Even in moderate-sized streams, the water level can rise by several meters and cover large areas. Also, new sediment sources open up during flooding.The sediment is deposited on flood plains and levees from water suspensions during flooding, when the discharge exceeds the amounts that can be contained within the normal channel.
Even in moderate-sized streams, the water level can rise by several meters and thus cover large areas. Also, new sediment sources open up during flooding. Thereby the origin of the suspended materials in the streams is diverse.
During the flooding and especially its last phases, some of the suspended material will deposit on the flood plain, out of reach of the ordinary stream channel. Thereby, nearly horizontal strata of overbank sediment are built up over time. The total thickness of overbank sediment can be up to several meters. Overbank sediments thus represent an important medium in which natural and anthropogenic toxic substances are transported and stored on floodplains.
Age of layers
Sampling both recent overbank sediments and sediments from the period of industrial development allows determination of the changes that have taken place.A vertical section through overbank sediment represents the history of sedimentation through time and will represent previous and existing erosion and sediment sources.
The overbank sediment presents a smart sampling medium because:
- The sediment suspended during floods has a widespread origin representing a large area.
- A composite sample through a vertical section of a floodplain represents a large number of sediment sources that have been active at different times and forms an intergrated sample of the entire catchment area over time. Therefore both man-made and natural chemical signals may be detected.
- Overbank sediment are available all over the globe
Sampling both recent overbank sediments and sediments from the period of industrial development allows determination of the changes that have taken place, as well as indicate future trends. 137Cs, 210Pb or 14C dating techniques can be used to identify the relative age of layers within the sedimentary sequences.
In several countries
Overbank sediments have been used in geochemical mapping in several countriesHistorical data about delta development and the sediment supply to deltas will be obtained where possible. In this way the changes in the fluxes of sediment-bound chemical elements may also be identified.
Overbank sediments have been used in geochemical mapping in several countries: China, Fennoscandia, Greece, Austria, Belgium, New Foundland, Spitzbergen and Norway.