Alpine Shire Council Flooding... · Alpine Shire Council Planning Scheme Amendment C38 Expert Flood Report February 2015

Alpine Shire Council Planning Scheme Amendment C38

Expert Flood Report

February 2015

GHD | Report for Alpine Shire Council - Planning Scheme Amendment C38, 31/32536 | i

Expert Witness Information Name and Address

Trevor Clark

Senior Water Resources Engineer within the Water Resources Service Group

GHD, Wodonga Office

105 Hume Street, Wodonga, 3690

Qualifications and Experience

BE (Civil Engineering), Monash University, 1986.

I have worked full time since completing my degree in 1986 as a civil engineer, specialising as a

water resources consulting engineer since 1991. During this period, I have worked on a large

number of projects for a range of government and private organisations including Council’s,

state government agencies and private developers in a range of roles.

Areas of Expertise

In relation to the preparation of this expert witness report, the following areas of expertise are

relevant:

Investigation and design of floodplain infrastructure, including mitigation measures to

reduce flood impacts.

Hydrologic and hydraulic modelling to simulate flooding conditions for flood studies and

development impact studies.

Past appearances as an expert witness in regards to flooding and drainage related issues

at planning panels and civil appeals.

Other Contributors to this Report

Gavin Hay (report reviewer)

Principal Engineer

GHD, Melbourne Office

ii | GHD | Report for Alpine Shire Council - Planning Scheme Amendment C38, 31/32536

Table of Contents Expert Witness Information ....................................................................................................................... i

Name and Address ......................................................................................................................... i

Qualifications and Experience ........................................................................................................ i

Areas of Expertise ........................................................................................................................... i

Other Contributors to this Report .................................................................................................... i

1. Introduction .................................................................................................................................... 3

1.1 Scope of this Report ............................................................................................................ 3

1.2 Basis of this Report / Limitations ......................................................................................... 3

2. June 2010 Flood Study Report ...................................................................................................... 4

2.1 Scope ................................................................................................................................... 4

2.2 Assessment Approach ......................................................................................................... 4

2.3 Expected Flooding Impacts – Runway Extension Options .................................................. 4

2.4 Adjoining Development Flood Issues / Risks ...................................................................... 6

2.5 Limitations ............................................................................................................................ 6

3. 2014 Master Plan Proposal ............................................................................................................ 7

4. Current Proposal ............................................................................................................................ 9

4.1 400 metres Runway Extension Option ................................................................................ 9

4.2 Development Layout Modification ........................................................................................ 9

5. Conclusions .................................................................................................................................. 10

6. Declaration ................................................................................................................................... 11

Figure Index

Figure 1 June 2010 Flood Study – Development Layout ................................................................... 5

Figure 2 2014 Master Plan ................................................................................................................. 8

GHD | Report for Alpine Shire Council - Planning Scheme Amendment C38, 31/32536

1. Introduction 1.1 Scope of this Report

The C38 Planning Scheme Amendment is for the rezoning of the Mount Beauty Air Park

development site to Special Use Zone. The site is currently zoned a mixture of Public Use Zone

(PUZ7), Farming Zone (FZ) and Industrial 1 Zone (IN1Z). The rezoning is being sought to

support the proposed land use change as per the development proposal for the Air Park site.

The purpose of this report is to provide an expert opinion on the following matters, as requested

by our client, the Alpine Shire Council:

With regards to the Mount Beauty Airpark Master Plan 2014 and the Mount Beauty Airpark

Precinct Plan 2014, whether there are any expected flooding impacts that would

compromise the ability to develop the site in accordance with the Master Plan and Precinct

Plan.

Assuming that the proposed runway extension is limited to 400 metres resulting in a total

runway length of 1,300 metres, what are the expected impacts on flooding conditions and

what mitigation measures could be implemented to minimise the impacts on flooding.

In regards to Lots 58 to 61, 63 to 74 and 80 shown on the Master Plan 2014, is

development of these lots acceptable given they are affected by 100 year average

recurrence interval (ARI) flooding.

1.2 Basis of this Report / Limitations

This report principally relies on the following report:

‘Mount Beauty Air Park Development – Flood Study’, prepared by GHD for the EDM Group,

30 June 2010.

The author of this Expert Witness report is also the author of the above 2010 report.

The following data was utilised by GHD in preparing the June 2010 report:

Streamflow data recorded at the Mongans Bridge gauging station.

Recorded flood height marks in the vicinity of the development site derived from a survey

commissioned by the North East CMA following a major flood in September 1998.

‘Flood Mapping Report – Kiewa River Basin’, GHD, 2000.

Ground survey data over the development site supplied by EDM Group.

This report should be read in conjunction with the above June 2010 Flood Study report.

4 | GHD | Report for Alpine Shire Council - Planning Scheme Amendment C38, 31/32536

2. June 2010 Flood Study Report 2.1 Scope

The June 2010 Flood Study report was prepared by GHD to:

Assess riverine flooding conditions over the Mount Beauty Air Park development site (i.e.

flooding associated with the East and West Branches of the Kiewa River).

Assess flood related impacts arising from the development and possible mitigation options

which could be implemented to mitigate the expected flood impacts.

The development layout provided by EDM which was the subject of the 2010 report is shown on

Figure 1. Two runway extension options were assessed, a 300 metres extension and a 700

metres extension.

2.2 Assessment Approach

The June 2010 report relies on hydraulic modelling using the then available ground survey data

to define pre development flooding conditions over the site and the expected impacts that the

development will have on flooding conditions. The hydraulic modelling was undertaken using

the HEC-RAS model, a one dimensional hydraulic model, which defines the floodplain geometry

using a series of cross sections orientated perpendicular to the direction of flood flows.

The hydraulic model was firstly calibrated to match the available recorded flood levels in the

vicinity of the development site. The calibrated hydraulic model was then used to identify the 5,

20 and 100 year ARI flood levels over the site and the 100 year ARI flood extent. The details of

this process including the adopted flow inputs into the hydraulic model are described in the June

2010 report.

Flooding conditions over the site are described in detail in the June 2010 report as determined

via the hydraulic modelling.

The 100 year ARI flood extent shown on the 2014 Master Plan (refer to Figure 2) is taken

directly from the June 2010 report. The majority of the development site is not affected by 100

year ARI flooding. The following parts of the development site are however subject to flooding:

All of the runway extension with the exception of the 150 metres extension closest to the

existing runway.

Portions of the adjoining proposed development as shown on the 2014 Master Plan (Lots

47 and 48, Lots 57 to 61, Lots 63 to 74 and Lot 80).

2.3 Expected Flooding Impacts – Runway Extension Options

The hydraulic model was used to assess the expected impacts on flooding conditions as a

result of the two runway extension options, assuming that the runway extension surface is

constructed above the 100 year ARI flood level.

Based on the hydraulic modelling results, an increase of up to 0.2 metres in the local 100 year

ARI flood level is predicted in the absence of any mitigation measures for both the 300 and 700

metres extension options. The report indicates there is no existing development located within

the areas expected to be affected by the increase in flood level.

GHD | Report for Alpine Shire Council - Planning Scheme Amendment C38, 31/32536 | 5

Figure 1 June 2010 Flood Study – Development Layout


Mitigation measures nominated to reduce the expected flood impacts are:

Subject to runway operation constraints, constructing the runway surface as low as possible

in order to minimise the resultant flood level increase.

Realignment of a shallow floodplain depression which is in the path of the runway

extension.

Provision of a culvert under the runway extension in the case of the 700 metres extension

option.

2.4 Adjoining Development Flood Issues / Risks

The Stage 1 area (refer to Figure 1 – portion of the development site on the west side of the

runway) is not subject to 100 year ARI flooding with the exception of a small area along its

northern boundary and a small area adjoining the Kiewa River West Branch towards its

southern end. With the exception of these two areas, the remainder of the Stage 1 area is

protected from 100 year ARI flooding by high ground which adjoins the West Branch river

channel.

The June 2010 report recommends in view of the identified flooding conditions over the Stage 1

area, the application of minimum floor level requirements for buildings and the retention of flood

storage volume in response to any filling inside the 100 year ARI flood extent line.

The Stage 2 area (refer to Figure 1) is free from 100 year ARI river flooding. There are

therefore no river flooding related constraints associated with the development of this area.

The northern portion of the Stage 3 area (refer to Figure 1) is subject to 100 year ARI flooding

from the Kiewa River East Branch. The Stage 3 area inundated includes a floodplain

depression.

To mitigate impacts on flooding, the floodplain depression could potentially be realigned down

the north side of the Stage 3 area. The realignment of the depression could potentially be done

subject to an assessment of other impacts aside from flooding (e.g. flora and fauna, cultural

heritage). Providing the realignment of the depression is undertaken appropriately (i.e.

replicates the size of the existing depression) and connects into the same downstream

depression, any residual impacts on flood levels and flood flow distributions are expected to be

minimal.

2.5 Limitations

The 2010 Flood Study assessed river flooding issues only. Stormwater (local runoff) flooding

issues are not covered.

The 2010 Flood Study report acknowledges that it is not possible to accurately predict the

magnitude of flooding impacts using a one dimensional hydraulic model given the largely two

dimensional nature of flooding at the development site. One dimensional modelling was

undertaken at the time given available ground survey data and project budgetary constraints.


3. 2014 Master Plan Proposal The 2014 Master Plan is shown on Figure 2. It shows a 700 metres runway extension. The

adjoining development layout is very similar to the layout assessed for the 2010 Flood Study

(see Figure 1). The 2014 Master Plan lot numbers are however different to the 2010 plan layout

lot numbers.

Given the similarities between the 2014 Master Plan and the development layout assessed by

the 2010 Flood Study report, the same conclusions are generally applicable.

The impacts on river flooding conditions as a result of the construction of a 700 metres runway

extension is expected to result in local flood level increases of up to 0.2 metres. This is based

on the June 2010 GHD report which assessed a 700 metres runway extension option and

assumes that the runway surface is constructed to above the 100 year ARI flood level.

The local increase in flood level caused by the runway obstructing flood flows will dissipate with

distance upstream of the obstruction. The closest existing development to the runway

extension appears to be located on the north side of Lots 88 to 92, approximately 300 metres

upstream of the runway extension.

The rise in flood level from the runway extension to the closest existing building, which is

located on the north side of Lot 88, is more than 2 metres (refer to Figure 4 of the June 2010

report). The relatively small increase in flood level immediately upstream of the runway is

expected to be fully or almost fully dissipated prior to this nearest building.

The 2014 Master Plan layout includes a number of lots which are subject to 100 year ARI

flooding. These lots are (refer to Figure 2):

Lots 47 and 48. These two lots are located on the fringe of a backwater depression

floodplain area with no through conveyance of floodwater. Most of the backwater

depression is located within the Reserve area shown on the Master Plan. Minimum floor

level requirements and no net loss of flood storage volume would seem appropriate flood

related development controls for these lots.

Lots 57 to 61. The 100 year ARI inundation is to a depth of generally not more than 0.3

metres on these lots. The application of development controls used within Land Subject to

Inundation overlay areas would therefore seem appropriate for these lots. These involve

minimum floor level requirements and no net loss of flood storage volume.

Lots 63 to 74 and Lot 80. This area includes a floodplain depression where the depth of

100 year ARI flooding is typically 1.1 metres. Outside the depression, the depth of flooding

does not generally exceed 0.5 metres. At least the deeper parts of this area are more

characteristic of a Floodway Overlay area which would generally result in the exclusion of

flood sensitive development. Developments would need to maintain the free passage and

temporary storage of floodwater.

The accuracy of all flood heights identified in the June 2010 report is not absolute and reflects

the accuracy of the available data and the modelling techniques used. For the setting of

minimum floor levels (e.g. 300 mm above the 100 year ARI flood level), two dimensional

hydraulic modelling is advisable given the complex hydraulic conditions present at the site.

LiDAR terrain elevation data (ground surface level information) is now available for the Kiewa

River floodplain which subject to verification should be utilised to establish the two dimensional

hydraulic model.


Figure 2 2014 Master Plan


4. Current Proposal 4.1 400 metres Runway Extension Option

The Alpine Shire Council has recently advised that the runway extension proposal now being

considered is for a 400 metres extension, resulting in a post extension runway length of 1,300

metres.

The impact on local flooding conditions of the 400 metres runway extension is expected to be

consistent with the findings of the June 2010 study. The HEC-RAS modelling undertaken for

the 2010 study identified that 100 year ARI flood level increases of up to 0.2 metres are

expected for the 300 metres and 700 metres long runway extension options assessed.

The increase in flood level will be greatest adjacent to the upstream side of the runway

extension. The increase in flood level will then dissipate with distance upstream away from the

obstruction (the runway).

The closest existing development is located on the north side of Lots 88 to 92, approximately

300 metres upstream of the runway. The rise in flood level from the runway extension to the

closest building on the north side of Lot 88 is 2 metres. The relatively small increase in flood

level immediately upstream of the runway extension is therefore expected to be fully or close to

fully dissipated prior to the nearest building.

There is no existing development on the north side of the Kiewa River East Branch opposite the

development site within or close to the 100 year ARI flood extent line.

Mitigation measures identified by the June 2010 report could be implemented to further reduce

the impact of the runway extension on flooding conditions. These potential measures are to:

Minimise the height of the runway extension (e.g. to the 20 year ARI flood level which is

approximately 0.2 metres below the 100 year ARI flood level).

Reinstate the floodplain depression which crosses the proposed runway extension down the

eastern side of the proposed runway.

Provide a cross drainage culvert structure under the runway extension.

4.2 Development Layout Modification

The Alpine Shire Council has recently advised that Lots 62 to 74 proposed on the 2014 Master

Plan (refer to Figure 2) are likely to be omitted from the proposed development.

This change removes 12 of the 13 lots which are within the 100 year ARI inundation extent on

the east side of the runway. The only remaining lot within the inundation extent on this side of

the runway is Lot 80.

Lot 80 could be developed subject to development controls to ensure that:

Building floor levels are sufficiently high.

Flood storage volume is retained.

These controls are consistent with the flood related development controls required for the

proposed lots on the western side of the runway which are affected by 100 year ARI flooding.


5. Conclusions In relation to the proposed Mount Beauty Air Park development, it is concluded on the basis of

the investigations documented in the June 2010 GHD Flood Study report that:

The extent of river flooding expected in a 100 year ARI event coincides with that shown on

the July 2014 Master Plan prepared by EDM Group.

The current runway extension proposal of 400 metres will lead to locally higher flood levels.

Hydraulic modelling undertaken as part of the 2010 study indicates 100 year ARI flood level

increases of up to 0.2 metres.

The localised increase in flood level due to the runway extension is expected to be fully or

close to fully dissipated prior to the nearest existing buildings located on the north side of

Lots 88 to 92.

The limited flood impacts associated with the runway extension could be further reduced by

limiting the height of the runway extension, realigning a shallow floodway depression which

crosses the runway extension route and by installing a culvert cross drainage structure

under the runway extension.

Seven lots on the western side of the runway are affected by 100 year ARI flooding. It is

considered that these lots are suitable for development subject to minimum floor levels and

flood storage volume retention requirements.

In relation to the 2014 Master Plan, thirteen (13) lots on the eastern side of the runway

extension are affected by 100 year ARI flooding. The recent change in omitting Lots 62 to

74 from the development proposal leaves only one lot remaining which is affected by

flooding on the east side of the runway. It is considered that this lot is suitable for

development subject to minimum floor level and flood storage volume retention

requirements.

Given the scale of the development (over one hundred lots) and the complexity of flooding

conditions over the site, detailed design of the development should include two dimensional

hydraulic modelling to confirm the local flooding conditions and to provide flood levels for

the setting of minimum building floor levels.


6. Declaration I, Trevor Clark, declare that I have made all the inquiries that I believe are desirable and

appropriate and no matters of significance which I regard as relevant have, to my knowledge,

been withheld from the Panel.

GHD

105 Hume Street Wodonga VIC 3690 Australia PO Box 992 T: 61 2 6043 8700 F: 61 2 6043 8711 E: [email protected]

© GHD 2015

This document is and shall remain the property of GHD. The document may only be used for the purpose for which it was commissioned and in accordance with the Terms of Engagement for the commission. Unauthorised use of this document in any form whatsoever is prohibited.

G:\31\32536\WP\7897.docx

Document Status

Rev No.

Author Reviewer Approved for Issue Name Signature Name Signature Date

A T Clark G Hay G Hay 18/2/2015

0 T Clark G Hay G Hay

23/2/2015

www.ghd.com

Documents

Alpine Shire Council Flooding... · Alpine Shire Council Planning Scheme Amendment C38 Expert Flood Report February 2015