**5. Discussion**

the flooded area, with profound geomorphologic consequences. The force of the water de‐ molished a bridge which a tourist was crossing at the time, leading to his death. A car park

**Figure 10.** Simulation of the maximum peak discharge in the 16th june (above) and in the 14th July (below). Compara‐

In fact, intense rainfall increases the erosive power of overland flow, resulting in deeply in‐ cised channels, such as rills and gullies (figure 13), and accelerates the removal of material from hill slopes. Increased runoff can also erode significant volumes of material from chan‐ nels. The net result of rainfall on burned basins is the transport and deposition of large vol‐ umes of sediment, both within and downstream of the burned areas. The following

tive analysis.

76 Research on Soil Erosion Soil Erosion

was partially destroyed by the water, causing a landslide, as can be seen in figure 12.

Wildfire is an important, and sometimes the most important, driving force behind landscape degradation in the Mediterranean region (e.g. Naveh, 1975; Andreu et al., 2001; Dimitrakopou‐ los and Seilopoulos, 2002; Alloza and Vallejo, 2006; Mayor et al., 2007). In fact, wildfire can have profound effects on a watershed. Burned catchments are at increased hydrological risk and respond faster to rainfall than unburned catchments (Meyer et al. 1995; Cannon et al. 1998; Ferreira et al. 2008; Stoof, 2012). Therefore, flooding and soil erosion also represent some of the most significant off-site impacts of wildfires, causing serious damage to public infrastructures and private property, as well as increased psychological stress for the affected population.

Wildfire alters the hydrological response of watersheds, including the peak discharge result‐ ing from subsequent rainfall.

Peak discharge is also directly related to flood damage, and it is therefore important to un‐ derstand the relationship between rainfall and peak discharge. The analysis of rainfall-run‐ off relations suggests that in the case of burned watersheds a rainfall intensity threshold exists, implying a critical change in the behaviour of the hydrological response. This thresh‐ old has been estimated at around 10 mm h\_1 (Krammes & Rice, 1963; Doehring, 1968; Mack‐ ay and Cornish, 1982; Moody and Martin, 2001). One of the main reasons for the existence of a critical threshold intensity could be the hill slope infiltration rate. Infiltration rates have been shown to decrease by a factor of two to seven after wildfires (Cerdà, 1998; Martin & Moody, 2001), meaning that post-fire rainfall intensities that exceed this infiltration rate and cause runoff may be lower than the pre-fire intensities required to produce a comparable runoff. Below approximately 10 mm h\_1 the rainfall intensity may be below the average wa‐ tershed infiltration rate, meaning that most of the rainfall infiltrates, with some transient runoff (Ronan, 1986) and some subsurface flow, which may either cause quickflow (Hewlett and Hibbert, 1967) in the channel or a lagged response. Above 10 mm h\_1 the rainfall intensi‐ ty may exceed the average watershed infiltration rate, so that the runoff is dominated by sheet flow, which produces flash floods. As an example, Martin and Moody (2001), consider if the rainfall intensity is 20 mm h\_1, the unit-area peak discharge response would be 27 times greater than the response if the rainfall-runoff relation had not exceeded the 10 mm h\_1 threshold. The same authors consider that if the rainfall intensity is 55 mm h\_1 the response will be 700 times greater.

The consumption of the rainfall-intercepting canopy and soil-mantling litter and duff, inten‐ sive drying up of the soil, combustion of soil-binding organic matter, and enhancement or formation of water-repellent soils are factors that reduce rainfall infiltration into the soil and significantly increase overland flow and runoff in channels. The removal of obstructions to flow, such as live and downed timber and plant stems, due to wildfire can increase the ero‐ sive power of the overland flow, accelerating the removal of material from hill slopes. In‐ creased runoff can also erode significant amounts of material from channels. The net result of rainfall on burned basins is often the transport and deposition of large volumes of sedi‐ ment, both within and downstream of the burned areas (Cannon et al., 2008; Cannon 2005).

**Figure 11.** Profile and different cross-sections of the Piodão river upstream of the village of Piodão and normal and flooded area in the event of 14th July.

**Figure 13.** Rills and gullies erosion as a consequence of the intense rainfall.

Post-fire debris flows are generally triggered by one of two processes: surface erosion caused by rainfall runoff, and landslides caused by the infiltration of rainfall into the ground. Runoffdominated processes are by far the most common, since fires usually reduce the infiltration ca‐ pacity of soils, which increases runoff and erosion. Infiltration processes are much less common, but prolonged heavy rain may increase soil moisture even after a wildfire. The wet

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79

According to (Johnson, 2005), although debris flows can occur in areas lying on almost any rock type, the areas most likely to produce debris flows are those lying on sedimentary or metamorphic rocks with more than around 65% of the area moderately or severely burned. In addition, debris flows are most frequently produced from steep (> 20 ), tightly confined drainage basins with an abundance of accumulated material, and are unlikely to extend be‐

soil may then collapse, producing infiltration-triggered landslides (Johnson, 2005).

**Figure 12.** A car park partially destroyed by the water, causing a landslide.

**Figure 13.** Rills and gullies erosion as a consequence of the intense rainfall.

significantly increase overland flow and runoff in channels. The removal of obstructions to flow, such as live and downed timber and plant stems, due to wildfire can increase the ero‐ sive power of the overland flow, accelerating the removal of material from hill slopes. In‐ creased runoff can also erode significant amounts of material from channels. The net result of rainfall on burned basins is often the transport and deposition of large volumes of sedi‐ ment, both within and downstream of the burned areas (Cannon et al., 2008; Cannon 2005).

**Figure 11.** Profile and different cross-sections of the Piodão river upstream of the village of Piodão and normal and

flooded area in the event of 14th July.

78 Research on Soil Erosion Soil Erosion

**Figure 12.** A car park partially destroyed by the water, causing a landslide.

Post-fire debris flows are generally triggered by one of two processes: surface erosion caused by rainfall runoff, and landslides caused by the infiltration of rainfall into the ground. Runoffdominated processes are by far the most common, since fires usually reduce the infiltration ca‐ pacity of soils, which increases runoff and erosion. Infiltration processes are much less common, but prolonged heavy rain may increase soil moisture even after a wildfire. The wet soil may then collapse, producing infiltration-triggered landslides (Johnson, 2005).

According to (Johnson, 2005), although debris flows can occur in areas lying on almost any rock type, the areas most likely to produce debris flows are those lying on sedimentary or metamorphic rocks with more than around 65% of the area moderately or severely burned. In addition, debris flows are most frequently produced from steep (> 20 ), tightly confined drainage basins with an abundance of accumulated material, and are unlikely to extend be‐ 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.

Despite the fact that the events studied occurred one year after the wildfires, it would be ex‐ pected that the stream flow and erosion response would be much lower after vegetation regrowth and the removal of some of the sediment by relatively smaller storms in the following autumn and winter. Nevertheless, post-fire threshold conditions change over time even though the sediment supplies are depleted and the vegetation recovers, and the net re‐ sult of intense rainfall on these burned basins was flash flooding in several areas and the transport and deposition of large volumes of sediment, both within and downstream of the burned areas. DeBano (2000) and Loaiciga (2001) consider that wildfires increase the magni‐ tude of runoff and erosion and alter the hydrological response of watersheds resulting from subsequent rainfall, creating a risk for downstream communities that lasts for 1-3 years after a fire. Several other authors (Rowe et al., 1954; Doehring, 1968; Scott and Williams, 1978; Wells et al., 1979; Helvey, 1980; Robichaud et al., 2000) extend the "window of disturbance"

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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‐

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

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‐

age to infrastructures and buildings and one human life was lost.

a longer (7-year) period of study (Shakesby, 2011).

to a much longer period of 3–10 years.

**6. Conclusion**

**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.

Despite the fact that the events studied occurred one year after the wildfires, it would be ex‐ pected that the stream flow and erosion response would be much lower after vegetation regrowth and the removal of some of the sediment by relatively smaller storms in the following autumn and winter. Nevertheless, post-fire threshold conditions change over time even though the sediment supplies are depleted and the vegetation recovers, and the net re‐ sult of intense rainfall on these burned basins was flash flooding in several areas and the transport and deposition of large volumes of sediment, both within and downstream of the burned areas. DeBano (2000) and Loaiciga (2001) consider that wildfires increase the magni‐ tude of runoff and erosion and alter the hydrological response of watersheds resulting from subsequent rainfall, creating a risk for downstream communities that lasts for 1-3 years after a fire. Several other authors (Rowe et al., 1954; Doehring, 1968; Scott and Williams, 1978; Wells et al., 1979; Helvey, 1980; Robichaud et al., 2000) extend the "window of disturbance" to a much longer period of 3–10 years.
