Assessing the remobilisation of recently deposited sediment from river flood plains during single overbank flood events, using caesium-134 and cobalt-60 as tracers

River flood plains can represent important stores of fine sediment and sediment-associated contaminants and there is therefore a need to develop an improved understanding of the rates and patterns of fine sediment deposition during overbank flood-events and its subsequent fate. Most existing studies have focused on deposition rates and only limited attention has been given to the potential for remobilisation of the recently deposited sediment during subsequent events. This communication reports an attempt to develop a tracing technique to address this problem. The approach employed involves placing small quantities of fine sediment, pre-labelled with artificial radioisotopes, on the flood plain surface and documenting the changes in the tracer inventories after subsequent overbank flood events. Reduced inventories provide evidence of sediment remobilisation. The magnitude of the reduction in the inventory provides information on the extent of the remobilisation, which can be related to the local physical conditions. This approach has been applied on two river flood plains in Devon, UK, where it was possible to document the change in tracer inventories after two subsequent overbank flood events. Remobilisation occurred at 80% and 87% of the measuring points on each flood plain during the first inundation event and at 67% and 73% during the second. The results confirm the potential importance of remobilisation of recently deposited sediment during subsequent overbank flood events.

[1]  D. Walling,et al.  New strategies for upscaling high-resolution flow and overbank sedimentation models to quantify floodplain sediment storage at the catchment scale , 2006 .

[2]  Saad Ali Khan,et al.  Sorption of the long-lived radionuclides cesium-134, strontium-85 and cobalt-60 on bentonite , 2003 .

[3]  B. Salbu,et al.  Cesium-134 as a tracer to study particle transport processes within a small catchment with a buffer zone. , 2001, Journal of environmental quality.

[4]  D. Walling,et al.  Use of floodplain sediment cores to investigate recent historical changes in overbank sedimentation rates and sediment sources in the catchment of the River Ouse, Yorkshire, UK , 1999 .

[5]  D. Walling,et al.  Lateral variability of overbank sedimentation on a Devon flood plain , 1998 .

[6]  D. Walling,et al.  The role of channel and floodplain storage in the suspended sediment budget of the River Ouse, Yorkshire, UK , 1998 .

[7]  D. Walling,et al.  The characteristics of overbank deposits associated with a major flood event in the catchment of the River Ouse, Yorkshire, UK , 1997 .

[8]  D. Walling,et al.  Investigating spatial patterns of overbank sedimentation on river floodplains , 1997 .

[9]  F. Nakamura,et al.  Some methodological developments in the analysis of sediment transport processes using age distribution of floodplain deposits , 1996 .

[10]  D. Walling,et al.  Floodplain Sedimentation: A Preliminary Investigation of Contemporary Deposition within the Lower Reaches of the River Culm, Devon, UK , 1987 .

[11]  M. Gerzabek,et al.  Vertical migration of 60Co, 137Cs and 226Ra in agricultural soils as observed in lysimeters under crop rotation. , 2005, Journal of environmental radioactivity.

[12]  D. Walling,et al.  Investigating Spatial Patterns of Overbank Sedimentation on River Floodplains , 1997 .

[13]  D. Walling,et al.  Rates of overbank sedimentation on the flood plains of several British rivers during the past 100 years , 1994 .

[14]  D. Simm,et al.  The deposition and storage of suspended sediment in contemporary floodplain systems : a case study of the River Culm, Devon , 1993 .