How to backflush cold water pipes using gravity pressure

[Alchemizt]

My pipes are filled with sand and the water pressure is really low. Can I backflush the pipes by running a garden hose from the water tower into the toilets, showers and sinks (their inlets)? If I cut the pipe that feeds the building, and run water backwards through the inlets, will this flush the sand out?

I need to mention that this is all gravity pressure from the water tower, so the water would run down from the tower, then need to come back up to be flushed out