In Las Vegas, the effect of climate change on flooding is not yet statistically noticeable. The main cause of the worsening flood damage observed over the past few decades is human action. : waterproofing of soils, construction on land bordering waterways, and non-compliance with the precautionary principles (flood-prone areas in principle not suitable for construction, red areas of the flood risk prevention plan). On the other hand, climate change will accentuate weather extremes, with heavier rains over a smaller number of days. This increase in the intensity of precipitation will amplify flooding phenomena and risk increasing the damage observed in vulnerable areas.

What is the role of natural spaces in flood mitigation?

When the rain falls heavily, natural spaces such as marshes, meadows, forests, absorb and store the water that comes from surface runoff. They can also reduce soil erosion, which in extreme cases can be the cause of mudslides or landslides, especially in the mountains.

Along a watercourse, these natural areas also play a fundamental role in the protection of goods and people. Because during a flood event, the river water can overflow, spill out and be stored there temporarily. The natural area behaves like a sponge: it absorbs water, thus reducing the intensity of the flood, the overflows of the river further downstream and then try to overcome through possible flood damage restoration that could be help there.

Apart from any restoration, the river can overflow wherever it sees fit in what is called the major bed. But when these natural areas are made impermeable by roads, houses, cities, water can no longer spread and penetrate the soil, accumulates on the surface, picks up speed and causes disasters in its path whose force does not stop surprising us.

Why let the flood go on?

Floods and floods are natural phenomena that have positive effects:

For agriculture: water and silt can improve soil fertility and grassland production.

For the drinking water supply: the flood makes it possible to recharge the water tables which are used for the production of drinking water.

To reduce the intensity of floods downstream: letting the river overflow often helps to mitigate the impact of floods downstream.

To spare the foundations of bridges and dikes: pebbles, pebbles and sand reduce the intensity of the flood since they serve to dissipate its energy (through transport activity): the force of the flood is broken and the foundations of these arrangements are spared.

For biodiversity: some animal or plant species need floods to survive. Installed on the bed where the flood can overflow, they wait for this influx of water to feed, reproduce … like the pike that comes out of the stream to lay eggs in the flooded meadows. The spring and autumn floods are a signal for the downstream migration of certain migratory fish (salmon, eels) towards the sea and for their ascent towards the watercourse. Overall, the environments associated with regular river overflows are rich in biodiversity.

For fishing and hunting: regularly flooded areas are good breeding grounds for pike, frogs and waterfowl. For landscapes, local identity:

Dikes, dams, channels … illusory protections?

It is legitimate to want to protect populations and property by all means. However, dikes, dams or even the modification of the route of the river are often illusory solutions: they do not provide a complete response to high-intensity floods. They can even be counterproductive. These solutions often have serious consequences for homes located downstream and give a false sense of security.

The dikes are designed to contain floods of a certain intensity; they can therefore be submerged during a higher flood. They can break, especially if they have been poorly maintained, poorly supervised or damaged. Many weirs and dams are used to raise the water level for human activities such as the production of electricity … They have no storage capacity and therefore no effect on flood flows and therefore flooding. downstream. Some dams aim to reduce flood peaks but when full, their effect is zero. Modifying the course of a watercourse (removing meanders to create a straight channel, for example) and its excessive digging can make it possible to lower the water level during small floods, but not during exceptional events. And these changes to the stream worsen flood peaks for people downstream.

Is the flood risk increasing?

Many flood-prone areas are not dangerous for property and people. The risk arises when homes, businesses or crops are established there. The rapid overflowing of rivers and streams in these territories can sometimes cause catastrophic flooding.