Sometimes, the storm hits years after a development is complete. Indeed, stormwater runoff can give rise to liability against a developer long after stormwater management facilities are constructed according to an approved final plan. Accordingly, it is important for developers to understand (1) how trespass is applied in the stormwater context and (2) that they might never be fully insulated from such claims.
Pennsylvania follows the “common-enemy rule” with respect to stormwater runoff. Under the Rule, an “upper” landowner may discharge its stormwater on the land of a “lower” landowner. However, water artificially diverted from its natural channel across a “lower” landowner’s property can be considered a trespass. In that case,