Tidal Bore and Importance of Tides

At a Glance

• Tidal bore Tide wave moving up a river
• Conditions Funnel estuary, large range, spring tide
• Famous bores Qiantang, Amazon, Severn, Hooghly
• Tidal power La Rance (1966), Sihwa Lake
• India Tidal potential in Gujarat and the Sundarbans
• Exam link UPSC Prelims, Geography: Ocean Movements, Temperature and Salinity

Where Does It Fit?

This is the sixth topic of the category on ocean movements, temperature and salinity. The previous topic described how tides are caused and classified, and this one covers the tidal bore and the uses and risks of tides. The next topic describes the temperature distribution of the oceans. Previous topic: Tides and Their Types. Next topic: Temperature Distribution of Oceans.

What Is a Tidal Bore and How Does It Form?

In short: a tidal bore is the incoming tide forced into a narrowing river mouth, so that it rushes upstream as a steep wave.

A tidal bore is a surge of the incoming tide that moves up a river as a wave or a series of waves, with a sharp front, against the river’s current. It is formed when the tidal range is large, usually more than about 6 m, and the estuary or river mouth narrows and shallows like a funnel, which squeezes the advancing tide and raises its height. It is strongest at spring tides, as in Tides and Their Types, and in the dry season when the river flow is low. After it has passed, the water level rises quickly and the river briefly flows upstream. Bores can be gentle undular waves, or steep breaking walls of water up to several metres high.

Why Are Tides Important?

• Navigation and ports: high tide allows large ships to enter shallow harbours and estuaries, and many ports, such as London, Liverpool and Kolkata, depend on the tide. Tide tables are used to schedule movement. The tide also scours the channels and keeps them from silting, and a strong bore can threaten small vessels.
• Fishing and coastal livelihoods: tides bring fish into the estuaries and expose shellfish beds, and fishermen time their work to the tide. Salt pans along the coast use the high tide to fill the evaporation ponds.
• Ecosystems: the intertidal zone, the area between high and low tide marks, is a rich ecosystem of mudflats, mangroves and salt marshes that nurse fish, filter water and protect the coast. The tidal flushing removes the waste from estuaries and mixes fresh and salt water, as in the Sundarbans.
• Tidal power: the rise and fall of the tide can drive turbines. The first big plant was La Rance in France, built in 1966, with about 240 MW, and the largest is Sihwa Lake in South Korea, with about 254 MW. Tidal power is predictable, unlike wind and solar power, but needs a large tidal range, costs a lot to build and affects the estuarine ecology. India has an estimated potential of several thousand megawatts, concentrated in the Gulf of Khambhat, the Gulf of Kutch and the Sundarbans, but no large tidal plant is working, because of the cost, as in Energy and Biotic Resources of the Oceans.
• Science and defence: naval operations, amphibious landings and the laying of cables depend on the tide.

How Do Tides Add to Coastal Risk?

High tide flooding, sometimes called sunny-day flooding, happens when a spring tide or a perigean tide pushes water above the street level in low-lying cities even without a storm, and it is increasing because sea level is rising. In a cyclone, the storm surge on top of a high spring tide is the greatest danger, as in Naming, Warning and Effects of Tropical Cyclones, which is why warnings give the time of landfall in relation to the tide. Mumbai’s monsoon floods become worse when heavy rain coincides with a high tide, which closes the outfalls of the drains. As an analytical point, the tide is the best-predicted natural force, so the damage from tidal flooding is a matter of planning, drainage and land use, and not of surprise.

Why Is This Topic Important?

• Examples Bores and plants asked
• Energy Predictable renewable
• Risk Tide plus surge or rain

What Came Next?

The next topic describes the temperature distribution of the oceans. Next topic: Temperature Distribution of Oceans.

Key People and Terms

• Tidal bore Tide wave moving up a river
• Tidal power Energy from tides
• Intertidal zone Area between high and low tide
• Barrage Dam across an estuary for tidal power
• Pororoca Amazon tidal bore
• High tide flooding Flooding without a storm

Exam Corner

Mains Angle

Questions ask for uses of tides and tidal energy prospects. These points add depth.

• Uses Ports, fish, energy
• Energy Predictable but costly
• Risk Flooding with sea-level rise