The town of Hemel Hempstead and the surrounding area are supplied by three groundwater pumping stations that take water from the chalk aquifer. During times of high demand water can also be imported from elsewhere through our network. The licences for the abstractions at the pumping stations are governed by the Environment Agency.
Through the National Environment Programme investigations were undertaken during AMP3 (Asset Management Period 2000 to 2005) by the Environment Agency into the cause of low flows in the River Gade. This work was combined with the development of a groundwater model of the Colne catchment and was undertaken by consultants Jacobs Gibb. This work identified that flow in the rural reach of the River (Gadebridge Park upstream) is reduced by public water supply abstraction, exacerbated by historical channel modifications. The project concluded that it was not financially viable to move the public water supply to another location. Funding was secured in AMP4 (Asset Management Period 2005 to 2010) to assess the current hydro geological situation in the catchment and come up with various options for improving low flows in the river.
The River Gade is an ephemeral chalk stream which in its natural state would exist as a small and shallow gravel-bed channel for most of its length. Throughout history human intervention has significantly altered the river's development. The Environment Agency found that the Upper River Gade suffers from low flows as a result of public water abstraction at Piccotts End and Marlowes. During AMP3 (2000 to 2005) a study concluded that it would be uneconomic to move the pumping stations to a different catchment and funding was secured for investigation into other sustainable options for improving flow conditions in the Upper Gade.
An extensive monitoring program has enabled the collation of ground and surface water data. A main point to note is the reduction in flow downstream of Bury Mill diversion weir even during times of low flow as shown in the spot flow gauging results.
A working group, established in autumn 2006, has enabled the involvement of interested parties with the detail of the project and has aided the development of the project with vital local knowledge.
HEC-RAS (1D computer flow simulation) modelling has shown that the Bury Mill diversion weir can be raised by 0.15m without causing major flooding whilst keeping flow within the main channel at other times.
The options appraisal and cost benefit assessment concluded that option 7 is the best option. Option 7 involves removing the Environment Agency gauging weir, re-grading the river and diverting groundwater to the existing channel.
Three Valleys Water have now completed the full options appraisal for the Upper River Gade Sustainability Study and have passed management of the implementation of these options onto the Environment Agency. Three Valleys Water will remain involved with the project and hope that in the course of time this work brings a phase in the future of the Upper River Gade.
Alex Back, Hydrogeologist, Three Valleys Wate