Chapter 14 - Flooding - Metro Mining · Skardon River Bauxite Project Chapter 14 - Flooding Page 14-4 The crossing must have a minimum (combined) culvert aperture width of 2.4 m or

CHAPTER 14 - FLOODING

GULF ALUMINA LTD – SKARDON RIVER BAUXITE PROJECT

Skardon River Bauxite Project Chapter 14 - Flooding

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TABLE OF CONTENTS

14.1 Introduction ..................................................................................................... 14-1 14.2 Environmental Objectives and Performance Outcomes ..................................... 14-1 14.2.1 Environmental Objectives ........................................................................................ 14-1 14.2.2 Performance Outcomes ........................................................................................... 14-1 14.3 Flood Modelling ............................................................................................... 14-2 14.3.1 Rainfall and Intensity-Frequency-Duration Relationship ......................................... 14-2 14.3.2 Extreme Rainfall Events ............................................................................................ 14-2 14.3.3 Flow Regimes and Runoff ......................................................................................... 14-2 14.3.4 Peak Flows ................................................................................................................ 14-2 14.3.5 Flood Model ............................................................................................................. 14-2 14.3.6 Namaleta Crossing.................................................................................................... 14-3 14.4 Flood Model Results ......................................................................................... 14-4 14.4.1 Mining Areas ............................................................................................................ 14-4 14.4.2 Namaleta Creek ........................................................................................................ 14-8 14.4.3 Kaolin Water Storage Pits ....................................................................................... 14-10 14.4.4 Skardon River ......................................................................................................... 14-11 14.5 Potential Impacts and Mitigation Measures .................................................... 14-15 14.6 Risk Assessment ............................................................................................. 14-15 14.7 Cumulative Impacts ........................................................................................ 14-16 14.8 Conclusion ..................................................................................................... 14-16

Tables

Table 14-1 Storm Event Peak Flows ........................................................................................... 14-3 Table 14-2 Flood Depths ............................................................................................................ 14-4 Table 14-3 Risk Assessment and Management Measures for Flooding Impacts ..................... 14-15

Figures

Figure 14-1 Namaleta Creek Crossing – Downstream View ........................................................ 14-3 Figure 14-2 1:50 Year Flood Event .............................................................................................. 14-5 Figure 14-3 1:100 Year Flood Event ............................................................................................ 14-6 Figure 14-4 1:2000 Year Flood Event .......................................................................................... 14-7 Figure 14-5 Flood Levels – Namaleta Creek Cross Section .......................................................... 14-9 Figure 14-6 Flood Levels – Namaleta Creek Schematic ............................................................. 14-10 Figure 14-7 Flood Lines for AEP 1:50 Year Event....................................................................... 14-11 Figure 14-8 Skardon River Flood Depth – PMP Flood 1:100 Year Flood ................................... 14-12 Figure 14-9 Skardon River Flood Depth – PMP Flood ............................................................... 14-13 Figure 14-10 Port Infrastructure Area Flood Depths and Contours ............................................ 14-14


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14. FLOODING

14.1 Introduction

This chapter describes the potential for flooding under different rainfall events and the potential impacts on Project infrastructure and activities. Flood modelling has been undertaken for Namaleta Creek and a flood model for the Skardon River has been reviewed. The influence of Namaleta Creek crossing on flood behaviour has been modelled.

Information in this chapter is primarily based on the information provided in Appendix 4. Metro Mining Ltd has prepared a flood model for the Skardon River as part of the approvals process for their Bauxite Hills Project (CDM Smith, 2015). This information has been available to Gulf Alumina through an information sharing agreement. The results of the Skardon River flood model are presented below in relation the Skardon River Bauxite Project. This flood model has not been reviewed in detail by Gulf Alumina and therefore Gulf Alumina makes no comment on the assumptions or accuracy of the flood model.

14.2 Environmental Objectives and Performance Outcomes

The environmental objectives and performance outcomes below are based on Schedule 5, Table 2 of the Environmental Protection Regulations 2008 (EP Regulation). The mitigation and management measures presented in this chapter are designed to achieve these environmental objectives and performance outcomes. The environmental management plan (EM Plan) presented in Appendix 13 provides a consolidated description of these mitigation and management measures.

14.2.1 Environmental Objectives

The activity will be operated in a way that protects environmental values of waters.

The location for the activity on a site protects all environmental values relevant to adjacent sensitive

uses.

14.2.2 Performance Outcomes

Mining activities and Port infrastructure will be located above the 1:100 year flood level.

Areas used for storing environmentally hazardous materials in bulk are located taking into

consideration the likelihood of flooding.

Containers used for the storage of hazardous contaminants are secured to prevent the removal of the

containers from the site by a flood event.

Contingency measures will prevent or minimise adverse effects on the environment due to unplanned

releases or discharges of contaminants to water.

Creek and drainage feature crossing structures will minimise change to existing flow regimes and flood

behaviour.


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14.3 Flood Modelling

14.3.1 Rainfall and Intensity-Frequency-Duration Relationship

Rainfall and the Intensity-Frequency-Duration (IFD) relationship are described in Chapter 13 and Appendix 4.

14.3.2 Extreme Rainfall Events

The approach described in Book VI Section 3.4 of the “Australian Rainfall and Runoff: A Guide to Flood Estimation, Vol. 1” (AR&R) was used to estimate the rainfall depths associated with extreme events for the Project area. The approach is recommended for storm durations in excess of 6 hours. The annual exceedance probability (AEP) = 0.0000001 event is regarded as the probable maximum precipitation (PMP) under the guidelines of AR&R (it is determined for a location on the basis of the catchment area).

14.3.3 Flow Regimes and Runoff

Flow regimes and runoff modelling and estimates are described in Chapter 13.

14.3.4 Peak Flows

The AEPs used in assessment of extreme flow behaviour across the area were derived from the IFD relationship. The methodology and formulas are described in Appendix 4. Based on the catchment, the appropriate rainfall intensities (mm/hr) were identified for storm recurrence intervals of 1 to 100 years.

14.3.5 Flood Model

The Hydrologic Engineering Centre River Analysis System software, version 4.1.0, January 2010 (HEC-RAS) model was used for hydraulic (flood) modelling. HEC-RAS is a one-dimensional package for hydraulic analysis of open channel flow to predict water surface profiles over a range of different boundary conditions. It allows incorporation of hydraulic flow control structures such as complex bridge deck and culvert groups for river crossings, and simulates channel and floodplain flow regimes. HEC-RAS requires geometric data of river channels and floodplains, geometric and physical properties of control structures (e.g. pipe roughness), flow boundary conditions and Manning’s coefficients (n) as inputs. The HEC-RAS model developed for the Project, including input assumptions, is described in Appendix 4.

High resolution data was obtained by Gulf Alumina for the Project area. Light Detection and Ranging (LIDAR) datasets measured within a river channel measure the water surface only and not the underlying river geometry. Since no ground survey cross-sections were available at the time of modelling, river channel geometries were assumed for Namaleta Creek. Depths of 2 m were applied to the channels in the cross-sections extracted from the LIDAR survey data.

It has not been possible to calibrate the model against any recorded extreme events in the local Creek catchment. In the absence of recorded local events, design flood flows for the catchment have been applied in the assessments.

Flood runoff for the hydraulic modelling was determined from storm depths derived for a range of extreme events using the AR&R approach referred to in Section 14.3.2 for the storm events shown in Table 14-1. The events with an AEP of >1:100 years were based on IFD information provided by BoM, and the flood peaks were calculated using the approach described in Section 14.3.4.


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Table 14-1 Storm Event Peak Flows

AEP event AEP (%) Risk of occurrence over proposed mine life* (%) Peak Flow (m3/s)

1 in 20 year 5.0% 39.3% 101.1

1 in 50 year 2.0% 18.1% 127.5

1 in 100 year 1.0% 9.5% 145.7

1 in 1000 year 0.1% 1.0% 309.3

1 in 2000 year 0.05% 0.5% 401.2

*for the expected project life of 10 years.

14.3.6 Namaleta Crossing

A schematic cross-section of the Namaleta Creek crossing is shown in Figure 14-1. The culverts and deck level of the crossing were sized for a 1:50 year AEP design flood standard. Figure 14-1 shows the preliminary sizings of the culvert groupings proposed to convey water through the haul crossing embankment, as applied in the model. Note that the culverts are distorted by the aspect ratio of the cross-section.

Figure 14-1 Namaleta Creek Crossing – Downstream View

Preliminary sizing of the culvert groups and bridge deck level required at the crossing was carried out with reference to the guidelines and recommendations of the Road Drainage Manual, Department of Transport and Main Roads, July 2015 (TMR-RDM).

Detailed design will be carried out in compliance with the TMR-RDM and with:

Roads in the Wet Tropics Manual, Transport and Main Roads, 1998)

design detail requirements of the Code for Self-Assessable Development; Minor Waterway Barrier

Works Part 3: Culvert Crossings, Code number: WWWBW01 (April 2013), Department of Agriculture,

Fisheries and Forestry (DAFF).

The following specific conditions are noted from the Code for addressing moderate impact waterways (applicable to Namaleta Creek):

Works must commence and finish within a maximum time of 360 calendar days and instream

sediment and instream silt control measures associated with the works must be removed within this

period.


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The crossing must have a minimum (combined) culvert aperture width of 2.4 m or span 100% of the

main channel width.

All new or replacement culvert cells must be installed at or below bed level.

The internal roof of the culverts must be >300 mm above ‘the commence to flow’ water level.

Where the cell is installed at less than 300 mm below bed level (potentially the case for the Namaleta

Crossing), the culvert floor must be roughened throughout to approximately simulate natural bed

conditions.

The culvert must be installed at no steeper gradient than the waterway bed gradient.

Apron and stream bed scour protection must be provided in line with the design requirements of the

Code.

It is expected that the Code requirements can be met for the Namaleta crossing design.

14.4 Flood Model Results

14.4.1 Mining Areas

The model was used to consider the potential flooding of mining pits adjacent to Namaleta Creek and for the drainage feature tributary of Namaleta Creek between Pit 14 and Pit 15. Modelled flood depths for the 1:50 year, 1:100 year and 1:2000 year flood events are shown in Figure 14-2, Figure 14-3 and Figure 14-4 respectively. These figures show topography in green, yellow and orange shades (ranging between 0 m and 12 m AHD) and flood depth in blue shades ranging between 0 m and 3 m.

There are some minor overlaps of modelled flood depth and mining areas on the margins of Pit 12 (upstream and downstream Namaleta Creek), Pit 14 and Pit 15. These are an artefact of the coarse scale of resource modelling. Economic bauxite is unlikely to be found in low lying areas subject to flooding and therefore mining areas will not be subject to flooding from the modelled flood events. Refinement of the resource model is unlikely to result in overlaps between mine pits and flood zones. By way of example, Table 14-2 shows modelled flood depths in the areas of overlap with the coarse scale resource model. It is reiterated that mining is not expected in these areas of overlap.

Table 14-2 Flood Depths

Location Flood Depth at Location (m)

AEP 1:50 year AEP 1:100 year AEP 1:1000 year AEP 1:2000 year

Mining Pit 12: Upstream 0.11 0.61 1.46 1.62

Mining Pit 12: Downstream - - 0.22 0.53

Mine Pit 14 - - - 0.15

Mine Pit 15 - - 0.08 0.10

If required following refinement of resource modelling, flood protection of the pits will be addressed in detail at design and managed as an operational activity with response planning in place to anticipate and prevent adverse effects of local flooding.

The proposed crossing of the drainage feature between Pit 14 and Pit 15 is located near the eastern side Pit 14 and Pit 15, which is upstream of all modelled flood depths and therefore not impacted by flooding. The haul road crossing between Pit 14 and Pit 15 will include appropriate cross-drainage to ensure that seasonal flow behaviour in the channel, which is often dry, is not impaired.

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14.4.2 Namaleta Creek

Figure 14-5 shows the predicted flood levels against haul road crossing (shaded brown), looking downstream. Figure 14-6 shows a schematic of the information in Figure 14-5, showing the crossing level at 4.5m RL and the modelled height of flood levels for different AEP rainfall events. The preliminary arrangement for the culvert conveyance is sufficient to meet design flood (AEP 1:50 year) estimated by the model. The model also demonstrates that the haul road crossing will meet a design flood level of 1:100 years (i.e. the haul road crossing will not be overtopped by a 1:100 year flood). The haul road crossing will be overtopped by a 1:1000 year flood event.

TMR-RDM makes recommendations of acceptable downstream velocities to avoid erosion which can occur over a significant distance downstream of the culvert outlets. For the conditions on the floodplain and in the channel section of the Namaleta Creek crossing, the target velocities recommended should lie in the range 1 to 1.5 m/s. Modelling shows that these target velocities are generally achievable in the floodplain section of the crossing but that in the main channel velocities could range between 2 and 4 m/s. Appropriate dissipation and protection measures will be installed to lower velocities in the main stream and possibly on the floodplain as well.

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Figure 14-6 Flood Levels – Namaleta Creek Schematic

14.4.3 Kaolin Water Storage Pits

Figure 14-7 shows flood lines for the AEP 1:50 year event, for which modelling predicts there will be no overtopping of the haul road crossing, the Claystone Pit or Water Pit. The model trial does indicate possible overtopping of the Fluvial Pit embankments at its downstream end near the outlet. For the higher magnitude events:

AEP 1:100 year - overtops the Claystone Pit, Water Pit or Fluvial Pit embankments, resulting in

potential uncontrolled discharge from these storages. However, the haul road will not be affected.

AEP 1:1000 year and higher - overtops Creek crossing, the Claystone, Water and Fluvial Pit

embankments, resulting in potential damage to the haul road, and to the storages.

The existing Plan of Operations described kaolin mine water storage management, rehabilitation and decommissioning. The current EA provides conditions of approval for the kaolin mine water storages and hence management of these storages is not considered further. The environmental management plan (EM Plan) (refer to Appendix 13) provides a summary of the kaolin mine water storage management, rehabilitation and decommissioning. During flood events flows in the receiving environment will be high and likely to have increased turbidity.


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Figure 14-7 Flood Lines for AEP 1:50 Year Event

14.4.4 Skardon River

The flood depths from the Bauxite Hills Project flood model for the Skardon River (CDM Smith, 2015) are presented in Figure 14-8 for the 1:100 year flood event, and in Figure 14-9 for the PMP flood event. Figure 14-10 shows the 1:100 year flood levels at the Port infrastructure area. The 1:100 year flood event approaches the 2.5 m AHD contour.

For the purposes of this assessment, the proposed Port infrastructure area and mine pit for the Project have been overlaid on the flood model images for the Bauxite Hills project.

Even under the PMP event, flood levels do not reach the Port infrastructure area (other than infrastructure within the estuary itself such as the wharf) or any mine pits. The proposed Port infrastructure, other than the wharf options, is above 3m AHD. The ground elevation rises fairly steeply from the level of the Skardon River estuary, with highest astronomical tide approximately equivalent to 2 m AHD. The bauxite plateau rises sharply from the low lying costal margins and which confines flooding even under a PMP flood event, thereby preventing interaction between flood waters and mine pits.

Storm surge within the Skardon River is described in Chapter 17, which concludes that storm tides are not considered to present a significant risk in the area and will be below the Port infrastructure area level.

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14.5 Potential Impacts and Mitigation Measures

Flooding of mine pits or infrastructure areas containing contaminants (e.g. hydrocarbons) has the potential to release sediments and contaminants into local waterways. Flood modelling demonstrates that flooding will not interact with, or impact on, Project infrastructure and activities (i.e. mining areas), which are situated above modelled flood levels, other than the Namaleta Creek crossing and wharf infrastructure at the Port.

All mine pits and Port infrastructure (other than the wharf) will be situated above the 1:100 year flood level. This will minimise the risk of release of:

hazardous materials or contaminants from the Port infrastructure area as a result of flooding of Port

infrastructure area by the Skardon River.

sediment from sediment ponds at the Port infrastructure area

sediment from mine pits.

Namaleta Creek crossing will be designed in accordance with the standards and guidelines described in Section 14.3.6. Appropriate dissipation and protection measures will be required to lower velocities downstream of culverts in the main stream and possibly on the floodplain as well.

The preliminary crossing design for Namaleta Creek predicts that the crossing will not impact on the existing kaolin mine water storages, other than the fluvial pit, for a 1:50 year flood event. For flood events greater than 1:50 years flooding may impact on the existing kaolin water storage pits. The kaolin mine water storages are already subject to the risk of flooding and management of these storages is undertaken in accordance with the existing Plan of Operations and environmental authority conditions of approval. In addition the EM Plan in Appendix 13 describes measures for kaolin mine water storage management, rehabilitation and decommissioning.

The existing crossing of Namaleta Creek consists of an earthen crossing (10 – 15 m wide), where two cylindrical pipes connect the upstream and downstream reaches of Namaleta Creek. The preliminary design for the crossing is likely to result in the hydrology of the crossing area more closely resembling its pre-disturbance condition, thereby improving ecological function, including fish passage.

Wharf infrastructure will be designed to withstand flooding in the Skardon River as well as tidal movements and storm surge.

14.6 Risk Assessment

A risk assessment of the likelihood and significance of flooding impacts is provided in Table 14-3. The risk assessment considers mitigated risk; that is, the impact from the Project with the implementation of management measures. The risks from flooding and to flood behaviour are low.

Table 14-3 Risk Assessment and Management Measures for Flooding Impacts

Environmental Value

Impacts / Emissions / Releases

Proposed Management Practices

Likelihood Consequence (Magnitude)

Risk Rating

Water quality Hydrology

Flooding interacts with, and impacts on, Project mining activities and infrastructure. Refer Section 14.4.

Refer Section 14.5

Rare Minor Low


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Environmental Value

Impacts / Emissions / Releases

Proposed Management Practices

Likelihood Consequence (Magnitude)

Risk Rating

The Project alters flood behaviour. Refer Section 14.4.

Refer Section 14.5

Unlikely Minor Low

14.7 Cumulative Impacts

Based on Figure 14-8 and Figure 14-9, flooding of the Skardon River is not predicted to impact Metro Mining’s Bauxite Hills project infrastructure and activities, other than sections of haul roads and potentially Port infrastructure in water. The Bauxite Hills project is not located within potential flood zones of Namaleta Creek. Therefore any flooding of Namaleta Creek and / or the Skardon River is highly unlikely to result in cumulative impacts.

The Project will have only minor impacts on flood behaviour from the crossing of Namaleta Creek, and no impacts on the flood behaviour of the Skardon River. The Bauxite Hills project may impact on flood behaviour at various haul road crossing locations of tributaries of the Skardon River, but will not impact flood behaviour of Namaleta Creek. Hence there will not be cumulative impacts on flood behaviour.

14.8 Conclusion

Independent flood models have been developed for the Skardon River and Namaleta Creek. Flood modelling demonstrates that flooding will not interact with, or impact on, Project infrastructure and activities (i.e. mining areas), which are situated above modelled flood levels, other than the Namaleta Creek crossing and wharf infrastructure at the Port.

Namaleta Creek crossing will be designed in accordance with relevant standards and guidelines to minimise changes to hydrological and ecological function, with potential improvement in hydrological and ecological function in comparison to the existing crossing design.

Wharf infrastructure will be designed to withstand flooding in the Skardon River as well as tidal movements and storm surge.

Flood modelling for Metro Mining’s Bauxite Hills Project demonstrates low risks from flooding of activities and infrastructure, and no cumulative impacts to flood behaviour with potential changes to flood behaviour limited to the Skardon River and not Namaleta Creek.

The proposed Project design, including selection of location for activities and infrastructure, and measures to mitigate impacts from flooding and to flood behaviour will result in the Project achieving the relevant environmental objectives and performance outcomes. There is low risk of impacts from flooding and to flood behaviour.

Documents

Chapter 14 - Flooding - Metro Mining · Skardon River Bauxite Project Chapter 14 - Flooding Page 14-4 The crossing must have a minimum (combined) culvert aperture width of 2.4 m or