Mr Nielsen's Evidence
118 Mr Nielsen held impressive academic qualifications as an engineer specialising in Coastal and Geotechnical Engineering. His professional career spanned a period of about 30 years. For approximately 14 years he had worked in Coastal Engineering with the New South Wales Public Works Department. He then became a specialist Geomarine Engineer in private practice working extensively in the field of coastal engineering consultancy. In the latter part of his career he joined Unisearch at the University of New South Wales where he was involved in a number of major projects including harbour proposals, beach erosion, management studies and estuary process studies. At the time he gave evidence he was working with the Snowy Mountains Engineering Corporation. In relation to this engagement, his work again embraced specialised coastal engineering and water studies.
119 Mr Nielsen's first report was Exhibit "E". It appears between paginations 436 and 534. This report dealt with three site inspections Mr Nielsen made at Coffs Creek. They were made on 10 November, 2 December and 9 December 1999. The first inspection was made in the context of a general body of material obtained by Mr Nielsen from the local office of the Department of Land & Water Conservation relating to the creek. The December inspections were made on a neap tide and a spring tide respectively. These particular tides were selected by Mr Nielsen "in order to understand better the range of conditions that may occur in the creek". A spring tide is one that occurs at or near the time of a new moon or a full moon. A neap tide is one that occurs in between the new moon and the full moon near the time of quadrature of the moon with the sun. The neap tidal range is generally around twenty per cent less than the mean tidal range and its occurrence is fortnightly.
120 The purpose of "understanding better the range of conditions likely to have occurred in the creek" on a neap tide and a spring tide was related to the known tidal conditions at the time of the plaintiff's accident. Mr. Nielsen had been able to obtain reliable information regarding meteorological conditions on 24 January 1999. He had been able to obtain information about the weather. In addition, he had obtained reliable data regarding the tidal stage and phasing which would have affected the creek at the relevant time.
121 A summary of this information (taken from Exhibit "E" pages 445-446) is as follows:
"Between 3:00pm and 6:00pm on 24 January, the air temperature was around 25 degrees C with a relevant humidity of around 70%. There was a light easterly wind blowing with a speed of around 5 knots. The barometric pressure was steady at 1008hPa.
Slight rain showers were recorded during the day but not of sufficient degree to have affected water clarity in the creek or to have caused any flooding conditions.
At around 5.pm on 24 January 1999, the tidal stage was 1.11m and falling from its highs of 1.7m to a low of 0.4m; that is, a falling stage of 1.3 metres. The tide was a "run out" tide, that is, an ebb tide.
The wave conditions in the ocean were around the annual average for the New South Wales coast. The seas were not unusually large or calm."
· (Later agreement between the experts gave greater precision to the tidal fall. Exhibit "T" paras 16 and 17 place the effective peak discharge as equivalent to that of a tide with a fall of approximately 1.43m.)
122 When Mr Nielsen made his first inspection on 10 November 1999, the tidal conditions were similar to, although larger than, they had been on 24 January 1999. The seas however, were considerably higher than the average and certainly higher than those recorded on 24 January 1999.
123 Using a face mask and snorkel, Mr Nielsen took the opportunity on this first occasion to inspect the creek during the mid-tide run out period. He took a number of rounds entering the creek from the southern bank opposite the western end of the training wall approximately, as well as in the vicinity where he understood the plaintiff had entered the water. This was a point a little further easterly down the creek from the western end of the training wall. He allowed himself to be taken down stream to the sea by force of the current.
124 Relevantly, he found on this occasion that the water was turbid with high concentrations of suspended sand. He observed that visibility was low. The bed of the creek portrayed relatively large undulations particularly on the southern side. Bed level variations reached values in excess of one metre along a line extending downstream equal distance some five to ten metres (approximately) from the southern bank. At some locations the depth of the water was in the order of two metres whereas at other locations it was as little as 0.5 metres. In other words, in some places the depth of the water was such that the water was over Mr Nielsen's head whereas in other locations it was about knee deep.
125 He observed that the bed of the creek varied in its softness. At some locations the bed was very soft underfoot whereas at other locations the bed was quite firm. The water surface in the channel area (that is, along the training wall), varied as a low relief wave and "boils laden with suspended sand" were visible at intervals along the water surface.
126 As the creek entered the sea, the water depth reduced to less than 0.5m, and the current speed increased significantly. In this area, the large seabed undulations disappeared and the current speed was high enough to make it difficult to walk across the stream.
127 The next inspection was made on 2 December 1999. This inspection was made at about mid-tide when the tide was on the ebb. It was a neap tide and the flow was not very strong. Mr Nielsen's observations were that the creek bed was relatively flat on this occasion. The water depth was fairly constant across the low tide channel at around 0.8 metres. The water was clear and the surface of the bed was covered with small current generated ripples. The current speed was measured consistently at 0.4 metres per second near the bottom of the creek and 0.6 metres per second nearer the surface. These speed measurements were taken at the centre of the creek about mid-way along the length of the training wall. A topographic survey of "the accident environs" was undertaken on the following day. This survey verified the field observations of the previous day in that the bed was relatively flat and of a consistent depth.
128 The third inspection was made on 9 December 1999. This date was selected for the purpose of measuring the flow velocities and bedforms in the creek for a similar tide to the one which had prevailed when the plaintiff had his accident. The tidal stage for the velocity measurements was mid-tide, around 1.0 metre ISLW, with the tide on the ebb (run out) falling some 1.50 metres from a high of 1.60 metres at 9.45am to 0.10 metres at 4.15pm. On the afternoon of 24 January 1999 the tide had fallen from 1.65 metres to 0.39 metres (Exhibit "E" paginated page 445 and 450). The similarity in tidal fall made for a fair comparison even though, in definitional terms, the tide on 9 December 1999 was a neap tide.
129 Mr Nielsen described his physical observations of the relevant geomorphology on 9 December 1999 in the following terms (Exhibit "E" paginated page 450): -
"At this time the creek bed expressed a pronounced undulation, with water depths varying in excess of 0.5 metres over short distances along the channel. This is shown in the sequence of photographs taken at the time and presented in figure 2.8. These show Mr Nielsen wading in thigh deep water at one moment to a depth up to his armpits immediately adjacent".
130 (These photographs were later supplemented by the tender of other photographs taken at the time. They became Exhibit "W" - "Z" and Exhibit "AA").
131 Again, the current speed was measured at the centre of the creek about mid way along the length of the training wall. It was measured "consistently" at 0.7 metres per second to 0.9 metres per second. Approaching the beach at the eastern end of the training wall, the current speed near the surface was measured at 1 metre per second. Mr Nielsen noted that during these measurements, the water was "boiling" bringing up to the surface "dense plumes of sediment" as shown in figure 2.9. This condition was, he said, in great contrast to the conditions he had experienced and measured in the channel on neap tides a week earlier.
132 Again, a topographic survey of the "accident environs in Coffs Creek" was made at low water from around 3pm to 4pm (Exhibit "E" Appendix D pages 518-520). This was done to obtain "objective measurements of the creek topography" further to the observations which were demonstrated in the photographs in figure 2.8. Both the survey done on the 3 December and that done on 9 December were carried out at low water. They are to be found in Exhibit "E" at paginated pages 514-520. Mr Nielsen explained that these surveys taken at low water do not duplicate the conditions which would have existed at mid-tide. The water depths were greater and the velocity in the stream was greater at mid-tide and hence the precise nature of the bedforms would have changed by the time of low water. Nevertheless, as Mr Nielsen explained, the low water survey demonstrates "a relict condition from the previous conditions" an hour to two before the survey was made. He noted, for example, that it may have been possible that the mid-tide bedforms were larger than those examined at low water survey. In this context he explained in general terms the phenomenon of sand dune formations in estuarine conditions, a phenomenon that is well known to marine engineers. He did so in these terms (T 170 lines 45-55):-
"One reason for that is that the maximum bedform size is related to the water depth, so the maximum size a bedform can reach will be controlled by the velocity of the water flow, the grain size of the material, as well as the actual water depth and in a given water depth there is a maximum height of bedform that can be achieved. Different researchers have studied this in river flows in unidirectional flows using flumes and measurements from rivers and those sorts of estimates vary from round about 30 per cent to some, even, 50 per cent of the water depth.
So, when the depth of water is deeper, which it would be at a mid-tide condition compared with the time of the survey taken at low tide, then it is possible that the maximum height of the bedform that could be developed at that time would be larger than one that may be relict and remaining at a low tide level.
It is also possible that a survey of a bedform…that is a bedform at mid-tide could be smaller than one at the very low tide. And while the water depth is greater, which means the potential for a larger bedform to form, it can be the case that the current speed is so strong that it in fact flattens out the bedform so that they can't reach that maximum level".
133 In the first of his written reports, Mr Nielsen (Exhibit "E" paginated page 452 and following) explained in more detail the hydrodynamics of an estuary such as Coffs Creek. The theory was expressed in the following terms: -
"Water flowing over an erodible bed of sand interacts with the bed and, as a result of an orderly pattern of scour and deposition, can deform it into a variety of bedform configurations (Henderson (1966); Kennedy (1963). The nature of the configuration depends on the depth and velocity of flow and the properties of the sediment and the fluid, with the principal types of bed features usually distinguished being ripples, dunes, flat bed and anti-dunes. (Kennedy 1963; Simons & Richardson 1961; see figure 3.1).
Under low flow regimes, if the speed of the current is great enough to move sand grains the bed is deformed into irregular features. Initially, with flow speeds at the threshold of sediment transport, ripple marks form on a sand bed. …To the casual observer under these very low flow conditions for all intents and purposes the bed would be relatively flat.
As the flow becomes stronger, larger bed features called dunes develop…in longitudinal section, dunes are approximately triangular with a relatively gentle upstream slope and a deep downstream slope. The dunes will move downstream relatively slowly (Kennedy, 1963). Most of the sand moves downstream close to the bed, with the dunes advancing downstream with some of the bed load avalanching down the face of the dune. The other part of the bed material load is carried onward; some to the surface in "boil" areas, which have a different colour due to the large concentrations of suspended sediment (Simons & Richardson 1961).
Dune size can vary with heights from 0.3 metres and spacing up to 6 metres in small alluvial channels to heights as large as 12 metres with lengths of around 100 metres in large rivers such as the Mississippi (Simons & Richardson 1961). Van Rijn (1989) presents field and laboratory flume data giving dune heights, on average, at about 20 per cent of the water depth with a range reaching up to around 40 per cent of the water depth".
134 Against the background of this theoretical knowledge, Mr Nielsen analysed the data he had collected on 9 December 1999. At paginated page 454 of Exhibit "E", he explained that the data was typical of spring ebb tide conditions in the lower reaches of Coffs Creek. Apparently this was so despite the fact that the relevant tide was, strictly speaking, a neap tide. He noted that the Weir survey taken at low water on 9 December showed dune bedforms with amplitudes of the seabed up to 0.6 metres.
135 The data, he said, was entirely consistent with the theoretical understanding of flow and bedform development in alluvial channels and was indicative of dune bedforms. He plotted the hydrodynamic and sediment data against the bedform predictions schema of Simons & Richardson (1961). The results are to be found in figure 3.3. This appears at paginated page 473 of Exhibit "E". The nature of the different types of bedforms in alluvial channels is best seen in figure 3.1 (paginated page 471 of Exhibit "E").
136 The assessment made by Mr Nielsen was that his physical observations of the bedforms in the creek at the time of inspection on 9 December 1999 were validated by the scientific methods utilised. The result was expressed very simply in the transcript at page 183 lines 10-20: -
"Q. On 9 December?
A. Well when I applied the measurements I made in the creek on 9 December, it resulted in an indication on this diagram that I would have expected to have found dunes on the bed of the creek at that time.
Q. Is that a method of, as it were, validating what you found in your own experience and measurement?
A. It is. In a sense it is indicating that the observations that I made were entirely not unexpected perhaps even predictable".
137 A second objective validating method was an examination of the historical aerial photography of this section of the creek. At page 474 of Exhibit "E" there appears figure 3.4. This is a copy of an aerial photograph portraying large scale dune bedforms in the relevant section of Coffs Creek during the middle of the spring tides that were occurring on the date of the photograph namely, 28 August 1977. The photograph plainly shows large scale dune bedforms spilling out in a southerly direction from the training wall.
138 Against the background of the observations he had made in November and December 1999 and the scientific verification he had made, Mr Nielsen turned his attention to the likely situation at about 5pm daylight saving time on 24 January 1999 at the relevant part of Coffs Creek. It needs to be stressed that Mr Nielsen made no physical observations of the creek on the day the plaintiff had his accident. Nor for that matter did Mr Druery, the scientific expert called on behalf of the second defendant. Apart from the objective physical data relating to the weather conditions, tidal conditions and ocean conditions, the only evidence of physical observation of the creek came from the plaintiff, Miss Brady and a gentleman who assisted the plaintiff from the water, a Mr McDonald. None of these three were scientifically trained to make observations of creek geomorphology. None of these were, in the tragic circumstances, concerned to do so.
139 Mr Nielsen however, was able to draw upon a number of positive matters. First, the tide on 24 January 1999 fell approximately 1.3 metres. This occurred between 2.15pm and 7.45pm. Secondly, this fall occurred over five and three quarter hours duration rather than the usual six and a half hours required for a tidal fall. That meant, as I have earlier indicated, that the effective peak discharge was equivalent to that of a tide with a fall of approximately 1.43m. Thirdly, the tidal fall occurred following a succession of spring tides. Fourthly, at the time of the accident the water level was around mid-tide level.
140 Given these factors, Mr Nielsen expressed his expectations in the following manner (paginated page 456 of Exhibit "E"):
"At mid-tide level on a falling 1.3 metre tide, the flow in Coffs Creek would have been strongly ebbing, with average flow velocities of around 1 metre per second. The average water depth across the creek would have been around 1 metre but the bed of the creek would have displayed large variation due to the formation of undulating subaqueous dune formations. With amplititudes of the dunes probably in excess of 0.5 metres and perhaps reaching 1 metre, the water depth was likely to have been varying along lines parallel to the thalweg of the creek from as deep as around 1.5 metres or more to as little as 0.5 metres or less. The wavelength of these undulations would have been around 5 metres.
The current speed would have been variable, with the flow accelerating over the shallower undulations in the bed, reaching speeds around 1 metre per second. Sediment laden boils of water would have been seen along the surface of the water, which itself would have been undulating slightly out of phase with the dune bedform undulations (see figure 3.1(c)). The visibility in the water would have been very low, due to the high concentration of suspended sediment, and it is unlikely that the bottom was visible beyond about calf depth.
Parts of the creek bed would have been soft, particularly those deeper parts where the sand was avalanching down the faces of the dunes into the deeper water, whereas other parts of the creek bed would have been firm or hard, particularly the shoaling depths on the consolidated upstream faces of the dunes from which the sand was being eroded."
141 Mr Nielsen's oral evidence (both in chief and in cross-examination) was given over a number of days. He adhered to his opinion as to the likely condition of the creek bed at the time of the accident although he was prepared to concede the possibility that the creek may have been flatbed or in transition. It very much depended on the precise conditions occurring on the afternoon in question.
142 Mr Nielsen prepared a second report on 29 January 2001. This also forms part of Exhibit "E" (paginated pages 535-554). A good deal of the report was disallowed. There remained in evidence however, a reference to beach or water safety signs at Wanda Beach and at Gunnamatta Bay. One of the signs at Gunnamatta Bay appears at page 546 of Exhibit "E". This is the sign which was shown to the plaintiff by Mr Murray QC and in respect of which evidence was given relevant to the causation issue (see para 80 page 47 of these reasons).
143 The remainder of the second report dealt with the signs which had been observed by Mr Nielsen on the northern and southern banks of Coffs Creek. There were no signs however warning swimmers of the potential risks of diving or jumping in the creek or warning that the creek was of variable depths in that location.
144 The third report of Mr Nielsen is headed "Historical Development of the Coffs Creek Entrance Channel". It appears in Exhibit "E" between pages 555 and 598. The theme of this report is the opinion expressed by Mr Nielsen that the construction of the rock training wall on the northern side of the channel and its later rebuilding not only stabilised the channel location but may have acted to improve the hydraulic conveyance characteristics of the channel thereby increasing tidal prism and channel velocities. This, as he explained, was likely to have had consequences in relation to the formation of bedforms in the creek
145 Mr Nielsen based his opinion on a number of matters. First, there was the history of the construction and improvement of the training wall itself. Secondly, there were the statements in a number of historical reports commenting on the stability of the entrance of the creek following the training wall construction. Thirdly, Mr Nielsen placed reliance on a number of aerial photographs taken from December 1976 to June 1996. These showed virtually no change in the general location of the inlet channel adjacent to the rock training wall. This was in marked contrast to the situation which had occurred in earlier times. Fourthly, he placed reliance on photogrammetric plotting and subsequent sand volume calculations. The latter demonstrated a lack of any large scale variation in the sand volumes downstream of the bridge since training wall construction. Finally, he placed reliance upon six photographs taken at various stages between 1942 and 1974. These were chosen for detailed examination based on the antecedent tidal conditions in the creek at the time each of the photographs were taken. Some of these photographs were taken at times when the antecedent tidal conditions were unlikely to have given rise to the creation of large bedforms. There were however, several photographs which were taken after suitable tidal conditions. It was Mr Nielsen's view that sand wave bedform features were not apparent on these photographs.
146 The argument was developed in the following terms (page 568 of Exhibit "E"):
"The construction of the northern training wall at Coffs Creek entrance and, particularly, its extension in 1988, would have reduced the influx of sand into the channel on the flood tide. This would have permitted the ebb tide scour of a larger equilibrium flow area and hence provided for a larger tidal prism. The tidal prism would have been increased further as a result of reducing the length of the entrance channel (Czerniack (1977); O'Brien & Dean (1972). By stabilising the location of the channel, the training wall construction has obviated the meander in the channel, thereby reducing its length and frictional resistance. That there can be high tidal velocities in the creek now is evidenced by measurements made during spring ebb tide discharges (Nielsen (2000)."
147 Mr Nielsen then brought to bear the descriptions and measurements he had made during November and December 1999 inspections. These had been detailed in his earlier report. He recalled (page 569 of Exhibit "E"): -
"Field data obtained in November and December 1999 attested to the flat bottom, clear and quiescent conditions observed in the creek during neap tides, which contrasted to the undulating, turbid and energetic conditions of the flow on the run out spring tide (Nielsen 2000). On spring run out tides, Nielsen (2000) observed the depths in the creek to have varied by up to 1.5 metres over short distances of only a few metres and bedforms as large as 0.6 metres in height were surveyed, subsequently, at slack water."
148 Mr Nielsen concluded his argument by noting that none of the photographic enlargements made of the channel prior to 1976 portrayed any bedform features. It was his view that, on the balance of probabilities, the training wall construction and, in particular the training wall extension in 1988 had a significant impact on the flow conditions in Coffs Creek. He summarises the impacts as follows: -
· "Stabilising the location of the entrance channel, thereby ameliorating dune erosion;
· Restricting beach sand transport into the channel on flood tides, particularly from the northern side of the inlet, thereby enhancing the net effect of ebb tide scour and hence, increasing the cross sectional area of the channel.
· Enhancing the hydraulic conveyance of the entrance thereby increasing tidal prism and velocities and ameliorating flooding in the creek; and
· Generating turbidity and causing large undulating and unstable sand waves to form in the creek bed on the run out spring tides."
149 During his evidence at the hearing, Mr Nielsen made a number of qualifications to matters mentioned in this report. There had been, during the hearing, commendable efforts by Mr Nielsen and Mr Bruce Druery to reach agreement on a number of matters in dispute between the experts. It is appropriate to record at this point that Mr Druery was a Director and Principal of Patterson Britton & Partners Pty Limited, a firm of consulting engineers. He had been in practice as a professional engineer for over 30 years particularly in the field of hydrodynamics and sedimentary processes of rivers and estuaries. Co-incidentally, he too had been employed by the Public Works Department for a considerable number of years before entering private practice. He had worked in the estuary management section of the department for 15 years. He had in fact directed the Coffs Harbour Creek Waterway Study in 1978/79.
150 Mr Nielsen and Mr Druery had conducted a joint examination of the aerial photography both pre and post 1975. The joint examination was able to utilise very sophisticated equipment to scrutinise the photography. Both experts agreed that the historical photos were generally of poor quality and, as such, made bedforms difficult to discern. The joint agreement (Exhibit "T") contained on page 1 the following matters: -
"Discussion of Features Related to Pre-1975 Photography
3. Sand dunes and larger sand bodies such as active sand shoals, are observable in the lower estuary.
4. The observed sand dunes, generally, appeared to be of lower relief (ie, dune height) compared with those observed in recent photographs in the area of the entrance wall."
151 During his evidence in chief, Mr Nielsen, following the joint examination of photos with Mr Druery, said that he had now been able to discern some bedform features in the pre 1975 photography. In his view however, the bedforms were not as extensive "as they are seen today in the area of the entrance channel adjacent to the rock wall", (T 195 lines 45-50).
152 Exhibit "T" contained other valuable matters of agreement between the experts. For example, in relation to the post 1975 photography, it was observed that there were observations of "large consistent dunes in the entrance channel adjacent to the entrance rock wall". Additionally, the photographs demonstrated that during neap tide conditions, sand dunes were observed in the area of the entrance rock wall "on four out of five occasions". Plumes of suspended sediment in the entrance channel adjacent to the rock wall "…were evident in those photographs taken while the ebb tide was still flowing significantly (for example, the photograph 16 May 2000)" [Exhibit "T" paras 7, 8 and 9].
153 Exhibit "T" contained a number of areas of agreement in relation to the issue of the impact of the entrance rock wall construction. This appears at section 3.4 of Exhibit "T" where a number of matters bear repetition: -
"24. The construction of the entrance rock wall in 1988 has resulted in a flattening of the flood tide gradient compared with 1977 which would tend to increase the tidal prism of Coffs Creek.
25. It is likely that the construction of the entrance rock wall has enhanced the natural scouring action of tidal currents, allowing a deeper channel to develop against the toe of the wall. The increased depths have the potential to increase bedform heights in the channel than otherwise would have been the case in the absence of the rock wall.
26. The entrance rock wall has increased the variability of depths in the entrance channel in the vicinity of the wall.
31. The construction of the training wall has stabilised the location of the inlet by preventing its tendency to meander to the north. While it is possible for present day gross transport of littoral sand, in the form of transient sand slugs driven by major storm activity, to impact on the entrance to Coffs Creek and cause periodic southerly shifts in the orientation of the outer eastern portion of the entrance channel, no major fluctuations in that regard have been observed to date. The enhanced natural scouring action of tidal currents flowing against the entrance rock wall has increased the propensity of the entrance channel to hug the wall."
154 In relation to the joint agreement reflected in paragraph 26 above, Mr Nielsen explained that the variability of depth in the vicinity of the wall would be propagated across other parts of the channel in a southerly direction. (T 212 lines 15-25).
155 Mr Nielsen concluded his third report in these terms (Exhibit "E" page 570):
"The changes to the entrance, undoubtedly, have been beneficial in many respects and it appears that they were designed to that effect. … The changes to the creek have been a result of training wall construction, which has increased the tidal prism of the creek and, hence, the run out tidal velocities, has generated turbidity and has caused the formation of large undulating and unstable sand waves on the bed of the entrance channel."
156 There is one final aspect of Mr Nielsen's oral evidence which requires recitation. This related to the matter that he considered provided justification for his views relating to the flattening of the flood tide gradation compared with 1977 and its consequences (Exhibit "T" para 24).
157 Mr Nielsen explained the particular matter in this way. In the Coffs Harbour Creek Waterways study Volume 1 (April 1979) the Department of Public Works reported a number of conclusions it had reached regarding the hydraulic investigation of the flooding and tidal characteristics of Coffs Harbour Creek. Figure Number 6 to the report is set out at page 321 of Exhibit "D". It is a graph of the tidal gradient measured on 1 September 1977. The graph demonstrates that the flood tide gradient fell from an ocean level of 0.56 to a level at the railway bridge of 0.49. This means that between the two locations, the high tide line was falling as demonstrated by the measurements taken in 1977.
158 Mr Druery in his second report (Exhibit 2 D9) dated September 2002, produced a document headed "Measured Tidal Gradients". This appears as figure A.1 in Exhibit 2 D9. The graph shows three measurements. The first is a repetition of the 1977 measured tidal gradient carried out by the Department of Public Works. The second is a measurement carried out by Bruce Fidge and Associates on 5 September 1991. The third is a measurement carried out by Mr Druery's firm on 10 August 2002. The measurements taken in 1991 and 2002 indicate that, between the ocean and the bridge, the high tide line was horizontal at the time of each of these measurements. This meant, Mr Nielsen explained, that the flood tidal gradient had "flattened" in the years since 1977 (T 206-207). This supported Mr Nielsen's view that the entrance rock wall had improved the conveyance characteristics of the creek so as to allow full penetration of the floor tide ie, the total volume of water getting into the creek had increased. In turn, the increase in tidal prism had the potential to increase bedform heights in the channel otherwise than would have been the case in the absence of the rock wall (T 208 line 45).