British Waterways were requested by the Environment Agency to dredge approximately 22,000 m³ of sediment from the River Lee Navigation so as to improve water quality. 

Land & Water were commissioned to sample and analyse the sediments and formally classify the material 6 months in advance of the intended start date. The sediment was found to be contaminated with heavy metals and oils which reduced the dissolved oxygen content of the water and caused unsightly duckweed and pennywort in the summer season. In addition the working area was within a source protection zone for a drinking water supply borehole.  The works needed to be complete within a short timescale in order not to interfere with the fish spawning season.

A sustainable solution for disposal was of primary importance to the client and therefore as an alternative to landfilling, bioremediation of sediments was undertaken at Biogenie site at Colnbrook as well as sediment blending with compost under a deployment of our mobile treatment license at a SITE facility.

The project was completed in 11 weeks and required close liaison with local interested parties such as the Lee Rowing Club and Canoe Club.

As a designated site of Special scientific interest the River Lambourn had been identified by both Natural England and the Environment Agency as not meeting the conditions required by the diverse fish species, aquatic invertebrates and stream flow normally found within a characteristic chalk stream river.

The main outlying cause of the conditions being impounded river sections by structures preventing fish movement and slow river flows. The slower river flows allow silts to deposit to the river bed smothering the natural gravels on which the fish, vegetation and invertebrates are all dependent. To address the situation Land & Water Services worked closely with Halcrow, the client’s appointed Engineer to undertake the works. 

The Land & Water contract called for a number of structures to be altered, reconfigured or completely rebuilt to high specifications. Temporary works included piled coffer dams and large scale river diversions in several areas of the site. Works to a former water wheel weir and a sluice structure were undertaken to allow the main river channel to flow 450mm below the original level. The main sluice downstream of the weir pool was extended and separated by the placement of a central wall, new stainless steel penstock valves were installed to allow full control of the water levels within the weir pool. A larinier fish pass was bolted to the new weir base in separate sections.

Provision was made for the installation of a new footbridge, the crest height to the weir pool was raised as required and a pre planted coir revetment installed. Detailed landscaping was undertaken to restore the areas affected by the works.