Riding the crest of the flood wave

Flooding in Kratie, Cambodia

Flooding in Kratie, Cambodia

28 February 2014 by Chris Hackney

Getting caught in one of the biggest floods Cambodia has seen in years wasn't part of Chris Hackney's fieldwork plans - but it turned out to be a great opportunity for scientists to learn how rivers behave in extreme conditions.

After two hours cramped under a tarpaulin, attempting to avoid the worst of the monsoon, I was starting to doubt I would ever dry off.

We had managed four days of fieldwork before we encountered any rain in Cambodia, but now near-zero visibility meant it was no longer safe to navigate on the Mekong River, and all work ceased. So here I was, hunkered down beneath a cover in a feeble attempt to keep myself and the equipment dry.

For about a week and a half after that point we were at the mercy of the annual monsoon and Typhoon Usagi, which made landfall in the northern part of the Mekong catchment, adding even more water to an already swollen river. Little did we know that we were soon to be caught up in one of the Mekong's largest floods.

At its peak, 63,000 cumecs (cubic metres per second) was flowing down the river. By comparison the largest flood recorded on the River Severn in the UK is 753 cumecs.

So, why was I sitting in a boat during the monsoon? The answer is that I was part of a fieldwork campaign doing research for the NERC-funded STELAR-S2S (Sediment Transport & Erosion in Large Alluvial Rivers - Source to Sink) project. This aims to understand the relationship between climate and sediment transfer on the Mekong - one of the Earth's largest rivers.

Every year, the world's major rivers carry around 19 billion tonnes of sediment to their deltas, which are home to around 14 per cent of the global population. Most deltas are under threat not only from rising sea levels, but also from dwindling supplies of the sediment needed to maintain them. If these supplies are cut off, large areas of land could be lost over the coming century. Most of this sediment is transported and deposited during the annual monsoonal floods, and their timing and magnitude are being affected by climate change as well as by human development, in the form of dams to provide hydro-electric power.

Setting the dGPS equipment up on the river bank

Setting the dGPS equipment up on the river bank

Along the Mekong, several such dams are currently being built, or have been planned for construction in the near future. Furthermore, the Mekong's delta is one of only three in the world classified by the UN Intergovernmental Panel on Climate Change as "extremely vulnerable" to future changes in climate. The STELAR-S2S project aims to provide the first quantification of the natural and human influences on sediment supply and deposition under changing climates. It is uniquely placed to consider the impacts dam construction will have on sediment transfer within the Mekong system.

The project is a collaboration between the universities of Southampton, Hull and Exeter in the UK and Illinois in the USA. For three weeks in September our team deployed high-resolution depth and flow surveying equipment along a 300km stretch of the River Mekong in Cambodia. The bathymetric data on the shape of the riverbed revealed 4m-high sand dunes within the main channel. By surveying these features every few hours we could track their movement downstream, and hence measure the amount of riverbed material being transported.

Later fieldwork will repeat these surveys, letting us see how much sediment the river is carrying at different times of the year; this will give us an idea of how important these flood cycles are in transporting sediment.

Measuring all the way to the bank

As well as the riverbed, we also surveyed the submerged parts of the banks in order to characterise their roughness. Our earlier work has already shown that the roughness of a riverbank can make an important difference to how fast it erodes. Bank erosion can deliver large quantities of sediment into the river, helping maintain the flow of sediment to the delta further downstream.

To understand more about how the Mekong's banks erode, we made detailed measurements of the flow of water near them, revealing complex patterns of behaviour that had never been identified before in large rivers. Specifically we are interested in how major events, such as the annual flood, affect the type and intensity of bank erosion and floodplain sedimentation in large rivers. Are the processes experienced around the riverbank different at the peak of the flood compared to periods of low flow? Or is it the changes in water level before and after the flood peak that erode the banks most?

The fieldwork's timing, coinciding with the 2013 flood peak, meant we could also gain unique data on the behaviour of a series of splay complexes - fans of sediment that are deposited by channels running off the main-stem through breaches in levees. These channels transfer large amounts of sediment and water to the floodplain, and are only active during high flows.

By monitoring how the water flows over these features and sampling the type and amount of sediment moving onto the floodplain, we hope to shed light on how these features connect the floodplain to the main channel.

We are now planning a further field season later in the year to retrieve sediment cores from these systems, once the flood waters have receded. This will let us analyse sedimentation rates and record the internal structure of the splay complexes. This in turn will mean we can see how these features have developed over time and link known flood events to individual layers of sediment. Eventually we want to understand if, and how, large flooding events deposit sediment on the floodplain.

All this data will ultimately enable us to say how large rivers like the Mekong respond to annual monsoonal floods. By quantifying the amount of sediment being generated through bank erosion and passed through to the delta, as well as the amount deposited on the floodplain, we will be in a position to understand how climate change and dam construction will affect the Mekong delta.

This could influence future dam designs so that they let enough sediment pass through to maintain delta construction - not only in the Mekong, but in all large rivers. We also hope our findings will allow local governments and intergovernmental institutions such as the Mekong River Commission to give better protection from riverbank failure to local communities and infrastructure.

By the end of the trip it began to sink in just how lucky we had been - this turned out to be a career highlight for everyone involved. Not only have we learnt so much about how large river systems behave and respond to extreme events; we also have first-hand experience of what extreme events look like, and how they affect the local environment and people. The team will head back out in 2014 to repeat the surveys - hopefully in drier conditions than before.

Chris Hackney is a research fellow at the University of Southampton.