**7. Long-Term River Relocation Rehabilitation**

The previous sections introduced the conflict of establishing natural values within river relocation channels whilst also ensuring engineering functionality and stability. River relocation channels can be criticised for their lack of long-term stability and lack of ecological and environmental attributes in comparison to the original channel. More recently, the importance of identifying river behaviour [95] and geomorphic processes [96] has been highlighted as a necessity for long-term stability of constructed channels.

The overall long-term objective of river relocation channels varies depending on the river's location, and previous modifications. There are many river relocation channels (such as the Twin Rivers at Heathrow, UK) that have had a long history of modification and are channelised or are constructed on restricted floodplains, so that it is challenging for them to possess all the attributes of a natural river system. Many of the constructed channels have substantially altered boundary conditions, and it may not be appropriate or feasible to rehabilitate the river to its pre-disturbance condition. Instead, it might be more relevant to strive to maximise the beneficial features of the river in its new setting if irreversible of systematic change has occurred [97]. This section focuses on river relocation for mining and the objective of long-term river rehabilitation.

Many environmental impact assessments (or equivalent thereof) now require evidence of long-term river relocation objectives. Rivers relocated for mining purposes are subject to long-term rehabilitation objectives, including a rehabilitation plan for the site to include channel stability and positive environmental outcomes. In Australia, mining river relocation licenses can only be relinquished (that is, returned to the responsibility of the government) once they have proven that the relocation has met the outcome-based conditions stipulated in the mining license [98]. However, difficulty arises, as river relocation channels have both a temporary and permanent role throughout the mine life-cycle. They represent a key element of engineered infrastructure to ensure both the functionality of the mine during its operation and its subsequent rehabilitation after mining has ceased. Stable river relocation designs are important throughout all stages of the mine-cycle and as such pose enormous challenges for water resource managers [82], not least of which is the danger that, at some time over centuries or millennia, the relocated channel could permanently divert into the mining pit.

Post-mining, most river relocation channels are left in their new position, a few are redirected back to their original course [99,100], or in some instances, the river channel is engineered into a pit lake as riverine through-flow to maintain or improve pit lake water quality [101,102]. Rehabilitation programmes are typically designed to ensure safety and minimise potential negative impacts of the closed mine [103]. Robust and stable engineered designs are crucial for flood conveyance during mine operation, with the ecological and geomorphic components of the river course developing more importance for the implementation of rehabilitation programmes.

Consideration of relocated river channel rehabilitation often begins in the design phase. Permanent river relocation channels present a new challenge in that they are designed for the long term, with channels now constructed with an attempt to replicate the natural channel they replace. This is typically carried out using a design criteria approach (e.g., ACARP geomorphic and hydraulic criteria) where available or a reference reach approach. The design criteria approach will use specific hydraulic and hydrologic targets to create a design standard designed to create the required hydraulic conditions within the channel to enable vegetation recovery and the establishment of geomorphic forms. The reference reach approach will use natural channels to establish closure criteria [104]. Blanchette et al. [104] sugges<sup>t</sup> that reference sites should lie within a river's normal variability and are both sustained and tracked over time. Both approaches advance the historic form of river relocation channels, which have tended to be trapezoidal and lacking geomorphic complexity.
