Port Fairy – Reedy Creek Flood Modelling Flood Risk Report ... · The works around Osmonds Lane have led to a significant reduction in flood extent and levels in the nearby upstream

Port Fairy – Reedy Creek Flood Modelling

Flood Risk Report – 88 Princes Highway

Prepared By:

Utilis Pty Ltd with HydroSpatial Pty Ltd

October 2019

Utilis Pty Ltd

ABN: 48324943836

42 Brown Street Hamilton, VIC, 3300

Contact: Brad Henderson

Email: [email protected]

Mobile: 0466 313 189

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Details Reedy Creek Flood Modelling – Flood Risk Report 88 Princes Highway Port Fairy

Client Max Atkins

Author Brad Henderson

Job Number 402019

Document Version Control

Version Type Release Author Reviewer Date

01 Draft Utilis Brad Henderson 24/6/2019

Cover Image: 88 Princes Highway 1% AEP Flood extent.

Copyright © This document has been produced by Utilis Pty Ltd for Max Atkins for their use only. The information contained in this document is the copyright of Utilis Pty Ltd. Use or copying of this document in whole or in part without written permission of Utilis Pty Ltd constitutes an infringement of copyright. Utilis Pty Ltd does not warrant that this document is free from error and does not accept liability for any loss caused or arising from reliance upon the information provided herein. Reference to this document in the provision of advice should not be made by persons other than those authorised by Max Atkins. Disclaimer No warranty is made as to the accuracy of any information provided in this report, which may change without notice. Utilis Pty Ltd disclaims all liability and responsibility for any direct or indirect loss or damage which may be suffered by any person through relying on anything contained in or omitted from this report. This report has been prepared for the sole use of Max Atkins and no responsibility is accepted by Utilis Pty Ltd with regard to any third party use of the whole or of any part of its contents. This information is subject to change as new information becomes available and as further investigations are carried out.

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Table of Contents 1 Scope of Works ............................................................................................................................... 4

2 Methodology .................................................................................................................................. 4

3 Results ............................................................................................................................................ 6

4 Conclusions and Recommendations ............................................................................................... 6

Table of Figures Figure 1. Existing vs Pre-Development Bathymetry ................................................................................ 5 Figure 2. Pre-Development Peak Flood Depth (1 in 100 Year ARI, 0.8m SLR)......................................... 7 Figure 3. Existing Case (1 in 100 Year ARI, 0.8m SLR) ............................................................................. 8 Figure 4. 1 in 100 Year ARI Afflux Map (Existing minus Pre-Development ............................................. 9 Figure 5. Flood extent – 88 Princes Highway (1 in 100 Year ARI, 0.8m SLR) ......................................... 10

PortFairy_Regional_Model_Update_88 Princes Highway.docx.docx Page 4 of 13

1 Scope of Works Port Fairy is a regional town along the south coast of Victoria. The town is subject to inundation from riverine flooding from the Moyne River and other smaller flowpaths, as well as storm surge. In 2008 Water Technology Pty Ltd completed the Port Fairy Regional Flood Study on behalf of Glenelg Hopkins CMA. One of the recommendations of the study was to remove a pipe culvert running from approximately the Princes Hwy to Osmonds Lane as it would significantly reduce flood depths upstream.

This Osmonds Lane upgrade has subsequently been undertaken, however the model that Council and CMA are using for development assessment processes has not been updated. Therefore, properties upstream would have an artificially high flood level when being assessed.

The scope of works for this project was to replicate the works as executed in the Osmond Lane upgrade in the existing MIKE FLOOD model and to produce flood mapping for the 1% AEP event showing the difference between the old and new model results, as well as the new flood extents upstream of the removed culvert. The purpose for this modelling update is to provide an assessment of flood risk relevant to 88 Princes Highway Port Fairy and the proposed construction of a shed at the rear (west) of the property.

2 Methodology

The main work has been undertaken on the MIKE FLOOD files provided by the CMA in the “2015 Port Fairy Remodelling_Models.zip”. The upgrades have been completed on the “MFx_100yr80cm_01b.couple” and associated files. The work completed includes:

- Updating the model to the 2016 sp3 version of MIKE FLOOD - Running the 100 Year ARI event “Pre-Development” case - Modifying the “STRUCTURE13” branch and couple from the old pipe setup to the currently

existing small culvert underneath Osmonds Lane. Note in Figure 1 the MIKE 11 channels are not correctly aligned.

- “Digging” out the area where the pipe was previously to a level matching supplied survey data. Note that all channels within the model are represented in the 10 m grid, rather than as coupled 1 Dimensional channels. Therefore the new channel has been represented as a 10 m wide channel with the surveyed invert.

- Re-running the model to represent the “Existing” case - Produce depth and afflux mapping from the two events and compare to the Water

Technologies report

Key parameters for the Osmonds Lane structure are presented in Table 1. A comparison of the pre-development and existing case bathymetries are shown in Figure 1.


Table 1 Osmond Lane Culvert Details

Parameter Value (Source)

Cross-Section Invert 0.67 m AHD (LiDAR)

Culvert Invert 0.685 m AHD (Survey)

Culvert Length 10 m (LiDAR)

Culvert Face Dimensions 1.2W x 0.5H (Field Measurement)

Road Crest Level 2.4 m AHD (LiDAR)

Figure 1. Existing vs Pre-Development Bathymetry


3 Results

The maximum depth for the pre-development scenario (i.e. what is modelled in the flood study) and existing scenario are shown as Figures 2 and 3 respectively. Afflux mapping (pre-development – existing) is shown in Figure 4.

A qualitative comparison to the figures within the Port Fairy Regional Flood Study show a good match between the result mapping.

The result mapping shows that 88 Princes Highway is not subject to flooding from the 100 year ARI, 0.8m SLR design event, refer Figure 5.

4 Conclusions and Recommendations

The Port Fort Regional Flood Study Model has been run for the 1 in 100 Year ARI scenario with 0.8 m sea level rise. The model was then modified to reflect the existing conditions in the Osmonds Lane area and re-run to determine the impacts.

The works around Osmonds Lane have led to a significant reduction in flood extent and levels in the nearby upstream areas. Given that this is the existing scenario, it is recommended that flood planning is based on this scenario, rather than the pre-development scenario.

The change in flood regime as a result of the works in Reedy Creek has reduced the flood extent significantly as it relates to 88 Princes Highway. The modelling suggests the property is now not subject to a flood risk for the 1 in 100 Year ARI scenario with 0.8 m sea level rise. The proposed construction of a shed at the property is therefore not considered at risk from the aforementioned design flood.


Figure 2. Pre-Development Peak Flood Depth (1 in 100 Year ARI, 0.8m SLR)


Figure 3. Existing Case (1 in 100 Year ARI, 0.8m SLR)




Figure 4. 1 in 100 Year ARI Afflux Map (Existing minus Pre-Development)



Figure 5. Flood extent – 88 Princes Highway (1 in 100 Year ARI, 0.8m SLR)

Documents

Port Fairy – Reedy Creek Flood Modelling Flood Risk Report ... · The works around Osmonds Lane have led to a significant reduction in flood extent and levels in the nearby upstream