Take a look at the essential concepts, terms, quotes, or phenomena every day and brush up your knowledge. Here’s your knowledge nugget for today.

Knowledge Nugget: DANA

Subject: Environment

(Relevance: Environmental phenomena and associated natural disasters are crucial components of the UPSC CSE syllabus. Questions related to floods and other ecological events are frequently asked in the exam. For example, in the 2020 mains examination, a question was asked regarding the interlinking of rivers as a viable solution to the complex, interrelated issues of droughts, floods, and disrupted navigation.)

The deadliest flash floods in Spain’s modern history have claimed at least 211 lives, with dozens still missing. The disaster has become Europe’s worst flood-related tragedy since 1967, when at least 500 people died in Portugal.

The recent flash floods have been attributed to torrential rains and the main reason for the heavy rainfall is likely an annual weather phenomenon known as a “gota fría,” or cold drop. This phenomenon is also referred to as a “DANA,” which stands for “depresión aislada en niveles altos,” meaning isolated depression at high altitudes.

1. DANA occurs when cold air descends over the warm waters of the Mediterranean Sea. This shift creates atmospheric instability, causing the hotter, moist air above the sea to rise rapidly. In just a few hours, this process leads to the formation of dense, towering cumulonimbus clouds. Eventually, these clouds release heavy rainfall.

2. The weather pattern’s occurrence is related to the polar jet stream — a fast-moving wind current at high levels of the troposphere which circulates from west to east and separates the cold polar air from the warm tropical air.

3. Often, a pocket of cold air gets separated from the polar jet stream and collides with the warmer air over the Mediterranean Sea, which results in DANA.

4. The DANA is a common phenomenon in Spain, usually occurring with the arrival of autumn and spring in the western Mediterranean. However, experts say that DANA has become more frequent and intense in recent years and has become more geographically spread.

5. Scientists say extreme weather events are becoming more common across Europe and beyond due to climate change. Meteorologists believe that warming in the Mediterranean, which increases water evaporation, plays a significant role in making heavy rainfall more intense.

1. Floods can be defined as ‘the submergence of the usually dry area by a large amount of water that comes from sudden excessive rainfall, an overflowing river or lake, melting snow or an exceptionally high tide’.

2. Excessive or continuous rainfall over a period of days, or during particular seasons can lead to stagnation of water and cause flooding. Flash floods refer to such a situation, but occurring in a much shorter span of time. For instance, the US’s meteorological agency, the National Weather Service, says flash floods are caused when rainfall creates flooding in less than 6 hours. It adds that flash floods can also be caused by factors apart from rainfall, like when water goes beyond the levels of a dam.

Since 2000, it is estimated that the proportion of people exposed to floods has increased by 24 percent.

3. Flash flooding commonly happens more where rivers are narrow and steep, so they flow more quickly, as per the Met Office, UK’s national weather service. They can occur in urban areas located near small rivers, since hard surfaces such as roads and concrete do not allow the water to absorb into the ground.

4. According to government data from a project by the Assam State Disaster Management Authority, India is the worst flood-affected country in the world after Bangladesh and accounts for one-fifth of the global death count due to floods.

5. In India, Flash floods have been commonly witnessed in cities like Chennai and Mumbai. Depression and cyclonic storms in the coastal areas of Orissa, West Bengal, Andhra Pradesh, and others also cause flash floods.

6. In India, flash floods are often associated with cloudbursts – sudden, intense rainfall in a short period of time. Himalayan states further face the challenge of overflowing glacial lakes, formed due to the melting of glaciers, and their numbers have been increasing in the last few years.

1. Greening cities and using blue-green-grey infrastructure: As our cities expand to become impervious concrete jungles, there is less place for water to percolate and flow. Conserving and protecting urban forests, wetlands, rivers and lakes are critical to addressing climate change-induced flooding, water scarcity and heat waves and improving livability.

For example, China is trying to transform 30 of its megacities into “sponge cities” that use green roofs to slow down run-off into drains, urban forests to enable percolation, groundwater recharge and wetlands to absorb and reuse two-thirds of their water. For over a century, East Kolkata’s wetlands have been an effective flood defence mechanism that help treat a large share of the city’s sewage, produce half of the city’s fresh vegetables, and provide livelihoods to one lakh people. Practical nature-based blue-green-grey infrastructure such as these hold the key to climate adaptation for many Indian cities.

2. Improving roads: The expansion of our urban areas faster than planned drainage systems means that many roads effectively become stormwater drains. To reduce local flooding, we need to improve the way city roads are built and repaired.

Every time a tar road is repaired, it gains a few inches, as tar is laid on top without milling down the existing road. In time, road level rises above surrounding areas, buildings, and drains which are not surprisingly inundated during a downpour. This is made worse when most flyovers, underpasses and sometimes metro lines built to address traffic, land up disrupting existing drainage, leading ironically to massive traffic bottlenecks post-flooding. This needs to be addressed with effective infrastructure planning and coordination by all concerned agencies, as has been demonstrated in many cities.

3. Improving early warning services: After a devastating series of urban floods in cities like Mumbai and Surat in the early 2000s, India has done well to improve its forecasting, early warning and evacuation systems in many large cities. This has to be extended to most places that are at risk along with strengthening critical cellphone, power and water supply services so that they are resilient and can recover rapidly from extreme events.

(Sources: Spain deploys 5,000 additional troops to tackle flood aftermath, What is DANA?, What are flash floods and why they may increase in the next few years, How to prevent disruptions by flood?)

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