Water retention in river plains is a common measure for flood routing. Discharge control is easily achieved by structures combining openings and weirs. Energy dissipation at the outlet is required to ensure the transition from supercritical to subcritical flow and to protect the downstream reach from scouring and erosion. Experimental investigations were carried out in a hydraulic flume at the Laboratory for Applied Hydraulics of hepia-Geneva with various patterns of alternate blocks on the ground sheet downstream of an adjustable sluice gate. The most relevant geometrical parameters of sharp edged blocks, width, height, longitudinal and lateral spacing and also the upstream and downstream flow conditions were systematically investigated. The main goal of the first order research was to provide a new easy-to-use formula for river engineers to estimate energy dissipation achieved by alternate immersed blocks downstream of an outlet and thus to design the most relevant arrangement of the blocks.