44 The evidence included a video of a flood event recorded from the dwelling at 15 Anthony Street in 1990. Mr Bewsher's evidence included his observations of a flood event at the site in February 2010. Mr Bewsher and Dr Martens agreed that both events were somewhere between a 1 in 5 and 1 in 20 year event. Both agreed that a flow similar to that in 1990 would move under the dwellings at the Anthony Street frontage. They agreed that there would be some debris, however Dr Martens was of the opinion that the gross pollutant trap on the other side of Anthony Street would mean that there would be a lesser volume than in a more free channel system. Mr Bewsher was of the opinion that the debris would require clean up, and would result in erosion, and that there is a possibility that the debris could cause obstructions to the flow path. Both experts agreed in a 1 in 100 year event the depths over Anthony Street would be hazardous to vehicles, and that it is possible that a vehicle could be washed off.
45 The experts agreed that because of the depth and velocity of the flow it would not be possible to drive out of the driveway or walk out onto Anthony Street in flooding; that the duration of flooding would be 1 to 2 hours; and that access would be cut for a half to one hour in a 1 in 100 year event. Both agreed that there would not be much difference between the 1 in 20 and 1 in 100 level. It was common ground that in small floods there would be no pedestrian or vehicle access to Anthony Street and that if residents are leaving it would be via the walkway to Eyles Ave, which is up to 100m from the Anthony Street frontage. The experts agreed that the extent of high hazard areas on the site could increase with the revised 1 in 100 year flood levels following their joint conferencing, and that the walkway to Eyles Ave would need to be raised.
46 In relation to the driveway to the basement car park, the experts agreed that if floodgates are not used, and the crest is raised, it would need to be at 99m AHD. Dr Martens' evidence was that floodgates are unusual for a basement car park for residential living, and that his preference is to engineer them out. The experts agreed that if the crest is raised, for the gradient not to change the length of the driveway down into the car park would have to increase. Dr Martens' evidence was that the details of the design for the car park configuration would not need modification, however the driveway would, and there is an option of elevating the driveway for some distance; if so, it would need to be constructed as a bridge so water could pass underneath. If the driveway was not elevated and at grade, water would pass over it. Dr Martens' opinion was that the outcome could be a floodgate, a modification to the basement car park arrangements, or a modification to the driveway entry point, and that this is an issue that can be resolved at the detailed design stage.
47 Dr Martens and Mr Bewsher agreed that there needs to be an emergency management plan prepared for the site which demonstrates how the occupants can be safely evacuated if required in the event of a major flood. They agreed that there needs to be a visual and aural warning within the basement car park in the event of flood waters entering the car park; that access to flood liable land should be suitably gated to restrict access; and that the basement carpark lift should be fitted with appropriate controls to ensure safe operation during a flood.
Findings - flooding
48 I accept the evidence of the hydrologists that in a 1 in 5 to 1 in 20 year flood event, water would flow beneath proposed units 1 and 2 of the development; that in a 1 in 100 year event the depth and velocity of water would make it unsafe for pedestrians to access Anthony Street, and that vehicle access would not be possible from Anthony Street. The proposed development is designed on the basis that flood waters will flow underneath the building under units 1 and 2. In their second joint report the hydrologists agreed that that the final levels would need to be set before construction certificate using a two-dimensional flood model; and that the extent of flooding and extent of high hazard areas would increase and are yet to be quantified. Even if the approach in Chisholm v Pittwater Council [2000] NSWLEC 143 is adopted, and the material contained in the original flood study, the two separate position papers, and the two joint reports, together constitutes a flood study, the evidence is that the final flood levels will be higher than currently agreed. The hydrologists agreed on a possible increase in flood level of about 0.5m. On the basis of their evidence, I accept that the structural stability of the proposed dwellings and the services located under the slab have been either addressed in the plans or could be addressed at the detailed design stage. The proposed development depends on access to the site or evacuation during a flood event using the walkway elevated above the floodwaters to Eyles Avenue. I accept the hydrologists' evidence that while the precise levels are not known at this stage, the walkway could be designed at an appropriate height above the relevant flood level, structurally stable, and with a gradient so as to be accessible. Mr Bewsher was of the opinion that the walkway should be covered.
49 Mr Bewsher and Dr Martens agreed that more stringent controls to mitigate flood damage and personal safety risks are required for a seniors living development than for normal residential development. The central difference of opinion between the experts related to social impacts.
50 Mr Bewsher's evidence was that he would expect some occupants of units 1 and 2 and their second storey counterparts "to become terrified during moderate flooding (eg 20 year flood event)", and that there remains an issue "related to amenity and particularly anxiety of the occupants of these units".
51 Dr Martens disagreed. In the first joint report Dr Martens stated that any reduced amenity below the buildings in relation to units 1 and 2 would not be unacceptable. In terms of resident anxiety, major flooding is highly infrequent and occurs generally only for a short duration. There is a possibility that some new resident may initially experience elevated anxiety should they observe flood waters passing under their dwelling; however flood awareness initiatives could be put in place including signage, flood information contained within the contract sale package, and which would be detailed in an emergency management plan; and once a resident had experienced and observed a flood event anxiety levels would be lower on the next event.
52 Mr Bewsher's position paper included the following statement:
21 In normal circumstances, medium density development of a significantly floodprone site, such as the one before the Court, would be considered unsuitable, or at best, heavily constrained by flooding. What makes the proposed development even more heavily constrained is that the subject site is proposed for seniors or those with a disability.
53 Mr Bewsher conceded in oral evidence that this passage was the same as that quoted by Roseth SC from his report as a single expert in Radray Constructions Pty Ltd v Hornsby Shire Council [2007] NSWLEC 34. Mr Bewsher stated that in contrast to the position in the previous proceedings, there is more information available in these proceedings including the high hazard map, and the extent of the high hazard area beyond the creek is now quantified.