On Saturday 17th October 2026 Parish Grasslands presented three talks on water drainage, supply and management within our parishes of Hewelsfield and Brockweir and St Briavels.
Here is a summary of the three talks which complimented each other.
1. Local Geology and Hydrology by Andy Stott
The two parishes of St Briavels and Hewelsfield and Brockweir comprise an undulating plateau at around 200m, deeply dissected by the Wye Valley gorge on the west side. There are two main streams draining to the west, the Slade/Mork Brook and the Brockweir Brook, both partly formed within ancient meander loops of the river Wye. There are a number of streams draining the plateau to the east, the most significant of which is the Aylesmore/Cone Brook. It is noticeable that springs generally occur on the edge of plateau and many parts of the plateau lack any surface water.
Beginnings – about 400 million years ago
To understand the current drainage of the two parishes it is necessary to go back 400 million years to the Devonian geological period. At this time, the land mass of Britain and Ireland was located on the southern edge of a large continent with chain of fold mountains to the north, the remains of which are found today the rocks of the Scottish Highlands. It was a semi-arid climate and large rivers flowed from the mountains and formed alluvial fans and floodplains stretching towards an ocean to the south. At the end of Devonian period the shallow seas inundated these coastal plains.
Geological map and stratigraphy
The 1:50,000 geological map shows a complex pattern of rocks across the parishes. In terms of effects on local drainage, there are three main groups of rocks: (1) Brownstones (dark grey) – these are hard, well-cemented sandstones comprising a mixture of fine sands and silts formed by slow moving rivers. They are resistant to erosion and underly the Brecon Beacons and Black Mountains in Wales to the west. They have poor permeability; (2) Tintern Sandstone Group (salmon pink) and Quartz Conglomerate (salmon pink with red spots) – these are quite variable sandstones, from coarse, pebbly conglomerates to finer silts formed in dynamic, fast moving river environments. They are generally softer that than the Brownstones, forming well-drained soils and gentler slopes, good for farming. The rocks are permeable and store water within the rock fabric, acting as an aquifer; (3) Lower Carboniferous Limestones (light blue and green) – variable limestones and shales formed in shallow seas, some of which has been dolomitized. The rocks are soluble and are characterized by underground drainage systems in fissures, along bedding plains and in caves.
Schematic Geological Sections of these rock types
When looked at in cross-section (top is south-north and bottom is east-west) the general layer structure (stratigraphy) is clear, with Brownstones at the bottom, followed by Tintern Sandstones, then limestones. But this generally horizontal layer structure is broken up by substantial faults and folding into a steep-sided syncline which trends north-south through the east part of the parishes. Consequently the geological map, previous slide, appears very fragmented and jumbled up. But a pattern emerges when you look at the geological map and drainage system together – the springs and stream headwaters are generally found at the boundary between the underlying Brownstone and the conglomerate/Tintern Sandstone above. In the following slides we will look at two examples in more detail – Hart Hill/St Briavels Common and the Hewelsfield village/Aylesmore areas.
Where do springs come from?
On Hart Hill and St Briavels Common, rainfall infiltrates through the soil into the permeable sandstones and conglomerate rock. The Brownstones below form a thick impermeable layer and a ‘perched’ water table builds up in the sandstones. Springs often occur on the surrounding hillsides and valleys along the boundary between the conglomerates and the Brownstones. During winter or other periods of heavy or prolonged rain the water accumulates, the flow of water from springs increases and springs may appear higher up the slopes.During summer or other periods of low rainfall the water drains out of the rock, flow of water in springs diminishes and some springs dry up
Water emerging in the Mere Brook, well below the usual the start of the stream, during the drought of 2026.
Lying to the east of Hewelsfield village the rocks are folded into a steep, asymmetrical syncline, with a very steep dip of strata on the eastern side of the fold axis. Here there are outcrops of limestone strata above the Tintern sandstone in the centre of the fold. These limestones, weathered over millions of years, have developed underground drainage systems along fissures and bedding plains, There are no streams on the surface and there are many large depressions where the surface rocks have collapsed into underground caverns. Below the limestone, sub-surface water drains into the underlying permeable sandstone strata. This water accumulates in the fold of the syncline, above the impermeable Brownstones, and emerges in caves and springs in the Aylesmore Brook near Clanna, draining east towards the river Severn
The aquifer below the limestone plateau was tapped as a water supply for the parishes of Hewelsfield and Brockweir and St Briavels in the 1950s, thus making the numerous wells and springs around the villages redundant as a water supply. The water is pumped from the St Anne’s Well borehole near Rodmore Mill to an enclosed storage reservoir near Sandy Lane. Having been filtered through the rocks the water is good for drinking with very little treatment needed, but it is also very hard having dissolved calcium on its way through the limestone strata.
Where the water emerges from cave systems in the limestone it has very high content of dissolved calcium carbonate. In aerated water, the calcium carbonate precipitates out forming small obstructions in the stream bed, which cause more aeration, and more precipitation and thus creating a series of tufa or travertine dams which are very well developed in the the Mork/Slade Brook. One of the best examples of these features found in the UK.
What for the future?
We have seen how the natural flow of water from springs varies seasonally between wet and dry weather, and these cycles can vary over periods of multiple years as the water reserves in aquifers can take a long time to replenish. We are now also experiencing longer term changes in patterns of rainfall as a result of climate change. Multiple modeling studies based on projected greenhouse gas emissions show that within 40-50 years from now summer rainfall is likely to be reduced by 10-40% and winter rainfall increased by 5-25%. Droughts, like that experienced in 2026, are likely to become more frequent but also periods of heavy rainfall in winter, causing flooding, are likely to increase.
Projected rainfall and potential evapotranspiration 1980s – 2070s (Thames at Kingston)
It isn’t only the amount of rainfall that effects the flow of springs but also temperature which increases water loss through evapo-transpiration. As well as generally drier summers we can expected hotter summers thus further adding to summer water deficit, without a compensating recharge effect in winter. This would be further exacerbated if rainfall becomes more intensive which means more water runs off the land quickly causing flooding and doesn’t infiltrate into the groundwater reserves.
The upper reaches of the River Thames near Kemble were totally dry for many miles in the summer of 2026
What can we do about it?
- Reduce our carbon emissions!
- Improve infiltration capacity so that more water goes underground
- avoid soil compaction (e.g. heavy machinery, over-grazing)
- increase surface roughness
- Slow the flow of water
- block drains (don’t drain wet areas)
- woody dams and ponds
- Look after your wet patches – they need you