“Seawater backflow” has become a hot search term. Why does the water from rivers flowing into the sea flow “backwards”?

Recently, seawater backflow has occurred in coastal areas such as Tianjin Binhai New Area, Tangshan City, Hebei Province, Yingkou City and Panjin City, Liaoning Province, Nantong City and Lianyungang City, Jiangsu Province, causing flooding of some streets, communities, and vehicles, water entering houses, and suspension of public transportation, causing considerable trouble to the lives of local residents. Some residents said that this high tide was the largest in decades and the magnitude of the tide was beyond their expectations.

The sudden high tide was very surprising, and the term "seawater backflow" quickly became a hot search on the Internet. What kind of disaster is this?

On October 21, 2024, seawater backflow occurred in Panjin City, Liaoning Province (Photo source: Xinhua News Agency)

01The strange phenomenon of seawater backflow

The great river flows eastward, and all rivers flow into the sea. This is an eternal law of nature. However, under certain conditions, rivers may flow backwards and seawater may flow back.

Seawater backflow, as the name suggests, is the phenomenon of seawater invading rivers originally dominated by freshwater, and even infiltrating into inland areas . This phenomenon usually occurs at the mouth of a river . When seawater backflow occurs, the originally clear river water suddenly becomes salty and astringent, and even some coastal areas are submerged by seawater, causing serious impacts on local agricultural irrigation, residents' drinking water, and the ecological environment.

In fact, this phenomenon is not uncommon in coastal areas, and relevant reports have been made all over the world. In November 2019, after days of bad weather and heavy rain, the water level in Venice, Italy, reached a maximum of 1.87 meters. The backflow of seawater caused about 85% of the city to be flooded, and the infrastructure was paralyzed. In February 2022, many European countries were hit by storms, which caused floods and serious traffic obstructions, large-scale power supply interruptions, and seawater backflow in coastal areas of Denmark, forcing rail traffic to suspend. In September 2023, due to the continued drought and hot weather, the water level of the Mississippi River in the United States was close to the lowest level in history, causing the backflow of seawater from the Gulf of Mexico for several months, approaching New Orleans, the largest city in Louisiana, and surrounding areas, threatening the safety of the local water supply system.

In September 2023, seawater backflow occurred in the Mississippi River in the United States (Photo source: Xinhua News Agency)

02When storm surge meets astronomical tide

The occurrence of seawater backflow is often closely related to a variety of natural factors, among which storm surge is one of the main causes of seawater backflow.

The so-called storm surge refers to the strong winds and sudden changes in air pressure caused by the passage of tropical cyclones, extratropical cyclones, marine squall lines (a type of severe convective weather composed of thunderstorms), etc., which are superimposed on the astronomical tide level and cause the sea surface to oscillate or rise (fall) abnormally non-periodically . There are two types of storm surges: typhoon storm surges and extratropical storm surges . In short, when strong winds (such as typhoons, hurricanes, etc.) blow over the sea, huge waves will be formed, pushing the sea water to the shore .

If a storm surge occurs at the same time as an astronomical tide (i.e. the gravitational pull of the moon and the sun on the earth causes the seawater to rise to the highest tidal level), the seawater will form a higher tidal level under the dual effects of the storm surge and the astronomical tide, and may even break through the seawall and invade inland areas. According to the 2023 China Marine Disaster Bulletin released by the Ministry of Natural Resources, storm surges are the most important marine disaster in China. In 2023, there were 14 storm surges along the coast of China, 7 of which caused disasters.

On October 20 (the 18th day of the lunar calendar) and 21 (the 19th day of the lunar calendar) this year, seawater backflow occurred in many parts of China. Meteorological experts believe that this may be related to the superposition of storm surges and astronomical tides . Astronomical tides usually occur one or two days after the new moon and full moon (new moon and full moon) of the lunar calendar, that is, around the second, third , seventeenth and eighteenth day of the lunar calendar. This is because on the new moon and full moon, the centers of the sun, the earth and the moon are almost in a straight line. At this time, the tidal force of the sun and the moon is the greatest, but the seawater will be affected by viscosity and inertia during the flow process, so that the rise and fall of the seawater cannot immediately respond to the changes in the position of the celestial body, but will lag for one or two days.

When the storm surge of Typhoon Dusurui occurred on July 28, 2023, the measured tide level, astronomical tide and storm surge at Dongshan Station in Fujian Province changed over time. The figure clearly shows that after the storm surge and astronomical tide were superimposed, the real-time tide level increased significantly.

(Source: 2023 China Marine Disaster Bulletin)

Qiantang River tide photographed on September 19, 2024 (drone photo). The best time to watch the Qiantang River tide is around the Mid-Autumn Festival every year, which is known as "the most spectacular tide on August 18th in the world." (Photo credit: Xinhua News Agency reporter Jiang Han)

03Tsunami creates huge waves

Tsunamis are another extreme natural disaster that can cause seawater backflow. Tsunamis are different from normal waves generated by wind, and also different from tides caused by the gravity of the moon and the sun. They are usually caused by submarine earthquakes, volcanic eruptions or submarine landslides , which will produce huge waves and spread in the ocean at a very fast speed, and then pour a large amount of seawater onto the land near the shore, causing seawater backflow. Sometimes, tsunamis can also cause huge waves to surge in and out repeatedly, causing huge losses in coastal areas.

On January 14-15, 2022, the underwater volcano on Honahāapai Island in the South Pacific island nation of Tonga erupted violently, triggering a large-scale tsunami, which not only affected 85% of the population of Tonga, but also the northern coast of Chile, more than 10,000 kilometers away, was hit by huge waves more than two meters high, and seawater backflow occurred in some urban areas.

04Land subsidence leads to seawater intrusion

Land subsidence is also an important cause of seawater backflow. In coastal areas, excessive exploitation of groundwater causes the groundwater level to drop sharply, forming a large-scale drop funnel. When the groundwater level is lower than the sea level, seawater backflow occurs, also known as seawater intrusion .

For example, the city of Venice in Italy is often threatened by seawater backflow. The city consists of 118 small islands and is known as the "City of a Hundred Islands". The islands are connected by more than 400 bridges of various sizes. Since the 1920s, factories around Venice have extracted a large amount of groundwater, causing large-scale ground subsidence, and it is still slowly sinking at a rate of 1 to 2 mm per year. As early as about 6,000 to 7,000 years ago, the intrusion of seawater in the late Ice Age flooded the coastal plain of the Adriatic Sea, forming this vast lake. At present, there are still three entrances to the ocean where the city of Venice is located, and many islands are often submerged by backflowing seawater.

05 Climate warming exacerbates seawater intrusion

The rise in sea levels caused by global warming is also exacerbating seawater backflow. According to the "China Sea Level Bulletin 2023" released by the Ministry of Natural Resources, the overall sea level in China's coastal areas is accelerating. From 1993 to 2023, the rate of sea level rise in China's coastal areas is 4.0 mm/year, higher than the global average of 3.4 mm/year in the same period. Affected by multiple factors such as climate change and the cumulative effect of sea level rise, below the ground, it manifests as seawater intrusion, among which the degree of severe seawater intrusion in some monitoring sections along the coast of northern Hebei, Shandong, and southern Jiangsu has increased (the criterion for severe seawater intrusion is chloride ion content > 1000 mg/L); above the ground, it manifests as saltwater intrusion, that is, the river sections that can be reached by the tide and whose flow and water level are affected by the tide experience seawater upstreaming at high tide.

In addition, continued climate drought can also lead to seawater backflow. According to a report by CCTV Finance Channel on March 18, 2024, due to climate warming, rising global sea levels and drought, many parts of Vietnam are facing the problem of seawater backflow, among which the Mekong Delta is the most serious, with the degree of soil salinization increasing and the fishery also suffering serious losses.

06How to prevent seawater backflow

Seawater backflow has many harmful effects on coastal areas. It not only damages the houses, water supply, power supply, transportation, ports, docks and other infrastructure of coastal residents, threatening people's lives and property, but also destroys the local ecological environment, leading to soil salinization, wetland degradation and other problems. In the face of the threat of seawater backflow, we need to take a series of measures to strengthen our defense and response capabilities.

The first is to strengthen the construction of seawalls . Seawalls are the first line of defense against seawater intrusion. Coastal countries such as the Netherlands and Japan have built waterproof dams up to more than ten meters high on the coast to block the invasion of waves, or built flood control ditches and gates to guide the large amount of seawater brought by tsunamis, but these engineering measures are usually very expensive.

The second is to implement ecological restoration projects . By planting salt-tolerant plants such as mangroves and reeds and restoring wetland ecosystems, the ecological defense capacity of coastal areas can be improved. These plants and wetlands can absorb and store large amounts of water and salt, thereby reducing the impact of seawater backflow on inland areas.

In addition, strengthening the dynamic monitoring of groundwater in coastal areas and strictly controlling the amount of groundwater extraction in coastal areas according to the hydrogeological conditions of different regions, or intercepting surface water or building underground reservoirs to increase the amount of underground fresh water recharge and raise the water level of freshwater aquifers can control the intrusion of seawater into terrestrial groundwater.

author:

Liu Hanbin, Senior Engineer, Shanxi Coal Geology Bureau, Doctor of Geology

Ma Zhifei is a popular science writer of geology and geography, and a member of Beijing Association for the Popularization of Science and Technology

Produced by: Science Popularization China

References:

[1] Earth Science Dictionary Editorial Committee. Earth Science Dictionary (Basic Subjects Volume)[M]. Beijing: Geological Publishing House, 2006.01.

[2] Ministry of Natural Resources. 2023 China Marine Disaster Bulletin. April 2024.

[3] Ministry of Natural Resources. China Sea Level Bulletin 2023. April 2024.