In environmental engineering, flood mitigation involves the management and control of flood water movement, such as redirecting flood run-off through the use of floodwalls and flood gates, rather than trying to prevent floods altogether. It also involves the management of people, through measures such as evacuation and dry/wet proofing properties. The prevention and mitigation of flooding can be studied on three levels: on individual properties, small communities, and whole towns or cities. The costs of protection rise as more people and property are protected. The FEMA, for example, estimates that for every $1.00 spent on mitigation, $4.00 is saved.