OR/15/038 Introduction

Gareth Farr (with contributions from Luz Ramos Cabrera). 2015. Nedern Brook Wetland SSSI Phase 1 hydrological monitoring. British Geological Survey Internal Report, OR/15/038.

The Nedern Brook Wetlands Site of Special Scientific Interest (SSSI), referred to from here on as ‘the wetland’ to avoid confusion with the Nedern Brook water course, was first notified in 1988 and covers 44.5 ha of the lower Nedern Brook and its adjacent floodplain. It is owned by both private land owners and Monmouthshire County Council. The wetland was designated as a SSSI for its importance to wading and overwintering birds including redshank, wigeon, and Bewicks swan. There are also populations of breeding birds including lapwing, shelduck and yellow wagtail (Countryside Council for Wales, 1988).

Although the site is designated for its bird interest, the hydrology is just as important as the wetland is subject to seasonally controlled groundwater flooding, creating a temporary freshwater lake about 1.5 km long. The bird populations are only supported when there are flood waters in the wetland, thus understanding of the mechanisms of flooding will enable better management of overwintering wading bird populations.

The Nedern Brook is a complex and heavily modified channel, which has been straightened and over-deepened along its course, however some remnant meanders can still be seen in the floodplain (River Restoration Centre, 2012). Natural Resources Wales (NRW) has identified the lower Nedern Brook as a main river that has poor water quality, ecology and hydrology in terms of its Water Framework Directive (WFD) classification. When a main river is considered at poor status, NRW are required to investigate options to improve the water course.

The Nedern Brook is classified as a main river however it has no permanent gauging stations or historic spot gauging data from which to characterize its flow. The lack of hydrological data coupled with the extensive and complex flooding regime have historically led to uncertainties in terms of flood prediction and modelling (Atkins, 2012) and options for river restoration (River Restoration Centre (2012) and Haskoning UK Ltd (2013).

The wetland is not always in a state of flood and thus understanding the mechanisms and duration of flooding and influence of groundwater are vital to support future management decisions. This may include a better understanding of the duration that the wetland can support wading bird populations based on the known flood duration of the wetland.

The wetland also lies within the Source Protection Zone (SPZ) for the ‘Great Spring’ (Lawrence et al 2013). The Great Spring is the name given to the large dewatering operation for the Severn Railway Tunnel (see Walker, 1888). Since 1887 groundwater has been continually pumped out of the underlying Carboniferous Limestone aquifer to reduce the risk of flooding within the Severn Tunnel. The wider impact of the dewatering and resultant lowering of the groundwater table is unknown as is its effect on the flow regime within the Nedern Brook. Connections with water loss from the Nedern Brook and the Great Spring have been known since the 1880’s and in a desperate bid to reduce the amount of water entering the tunnel during construction Walker ordered his men to concrete 4 km of the Nedern Brook, large sections of which are still visible today. Drew et al (1970) proved this connection using tracers injected at a known sink located on the Castrogi Brook called the ‘Cwm’ and detecting them again at the Great Spring.

This report represents the first attempt to characterise the hydrology of this wetland using new surface water and groundwater data.