They enable ships to enter shallow harbours.
Explanation
Tides are the periodic rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth. High tides result in a temporary increase in the vertical depth of water along coastlines and in estuaries.Significance for Harbour Operations:
- Navigation and Access: The most critical benefit of high tides is that they raise the water level close to the shores. This enables ships, especially those with deep drafts, to enter harbours that are otherwise too shallow during low tide.
- Tidal Ports: Certain ports, known as tidal ports (e.g., Kandla in India or London in the UK), are situated in areas where water depth fluctuates significantly. Vessels must wait for the high tide to cross sandbars or shallow entrances to dock safely.
Analysis of Other Options:
- They prevent the accumulation of silt at river mouths.: While tidal currents can assist in desilting river mouths (tidal flushing), this is a secondary maintenance benefit. The primary operational necessity for a harbour is the immediate ability of ships to enter, which is dependent on water depth.
- They reduce the impact of storm surges.: Tides do not reduce the impact of storm surges. In fact, if a storm surge coincides with a high tide (a "storm tide"), the potential for coastal inundation and damage is significantly increased.
- They increase the water density for better buoyancy.: Tides are movements of water masses and do not alter the intrinsic density of seawater to an extent that would materially affect ship buoyancy.
Key Takeaway: High tides facilitate navigation by increasing the water depth in shallow harbours and riverine ports, allowing large vessels to enter and exit safely.