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A strategic programme for NERC Lowland catchment research
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Fine sediment budgets, nutrient & contaminant transfer

Title

The fine sediment budgets of lowland permeable catchments and their implications for nutrient and contaminant transfer

Overview of project

Increased suspended sediment concentrations result in increased turbidities; reduced light penetration; the accumulation of sediment; siltation of salmonid spawning gravels; contaminant loading and degradation of aquatic habitats. Scientists frequently cite increased fine sediment fluxes as important contributors to the degradation of Chalk stream ecosystems.

Current understanding of sediment sources and sediment mobilisation, transfer and storage within lowland groundwater-dominated catchments is limited. Detailed information on the fine sediment budgets of such catchments is an essential requirement for the development of effective sediment management strategies. In this project a novel integrated approach to data collection was used to assemble the information required to establish a catchment sediment budget. This approach involved sediment monitoring, sediment source fingerprinting and the use of falloutradionuclides to document sediment mobilisation and storage It has been successfully employed to establish the fine-grained sediment budgets of the Pang and Lambourn catchments.

This project found that surface erosion from cultivated land is generally the dominant sediment source in the study catchments, and that the contribution from channel banks and other subsurface sources is commonly low. A large proportion of the mobilised sediment is sequestered within the fields, between the fields and the stream channels and within the channel systems and only a very small proportion of the mobilised sediment reaches the catchment outlet.

Aims

This project assembled hitherto unavailable information on the fine sediment budgets and associated process dynamics of the three LOCAR catchments. This included information on suspended sediment yields and sources and their variation according to catchment characteristics, rates of soil redistribution on slopes, slope-channel transfers, channel routing, and channel and floodplain storage. In addition, the role of fine sediment transfer and storage in the transport and fate of nutrients and contaminants in lowland permeable catchments was also considered.

The ultimate aim was to use this assembled information as the basis for developing a sediment budget model for lowland catchments, which could be used to support the design and implementation of effective sediment control and management strategies in such catchments.

Main findings

  • Sediment budgets between the two study catchments are broadly similar.
  • Gross surface erosion is higher on cultivated land than on pasture.
  • A substantial proportion of the mobilised sediment is sequestered within the fields and between the individual fields and river channel network.
  • The sediment contribution from banks and subsurface sources is relatively low and generally represents only about 1-5% of the suspended sediment output from the study catchments.
  • A large proportion of the fine sediment moving through the channel system enters temporary storage within the channel and is remobilised during high flow events
  • The estimated sediment delivery ratios for the study catchments are low (< ca. 5%).

Description of activities

Documenting soil erosion and sediment redistribution

  • The fallout radonuclide caesium-137 (Cs-137) was used to provide spatially distributed information on medium term (ca. 45 years) soil erosion and sediment redistribution rates.
  • Cs-137 reference inventories were established for all three study catchments.
  • A comprehensive programme of soil sampling along slope trandsects was used to characterise the spatial variability of caesium-137 inventories and to estimate gross and net erosion rates.

Fingerprinting approach

  • The sediment transported by the study rivers was 'fingerprinted' by relating the geochemical properties of the suspended sediment to those of the potential sources.
  • Four potential sediment source types were identified within the study catchments. These represented surface soils under the three major land use types (i.e. arable, grassland and forest) and channel banks and other subsurface sources such as ditches. Representative samples of these potential source materials were collected and analysed geochemically, in order to establish composite fingerprints which successfully discriminated between the four potential sources.
  • Representative time integrated suspended sediment samples were collected at water quality monitoring stations at the outlets of the two study catchments and smaller subcatchments.
  • The geochemical fingerprints of the suspended sediment samples were determined and the relative contribution of the four source types to the suspended sediment yield at the catchment outlets was established using a mixing model.

Channel bed sampling and river monitoring

  • The amount of fine grained sediment stored on the channel bed of each study river was determined for representative river reaches at regular intervals, using the resuspension cylinder technique.
  • Suspended sediment fluxes were monitored using field-calibrated turbidity probes to provide a continuous record of suspended sediment concentration at the flow gauging stations associated with the study catchments.
  • Suspended sediment loads were calculated for the flow gauging stations.

Researchers' details

Principal Investigator:

Co-investigators:

Other researchers:

Publications

All publications from this and other LOCAR projects are listed in the publications database.

Refereed journal papers

Collins A L and Walling D E. 2007. The storage and provenance of fine sediment on the channel bed of two contrasting lowland permeable catchments, UK. River Research and Applications, 23, 4, 429-450.

Collins A L and Walling D E. 2007. Fine-grained bed sediment storage within the main channel systems of the Frome and Piddle catchments, Dorset, UK. Hydrological Processes, 21, 11, 1448-1459.

Walling D E, Collins A L, Jones P A, Leeks G J L and Old G. 2006. Establishing fine-grained sediment budgets for the Pang and Lambourn LOCAR catchments, UK. Journal of Hydrology 330, 1-2, 126-141.

Refereed book sections

Collins A L and Walling D E. 2006. Investigating the remobilization of fine sediment stored on the channel bed of lowland permeable catchments in the UK. In Sediment Dynamics and the Hydromorphology of Fluvial Systems, IAHS Publication no. 306, IAHS Press, Wallingford, 471-479.

Recommended reading

Ballantine D, Walling DE and Leeks G J L. 2006. The deposition and storage of sediment-associated phosphorus on the flood plains of two lowland groundwater fed catchments. In Sediment Dynamics and the Hydromorphology of Fluvial Systems, IAHS Publication no. 306, IAHS Press, Wallingford, 496-504.

Bogen J, Fergus T and Walling D E (Eds.). 2003. Erosion and Sediment Transport Measurement in Rivers: Technological and Methodological Advances. International Association of Hydrological Sciences Publication no.283. IAHS Press, Wallingford, viii + 238 pp.

Golosov V, Belyaev V and Walling D E (Eds.). 2004. Sediment Transfer through the Fluvial System. International Association of Hydrological Sciences Publication no.283. IAHS Press, Wallingford, viii + 238 pp.

Horowitz A J and Walling D E (Eds.). 2005. Sediment Budgets 2 - Proceedings International Symposium on Sediment Budgets, Foz do Iguacu, Brazil, April 2005. International Association of Hydrological Sciences Publication no. 292, IAHS Press, Wallingford, UK, 342p.

Lambert C P and Walling D E. 1988. Measurement of channel storage of suspended sediment in a gravel-bed river. Catena 15, 65-80.

Walling D E. 2007. Tracing versus monitoring: New approaches to studying the fine sediment dynamics of catchments and river basins. In: Proceedings of the International Conference on Erosion and Torrent Control as a Factor in Sustainable River Basin Management, September 2007, Belgrade, Serbia.

Walling D E and Horowitz A. (Eds.). 2005. Sediment Budgets 1 - Proceedings International Symposium on Sediment Budgets, Foz do Iguacu, Brazil, April 2005. International Association of Hydrological Sciences Publication no. 291, IAHS Press, Wallingford, UK, 366p.

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