Investigating spatial patterns of medium-term overbank sedimentation on floodplains: a combined numerical modelling and radiocaesium-based approach

A numerical model of overbank deposition of fine-grained suspended sediment is presented and employed to predict spatial patterns of mean annual floodplain sedimentation within a small reach of the River Culm, Devon, UK. The model incorporates a deposition function which is calibrated with the aid of a dataset of measured deposition amounts obtained from nine flood events over a 13 month period. Model predictions of mean annual deposition rates are then compared against an independent dataset comprising estimates of longer-term (ca. 35 years) average sedimentation rates derived from caesium-137 measurements on 200 floodplain sediment cores collected from two floodplain locations. The numerical model is found to adequately reproduce the spatial patterns of overbank sedimentation documented by the 137Cs data. However, where maximum rates of deposition occur, the model is found to seriously underpredict the 137Cs derived estimates. The simulation model employed here might be improved upon by incorporating more sophisticated 2D or 3D numerical treatments of flow hydraulics and suspended sediment transport and settling processes. Such numerical schemes may offer the potential to provide not only more accurate predictions of sedimentation rates, but also more realistic representations of overbank processes. While models of this type are now becoming more widely available, in order for the benefits associated with increased model complexity to be realised, in future, greater emphasis must be placed upon testing models against distributed datasets of hydraulic and sedimentological variables obtained through intensive field monitoring programmes. The use of 137Cs measurements appears to offer considerable potential in this respect.