Our scheme is called run of river, which means we are using the river as it is, not diverting it, or adding a dam to create a reservoir. The same amount of water will flow as before, except that some of the water that would have gone through the weir goes through the screws instead.
So the amount of power we can generate depends on how much water is flowing, and that varies widely. We have to decide how much water our generator should be designed to take. Make it too big and most of the time it will be running well below capacity.
Another limit is that we cannot take all the flow. Some of it is needed to keep the other waterways alive, such as the Swift Ditch, and there would be some environmental concerns if the water flowing over the weir were reduced too much.
Another factor that affects the design is that as the river flow increases, the downstream water level rises, so the head decreases. Then even if the generator is taking the maximum amount of water, less power is generated. We have to take that into account when calculating how much energy we can generate.