Wildfires, Floods and Sediment Delivery at the Wilderness-Urban Interface in coastal Southern California

Clifford Riebe
University of Wyoming

Description

Wildfires pose recurrent, news-making threats to life, property and livelihood throughout the arid American West. They often also contribute substantially to hillslope erosion and flooding by increasing runoff and destabilizing slopes against natural erosive processes such as dry ravel, rainsplash, sheetwash, gullying and shallow landsliding.

Wildfires and their effects on erosion have exacted a devastating toll in coastal Southern California, thanks in part to a decades-long history of aggressive urbanization and fire suppression. This has placed lives and structures in increasingly close proximity to abundant natural fuels and steep slopes that become highly unstable when protective vegetation is burned away.

In coastal southern California, the risks posed by urbanization and fire suppression are further exacerbated by the timing of the hot, dry "Santa Ana" winds, which originate katabatically in highlands to the north-northeast between August and November, when the region's pervasive, highly flammable chaparral vegetation is at its driest. The Santa Ana winds ensure that fires are both more likely to start and more difficult to control when they do.

Chaparral seeds are fire-resistant and germinate into highly flammable bushes that restore pre-fire fuel conditions in as little as 30 years, making frequent fire-recurrence likely. Organic compounds within the chaparral itself may enhance post-fire erosion. In hot fires, organic compounds in burning vegetation within soils can literally vaporize and then condense at a cooler depth to form a water-repellent, or hydrophobic, layer. Hydrophobicity is thought to be largely responsible for the characteristic post-fire development of dense networks of narrow channels, or rills, on southern California hillslopes; the rills appear to form when soil overlying the hydrophopic layer becomes saturated and unstable to downslope transport.

Increased runoff from fire-related rills tends to promote downstream mobilization of any sediment that has accumulated in channels during the dry season by raveling and other processes. In addition to affecting runoff, fires can also promote erosion from hillslopes in several ways. For example, slopes that burn in the fall are often prone to enhanced erosion in the winter wet season because they are devoid of protective vegetation when the first intense rains arrive. On steep slopes, vegetation can form organic dams, effectively retaining sediment that originates upslope. When fire incinerates this vegetation, sediment that was impounded behind it may be quickly mobilized downslope by dry ravel and overland flow. Newly burned surfaces are also prone to efficient erosion by rain impact. Fires can also accelerate erosion by causing inter-particle fusion, which makes soils coarser, and thus increases their vulnerability to raveling. Fires can also accelerate erosion by changing soil permeability. Field studies in California chaparral woodlands indicate that fires can lead to ten-to hundred-fold increases in sediment transport rates, due mostly to increases in sediment delivery along post-fire rills and increases in dry raveling rates, both during and immediately after fires.

The increase in runoff and destabilization of slopes after fires has been dubbed the "fire-flood" sequence by the US Forest Service (check citation). The convergence of intense urbanization and fire suppression, together with regional factors, such as the Mediterranean climate, the Santa Ana winds, and the prevalence of chaparral vegetation and steep slopes, all contribute to making the erosive destructiveness of southern California's "fire-flood" sequence intense in comparison with the post-fire responses that have been observed in other wildfire-prone landscapes.

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