*4.4. Alluvial River Relocation Channels*

Alluvial river relocations are carried out using natural channel materials, such as in situ alluvial sediments, and they cut across a floodplain rather than into bedrock. In some instances, they can be sculpted to maintain similar channel dimensions and bed grade to the natural channel. Other alluvial river relocation channels also incorporate the floodplain into the channel design.

### 4.4.1. Case Study: Twin Rivers Relocation, Heathrow Terminal 5, UK

Rivers are frequently relocated for construction purposes, either as a temporary measure while building bridges or dams, or permanent diversions for development. There are many examples of airport river relocations, including the River Mole diversion for Gatwick Airport, UK [73], Sugar Brook relocation for Manchester Airport, UK [28], the Twin Rivers relocation for the expansion of Heathrow Terminal 5 [74], and the planned relocation of the Ulwe and Gadhi rivers for the construction of Navi Mumbai airport, India [75].

Many airports are located in areas with limited available land for expansion, resulting in increased pressure to utilise river corridors for continued airport growth, with the economic return of expansion outweighing the cost of river relocation. There are specific managemen<sup>t</sup> issues for the relocation of rivers for airport construction. As part of the construction process, valleys are backfilled to bring ground levels up to required elevations for runway construction [23]. In addition, contaminants, such as jet fuel and de-icer, can flow into the river system, presenting a significant source of pollution. Birds present an additional challenge, as open bodies of water such as rivers attract avian communities but represent a hazard to aircraft safety.

The expansion of Terminal 5 at Heathrow airport in the UK required two rivers to be relocated. The Duke of Northumberland's River and the Longford River (known collectively as the Twin Rivers) flowed through the middle of the Terminal 5 project site. Both rivers have a long history, and were originally man-made, constructed to supply royal estates located on the banks of the River Thames [74,76].

To facilitate expansion, the Twin Rivers were relocated around the western perimeter of the airport. The relocated channels were designed to ensure they had the capacity to convey peak flows of 3 m3/s and 1.5 m3/s for the Duke of Northumberland and the Longford, respectively [74]. River relocation developments need to comply with local and national policies, where present. In this instance, the Twin Rivers relocation needed to comply with the EU Floods Directive (Directive 2007/60/EC) and the Flood and Water Management Act (2010), which relay the overarching message that all development must consider and mitigate flood risk, ensuring that this risk is not increased because of river relocation construction [77].

The new channel design saw an increase of open channel, with 95% of the relocation channel occurring in an open channel, compared to 50% in the previous diversion design [78]. This 'daylighting' of the channel aimed to enhance the environment, with the inclusion of habitat features designed to provide a minimum of environmental equivalence compared to pre-diversion standards [74]. Habitat features included modifications for fish passage, the addition of in-channel wood, 8000 m<sup>2</sup> of pre-planted vegetation, and the provision of alternating berms with rock-filled gabions and logs [74]. A bird exclusion net was also added throughout the entire length of the Twin Rivers channel (Figure 10). A 0.3 mm diameter, lightweight polypropylene netting with a mesh size of 75 mm was selected, as it excludes all hazardous birds but allows exit and entry for a range of invertebrates, such as the emperor dragonfly [74].

 **Figure 10.** *Cont*.

**Figure 10.** Twin Rivers relocation at Heathrow. (**a**) Aerial view of the Twin Rivers Diversion, UK; (**b**) bird exclusion netting across channel (Source: HAHL Airports Limited).

### 4.4.2. Case Study: Kaituna River Relocation, New Zealand

The Kaituna River is an example of a laterally active river located on the Bay of Plenty, New Zealand. This case study illustrates the cumulative impacts that can arise from river relocation. The Kaituna River is a 50 km long [79] modified river that has been relocated on several occasions. The original course of the river passed through the Papahikahawai Channel into the Ong ¯ atoro/Maket ¯ u¯ Estuary. In 1907, during a flood, the Kaituna River broke from its original course due to an avulsion through a sandspit at Te Tumu [80], stopping flow to an adjacent coastal estuary.

Between 1926 and 1928 [81], two parallel chutes were cut to relocate water back into the estuary, becoming known as Ford's Cut (Figure 11a). Ford's Cut enabled the river to return to the estuary, however, during the same period, natural channel migration caused the river's flow to migrate eastward, returning to the estuary via the Papahikahawai Channel [81].

**Figure 11.** (**a**) Ford Twin Cuts in 1948. This photo was taken before the Te Tumu cut was opened to the sea in 1957; (**b**) Ford Twin Cuts in 1957, this photo is 2 years after the Te Tumu cut was opened in 1959 (Source: The Ford Collection).

Additional serious flooding occurred again in 1949 and 1951 [80], which resulted in a managemen<sup>t</sup> response to construct a new mouth for the Kaituna River. The new mouth of the river was

commissioned in 1956 and was named the Te Tumu Cut (Figure 11b). The objective of this river relocation was to reduce the frequency and severity of flooding on the Te Puke lowlands, former wetlands that are now surrounding agricultural land [79,82]. The Te Tumu relocation channel could reinforce the natural 'second mouth' that occurs during flood events [82].

The previously engineered Ford's Cut and the Papahikahawai Channel were blocked with a causeway at their upstream ends to maintain full river flow throughout the Te Tumu cut and to potentially reclaim the Maketu Estuary [ ¯ 81]. However, a secondary response to the blocking of Ford's Cut and the Papahikahawai Channel was the reduction of flow through the old river estuary. The flow reduction caused tertiary issues, such as an increase in salinity which destroyed wetland and reduced the estuary's ability to flush out sand and mud [82]. The Te Tumu river relocation contributed to sediment infilling and general ecological decline of the estuary [79] and has been described as a venture carried out for the benefit of the farming community at the expense of another community: estuary users [80].

Since the Te Tumu relocation, there have been attempts to restore flow to the estuary and growing support for another channel relocation to combat increased sedimentation and the closing of the estuary mouth [83]. In 1995, the construction of four culverts was undertaken at Ford's Cut [81] to resupply water back to the Maketu estuary via flapgates following years of concern about the ¯ closure of all previous river paths. Domijan [84] estimated that this flow restoration resulted in an additional volume of 100,000 m<sup>3</sup> of water entering the estuary per tidal cycle. The addition of water into the estuary was hoped to reduce sediment infilling and restore some of the declining habitat and restore fish stocks or "kaimoana" [85]. The addition of water did assist in reducing the salinity in the upper estuary but there has been no measurable reduction in sedimentation rates [28], with continued poor overall hydrodynamic and ecological improvement [83]. There are plans to construct an additional relocation channel through to the Maketu Estuary to create new wetlands and maximise ¯ both community and ecological benefits [85].

### **5. Implications and Challenges of River Relocation**

Lined, bedrock, and alluvial channel relocations have been introduced, with case studies providing examples of each of these types. Each case study highlights some of the challenges surrounding the design, construction, and performance of river relocation channels. In the past, relocated channels were considered successful if they passed all flood flows, did not erode excessively, and did not degrade the river reaches up and downstream. Since the 1990s, higher standards have been demanded of the relocated channels, and they are now expected to maintain biological and aesthetic values in both the channel and adjacent reaches up-and-downstream. In general, relocation channel construction can cause a series of primary changes (defined here as physical changes around the diverted channel), which can then lead to the generation of secondary issues (defined here as physical and biological connectivity issues caused by the primary changes), and then tertiary issues (defined as linked, but perhaps surprising, consequences on biology and human communities caused by secondary issues) (Table 2). The issues arising from channel relocation can be broadly characterised as either fundamental engineering problems, or issues relating to the ability of the relocated channel to behave in a comparable way to a natural channel. As such, the performance of river relocation channels can be considered through the lens of successful engineering, but also in relation to the natural characteristics of the channel that they replaced.



