An industry norm for the acceptability of non-core flood risks is required.
image
Floodplain management plans only fully consider some flood risks, including over-floor flooding and risk to life. This allows simple lines to be drawn on maps for town planning purposes, but overlooks what needs to be considered when fully evaluating flood risks. By Steven Molino.

It also means there is no nationally adopted standards to guide developers, planners, regulators and the courts around the acceptability of non-core flood risks. These include isolation, roads being cut, infrastructure shutdown and building failure, which have been less explicitly dealt with in floodplain management plans.

As part of Moreton Bay Regional Council’s (MBRC) development of comprehensive, consistent floodplain management across its local government area, GHD and Molino Stewart were engaged to develop a framework for floodplain risk management.

A set of risk tables was developed showing what combinations of hazard and probability were acceptable, tolerable and unacceptable. It considered the following risk categories:
•    Risk of isolation
•    Risk to road access
•    Risk to life in residential buildings
•    Risk to life in non-residential buildings
•    Risk to residential property
•    Risk to non-residential property
•    Risk to critical infrastructure

It is common to define risk as being a function of both probability and consequence (or hazards). Most possible contributors to flood hazards are being considered in the MBRC project, but the point at which they are considered in the risk assessment process varies.

Hydraulic hazard is recognised as a major contributor to flood hazard. There are recognised thresholds where the combination of depth and velocity have different consequences for different things placed in the floodplain.

The MBRC project is defining the hazard using both the hydraulic hazard category and, where relevant for that particular type of risk, the other hazards which contribute to the overall hazard rating. So what are they?

Flood duration, or the period a building or community is cut off by floodwaters, can have a significant impact on the costs and disruption associated through stress, supplies and medical requirements.

Another consideration in assessing consequences is vulnerability. This is taken into account to some extent in hazards diagrams, which recognise there are thresholds above which all people are vulnerable or timber framed buildings are vulnerable to flooding, but not all people and all timber framed buildings are the same.

The consequences will also differ depending on the use to which an asset is put. For example, the consequences for a community will be different if a hospital is closed than if a commercial operation is closed, at least in the short term.

The issue of cumulative consequences must also be taken into consideration. If one home is flooded during a major storm event, the consequences are different at a societal level than if 1,000 buildings are flooded, even if the chance of them being flooded were the same.

Taking the above hazard and probability issues into consideration, draft risk tables were created for each risk category.



Risk to Life - all residential buildings in the floodplain.


The process proposed for MBRC is the risk tables be populated with appropriate risk management measures to determine whether additional risk management measures are required for existing risks and to manage future risks.

The potential options to mitigate 'Risk to life - all residential buildings in the floodplain' are:

1.    Warning system, community education program and evacuation plan which can be demonstrated to evacuate all people to a location outside of the flood affected area before evacuation routes are cut by H2 flooding.
2.    Able bodied occupants are able to walk to a flood free location ahead of rising floodwaters should they not evacuate until floodwaters enter the premises
3.    The building is flood resistant, there is a flood free refuge within the building and there is sufficient clean water, food and emergency power supply for the duration of the flood and there is a practical means of medical evacuation
4.    Voluntary purchase of building
5.    The building is flood resistant and ground floor level is above peak flood level
6.    The building is flood resistant and there is a flood free refuge within the building



Mitigation measures: Risk to Life - all residential buildings in the floodplain


This work is currently in its early stages and more discussion is needed within the floodplain management profession to better define acceptable and tolerable risk thresholds.

The advantages of the proposed methodology are that it is suitable for existing and future development, it considers existing and residual risk and it is outcomes focused rather than prescribing risk management measures.

Based on a paper to the Floodplain Management Association Conference in February by Steven Molino (Molino Stewart), Steve Roso (Moreton Bay Regional Council) and Giorgis Hadzilacos (GHD). Find the full paper at www.molinostewart.com.au