Collecting sediment cores in the Limpopo

Our fieldwork for CONNECT4 has begun! Our co-I, Fulvio Franchi, is leading a team of researchers on a trip to collect sediment cores from a number of dams across the Limpopo. The team tested out their equipment in Botswana last week and are now in Zimbabwe, collecting the first cores in the waters of the Zhove dam. In this post, they let us know how they built the raft that that will carry them on this adventure. Good luck Fulvio and team!

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Our first step, before we collect any data, is to build a safe, stable raft on which to set off into the dams of the Limpopo. The raft really needs to be stable, because we will spend a few days on it, floating in the middle of a number of dams, maneuvering a gravity corer that can weight up to 150 kg! We are happy to report that we tested the raft this week in the waters of the Bonkwakathako dam (Lat. -22.484626° – Long. 27.223099°, near Palapye in Botswana) and it worked really well!

What data will we collect?

We will be collecting cores of sediments from a number of dams. This is part of our first work package, which aims to explore how cycles of droughts and flood propagate across the river basin and how they affect water resources distribution. Sediments provide key archives for such events and their impacts in the past.  

Our coring work is novel – no one has collected or analyzed these sediment records in the Limpopo before. These dams have been built in the last 50 to 80 years and not been ‘disturbed’ since then, either by dredging or by sampling. Thus, everything that happens in the sub-catchments is recorded in the sediments that accumulate at the bottom of the dams. So basically, studying those samples is like reading the story of the area for the past 50-80 years. This includes past floods, droughts and any event that might have indirectly affected ecosystems and communities that are relying on those dams for ready available water.

We will collect the samples from the bottom of the dams using a gravity corer. The chemical analyses of these sediments will shed light onto the sedimentation rate into each dam, the presence of layers deposed by floods, the sediments hydraulic properties and, eventually, on the long term records of floods and drought cycle, the effects of these in the distribution of pollutants, and the infiltration capacity towards the underlying aquifers.

In conclusion, it is worth the labour of building a raft to do this for the first time!

Getting ready to collect sediment cores

In order to collect the cores, we need to find a safe and stable way to go out into the dams. To do this, we built a special raft, more like a platform floating on plastic drums. The launch in the Bonkwakathako went smoothly so we are confident we will complete the surveys safely!

Taking it step by step to build our raft

Step 1 – The preparation

Safety was our first priority, so that the raft would be as stable as possible. We started by drawing a 3D model of the raft based on the experience and drawings from our project collaborators from DABANE in Zimbabwe. The BSc students at BIUST did an awesome job with this! This was followed by a careful cutting and assembling process, which took a week, and involved some sore hands and arms!

Working on our raft design, before putting our materials together. Photo credit: Fulvio Franchi, 2019.
The team with the completed raft at the BIUST Engineering worshop. Photo credit: Fulvio Franchi, 2019.

Step 2: Finding some more equipment!

The next step was to find oars and life vests, which isn’t an easy task in a landlocked country covered in deserts and bush! But here at BIUST we never give up… we found our equipment quite unexpectedly, in a hunting shop!

Florian, an Italian MSc student from Università dell’Insubria, who will follow us in this crazy adventure. Here are the oars which we found in a hunting shop! Photo credit: Fulvio Franchi, 2019.

Step 3 – Building up our expectations

If we are lucky, we will be able to retrieve cores such as the one shown below (courtesy of previous work for the NERC-funded PULA project). This shows a flood event (or flood couplet) in the Notwane dam, in southern Botswana. Once you are able to pinpoint a flood event you can run some analyses and see if the flood has brought into the basin pollutants, for instance. And, if you compare the record of different dams in the same basin you will be able to see if extreme weather events such as cyclones and heavy rains are really affecting the whole basin or if there are local buffering factors instead. Measuring the permeability of these sediments also gives information on whether dam contributed to recharge the aquifer, through leakage.  

To cut a long story short, sediments can tell you a lot about the environment. In other parts of our project, we’ll be talking to village elders and residents about what these environmental changes mean for the communities who live around the dams, and how people cope.

The next step for us will be to characterize land use changes because this is a key factor that can enhance erosion and affect sediments yield and contaminates dispersal.

Step 4 – Preparing our team!

Before departure you also have to deal with the expectations of your students. Our Italian student, Florian, has this to say about the challenge of floating on a raft in Limpopo dams:

For sure dealing with the wilderness of Africa will be surprising! I hope that hippoes and crocodiles will be busy with their ordinary habits! I trust the work we have done so far, and I know the raft will carry us safely through the dams.”

Having tested the raft, the team is now in Zimbabwe, first stop Bulawayo and then Zhovhe dam (21°48’44.35″S – 29°42’40.83″E). We will proceed to the unnamed dam on the Bubi river (21°19’43.66″S – 29°56’9.85″E) (the wildest of them all, on a big game hunting farm!)

A view from the office! Photo credit: Fulvio Franchi, 2019.
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