**3. Methodology**

20% and in the Piodão river more than a half of the watershed has slopes of over 50%. A comparative analysis shows the basin ruggedness and coefficient of torrentiality to be slight‐

> Basin area (km2) 34.3 44.7 Basin gradient (m) 1047 (295-1342) 1069 (211-1280) Basin ruggedness1 1.13 1.84 Drainage density2 (km /km2) 4.13 4.42 Coefficient of torrentiality 29.48 41.39

Burnt area, June 2005 (in%) 100 60 1. maximum change in elevation within a basin, divided by the square root of the basin area (Melton, 1965);2. the total length of all channels within a basin, divided by the

Important demographic and socio-economic changes have affected the mountain areas of Portugal for at least the last five to six decades. The population of the mountain areas de‐ creased substantially during the second half of the 20th century, leading to the abandon‐ ment of agricultural land and a reduction in the size of herds and the amount of forest fuels consumed by grazing and the collection of firewood (Rego, 1992; Moreira et al.,

Consequently, the landscape has been drastically modified due to the sequential abandon‐ ment of traditional land use throughout the second half of the 20th century. The increase in uncultivated land has led to a secondary vegetation succession and modification of the veg‐ etation structure, favouring horizontal and vertical fuel continuity and a consequent in‐ crease in flammable biomass. The unmanaged accumulation of large quantities of fuel and the exclusion of fires from forest management has led to a dramatic increase in the magni‐

In addition, afforestation has focused primarily on highly inflammable species, mainly pines (predominantly *Pinus pinaster*) which also favours the proliferation of forest fires (Shakesby et al., 1996). Once fires break out under these highly dangerous conditions, they spread more easily and cannot be stopped. The low population density, delays in detecting fires, and diffi‐ culties in gaining access to the sites where fires tend to start, due to the rugged topography, are

The Mediterranean characteristics of the Portuguese climate (warm, dry summers and rela‐ tively wet winters) make it prone to wildfires and post-fire soil erosion. In Portugal, the ma‐

other factors that explain the large burnt areas in the central mountain area of Portugal.

tude and frequency of forest fires (Carvalho et al., 2002; Moreira et al., 2011).

43.9 52.4

Basin area with slopes greater than 20 percent Basin area with slopes greater than 50 percent

basin area, (Horton, 1945)

**Table 1.** Main characteristics of both basins.

2011; Lourenço, 1996, Nunes, 2012).

**Piodão river Pomares river**

54.7 38.6

ly higher in the Pomares basin.

68 Research on Soil Erosion Soil Erosion

The post-wildfire hydrological and erosional responses are based on intensive post-event fieldwork to determine the geomorphological impacts and socio-economic implications by collating, collecting and analysing data from field studies that was essential to understand‐ ing the meteorology, hydrology and hydraulics of the event.

The meteorological characteristics of the storms that affected the basins were determined us‐ ing data from a rain gauge installed in the Piodão basin. Daily and 30-minute rainfall inten‐ sity measures (I30) were chosen for each event, since rainfall frequency studies (Hershfield, 1961; Miller et al., 1973) indicate that in mountainous terrain 79% of the hourly rainfall oc‐ curs within 30 minutes and this type of storm has a short duration, lasting between 10 and 60 minutes (Moody and Martin, 2001).

The fieldwork took place a few days after the events occurred and was based on identifying certain variables:

Indicators of the peak discharge values, mainly cross-section surveys based on flood marks, in addition to signs of flow velocity (witness observations and water super-elevations in riv‐ er bends or in front of obstacles). High water marks on channel banks, mostly indicated by the deposition of vegetation fragments and silt, were visible in the sites. These marks are very important and provide approximate estimates for reconstructing peak discharges for ungauged cross-sections of rivers affected by floods.

Sediment transfer processes (erosion and deposition on slopes and in river beds, hypercon‐ centrated mud or debris flow), which may give an indication of local runoff generation proc‐ esses and flow energy and velocity.

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‐ sessing the return period of the flood.

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

Soil Erosion After Wildfires in Portugal: What Happens When Heavy Rainfall Events Occur?

http://dx.doi.org/10.5772/50447

71

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‐

**Figure 3.** Effects of the intense rainfall after the wildfires.1. Area of the basin not affected by the wildfire of 2005; 2.

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

Areas worst affected by the intense rainfall; Piscina fluvial="river beaches".

between 5 pm and 6 pm.

marises the areas worst affected by these processes.

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 areas affected by flash floods.
