Managing sedimentation in a unique water supply system4 July 2002
BRITISH WATERWAYS (BW) MANAGES AND conserves 3200km of canals and inland waterways nationally, including a number of major rivers plus 89 feeder reservoirs.
Managing such a unique water supply system is a skill which BW has developed over a number of years. The water supply system interfaces with the major river basins of the UK and includes reservoirs, direct river abstractions, controllable and uncontrollable feeder streams, groundwater, and re-circulating pumping schemes. During summer months reservoirs can make up to 45% of the water available to the canal system and it is vital that any changes to the volume of water in them are understood.
British Waterways Technical Services' (BWTS) hydrographic survey team specialises in surveying, sedimentation studies and material sampling. Utilising the latest DGPS technology and multi-beam sonar, BWTS has completed 30% of a three-year programme to re-survey and produce depth capacity charts for all their reservoirs.
Its environmental investigation capabilities include both field data collection and analytical and experimental work. With extensive equipment, four survey vessels and staff resources, intensive biological and physical/chemical investigations of fresh, estuarine and marine water bodies, as well as investigations of environmentally sensitive areas can be conducted.
The 2001 survey of Bosley reservoir in Cheshire is an example of how BWTS conducts hydrographic surveys. Bosley covers an area of 34.4 ha, and feeds the Macclesfield Canal. When the reservoir was built in 1829, the reported storage capacity was approximately 2M m3, but subsequent surveys show a reduction in capacity.
1829 2,000,000 m3
1931 1,823,513 m3
1959 1,666,180 m3
2001 1,506,254 m3
Extensive research and analysis is under way to determine rate and type of sediment build up.
The methodology for surveying Bosley is the same as used for all BW reservoirs - first of all the lake boundary is digitised and then a contour map is developed. Survey transect lines are superimposed on this map at 30m intervals and the bearing computerised to guide the survey boat. Benchmarks are selected at key reference points and GPS co-ordinates are established using satellite data. For two days the hydrographic survey team continuously collected data along 700 transect lines covering the reservoir surface. Bathymetric data was measured with a depth sounder and stored electronically on the boat's computer. Later, a three-dimensional digital terrain model was created of the reservoir's bottom surface, and then a depth capacity chart. Detailed comparisons are then undertaken using satellite imaging.
To dredge or not to dredge? is the big question in managing BW's water resources. It owns around 30 disposal sites for dredgings and also some transfer sites (sites where sediment is deposited and allowed to de-water before it is re-excavated and taken to landfill on third-party licensed sites or re-used). Dredged sediment contains a lot of water which can form a significant proportion of its weight and sending it straight to landfill without de-watering would be costly.