River Relocation Channel Dimensions

Channel conveyance, alongside the sizing of hard engineering materials and culverts, is ultimately determined by the discharge of the river. Most modern relocation channels continue to be trapezoidal in design, developed from size and stability criteria derived from European or North American rivers. These designs focus on the relocation channel being robust and capable of conveying a certain flow efficiently.

Relocated channels are often designed to convey the 100-year average recurrence interval (ARI) flood without overtopping [36]. For river relocation channels designed for mining, a more conservative estimate of rainfall and discharge is typically used to avoid water entering the mining pit. River relocation channels constructed in and around mine sites are designed to withstand a flood with a 100-year average return interval, or even an event once every 1000 or 10,000 years [91]. Conservative design flood standards can lead to artificial channels that are constructed with enlarged flood protection bunds, and channel dimensions that exceed the size of the original channel. Engineering failure within river relocation channels often occurs when the artificial channel is poorly sized, or with materials that do not withstand large floods.

All river relocation channels present an artificial discontinuity between natural sections of a river. This artificial channel seldom has the identical physical characteristics of the adjoining upstream and downstream reaches [64]. River relocation channels tend to be straighter and shorter than the original channel, with a higher bed slope and different channel dimensions (width and depth). River relocation is often expensive, particularly when cutting through bedrock or reinforcing the channel with artificial structures. Because of this, engineers will often attempt to minimise the length and cross-sectional size of the relocated channel, resulting in a new channel that is often substantially shorter and smaller than the original.

Even if the channel dimensions and boundary materials are the same (which might be the case with an alluvial river relocation), the channel will usually be straighter, steeper, or feature a reduced floodplain width [57], prompting heightened erosion within the channel. These issues can be further intensified through the feedback loops of secondary and tertiary problems [76,79]; in other words, a change in channel dimensions can cause increased erosion and unstable banks. These unstable banks can fail, prompting vegetation loss, lower channel roughness, and further channel erosion.

Increased erosion within the channel can lead to amplified incision of adjoining tributaries alongside erosive tributary junctions (where the artificial channel re-joins the natural channel). This can cause sustained secondary issues, such as knickpoint migration from hanging tributaries [65], and increased sediment supply to the main channel. These changes produce tertiary issues, such as disruption of fish passage [36]; loss of habitat [74], species diversity, or assemblages [90]; and reduced water quality [85]. Secondary and tertiary issues can impact adjoining reaches, propagating the impacts of channel relocation both upstream and downstream. In the past, diversion channels were expected to remain as simple engineered channels that carried major floods. Vegetation would typically be removed from the channels to maintain conveyance. More recently, channels have been designed to gradually develop more natural morphology and vegetation, and to have more natural rates of erosion. We now turn to this issue of designing more natural channels.
