Project background

Geophysics can be used to reveal a wealth of information in addition to the modern-day survey. In this study, RSK Geophysics was commissioned by Affinity Water to locate and map a hidden palaeochannel as part of a river restoration project. The River Misbourne in the Chilterns is a globally rare chalk stream. At the location of interest, the river’s course had previously been altered and diverted by human milling activities. As part of works to improve flow and ‘renaturalise’ the river back to its original location, a geophysical survey was conducted to identify the former position of the channel in 17 ha of arable fields alongside and west of the current river course.

Geophysical survey

The survey involved ground-penetrating radar (GPR) and electromagnetic (EM) ground conductivity mapping across the entire area. Four additional profile lines of electrical resistivity tomography (ERT) were used to further calibrate the results. The GPR surveys provided high-resolution data in the vertical and the horizontal, with a depth of penetration of up to 4 m. GPR features indicative of buried gravel-filled channels were clearly identified in plain view by areas with reflections of higher amplitude than the surrounding ground. The ERT data were also used to estimate the depth and width of channels to investigation depths of up to 15 m.

The combination of three geophysical techniques increased the confidence of the location of the former buried river channels. As a result, a targeted and lower cost follow-up borehole investigation was subsequently undertaken to verify the geophysics. In the location of the anomalous features, a gravel thickness of up to 6 m was encountered, confirming the geophysical data and most likely palaeochannel location for restoration.

GPR amplitude depth-slice data

Figure 1: GPR amplitude depth-slice data across part of the area at 1m depth. The darker regions highlight areas of higher amplitude interpreted as the buried river channels. Area shown is 1000m in length

High resistivity areas
Figure 2: High resistivity areas in the ERT data were interpreted as the granular channel fill.

Excavation of the paleo channel based on the geophysics results

Figure 3: Excavation of the paleo channel based on the geophysics results. The result is a newly restored channel which meanders, flows at a higher rate, is biodiverse and floodplain connected.