The driving forces behind the flow of water in the recently depoldered area, the ‘Kleine Noordwaard’

Abstract

The fieldwork area ‘De Kleine Noordwaard’ is an area that is reopened for rivers and tides (depoldered) as part of the Room for the River project in the Netherlands. The project is initiated to decrease flood risk along the river Rhine, by capturing and storing water in the newly created channels and floodplain. The area is being monitored and works as a study and pilot area to find out what happens to a polder when it is given back to nature. It is expected to catch sediment and grow, keeping up with sea level rise. The driving force behind the sediment distribution and deposition in the area is the water flow. The water flow is monitored using H-ADCP’s at both the in- and outlet of the area. Using field data, the data of these H-ADCP’s is used to construct a flow volume balance for the area. Analysis of this flow volume balance shows that the area is mainly functioning as a side channel with a small storage capacity. The water flow through the area is determined by water levels at the up- and downstream boundary of the area, creating a gradient over the area. The gradient in water level over the area is being determined by the discharge of both the Rhine and the Haringvliet sluices, and transformed by the tidal wave. How much influence the tidal wave has on this gradient depends strongly on the original magnitude of the gradient. High gradients are only slightly changed by the tidal wave while low gradients are suspect to a much larger change. This causes situations at which the water flow through the area is going in the upstream direction instead of the regular downstream direction.