River Tarrant Preservation Society

Review of the Impact of Abstraction

on River Flows

5th January 2012

John Lawson, FREng FICE FCIWEM

March House

Ogbourne St George

Marlborough

Wiltshire

SN8 1SU

1

River Tarrant Preservation Society

Review of the Impact of Abstraction on River Flows

Contents

Summary ................................................................................................................................................. 3

1. Introduction .................................................................................................................................... 4

1.1 Objective ................................................................................................................................. 4

1.2 Scope of work.......................................................................................................................... 4

2. Review of previous investigations .................................................................................................. 5

2.1 Marcus Hodges reports (1995 & 1996) ................................................................................... 5

2.2 John Stowell report on hydrological study (1998) .................................................................. 6

2.3 ESI report on Shapwick and Stubhampton abstractions (2003) ............................................. 6

2.4 Entec report on the Wessex groundwater modelling study (2011) ....................................... 8

2.5 Conclusions from review of existing reports ........................................................................ 13

3. Further analysis ............................................................................................................................. 15

3.1 The relationship between river flows and groundwater level .............................................. 15

3.2 Further analysis of modelling output .................................................................................... 16

4. Conclusions and recommendations .............................................................................................. 20

4.1 Conclusions ........................................................................................................................... 20

4.2 Recommendations ................................................................................................................ 21

Appendix A – List of documents reviewed ........................................................................................... 22

Appendix B – River Tarrant Water Body Classification ......................................................................... 23

Figures

Figure 1 - Calibration of modelled groundwater levels .......................................................................... 9

Figure 2 - Model calibration for flows at Tarrant Monkton in 1998 ..................................................... 10

Figure 3 - Impact of abstraction at Tarrant Monkton (as presented by EA) ......................................... 12

Figure 4 - Impact of abstraction at Tarrant Monkton in 2004/05 ........................................................ 12

Figure 5 - Relationship between river flows and groundwater levels .................................................. 15

Figure 6 - JDL/Entec modelling of impact on winterbourne flow at Tarrant Hinton ............................ 16

Figure 7 - JDL/Entec modelling of the impact on flows at Tarrant Keynestone ................................... 18

2

Source: ESI report Figure 4.2

Location map

Tarrant Keystone Manor OBH

Stubhampton OBH

Crichel Chitterwood OBH

Black Lane

Shapwick

Stubhampton

Sturminster Marshall

Water supply borehole

Observation borehole (OBH)

3

Summary

This report presents the findings of a brief review I have undertaken for the RTPS of the possible

impact of abstraction on the River Tarrant. It is based on about one week’s work, drawing mainly

on the output of a number of investigations undertaken since 1995.

In my opinion, the studies undertaken by Marcus Hodges in 1995/96 and ESI in 2003 raised

concerns over the likelihood of abstraction impacts that have not yet been properly addressed.

The Wessex regional groundwater model, prepared by Entec Ltd in 2011, seems excellent in

many respects, but it has not been calibrated to fit flows in the River Tarrant. The calibration for

Tarrant flows is so poor that the model should not be used, in its present condition, as a tool for

assessing impacts on river flows. However, I believe that the model could be readily improved to

give reliable results for the Tarrant.

I have shown that there are clearly defined relationships between recorded groundwater levels

and river flows that can be combined with the model output for predicted groundwater levels

(which are more reliable than its predictions of flows) to give indications of the likely impacts of

abstraction on flows. These are:

1. In the winterbourne around Tarrent Hinton, flows in the spring are probably being

reduced by about 15% to 50% each year. The river dries about one week earlier than

naturally. The head of the river is moved downstream by about 500 m.

2. In the lower river around Tarrant Keynestone, flows are probably being reduced by in

the region of 20% to 60% in the late spring and summer every year. The recession of

river flows is about 10 days earlier than with the natural condition.

3. In my judgement these flow changes are likely to have a significant impact on river

ecology, particularly on river plants, aquatic invertebrates and juvenile fish. The timing

of the flow changes, mainly in the spring and early summer, is likely to be especially

damaging.

My conclusions on abstraction impacts are based on a fairly crude method of assessment.

However, I believe they are sufficiently robust to justify re-opening the case for restoring

sustainable abstraction in the Tarrant.

This should start with a three month desk study involving re-calibration of the existing

groundwater model to fit flows in the Tarrant. The model can then be used to assess the impacts

of the abstractions at Stubhampton, Shapwick and Black Lane, separately and together. A report

should be prepared to address the unanswered questions from earlier investigations and the

findings of this report. Flow gauging in the River Tarrant, which has not taken place since 2006,

should be resumed.

In my opinion, the case for dropping the concerns raised in earlier investigations of the Tarrant

has not been properly made. I believe that there is evidence that abstraction is probably having

a significant impact on the river. The long-running complaints of local people, which are based

on intimate knowledge of the river over the past 50 years or more, are justified and have not

been satisfactorily answered.

4

1. Introduction

1.1 Objective

The River Tarrant Preservation Society has for many years been concerned about the impact of

abstraction on the River Tarrant. There have been various reports and papers produced by or for

RTPS, Wessex Water, the Environment Agency and their predecessors over the past 30 years, as

listed in Appendix A. These have led the Environment Agency to conclude that abstraction is not

having a significant impact on the river.

RTPS now wish to have an independent opinion on this conclusion. They seek advice on whether

the present condition of the river is adversely affected by abstraction and whether they should

continue to express concerns and lobby for improvements.

1.2 Scope of work

The Environment Agency and Wessex Water have been very helpful in providing the documents

and data listed in Appendix A. I have reviewed the information focusing on:

The adequacy of the data on which conclusions are based

The quality of the analysis presented

The robustness of the conclusions reached

I have looked at the impact of abstractions within the Tarrant catchment, including

Stubhampton, and at the Shapwick abstraction. I have also looked at the wider issue of whether

River Tarrant flows are affected by a general lowering of the water table in the regional chalk

aquifer, caused by abstractions in adjacent valleys. To this end I have obtained and reviewed the

output of the Wessex regional groundwater model which was used recently as evidence that

abstraction is not adversely affecting the river (item 29 in Appendix A and The Environment

Agency’s letter to the RTPS dated 22nd June 2011).

Although not part of my original terms of reference, I have also undertaken additional analysis

incorporating some of the output from the regional groundwater model and using a simplified

model I have developed of the relationship between recorded groundwater levels and flows in

the Tarrant catchment.

This report sets out my findings and makes recommendations for further actions by RTPS, in a

format suitable for discussion with the Environment Agency and Wessex Water.

5

2. Review of previous investigations

I have reviewed the documents listed in Appendix A. I have focused my review on four main

investigations undertaken over the past 15 years:

The investigations undertaken by Marcus Hodges Environment Ltd for the National

Rivers Authority in 1995 and for the Environment Agency in 1996.

The hydrological study undertaken in 1998 by John Stowell, consulting hydrologist, for

RTPS and North Dorset District Council

The study “Assessment of the effects of Shapwick and Stubhampton PWS abstractions

on flows in the River Tarrant” undertaken by ESI Ltd for the Environment Agency in 2003

The Wessex basin groundwater modelling study undertaken by Entec Ltd, reported in

2011.

I have reviewed these in the following sections.

2.1 Marcus Hodges reports (1995 & 1996)

Marcus Hodges Ltd’s reports were in the form of short technical memoranda prepared by Ian

Colley, now with Wessex Water. They addressed concerns expressed by local residents that

abstraction adversely affects flows in the Tarrant and causes the spring head in summer to

migrate below Tarrant Monkton.

The 1995 report focused on the Shapwick abstraction. It compared flow data in the Tarrant in

the early 1970s, when there was no abstraction at Shapwick, with flows in the early 1990s when

there was abstraction. It concluded that the Shapwick abstraction (and possibly abstraction for

the Spettisbury water cress beds) is likely to be affecting flows in the lower Tarrant, aggravating

the natural effect of droughts. It noted that the Shapwick abstraction at that time was only

about half the licensed amount. It recommended more investigation to quantify the impact.

The 1996 report focused on the impact of the Stubhampton abstraction in the period between

1992 and 1995, including the drought of 1995. It compared effective rainfall, groundwater levels

and limited data on river flows each year and concluded:

1. The Stubhampton borehole has a negligible effect on river flows and the source of the

Tarrant throughout most of the year (my emphasis).

2. At times of low flow the abstraction may cause the spring head to migrate further

downstream.

3. In 1995, the abstraction at Stubhampton may have caused the river to dry up two weeks

earlier than would have taken place naturally. However, the drying of the river was mostly

the natural result of low rainfall in the spring of 1995.

4. Groundwater abstractions in neighbouring valleys may affect summer flows in the Tarrant,

by moving the groundwater divide between the valleys, effectively reducing the Tarrant

groundwater catchment.

6

The Marcus Hodges reports recommended further investigations of whether abstractions in

neighbouring valleys (for example at Shapwick and Black Lane) could account for the perceived

changes in river flows reported by residents in the early 1970s.

In my opinion, the two reports’ conclusions were a reasonable reflection of the data available at

that time and the fairly limited analysis undertaken. They acknowledged that both the Shapwick

and Stubhampton abstractions were probably having some effect on Tarrant flows in droughts.

However, the level of analysis was not sufficient to quantify the significance of the impact or to

propose any remedies.

2.2 John Stowell report on hydrological study (1998)

John Stowell’s brief was to “establish whether there are detectable changes in the dependence

of groundwater levels and river flows on the overall rainfall pattern which might be attributable

to changes in land use, abstraction or other effects”.

The report provides a useful accumulation of data and some general description of the

hydrological behaviour of the river. However, it failed to address the main thrust of the brief – is

the Tarrant affected by abstraction? – or to reach any conclusions, or even to make any clear

recommendations for more investigations. The Marcus Hodges reports of 1995 and 1996 were

not included in the report’s list of references and it appears that the author was not aware of

their existence.

In my opinion, this report contributed little to the understanding the Tarrant’s problems or to

providing any solution.

2.3 ESI report on Shapwick and Stubhampton abstractions (2003)

ESI Ltd’s brief for Wessex Water was to provide a conceptual understanding of flows within the

Tarrant catchment and to provide an initial assessment of the impact that the abstractions at

Stubhampton and Shapwick could have on flows in the Tarrant. The report presented a

comprehensive assembly of the information and data available at that time, including some

assessment of geological context (which was notably absent from the John Stowell report).

The Shapwick abstraction

The conclusions presented on the impact of the Shapwick abstraction were:

1. Based on groundwater level observations during about five periods of prolonged pump

shut-down at Shapwick from 1997 to 2001, each lasting several months, the Shapwick

abstraction had no discernible effect on groundwater levels at Tarrant Abbey, Tarrant

Keynestone or Crawford Drove.

2. However, some pump test data (not presented in the report) had suggested that only

50% to 66% of Shapwick abstraction is drawn from the River Stour, leading to a tentative

conclusion that some of the water could be coming from the lower Tarrant.

3. There was insufficient information to determine whether, or by how much, Tarrant

flows are affected.

7

4. There are concerns that the MORECS data used in the analysis does not correctly

estimate effective rainfall in the summer months, tending to under-estimate the effect

that heavy summer rain can have on groundwater levels and river baseflows.

5. Although the data analysed was not able to demonstrate a clear impact on Tarrant flows

from the Shapwick abstraction, it concluded that more investigation should be

undertaken, including

Expansion of the surface and groundwater monitoring network

Detailed testing and monitoring of catchment behaviour

Detailed analysis of the water balance for the Shapwick and Tarrant catchments

Groundwater modelling

In my opinion, these conclusions were a reasonable interpretation of the available data. There is

sufficient evidence of some impact on the lower Tarrant, but insufficient evidence to quantify

the impact. Therefore, some more investigation is justified.

The Stubhampton abstraction

Despite the reference to Stubhampton in its title, the ESI report did not address the impact of

the Stubhampton abstraction with any substance. It merely says “The abstraction is fairly small

at about 2 Ml/d and there is no evidence to suggest that it is having a significant impact on flows

in the River Tarrant. However, the abstraction might be expected to slightly increase the length

of time the upper reaches of the winterbourne section remain dry in summer “.

In my opinion, the ESI report failed to address the impact of the Stubhampton abstraction in any

substantial way. It provides no evidence, one way or the other, on whether the abstraction has

any significant impact.

Sturminster Marshall and Black Lane abstractions

Although not formally part of their brief, ESI made some comments on the possible impact of

the Sturminster Marshall and Black Lane abstractions.

The Sturminster Marshall abstraction (licence 5.75 Ml/d), about 3.5 km to the south-east of the

Stour/Tarrant confluence, was found by Wessex Water pump tests to take only about 50% of its

abstraction from the River Stour. This implied that the other 50% could be drawn from an

adjacent catchment like the Tarrant, particularly from its lower reaches. There were said to be

pump test impacts recorded at Crawford Farm, about 1 km from the lower Tarrant.

The Black Lane abstraction (licence 10.5 Ml/d), about 4km north-west of Tarrant Keynestone,

was also found to take most but not all of its water from the River Stour. Again, there is an

implication that some of the abstraction is drawn from the Tarrant, possibly affecting the middle

reaches, as suggested by Marcus Hodges.

ESI comment that their recommended groundwater modelling could also investigate the impacts

of other abstractions in the Tarrant vicinity, including Black Lane. I agree.

8

2.4 Entec report on the Wessex groundwater modelling study (2011)

The model generally

Entec Ltd’s brief was to develop a regional groundwater model covering much of the Wessex

region including the catchments of the rivers Hampshire Avon, Stour, Piddle and Frome. The

objective was to provide a tool to help the Environment Agency and water companies to manage

water resources across all of their chalk catchments, determining the impact of abstractions and

assessing the effects of licence changes.

This has been a major study, producing, it seems to me, an excellent modelling tool. Its strengths

include:

A 250m modelling grid – this is a small grid size for such a large model, which should be

adequate for assessing localised effects in sufficient detail

A strong, detailed geological assessment, which demonstrates good understanding of

the realities of the widely varying nature of the chalk aquifer

Reasonably comprehensive and successful calibration of the model for many

groundwater locations and at many points on the main rivers, but not for the Tarrant

catchment.

Supervision of Entec’s work by a Technical Working Group from the Environment

Agency, Wessex Water and another consultant, Mott MacDonald, overseen by a steering

group including representatives from other water companies and Natural England.

From my brief review, it seems to me that this has been a comprehensive exercise undertaken

with great professionalism and well presented in the report. That said, there are the usual

doubts over the accuracy of the modelling, particularly in dealing with smaller catchments like

the Tarrant. As the report says on Page xix of the executive summary “as is typical for many

groundwater models, predictions become less reliable as the upstream catchment area becomes

smaller”.

Calibration of the model in the Tarrant catchment

Overall, the Wessex groundwater model should be an appropriate tool for assessing the impact

of abstractions in the Tarrant catchment and the surrounding area. It should be fit for the

purpose of the undertaking the investigations recommended in the earlier Marcus Hodge and

ESI reports. However, to give reliable results, the model would need to have been calibrated

specifically to give a good fit to historic groundwater levels and river flows in the Tarrant

catchment. As acknowledged in the Environment Agency’s letter to the RTPS of 22nd June 2011,

the model has not been specifically calibrated for the Tarrant.

The model output which was provided to me by Ian Colley of Wessex Water gives details of the

calibration of the modelled groundwater levels compared to recorded water levels in a number

of observation boreholes in the Tarrant catchment. Some examples are shown in Figure 1.

9

Figure 1 - Calibration of modelled groundwater levels

60

65

70

75

80

85

90G

rou

nd

wat

er

leve

l m

AO

D

Modelled v observed groundwater levels at Stubhampton

Modelled water level Observed water level

40

45

50

55

60

Gro

un

dw

ate

r le

vel

mA

OD

Modelled v observed groundwater levels at Crichel Chitterwood

Modelled water level Observed water level

26

27

28

29

30

31

32

33

34

Gro

un

dw

ate

r le

vel

mA

OD

Modelled v observed groundwater levels at Tarrant Keynestone Manor

Modelled water level Observed water level

Observation borehole

stops at 69.3 mAOD – no

records below

Model may under-

estimate low summer

levels?

Poor calibration - Model

under-estimates low

summer groundwater

levels by about 6 m

Quite good calibration -

Model slightly over-

estimates level

fluctuations

Good calibration fit at

high groundwater levels

10

Figure 1 shows that the model provides quite good predictions of water levels at Stubhampton

(at the observation borehole, not the abstraction borehole) at high and middling water levels.

However, the model may over-predict the fall in water levels in summer, although the borehole

stops at 69.3 mAOD, so cannot record lower levels. At Crichel Chitterwood the model calibration

is poor, with summer water levels predicted to be about 6 m lower than recorded. At Tarrant

Keynestone Manor, the modelled groundwater levels calibrate quite well, although there is a

tendency to over-predict water level fluctuations.

The report presents no calibration details for River Tarrant flows, because the model has not

been specifically calibrated for the catchment. From the perspective of the report’s primary

purpose of presenting a regional model this is understandable, because there are many other

catchments in the Wessex region with larger abstractions than the Tarrant and with known

problems. Nevertheless, as the model output has been used as evidence that flows in the

Tarrant are not significantly affected by abstraction, it is important to understand how well

modelled flows fit historical reality.

I have used the information EA provided on spot flow gaugings for the Tarrant and the model

output for historic flows to look at the calibration fit for T. Keynestone and T. Monkton. I have

shown some results for Tarrant Monkton in 1998 (a year of normal flows) on Figure 2.

a) Showing full range of flows

b) Showing low summer flows (vertical scale enlarged)

Figure 2 - Model calibration for flows at Tarrant Monkton in 1998

0

50

100

150

200

Flo

w M

l/d

Tarrant Monkton in

Gauged flow Historic flow

1998

0

5

10

15

20

25

30

Flo

w M

l/d

Tarrant Monkton in

Gauged flow Historic flow

1998

Modelled flows give good representation

of overall pattern of flows

Modelled low flows

much lower than actual

Duration of dry river

grossly over-estimated

11

Some comments:

1. The upper graph in Figure 2 shows calibration over the full range of flows. The general

impression is that the model is a good reflection of actual flows. Certainly, modelled flows

are high when they should be high and low when they should be low.

2. The lower graph, showing the same data with a larger vertical scale, ie emphasising summer

low flows, gives a different picture:

Modelled flows in early summer are much less than recorded flows

The modelled durations of zero flow are grossly over-estimated, eg in 1998 the

model shows the river dry for four months at Tarrant Monkton, whereas the river

did not actually dry up at all.

3. The same is found in all years and all locations where spot gauging data are available, ie

1991 to 2005 – the model substantially underestimates summer flows and grossly

overestimates the duration of zero flows. At Tarrant Keynestone, where the river only dries

up very occasionally, the model shows zero flow for several months every year. Overall, the

modelling of low summer flows bears little relationship with reality.

4. I do not agree with the statement at the top of Page 58 of the Entec report – “Whilst there

are limited data [for the Tarrant], the model performs adequately across the range of flows

available for comparison”. In my opinion, the model does not perform adequately for low

flows in the Tarrant and therefore is not fit for the purpose of assessing the impact of

abstraction on summer flows, at the moment.

5. However, in view of the consistency of the mis-calibration at low flows and the generally

good overall calibration, it would appear that calibration for Tarrant low flows should be

readily improvable by adjusting model parameters.

6. The under-estimation of summer flows may be due to inadequacies in the MORECS data

identified in the ESI report (see point 4 at the top of Page 7 of their report) and should be

investigated.

7. In the gauged flow data provided to me (reference 30 in Appendix A), there have been no

spot flow gaugings since early 2006. This seems to me an unfortunate omission, which

carries an unjustified implication that there are no concerns over flows in the Tarrant. With

still unresolved doubts over the impact of abstraction on flows, spot gaugings should not

have been stopped and should now be resumed.

In my opinion the poor calibration of the model for low flows in the Tarrant makes the use of the

existing model for assessing the impact of abstraction highly questionable, as discussed below.

Assessment of abstraction impacts

In the letter to RTPS of 22nd June 2011, the Environment Agency used the modelling results as

evidence for the very low impact of abstraction. Graphs were presented of the impact of

abstraction from 2000 to 2008. An example is shown in Figure 3.

12

Figure 3 - Impact of abstraction at Tarrant Monkton (as presented by EA)

With the information presented in this way, it is difficult to see any difference in the flows,

particularly with the all important low flows. Therefore, the information for 2004/05 is re-plotted as

shown in Figure 4, which also shows the gauged flows and the RTPS records of when the river was

dry at Monkton Ford.

Figure 4 - Impact of abstraction at Tarrant Monkton in 2004/05

Figure 4 again shows the very poor calibration at low flows – the model severely underestimates

the flows and grossly over-estimates the duration of zero flows.

In the Environment Agency’s letter to RTPS of 22nd June 2011, they state that what the graphs

show (ie the information presented in Figures 3 and 4) is that “the impacts of abstraction are

0

5

10

15

20

25

30

35

40

45

50

01/01/2000 01/01/2001 01/01/2002 01/01/2003 01/01/2004 01/01/2005 01/01/2006 01/01/2007 01/01/2008

Flo

w (M

l/d

)

River Tarrant at Monkton (Ford)

Historic (Run 70)

Natural (Run 77)

0

5

10

15

20

25

30

35

Flo

w M

l/d

Tarrant Monkton in

Gauge Historic Natural

2004/2005

Periods of dry river as

recorded by RTPS

Model shows flows

with abstraction

about 10% lower

than natural

13

very small”, but are “slightly more noticeable at Tarrant Monkton”. Based purely on what the

graphs show and setting aside concerns over the model’s accuracy, the Environment Agency’s

statements are correct – the maximum drop in flow due to abstraction is shown to be only 10%

and the duration of the river drying up is hardly changed, according to the model output.

The EA’s letter also mentions some caveats “The model doesn’t represent winterbournes as well

as perennial rivers” and “it cannot be relied upon to accurately assess when a river will dry”. In

my opinion, these are honest statements of valid concerns.

The gross over-estimation by the model of the duration of zero flows makes it inappropriate, in

its present form, for estimating the impact of abstraction on low flows. At times when flows in

the river were falling but still flowing in the spring and summer, the model usually predicts flow

would have been zero, both naturally and with abstraction. The conclusion of negligible impact

of abstraction on flows at such times is not justified.

It is a particular concern that the times when the modelling of flows is most inaccurate in spring

and summer are also the times when the modelling shows that the impact of abstraction on

groundwater levels is greatest. As river flows are almost entirely dependent on groundwater

levels, abstraction could be having a substantial impact on river flows but the model is incapable

of showing it. This concern is borne out by the further analysis that I have undertaken in Section

3 of the report.

2.5 Conclusions from review of existing reports My conclusions from reviewing the existing reports are:

1. The Marcus Hodges reports of 1995 and 1996 established that both the Shapwick and

Stubhampton abstractions were probably having some effect on Tarrant flows in

droughts. However, the level of analysis was not sufficient to quantify the significance of

the impacts.

2. The John Stowell report did not provide any substantiation of abstraction impacts.

3. The ESI report provided more evidence that the Shapwick abstraction is probably having

some impact on the lower Tarrant, but was not able to quantify it. It made

recommendations for more investigation including groundwater modelling. The ESI

report did not meaningfully address the Stubhampton abstraction.

4. The Entec groundwater modelling report provided no valid evidence that the impacts of

abstraction are not significant.

5. Although the groundwater modelling recommended in the ESI report has been

undertaken by Entec (but not calibrated properly for the Tarrant), other important work

recommended by ESI in 2003 has not been done (to my knowledge):

Expansion of the surface and groundwater monitoring network

Detailed testing and monitoring of catchment behaviour

Detailed analysis of the water balance for the Shapwick and Tarrant catchments

14

Overall, in my opinion the impact of abstraction on the Tarrant has still not been adequately

addressed. Concerns raised in the Marcus Hodges reports of 1995/96 and the ESI report in 2003

have not been followed through. The recommended investigations have mostly not been

undertaken. The regional groundwater modelling has not specifically addressed the Tarrant

problem and its poor calibration for Tarrant flows makes its results currently un-useable.

I appreciate that these conclusions are contrary to current Environment Agency and Wessex

Water views and that they have unwelcome implications in terms of costs, Water Framework

Directive compliance and future planning of water supplies. Therefore, in the following section I

have undertaken some more analysis to support my conclusions and to quantify what the impact

of abstraction might be.

15

3. Further analysis

3.1 The relationship between river flows and groundwater level As is normally the case with chalkstreams, there is a strong relationship between groundwater

levels and river flows in the Tarrant catchment, especially in the upper valley. This is illustrated

in Figure 5.

Figure 5 - Relationship between river flows and groundwater levels

This shows the dependency of river flows on groundwater levels. So, for example, when the

groundwater level at Stubhampton falls below about 74.5 mAOD, the river ceases to flow at

Tarrant Hinton. If the groundwater level at Stubhampton drops from 78 mAOD to 76 mAOD, the

river flow falls from about 12 Ml/d to 3 Ml/d. The relationship between river flow at Tarrant

Hinton and groundwater level at Stubhampton can be represented by the formula on the upper

part of Figure 5. This formula, which is entirely empirical but closely matches the gauged flows,

has been used in combination with Entec’s modelling of groundwater levels, as described in

Section 3.2.

There is a similar relationship between groundwater levels at Tarrant Keynestone Manor and

river flows at Tarrant Keynestone, as shown in the lower part of Figure 5.

687072747678808284

0 20 40 60 80 100

Stu

bh

amp

ton

g/w

leve

l

mO

D

Tarrant Hinton flow Ml/d

Tarrant Hinton flow v Stubhampton groundwater level

Gauged flow Formula Flow =1.9(h-74.5)1.5

27.5

28.0

28.5

29.0

29.5

30.0

30.5

0 50 100 150

g/w

leve

l mA

OD

Tarrant Keynestone flow Ml/d

T. Keynestone flow v T. Keynestone Manor groundwater level

Gauged flow Formula Flow =10.0 (h-27.5)2.2

16

3.2 Further analysis of modelling output The relationships shown in Figure 5 allow some further assessment of the implications of

modelled groundwater level changes on river flows. The analysis that I have presented below

uses a crude methodology that I do not intend to be treated as a definitive assessment of the

impact of abstraction on flows. However, in my opinion, it provides a good indication of what

the impacts could be and whether they need to investigated in more detail.

Figure 6 - JDL/Entec modelling of impact on winterbourne flow at Tarrant Hinton

60

65

70

75

80

85

GW

L m

AO

D

Stubhampton groundwater levels in 1995/96as modelled by Entec

Natural Historic Full licence

0

10

20

30

40

50

60

Riv

er

flo

w M

l/d

JDL modelled river flows at Tarrant Hinton

using Entec modelled g/w levels and formula from Figure 5

Natural flow Historic flow Full licence Gauged flow

0

10

20

30

40

50

Riv

er

flo

w M

l/d

Natural flow Historic flow Full licence

Flow =1.9(h-74.5)1.5

Flow about 15% - 50%

lower than natural

throughout the spring

1 week earlier

17

The upper part of Figure 6 shows Entec’s modelled groundwater levels for 1995/96 under

natural conditions, with actual abstraction in 1995/96 and if the existing licences were fully used.

The middle part of Figure 6 converts the modelled groundwater levels to river flows at Tarrant

Hinton, using the formula shown on Figure 5. The lower part of Figure 6 shows the flows from

January to June 1996 – the winterbourne flows in the season following the drought of 1995.

It should be noted that this modelling accurately predicts when the river actually dried up in

1995 and 1996, but over-estimates the extent of flow recovery in the winter of 1995/96. This is

because of the only moderately good calibration fit of Entec’s modelling of water levels at

Stubhampton. Nevertheless, I feel that it can be used to give a reasonable indication of the scale

of flow changes due to abstraction.

The lower part of Figure 6 shows that abstraction reduced winterbourne flows by in the region

of 15% to 50% throughout the spring of 1996. This is a crucial time of year for winterbourne

ecology and, in my judgement, such a reduction in flows is likely to have a significant impact on

river plants, aquatic invertebrates and juvenile fish.

The modelling in Figure 6 shows no change in the date that the river would have dried up at

Tarrant Hinton in 1996. However, this is because the model time step is 10 days. Comparing the

horizontal distance between the green and the red/blue lines in the lower part of Figure 6

suggests that abstraction would have caused the river to dry up about 1 week earlier than

naturally. The upper part of Figure 6 shows that abstraction causes groundwater levels in spring

and summer to drop by about 2 m. Noting that the gradient of the river bed is about 1:250

between Tarrant Hinton and Tarrant Monkton, this implies that abstraction causes an additional

500 m of river to be dry.

In my opinion, the major impact of abstraction on winterbourne flows is the reduction in flow by

15% to 50% when the winterbourne is flowing, rather than increased drying up of the river.

A similar analysis of the impact of abstraction on flows at Tarrant Keynestone is shown in Figure

7.

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Figure 7 - JDL/Entec modelling of the impact on flows at Tarrant Keynestone

The modelling shown in Figure 7 gives a quite good calibration fit to gauged river flows in

1995/96. The calibration is better than for my modelling of Tarrant Hinton flows shown in Figure

26

27

28

29

30

31

GW

L m

AO

D

Tarrant Keynestone Manor groundwater levels in 1995/96as modelled by Entec

Natural Historic Full licence

0

20

40

60

80

100

120

Riv

er

flo

w M

l/d

JDL modelled river flows at Tarrant Keynestone using Entec modelled g/w levels and formula from Figure 5

Natural flow Historic flow Full licence Gauged flow

0

10

20

30

40

50

60

70

80

Riv

er fl

ow

Ml/

d

Natural flow Historic flow Full licence Gauged flow

Flow =10.0 (h-27.5)2.2

Flow about 20% - 60%

lower than natural from

May to July

10 days

earlier

19

6. This is because the calibration fit for Entec’s modelling of Tarrant Keynestone Manor water

levels is better than their calibration of Stubhampton water levels, as shown on Figure 1.

Figure 7 shows that abstraction could have a very substantial impact on spring/summer flows at

Tarrant Keynestone. The analysis suggests that the recession of flows in spring and summer is

brought forward by about 10 days. Flows are reduced by in the region of 20% to 60%. I would

expect this extent of flow reduction to have a major impact on river ecology – aquatic plants,

particularly the flow sensitive ranunculus, invertebrates and salmonid fish, especially juveniles.

In the Environment Agency’s river basin management plan for the South West region, the River

Tarrant has been classified as having good ecological potential. The water body sheet is attached

as Appendix B. I have several concerns about this:

1. The water body (ie River Tarrant) is classified as “heavily modified” on the grounds of

“urbanisation”. I find this difficult to accept – in my judgement, the river is as close to

natural as any small chalkstream in southern England and should not be classified as

heavily modified. The significance of the mis-classification is that it allows less stringent

standards to apply.

2. The quantity and dynamics of flow are said to support “good” status. In the light of the

analysis I have presented above, I think this is highly questionable.

3. In the biological assessment, the invertebrate status is high and the fisheries status is

“moderate”. The status of aquatic plants is not specified. In view of the concerns over

flows, I would query these assessments and would wish to see the scientific evidence on

which they are based.

The assessment of the impact of abstraction that I have presented in this section is based on a

fairly crude methodology. Nevertheless, I consider that it presents a compelling case that the

impacts of abstraction are likely to be significant and need to be properly addressed.

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4. Conclusions and recommendations

4.1 Conclusions My conclusions from the review of previous investigations and my own further analysis are:

On the quality of investigations and reliability of their conclusions

1. The investigations undertaken to date by the Environment Agency and Wessex Water

have established that the Stubhampton and Shapwick reports are having some impact

on Tarrant flows, but have not been able to quantify the impact.

2. Further investigations recommended by ESI in 2003 have not been undertaken.

3. The Wessex regional groundwater model prepared by Entec in 2011 is potentially an

excellent tool for assisting for assessing impacts on Tarrant flows. However, the model

has not been specifically calibrated for the River Tarrant.

4. The fit between modelled historic flows and gauged flows in the Tarrant is so poor that

no credibility can be attached to the assessment of impacts on flows using existing

model outputs.

5. The model fit for groundwater levels in the Tarrant is better than the fit for flows. There

is a strong relationship between recorded groundwater levels and gauged flows. By

combining modelled groundwater levels with the recorded groundwater level/flow

relationship, the groundwater model can be used to predict flows with more confidence.

On the impact of abstraction on the River Tarrant

6. The abstractions probably reduce winterbourne flows at Tarrant Hinton by in the region

of 15% to 50% every spring. The river dries up about 7 days earlier each year. The river

head is moved downstream by about 500 m.

7. Abstractions have a bigger effect at Tarrant Keynestone, reducing flows by typically 20%

to 60% in the spring and summer. The flow recession is brought forward each year by

about 10 days.

8. These impacts on flows are from the combination all abstractions. My analysis is not

able to differentiate between the effect of abstractions at Shapwick, Stubhampton and

Black lane.

9. Although ecological assessment is not part of my brief, I would expect that the scale and

timing of abstraction impact is sufficient for substantial ecological damage, especially to

river plants, aquatic invertebrates and juvenile fish.

10. The scale of impact is likely to mean that the River Tarrant should not have been

classified as being of good ecological potential under the terms of the Water Framework

Directive. Nor, in my opinion, should the river have been classified as a heavily modified

water body, with associated less stringent standards.

11. My conclusions on abstraction impacts are based on a fairly crude method of

assessment and one week’s work for RTPS. However, I believe my findings are

sufficiently robust to justify re-opening the case for restoring sustainable abstraction in

the Tarrant.

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In my opinion, the case for dropping the concerns raised in the Marcus Hodges and ESI reports

has not been properly made. The long-running complaints of local people, which are based on

intimate knowledge of the river over the past 50 years or more, have not been satisfactorily

answered.

4.2 Recommendations I recommend that the restoration of sustainable abstraction in the River Tarrant should start

with the following investigations:

1. Re-calibration of the existing Wessex regional groundwater model to improve the fit of

modelled flows to gauged flows in the river Tarrant and taking account of ESI’s concerns

over the adequacy of MORECS effective rainfall data.

2. Use of the re-calibrated model to look at the predicted impacts of abstraction on the

Tarrant, differentiating between the impacts of the Stubhampton, Shapwick and Back Lane

abstractions.

3. Use of the new model output and all recorded data from the drought of 2011 to up-date the

findings of the Marcus Hodges reports of 1995/96 and the ESI report of 2003 (also

addressing the findings of this report).

4. Production of a report combining the findings of the above three actions and justifying

whether or not further action is needed to deal with over-abstraction in the Tarrant.

I recommend that these actions are undertaken as a desk exercise, before any further on-the-

ground investigations of either the hydrogeology of the ecological impacts of abstraction. The

aim should be to complete the investigations within three months.

In view of the unresolved concerns over the impact of abstraction, I recommend resumption of

spot flow gaugings on the Tarrant, starting with a record of recovery of flows following the

drought of 2011 and close monitoring of the flows in the spring and summer of 2012, which are

likely to be low following the drought.

I also recommend that RTPS should press for review of the classification of the River Tarrant as a

heavily modified water body with good ecological potential. The re-classification as an

unmodified water body at less than good status would enable the Water Framework Directive to

be used as a driver for improvements.

John Lawson, FREng, FICE, FCIWEM

5th January 2012

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Appendix A – List of documents reviewed

Supplied by the Environment Agency

1. River Tarrant preliminary review by Marcus Hodges for NRA 2. River Tarrant ongoing assessment by Marcus Hodges for NRA 3. R Tarrant winterbourne monitoring locations 4. Field survey notes 5. Field survey notes 6. Historical summary 7. 1995 groundwater monthly dips 8. Newspaper article – Death on the Tarrant 9. Winterbourne signatures 74 -94 10. Tarrant meeting (assume internal) 11. Comments by WW to Stour LEAP with reference to Tarrant 12. Tarrant update 13. Newspaper article – A Land Where No Water Is 14. Letter stating that there will be a study by North Dorset District Council 15. NDDC minutes 16. Update on EA work on Tarrant 17. Rainfall stations in surrounding area 18. NDDC report – River Tarrant 19. EA comments on NDDC report 20. Communication with David Panter 21. EA leaflet on winterbournes and the Tarrant 22. More winterbourne signature data 23. Winterbourne signature points 24. Restoring Sustainable Abstraction – River Tarrant 25. The ESI report on the effects of Shapwick & Stubhampton PWS abstractions on flows in the R

Tarrant 26. RTPS – an update 27. Invitation to a public meeting to discuss the Tarrant 28. Summary of all winterbourne signature data up to 2008. 29. Output from the chalk basin groundwater model. (red historic and blue full licence). Sites go

upstream so TK lowest and TM closer to head of the water course. 30. Spot flow gaugings in Excel file “JLawson Tarrant SpotFlows 170811” 31. Entec Ltd report on Wessex regional groundwater model

Supplied by Wessex Water

1. Data in Excel file “Tarrant modelled flow 70_75_77”

2. Data in Excel files “MonthAvGWLsmAODRun70, 75 & 77”

Supplied by River Tarrant Preservation Society

Bundle of papers under cover of David Panter’s letter dated 12th August 2011

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Appendix B – River Tarrant Water Body Classification