Friday, 5 July 2013

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.

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