**6. Conclusion**

yond the mouths of basins larger than about 25 square kilometres (Johnson, 2005). The nu‐ merous instances of debris flows found in the study area suggest that the bedrock must

have been highly fractured and weathered in order to be transported by the flow.

80 Research on Soil Erosion Soil Erosion

**Figure 14.** The powerful capacity to transport materials along the main channel during the event of 14th July.

**Figure 15.** A trout pond is crammed with material transported by the flood.

The hydrogeomorphic consequences of the 2006 events were identified during the field sur‐ vey and it was found that there were widespread effects in the valleys of the watershed as well as in the main river channel and tributaries. In the Piodão and Pomares river basins, there were many instances of bed lowering, channel widening, avulsion and deposition. In several valleys there were flood marks, shallow landslides, slope failures and erosion gullies due to the intense rainstorm registered in both events. There were several instances of dam‐ age to infrastructures and buildings and one human life was lost.

Fires, floods and intensive erosion are a regular part of the landscape in mountainous re‐ gions around the world (Tryhorn et al., 2007) and are particularly significant in the Mediter‐ ranean basin, where forest fires have been increasing (JRC, 2005) and the climate is characterised by intense rainfall as a consequence of strong cyclogenesis (Kostopoulou, 2003). However, this intense rainfall has also been associated with factors other than cyclo‐ genesis. Estrela et al. (2000) show that orographically induced thunderstorms caused by the Iberian thermal low can produce large volumes of precipitation. Post-fire floods may be as‐ sociated with several different meteorological mechanisms and may either occur immediate‐ ly after the fire or be delayed by several weeks or even years. Delayed floods are more likely to be caused by surface modifications that reduce infiltration, with precipitation due either to a large-scale drought break or localised thunderstorms. In combination, these processes can create a greater potential for severe flooding and intense erosive processes. A single in‐ tense rainstorm can generate peak flows which produce 75% of the sediment eroded during a longer (7-year) period of study (Shakesby, 2011).

In Portugal, several mountain areas have been affected by flash floods and landslides after forest fires. As an example, about 2 decades previously a major fire, which occurred in Sep‐ tember 1987 and burnt an area of 10900 ha, affected most of the Pomares and Piodão basin area (Lourenço, 1988; 2006a b). A storm with similar characteristics to those in this study oc‐ curred in 2006, generating flash floods and severe erosion. Lourenço (1994) also studied a landslide which occurred in the Serra da Estrela mountains (in granitic lithology), after a se‐ vere rainfall event in October 1993, in an area burnt in August 1991. In the northern region of Portugal, Pedrosa et al. (2001) also studied a landslide that destroyed the great part of vil‐ lage of Frades (Arcos de Valdevez). This landslide also occurred in a granitic soil and was linked with a fire that occurred a few months before and destroyed the plant cover.

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There is therefore a need to develop tools and methods to identify and quantify the potential hazards posed by flash floods and landslides generated by burned watersheds. An analysis of data collected from studies of flash flooding and debris flows following wildfires can an‐ swer many of the questions that are fundamental to post-fire hazard assessment—what and why, where, when, how big, and how often?

In fact, it is necessary to improve predictions of the magnitude and recurrence of the flood‐ ing that follows wildfires, due to the increased human population at risk in the wildland– urban interface. By understanding the magnitude of the runoff response and the erosion and deposition responses of recent wildfires, we can minimise loss of life and damage to proper‐ ty and provide data for landscape evolution in areas prone to wildfire. Moreover, water‐ shed-scale predictions of erosion and deposition caused by these natural disasters can be used by land managers to prioritise forestry measures based on the erosion potential before and after wildfires.
