- 17th December 2020
- Posted by: Miranda Pont
- Category: Projects
Location: Bulgaria Client: World Bank Partner: Aberon
The World Bank and Bulgarian Government have agreed to develop methodologies and deliver the second cycle Floods Directive work for Bulgaria. We were commissioned to develop updated methodologies for Flood Hazard and Risk Mapping.
The challenge is to update the national methodology for estimation of flood hazard and flood risk, according to the requirements of Directive 2007/60/EU (NIMH 2013). To respond to requirements from the European Commission regarding the integration of climate change and the emerging need to incorporate an approach to assess the risk of pluvial flooding and flash-floods, and also to update the vulnerability and risk assessment.
The methodology is data driven and is based on a process chart structure. The methodology describes each of the stages and decisions in the process. In this way, based on data availability and quality, the user of the methodology can follow the decision trees and take the most appropriate decisions.
Depending on whether the hazard is flash flooding or pluvial surface water flooding, different method processes have been proposed.
The proposed method for the assessment of pluvial hazard is direct rainfall (rain-on-grid) two-dimensional hydrodynamic modelling, which will dynamically model overland flow paths and surface ponding. The concept is for rainfall to be applied to the hydraulic model grid and the flood water routed overland. This approach is suited to lowland and urban catchments where point inflow estimation is challenging, lowland areas with standing water, and more impermeable urban surfaces reduce the contribution of soil throughflow to flood flows. The models allow for analysis of the spatial and temporal distribution of rainstorms and dynamic interactions between different topographic catchments and any river tributaries.
Flash flooding methodology
The methodological approach to flash flooding reflects the linear nature of the identified Areas of Potential Significant Flood Risk (APSFRs) and can take a 1D only, 1D-2D linked and 2D only approach.
The proposed updated coastal methodology includes details on applying joint probability considerations to different coastal boundaries i.e. tide, surge and wave, and in combination with other sources i.e. fluvial and pluvial. The updated methodology also includes improved inundation modelling approaches with an option to suit APSFRs with higher and lower quality data.
Vulnerability and risk assessment is also approached in the methodology and makes the transition from a qualitative assessment to a semi-quantitative approach.
The improved methods will not only allow the second cycle flood hazard and risk maps for Bulgaria to incorporate flash and pluvial flooding sources, but will also improve the modelling and mapping of fluvial and coastal flood sources. The improved mapping will enable more robust flood risk management plans to be developed.