Sand filter washing
On the old installations, there was what is called a high flow valve.
Let me explain:
At pump discharge (without VSD), there is a discharge pipe that is equipped with an automatic valve.
On either side of it is a smaller diameter bypass pipe.
For small flow washing, the valve is closed and the flow is constrained by the bypass section. During high flow washing, the valve is opened, and the larger section of the line allows a greater flow to pass through.
No control valve or flow meter is required, except if you need to compensate for the drop in flow due to the drop in the level of the washing water holding tank.
Hope this helps.