INNOVATION-July-August-2020

F E A T U R E

A lluvial fans and fan deltas are landforms that gradient compared to the surrounding uplands, and comprise mostly perennial streams as a reliable water source, good foundation conditions, and a scenic outlook. Consequently, fans have become much-desired locations for dwellings in mountain valleys in BC. Their appealing aesthetics, however, mask a moody demeanor when torrential downpours occur, creating flows that are sometimes worsened by melting snow. Torrent flows often ignore the confines of their banks and seek a new path (i.e., avulse), which, over time, leads to the growth of the fan. This is nothing new to those who have lived for a long time on an active fan, nor to engineers or geoscientists working with local governments or owners trying to tame the troubled torrents. However, despite numerous well-intentioned efforts to mitigate the effects of floods, debris floods, and debris flows, some common misconceptions persist which have led, and will continue to lead, to losses that are sometimes catastrophic. FAN FORMATION 101 Fans are subject to a variety of potentially dangerous events. In general, one differentiates between so-called “clearwater” floods, debris floods, and debris flows. The stage, discharge, and sediment concentration increases from floods to debris floods and debris flows. Debris floods transport unusually large volumes of sediment because almost all grains on the bed are in motion. A fan may be subject to clearwater floods up to a certain annual (e.g., a 1-in-20-year) probability. With lower probability events (e.g., a 1-in-100-year rainstorm), a debris flood may be unleashed causing substantially more damage due to flooding and bank erosion than the 1-in-20-year clearwater flood. In some cases, debris floods may stem from outbreaks of beaver dams, landslide dams, glacial or moraine dams, or human-made dams. These types of debris floods are particularly dangerous because their discharges can be many times that of clearwater floods. Debris flows, by definition, are a landslide process and carry more sediment than water. Their discharge may be 10 times or even up to 100 times greater than that of a clearwater flood of comparable probability. Due to their high flow velocities (i.e., sprinting speed) and high develop when steep creeks debouch from their watersheds onto floodplains or valley flats, or into standing bodies of water. They feature moderate

density, these floods can carry boulders several metres in diameter, resulting in tremendous impact forces. When floods, debris floods, or debris flows arrive on fans, they tend to deposit their sediment load. The more load these events carry, the more likely they will jump out of their channels—a process known as avulsion. With avulsion, further sediment is deposited and the cycle of fan formation continues. This narrative, while generally true, misses some key nuances. Some fans are a legacy of the early postglacial era when rates of sediment delivery were much higher, or the receiving river, floodplain, ocean or lake was at a higher elevation than today. Such fans are termed paleofans. They feature incised channels and fan surfaces no longer prone to the processes that created them. Fan-deltas, which are fans depositing sediment into standing water, are subject to their own suite of hydrotechnical issues, such as hydraulic and backwater effects that invite sedimentation near the creek’s mouth. Here, the arrival at still water can lead to enhanced sedimentation and creation of an upstream propagating sediment wedge that may eventually lead to avulsions and impacts to fan infrastructure. Any blockages on such fan deltas—caused by, for example, log jams or a failed bridge—will enhance avulsions and impacts. Equally, artificial or natural-flow constrictions will invite upstream buildup of sediment and increase the potential for the stream to jump its banks. THE CYCLE OF SEDIMENT EROSION AND INFILL As water rushes down the steep upper channel, it scours loose material, including rock, soil, and mud. Bank collapse may occur concurrently. As discharge increases during the rising stage of the flood, channel bed scouring and bank erosion typically occur. When the sediment reaches the fan, the flow encounters stream banks comprising material similar to that in the bedload. Since gravity assists the force of the flow, bank erosion is favoured, the channel widens, and flow becomes less deep. Additionally, channel gradient typically declines down-fan. The result of these conditions is that the driving force of the flow declines and sediment is deposited. Bank erosion dominates after the flood peak, when the peak sediment load is overrun by declining flows. Heavy

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