Tenacity Investments Pty Ltd v Ku-ring-gai Council

Tenacity Investments Pty Ltd v Ku-ring-gai Council - [2007] NSWLEC 246 - NSWLEC 2007 case summary — Zoe

[24]

4 The plan at the end of this judgment shows the local area, proposed development site and public drainage system. The development site is known as 2 Pymble Avenue and 2 - 4 Everton Street, Pymble. It is located on the corner of Pymble Avenue (to the west) and Everton Street (to the north). Everton Street connects with Livingstone Avenue which runs parallel to Pymble Avenue. 5 The development proposal consists of the re-development of three residential lots containing free-standing houses into a consolidated development of four townhouses and 25 units. The proposal incorporates on-site rainwater detention mechanisms, in association with rainwater storage for reuse on the site, to mitigate adverse impacts created by increased imperviousness on the site. 6 The stormwater design includes connection of the piped drainage from the site to the existing public drainage system, which is a council underground trunk drain running downstream from Everton Street. The trunk drain eventually discharges into a landscaped open watercourse within 15 Livingstone Avenue. Inflows to the trunk drain include flows from the catchment above the inlet pits in Everton Street (located between 2 Everton Street and 3 Livingstone Avenue), stormwater pipes from 3 and 5 Livingstone Avenue and any other downstream stormwater pipe that connects to the drain between Everton Street and the open watercourse. There are no open pits that will allow surface runoff to enter the underground trunk drain or that will permit surcharges from the drain. 7 Mr Wall's earlier report of December 2006 to the Court expressed his opinions, which I accept, (a) that discharges from the development site to the existing public drainage system for storms of 20 and 50 year Average Recurrence Interval (ARI) will be reduced following the development; and (b) that the pipeflow from the proposed development site to the trunk drain will be reduced following development from 45 litres per second to 29 litres per second, a reduction of some 36 percent.

[28]

14 Mr Lawrence in his report of 17 April 2007 generally agreed with Mr Wall. 15 Thereafter, at the request of the Court, Mr Wall re-examined his drain modelling to consider a 50 year ARI storm event. Mr Lawrence and he on 20 April 2007 inspected the downstream section and prepared a joint inspection report dated 23 April 2007. The plan which formed part of that report identified the information gathered which consisted essentially of the location of pits and, wherever possible, measurement of pit dimensions and pipe diameters. The inspection indicated that council records of pipe diameter relied upon by Mr Wall in his original computer model construction were in error. These were corrected in Mr Wall's report of 27 April 2007, which gave the results of further computer model runs with amended pipe diameters. The pits had heavy concrete covers which indicated to Mr Lawrence that the computer model results obtained by Mr Wall are conservative; that is, that the actual capacity may be higher than the results indicated due to the possibility of pressure heads being generated at the pits. Though no further investigations were carried out, the orientation of the site connections implied to Mr Lawrence that impervious areas from several of the properties were drained directly to the trunk drain. 16 Table 3.1 of Mr Wall's report of 27 April 2007 showed that during the 50 year ARI design storm event the existing trunk drain has the capacity to accept the piped runoff from the development site. At Pit F, where the development site pipe would enter the trunk drain system, the modelled flow in the drain would be 425 litres per second and the estimated maximum capacity of the drain 588 litres per second. Hence at that point the trunk drain is operating at 72 percent of capacity. 17 In this report of 27 April 2007, Mr Wall did not take into account the inflow to the trunk drain from the downstream properties. As explained in oral evidence, that was because he did not consider that they were relevant to the preliminary issue.

[37]

Therefore it can be seen that not only is the model being used for a different purpose to the Penrith modelling exercise but the rainfall input data is truly representative of the rainfall to be expected over the catchment area for a particular ARI event. 24 In my view, this is an effective rebuttal of the fourth respondent's contention concerning alleged inaccuracy of the DRAINS model. Mr Lawrence and Mr Wall were unshaken in cross-examination. 25 In the opinion of the experts in this case, which I accept, the DRAINS model results provide the best estimate based on available data. ALLEGED LOGICAL INCONSISTENCIES OF MIXING 20 YEAR AND 50 YEAR STANDARDS 26 The fourth respondent criticised Mr Wall's initial approach of applying a 20 year ARI standard for the trunk line design and a 50 year ARI standard for the development site, submitting that it was anomalous. The fourth respondent's submission was that if there is a 50 year storm event, there may be no capacity in the existing public drainage system for any discharge into the public drainage system from the development site, without overland surcharges. 27 Mr Wall and Mr Lawrence had rationalised using the 20 year ARI storm event because that was in accordance with the council's DCP 47 requirement for its trunk drain. It could also be rationalised by reason of the applicant's undertaking at the mediation on 1 March 2007, whereby the applicant undertook to design an on-site detention system to provide sufficient detention capacity to ensure that no spillage from the system occurred during the 50 year ARI storm event. In any event, Mr Wall did later model a 50 year ARI storm event with which Mr Lawrence agreed (as explained above). It showed that the trunk drainage system had the capacity to accept the anticipated runoff from the proposed development. 28 To put the matter into perspective, the DRAINS model indicates that the critical storm duration varies from section to section of pipe but is generated by a storm duration not exceeding two hours. The 20 year ARI storm can be expected to occur, on average, five times in 100 years. If the catchment flows exactly match the pipe capacity, then the cumulative time that the pipe flows full is ten hours in 100 years. ALLEGED ERRORS IN INPUT DATA 29 The input data for the computer model used to estimate flows within the pipeline upstream of and within the development site, consisted of rainfall intensity frequency data supplied by council, survey data supplied by a registered surveyor, hydraulic characteristics of pipe/inlet pipe systems, and assumptions regarding the local soil characteristics. In Mr Lawrence's opinion, there is no basis on which to challenge the first and second inputs and the third and fourth inputs would be accepted by all experienced practitioners. After consideration of the respondent's submissions referred to below, I accept his opinion. 30 The fourth respondent submitted that there were input data errors by Mr Wall in two respects: first, in relation to pervious (ie non-paved) areas in the Everton Street catchment area; and, secondly, in relation to rainfall data. 31 As regards the first point, which concerns the volume of water inflow into the upstream Everton Street pits, Mr Lawrence concluded that the Everton Street catchment area was 47 percent pervious and Mr Wall initially concluded that it was 40 percent pervious. The fourth respondent submitted that photographic evidence would indicate to a reasonable person that it was closer to 90 percent impervious. Mr Wall responded that the catchment of the Everton Street pits included the streetscape as well as the development between Everton Street and the Pacific Highway. He considered Mr Lawrence's estimate of 47 percent pervious to be acceptable. Mr Lawrence's response in his report of 27 April 2007 was that whether the value of 40 or 47 percent for pervious areas is adopted makes no significant difference to the model results. His reason was that during a more severe storm event or a greater ARI, the rainfall intensity increases and the differences between rates of runoff from pervious and impervious areas decrease due to the fact that infiltration capacity of the soil is very limited. 32 Secondly, as regards the erroneous input rainfall data allegation, the fourth respondent said that council data had been taken from Australian Rainfall and Runoff (AR&R). He assumed that the bulk of the data was from weather stations such as Observatory Hill, given (he said) that short duration rainfall data is not available from the closest Bureau of Meteorology station at Turramurra. He then submitted that rainfall at Turramurra is historically higher than at Observatory Hill. It appears that he was submitting that data derived from AR&R should be rejected in favour of a local rainfall station. 33 Mr Wall responded that storm characteristics were sourced from council's DCP 47 appendix 10 - Ku-ring-gai Rainfall Intensity Frequency Duration data and was therefore valid for the Pymble area. He said the particular local rainfall station data is generally used by the Bureau of Meteorology to assist councils to develop such rainfall data for their council area. Mr Lawrence responded that rainfall data from one local daily read rainfall station is of no assistance to the matter in issue, which involves critical storm durations varying from 25 to 120 minutes. He referred to and supported the following quotation from AR&R: "Analysis of data from single stations is often unreliable, it is not temporally or spatially consistent and should generally not be used for design purposes". 34 Mr Lawrence and Mr Wall were unshaken in cross-examination. I accept their views and do not accept the fourth respondent's submissions concerning alleged errors in input data. DOWNSTREAM PROPERTIES 35 As the fourth and tenth respondents own properties downstream from the development site, they are concerned about any risk of water inundation to their properties caused by the development. This is notwithstanding, as I have noted earlier, that the pipeflow from the proposed development site to the trunk drain will be substantially reduced following development. 36 The plan included in this judgment shows closed pits along the trunk drain. The oral evidence of Mr Wall indicates that there is a pipe from the fourth respondent's property to Pit F, a 225 millimetre pipe to Pit G and two 110 millimetre pipes to Pit J. These pipes were observed by Mr Wall to be clean, indicating water passed through them. Therefore, it appears that they drain paved areas, although their catchment areas are unknown. Mr Wall's judgment was that, even taking them into account in a 50 year ARI event, the trunk drain still has capacity to accommodate these downstream flows.

[39]

38 Secondly, the tenth respondent submitted that if downstream properties were flooded by reason of their inflow exceeding the capacity of the trunk drain at their points of connection, then that would have a deleterious affect on the downstream properties. It was said that any such deleterious effect was within the scope of the preliminary issue. I disagree that it is within the scope of the preliminary issue. That issue is only concerned with whether the existing public drainage system has the capacity to accept the anticipated runoff from the proposed development. In any case, the evidence before me does not establish that the downstream properties will suffer such a deleterious effect. The tenth respondent suggested that a deleterious effect was indicated by the statement in Mr Lawrence's report of 27 April 2007, that during more severe storms the system becomes overloaded and overland flow occurs along the line of pipe and easement. This appears to be a reference to a storm event in excess of the 50 year ARI because the applicant's undertaking of 1 March 2007 would contain spillage up to a 50 year ARI storm event. In the event of a storm in excess of the 50 year ARI, Mr Lawrence's report went on to preclude deleterious effects because he said that he had inspected the trunk drain through and downstream of the development site as far as 15 Livingstone Avenue and assessed that there was no risk of property damage or danger to persons which would warrant applying a design standard in excess of the 20 year ARI event, given that all dwellings within the properties through which the drain passes are several metres distant from the pipe and floor levels are elevated a considerable height above the line of the pipe. I accept his opinion. 39 In my opinion, the evidence before the Court does not indicate that downstream inflows bear on the preliminary issue. CONCLUSION 40 The preliminary issue is: whether the existing public drainage system referred to in Condition 89A(d) of the development consent granted 2 May 2006 and modified by this Court on 17 October 2006, has the capacity to accept the anticipated run-off from the proposed development at 2/4 Everton Street and 2 Pymble Avenue, Pymble. 41 I answer the preliminary issue: yes, on the basis of 20 and 50 year Average Recurrence Interval storms events. 42 The exhibits may be returned. The proceedings will be listed before the list judge for directions on 18 May 2007.

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