WINTER 2019 AVENUES 33 AGGREGATES 101 Transported sediments in water can signifi- cantly alter streambed composition. Exces- sive sedimentation can impact the quality of habitat for aquatic species, which disrupts their lifecycles. For this reason, the Ministry of the Environment, Conservation and Parks sets limits to suspended solids in aggregate site discharges. In the built world, erosion can diminish the carrying capacity of water courses. This inability to transport water efficiently then increases issues with flooding. A plugged culvert, for instance, conveys little water. Erosion and flooding at operating sites can damage access roads, making them impas- sible. The damage can significantly compli- cate extraction and rehabilitation efforts. Aggregate sites are particularly suscep- tible to erosion for a number of reasons. When vegetation is cleared ahead of extrac- tion, it makes soil more vulnerable to erosion forces. Plant roots reinforce and act to bind the soil, and vegetation intercepts rainfall, which reduces the volume of runoff and slows its velocity. Plants also extract moisture from the soil, and this reduces the time the soil remains saturated, making the soil less prone to erosion. Secondly, erosion can be exacerbated by the tall slopes typical of many rehabilitated aggregate sites. Precipitation that falls on slopes can concentrate and gain velocity as it moves downhill. The erosive forces of this flowing water intensify with increased gradient and slope length. This often results in the formation of rills and gullies on reha- bilitated slopes that don’t have adequate protection. These gullies are often deepest and widest near the base of the slope where water has reached its maximum velocity. Thirdly, the quality of rehabilitated slopes can impact the severity of erosion. Fine particles left over from aggregate processing or from natural parts of the resource that are not marketable are commonly used for sloping at pits and quar- ries. Unlike engineered backfill used in construction projects, this fine material is often placed with less emphasis on compac- tion and slope construction. As a result, loose and fine-grained material, common at aggregate sites, is more sensitive to the forces of nature. WHAT CAN BE DONE? Aggregate producers in Ontario have a good record of employing practices to limit erosion. One option is to have facilities in place that slow water velocity, improve retention times and encourage particulate settlement. In this way, aggregate producers can improve water quality and reduce the potential for erosion on and further down- stream from their facilities. Attendees at OSSGA’s rehabilitation tours have seen a number of their colleagues address erosion challenges, including some in very difficult conditions. Below are examples of effec- tive strategies that aggregate producers can follow. 1)  Good planning goes a long way, and a rehabilitation plan needs to consider erosion from the start. For example, any time a producer modifies a drainage feature or alters the topog- raphy on a site, erosion and sedimenta- tion protection should be considered. A good mining plan will limit the amount of time a stripped area is exposed and can significantly reduce erosion effects. Planting windows are also important in order to quickly establish vegetation. Scheduling extraction and rehabilita- tion needs to be an integral part of a well-designed mining plan. 2)  The areal extent of exposed terrain is also critical. Unmitigated problems tied to erosion usually grow over time as rills and gullies from runoff become deeper and wider with every new storm. Producers need to keep erosion-prone areas limited in size and they need to quickly rehabilitate these areas. One of the best defences against erosion is to seed and encourage vege- tation on slopes, and this vegetation can take time to establish. 3)  To slow the velocity of runoff, the industry employs a number of different methodologies. In the Oak Ridges Moraine, where there are some of the tallest aggregate slopes in the province, it is common to “break up” slopes with short steps or terraces. Although these flattened areas make the slope longer or possibly steeper over short distances, they act to break up the flow of water and can redirect storm water to limit concentration. As a bonus, steps are particularly favourable for establishing vegetation, as seeded material is less likely to move downslope in flat- tened areas. 4)  Directing runoff away from the top of slopes will often prevent the concentra- tion of flow. Given that a deep excava- tion is often the nature of aggregate extraction, it is not always possible to direct overland flow away from slopes. Engineered diversion chan- nels, however, can be installed in steep areas to direct water flow. These are often reinforced with riprap or through some form of innovative engineering. Civil engineers have designed tools for erosion control in construction that provide separation, filtration, drainage and reinforcement. These include geotextiles, walls, mats or the place- ment of specialty aggregates. These solutions are sometimes used in partic- ularly problematic areas at aggregate sites – the Herber Down site in Durham County is a good example. Understanding that erosion is a natural process and that some erosion is unavoidable is key. The goal for operators is not to elimi- nate all erosion but to control problems and to find solutions to site-specific needs. Plan- ning ahead to address erosion concerns and choose the right erosion-control methods is critical. During an aggregate site’s active operation, inspection and maintenance of storm water and erosion control facilities should be part of the routine work. • “The goal for operators is not to eliminate erosion but to control problems and find solutions”