Diversion ditches and channels
Anniina Kittilä, ETH Zürich, Institute of Geophysics, Geothermische Energie u. Geofluide. Sonneggstrasse 5, 8092 Zürich, Switzerland e-mail: anniina.kittila(at)erdw.ethz.ch
At closed mine sites it is important to keep undisturbed and contaminated waters separated to avoid contamination of natural waters and to minimize the mine water volumes. It can also be desirable to divert mining influenced water to treatment or collection systems and away from sensitive environments and surrounding waters entering closed mining operations. One of the solutions to keep waters with different quality separated is to construct diversion ditches and channels (Lottermoser 2007, Wolkersdorfer 2008, ITRC 2015). The diversion structures divert surface water around potential inflow zones, such as seeping places or fracture zones, and can be extremely useful to minimize post closure impacts. They are designed on a case-by-case basis, possibly extending several kilometres, and need to be securely sealed to prevent percolation into the mine. Their discharge capacities also need to be evaluated based on local hydrological conditions, so that even larger quantities of water can be diverted, for example during snowmelt, and so that the structures are not destroyed in more extreme conditions (Wolkersdorfer 2008).
Description of the method
The capacities of diversion ditches and channels must be evaluated by hydrological investigations or calculations and models, because higher discharge conditions than what the channel was designed for might destroy it (Wolkersdorfer 2008). The total amount of runoff and its temporal variations are affected by three main factors: climatic, areal and anthropogenic. From climatic factors the most important are precipitation and evapotranspiration. Precipitation can be described in various ways; with intensity, duration, temporal and areal distribution, and the direction of the rain front. In addition to evapotranspiration, the amount of runoff is reduced or controlled by areal factors, such as the basic features of the catchment (size, shape, the amount of lakes, swamps and other larger water bodies from the total area), topography, soil type and bedrock, vegetation, and the characteristic of the streams and channels. The anthropogenic factors are especially important when considering natural streams and the formation of runoff in their catchment (Mustonen 1986); the diversion ditches and channels themselves are man-made water management solutions employed in the mining sector.
In general, diversion ditches and channels can be characterised based on the material used in channel lining. There are dug channels without any specific lining (earth or grass-lined), but the use of natural or man-made lining materials greatly decreases the infiltration of water through the channel walls. Possible lining materials are, for example, on-site clay, bentonite, sheet plastic or other geosynthetic materials, and cement (concrete). Also erosion should be taken into account where water velocities are sufficient to cause it, and the lining material should be erosion-resistant (EPA 2000). The channel walls can also be reinforced with gravel or stones, and locally with concrete along steep slopes to increase the hydraulic stability (Barnekow et al. 2011).
Concrete and geosynthetic linings are to be considered when flow velocity is relatively high and erosion needs to be avoided. These lining solutions also basically prevent any infiltration of water through the channel walls, and are effective in conveying water out of the area (Suomen ympäristökeskus 2002, Tiehallinto 2004, NCDENR 2009). These smooth-surfaced linings, however, introduce high energies and high flow velocities that must be controlled and dissipated to avoid damage to channel outlets and receiving streams (NCDENR 2009). From geosynthetic materials particularly geotextiles, geonets and geocomposites are suitable for drainage structures and erosion control. However, because of the variety of different materials, also their durability and installation recommendations are different (Suomen ympäristökeskus 2002, Stevenson 2008).
If passive treatment of the water (increasing alkalinity) in diversion channels is required, they can be lined with limestone. This is suitable particularly for acid mine drainage, although the limestone must be replenished, on an approximately annual basis. The rise in pH, however, removes metals from the water as precipitates, possibly filling the pore spaces in the limestone lining and greatly attenuating the dissolution rates. Thus regular maintenance of channels lined with limestone is necessary. Construction criteria for limestone channels are determined from the flow rate, channel slope, and acidity concentration. (Ziemkiewicz et al. 1994, EPA 2000)
Clean surface water may flow into mines and mine waste areas, where it can become contaminated, increasing the amount of contaminated waters that require treatment in the mining area. Once mining influenced water is produced, it can flow into sensitive environments and damage or destroy the ecosystem, or affect downstream groundwater supplies. Diversion ditches and channels can be used in diverting both clean and contaminated waters, reducing risks and damages that could occur due to incorrect water management (ITRC 2015). The two main advantages of diversion ditches and channels can be listed as: i) possibility to design them for a wide variety of flow conditions and sites, making them suitable also for a variety of water types, and ii) reduction of pollution load (EPA 2000, ITRC 2015). The disadvantages of the diversion structures are, for example, that they might be prone to erosion or destruction by structural failures, overflow in case the channels have not been designed for larger discharges, and the need for regular maintenance (EPA 2000, Wolkersdorfer 2008, ITRC 2015).
Regular inspections and maintenance of diversion ditches and channels are highly recommended to avoid structural failures and reduction in the channel’s capacity by, for example, excess vegetation growth, precipitation or sedimentation (EPA 2000, Wolkersdorfer 2008). Another serious problem associated with diversion ditches and cannels is erosion, which is strongly dependent on the flow velocity in the channel. For velocities up to approximately 1.8 m/s a well-established grass lining can protect the channel from erosion, but it does not fully prevent it. Typically the vegetative linings begin eroding from the base of the channel, which most likely stops just when an erosion-resistant layer is encountered. The eroded areas can be patched, for example, with rock or stone rip rap, but the maintenance costs might increase rapidly if extensive repairs are needed. In general, however, vegetated channel linings are easy to construct and they have low initial costs (EPA 2000).
Channels lined with concrete, geosynthetic materials or rip rap can be considered permanent structures that can convey large volumes of water without eroding. Rip rap is a low cost solution to prevent erosion, and in some extent it has an ability to adjust in the channel foundation and its failures. In case of failure of the rip rap lining, its repair is relatively inexpensive, compared to structural failures in concrete or geosynthetic linings. The rough surface of rip rap also reduces flow velocity in the channel, and allows infiltration (NCDENR 2009). If diversion channels are to be used to convey contaminated or polluted water, rip rap would not be best solution because of the infiltration. The material costs and need of personnel in installation of channels lined with concrete or geosynthetic materials are higher than for other lining solutions, but they have lower maintenance needs. However, if this kind rigid lining material is damaged, its repair could be expensive. On the other hand, concrete and geomembrane linings are very durable compared to other lining materials, and are often chosen because of this property (Stevenson 2008, NCDENR 2009).
Limestone channels are not permanent structures, but they are relatively simple and inexpensive systems to construct. However, the material needs to be replenished in order to treat the acidity, and the neutralization ability may be less than reported values. The efficiency of the system can also be reduced by certain water quality parameters. Additionally, some thousands of tons of limestone might be required to treat large discharges with considerable acidity concentrations, and therefore this structure and method may not be applicable, especially in space-limited sites. Another construction limitation is that these channels require at least 10% slope to prevent settling of precipitates and clogging, so they cannot be constructed in areas without sufficient, steep topography. Also, as the channels flush metal precipitate particles (flocs) downstream, the construction of settling pond at the outlet point of the channel will allow the metal flocs to be concentrated at one point. To maintain the efficiency of this system, also the settling ponds should be cleaned periodically (Ziemkiewicz et al. 1994, EPA 2000).
The dimensions of diversion ditches and channels must be measured so that they have a capacity to collect large amount of water, especially during storms or snowmelt (cf. Flow rate measurements; Wolkersdorfer 2008). The design of suitable structure also requires choosing the most appropriate geometry of the channel. The three most common cross-sections are: V-shaped, parabolic, and trapezoidal (NCDENR 2009). There might also be some concerns about land use, as the construction of diversion structures usually involve significant surface disturbance (ITRC 2015). In proper design of diversion structures hydrological characteristics of the area must be taken into account, in addition to the changes that the structures cause to the original flow and runoff conditions. The routing of a diversion channel is most often affected by local topography, which determines, for example, the upstream starting point for the channel in order to achieve a sufficient gravity route around the mine area. According to Hustrulid et al. (2000), the upstream part of a diversion channel has typically flatter slopes than the downstream part, where also the velocity is higher (because of steeper slopes). High flow velocities might require erosion-resistant lining. One major factor when designing diversion ditches and channels in a closed mine site is the overall erosion resistance and maintenance needs, which affect the economical aspects of the closure and in case of a breakdown also environmental aspect. A major hydrological factor to be considered in designing the capacity of the channel is peak discharge. The design discharge, which the diversion channel should have the capacity to convey, corresponds to a certain probability of occurrence or return period. For smaller channels a design flood return period in the range of 10 years and for larger channels in the range of 100 to 200 years should be reasonable, in case no significant interruptions to mine operations or damage to environment would result from structural failure. The risks and consequences of failure should, however, be balanced against the capacity and cost of the channel on a case-by-case basis (Hustrulid et al. 2000).
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