Alert: flash floods

The river systems remind, in a certain way, of the circulatory system. Blood carries the elements that the body needs, and rivers and streams transport the water and nutrients that make life possible.

But rivers, streams, and creeks also know how to display their destructive potential through flash floods. These phenomena, also known as flash floods or flash floods, ravage territories and claim human lives.

What is a flash flood?

A flash flood occurs when a river or stream overflows its usual channel and inundates the surrounding areas. Their uniqueness lies in the speed with which events unfold (<6 hours in many cases).

Their origin is often the result of intense rainfall events or rapid snowmelt. However, other factors also influence their occurrence and severity, as Tucci (2007) explains in reference to floods in general:

  • Topography of the terrain, i.e., whether the area is mountainous, undulating, or flat, which affects the speed at which water can travel.
  • Presence of vegetation, which reduces runoff or water that freely flows down a slope. It also aids in water infiltration into the soil.
  • Soil drainage capacity, which defines the water absorption capacity. The previous saturation level or the impermeability generated by infrastructure occupying flood-prone areas are crucial aspects.

How to reduce the risk of a flash flood?

When talking about floods, it is important to understand that there is no zero risk. Danger can lurk anywhere. Thus, a dry riverbed can suddenly turn into a raging torrent, or a stormwater collection system can become overwhelmed and fail to drain the water. Building structures in flood-prone areas or infrastructure that obstructs the free flow of water, as well as the aforementioned land impermeability, also do not minimize the danger.

An example of how quickly a situation can change were the floods caused by the Cidacos River in Tafalla (Navarra) and nearby areas in July 2019. In less than 4 hours, the water level rose from 0.11 meters to nearly 4 meters, with rainfall exceeding 150 l/m2 upstream (1).

Another similar example is the floods that affected the city of Washington, also in July 2019, where it became evident that the drainage systems did not function as needed and caused numerous problems.

What can be done in this situation? As of today, there is no magic solution beyond the protection offered by flood defenses or measures such as building at higher elevations. However, these options, common in areas accustomed to periodic flooding, are costly and not feasible in all circumstances.

An alternative could be the establishment of an early flood warning system. It may not prevent water overflow, but it can provide time to deploy emergency measures. Additionally, it can be used in combination with other systems mentioned in the previous paragraph.

What is an early flood warning system?

The United Nations Office for Disaster Risk Reduction (UNISDR) defines an early warning system as “the set of capacities needed to generate and disseminate timely and meaningful warning information, enabling individuals, communities, and organizations threatened by a hazard (in this case, a flood) to prepare and to act appropriately and in sufficient time to reduce the possibility of harm or loss” (2).

In general, early warning systems focus on the community with the aim of reducing losses and severe damage to the population and properties. It is essential for these systems to encompass four key points:

  • Risk assessment and knowledge
  • Monitoring and risk alert
  • Risk communication
  • Response development

How does an early flood warning system work?

The system, installed, configured, deployed, and maintained by Arantec through its Smarty River solution, involves the installation of a series of sensors along a watercourse. These devices monitor changes in water level and transmit real-time data to a central control. Based on the received information and configured thresholds, the system determines the level of emergency and communicates the risk, initiating the response procedure.

The sensors are typically powered by solar panels, making them autonomous and facilitating installation in remote locations without access to electricity.

Vigicrues, a practical example of an early warning system for flash floods

The Vigicrues system has been operating in France since 2003. It remotely monitors approximately 22,000 km of rivers and streams through a network of flow measurement stations. It is a publicly accessible service available through the internet, allowing users to receive alerts via RSS (Javelle, Saint-Martin, Garandeau & Janet, 2019).

Source: Vigicrues ( Fecha de acceso 05/08/2019

Since 2017, Vigicrues has also incorporated a specific system for flash floods, called Vigicrues Flash, which covers approximately 30,000 km of river networks. It is a service designed to provide specific warnings to subscribed local authorities. Alerts are delivered through SMS, voice messages, or email, with alerts only issued when the risk is high or very high, thus avoiding the generation of unfounded alerts that can be counterproductive (3).

The intense rainfall experienced in the country in June 2018 allowed the system to be tested, and the results were more than acceptable. However, Javelle, Saint-Martin, Garandeau & Janet (2019) acknowledge the need for improvements in areas such as risk assessment and geographical coverage.

A compelling reason not to ignore technology

This factor is none other than climate change. Numerous studies suggest that this phenomenon is accompanied by an increase in extreme events such as droughts, heatwaves, and heavy precipitation (Barros, Field, Dokken, Mastrandrea & Mach, n.d.), with flash floods being one of the threats expected to increase in frequency (EEA, 2017).

The reason lies in the atmospheric functioning mechanisms themselves: climate change causes the air to warm up, and a warmer atmosphere has a greater capacity to hold moisture. In fact, for each degree Celsius of warming, the percentage of water vapor increases on average by 7% (4). The problem is that the release of water is uncontrolled and quite unpredictable, similar to pouring out a bucket of liquid, generating some of the issues we have already mentioned throughout the article.

In summary, climate change is already disrupting everything we took for granted or assumed as normal. In many parts of the world, rainfall is no longer what it used to be, and it does not occur in the same way. These consequences are further exacerbated by urban planning that can be described as irrational in some cases, coupled with poorly designed or sized infrastructures. Early warning systems are not a magic solution, but what if half an hour could mean the difference between keeping your family safe or mourning their loss?

Sources consulted:

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