Another summer of high rainfalls, full dams and flooding rivers around Australia has once again highlighted the catastrophic damage that can be caused to communities and businesses by floods and inundation.
Mining operations are certainly not immune to the risk of flooding from high rainfalls or inundation from surrounding water courses – in fact the nature of our sub-surface operations means we can be considerably more exposed to initial impacts, and subsequently impacted for a longer period.
Impacts of Flooding
The impacts of flooding rains come in a number of forms, and will affect different mines in different ways depending on their geographical location, type of operation, and access routes for logistics and transport to and from site. Impacts could include:
- Suspension of operations due to road conditions in the mine. Usually restricted to open pit mines, and often somewhat accounted for in budget operating assumptions.
- Water building up at working faces and mining areas from rain fall directly onto the mine site. The duration of interruption caused will depend on the on-site storage and pumping capabilities of the mine. Extended periods of rain increase the amount of water stored on site, hence reducing the available volume to remove any further build up of water from working areas.
- Inability to discharge water from storage areas into water courses. The requirements here vary by state, and are in many instances under review. Inability to discharge offsite in conjunction with full onsite storage areas means mining voids may have to be used as emergency storage areas.
- Road and rail systems off site being cut by flooding or damage, preventing personnel and supplies from getting to the mine, and preventing product from being shipped from the mine. For an underground mining operation, these impacts are likely to be the most important in terms of lost production and sales.
- Inundation of the site from surrounding water courses. Flooding of mining pits (underground or open cut) due to inundation from creeks or rivers has the potential to be the largest and most critical impact on an operation. Impact could be caused by inadequate protection from water courses, failure of protection levees, or extreme rain falls overtopping designed protection systems. The duration of the impact will depend on the mine’s capability to pump the water from the working areas to storage areas or back into water courses. Losses of equipment and infrastructure, and geotechnical instability may also occur.
Assessing the Risk
Mining operations should assess their risk and exposure to flood related interruptions and damage on a regular basis. Like any risk assessment, the site should consider the effectiveness and adequacy of their controls, both preventative and mitigating. The site should consider their exposure and likely impacts at the different levels of flooding, i.e. 1:10 year, 1:50 year, 1:100 year, 1:1000 year recurrence period flood events, and at the Probable Maximum Flood event level.
The risk assessment should consider impacts to onsite operations, as well as potential downstream and offsite interruptions. Different types of consequences should also be assessed – financial, production, health and safety, and environmental impacts.
Complications in Assessing Flood Risk
One complication that often arises in the assessment of flood risk is the dependence on the recurrence period estimates (i.e. 1:10, 1:100 years) of rain fall quantities and water course heights related to these events. If information based on these flood and rain fall levels is to be used when planning flood protection controls, it is critical that the site understands the basis for these numbers. For example, a 1:100 year rain fall event on a single day may not equate to a 1:100 flood event in the local area. Likewise, several days or weeks of 1:5 year rainfall over a large area could culminate in 1:500 year flooding levels further downstream.
There is an important differentiation to be made between a sudden intense dump and a long, slow build up of waters. Likewise the amount of rain in proceeding days and months makes a substantial difference to the impact from a particular amount of rain fall in a single event.
While recurrence intervals are useful when calculating the “likelihood” side of the risk equation, the risk assessment team should not place too much dependence on impact information associated with the particular intervals. The team should instead consider the different key levels of rain fall or flooding from minor up to the Probably Maximum Flood level, and then consider what controls are in place to manage the risk at each level. The controls should include predictive, monitoring, preventative, mitigating, and reinstatement controls.
In hindsight it is easy to assess what recurrence interval should be assigned to a particular event which has already occurred. Calculations can provide a 1 in X years estimate for a flood or rainfall event in the past, but no one can yet predict what numeric level an approaching weather system will reach. If only they could tell us that the low approaching next week was going to be a 1 in 125 year event, we could probably prepare a lot better!
Mine sites should have a written system (Trigger-Action-Response Plans) for what will be done in preparation for and during rain fall events, and what exactly will trigger them to take those particular actions. These response plans must relate specific triggers to specific actions, and allocate those actions to specific people. The triggers may come from weather forecasts or actual measurements during an event.
Response and Reinstatement
Mining operations should have sound and thorough reinstatement plans in place to return assets and operations to service as soon as possible, with minimum risk to safety. The plans may be known as Asset Reinstatement Plans, Business Interruption Plans, Contingency Plans, or Continuity Plans. These plans should be based on risk assessment, and draw on recent industry experience in developing the roles and activities to be undertaken following a major flood or rainfall event.
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All photos courtesy of www.MiningMayhem.com