**4. Results**

#### **4.1. The event of 16th June 2006**

A rain gauge installed in the Piodão basin registered high levels of precipitation roughly one year after the July 2005 wildfire for two main events on 16th June and 14th July 2006. Figure 2 shows the 24 hour precipitation registered by the rain gauge during the month of June, total‐ ling 58mm, distributed over 5 days (Figure 2).

**Figure 2.** Daily distribution of rainfall in June and bi-hourly distribution on 16th June.

However, around 50% of the total rainfall was concentrated on 16th June. A more detailed analysis of the hourly distribution of rainfall on that day shows that 22 mm were recorded between 5 pm and 6 pm.

The post-wildfire hydrological and erosional research benefited from the cooperation of lo‐ cal authorities and organisations that knew the area and had information about the catch‐ ment and the event. They provided useful information on the rainfall runoff processes (observation of surface runoff, origin of the runoff) and the local flow characteristics (type of flow – i.e. flood water, hyperconcentrated or debris flow, the presence of woody debris in the flow, approximate surface water flow velocities, blockages formed during the flood and their possible breakup, time and the effect of the collapse of bridges or dykes). The local au‐ thorities also provided important information on previous floods, which was relevant in as‐

After compiling the information using a Geographical Information System (GIS), detailed information was produced (mainly in the form of maps) which identified the areas heavily affected by water erosion (splash, rill and gully erosion) and sedimentation, as well as the

A rain gauge installed in the Piodão basin registered high levels of precipitation roughly one year after the July 2005 wildfire for two main events on 16th June and 14th July 2006. Figure 2 shows the 24 hour precipitation registered by the rain gauge during the month of June, total‐

sessing the return period of the flood.

areas affected by flash floods.

**4.1. The event of 16th June 2006**

ling 58mm, distributed over 5 days (Figure 2).

**Figure 2.** Daily distribution of rainfall in June and bi-hourly distribution on 16th June.

**4. Results**

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This event was caused by a high altitude cyclone in the southwest of the Iberian Peninsula which affected the weather in the Portuguese mainland during this period. In mid-latitudes, a 'cyclone' refers to the low pressure centres formed by baroclinic instability, with a typical scale in the order of 1000 km. However, cyclones or cyclonic centres also include any kind of surface depression, even small, weak, shallow low centres of orographic or thermal origin.

Following the high concentration of precipitation recorded on 16th June, several areas in both basins were affected by flash floods, soil erosion and sedimentation processes. Figure 3 sum‐ marises the areas worst affected by these processes.

**Figure 3.** Effects of the intense rainfall after the wildfires.1. Area of the basin not affected by the wildfire of 2005; 2. Areas worst affected by the intense rainfall; Piscina fluvial="river beaches".

The figures 4 and 5 confirm the super-elevation of the flow at the Pomares Bridge (in the Pomares river basin) as well as the flooding of the right bank of the river. In fact, the stream flow created a 2.5 meter waterfront, although the floodgates were open. The impossibility of draining off the volume of water that had accumulated during the intense rainfall, as well as the power of the runoff and stream flow to transport materials obstructed the flow of the water and enlarged the flood area. Figure 5 simulates the peak discharge level and shows the tonnes of material, mainly branches of trees and shrubs, carried downstream, which cre‐ ated a blockage at the bridge.

ble, particularly affecting the Piodão, Foz da Égua and Vide river beaches, where flash

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**Figure 6.** Simulation of the peak bank flood at Soita da Ruiva in the Pomares basin.

**Figure 7.** Deposition of sediment at Soito da Ruiva, in the Pomares basin.

During reconnaissance of the watersheds, widespread geomorphological consequences of the event were identified. In fact, high volume discharges have great erosional energy and the natural and man-made structures (dykes and bridges) along the rivers created obstacles to the transport of sediment and led to deposition throughout the main river channel and

floods were recorded (Figure 3).

**Figure 4.** The super-elevation of the flow at Pomares Bridge and the flooding of the right bank of the river.

**Figure 5.** Simulation of maximum peak discharge and the blocked organic and sediment debris (Pomares Bridge).

Upstream, at the of Sobral Magro and Soito da Ruiva river beaches the flood marks were also evident, as can be seen in figures and 6, 7 and 8. At Soito da Ruiva, the stream over‐ flowed on both banks (Figure 6). In the Piodão basin the hydrological effects were also visi‐ ble, particularly affecting the Piodão, Foz da Égua and Vide river beaches, where flash floods were recorded (Figure 3).

**Figure 6.** Simulation of the peak bank flood at Soita da Ruiva in the Pomares basin.

the tonnes of material, mainly branches of trees and shrubs, carried downstream, which cre‐

**Figure 4.** The super-elevation of the flow at Pomares Bridge and the flooding of the right bank of the river.

**Figure 5.** Simulation of maximum peak discharge and the blocked organic and sediment debris (Pomares Bridge).

Upstream, at the of Sobral Magro and Soito da Ruiva river beaches the flood marks were also evident, as can be seen in figures and 6, 7 and 8. At Soito da Ruiva, the stream over‐ flowed on both banks (Figure 6). In the Piodão basin the hydrological effects were also visi‐

ated a blockage at the bridge.

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**Figure 7.** Deposition of sediment at Soito da Ruiva, in the Pomares basin.

During reconnaissance of the watersheds, widespread geomorphological consequences of the event were identified. In fact, high volume discharges have great erosional energy and the natural and man-made structures (dykes and bridges) along the rivers created obstacles to the transport of sediment and led to deposition throughout the main river channel and tributaries. However, the volume of off-site eroded sediment after a wildfire is difficult to assess because its response to rainstorms and runoff has different characteristics. The debris that was transported was mainly sediment from the thalwegs of tributaries that had been loosened by daily weathering and erosion, but could only be moved by large events.

According to Jarrett (2001), convective thunderstorms are known to have sharp rainfall gra‐ dients and rainfall intensities and vary in size, so that entire watersheds are not necessarily

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The natural consequence of these precipitation patterns, which are relatively common in this climate, is that neighbouring watersheds receive different amounts of rainfall and therefore respond differently to the event. In fact, this event was more localised in comparison with the event of 16th June, mainly affecting the headwaters of the Piodão stream. The heavy rain‐ fall significantly increased the amount of streamflow, resulting in a stronger and faster re‐ sponse and generating downstream floods and serious damage due to sediment transport. In addition to the substantial damage to human infrastructures, one death was recorded.

subjected to the same rainfall intensity.

**Figure 9.** Daily distribution of rainfall in July and bi-hourly distribution on 14th July.

was flooded to a depth of 1 metre.

Figure 10, provided by a local resident and showing the volume of accumulated water, dem‐ onstrate that the peak discharge was higher during this event than the previous one. The flood marks on the house used to estimate the peak discharge level show that the ground floor was not flooded in the 16th June event, whereas during this flash flood the building

The diagram in Figure 11 shows the longitudinal profile and different cross-sections of the Piodão river upstream of the village of Piodão, defines the stream bed and simulates the flood bed on the basis of flood marks, for the event of 14th July. Overall, the stream over‐ flowed its banks and doubled in size in comparison to the "normal" bed. Immediately up‐ stream of the village of Piodão, the flooded area was triple the size of the stream bed. This expansion of the flooded area was associated with a man-made structure designed as a channel for the bed stream. The inability to drain off the flow of water led to an increase in

Figures 7 and 8 show a plan of the debris flow deposition area caused by the inability of the drainage ditches to cope with the increased run-off generated in the upstream areas and the soil erosion, which led to flooding and the accumulation of large boulders and woody debris.

**Figure 8.** Wood accumulation following the wildfire at Sobral Magro, in the Pomares basin.

#### **4.2. The event of 14th July 2006**

In July, the precipitation was higher than the precipitation recorded in June, totalling 95 mm (Figure 9). This second event also registered very intensive rainfall. In fact, about 70mm fell in two days, on 13th and 14th July, registering 30mm and 39mm, respectively. The rainfall record‐ ed on 14th July was concentrated in one single event that occurred between 4 pm and 5 pm. The total precipitation in the first half hour was 14mm, followed by 24mm in the next 30 minutes.

According to the Portuguese Meteorological Services, the heavy rainfall in several areas of inland Portugal was associated with "high atmospheric instability" related to the formation of a thermal low in the interior of the Iberian Peninsula, typical of the summer months. The summer heat in the Iberian Peninsula causes the surface pressure low (Alonso et al., 1994). If the Iberian thermal low draws air from the Atlantic rather than Africa, incursive winds can become humid, conditions become unstable, and intense thunderstorms may occur (Linés, 1977), sometimes leading to torrential rain.

According to Jarrett (2001), convective thunderstorms are known to have sharp rainfall gra‐ dients and rainfall intensities and vary in size, so that entire watersheds are not necessarily subjected to the same rainfall intensity.

tributaries. However, the volume of off-site eroded sediment after a wildfire is difficult to assess because its response to rainstorms and runoff has different characteristics. The debris that was transported was mainly sediment from the thalwegs of tributaries that had been

Figures 7 and 8 show a plan of the debris flow deposition area caused by the inability of the drainage ditches to cope with the increased run-off generated in the upstream areas and the soil erosion, which led to flooding and the accumulation of large boulders and woody debris.

loosened by daily weathering and erosion, but could only be moved by large events.

**Figure 8.** Wood accumulation following the wildfire at Sobral Magro, in the Pomares basin.

In July, the precipitation was higher than the precipitation recorded in June, totalling 95 mm (Figure 9). This second event also registered very intensive rainfall. In fact, about 70mm fell in two days, on 13th and 14th July, registering 30mm and 39mm, respectively. The rainfall record‐ ed on 14th July was concentrated in one single event that occurred between 4 pm and 5 pm. The total precipitation in the first half hour was 14mm, followed by 24mm in the next 30 minutes.

According to the Portuguese Meteorological Services, the heavy rainfall in several areas of inland Portugal was associated with "high atmospheric instability" related to the formation of a thermal low in the interior of the Iberian Peninsula, typical of the summer months. The summer heat in the Iberian Peninsula causes the surface pressure low (Alonso et al., 1994). If the Iberian thermal low draws air from the Atlantic rather than Africa, incursive winds can become humid, conditions become unstable, and intense thunderstorms may occur (Linés,

**4.2. The event of 14th July 2006**

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1977), sometimes leading to torrential rain.

The natural consequence of these precipitation patterns, which are relatively common in this climate, is that neighbouring watersheds receive different amounts of rainfall and therefore respond differently to the event. In fact, this event was more localised in comparison with the event of 16th June, mainly affecting the headwaters of the Piodão stream. The heavy rain‐ fall significantly increased the amount of streamflow, resulting in a stronger and faster re‐ sponse and generating downstream floods and serious damage due to sediment transport. In addition to the substantial damage to human infrastructures, one death was recorded.

**Figure 9.** Daily distribution of rainfall in July and bi-hourly distribution on 14th July.

Figure 10, provided by a local resident and showing the volume of accumulated water, dem‐ onstrate that the peak discharge was higher during this event than the previous one. The flood marks on the house used to estimate the peak discharge level show that the ground floor was not flooded in the 16th June event, whereas during this flash flood the building was flooded to a depth of 1 metre.

The diagram in Figure 11 shows the longitudinal profile and different cross-sections of the Piodão river upstream of the village of Piodão, defines the stream bed and simulates the flood bed on the basis of flood marks, for the event of 14th July. Overall, the stream over‐ flowed its banks and doubled in size in comparison to the "normal" bed. Immediately up‐ stream of the village of Piodão, the flooded area was triple the size of the stream bed. This expansion of the flooded area was associated with a man-made structure designed as a channel for the bed stream. The inability to drain off the flow of water led to an increase in 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 was partially destroyed by the water, causing a landslide, as can be seen in figure 12.

photographs illustrate its powerful capacity to transport materials along the main channel and its highly destructive force (Figures 14 and 15). In figure 14 a large block can be ob‐ served abandoned in the river bed. In figure 15 a trout pond is crammed with material transported by the flood. The power of the stream affected sediment transport processes

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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‐

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

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

during the flood, also influencing the morphology of the river.

**5. Discussion**

ing from subsequent rainfall.

will be 700 times greater.

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

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 photographs illustrate its powerful capacity to transport materials along the main channel and its highly destructive force (Figures 14 and 15). In figure 14 a large block can be ob‐ served abandoned in the river bed. In figure 15 a trout pond is crammed with material transported by the flood. The power of the stream affected sediment transport processes during the flood, also influencing the morphology of the river.
