Nedern Brook Wetland SSSI. Phase 1, hydrological monitoring

Farr, Gareth. 2016 Nedern Brook Wetland SSSI. Phase 1, hydrological monitoring. British Geological Survey, 45pp. (OR/15/038) (Unpublished)

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This report provides a description of the first targeted hydrological and hydrogeological investigation at the Nedern Brook Wetland SSSI (described as ‘the wetland’) South Wales. The wetland is designated for its importance for overwintering and wading birds. The Nedern Brook – the water course that flows through the wetland from north to south is classified as a main river, however it has been heavily modified in its lower reaches. Historical alterations to the Nedern Brook, such as straightening and over deepening, have resulted in a ‘Poor’ ecological and hydrological status classification for the Water Framework Directive (WFD). This investigation collects data that has previously been absent from other studies and will support decision making in terms of management and potential restoration of the Nedern Brook to meet WFD targets. The hydrology of the wetland and the brook are interlinked and both are heavily influenced by changing groundwater levels within the underlying aquifers. In the summer, water is only visible in the over-deepened Nedern Brook channel that flows through the wetland. In the winter, flooding from groundwater discharge along the floodplains and discrete springs and seepages contributes to the formation of a freshwater lake approximately 1.5 km in length, 1.5 m in depth, covering an area of over 30 ha. Flooding in the Nedern Brook starts with groundwater discharge onto the floodplains rather than over-bank fluvial flooding from the Nedern Brook. The Nedern Brook is over-deepened and acts primarily as a drain, directing water away from the floodplains. During the study there was no evidence that fluvial flooding, from overtopping of the Nedern Brook, was the initial cause of flooding. During the study there was no evidence that flow within the Nedern Brook, especially downstream of the wetland, was inhibited and on all site visits visible flow was reported from Caldicot Castle to the mouth of the brook in the estuary. Water levels were recorded during one ‘fill and empty’ cycle between September 2014 and May 2015. Monthly field observations and detailed elevation surveys were undertaken to improve the understanding of the flooding mechanisms in the wetland and to identify areas where groundwater discharge enters the wetland, contributing to flooding. Spot gauging to calculate flow within the Nedern brook was undertaken both above and below the SSSI. The flow measurements show that there is a greater volume of water in the Nedern Brook downstream of the wetland (outflow) than there is upstream of the wetland (inflow). This difference, which can be as much as 225 l/s in January 2015, can be attributed mainly to groundwater discharge into the wetland area, although direct rainfall and other surface water inputs are likely to contribute to the flood waters. Further work is needed to translate existing river stage data and spot gauging data into stage discharge curves. Further north of the wetland the Nedern Brook loses its water both to a discrete sink at a location called the ‘Cwm’ and it continues to do so along its course towards the M48 road bridge. The concrete lined channel installed by Victorian engineers, in an attempt to reduce water inflow into the Severn Tunnel, is reported to be in poor condition and ineffective in retaining water in the brook.

Item Type: Publication - Report
Funders/Sponsors: Natural Resources Wales
Additional Information. Not used in RCUK Gateway to Research.: This report has been internally reviewed but not externally peer-reviewed
Additional Keywords: GroundwaterBGS, Groundwater, Surface water interaction
Date made live: 10 Aug 2016 15:31 +0 (UTC)

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