Extreme Weather Layer (EWL)

The EWL is a tool for spatial planning and flood risk mitigation in urban areas. It is built on the model of an existing stormwater system and is a combination of hydraulic modelling, climate scenarios and other urban planning datasets.

Quick guide

The EWL is one of the main outputs of BSR NOAH – a transnational project that aims to protect the Baltic Sea from untreated wastewater spillages during flood events in urban areas. The tool has been developed by experts at the Tallinn University of Technology.

As extreme weather events increase due to climate change, effective flood risk mitigation and prediction become more important. The EWL flood risk maps can depict a stormwater system’s response to extreme weather events, taking into account changes in urban characteristics and different climate change scenarios. The tool allows users such as urban planners to generate an additional layer to an existing GIS map and identify the areas that may experience flooding in the present and future. The severity of the flood risk is displayed in traffic light colors on the map (either 2D or 3D) from low risk to high risk.

In practice, the EWL has to be generated based on the hydraulic model of the urban drainage system (UDS) and the GIS of the city. The EWL is a set of procedures for combing hydraulic models and GIS data. As many different hydraulic software for both hydraulic modelling and GIS are applied, the tool aims at simplifying the data tranfer between the model and GIS. The manual created in the project describes the procedure.


The EWL helps experts to prepare for future challenges in the field of water management and to develop the climate-resilience in urban areas. With the assistance of the new planning layer, the most suitable solutions for flood mitigation can be implemented in the areas with the highest flood risk. Further, the effects of the solutions can be analyzed – how the new developments change the flooding risks on plot-level, district-level or city-level in the selected area.

In the BSR NOAH project, the EWL has been piloted in eight pilot sites in the Baltic Sea region. With the information acquired by testing the tool in a flood risk area, the selected areas have been equipped with suitable water management and/or measurement technologies. By enhancing the water management conditions in an urban area, spillages of wastewater in flood events can be decreased – which helps to reduce overflows of nutrients and harmful substances into receiving water bodies, especially the Baltic Sea.

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