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In 1920, the Lee Conservancy Catchment Board (in addition to its existing functions of controlling navigation and fishing and preventing pollution) was empowered to spend money on a scheme to prevent floods in the Lea Valley, and to levy a precept on the local authorities affected by the scheme.
The Board prepared a scheme that included the stopping up and diversion of several old sewers, and the cutting of a broad additional channel leaving the River Lea (New Cut) at Tottenham (near the Banbury Reservoir) and running through Walthamstow and Leyton (eastward of the reservoirs) to rejoin the river just below the filter beds of the Metropolitan Water Board at Lea Bridge.
Parliamentary sanction was obtained in 1938, but owing to the War, the work was deferred. Serious flooding in 1947 that, among other things, put the filter station out of action for several days, showed the importance of the project.
In 1950, work eventually began on the construction of the new River Lea Flood Relief Channel (RLFRC) and was completed in either 1960 or 1976 (accounts differ).
The channel has significantly improved flood defence in the Valley. It was constructed to give protection during events, with return periods of up to 1 in 70 years. Since 1976, it has been successful in containing flooding along the Lower Lea, although it was almost full in the storms of October 1987, October 1993 and October 2000.
The River Lea flows through the RLFRC for 25 km downstream of Feildes Weir, before becoming part of the River Lea again south of Friends Bridge, and travelling the remaining 9 km to the Thames.
For most of its length, the RLFRC flows parallel to the Lea Navigation Channel and the old River Lea system. There are several links between the Navigation Channel and the flood relief channel control structures that keep the water level in the relief channel constant, except in times of flood when the sluices open to allow discharge of flood waters.