Based on observations and our first test runs, we can get some clues on how a river transports litter. Three basic elements must be considered:
- The river
- The weather
- The litter
1 The river
The Meuse is a river that is completely determined by meteorological events in its catchment area. It starts in the north-east of France, flows through Belgium and the Netherlands.
It’s throughput is quite fluctuating through the seasons as the graph below shows:
This difference in throughputs implies that there is an intensive interaction with the river banks. Every peak leaves a mark on the bank, washed away whenever a higher peak follows. These lines on the picture below have been deposited during the peaks in the winter periods of the last couple of years.
At high throughputs the river is very turbulent. This turbulence pulls a lot of debris under the surface, only a small part of the debris stays afloat. Trees and bushes on the banks catch these suspended litter fragments, resulting in highly decorated trees after a period of high waters.
2 The weather
Rain and wind contribute to the transport of litter by a river. When it’s raining streets are washed clean, litter is transported through sewer systems and smaller rivers to the Meuse. Small litter passes through purification plants, where only big parts are collected.
Some installations catch the “first flush”, but most of the “clean” rainwater from the streets is directly discharged into a stream.
Wind creates small waves, picking up litter deposited on the banks, mixes it in the top water layer of the river, or pushes very buoyant parts onto another part of the riverbank. This intermittent way of transportation is very typical for the way a river transports litter.
3 The litter
With our first catches and a lot of searching along the riverbanks, we get a first glimpse of the kind of litter transported by the river. There are numerous divisions you can make, but one is most certainly related to the “surface to volume”-ratio. The larger this ratio, the more suspended a piece of litter will be, depending on the turbulence of the river.
A simple demonstration with plastic films with different thickness demonstrates shows clearly the impact of this “specific surface” aspect. The thinner the product is, the longer the product stays in suspension.
When the river is calm, like in summer with no wind, almost all floating (specific mass < 1) products will slowly drift to the surface. As soon as the wind starts to blow or the speeds and turbulence of the water increases, ever more litter will go in suspension at ever greater depths.
Litter with a high specific surface like leaves, packaging films, thin walled plastic products will disappear from the surface. It can be entangled in the flooded parts of the trees or just flow downstream towards the sea.
Litter with a low specific surface, like twigs, treestumps, PET and glass bottles, EPS foam and metal cans stay on the surface, even at the highest throughput rates. They get deposited on downwind banks or get concentrated in calm spots next to the current. Together with the films caught in the trees, they create the very visible litter fraction people get annoyed about in early spring.
Another characterisation is between big, small and very small. The next picture shows the results of a mechanical clean-up of the river bank. You can clearly see both the small and the big pieces, all very buoyant and therefore present on the river bank.
In this pile hardly any flexible plastics are found. The larger film pieces (like bags) hang in trees or might have entered the North Sea, together with all the other small pieces of flexible film products.
These are some of the first qualitative observations we have made. Now our job is to get a more quantitative and statistically valid approach towards all these phenomena we are discovering.
Interesting stuff for students!