10. Cardiff Bay barrage and groundwater rise

Abstract

Plans to construct a barrage across the entrance to Cardiff Bay were evolved by the Welsh Office in 1986. The proposed barrage would impound the rivers Taffand Ely at about the normal mean high water level to create a 250 ha freshwater lake in Cardiff Bay. A Parliamentary Bill was required to permit the construction of the Barrage in tidal waters. Preliminary studies identified rise in groundwater level as being potentially the most significant adverse impact of the barrage. More detailed impact studies were in turn followed by an extended range and period of collection of field data. Conceptual and numerical modelling was undertaken to identify the magnitude and extent of any rise in groundwater level related to impoundment, the cost of protection and remedial works and the need to provide clauses in the Bill for the protection of householders and other organizations. Initial numerical modelling showed that control of groundwater levels by infiltration to sewers was a likely process. Detailed modelling, combined with examination of piezometer and sewer responses to individual rainfall events, confirmed this hypothesis and allowed the range of parameters in the numerical model to be constrained. The model could then be used to predict the effect of the barrage on the groundwater regime. To address the concerns of householders in the area, the Corporation, in drafting the Bill, included bespoke protective clauses giving wide-ranging protection to properties. The implementation of protection will be based on the assessment of pre- and postimpoundment property surveys and groundwater level monitoring. Introduction Geology Hydrogeology Data collection Predictive modelling Impact of groundwater rise Legal protection Physical protection Preventive measures Acknowledgements References