Operational Flood Forecasting System in Denmark – Integrating Groundwater and Surface-water

Jun Liu , Julian Koch, Simon Stisen, Lars Troldborg, Raphael Schneider

Most operational flood forecasting systems provide predictions of pluvial and fluvial floods, often neglecting groundwater flooding processes. Groundwater flooding occurs when natural drainage system cannot drain water away quick enough, causing the water table to rise above ground. This study presents an operational integrated flood forecasting system that combines surface water and groundwater components. It provides key variables - such as river discharge and shallow groundwater levels - for assessing flood risk in Denmark. During the winter of 2024, the system predicted relative shallow groundwater levels with an average error of 0.27 m at a 5-day lead time at selected wells. Moreover, the system effectively captures peak flows in rivers alongside high groundwater levels, as exemplified for a specific local flood event in Varde in January 2024. This work demonstrates how groundwater flooding, often neglected in operational forecasting, can be included at the national scale.