If the property is not yours then I think you are restricted to being able to do anything with it. Check with the property manager.

Can you build a wall around it and fill it with potting soil for a raised bed? Just using the rock for the drainage instead....

If the property is not yours, why not do your flower gardening in containers? You can take them with you should you move. Go for the plastic or resin/fiberglass type pots, they are a lot lighter than terracotta or clay. I bought a cheap plastic window box from Big Lots last Fall and grew planted pansies in it and they bloomed all winter and are still blooming. By June they will probably not be able to survive the hot summer and that's okay because they have given color during the winter months and this Fall, I will buy more pansies and the smaller violas..

Try to find a copy of Beth Chatto's Gravel Garden for inspiration.

Here is a link that might be useful: Beth Chatto's Gravel Garden