Coanda screens have the great advantage of being passively self-cleaning, because as debris builds up on the upper surface of the screen it is washed downstream by the water flowing over the screen.
As well, the efficiency of these wheels is highest when the speed of the movement of the cups is half of the speed of the water jet. These screens are built into the main channel so that the water flows over them, as shown in Figure You can read more about hydropower in our Hydro Learning Centre.
For this reason it is normally made from tungsten carbide which is incredibly hard and able to resist erosion, and even then is designed to be easily replaceable during servicing. In the process, the water jet's momentum is transferred to the wheel and hence to a turbine.
Note that this Turgo turbine is using a fixed jet as opposed to a spear-jet, so it will only operate on a fixed flow rate. Maximum power and efficiency are achieved when the velocity of the water jet is twice the velocity of the rotating buckets.