Channel (geography) | EPFL Graph Search

In physical geography, a channel is a type of landform consisting of the outline of a path of relatively shallow and narrow body of water or of other fluids (e.g., lava), most commonly the confine of a river, river delta or strait. The word is generally used to refer to a natural formation of a narrow and is cognate to canal, which is more commonly used to denote an artificial formation. Channels are important for the functionality of ports and other bodies of water used for navigability for shipping. Naturally, channels will change their depth and capacity due to erosion and deposition processes. Humans maintain navigable channels by dredging and other engineering processes. The term is also traditionally used to describe the waterless surface features on Venus. Channel initiation refers to the site on a mountain slope where water begins to flow between identifiable banks. This site is referred to as the channel head and it marks an important boundary between hillslope processes and fluvial processes. The channel head is the most upslope part of a channel network and is defined by flowing water between defined identifiable banks. A channel head forms as overland flow and/or subsurface flow accumulate to a point where shear stress can overcome erosion resistance of the ground surface. Channel heads are often associated with colluvium, hollows and landslides. Overland flow is a primary factor in channel initiation where saturation overland flow deepens to increase shear stress and begin channel incision. Overland flows converge in topographical depressions where channel initiation begins. Soil composition, vegetation, precipitation, and topography dictate the amount and rate of overland flow. The composition of a soil determines how quickly saturation occurs and cohesive strength retards the entrainment of material from overland flows. Vegetation slows infiltration rates during precipitation events and plant roots anchor soil on hillslopes. Subsurface flow destabilizes soil and resurfaces on hillslopes where channel heads are often formed.

Influence of consecutive sediment replenishment on channel bed morphology

Anton Schleiss, Mário Jorge Rodrigues Pereira Da Franca, Elena Battisacco, Lena Bösch

Dams interrupt the longitudinal connectivity of a river as they store water and trap sediment. When transport capacity downstream of dams exceeds sediment supply, water becomes hungry and channel incision, bed armouring and reduction of morphological diversity are the consequences. Sediment replenishment is an increasingly common measure to restore the sediment regime of such disturbed river reaches. In the present study the influence on channel bed morphology of different geometrical configuration of replenishments and of consecutive gravel augmentation is assessed by means of systematically laboratory experiments. The typical features of a straight armoured alpine gravel channel are reproduced on the model in terms of slope, cross section and bed grain size. The total amount of replenished sediment is deposited in four identical volumes following two different geometrical configurations (parallel and alternating). The reaction of the channel bed is analysed on image and laser data. Persistency of the first replenishment is high and it covers a large portion of the channel bed, which consequently decreases channel bed roughness remarkably. The persistency of the second replenishment is low and the effect on the depositional pattern established after the first replenishment only minor. This is explained by the smoothing of the channel bed during the first replenishment, resulting in higher transport capacities. For the alternating configuration an increase in the deposition heights in the very downstream channel reach can be observed due to the second replenishment. Thus it is assumed that consecutive replenishment can increase the impact length of restoration projects. The parallel configuration results in very spread depositions, while for the alternating configuration a clear bed form pattern could be observed with a wavelength of depositions corresponding to the length of the replenished volumes. These depositions are understood as mounds, forming the initial condition of alternating bars. Thus in order to enhance channel bed topography of an armoured layer downstream a dam, sediment replenishment added by the alternating configuration is most favourable and effective.

2016

Influence of consecutive sediment replenishment on channel bed morphology

Anton Schleiss, Mário Jorge Rodrigues Pereira Da Franca, Elena Battisacco, Lena Bösch

2016