulent kinetic energy seems to be considerably lower for a same dissipated power in brush fish pass than other conventional fish passes (vertical slot and nature type), (iii) the turbulence intensities in the lateral velocity component were an important contributor to turbulence, and (iv) when the bristles are submerged the maximum turbulent kinetic energy value is tripled. The flow is quasi-uniform and in subcritical regime which provides different migration corridors with typical hydraulic conditions and, very important for the fish, these corridors continue through the complete fish pass. The results from this study would be useful to fish-pass designers
