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.

Experimental physical modelling

What does a geographer do when the real world is too big?

By Lucy Clarke (@DrLucyClarke)

Rivers change due to the interactions between water, sediment
& vegetation in different environments. Merced River, Yosemite, USA

I am a fluvial geomorphologist, which means that I am interested in how rivers work. This means that I spend a lot of my time thinking about how water, sediment and vegetation interact with each other in these systems and what impact this has on the river channel and processes occurring through time and especially during extreme flood events. Trying to look at these variables in the real world can often be problematic, as it is difficult to isolate the one variable you are interested in from everything else going on and time can be a real issue - accurately predicting when a flood will occur to measure the affect it will have is a game of chance, and often the things that I am interested in exploring happen over such long timescales that it would take a lifetime to monitor them in the field. Saying that, field work is still an important part of my research, but to help answer some of my research questions I use experimental physical modelling – so basically creating my own miniature landforms in controlled laboratory conditions.

An example of a braided river. The 

Waimakariri River, New Zealand

An example of an alluvial fan. Centre 

Creek fan, New Zealand

Obviously there are a number of factors you have to consider when you use experimental models, such as: how realistic is the model? What do the results mean in the real world? And how do you scale the model? The answers to these vary depending on the research aims of the project in question. To take the last question as an example, scaling is probably the most important consideration. Some things in nature cannot be scaled – such as the influence of gravity or water – but we can control a number of variables used in the model, like the size of the sediment and the quantity and speed of water delivered through a river system. Then you have to decide whether you want to build an exact prototype of a field example (a ‘scaled’ model) which will allow you to take accurate measurements that can be scaled up to the field site, or instead use a ‘similarity of processes’ model where realistic values are chosen for the model but they don’t relate to a specific field site, instead you observe what happens during the experiment and use this to help improve your general understanding of that landform.

Experimental channel used to explore
flooding on braided rivers. Run at the TotalEnvironment Simulator
(@TotEnvSimulator) at the University of Hull    

I have used both types of model and they each have their own advantages and disadvantages, but both have generated interesting and useful results. First of all, I have used scaled models to explore braided river systems – rivers that have an interconnected network of small channels rather than one main channel flowing through them. Using an experimental channel 10m long and 1.5m wide we scaled the sediment size and flow rates to replicate the Tagliamento River in Italy and explored how different size floods change the behaviour of the river. This is work that we are currently analysing the data from so I will hopefully have some results to report in the next year.

A lot of my research has looked at alluvial fans – these are fan-shaped deposits formed by water and sediment that are often formed where small streams lead into larger rivers or when rivers enter lakes and other water bodies. I have used ‘similarity of processes’ models to try to get a better understanding of the processes operating on these through time and my recent experiments have used live vegetation (alfalfa) to explore how vegetation can influence this.

An experimental alluvial fan: the water was dyed red to aid in 

identification of areas of flowing water. Experiments were carried out in a 

3 x 3m plot at the Sediment Research Facility at the University of Exeter

Live vegetation (alfalfa) being grown on an experimental alluvial fan. 

The experimental plots were 2x2m and were carried at the Total 

Environment Simulator (@TotEnvSimulator), University of Hull.

Some of results of my experimental work will follow in future blog posts, so if you are interested keep an eye out for the kind of questions that I am trying to answer using this technique.