The impact of forest fires on the vulnerability of peri-urban catchments to flood events: The case of the Easter Attica Region, Greece

Abstract

Floods are natural disasters that pose a direct threat to human life and often cause significant economic loss. River floods are caused by heavy and/or prolonged rainfall, causing rivers and streams to overflow and sometimes also burst dams or levees. Forest fires increase the risk factor to which humans and properties are exposed during flood events, by intensifying and accelerating the peak volume of the overflow water. Wildfires alter significantly the geomorphological characteristics of a river basin, thus affecting both directly and indirectly its hydrological behavior. A forest fire, followed by a rainfall event, may cause a significant flood downstream. This paper presents a hydrological analysis of the eastern Attica region, which is performed both prior to and after forest fire events, in order to assess the change in the consequent flood risk. The study area is the eastern part of the greater Athens area in Attica (Greece). This is a peri-urban area experiencing rapid urban growth, and its hydrometeorological conditions are being monitored though a dense hydrometeorological network for the past 10 years. A fire model is set up to simulate the characteristics of the development of three hypothetical fire events of low, medium and high severity accordingly. The parameters that affect fire intensity and rate of spread (e.g. topographic factors such as slope steepness, elevation, aspect, and configuration of land) are taken into consideration and the temporal and spatial distribution of a forest fire is studied. Of additional interest in this study is the fact that a recent forest fire event devastated a significant part of its northern, mountainous area and the consequences of this event are further examined. A detailed simulation of the three hypothetical fire scenarios and the real fire event is performed and the outcomes of the analysis are used as an input in a rainfall – runoff model that allows for an enhanced hydrological study of the affected area. Land use changes and geomorphological and hydrological alterations as a result of the fire event are taken into consideration during a post-fire hydrological analysis, which eventually demonstrates the impact of fire on the hydrological response of the study area. This impact is assessed by means of application of a methodological framework for the estimation of post-fire values for three hydrological parameters (CN, initial abstraction and lag time) and the paper concludes that the fire impact has rendered the downstream areas more prone to floods.