Adding Fuel to the Fire – When Wildfires and Flooding Interact

Natural disasters becoming more frequent and severe is our reality. From devastating wildfires engulfing the west coast to hurricanes battering the Gulf and east coasts, communities across the country face the profound impacts of these events. The consequences extend beyond immediate destruction, encompassing economic loss, displacement of populations and long-term environmental damage. As these disasters become more prevalent, the urgency for proactive measures in disaster preparedness, mitigation and climate resilience to safeguard lives and livelihoods grows.

The interaction between wildfires and flooding can be a reoccurring problem in prone communities. Wildfires can significantly increase the severity of flooding in burned watersheds, which can remain elevated for years until the watershed recovers. Additionally, wildfires can increase the risk of mud and debris flows. This cycle may repeat over time, and mitigation efforts are particularly beneficial for protecting communities immediately and in the future.

One such community is Montecito, California. Historic records show a repetitive cycle of fires in the steep mountains above the community, followed by flooding and debris flows in the developed area downstream. The most recent event was the Thomas Fire which occurred in late 2017. While the fire was still burning, an intense rainstorm triggered a massive debris flow causing 23 fatalities, numerous injuries, hundreds of damaged or destroyed structures, and more than 1 million cubic yards of rock, woody debris and soil deposited in the community.

The community was eager to recover from the disasters. However, the disaster changed landscape and existing risk models were now inaccurate. Performing detailed risk studies can take time, but delaying the recovery process was not a favorable option. It was an opportunity to leverage new technology to quickly assist the community in rebuilding with a focus on resilience.

New technology

Shortly after the 2018 disaster, FEMA, the Federal Emergency Management Agency initiated a hazard identification project for the community. We were a key team member to perform the work. The goal was to provide expedient information for the County to use during the recovery and rebuilding process. Recent advances in computing power, data and modeling provided a unique opportunity for enhanced risk evaluation.

The team utilized technology that considered movement of water and debris through the landscape in multiple directions on a gridded basis. Significant data was obtained, including detailed ground surveys performed soon after the disaster using airborne LiDAR, with deposition and damage measured by field crews. The information allowed the engineering team to create and validate models with enhanced detail to aid the rebuilding process.

The team performed computations using HEC-RAS flood routing software from the US Army Corps of Engineers, USACE, that included 2D modeling. The model simulates flow over the terrain and allows water to move in multiple directions. The ability to perform this type and scale of 2D modeling is a relatively recent advancement in the industry. While the underlying equations governing the modeling are not new, the models require fast computing speeds and gigabytes of available data storage space which has been widely available only since the early 2000s. This modeling advancement allowed for more accurate risk and rebuilding identification.

The model was used to create data and maps of potential flood inundation, water surface elevations, depths, and velocities of water and muddy water on the post-burn, post-disaster landscape conditions. These “recovery maps,” considered multiple hazards, including intense storms, flooding, sediment accumulation and debris flows.

Recovery Mapping as a tool for resilience

The community immediately recognized the value of the data and maps. The County officially adopted the data as the basis for rebuilding, with structure reconstruction to include protective measures such as elevation of structures and structural enhancements. Recognizing that the wildfire/flood cycle may repeat, the result is that rebuilt structures have improved resiliency against potential future events.

The value of the recovery maps goes beyond rebuilding. Since their creation, the data has been used by emergency personnel to develop disaster evacuation plans and determine areas with increased hazards. Meteorologists may use the data to help storm forecasting. Public works and transportation officials can assess infrastructure such as roads, culverts and bridges for risk. And local residents can know their risks where they live and work. The County has also leveraged the data to develop a comprehensive flood mitigation master plan for the community to provide additional resilience.

Now and in the future

On the 5-year anniversary of the 2018 debris flow, another intense storm happened in the community. While not as intense as the 2018 storm and with more upland vegetation, debris movement and flooding occurred in the watershed, but on a lesser scale. The predictions of the model matched the event and validated the efficacy of the data.  The community was prepared to respond and recover.

While the vegetation in the watershed has seen much regrowth since 2018, it may take many more years for full recovery. Until the next fire…

Want more?

Listen to the Built + Beyond podcast Floods + Fire – Mitigating Risk in Montecito episode as our experts discuss the growing urgency for proactive measures in disaster preparedness, mitigation and climate resilience to safeguard lives and livelihoods.