Tuesday, April 14, 2015

Can flooding be prevented by rebuilding the soil?

Living close to the River Thames, I am well aware of the reality and presence of flooding. Although this house has not been flooded since 1947, some, closer to the river and on lower ground, have. On periodic occasions, we have been presented with television-footage of the Somerset Levels, utterly overwhelmed by water, and it is apparent that the suffering of those living there and all whose lives are also disrupted by flooded roads and railway tracks, may not be abated beyond temporary "fixes", according to the likely weather patterns that we may anticipate in the coming decades. For hundreds, or perhaps thousands, of years, the regions of the Somerset Levels were marshlands, until humans encroached upon them, in days before the Domesday Book was written http://en.wikipedia.org/wiki/Somerset_Levels. Similarly, here and elsewhere, many houses have been built on the floodplain. The floodplain does the job that ecology might expect it to, which is to take-up the excess water when the river is in flood.

At the end of our road used to be a farm, and every winter that I can recall, the riverside field there used to flood, and I did wonder if it was such a good idea to graze cattle there, since they might pick-up some contagion such as liver-fluke from the sodden ground. Then, perhaps twenty years ago, the farm was sold to developers, who created a rather picturesque complex of houses, each of which sold for a pretty penny, and I imagine that the farmer is still laughing his socks off. In contrast, those who now occupy the complex are less amused, since every winter, from where the cows used to graze, people have to make their way gingerly along the single narrow track out, while the former fields are flooded not only as they were, but more heavily because the water can no longer soak into the soil, being debarred by the impermeable foundation of concrete and tarmac on which the houses are built. The name of that narrow track remains as "The Causeway", which probably sounds rather twee, but by definition does give a heavy hint as to the prevailing geography: i.e. "A causeway is a road or railway route across a broad body of water or wetland raised up on an embankment." http://en.wikipedia.org/wiki/Causeway. Interestingly, there is a house that has stood on ground to the other side of the causeway for around two hundred years, that is protected from flooding, since not only is access provided to the farm/development but its raised structure acts as a fortification against the floodwater from the river. This, they knew to do, in days past when people observed and understood the lay of the land, and worked with its natural topology.

In the midst of such reflections, a friend alerted me to an article by George Monbiot http://www.monbiot.com/2014/01/13/drowning-in-money/ which refers to a study in which the replanting of trees enhanced the rate at which soil could take-up water by 67 times. While this is something of a revelation to read, sitting here in an English village, Britain has in fact been involved with and has funded research across the globe whose results show similar effects. I know that soil erosion/degradation runs hand in hand with poor absorption of water into the soil, and hence flooding. Planting cover crops is one way to curb erosion, otherwise the soil is more strongly eroded in the winter: (1) when it is left bare (on fields harvested earlier in the year), and (2) when the elements are at their most forceful. Through erosion, soil is washed away and contributes to the silting of rivers, so reducing their flow capacity and further exacerbating the problem of flooding.

Planting trees acts to shield soil from the winds, and the mulch from fallen leaves forms a layer to protect the soil on the "forest" floor. The roots, and associated fungi, also help to hold the soil together. The mulch furthermore contributes to building soil organic matter (SOM), when it is taken down into and processed by the soil food web (the ecosystem of microbes and other creatures, such as earthworms and beetles that live in soil), and so the soil structure improves, meaning that it can absorb water and drain properly. In combination, this reduces run-off, at least until the soil becomes saturated. The study suggests that the roots can act as conduits for water into the soil, which makes sense because tree roots are very deep and can access soil regions that roots from grass, say, cannot. There is a rough symmetry between the depth of the roots and what grows above the surface. Hence, a greater "volume" of soil can be made available to absorb water, and so that sixty seven-fold improvement in water absorption rate is perhaps accounted for. Impact by the hooves of animals can certainly compact soil, raising the volume of runoff, and particularly when combined with overgrazing accelerates erosion of the soil. That said, in arid regions of Australia, soil has been brought back to life by the deliberate introduction of grazing animals, whose hooves drive seeds into the soil, while their manure assists the creation of SOM http://www.sciencereviews2000.co.uk/blog/view/science-progress-news/62/feeding-and-healing-the-world/650. 

The study is fascinating, and if funding has been withdrawn from tree planting projects, as Monbiot says, the policy might appear misguided. Perhaps those allocating the cash are unaware of the potential benefits of having trees rather than ground that is otherwise left "open". Certainly more work should be done, as it seems such a cheap and easy option - a "back to nature" approach.To be sure, the situation on the Somerset Levels and elsewhere is complex, but I was struck recently by another news item about mitigating flooding at lower levels by cutting trees on higher ground and placing them to act as partial barriers to slow down the flow of water, which can be described as "engineering nature's way"  http://www.engineeringnaturesway.co.uk/category/blog/flood-alleviation/. Rather than getting "rid" of the water as quickly as possible, as has been done by technology, holding it back using natural defences has proved successful in preventing flooding in property that has previously been inundated http://www.theguardian.com/environment/2014/jan/10/flood-defence-nature-experiments

It seems likely that other human actions are contributing to our flooding problems, which are expected to continue into the coming decades http://www.ceh.ac.uk/news/news_archive/the-recent-storms-and-floods-in-the-uk_2014_06.html  However, there are many ways by which the issues of degraded and flooded land might be addressed, particularly through the observation and partnership of Nature  http://www.sciencereviews2000.co.uk/blog/view/science-progress-news/62/feeding-and-healing-the-world/650. Hence, surely, to take care of the soil, the land and the landscape must be the certain path away from an emerged human condition that is all too apparently, and in all respects, non-maintainable. The importance of earth-works such as swales, ponds and dams should not be forgotten either: "slow it, spread it, sink it!" as they say in permaculture circles