'No Adverse Impact' floodplain management

Missoula County manages the regulated flood hazard under a principle that the actions of one property owner should not be allowed to adversely affect the rights of other members of the community. The adverse impacts of unwise development decisions can be measured by:

• increased flood levels
• higher flood velocities
• increased erosion and sedimentation
• deterioration of natural floodplain functions
• increased risk for emergency response personnel
• higher infrastructure maintenance costs
• other measurable adverse impacts to a community’s well-being

A concrete example of an adverse impact would be a homeowner in the floodplain having an unsecured structure that floats away in a flood and damages a public bridge downstream, impacting their neighbors’ ability to evacuate.

The approach of No Adverse Impact floodplain stewardship protects the rights of property owners and protects the natural systems that provide flood mitigation benefits to the community, such as slowing runoff and storing floodwater. Implementing strategies for no adverse impacts does not mean that no development can take place in the floodplain. It means that adverse impacts generated by land use activities are identified and mitigated on a case-by-case basis. This principle gives communities a way to promote responsible development measures through community-based decision making.