A synonym for ash heaps could be ash piles or ash mounds, as these terms still convey the same idea of a large accumulation of ash. Another option might be ash dunes, which might better describe the undulating shape of the ash heaps. Ash banks or ash berms might also work, as they give the impression of a raised area of ash. Ash hillocks or ash knolls could be used to describe smaller mounds of ash, while ash rises or ash elevations could describe larger formations. Ultimately, there are many synonyms for ash heaps that could be used depending on the context and the particular image the writer wants to convey.