PhD Defence (Layman’s talk), Wageningen, The Netherlands
Halfway the 20th century, groundwater management in agricultural areas led to channelisation of the majority of lowland streams in the Netherlands. This has led to degradation of the aquatic and terrestrial ecosystems, characteristic for lowland streams. Over the past 25 years, water authorities in the Netherlands have aimed at restoring these degraded streams. Historical maps show that many lowland streams consist of a meandering planform. Re-meandering is the common practice regarding stream restoration in the Netherlands. Little is known about the morphological processes following the completion of such stream restoration projects. The aim of this thesis is to characterize the morphodynamic developments of restored lowland streams, with a focus on meander processes. Field data was obtained from five study areas, located in the Netherlands. Two chapters focus on the processes related to initiation of meandering of lowland streams. A historical analysis is of a once straight man-made canal and alternate bar dynamics are studied in a straight lowland stream. The results from these two field sites show that it is likely that the sinuous planform observed on historical maps is a result from exogenous influences, rather than autogenous processes. Three chapters focus on the short-term morphological response of re-meandered streams. In general, the studied lowland streams show little morphological activity. The observed morphodynamics occur mainly in the first year after construction and are caused by backwater effects and floodplain heterogeneity. After initial morphological adjustments, the channel planform remains stable. The rapid establishment towards a near-equilibrium state within the reconstructed streams is at odds with the view on lowland streams as small rivers migrating actively in their own deposits. The term re-meandering may be misleading, because of the connotation with channel meandering in time. The steadiness of the channel planforms has not been extensively demonstrated over geological time scales. This study has focused on short-term observations, since engineering time scales are more relevant for stream restoration practitioners.