The groundwater modelling studies, undertaken prior to construction of the Cardiff Bay Barrage, predicted that the primary effect of impoundment would be observed in the gravel aquifer below Cardiff. The gravel aquifer in most areas is separated from the overlying made ground by a highly impermeable clay layer (estuarine alluvium). This clay would normally stop groundwater from the gravel aquifer rising into the made ground layer.

In certain areas however, an absence of the clay layer was identified which creates a potential for raised groundwater levels in the made ground.

This made ground layer is where the majority of house basements and cellars are situated and it is these basements or buried structures that could be adversely affected by any long-term rise in groundwater levels.

Six areas in south Cardiff were identified as being 'at risk' from post-impoundment groundwater rise. These areas were Riverside, the Millennium Stadium area, Merches Gardens, Central Grangetown, the Marl and south Butetown.

It was therefore decided to install groundwater control systems, comprising pumped vertical wells, pumped horizontal collectors and field drains in these areas. These control measures were designed to maintain groundwater levels within the pre-impoundment range, in order to protect properties within these areas.

The implemented groundwater control measures are summarised below.

Field Drains (Pump Assisted and Gravity)

 Horizontal Collectors

These systems have been in continuous operation since the impoundment of the Bay.

Groundwater levels in the pumped wells and collectors and observation wells are recorded at 10 minute intervals. The performance of the systems (with the exception of the Millennium Stadium and the Marl) is monitored continuously via telemetry (data sent over telephone lines) by Cardiff Council’s Drainage Services, who have responsibility for the operation and maintenance of the groundwater control systems, under a Service Level Agreement with the Harbour Authority. Each system can automatically transmit alarms indicating high groundwater levels in local monitoring wells, pump malfunction, power failure and unauthorised entry into pump control kiosks.

Data is subsequently transferred to and stored in the Harbour Authority's ground water database where it is analysed and graphical presentations of the data are produced to show the variation in groundwater levels due to pump operation.

These graphs are used to determine whether any actions are required to adjust the performance of the system e.g. amending control levels of the pumps or discharge rates.

The operational data collected is reviewed and discussed every six months by a panel including the independent hydrogeology expert Professor John Lloyd. The principal function of the review is to confirm satisfactory operation of the the systems and to identify appropriate operational adjustments and maintenance requirements.