國立成功大學 近海水文中心
Coastal Ocean Monitoring Center, National Cheng Kung University (COMC, NCKU)
Coastal Ocean Monitoring Center, National Cheng Kung University (COMC, NCKU)
Tide
1. What are tides?
When children play on the beach, they often use small shovels to build sandcastles, imagining themselves walking through them or defending them from attack. A few hours later or even the next day, the sandcastle may have disappeared. This is often due to the rising and falling of the sea level caused by tides. Similarly, news reports sometimes describe tourists who become stranded because they failed to notice the incoming tide while exploring the shore during low tide. These rising and falling sea levels are known as tides. Tides are the periodic rise and fall of sea level caused by the differential gravitational forces exerted by celestial bodies on different parts of the Earth. The force that generates tides is called the tidal force. The primary contributor is the Moon, followed by the Sun. The tidal force exerted by the Moon on the Earth is more than twice that of the Sun, while the effects of other planets and stars are negligible.
Since tides are driven by the motion of celestial bodies, they exhibit regular and predictable patterns. Among various methods for tide prediction, harmonic analysis is the most widely used. After collecting long-term water level observations at a target location, harmonic analysis is applied to the time series data to extract tidal constituents. These constituents are then combined to reconstruct tide predictions at that location, known as astronomical tides.
2. The impact of tides on human activities
Tides have a significant influence on fisheries. On one hand, the flood and ebb phases of tides drive water movement that may enhance fish availability. On the other hand, tidal levels directly affect the accessibility of harbors for fishing vessels. In western Taiwan, where tidal ranges are relatively large, water levels in some harbors may become too shallow or even exposed during low tide, preventing vessels from entering or leaving until the water depth during high tide becomes sufficient for vessel navigation. Therefore, accurate tide forecasts are crucial for fisheries operations.
Fishing vessels resting on exposed sand at low tide in Taixi Fishing Harbor (Photo by Lin Kuo-Hsien, reporter)
In addition, sea level variations may deviate from astronomical tide predictions due to meteorological conditions such as wind and atmospheric pressure systems (e.g., typhoons or tropical depressions). This deviation is known as a storm surge. During typhoon events, low atmospheric pressure causes sea level rise. If this occurs during high tide, low-lying coastal areas may experience flooding or drainage failure, posing serious risks to life and property.
Tides also play important roles in various daily activities. For example, the Twin Heart Stone Weir in Penghu uses tidal motion for fishing: fish enter the stone enclosure during high tide to feed and become trapped as the tide recedes, allowing fishermen to harvest them. For recreational diving and snorkeling, high tide conditions are generally more favorable due to deeper water levels. In contrast, activities such as ecological exploration at Gaomei Wetlands, clam digging in Qigu, or crab catching on Jibei Island are better suited to low tide conditions. The rise and fall of sea level also represent a conversion between potential and kinetic energy, known as tidal energy. Tidal energy is a large-scale, clean, and renewable energy source. Some countries have constructed tidal power plants in energy-rich bays or estuaries to generate electricity from tidal motion.