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Groundwater Risk

Groundwater Flooding Groundwater flooding normally occurs where the table meets the ground surface in low lying areas which are underlain by permeable rock known as . This tends to follow long periods of sustained rainfall, but can also be caused as a result of local obstructions to (e.g. following the placement of engineering structures or buildings with foundations) or by the rebound of groundwater levels after a decrease in abstraction or dewatering.

Groundwater tends to flow from areas where ground level is high, to areas where the ground level is low. In low-lying areas the is usually at shallower depths anyway, so the additional groundwater flowing towards these areas can cause the water table to rise up to the surface causing groundwater flooding.

Groundwater flooding typically takes longer to dissipate because groundwater moves much more slowly than and will take time to flow away underground. The rate at which it recedes depends on the lithology of the and the magnitude of the event, but may take weeks or even months.

It is important to recognise the risk of groundwater flooding is typically highly variable and heavily dependent on local geological, topographical and weather conditions. Groundwater flooding is hard to predict and challenging to mitigate.

Geology and

Atkins (2009) collected and analysed data relating to the underlying geological strata and hydrogeology. The data utilised included historical borehole records, ground investigations undertaken in January/February 2009 and BGS Map sheet 141.

 This report indicated that the strata likely to underlie the site are as follows, in a general downward succession:

 Infilled Ground

 Made Ground

 Alluvium (Drift)

Terrace Deposits; Hemington Terrace Deposits (Drift)

 River Terrace Deposits; Holme Pierrepont SG (Drift)

 Mercia Mudstone Group (Solid)

 Sherwood Group (Solid)

One major system, the Burton Fault, is identified on the 1:50,000 scale BGS Geological Map Sheet 141, which dissects the STW site in an approximate north to south alignment. To the northwest of this fault the Drift deposits are underlain by Mercia Mudstone, and to the southeast the Drift deposits are underlain by Bromsgrove Sandstone (part of the Sherwood Sandstone Group).

From the ground investigation (Atkins, 2009) the topsoil was reported to be 0.1 to 0.5m thick. The Made Ground varied between 3 and 6.8 m below ground level (bgl) (37.46 to 42.19 mAOD). The River Terrace deposits ranged from soft to firm slightly gravelly clay to gravelly very clayey medium to coarse sand / dense sand and gravel. The thickness of these River Terrace Deposits varies across the site, with a maximum depth of 8.5 m bgl (36.46 mAOD).

The Drift deposits are underlain by the Mercia Mudstone to the west of the fault. The top of this strata is a weathered layer comprising firm occasionally sandy and gravelly Clay. Beneath this is the Mudstone with rare beds of siltstone. The depth of this deposit was unproven. The Bromsgrove Sandstone is medium- grained sandstone and the depth to the base was not proven.

Shallow groundwater strikes were encountered within the Made Ground at depths between 0.40m and 1.20m bgl which rose to depths between 0.10m and 1.00m bgl. Groundwater strikes within sand and gravel horizons ranged between 1.40m and 6.70m bgl with a maximum rise of 1.50m recorded where a rise occurred. Three groundwater strikes occurred within clay deposits at depths of 1.20m, 1.30m and 5.50m bgl, with no rise, a rise of 0.05m and a rise of 0.50m respectively.

From the levels of groundwater strikes, there appears to be three distinct horizons where groundwater was encountered within the Mudstone. The first horizon is between 6.20m and 6.90m bgl, the second at 9.0m bgl and the third at 15.90m bgl. The monitoring data shows groundwater standing at depths between 0.32m and 2.76m bgl with levels across the site standing between 43.79m and 42.42m AOD. No monitoring data was available for this study from the Bromsgrove Sandstone to the east of the Fault.

The Bromsgrove Sandstone is highly permeable and is classed as a major aquifer (Allen et al., 1997) and as a Principal aquifer by the Environment Agency1. Generally the Mercia Mudstone acts as an aquitard confining the Sherwood Sandstone Group (of which the Bromsgrove Sandstone is a part) and is classified by the Environment Agency as a ‘Secondary B’ aquifer. However, as described above, (Atkins, 2009) reports that due to the presence of the Burton Fault, to the east of this fault, the drift deposits directly overlie the Bromsgrove Sandstone. The drift deposits across the site have been classified as a minor aquifer (Secondary A) with variable permeability potential. It is likely in the river that the drift deposits are in hydraulic connectivity with fluvial levels in the River Trent. The site is not located within a Source Protection Zone.

Faults commonly create zones of broken, disturbed ground. The fault that usually comprises coarse broken rock debris within the fault zone is commonly a source of groundwater flow therefore excavations that intersect the presumed fault systems may encounter strong groundwater seepages.

Groundwater Flood Risk

East Staffordshire Borough Council (2008a) report that the Environment Agency’s groundwater team was consulted and confirmed that there have been very few recorded incidences of groundwater flooding within East Staffordshire. The only events that have occurred are as a result of the cessation of the quarrying of gravel and sand in the area and thus the abstraction of water from the pits. Once the abstraction cessed, the groundwater levels rose and filled some of the pits. As a result of this, there has been one report regarding the occurrence of minor cellar flooding. The SFRA illustrates location with historical flooding from all sources. This shows there no known events of groundwater flooding either on the site or in the neighbouring areas. East Staffordshire Borough Council (2008b) considered groundwater flooding to be of minor risk.

For this FRA the Environment Agency were contacted about groundwater flood risk and in the response dated 1st November 2010 and it was reported that they have no information on groundwater flooding issues in these areas.

There have been no recorded incidences of groundwater flooding in the vicinity of the site and there are no natural springs noted on the 1:50,000 BGS map or historical maps, however, there is still a risk of groundwater flooding due to the permeable of the underlying geology. To the west of the Fault the Mercia Mudstone acts as an aquiclude to the Sherwood Sandstone Group, so the risk of groundwater flooding is low and relates only to the drift deposits which have been classed as a minor aquifer. Ground water levels have been recorded within 0.32 to 2.76 metres of the ground surface in this area. The risk of groundwater flooding surrounding the fault and to the east of the Fault is potentially slightly higher, as here

1 www.environment-agency.gov.uk Accessed 09/12/2010 Principal Aquifers - These are layers of rock or drift deposits that have high intergranular and/or fracture permeability - meaning they usually provide a high level of . They may support and/or river base flow on a strategic scale. In most cases, principal aquifers are aquifers previously designated as major aquifers. These include a wide range of rock layers or drift deposits with an equally wide range of water permeability and storage. Secondary A - permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to . These are generally aquifers formerly classified as minor aquifers. Secondary B - predominantly lower permeability layers which may store and yield limited amounts of groundwater due to localised features such as fissures, thin permeable horizons and . These are generally the water-bearing parts of the former non-aquifers. the highly permeable Bromsgrove Sandstone is unconfined, and in the vicinity of the Fault itself there is the potential for strong groundwater seepages. In this eastern area of the site there are no groundwater level records to determine the current standing groundwater levels.

The proposals for the redevelopment of this site as reported in Section 1.2 involves two aspects; the extension of the existing site to the south to increase treatment capacity of the site and the replacement of existing facilities within the current site boundaries. The extension is proposed through defence realignment and is approximately only 60m west of the existing flood defence location and will be designed in accordance with the Environment Agency guidance for formal flood defences in line with the current defences at this location. It is therefore understood that there will not be any significant impact on flood risk from groundwater.