Ecohydraulic flows in shallow waterways with large bed rougness

Large variations in bed topography can have important functions regarding the biodiversity of the river. One example is habitat for different species of fish which has been shown to depend on hydraulic parameters such as local water depth, flow velocities and substrate size. In areas where there are large variations in bed topography, there will also be large fluctuations in these parameters and therefore the idea of complex habitats that suits multiple species could exist simultaniously. Understanding the detailed flow over complex geometric structures could therefore be a key component in locating and designing habitats which enables species diversity and density.

There are many waterways in regulated rivers where the size of the structures of the river bed are on the same order or larger than the water depth, which obstructs the flow and creates highly disturbed flow fields. This also makes empirical assumptions of flow resistance insufficient to describe the hydraulic behavior in such streams. Modern methods for topography measurements such as LiDAR and drone based photogrammetry have developed to a point where it is now possible to capture the geometry of these large structures on the river bed, but hydraulic modelling tools are typically not of high enough resolution to take advantage of this information. It is therefore of great interest to understand the effects of large scale natural roughness on local flow properties both in order to determine how they correspond to ecological responses in rivers and to develop new ways for hydraulic tools to include additional roughness descriptions in order to better predict the flow in shallow waterways in terms of water depths and local flow properties. Besides providing fundamental information for the hydraulic behaviour of shallow streams, this knowledge can also be utilized to further develop the hydraulic tools used for environmental applications and ecohydraulical design. Examples include modelling currently deployed or planned natural spillways and fish passages, numerical simulations of minimum mandatory spilling, as well as optimizing the hydraulic data used as input to habitat- and population models.