appendix h erosion and sediment control strategy - · pdf fileerosion and sediment control...

APPENDIX H

EROSION AND SEDIMENT CONTROL STRATEGY

March 2014

JEJEVO/ISABEL B PROJECT

Erosion and Sediment Control Strategy

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Report Number. 137633001-3008-R-Rev0-2400

JEJEVO/ISABEL B PROJECT EROSION AND SEDIMENT CONTROL STRATEGY

March 2014 Report No. 137633001-3008-R-Rev0-2400 i

Table of Contents

1.0 INTRODUCTION ........................................................................................................................................................ 1

1.1 Compliance Criteria ...................................................................................................................................... 1

1.2 Method .......................................................................................................................................................... 2

2.0 DESCRIPTION OF EXISTING SITE .......................................................................................................................... 2

2.1 Location ........................................................................................................................................................ 2

2.2 Topography................................................................................................................................................... 4

2.3 Climate ......................................................................................................................................................... 4

2.3.1 Rainfall .................................................................................................................................................... 4

2.4 Background Total Suspended Solids Concentration ..................................................................................... 6

2.5 Geology and Soils ......................................................................................................................................... 6

2.5.1 Particle Size Distribution ......................................................................................................................... 7

2.5.2 Settling Columns ..................................................................................................................................... 8

2.5.3 Hydraulic Conductivity ............................................................................................................................. 9

3.0 EROSION PROCESS .............................................................................................................................................. 10

3.1 Erosion Processes ...................................................................................................................................... 10

3.2 Road Erosion .............................................................................................................................................. 10

3.3 Active Mine Area Erosion ........................................................................................................................... 10

3.4 Disposal/Revegetated Area Erosion ........................................................................................................... 11

3.5 Stockpiles and Jetty Facilities ..................................................................................................................... 11

4.0 CONCEPTUAL EROSION AND SEDIMENT CONTROL MEASURES ................................................................... 12

4.1 Design Principles ........................................................................................................................................ 12

4.2 Conceptual Control Measures Overview .................................................................................................... 12

4.3 Sediment Basins ......................................................................................................................................... 17

4.3.1 Maintenance ......................................................................................................................................... 18

4.4 Pocket ponds .............................................................................................................................................. 18

4.5 Road Water Management ........................................................................................................................... 18

4.6 Active Mine Area Water Management ........................................................................................................ 19

4.7 Rehabilitation of Disturbed Areas ............................................................................................................... 19

4.8 Jetty Site Water Management..................................................................................................................... 20

5.0 DESIGN ................................................................................................................................................................... 20


March 2014 Report No. 137633001-3008-R-Rev0-2400 ii

5.1 Design Standards and Methodology ........................................................................................................... 20

5.2 Sediment Basins ......................................................................................................................................... 20

5.2.1 Basin Overview ..................................................................................................................................... 20

5.2.2 Rainfall Depth ....................................................................................................................................... 20

5.2.3 Basin Sizing .......................................................................................................................................... 21

5.2.4 Basin Volumes ...................................................................................................................................... 22

5.2.5 Alternative Designs ............................................................................................................................... 22

5.3 Pocket Ponds.............................................................................................................................................. 23

5.3.1 Pocket Pond Overview .......................................................................................................................... 23

5.3.2 USLE Factors ........................................................................................................................................ 23

5.3.3 Design Rainfall Depth ........................................................................................................................... 23

5.3.4 Pocket pond Sizing ............................................................................................................................... 24

5.4 Roadside Channels .................................................................................................................................... 24

5.4.1 Channel Overview ................................................................................................................................. 24

5.4.2 Design Assumptions ............................................................................................................................. 24

5.4.3 Dimensions ........................................................................................................................................... 25

6.0 MODELLED SEDIMENT YIELD .............................................................................................................................. 26

7.0 CONCLUSIONS ....................................................................................................................................................... 26

8.0 REFERENCES ......................................................................................................................................................... 27

TABLES Table 1: Historical Climate Stations .................................................................................................................................... 4

Table 2: Hydraulic Conductivity and Hydrologic Soil Group Results ................................................................................... 9

Table 3: Sediment Basin Type Selection Criteria .............................................................................................................. 17

Table 4: Preliminary Sediment Basin Design Criteria ........................................................................................................ 17

Table 5: Conceptual Volume Requirements for the Sediment Basins Located in the Active Mine Areas .......................... 22

Table 6: Conceptual Drainage Sizing for the 2 Year ARI Rainfall Event ........................................................................... 25

Table 7: Conceptual Drainage Sizing for the 5 Year ARI Rainfall Event ........................................................................... 25

Table 8: Conceptual Drainage Sizing for the 10 Year ARI Rainfall Event ......................................................................... 25

FIGURES Figure 1: Freshwater Quality Local Study Area ................................................................................................................... 3

Figure 2: Average Monthly Rainfall - Santa Isabel Island Climate Stations IWS01 to IWS04 ............................................. 5

Figure 3: Cumulative Total Rainfall – Project Site, at Nuha (IWS04) and the Heple Catchment (JRG02) .......................... 5

Figure 4: Dry Particle Size Distribution Results ................................................................................................................... 7

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March 2014 Report No. 137633001-3008-R-Rev0-2400 iii

Figure 5: Hydrometer Particle Size Distribution ................................................................................................................... 8

Figure 6: Settling Column Turbidity Results ........................................................................................................................ 9

Figure 7: Example Mining Profiles Demonstrating Water Flow ......................................................................................... 13

Figure 8: Conceptual Sediment Basin Layout – Aerial Perspective 1 ............................................................................... 14

Figure 9: Conceptual Sediment Basin Layout – Aerial Perspective 2 ............................................................................... 15

Figure 10: Conceptual Sediment Basin Layout – Aerial Perspective 3 ............................................................................. 16

Figure 11: Five Day Rainfall Depths for IWS01 and IWS04 for a Range of Percentile Storm Events .............................. 21

ATTACHMENTS Attachment I Water Management and Erosion and Sediment Control Toolbox

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March 2014 Report No. 137633001-3008-R-Rev0-2400 1

1.0 INTRODUCTION

SMM Solomon Ltd. (SMM Solomon) is developing the Solomon Islands Nickel Project (SINP) on five tenements on two islands in Solomon Islands. The islands and tenements are:

Choiseul Island (Choiseul tenement)

Santa Isabel Island (Jejevo, Isabel B, D and E tenements)

Environmental and Social Impact Assessments (ESIA) were completed and approved by the Solomon Islands government for the Choiseul, and Isabel D and E tenements in 2012. SMM Solomon is now submitting an ESIA and supporting documents for the Jejevo/Isabel B (the Project).

The Project includes:

mining area

mine haul road

ore stockpile

jetty

accommodation camp

mine administration buildings

transhipment mooring

SMM Solomon will mine two ore types for the Project, limonite and saprolite. The limonite and saprolite will be mined and stockpiled separately, then limonite will be transported to elsewhere and saprolite will be shipped to Japan for further processing. The Project will have a production of about 0.685 Mt per year of ore and will operate for about 14 years.

1.1 Compliance Criteria

The environmental governance for the Solomon Islands consists predominantly of the Environment Act 1998 (the Act). This legislation is “An Act to make provision for the protection and Conservation of the environment, the establishment of the Environment and Conservation Division and the Environmental Advisory Committee and for matters connected therewith or incidental thereto.” The Act provides for an integrated system of development control, ESIA and pollution control.

No water quality discharge standards are presently documented for Solomon Islands, as the regulations under the Act are currently being drafted. The International Finance Corporation (IFC) Guidelines (2007) recommend that site runoff released to the environment under normal operating conditions should achieve a Total Suspended Solids (TSS) concentration of less than 50 mg/L 95% of the time. The IFC (2007) allows for variation of these guidelines if alternative targets suitable for specific local project conditions can be adequately justified.

The background water quality monitoring data and sediment transport modelling results indicate that the existing river sediment concentrations exceed the IFC (2007) guidelines during periods of high flow. Therefore it is proposed that sediment control measures act to mimic the flow regime and sediment load of the pre-Project environment. The background water quality data collection is ongoing, and more specific TSS targets will be developed once baseline studies are completed.

The erosion and sediment control strategy (ESCS) therefore aims to provide best practice erosion sediment control (ESC) measures to minimise effects due to soil loss and sedimentation. The primary design guidelines used for sediment capture devices is the International Erosion Control Association (IECA) Best Practice Erosion and Sediment Control (2008) guidelines.



1.2 Method Four stages of control measures are proposed for the Project:

Stage 1 – erosion control at source by minimising exposed soils, revegetating and rehabilitating disturbed areas.

Stage 2 ― drainage ditches with appropriate armouring, clean and sediment laden water diversions, chutes with aprons or stilling basins to convey flows down steep slopes, pocket ponds and infiltration trenches and basins.

Stage 3 ― sediment basins to reduce the amount of sediment discharged from these areas into the natural environment.

Stage 4 – slope stabilisation using hard engineering or bioengineering (when applicable).

The control measures for the Project will primarily consist of Stage 1, 2 and 3 measures. Stage 4 measures may be required in specific circumstances, and will be detailed in a later design phase.

This report outlines these control measures and design considerations applicable to the ESCS.

2.0 DESCRIPTION OF EXISTING SITE 2.1 Location The Local Study Area (LSA) consists of the Isabel B and Jejevo tenements and includes the catchment areas (watersheds) and rivers within and downstream of the Project (Figure 1).

There are three major waterways and a number of smaller waterways located within the LSA. The major waterways are the Jejevo, Nuha and Heple Rivers. The smaller waterways consist of the Ola, Hughukapote, Sivoko, Jihro, Kolosighoni and Gajuhongari Rivers.

The LSA and infrastructure associated with the Project includes:

mining area

roads (existing and mine haul roads)

ore stockpile facilities

run of mine (ROM) pad

jetty

accommodation camp

mine administration buildings

transhipment mooring

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SCALE (at A3)DATUM WGS 84, PROJECTION UTM Zone 57 South

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03 MAR 2014

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FIGURE 1

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1. Tenement boundaries supplied by Client.2. Base data copyright © Solomon Islands Government, Ministry ofLand.3. Key Inset Bathymetry copyright © National Oceanic andAtmospheric Administration (NOAA), 2009.4. Key Inset Terrain copyright © Consultative Group onInternational Agricultural Research (CGIAR), 2013.

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2.2 Topography The mining area is typically steep, with over 50% of the LSA consisting of grades of above 20% (Appendix G). The steep terrain includes elevation changes between the coastal fringes and the nearby mountain ranges, with peaks over 1,000 m. The mountainous terrain is incised with steep valleys, many of which contain waterways that discharge to the ocean.

2.3 Climate Solomon Island’s proximity to the equator has resulted in a tropical climate, with uniform temperature and humidity and abundant rainfall in most months. Rainfall distribution is largely affected by the formation and migration of the South Pacific Convergence Zone and can vary considerably from one location to another.

The location of the historical climate stations considered in this analysis, and respective period of rainfall record, is provided in Table 1. SMM Solomon stations on Santa Isabel Island have been installed at a range of elevations to monitor the expected increase in rainfall totals at elevated sites. The SMM Solomon stations are located at IWS01, IWS02, IWS03, IWS04 and JRG02. The Solomon Islands Meteorological Service (SIMS) has long term monitoring stations at Henderson, Honiara and Auki. All of the SIMS weather stations are located in populated coastal areas (at low elevation).

Table 1: Historical Climate Stations

Station ID Owner Island Distance to Nuha

[km] UTM Easting Northing Elevation

[m] Rainfall Record Period

IWS01 SMM Santa Isabel 65 57 L 577 207 9 072 261 195 3 years IWS02 SMM Santa Isabel 55 57 L 568 212 9 081 016 425 2.5 years IWS03 SMM Santa Isabel 60 57 L 573 778 9 076 670 347 2.5 years IWS04 SMM Santa Isabel 0 57 L 518 722 9 099 962 15 10.5 months JRG02 SMM Santa Isabel 11 57 L 530 165 9 098 537 280 2 months Henderson SIMS Guadalcanal 170 57 L 615 826 8 957 654 8 32 years Honiara SIMS Guadalcanal 160 57 L 604 112 8 957 423 55 32 years Auki SIMS Malaita 190 57 L 690 620 9 028 698 11 32 years

The climate conditions are discussed in detail in the Climate Baseline Report (Appendix F). Rainfall is directly relevant to soil erodibility and development of the ESCS, as discussed in the following section.

2.3.1 Rainfall Rainfall in Solomon Islands in coastal (low elevation) stations ranges between 2,000 mm and 5,000 mm annually (Appendix F). Distinct wet and dry seasons are evident in some locations of Solomon Islands with long term data records. However on the southern side of larger islands such as Santa Isabel Island, and as is the case at the Project site, there is little seasonal variation in rainfall. The absence of a dry season on the southern side of larger islands is due to the influence of the rain bearing southeast trade winds from May to October. The long term record at Auki (southern Malaita) demonstrates much higher average annual rainfall (3,036 mm) over the 32 years of record compared to Honiara and Henderson (northern Guadalcanal), (1,845 mm and 1,924 mm respectively). The cumulative rainfall total at Nuha for the period of record (10.5 months) is 4,065 mm (Appendix F).

Average monthly rainfall for climate stations IWS01 to IWS04 for the available data period is provided in Figure 2.



Figure 2: Average Monthly Rainfall - Santa Isabel Island Climate Stations IWS01 to IWS04

Cumulative rainfall totals are higher at elevation than at coastal sites. Over a two month period at the Project site, rainfall at elevation 280 m (Heple catchment) (JRG02) was 1.16 times the rainfall at Nuha (IWS04) (note limited data record). The comparison of cumulative rainfall between the two sites is provided in Figure 3.

Figure 3: Cumulative Total Rainfall – Project Site, at Nuha (IWS04) and the Heple Catchment (JRG02)

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2.4 Background Total Suspended Solids Concentration Baseline freshwater quality within unmodified catchments in the LSA was generally of high quality. Water quality parameters were within the applicable water quality guidelines except at two sites, the Heple River and Gajuhongari River, which were relatively more turbid than other sites monitored. Median turbidity values for these sites exceeded the upper limit of the Australian and New Zealand Environment and Conservation Council and the Agriculture and Resource Management Council of Australia and New Zealand (ANZECC and ARMCANZ 2000) guidelines during normal flow conditions during the baseline investigation (Appendix C). These catchments are actively logged and this commercial logging activity is likely to be the cause of these increased turbidity levels.

Under normal flow conditions, it was found that TSS concentrations are below the IFC and ANZECC and ARMCANZ (2000) guidelines (<5 mg/L to 615 mg/L) for all sites. However, TSS concentrations exceed the guidelines during high flow events (Appendix B).

It therefore becomes unrealistic to develop an ESCS to reduce sediment concentrations to a state within the guidelines. The ESCS shall aim to maintain sediment concentrations which mimic the river conditions before commencement of mining activities.

2.5 Geology and Soils The nickel laterite deposits of Solomon Islands have developed under tropical conditions over ultramafic rocks. The laterite profile overlying the ultramafic rocks can be subdivided into six soil types that form two distinct zones. The lower silicate zone consists of saprolitic rock, rocky saprolite, and saprolite. The upper oxide zone consists of ferruginous saprolite, limonite, ferruginous zone and soil types. For the purposes of assessing site conditions and determining erosion risks, the soils at the resource areas have been classified as:

Top Soil

Limonite 2 and 3

Transitional

Decomposed

Highly Weathered

Bed Rock

A thin top soil lithology which is highly weathered with high nutrient content makes up the topsoil/overburden zone.

The limonite zone consists of the Limonite 2 and Limonite 3 lithologies. Limonite is characteristically soft and earthy with a high proportion of the minerals present which are very fine grained. There is a high proportion of pore space and consequently the water contents are relatively high.

Colours range from mid-brown (goethite) at the base of the limonite to deep red-brown (goethite-hematite) near ground surface. The red-brown limonite has a more sandy texture, lower water content and a higher load bearing capacity than the underlying brown limonite. As a consequence, roads constructed upon the natural surface require less paving than those constructed on deeper levels of exposed limonite.

The transitional zone lies between the saprolite and limonite zone. It consists of a mixture of these two zones.

The saprolite zone consists of the Decomposed and Highly Weathered lithologies, including boulders of bedrock in a soft, earthy matrix.



A detailed description of the geology and soils can be found in the Geology and Soils Baseline Report (Volume 3, Appendix A). The following sections summarise the main soil parameters that impact on the development of the ESCS.

2.5.1 Particle Size Distribution Particle size distribution (PSD) describes the relative amounts of gravel, sand, silt and clay within a sample of soil or rock. The proportion of different sized particle fractions in each sample is important, as it can have a large effect on the properties of the sample including the settling time of the soil particles. Soil samples, representing each of the seven lithologies identified in Section 2.5, were obtained from the resource area, as described in the Waste Management Baseline Report (Volume 5, Appendix C). Dry PSD analysis was conducted on these samples and the results are presented in Figure 4. The figure shows that the samples have similar distributions of gravel, sand and clays. The Top Soil sample has the largest proportion of medium gravel in comparison to other samples and no fine grained silt clay material. The results for the Top Soil, Limonite 2 and Limonite 3 material do not correspond with the results for the hydrometer PSD analysis, which is presented in Figure 5. This may indicate that this material is readily soluble in water and produces more fine sized particles. Further investigation into potential soil dispersion should be completed at a later design phase.

Figure 4: Dry Particle Size Distribution Results

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WM_003 WM_004 WM_005 WM_041 WM_011 WM_006 WM_038 Composite 1 WM_007 Composite 1 WM_008

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Dry Sample Particle Size Distribution

< 75 µm - Silt and Clay 75 µm - 200 µm - Fine Sand 200 µm - 600 µm - Medium Sand600 µm - 2 mm - Coarse Sand 2 mm - 6 mm - Fine Gravel 6 mm - 20 mm - Medium Gravel

Top Soil Limonite 2 Limonite 3 Transitional Decomposed Highly Weathered Bed Rock



Figure 5: Hydrometer Particle Size Distribution

2.5.2 Settling Columns Settling column analysis was conducted on four samples, Limonite 2 (WM_004), Limonite 3 (WM_005), Decomposed (WM_006) and Highly Weathered (WM_007) and the results are presented in Figure 6. For each of the soil types, four sample concentrations were used; 50 g, 5 g, 0.5 g and 0.05 g, corresponding to initial TSS concentrations of 50,000 mg/L, 5,000 mg/L, 500 mg/L and 50 mg/L, respectively.

Generally the 50 g, 5 g, 0.5 g and 0.05 g samples recorded consecutively decreasing turbidity values (i.e., the 50 g samples had the highest turbidity and the 0.05 g samples the lowest turbidity). However, the 50 g Decomposed (WM_006) sample showed a larger decrease in turbidity over 24 hours than the 5 g and 0.5 g Decomposed (WM_006) samples. The 50 g sample for Limonite 2 (WM_004) and Limonite 3 (WM_005) did not decrease in turbidity over 24 hours and remained greater than 2,000 NTU (Volume 5, Appendix C).

The 0.05 g samples represent an initial TSS concentration of 50 mg/L. It can be assumed that the initial turbidity recorded for this sample represents the discharge criteria for sediment basins. The results indicate that the 0.5 g (500 mg/L) samples of limonite 2, decomposed and highly weathered, experienced sufficient settlement to reach the discharge criteria of 50 mg/L within 24 hours. Initial TSS concentrations higher than this would require more than 24 hours or addition of flocculants for sufficient settlement to occur to meet the discharge criteria (Volume 5, Appendix C).

The results of the hydrometer tests also indicate that there is not a constant correlation between turbidity and TSS. As expected, each soil type has parameters that alter the turbidity reading. These parameters include aspects such as particle size and particle shape.

0%

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WM_002 WM_003 WM_004 WM_005 WM_011 WM_006 WM_007 WM_008

Hydrometer Particle Size Distribution

<2 µm - Clay 2 µm - 60 µm - Silt 60 µm - 200 µm - Fine Sand

200 µm - 600 µm - Medium Sand 600 µm - 2 mm - Coarse Sand 2 mm - 6 mm - Fine Gravel

6 mm - 20 mm - Medium Gravel 20 mm - 63 mm - Coarse Gravel 63 mm - 200 mm - Cobbles

Top Soil Limonite 2 Limonite 3 Transitional Decomposed Highly Weathered

Bed Rock



Figure 6: Settling Column Turbidity Results

2.5.3 Hydraulic Conductivity Five soil types were characterised against reference soil types using laboratory test results for hydraulic conductivity and PSD. The five soil types are Top Soil, Limonite 2, Limonite 3, Decomposed and Highly Weathered samples. The hydraulic conductivity results, obtained from the Waste Management Baseline Report (Volume 5, Appendix C), are provided in Table 2. The Top Soil sample had the highest hydraulic conductivity, while the Limonite 3 had the lowest hydraulic conductivity and produced high runoff.

The relatively high hydraulic conductivity and resulting hydrologic soil group indicates that a large portion of precipitation will enter the soil matrix and will not become surface runoff. Surface runoff would be likely to develop once the top soil profile is saturated, the rainfall is of sufficient intensity and duration and/or the soil profiles below the top soil are exposed.

Table 2: Hydraulic Conductivity and Hydrologic Soil Group Results

Sample Top Soil Limonite 2 Limonite 3 Decomposed Highly Weathered

Saturated Hydraulic conductivity (m/s) 1.5×10-3 3×10-5 1.4×10-6 4×10-6 2×10-5

Hydrologic soil groupa A B B/C B B (a) Based on definitions provided by the U.S. Natural Resource Conservation Service Soil Survey Staff (1996).

The hydrologic soil group definitions provided by the Natural Resource Conservation Service Soil Survey Staff (1996), for thoroughly wetted soils, are:

Group A: High infiltration rate; predominately consisting of deep, well to excessively drained sand or gravel; and a high rate of water transmission.



Group B: Moderate infiltration rate; predominately consisting of moderately deep to deep, moderately well-drained to well-drained soils of moderately fine to moderately coarse textures; and a moderate rate of water transmission.

Group C: Slow infiltration rate; predominately consisting of impeding downward flow layer, moderate fine to fine texture; and a slow rate of water transmission.

Group D: Very slow infiltration rate; predominately consisting of clay soils; and a very slow rate of water transmission.

3.0 EROSION PROCESS 3.1 Erosion Processes When rain falls on unvegetated soil or disturbed overburden material it produces sediment in a mix of two primary modes: surface erosion and mass erosion (also known as gravity erosion). Surface erosion occurs when rain-induced runoff passes over the soil surface and dislodges the loose particles that can be mobilised by the velocity of the running water. As this process continues, surface water begins to concentrate and forms sheet flow with more erosive power. The concentrate flow rivulet develops into a rill and then into a small gully, with increasing soil removal. As a gully forms, the banks of the gully give way and provide a mass erosion of sediment to the transporting runoff. In many instances when water runs over an edge, for example, for most of the road runoff, there is often a large mass of sediment released.

The consequences of erosion are numerous but primarily relate to water quality issues in which terrestrial and aquatic flora and fauna are negatively affected. Sediment transported from the mining areas accumulates in downstream tributaries, estuaries and ultimately the ocean, where there may be effects to environmental and/or social values. Erosion can also be a hazard to infrastructure where road beds, culverts, or bridge abutments may be affected by either scour erosion or by sediment deposition.

3.2 Road Erosion Erosion can occur during construction, operations and decommissioning of roads. Haul roads, port roads and mine access roads are identified as potential erosion sources. The required erosion control measures for temporary (i.e., construction or exploration) roads are generally less extensive than permanent or long term mine haul roads, as smaller areas of disturbance are involved. Roads in areas of environmental value may require additional controls, which should be assessed on an as needed basis.

Roads will typically require Stage 2 erosion control measures (Section 4.1). Stage 2 measures includes road side ditches with appropriate armouring and velocity controls, cross-road drains to convey flow across the road, pocket ponds, check dams and grading.

Major roads may require Stage 3 control measures if they are in high risk areas or need specific requirements for improved water quality (Section 4.0 and Attachment I). Stage 3 measures include sediment basins.

3.3 Active Mine Area Erosion Active mine areas have potential to generate sediment due to the high rainfall combined with areas with reduced vegetative cover, loose disturbed soils and steep slopes. During operations, temporary structures will be required to minimise the volume of generated sediment laden stormwater, including:

diversion bunds above the active mining area to divert clean water around the disturbed area, as much as practical

armoured chutes to convey concentrated flows through the disturbed areas

diversion bunds at the lower end of the active mining areas (adjacent to the access roads) to divert sediment laden water into medium sized sediment basins



medium sized sediment basins at the lower end of the active mining area

large end of line sediment basins located offsite downstream from the mining area, and

additional measures such as minimising clearing, prompt stabilisation and revegetation

The implementation and design of these measures are discussed in greater detail in Section 4.0 and 5.0.

3.4 Disposal/Revegetated Area Erosion The mine operations will not require a dedicated disposal area due to the mining methodology, where overburden is stabilised once it is deposited in previously mined areas. This overburden material is a potential source of sediment if adequate controls are not adopted. Proper drainage design on the slopes incorporating armoured ditches, clean water diversions, chutes with aprons or stilling basins, pocket ponds, infiltration measures and disposal area berms with rock drains for drainage can help minimise erosion. Stage 3 practices such as sediment basins can further reduce the amount of sediment released to the environment from disposal areas. The need for Stage 4 measures for slope stabilisation will be determined at a later design phase.

Revegetation and rehabilitation of the disposal area can decrease site erosion, and should be conducted as soon as practical after material is placed. Bioengineering techniques such as wattles, coir rolls, live staking, hydromulching and brush layering can provide both vegetation and structural stability on the slope, reducing erosion and increasing slope stability. Shaping the placed material into benches to reduce runoff velocities and increasing infiltration may also be beneficial.

Practices to increase infiltration on rehabilitated surfaces, such as infiltration basins or trenches, are also effective. This is only recommended if a geotechnical investigation has been conducted at the disposal area to determine the site’s suitability for infiltration. Risks include slope instability and potential mass erosion slope failures.

Mining areas which are inactive for an extended period of time should also be treated as a revegetation area, and have additional erosion control measures installed. The control measures should be suitable to the anticipated length of inactivity and could include the measures discussed in this section.

Section 4.0 and Attachment I outline these practices and design considerations.

3.5 Stockpiles and Jetty Facilities The stockpiles located adjacent to the jetty facilities will require management to minimise the risk of sediment loss into the ocean. The primary controls for the stockpiles will include cover to protect against raindrop impact and wind erosion, and drainage design to prevent the stockpiles becoming inundated with stormwater during rainfall events. The use of Stage 3 controls may be required depending on the final location and layout of the stockpiles and the available space for constructing sediment basins. Regular watering of stockpiles and haul roads may also be required to manage dust.

The jetty and ship loading facilities will also require management to minimise the risk of sediment loss into the ocean. Possible sources of marine sedimentation include:

direct spills into the ocean while loading and transporting material

materials deposited on the jetty and haul roads being mobilised into the ocean; and

wind blowing material into the ocean during loading activities

Management controls will need to be developed when the location and type of jetty facilities have been finalised. The applicable controls may include continual cleaning of the jetty area, watering of material to control dust, managing load sizes to reduce instances of spills, staff training and spill cleanup procedures.



4.0 CONCEPTUAL EROSION AND SEDIMENT CONTROL MEASURES 4.1 Design Principles At the current design detail level of mine plan and infrastructure planning, a fully resolved erosion sediment control plan (ESCP) is unachievable at this time. This ESCS has been prepared based on the philosophy of utilising Best Management Practices (BMP) to achieve the desired outcomes and meet the compliance criteria. These BMP have been developed from industry accepted guidelines and manuals (Section 8.0). The control measures have been split into three stages:

Stage 1 – staging mining works to minimise the area and duration of exposed soils, including initiating revegetation and rehabilitation works.

Stage 2 ― practices and methods to control erosion processes generated closer to the source from small catchment areas, such as drainage ditches with appropriate armouring, clean and sediment laden water diversions, chutes with aprons or stilling basins to convey flows down steep slopes, pocket ponds, and infiltration trenches and basins.

Stage 3 ― practices and methods to control water and sediment generated from much larger catchment areas, such as sediment basins to reduce the amount of sediment discharged from these areas into the natural environment.

In general, the design of sediment reduction and control facilities take into account the following:

amount of sediment generated on site

amount of sediment transported to the facilities

acceptable amount of sediment that can released from these facilities

short and long-term safety of the structures (to the public and environment)

geotechnical and hydrotechnical (hydrologic and hydraulic) factors corresponding to the acceptable risk associated with the structure

Risk is a measure of the probability and severity of an adverse effect to health, property, or the environment.

The ESCS, where possible, minimises the use of large sediment basins due to the potential geotechnical and hydrotechnical risks associated with the mine site, namely steep terrain and high rainfall. These risks may require additional investigations as the mine plan progresses.

4.2 Conceptual Control Measures Overview The design principles presented (Section 4.1) have been used to develop an overall conceptual strategy, which will provide guidance on developing the construction and operational ESCP for the staged mining activities. The general strategy has been outlined below. Further detail is provided for specific structures or management areas in the following sections.

The main haul roads and access roads will be constructed. Road drainage will include inside and outside drainage ditches, pocket ponds to capture larger sediment particles close to source, armouring of the drainage channels (as required by gradient) and rock check dams for velocity control.

Large end of line sediment basins will be constructed at suitable locations downstream of the mining area. Where topography prevents discharging all runoff from the mining area to report to a single basin location, multiple end of line basins will be required.

Access roads will be constructed above and below each panel prior to stripping activities commencing. These roads will be constructed with the proposed Stage 2 ESC measures, and will also include



associated drainage to capture sediment laden stormwater from adjacent catchments (if required) to the end of line sediment basin(s) and divert clean stormwater around the active mining areas.

Smaller sediment basins will be constructed on each panel at the base of the active mining area prior to stripping activities commencing (Figure 7). These sediment basins are designed to capture medium to coarse sized sediment particlesclose to source and will discharge into natural or constructed channels that drain to the end of line sediment basin(s) for further treatment. These smaller basins will increase sediment capture close to source as well as reduce maintenance requirements at the end of line sediment basin(s).

Once mining activities are completed on each panel, new access roads and drainage structures will be progressively constructed along the base of the next panel. Overburden from the new panel will be placed over the previously mined area and stabilised/revegetated. The small sediment basins servicing the revegetated areas will be maintained until the catchment they service has been suitably stabilised with minimal observed sediment loss. When each panel has been successfully stabilised and revegetated, the small sediment basins may be rehabilitated.

The diversion drains located outside of the catchment for the end of line sediment basin(s) should be removed at this time, as runoff from these areas should then be ‘clean’ and should not be diverted into the large end of line sediment basin(s) to improve their operating efficiency.

This process is repeated as mining activities progress from one panel to another.

If any isolated mining panels are located outside the catchment of the end of line sediment basins, additional emphasis on erosion control will be required, as well as adequately sized small sediment basins.

Figure 7: Example Mining Profiles Demonstrating Water Flow

The layout of the general strategy, discussed above, is provided in Figure 8, Figure 9 and Figure 10. Note that the conceptual figures do not represent a specific location within the Project and hence do not have locally identifiable features included.

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4.3 Sediment Basins The International Erosion Control Association (IECA) (Australasia) has developed a series of publications on best practice erosion and sediment control techniques, including the design of various types of sediment basins. Conceptual sizing of the sediment basins were calculated based on the ICEA (2008) guidelines for best practice. Conservative assumptions based on professional judgement and experience has been applied to the design where information gaps or uncertainty in data was present. It is believed the demonstration designs meet best practice standards for water quality at their discharge point when considered as part of the entire treatment train. Further refinement and design checks should be completed during the detailed design phases.

The IECA provide design guidance for three types of sediment basins applicable to particular catchment conditions; Type C, F and D basins. The basins differ in their function and design, with Type C basins designed to capture coarse grained materials, Type F to capture fine grained non-dispersive materials and Type D for dispersive materials. The appropriate basins for different soil types are outlined in Table 3.

Table 3: Sediment Basin Type Selection Criteria Soil and/or catchment conditions Basin type

Less than 33% of soil finer than 0.02 mm and no more than 10% of soil dispersive Type C basin (wet or dry basin)

More than 33% of soil finer than 0.02 mm and no more than 10% of soil dispersive

Type F basin (wet basin)

More than 10% of soil dispersive, or when adopted water quality objectives specify strict controls on turbidity levels and/or suspended solids concentrations for discharged waters

Type D basin (wet basin with flocculation)

(IECA 2008)

Based on the soil properties outlined in Section 2.5, Type F basins are appropriate for the Project. The design criteria used for the preliminary sediment basin designs have been summarised in Table 4.

Table 4: Preliminary Sediment Basin Design Criteria Criteria Values Comments

Basin settling zone volume Based on 85th percentile 5 day rainfall event

Suitable for basins discharging to sensitive environments

Sediment storage volume 50% of settling zone — Length to width ratio Between 3:1 to 5:1 — Inlet and outlet structure — To be determined in detailed design Emergency spillway — To be determined in detailed design

Type F basins are operated as wet basins without a chemical flocculation system to achieve sediment capture. The basin design comprises a settling zone, and a sediment storage zone. The settling volume is calculated as the anticipated runoff from the 85th percentile 5 day rainfall event, as per the guidelines for basins discharging into sensitive receiving waters (ICEA 2008). The sediment storage was calculated as 50% of the settling volume as per their recommendations.

The performance of Type F sediment basins can be improved, if required, through the addition of a flocculation system as per a Type D sediment basin. Monitoring of sediment basin performance for the initial mining areas will determine if a flocculation system is required. Type D and F basins are identical in design with exception of the flocculation system. Note that it may prove impractical to achieve a 5 day retention period in the large ‘end of line’ sediment basins due topographic limitations and/or the high frequency of rainfall events.



The ideal length to width ratio (3:1 to 5:1) and maximum depth (1.5 m) are stated in the design guidelines, however it is unlikely that all the basins will be feasibly constructed to these dimensions. To improve the efficiency of the basins if these guidelines cannot be met, additional controls, such as baffles, decant systems, inlet and outlet systems and chemical flocculation, may be required.

Alternative sediment basin locations and arrangements were investigated during the ESCS development, including locating significantly larger sediment basins at lower elevations nearer the coastline. These alternatives were not pursued due to the significant increase in potential environmental impacts due to construction, safety factors relating to operation and potential failure and large construction costs associated with large basin structures. The design of the sediment basins is discussed in greater detail in Section 5.2.

4.3.1 Maintenance It is recommended that sediment basins are inspected for safety and performance aspects after every major rainfall event and rectification measures be undertaken as appropriate. Extraction of settled settlement should be undertaken approximately every two months. This is to ensure the sediment basins maintain sufficient storage capacity.

4.4 Pocket ponds Pocket ponds are small sediment traps that are designed to trap coarse sediment but not to retain water. They are typically used to capture the sand and gravel particles likely to be washed off unsealed roads such as the haul and access roads. They also moderately attenuate initial runoff from the road catchment.

The pocket ponds are not designed to capture fine particles, which will be carried through the basin with only a minimal fraction settling out. Where practical, pocket ponds should discharge into the catchment of large end of line sediment basin for treatment of the fines.

Pocket ponds should be sized according to sediment trapping efficiency and maintenance frequency. The best estimates for soil parameters should be used to calculate sediment yield site-specific data. To account for fine particles remaining entrained in the flow, the sediment yield per maintenance period should be reduced, which will give an estimate of the coarse particle fraction of sediment trapped in the pocket pond between cleanings. This value can be used to convert the mass of sediment accumulated to the required sediment volume.

In addition to this sediment volume, the pocket pond will have additional depth to accommodate a design runoff volume in the basin (calculated as for sediment basin, including all catchment areas which flow to the pocket pond), as well as a 300 mm freeboard. Freeboard is a design factor of safety relating to the added height above a calculated water level to compensate for factors outside of the design analysis, such as wave action, localised hydraulic behaviour and settlement of levees (CSIRO 1999). Further discussion on the design methodology for pocket ponds is provided in Section 5.3.

4.5 Road Water Management The ESCS for the road water management will include Stage 2 and 3 practices. The proposed control measures are:

Stabilising all exposed cut and fill surfaces by revegetation or rock armouring.

Construction of upstream diversion drains to divert clean water into creeks.

Construction of suitably sized and stabilised roadside drainage channels, with velocity controls as required.

Construction of pocket ponds along all roads. Sizing methodology is discussed in Section 5.3. The basins will drain through cross-road culverts or shallow spoon drains, and discharge into level spreaders. Typical drawings of pocket ponds can be found in Attachment I.



Constructing small sediment basins into the switchbacks of the main haul road. This may be achieved by excavating enough material at the toe of the natural surface, at each location. The flow could be conveyed across the road and discharged into an existing watercourse. The location and sizing of these basins will be considered in the future studies.

4.6 Active Mine Area Water Management To control runoff in and around the active mining areas, a combination of Stage 1, Stage 2 and Stage 3 practices should be implemented to:

Divert clean runoff away from the disturbed areas wherever possible.

Convey water through the disturbed areas using armoured chutes or implement velocity controls to minimise erosion of the watercourse.

Divert sediment laden stormwater to catchments with end of line sediment basins wherever possible and practical.

Capture sediments as close to the source as practical.

Sediment laden runoff from the active mining areas will pass through pocket ponds and small sediment basins before being discharged offsite towards the end of line sediment basin. This will require engineered diversion bunds as discussed in Section 4.5.

The staged construction of active mine area sediment basins will represent a reduced cost to the Project as any required earthworks will be included in the stripping earthworks. Additionally, omitting the small sediment basins in preference for a larger downstream basin will present increased risks due to limitations of the required wall and high flow volumes during intense rainfall events and spillway requirements. Details regarding the design of these basins are provided in Section 5.2.

4.7 Rehabilitation of Disturbed Areas The overburden on previously mined areas will be stabilised through revegetation. This will be essential in reducing sediment loads generated from these areas. The engineering measures implemented during the mining phase (drainage works, pocket ponds and large sediment basins) will be maintained. Additional stabilising works may be introduced:

Mulching, hydromulching and/or hydroseeding - utilising vegetative material cleared during stripping works.

Topsoil management – utilising topsoil stripped from adjacent mining areas to assist in re-establishing vegetation and adding soil amendments if required.

Surface roughening to reduce runoff and increase infiltration into the soil.

Bioengineering techniques such as wattles, coir rolls and live staking.

Placement of large woody debris – large logs and branches placed on exposed soil will assist in holding surface soils during runoff events, allowing vegetation to establish and spread over slopes.

Infiltration measures such as infiltration trenches to increase infiltration into the soil, which will naturally filter out sediments and reduce peak runoff during rainfall events.

Once an area is stabilised, the drainage devices at the base of the area may be removed if no further mining is occurring on a downstream mining panel. The suitability of rehabilitation measures for each mining area should be assessed based on the soil types, materials available from stripping operations and the observed success of the measures onsite. Further details have been provided on rehabilitation techniques in Attachment I.



4.8 Jetty Site Water Management Water management at the jetty and stockpile facilities will be achieved using a combination of Stage 1, Stage 2 and Stage 3 methods, in addition to in-water sediment curtains, where necessary. Rehabilitation and best management practices and methods will be used on excavated and battered slope areas to prevent erosion into the surrounding sensitive marine ecosystem. During operations, the stockpiles should be covered with tarpaulins or similar to protect against raindrop impact. Drainage design around the stockpile should be free draining to prevent damming under or adjacent to the stockpiles.

The use of Stage 3 controls, such as sediment basins, may be required. This will depend on the final location and layout of the stockpile and jetty facilities, and also the feasibility of constructing a sediment basin in proximity to these facilities. If a sediment basin is required, the surface drainage should report to a single location where the sediment basin will be located.

5.0 DESIGN 5.1 Design Standards and Methodology Design standards and methodologies specific for Solomon Islands have not been identified for hydrology, hydraulics and ESC design. As such, a number of design methodologies commonly used in Australia have been implemented. The design procedures adopted include:

The Sediment and Erosion control assessment and design methodologies outlined in the Best Practice Erosion and Sediment Control guidelines published by the IECA (IECA 2008).

Rational Method calculations for peak flows follow the methodology outlined in the Queensland Urban Drainage Manual (Department of Natural Resources and Water 2008).

Soil erosion prediction estimation using the USLE methodology, as per the Institute of Engineers Australia (1996) guidelines.

Hydraulic design of drainage structures follows the standard Manning’s equation.

Where design standards applicable to the Solomon Islands have not been available, assumptions based on previous experience and conservative estimates have been utilised. These assumptions and estimates are stated in each relevant section within this report.

It should be noted that only a limited length of site-specific climate data was available for the development of this ESCS. All calculations should be revised during later design stages when longer datasets are available.

5.2 Sediment Basins 5.2.1 Basin Overview A Type F basin type was sized for the conceptual design. These basins are wet basins, and may have a flocculation system added to improve treatment efficiency depending on the operating conditions. The overall basin dimensions are identical for flocculation and non-flocculation basins. Therefore, differentiating between the two types is not required for conceptual sizing purposes.

5.2.2 Rainfall Depth When the receiving waters are environmentally sensitive and dispersive soil particles are expected, the design rainfall depth for the 85th percentile 5 day storm event should be utilised as per the IECA guidelines. The closest weather station to the LSA, IWS04, only has approximately 10 months of records for rainfall analysis. IWS01, located on Santa Isabel Island, has approximately 53 months of records and has been used for calculation of the 5 day rainfall depths. The range of 5 day percentile rainfall depths for IWS01 and IWS04 are indicated in Figure 11. The 5 day rainfall depth utilised for this concept design is 81.6 mm. Note that as further rainfall records become available from IWS04 and IWS01, the 5 day rainfall depths should be re-assessed and the design adjusted accordingly.



Figure 11: Five Day Rainfall Depths for IWS01 and IWS04 for a Range of Percentile Storm Events

5.2.3 Basin Sizing The dimension of the settling zone within the sediment basin is determined using the following formula:

ACvRVs ×××=10

Where:

Vs is the volume of the settling zone

R is the total rainfall depth of the selected percentile, 5 day rain event

Cv is the volumetric runoff coefficient

A is the effective catchment surface area connected to the basin

An allowance of 50% of the settling zone is added to provide for sediment storage between maintenance activities.

Two types of sediment basins are proposed: the large end of line basin and small basins. These basins operate in series, where the end of line basin is last. The cumulative volume of the small sediment basins contributes to the overall treatment capability of the large end of line sediment basin. This effectively reduces the required volume of the larger end of line sediment basin. The reduction in size reduces the potential environmental effect associated with developing a large sediment basin in steeply sloping terrain, as well as the ongoing maintenance requirements of the end of line basins.

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ISW01 5 Day Rainfall Depth ISW04 5 Day Rainfall Depth 85th Percentile



A number of assumptions had to be made regarding the sizing of the sediment basins:

It has been assumed that the daily rainfall records for the climate station IWS04 are representative of rainfall on the site.

The soil types within the catchment are assumed to be more than 33% of soil finer than 0.02 mm and no more than 10% of soil dispersive, as per the specifications for Type F sediment basins.

The end of line basin and small sediment basins were sized based on the 85th percentile 5 day storm event, which is the recommended standard for basins discharging to sensitive receiving waters (IECA 2008).

The soil infiltration properties for the tenement will be as per the U.S. Natural Resource Conservation Service Soil Survey Staff (1996) hydrologic soils group definitions, as discussed in Section 2.5.3.

It has been assumed that only the small basins servicing the active mining panel and previously mined panel will be in operation at one time, basins from previous mining panels will be progressively removed and no longer contribute to the overall treatment of sediment laden runoff.

The preliminary design is for the basin volume only, no design has been completed for inlet or outlet systems, emergency spillways, baffles, maintenance access or other sediment basin features. The small sized basins should have a maximum water depth of approximately 1.5 m and a length to width ratio of between 3:1 to 5:1, however can be constructed to suit the available space and topography.

Where basins are to be constructed with fill embankments, the basins should be designed with 0.8 m freeboard from the full water surface to the top of the embankment (assuming 0.5 m depth of flow over the emergency spillway with an additional 0.3 m freeboard).

Internal embankments were assumed to be 3:1 (H:V), based on the assumption the embankments will be constructed from either good, erosion resistant clay, clay-loam or sandy-loam soils.

The assumptions will need to be re-assessed and the results updated when more data becomes available and the mine plan is progressed.

5.2.4 Basin Volumes Indicative sediment basin volumes required for the active mining areas have been provided in Table 5, based on the design methodology outlined above. Sizing for the larger end of line basins will be determined during detailed design when suitable locations for placing the sediment basins have been identified.

Table 5: Conceptual Volume Requirements for the Sediment Basins Located in the Active Mine Areas

Location Disturbed Area [ha]

Settling Volume per Basin [m3]

Sediment Storage per Basin [m3]

Individual Basin Volume [m3]

Panel/Pit 0.5 286 143 428 Panel/Pit 1.0 571 286 857 Panel/Pit 1.5 857 428 1285 Panel/Pit 2.0 1142 571 1714 Panel/Pit 3.0 1714 857 2570

5.2.5 Alternative Designs Alternative sediment basin designs may be required to suit the mine plan, individual catchment topography and/or geological conditions. The total basin volume required to capture sediments is unlikely to change significantly with alternative designs (unless site specific data alters design parameters). Alternative designs may relate to ancillary features, such as addition of flocculation systems or decant systems.



The need for alternative designs will be investigated further in a later design phase when more site specific data and the mine plan is available.

5.3 Pocket Ponds 5.3.1 Pocket Pond Overview Pocket ponds are sized based on the anticipated soil loss of the catchment using the Universal Soil Loss Equation (USLE), sediment capture ability of the basin and a design rainfall volume. Due to the highly variable catchment types anticipated along the haul and access roads, it is not feasible to provide designs for all possible pocket pond locations at this stage. The design methodology has been provided so that these devices can be designed on an as needed basis during detailed design.

To size the pocket pond, the USLE value times three should be used, divided by the number of times per year the basin will be cleaned out. This value is reduced by half to account for the fine particles which are not captured within the basin. It is important that only catchment areas flowing to the pocket pond are included in the calculation. Flows from areas which are first routed through another sediment control structure before reaching the pocket pond, should not be included in the sediment calculation, as the majority of coarse particles should have already been removed from the water.

5.3.2 USLE Factors The USLE is a multiplicative relationship of:

PCLSKRA ××××=

Where:

A is the average soil loss per unit of area

R is the rainfall erosivity factor

K is the soil erodibilty factor

LS is the topographic factor derived from slope length and slope gradient

C is the cover and management factor, and

P is the erosion control practice factor.

Many of these factors can be assumed from textbook values, however the K factor, LS factor, C factor and P factor will depend on site specific conditions for each basin and should be determined when more site data is available.

5.3.3 Design Rainfall Depth The rainfall depth to be utilised is discussed in Section 5.2.2. Where there is insufficient room available to construct a basin to contain the 85th percentile 5 day rainfall depth, a lower percentile storm event may be utilised providing the pocket pond ultimately discharges into an end of line sediment basin.



5.3.4 Pocket pond Sizing The pocket pond sizing methodology is based on experience in ESC in tropical environments. The sizing calculation can be represented as:

MAsLVs 5.1××=

Where:

Vs is the sediment storage volume of the basin (m3/yr)

L is the calculated soil loss per hectare per year (m3/yr)

As is the catchment area contributing sediment (ha)

M is the maintenance period (# of cleans per year)

ArRVt ×=10

Where:

Vt is the treatment volume of the basin (m3/yr)

R is the design runoff depth (mm)

Ar is the catchment area contributing runoff

VtVsVp +=

Where:

Vp is the total volume of the basin (m3/yr)

Vs is the sediment storage volume of the basin (m3/yr)

Vt is the treatment volume of the basin (m3/yr)

The pocket pond will also require a 300 mm freeboard allowance in the design. These basins will be designed during a later design phase when more site specific information is available.

5.4 Roadside Channels 5.4.1 Channel Overview For the demonstration design sites, the roadside drainage will provide conveyance of sediment laden runoff to pocket ponds and sediment basins. To achieve this, the roadside drainage will need to be of sufficient size to convey the design flows without overtopping. Longitudinal slopes of the roads will be carefully controlled to minimise the flow velocities in the adjacent drainage structures.

5.4.2 Design Assumptions The drainage was designed based on the following assumptions and parameters:

A Manning’s n of 0.025 was adopted, as recommended by Queensland Department of Transport and Main Roads (2010) for open channels constructed from earth with uniform section.

Longitudinal slope of 1%. Steeper channels may require additional controls as discussed in Attachment I.



The design standard adopted for temporary haul roads was the 2, 5 and 10 year average recurrence interval (ARI) storm event, depending on the type of road and intended length of use. The ARI is the average value of the periods between exceedances of a given rainfall total accumulated over a given duration. The adopted design ARI periods are a lower design standard than the 1:25 year ARI event recommended by the IFC, however roadside drainage is considered to be of low consequence if a failure occurs. Therefore, the adopted design standard is considered to be sufficient for the purposes of this study.

Velocity controls (such as rock check dams) or armouring (with rock, gravel or grasses) will be implemented on long drainage lines to reduce scour erosion.

All drains were sized as a trapezoidal channel, with a 0.5 m base width and 2:1 (horizontal to vertical) side slopes.

A freeboard allowance of 300 mm was made for all drains. Freeboard is the additional vertical height between the design high water level and the top of the drain.

The Intensity Frequency Duration (IFD) data provided by Hatch in Hatch (2011) is suitable for conceptual design purposes, as site specific data has insufficient length to develop IFD curves.

5.4.3 Dimensions Indicative sizing for the roadside drainage has been provided for Isabel Island in Table 6, Table 7 and Table 8 below for a range of design rainfall events.

Table 6: Conceptual Drainage Sizing for the 2 Year ARI Rainfall Event Catchment Area [ha] Design Flow [m3/s] Depth of Drain [m] Total Width [m] Velocity [m/s]

0.5 0.18 0.50 2.49 1.01 1 0.36 0.58 2.81 1.22 2 0.64 0.67 3.17 1.42 5 1.27 0.80 3.70 1.69


0.5 0.23 0.53 2.60 1.09 1 0.47 0.62 2.97 1.31 2 0.84 0.72 3.36 1.52 5 1.68 0.87 3.97 1.81


0.5 0.27 0.54 2.66 1.13 1 0.53 0.64 3.05 1.35 2 0.97 0.74 3.47 1.58 5 1.94 0.90 4.12 1.88



6.0 MODELLED SEDIMENT YIELD The Jejevo, Hughukapote, Nuha and Sivoko catchments were identified as the catchments most greatly affected by mine activities as the Project footprint overlaps these catchments. A Soil and Water Assessment Tool (SWAT) model has been developed to better understand the increase in sediment yields within each catchment resulting from mining activities, as detailed in the Water Balance and Sediment Transport Model report (Appendix G). The SWAT analysis was undertaken in order to determine the effectiveness of sediment control measures in mitigating the impacts of commencement of mining activity on sediment concentrations and yield within the LSA.

The SWAT analysis incorporated four scenarios to represent the different stages of the Project, including:

Baseline scenario representing the current infrastructure and features, including logging area, existing roads and residential areas.

Construction phase incorporating construction infrastructure required prior to the commencement of the mining stage, including mine haul roads, ROM pad, ore stockpile facilities and accommodation camp facilities.

Operations phase representing the full commencement of mining activities and incorporating end of line sediment basins downstream of the resource area in the Jejevo, Hughukapote, Nuha and Sivoko catchments.

Decommissioning and closure phase consisting of the rehabilitation of all affected mining and associated infrastructure features/areas into natural vegetation with the post mining landform.

The sediment control measures proposed in this ESCS, however, cannot be fully modelled in SWAT due to the models limitations, as discussed in the water balance and sediment transport model report (Appendix G). Local small-scale sediment control measures cannot be represented in the SWAT model. The construction and operations phases are modelled only to indicate the ability of sediment control measures in reducing sediment concentration and yield arising from construction and mining activities.

The SWAT analysis confirms that mitigation of the effects of mining activity can be successful with implementation of sediment control measures. Further analysis and optimisation is required in order to determine the size and frequency of sediment control measures appropriate to reduce sediment concentrations to that of the natural environment.

7.0 CONCLUSIONS Erosion and sedimentation pose a key risk to the Project. The Project site is subject to high erosion potential due to its steep terrain and intense rainfall. Also the current regulatory guidelines are very broad and will need to be clarified so that the environmental design criteria can be developed to enable further design work to be undertaken.

This ESCS aims to limit where possible the erosion potential and sediment generation from exposed surfaces. This will be completed through the use of best management practices. The measures are discussed in detail in Attachment I.

Where these measures are not sufficient, a number of small sediment basins have been proposed as close to the source as possible. This philosophy aims to limit the total required sediment basin size by eliminating the need to treat clean water and provide access and egress for regular maintenance.

This ESCS has been developed with limited site specific data. It is recommended that the ESCS be updated when more site specific data and the mine plan is available. This will enable the design calculations to be revised prior to any construction commencing.



8.0 REFERENCES ANZECC and ARMCANZ. 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality. Australian and New Zealand Environment and Conservation Council (ANZECC), 2000.

CSIRO (Commonwealth Scientific and Industrial Research Organisation). 1999. Floodplain Management in Australia: Best Practice Guidelines. Primary Industries Report. Issue 73 of SCARM report. Standing Committee on Agriculture and Resource Management (SCARM) Series. CSIRO Publishing. Australia.

Department of Natural Resources & Water. 2008. Queensland Urban Drainage Manual. Second Edition 2008.

Queensland Department of Transport and Main Roads. 2010. Road Drainage Manual. 2nd Edition, March 2010.

Hatch. 2011. Santa Isabel Site Hydrology & IFD Data. Project Memo. H339181-0000-00-220-0004, Rev.A. Dated 1 November 2011.

Institute of Engineers Australia, Queensland Division. 1996. Soil Erosion and Sediment Control Engineering Guidelines for Queensland Construction Sites. June 1996.

IECA (International Erosion Control Association). 2008. Best Practice Erosion & Sediment Control. November 2008.

IFC (International Finance Corporation). 2007. Environmental, Health and Safety Guidelines for Mining. December 2007.

Maidment, D.R. 1993. Handbook of hydrology. Civil engineering. McGraw-Hill. USA.

U.S. Natural Resource Conservation Service Soil Survey Staff. 1996. National soil survey handbook. Title 430-VI. U.S. Department of Agriculture Natural Resources Conservation Service. U.S. Government Printing Office, Washington, DC, USA.


March 2014 Report No. 137633001-3008-R-Rev0-2400

Report Signature Page

GOLDER ASSOCIATES PTY LTD

Greg Hansell Russell Merz Project Water Resources Engineer Principal, Principal Water Engineer Ian Gilchrist Associate, Principal Environmental Consultant

GH/RM/IG/kg

A.B.N. 64 006 107 857 Golder, Golder Associates and the GA globe design are trademarks of Golder Associates Corporation.

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control strategy.docx


March 2014 Report No. 137633001-3008-R-Rev0-2400

ATTACHMENT I Water Management and Erosion and Sediment Control Toolbox

ATTACHMENT I Water Management and, Erosion and Sediment Control Toolbox


EROSION AND SEDIMENT CONTROL PLAN The philosophy of an Erosion and Sediment Control Plan (ESCP) is to manage the erosion processes through active preventative measures rather than remedial methods. Through this application, the requirement for construction of large sediment basins is reduced. This is important because of the intense rainfall and steep terrain encountered on the mine site. The measures have been split into the following stages:

Stage 1 and 2 practices and methods are designed to control erosion processes and sediment and water generated from small catchment areas; and

Stage 3 practices and methods are designed to control water and sediment generated from much larger catchment areas.

In general, a critical element to the control of erosion and sedimentation is the need to promote efficient management of site drainage, surface protection and land disturbance at all times.

Erosion Prevention Land disturbance as a result of mining activity will inevitably increase the potential for erosion to occur. Erosion prevention measures are intended to limit the amount of soil dislodged by rain and flowing water. On site, these preventative measures generally concern methods of:

binding soil in place with vegetation

surface protection of disturbed areas

reducing slope lengths to limit accumulation and acceleration of overland flow

providing enhanced drainage

scheduling land disturbance for dry periods or periods of less severe rainfall

minimising land disturbance

The technologies used to implement these measures include ground covers such as vegetation, mulch, riprap and blankets that absorb rain energy to reduce sheet erosion. Diversions, check dams, chutes, and ditch liners may trap sediment, but also have value in preventing rill and gully erosion which result in greater volumes of sediment leaving the site.

Sediment Control Where sediment is generated following land disturbance, it is necessary to contain it and prevent it from moving off-site. There is a range of practices available for entrapment and containment of sediment. For purposes of efficiency in implementing sediment control measures on site, the available practices are classified into two stages according to whether the necessary works are minor or major in scale.

Stage 1 and 2 sediment control practices include the use of pocket ponds, check dams, modified site drainage, and revegetation.

Stage 3 measures include sediment basins (i.e., larger facilities for the capture and storage of sediment generated over wider areas of the site and to prevent movement to points off site).

Rehabilitation of the landscape after mining activities in the area have been completed, can be an effective erosion and sediment control approach, and contributes to both the functionality and aesthetics of the post-mining landscape. Rehabilitation techniques include slope grading, drainage, channel design, revegetation and bioengineering techniques, which provide both vegetation and structural stability on slopes.



Best Management Practices A summary of best management practices and their relevance to each level of erosion and sediment control is provided in Table I-1. Note that this is a summary only, and alternative BMPs may be applicable to specific conditions not listed here.

Table I-1: Best Management Practices

Practice Temporary Permanent Erosion Prevention

Sediment Control Rehabilitation Stage 1

and 2 Stage 3

Site Drainage X X X X – X Surface Roughening X X X X – X Slope Stabilisation – X X – – X Gradient Treatment X X X – – X Erosion Control Mats X X X – – X Revegetation X X X X – X Bio-Engineering X X X X – X Channel Protection X X X X – X Check Dams X X X X – X Channel Head cutting protection – X X – – X

Road Drainage X X X X – – Infiltration Basins X – X X – – Pocket ponds X – X X – – Disposal Area Berms X X X X – – Basins X – X – X – Dams X – X – X – – = not applicable

Site Drainage Site drainage is conducted by mine operations to maintain efficient and safe working conditions. Drainage is generally in the form of excavated drainage ditches along the perimeter of work areas and along roads. Revegetated areas that are being reclaimed are generally provided with more structured drainage that minimise erosion. These practices include cross-slope, or contour drains, and rock-lined steeper channels. The drainage water will generally be treated using Level 1 practices, and discharged into the main drainage channels, which are often part of natural watercourses. Where present these water courses will convey runoff to sediment control facilities in the catchments. Refer to Figure I-1 for typical drainage ditches.

In the rehabilitated landscape, the overall drainage design should reflect natural patterns in pre-mining conditions. This includes drainage density (or the number of channels) and stream order (or the number of tributaries in each stream system). Pre-mining conditions may be estimated by evaluating the surrounding landscapes. A site-specific study is required to accurately determine the natural drainage conditions in the area.

1V1V

1V1V

Figure I-1



Surface Roughening Surface roughening reduces erosion by creating shallow soil depressions, breaking up the overland flow and slowing over the rate at which small amounts of surface water flows, allowing sediment to be deposited. This practice can be applied as a temporary measure, to assist in reducing erosion on cleared slopes or graded areas, and as a permanent measure on rehabilitated slopes. This practice is ideally applied to sites that will be revegetated, as the surface roughness will increase water retention after rainfall, allowing more water to infiltrate and promoting vegetative growth.

Running a tracked vehicle up and down the slope leaves divots perpendicular to the slope length that can roughen the hill slope.

Slope Stabilisation Slopes can be stabilised and protected from excessive erosion using a number of methods, including gradient treatment, erosion control mats, riprap bioengineering techniques and vegetation. Steep, uniform, unprotected slopes yield the highest amount of suspended sediment, as water is allowed to reach high velocities and soil particles are easily detached and washed away.

Grading Grading involves reshaping of the ground surface to prepare the site for processing equipment, stock pile areas and for post-mining reclamation. The purpose of grading is to:

provide suitable topography for post-mining land uses

facilitate equipment operation and stockpiling

control surface runoff

minimise soil erosion and sedimentation, during and after development of the site or aggregate extraction

Before grading begins, a decision must be made on the steepness of cut-and-fill slopes and how they will be protected from runoff, stabilized and maintained. A grading plan can establish drainage areas, direct drainage patterns and affect runoff velocities. A grading plan can form the basis of an erosion and sediment control plan. Considerations that affect erosion and sediment control include deciding which slopes are to be graded, when the work will start and stop, the slope angle, the length of finished grades, where and how excess material will be stored or disposed of, and where borrow material will be needed. Early completion of grading work will allow for prompt re-vegetation for erosion control and reduces or eliminates temporary seeding expense.

Careful shaping of a site for mining operations and for post-reclamation activities reduces the potential for erosion and the cost of installing erosion and sediment control measures. Undisturbed temporary and permanent buffer zones may provide an effective and low-cost erosion control measure for adjacent grading work. Stormwater should be intercepted and redirected to avoid flows on newly graded slopes.

Using appropriate slope breaks, such as diversions or benches to reduce the length of a cut or fill slope, will limit sheet and rill erosion and prevent gullying. Slopes and benches must be designed in consultation with a geotechnical engineer to ensure the designed slopes will be stable under site conditions. Slope stability considerations will override slope angles for erosion protection (Figure I-2).

Slope stabilisation after mining may include stacking remaining boulders, cobbles and gravel along contours to form terraces. These terraces may assist in soil retention and development on the slope, and will increase the stability of the slope. This practice should be assessed by a qualified engineer to determine the stability of the slope to support the stacked rock, and to assess the need for a geotextile layer within the terrace walls.

Figure I-2



All graded areas should be stabilised with hydroseeding, vegetation, crushed stone, riprap, erosion control blankets (ECBs) or other appropriate ground cover as soon as grading is completed. Using mulch or rice straw to temporarily stabilize areas can be undertaken where final grading must be delayed. Timing of grading should account for the conditions to revegetate the site after the machine work is completed. For example, grading should not be done immediately following an extreme rainfall event.

Roughing up the surface of all slopes should be undertaken during grading to retain water, increase infiltration and facilitate vegetation. Running a tracked vehicle up and down the slope leaves divots perpendicular to the slope length that can roughen the hill slope. Scarifying the surface to a minimum depth of eight centimetres before placing topsoil should be undertaken. Excessively compacted areas should be thoroughly ripped/subsoiled to facilitate drainage and root growth.

Fill should not be placed adjacent to a channel bank, where it can create bank instability and failure, or result in deposition of sediment downstream. Placing fill in places where it will block or limit natural flooding should also be avoided. All graded areas should be inspected periodically, along with supporting erosion and sediment control measures, especially after heavy rainfalls. If washouts or breaks occur, they should be repaired immediately. The incident should be documented and remedial measures taken. In the reconstructed landscape, land should be formed with non-uniform slopes and concave down gradient sections to reduce erosion and allow for sediment deposition on the flatter sections.

Erosion Control Blankets An ECB is a temporary protective blanket placed on bare soil that is used on slopes to absorb rainfall impact energy and protect the soil from erosion. The mats are typically made of natural, degradable materials such as mulch, wood fibre, coconut fibre, or jute, or synthetics, and used in conjunction with site revegetation can provide an effective erosion and sediment control.

The purpose of an ECB, when placed onto prepared, seeded soils, is to prevent washing away of the seed and erosion of the prepared seedbed. Erosion control mats assist revegetation by helping maintain moisture and temperature, as well as inhibiting weed growth. After vegetation is established, the blanket degrades over time, leaving only the vegetation in place.

An ECB can be used on steep slopes where severe erosion control problems are anticipated. An ECB is superior to hydroseeding, when the growing season is short and plants cannot stabilize the slope quickly; when at high altitudes; or where major storms are a frequent occurrence.

An ECB can be made from a variety of materials including mulch, vegetable matter, wood fibre, synthetics or combinations. Staples, stakes or anchors are required to secure the ECBs until the vegetation takes root. Installation requires hand tools and labour. Cost estimates are considered to be medium to high, dependant on channel length and delivery cost, if commercial ECBs are used.

Applying erosion control blankets over large areas can be prohibitively expensive. However, small applications in areas that are especially steep and/or prone to erosion can be very effective, in conjunction with less expensive forms of erosion control. Proper anchoring and ground preparation are essential if jute or coconut fibre is used as gullies can form underneath the mats. Manufacturer's instructions should be followed.

Close inspection after storms and major runoff occurrences is essential, as ECBs can mask slope failures until erosion is so far along that the slope can no longer be effectively treated with spot methods.



Bio-Engineering Techniques Bio-engineering techniques provide two functions on a slope:

structural stabilisation against erosion and land movement

assisting in revegetating the slope

Four methods have been outlined in this manual: wattles, coir rolls, live staking and brush layering. These practices involve the use of live branches of an easily-rooted woody plant, such as a fast growing native shrub, which function as stakes or bundles anchored into the soil. Coir rolls are fibre rolls held in place on the slope by stakes that allow water retention and sediment deposition behind the roll. Bioengineering techniques help provide an environment conducive to vegetation establishment. They also provide plant cuttings on the slope that, under appropriate conditions, will root and grow, to vegetate the slope themselves.

Mine Revegetation Mine revegetation uses landforming, constructing contour (cross-slope) drains, and hydro-seeding with a grass/legume seed mix. This type of reclamation is heading towards best practice for reducing erosion and minimising sediment delivery. When combined with disposal areas, landforming and construction of contour drains, it should be able to be implemented as a management practice earlier in the cycle.

Revegetation in this climate faces significant timing challenges due to rainfall frequency and intensity. The practice of revegetating small blocks assists in minimising the amount of land that is disturbed at any one time, and this can present significant improvements on minimising sediment leaving the site.

Site drainage within reclaimed/revegetated areas is critical to minimise erosion both in the short term, while the vegetation is being established, as well as in the long term for stable land forms. Channel stabilisation can be accomplished with a variety of methods, including hydraulic design for optimal channel dimensions and slope. Numerous materials are available to assist in stabilising drainage channels, ranging from vegetated waterways to erosion mats and riprap.

Trees and other woody plants should not be planted on basin embankments as part of an erosion control plan. Roots from these plants may cause cracking and/or seepage pathways through the basin, compromising the stability of the structure. Grasses are acceptable vegetation for planting on basin faces to help reduce erosion.

Channels Flow in the channel can be estimated from the channel catchment area using the Rational Method (Maidment, 1993):

Q = 0.278 * A * C * I;

Where:

Q = Flow (m3/s)

A = Catchment Area (km2)

C = Runoff Coefficient = to be adopted by design team; and

I = rainfall intensity (mm/hr) for a design storm of a given return period, with a duration equal to the catchment time of concentration.

The time of concentration is an idealized concept and defined as the time it takes for water to travel from the outermost part of a catchment to the outlet. It should be calculated according to the Bransby-Williams equation (Maidment, 1993):



tc = 58.5 * L (S0.2 * A0.1);

Where:

tc = Time of concentration, (min);

L= Main drainage length (km);

S = Slope of main drainage (m/km); and

A = Area (km2).

Channels should be designed using the following design storm return periods:

Temporary Roads: 1:2 year Operational Roads (Minor Drains): 1:5 year Operational Roads (Major Drains): 1:10 year Roads with downstream low consequence failure impact: 1:100 year

An additional 300 mm freeboard should be provided in the channel design. Channel designs with bottom widths or batter slopes other than those presented in the figure can be calculated using the Manning’s Equation, or software such as FlowMaster.

Channel slopes should be kept to the minimum gradient possible. Small gradients reduce the flow velocity, increase bed stability, reduce erosion and allow for smaller diameter riprap armour to be placed for channel stabilisation. In the terrain of the Solomon Islands, gradients are steep and constricted by natural topography. Where possible, cross-slope drains should be used to reduce channel slope.

Velocities in the channels should be a minimum of 0.6-0.9 m/s to prevent deposition of sediment in the channel.

The slopes of channel batters should be designed to be as flat as possible, to increase capacity of the channel, increase bank stability, requiring a smaller diameter riprap armour. A balance must be achieved between engineering and practicality when available space and/or economics restrict the design batter slopes.

Planting vegetation and a vegetated buffer panel alongside ditches can be highly beneficial, creating small wind breaks to reduce soil erosion and dust. Vegetation growing on the bank of the ditch can help to remove sediment as surface run-off flows through it.

Channel beds and banks must be protected against erosion. Protection measures include riprap, grouted rock, reno (gabion) mattresses and erosion control blankets. The type and depth of protection required is based on the channel slope and water depth and velocity in the channel. A rock lined or concrete chute is required for channel sections with slopes greater than 10%. Structures such as a channel confluence structure, rock apron, stilling basin and flow spreaders should be used to minimise erosion and scour at the bottom of slopes and at locations where flows or velocities suddenly change, and reduce the erosion potential at outlets to natural channels (Figure I-1).

There are several useful tools for sizing culverts for channel road crossings, including software programs such as Culvert-W and Haested Methods’ CulvertMaster.

Constructed drainage channels should discharge to existing stable watercourses. If the existing channel is not stable (i.e., if erosion or scour is observable in the channel), drainage should not be discharged to the channel without providing additional protection such as a riprap liner. If gullying is evident in the downstream channel, drainage should not be discharged at that location. Instead, the gullied channel should be filled in and a new channel constructed based on the hydrology of the catchments of the gullied channel and the discharging ditch. Adequate erosion protection must be provided, incorporating head cutting protection to prevent gullying from occurring in the new channel.



Regular removal of debris and sediment, especially at culvert inlets and outlets and sediment accumulation areas, is required. It is best to work in or near ditches during dry weather. Look for areas of the ditch that consistently fill in overtime and constrict water flow, usually at an obstruction or a sudden decrease in gradient. These sections should be cleaned out first in order to see if the resulting improvements to water flow are adequate. If ditches and sumps are refilling with sediment on a chronic basis, erosion control measures upstream need to be reassessed and improved (i.e., erosion control blankets, check dams and pocket ponds).

Check Dams Check dams are small dams constructed in channels that act to slow flows in the channel, reducing channel bed erosion, and allowing sediment to be deposited behind the dam. Check dams should be used in channels that are not grouted and have slopes greater than 4%. They are typically constructed of rocks, sand bags, gabions, straw bales, logs, lumber, or interlocking pre-cast concrete blocks. Materials for construction should be based on what is locally available and effective for local conditions. If heavily sediment-laden flows are regularly expected, a sump should be constructed immediately upstream of the check dam.

Check dams should be spaced so that the toe of the upstream check dam is the same elevation as the crest of the downstream check dam. Riprap should be placed downstream of the check dam to prevent erosion. Care must be taken to prevent failure caused by water undermining or side cutting the structure.

Sediment deposition will occur on the upstream side of the dam, and erosion may occur at the sides and bottom of the dam. Accumulated sediment should be removed when it reaches 1/2 the dam height and disposed of in accordance with local regulations, and riprap repaired as necessary. The check dams should be periodically inspected, particularly after large rainfall events (Figure I-1).

Channel Head Cutting Protection Steep channels constructed in erodible soils in the rehabilitated landscape may be susceptible to head cutting, an erosion process where the channel bed severely erodes at a downstream location, such as where a steep slope suddenly flattens, and the erosion plane moves continually upstream, creating a gully. This condition may be prevented in constructed channels by incorporating pockets of riprap in the channel bed. If erosion occurs and a headcut reaches the riprap pocket, or “sacrificial zone”, the riprap will fall into the eroded section and provide protection against further head cutting. The depth of the riprap pockets must be at least the anticipated scour/erosion depth.

Road Drainage Surface water should be designed to drain off roads into collection ditches and sumps, and conveyed across roads where necessary, using surface or culverted cross road drains. All water collected in ditches and sumps should pass through a pocket pond, or small sediment trap, before discharging to an existing stable watercourse or another drainage structure. Ditches are to be designed using accepted hydrological and hydraulic analysis methods, such as the rational method for estimating flow from the catchment, and the Manning’s equation for sizing the channel.

Temporary roads (i.e., exploration and construction roads) require less rigorous armouring and sediment control structures than permanent or long term roads, as the cost may not be justified by the length of time the structures will be in place. Erosion and sediment control measures, are still required on temporary roads (Figures I-1 to I-5.).

Figure I-3

WIDTH = W (m)

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ROCKFILL LINING 250mmNOM. SIZE ROCK 300mmTHICK

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ROCKFILL CONFINING BUND(TO CONTROL SIDE SPILLAGE)

ROCK APRON

SAFETY BARRIERHEADWALL

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PRECASTCONCRETE SILL

ROCKFILL LINED FLOW SPREADER

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SAND AND ROCK450mm MINUS ROCK

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Application:For all long term roads (i.e., those service for 5 or more years) with longitudinal slopes of less than 12%, culvertsshould be used to convey water under the road.

CROSS SECTIONNTS

PLANNTS

1-5

TYPICAL CROSS ROAD CULVERTDRAIN AND POCKET POND

General Design Notes:

(a) Cross road culverts should be used for all long-term roads (ie. Those in service for greater than 5 years) when thelongitudinal slope “S” is less than 12%.

(b) Pocket pond base plan area (L x W) to be a minimum 4m2. Pocket pond may be elongated parallel to road (L > W) toreduce cut into hillside, with a minimum W = 1m.

(c) Pocket pond depth “D” to be nominally 2m or maximum practicable given site constraints. For example, flatter excavationbatters in laterite may limit “D” so that surface area of pocket pond is not excessive. Notwithstanding, depth “D” should be atleast 1m greater than drainage ditch depth. Pocket pond may need to be enlarged if excessive sediment accumulates inwet season.

(d) The total required volume for the pocket pond may be divided between 2 or more ponds in sequence to reduce the arearequired for each pond. As pocket ponds are a Stage 1 sediment control structure, they may be constructed smaller thanthe calculated required volume, which will pass a greater sediment volume downstream. If pocket ponds are designedsmaller than required by calculation, this must be compensated for by increasing the sediment capacity of the downstreamStage 2 sediment pond/dam.

(e) A precast concrete sill should be located along the downstream (outfall) edge of the cross road drain or culverts, with a levelcrest, to distribute the flow evenly over the downstream rockfill flow spreader.

(f) A French drain constructed of 0.5m depth of rock and sand covered by 0.5m depth of 450mm Minus rock and wrapped ingeotextile to provide passive dewatering of pocked pond. The outlet may be covered by the rock lined flow spreader.

(g) The sill and rockfill flow spreader may be omitted if discharging to an existing watercourse or for temporary roads.

(h) Where pocket pond discharge does not occur to an existing watercourse, the maximum catchment area for each pocketpond should not exceed approximately 0.5ha.

(i) Erosion protection to comprise of rockfill as shown or an approved equivalent to provide effective long-term protection.

Figure I-4

Figure I-5



Infiltration Basins Infiltrating mine runoff water, which may be relatively high in both sediment and metals, reduces the amounts of both sediment and metals entering the natural streams and surface water bodies downstream. Infiltration basins and trenches placed in the mine pits allow infiltration of mine runoff to the fractured bedrock. Sizing of infiltration basins is based on a maximum permeability of the design area. To size the infiltration basin, a catchment water balance must be calculated. Water that does not infiltrate or evaporate from the catchment runs off and flows into the basin. A water balance must then be calculated for the basin, so that the rainfall on the basin and the runoff from the catchment entering the basin, is equal to the infiltration to the Blue Zone and evaporation from the basin surface. Infiltration basin sizes should fall within design parameters, which should be developed for a range of permeabilities.

Using infiltration basins or trenches on waste dump surfaces or steep slopes to increase infiltration to assist with revegetation, is not recommended. Due to concerns of land movement, infiltration basins and/or trenches must only be built on rehabilitated or constructed slopes if a satisfactory geotechnical investigation has been completed at the proposed location.

Pocket Ponds Pocket ponds are small sediment traps intended to infiltrate water into the ground, and also provide spill provision and/or an outlet. Pocket ponds should be constructed in the immediate proximity of operational areas, in line with ditches and/or at cross road drain locations to provide a first line of defense in the containment of sediment. Through localised storage and regular maintenance, they are intended to limit or prevent the possibility of large-scale sediment mobilising over great distances.

Sizing of pocket ponds is addressed in Section 5.3.

Maintenance is typically limited to removing accumulated sediment when it reaches 1/3 to 1/2 of the pocket pond depth. Larger pocket ponds will function for a longer period before requiring maintenance. A typical pocket pond is designed to be cleaned out four times per year in areas with significant disturbance. Periodic inspection is required to ensure proper functioning of the pocket pond, especially during periods of high rainfall and potential sediment movement.

Surface water infiltration should not be used where oil and grease are present, as they can contaminate the groundwater (Figures I-4 to I-6).

Disposal Area Berms The disposal area berm refers to a berm constructed with overburden material around the disposal area that contains surface water runoff from the dump. It allows sediments to collect naturally at low spots along the berm in the collection basins. Water is discharged from the disposal area through geotextile-wrapped rock underdrains or rock berm drain sections located at low spots along the berm. An emergency spillway is required to protect the berm against failure in the case of overtopping. The berm also protects surrounding areas by containing minor waste dump slope failures or dump material runouts.

Sediment Basins Both sediment basins collect runoff water and allow sediment to settle before discharging to a constructed or natural channel. A sediment basin is considered to be an excavated storage area contained by a small berm, whereas a sediment basin is considered to be a larger, engineered embankment that retains behind water and sediment behind it.

Basins are constructed in flatter topography and are comprised of a large excavation of overburden, bounded on the downslope side by a berm or dyke. Basins have the added advantage of significantly reducing the environmental consequence of failure, as the accumulated sediment is stored below the elevation of the natural bed of the watercourse at the downstream toe of the barrier.

Sediment basins can be used in low areas or natural drainage ways. They should be located where surface water naturally flows or collects. They should not be constructed where failure can endanger fish habitat,



human safety or property. Sediment basins should be located to intercept the largest possible amount of runoff from the site. Drainage into the basin can be improved by diversion dikes and ditches.

Sediment basins are typically only 70 to 80 percent effective in trapping sediment that flows into them. Therefore, they should be used with other upstream erosion control practices such as seeding, ECBs and check dams to reduce the sediment load to the basin. A common approach is to construct a number of basins in series, with the first to take out the coarsest material, and subsequent basins to capture progressively finer suspended solids. A series of basins allows one or more basins to operate while another is being cleaned.

Drainage options to restore available capacity in the basin following periods of rainfall include armoured spillways, floating skimmers, stand pipes and infiltration. A floating siphon or skimmer is the recommended method, as it continuously dewaters the basin by removing the cleanest water from the top of the water column at the surface of the basin. A Faircloth Skimmer is a commercially available floating siphon system. As the basin level rises or falls, the skimmer, which is attached to a movable arm, rises and falls with it.

A staff gauge should also be installed in the basin. This will allow the water level to be read during or after rainfall, as well as enabling an accurate reading of the depth of sediment accumulated in the basin to be taken during dry periods.

Sizing of sediment basins is addressed in Section 5.2. Refer to Figure I-6 for typical drawings.

Emergency Response An emergency response team should be identified and trained for emergency scenarios, in the case of a failure of an erosion or sediment control structure. A failure may comprise a channel bank failure, a sediment basin breach, a slope slide, or another mechanism of failure. The actions to be taken in an emergency depend greatly on the type of failure and the degree of threat imminent to human life, property or the environment.

All failures, emergency situations, near-failures and remedial measures undertaken must be fully documented.

Typical Drawings Typical drawings of the ESC design features discussed in this report have been provided to provide an indication of the required devices. An overview of the figures is provided in Table I-2.

Table I-2: Typical ESC design figures STAGE 1 AND 2 PRACTICES

I-1 Typical Drainage Ditches I-2 Typical Road Water Management Arrangement I-3 Typical Uphill Pocket pond and Cross Road Drainage I-4 Typical Cross Road Culvert Drain and Pocket pond I-5 Typical Cross Road Surface Drain and Pocket pond

STAGE 3 PRACTICES

I-6 Typical Stage 2 Excavated Sediment Basin

INLET CHANNEL

EMERGENCY SPILLWAY

SKIMMER OUTLETTOP OF EMBANKMENT

FAIRCLOTHSKIMMER

NATURAL GROUNDSURFACE

STAFF GAUGE

BASIN INLET CHANNELROCK LINED (REFER FIG

1-2) EXTENDS TODEWATERED BASIN LEVEL

DEWATERING DEVICEFAIRCLOTH SKIMMER (FLOATING

SIPHON) REFER FIGURE 2-6

ANTI-FLOTATION BLOCK ANTI-SEEP COLLARTYPICAL OF 2

SPILLWAY BARREL

STABILIZEDOUTLET

SPILLWAY ELEV.

S (2H:1V MIN.)

EMBANKMENT STABILIZEDWITH VEGETATION

300m

m

FRE

EB

OA

RD

300

mm

MIN

EMERGENCYSPILLWAY

SEDIMENTBASIN

STABILIZEDOUTLET

ENGINEERED SELECTEDFILL PLACED IN LAYERS

AND COMPACTED

11.5

12 (MIN)

HIGH WATER LEVEL

BASIN LEVEL AFTERDEWATERING

"LIVE" STORAGE

'DEAD' STORAGEFOR SEDIMENT

Golder Associates

Design Notes:(a) Final layout and dimensions are specific to each external Stage 2 structure.(b) Refer to report for basin sizing procedure. Spillway should be sized using the Broad-Crested Weir Equation, with a

weir coefficient of 1.7 and the applicable design ARI event.(c) Spillway channel slope to be 2H:1V unless downstream topography is limiting.(d) Basin side slopes are to be 1.5H:1V or flatter.(e) Basin inlet design to be determined at a later design phase.(f) A passive dewatering device (e.g., Faircloth Skimmer) may be installed in the basin.

Application: An excavated sediment basin can reduce the embankment height required to retain the sediment, as well as reduce the surface area required for the pond at high water level.

PLAN VIEWNTS

SECTIONNTS

2-1

TYPICAL EXTERNAL STAGE 2 EXCAVATEDSEDIMENT BASINFigure I-6

appendix h erosion and sediment control strategy - · pdf fileerosion and sediment control...

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