British Westinghouse Company v Underground Railway [1912] AC 673

Corbett v Pallas (1995) 86 LGERA 312

Davey v Harrow Corporation [1958] 1 QB 60

Dodd Properties (Kent) Ltd v Canterbury City Council & Ors [1979] EWCA Civ 4; (1980) 1 All ER 928

Fullwood v Fullwood (1878) 9 Ch 176

Jones v Dunkel [1959] HCA 8; (1959) 101 CLR 298

Lawrence v Kempsey Shire Council (1995) ATR 81-343

Paramatta City Council v Lutz (1988) ATR 80-159

Proprietors of Strata Plan No 14198 v Cowell (1989) 24 NSWLR 478

Travis v Vanderloos (1984) 54 LGRA 268

Watts v Rake [1960] HCA 58; (1960) 108 CLR 158

1 OWEN J: This case involves the aftermath of the construction of a dam by the defendants on their land at Gidgegannup. The plaintiff is the owner of the adjoining property. At an earlier trial the dam was found to be defective and to have caused damage to the plaintiff's property. These reasons cover the remedies to which the plaintiff is entitled.

2 The plaintiff and defendants are neighbours, owning respectively Lots 30 and 34 Quenda Glade, Tilden Park, Gidgegannup. The defendants' land ("Lot 34") is higher than that of the plaintiff ("Lot 30") and drains naturally on to the plaintiff's land. On Lot 30 there are, relevantly, a pine forest ("the pine forest"), a dam and a saltwater diversion channel ("the channel") which was constructed some years ago. It is designed to prevent the flow of saline ground-water from entering the plaintiff's dam from Lot 34. The channel, which is wholly within Lot 30, lies, in part, close to the boundary between Lots 30 and 34. I should add that the channel was constructed before the land was subdivided. Accordingly, the proximity of the channel to the boundary is not the fault of either party.

3 The channel is roughly horseshoe shaped and, in effect, surrounds on three sides the area of the plaintiff's dam. The channel is designed to protect the plaintiff's dam against the intrusion of saline water. The top section of the horseshoe goes in a north-easterly direction towards Lot 34. It then turns, roughly at a right-angle, in a south-westerly direction. It then deviates slightly to the south-east to discharge water into a local creek system. The north-east and south-east sections of the channel are not material to this dispute. Critical to the resolution of this problem is the fate of part of the south-west section. The part in question is itself divided into two sections. The first (which I will call "Section 1") starts at the corner where the north-east and south-west sections meet and travels about 60 metres adjacent to the boundary between Lots 30 and 34. The second (to which I will refer as "Section 2") commences where Section 1 finishes and travels for a further 60 metres or thereabouts adjacent to the pine forest.

4 In or about February 1996, the defendants constructed a dam on Lot 34. It will be convenient to refer to it simply as "the dam" and to avoid confusion I will refer to the dam on the plaintiff's land as the "the plaintiff's dam". The dam initially had a capacity of some 4,500 cubic metres but it has since been extended and now has a capacity of something in excess of 5,000 cubic metres. The dam leaked. In May 1997, with a view to carrying out remedial work on the defendants' dam, the defendants pumped the water from it with the result that a mixture of water and soil flowed on to Lot 30, into the pine forest and, eventually, into the channel.

5 This case has had a tortuous history. The genesis of the litigation lies in the de-watering of the dam in May 1997. The plaintiff issued a writ and sought an injunction to prevent the defendants from taking steps that would result in the contents of the dam (save for natural overflow) flowing onto Lot 30. The plaintiff obtained, ex parte, an interim injunction in much those terms. Later, the defendants gave an undertaking to similar effect and the injunction was discharged.

6 In the statement of claim the plaintiff alleged that the emptying of the dam by the defendants caused injury to her land, predominantly accelerated erosion to the embankments of the channel. The injury, the plaintiff pleaded, was aggravated by the defendants' further conduct in ploughing the defendants' land below the dam. She said that the ploughing increased the sub-soil saturation of the defendants' land with the result that there was a sub-soil flow of the liquids from the dam through the embankment wall to the channel. The plaintiff also pleaded that the dam was a continuing danger to her land. The defendants' conduct was said to constitute negligence, nuisance and a trespass and to have caused loss and damage to the plaintiff.

7 By the time the matter came to trial the plaintiff's prayer for relief included claims for:

(b) a mandatory injunction requiring the defendant to remove the dam and reinstate the natural contours and stability of Lot 34; and

(c) an order that the defendants pay the cost of restoring and reinforcing the western embankment of the channel.

8 On 21 October 1997 a Judge (who was not the trial Judge) made an order that "[t]he trial shall be a trial as to liability only". The trial occurred over six days in April and June 1998. In August 1998 the trial Judge handed down judgment in which he found for the plaintiff: Conti v Chenery, unreported; SCt of WA; 5 August 1998; Library No 980432 (which I will call "Conti"). His Honour found that the construction of the dam was not a reasonable use by the defendants of their land and that the dam constituted a danger to the plaintiff's land. He also found that the defendants' conduct had caused injury to the plaintiff's land. I will describe the critical findings in more detail later. Judgment was entered for the plaintiff, relevantly, in these terms:

"3. The defendants do by not later than 30 March 1999 fill in [the dam] and restore the contours of Lot 34 at the site of the dam on Lot 34 as nearly as possible to the contours prevailing thereat prior to the construction of the dam on Lot 34 such filling in and restoration to be in accordance with the directions, and under the supervision, of a duly qualified geotechnical engineer.

.........

5. The determination of damages including damages for the restoration and reinforcement of the drainage trench on the plaintiff's land downslope of the [dam] and including general damages, and interest, be adjourned for assessment by a Master ... ."

9 It is apparent that the trial Judge took the view that the pre-trial order made in October 1997 segregated the issue of damages only.

10 The defendants appealed against that decision. The gravamen of the appeal was that the trial had been as to liability only. The grant of a mandatory injunction was a matter going to remedies, not liability. The defendants had not had a proper opportunity to address the issue whether a mandatory injunction was an appropriate remedy in the circumstances. The defendants also challenged the trial Judge's finding that the construction of the dam was an unreasonable use by them of their land.

11 In November 1999 the Full Court (of which I was a member) ruled that the trial process had miscarried because, in granting a mandatory injunction, the trial Judge had gone beyond "liability" and into an area of "remedies" which the defendants had not had a proper opportunity to confront: Chenery v Conti [1999] WASCA 258 (which I will call "Chenery"). However, the appeal against the trial Judge's finding that the construction of the dam was an unreasonable use of the land was rejected. The Full Court ordered that the matter go back to the trial Judge for further consideration concerning remedies. In so doing the Full Court made it clear that critical findings made by the trial Judge on issues of liability (not remedies) could not be re-opened at the further hearing: Chenery per Owen and Steytler JJ at [74]. As things have turned out it was not possible for the trial Judge to re-hear the matter. It has therefore fallen to me to do so.

12 In this re-hearing, I am concerned, and only concerned, with two broad issues. First, should the Court compel the defendants to fill in the dam or is there some other remedy that will eliminate or reduce, to a reasonable and acceptable level, the danger which the dam represents to the plaintiff's land? Secondly, what is the measure of damages to which the plaintiff is entitled to compensate her for the injury that has already occurred (and which may continue to occur) to her land? There may, of course, be consequential orders, but they are the main issues.

13 I repeat what I have already said. In determining these matters the parties are bound by the critical findings of the trial Judge on issues relating to liability. For this reason, I think it is necessary to spell out what I see as the critical findings. The consideration by me of the evidence adduced at the re-hearing must be seen against that background.

14 The trial Judge identified, from the pleadings, a number of discrete, although in many instances related, questions and he decided the case by answering those questions. It will be convenient for me adopt such of the trial Judge's answers to questions as are necessary for present purposes. Some of them will, however, be reformulated, to fit the present circumstances.

15 1. Issue 3(a) and (b): When the defendants' dam was being emptied in May 1997, did salty water and sludge enter Lot 30? The trial Judge's ultimate conclusion on this issue appears in Conti at 14:

"I find that the water in the defendants' dam is reasonably fresh and that the water entering the channel from Lot 34 is salty. I find that the probability is that water seeping from the dam is mingled with saline ground water before entering the channel on Lot 30. Accordingly, while I find that the contents of the defendants' dam is not salty water or sludge, the water entering Lot 30 from Lot 34 was salty and sludge was deposited by such water on to Lot 30."

16 2. Issue 3(c): Did the defendants thereby cause injury to Lot 30?

"I find that the emptying of the defendants' dam did cause some structural damage to the walls of the plaintiff's channel on Lot 30 by way of accelerated erosion.

I am satisfied on a balance of probability that the emptying of the water from the defendants' dam caused some damage to the pine forest on Lot 30. The photographic evidence shows the deposit of mud within the pine forest and that the angle of some of the pine trees departs from the vertical. I accept that this latter effect was caused by a loss of foundations which is to be ascribed to the action of the large volume of water discharged from the defendants' dam." [Conti at 16]

17 3. Issue 4(a): Was the injury aggravated by the ploughing of Lot 34 downslope of the defendants' dam?

"The result of the ploughing was, I find on a balance of probability, to reduce the surface flow of water from the defendants' dam downslope to Lot 30 and, at the same time, to increase the sub-surface drainage to Lot 30.

However well-meaning, the ploughing did not minimise the damage to Lot 30 and, indeed, aggravated the position. The sole purpose of the ploughing was to reduce the flow of water from Lot 34 to Lot 30. It did not achieve that purpose."

The damage caused to the plaintiff's channel is, I find, substantial and I find that the ploughing was not a necessary aspect of the occupation of Lot 34. While, no doubt, the ploughing of a farm for the purpose of planting crops is a normal and reasonable activity, the ploughing in question was not in that category."

18 4. Issue 5: Has the plaintiff suffered and will she continue to suffer loss and damage as a result of such injury?

"I find that the plaintiff did suffer loss and damage as the result of the emptying of the dam on Lot 34, such loss and damage being as pleaded [in par 11 of the statement of claim] as follows:

(a) accelerated erosion of [the channel] embankment;

(b) in and about the area of the plaintiff's land at which the contents of the defendants' dam entered the plaintiff's land including the pine forest floor:

(v) growth on the plaintiff's land of foreign pasture species;

(vi) destabilisation of tree root systems on the plaintiff's land;

(c) undercutting and slip failure of the [channel] embankment;

(d) undercutting of the pine forest boundary fence;

(e) development of a zone of failure in the general area of the pine forest;

(f) reduced resilience of the plaintiff's land to the effects of the risks [of hydraulic shock and destabilisation of the embankment walls];

(g) loss of access to the pine forest adjacent to the Pine Forest around the [channel];

(h) slumping of the [channel] embankment wall;

(i) loss of mass from [channel] embankment wall;

(j) reduced access along top of drainage embankment adjacent to boundary with Lot 34;

(k) endangerment arising from destabilised pine forest trees.

If the water from the defendants' dam is again discharged in a similar manner, I find that the plaintiff is likely to suffer similar loss and damage in the future. Accordingly, I find the defendants liable in damages for nuisance." [Conti at 20 - 21]

19 5. Issue 6: Was the defendants' conduct negligent and, if so, has the plaintiff suffered and will she continue to suffer, loss and damage as a result of such negligence?

"The defendants were aware, before they purchased Lot 34, of the existence of the channel on Lot 30 and of its fragility. In these circumstances, the construction of the defendants' dam so close to the boundary has given rise to the problems deposed to during the trial. The defendants have contended that the dam is sited in the optimum position to gather water on Lot 34. However, the evidence suggests that the dam is kept partly empty (and this is part of the basis for the contentions by the defendants' experts that the defendants' dam poses no threat to Lot 30) so that it is not apparent that the defendants require a dam of the capacity of that presently on Lot 34. As [the defendants' expert] said, with the benefit of hindsight, he would have advised locating the dam a further 5 or 10 metres up the hill. That would, in my opinion, have been wise.

In my opinion, and I so find, the construction of the dam in the position in which it has been located was negligent and the defendants are liable to the plaintiff for damages in negligence." [Conti at 21 - 22]

20 6. Issue 7: Has the seepage from the defendants' dam caused damage to Lot 30 on the western embankment of the channel?

"The evidence satisfies me that the seepage from the defendants' dam has indeed caused the damage complained of. While the evidence indicates that, over the years since it was first constructed, the walls of the channel have been subjected to erosion, there has been a marked acceleration of erosion since the construction of the defendants' dam. In my opinion, that acceleration is the result of the saturation of the land below the defendants' dam and is directly attributable to the fact that that dam has been located in its present site." [Conti at 22]

21 7. Issue 8(a): Is the defendants' dam a danger to Lot 30 and likely to do further damage and irreparable harm to Lot 30 by flooding the channel and the pine forest?

"Dr McInnes, whose evidence I accept, gave the following evidence in chief in this regard:

'8. In my opinion:

(a) the construction of the dam in the location chosen for it so close to the drain on Lot 30 and the ponding of water within the dam presents a very significant risk to the point of inevitability of the dam failing due to:

(i) backwards erosion of the open drain trench towards the dam; or

(ii) by water escaping from cracks in the downstream dam wall due to ground movement of the foundations beneath the wall of the dam;

(b) dam failure due to one or other of the abovementioned mechanisms can be expected to occur within years (not decades) at most and could easily occur with the effects of only one rainfall season or even of only one severe rain event such as the rare cyclonic event of February 1992;

(c) the presence of the dam on Lot 34 has created circumstances which must be considered as very serious requiring urgent implementation of remedial measures.' " [Conti at 25]

22 8. Issue 10: Is the plaintiff's loss and damage due to her own negligence in failing to carry out remedial work on the channel made necessary by natural erosion over time?

"In my opinion, the damage to the channel is not attributable to any negligence on the part of the plaintiff. If the existence of the defendants' dam prevents the plaintiff carrying out remedial work, that can hardly be laid at her door. [Conti at 32]

..........

The defendants' expert witnesses deposed to the fact that the walls of the channel are too steep to be effective and that the steepness of the walls encourages their collapse under the influence of the ground water entering the channel from Lot 34. However, over the years preceding the construction of the defendants' dam, the extent of the erosion seems not to have been great, according to the evidence. I find that the profile of the channel is such that it is prone to damage resulting from the action of ground water seepage, albeit, prior to the construction of the defendants' dam, this was not a serious problem. However, I consider that the presence of the defendants' dam has resulted in accelerated erosion and the creation of a serious condition." [Conti at 32 - 33]

23 9. Issue 11: Does the construction of the defendants' dam comprise a reasonable use of Lot 34?

"In my opinion, the construction of the defendants' dam in the position in which it is located and in the circumstance that it was known to the defendants that the channel on Lot 30 was fragile and likely to be damaged by additional seepage of water, was not a reasonable use of Lot 34." [Conti at 40]

24 These findings cannot be re-opened. However, there may be some instances where it is permissible to look at some of them, for example, the vulnerability of the channel to erosion over time in any event, as part of the argument on remedies.

25 Before I embark on a consideration of the evidence, I think it is necessary to stand back, as it were, and look at the broad context in which the selection of remedies falls to be decided.

26 The dam was built too close to the boundary between Lots 30 and 34. It was constructed by a contractor who was experienced in the construction of dams, but not in accordance with engineering specifications and, when constructed, it was defective. It leaks, although it is accepted that all dams have some seepage. The dam, in its present state, represents a danger to Lot 30. It is, I think, important to understand exactly why it represents a danger.

27 The de-watering of the dam and the ploughing of the paddock in which the dam is situated resulted in an artificial saturation of the soil profile. The presence of the dam, with the weight of 5000 cubic metres of water in it has resulted in excessive hydraulic pressure resulting in preferred drainage pathways in the sub-soil. The surface flow from the de-watering itself caused problems. In addition, the dam on Lot 34 has exacerbated sub-surface flow by concentration of water with the effect that what would otherwise have been a seasonal flow is now year round. These factors contributed to the collapse of the channel embankment and have enhanced the embankment failure mechanisms. Put in a slightly different way, the excess water flow from the de-watering operation and from seepage in the saturated soil found its way into the channel and contributed directly to erosion of the channel wall. It also made the channel wall vulnerable to further accelerated erosion from the continuing action of water seeping into the channel.

28 The reason why the dam represents a danger to Lot 30 is linked to the issue of erosion. If the western embankment continues to erode towards, and over, the boundary between Lot 30 and Lot 34, it will approach the toe of the dam. There will be erosion of the foundations beneath the dam wall. This will reach a point where the dam is unable to withstand the water pressures that are placed on it. It will collapse, or at very least, there will be discharge of water from beneath the dam wall.

29 There is a second aspect. The purpose of the channel is to trap saline water that is moving down the hill through the sub-soil so as to divert it away from the plaintiff's dam. As the embankment erodes, sediment is deposited in the base of the channel, reducing its efficiency as a means of diverting saline water. The increase in sub-surface flow caused by the presence of the dam exacerbates these problems.

30 In the statement of claim the plaintiff seeks a mandatory injunction for the removal of the dam and the reinstatement of the land surrounding the dam site to its natural contours. The plaintiff also seeks damages.

31 The question that I have to decide is whether, in order to remove the danger that the dam represents to Lot 34 and to preserve the integrity of the channel for its designated purpose, I should require the defendants to fill in the dam and reconstruct it elsewhere. This involves a consideration as to whether something short of filling in and re-siting the dam will reduce the danger to an acceptable level. I must also decide what damages are to be paid to the plaintiff to compensate her for the injury caused to the channel and to her land.

32 In relation to monetary compensation, the trial was conducted in such a way that the proper award is limited to the cost of remedial works on the channel. The plaintiff also sought compensation for the damage to her land generally, such as the destabilisation of the pine forest, the loss of amenity due to access difficulties as a result of erosion and so on. However, no evidence was led that would permit an award of that nature to be made. Nor was any evidence led as to the diminution in value (if any) of the plaintiff's land due to the injury to the channel and its surrounds. I raised this with counsel for the plaintiff before the cases were closed. He effectively conceded that the level of damages would have to be confined to the proper cost of remedial works.

33 It needs to be clearly understood that the Court could not order, and is not ordering, the plaintiff to carry out any remedial works on her land: see Chenery at [76]. However, it is inevitable that the plaintiff will be required to carry out remedial work. Unless she does so she will almost certainly lose further land to erosion, the efficiency of the channel for its designated purpose will deteriorate and the prospect of a catastrophic collapse of the dam on Lot 34 will increase. Accordingly, the reasonable cost of remedial works constitutes a proper basis on which to assess damages.

34 The last thing that I wish to say by way of overview is this. While, to some extent, the issues of injunctive relief and damages are separate and discrete, in the circumstances of this case they need to be considered together. The identification of the fairest means of reducing the danger that the dam represents cannot be divorced from issues related to the integrity and functional utility of the channel. The level of damages is directly related to restoring and preserving the functional utility of the channel. Accordingly, the two issues are intertwined.

35 Mandatory injunctions are granted on the same general basis and principles as prohibitory injunctions, although it must be said the jurisdiction to order a mandatory injunction is exercised cautiously: see Redlands Bricks Ltd v Morris [1970] AC 652. Nonetheless, it is essentially a matter of discretion so as to do justice in the circumstances of the particular case.

36 As a general statement, the evidence was that provided certain works were carried out on the dam and rectification works were applied to the channel, it would not be necessary to remove the dam so as to correct the danger that the dam represents. There was evidence that it would cost approximately $40,000 to fill in the dam and reconstruct it at a different location. There was a difference of opinion between the expert witnesses as to the works that would have to be carried out on the dam and on the channel to achieve this result. It was common ground that something would have to be done to improve the overflow mechanisms of the dam. There was no unanimity as to whether it would also be necessary to seal the dam. In relation to the channel, it was common ground that some work would have to be carried out on Section 1 but there was an argument as to whether work was also required in Section 2. The expert witnesses for the plaintiff put forward a proposal for the remedial works for the channel. The defendants' engineers put forward alternative proposals at a significantly lower cost. The plaintiff's experts expressed reservations about the defendants' proposals.

37 In the end, I have come to the view that justice does not require the removal of the dam. This is on the understanding that remedial works are carried out so as to make the dam more functional and less likely to fail and provided also that the plaintiff is compensated, by an award of damages, for works that are necessary to stabilise the channel. In my view, the central issue in the case is the identification of the necessary works and, in relation to the channel, the cost of those works.

38 I need to describe in summary form the remedial works which the expert witnesses suggested should be carried out to the channel.

39 The plaintiff called two expert witnesses. The first was Douglas McInnes, a Geotechnical Engineer with the firm Golder Associates. The second was Douglas Blandford, an Environmental Scientist and a principal of D C Blandford and Associates Pty Ltd. Michael O'Neill also gave evidence for the plaintiff. He is an Earthmoving Contractor trading under the name Darling Earth Movers. In the main, his evidence went to the costs of the remedial works. The main expert witness for the defendants was Geoffrey Cocks, a Geotechnical Engineer and a principal of Coffey Geosciences Pty Ltd. Kenneth Ash, an Engineering Surveyor and Earthworks Contractor and a principal of Ash Engineering Surveys Pty Ltd, gave evidence on behalf of the defendants. His primary role was to provide a costings estimate. However, during his inspection of the site he put forward an alternative proposal.

40 McInnes produced drawings for remedial that he said were necessary to secure the channel in Section 1. I will call it "the Golder design". He testified that his plan was "necessitated by the effects on Lot 30 of the dam on Lot 34 and [did] not exceed the requirements for remedial work necessitated by those effects".

41 The Golder design is to stabilise the walls or banks and the floor of the channel but to leave it open. The base of the channel is to be excavated to about 300mm below the present surface to remove all weak material. A non-woven geotextile is to be placed over the base and covered with 5mm aggregate. The banks are to be supported by gabions and rockfill. Gabions are wire baskets of about 1m by 1m in size. They are filled with rock spalls, which are individual pieces of rock of about 150mm in size. The gabions are positioned in a continuous formation down each side of the channel. Generally speaking, the gabions will be about 2m in height. On the east bank they will be 1m wide. On the west bank they will be dug into the wall of the channel and will be 2m wide at the base and 1m wide on top. On both sides rockfill will be placed between the trench bank and the back side of the gabion. Geotextile will extend up the back face of the gabion baskets to the ground surface at the top of the gabion baskets. The geotextile will assist in preventing fine soil material entering into the gabion baskets. A covering of soil to a depth of about 200mm will be placed on the top of the rockfill. Erosion gullies and depressions in the banks of the channel are to be filled with 10-20mm aggregate to form an even surface against which the rockfill is to rest. The flow channel (that is, the gap between the gabions on each side) will be about 1.5m in width.

42 Blandford produced a design commentary and associated drawings (which I will call "the Blandford design") for Section 2. He described the works as "no more than what is requisite to remedying the adverse effects of the dam on Lot 34 on the area below the pine plantation".

43 The aim of the Blandford design is to arrest basal erosion, enhance sediment deposition to build up the floor of the trench to a width of 2m, reinstate lost ground below the pine plantation and stabilise and protect the embankment from further damage. Gabions sized at 2m x 1m x 1m are to be filled with rock and embedded in the floor of the channel to a depth of about 0.5m. The gabions will not form a continuous line but will be located at every 0.5m change in floor height. Gabions of a similar size will be placed on the eastern embankment wherever a basal gabion has been located. The western embankment will be deep-ripped up to 400mm to destroy drainage pathways and help bond the fill material. Fill material is to be placed in the ripped area. The top of the bank is to be reinstated to a width of 1m. The slope of the western embankment is then to be covered by reno mattresses. They are like gabions, except that they are flatter, flexible and come in various dimensions in terms of width, depth and length. Reno mattress are generally designed to be about 300mm deep, 2m in width, and from 4m to 12m in length. They are filled with rock and because they are flexible they tend to settle into the shape of the ground under them. The top of each reno mattress will be flush with the top of each basal gabion.

44 A geotextile is to be placed under the sidewall and basal gabions and is to extend the length of the fill under the reno mattresses. This will allow the movement of water while retaining soil and sediment on the slope and will increase friction between the reno mattress and the underlying fill material. The upstream face of the basal gabions is also to covered by geotextile.

45 Topsoil is to be placed over the fill material at the top of the embankment to a depth of 300mm and seeded with a grass or clover species suitable for the Gidgegannup area. Once seeded, the topsoil is to be covered with a fine mat of a biodegradable substance. The purpose of the mat is to prevent erosion while vegetation is being established.

46 The defendants main expert was Cocks and I will call his plan "the Coffey design". It involves the installation of a 150mm slotted pipe subsoil drain in the base of the channel. The slots in the pipe are about 2mm wide and 200mm in length. A non-woven geotextile is laid underneath the pipe. The pipe is then covered by aggregate to create an intercept zone of about 1m by 0.8m. The geotextile is then drawn over the aggregate and the channel is back-filled with soil from the original channel construction thus lowering the top of the embankment to its original level. The channel would therefore be filled in. The back-filling would be done so that the surface would slope down from each side with a swale in the middle. One of the most controversial features of the Coffey design is that it is confined to Section 1 and does not include the area adjacent to the pine forest.

47 In his oral evidence McInnes suggested that the Coffey design would be improved if the pipe were to be positioned below, rather than on, the current floor of the channel. In his evidence, Cocks agreed that this would be desirable as it would allow the pipe to catch more of the saline water that was flowing into the trench. McInnes also suggested that the intercept zone should be increased in size by using more aggregate closer to the surface. Cocks was less enthusiastic about that suggestion.

48 Ash advanced an alternative rehabilitation treatment which I will call "the AES design". It involves excavating a new salt water diversion trench approximately parallel to, but about 10m to 15m to the east of, the existing channel. The new trench would cover the length of Section 1 and Section 2 and eventually join up with the existing channel at a point south of the pine forest. The base of the trench would be flat, about 2.5m in width and of a similar depth as the channel so as to maintain the salt water diversion function. The sides of the new trench would have a flatter slope than those of the channel. Newly excavated material as well as previous heaps of excavated soil would be used to back-fill the channel. The soil would be compacted to ensure stability.

49 Part of the Coffey design envisaged some remedial works to the dam. The Coffey design has a 150mm pipe coming out of the western wall of the dam near the south-western corner of the dam and about 1.5m below the level of the spillway. The pipe curls around so that it would discharge in the area of the firebreak at the boundary between Lots 30 and 34 adjacent to Section 1 of the channel. According to Cocks this would control the water level in the dam without the need for pumping and would return any overflow to the path that rainfall run-off followed prior to construction of the dam.

50 McInnes and Blandford pointed out, and I accept, that there are defects in this plan. The 150mm pipe would concentrate the water and it would come out at such a velocity to create a risk of erosion. The erosion could be reduced by having the water discharge into a stilling basin. However, even then, when it overflowed the stilling basin it would pick up waste material and flow towards the boundary between Lots 30 and 34. It would collect on the fence and create a mini-dam. If it did flow over or through the fence it would go into the channel. The deposit of waste material in the channel would reduce its effectiveness.

51 Blandford suggested an alternative to the Coffey design (so far as it affected the dam). He could not see that there was a need for an outlet pipe at all. He suggested that there be a diversion or contour bank starting at the level of the existing spillway and following the natural contours away to the south-west. Water which was discharged from the dam would be directed along the bank and flow out over the paddock and eventually permeate into the ground and become a sub-surface flow. This would not affect Lot 30 at all.

52 Having considered the matter further, Cocks agreed that the Blandford suggestion had merit. I think Cocks has it in mind to lower the existing spillway slightly and to position a large stilling basin at the end of the diversion bank. All witnesses seemed to agree that this would be an inexpensive and cost-effective solution to that particular problem.

53 A separate question is whether the dam should be lined or sealed. Cocks thought this was entirely unnecessary. He said that dams tend to "self-seal" and, from his observations, this one had done so. When he said this, Cocks was being cross-examined about evidence he gave at the original hearing. The inference is that, at the time of the first hearing, he thought leakage was not a problem and he was still of that view. It is beyond question that all dams leak to some extent. However, I believe that White J's comments in relation to Issue 7 constitute a finding that the seepage from this particular dam created a problem in that it resulted in, or contributed to, saturation of the sub-soil. That is a finding that cannot now be challenged. I could not help but feel that Cocks' views on this point stemmed from a belief (I have no doubt conscientiously held) that his Honour was wrong. In any event, there was evidence before me (which I accept) to the contrary. McInnes said there remained a risk of seepage from the dam. He said: "... if appropriate and well-controlled works were conducted on the base of the existing dam then that would further minimise the volume of seepage which was reporting to the [channel]".

54 The preponderance of evidence was that if the channel were stabilised it would not be necessary to fill in the dam. Blandford said his design had been developed on the assumption that the dam would remain. That, of course, was Cocks' view. McInnes was a little more ambivalent but he, too, thought that provided remedial measures were taken on the dam and the channel, the dam could remain in its present location. He said:

"It would be correct to say, would it not, that it's not necessary to infill the dam to prevent the erosion or the continued erosion of the channel?---That's correct.

Nor is it necessary to fill the dam to remove the danger of the dam failing, which currently exists because of the interaction between dam and trench?---It would be necessary to conduct other measures or to put in place other measures to prevent the failure of the dam if it was not filled in."

55 There are other factors that, although of lesser significance, also point against the filling in of the dam. I accept Mrs Chenery's evidence that the defendants have spent around $19,000 on the construction of the dam and that they require the fresh water it collects for their livestock and for firefighting, irrigation and domestic purposes. I also accept the evidence of Ash that it would cost approximately $40,000 to fill in the dam and construct another one in a different position. I also note the evidence of Cocks that to fill in the dam and construct a new one further up the slope on Lot 34 "may do more harm than good, assuming that one of the treatments is applied to the [channel]". I have also taken into account that the cost of re-sealing the dam will be in the region of $10,000 to $20,000.

56 In my view, and provided that remedial works are done on the dam, it is not necessary to fill in the dam and I would not be disposed to grant a mandatory injunction that would have that effect. I also find that the remedial works necessary to handle excess water in the dam are those suggested by Blandford. I also find that it is necessary to line or seal the dam, as suggested by McInnes. In my view, this will reduce the quantity of sub-surface flow with which the channel has to cope. It will increase the effectiveness of the remedial works to the channel that have to be undertaken.

57 Looking ahead to an argument about costs, I should say that had the defendants adhered to the plan to position an overflow pipe so that it discharged water at a point near the boundary between Lot 30 and Lot 34, I would have had serious concerns about the efficacy of the proposed remedial works. In other words, the evidence contained in the initial Cocks proposal for the dam probably would not have been sufficient to persuade me not to order that the dam be in-filled.

58 I can see little point in an in-depth analysis of the juridical nature of damages. In the context of this case it seems to me the level of damages should be the reasonable cost of restoration works that are appropriate in order to:

(a) prevent further erosion of the channel in Section 1;

(b) prevent further erosion of the channel in Section 2; and

(c) provide the same saline water diversion facility that the channel was providing before these events occurred.

59 Generally speaking, if there is more than one method of achieving the goal, the less or least expensive of them should serve as the appropriate cost on which to base the award. The Golder design and the Coffey design would be of equal utility in achieving objective (a). I will come to the actual costings later, but the Coffey design comes at a significantly lower cost. The Coffey design does not achieve the second objective, although it could be extended into Section 2. However, I think the Coffey design would be more expensive than the Blandford design in that area. There is a real issue whether either the Coffey design or the AES design would be as effective as the Golder design in providing the same level of salt water diversion as the plaintiff has enjoyed in the past.

60 Significant credibility attacks were made by the plaintiff against Cocks and by the defendants against McInnes and Blandford. When I say "credibility attacks" I do not mean that the honesty of the witness was attacked. Rather, the emphasis was on alleged shortcomings in their respective testimonies such that I should reject one set of solutions and accept another. It would have been rather more easy to resolve the matter had I been able categorically to reject one set of expert opinions and accept the other.

61 I did not find it useful to examine in minute detail whether, for example, placing geotextile under the reno mattresses would increase or decrease friction between the surfaces and, if I were to decide that the latter was the case, to regard Blandford's opinions generally as less reliable. What I have done is to take the various opinions expressed and to consider them in the light of the evidence overall and to see which ones best fit with the entire circumstances of the case.

62 In my view the appropriate way to approach this problem is to start with the proposition that the Golder and Blandford designs will achieve the three objectives that I have set out above. I will then consider whether either of the Coffey or the AES proposal will achieve the same or a substantially similar result such that the lesser costs represented by those options should form the basis of a reasonable award. The easiest way of doing this is to focus on the several concerns that Blandford and McInnes expressed about the alternative proposals. However, before I do that I want to comment on two particular issues. One is whether there should be an award for the cost of remedial works in Section 2. The other is the adequacy of the AES design.

63 Counsel for the defendants submitted that the award of damages should not take into account any remedial works in Section 2. The approximate cost of the Blandford design covering Section 2 is $31,000. If the Coffey design were continued along Section 2, the cost would be about $53,000.

64 First, the Blandford treatment is not a necessary factor in rendering the dam structurally sound and no longer a danger to the channel. That is so. To achieve that objective, remedial works must be carried out in Section 1. McInnes seemed to accept that if the Golder design were implemented in Section 1 it would not be necessary to construct the Blandford design in Section 2 in order to render the dam safe. However, that is not the point. The issue here is the integrity of the channel. In the area of Section 2 it has been rendered susceptible to erosion and the western bank has eroded. In other words, the plaintiff has suffered damage to her land in the area of Section 2. That damage has a causal link to the tortious conduct of the defendants. This follows from the findings of White J on Issues 3(a), (b) and (c) and 5, among others. It is difficult to see why the plaintiff should not be compensated for this loss.

65 Secondly, if the Blandford treatment is not implemented there will simply be a gradual loss of the plaintiff's land in the vicinity of the pine forest. Cocks thought that the erosion would be very gradual and may lead to the loss of only two or three trees in the next 20 years. I think that stands in somewhat marked contrast to his evidence (admittedly in another context) that had the dam not been constructed at all, the channel would have deteriorated to the state in which it now is in about another eight or ten years. On the other hand, Blandford's evidence, which I accept, is that once destabilisation has taken place, the tendency for the embankment to fail is increased. Erosion would be accelerated. Once again, this follows from the findings of White J, especially on Issue 5. His Honour made it clear that his findings there encompassed damage to the area of the pine forest as well as to the area which I am now describing as Section 1. White J described the defendants' tortious conduct as, among other things, contributing to the "development of a zone of failure in the general area of the pine forest".

66 On this basis, counsel submitted that it would be unreasonable for the defendants to have to pay about $30,000 effectively for the loss of two or three pine trees. This was especially so, counsel said, when the plaintiff had adduced no evidence of the diminution of the value of her property caused by the fact that this land was eroding. I acknowledge that evidence of diminution in value can be important. But this is a case of a small portion of land which forms part of a larger whole. In a case where there had been injury and that injury is likely to continue, unless remedial steps are taken, I do not think that failure to show an actual diminution in the value of the property is fatal to the claim.

67 There is another factor. If it is accepted (and I do accept) that there is an undeniable need for work to be done in Section 1, the question arises whether the integrity of the design in Section 1 would be compromised unless something was done in Section 2. McInnes said (and I accept):

"I can indicate that unless the work downstream of this design that I'm looking at, unless that downstream work - that is, the Blandford design is put into place then there will be serious build-up of sediment in the base of the channel in front of the pine plantation and that will reduce the effectiveness of the trench."

68 Clearly, the Blandford and Golder designs are compatible. They were developed separately but they fit together. No witness was asked whether the Blandford design would fit with the Coffey design, assuming the latter was constructed in Section 1. Accordingly, if the integrity of the work in Section 1 could be threatened without work being carried out in Section 2, the defendants can obtain little comfort from this line of attack.

69 I find, therefore, that the award of damages should include an element for the cost of remedial works in Section 2.

70 I am not satisfied on the evidence that the AES design form an adequate basis from which to proceed in assessing damages. It was very much a concept plan. No site investigations had been done. The path that the new trench would follow had not been selected. I am not pointing these things out in a pejorative sense. It is just that there is too much uncertainty surrounding the proposal.

71 The area between the channel and the plaintiff's dam has a large number of mature eucalypts. Depending on the path that the trench was to follow, some of those trees might have to be sacrificed. Blandford testified that there was a dolerite dyke in the general area. If the new trench intersected it, there would be a leakage problem that might affect the plaintiff's dam. I realise that Cocks took a different view both of the difficulties of constructing a trench and of the likelihood of significant leakage if it did occur. Nonetheless, the potential is there and without a clear indication of the path the trench would follow, there is a problem with the proposal.

72 McInnes also thought that the new trench could itself lead to erosion problems. He described it as "a very simple approach and as a result of ongoing effects that are at work in this particular locality we will finish up with a trench that needs to be stabilised". He thought that in ten years or so the potential was there for this very situation to recur.

73 In my view there are simply too many uncertainties with the AES design for it to form the basis of costings which would, in turn translate into fair and reasonable compensation to the plaintiff.

74 I return now to the Coffey design and to the problems that Blandford and McInnes raised in relation to it. McInnes, and to a lesser extent Blandford, proffered the view that the Coffey design would be less effective than the Golder design in capturing and diverting saline water. Cocks was of a different view. I need to look at particular issues.

75 McInnes raised a concern that the slots in the pipe might become clogged. He said it would be difficult to clear clogged slots because fine material would be washed out but would remain in the vicinity and return to clog again. He said this was a particular problem with saline water because the chlorides that were left behind as the water evaporated would lodge in the pipe as solids. The salts would dissolve again once further water entered the system but would still be deposited in the pipe. McInnes thought that the slotted pipe solution was more effective where the water was fresh and that it did not work well in "extremely saline conditions".

76 Blandford, too, had concerns about the efficiency of the pipe. He had less of a problem when it was explained (contrary to a strict reading of the design sketch) that the pipe was to be laid below, rather than above, the current floor of the channel. Blandford did not mention the deposit of salts as creating a problem. He was more concerned with the slots clogging with soil material. He agreed that the aggregate (suggested to be 14mm in size) would aid the filtration process. But the difficulty was in ensuring that the aggregate did not itself become clogged with soil particles In that respect, Blandford was concerned that the geotextile did not go up the western embankment. I think the inference to be drawn is that if the aggregate was completely enclosed by the geotextile fabric, the chances of it becoming sullied by soil particles would be significantly lessened.

77 In his examination-in-chief, Cocks answered these criticisms in this way:

"Dr McInnes also said he would have a concern about whether the slots of the pipe could clog either by soil particles or sale. Could you comment on that?---I think that's very unlikely. The slots proposed would probably be about 2 millimetres cut into it. That's about 10 times the size of the - and that's the least dimension, because they are long, the slots are long. They are one or two hundred millimetres long. I don't exactly recall but they are very much larger than the pore size of a geofabric. The Bidum A34 that Mr Blandford referred to I think had a pore size of 0.18 millimetres. This is a slot width of 2 millimetres, so it's more than 10 times the size, so I think it's much less likely to block than the fabric and in any event I think it's a very unlikely event.

What about blockage from salt?---Again I think that's a very unlikely event. Even with the trench exposed there doesn't appear to be a lot of signs of salt deposition and taking the absolute worst scenario, that some slots did become blocked, then the water would still flow through the aggregate parallel to the pipe."

78 He also said that as there was no source of evaporative energy (that is, no light, no heat and no air flow) it was unlikely there would be any significant imposition of salt. He also said that the decision not to place geotextile up the western embankment was deliberate. He said that his past experience indicated that the geotextile tended to block, thus preventing the water from entering the aggregate and from there the pipe. On occasions it had become necessary to dig out the geotextile and put cuts or slits in it to allow water to pass through.

79 A significant factor in my assessment of the evidence on this aspect is that Cocks testified to practical experience with this type of construction. He said he had worked for Main Roads for 20 years and that it was common practice to use the slotted pipe mechanism for sub-soil drains. I am not sure whether McInnes or Blandford had enjoyed the same level of practical experience. In this instance, I find Cocks' answers to the queries raised by McInnes and Blandford to be satisfactory. In any event, if Blandford is right in believing that the filtration ability of the aggregate would be improved by having geotextile on the western embankment, the cost can be accommodated in the award.

80 McInnes said: "... evaporation from an open trench will be significant and particularly during the summer months evaporation will draw water towards the trench. When the trench is infilled, my estimation would be that you would lose the effect of evaporation from the trench base". Blandford did not comment on this aspect. Cocks responded as follows:

"Dr McInnes raised a concern as to a loss of evaporation effect, that effectively if you close the trench then you lose the prospect of evaporation of the saline water going through that trench. Could you comment on that?---Yes. The evaporation rate for saline water is much lower than the evaporation rate for fresh water and it would be quite minor, and in fact salt is sometimes used as a dust suppressant on roads because it attracts moisture. It's not commonly used in Western Australia but is used in Africa as a treatment in places like Botswana and arid areas to actually salt-treat roads. Salt stabilisation is what it's referred to and that's because in very saline conditions it actually attracts moisture rather than evaporates. In Western Australia we see that with mine tailings dams. If salt water is used for the processing of the gold, the tailings stay wet for a very long time. So I think that would be a negligible effect."

81 Once again (and this is not meant as a criticism of McInnes) the explanation seems more considered and it relies on practical examples. I find Cocks' explanation satisfactory.

82 McInnes thought, even given that the pipe would be placed below the current floor level of the channel, that the Coffey design would collect only about 50 per cent of the flow that is currently being intercepted by the channel. He conceded that he had not done any tests to ascertain the quantity of water entering the channel. He also said that water entered the channel through the base and the east embankment as well as through the western embankment. He explained the process as follows:

"[Water also enters through] the base and the eastern side because, if I may elaborate on that, the trench is cut into the groundwater and water flows into the trench both from the uphill side but also from the base and from the eastern side. So it's like a sink into which water is seeping and because of its width, because of the width of the trench, it intercepts a significantly more proportion of the flow of groundwater that's in the ground than what a single slotted pipe could ever do."

83 Blandford was also concerned with the loss of what he described as "the basin effect". However, he did not ascribe a figure to the loss of utility and he seemed to accept that lowering the pipe into the floor of the channel would improve it significantly.

84 Cocks thought the McInnes' estimate was unduly pessimistic and put the figure at about 95 per cent. He sought to answer these concerns as follows:

"I don't agree. From my experience with the use of subsoil drains in roads, I think any change in the rate of flow of water would be very small. The pipe and aggregate would form effectively a free surface with air to which the water could flow and I think the amount of bypass, for lack of a better term, of salt water passing it would be not significantly greater than that which already occurs. Some salt water would already pass under that trench in the natural scheme of things because that's the direction of flow. I don't believe there would be a significant difference with the Coffey solution versus an open trench or versus a trench with gabions."

85 Cocks did not agree with the "basin" or "sink" explanations put forward by McInnes and Blandford. However, in this instance, McInnes testified to actual observations of water entering the channel through the base and the eastern embankment and I have no reason not to accept what he said in that regard. However, what was not explained was how or why the Coffey design, which provides for aggregate in at least part of the channel, would not have some of the same properties and produce a similar effect. Again, I am not satisfied that the Coffey design is so devoid of merit in relation to its ability to collect and disperse water, that it would materially threaten the integrity of the mechanisms to divert sub-surface saline water away from the plaintiff's dam.

86 On the basis of the design sketch McInnes thought there might be a flaw in the Coffey design because "there's no reference to what level it should be set at or what gradient it should be set at. It could, for instance, slope upwards instead of downwards". It goes without saying that unless the pipe (or for that matter the open trench) were set at a downwards gradient, it would not be effective to disperse water. However, in his evidence Cocks clarified this point by reference to the site plan, which is exhibit 1:

"The treatment proposed in filling in the trench and then recutting to place the pipe and aggregate would be to create a base with a consistent slope down towards to rejoin the existing trench.

Dr McInnes, I think, wasn't sure as to the base of the trench and whether it did slope downhill to the south or not. Have you made any observations or checked that on a contour map?---Yes, I have ... Looking very carefully one can see the contours on the map actually cross the base of the trench, so it is actually possible to record, for each location where the contour crosses the trench, an elevation to the datum. ... It can be regarded as metres above the mean sea level for the purposes, so the elevation is dropping consistently down the trench. From that point in the trench, the bend, down to the next bend, there is probably about ... a 4-metre fall."

87 This evidence dealt only with Section 1. However, reference to exhibit 1 indicates that in the area of Section 2 there is a continuous downward slope towards the south. Accordingly, if the Coffey design were to be continued into Section 2, the pipe could be positioned to taken advantage of the natural contours.

88 McInnes thought the Coffey design would be enhanced if the intercept zone were increased by extending the aggregate further towards the top of the channel. Cocks provided this answer:

"I see a disadvantage of that as introducing rainfall run-off into the system, which are conditions that could overload the pipe in terms of the capacity, so I was trying to separate here the surface run-off from the salt water seepage. The same thing is done with subsoil drains for roads. I worked for Main Roads in Western Australia for about 20 years and it was common practice - and in subdivision design, which I've worked in since - to have some sort of a cap over the subsoil drains so that surface water and subsurface water are kept separate."

89 This seems to me to make sense. The idea of the channel is to remove sub-surface water that is flowing down the slope from Lot 34. It is not to divert the flow of surface water. Indeed, the whole idea of a dam (and I am here referring to dams that are not in a watercourse or which are not spring fed) is to catch surface run-off. If the volume of surface water going into the trench is increased there must inevitably be more water which the pipe (or trench) is required to handle.

90 Both McInnes and Blandford criticised the Coffey design as being vulnerable to further erosion. As I understand the concern, it is that surface run-off into the swale could cause surface erosion to the backfill. McInnes said this would require significant maintenance to eradicate the undercutting as a result of surface water concentrating in the low point of the swale and running down the new invert level. He said: "I don't believe that this design has been detailed to the extent required to remove those significant engineering concerns". Blandford's concern was how the run-off from Lot 34 would be controlled when it passed into Lot 30 and started to move down the infill. He thought the water would spread out; unless it was in a controlled situation, and would then start to erode the surface of the backfill.

91 Cocks agreed there would be some erosion of the swale but he thought it would be relatively easy to control it by seeding. He pointed to the natural slopes and broad valley in the area that with seeding do not erode. He also said that at the "downstream end" of the pipe construction there would be some rock protection. He said that the extent of any erosion to the area of the swale would not interfere with the effectiveness of the Coffey design. He conceded that it was not a perfect treatment, that there would be some erosion, as there had been in the past. As he put it: "the intention is to remediate rather than to improve". Finally, he did not believe that erosion in the area of the swale would not move back towards the dam and would not affect the safety of the dam. I can imagine that if the erosion caused serious undercutting that threatened the embankments once again, then the integrity of the dam might be in jeopardy. However, I do not think that is what McInnes and Blandford were saying.

92 In answer to questions that I asked, Cocks agreed that the use of Jutemaster in the surface of the trench "would be a design improvement". Ash thought it was "a bit of an overkill". He thought that if the surface was scarified and spread with topsoil obtained locally there would be a better regrowth naturally with the seeds that are native to the area. However, he conceded that he had no experience with the use of Jutemaster. Cocks was responsible for the design and I prefer his evidence.

93 Erosion is, and always has been, a problem. It is a problem in most of the south of Western Australia. It is very difficult to eradicate it entirely. The question here is whether the surface run-off from Lot 34 into Lot 30 will, be sufficiently captured by the swale so as not to flow to the east of the trench and do damage to that area. The second question is whether the surface run-off into the swale will, or is likely to, cause erosion that will, in time, threaten the integrity of the in-filled trench. On the basis of Cocks' evidence, and with a further design improvement by the use of Jutemaster, I think the answers to those questions are "yes" and "no" respectively.

The Coffey Design of the Golder/Blandford Design?

94 What I am about to say is going to sound like a conclusion on the merits of the various engineering proposals as between themselves. That is not the purpose but it is easier to phrase this section of the reasons in that way. The true question remains the level of damages that should be awarded rather than the design concepts themselves.

95 I have already indicated that I have discounted the AES design and that I believe that there should be remedial works in Section 1 and Section 2. I have no doubt that the Golder design and the Blandford design are without engineering or environmental defects and would adequately achieve the three objectives that I set out earlier in these reasons. However (and subject to what I am about to say) I have also been persuaded that the Coffey design would adequately achieve those same objectives.

96 In relation to the Coffey design, I think there are some variations that would enhance its effectiveness. First, it must continue into Section 2 as well as Section 1. Secondly, it may be (as Blandford seemed to feel) that geotextile on the western embankment would assist in minimising the movement of soil particles into the aggregate. If the plaintiff were to carry out works on the channel that are, or are similar to, the Coffey design, further investigations would need to be done to take into account the views expressed by Cocks on that issue. For the purpose of assessing damages, I intend to take into account the cost of geotextile on the western side. Thirdly, I think the use of Jutemaster on the surface of the in-filled trench would assist by arresting erosion while the seeding process takes effect. Fourthly, the pipe should be placed below, rather than on or above, the floor level of the channel. As I understand the evidence of Ash, this will make no difference to the cost of construction.

97 I accept Cocks' evidence that it would not be wise to fill the trench with aggregate to, or almost to, the surface. Accordingly, the additional cost that would entail will not be taken into account in assessing damages.

98 Finally, the remedial works that I have in mind for the dam have been a significant influence in the decision to which I have come as to the appropriate measures to be taken to stabilise the channel. If the seepage from the dam is arrested there will be a lesser volume of sub-surface flow with which the channel has to cope. Of course there will still be some sub-surface flow. After all, that is why the channel was constructed in the first place. If, as McInnes suggests, an open trench will be more effective than the pipe proposal, then measures to lessen the volume of water with which it has to cope will go some way towards alleviating the problem. I think the difference in facility between the Golder design and the Coffey design in that respect is not quite as great as has been suggested but it is conceivable that the in-filled trench will miss some of what an open trench would catch. If that is so then, once again, arresting seepage from the dam will minimise the ongoing problem.

99 In light of this conclusion I do not need to consider what was, I think, the most serious attack made by the defendants on the Golder proposal, namely whether the work on the eastern side of the channel was necessary and justified.

100 This brings me, finally, to the monetary sum that I believe will be fair and reasonable compensation to the plaintiff for the injury that she has suffered to her land. In relation to the award of damages there are a couple of preliminary matters that need to be addressed.

101 The defendants argued that whatever damages were to be awarded to the plaintiff should be reduced because she had failed to mitigate her loss. The argument about mitigation is essentially one of timing. The answer which White J gave to Issue 10 precludes the defendants from arguing that some of the injury which has been done to the plaintiff's land was due to her own negligence in the initial construction of the channel or in failing to carry out remedial work made necessary by natural erosion over time.

102 However, in par 10B of the defence the defendants contend that the plaintiff has "failed to mitigate her loss by taking ... steps to alleviate or minimise the effects of the defendants' dam on [the channel] to the susceptibility of [the channel] to erosion [namely] the stabilising, strengthening and/or reinforcing of the [channel] embankment, whether by the sue of gabions or otherwise".

103 The defendants now say that the remedial work could have, and should have, been carried out in the Summer of 1997/98. Costs have increased since then and the defendants should not have to bear the additional impost. Counsel for the plaintiff argued (vigorously) that such an argument was not within the pleading. I do not agree. I think the pleading is broad enough to countenance the arguments that have been raised and I intend to proceed accordingly.

104 Mitigation of damages is a well-known concept. This is essentially an action in nuisance. A claimant has the same duty to mitigate the loss in tort as it has in contract: Thynne v Petrie [1975] QR 260 at 265. However, the burden is on the defendants to establish that some action on the part of the plaintiff would have reduced the loss and that the failure to take that action was unreasonable: Munce v Vinidex Tubemakers Pty Ltd [1974] 2 NSWLR 235 at 239.

105 Dodd Properties (Kent) Limited v Canterbury City Council [1979] EWCA Civ 4; [1980] 1 WLR 433 is a case concerning the time at which damages should be assessed for the rectification of a building that had been damaged by a tortious act. Browne LJ said, at 454 - 455:

"The general rule, both in contract and in tort, is that damages should be assessed as at the date when the cause of action arises, but they may be assessed at some later date. In my view [the trial judge] was right in saying: 'The appropriate damages are the cost of repairs at the time when it is reasonable to begin repairs'."

"... in a case in which the plaintiff has reinstated his property before the hearing, the costs prevailing at the date of the operation which were reasonably incurred by him are those which are relevant. Equally, in a case in which the plaintiff has not effected reinstatement by the time of the hearing, there is a prima facie presumption that the costs then prevailing are those which should be adopted in ascertaining the cost of reinstatement. ... This is, however, only a prima facie approach. It may appear on the evidence that the plaintiff, acting reasonably, should have undertaken the reinstatement at some earlier date than that in fact adopted or, as the case may be, earlier than the hearing. If so, the relevant costs are those ruling at the earlier date."

107 The question in this case is whether the defendants have displaced the "prima facie presumption" by establishing that the plaintiff, acting reasonably, should have undertaken the reinstatement at the end of the Summer of 1997/98.

108 McInnes gave clear evidence that the remedial work on the channel could have been done in December 1997. As the case unfolded, I think it became common ground that the most appropriate time to attempt the work was at the end of Summer, around March. This answer should therefore be regarded as if he had said March 1998. The plaintiff gave no evidence as to why some remedial work was not carried out at that stage. It might have been, for example, impecuniosity. But I have no evidence on which to base any finding that it was reasonable for the plaintiff to have delayed taking the necessary steps to stem the loss.

109 It is true that by March 1998 the parties were locked in litigation. By that time the plaintiff had obtained an injunction to prevent further de-watering of the dam. However, I do not think that the mere fact the litigation was proceeding and that there was uncertainty as to the level of comfort the plaintiff would obtain, render it reasonable for her to have refrained from taking remedial steps. I have also considered whether it was reasonable to delay because the fate of the dam (that is, whether or not it was to remain in its present location or be in-filled) may have had an impact on the type or range of remedial works necessary to stabilise the channel. However, the evidence does not support such a conclusion. McInnes said:

"Is it also correct to say that if the rectification work to the trench had been commenced earlier but after that dewatering and the dam being built, there would have been less rectification work required?---.... No, I believe that the rectification work that has been proposed in the plans in front of me would have still substantially been required. There would be no diminution in the extent or in the manner in which these rectification events should proceed.

From what period do you say that is so? In other words, at what point, on your evidence, had one reached a situation where this extent of rectification work was required to the trench below the dam?---My contact with the trench extends back over several years, about 3 years now, and it would be from that point. I'm not sure of the exact date."

110 I therefore find that the plaintiff has failed to mitigate her loss and that the award of damages should be reduced accordingly. Ash was not asked as to the impact of the delay on his costings. However, in his quotation of 3 December 2000 he says that his quotation is exclusive of the Goods and Services Tax ("GST"). When he was being cross-examined about his cost estimates for the Golder and Blandford designs, O'Neill said that since 1997 labour costs had increased by about 2 per cent, material and machinery costs had not moved much at all but there had been the introduction of the GST at the rate of 10 per cent. There is no evidence on which I could assess the proportion of the Ash estimates that are attributable to labour. Ash was not asked whether his estimate would have been different had he been calculating the costs as at the Summer of 1997/98. In making an allowance for the failure to mitigate, I will work on the figure of 10 per cent representing the GST.

111 Counsel for the defendants also argued that the damages ought to be adjusted by applying a discount rate of 6 per cent to reduce the dollar figure back to the net present value. Counsel argued that something like the Golder treatment would inevitably have been required to the channel at some time in the future even if the defendants tortious conduct had not occurred. This is because the channel was already eroding. The defendants should not, therefore, be liable to compensate the plaintiff for a cost that she would inevitably have incurred at some time in the future. The proper way to recognise this factor was, counsel submitted, to identify the time at which the expenditure would be incurred and to calculate what it is worth to the plaintiff to keep as a resource the amount that she would have to spend in 2001 dollar terms for the period so identified.

112 Counsel for the plaintiff argued that it was not a proper exercise and, in any event, there was no reason in logic to select a 6 per cent discount rate.

113 I think it is clear that the channel was, prior to the construction of the dam and its dewatering, suffering from erosion to some extent. There was a gradual loss of amenity to the west and, in time, the integrity of the channel would have come under threat. White J found, in the answer to Issue 7, that the walls of the channel had been subjected to erosion but that there had been a marked acceleration of erosion since the construction of the dam. All of the expert witnesses seemed to agree that at some stage in the future the channel would have eroded to the same point that it had reached as the time of the hearing. There was no unanimity as to when that stage would be reached.

114 Cocks thought it would occur within eight to 10 years. McInnes thought it would be about 30 years. Blandford proffered the view that it would be 10 to 20 years. In this instance, I think McInnes and Blandford have had an advantage that Cocks has not enjoyed. They have actually been on site on Lot 30 on a greater number of occasions. They have had a better opportunity to assess the channel and the rate of degradation. I prefer their evidence and will take the medium point of 15 years.

115 Counsel for the defendants conceded that if I were to form that view, the economic analysis argument would be of no practical effect. I think that concession is correct.

The Cost of the Coffey Design and Necessary Adjustments

116 In cross-examination, counsel for the plaintiff did not seriously challenge the accuracy or adequacy of the estimates given by Ash concerning the Coffey design. Counsel certainly challenge whether the Coffey design should be considered at all but did not condescend to particular challenges to individual cost items or the estimate overall.

117 The cost estimate for Section 1 is $58,657. But that includes an amount of $3,350 for some of the remedial works on the dam. These costs will be incurred directly by the defendants and so should be deducted. That leaves a figure of $55,307. To this must be added an amount of $1,000 to take the geofabric up the western embankment to the level of the aggregate (see T 294 - 295). It would also be necessary to add $300 to accommodate the use of 150mm pipe (T 303). The figure for Section 1 is, therefore, $56,607. That is exclusive of GST and no further adjustment needs to be made.

118 Ash's evidence was that if the Coffey design were to be taken further down the channel it would be at the rate of $878 per metre. The best evidence is that Section 2 is 60 metres in length, giving a base cost of $52,580. Again it would be reasonable to add the cost of additional geofabric and for the larger pipe. The figure for Section 2 is $53,880. There was no evidence that Ash's estimate of $878 per metre included GST. Once again, no further adjustment is necessary.

119 Ash's estimate does not include the cost of Jutemaster on the surface or the cost of seeding. There is no evidence before me of the cost of those items. However, it seems to me to be fair and reasonable to compensate the plaintiff in that regard because they are an integral part of the design enhancement necessary to minimise the impact of surface erosion in the swale. I have no evidence to go on but I will simply do the best I can. I will add a further $10,000 to cover those items and to provide some contingency amount. All of the witnesses agreed that a contingency factor was usual in construction contracts of this type and it was often 10 per cent. I acknowledge that Ash said he had already made a contingency allowance but he was a little short on specifics in that regard. He said he would "probably" have added 10 per cent in this case.

121 At the hearing no argument was addressed to the issue of pre-judgment interest. I will hear the parties concerning it. What follows are some tentative views to assist the parties.

122 The Court has a broad discretion under s 32 of the Supreme Court Act 1935 (WA) to award interest on damages. It may seem odd if I were to do so in this case because it might, in effect, be seen to reverse the effect of the finding that the plaintiff failed to mitigate the loss. However, I think the two things are different. The purpose of the rule requiring the plaintiff to mitigate the loss is to identify that part of the injury which is truly the responsibility of the tortfeasor in terms of compensation. An award of interest serves a different purpose. It is (among other things) to recognise that the money has been outstanding for a period and one side has had the benefit of it and the other has not.

123 The discretion is to be exercised having regard to what is fair between the parties. The object is not to punish the defendants for delay in settling the claim but to compensate the plaintiff for having been kept out of money that theoretically was due to her at the time when the action was commenced.

124 In this case there are complicating factors. The parties agreed to an order splitting issues of liability and damages. They therefore agreed to defer the time at which the money would be payable if indeed the defendants were found liable. As a result of the decision of White J, confirmed by the Full Court, the plaintiff has undoubtedly been entitled to damages since at least 5 August 1998. From that date the entitlement of the plaintiff, although not the amount, has been undeniable. In my view, it would be appropriate to recognise that she has been kept out of her money since then.

125 I can see no reason why the rate of interest payable on judgments should not be applied so as to calculate pre-judgment interest. Throughout the relevant period the applicable rate has been 6 per centum per annum. Interest on $120,487 at 6 per centum per annum calculated on a simple basis is $7229.22 per annum or $19.81 per day. Interest from 5 August 1998 to 4 August 2000 would be $14,458.44. Interest from 5 August 2000 to 30 April 2001 would be $5,327.84, being 269 days at $19.81 per day. This makes a total interest component of $19,786.

126 I will hear the parties as to the appropriate orders that are to be made in accordance with these reasons. What I have in mind concerning the remedial works to the dam is as follows:

"1. Subject to pars 2 and 3 the defendants do, by not later than, [date] empty (in a way that will not cause water to flow onto the plaintiff's land) and then fill in the dam on Lot 34 and restore the contours of Lot 34 as nearly as possible to the contours prevailing on the land prior to the construction of the dam on Lot 34, such filling in and restoration to be in accordance with plans and specifications prepared by, and under the supervision, of a duly qualified geotechnical engineer.

2. It shall not be necessary for the defendants to comply with par 1 if, prior to [date], the defendants have carried out remedial work to the dam on Lot 34 in accordance with the following conditions:

(i) to lower the spillway slightly and build a diversion bank and trench moving roughly in a westerly or south-westerly direction consistent with the existing contours and with a stilling basis at the end of the trench;

(ii) to empty the dam (in a way that will not cause water to flow onto the plaintiff's land) and seal the base and walls of the dam so as to eliminate or reduce to a minimum extent seepage or leakage from the dam;

(b) the remedial work is to be carried out in accordance with plans and specifications prepared by, and under the direction of, a duly qualified geotechnical engineer.

3. In relation to the both pars 1 and 2, no work is to be carried out without the prior approval of the plaintiff, such approval not to be unreasonably withheld. To facilitate the process of approval:

(a) before any work is done the plans and specifications shall be submitted to the plaintiff;

(b) the plaintiff shall, within [number] of days, advise the defendants whether the plans are approved and, if not, giving precise details of the objections;

(c) if there are objections the parties and their engineering and other advisers shall meet as soon as is practicable to resolve the differences;

(d) if the parties cannot resolve the differences then [insert a conventional arbitration clause for the dispute to be submitted for binding determination by an independent geotechnical engineer];

(e) while work is being carried out the defendants must permit the plaintiff and persons authorised by her to have reasonable access to the area of the dam and its immediate surrounds to inspect the works.

127 In relation to damages, an appropriate award would be $120,487. This figure will need to be adjusted to take into account interest under s 32 of the Supreme Court Act 1935 (WA), if appropriate.