Reconstruction of floodplain sedimentation rates: a combination of methods to optimize estimates

Reconstruction of overbank sedimentation rates over the past decades gives insight into floodplain dynamics, and thereby provides a basis for efficient and sustainable floodplain management. We compared the results of four independent reconstruction methods - optically stimulated luminescence (OSL) dating, caesium-137 (Cs-137) dating, heavy metal analysis, and flood bed interpretation - applied at three embanked floodplain sites along lower Rhine River distributaries in the Netherlands. All methods indicate significant sedimentation rates on the floodplains, varying between 2-7 mm/a in the distal zones and 3-9 mm/a in the proximal zones. On a rapidly developing sand bar along a natural levee sedimentation rates of 9 to 25 mm/a were found. Except for some minor inconsistencies in Cs-137 dating results, all methods show decreasing sedimentation rates with increasing distance from the river channel. Intercomparison of the results of the different dating methods revealed the potential errors associated with each method, particularly where disagreement among the results were found. Uncertainties may arise due to (1) grain-size dependent downward migration of Cs-137, (2) smoothing of the vertical heavy metal and Cs-137 profiles, (3) delayed sediment-associated input of Cs-137 in addition to direct atmospheric fall-out, (4) overestimation of the burial age in OS L dating due to incomplete resetting of the OSL signal, or (5) non-linear relationships between sediment deposition and flood magnitude in the count-from-the-top correlation between sediment lamination and past observed flood records. Still, taking the uncertainties associated with each method into account, the results are generally in good agreement. Using the results we indicate the optimal spatial range of application of each method, depending on sediment texture and sedimentation rate. The optimal spatial and temporal ranges differ for each method, but show significant overlap. A combination of the methods will thus provide maximum information for accurate estimation of sedimentation rates on a decadal time scale.