High risks for MAAS on Parramatta floodplain – Debbie Rudder

This paper, compiled by Debbie Rudder in October, 2017, identifies problems raised by experts who consistently draw attention to the extremely high flood risk at the proposed riverside site for the new MAAS in Parramatta, through the following documents: 

  1. Parramatta Council paper re flood risk (current document)
  2. SES submission to Greater Sydney Commission, 2017
  3. Draft Updated Parramatta Floodplain Risk Management Plans, 2015
  4. Parramatta Council Development Control Plan, 2011
  5. Acid sulphate soil map, Parramatta City Centre Environmental Plan, 2007
  6. Soil contamination over 64 years; refer The Cumberland Argus, Sept 8, 1954.

 Summary of issues:
The site for the proposed location of MAAS on Parramatta floodplain is in the highest flood risk category and within the 1 in 20 year flood zone.
Its Probable Maximum Flood Level (the metric preferred by SES and others) is 4 metres, with a flood duration of around nine hours.
It is subject to both direct flooding by the river and overland flash flooding.

Nearby roads would also flood quickly, blocking evacuation routes and ambulance access.
Evacuating the museum whenever storms are forecast would severely disrupt its operation and particularly impact school groups, whose visits involve much planning.

Development should allow buffer areas for the preservation and maintenance of floodway, riparian corridors and habitat protection, and should not contaminate groundwater.
Disturbing the site’s acid sulphate soil to create foundations would release sulphuric acid into the river, and its use as a carpark for 64 years has probably contaminated soil with carcinogenic or neurotoxic products of hydrocarbon degradation.

Maps in the Draft Updated Parramatta Floodplain Risk Management Plans of 2015, on pages 20, 21, 25 and 43, are of particular relevance to this issue.

[The comments in square brackets are by the author.]

1. Parramatta Council paper re flood risk (current document)
https://www.cityofparramatta.nsw.gov.au/business-development/development-application-da/application-requirements/developing-in-a-flood-area

A High Flood Risk Precinct is generally defined as the area of land below the 100 year flood that is either subject to a high hydraulic hazard or where there are significant evacuation difficulties. Most land uses (with the exception of Open Space & Non-Urban and Concessional Development) would not be suitable within this precinct. [The proposed site is within this risk category as shown in later documents.]

2. SES submission to Greater Sydney Commission, 2017
https://gsc-public-1.s3-ap-southeast-2.amazonaws.com/s3fs-public/submissions/StateEmergencyServices_Open_Submission.pdf

p2. ‘The NSW SES recommends that…there should be appreciation and consideration of full risk up to the Probable Maximum Flood (PMF)…flood risk does not disappear at the flood planning level, but rather creates a cumulative and catastrophic risk scenario at the 1 in 101 level and above…the risk is not necessarily “low” as in most cases emergency management becomes more complex and the risk to life is often higher.’

p5. ‘Parramatta (the future “river city”) has significant flash flood and riverine flooding risks…development on the scale proposed is likely to impair the operational capability of the SES…recent flood events on the north coast of NSW show the pressure that the NSW SES volunteers are under in a relatively frequent flood event, which was less than a 1% AEP (1 in 100 year ARI) event.’ A more severe event across Greater Sydney could lead to ‘large-scale loss of life’.

3.   Draft Updated Parramatta Floodplain Risk Management Plans, 2015
https://www.cityofparramatta.nsw.gov.au/sites/council/files/inline-files/Appendix_10_Draft_Updated_Parramatta_Flood_Risk_Management_Plans.pdf

p18. ‘The primary source of flooding is from the Parramatta River, [which] expands laterally into the floodplain through the CBD area. Some areas within the CBD can also be flooded by local overland flow from intense rainfall overwhelming the drainage system…the flooding spreads throughout the CBD, cutting off many evacuation routes.’

p19. ‘Depths are greatest in the areas directly adjacent to the river and on the roads…The flood rate of rise in the Parramatta River is relatively quick, particularly for the Probable Maximum Flood (PMF)…The average flood rate of rise…1.6 metres per hour for the PMF.’

p20. Map shows the proposed museum site lies mostly within the 1 in 20 year ARI area, as do major portions of surrounding roads.

p21. Map shows PMF peak depth at the proposed site and nearby roads is greater than 4 metres.

p24. Flooding in the Parramatta CBD is typical of flash flood catchments. Flooding arrives quickly and without significant warning time.

p25. Map shows the PMF duration at the proposed museum site is around 9 hours.

p31. ‘the evacuation of Parramatta would be problematic for a multitude of reasons…within two hours the flood level could already be greater than the peak of the 100 Year ARI event and many roads in the CBD would be too dangerous to use.’ Given the rapid rise of flood waters, there would not be time for the SES to mobilise for traffic control operations or for people to prepare to evacuate.

[Could the museum handle the orderly and safe evacuation of school groups, elderly and disabled visitors, other visitors and staff, most of whom have arrived on foot from the railway station or bus interchange? How would staff handle a medical emergency if floodwaters blocked ambulance access?]

p32. ‘There are drainage capacity issues within the CBD which would likely flood the local streets prior to…floodwaters arriving directly from the river…this localised flooding in a 20 year ARI flood could prevent vehicular evacuation earlier than river flooding does…If evacuation triggers were set at a lower river level to allow sufficient time for evacuation…there would be many circumstances where evacuations would be called and then turn out to be unnecessary.’

[It would be particularly disruptive to the operation of a museum if visitors were evacuated, or school groups advised to stay away (after much planning and preparation for each visit), whenever a severe storm warning was issued by BoM.]

p43. Map shows the proposed museum site is in the highest Flood Risk Category. This classification summarises all the risks and issues that affect each site, so the map clearly indicates that the site is unsuitable for such a use.

4.  Parramatta Council Development Control Plan , 2011
https://www.cityofparramatta.nsw.gov.au/sites/council/files/inline-files/DCP%202011%20-%20Part%202%20-%20Site%20Planning.pdf

 Flooding: Design Principles (page 16)
P.9 Development is to be compatible with any relevant Floodplain Risk Management Plan, Flood Studies, or Sub-Catchment Management Plan.

P.10 Development must not divert flood waters, nor interfere with floodwater storage or the natural function of waterways.

P.11 Filling of land up to 1:100 Average Recurrence Interval (ARI) (or flood storage area if determined) is not permitted. Filling of and above 1:100 ARI up to the Probable Maximum Flood (PMF) (or in flood fringe) must not adversely impact upon flood behaviour.

P.12 New development must consider the impact of flooding resulting from local overland flooding whether it is a result of Local Drainage or Major Drainage.

 Protection of Waterways: Design Principles (page 21)
P.1 Development is to make provision for buffer areas for the preservation and maintenance of floodway, riparian corridors and habitat protection.

P.3 The piping, enclosing or artificial channelling of natural watercourses and drainage channels is not permitted. Consideration is to be given to re-opening piped or lined drainage systems wherever feasible.

 Protection of Groundwater: Design Principles (page 22)
P.1 Operating practices and technology including dewatering shall not contaminate groundwater or adversely impact on adjoining properties and infrastructure.

NOTE: The potential risk to groundwater is high when construction involving excavation is below the water table and is within alluvial areas and sandstone environments.

5.  Acid sulphate soil map, Parramatta City Centre Environmental Plan, 2007
https://legislation.nsw.gov.au/maps/8d7a43a6-1b1e-11dd-8fae-00144f4fe975/6250_CEN_ASS_001_010_20071205.pdf

[The proposed museum site is class 4, meaning acid sulphate soil is likely to be found beyond 2 metres below the natural ground surface. Any works that extend beyond that level (surely a necessity to support a large museum on this flood-prone site) will require assessment and may require management. Disturbing such soil can release sulphuric acid into the local environment, posing problems for the river ecosystem and jeopardising plans to make the river swimmable for humans by 2025.]

6. Soil contamination over 64 years: refer The Cumberland Argus, Sept 8, 1954http://nla.gov.au/nla.news-article132140615

[As the site has been used as a carpark since 1953, it is highly likely that the soil is contaminated with hydrocarbons and their breakdown products, which can be carcinogenic or neurotoxic.]