Information Sheet1 (Based on the BMT Flood Study Report for the Ambiwerra Master Plan)

Background St Aidan’s Anglican Girls’ School has provided school facilities in Corinda for over 90 years and continues to provide an excellent educational experience for its students and families. Ambiwerra plays an important role in the St Aidan’s educational experience, with the School’s sports and recreational facilities located at Ambiwerra since 2002.

Identified improvements to existing facilities at Ambiwerra are outlined in a masterplan. These improvements include the development of covered/indoor sports courts and swimming facilities. Specialist technical studies have informed the preparation of the master plan, and informed ways to mitigate impacts from the proposed additional facilities.

Purpose of the Flood Study BMT have completed a flood study to assess the flood implications of the proposed master plan for Ambiwerra. This has included a flood risk assessment, identifying the levels relevant to the site with respect to River and Oxley Creek flooding and the currently proposed levels for the works associated with each stage. The reporting has also considered the impacts of the works on flood levels on the site and surrounding areas.

Background Ambiwerra is affected by flooding from the and Oxley Creek.

The location of sporting facilities within areas is considered to be an appropriate use of such areas provided the associated flood risk can be minimised and managed acceptably.

In this case, it is not proposed to fill the site to above flood level as such works would reduce the floodplain storage of Oxley Creek. The cumulative impact of filling the site and other sites in the floodplain would be to raise flood levels associated with Oxley Creek and potentially adversely affecting flooding in existing areas of development.

Instead, it is proposed to undertake a balanced earthworks exercise (lowering parts of the site in order to allow other parts to be raised) to ensure that there is no reduction in floodplain storage at the 1% Annual Exceedance Probability (AEP) flood level in Oxley Creek. The 1% AEP event is equivalent to an event with an average recurrence interval (or ARI) of 100 years.

Given the ground levels that occur across the site, insufficient fill is available to raise development levels above the governing flood levels associated with Brisbane River flooding. In addition, the height required for facilities to be above Brisbane River flood

1 Note – this is a summary only. The full report is available on the St Aidan’s or Department website.

levels would be problematic. Instead, it is proposed to adopt a risk management approach with respect to flooding in the design and operation of the facility, and a achieve a level above the 1% AEP for Oxley Creek.

Flood Levels Current best practice is to refer to in terms of their Annual Exceedance Probability, or AEP. This terminology expresses the potential for a flood of a particular magnitude to occur within a given year. For example, a 1% AEP event has a one percent chance of occurring in a given year. This approach is used in order to highlight the fact that major floods could occur in successive years and the fact that if a major flood occurs in one year does not mean that a similar or larger flood could not occur the following year.

Brisbane River Flooding In City Plan 2014, Brisbane City Council’s planning scheme, areas affected by Brisbane River flooding have been mapped and divided into flood planning areas based on the severity of flooding that occurs.

The flood mapping for Brisbane River flooding of the site is shown on figure 1 below. The lower parts of the site are located in flood planning area 1 and affected by the 10% AEP (10-year ARI) event in the Brisbane River. The majority of the remainder of the site is located in flood planning area 2a, where the depth of flooding is in excess of two metres at the Residential Flood Level (RFL, refer below).

A small part of the site is located in flood planning areas 2a, 3 and 4, with flood planning area 4 extending to the limit of inundation for the River Flood Level (RFL). Flood planning area 5 relates to flooding up to the 0.2% AEP (500-year ARI) event.

Figure 1 City Plan v20 Brisbane River Flood Mapping for the Site

Source: Brisbane City Plan v20 https://cityplan.brisbane.qld.gov.au/eplan/#/Property/0

Oxley Creek Flooding Oxley Creek flooding produces lower flood levels than flooding in the Brisbane River. A small part of the lower playing fields is located within creek flood planning area 1, indicating that the area is inundated by the 10% AEP (10-year ARI) event. The remainder of the playing fields are located in the creek flood planning area 2, where flooding occurs to a depth greater than 1.2 metres in the 1% AEP (100-year ARI) event).

The tennis courts are located in creek flood planning areas 3 and 4, indicating that they are affected by the 1% AEP (100-year ARI) event. The remainder of the site is either located in flood planning area 5 (areas flooded by events up to the 0.2% AEP (500-year ARI) extreme flood event) or above the 0.2% AEP level).

Figure 2 City Plan v20 Oxley Creek Flood Mapping for the site

Source: Brisbane City Plan. https://cityplan.brisbane.qld.gov.au/eplan/#/Property/0

Review of Development levels The City Plan’s Flood Overlay Code identifies minimum development levels. Minimum development levels are related to the class of building being considered according to the Building Code of .

In all cases it will be necessary to locate essential electrical services at a level that meets the requirements of the Flood Overlay Code. No relaxation would be proposed in relation to the provision of acceptable immunity to essential services. Essential electrical services include any room used for fire control panels, telephone PABX, substations and associated electrical equipment, communications equipment, low and high voltage cables and pump controls.

In general, the adopted levels comply with the recommendations of the Flood Overlay Code, subject to an appropriate risk management strategy being defined relative to Brisbane River flooding. Areas where a lower level of immunity is proposed and considered to be acceptable are discussed below.

Undercover PE Facility The proposed facility includes courts set above the adjacent ground level, with the ground level area used for a gym and other gathering purposes. The courts are to be either covered or enclosed, depending on final detailed design, which also offers flexibility and potential staging. For an indoor court facility, the Flood Overlay Code nominates a floor level equal to the 1% AEP (100-year ARI) flood level plus 500 mm freeboard. The need for such a high level of immunity is difficult to justify given the relatively low frequency of patronage of the courts and the uses to which it will be put. Further, a similar use without a roof would not require such a level of immunity.

It is recognised that different approaches are applicable for open and enclosed areas with respect to the limited warning times associated with creek flooding and the potential for people to remain within an enclosed space. However, in cases such as Ambiwerra where the creek level criterion is satisfied and a covered / enclosed area is being created, the significantly longer warning time associated with Brisbane River flooding should allow a risk-based approach to be adopted with respect to setting levels relative to river flooding.

It is considered reasonable to adopt a reduced level of immunity for a covered . enclosed school sports courts with respect to Brisbane River flooding. In this case, the level of the covered courts is to be set well above the 1% AEP (100-year ARI) flood level in Oxley Creek and above the 2% AEP (50-year ARI) flood level in the Brisbane River.

As the main school is located away from the site, it would be possible to continue school operations while the sports facilities are repaired following a flood. It is therefore considered appropriate to adopt a risk management approach with respect to educational activities on the site in this case.

Flood Risk Summary

The flood risk assessment has confirmed the nature of the flooding that affects the site. While existing sporting fields are affected by flooding from both Oxley Creek and the Brisbane River, the proposed buildings will be set above the 1% AEP (100-year ARI) flood level in Oxley Creek and will only be subject to inundation as a result of moderate to major flooding in the Brisbane River.

It is considered that the development generally complies with the immunity levels nominated by Brisbane City Council in City Plan 2014 and that uses for which a reduced immunity is proposed are acceptable given the intended use of the facility and the inappropriate nature of the nominated immunity level. Similarly, it is considered that the use of the site is tolerable with respect to the hydraulic risk criterion nominated in the Brisbane River Strategic Floodplain Management Plan for Brisbane River flooding and generally acceptable with respect to Oxley Creek flooding.

In the design phase, it will be necessary to adopt materials that are resistant to flood inundation (for example concrete or tiled rather than carpeted floors) that can be readily restored after a flood event and providing structures that are resilient to hydrostatic forces.

Although Brisbane River flooding is associated with greater flood depths, the associated velocity of flow is low and it is expected that an appropriate structural design will be readily achievable. In the operational phase, it will be necessary to operate the site in accordance with a Flood Emergency Management Plan that applies a precautionary approach to the closure or evacuation of the site. As the site will be operated by the school, management of flooding can be achieved as part of site operations. Further, the area will not be in use 24 hours a day and will have limited use over a significant portion of the wet season when the school is not operating.

Managing Flood Impacts Beyond Site Boundaries

Oxley Creek Flooding As stated above it is not proposed to fill the site to above flood level as such works would reduce the floodplain storage of Oxley Creek. Instead, it is proposed to undertake a balanced earthworks exercise (lowering parts of the site in order to allow other parts to be raised) to ensure that there is no reduction in floodplain storage at the 1% Annual Exceedance Probability (AEP) flood level in Oxley Creek. The 1% AEP event is equivalent to an event with an average recurrence interval (or ARI) of 100 years.

Predicted flood impacts are presented for a range of design flood events based on flooding within the Oxley Creek catchment.

Brisbane River Flooding The area is located in a backwater area with respect to Brisbane River. As a consequence, changes in landform associated with earthworks and buildings will not affect the flow capacity of the Brisbane River. Similarly, a balanced earthworks solution

will result in no loss in storage capacity of the Brisbane River floodplain, and the earthworks will occur at levels that are well below the peak flood levels associated with Brisbane River flooding.

As the works will not affect the conveyance of or storage in the Brisbane River, the development of the site will not affect Brisbane River flooding to any discernible degree. Given this, no modelling is necessary with respect to flooding in the Brisbane River.

Flood modelling has been undertaken using Council’s TUFLOW model of Oxley Creek.

Existing Case For the assessment of the existing case design flood events, the TUFLOW model was updated with current site survey. The site survey was provided in the form of ground survey of the existing natural surface.

Having incorporated the model refinements, the TUFLOW model was run for a range of ARI design flood events from the 39% AEP (2-year ARI) design event through to the 1% AEP (100-year ARI) design event for the critical 18 hour storm duration (i.e. the storm duration that produces peak flood levels and flows in the vicinity of the site).

Proposed Development Case Utilising the refined existing case model as described above, the proposed development was incorporated into the TUFLOW model.

The proposed development was based on masterplan drawings provided to BMT and consisted of elements representing the proposed fill area, buildings, structures and tennis court areas. The model therefore conservatively includes a greater amount of fill than will occur in practice due to the intent to complete a balanced earthworks solution.

A flood impact assessment was conducted by comparing the developed case peak water levels and existing case peak water levels.

The assessment was conducted using an updated version of Council’s Oxley Creek TUFLOW model to assess the potential flood impacts associated with the proposed development works over a range of design storm events. Figure 3 shows the results of the flood modelling.

Figure 3 – 1% AEP Water level impacts – Developed case

(Source: BMT)

Based on the flood impact assessment detailed within this report it can be concluded that the proposed development results in no adverse offsite flood impacts and is therefore acceptable. The predicted flood impacts demonstrate that the proposed development results in no adverse flood impacts beyond the site boundary and are limited to the 1% AEP (100-year ARI) flood event.

Predicted impacts that occur during this event are located within the site and occur over areas of open space.

For the remainder of the design flood events (2% AEP to 39% AEP or 50-year ARI to 2- year ARI) flood impacts are reported to be less than 5mm. Given this outcome, it is considered that the impact of development on flooding in Oxley Creek will be both negligible and acceptable.