## 2 Methodology
### Experimental Setup  
Data obtained for this research are collected from the series of flow simulations on the physical fishway model built in the Department of Hydroscience and Engineering laboratory under the University of Zagreb. Experimental setup included an adaptive fishway geometry adjusted for two technical fishway types (pool-orifice and vertical slot fishway) in order to simulate flow conditions under varying flow boundary conditions, bottom slope, pool length and dimension of the orifices (for the pool-orifice type) or slots (for vertical slot type). Fishway geometry was designed according to the physical design standards [54, 55] and state-of-the-art literature [20, 56, 57, 58]. Both fishway types considered in this research have been built with a matching geometry: bottom slope, pool width, and pool length. The simulations were made with three different bottom slopes (7.5%, 10%, and 12.5%), three different pool lengths (45 cm, 60 cm, and 90 cm) and a constant pool width (45 cm). For a pool-orifice-type fishway, three different orifice dimensions (8 cm \(\times\) 8 cm, 10 cm \(\times\) 10 cm, 12 cm \(\times\) 12 cm) were tested in an offset configuration, alternating positions in the cross-walls from side to side in the pools. Orifice size was selected using Larniers [44] guidelines for migratory salmonids and cyprinids, considering their widespread population in Croatian rivers [59]. Taking into account all geometry variations, 27 different physical model setups combining the bottom slope, pool length, and orifice size were investigated. For a vertical slot fishway, 9 different physical model setups were investigated, combining the bottom slope and pool length. Vertical slots were aligned along the right-hand side of the fishway, formed by an L-shaped baffle protruding from the cross-wall into the upstream pool.  
The fishway physical model was built in the flume according to the schematic below (Figure 2) in an 18 m long, 0.9 m wide, and 0.9 m deep recirculating hydraulic flume with a rectangular cross-section. The working section of the flume is 13 m long, 0.9 m wide, and 0.7 m deep (Figure 3). Flow rate in the flume is controlled by a frequency-regulated pump which has a maximum capacity of \(\sim\)30 l/s, while a hinged flap gate controls the water level at the flume outlet. A flow rectification system was built on the flume inlet to streamflow the jet from the pump into the working section of the flume. The fishway physical model was built in a 1:3 scale to the prototype, according to the Froud similarity principle [60]. The fishway model was built with at least three pools for the tests, depending on the pool length: a head pool that is receiving the water from an upstream reservoir, at least one typical pool for the measurements and a tail pool. Velocity measurements were collected in the typical pool of each configuration, positioned in the middle section of the fishway where the hydraulic balance was attained, unaffected by the upstream and downstream boundary conditions. For the fishway configurations with pool lengths of 45 cm and 60 cm, the third pool from the upstream was selected as the representative, while the second pool from the upstream was selected for the configuration with a pool length of 90 cm.