Legal Definitions - common-enemy doctrine

The common-enemy doctrine is a legal principle that addresses how landowners can manage unwanted surface water, such as rainwater runoff or floodwaters. Under this doctrine, a property owner is generally permitted to take reasonable steps to protect their land from surface water, even if those actions inadvertently cause the water to flow onto or accumulate on a neighboring property. The underlying idea is that uncontrolled surface water is considered a "common enemy" that every landowner has the right to defend against, without being held liable for the resulting impact on others.

Here are some examples illustrating the common-enemy doctrine:

• Residential Property Protection: Imagine a sudden, intense thunderstorm causes significant surface water to accumulate rapidly in a suburban neighborhood. Mr. Chen, fearing his basement will flood, quickly digs a shallow trench along the edge of his property to divert the rushing rainwater away from his house and towards the street.

  Illustration: This action successfully protects Mr. Chen's home, but the diverted water flows directly onto Ms. Davis's adjacent lawn, causing a larger puddle than would have otherwise formed there. Under the common-enemy doctrine, Mr. Chen would likely not be held responsible for the increased water on Ms. Davis's property, as he was taking necessary steps to protect his own land from the "common enemy" of floodwater.

• Agricultural Land Defense: A prolonged period of heavy rainfall causes a nearby creek to swell and threaten to overflow its banks, potentially inundating Farmer Rodriguez's fields of newly planted corn. To prevent crop destruction, Farmer Rodriguez uses a small tractor to quickly build a temporary earthen berm along the creek bank bordering his property.

  Illustration: While the berm successfully protects Farmer Rodriguez's corn, it causes the overflowing creek water to be redirected and accumulate more heavily on Farmer Lee's adjacent pastureland, which is at a slightly lower elevation. The common-enemy doctrine would likely shield Farmer Rodriguez from liability for the increased flooding on Farmer Lee's land, as he was acting to protect his crops from the encroaching surface water.

• Commercial Property Flood Barrier: A severe flash flood event is predicted for a downtown area. The owner of "The Book Nook," a bookstore located on a street prone to flooding, quickly installs sandbags and a temporary flood barrier across the front entrance to protect her inventory.

  Illustration: These protective measures effectively keep water out of "The Book Nook." However, the displaced floodwaters flow more intensely down the sidewalk and pool significantly in front of "The Coffee Bean," a neighboring café, causing minor damage to their outdoor seating area. According to the common-enemy doctrine, the bookstore owner would likely not be liable for the increased water at "The Coffee Bean," as her actions were a reasonable defense against the "common enemy" of the flood.