Hat Head Flood Levee Audit

Transcription

1 KEMPSEY SHIRE COUNCIL Hat Head Flood Levee Audit April 2014

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3 Executive Summary This report represents a complete and thorough Audit of the Hat Head Flood Levee System as at April, It forms part of a widespread audit of the entire Lower Macleay Flood Levee System. The Scope of this Audit included but was not limited to: 1. Compilation of all the known reports and information available on the levees and a summary of their currency, findings and recommendations. 2. Comparison of the Levee Design Levels with the Existing Levels and recommendations to reinstate the levees to the Design Levels. 3. A Geotechnical Investigation to assess the structural adequacy of the levees during prolonged flooding and recommendations as to any remediation measures required 4. A Defects Inspection of the levees to identify areas of anomalies, such as missing levees, low sections, localised depressions, and signs of erosion/scour or instability. Specifically, the following items were not included in the Scope of the Audit and should be carried out at later date: 1. Preparation of an estimate of cost to remediate each individual defect and a total cost to repair the levees. 2. Preparation of a condition rating for each levee based on the risk of failure and the likely severity of a failure. 3. Review of the Flood Structures including the Flood Gates. This was already carried out in May 2012 and no major issues were found. 4. Preparation of an Operation & Maintenance Manual including a Maintenance and Inspection Plan. It is proposed to develop one document for the entire Lower Macleay Floodplain. As part of the Audit, Consultants JK Geotechnics were engaged to Carry out field and laboratory work to assess the structural integrity of the levees under prolonged flood events. Evaluate the likelihood of failure of the levees under prolonged flood events including the stability of the levee cross section and protection against seepage. Undertake an inspection of the general defects of the levees Provide recommendations to remediate any inadequacies found. JK Geotechnics complete reports are attached as Appendix 6. The findings of the entire Audit are summarised in the Combined Action and Defects Inspection Report (Appendix 7). Specific issues identified included, but were not limited to, the need to address the effectiveness of Rowes Cutting - an emergency overflow spillway through the coastal sand dunes determine a revised design flood height and associated requirements to raise the levees repair the general defects in the levees Hat Head Flood Levee Audit ii

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5 Table of Contents 1.0 INTRODUCTION AUDIT SCOPE DESCRIPTION AND CONDITION OF LEVEE SYSTEMS LEVEE CONSTRUCTION HISTORY CONTROL LEVEE VILLAGE LEVEE SOUTH WESTERN LEVEE OCEAN BREAKOUT SPILLWAY (ROWES CUT) PREVIOUS REPORTS AND INFORMATION - SUMMARY OF CURRENCY, FINDINGS AND RECOMMENDATIONS COMPARISON OF LEVEE DESIGN LEVELS WITH THE EXISTING LEVELS LEVEE DESIGN LEVELS EXISTING LEVEE LEVELS COMPARISON OF LEVEE DESIGN LEVELS AND EXISTING LEVELS Village Levee South West Levee Control Levee CONFIRMATION OF DESIGN LEVELS RECOMMENDED ACTIONS TO REINSTATE THE LEVEES TO THE DESIGN LEVELS GEOTECHNICAL INVESTIGATION OF LEVEES COMBINED ACTION AND DEFECTS INSPECTION REPORT AND AUDIT FINDINGS JK Geotechnics Report Rowes Cut Channel Flood Damage Claim for Riverbank Erosion of Levee along Korogoro Creek Erosion of Village Levee Crest by Pedestrians, Cyclist and Vehicles Preparation of an Operation & Maintenance Manual including a Maintenance and Inspection Plan APPENDICES Hat Head Flood Levee Audit iv

6 1.0 INTRODUCTION The Audit of the Hat Head Levee System forms part of a widespread audit of the entire Lower Macleay Flood Levee System. The village of Hat Head is located at the mouth of Korogoro Creek, 32 kilometres east of Kempsey. Korogoro Creek forms part of the flood mitigation and drainage system for the Belmore- Kinchela area of the Lower Macleay River floodplain. The Hat Head flood mitigation system comprises the following works: 1. Floodgates on a 9 cell box culvert (3000 wide and 1800 high) across Korogoro Creek at Hat Head Road approximately 3 kilometres upstream of Hat Head Village 2. A Control Levee fitted with 7 control box culverts (2400 wide and 1800 high) and gates constructed across Korogoro Creek between the coastal and inner sand dunes The levee is constructed approximately 2 kilometres upstream of the village 3. An emergency overflow spillway known as Rowes Cut excavated through the coastal sand dunes some 300 metres upstream of the Control Levee 4. Flood Levees on both sides of Korogoro Creek through Hat Head village known as the town levee and the South West Levee. The locations of the levees investigated as part of this Audit are shown in the Plan in Appendix AUDIT SCOPE The Scope of the Hat Head Audit included but was not limited to: 1. Compilation of all the known reports and information available on the levees and summarise their currency, findings and recommendations. 2. Comparison of the Levee Design Levels with the Existing Levels and provide recommended actions to reinstate the levees to the Design Levels. This was undertaken by a. Obtaining the Levee Design Levels from either: i. The original design plans, or Hat Head Flood Levee Audit 1

7 ii. Previous flood studies b. Undertaking a desktop review of the current Laser Imaging Detection and Ranging (LIDAR) information to ascertain Existing Levels. c. Surveying of sections of the levees to confirm Existing Levels where LIDAR information was not considered accurate. d. A Geotechnical Investigation to assess the structural adequacy of the levees during prolonged flooding and provide recommendations as to any remediation measures required. e. A Defects Inspection of the levees to identify areas of anomalies, such as missing levees, low sections, localised depressions, and signs of erosion/scour or instability. Specifically, the following items were not included in the Scope of the Audit and should be carried out at later date: 5. Preparation of an estimate of cost to remediate each individual defect and a total cost to repair the levees. 6. Preparation of a condition rating for each levee based on the risk of failure and the likely severity of a failure. 7. Review of the Flood Structures including the Flood Gates. This was already carried out in May 2012 and no major issues were found. 8. Preparation of an Operation & Maintenance Manual including a Maintenance and Inspection Plan. It is proposed to develop one document for the entire Lower Macleay Floodplain. 3.0 DESCRIPTION AND CONDITION OF LEVEE SYSTEMS 3.1 LEVEE CONSTRUCTION HISTORY The history of the construction of the Hat Head Levees is detailed in Table 1 below. Hat Head Flood Levee Audit 2

8 Table 1 Flood Mitigation Works within the Study Area Flood Mitigation Work Hat Head Control Levee and Headworks and Village Levee Date of Construction 1968 Ryans Cut Ocean Outlet, Headworks and Drain 1973 Hat Head Control and Village Levees raised and reinforced 1999 Maintenance works have been carried out over the years on an ad hoc basis. 3.2 CONTROL LEVEE The control levee is located approximately 2 kilometres upstream of the Korogoro Creek Bridge. The levee is approximately 500 metres long and runs perpendicular across the Creek tying Hat Head road into the coastal sand dunes. The levee is an earth bank fitted with a 7 cell 1.83m H x 2.44m W box culvert designed to limit the amount of flood water flowing through Korogoro Creek to the ocean outlet. Flood water flows from the Swanpool plain into Korogoro Creek and builds up behind the control levee. Excess floodwater behind the control levee should result in a breakout of the ocean spillway to allow the flood water to reach the ocean and prevent the control levee from over topping. This ocean spillway is known as Rowes Cut. The levee s existing surface level averages at AHD 4.0m which is approximately 1.7 m above the natural surface level. It has an average crest width of 3.2 m. Batter slopes are approximately 1 in 5 on the water side and 1 in 8 on the town side. There is a rock retaining wall that varies in height between 0.5m and 1.0m at the toe of the town side batter. The vegetation at the top of the control levee is generally limited to native and exotic grasses with a few small trees along the toe on the town side. In comparing heights and embankment materials listed in the 1993 Audit of Flood Levees by the Department of Public Works with the current position, it appears that the levee has been subsequently raised with a layer of clay. This was confirmed by the Geotechnical Report prepared by specialised Flood Levee Geotechnical Consultants JK Geotechnics (Appendix 6). The control levee ties into the coastal sand dunes at its Eastern end. Aerial LIDAR survey have identified that there are points in these coastal sand dunes that are lower than the levee height by approximately 500mm. This can be seen in Figure 2 of Appendix 4. These low points may have always existed or it may be more likely that they have been caused by wind and rain erosion or when the levee was raised the sand dunes were not. In any event, there Hat Head Flood Levee Audit 3

9 is a potential low point in the coastal sand dunes which may result in water escaping into Hat Head Village. Erosion is present around the wing walls of the control culvert and the geo-fabric designed to cover the embankment material behind the wing wall is failing. Due to vehicle and pedestrian access along the top of the levee, some undulations are evident in the crest of the levee. These undulations do not affect the crest height of the levee, however, they store water when it rains and allows the water to seep down into the levee embankment. 3.3 VILLAGE LEVEE The Village Levee runs from the Korogoro Creek Bridge (on Hat Head Road at the entrance to the Village) along the Northern bank of Korogoro Creek and terminates at high ground within the Caravan Park. The levee is approximately 1.6 kilometres long and its crest varies between 2.4m to 2.9m AHD (1 and 1.5 m above natural surface level) It has a rounded crest which varies in width between 1.5 to 2.0 m. Batter slopes are approximately 1 in 3 on the water side and 1 in 3 to 1 in 4 on the town side. The batter slopes are steeper on the water side where the creek has eroded the bank/ levee. The levee embankment materials and subsurface conditions are outlined in the consultant s report by JK Geotechnics (Appendix 6). The levee batters are for the most part covered in native and exotic grasses. The crest of the levee however is often bare exposing a layer of clay material and occasionally sand. In comparing heights and embankment materials listed in the 1993 Audit of Flood Levees by the Department of Public Works with the current position, it appears that the levee has been raised with a layer of clay. This was confirmed by the Geotechnical Report prepared by Consultants JK Geotechnics (Appendix 6). As the levee runs alongside Korogoro Creek, it is often used by the residents as a bike and walking trail. Because of this, over time the crest has been eroded into a non-uniform shape as can be seen in the long section and cross sections in Appendix 5. Localised erosion of the levee crest and batters is also caused by vehicles driving across or along the levee to access the creek. Erosion of the levee as described above is severe in some points creating low points and destabilised sections of the levee. The water side batter is highly vegetated and for the majority of the levee is unmaintained. This makes inspecting the batters of the levee very difficult and critical defects such as burrows and erosion could be missed. There are a number of mature trees growing on and at the toe of the levee along its entire length. The roots of these trees are visible in the levee crest and batters at numerous locations along the levee. An impermeable plastic membrane within the levee is designed as a cut-off to prevent seepage of water through the levee. Due to erosion of the levee crest the plastic liner is visible in a number of locations along the levee. Tree roots are also visibly penetrating the Hat Head Flood Levee Audit 4

10 plastic barrier and it can be assumed that due to the number of trees along the levee the condition of this plastic layer is poor or at the very least, its purpose has been compromised. There are a number of concrete pipe culverts that run beneath the levee to remove stormwater from the town into Korogoro Creek. The headwalls of these culverts are often close to the levee embankment and erosion is present around the headwalls in some cases. A number of the headwalls are also cracked and in need of repair. The flood flaps at the outlets are generally in good condition; however the outlet drains are generally silted up resulting in the flood flaps being unable to operate effectively. 3.4 SOUTH WESTERN LEVEE The South-Western Levee extends some 1.8 kilometres downstream from the Korogoro Creek Bridge. The levee is effectively the embankment for Gap Road which is a two lane sealed road. The crest of the levee is approximately 7 metres wide with a 5 metre wide sealed road pavement. The road surface is in relatively good condition with only minor patches. No significant subsidence of the road formation is evident. As the road/levee is generally maintained in good condition, any defects associated with the structural integrity of the levee are minor and accordingly are of very low priority The levee was therefore not inspected in detail. However, a visual inspection of the road/levee was carried out and revealed no defects of significance. 3.5 OCEAN BREAKOUT SPILLWAY (ROWES CUT) The Hat Head Flood Levee System was designed to have an ocean breakout spillway located in the sand dunes upstream of the control levee. The original design allowed for floodwaters to build up behind the control levee and spill into the coastal storage area. Once the water level behind the control levee reaches 2.4m AHD, the floodwater should then breakout through the sand dunes into the ocean. A pilot channel, known as Rowes Cut, was constructed through the sand dunes to the ocean. The base level of the pilot channel at Korogoro Creek was at 2.4m AHD and had a 4m base width with sides sloping at 1 V and 2H. A plug at the ocean end of the channel at 2.3m AHD with a 10m wide base was constructed to enable Council to remove it during a flood. The ocean breakout spillway was modelled in 1994 and 1996 by Webb, Mckeown and Associates assuming an ocean outlet width of 10m with a base level at 2.3m AHD. This model produced a 1 in 100 year flood height of 3.63m AHD at the control levee. The design level of the control levee is 4.03m AHD. The levee s existing surface level averages at 4.0m AHD. Hat Head Flood Levee Audit 5

11 The original formalised channel has eroded and is now overgrown by vegetation. This is highlighted in the comparison of aerial photographs from when it was constructed in 1969 and in 2009 (Appendix 4). Rowes Cut channel can be clearly seen in the 1969 image extending from the bend in Korogoro Creek approx. 300m upstream of the control levee into the sand dunes. In the 2009 photograph this channel can no longer be seen and the area is densely vegetated. An aerial LIDAR survey was used to determine whether an ocean breakout was still possible at Rowes Cut. Figure 1 of Appendix 4 is a terrain model that highlights the ground height that exceeds 2.4m AHD upstream of the control levee. Rowes Cut is designed to breakout to the ocean at a flood height of 2.4m. For this to occur there must be a flow path at 2.4m AHD or lower from Korogoro Creek to the frontal sand dunes. It can be seen clearly that the high point of the sand dunes (highlighted in red) creates a continuous barrier significantly higher than 2.4m AHD which will stop water flowing through Rowes Cut from reaching the ocean. All flood modelling conducted has assumed that the ocean breakout spillway is functional. If in a large flood event the ocean spillway was not opened, the protection of Hat Head against the 1 in 100 year flood level could not be guaranteed. 4.0 PREVIOUS REPORTS AND INFORMATION - SUMMARY OF CURRENCY, FINDINGS AND RECOMMENDATIONS The following activities were carried out in order to correlate the existing information: All of the known information and reports available on the levees were compiled. This included a thorough search of Council Records together with enquires from Council employees working in the area. All reports were than electronically scanned, filed and summarised (see Table 2 below).. Hat Head Flood Levee Audit 6

12 TABLE 2 Summary of Scanned Hat Head Reports Report Macleay River County Council Swanpool Drainage, Korogoro Creek Scheme Design Drawings, Drawing Nos to Macleay River Flood Study Webb, Mckeown and Associates Pty Ltd NSW Public Works, Report No , Audit of Flood Levees for NSW Village of Hat Head NSW Public Works Hat Head Levee Hydraulic Review Webb, Mckeown and Associates Pty Ltd Date of Summary of Aims Findings / Recommendations Currency Report 1962 Not applicable as only Design Drawings Not applicable as only Design Drawings Current April 1989 November 1993 October 1994 Flood Study of the Lower Macleay River to determine the design conditions for the 1 in 20, 1 in 50 and 1 in 100 probability floods and an extreme event Appraise the flood security and structural adequacy of the levee system at Hat Head No investigation of the scour protection at the mouth of Korogoro Creek or the effectiveness of the escape spillway to carry excess flood water Determine the 1% AEP Design Flood Levels Provide an indication of the extreme flood levels which would occur during an extreme ocean surge Flood behaviour throughout the study area for the 1 in 20, 1 in 50 and 1 in 100 probability floods and the extreme flood calculated and results provided on plan views Numerous recommendations none of which appear to have been implemented 1% flood level is approximately 2.39m and extreme flood level is 3.0 m AHD (both downstream of Korogoro Creek Bridge) Not Current Superseded See Section 5 of this Audit Report for the flood levels adopted and the reasons thereof Hat Head Flood Levee Audit 7

13 Report Investigation of Hat Head Flood Levees Final Report, Paterson Consultants Pty Ltd Date of Report June 1995 Summary of Aims Findings / Recommendations Currency Determine the flood levels at which there is a definite possibility of levee failure including the impact of overtopping Remedial works to be undertaken to provide a nominated level of protection There appears to be no definition of the risk of overtopping of the levee system as built to the design crest level of 2.35m (includes 0.3m freeboard) Foundation structurally adequate to support levees Plastic internal liner in levees in good condition Significant risk of levee failure due to patchy grass Current To be updated as part of this audit Hat Head Levee Hydraulic Review Supplementary Report Webb, Mckeown and Associates Pty Ltd May 1996 Carry out further studies to the October 1994 Report to determine flood levels in Korogoro Creek for various combinations of tide, gate openings in the throttle control and the size of Rowes Cut ocean breakout Reducing the gate openings in the throttle control from 7 to 5 has a small benefit downstream with no dis-benefit upstream Any reasonable combination of design floods will not exceed the critical level Current Variations in the dimensions of Rowes Cut have no discernible impact on flood levels It is noted that the report did not investigate the implications of Rowes Cut not function in a flood event Hat Head Flood Levee Audit 8

14 Report Seepage analysis of the Village Levee Department of Land and Water Conservation Date of Report August 1997 Summary of Aims Findings / Recommendations Currency Review of Critical flood level envelope and survey data to determine the critical; levee sections for analysis Determine levee bank and foundation representative permeability and reassess the seepage analyses to determine required remedial works Review remedial options and revise costs in the light of field survey and permeability data Raise the levee crest levels to obtain a uniform freeboard of 400mm above the expected flood level, a crest of 2m and establish batters of 1 vertical to 3 horizontal On completion of above and where differential heads exceed 850mm, undertake one of four recommended remedial options to protect against seepage In areas of confined width, incorporate gabion retaining walls and seepage control measures into the levee formation to provide a safe levee configuration Not current / superseded by JKGeotechnics Current Report Revised seepage analysis undertaken as part of this Audit NSW Levee Study for Emergency Management SES September 2008 Provide current information of a number of levees throughout NSW including those of the Lower Macleay Difficult to determine whether any of these recommendations were implemented but only of historical significance as report superseded by JKGeotechnics current report Specifically with respect to Hat Head Summarised all available information on the levee system Current Levees structurally adequate Overtopping Crest Height of 2.35 Hat Head Flood Gates and Culvert Inspection Reports May 2012 Inspect Flood Gate and Culvert for structural and maintenance issues No accessible high ground for evacuation No major issues found Current Does not form part of the Scope of this Audit Hat Head Flood Levee Audit 9

15 5.0 COMPARISON OF LEVEE DESIGN LEVELS WITH THE EXISTING LEVELS 5.1 LEVEE DESIGN LEVELS The Hat Head levee system is designed for a 1 in 100 year flood. Design levels for the Hat Head levees have been determined by Webb, Mckeown and Associates Pty Ltd using a model developed in 1994 which was then supplemented in Two flooding cases considered in the 1996 report are based on the following: 1in 100 year flood with ocean level of 2.0m AHD 1 in 20 year flood with an ocean level of 2.1m AHD The levee design levels were based on the maximum flood envelope from these two flooding cases, being the 1 in 100 year flood with ocean level of 2.0m AHD. The 1 in 100 year flood levels assume that all 7 flood gates on the Control Levee Culverts are open and Rowes Cut has been opened to a width of 10m with a base level at 2.3m AHD. These flood levels are summarised on Table 3 below. Based on the recommendations of the 1997 Seepage Analysis Report (see above in Table 2), Council has adopted a freeboard of 400mm above the 1 in 100 flood height. This results in levee design levels set out in Table 3 below. Hat Head Flood Levee Audit 10

16 Table 3 Adopted 1 in 100 Year Flood Levels for the Village, South West and Control Levees Location 1 in 100 Year Flood Height (m AHD) Design Freeboard (m) Levee Design Levels (m AHD) Control Levee Control Levee U/S Village and South Western Levee Korogoro Creek Bridge U/S Korogoro Creek Bridge D/S Dodds Street Mason Street Vine Street Myrtle Street Footbridge Boat Ramp These levels are set out in the long sections in Appendix EXISTING LEVEE LEVELS A desktop review of the current Laser Imagining Detection and Ranging (LIDAR) information was undertaken to attempt to ascertain the Existing Levee Levels for the Village, South West and Control Levees. Ground cover (trees and long grass) affected the accuracy of the survey readings. LIDAR generally requires flat open ground for accuracy. It was therefore necessary to physically survey all the levees to confirm the Existing Levels. A survey was carried out using Global Navigation Satellite System (GNSS) Surveying Equipment which can be considered accurate to approximately 30mm. The survey plans produced are attached in Appendix COMPARISON OF LEVEE DESIGN LEVELS AND EXISTING LEVELS Long sections were prepared showing the Design, Flood and Existing Levels of the Village, South West and Control Levees. Hat Head Flood Levee Audit 11

17 The Design Level is the 1 in 100 year Flood Level plus 400mm freeboard. Village Levee The Existing Levels are below the Design Levels in various sections for a total length of approximately 1017m which equates to 58% of the length. However, the Existing Levels are above the 1 in 100 year Flood level for the total length. The Minimum Actual Freeboard (existing level minus 1 in 100 year flood level) is 150 mm but ranges between 300 mm to 400 mm for the majority of the levee. South West Levee The Existing Levels are below the Design Levels in various sections for a total length of approximately 80m which equates to 5% of the length. However, the Existing Levels are above the 1 in 100 year Flood level for the total length. The Minimum Actual Freeboard (existing level minus 1 in 100 year flood level) is 280mm but ranges between 400 mm to 700 mm for the majority of the levee. Control Levee The Existing Levels are below the Design Levels in various sections for a total length of approximately 290m which equates to 53% of the length. However, the Existing Levels are above the 1 in 100 year Flood level for the total length. The Minimum Actual Freeboard (existing level minus1 in 100 year flood level) is 170 mm (at the Headworks Control Culvert) but ranges between 300 mm to 400 mm for the majority of the levee. 4.4 CONFIRMATION OF DESIGN LEVELS JK Geotechnics Report (Village Levee Report Section 4.2 and Control Levee Report Section 4.1.2) has identified different design levels to the above. This report went on to recommend that a hydraulic/water resources engineer be engaged to revisit the1% AEP design flood levels, extreme flood events and minimum freeboard requirements. The reasons put forward for this recommendation were: the age of the previous reports (mid 1990s) the different Council s Design Flood Levels compared to JK Geotechnics findings the need to now consider the effects of global warming. This is an important issue. Design levels must be determined as accurately as possible before any work is carried out on the levee system. The Combined Action and Defect Inspection Report has noted this. Hat Head Flood Levee Audit 12

18 6.0 RECOMMENDED ACTIONS TO REINSTATE THE LEVEES TO THE DESIGN LEVELS In order to provide a consistent satisfactory factor of safety for the flood mitigation system, it is recommended that the existing levels be raised to the design levels on the entire levee system. Recommended actions to reinstate the existing levels to the design levels were prepared by Consultants JK Geotechnics. The raising of the levees should not take place until a hydraulic/water resources engineer is engaged to revisit the1% AEP design flood levels, extreme flood events and minimum freeboard requirements as outlined previously. The details of the above are set out in JKGeotechnics Report (Appendix 6) and are also summarised in the Combined Action and Defects Inspection Report (Appendix 7). 7.0 GEOTECHNICAL INVESTIGATION OF LEVEES Consultants JK Geotechnics, were engaged to carry out: Field and laboratory work as required to assess the structural integrity of the earth levees under prolonged flood events. Evaluation of the likelihood of failure of the levee under prolonged flood events including the stability of the levee cross section and protection against seepage. An inspection of the general defects of the Levees Provide recommendations to remediate any inadequacies found. JK Geotechnics complete reports are attached as Appendix 6. The findings of the report have been incorporated into the Combined Action and Defects Inspection Report (Appendix 7). 8.0 COMBINED ACTION AND DEFECTS INSPECTION REPORT AND AUDIT FINDINGS An important part of this audit is the Combined Action and Defects Inspection Report (Appendix 7). This Combined Action and Defects Inspection Report incorporates the defects from the following areas: The findings and recommendations of this Audit Defects identified as part of JK Geotechnics report. Hat Head Flood Levee Audit 13

19 Detailed inspections were carried out on the Village and Control Levees. The South Western Levee is generally in good condition and accordingly a quick visual assessment was only required to confirm this fact. Defects such as geotechnical issues missing levees, low sections, localised depressions, trees in the levee and signs of erosion/scour or instability were compiled The defects identified in the Levee Levels Investigation, The Combined Defects Inspection Report therefore provides a summary of all defects associated with the Levee Levels, Geotechnical Issues and General Defects. A cost estimate will need to be prepared to rectify each of these defects. The recommended actions will than need to be prioritised. Specific issues are detailed below. JK Geotechnics Report The findings and of the JK Geotechnics Report and the associated Defects Inspection Report are concurred with. They form part of the Combined Defects Inspection Report (Appendix 7). Rowes Cut Channel The channel is effectively non-existent and will need significant rehabilitation to ensure its effective operation. Detailed survey, design and environmental approvals would be required before physical work could commence. These activities would be difficult due to the extensive vegetation existing within the channel and the fact that the channel is located within a National Park. All flood modelling conducted to date has assumed that the ocean breakout spillway is functional. In the light of the significant time and cost in carrying out the design, obtaining approvals and physically reconstructing the channel, it is recommended that additional flood modelling work be carried out based on Rowes Cut Channel not providing any flood relief (ie no flood water escaping to the ocean via the channel). This additional modelling will ascertain whether the reinstatement of Rowes Cut Channel is necessary to provide adequate protection to Hat Head Village. Alterative protective measures may be found to be more appropriate such as raising the levee heights Hat Head Flood Levee Audit 14

20 It is also recommended that an investigation be carried as to the Environmental Approvals required to reinstate Rowes Cut. This investigation should also ascertain whether State Environmental Planning Policy (Infrastructure) 2007 is applicable. Depending on the findings of the above, it may also be necessary to have a Contingency Operational Plan for when it is necessary to open Rowes Cut. The plan should detail: 1. when and how plant such as excavators are to be bought to site and placed on standby 2. the environmental requirements of the work 3. the exact trigger for the opening of Rowes Cut. Flood Damage Claim for Riverbank Erosion of Levee along Korogoro Creek JK Geotechnics has identified riverbank erosion of the levee along sections of Korogoro Creek. It appears that this damage has been caused by previous natural flood disasters. It is recommended that funding for the repair of this river bank/levee erosion be sought from the appropriate NSW Department as a late natural disaster claim. Erosion of Village Levee Crest by Pedestrians, Cyclist and Vehicles Erosion of the levee by pedestrians, cyclists and vehicles has been described in Section 2.3 and is also discussed in JK Geotechnics Report. It is severe in some points creating low points and destabilised sections of the levee. Once the levee design levels are finalised, it is recommended that consideration be given to provide a concrete pedestrian / cycleway on the crest of the levee to eliminate any further erosion issues. This would form part of the overall Hat Head Cycleway. Preparation of an Operation & Maintenance Manual including a Maintenance and Inspection Plan An up to date Operation & Maintenance Manual which includes Maintenance and Inspection Plans needs to be developed as one document for the entire Lower Macleay Floodplain. In respect to the Operation of Rowes Cut, it is understood that there is a requirement to close the two outer gates of the Korogoro Creek Choke when Rowes Cut is operating. This requirement needs to be fully investigated and if found correct needs to be incorporated into the Manual. The preparation of this Manual was not part of the Scope of this Audit. It has not yet commenced. It has been listed as an action in the Combined Action and Defects Inspection Report. Hat Head Flood Levee Audit 15

21 9.0 APPENDICES The following Appendices are attached: Appendix 1 - Hat Head Levee Site Plan Appendix 2 - Original Design Plans Appendix 3 Photographs of Levees Appendix 4 - Rowes Cut Terrain Maps and Aerial Photography Appendix 5 Levee Plan and Long Sections Appendix 6 - JK Geotechnics Reports Village Levee and Control Levee Appendix 7 - Combined Action and Defects Inspection Report Hat Head Flood Levee Audit 16

22 Appendix 1 Hat Head Levee Site Plan Hat Head Flood Levee Audit 17

23 Appendix 2 Original Design Plans HAT HEAD ORIGINAL DESIGN PLANS COMBINED.pdf See separate PDF File (HAT HEAD ORIGINAL DESIGN PLANS COMBINED) Hat Head Flood Levee Audit 18

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25 Appendix 3 Photographs of Levees Hat Head Flood Levee Audit 20

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27 Hat Head Control Levee Photos Hat Head Flood Levee Audit 22

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29 Hat Head Village Levee Photos Hat Head Flood Levee Audit 24

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33 Hat Head South West Levee Photos Hat Head Flood Levee Audit 28

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35 Rowes Cut Photos Hat Head Flood Levee Audit 30

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39 Appendix 4 Rowes Cut Terrain Models and Aerial Photography Hat Head Flood Levee Audit 34

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41 Figure 1: Terrain model showing location of low points in coastal sand dunes Hat Head Flood Levee Audit 36

42 Figure 2: Terrain model showing ground surface at Rowes Cut Hat Head Flood Levee Audit 37

43 Hat Head Control Levee Aerial Photograph 1969 Hat Head Control Levee Aerial Photograph 2009 Hat Head Flood Levee Audit 38

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45 Appendix 5 Levee Plan and Long Sections Hat Head Flood Levee Audit 40

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47 Plan and Long Sections Hat Head Control Levee CONTROL LEVEE PLAN.pdf See separate PDF File (Hat Head Control Levee Plan) Hat Head Flood Levee Audit 42

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49 Plan and Long Sections Hat Head Village Levee Hat Head Village Levee.pdf See separate PDF File (Hat Head Village Levee Plan) Hat Head Flood Levee Audit 44

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51 Plan and Long Sections Hat Head South West Levee Hat Head SW Levee PLAN.pdf See separate PDF File (Hat Head SW Levee Plan) Hat Head Flood Levee Audit 46

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53 Appendix 6 JK Geotechnics Reports Hat Head Flood Levee Audit 48

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55 JK Geotechnics Report Village Levee 27111LA5rpt-Rev1 Hat Head Village Levee JK Getechnics Final Report 9 April 2014.pdf See separate PDF File (27111LA5rpt-Rev1 Hat Head Village Levee JK Getechnics Final Report 9 April 2014) Hat Head Flood Levee Audit 50

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57 JK Geotechnics Report Control Levee 27111LA4rpt Hat Head Control Levee JK Getechnics Final Report 9 April 2014.pdf See separate PDF File (27111LA4rpt Hat Head Control Levee JK Getechnics Final Report 9 April 2014) Hat Head Flood Levee Audit 52

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59 Appendix 7 Combined Action and Defects Inspection Report Hat Head Flood Levee Audit 54

60 Hat Head Flood Levee Audit 55

61 Combined Action and Defects Inspection Report General Findings Defect No. Description Comments A Prepare estimates, prioritise and obtain funding to rectify the defects identified in the Combined Defects Inspection Report. Not Commenced. B Preparation of a condition rating for each levee based on the risk of failure and the likely severity of a failure. Not Commenced. C D E F G H Preparation of an Operation & Maintenance Manual including a Maintenance and Inspection Plan. Specifically with respect to the Operation of Rowes Cut investigate the requirement to close the two outer gates of the Korogoro Creek Choke when Rowes Cut is operating. A hydraulic/water resources engineer be engaged to revisit the1% AEP design flood levels, extreme flood events and minimum freeboard requirements of the entire levee system. Modelling work should also be carried out based on Rowes Cut Channel not providing any flood relief (i.e. no flood water escaping to the ocean via the channel) and the levee design levels adjusted accordingly. Depending on the findings of Defect No D, and if necessary, raise the existing levee levels to the design levels on the entire levee system in accordance with the recommended design/actions of JK Geotechnics. Depending on the findings of Defect No D, investigate the potential low point in the coastal sand dunes which may result in water escaping into Hat Head Village. Investigate the Environmental Approvals required to reinstate Rowes Cut. This investigation should also ascertain whether SEPP (Infrastructure) 2007 would be applicable. Depending on the findings of Defect No D, prepare a Contingency Operational Plan for when it is necessary to open Rowes Cut The plan should detail: 1. when and how plant such as excavators are to be bought to site and placed on standby 2. the environmental requirements of the work 3. the exact trigger for the opening of Rowes Cut. Not Commenced.. Proposed to develop one document for the entire Lower Macleay Floodplain. Not Commenced Not Commenced Not Commenced Discussions with NPWS commenced Not Commenced I Submit a late Natural Disaster Claim for funding for the repair of the Korogoro Creek bank/levee erosion. Not Commenced J Once the levee design levels are finalised, provide a concrete pedestrian / cycleway on the crest of the levee to eliminate any Not Commenced further erosion issues. This would form part of the overall Hat Head Cycleway.

62 Combined Action and Defects Inspection Report Village Levee Defect No. Chainage Description J K Geotechnics Report Section 1-80 Large tree in upstream shoulder & Flood Gate: Concrete headwall surrounded the flood gate. The gate was not completely closed. A crack was present in the assembly. The concrete connection was cracked, contained a penetration as well as concrete cancer Stormwater Inlet: The inlet comprised a 450mm diameter concrete pipe and contained leaf litter and eroded debris. Priority Level 4.5 & (repair flood gate) 3 (repair head wall) 4.2 (Item 2), 4.5 & Two large trees in upstream shoulder. 4.2 (Item 2), & Stormwater Inlet: The inlet comprised a 450mm diameter concrete pipe surrounded by a concrete headwall. Flood Gate: Concrete headwall surrounded the flood gate. The gate was closed. The concrete encasement surrounding the pipe outlet had broken off in some sections, revealing voids Stormwater Inlet: The inlet comprised a 375mm diameter concrete pipe surrounded by an ad-hoc dry stacked boulder wall. A 100mm diameter ag pipe daylighted adjacent to the crown of the concrete pipe. Both inlets contained leaf litter and grass clippings. Flood Gate: Concrete headwall surrounded the flood gate. The gate was closed. Debris had accumulated in front of the gate however it did not appear to impede operation. The concrete encasement surrounding the pipe outlet had broken off in some sections, revealing voids. 1 (investigate creek bank stabilisation) 2 (for clean out) 3 (for improvement) 1 (investigate creek bank stabilisation) 3 (for reconstruction) 4.5 & (for clean out and CCTV inspection & treatment of ag pipe) 3 (for repair/improvement) Frangipani tree on downstream shoulder Stormwater Inlet: The inlet comprised a letterbox pit. To the south-west of the pit, adjacent to the tree, a low spot existed. Leaf litter and debris were observed at the letterbox pit opening. Flood Gate: Concrete headwall surrounded the flood gate. The gate was closed and partially buried/obstructed by debris. The condition of the head wall and concrete surrounds to the pipe could not be observed due to dense vegetation (for clean out and topping up of low spot )

63 Combined Action and Defects Inspection Report Village Levee Defect No. Chainage Description J K Geotechnics Report Section to 485 Tree root, approximately 100mm diameter, extended over levee crest. The tree root most likely belonged to the gum tree located 2.5m beyond the downstream toe. Black builder s plastic exposed along levee crest. Priority Level Tree roots extended over levee crest Stormwater Inlet: The inlet was completely blocked by debris and obscured by long grass. Flood Gate: Concrete headwall surrounded the flood gate. The gate was closed and partially buried/obstructed by debris. The condition of the head wall and concrete surrounds to the pipe could not be inspected due to the accumulated debris Tree roots up to 80mm diameter extended over crest for a 13.5m length of levee & Stormwater Inlet: The inlet comprised a letterbox pit. The letterbox pit opening was partially buried by leaf litter and debris. Flood Gate: Concrete headwall surrounded the flood gate. The gate was closed. The head wall and concrete surrounds to the pipe appeared to be in good condition Stormwater Inlet: The inlet comprised a letterbox pit. The letterbox pit opening was mostly obstructed by (uncut) long grass. Flood Gate: In this area, the upstream toe of the levee had eroded out and was undercut. Despite the upstream shoulder being covered with dense grass, there was evidence of erosion and subsidence on the shoulder itself. A concrete headwall surrounded the flood gate. The erosion had exposed the back of the headwall. Debris had jammed open the flood gate to 745 Riverbank Erosion: An approximately 40m length of riverbank had eroded. The erosion, which included soil slumping, had undercut the upstream toe of the levee. Some rock armour was observed along the riverbank to 810 Riverbank Erosion: An approximately 65m length of riverbank had eroded but to a lesser extent than observed between CH705 & CH745 (Item 20). The erosion had undercut the upstream toe of the levee. Rock armour was present along the riverbank to 810 Large Trees: Three large trees were located on the downstream shoulder. Along this 40m length of levee, tree roots extended over the crest. Black builders plastic exposed along the crest was quite noticeable along this root affected section of levee (Item 2) 1 (investigate creek bank stabilisation) 4.2 (Item 2) 1 (investigate creek bank stabilisation) & 4.8 3

64 Combined Action and Defects Inspection Report Village Levee Defect No. Chainage Description J K Geotechnics Report Section Birds Nest: A 100mm diameter horizontal nest hole was observed mid-slope on the upstream shoulder. The nest hole was 800mm deep when probed with a steel tape measure Stormwater Inlet on Town Side of Creek Street: The inlet comprised a letterbox pit. The letterbox pit opening was partly obstructed by (uncut) long grass. Stormwater Inlet on Creek Side of Creek Street: The inlet comprised a letterbox pit. The letterbox pit opening was partly obstructed by (uncut) long grass. Flood Gate: Concrete headwall surrounded the flood gate. The gate was closed. The head wall appeared to be in good condition. Debris had accumulated in front of the gate however it did not appear to impede operation Stormwater Inlet: The inlet comprised a letterbox pit. The letterbox pit opening was partly obstructed by (uncut) long grass. Flood Gate: The flood gate could not be found amongst the dense fern and leaf litter coverage. Priority Level to 1038 Tree roots extended over crest for an approximate 20m length of levee & to 1085 Birds Nests: Five 50mm diameter horizontal nest holes were observed mid-slope on the upstream shoulder. The nest holes ranged in depth between 50mm and 900mm when probed with a steel tape measure Stormwater Inlets: Grated stormwater pits on either side of Creek Street. Flood Gate: Located approximately 10m beyond the upstream toe of the levee, within the creek inter-tidal zone. The gate was closed. Concrete headwall surrounded the flood gate and appeared to be in good condition. There was some erosion of rock fill/armour adjacent to the headwall Stormwater Inlet: The inlet comprised a letterbox pit. The letterbox pit opening was partly obstructed by (uncut) long grass and litter. Flood Gate: The flood gate was located on the creek bank, within the inter-tidal zone. The gate was closed. The pipe behind the flood gate was surrounded by concrete which appeared to have been placed in an ad-hoc manner. The bank material surrounding the concrete had eroded out, resulting in cracking of the concrete (for clean out) 3 (for improvement) Crest erosion due to pedestrian/bicycle traffic. 4.7 & Fig tree at toe of downstream shoulder & 4.8 3

65 Combined Action and Defects Inspection Report Village Levee Defect No. Chainage Description J K Geotechnics Report Section Priority Level Gouge in downstream shoulder of levee. 4.2 (Item 8) 2 or Stormwater Inlet: Grated stormwater pit at downstream toe of levee. Flood Gate: Located approximately 7m beyond the upstream toe of the levee. The gate was closed. Concrete headwall surrounded the flood gate. The headwall was in poor condition with a vertical crack adjacent to the flood gate. There was faulting across the crack by approximately 10mm. Debris and leaf litter had accumulated in front of the gate however it did not appear to impede operation (for clean out) 3 (for improvement) Tree root up to 100mm diameter extended over levee crest Stormwater Inlet: Grated stormwater pit at downstream toe of levee. Flood Gate: Located approximately 6m beyond the upstream toe of the levee. Concrete headwall surrounded the flood gate and appeared to be in good condition. The gate was closed and partially buried/obstructed by debris. Beyond the flood gate was an unlined drainage channel through the beach. Water was ponding in the channel to 1578 Tree roots extended over crest for an approximate 18m length of levee. Large trees were located within 2.5m of the upstream toe Stormwater Inlet: The inlet comprised a concrete pipe and appeared to be partially obstructed by vegetation, eroded debris, leaf litter and litter. The diameter of the pipe could not be measured as a campsite had been set up over the inlet. Flood Gate: Concrete headwall surrounded the flood gate. The flood gate was almost completely buried by debris and was forced closed. General Various Dense tree coverage adjacent to upstream toe of levee. Chainages -110 to -20, 0 to 50, 90 to 510, 675 to 710, 810 to 1070, 1570 to (for clean out) 3 (for improvement) General - Increasing the height of the levee where required. 4.7, 4.8 &

66 Combined Action and Defects Inspection Report Control Levee Defect No Chainage Description J K Geotechnics Report Section(s) Box culvert investigation (if required). 4.2 (Item 4) 1 Priority Level Timber plank protection above box culvert. 4.2 (Item 4) Patchy tree coverage along downstream toe Erosion behind south-western wing wall of box culvert Restricting vehicular access onto the levee 4.2 (Item 5) Increasing the height of the levee and box culvert where required and construction of an unsealed access road The recommended priority levels are based on the following criteria: Priority Level Description 1 The hydraulic assessment and additional geotechnical investigations should be commissioned as soon as possible. 2 Preliminary works that should be completed as soon as practicable by KSC whilst the hydraulic assessment and additional geotechnical investigations are being completed. 3 Specific remedial works that will likely be included in the raising or reconstruction of the levee. Once the hydraulic assessment, additional geotechnical investigations and detailed design have been completed, this work must be carried out as soon as practicable.