New geological mapping underway to help manage flooding along the River Tweed

It is a common assumption that flooding arises as a result of intense rainfall. Whilst this is true in many cases, another common cause is actually groundwater flooding, where groundwater emerges at the surface. Groundwater flooding can occur in a variety of geological settings and poses a significant risk around the UK. In England and Wales alone, it’s estimated that groundwater flooding accounts for around £530 million in damages per year (ESI, 2016). The impacts can be devastating for farmers, local businesses and homeowners, and cause significant disruption across regional transport routes.

BGS geologists have started a four-year project that will enhance knowledge of ground conditions and undertake flood-pattern investigations around the River Tweed. This survey will provide crucial geological data for local authorities, organisations and others involved in land-management initiatives to the benefit of the 450 000 people who live in or around the flood-prone Borders region.

Much of the region was last surveyed in the 1920s (or earlier) and this latest project forms a central part of BGS’s current national geological mapping programme. The picturesque landscapes so synonymous with the famous salmon river have been formed over millions of years. Most recently, they were sculpted by ice during the last glaciation, which ended around 19 000 years ago in the Tweed catchment.

Existing maps indicate that the area is dominated either by glacial till or bedrock but, in reality, it is a lot more complex than this, with a continuum from bedrock through weathered bedrock, slope deposits and till. These nuances will be a particular focus for geologists as they are likely to have an effect on how groundwater moves through the shallow subsurface, due to the more open pore space of slope deposits compared to till or bedrock. This enhanced understanding will help to provide baseline data for flood monitoring and forecasting for those in the environmental modelling and policy communities, as well as landowners, to make more informed decisions about the management of land in the catchment area.

Our understanding of glacial systems has evolved enormously in the hundred years since the Tweed catchment was last surveyed. In the 1920s, geological surveys were performed solely by walkover surveys, but now we are able to take advantage of high-resolution Earth observation data to understand and model these systems.

Creating new maps using a modern understanding of how ice sheets develop combined with this new data will improve our understanding of this crucial waterway and may enable us to use additional knowledge of ground conditions to assist with land-planning initiatives that will help manage flooding.

Sam Roberson, BGS Quaternary geologist and Tweed project lead.

The Tweed project is exploring new technologies to create geological maps, combining artificial intelligence, high-resolution terrain data and field observations to understand how the different deposits affect the shape of our landscapes. Field studies began in the Cheviot Hills in spring 2025, before heading to the Tweedsmuir Hills during summer 2025 and around Melrose and Galashiels during autumn 2025. All fieldwork is completed on foot, making observations about the sedimentary exposures and the form of the landscape.

Geological maps produced from this work will form part of the national geological map, which can be freely accessed on the BGS maps portal or via the GeoIndex (onshore) on the BGS website. Research papers and reports will also be accessible via the BGS website and the NERC Open Research Archive service.