Silt – a dirty word?

The dramatic and far reaching UK flooding events of last winter have been well documented in the media and discussed at large across public and private platforms. One factor that has received little attention is silt. George Monbiot highlighted the problem back in 2014 but there has been, what seems to me, a worrying lack of discourse on the topic in popular media since.

There has been mention of silt in passing, typically in connection with drains blocking or in footage of people sweeping the stuff out of their homes and shops. It has probably gone by under represented in the national conversation as its impact has been overshadowed by the sheer volumes of water and scale of debris left behind. The importance of silt may not be felt now, but it will soon.

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Sediment deposited after flooding in Dumfries.

Satellite images of the country during wet weather show tonnes of silt being washed off our land and in to the sea. This is of course a natural part of the nutrient cycle but not on this scale and not without replenishment. Soil is made by the decomposition of plant and animal matter. Soil is also moved across the land by wind and water, so certain geographical areas have top soil topped up through processes such as flooding.

Think back to school text books with illustrations of the fertile plains of the Nile delta. The reason they were so fertile was the constant replenishment of soil as a result of regular flooding. Far from being held back by dykes and levees, the waters of the Nile were allowed to spread seasonally across the plains. As the water spread out the speed of the water decreased and silt carried by the water deposited.

Look at most hedgerows in fields and you will see the level of the field is typically lower than the base of the hedge: an indicator of the amount of soil lost already.

Look at most hedgerows in fields and you will see the level of the field is typically lower than the base of the hedge: an indicator of the amount of soil lost already.

Our current tactic with rivers and flooding is to hold these waters back at all costs, even on farmland which would benefit from regular fluvial inundation. It’s one of the reasons valleys were farmed in the first place, for their rich soil, in part put there by flooding.

We are already suffering a problem of lack of soil. The Soil Association estimate 2.2 million tonnes of topsoil are lost every year, or 30 football pitches worth of the stuff every minute. According to the Soil Association Report around one-third of the world’s arable land has been lost since 1960 as a result of soil degradation. Considering that 95% of our food currently comes from soil, it looks like we are on the brink of a crisis.

It’s a complex problem tied up with land use across a whole river catchment. A catchment that has many land uses; housing, transportation, forestry, farming. It may span counties and even countries, bringing in multitudes of landowners, stakeholders, councils and governments. There are conflicts. But ignoring it is not going to help.

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Rhian

Rhian

With a degree in Natural Sciences (Biology and Geography) from Durham University, I went on to volunteer in conservation before working short term contracts with several of the main UK conservation organisations. As well I have worked in sustainable transport with two of the leading UK charities. With experience in practical conservation, environmental education and public engagement, I have a strong belief that society and the environment must always be considered in conjunction. The need to preserve functioning ecological processes rather than ecological states is also something I am keen to see conservation move towards.
Rhian

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