[2023] UKUT 00282 (LC)

This section is structured as follows:

The expert witnesses and their positions

Evidence about hydrology was given for the claimants by John Evans, consultant to and former managing partner of Evans & Langford LLP, Consulting Civil and Structural Engineers in Maidstone. He has worked on a number of large-scale projects involving dams and tidal defences. For the respondent evidence on hydrology was given by Alasdair Groves, Technical Director of Hydrogeology at Arcadis, an international company specialising in large infrastructure projects.

Ms Ward suggested that Mr Groves by virtue of his expertise in hydrogeology is better qualified than Mr Evans to express an opinion on the matters at issue here. We note that the two experts do not have identical expertise and experience; Mr Evans’ experience in projects relating to dams, harbours and flood defences is extensive and we accept that both he and Mr Groves are eminently qualified to give evidence about hydrology of the ground surrounding The King’s Lodging and the old and new walls. Both experts are of course giving opinion evidence about matters that are outside the Tribunal’s expertise, and we have assessed their opinions on the basis of the available data, and of their explanations of that data, rather than by weighing the qualifications of one against those of the other.

Mr Hussain KC in cross-examining Mr Groves suggested that he should have disclosed the framework agreement between Arcadis and the respondent, under which Arcadis has been engaged to support the respondent over a period of years. He appeared on occasion to suggest that Mr Groves had failed to take proper account of the data as a result of that contractual relationship. In closing he argued that while he does not suggest any impropriety on Mr Groves’ part, his credibility should be regarded as impaired by the relationship his firm has with the respondent, and that “if it comes down to credibility” we should therefore prefer the evidence of Mr Evans.

We reject any suggestion of impropriety on Mr Groves’ part. It would have been helpful to have the framework agreement disclosed, but we accept that the omission was innocent. Public authorities engage the services of experts by the use of such agreements; we do not regard the arrangement as being a source of bias on Mr Groves’ part, any more than Mr Evans or Mr Corcoran are biased by their long involvement on the claimants’ behalf before the reference to the Tribunal was made. And the Tribunal’s decision will not “come down to credibility”; it is our task to make an objective assessment of the scientific evidence put forward by the experts, not to make a guess on the basis of conjecture about credibility.

Turning now to the expert evidence, Mr Evans and Mr Groves have exchanged reports in April 2022, supplemental reports in February 2023 and addendum reports in September 2023, as well as completing a joint report in August 2022, two rounds of questions and answers and finally a revised version of their joint report a week before the resumed hearing in October. The Tribunal was greatly assisted by the hydrologists’ giving concurrent evidence (see paragraph 3 above), which enabled it to focus on the details of the physical data on which the experts have worked and to hear both their views together on each point in issue. Much time was saved by their being able to agree that certain points were irrelevant or not decisive (hence for example the brevity of our discussion of the drains, see below). We pay tribute to the measured and constructive way in which the two experts answered the Tribunal’s questions.

Before we turn to the detail it may be helpful to summarise the positions of the two hydrology experts, as we understand them after the hearing.

Mr Evans’ opinion is that the respondent’s works have raised groundwater levels at the property, by between 0.5 and 0.9 metres, and he has suggested a mechanism which might cause that to happen and which we discuss below. He also says that the extra-high tidal inflows (see paragraph 34 above) will recur when river levels at high tide reach about 3.1m AOD in the future as a result of inflow through the new wall at the clutches (where SSPs are joined) and at the dock corner; it is not in dispute that the river has not reached that level since January 2019. Mr Evans’ opinion is that the house is at risk primarily because water levels have risen but also because of future extra-high tidal inflows, and therefore that serious remedial work is still needed.

Mr Groves’ position is that groundwater levels have not risen. He agrees that the extra-high tidal inflows will recur in the future (although they have been mitigated to some extent by the remedial works), but says that such inflows will drain away again and will not reach the house.

It will be apparent therefore that there are two potential sources of water damage. One is raised groundwater levels, which is the primary basis of the claimants’ case. The other source of damage is future extra-high tidal inflows; if we find that groundwater levels have not risen then the question is whether these occasional inflows might cause damage, in particular to the garden.

Hydrological processes

Both hydrologists provided helpful, and consistent, explanations about what generally happens to water in the ground close to rivers.

We said earlier that Sandwich stands in the flood plain of the River Stour and that all the land round about is at very low levels. The hydrologists agree that in these circumstances the groundwater levels near the river would be at or slightly higher than the level of the river. Mr Evans said at paragraph 14.1 of his first report:

“Ground water levels close to rivers would normally be expected to be at about mean river level, rising with distance from the river to give the hydraulic gradient that drives drainage to the river.”

Mr Groves at paragraph 51 of his first report said:

“The water table [at the property] can be expected to have been present at shallow depth and to have shown a slight variation around a mean elevation maintained slightly above mean river level, which will approximate to Mean Sea Level adjacent to the property, estimated to be approx. +1.2m AOD.”

That water level is, however, changeable, as Mr Groves explained at his paragraph 48:

“48.

In most circumstances, when it rains at any site, a proportion of the water will be lost back to the atmosphere as water vapour, either through evaporation (caused by the action of sun and wind) or transpiration (by respiring vegetation). Hydrologists generally combine these two processes and refer to “evapotranspiration.” In most circumstances, a proportion will run over the surface of the land as runoff into ditches or other surface water features, leaving the residual proportion to infiltrate into the ground, from where it will move under gravity until it meets the water table, which is defined as the upper limit of the saturated zone. This causes the water table to rise in response, which induces a lateral movement of water. The water table slopes towards groundwater discharge zones, which are often surface water features. The slope of the water table is referred to as the hydraulic gradient.”

Turning to this particular property, importantly the hydrologists agree that before the respondent’s works took place the old wall will have been permeable. Mr Groves put it this way at his paragraph 51:

“At times of higher river level, some leakage into the Property will have occurred through the imperfections within the river wall, causing the water table to rise locally. At lower river levels some drainage out of the ground will have occurred, causing it to fall. These changes will have been greatest immediately adjacent to the river wall and decreased progressively inland, the amount of leakage being controlled by the permeability of the river wall and the material immediately behind it. A seepage face will have been present at the river wall at times of falling river level.”

Because of that, one of the concerns expressed by the claimants and shared by CH2M before the works started was the risk that the new wall would cause the land at The King’s Lodging to dry out. We have been shown one of the diagrams produced by CH2M at the planning stage from which it is clear that groundwater levels were assumed to be about 1m AOD; flap valves in the new wall were to be placed at about the same level so as to maintain that level within the property.

The hydrological data

A number of reports, studies and events comprise the data that the experts took into consideration.

The earliest measurements were made by GL Martin Ltd, an engineering firm, in 2007. At that date the claimants were considering extending their garage by a first floor extension, and GL Martin were instructed to investigate the ground conditions prior to the structural design of the extension. The extension was never built; Mr Brookhouse explained that the project was postponed following publicity for the new tidal defence scheme. The study was particularly concerned with the consistency of the ground. It looked at whether the garage had been affected in the past by foundation movement problems, and at the possible consequences of putting an extension on top of it and whether it would cope with the additional load.

GL Martin made a borehole behind the garage, and two trial pits beside the garage known as TPA and TPB, shown on the diagram below (Footnote: ⁶). Groundwater levels in the borehole and TPA were found to be at around 1.3m AOD, and no water was found in TPB at 1.6m AOD. GL Martin noted that that would not be a static level and would vary “due to seasonal or other effects”. Mr Evans reported the 1.3m level without discussion in his first report, perhaps on the basis that it was unsurprising in light of the general expectations about water levels in the area (see paragraphs 87 and 88 above), albeit perhaps a little higher than expected - we note that GL Martin regarded this as a “comparatively high water table” (GL Martin paragraph 5.1). We shall look in detail later at the reasons why Mr Groves does not agree that the findings of the GL Martin survey were accurate.

The claimants themselves installed a number of pipes to measure water levels in November 2017.

In January 2018, at the claimants’ instigation and with the respondent’s agreement, Eldred Geotechnics were instructed to carry out a study of the water levels in the garden; it will be recalled that by that date there had been a series of extra-high tidal inflows and a number of phases of remedial work. In February 2018 Eldred installed boreholes which were continuously monitored from 28 February to 9 May 2018. That exercise produced a rich dataset (“the Eldred data”) showing the movement of water in the ground in response to the tide and to rainfall at various points in the garden. On the previous page is a diagram produced by Mr Evans which gives an indication of the thoroughness of the investigation. (Footnote: ⁷)

Eldred provided a series of plots showing the behaviour of each borehole and of the claimants’ pipes throughout the monitoring period, on which both hydrologists commented in their first reports. Later Eldred provided them with the data on which those plots were based, which meant that the two expert witnesses were able to go into further detail. Because the Eldred data are of such good quality they have given rise to rather more agreement than disagreement, and so we do not need to rehearse all the detail studied by the experts. They agree that the boreholes were correctly placed and that (with some caveats about a couple of the boreholes) the Eldred data are an accurate measure of water levels during that period. They agree that water levels in all the boreholes rise and fall with the tides but do so to a far greater extent nearest the river (up to a maximum of 1m at boreholes 4, 5 and 6 at spring tides, according to Mr Groves) and that they all respond to rainfall.

Most importantly, they agree that the Eldred data showed groundwater levels near the house – in the area bounded by boreholes 1, 2 and 9 and by pipes 2, 3, 4 and 5 to be around 2m AOD during the period of monitoring. Mr Evans spelt that out further by saying that those boreholes and pipes 2 and 4 all show groundwater levels maintained above 1.75 to 2m AOD and rising to 2.5m AOD during periods of heavy rainfall.

That level of around 2m AOD is an important point of agreement; the central issue between the hydrologists is whether water levels near the property (where tidal response is weak) were at or near that level before the respondent’s works commenced. The claimants’ case is that levels were 0.5m to 0.9m lower, and the respondent says they were not.

A further point extracted from the Eldred data relates to the hydraulic gradient at the property. Hydraulic gradient is the slope of the water table (according to the glossary in Mr Groves’ first report). Normally the gradient in land next to a river is expected to be towards the river. Mr Groves’ view about the hydraulic gradient at the property changed in the course of the litigation. In his first report he concluded on the basis of the Eldred data that “There is no evidence within the data for the presence of a sustained hydraulic gradient towards the main King’s Lodging building.” In his supplemental report he said that the Eldred data indicated that there was a hydraulic gradient towards the building for 5.4% of the period of record, and that that occurred only at the highest tides. In his addendum report, after the release of the raw data on which the Eldred plots were based, he presented an analysis of levels around a high spring tide and found that the gradient was towards the house for about 20% of the time. That is not what the experts would have expected to see on the basis of the hydrological processes normally at work in land next to a river.

There was some discussion about gradient at the hearing. The experts disagreed about the effect of underground structures in the garden which Mr Evans, if we have understood correctly, said must have the effect that hydrological gradient is not a straight line. At any rate on the basis of Mr Groves’ evidence we can conclude that the Eldred data indicate that there was a hydraulic gradient towards the house for about 20% of the time during spring tides. Mr Groves was adamant that nevertheless river water was not able to reach the house; yet he agreed that water levels in the area of the house are directly affected by rising tides in the river, and that so far as any effect upon the house is concerned it does not matter whether that water comes directly from the river or is groundwater that has been pushed landwards and upwards by water from the river.

Finally in September 2022 Mr Groves re-measured the water levels in the boreholes at the property, in order to check the then current groundwater levels. Mr Evans declined to take part in the exercise because he regarded it as unreliable; he explained in his addendum report that it is likely that the boreholes will have silted up over time. He took the view that such data should have been collected by an independent technician, and surmised that that had not been done because no-one was willing to do it because of the inevitable inaccuracy. Mr Groves disagreed and expressed the view that the boreholes were well within their useful life.

It is not possible to know whether the data collected in September 2022 were accurate; but that turns out to be unimportant because the two experts agree that the water levels were considerably lower in September 2022 than they had been at the time of the Eldred data, and that that was because there was a prolonged drought that summer.

So those are the steps taken to measure water levels in the garden. Further evidence of those levels is provided by the swimming pool. The claimants have expressed the view that the swimming pool is of a type that cannot be installed if groundwater levels are above the level of the bottom of the pool, which is around 1.6m AOD. We discuss this further at paragraphs 127 and following below.

The drains

The hydrology experts compiled their initial reports on the assumption that rainwater from the roof of the house and from hard surfaces in the garden went to soakaways, but it is now clear that that is not the case and that the property benefits from a combined drainage system taking rainwater and foul drainage to the sewer in Strand Street. The drains run around all sides of the building, and the foul drainage from The King’s Lodging and from Giles Quay, the property immediately to the north, passes round the back of the house, across the lawn and out to Strand Street; there are also drains on the street side of the house which leave the property at the same place. A brief report for the claimant by The Drainage Team following an inspection in September 2014 reported that clay pipes had been damaged by ground movement and that if they were left unrepaired structural damage to the building could occur. The same firm carried out a further inspection in August 2016 and again recommended remedial work, noting that roots had caused further damage to the pipes and that one of the manholes was blocked with rubble and silt.

In the autumn of 2022 further inspections of the drains were carried out, one in September which was not properly recorded and one in November by Thanet Drainage. It is not in dispute that the drains were found to be in poor condition with at least some blockages; unfortunately Thanet Drainage produced a report on the state of the drains after blockages had been cleared but provided no evidence of what they found when they initially inspected. The inspection was witnessed by Mr Evans, and by Mr Nicholas Paige who is a catchment engineer employed by the respondent. They both described what they saw. In short, Mr Paige’s evidence was that the drainage system was “in a state of severe disrepair” and that “every drain run that was inspected was blocked … there was evidence of significant joint displacement in the drainage as well.” Mr Evans on the other hand regarded that as a gross exaggeration, although he agreed that some of the drainage runs were slow-moving.

Evidence about the state of the drains was heard from Mr Paige during the first part of the hearing in May 2023, and the hearing was then adjourned before we had heard from Mr Evans. It occurred to us at that stage that it was unlikely that if the drains had been totally blocked that would have gone unnoticed, unless the property had been empty for long periods. In answer to our question about this Mr Lewis confirmed at the hearing in October, on instructions, that the property had been lived in by the claimants (not by tenants) for most of the time from 2014 onwards; the longest period it had been unoccupied was one instance of two months. The respondent did not seek to challenge that information. The claimants also produced a letter from their neighbours at Giles Quay confirming, in September 2023, that they had not been aware of any fault with their shared drain since they moved in in 1987. That is helpful because if the foul drainage had not been getting through the drains in the garden of The King’s Lodging we think it is inevitable that the neighbours at Giles Quay would have had problems.

It is not possible to say exactly how bad the state of the drains was before they were cleared in 2022. We accept that both Mr Evans and Mr Paige gave honest descriptions of what they saw, but we regard Mr Evans’ description as the more likely to be accurate. That the system as a whole was entirely blocked is not plausible over a period when the house had been lived in for most of the time. Certainly some clearing was needed in 2022. Moreover, the report by The Drainage Team in 2017 certainly did not say that the system was totally blocked or mostly blocked, and we infer therefore that it was not totally or mostly blocked when the Eldred data were recorded.

In his supplemental report Mr Groves expressed the view that in light of recorded rainfall during the period when the Eldred data were gathered, rainfall could not explain the magnitude of the rise in water levels during the two very wet days of 28 and 29 April 2018. He concluded that the excess was accounted for by water running off the roof of the property and not being taken away by the faulty drains. It seemed at the hearing in May that the respondent might be arguing that water levels in the garden, insofar as they were higher than expected, were attributable to the state of the drains. If that position had been maintained we would have rejected it because we prefer the evidence of Mr Evans to that of Mr Paige for the reasons we have explained. In any event such an argument would have been problematic because of the engineers’ evidence that the drains may have been damaged by the respondent’s work.

However, it became clear that that was not the respondent’s case. In their revised joint statement of September 2023 the hydrology experts agreed that the condition of the drains was relevant to understanding the hydrological processes at work at the property but said that the condition of the drainage system did not “influence”, by which we think they meant did not answer, what they had been asked, namely whether the respondent’s works have caused a change in the groundwater levels at the Property. That must be right because the drains have been a constant feature; their condition was documented in 2014 and had not changed much by 2017.

Beyond that, the experts disagreed as to how much the condition of the drains influenced the ground water levels; Mr Evans thought hardly at all, Mr Groves expressed the view that “the drains were in poor condition and influenced groundwater levels beneath the property.” Mr Evans produced a calculation to show (on the basis of rainfall records) that even if all the rainfall soaked into the garden rather than being carried away by the drains, that would not be sufficient to raise water levels from 1.3m to 2m AOD. Mr Groves had no specific criticism of that calculation beyond saying that it was “too average”, and we therefore accept it.

However, even Mr Groves is not saying that the state of the drains could have caused groundwater levels to rise from 1.3m AOD (if that is indeed the pre-works level) to 2m AOD. The drains were therefore something of a red herring. They have been problematic and are now in a better condition than they were, but the respondent is not saying that, if groundwater levels were raised, the cause was the state of the drains.

The pre-works groundwater levels

So we come to the crucial point of difference between the hydrologists: whether by the time the Eldred data were recorded the groundwater levels had risen as a result of the works. In this section we examine in detail the evidence that Mr Evans puts forward for his proposition that levels have risen, and Mr Groves’ criticisms of that evidence. And that is all we can do, because of the evidential tragedy in this case which is that the respondent did not carry out a hydrological survey before the work commenced.

For the claimants it was said that it was a condition of the Listed Building Consent that the respondent do so, but that is not the case; the condition requires that the work be carried out in accordance with the Design and Access Statement submitted by the respondent, which recommended but did not require that such a study be carried out. The wisdom of carrying out a hydrological survey before starting work on an historic and sensitive property next to a river is obvious and the decision not to do so may now appear to be regrettable, but it was not a breach of any obligation on the part of the respondent.

The decision is in a way understandable because the respondent and CH2M took the view that groundwater levels at the property were likely to be at or not far above the river level. The topography and surrounding watercourses all pointed to a groundwater level of 1.1 or 1.2m AOD, the garden was thriving and mostly dry (according to Mr Brookhouse’s unchallenged evidence), GL Martin’s measurements taken after a wet season showed a level of 1.3 which was regarded as high. We suspect that if anyone had suggested to CH2M in the autumn of 2014 that water levels at the property were generally in the region of 2m AOD the response would have been dismissive. The respondent is now in the uncomfortable position of arguing strenuously against the accuracy of the assumptions that it made and that its expert contractors made before starting work.

In the paragraphs that follow we look at the items of evidence relied upon by Mr Evans one by one, set out Mr Groves’ view, and express our finding.

Mr Evans relies on eight items of evidence, which he and Mr Groves helpfully went through in the joint statement. The first three relate to the work done by GL Martin in 2007.

1 - 3. The GL Martin borehole and trial pits As we said above, Mr Evans in his first report referred to the groundwater level of 1.3m AOD as found by GL Martin, and inferred from that that levels have risen as a result of the work. Later he was able to access rainfall records and reported in his addendum report in September 2023 that rainfall in the month before GL Martin carried out their investigation was 92mm; so it was not a dry period. He thought it likely therefore that the water table found by GL Martin was, if anything, slightly above normal. It will be seen from the diagram above that the GL Martin borehole is just slightly nearer the river than the Eldred boreholes 1 and 2 and at the same distance from the river as pipe 5, and therefore in an area where the tide has very little effect upon the water table. (Footnote: ⁸)

Mr Groves in his first report expressed the following misgivings about GL Martin’s measurements. He took the view that measurements from the trial pits (extended by hand auger when digging by hand became impracticable) would be more accurate than those from the borehole because the type of auger used for the borehole would have disrupted the soil column and its structure. He pointed out that GL Martin reported the level at which water ingress occurred in the borehole and TPA but did not say whether drilling was suspended at that point nor whether water levels were monitored at regular five-minute intervals for a minimum of 20 minutes “which is standard industry practice”.

The experts exchanged questions and answers in June and July 2022. Mr Evans asked Mr Groves if he had taken account of the fact that drilling was suspended in the borehole to carry out cone penetration tests and to take samples, and that time was taken when the trial pits were extended with a hand auger. Mr Groves said his position had not changed. He questioned whether the results could be extrapolated across the whole property; he repeated his misgiving about the type of auger used for the borehole, noted that it is difficult to assess the sequence of drilling operations, and conjectured (his word) that drilling operations in the borehole were only partially supervised because the supervising engineer would have been busy with the trial pits.

Mr Groves further pointed out that groundwater was encountered at the junction between made ground (that is, ground that has been subject to human intervention) and clay; he would have expected a zone of saturation above the clay which could not have been detected by the unsophisticated drilling method used. A reliable equilibrium could only have been established by multiple readings. He suggested that measuring the water level properly “was not considered as a key objective by the driller as he was only 2.3m into his target drill depth of 15m and was pushed for time.”

At the hearing the Tribunal asked Mr Groves what his view would be if he knew that standard industry practice had been followed when the GL Martin measurements were taken, and he said that he would then accept that they indicated an equilibrium water level.

We are not persuaded by Mr Groves’ criticisms. They are largely speculative: perhaps standard industry practice was not followed, but why would it not have been? Perhaps the driller was pushed for time, but there is no evidence whatsoever to suggest that. Perhaps he was unsupervised, but again there is no of evidence to that effect beyond Mr Groves’ assertion that detailed records of the drilling were available for the trial pits. Perhaps measurement of water levels was “not a key objective”, but that is highly unlikely since GL Martin were instructed to study the consistency of the ground and its capacity to bear an extra load and it seems to us that water levels would have been central to that objective.

Aside from those conjectures, the only scientific point Mr Groves made was that he regarded it as unlikely that if there was water present in clay it would not have saturated the made ground above it. We were not shown any evidence or data or published work to substantiate that assertion and therefore, despite Mr Groves’ expertise, we are not able to rely upon it.

Mr Groves said at the hearing that the sort of inaccuracy he was alleging might make up to 20cm difference in levels. Accordingly, at highest, the level was 1.5m AOD. Mr Groves’ strongest point was that there might be all sorts of inaccuracies in the measurements and that they could not be taken to indicate a single static level across the property. That of course is right, but nothing like that is being claimed. What is claimed is they indicate a very different water level from that seen at nearby positions within the Eldred data.

In our judgment the GL Martin measurement of the groundwater level at their borehole and in TPA, and the absence of water in TPB, indicate that at those points, and on that date, the groundwater level was at about 1.3m AOD, plus or minus 0.2m. It is not claimed that that alone justifies the further inference that water levels across the property in 2007 were between 0.5m and 1m lower that that seen in the Eldred data, but that it is one of a number of items that justify that inference when taken together.

4. The swimming pool The next item of evidence relates to the other end of the property around the swimming pool in the northern end of the garden. It was installed in the 1980s, and the claimants had it re-lined in 2004 by a firm then known as Hollingworth Pools and now Oasis, but in the same family ownership. Mr Brookhouse met with Mr Peter Hollingworth, the firm’s director, in June 2022 and has produced (and exhibited to his second witness statement) a note of his meeting. Mr Hollingworth recalled the re-lining, carried out in early May 2004. The bottom of the pool is at about 1.6m or possibly 1.5m AOD. For the re-lining the pool had to be drained so that the shape could be measured, and was then left empty and unlined for three weeks while a new lining was made. No groundwater was seen in the pool during that time. The pool is of a construction that could not have been installed, nor drained for re-lining, had the water table been above the bottom of the pool.

Mr Groves initially refused to regard the evidence about the pool as significant in the absence of any direct evidence from Mr Hollingworth. Mr Brookhouse therefore also supplied a letter from Peter Hollingworth dated 24 January 2023 confirming the accuracy of those notes.

Mr Groves in his supplementary report in February 2023 then argued at his paragraphs 58 and 59 that the information about the pool could not be used to infer that water levels have risen at the property because the water table at the property is neither constant nor static, and that at a period of low rainfall water levels could have fallen to at least 0.35m below the pool base as they did in September 2022. In response to that Mr Evans pointed out that rainfall records for the period leading up to May 2004 show that rainfall was 114% above average, and that in the first ten days of May when the hole was open 51mm of rain fell, more than the average for the whole of May, and asked if he did not therefore regard Mr Hollingworth’s evidence as indicating that water levels had risen. Mr Groves in response said that he was making only a general point about water levels varying.

It is difficult not to feel that Mr Groves ducked that question. No-one has called into question what Mr Hollingworth said about the impossibility of re-lining that sort of swimming pool if the water table was above the pool base. It was not above the pool base for three weeks in 2004. Mr Groves has not explained why that does not support Mr Evans’ argument.

Mr Groves’ comment about the pool in the revised joint statement in October 2023 was curious. He said:

“With respect to the swimming pool, he accepts the views of Mr Hollingworth regarding relining of the pool in 2004 but considers there to be an atypical response of the ground to rainfall in its immediate vicinity.”

That is puzzling. If the garden around the pool was a particularly dry area by contrast with other parts of the garden then that would support Mr Groves’ reluctance to attach significance to the evidence about the pool. But the Eldred data show the corner of the property as a particularly wet corner. Even so, levels there were below 1.6m AOD in a wet May in 2004. It is difficult to understand how that casts any doubt on the inference Mr Evans seeks to draw from the evidence. When asked about this at the hearing, Mr Groves said that he accepted what Mr Hollingworth said about water levels in the ground when the pool was re-lined, but that because the Eldred data showed groundwater levels in the pool area rising too quickly and by too much in response to rainfall they therefore should not be regarded as a reliable indicator that water levels have risen.

That seems to miss the point. Even if the water levels in the Eldred data are less than reliable in the vicinity of the pool, the hydrologists agree that overall the Eldred data are reliable and indicate groundwater levels of around 2m AOD near the house. What is at issue is what those levels were in 2004; and Mr Groves has nothing to say to counter the evidence that water levels in the ground where the pool is were below 1.6m AOD for three weeks when the pool was re-lined in 2004.

We note that this evidence is independent of, and consistent with, the evidence from the GL Martin investigations in 2007.

5. The Ruskins letter In February 2018 Mr Keith Morley of Ruskins Trees and Landscapes visited the property with a view to re-planting the garden; it will be recalled that a strip of garden inside the river wall has had plants and topsoil removed and has been not only a working corridor for the respondent but also the site of considerable excavations in the course of the remedial works. Mr Morley said:

“It was clear from my visit that this site is wholly unsuitable for any replacement planting at present. The top soiling and seeding should not be undertaken until the structural drainage issues are resolved.

There is surface water over the entire site and the ground is too boggy to walk on. There is a slight green tinge over the soil that looks like algae and a smell to the soil, suggesting anaerobic conditions. Furthermore it appears from the previous documented history that this water has a high saline content that the agency had been trying to deal with by applying a clay cap to stem the leaching of the tidal water into this area.

I am concerned that the area is not free draining therefore planting of almost any tree or shrub will fail very quickly.”

It is the claimants’ unchallenged evidence that before the work was done the garden was thriving, there were no incursions of water and the ground was not water-logged; Mr Brookhouse said “the land was not boggy when we were in residence”. Mr Evans takes the condition of the garden as observed by Mr Morley in February 2018 as evidence that water levels have risen, because visibly raised water levels like this were not seen before the respondent’s work was done.

Mr Groves’ response is that this is not reliable evidence “because the statements made are unrepresentative of average conditions. This correspondence states that the ground immediately landward of the sea defence wall was saturated, conditions which were not present several weeks later or maintained since.”

This does not seem to us to answer the point. It is not being said that the water levels in February 2018 happen all the time or in average conditions; indeed, the garden had dried out considerably by the time the Eldred boreholes were installed at the beginning of March. But wet conditions happen every winter, and before the work was done the garden was not seen to respond in this way. The evidence supports the view that something has changed.

In further discussion at the hearing Mr Groves expressed the view that the conditions observed by Mr Morley were the result of the recent water incursion. But the parties’ agreed chronology records the last water incursion before that date as being in May 2017; if the garden had been waterlogged from May 2017 to February 2018 we would doubtless have other evidence about that. We find that the state of the garden in February 2018 was caused by water levels in a wet period over the winter, which rose higher than they used to do before the respondent’s works.

6. The CH2M re-charge calculations

We mentioned above the work done by CH2M at the design stage when the concern was that the works would dry out the land, rather than making it wetter. Mr Evans relies on the estimate made by CH2M that the pre-existing water table was around 1m AOD. No measurement was taken. The plan produced by CH2M showed a water table sloping away from the river from a point around river level next to the wall, which cannot be right because for the most part the gradient would be towards the river as landward water drains towards it. Mr Groves’ response to this point was that because CH2M’s calculation contained “errors in the conceptualisation of the hydrology” it could not be relied upon.

Again that is to miss the point. The experts agree that the diagram is not right; but the fact remains that CH2M expected the groundwater level to be at or just above river level. As Mr Brookhouse said in his unchallenged evidence, that was the level the respondent (with its considerable expertise) and its contractor expected when it planned the works. It is also the level the hydrologists themselves would have expected absent any special feature (see paragraphs 87 and 88 above), and there is no evidence of any special feature before the works took place.

7. The landward watercourses Next Mr Evans relies on a survey carried out by Evans & Langford of water courses near the property, which are maintained at a level of 1m or 1.1m AOD. His argument is therefore that it is difficult to see how water levels at the property could be higher. This gave rise to some debate at the hearing; Mr Groves accepts the measurements of the landward watercourses but surmises that water levels just outside the property in Strand Street may be higher.

Mr Groves accepted that it is generally right that the gradient of water runs towards the river and that levels in the garden of not far above 1m AOD would have been consistent with both river levels and levels in the nearby watercourses. His point was that localised features may have given rise to atypical levels at the property. That is also right but we have no evidence of any such features. We accept that the levels of local watercourses make it likely that groundwater levels at the property, before the respondents’ works, were much nearer to 1m AOD than to 2m AOD.

8. Mean river levels adjacent to the property The claimants have calculated, from open source data, that mean river levels adjacent to the property have been generally in the range of 0.9m to 1.1 m AOD. This is not in dispute. In the absence of any special features in the ground outside the property, this supports Mr Evans’ view.