3.1 Endorsement of Flood Mapping Study...3.1 Endorsement of Flood Mapping Study Abstract The City of Boroondara Flood Mapping Study (the study) was commissioned in 2016 to improve

Services Special Committee Agenda 08/10/18

City of Boroondara 3

3 Presentation of officer reports

3.1 Endorsement of Flood Mapping Study

Abstract

The City of Boroondara Flood Mapping Study (the study) was commissioned in 2016 to improve the understanding of flood risk within the municipality.

The draft report was completed in 2017 and identified approximately 6,200 properties within the municipality that would be affected by flooding during a 1% Annual Exceedance Probability (AEP) rainfall event. The affected property owners and occupiers were notified of the results and offered opportunities to provide feedback. The consultation outcomes are summarised in this report. The study has now been completed and property owners and occupiers who engaged with Council during this process or recently purchased a property in one of the affected areas have been invited to comment on the study outcomes via verbal and written submissions to Council.

A Council resolution endorsing the results of the study is required before the information can be used to officially manage buildings and works in affected areas or to proceed with preparation of a planning scheme amendment required to identify areas of flood risk within the Boroondara Planning Scheme.

Officers' recommendation

The Services Special Committee resolve:

1. To receive and acknowledge the results of the Flood Mapping Study as shown in Attachment 1 and annexed to the Minutes.

2. To receive and note the verbal and written submissions made by property owners and occupiers regarding the outcomes of the Flood Mapping Study.

3. To thank the submitters for their verbal and written submissions.4. To refer the Flood Mapping Study to a future Ordinary Council Meeting for

consideration and determination.



Responsible director: Shiran WickramasingheActing Director Environment and Infrastructure

___________________________________________________________________

1. Purpose

To receive and acknowledge the results of the City of Boroondara Flood Mapping Study (the study) and receive and note verbal and written submissions from property owners and occupiers regarding the results of the study.

2. Policy implications and relevance to community plan and council plan

The study identifies flood risks associated with Council’s drainage catchments and is relevant to the Boroondara Community Plan Theme 1 Your Community, Services & Facilities and Theme 3 The Environment. The results of the study will be used to guide future buildings and works on affected properties in order to minimise the impact of infrequent but intense storms and to inform future capital works programs for stormwater management. This work is reflected in 2018/19 annual commitment "Commence the process of introducing a Special Building Overlay to identify properties at risk of flooding during a 1 in 100 year storm".

3. Background

Overall responsibility for floodplain management within Greater Melbourne rests with Melbourne Water. Areas in Boroondara that are prone to flooding in major storm events were included in a Special Building Overlay (SBO) in the Boroondara Planning Scheme in 2006. However, the areas included in the SBO only cover areas for which Melbourne Water provides drainage infrastructure. In addition to these designated areas, it has been identified that drainage catchments upstream of Melbourne Water’s drains and watercourses are also prone to flooding. The flood mapping study project identifies and addresses flood risks in these catchments.

Council’s Integrated Water Management Strategy 2014 (IWMS) was developed to set the strategic direction and implementation approach for improving water cycle management across the municipality over the next decade, including delivery of a flood modelling program.

In response to IWMS strategies, officers engaged Engeny Water Management (Engeny) to undertake the flood mapping study in 2016. The study represents the first comprehensive hydraulic and hydrologic assessment of Council’s stormwater drains and catchments and provides Council with an understanding of how its drainage systems perform under both frequent and infrequent rainfall events.

The study identified approximately 6,200 properties affected by flooding during a 1% AEP rainfall event. A copy of the study report is included in Attachment 1 to this report.



4. Outline of key issues/options

The study results provide an industry best practice assessment of the risk of flooding associated with a 1% AEP rainfall event. It is therefore important that Council takes necessary steps to ensure property owners and occupiers are made aware of this risk and provides appropriate advice on how to mitigate the risk of flooding.

A resolution by Council to endorse the study results will trigger a notification to the Victorian Building Authority (VBA) of land designated as subject to flooding in accordance with Building Regulation 153.

Once designated, building surveyors must obtain report and consent from Council prior to the issuing of permits for buildings or works on affected land. Consent from Council for the issue of a building permit on affected land will be provided once:

Applicants apply to Council for a flood level assessment; and The design of the proposed buildings or works appropriately addresses

the risks identified by the flood level assessment.

Council’s resolution to endorse the study results will also initiate an amendment to the Boroondara Planning Scheme to identify affected areas within a Special Building Overlay (SBO) for flood management. This project is included in Council's 2018/19 annual commitments as "Commence the process of introducing a Special Building Overlay to identify properties at risk of flooding during a 1 in 100 year storm". Assuming the planning scheme amendment process is initiated in March 2019 and the project progresses without significant delays, the amendment is likely to be gazetted in mid 2020.

The SBO introduced into the planning scheme by Melbourne Water in 2006 is based on flood modelling conducted over 10 years ago. Melbourne Water has since conducted extensive re-modelling of their systems and intends to use their current assessment of flood extent to update the SBO. In order to avoid multiple planning scheme amendments within a short space of time, Melbourne Water has agreed to partner with the City of Boroondara to introduce a joint planning scheme amendment. This amendment will combine the introduction of an SBO2 to represent areas subject to flooding associated with Council’s catchments and an update to the existing SBO representing areas subject to flooding associated with Melbourne Water’s drains and water courses.

Council is currently using the the draft results from the study to advise property owners planning or proposing buildings and works within affected areas of the flooding risk.

5. Consultation/communication

Owners and occupiers of affected properties were notified by direct mail of the draft study results in October 2017. Council officers made a deliberate decision to inform affected property owners and occupiers prior to the completion of the study, to enable them to comment or respond to the study results.



The information provided to affected property owners and occupiers invited them to contact Council officers to discuss the specific impacts to their property. In addition to receiving telephone enquiries, meeting with property owners and occupiers at Council’s office and responding to written enquiries, officers were available at five pop-up council events at various locations throughout the municipality. In total approximately 440 interactions between property owners and occupiers and Council staff were documented between October 2017 and January 2018 via the following channels:

150 - Direct mail or email registered as a CRM case 80 - Attendance at one of the five Pop-Up Council events 210 - Phone or attendance at Council office (mostly by phone)

The main issues raised by property owners and occupiers are outlined below along with the responses put forward by Council officers to each issue:

1. Concerns regarding the inclusion of marginally affected properties and reports of physical attributes within or surrounding the land in question that may alter the forecast flood extent.

The matters raised for each site were investigated and assessed, resulting in minor changes to the flood extents in some instances.

2. Concern that property values will fall. Analysis of property valuations before and after the introduction of the Melbourne Water SBO has revealed that over four years, no discernible difference in value growth occurred between properties within the SBO vs those outside the SBO. This is because identification of a flood risk typically does not prevent the development of land but ensures that properties are developed in a manner that does not exacerbate the flood risk.

3. Concern that insurance premiums would rise. The Insurance Council Australia advised that additional flood insurance premiums only apply where properties are subject to riverine flooding. Domestic insurance policies already cover policy holders for the storm damage associated with flash flooding which is the phenomenon described by the flood mapping study.

4. Concern that drains aren’t properly maintained. The flood risk determined by the study assumes relatively clean drains so the flood extent forecast by the flood mapping assumes that drainage systems are performing to their design standard.

5. Concerns that the problem may be caused by over-development. Flood mapping forecasts the risks associated with extreme rainfall. During such extreme events the ground quickly becomes saturated and the runoff associated with developed land will not differ greatly from the runoff associated with undeveloped land. Nevertheless, Council also requires all new developments to restrict their stormwater discharge into Council systems and detain stormwater on site in underground storage. This on-site detention reduces the impact of the development on the drainage systems during frequent rainfall but also acts to replicate ground absorption during infrequent storms.



6. Concern that drains are too small. Drains are designed for frequent rainfall and it is not efficient or effective to build drains to contain infrequent rainfall events. Such drains require more space than normally available in easements and road reserves and require oversized collection systems that would permanently encroach on road or open space and would be prone to blockage.

7. Commentary that flooding has never happened. The flood mapping study is an assessment of risk. The risk exists even if it has never been realised and it would be negligent to wait for a risk to be realised before acting.

8. Perception that it can’t happen because the built form won’t allow it.It is true that, in some instances, solid structures within the catchment may prevent the flooding to the extent forecast by the study, but such structures are not permanent. The management of redevelopment in affected areas will be aimed at removing structures that are restricting the movement of stormwater flows to restore natural flow paths wherever possible.

9. Commentary that flooding used to happen, but was fixed by the property owners.

Council’s inability to manage flood risks in the past has resulted in property owners taking matters into their own hands and filling over natural flow paths. In many cases this has been done without consent from Council. Such ad hoc flood management is often to the detriment of neighbouring properties.

The consultation led to a comprehensive review of the study results. This review resulted in 35 land parcels containing 58 individual properties being removed from the flood extent and 3 properties being added.

On 12 September 2018, all individuals who have engaged with Council officers during the consultation process were advised in writing of the completion of the study and the intent to request a Council resolution to endorse the study results. The letters also invited verbal and written submissions regarding the flood mapping study outcomes to this SSC meeting. Similar information and invitations have aslo been sent to 267 owners who purchased affected properties between October 2017 and August 2018 and may not have had an opportunity to engage with Council during the consultation process.

6. Financial and resource implications

The cost of the flood mapping project was included in the 2017/18 budget. The future cost of providing flood level assessments will be recovered by the charging of a flood assessment fee for each application.

In the event Council adopts the study, the cost associated with the introduction of the new SBO controls will be covered by the current 2018/19 budget allocation.

The results of the study will also be used to develop and prioritise future drainage capital improvement works.



7. Governance issues

Staff involved in preparation of this report do not have any direct or indirect interest in the outcomes of the flood mapping study.

No human rights would be impacted or infringed as a consequence of the Outcomes and Next Steps of this report.

8. Social and environmental issues

The flood mapping study provides Council with a best practice assessment of the flood risk associated with Council’s stormwater drainage network. As such, it allows informed decisions to be made which will improve the amenity and sustainability of future development and works within the City of Boroondara.

9. Conclusion

The completion of the City of Boroondara Flood Mapping Study provides Council with an industry best practice assessment of flood risk associated with Council’s stormwater drainage systems. This assessment will be used to develop and prioritise future capital works for drainage improvements and ensure that future buildings and works proposed in affected areas are designed to mitigate flood risk.

Council resolutions are required to endorse the study results and support the preparation of a Planning Scheme Amendment to designate areas of flood risk in the Boroondara Planning Scheme. Councillors are requested to review and note the submissions made from property owners and occupiers to the study results prior to consideration of these resolutions which will be proposed at a subsequent Council meeting.

Manager: Zhanna Sichivitsa, Asset Management

Report officer: Stephen D’Agata, Team Leader Drainage

Boroondara City Council

Catchment Modelling in BoroondaraTechnical Report

July 2018

V2002_002


City of Boroondara Attachment 3.1.1 9

BOROONDARA CITY COUNCIL CATCHMENT MODELLING IN BOROONDARA

:

DESCRIPTION AUTHOR REVIEWER APPROVED BY DATE

DISCLAIMER

This report has been prepared on behalf of and for the exclusive use of Boroondara City Council

and is subject to and issued in accordance with Boroondara City Council instruction to Engeny

Water Management (Engeny). The content of this report was based on previous information and

studies supplied by Boroondara City Council

Engeny accepts no liability or responsibility whatsoever for it in respect of any use of or reliance

upon this report by any third party. Copying this report without the permission of Boroondara City

Council or Engeny is not permitted.



CONTENTS

Appendices






1. EXECUTIVE SUMMARY







2. INTRODUCTION

2.1 Study Overview

2.2 Study Objectives




2.3 Study Area



Figure 2.1




3. BACKGROUND INFORMATION

3.1 Technical Methodology Overview

3.2 Available Data




3.3 Site Inspection

Appendix A




4. HYDROLOGY

4.1 Overview

4.2 RORB Model Development

Appendix B




5. HYDRAULIC MODEL DEVELOPMENT

5.1 Overview




5.2 TUFLOW Model Development

Appendix C

Appendix C1




6. FLOOD MAPPING

6.1 Flood Extents

Appendix D




6.2 Other Mapping Deliverables




7. COMMUNITY CONSULTATION AND RESULTS VERIFICATION







8. CONCLUSIONS AND RECOMMENDATIONS




9. QUALIFICATIONS




10. REFERENCES



Documents

3.1 Endorsement of Flood Mapping Study...3.1 Endorsement of Flood Mapping Study Abstract The City of Boroondara Flood Mapping Study (the study) was commissioned in 2016 to improve