Determining the short-term sediment budget in a tidal freshwater wetland: a case study in the Kleine Noordwaard, the Netherlands.

Abstract

Due to the predicted rise in sea level, it is quite possible that river deltas in low-lying areas will start to drown. When the rate of sea level rise exceeds sediment accretion in a wetland, loss of delta land is inevitable. The Netherlands has many wetlands and the Biesbosch in the south-western part of the Netherlands is one of the best known examples. The area investigated in this thesis is called the Kleine Noordwaard. In this area, a recently inundated tidal freshwater wetland provides an ideal opportunity for investigating the sediment budget of such a wetland. This can give information on the survival chances of such a wetland. The main focus in this thesis is to gain information on the short term sediment budget in the Kleine Noordwaard. In order to answer this question, stream velocities and suspended sediment concentrations were measured. Using the suspended sediment measurements, maps of suspended sediment concentration were made. From the stream velocities, discharges were calculated which in turn were used for flux calculations. These flux values were used for sediment budget calculation in order to answer the main research question. Fall velocities were computed as well and used for the calculation of the trapping efficiency of the Kleine Noordwaard. Temporal differences in suspended sediment concentrations were seen on the maps which were made, the flux calculations and meteorological data. It has been found that the research area was a source of sediment during low river discharges and strong SSW winds. The area acted as a sink of sediment during high river discharges and low SSW winds and during average river discharges and strong NE winds. The mean median settling velocity of the suspended sediment was 0.04 mm/sec. The trapping values that were calculated using two different formulas varied between 51% and 71%. In contrast to the positive trapping efficiency of the Kleine Noordwaard, the calculated accretion rate of the area shows a smaller value (0.014 mm/yr) than the rate of sea level rise (1.6 mm/yr). This means that even though the trapping efficiency is generally positive, the wetland’s accretion rate is insufficient to keep up with present sea level rise, let alone any possible future enhanced sea level rise.