Determining Surface Velocity Coefficients in Headwater Streams for Natural Flood Management

With concerns about climate change, nature-based solutions for flood risk management have been increasing in popularity across Europe, and targeting headwater streams can be an effective approach by increasing forest floodplains through inundation of surrounding areas. However, accurately measuring stream discharge remains a challenge. One promising method is to measure the surface velocity and apply a surface velocity coefficient. This study investigates the determination of surface velocity coefficient in headwater streams using the float and current meter methods. The relationship between surface velocity coefficient and relative submergence (ratio of water depth to roughness height) is analysed. The results support prior modelling results that the surface velocity coefficient decreases with relative submergence and extend the range of prior work for very small relative submergence, using experimental and field observations. The surface velocity coefficient was observed to decrease from 0.85 to 0.53 as relative submergence decreased from 270000 in a smooth glass laboratory channel to 0.093 in a field setting. Field discharge measurements upstream of a channel-spanning engineered logjam were used to show used to predict local inundation and the uncertainty is reported.