OR/15/038 Results and discussion

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.

Surface water and groundwater

Prior to this study the absence of hydrometric data, including flow and flood levels, for the Nedern Brook had resulted in uncertainties for, flood prediction and modelling (Atkins, 2012) and potential options for river restoration Haskoning UK Ltd (2013). The paucity of hydrometric data was highlighted as the ‘most significant data gap’ by Haskoning UK Ltd (2013). It is this lack of data on the Nedern Brook that data within this survey is hoping to address.

Surface water spot gauging

Spot gauging results (Table 1 and Table 2 Flow gauging in the Nedern Brook) show that temporal variations in flow can range from 0.086 to 0.256 m³/s (Nedern at Tyne Cottages Bridge upstream of the wetland) to 0.151 to 0.481 m³/s downstream of the wetland (Nedern DS). The difference between the upstream (Nedern at Tyne Cottages) and downstream monitoring points (Nedern DS) can reach 0.255 m³/s (31st Jan 2015). The difference in flow is related to additional discharge into the wetland from either groundwater discharge onto the floodplain, baseflow to the brook, surface water from small tributaries, field/surface drains and direct precipitation. It is proposed that the majority of this additional flow originates from groundwater that discharges into the wetland from discrete inflows such as the Upper and Lower Whirly Hole or across more diffuse areas where groundwater upwells onto the floodplain or where it can be seen discharging from bedrock outcrop. To better understand the additional contribution into the wetland area future work should focus on forming a stage discharge relationship between the stage at the DS gauging station spot gauging in the brook.

Table 2    Flow gauging in the Nedern Brook.

SITE	NGR	Date	Time	Flow m3s-1 (cumecs)	Date	Time	Flow m3s-1 (cumecs)	Date	Time	Flow m3s-1 (cumecs)
Nedern at Tyne Cottages Bridge	ST 47199 89998	30/11/2014	10:10	0.193	06/12/2014	10:10	0.086	31/01/2015	10:34	0.256
Nedern US	ST 48427 89489	30/11/2014	11:15	not possible	06/12/2014	11:15	0.083	not possible	n/a	n/a
Small trib	ST 48654 89452	30/11/2014	11:55	not possible	06/12/2014	11:55	0.042	not possible	n/a	n/a
Nedern DS	ST 48676 88593	30/11/2014	12:30	0.289	06/12/2014	12:30	0.151	31/01/2015	12:30	0.481
Nedern Castle Car Park	ST 48771 88369	30/11/2014	13:40	0.319	06/12/2014	13:40	0.138	31/01/2015	12:50	0.473

Base flow index

Base flow is the percentage of water in a stream or river that is not derived from surface runoff, and high base flow values indicate a strong groundwater control. A modelling exercise (Atkins, 2012) estimated BFI-HOST (base flow index values using the HOST soil classification) values of between 0.677 and 0.739% suggesting that flow within the Nedern comprises of 68–74% baseflow from discharging groundwater. Although there was no hydrometric data to base this upon the assumption that BFI is high is not disputed and could provide an explanation for the observed increase in flow upstream and downstream of the Nedern SSSI.

Precipitation

Rainfall data was supplied from NRWs monitoring points at Collister Pill and Llanvaches as 15 minute data converted into daily totals. No on site data was collected as part of this project.

Observations on the influence of the Nedern Brook during flood events

During each site visit observations were made on both water levels in the wetland and in the Nedern Brook. The key observation is that the over deepened Nedern Brook acts as a drain, taking flood waters away from the adjacent floodplain. Flooding does not appear to be of ‘fluvial flooding’ type and does not initiate from over topping of the Nedern Brook. Evidence for this can be seen in numerous locations, throughout the wetland, both above and below the M48 road bridge. Drains installed into the river banks (P915242) to take water from the floodplains into the brook, were further evidence that flooding initially occurs on the floodplain and then drains into the Nedern Brook.

The second key observation was that, during the flood period, flow was observed in the Nedern Brook below the wetland area all the way to its mouth in the Bristol Channel. This flow observed in the brook is evidence that during this study, water was actively draining from the wetland area, and was not impeded. The monitoring period could be considered relatively dry and 2014–2015 was certainly not a winter of excessive rainfall when compared the stormy weather of the previous winter (MetOffice Winter 2014/15 summary). Flow conditions and the likelihood of impeded flow within the Nedern Brook have not been observed during more prolonged wet periods.

File:P915242 fig4.jpg

P915242     Flooding initiated on the banks and floodplains drains into the Nedern Brook © BGS NERC.

Observations on groundwater discharge

During the walk over surveys it was possible to observe areas where groundwater was discharging into the wetland, the key areas are illustrated on Figure 5. The Lower Whirly Hole was actively discharging groundwater for most of the monitoring period and during recession other small seepages and springs appeared nearby. The electrical conductivity of the spring water was 580–670 µs/cm, indicative of groundwater. A large spring head/seepage area can be found in a woodland area just to the north of the Lower Whirly Hole however it was only actively discharging water during very high flood levels, remaining dry for the majority of the monitoring period.

File:OR15038fig5.jpg

Figure 5    Key groundwater and surface water discharges in the Nedern SSSI (P502171, P915237, P915234, P915243). © BGS NERC.

The Upper Whirly Hole remained dry for the majority of the monitoring period only becoming flooded during January–February 2015. It is associated with a spring head, near the large Oak Tree and is also in very close proximity to the Tyne Cottages NRW monitoring borehole. On a previous visit in 2012 groundwater could be seen seeping upwards through the very sandy soil near the Upper Whirly Hole.

To the south of the M48 road bridge an outcrop of Carboniferous Limestone occurs between ST 48365 89487 and ST 48211 89555. This appears to be an important area for groundwater discharge into the wetland (see video in Appendix). The electrical conductivity of the water was measured at 740 µscm, indicative of groundwater from the Carboniferous Limestone. Flow across this area was estimated in a small channel draining into the Nedern Brook at 10 l/s (18.3.2015) however the true volume of groundwater seepage across this area is likely to be much greater. Eventually the water is intercepted by the channelised Nedern Brook to the east flowing through the remainder of the wetland.

Diffuse areas of groundwater discharge occur across the floodplains of the wetland and are most notable to the north of the M48 road bridge near ST 4787 8989 but also occur south of the bridge in areas centred at ST 4844 8952, ST 4872 8940 and ST 4829 8953.

Groundwater and flood levels in the Nedern Brook Wetland

Groundwater in the underlying Carboniferous Limestone aquifer is monitored by NRW as part of routine monitoring within the Great Spring Source Protection Zone (SPZ). The general overall trend of groundwater levels (Figure 6) within the limestone aquifer is very similar. There is a significant groundwater abstraction at the ‘Great Spring’, located about 2 km to the south east. The Great Spring is a dewatering operation to keep the Severn Tunnel from flooding. It is monitored by Network Rail (Figure 6). Pumping at the Great Spring has to respond to increasing groundwater levels in order to maintain groundwater at a set level within the tunnel and thus also shows a similar trend to the groundwater hydrographs. Figure 6a illustrates groundwater levels in the Caldicot Country Park borehole, during September when groundwater levels are <0 maOD. Groundwater levels would not normally be <0 maOD under natural conditions and it is prosed this is a dewatering effect of the Great Spring. Small changes in the same hydrograph (Figure 6b) are also possible responses to pumping at the Great Spring. The Tyne Cottages borehole, Figure 6c, is geographically closest to the wetland and has a range of nearly 6 m.

File:OR15038fig6.jpg

Figure 6    Groundwater levels in the Carboniferous Limestone aquifer compared to pumping rates at the Great Spring.

Groundwater levels recorded from a piezometer ‘P3’ located within the wetland are plotted against surface water levels collected from the stilling wells on the Nedern Brook and groundwater levels from the NRW Tyne Cottages borehole(Figure 7). Tyne Cottages is the closet bedrock borehole to the wetland that monitors the underlying Carboniferous Limestone aquifer (Figure 7). The ground level near the dipwells and piezometers, approximately 4.6 maOD, is marked by the black dashed line (Figure 7 and Figure 8). The rapid rise in groundwater levels within the limestone aquifer, in response to precipitation, is reflected by a rise in Piezometer P3 (Figure 7a) and also the stage readings within the Nedern Brook US and DS monitoring points. During the flooding period the piezometric head in the limestone aquifer (Tyne Cottages borehole) is lower than that of the flood water in the wetland suggesting that there is a limited vertical movement of groundwater from the limestone during the initial flooding period. This could highlight that springs and seepage from shallower sources such as the River Terrace Gravels (which are not instrumented) are in part responsible for initial flooding within the wetland. However in late November the piezometric head in the limestone aquifer reaches 7 maOD (Figure 7b), higher than the ground level within the wetland and greater than the flood waters within the wetland, suggesting that vertical flow of water upwards into the wetland might be possible if a low permeability pathway (such as a sand or gravel horizon) exists. The flood depth reaches about 1.5 m in the vicinity of the dipwell and piezometer nests and covers an area of over 30 ha (see Appendix 5 field maps for the 1st January 2015).

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Figure 7    Groundwater and surface water levels in the wetland.

Precipitation at Collister Pill gauging station is compared to groundwater levels in the Carboniferous Limestone (Tyne Cottages Borehole) and flood levels in the Nedern wetland, Piezometer P3 (Figure 8). The black dashed line represents ground level within the wetland next to the peizometer, and not at the Tyne Cottage borehole. It is clear that flooding in the Nedern Wetland occurs before the piezometric head in the limestone aquifer is great enough to cause surface flooding (Figure 8a), suggesting either an input from another source such as the overlying river terrace gravels or impediment of downwards flow by low permeability infill within the Nedern Brook Wetland.

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Figure 8    Rainfall compared to flood depth in the wetland (P3) and groundwater levels in the Carboniferous Limestone (Tyne Cottages).

Flood depth maps

The following flood depth maps were produced to illustrate the maximum and mean flood depths during the monitoring period (Figure 9; Figure 10). The existing and historic meanders of the Nedern Brook are clearly visible, the deeper areas tend to be those that flood first and retain water longest.

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Figure 9    Maximum Flood levels based on 5.92 maOD elevation of maximum flood depth between 30/09/2014 and 1/05/2015.

File:OR15038fig10.jpg

Figure 10    Mean flood depth, based on 5.15 maOD elevation of average flood during between 30/09/2014 and 1/05/2015.

Groundwater chemistry

The groundwater chemistry provides only a snapshot of the ionic composition of the water in the brook and in the flood waters during December 2015. Due to the flood waters it was not possible to sample directly from the piezometers or dipwells, nor from the discharge from the Whirly Holes. Samples from the Great Spring abstraction are provided, to illustrate the composition of groundwater from the Carboniferous Limestone aquifer (Table 3 and Figure 11).

Calcium bicarbonate dominates the water types, however the Great Spring outflow, that represent groundwater from the Carboniferous Limestone aquifer, has higher levels of sulphate especially when sampled at the outflow to the River Severn. The groundwater from the Great Spring is more mineralised that the waters in the Nedern Brook and the wetland, however this could representative of a longer residence time of groundwater within the Carboniferous Limestone aquifer before it reaches the Great Spring. In the upper part of the Nedern Brook (nr Caldicot Castle) nitrate (5.45 mg/l) concentrations reflect that of local groundwater in the Great Spring (5.61 mg/l), which may reflect the high amount of baseflow that the upper course of the brook receives from the underlying limestone aquifer. The effects of dilution, from direct precipitation or from groundwater from the river terrace gravels are not understood.

File:OR15038fig11.jpg

Figure 11    Major ions of water samples taken from Table 1, shown in a ‘Piper Trilinear Diagram’. Most samples are of CaHCO₃ type as highlighted by the red dashed circle in the upper triangle.

Table 3    Water chemistry analysis from the Nedern Brook SSSI (wetland), Nedern Brook (surface water) and the Great Spring (groundwater). All data stored and accessible on the Natural Resources Wales ‘WIMS’ database.

Location		Nedern Brook at Tyne Cottage	Nedern Brook nr Castle	Flood water nr limestone outcrop	Flood water near dipwells	Surface drain from housing estate		Great Spring: Outflow	Great Spring: Sudbrook Pumping Station
Date		10-Dec-2015	10-Dec-2015	10-Dec-2015	10-Dec-2015	10-Dec-2015		30-Sep-2014	22-Oct-2014
Time		12:42	10:48	12:03	10:24	11:47		12:12	15:45:00
Type		Surface water	Surface water	Flood water	Flood water	Drain		Groundwater	Groundwater
NRW ‘WIMS’ Code		660078	660078	660078	660078	660078		51260	48420
E		347190	348674	348293	348658	348415		350838	350701
N		190000	188592	189500	189270	189460		187472	187431
Temp	°C	9.9	8.6	9.3	8.6	11.2		13.4	11
EC @25°C	uS/cm	236.2	255.3	236.8	273.8	178.9		1177	825
Ammoniacal Nitrogen as N	mg/l	<0.03	0.04	0.07	0.039	<0.03		<0.03	<0.03
Nitrogen Total Oxidised as N	mg/l	2.42	5.47	1.70	2.04	0.80		5.61	5.50
Nitrate as N	mg/l	2.42	5.45	1.69	2.02	0.78		5.61	5.50
Nitrite as N	mg/l	<0.004	0.017	0.009	0.016	0.027		0.004	<0.004
Hardness as CaCO3	mg/l	103	114	103	124	52.8		403	374
Alkalinity as CaCO3	mg/l	89.8	97.7	89.1	106	46.5		290	282
Chloride	mg/l	11.8	12.6	11.8	13.8	10.6		283	61
Orthophosphate reactive as P	mg/l	0.021	0.07	0.029	0.104	0.12		0.039	<0.0.2
Sulphate as SO4	mg/l	13.7	14.3	13.8	14.2	11.2		60.1	41
Phosphate TIP	mg/l	0.0339	0.0807	0.0418	0.123	NR		NR	32
Sodium	mg/l	7.76	8.23	7.72	8.72	7.73		90.5	32.0
Potassium	mg/l	2.02	2.61	2.19	3.08	2.5		4.88	2.6
Magnesium	mg/l	8.77	9.36	8.75	10.1	1.85		44.1	37.1
Calcium	mg/l	26.7	30.3	26.9	32.9	18.1		92.2	88.1
pH in Situ	pH	7.93	7.31	7.54		7.67		7.58	7.5
Manganese	ug/l	22.2	11.6	<10	14.5	33.2		<10	<10
Iron	ug/l	117	116	72	189	66.4		34.6	<30
Manganese Dissolved	ug/l	<10	<10	<10	<10	24.8		<10	<10
Iron Dissolved	ug/l	<30	51.6	<30	75.1	<30		<30	<30
Ionic balance	%	-2.85	-5.04	-1.38	-0.324	-2.13		-11.3	1.7
Bicarbonate as HCO3	mg/l	110	119	109	129	56.7		354	344
Oxygen Dissolved %	%	106	89.9	72.8	84.7	92.8		104.5	90.8
Oxygen Dissolved as O2mg/l	mg/l	12	10.5	8.34	9.87	10.2		10.9	<0.02

Site walkover of the nedern brook concrete lined channel

The concrete river channel was installed to minimise water loss to ground and to reduce flow to the Great Spring. It was constructed in just a few months between August and October 1883, (Walker, 1888). The channel was constructed on the upper part of the Nedern Brook from the Cwm (ST4591093175) and Rodge Farm (ST4609509461) for a distance of 3 km. There are no concrete sections within the wetland SSSI boundary. Haskoning UK Ltd (2013) recommended that a survey was undertaken to assess the state of the concrete channel. Although this was not done during this project a similar survey had been undertaken for Environment Agency Wales as part of the Great Spring work (Lawrence et al 2013). The survey showed that the concrete channel was still visible over much of the original 3 km reach however the concrete bed ‘is now in a poor state of repair and it is considered unlikely to prevent recharge to the aquifer from the Castrogi or Nedern Brooks’(Lawrence et al 2013). The Nedern Brook is known to have a discrete sink at the ‘Cwm’ (P915235) and will also dry up along much of its lower reach (P917085).

File:P915235fig12.jpg

P915235    Nedern Brook sinking to the base of the river, at the Cwm (1.5.2015). The concrete river channel is still visible however it does not restrict the water from sinking to ground. © BGS NERC.

File:P917085 fig13.jpg

P917085     Nedern Brook (dry) looking south towards the M48 road bridge 1st May 2015. © BGS NERC.

Classification

The classification of the Nedern Brook Wetlands SSSI is not the main purpose of this study however it is worth some consideration in light of the information collected. This study has shown that the wetland is ephemeral, fed by springs and groundwater seepages and that it responds to changes in groundwater levels in the underlying aquifers. The wetland should be considered to be a ‘Groundwater Dependent Terrestrial Ecosystem’ (GWDTE). The current SSSI Citation (CCW, 1988) does correctly note that groundwater levels control the flooding regime, however the site is only classed as ’productive meadows’.

One possibility is that the Nedern Brook SSSI could fit the description of the Priority Habitat ‘aquifer fed naturally fluctuating water body’ (UKBAP, 2008), however additional data would be needed to confirm that the vegetation displayed the characteristic zonation of these habitats. Aquifer fed naturally fluctuating water bodies are rare in the UK with only 10 ha in Northern Ireland, 1 ha in Wales and 20 ha in England (UKBAP, 2008). In comparison the Nedern Brook Wetland when flooded covers an area over 30 ha. Currently the wetland fits some, but not all, of the classification criteria leaving several grey areas in terms of any potential future re-classification.

The Nedern Brook SSSI fits the UKBAP priority habitat criteria including:

• Natural water body that has an intrinsic regime of extreme fluctuations
• Periods of complete or almost complete drying out occur
• Water flooding exceeds 0.5 m depth

However it does not fit the following criteria:

• The wetland should not have an inflow and outflow stream
• Aquatic vegetation should not be present

The following criteria need further data collection to allow reassessment:

• It is unknown if specialist semi-aquatic bryophytes capable of withstanding fluctuating water levels are present (survey required)
• There is no NVC map and concentric zonation of vegetation (if any) has not yet been identified
• The aquatic fauna is currently unknown and the wetland may not include any key species often associated with the priority habitat
• Nutrient status reflects that of local groundwater (requires more detailed sampling and analysis)

In conclusion the Nedern Brook Wetlands SSSI has several key features that are similar to the UKBAP Priority habitat ‘aquifer fed naturally fluctuating water body’ and also several features that remain unassessed due to the lack of information, thus is it not currently clear if the wetland fits the UKBAP description for this habitat. Consideration of these features could be beneficial should the classification of the wetland be updated in the future as and when information become available.