Drainage Tubes versus Sediment

Abstract

The study starts with possible working mechanisms of the tubes within the complex beach system. In addition, huge amounts of data are collected which tells us more about the sedimentology of the beach and soil structure. This data is used later on to study influenced beach processes and morphological change. For this study both the beach near Egmond aan Zee as the beach near Hvide Sande are looked at. At these coastal areas several drainage tube fields are installed. A large-scale mechanism is described which describes a hypothesis for the influence of the drainage tubes on the beach sediment. This mechanism consists of five possible processes that could be categorized into three groups: pressure changes, air ventilation and vertical transport of water. It starts with the flow of water through the tubes due to a continuous variation of the sea level. A starting point or ‘trigger’ could be the measured outflow of groundwater by Pieterse (2009). Water stored in the beach subsoil flows from the high water line to the low water line by a groundwater gradient. The drainage capacity of the tubes is largely determined by the outflow capacity of water. As the water chooses the path of least resistance the very permeable shell layers at Egmond aan Zee or gravel layers near Hvide Sande could increase the flow through the tubes. Levelling of the groundwater gradient increases the storage capacity of water in the beach. As the moisture level of the beach lowers, the beach becomes more dry in time. Aeolian transport increases which sorts the sediment at the beach. Fine grains are blown away along the coast and into the dunes. The beach becomes more stable.