Andrew Pepper, BSc(Hons), CEng, MICE, FCIWEM, C.WEM



The Hertfordshire and North London Area of the Environment Agency has 18 flood storage reservoirs (FSRs) which come within the remit of the Reservoirs Act 1975. Many have fixed flow control orifices, which cannot be changed when the reservoir is in operation.

A review was carried out to determine the ability of the flow control systems to provide drawdown at these reservoirs, considering both the latest guidance on drawdown rates and the ability of the reservoirs to empty quickly to accommodate subsequent floods. The ratio of full storage volume to maximum allowable outflow was used to rank the 18 reservoirs, in order to identify whether investigations into additional drawdown facilities were needed.

Three reservoirs were found to require alterations to their outflow controls so that they could provide adequate emergency drawdown. The works required at these three FSRs are described, all of which involve penstocks with differing methods of operation – two being remotely operated.



dams, barrages & reservoirs floods & floodworks hydraulics & hydrodynamics