What is a flood study and why is it important?

What is a Flood Study?

The Lower Macleay Valley is subject to significant flooding. This study determines the nature and extent of the flood problem and will inform the development of a Floodplain Risk Management Study and Management Plan to be developed by December 2020

This is accordance with the NSW Government's "Floodplain Development Manual: the management of flood liable land" (April 2005)

Why is a flood risk management plan needed?

Under the NSW State Government’s Flood Policy, the management of flood liable land is the responsibility of local government (Council). As part of their normal planning responsibilities, Councils need to plan and manage flood prone land in accordance with its flood exposure.  Council is also responsible for land use planning and management under the Environmental Planning and Assessment Act (EP&A 1979).  A flood risk management plan provides input into strategic and statutory planning roles of Council.

Why update the old flood planning documents relating to the Lower Macleay ?

Previous studies and plans produced for the Lower Macleay River floodplain were done prior to the adoption of the NSW Flood Policy and Manual 2005 and therefore this requires Council to update the study and associated documents 

The Manual sets out the mandatory processes required for preparing Flood Studies, Floodplain Risk Management Studies and Plans as well as detailing roles and responsibilities for the management of flood prone land.

Additionally, significant computer technology advancements have occurred, tidal and ocean storm surge data has been revised, and higher resolution base terrain and bathymetry (below water surface) level datasets are now available all of which can be deployed to develop a model to estimate flood behaviour in detail far greater than previously undertaken. For instance, the old model produced data at a scale of kilometres whereas the new model produces data across a 5m grid in urban zones and 20m grid in rural zones.


How will the study results be used?

The results of the study will inform the development of a Floodplain Risk Management Study that will evaluate the management options with consideration of social, ecological and economic factors relating to flood risk. This will cover both existing and future development. For example the option and costs associated with raising existing flood levees. 

This will in turn inform a plan of management for the Floodplain, and determine the actions Council will take to reduce the risks associated with flood events. For instance this may include mitigation works, planning controls, flood preparedness and warnings and rehabilitation. 

Where does this Flood Study cover?

The Draft Flood Study for the Lower Macleay River floodplain from Frederickton to South West Rocks to Stuarts Point in the north and Crescent Head in the south. 

Who has been involved in this project so far?

In accordance with the guidelines Council has formed a Floodplain Risk Management Committee. This committee is representative of Council, NSW Department of Planning, Industry and Environment, NSW State Emergency Services, and four private long-term landowners on the Lower Macleay. 

The Draft Flood Study was prepared by Specialist Consultant Engineers (JACOBS) and developed in accordance with the NSW Government’s Flood Prone Land Policy and Floodplain Development Manual (2005).

What happens next ?

Following the exhibition period, all submissions received will be reviewed by the Committee and consultant to determine responses and amendments required to the Draft Flood Study. The Final Flood Study Report will be presented back to Council during the December 2019 Ordinary Meeting with a recommendation for adoption. Once adopted, Council can proceed to future stages in the process.

We are expecting a Lower Macleay River Floodplain Risk Management Plan to be completed late 2020. 

Other FAQs

What’s the difference between a flood risk management study and a flood risk management plan?

A flood risk management study draws together the results of the hydraulic report and data collection process. It provides information and tools to allow strategic assessment of the impacts of management options for existing, future and continuing flood behaviour and risk. 

A flood risk management study evaluates management options for flood prone land in respect of both existing and proposed development having regard for social, ecological, economic factors which relate to flood risk. It also provides the basis for robust decision making in the flood risk management plan. A flood risk management plan is the formalisation of the flood risk process. It is based on evaluation of all factors that affect and are affected by flood events and represents considered opinion on how to best manage flood risk. The objective of the flood risk management plan is to present the prioritised and costed flood risk management measures recommended for implementation.


Does the current Flood Mitigation Scheme actually work ?

Yes to an extent. The results indicated that the infrastructure (levees, major gates, control structures and cut diversion drains) serves a critical role as intended during frequent minor floods like the 0.2 EY event. However they have less influence on major floods such as the 1% AEP due to submergence effects

Why is flooding in the Lower Macleay River floodplain so severe ?

The catchment upstream of Kempsey is very steep with extensive gorges. The runoff potential is high and all flood flow is funnelled towards Kempsey where upstream near Aldavilla the topography/terrain forms a constriction that concentrates high energy flows. 

Major overtopping of the riverbank occurs within Kempsey, Hampden Hall, and the Seven Oaks bend distributing enormous flow rates and floodwater volumes to the floodplain. The river entrance channel acts as a hydraulic control and it was found that the dominant source of flooding within the entire lower floodplain is riverine inflows. 

Tidal ocean storm surge and Maria / Hastings River back-flooding mechanisms were found to be less dominant in producing peak flood levels.


Has Climate Change been considered ?

Yes. The results indicate significant increases in flood level and hazard that needs to be considered during the Floodplain Risk Management Plan stage of which will identify considerations to revise current land-use planning and controls

The river appears to be silting up – does this impact on flooding ?

Yes. There is an increase in sediment in the river due to changes in land-use from forests to grazing, degrading riparian zones. This increases the risk of flooding in an event. Dredging of critical zones will be assessed as a management option in the development of the Flood Plain Risk Management Study. 

Glossary

The terminology is confusing – what does EY and AEP mean ?

This flood study report discusses flooding events in relation to the probability of a certain magnitude flood event occurring during any given year. The main terms used to describe design flooding are:

·  Exceedances per year (EY): the number of times an event is likely to occur or be exceeded within any given year.

·  Annual exceedance probability (AEP): the probability or likelihood of an event occurring or being exceeded within any given year, usually expressed as a percentage.


What is “m AHD” ?

Height in metres above the Australian Height Datum. The datum that sets mean sea level as zero elevation. Mean sea level was determined from observations recorded by 30 tide gauges around the coast of the Australian continent for the period 1966–1968. All flood levels reported within the Study are stated to “m AHD”.


What is a Hydraulic Report?

A flood hydraulic report is a technical investigation of flood behaviour that determines the nature and extent of flood problems. It constitutes the major foundation for flood risk management studies and plans.