- 1st May 2019
- Posted by: Joanne Woodhouse
- Category: News
The Hydrogeological Group and Contaminated Land Group of the Geological Society conference is taking place today and tomorrow. Alex Jones and Eleanor Haresign, both Senior Hydrogeologists based in our Saltaire office, are presenting talks and a poster within today’s session covering Flood Management/Wetland and Hydroecology Support. The three projects being highlighted are as follows:
Flag Fen – the archaeological significance of superficial aquifer groundwater levels
Concern exists regarding the long-term viability of preserving archaeological remains in the Flag Fen basin near Peterborough, including the Bronze Age Scheduled Ancient Monument timber platform and post alignment. The delicate organic remains are degrading due to desiccation of the underlying soils and peat deposits.
On the basis of a hydrogeological conceptual model for the site, a numerical groundwater model has been used to investigate the relationship between groundwater levels in the superficial deposits and preserved remains.
By calibrating the modelled groundwater levels with observed data, the model has been used to identify three characteristic zones:
- Zone 1: dry – above the seasonal maximum water table
- Zone 2: intermittent – seasonal water table fluctuation
- Zone 3: wet – deeper zone of permanent saturation.
Although archaeological wood is best preserved in Zone 3, many features at Flag Fen are located within Zones 1 or 2. Artificial agricultural drainage controls groundwater levels at Flag Fen and has lowered these levels in otherwise naturally wet fenland.
Modelled future scenarios include both external threats (climate change and development) and water level management options. In 1987 an artificial pond was constructed over the timber platform to raise groundwater levels by artificial recharge.
Modelling outputs suggest that leakage from the pond may be successful in maintaining locally high groundwater levels. Further work explored the potential for ‘top-up’ water to introduce oxygenated water to the groundwater system and inadvertently increase the rate of degradation to the underlying archaeological wood.
Finally, blocking ditches to create a wetland to the southwest of Flag Fen was identified as a way to raise groundwater levels with additional benefits for local biodiversity.
River Colne and Staines Moor – finding the connection
Staines Moor, 1.5km south of Heathrow, is an area of common land designated as a Site of Special Scientific Interest (SSSI) for its botanical intrigue. The River Colne flows through it from north to south and suffers from several issues including fine sediment input, bank erosion, over-deepening, over-widening and over-grazed banks. By understanding the hydrological interactions between the watercourses and the SSSI, the project sought to identify options to improve these features.
The underlying gravels are directly connected to the River Colne, and river stage determines the levels of groundwater across the site. An eco-hydrological conceptual model for the site showed that topographical variations in the flood plain control relative depth to the water table. This resulted in four distinct eco-hydrological levels, which in turn control the distribution of habitats across the site.
In the 1990s, a weir downstream was modified resulting in both river and groundwater levels being raised within the site. By comparing historic and recent National Vegetation Community surveys, the effect of the modification was shown through the replacement of the drier communities (which dominated the site in the 1980s) by the current wetter communities.
Alongside the eco-hydrological conceptualisation, geomorphological and ecological surveys were used to develop vegetation management plans and to outline a river restoration plan sensitive to requirements of the SSSI. River restoration recommendations include narrowing of the channel, using deflectors and gravel augmentation, to maintain the present botanical diversity whilst improving the morphology of the channel.
However, these works have the potential to raise the river stage further and, therefore, affect the eco-hydrological conditions of the SSSI communities. Lowering the downstream weir invert then provides mitigation to reduce inundation in the southeast of the site.
Hydrogeological conceptualisation of a tufa spring, Dún Laoghaire-Rathdown, Ireland
The Cherrywood Strategic Development Zone was proposed to the south of Dublin and lies within the catchment of a nearby tufa spring that supports a rare and protected range of species, including a range of bryophytes and other lime-loving floral species.
Tufa is a superficial deposit associated with hard water springs that are formed through the precipitation of calcium carbonate. The characteristic flora that is supported by the particular hydrogeological and geochemical setting of tufa springs are protected under the Habitat Directive.
Any development within a tufa catchment has to be carefully considered and will not successfully achieve planning permission if it either affects the supply of water to the tufa spring or changes the geochemical signature of the groundwater.
The preliminary hydrogeological conceptual model indicated that recharge through limestone-rich till in the headwaters of the tufa springs catchment enriches the water with calcite and flows into a gravel band overlain by low permeability tills. Multiple stages of site investigation allowed model refinement and comprised long-term borehole monitoring of groundwater levels as well as catchment-wide trial-pitting.
The conceptual understanding was used to identify potential impact mechanisms and classified the catchment into various zones regarding which were appropriate for different types of development. This has provided the council with an effective framework for assessing multiple applications within the catchment.
Want to know more?
Email Eleanor Haresign or Alex Jones for more information on these projects and their Hydrogeology of the Superficial Deposits conference attendance. You can also find out more on our environmental services web pages.