OR/15/038 Monitoring

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.

Monthly monitoring visits were undertaken between October 2014 and May 2015. Observations on the brook and the wetlands were made from the ‘Cwm’ in the north to its mouth in the Bristol Channel (Figure 2). Information on the location of key groundwater discharge areas was also collected. Detailed survey elevations were collected from repeatable locations near all water level data loggers in both the dipwells and the stilling wells in the brook (see Appendix 2 - Elevation survey data).

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Figure 1    Flow gauging at ‘Nedern DS’ using a Sontek M9 ADCP view south (Photograph with permission of Hydrometry & Telemetry Team South East Wales, Natural Resources Wales).

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Figure 2    Hydrometric Monitoring Locations within the Nedern Brook catchment. Contains Ordnance Survey data © Crown Copyright and database rights 2015.

Surface water

Spot flow gauging was undertaken by Paul Griffiths, John Evans, Ross Adamson and Kris Tomsett staff from Natural Resources Wales’ Hydrometry & Telemetry Team, South East Wales. Spot flow gauging was undertaken at three sites on the Nedern Brook (Table 1, Figure 2, Figure 3) during low, medium and high flow conditions using a Sontek M9 ADCP (Figure 1). The sites represent inflow to the top of the SSSI (‘Nedern Brook at Tyne Cottages’) and outflow (Nedern DS and Nedern Castle Car Park).

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Figure 3    Hydrometric Monitoring Locations within the Nedern Brook Wetland SSSI catchment. Contains Ordnance Survey data © Crown Copyright and database rights 2015.

Stilling wells were installed at the start of the project at two locations called Nedern Brook US and DS (Figure 2). Later on, in January 2015, an additional stilling well was installed further up the Nedern Brook underneath the bridge near Tyne Cottages to complement the spot gauging and to allow for stage-discharge relationships to be calculated in the future. Due to this stilling well being installed part way through the project the data from this site only covers part of the study period.

Surface water gauging was also attempted within the SSSI boundary at the small tributary (ST 48654 89452) and at the ‘Nedern US’ monitoring point (ST 48427 89489). However both were ruled out for further monitoring due to access. Both sites can be entirely flooded and conversely experience extremely low flow velocities during dry periods.

Visual observations of surface water flow were undertaken during the monthly monitoring visits, starting upstream on the Castrogi Brook ‘Cwm’ where the surface water is known to sink and has been traced to the Great Spring (Drew et al 1970, Clarke & Aldous, 1987 and Lawrence et al, 2013). The Nedern Brook was observed throughout the SSSI section and then south through the industrial estate to the outflow.

Groundwater

Groundwater levels in the bedrock Carboniferous Limestone aquifer are monitored at boreholes installed by Natural Resources Wales at Caldicot Castle Car Park, Tyne Cottages, Five Lanes and Dewstone Road (Figure 2, Table 1, Appendix 4 - Borehole logs). Each borehole is instrumented with a vented pressure transducer recording changes in groundwater levels every 30 minutes with the data corrected to maOD. To complement the groundwater level data within the Carboniferous Limestone aquifer a non-vented Solinst Level logger™ with a 10 m range was installed in Piezometer P3 monitoring the shallow clays and peats within the wetland (Figure 2, Figure 3). It was not possible to dip P3 during the majority of the study as it was fully submerged by flood waters.

Precipitation

Precipitation data on a 15 minute basis was provided by Natural Resources Wales from weather stations at Collister Pill and Llanvaches, the data was then converted to daily total (mm).

Elevation survey and flood map

Dipwells, piezometers and stilling wells were surveyed to maOD using a Leica Smart Rover CS10/CS15 & GS 14 Sensors, capable of surveying elevation to an accuracy of <5 cm. Flood levels of the Nedern Brook were also surveyed on a monthly basis to allow for corrections of the water level data loggers installed within the flooded area (Nedern US, Nedern P3 and to a lesser extent Nedern DS). The survey data is included in Appendix 2 - Elevation survey data. The water level data and survey data were combined, and the maximum and mean flood values were input into ArcView, with the 1 m LIDAR data for the area. The LIDAR was ‘flooded’ in order to create two flood maps.

Water chemistry

Water chemistry sampling was undertaken in Dec 2015, with samples collected from the Nedern Brook and flood waters. The samples were analysed at the Environment Agency National Labs. Due to the flood levels it was not possible to sample groundwater directly from the Piezometers, or from discharge from the Whirly Holes, both of which were under water. Field observations of temperature, pH and electrical conductivity were collected during site visits on the 18th December 2014 and 16th January 2015 using a Hannah HI98312 hand held temperature and electrical conductivity meter with an accuracy of ± 2% for electrical conductivity and ± .5°C for temperature. Results for the field parameters are presented in Appendix 3 - Field water chemistry, and the ion analysis in Table 3.

General observations

During each site visit a sketch map was made of the extent of flooding (Appendix 5 - Field maps with sketches of flood extent). Observations on the flow of water in the Nedern Brook were made from the Castrogi Brook ‘Cwm’ monitoring point upstream to the outflow into the Bristol Channel.

Table 1    Monitoring points and monitoring frequency.

Monitoring Point	Source	Type	Method	Depth (mbgl)	Response Zone (mbgl)	Frequency	Easting	Northing
Dipwell DW2	Groundwater	Dipwell	Manual Dip	0.86	0–0.86	60 minutes	348674	189360
Piezo P3	Groundwater	Piezometer	Data logger	1.78	1.815–2.115	Monthly	348675	189361
Dipwell D1	Groundwater	Dipwell	Manual Dip	0.76	0–0.76	Monthly	348638	189368
Piezo P1a	Groundwater	Piezometer	Manual Dip	2.265	1.965–2.265	Monthly	348638	189368
Piezo P1	Groundwater	Piezometer	Manual Dip	3.07	2.77–3.07	Monthly	348638	189369
Barologger	Atmospheric Pressure	Barologger	Data logger	n/a	n/a	30 Minutes	348685	189307
Nedern US	Surface water	Surface water	Data logger	n/a	n/a	30 Minutes	348430	189486
Lower Hole Whirly	Groundwater	Spring/rising	Visual	n/a	n/a	Monthly	348163	189925
Upper Hole Whirly	Groundwater	Spring/rising	Visual	n/a	n/a	Monthly	347354	189943
Caldicot Country Park BH	Groundwater	NRW Observation Borehole	Data logger	70	56–70	30 Minutes	348900	188200
Five Lanes BH	Groundwater	NRW Observation Borehole	Data logger	55	22–55	30 Minutes	344800	190900
Tyne Cottage BH	Groundwater	NRW Observation Borehole	Data logger	65	17–65	30 Minutes	347200	189900
Dewston Road BH	Groundwater	NRW Observation Borehole	Data logger	60	5.1–60	30 Minutes	347100	188500
Great Spring	Groundwater	Abstraction Monitored by Network Rail	Abstraction Monitored by Network Rail		n/a	Daily	350721	187374
Nedern at Tyne Cottages	Surface water	Nedern Brook	NRW Spot Gauging and Data logger	n/a	n/a	30 Minutes	347195	189997
Nedern DS	Surface water	Nedern Brook	NRW Spot Gauging and Data logger	n/a	n/a	30 Minutes	348674	188591
Nedern Castle Car Park	Surface water	Nedern Brook	NRW Spot Gauging and Data logger	n/a	n/a	30 Minutes	348771	188369
Nedern Outflow	Surface water	Nedern Brook	Visual	n/a	n/a	Monthly	348985	187258
Castrogi Brook Sink at the ‘Cwm’	Surface water	Castrogi Brook	Visual	n/a	n/a	Monthly	345879	192739