Wednesday, 7 September 2016

A new lake for Wales?...by Gareth Farr

The low lying valley in which the ephemeral lake 'Tir-llyn Nedern' forms, the
gate in the middle is part of a footpath when water levels recede. 
A recent hydrological study sheds light on an unusual wetland in South Wales. During the autumn and winter a shallow lake, up to 2m deep and 1.5km in length can form, only to dry up completely during the spring and summer.  The lake is not marked on any maps, neither does it have a name. Gareth Farr (BGS Wales) and Catrin Grimstead (Natural Resources Wales) explain why this type of wetland is rare and what causes this unusual flooding.

The unnamed lake is partially located within the Nedern Brook Wetland SSSI, located near the picturesque Caldicot Castle in Monmouthshire. The wetland is a popular site for overwintering birds, especially waders and waterfowl including Bewick's swans, redshank and widgeon.

Situated in a broad valley, the wetland is underlain by glaciofluvial sand and gravels and Carboniferous limestone bedrock. A small main river, the Nedern Brook, flows North-South through the wetland. The brook has a long history of alteration (too long to describe in this blog post), being both over-deepened and canalised, resulting in a loss of morphological and ecological status.  It was due to these negative impacts that Natural Resources Wales were required by European Law to consider options for restoration of the water course. Previous studies in the area highlighted a lack of hydrological evidence (in both the book and associated wetland) as the main knowledge gap prohibiting restoration plans. To help fill in the knowledge gaps we undertook a year of groundwater and surface water monitoring, recording the levels and flow to better understand the hydrological regime.

Tir-llyn Ndern in flood (L) and after the flood waters have receded (R). 
 Our study, funded by Natural Resources Wales, suggested that as water levels rise in the Carboniferous limestone aquifer below the site (eventually reaching the surface), springs and seepages result in the formation of a temporary (or ephemeral) freshwater lake. The process by which the temporary lake occurs is not due to ‘fluvial’ flooding caused by the over-topping of river banks but is a result of rising groundwater levels. This process is entirely natural and is dominated by the seasonal changes in groundwater levels coupled with the topographically low setting of the Nedern (it is just 5m or so above sea level).  The lake dries up as groundwater levels in the underlying aquifer recede during the spring and summer.

Flood depth map for Tir-llyn Nedern. The old river meanders can
still be seen (yellow) adjacent to the more linear present day
channel (also in yellow). The map is based on maximum flood
levels (Ordnance Survey Data © Crown Copyright and database
rights 2016). 
Naturally fluctuating wetlands, that form ephemeral open water bodies, are rare in the UK. They are rarer still when groundwater controls the flooding. Currently the only recognised examples of this ‘aquifer fed naturally fluctuating wetlands’ habitat in the UK are the Norfolk Breckland Meres, a single turlough in south Wales and three turloughs in Northern Ireland, covering just 31ha combined. The Nedern Brook Wetlands, when flooded, covers an area of nearly 40ha and could potentially be a significant addition to this rare UK habitat. It is tempting to suggest that the Nedern Brook Wetlands is referred to as a ‘turlough’, but in Wales we need a name to distinguish our fluctuating wetlands. Thus we propose a new name for this ephemeral lake; ‘Tir-llyn Nedern’. (Tir-llyn translating as ‘land lake’ in Welsh).

Acknowledgments: Natural Resources Wales for funding of the study and numerous staff at NRW including Catrin Grimstead, Liz Lawrie-Meddins, Rob Bacon, Alan Price and Julian Woodman.

Reference: Farr, G. 2015. Nedern Brook Wetlands SSSI Phase 1 hydrological monitoring. British Geological Survey Open Report OR/15/038 for Natural Resources Wales.

@NatResWales @BGSWales @garethfarr1 @Caldicot Castle

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