The hydrological impact of low-flow alleviation measures

The River Ver, a chalk stream in Hertfordshire, was identified as one of the ‘top 40’ low-flow rivers of England and Wales in 1992, and measures to improve flows were subsequently proposed under an alleviation of low flows (ALF) scheme. Consequently, the largest groundwater abstraction within the catchment at Friars Wash was closed, other than for emergencies. The impact of this closure on river flows as measured at a gauge near the outlet of the Ver catchment is evaluated. A range of methods was employed: analyses of hydrographs, flow duration curves, flow percentiles, baseflow index, correlations with data from an analogous catchment and rainfall–runoff modelling. Results suggest an apparent increase in river flow in the 11 years since the closure at Friars Wash; however, this cannot be attributed entirely to the ALF scheme. Further modelling investigations are recommended to support the assessment and to allow more localised effects on stream flow to be investigated.