Wednesday, 28 August 2013

What drives change on alluvial fans?

By Lucy Clarke (@DrLucyClarke)

So first of all you are probably wondering what an alluvial fan is? An alluvial fan is a landform that is created when a small river feeds into a larger one or when a large river flows into a lake or sea, they form in characteristic fan shapes – hence the name. Changes on an alluvial fan system are driven by the amount of available sediment and water, and two main types of alluvial fan can be distinguished based on whether a fan is formed primarily by sediment movement (‘debris fan’) or by the action of water (‘fluvial fan’). It is the latter, fluvial fans, that I am interested in – these tend to have a shallower slope and lower grain size than their debris counterparts.

Debris fans in Canada – formed by the movement of rock and sediment due to gravity, 
these characteristically have steep slopes and large sediment on them

Fluvial fan in New Zealand – formed primarily by water flow these 
have shallower slopes and lower grain size than debris fans.

So, you may be asking why am I interested in these landforms at all? First of all, alluvial fans have a global distribution and are often prime locations for settlements and road networks. In many temperate and humid environments these fans are dynamic systems that are prone to rapid change and due to their steeper slopes (compared to the surrounding area) they are prone to flooding, so understanding how they respond to changing conditions is important in their management. A recent example of this can be seen in the floods that hit Alberta, Canada in June 2013 – one of the worst affected areas was the town of Canmore located on the Couger Creek alluvial fan, shown in the photos below. Alluvial fans are also important on a longer timescale. Fans trap sediment and therefore preserve a record of environmental change. Changes in the climatic conditions can be reflected in the amount of sediment produced; the amount of rainfall can influence erosion rates, whilst also affecting the density of vegetation growing in an area (denser vegetation traps sediment and the roots stabilise soils lowering the sediment delivery to the fan). So periods of growth and decline on the fan can help us to know what the environment was like at different stages through its formation.

Cougar Creek, Canmore, Canada: (a) Photograph I took of Cougar Creek in June 2007 showing the low flow conditions, (b) flooding in the same area in June 2013 (image courtesy of the Calgary Herald) and (c) the damage caused by the floods – the house shown is the same that is circled in Photo (a) (image courtesy of the Calgary Herald).

To understand the response of an alluvial fan during its evolution we need to look at the sediment and water delivery to the fan system and how these alter the processes that are operating on the fan. The impact of fluctuating climate and tectonics in changing the relative amounts of sediment and water and how these drive change are pretty well understood, but lots of work has shown that reconstructing just these variables doesn’t give a complete picture of what is happening on the fan. As well as these ‘external’ controls, there seems to be something else going on, an internal reaction in the fan system itself that is promoting change. And it is this that I am interested in trying to look at.

It is impossible to try to isolate these variables out in the field, as there are too many complex interactions taking place on a field fan to determine what is driven by climate, tectonics or internal processes. So I used a physical model, or a miniature landform, in which I could create my own scaled alluvial fans and control the conditions that were feeding them (if you are interested in learning more about using physical models in geomorphology see my blog post from 5 July 2013). So I ran lots of experiments where I kept the sediment and water supply constant, so there were no external factors impacting the experimental fans, so any changes that I saw must have been driven by internal processes.

Experimental plot used in these experiments; experiments were carried at the Sediment
Research Facility at the University of Exeter.

The experiments I ran were not scaled to a specific fan in the field, but I was instead interested in learning more about the general trends that occurred using what is known as a similarity of processes model. The experimental fans behaved as we would expect fans in the field to, which was a good indication that we were replicating natural processes. The initial results of these experiments were published in a paper (Clarke et al., 2010) and demonstrated that independent of any change in the external conditions the shape and flow patterns on the fans changed through time. I will highlight two of the main findings. First of all I calculated the fan volume at various points through time, to show the overall size of the fan. These are shown for three example experimental runs below, Run 1 has the lowest sediment and water rates fed onto the fan with Run 3 having the highest. Fans grow rapidly in the initial stages (Stage 1) and then begins to stabilise (Stage 3), this is because the fan fills up all the available space and so starts moving sediment out of the system rather than storing it. The higher the discharge rates (increases from Run 1 to Run 3) the quicker the space is filled and so the sooner the fan stops building, therefore lowering the overall volume.

Changes in fan volume through the experiments driven by internal processes. 

The experimental fans also displayed a change in flow patterns through time. Four stages were observed: (1) at the beginning sheetflow dominated, this is when over 50% of the fan area is covered in water; (2) unstable channelised, with multiple channels covering wide areas of the fan; (3) formation of 1-2 main channels that continually move across the fan surface; and (4) a single channel forms that erodes (cuts into) the fan surface.

Changes in the flow conditions through the experiments driven by internal processes on the fan. From left to right: (1) sheetflow dominated, (2) unstable channelised, (3) formation of 1-2 main channels, and (4) single channel. 

This paper highlighted the importance of internal processes in driving change on alluvial fans. I have recently submitted a paper exploring the quantitative data from the flow patterns from these experiments and I will hopefully talk more about that in a later blog. I am now working to try to understand more about the triggers behind these processes and how to identify these features in the field.

Reference: L Clarke, T Quine and A Nicholas (2010) An experimental investigation of autogenic behaviour during fan evolution. Geomorphology, 115, p 278 – 285.

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