Greenwell Point Floodplain Risk Management Options Feasibility Study

Greenwell Point Floodplain Risk Management Options Feasibility Study

A Floodplain Risk Management Options Feasibility Study is currently being prepared for Greenwell Point.  Shoalhaven City Council has appointed WMAwater, Water Engineers, to undertake this Study.

Community Workshop Presentation

On the evening of Thursday 26 June 2014, Council held a community workshop at Greenwell Point Community Memorial Hall to discuss the progress of the study and to receive feedback from the community regarding the proposed options.  The workshop was attended by over 60 Greenwell Point residents and interested parties.

Consultants WMAwater provided a 30 minute presentation which was followed by questions and discussion with Council staff and the Consultants from those in attendance.  A copy of the presentation slides can be downloaded from the following link.

Community Workshop Presentation - 26 June 2014 

The Floodplain Management Process

The State Government's Flood Policy aims to reduce the impacts of flooding and flood liability on individual owners and occupiers, and to reduce private and public losses resulting from flooding.  Under the Policy, local government is responsible for managing flood liable land.

The Policy encourages the development of:
solutions to existing flood problems in developed areas and
strategies for ensuring that new devleopment is compatible with the flood hazard and does not create additional flooding problems in existing devleoped areas.

The State Government's Flood Policy provides for technical and financial support for a number of floodplain management activities.  Funding for this study was provided from the State Government's Flood Risk Management Program and Shoalhaven City Council.

What's Happening Now?

The Lower Shoalhaven River Floodplain Risk Management Study and Plan Climate Change Assessment (FRMSP) was adopted in August 2011 which recommended flood mitigation measures for the Greenwell Point area.  Much of Greenwell Point is shown to be at risk from flooding in events as small as the 10% Average Exceedance Probability event (an event which has a 1 in 10 chance of occurring in any one year).

The FRMSP recommended options in response to the flood risk within the Greenwell Point area.  These included flood modification measures (a possible levee around the Greenwell Point waterfront), response modification measures (the raising of Greenwell Point Road to Nowra to provide higher access during a flood event) and property modification measures (changes to planning controls and/or implementation house raising strategies).  This next stage is the preparation of the Floodplain Risk Management Options Feasibility Study which will assess these options, and other potential flood relief measures.

Flooding has not significantly affected Greenwell Point in recent times although there have been a number of minor floods.  Flooding in June 2013 caused flooding above floor to some properties on Adelaide Street as well as flooding of some roads.  Although you may not have personally experienced flooding, flooding will occur in the future and could be several metres deep.

How is Greenwell Point Affected by Flooding?

Greenwell Point is affected by flooding from two sources, the Shoalhaven and Crookhaven River Catchments as well as ocean conditions. A combination of a heavy rainfall event with elevated ocean levels could have devastating effects.  In the future climate change is likely to cause elevated ocean levels which could result in permanent inundation of low lying areas.
Greenwell point is particularly at risk due to its low lying ground levels and waterfront location.  Furthermore, the areas inland of Greenwell Point towards Nowra area also low lying and flood prone and the village would become isolated if these areas were to become inundated.

There does not need to be a very rare event to cause flooding in Greenwell Point.  Flood modelling shows us that in a 10% AEP event flood depths could exceed 1m in places and exceed 2.5m in a 1% AEP event within the village.

Greenwell Point Road, the main access to the village, can be inundated to depths of over 1.5m in the 10% AEP event and depths of up to 3m in the 1% AEP event, making access to and from Greenwell Point impossible.

What does AEP mean?

When referring to the likelihood of a particular flood event occurring, we use a percentage AEP (Average Exceedance Probability) to describe an event. The AEP is the percentage probability that an event of a certain size or larger will occur in any one year.  For example:

  • 10% AEP event is an event which has a 1 in 10 chance of occurring in any one year
  • 5% AEP event is an event which has a 1 in 20 chance of occurring in any one year
  • 1% AEP event an event which has a 1 in 100 change of occurring in any one year
Just because a 1% AEP event may have happened last year, it does not mean that it cannot happen again tomorrow.  For example, in 1949 and 1950 Kempsey, NSW, suffered two 1% AEP events in just two years.

The Floodplain Risk Management Study and Plan Climate Change Assessment mapped flood depths and velocities as well as flood hazard classifications.  This information is useful in flood and emergency management planning such as identifying flooded roads and alternate evacuation routes. A key part of the work is liaising with the SES and updating their response plans to take into accout any new information.

What can be done to reduce flood risks?

Flood Modification Measures
Levees are often used to exclude flood waters form areas and, for a levee to be successful, there needs to be no gaps in it. Levees can be constructed of earth embankments (see picture) or simply walls that can be tied into existing property walls. Through much of Greenwell Point there is public land between private properties and the water line where a levee could be constructed.  However, in some locations negotiations may be required with existing land holders.  A levee surrounding the village could essentially prevent flooding in events up to a specified design event, for example the 10% AEP event.  Levee design would also include consideration of drainage outlets to ensure local runoff ponding behind the levee would not cause significant flood issues.  A significant advantage of levees systems is the safety provided to residents who would otherwise be flooded and the reduction of damages and cost to residents and Council in a flood event. However, levees can affect the aesthetics of and access to a waterfront area.

Flood Response Modification
Raising Greenwell Point Road would mean that the road is inundated for a shorter period of time and will therefore improve evacuations from and access to Greenwell Point during a flood event. Raising the road to a level above the 1% AEP flood level would not be possible as it would involve increasing the road height by several meters.  However, raising the road by a smaller amount to provide longer time for evacuations could be feasible.

Other flood response measures include improving warning and evacuation procedures.

Property Modification Measures
House raising can reduce flood damages by raising people and their belongings above the flood level however, does not solve the issue of isolation. Some houses cannot be raised due to their construction.

Changes to planning controls, such as requirements for minimum flood levels, can be used to protect all new development from flooding.

What will the Floodplain Risk Management Feasibility Study tell me?

Council wants to determine if any of the flood management options are feasible in terms of:
  • How much will the option reduce flood risks and damages?
  • Are the options acceptable to the community?
  • What are the impacts of the options of flood levels in the surrounding area?
  • Is the benefit-cost ratio feasible to warrant any works?
  • Are there any other measures that could be considered to provide flood relief to exsiting development and residents?
  • Are there any additional measures that reduce the stress on emergency services and SES during a flood event?
  • How can future development be protected from flood risk?

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