Anticipatory Flood Risk Assessment Considering Potential Land Use Scenarios (PAP014731)
Hans Peter Nachtnebel
Land use and Floods, landslides and erosion
The implementation of structural flood protection measures is often followed by an intensive land development in the former flood plain. As a consequence the damage potential and thus the flood risk are substantially increased and may become even larger than in the initial state. This is also reflected by the reported flood damages which have increased tremendously and continuously in the last decades, although substantial investment in protective measures took place. Structural design principles are based on historic observations and neglect mostly dynamic changes of the flood probability distribution and the damage function. Land use changes have impacts on both risk elements. Structural measures reduce the flooding probability of the Hinterland up to the design level. Due to an intensified land use the value of properties at stake increases. Additionally, levees may fail or floods may occur exceeding the design level during the life time of a hydraulic structure. And considering that these measures have a long physical lifetime, there is an urgent need for anticipatory flood risk assessment considering potential land use scenarios during the life time of the structure. Secondly, risk mitigation measures have to be identified including restrictions imposed on future land development in the former flood plain. To be able to account for this changes anticipatory flood risk assessment and management is required. This paper discusses approaches, time scales and potential land use development scenarios based on three case studies in Austria to underline the necessity of assessing the dynamic features of flood risk. The development of flood risk is analysed based on ex-post and ex-ante approaches. Case study 1 (city of Gleisdorf in Southern Austria, Styria; ex-post approach) analyses the influence of Hinterland development on the overall flood risk after protection measures were implemented. Subsequently, large parts of the former floodplain were transformed into industrial and residential areas. As a consequence, the numbers of exposed objects and people at risk increased substantially. Results clearly indicate the increase in expected annual losses triggered by hinterland development. Case studies 2 (Mattig, a rather agricultural catchment in Upper Austria) and 3 (Gr. Rodl, also in Upper Austria, a catchment with increasing number of residents) aimed at developing ex-ante approaches to assess anticipatory flood risk enabling strategic regional development within flood prone areas. Typically applied methodologies - starting with the assessment of the current state - are extended by integrating future potential land use development scenarios in the assessment procedure. Several data sources referring to demography, land use, classification of buildings, settlement characteristics, historic data, etc were utilised to define land use scenarios. Two from the three case study sites show remarkable increase of the overall flood risk, ranging from 40 % up to five times until 2100. The results clearly indicate that the consideration of potential land use development scenarios is needed to enable comprehensive flood risk assessment providing a reliable basis for adequate flood risk management strategies for the next decades.