*4.3. Bedrock Relocation Tunnels*

The simplest river relocation channels are found when a new channel has been blasted through bedrock, commonly as a tunnel through horseshoe bends within sections of river. Thirteen such channel relocation tunnels were constructed for historical gold mining purposes in Victoria, Australia (Figure 7) [62]. The purpose was to dry out the meander bend to allow easy access for alluvial mining. These relocation channels were typically short and utilised the natural features of the watercourse to minimise the cost or distance of the relocated channel. The introduction of dynamite in 1867 [63] allowed for more substantial channels to be constructed. These tunnels disrupt sediment supply through the reach (sediment tends to build-up upstream of the relocation). They also act as barriers to fish passage. In this case, the nationally threatened Australian grayling cannot traverse the high flow velocities in the steep bedrock channel [64].

**Figure 7.** (**a**) Thomson River relocation, also known as Horseshoe Bend [62] (Source: Victorian Heritage Database); (**b**) diagram of residual reach and relocation tunnel (Source: West Gippsland CMA, 2010).

One of the most common types of river relocation are open channels cut through bedrock. This type of relocation is most common in open-cut mining operations. This type of mining usually takes place higher in the catchment, where floodplains are narrower, and any channel has to be cut into the bedrock valley walls. The purpose is, firstly, to divert tributaries around the mine to avoid flooding, and secondly, to divert the river away from areas that can be mined. The Goulburn River diversion in New South Wales (Australia) is an example of a bedrock river relocation constructed in 1981 to relocate 4 km of the Goulburn river around a coal mine (Figure 2c). The relocated channel is cut 10–20 m deep into a deeply weathered saprolite [65]. The central reaches of the channel relocation were constructed to have a box-like canyon form with benches constructed on the channel banks [66].

The relocated bedrock channel is a simple rectangular channel with vertical walls. Compared to the natural reaches upstream, the relocated channel is steeper and hydraulically smooth, with high stream power. As a result, the channel experiences high erosion rates with dispersive subsoils exposed throughout the channel [67]. The new channel also has simple morphology, with a flat floor, and an unnaturally dense covering of reeds [66]. Also, the bedrock channel cuts across tributaries, producing 'hanging' tributaries at each junction. These hanging tributaries become waterfalls during storms and can form gullies. They also completely disrupt up-and-downstream migration of fish, and any form of riverine connectivity between the river and the tributaries. Current rehabilitation strategies are being implemented to improve the stability and design of this relocated channel [68].

### Bedrock Diversions for Coal Mining in the Bowen Basin, Queensland, Australia

Since the 1970s, over 60 full bedrock river relocation channels have been constructed in the Bowen Basin, Queensland (Figure 8), a major coal mining region [69]. Dynamic meandering channels were replaced with relocated channels that were straight and of trapezoidal form to reduce construction costs and maximise the discharge capacity of the river channel [69]. Designs from the 1980s onwards incorporated drop structures to compensate for reduced channel lengths and the accompanying increase in bed slope [69,70].

**Figure 8.** The Bowen Basin (Queensland) with locations of river relocation channels [69].

The Australian Coal Industry Research Program (ACARP) reviewed the performance of river relocations [69,70]. Some relocated channels experienced high erosion rates due to inadequate design widths, increased bed slopes from shortened channel lengths, and increased velocities exacerbated by an absence of vegetation, but conversely, a smaller number experienced high sedimentation [69]. Some were at risk of eroding into adjacent open-cut pits or associated mining infrastructure. The poor performance of the relocated channels (Figure 9) led to a temporary moratorium on the approval of river relocation construction by the Queensland governmen<sup>t</sup> [71] which lasted for 5 years.

Overall, five key factors were identified that consistently limited the performance of the Bowen Basin relocated channels. These factors were sediment supply and transport, vegetation condition, the occurrence of major flood events in the early years of diversion establishment, overland flow drainage, and the transition between the relocated channel and the natural watercourse [72]. Improved design standards have dramatically improved the performance of Bowen Basin relocations, and these are discussed in Section 6.2 below.

**Figure 9.** Changes in the design of relocated channels over time in the Bowen Basin, Queensland: (**a**) 1970s river relocation with a trapezoidal design and exposed banks; (**b**) 1990s river relocation with limited vegetation establishment, increased channel dimensions, and rill erosion on batter banks; (**c**) 2000s river relocation with a more natural bank batter and more natural morphology in the bed [69,70].
