What is the salt tide, its causes and typical time?

Salt tide (also known as the upper reaches of salt tide and salt water intrusion) is a natural hydrological phenomenon, which is caused by the attraction of the sun and the moon (mainly the moon) to the surface seawater. When the flow of Danshui River is insufficient, seawater flows backwards, and the mixing of salt and fresh water leads to the salinization of the upstream water body, that is, the salt tide is formed. Salt tides generally occur in winter or dry season, that is, from 10 to March of the following year, at the intersection of rivers and seas, such as the Yangtze River Delta and the Pearl River Delta.

The main factors that affect the salty tide are the weather change and the ebb and flow of the tide. Especially in the astronomical tide, the salt tide is more serious. In addition, the process of sea level rise caused by global climate change makes the salt tide increase very slowly, but the long-term accumulation gradually appears.

Salinity (also called salinity) is measured in degrees. Once a liter of river water, it contains a lot of chloride (including sodium chloride) at microgram level. According to China's Water Quality Standard for Drinking Water Sources (CJ3020-93), the chloride content should be less than 250 mg/L (that is, 250 degrees). If the salinity of the river exceeds 250 degrees, it will not meet the water quality standard. For example, the total salt content of iron and steel industry production should not exceed 200 degrees, and the salt content of boiler water in power plant should be below 300 degrees; The salinity of rice seedling is required to be below 600 degrees.

Cause of salt tide

1. Less precipitation is the main reason.

In 2003, the rainfall in the whole basin decreased by more than 20% on average. The amount of water in Xijiang River basin in the upper reaches of Shawan Waterway in Panyu District, Guangzhou has decreased by 60%, and the rainfall has dropped sharply since the winter of 2004, resulting in a sharp drop in the water levels of rivers, lakes and reservoirs in southern Guangdong. At present, the total storage capacity of 30 large reservoirs in Guangdong Province is 1 10484 million cubic meters, a decrease of 336.36 million cubic meters or 23% compared with the same period in 2003. The decrease of rainfall leads to a serious decrease of river flow. At the beginning of 2005, the water level of Gaoyao Station in Xijiang River was -0.06 meters. There is little rain in the upper reaches of the Pearl River, and the source water is reduced, and the downstream is affected by seawater tides, forming a salty tide. The direct cause of the salt tide is the drought in South Guangdong for years.

2. The disorderly dredging of sand in the Pearl River also contributed to the formation of extra-large salt tide.

The average annual sediment transport in the whole Pearl River Estuary is 80 million tons, including suspended sediment, and the coarse sediment at the bottom of the river only accounts for 5% ~ 6% of the total. Most of the silt excavated by people is coarse sand, and the annual exploitation amount exceeds 80 million tons 15, and the river sand accumulated in history is excavated, and the upstream sediment is not enough to supplement the excavated river sand.

At present, there are more than 100 illegal sand mining vessels in the whole Pearl River Delta, resulting in no river sand. No river or sandy reach is from bottom to top along the river; Over-exploitation of river sand caused serious downcutting of river bed, which led to upward movement of salt tide.

3. The sea level rise has intensified the spread of salt tide.

The relationship between sea level rise and salt tide is remarkable. Recently, a study completed by China Academy of Sciences, Guangdong Academy of Sciences and other 13 units and 100 scientific and technical personnel over 8 years shows that by 2030, the sea level in the Pearl River Delta region may rise by 30 cm. A researcher of the research team said that if preventive measures are ignored, the most developed Pearl River Delta region in China will be hit by more serious floods, storm surges, waterlogging and salt tides, and it is in danger of being submerged.

4. The increase of production and domestic water has aggravated the salt tide.

With the rapid economic development, the expansion of industrial production scale, the growth of permanent population and the sharp increase of production and domestic water consumption in the southern coastal areas of China, the river flow decreases, leading to the increasing intrusion of local salt tide.

Harm of salt tide

The chloride concentration in seawater is generally higher than 5000 mg/L. When salt tide occurs, the chloride concentration in river water rises from several mg/L to more than 250 mg. If the salinity in the water is too high, it will do harm to the human body and is not suitable for the elderly and patients with hypertension, heart disease and diabetes. The high salinity in water will also pose a threat to the production of enterprises, and the production equipment is easy to oxidize and the boiler is easy to scale. In the salt tide disaster, chemical raw materials and chemical products manufacturing, metal products, textiles and clothing industries, which consume a lot of water, were hit hard, and some industries had to stop production.

Salt tide will also increase the salinity of groundwater and soil, seriously affect the agricultural production in the Pearl River Delta, a land of plenty, and endanger the survival of local plants. The situation seen from the rural areas of Panyu District, Guangzhou is shocking. On the edge of some rice fields in Shilou Town, Panyu, although there is some water in the ditch, the fields have cracked. Because of the salty tide in this town, the salinity of the water in the ditch has reached 0.5%. If crops "drink" water with salinity exceeding 0.4%, they will stop growing or even die after half a month.

The impact of water shortage on local agriculture is obvious. According to the statistics of the statistics department, the area of early rice in Panyu District of Guangzhou is planned to be 65,000 mu in 2004, which is 2 1 10,000 mu less than that of the same period of last year, and nearly13 of paddy fields cannot be planted. The sugarcane area is 52,000 mu, a year-on-year decrease of 0. 1 10,000 mu; Perennial vegetable area 1 1 10,000 mu, a year-on-year decrease of10.8 million mu.

Salt tide prevention and control

1. Establish an early warning mechanism

Strengthen the research on the formation mechanism of salt tide, use advanced equipment and technology such as ultrasonic velocity profiler to monitor the salt tide synchronously, establish early warning mechanism and coordination mechanism, and plan ahead before the salt tide comes. For example, in mid-September 2004 and early 10, a rare salt tide appeared in the Pearl River Estuary, which attacked Zhuhai and Zhongshan successively. After that, the water conservancy, three prevention and hydrological departments of Guangdong Province intervened in the salt tide forecast and early warning in advance. Only in this way can we deal with the invasion of rare salt tide freely, methodically and methodically.

2. Take water transfer to dilute the salt.

Because the salt tide activity is mainly controlled by tidal activity and upstream water. The adjustment space of tidal activity is limited, while the adjustment of upstream runoff is promising. In 2 1 century, the operation of water control project urgently needs to resist salt tide. At present, transferring water from brackish water to salty water is an effective emergency measure. According to experts, most hydropower stations in the central and western rivers of the Pearl River system have small regulating capacity, limited capacity and are relatively scattered. Therefore, emergency dispatching of water pressure and salinity should rely on Tianniuqiao I and Yantan Reservoir in the main stream of Xijiang River. In view of the fact that Feilaixia of Beijiang River is close to the Pearl River Delta, and the process takes about 1 ~ 2 days, priority should also be given to the adjustment of its water pressure and salinity, so as to give full play to the comprehensive benefits of water resources in the Greater Pearl River Basin through water transfer.

3. Strengthen the management of river sand mining.

In view of the fact that excessive exploitation of river sand in the Pearl River Delta has caused serious river bed undercutting, leading to the upward movement of salt tide, relevant departments should strengthen sand mining management in the whole Pearl River Basin and severely crack down on illegal sand mining by legislative means.

Please?Save?Water

According to the statistics of Guangdong Provincial Water Resources Department, the total annual water consumption in Guangdong Province has been increasing for many years, with an annual growth rate of about 5%, ranking first in the country. The water consumption (waste) in the province is 65.438+06.749 billion cubic meters, and the waste rate accounts for 37.5% of the total water consumption. This indicator also ranks first in the country, among which agriculture is the first wasteful household, accounting for more than 70% of the total consumption. The comprehensive water consumption per capita in the province reached 584 cubic meters, higher than the national average. Other provinces in the Pearl River basin waste water resources like Guangdong Province. The serious waste of water resources leads to the decline of river water level, which intensifies the harm of salt tide. Therefore, people should be encouraged to save water and improve the efficiency of water utilization, so as to reduce the harm of tsunami.