Scale-dependent lithological variations and their control on water resources and flooding in the Eden Valley

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The Eden Valley (Cumbria, UK) has been involved in two of the worst flooding events this century. It is also an important local water resource. The River Eden courses over a complex geological terrain. Rising on the volcanic rocks of the Lake District, it flows over the Carboniferous limestones of the north Pennines and on to the Permo-Triassic sandstones in the Vale of Eden, a largely rural area where agriculture and tourism are the main sources of income. Comprising the Penrith Sandstone Formation and the Sherwood Sandstone Group, the Permo-Triassic sandstones lie in a fault-bounded basin (approximately 50 km long and 5-15 km wide) and are the region's primary aquifers. Their structure, however, is highly complex due to the heterogeneous nature of the rocks, due to primary and secondary lithological variation. Consequently, an improved understanding of this complexity and its influence on water flow is needed to manage the aquifer's potential as a water resource, and understanding its role in flooding. A key part of this is the role of low permeability features, which are highly scale-dependent. These include the occurrence of thick silicified (depositions of silica) layers. Individual units are relatively small but can form discontinuous bands up to tens of metres thick that extend over several kilometres (Fox, 2016) and affect groundwater recharge and water level dynamics (Lafare et. al., 2016). Another important feature, the Armathwaite dyke, an igneous intrusion that crosses the entire Eden valley, appears to act as a major barrier to groundwater flow on a variety of scales. With increased awareness of this scale dependent complexity, there is a need to incorporate this knowledge into a new hydrogeological conceptual model of the region that can be tested using groundwater flow models, which are able to reproduce groundwater level and surface water flow responses and provide new insights for the management of this geological system and its role in the wider Eden valley. A key aim of the research will be to demonstrate the importance of heterogeneity at a variety of scales in controlling flow in Permo-Triassic sandstones. Not only will this improve our understanding of the relationship between scale dependent geological features and groundwater response, due to their widespread occurrence elsewhere in the UK and internationally, this work is highly transferrible. The basic outline of the project will be to:- Collate available geological data and, working with BGS geologists, enhance a geological model to develop hydrogeological conceptual models;- Encapsulate the conceptual understanding into regional and local scale numerical models of groundwater flow that simulate flow processes and observation borehole hydrograph response;- Examine how this scale-dependent heterogeneity controls groundwater flow and influences water resources and flooding in the catchment ReferencesFox K (2016). Development of a numerical groundwater model of the Eden Valley, using conceptual understanding derived from seasonal trend decomposition. MSc thesis, Imperial College London.Lafare AEA, Peach DW & Hughes AG (2016) Use of seasonal trend decomposition to understand groundwater behaviour in the Permo-Triassic Sandstone aquifer, Eden Valley, UK. Hydrogeology Journal. 24 (1), 141-158.