Seven Mile Beach Drainage and Groundwater

William C. Cromer Pty Ltd was engaged to report on groundwater atSevenMileBeachand make recommendations to Council on actions to address and mitigate potential flooding due to groundwater table rise.  Mr Cromer has produced a report which further analyses groundwater levels and based on the findings in the report, Mr Cromer made a series of recommendations.  Council, as part of its 2012/2013 Operating Plan, has set aside funds to implement these recommendations with the following specific works to be undertaken in the Seven Mile Beach Township this financial year;

  • Install an underground pipe inLewis Avenueat the corner ofWoodhurst Roadto prevent the need of installing a road hump if pumps are required again;
  • Construct a gravity stormwater pipeline from theLewis Avenue/Woodhurst Roadintersection across the Royal Hobart Golf Club practice fairway to Council land in the Acton Creek tributary. (This is subject to agreement with the Royal Hobart Golf Club).
  • Install additional bores as per the Cromer Report (these have already been installed);
  • Undertake extended monitoring of all bores in theSevenMileBeachTownshipand
  • Develop a numeric groundwater model for the aquifer underlying theSevenMileBeachTownship.

In addition, Sloane Geoscience Pty Ltd was engaged to monitor the groundwater quality in the township; with particular emphasis on assessing any impacts from domestic wastewater systems.

A copy of both these reports are available to download below:

The details of the recommendations from Mr Cromer’s report “Groundwater Level Monitoring 4th Progress Report July 2012” along with Council’s response are shown in ATTACHMENT 1 - Groundwater Level Monitoring 4th Progress Report(10 kb).  As part of the original 2010 report there was a recommendation to undertake a benefit/cost analysis of permanent pump stations to lower ground water.  In order to determine if benefits would accrue it was necessary to obtain more data on the groundwater system and in particular how it responded to rainfall events.  Essentially the numerical groundwater model was developed to provide some additional certainty in determining which options will give the best result in terms of lowering groundwater during extreme events. 

Mr Cromer has now completed a further report “Seven Mile Beach Groundwater Level Monitoring 5th Progress Report, March 2013” which updates groundwater level monitoring results at Seven Mile Beach since June 2012.  The report details:

  • the installation of two extra water level data loggers in new bores 15d and 16 at the south western end of the Royal Hobart Golf Club eastern soak in November 2012,
  • the relationship between rainfall and surveyed water levels in monitoring bores and soaks,
  • the relationship between rainfall and groundwater levels in the Royal Hobart Golf Course eastern soak,
  • the relationship between rainfall and water levels at the mouth of Acton Creek,
  • results of November 2012 drilling near the Royal Hobart Golf Club eastern soak,
  • the results of a seven day pump test of the eastern soak in early December 2012, during which time about 3.3ML of water were discharged to Acton Creek, and
  • the results of March 2013 sampling and analysis of water in Acton Creek.


A separate accompaniment to Mr Cromer’s report is the “Seven Mile Beach Groundwater Model Report, February 2013” produced by Mark Hocking which describes the development of the Seven Mile Beach groundwater model. 

A copy of both these reports are available to download below:

In summary the findings of the latest monitoring and modelling reports are:

  • Flooding at Seven Mile Beach is caused by a mixture of:
    • rapid increase in groundwater levels due to short-term high intensity rainfall events,
    • low lying flat land,
    • a shallow groundwater system and
    • the slow rate of fall in groundwater levels of about 2 millimetres/day.
  • Direct rainfall is the primary contributor to groundwater level, and evapotranspiration is the primary method for decreasing the groundwater levels,
  • Mean annual simulations show flooding is not a regular issue,
  • One management activity alone is unlikely to stop flooding,
  • Constantly lowering of the Royal Hobart Golf Club soak via pumping is a cost effective means of lowering the groundwater level in the area most effected by flooding,
  • Anticipated sea level rise (0.3m by 2050) will raise groundwater level.  The impact will extend inland approximately 300 metres and cause the backup of water in Acton Creek, and
  • Dams do not contribute to flooding issues at Seven Mile Beach.

Following the development of the groundwater model and the finding that lowering of the Royal Hobart Golf Club soak via pumping is a cost effective means of lowering the groundwater level in the area most effected by flooding Council is now finalising the design details for the pump station and associated rising main from the Royal Hobart Golf Club soak to the Acton Creek tributary.  The works involve:

  • Installation of a pump station at the Royal Hobart Golf Club soak;
  • Installation of a pump station in the vicinity of the Lewis Avenue/Woodhurst Road intersection, and
  • Construction of a rising main from the Royal Hobart Golf Club soak to connect to the pump station in the vicinity of Lewis Avenue/Woodhurst Road intersection and then to the Acton Creek tributary.

It is anticipated that works will commence in late April subject to weather and availability of plant and equipment to undertake the project.

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