Abstract. This paper analyses the morphological effects of extreme floods (recurrence interval > 100 years) and examines which parameters best describe the width changes due to erosion based on 5 affected alpine gravel bed rivers in Austria. The research was based on vertical aerial photos of the rivers before and after extreme floods, hydrodynamic numerical models and cross sectional measurements supported by LiDAR data of the rivers. Average width ratios (width after/before the flood) were calculated and correlated with different hydraulic parameters (specific stream power, shear stress, flow area, specific discharge). Depending on the geomorphological boundary conditions of the different rivers, a mean width ratio between 1.12 (Lech River) and 3.45 (Trisanna River) was determined on the reach scale. The specific stream power (SSP) best predicted the mean width ratios of the rivers especially on the reach scale and sub reach scale. On the local scale more parameters have to be considered to define the "minimum morphological spatial demand of rivers", which is a crucial parameter for addressing and managing flood hazards and should be used in hazard zone plans and spatial planning.

Meander reconnection is becoming a widely applied river restoration measure, aiming to enhance the ecological status of river systems. Based on a physical model study and a 3D hydrodynamic-numerical simulation, the effects of different forms of meander reconnections on flow dynamics, water level and river morphology are investigated in present study including retention of the old main channel for flood protection when the bankfull discharge is exceeded. The distribution of total discharge between the reconnected meander bend and the flood channel during floods decreases flow velocity and bed shear stress and leads to aggradation. At higher discharges, the flow velocity decreases in the meander bend whereas it increases in the flood channel. The controlling parameter for flow diversion-hence for flow velocity and bed shear stress-is the elevation of the ramp diverting the flow.Using the hydrodynamic model RSim-3D a particle tracing study simulated the trajectories of conservative particles at six vertical levels. The vertical movements of the particles on trajectories going through the reconnected meander were investigated by calculating the number of level changes of each particle. This novel approach in simulating particle movement in channels facilitating qualitative decision-making in the framework of river restoration issues. In particular, it reveals information regarding heterogeneity and turbulence of the water and also nutrient movements under variable flow conditions and can thus help answer restoration-related ecological questions.

New approaches to integrated flood management: Floodplain Evaluation Matrix (FEM), minimum river morphological space demand (FMRB), and spatially variable vegetation management (VeMa FLOOD )Summary: This paper presents three new approaches within the integrated flood management, which have been developed or optimized in FloodRisk II. (I) The Floodplain Evaluation Matrix (FEM) allows an objective evaluation of inundation areas within a river basin and/or river reach from hydrological/hydraulic, ecological and sociological viewpoint. (II) The minimum river morphological space demand is to be seen especially after large floods and was defined, based on the analysis of the recent extreme floods, as three to sevenfold the existing river bed width (measured from the river banks), where no buildings or other higher value usages are allowed. If possible, the potential river morphological space demand, defined by the potential river floodplain system, should be aimed for. (III) The spatially variable vegetation management suggests a differentiation of the river into vegetation dynamic zones, transition zones and sensitive zones with different widths and densities of the vegetation, in order to improve the retention (hydrological) and minimize the water level (hydraulic). Einleitung 1.