The UK’s Most Famous Chalk Stream

The National Trust and Atkins commissioned Haycock to conduct an investigation into the current soil and hydrology regimes at Mottisfont Abbey Estate. It is a 665 hectare site, situated in the Test Valley, Hampshire, immediately north of the confluence of the River Dun with the River Test. The centre of the estate was the site of a 12th century Augustinian Priory, which following the Dissolution of the Monasteries in 1536 was converted into a house. The name of the estate stems from the Old English ‘motes funta’ a spring where meetings or ‘moots’ were held.

Haycock’s role was to develop a conservation management strategy for Mottisfont Abbey Estate that would not only drive environmentally sustainable management of activities such as fly fishing on the River Test, but also provide long term re-naturalisation objectives of the heavily human-modified River Test floodplain. An understanding of the mechanisms that have driven historical land management and shaped the current landscape was developed. Combining historical records with current data-sets, it is possible to assess the significance of each component that make the site unique, both nationally and internationally. Strategies have been developed which will aid The National Trust to make informed decisions to protect and preserve the site, and restore it to a more natural and sustainable system.

Combining primary data obtained from coring of the peat soils with water network auditing and sediment analysis, along with secondary data obtained from geological, hydrogeological, archaeological and historical data and LiDAR imagery within a complex GIS system, Haycock have been able to conduct a holistic appraisal of the site’s significance.

Key to the natural development of the site is the chalk bedrock underlying the whole estate. Groundwater upwelling enhanced by folding structures in the chalk at this particular point in the Test Valley is a key factor in the continued upwelling of the spring. This upwelling combined with denudation of the anticline within the chalk bedrock has improved the hydraulic conductivity between the aquifer and River Test floodplain, driving the upwelling and saturation conditions that have promoted peat accretion. The sustainable management of the site depends on this groundwater upwelling and it is advised that The National Trust follow the issuing of groundwater abstraction licenses in the local area very closely.

Modification of this landscape has seen the drainage of these groundwater fed water meadows, driving shrinking and desiccation of peat layers. Channelisation of feeder leats has forced sediment deposition within the channel, riverbed accretion and the subsequent precarious elevation of the stream above the floodplain. Haycock’s conservation management plan highlights the need to protect the current peat layers, renaturalise the floodplain to promote more peat growth and to preserve the current historical features such as the ornamental water spiral within the duck grounds. Our recommendation is to trial a series of ditch blocking to promote rewetting of the water meadows. This will not only help to tackle climate change through carbon storage within the peat, but will increase the environmental attraction of the site to visitors.

Sensitivity of future management strategies in the face of uncertainties such as changing climate can only be achieved through a sound understanding of how the direct and indirect variables such as summer temperatures and precipitation might impact upon the different components of the hydrological system operating at Mottisfont. To achieve this, a program of ground- and surface-water monitoring has been proposed, that will bridge the gap in our understanding of the interaction between these two regimes and how this affects the physical environment within the River Test and Mottisfont Abbey Estate.