OR/15/038 Conclusions

Gareth Farr (with contributions from Luz Ramos Cabrera). 2015. Nedern Brook Wetland SSSI Phase 1 hydrological monitoring. British Geological Survey Internal Report, OR/15/038.

For the first time a complete flood cycle has been characterised at the Nedern Brook Wetland SSSI. A hydrological monitoring network of stilling wells, piezometers, boreholes were instrumented with data collected every 30 minutes.

• The Nedern Brook has been heavily modified in the past. It has been straightened and over deepened and acts primarily as a drain for groundwater that discharges onto its floodplains.
• The Nedern Brook was not seen to be ‘overtopping’ or causing fluvial flooding during the study, but only acting as a drain taking water away from the wetland.
• Surface water gauging both upstream and downstream of the wetland proved that flow within the Nedern Brook can be highly variable. Flow downstream of the wetland is often much greater than the flow recorded upstream of the wetland. This difference (up to 225 l/s in January 2015) is attributed principally to groundwater discharge into the wetland.
• A walkover of the Nedern and Castrogi Brook showed that the Victorian concrete lined channel was in a poor state of repair and is very unlikely to prevent surface water loss into the aquifer.
• The wetland can flood to a depth of 1.5 m (based on depth near piezometer P3) and flooding can cover an area nearly 1.5 km in length and cover an area greater than 30 ha.
• Groundwater plays a principal role in the flood regime of the Nedern Brook Wetland and it should be classified as a Groundwater Dependent Terrestrial Ecosystem (GWDTE).
• Key discrete groundwater discharges were identified namely the Upper and Lower Whirly Holes an area of discharge from the limestone outcrop south of the M48 and an unnamed spring that only flows when groundwater levels are high.
• Larger diffuse areas of groundwater discharge on the floodplains were identified within the wetland.
• It is possible that the Nedern Brook Wetland should be reclassified as the UKBAP Priority Habitat ‘aquifer fed naturally fluctuating wetland’ however further information, especially about vegetation zonation, is required.