In Huangbu Town of Huizhou, Guangdong Province, residents witnessed such an intrusion on November 18. Some coastal villages in the town experienced ankle-deep flooding. A local snack shop owner, surnamed Tang, told press that she saw fish swimming in the streets that day after seawater had flooded low-lying areas. Most residents there had to move their belongings and vehicles to higher ground. The water receded within two hours.

Similar incidents occurred across multiple coastal provinces in October and November. On October 21, Hebei Province in north China and Liaoning Province in northeast China both experienced seawater intrusions. In November, seawater flooded some parts of Huizhou, as well as Shenzhen and Shanwei in Guangdong Province, Haikou in Hainan Province and Beihai in Guangxi Zhuang Autonomous Region.

Seawater intrusion, as the name suggests, is the phenomenon of seawater invading coastal and inland regions, including freshwater waterways. The consequences of seawater intrusion extend beyond temporary inconvenience. Homes, water supplies, power grids, transportation, ports, and wharves are all vulnerable. It jeopardizes lives and livelihoods and causes severe environmental impacts including soil salinization and wetland degradation.

The severity of the consequences was illustrated in Panjin City of Liaoning, where sudden seawater intrusion on October 21 submerged homes in Erjiegou Street, trapping residents. Firefighters conducted a seven-hour rescue operation, evacuating 125 people and rescuing 72 others after searching 231 affected houses. One local restaurant owner described waking to the sound of crashing waves and finding his restaurant flooded, which resulted in two submerged vehicles and the loss of appliances and thousands of yuan (hundreds of U.S. dollars) worth of food. He told newspaper Heilongjiang Daily that he had never witnessed such a high tide in his lifetime.

While not unprecedented, the increased frequency and altered timing of these events call for enhanced coastal protection measures and improved preparedness to address the escalating threat.

The causes 

The recent surge in seawater intrusion events is a complex issue with multiple causes, according to experts. Fu Cifu, head of the forecast team at the National Marine Environmental Forecasting Center's Storm Surge Forecast Division, points to storm as a primary driver. He told Xinhua News Agency that these abnormal rises in sea level, caused by storm winds pushing water onshore, were amplified as they coincided with astronomical high tides—periods of exceptionally high tides resulting from the high gravitational pull of both the sun and the moon.

Liu Qiuxing, Deputy Director of the same division, told China Central Television that while past typhoon-induced storm surges typically occurred between July and September, the recent November events are unusual. He cited the 2014 Typhoon Kalmaegi and the 2018 Typhoon Mangkhut as examples of previous storm surges causing similar damage, but noted that both occurred in the typical summer months.

Fu attributed the seawater intrusions in southeastern coastal areas in November to the increased frequency of autumn typhoons. He explained that these typhoons, in combination with astronomical high tides, had caused the significant inland flooding, while astronomical high tides alone typically do not cause seawater intrusion.

Experts also point to several underlying factors exacerbating the problem. Rising sea levels, a direct consequence of global warming, are increasing the vulnerability of coastal areas. The 2023 China Sea Level Bulletin, released by the Ministry of Natural Resources this April, reported a sea level rise along China's coast of 4 mm per year between 1993 and 2023, exceeding the global average of 3.4 mm per year during the same period. This upward trend increases the likelihood and frequency of future seawater intrusion, Wang Hui, a researcher at the Sea Level Department of the National Marine Data Information Center, told Southern Weekly newspaper.

Excessive groundwater extraction also plays a critical role. Excessive groundwater extraction in coastal regions lowers the water table, making these areas more susceptible to seawater intrusion once the groundwater level falls below sea level, according to an article written by Liu Hanbin, a geological engineer with the Coal Geological Bureau of Shanxi Province, and Ma Zhifei, a popular science writer specializing in geology and geography, which was published on Xinhuanet.com in late October.

Solutions 

Addressing the escalating threat of seawater intrusion requires a multifaceted approach, according to Liu Hanbin and Ma. Increasing the use and size of seawalls is paramount. These structures serve as the primary defense against seawater intrusion. Countries like the Netherlands and Japan have demonstrated the effectiveness of robust seawalls, often exceeding 10 meters in height, in protecting against wave action.

Implementing ecological restoration projects is also crucial. Planting salt-tolerant vegetation, such as mangroves and reeds, and restoring degraded wetland ecosystems can enhance coastal resilience. These natural buffers can absorb and retain significant amounts of water and salt, mitigating the impact of seawater intrusion when it occurs.

Rigorous monitoring and management of groundwater in coastal areas are essential. Strict control of groundwater extraction rates, tailored to local hydrogeological conditions, is necessary to prevent further land subsidence and increase the resilience of the coastal aquifer.

Liu Qiuxing advocates for a notable increase in investment in the research and development of advanced forecasting technologies and systems. Improved forecast capabilities will enable earlier warnings of potential seawater intrusion, enhancing disaster prevention and mitigation efforts, and reducing public anxiety.

(Print Edition Title: Rising Tides, Rising Concerns) 

Copyedited by G.P. Wilson 

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