Grain-size trends used to assess the effective discharge for floodplain sedimentation, river Waal, the Netherlands

The importance of hoods of different magnitude on floodplain sedimentation can be estimated from sedimentation measurements carried out during a series of hoods, However, because these measurements represent sediment accumulation during the entire hood, they cannot be used to estimate the importance of individual hood stages on sediment deposition. In this study, the importance of different hood stages on floodplain sedimentation was assessed from analysis of spatial variations in grain-size distributions, which are indicative of sediment transport directions. As a secondary objective, the applicability of grain size trend analysis to fine-grained floodplain sediments was tested. The grain-size trend analyses were carried out in a Geographical Information System, and applied to sediment deposited on an embanked floodplain section along the river Waal in the Netherlands, during a large flood in December 1993, Because flow patterns at the studied floodplain section vary with flood stage, comparison of the identified sediment transport pathways with computed flow patterns indicates at which discharge most sediment is deposited. The results indicate that large discharges with a recurrence time of more than two years are most effective in depositing suspended sediment on floodplain areas enclosed by a minor river dike. On floodplain sections that directly border the main channel, moderate discharges with a recurrence time of about one year seem to be more efficient. Contrary to the results of floodplain sedimentation models, low but increasing discharges with a recurrence time of less than one year and during which only low lying areas near residual channels are inundated appear important for deposition of suspended sediment in these low-lying areas. This observation is confirmed by measurements of spatial concentration gradients and by floodplain sedimentation measurements, Comparison of the trends with additional information on sediment transport directions indicates that trends in which sediment becomes finer, better sorted, and more negatively skewed (when measured in phi units) are most suitable for defining transport pathways on river floodplains. Trends may, however, deviate up to 90 degrees from true transport directions. Flocculation does not seem to have any negative effects on the applicability of grain-size trend analysis.