What’s The Real Cause Of Flooding?

What's The Real Cause Of Flooding?

It seems like every time it rains for more than a few hours in the Lake District we are seeing roads flooded and standing water in pastures and meadows. So what’s the real cause of flooding?

We’ve always had floods but never have we seen such disruption from relatively ‘normal’ amounts of heavy rain.

For a landscape to be flood and drought resistant, rain must be absorbed slowly into the various layers of the soil structure. Once the water has infiltrated the upper soil layers it should either pass into underground aquifers, streams, be transpired back through living plants, utilised by animals or evaporate naturally over a prolonged period.

The key to an effective water cycle is slowing this process down enough so water doesn’t rush over the soil into streams too quickly and moisture is regulated in the soil keeping our rivers flowing more consistently year-round.

If you take a walk on much of the grassland in the uplands during a ‘business as usual’ torrential downpour, you will see a very different picture!

So, why is it all going wrong? To understand the real cause of flooding we need to follow the journey of a raindrop:

Where a raindrop lands is critical to how the rest of this story unfolds. If the raindrop lands on either a tree leaf, or area of tightly spaced grassland plants with long foliage and thick dense decaying plant matter it will take considerable time to pass from the canopy into the soil surface.

Here is an example of a fairly typical pasture near my home. The grass plants are cropped short, there are large bare soil patches and minimal leaf litter to slow and absorb water flow.

What is the cause of flooding?

While walking in the rain the other day my son stopped me and pointed to a beech tree, the trunk of the tree was literally flowing with water – it was beautiful and a great example of how much a tree or tall plant slows the speed of water from leaf to root tip.

Reaching the soil surface, water soaks quickly into deep soil layers if there’s a high percentage of healthy soil organic matter. If the surface is degraded and either hard, capped or slimy, up to 90% will run off. If under the plants there’s lots of decaying leaf litter (where plants have been allowed adequate recovery periods between grazing spells) water will be slowed by these ‘micro-dams.’ Closely grazed grass plants offer little to stop the water flow rushing toward the nearest low point.

It’s easy to underestimate the effect of the gradual and insidious loss of soil organic matter from our farmland but DEFRA estimates soil degradation costs England and Wales £1.2 billion annually. Most people think this is happening ‘somewhere else’ but I see it in most UK fields as I drive around – it’s endemic.

The global picture is terrifying; for every 1 tonne of food produced, 20 tonnes of soil are degraded. It’s clear we’re doing something very wrong. Good land and habitat management and regenerative grazing such as Holistic Planned Grazing can restore soil organic matter quickly.

“Each 1 percent increase in soil organic matter helps soil hold 20,000 gallons more water per acre.” (1)

The variety and root depth of a plant matter a great deal too. A good range of broad and narrow plants will help catch and slow the flow. The taller the above ground foliage, the deeper the root system; as water hits a plant, the capillary action of water means it will naturally trace the roots into the deeper layers of the soil structure. Grassland with a tightly nibbled sward has reduced root depth so wont divert much water underground.

Even where deep litter or dense canopy occurs (in the case of bracken or purple moor grass) there is often bare soil underneath which will offer little to slow the rush of water. We need diversity of species and dense spaces growing plants.

 

Over the years, I’ve seen a significant increase of rush in the pastures in the UK. Rush tends to start in the low points of fields when the water is no longer being ‘held’ in the soil further up the pasture or hillside so land becomes saturated year-round. I covered this issue HERE. If an area of land is saturated it’s no longer absorbing water, so the ever-increasing areas of wetter ground contribute significantly to the overall water-shedding effect.

On a landscape with a poorly functioning water cycle, the small streams will form on the surface instead of entering rivers underground and will collect any degraded or loose soil in the process accelerating soil organic matter loss. Streams will rise fast after a rain shower with rivers receiving a whole catchments water very quickly. In a flood, if rivers and streams are dark in colour instead of clear then you are losing soil from the catchment; it’s further evidence of a dysfunctional water cycle.

Flooding – What’s the real cause?

We recently visited ‘Wild Ennerdale’ in the Western Lakes and camped overnight in the wooded Valley. It rained hard and we purposely chose a camping site away from the river to avoid being washed into Ennerdale Water!

 

 

We needn’t have worried however, the river hardly rose at all and remained crystal clear – the water was being slowed by trees and tall fully grown plants, held in deep litter, soils rich in organic matter and diverted underground by a deep network of roots.
The other noticeable difference in valleys where nature has been fully accommodated is that rivers can meander and change course. This is a natural dynamic process that reduces the energy of a river and allows it to ‘spill out’ to further to reduce volume and speed – causing little permanent damage to higher land and greatly reducing the impact on the infrastructure in villages and towns. If a river swaps sides in a valley, no land is lost, you simply gain land where the river was and lose it in exchange where it now chooses to flow. The issues with this natural accounting systems only occur when land ownership or cultivated land is affected.

Well-intended protective measures of raising the banks of a river and or dredging it may protect the owner of the land immediately surrounding the remedial action but it simply shifts the issue further downstream – gaining speed all the while.

By the time this dirty torrent of destruction reaches towns and villages and lowland grassland it’s no wonder we are seeing such life changing consequences. By then it’s too late.

Millions of pounds is going into ever more clever flood defence systems when all the time the answer to this costly issue lies in the management of the hills and fields in the distance!

By using regenerative farming methods and a holistic approach to land and habitat management, significant and fast improvements could be made to the water holding capacity of our catchments whilst still managing them to produce healthy food. In a climate that is becoming ever more erratic, it’s critical to be better prepared for the inevitable record-breaking rainfall events; we need to act now.

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Caroline Grindrod

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