Artificial floods in residual flow reaches – How do they affect hydraulic habitats

Floodplains in hydropower-controlled rivers, where a constant residual flow discharge is imposed, often lack sediment supply and periodic inundation due to the absence of natural flood events. In this study we compare two residual flow systems. The first river is the Spöl, located in the Swiss National Park close to the boarder with Italy. Since the year 2000, artificial floods are released from the Ova Spin Dam on a yearly bases, with the aim to restore river dynamics and habitats. The second river, is the Sarine in Western Switzerland where an artificial flood was released from the Rossens Dam for the first time. At one location this flood event was combined with sediment replenishment. In this study, an analysis of the two different systems before and after the event using the Hydro-Morphological Index of Diversity (HMID) is presented. The index is based on the variability of flow depth and flow velocity and represents the habitat diversity on a reach scale. Results reveal that a flood pulse can significantly increase hydraulic habitat diversity in a river reach and that sediment availability is crucial. Nevertheless, the change in habitats of a single flood event has a minor effect if artificial floods are periodically released. Under such conditions, the river has already reached an equilibrium condition considering habitat diversity.