PACA-TRENDS

The PACA-TRENDS project investigates forest fires and flooding hazards in South-East France.

Forest fires depend on both weather conditions and housing densities in the Wildland Urban Interface (WUI). It is commonly expected that forest fire hazard will increase in the Euro-Mediterranean zone with warmer weather conditions and WUI expansion. However, previous studies have shown that the WUI-fire related risk has remained unexpectedly stable in South-East of France since about 1990. This is due to an increase in housing density over time: progressively, high risk isolated and scattered housing has evolved into denser, lower risk communities. In addition, firefighting efforts have had a major positive impact on fire occurrence despite warmer and drier summers. Big fires do still occur, however, in conditions of high winds, as was the case in 2003 and again in 2017. Although wind speed is of critical importance for the occurrence of large fires, nothing is known about how wind speeds have been evolving with climate change in South-East France. We thus tackle this question here. Daily wind speeds were obtained for the months of July and August for 5 weather stations located in the PACA region of France between 1960 and 2017. Wind speeds were classified into 3 categories: Low (<5 m/s), Moderate (5-10 m/s), and High (>10 m/s). We show that the number of days with High and Moderate wind conditions has decreased over time in SE France since 1960. Our results thus suggest that even though global warming might lead to more frequent fires overall, the number of very large fires in South-East France might not increase as dramatically due to fewer windy days in summer.

The recent catastrophic floods in Draguignan (2010) and Cannes-Biot (2015) have once again raised the question of the role of urban planning in flood severity. As natural areas are converted to impervious surfaces, infiltration of rainwater into the soil diminishes and runoff increases, thereby potentially contributing to a greater flood risk. To examine this common hypothesis, we studied 2 catchments in South-East France (Frayère and Mourachonne). Building extent and imperviousness were mapped for each catchment for 3 dates: 1988, 1999, and 2014. Runoff simulations were performed using the HEC-HMS hydrologic model. We found that changes in imperviousness over the 26-year interval increased storm runoff only marginally (about 5%). However, the built area in the flood plain increased substantially, almost twice as fast as in the catchments as a whole. We thus conclude that urban development in the catchments had little impact on flooding hazard but exposed far more buildings and people in the flood plain to potential damage, injury and loss of life.