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Project Construction

TECH GUIDE

Water Controls: Surface Water Controls

The key objectives of surface-water control structures are to: (i) protect the site from runon that may come into contact with tailings and waste rock by diverting water around the site, (ii) protect engineered covers from erosion and exposure of contaminates, and (iii) protect water treatment systems from surface water runon and possible overtopping.

Diversion Structures

Surface water diversion technologies help protect reclaimed lands from the consequences of water erosion and large precipitation events. Water erosion from runon and large storm events can damage or destroy the workings completed during mine reclamation. Surface water diversion technologies help keep reclaimed hillslopes, containment structures, and water treatment structures from structural failure or erosion damage and flooding which may cause the release of significant concentrations of acid and metals into the environment. These diversion technologies are considered permanent since they protect the reclaimed areas over the long term. They differ from the diversion technologies discussed in the site preparation section which are constructed on a temporary basis to divert clean water around contaminated sites before and during reclamation. Different permanent diversion structures include dikes and berms, terraces and benches, ditches and drainways, and chutes and downpipes. The permanent diversion structures are often similar in design to the temporary structures except the permanent structures must be able to carry a lower frequency, higher duration storm than the temporary structures. This entails larger, better-designed structures with a larger factor of safety.

Dikes and berms reshape the surface of the ground to form earthen walls that prevent runoff and off-site runon from reaching sensitive areas. Dikes and berms are often constructed around the base of containment structures and water treatment systems. Behind the berm there may be a diversion ditch to direct the flow to a nearby surface water body, sedimentation basin, or water treatment system. These measures prevent degradation of engineered covers and subsequent release and transport of contaminants to off-site locations. They also prevent flooding and overtopping of water treatment systems during large storm events.

Benches and terraces help protect hillslopes from excessive water erosion by creating barriers along the contour which captures flow and prevents excessive erosion. Surface roughening techniques such as dozer basins, pittes, and gouges also protect the hillslopes from excessive water erosion and subsequent instability, settlement, or sudden failure problems. For more information on these topics, see the hillslope erosion control section.

Ditches and drainage ways are constructed to collect and direct overland flow and runoff/runon captured by dikes, berms, benches, and terraces away from reclaimed hillslopes, containment structures, and water treatment systems. Water is diverted in ditches to nearby surface water bodies, sedimentation basins, and/or water treatment facilities. Ideally, a ditch has minimal and consistent gradient. Ditches are intended to convey the peak flows from the design event with adequate "freeboard", the distance between the calculated surface of the peak flow and the top of the constructed channel. For more information on ditch and drainage way design, see the site preparation section.

Chutes and downpipes are used to collect precipitation and runon and convey the flow down steep gradients, beneath roads or other structures and across areas that would obstruct the flow. The pipe material must be compatible with water quality. For example, corrugated metal pipes should not be used to convey acidic drainage. Pipes generally are designed with inlet controls to prevent complete filling and excessive joint pressure on the pipe. They are also designed with seepage collars to prevent undermining of pipes and with trash racks to prevent clogging. For more information on chutes and downpipes, see the site preparation section.

Water collected from ditches, drainage ways, chutes, and downpipes are commonly directed to sedimentation and detention basins where the water is detained for a given residence time in order to capture and drop out eroded sediment from the surface runoff. Depending on the water quality of the drainage, the water may then be diverted to a water treatment system or it may be released to a nearby surface water body.

For more information on diversion and collection, see:

Dams

Dams are created by building an embankment wall that is used to impound water. They store water during times of high runoff which allows for continuous flow downstream during times of low flow. An outlet is constructed for the controlled release of water from the dam. Spillways are a feature of the dam which discharges excess water when the reservoir threatens to become dangerously high.

Dams have various applications in mine reclamation. A dam may be built to: (i) store water for use on the reclamation site, (ii) impound contaminated water for treatment, (iii) divert waterways away from contaminated sites to prevent the migration of contaminants off-site, or (iv) dampen the effects of peak flows to reduce the effects of erosion.

Most reclamation dams are earthen dams made from indigenous materials. This material may not always be suitable for the structural stability of the dam therefore, in certain cases, it may be necessary to import some or all of the dam material. Maintaining structural stability of a dam is the most important design component since the pressure of the water against the embankment wall is always substantial. Embankments made of earthen material may become saturated due to the porous nature of the material. Once the embankment becomes saturated, the structural integrity of the dam is low and the dam could fail. To prevent this, most earth-filled dams have an impermeable core that allow a substantial amount of the downstream portion of the dam to be unsaturated. Often the toe of the dam on the downstream side is made from large rocks (rip rap) that ensure large, well-drained pores which aid in keeping the downstream portion of the dam dry.

For information on the environmental impacts of damming, see the environmental impacts section. Also, check out the following websites:

 

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