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

Abstract

Reconstruction of overbank sedimentation rates over the past decades gives insight into fl oodplain dynamics, and thereby provides a basis for effi cient and sustainable fl oodplain management. We compared the results of four independent reconstruction methods – optically stimulated luminescence (OSL) dating, caesium-137 (137Cs) dating, heavy metal analysis, and fl ood bed interpretation – applied at three embanked fl oodplain sites along lower Rhine River distributaries in the Netherlands. All methods indicate signifi cant sedimentation rates on the fl oodplains, 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 137Cs 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 137Cs, (2) smoothing of the vertical heavy metal and 137Cs profi les, (3) delayed sediment-associated input of 137Cs in addition to direct atmospheric fall-out, (4) overestimation of the burial age in OSL dating due to incomplete resetting of the OSL signal, or (5) non-linear relationships between sediment deposition and fl ood magnitude in the count-from-the-top correlation between sediment lamination and past observed fl ood 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 signifi cant overlap. A combination of the methods will thus provide maximum information for accurate estimation of sedimentation rates on a decadal time scale.