Swales are shaped grassed or planted ditches or berms that filter out contaminants carried in stormwater. The grass or plants also slows down stormwater flow and provides temporary storage during heavy rainstorms. Swales also play an important role improving stormwater quality. 

Parking vehicles over swales should be avoided as this causes rutting and disturbs the flow of stormwater through the swale. Like all stormwater quality devices that trap contaminants, the material that accumulates over many years needs to be removed. This means that at some stage the swale should be dug out, the contaminated earth removed to a safe site and the swale re-established again.