Flooding is becoming more frequent across the world, and more severe when it happens. In 2014 alone, Nicaragua, Bolivia and Romania experienced rainfall and subsequent flooding on the worst scale in nearly a century. But in many other countries flooding is a more regular and often more devastating occurrence in recent decades. There can be several reasons why this occurs. The increased urbanization of many nations, with higher proportions of the population living in cities than in the countryside, means a greater degree of impenetrable land surface. Concrete buildings, tarmac roads and sidewalks cause rainfall to run off rather than sink into the land, as it would in rural and wilderness areas. If municipal drainage systems are not able to cope with excessive rainfall, the water floods the city (in 2011 Brisbane in Australia suffered severe flooding throughout the city center because of this).
In rural areas as well, though, flooding is a more common phenomena than it was centuries ago. Some of the reasons for this include the increase prevalence of monoculture agricultural practices that strip the land of biodiversity and make it more prone to flooding. This is exacerbated by the fact that such practices typically make erosion of the topsoil – the part of the soil profile best suited to the absorption of moisture – more likely. The increased density of livestock production is another factor. The tamping down of the earth by animals’ hooves means water is unable to percolate down into the soil and so runs off the land after heavy rainfall. There is also the impact of global warming. Rising temperatures mean more moisture evaporates from water bodies and so more rain – following the cycle of water in the atmosphere – falls on the land. Combined with the other factors listed above, this can have damaging effects on the land and people.
Of course, in permaculture, one of the principle practices when designing is to slow the run off of water from the land. So permaculture gardeners seek to increase biodiversity, institute ground covers and even build swales to allow the water to percolate into the soil. But even with a plot designed to use water as efficiently as possible, flooding can still occur. Here are some techniques to revivify your space should it suffer from a flood.
The first step with flooded soils is to let them drain naturally. How long this will take will depend on the amount of moisture that has affected the permaculture plot and the makeup of the soil, with those containing a greater proportion of clay taking longer to drain than loamy or sandy soils. While it can be tempting to try to improve the soil before the water is all drained, this tends to cause problems of compaction of the soil, as it is still heavy with moisture, and so starting your repairs too early can limit their effectiveness. In many cases, floodwater will drain within a few days, and most plants can recover if standing in water for just that long. If the floodwater remains a week after the flood, consider digging channels to aid drainage, as after more than a few days the lack of soil aeration and inhibition of photosynthesis the water causes will mean plants start to die.
The primary damage that flooding – and the resultant waterlogging – does to soil is removes the nutrients from the soil. This is done by the floodwater eroding the humus-rich topsoil, and by the water that seeps into the soil washing the nutrients through the soil profile and out of reach of plant roots. Once the floodwater has drained away, reinvigorate the soil by adding lots of organic manure. This will help resupply the soil with much-needed nutrients, and will become the basis of a new layer of topsoil, which may well have been eroded by the flood.
Because floodwaters can leach such a high proportion of the soil nutrients through the soil profile, augmenting the compost with some manure will really help to kickstart the soil nutrient levels back to life. Some well-rotted chicken manure will help restore nitrogen levels – essential to plant growth – while adding some blood and bone will help microorganism activity increase.
Flooded soils will typically become acidic, due to soil fermentation. Adding organic agricultural lime can
help bring the pH level of the soil back towards neutral, or to the level that best suits your plants and normal climate conditions. Test the soil for pH and add lime as required to stabilize the acidity and alkalinity levels.
Having added compost, manure and lime as required by the condition of the soil, add a layer of organic mulch. Not only will the mulch further enhance the nutrient levels of the soil, it will also help to protect the soil additives you have used, and create a more stable environment for your plants to recover in.
Check the plants on your site for stress and damage caused by the floodwater. Foliage that has been covered with debris should be gently hosed or sponged to allow the leaves to access sunlight and resume photosynthesizing. Leaf curling and yellowing in plants is a common symptom of flooded soils. Applying an organic seaweed fertilizer to roots and foliage can help recovery. Food crops that have come into contact with floodwater may be contaminated as floods can cause sewage discharge. Many local authorities advise discarding leafy green vegetables that have been in contact with floodwater, and avoiding eating other crops for a month after the water has subsided.
Unfortunately, floods can cause you to lose some of the plants on your permaculture plot. Severe floods can uproot and carry away plant specimens, while the secondary effects of submersion, lack of aeration and nutrient deficiency can mean other specimens die. When considering which species to replant your site with, consider the likelihood of further floods in the future. In flood prone areas, you may wish to use species that are better able to cope with extremes in climate conditions, such as bamboo, ginger and bananas. Of course, in a biodiverse permaculture plot, you are also likely to want to grow annual vegetables and fruits, so consider deigning defensively, with swales and channels diverting runoff to creeks and rivers. If that is not possible, you may simply need to accept the effects of nature reacting to the environment we have created.