Establishing a sediment budget in the ‘Kleine Noordwaard’ area of the Biesbosch inland delta

Eveline van der Deijl, Eelco Verschelling, Marcel van der Perk, Hans Middelkoop

Tuesday 30 june 2015

9:30 - 9:45h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 4B. Sediment - River

Many deltas in the world cope with drowning and loss of delta land by sediment starvation and accelerated soil subsidence as a result of embankment of channels and drainage of land. The urgency of the problem is enhanced by sea level rise. Effective delta restoration requires a thorough understanding of the rates and mechanisms of delta aggradation and their controls. This study aims to quantify the rates and patterns of aggradation in the ‘Kleine Noordwaard’, a former polder area in the Biesbosch inland delta in the south-west of the Netherlands, in which water and sediment have been reintroduced since 2009. A sediment budget was established using existing bathymetric data collected using a multibeam echosounder and LiDAR digital elevation models collected since the opening of the polder, supplemented with field observations of the location and height of cut banks. Consecutive measurements of channel bathymetry showed a positive sediment budget in the channels between march 2009 and march 2013. During this period 59533 m3 of sediment was retained in the channels in the ‘Kleine Noordwaard’, which corresponds to an average sedimentation rate of 16.8 mm/year in the channels and 3.4mm/year over the entire area. Sedimentation and aggradation mainly took place in the channels in the central part of the former polder area, whereas the areas near the entrance and exit of the system were subject to channel erosion. It is therefore likely that a part of the sediment deposited in the central part of the system was supplied internally by upstream erosion of the channels. In addition, erosion of an island and old dikes also significantly contributes to internal sediment redistribution. Near-future research will focus on the sources of the sediment deposited in the area.