Quantifying flood risk reduction strategies for Tirana

Abstract

Global economic losses due to flooding have increased over the last 50 years, with damages estimated in the billions of dollar each year. Governmental institutions face great challenges to reduce flood risk under the influence of population increase, urbanization and climate change. Interdisciplinary urban planning becomes more important to mitigate flood risk with a multidisciplinary approach and insights in conceptual designs is important to support interdisciplinary cooperation. An example of a country facing great challenges is Albania, where on average 23 flooding events occur each year and at least 54% of the regions have been affected. In addition, the country faces challenges in the disciplines of urban planning, transport and water quality. The capital city Tirana and surroundings were highly affected by flooding events in 2017. The city is prone to flash floods and, especially in the winter months, river flooding. It is crucial to identify areas susceptible to flooding and to quantify the impact on flood risk reduction of potential measures. Data availability imposes a major constraint to quantify flood risk in urban areas with currently available assessment methods. Therefore a flood risk assessment method needs to be identified that is applicable in areas with poor or scarce data. Once the method is identified it can be applied in the Tirana region to identify flood risk and to quantify the impact of measures. Potential measures were developed in an interdisciplinary workshop with students from the TU Delft and Tirana and focussed on the Tirana region, including the Lana river area. The proposed measures for the Lana river are quantified in the risk assessment in this thesis. The identified method defines flood risk as the product of hazard, exposure and vulnerability expressed in Expected Annual Damage (EAD) in Euro. European depth-damage functions for four land use classes are used, in combination with maximum damage values, to express the exposure and vulnerability component. The hazard is described by inundation maps and the development forms the core of this thesis. With the use of a Digital Elevation Model (DEM) a Height Above Nearest Drainage (HAND) map is developed. A Synthetic Rating Curve (SRC) was constructed based on the application of the Manning equation with reach-averaged geometries derived from the HAND map. With a time series analysis of measured discharges, inundation maps could be constructed linked to a return period. The identified method was applicable in the Tirana region. Major benefits are the low computational requirements, easy applicable GIS operations and low data requirements, which make the method easily transferable to other regions. Constraints were found in the use of the coarse DEM with 25 meter resolution, which did not cover river geometries and local measures with high enough accuracy. This was solved by the manual inclusion of river geometries and verification of the developed inundation maps. The Lana catchment is identified as the area most susceptible to flooding with an EAD of approximately one million Euro. One of the proposed measures, a multifunctional cross-section, proved to be most effective with a reduction of 65%. An urbanization scenario, in which the risk increased with 84%, demonstrated the need of the measures. Finally, the assessment revealed potential adverse effects of upstream measures on other areas and proved to add beneficial insights.