INNOVATION-July-August-2020

sedimentation typically commences on the lower fan, and then backfills upstream. Sufficient bank scour on the upper fan or mid- to lower fan channel filling leads to overflow out of the channel and to channel avulsion. ENGINEERING DESIGN ON FANS The most common method of dealing with fan processes in the engineering world in Canada is to straighten, deepen, and armour (SDA) a channel. The idea is to make the channel more hydraulically efficient to funnel water and sediment quickly through the fan into the receiving river, lake or reservoir. However, this method has some flaws whose repercussions are not widely understood. First, the SDA method leads to higher flow depths than those found in the natural channel. Higher flow depths mean higher forces acting on the bed, which results in larger stones being moved. In other words, the deeper and narrower a channel, the more it needs protection to avoid scour and bank erosion. Second, an SDA channel does not prevent sediment from depositing. When shear stresses decline, sediment will deposit, even in an SDA channel, particularly as flows decline. Increasing bed level jeopardizes bridge and culvert freeboard and increases the likelihood of future avulsions. It also buries the bank protection, which, if placed only along the lower channel banks can nullify its usefulness. This necessitates cleanout, complicated by the regulatory constraints of in- stream channel works. Storms with two distinct peaks may limit the An aerial view of the fan portion of Cougar Creek, in Canmore, AB, which shows the dramatically widened channel and damage to nearby residences in the aftermath of the June 18 to 21, 2013 debris flood. P hoto : T own of C anmore .

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