Informing zoning ordinance decision-making with the aid of probabilistic debris flow modeling

Globally, population and infrastructure continue to grow in the Wildland Urban Interface (WUI) despite the recognition of wildfire and secondary geohazards (i.e.- landslides, debris flows, flooding). In urban and rural settings, expansion into the WUI is the result of a need for space, the subsequent development of cheap marginalized land or, paradoxically, the development of expensive and highly desirable land. As development continues, updated zoning ordinances that reflect the dynamic environment can reduce the risks associated with wildfire and debris flow hazards. Here, we focus on the growing issue of debris flows in the USA. We integrate two case studies from our recent work to illustrate a new approach to developing zoning ordinance boundaries that can reduce debris flow hazards to communities and infrastructure.

The case studies use historical data, field observations, and debris flow modeling to provide new information on the hazard in space and time that can be used to define development or prospective development areas prone to debris flows in the WUI. The results provide definitive data for establishing zoning ordinances where debris flow hazards exist, and data that is useful to mitigate these hazards. These diverse case studies show the broader applicability of this approach in locations where the WUI encroaches on steep, fire- and debris flow-prone terrain is highlighted in the case studies. Results and recommendations from this work can ultimately lead to the development of safer and more resilient communities.