Designing Nature-based Solutions for hydro-meteorological risk reduction by coupling surface water modelling with participatory sciences in a peri-urban neighbourhood in El Salvador

Semi-formal neighbourhoods face complex social, economic and environmental challenges, of which many are related to poorly coordinated land and water management. Reduced vegetation and surface sealing combined with the lack of a sewage system lead to high surface water runoff, which results in increasing pluvial flooding, erosion, and landslides. Mitigation measures are often implemented by individual residents in the form of low-cost Nature-based Solutions (NbS), but implemented measures frequently lead to negative impacts further downhill. The effects of climate change are increasing hazard intensities and therefore, hydro-meteorological risks.

The aim of this study is to support the design of NbS by coupling pluvial flood modelling with participatory methods. The use of numerical models to describe surface water runoff and to analyse the effects of NbS has been applied often. Nevertheless, the calibration and validation of urban surface runoff models and the choice of appropriate solutions remains challenging.

This study uses the coupled hydrological-hydrodynamic model HEC-RAS to simulate pluvial flooding in selected precipitation events by utilizing Rain-on-Grid modelling, and to quantify the potential effects of selected NbS. The validation and calibration of the model is supported by flow paths, identified problem sites and flooding depths during historical precipitation events, which were determined during a participatory mapping workshop.

To find appropriate solutions interviews and transect walks were conducted along with the participatory mapping workshop as a basis to discuss, design, and locate low-cost, self-implementable NbS. These were implemented in the model to evaluate the effectiveness of potential solutions.

The results are expected to demonstrate the ability of this conceptual approach to utilize local knowledge to design implementable and effective NbS to reduce hydro-meteorological risks. Furthermore, this study shows how local knowledge and participatory mapping can be used to validate urban surface runoff models. The quantification of proposed NbS effects can support the implementation of suggested measures.