Flooding and Landuse Planning

Flood and Sea Level Rise Policy and related amendment to Bega Valley Development Control Plan 2013.

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Flood Information

Council’s flood mapping shows the extent of land identified as flood prone throughout the Bega Valley Shire. 

Flood Planning Area (FPA) means the area of land where there is a 1% chance of a flood of a given or larger size occurring in any one year. This includes 2100 sea level rise (where relevant)  and rainfall increases, as well as 0.5m freeboard for flood risk management purposes.

Probable Maximum Flood (PMF) means the largest flood that could conceivably occur at a particular location, usually estimated from probable maximum precipitation, and where applicable, snow melt, coupled with the worst flood-producing catchment conditions.

Flood prone land is divided into four flood planning constraint categories (FPCCs) defined in Table 1:

• FPCC1: The majority of development types are likely to be excluded from FPCC1 due to the high flood risks to the development and potential for the development to impact flood behaviour.
• FPCC2: Next least suitable area for more intense development due to the impacts of flooding on the land and the consequences to the development and its users.
• FPCC3: More suitable for increased density of development and expansion of existing development when flood related development controls are met. Consideration may be given to restricting emergency response facilities and vulnerable land uses in some of these areas.
• FPCC4: Special flood considerations to certain types of development may apply to some areas in FPCC4.

Table 1 – Flood Planning Constraint Categories (FPCCs)

Flood planning constraint category	Where no FPCC mapping is available, development controls apply to:
FPCC1	DFE* Floodway and Flood Storage DFE H6 Hazard
FPCC2	Outside of FPCC1 0.5% AEP Floodway DFE H5 Hazard Isolated areas (if known)
FPCC3	Outside of FPCC2 Within the Flood Planning Area (FPA) (DFE plus a Freeboard of 0.5m)
FPCC4	Outside of FPCC3 Within the Probable Maximum Flood (PMF)

^* Defined Flood Event (DFE) is equal to the 1% AEP event including 0.9m sea level rise

When is a Flood Management Report required?

A Flood Management Report must be submitted with any Development Application on flood prone land (with exceptions noted below), for Council to consider the potential flood impacts and applicable controls. For Residential or Commercial development, it is required for development on land identified within the FPCC1, FPCC2 and FPCC3 areas. For Vulnerable or Critical development, it is required if it is within any FPCC.

There are some circumstances where a formal Flood Management Report undertaken by a professional engineer may not be required. However the relevant parts of Bega Valley Development Control Plan 2013 (DCP 2013) and Bega Valley Local Environmental Plan 2013 (LEP 2013) would still need to be addressed, so as to demonstrate compliance. Examples where this may apply include:

• If all proposed works are located outside the relevant Flood Risk Precinct extent
• First floor addition only, where the floor level is above the Probable Maximum Flood level
• Internal works only, where habitable floor areas below the FPL are not being increased

Note that development on flood prone land will still be assessed for compliance with the relevant DCP and LEP and may still be subject to flood related development controls.

What is the purpose of a Flood Management Report?

The purpose of a Flood Management Report is to demonstrate how a proposed development will comply with flood planning requirements, particularly the development controls outlined in the relevant LEP and DCP clauses. The report must detail the design, measures and controls needed to achieve compliance, following the steps outlined below.
A Flood Management Report should reflect the size, type and location of the development, proportionate to the scope of the works proposed, and considering its relationship to surrounding development. The report should also assess the flood risk to life and property.

Preparation of a Flood Management Report

The technical requirements for a Flood Management Report include (where relevant):

1. Description of development

• Outline of the proposed development, with plans if necessary for clarity
• Use of the building, hours of operation, proposed traffic usage or movement
• Type of use as per the DCP use categories, i.e. Critical Uses and Facilities, Sensitive Uses and Facilities, Land Subdivision, Increasing dwelling density, Residential, Commercial and Industrial, Recreational and Non-Urban, and Alterations and Additions.

2. Flood analysis

• 1% AEP flood level (with and without 0.9m sea level rise)
• Flood Planning Level (FPL)
• Probable Maximum Flood (PMF) level
• Flood planning constraint category
• Mapping of relevant extents
• Flood characteristics for the site, e.g. depth, velocity, hazard and hydraulic category, and the relevance to the proposed development

3. Assessment of impacts

• Summary of compliance for each category of the DCP:
  • Minimum Floor Level
  • Minimum Ground Level
  • Building Components
  • Structural Soundness
  • Flood Affectation
  • Emergency Response
  • Management and Design
  • Parking and Driveway Access.
• Demonstration of how the development complies with any relevant flood planning requirements from the DCP and LEP.
• For any non-compliance, a justification for why the development should still be considered.
• Calculations of available flood storage if compensatory flood storage is proposed.
• Plan of the proposed development site showing the location of the development, predicted 1% AEP (with and without sea level rise) and PMF flood extents, as well as any high hazard or floodway affectation
• Development recommendations and construction methodologies
• Qualifications of author - Council requires that the Flood Management Report be prepared by a suitably qualified Engineer with experience in flood design / management who has, or is eligible for, membership to the Australian Institute of Engineers.
• Any flood advice provided by Council
• Any other details which may be relevant.

Planning Requirements for Development on Flood Prone Land

Development must comply with requirements set out in the relevant Local Environment Plan (LEP) and Development Control Plan (DCP).

The clauses specific to flooding in the LEPs and DCPs are as follows:

LEP Clauses:

• 5.21 – Flood Planning
• 5.22 – Special Flood Considerations

DCP Section:

• 5.8.1 – Flood Planning

Information on some specific controls is presented below.

Floor Levels

Residential floor levels are generally required to be set to the FPL (1% AEP including 0.9m SLR plus 0.5m freeboard). While the minimum floor level requirements for critical and sensitive uses should be set at the PMF level (where it is higher than the FPL).

There are some concessions provided for minor alterations to existing developments and commercial developments.

Minimum Ground Levels

In low lying areas that may be impacted by more frequent tidal inundation with sea level rise, minimum ground levels have been included in the controls to ensure the ongoing viability of the land and contributing to adaptation of the wider area landform over time (e.g. raising of roads and services).

Building Components and Structural Soundness

Building components for all development types are required to be flood compatible above the 2100 1% AEP + 0.5m. Where the PMF is higher than this level, the PMF is applied to Critical and Sensitive Uses and Facilities.

Likewise the proposed structure for all development types must be able to withstand the forces of floodwater, debris and buoyancy up to and including the 2100 1% AEP + 0.5m. The PMF should also be considered for:

• Residential, Commercial and Alterations / Additions within FPCC2, due to the high risk flood conditions present, and to allow for safe shelter in place conditions in events greater than the 1% AEP.
• Critical and Sensitive Uses and Facilities.

Flood Affectation

All development types may not result in significant increase of flood risk elsewhere in the floodplain for a full range of flood events up to the 1% AEP flood event (with and without 0.9m sea level rise), having regard to: a) loss of flood storage, b) changes in flood levels, flows and velocities upstream, downstream and adjacent to the site, c) cumulative impact of multiple development in the vicinity.

Emergency Response

The objective of the LEP 2013 Clause 5.21 is to enable the efficient evacuation of people in the event of a flood. Further the LEP 2013 states that development consent must not be granted to development on land unless it will not adversely affect the safe occupation and efficient evacuation of people or exceed the capacity of existing evacuation routes for the surrounding area in the event of a flood.

To this effect, the following development controls have been recommended:

• No increase in development density within high risk flood areas (FPCC1 and FPCC2)
• Flood free access to be provided up to the PMF for proposed Critical and Sensitive development and for any development that increases dwelling density in the floodplain.
• Reliable egress for pedestrians or vehicles is required up to the PMF for all other residential, commercial and industrial development types. Reliable egress refers to H1 and H1 Hazard Classifications up to and including the PMF.
• Shelter in place may be considered where the flood free or reliable access requirements cannot be met, and the following conditions can be satisfied:
  • The duration for flood inundation is less than six hours
  • The development is not located in an area of high-risk (e.g., floodways and H5 or H6 flood hazard areas up to the PMF)
  • Access to on-site systems to provide power, water and sewerage services during and beyond the event for the full range of flooding
  • The location of storage of food, water and medical emergency for SIP purposes should be above the PMF level and available during and beyond the event for the full range of
  • flooding
  • Shelter in place floor level is above PMF
  • Shelter in place provides a minimum 2m² floor space per person
  • Shelter in place must be structurally safe and accessible during floods up to the PMF.

Management and Design

Ongoing consideration of flood planning and response is required. These controls relate to developing a suitable Flood Safe Plan for homes and businesses or a site specific Emergency Response Flood Plan for high flood risk location, commercial, critical and sensitive land uses.

Parking

The minimum floor level of parking is generally required to be as high as practical but no lower than the 5% AEP flood, 2m AHD or crest level of the road at the location where the site has access (whichever is higher). Higher levels are required for some garage types and consideration of lower levels is provided for minor developments (e.g. Alterations and Additions).

Where basement carparking is proposed, the entry crest level shall be as high as possible but no lower than the  1% AEP flood, 2m AHD or 300mm above the road crest level at the location where the site has access (whichever is higher). If the level of the PMF is higher than the proposed entry crest level, than a flood emergency response plan should also be prepared to demonstrate how flood risk in the car park in the car park can be managed. This should include  signage and access/egress for the full range of floods and may incorporate adequate flood warning.

Flood Impact and Risk Assessment

A flood impact and risk assessment (FIRA) may be required by Council to support a development application. A fit for purpose FIRA can examine flood constraints and how to manage the flood risks posed to and by new development.

A FIRA may be required when development is proposed on land that is considered to have flood risk but no existing flood study exists, or in locations where the overland flow hazard is considered high. In these cases the FIRA would need to demonstrate that the proposed development complies with the development controls set out in Council’s LEP and DCP.

Additionally, a FIRA may be required where development is proposed within the floodplain and may have adverse impacts on flood risk (e.g. an increase in flood level or hazard on an adjacent property).

The aim of a FIRA is to support a development proposal to identify and analyse:

• the impacts of the proposed development on the flood risk to the existing community
• the impacts and risks of flooding on the development and its users
• how these impacts can be managed to minimise the growth in risk to the community due to the development.

A FIRA may also be required to provide information and define flood behaviour and constraints where there are gaps in available information.

A FIRA should be undertaken in accordance with the guidance provided in the Flood Risk Management Guide LU01: Flood Impact and Risk Assessment.

A FIRA should be undertaken by an appropriately qualified professional engineer. They should have experience and advanced skills in catchment hydrology, floodplain hydraulics and have a good working knowledge of FRM practices and guidance in New South Wales. The qualifications needed to undertake a FIRA may depend on the assessment requirements for the development proposal.