Agenda of City Infrastructure Committee Meeting

Agenda (Open Portion)

City Infrastructure Committee Meeting

Page 5

26/8/2020

City Infrastructure Committee Meeting (Open Portion) held Wednesday, 26 August 2020 at 5:15 pm.

This meeting of the City Infrastructure Committee is held in accordance with a Notice issued by the Premier on 3 April 2020 under section 18 of the COVID-19 Disease Emergency (Miscellaneous Provisions) Act 2020.

COMMITTEE MEMBERS

Harvey (Chairman)

Lord Mayor Reynolds

Deputy Lord Mayor Burnet

Behrakis

Ewin

NON-MEMBERS

Zucco

Briscoe

Sexton

Thomas

Dutta

Sherlock

Coats

Apologies:

Leave of Absence: Nil.

1. Co-Option of a Committee Member in the event of a vacancy

2. Confirmation of Minutes

The minutes of the Open Portion of the City Infrastructure Committee meeting held on Wednesday, 29 July 2020, are submitted for confirming as an accurate record.

3. Consideration of Supplementary Items

Ref: Part 2, Regulation 8(6) of the Local Government (Meeting Procedures) Regulations 2015.

Recommendation

That the Committee resolve to deal with any supplementary items not appearing on the agenda, as reported by the General Manager.

4. Indications of Pecuniary and Conflicts of Interest

Ref: Part 2, Regulation 8(7) of the Local Government (Meeting Procedures) Regulations 2015.

Members of the Committee are requested to indicate where they may have any pecuniary or conflict of interest in respect to any matter appearing on the agenda, or any supplementary item to the agenda, which the Committee has resolved to deal with.

5. Transfer of Agenda Items

Regulation 15 of the Local Government (Meeting Procedures) Regulations 2015.

A Committee may close a part of a meeting to the public where a matter to be discussed falls within 15(2) of the above regulations.

In the event that the committee transfer an item to the closed portion, the reasons for doing so should be stated.

Are there any items which should be transferred from this agenda to the closed portion of the agenda, or from the closed to the open portion of the agenda?

Item No. 6.1

Agenda (Open Portion)

City Infrastructure Committee Meeting

Page 13

26/8/2020

REPORT TITLE: Weerona Avenue, West Hobart - Response to Petiton - Stormwater Concerns

REPORT PROVIDED BY: Program Leader Stormwater Assets

Manager Stormwater

Director City Amenity

1. Report Purpose and Community Benefit

1.1. The purpose of this report is to respond to a petition of 23 signatories, tabled at the Council meeting held on 21 October 2019 calling on the City to

…construct open stormwater drains along the back of Weerona Avenue residences on the western side of the street and any other necessary measures to mitigate the threat of flood damage for all residences in the street.

2. Report Summary

2.1. The petition relates to resident concerns with respect to stormwater overland flows entering properties from Knocklofty Reserve, located behind the properties.

2.2. Investigations identified no existing infrastructure with the potential to concentrate flows upslope from the Weerona Avenue properties (aside from 6 stormwater pits/grill located along a section of a right-of-way walking track) and therefore any overland flows are result from only the natural non-concentrated sheet flow from the Reserve.

2.3. Accordingly, the City is not liable or required under the Urban Drainage Act 2013 to implement stormwater mitigation measures.

3. Recommendation

That:

1. In accordance with the Urban Drainage Act 2013, as any overland flows arising from Knocklofty Reserve (located upslope from Weerona Avenue, West Hobart), are non-concentrated sheet flow originating the non-urban (natural bush catchment). Therefore the City is neither liable nor required to intervene with stormwater mitigation measures, as sought by the petition tabled to the Council at its meeting held on 21 October 2019.

2. The Petitioners be advised of the Council’s decision.

4. Background

4.1. A petition of 23 signatories, was tabled at the Council meeting held on 21 October 2019 calling on the City to

…construct open stormwater drains along the back of Weerona Avenue residences on the western side of the street and any other necessary measures to mitigate the threat of flood damage for all residences in the street.

4.2. Further to this petition a resident response was submitted on the 28 of July 2020, being detailed in Attachment A.

In reply to this response, the report originally tabled to the City Infrastructure Committee has been corrected and updated to address the additional points raised in the response.

4.3. The petition relates to resident concerns with respect to overland flows entering properties from Knocklofty Reserve.

4.4. Prior to the receipt of the petition, the City was not aware of any detrimental overland flows impacting the petitioning properties outside of a known land subsidence at 27 Weerona Avenue, above an existing right-of-way.

4.4.1. The City actively monitors the flood levels in its major rivulet systems, however it is not practical or realistic to monitor overland flows outside of waterways, or surface runoff from bushland, or seepages from top soil or subsoil.

4.5. To investigate the petition, Officers undertook a series of desktop and field investigations, including review of historical work orders, site and asset inspections, and hydraulic modelling of the infrastructure requested by the petitioners.

4.6. The investigations identified no existing infrastructure with the potential to concentrate flows upstream from the Weerona Avenue properties (aside from 6 stormwater pits/grill located along a section of a right-of-way walking track, that also houses a TasWater main). Any overland flow from the Reserve therefore is the result of natural non-concentrated sheet flow from the vacant bushland area.

4.7. Accordingly, the proposition that liability is carried by the City is not supported.

The Urban Drainage Act 2013 places obligation on the City to manage urban area, such an area is defined under the Urban Drainage Act 2013 as the inner city plus built-up environs.

4.8. Should the Council decide to undertake an intervention to divert flows from the requested cut-off drain, there could be detrimental impact on the downstream network, with substantial additional pipe upgrades required to prevent increasing risk to downstream residents.

4.9. The cut-off drain would also impose a future maintenance burden and new ongoing liabilities on the City.

Obligations under the Urban Drainage Act 2013

4.10. The City is obliged to maintain its stormwater infrastructure and manage the risk of urban flood associated with concentrated stormwater flows.

Maintenance of nearby stormwater system

4.10.1. Following the May 2018 flood event that resulted in damage to the right-of-way track and occlusion of the stormwater pits, the pits were subsequently cleared with an enhanced maintenance program now established to preserve their functioning.

4.10.2. In respect to Mount Stuart Road and the Fiona Allan Memorial Walkway, previous independent investigations confirmed that neither asset results in an increase in the collection nor concentration of stormwater flows, with the grading of both assets diverting flow away from the petitioning properties.

Mitigation works at nearby properties

4.10.3. The right-of-way, listed to both the City and TasWater, was established to provide maintenance access to the water main and existing stormwater pits, but is subsequently also used by the public for access to Knocklofty Reserve.

4.10.4. The stormwater system in the right-of-way, consisting of five grated inlet pits and a grated overflow pit and including the works installed as part of the subdivision at 66 Summerhill Road, was constructed to collect the concentrated surface run-off along the right-of-way before it enters Weerona Avenue.

4.10.5. This work, including the work done in collaboration with the developer of 66 Summerhill Road, was undertaken because the City deemed that the right-of-way created the potential to concentrate run-off - resulting in obligations to the City under the Urban Drainage Act 2013.

27 Weerona Avenue - Subsidence Risk

4.10.6. Previous investigations have identified that the risk of subsidence (located above the right-of-way) is the result of the area’s geology and the removal of native vegetation by the private property owner within the property boundary.

4.10.7. TasWater has undertaken its own investigations and determined that the water main located within the right-of-way is stable and will not contribute or increase any future risk of subsidence.

5. Response to Petitioner’s Submission

5.1. On 28 July 2020 the petitioners lodged a supplementary submission to the Council (Attachment A).

5.2. This report has been reworded to clarify the City’s position in response to the points raised in the submission, including correcting the clerical error of the date of the Urban Drainage Act 2013 (erroneously referred to as 1993 in the earlier report).

5.3. In further response to the supplementary submission: the primary point of contention relates to the responsibility of the City to manage non-urban natural non-concentrated sheet flow (in this instance from the from bushland area).

In response the following is noted:

5.3.1. The City’s power and obligation under the Urban Drainage Act 2013 extend to stormwater only – that is runoff that has been concentrated by means of a drain or formed surface.

5.3.2. The runoff experienced at the Weerona Avenue properties is natural, non-concentrated sheet flow. This has been verified by an independent report.

5.3.3. The City does not have the power or the obligation under the Urban Drainage Act 2013 to manage non-urban natural non-concentrated sheet flow – in the same way that the City does not have the power or the obligation to manage ground water, even though both these phenomenon can at times cause issues at individual properties.

5.3.4. Based on the City having no obligation under the Urban Drainage Act 2013 subsequent obligation under Guide for Local Government in Tasmanian on Stormwater System Management Planning do not apply.

5.3.5. Where runoff has been concentrated by means of a drain or formed surface (such as above 51 Summerhill Road or the 66 Summerhill Road subdivision) the City has taken steps to mitigate the risk in accordance with its powers and obligations under the Urban Drainage Act 2013.

6. Proposal and Implementation

6.1. That in accordance with the Urban Drainage Act 2013, as any overland flows arising from Knocklofty Reserve (located upslope from Weerona Avenue, West Hobart), is natural non-concentrated sheet flow, the City is neither liable nor required to intervene with stormwater mitigation measures.

6.2. It is proposed that the Petitioners be advised of the Council’s position on the matter.

7. Strategic Planning and Policy Considerations

7.1. The City Stormwater Strategic Management Plan (SMP) and supporting documents outlines the City’s obligations under the Urban Drainage Act 2013.

7.2. The recommendation of this report is consistent with decisions for other areas at the bush-urban interface subject to non-concentrated natural overland flows.

8. Financial Implications

8.1. Funding Source and Impact on Current Year Operating Result

8.1.1. Not applicable.

8.2. Impact on Future Years’ Financial Result

8.2.1. Not applicable.

8.3. Asset Related Implications

8.3.1. The City’s existing stormwater assets are scheduled for ongoing clearance and maintenance.

9. Legal, Risk and Legislative Considerations

9.1. The City holds no liability to create additional stormwater assets in the area sought by the petition, aside from its obligations to maintain the existing systems.

9.2. The Urban Drainage Act 2013 requires Council to ‘minimise the risk of urban flooding due to stormwater flows’.

9.3. Stormwater is defined in the Urban Drainage Act 2013 as ‘run-off water that has been concentrated by means of a drain, surface channel, subsoil drain or formed surface’.

9.4. The overland flows from Knocklofty Reserve are non-concentrated sheet flows from a bushland area. They cannot be considered stormwater flows under the definition in the Urban Drainage Act 2013.

9.5. The Urban Drainage Act 2013 does not provide a legislative head of power to mitigate flood risk from non-concentrated natural overland flows as requested by the petitioners.

9.6. Construction of open drains as requested by the petitioners would result in the delivery of now concentrated stormwater to the downstream piped network which hydraulic modelling has demonstrated will not have sufficient receiving capacity.

9.7. Any increase in flood risk to downstream properties as a result of the works requested by the petitioners would become the responsibility of Council.

10. Delegation

10.1. As the matter responds to a petition, the matter is for the Council to determine.

As signatory to this report, I certify that, pursuant to Section 55(1) of the Local Government Act 1993, I hold no interest, as referred to in Section 49 of the Local Government Act 1993, in matters contained in this report.

Jennifer Flanagan

Program Leader Stormwater Assets

Aaron Smith

Manager Stormwater

Glenn Doyle

Director City Amenity

Date: 21 August 2020

File Reference: F20/85885

Attachment a: Residents' Response to Weerona Ave Flood Protection Petition ⇩

Item No. 7.1

Agenda (Open Portion)

City Infrastructure Committee Meeting - 26/8/2020

Page 48

ATTACHMENT a

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