Hydromorphological assessment of sediment augmentation measures in gravel-bed rivers

In the riverine environment, the riverscape, sediment and flow regime are essential drivers for natural habitat dynamics. Today, most water courses in Europe are regulated, and their natural dynamics are impaired. Flood releases coupled with the artificial addition of sediment, called sediment augmentation, downstream of obstructing hydraulic structures can help to restore ecological functions and mitigate adverse morphological effects from sediment deficit and an invariable, low discharge. Although many scientific studies have emerged on some design criteria and related physical processes of sediment augmentation measures, there is still a lack of knowledge regarding different restoration phases and site characteristics. This thesis investigates hydromorphological assessment strategies and design optimisation. It focuses on alternating in-channel deposits in low-gradient, sediment-starved, gravel-bed rivers. The influence of hydrograph shape and sediment augmentation repetition frequency on sediment transport dynamics and morphology evolution were investigated in different experimental series in the laboratory. Hydrograph skewness was distinguished by left skewed, right-skewed and symmetrical shapes. Symmetrical hydrographs increased overall bedload mobilisation and transport rates, and left-skewed hydrographs increased the mean transport distance compared to right-skewed hydrographs. Repetition frequency was separated in augmentation scenarios before every and every second mobilising flood and was performed with up to four consecutive augmentations. Increasing the repetition frequency led to increased persistence of sediment patches and deposition rates in the downstream section. Calculations based on numerical simulations showed a significant increase in hydromorphological diversity until the fourth repetition. The influence of channel geomorphic units on river morphology and the validity of different indicator types for assessing physical habitat were investigated in the field. Tagged bedload particles (tracers) from a sediment augmentation measure were tracked in the river before and after a low-magnitude bed-forming flood. Channel geomorphic units were mapped according to qualitative definitions. The physical habitat assessment was performed in parallel at the restoration and an upstream control reach. Channel geomorphic units influenced tracer distribution and persistence during a flood. The persistence of tracers was highest in riffles and lowest in runs. The investigated flood did not significantly restructure the channel geomorphic units' organisational patterns. Not all assessment indicators proved to be valid for quantifying the impact of the sediment augmentation measure on habitat diversity. Riverbed structure, substrate mobilisability, water depth and velocity distributions were best suited for the impact assessment at the study site. The thesis provides important design recommendations for adapting sediment augmentation measures