Questions raised at public information sessions

The questions and statements on this page were raised by community members that attended our information sessions about the Narooma Coastal Inlet Flood Study in July 2015. Our responses are included below each question.

What are the authors' qualifications? Who were the consulting hydrologists, hydraulic engineers and computer modellers?

Our response: The report was prepared by WMA water, a consulting firm that specialises in flood modelling and flood management. WMA water were awarded the project following a competitive tender process that required the successful firm to demonstrate their qualifications and experience.  The consultants managing the project attended both community consultation sessions.

The report contains no executive summary, no conclusions and no recommendations. Why?

Our response: The flood study is Step 1 in the floodplain risk management process. The flood study is a technical investigation that aims to provide information on the extent, level and velocity of flood waters. The flood study is a technical document only and does not include a discussion on management actions.

The stages of floodplain risk management process examine the risk to life and property. The subsequent stages identify and discuss management options which includes recommendations on the most viable measures to reduce flood risk.

Council will continue to implement flood mitigation measures and stormwater drainage maintenance in accordance with the scheduled maintenance programs.

When did local consultation take place and who in Dalmeny was consulted?

Our response: The following local consultation occurred:

  • media release 'Flood Stories wanted for Narooma catchment study' distributed 12 September 2012
  • questionnaire and information sheet distributed to local residents and businesses September 2012
  • online 'survey monkey' publicised in information sheet
  • community drop-in session 17 September 2012
  • Council report seeking endorsement to exhibit draft plan 9 June 2015
  • website notice of exhibition and public meeting 12 June 2015
  • media release that appeared in the Narooma News on 10 June 2015
  • mail out to all residents within identified flood planning area 15 June 2015
  • article in Eurobodalla News, Council's online newsletter, with advice on public information sessions and exhibition 3 July 2015
  • public meetings listed on Council's website
  • follow-up media release that appeared on the Narooma News website on 8 July 2015
  • public information sessions 8 and 9 July 2015
  • business information session 8 July 2015.

Many residents attended the 2012 public information session. Nearly 60% of survey responses came from the Dalmeny catchment.

Residents do not accept the problem of poor base rainfall data was overcome by using Tuross Head and Captains Flat radar.

Our response: The rainfall patterns for the eastern catchment areas of Mummaga Lake were calibrated by recordings at pluviometers located at the Narooma (1999 event) and Barlows Bay (2007, 2010 events).
A number of additional rainfall gauges (listed below both daily and pluvio) informed event rainfall applied across the Mummaga Lake catchment:

  • Bodalla Post Office
  • Tuross Head (Nelson Pde)
  • Tuross River at Eurobodalla
  • Narooma RVCP
  • Barlows Bay
  • Cobargo
  • (Wandella) Central
  • Tilba (Braeside)

Rainfall patterns for the western sub-catchments of Mummaga Lake were calibrated with the Tuross recordings.

The recorded rainfall patterns from the Captains Flat radar were analysed to identify when the storm burst occurred and to confirm the appropriate rainfall pattern to be applied.  The recorded rainfall patterns were found to have replicated well across the catchments.  The radar is useful in identifying rainfall duration and intensity and is applied as an additional analysis tool to support the recorded rainfall data.

We understand you may be able assist with providing rainfall recordings taken by local residents. This information would assist to validate the total rainfall depths but would not increase the validation of the applied rainfall patterns. The radar was used to validate the pattern.

The modelled flood levels are far higher than the actual levels recorded by many local residents. They are grossly over stated.

Our response: As part of the exhibition process, we are seeking information from the public that may assist to more accurately calibrate the model runs.

Discussions with residents during both consultation sessions confirmed a strong correlation between observed flood behaviour and the flood model. For example, a resident observed the entrance to Mummaga Lake as open at 7:30am during the 2010 event. The model calibrations calculated an open entrance at 7:40am.

The model in many cases has confirmed the residents' observations of only a portion of their land being flooded during the 2010 event while the houses were not impacted.

The modellers make two very inaccurate assumptions in modelling the 1999 and 2010 storm events:

  1. They have assumed that in each case the sand berm was opened at the lake trigger point of 1.175.
  2. The starting level of the lake was 0.6m AHD.

Our response:

  1. No survey data were available to determine the precise height of the Mummaga Lake sand berm at the commencement of the 2010 flood event. It is known the entrance was closed at the commencement of the event and a natural break out occurred. Given no survey data were available, it was assumed the entrance opened at the management trigger level of 1.175mAHD. The assumed level is also supported by the validation of the entrance opening time.
  2. No survey data were available to accurately determine the lake level at the commencement of the event therefore an assumption needed to be made.  The model indicates Mummaga Lake is relatively insensitive to variations in the Initial Water Level as demonstrated in Table 26 on page 61 of the draft study.  Varying the starting level of the lake would therefore have little difference in the model results and we are seeing a good level of correlation between the observations of local residents and the modelled levels for the 2010 flood event.

The report ignores the October 2014 event when 207mm of rain fell in Dalmeny when the lake was opened to the sea.
Narooma had major flooding but Mummaga Lake was barely affected. This demonstrates that the channel can handle any flood event if it is open.

Our response: The study modelled a range of flood events for Mummaga Lake. This included flood scenarios with predominantly closed entrance conditions and a range of ocean conditions that can also influence peak flood levels. Notwithstanding this, peak flood levels during large flood events are not strongly influenced by entrance conditions which break out naturally during these events.  Furthermore, large flood events peak at levels higher than the entrance opening management trigger.

It is acknowledged however, that entrance conditions during smaller more frequent events have a strong influence of peak flood levels.  For example, the 2010 event was almost double the rainfall recorded for the 2014 event when you observed the influence of an open entrance.

The report fails to comment on adequacy of culverts in the flood affected areas.
Flooding on corner of Binalong and Mort Ave, Acacia and Tatiara is a direct result of inadequate stormwater and not lake flooding.
This is a Council problem that is not addressed in the report.

Our response: The aim of this study is to identify flood behaviour under existing catchment conditions. We have passed on your concerns about the adequacy of culverts in this and other locations has been passed on Council's Infrastructure Services.

The report fails to examine the management of the lake berm under closed conditions.

Our response: The flood study modelled a range of entrance conditions. Please refer to page 50 and page 60 of the report that describes the entrance conditions modelled.

The National Parks and Wildlife Service (NPWS) has management responsibility for the entrance of Mummaga Lake as part of the Eurobodalla National Park. This flood study may provide additional information to assist NPWS with managing the entrance.

How do the authors propose to combine a flood policy with a Sea Level Rise policy?
Sea Level Rise is permanent and flooding intermittent. Mitigation is different. The report should include a full explanation.

Our response: The study reports on flood behaviour. The appropriate management response and mitigation options are examined and discussed in subsequent stages of the floodplain risk management process.

Council's current Interim Coastal Hazards Adaptation Code guides development in areas potentially impacted by sea level rise. The draft flood study represents progress towards longer term adaptation guidelines that will replace the interim code.