What should we do about graves collapsing due to heavy rains, and how can we prevent this from recurring? Is it permissible to place plastic over graves?

Building structures over graves is forbidden. It is not prescribed to raise graves more than a handspan. It is sunnah for graves to be humped. If there is fear for the graves from floods, demolition, or being dug up by wild animals, it is permissible to put something to protect them as needed, such as placing a fence of bricks and cement no higher than a handspan. As for secure graves, nothing should be placed on them except for the humped earth and some pebbles.