PhD defence

Water level and discharge dynamics in human-affected lowland rivers


The current water policy aims to accelerate discharge, which is causing problems, such as eutrophication, reduced (aquatic) biodiversity, drastic decline in groundwater levels, increased water levels during floods and prolonged droughts, predominantly in urban areas and densely populated lowland areas. To address and prevent these problems, the water policy should concentrate more on water retention. Few of the existing strategies towards water retention have been evaluated regarding their functionality and quantified impacts on the regional water system. Specifically, regional water systems in lowland areas are defined as areas that freely drain in large rivers and where groundwater is an important component in the water system. In this context, the research focusses on the local and regional effects of increasing water levels and delaying discharges in regional water systems. The current practice of stream restoration involves the construction of meander bends and insertion of wood in streams, asserting local controls on stream conveyance capacity and on water level and discharge dynamics. Backwater effects resulting from sharp bends and the obstruction by wood in streams are currently poorly understood, and are considered key sources of uncertainty in predicting the effects of stream restoration and water retention measures. Regionally, streams are typically analysed in isolation from the downstream river system, with the risk of simultaneous occurrence of discharge peaks in the river and its tributaries. Within a catchment, groundwater variation near rivers is rarely being analysed as a response to surface water dynamics. Progress in understanding the impacts of stream restoration on discharge dynamics requires more knowledge about simultaneous occurrence of discharge peaks in a river and its tributaries, and a deeper understanding of response time dependence on open water-subsurface water exchange. The thesis discusses the topics wood in streams (chapter 2), flow in sharp bends (chapter 3), groundwater reaction as result of instant lowering of river water levels (chapter 4) and the simultaneous occurrence of discharge peaks in a main river and its tributaries (chapter 5).