Harbours & Ports

Water transportation

Water transport is the cheapest mode for bulk and long-distance freight — most of India's international trade by volume moves through seaports. It includes ocean (maritime) transport, coastal shipping and inland waterways (rivers, canals).

Advantages: low cost per tonne-km, high capacity, energy efficient, natural routes. Limitations: slow, depends on ports and navigable depths, weather and seasonal restrictions, needs transhipment to other modes.

Definitions

Every port has a harbour, but not every harbour is a port.

Classification

Harbours

Ports

• Ocean (sea) ports, river ports, inland ports (dry ports/inland container depots).
• By function: cargo ports (bulk, general, container), passenger ports, transhipment ports, free ports.

Ports in India

India's ports are grouped as major ports (under the Union Government, administered through major port authorities) and non-major (minor) ports (under state maritime boards/governments). There are twelve operational major ports on the west and east coasts; Vadhavan in Maharashtra has been approved as a new major port. The Sagarmala programme promotes port-led development, and the Indian Ports Act and the Major Port Authorities Act, 2021 govern port administration.

Site selection for a harbour

1. Natural protection from winds, waves and currents; minimum breakwater length.
2. Adequate depth of water (minimum dredging) and sufficient area for manoeuvring and future expansion.
3. Good holding ground for anchorage and suitable foundation conditions for structures.
4. Minimum littoral drift and siltation.
5. Favourable tidal range, currents and climate.
6. Hinterland — productive area with trade potential; good rail, road and pipeline connections.
7. Availability of construction materials, fresh water, power, labour.
8. Defence considerations and environmental impacts (coastal ecology, CRZ).

Harbour planning

• Size — based on the number and size of ships, berths needed, anchorage and manoeuvring areas.
• Depth below the lowest low water (chart datum) = maximum draft of the design ship + squat (sinkage when moving) + trim + allowance for wave-induced motion + keel clearance (depends on bottom type) + siltation allowance.
• Entrance — wide enough for safe navigation but narrow enough to limit wave penetration; oriented to avoid beam seas; located away from zones of heavy littoral drift.
• Turning basin — area for ships to turn; diameter commonly about 1.5 to 2 times the length of the largest ship (smaller with tug assistance).
• Approach channels — width based on ship beam, one-way or two-way traffic, currents and cross-winds.

Natural phenomena

Winds

Winds generate waves and currents and influence harbour layout (entrance orientation, berth alignment). Wind speeds are described by the Beaufort scale; wind roses summarise directions and intensities.

Waves

• Wave height ($H$) — vertical distance between crest and trough; wavelength ($L$); period ($T$).
• Wave height increases with wind speed, duration and fetch (the open-water distance over which wind blows).
• Deep-water waves — water depth $d>L/2$; shallow-water waves — $d<L/20$; transitional in between.

• As waves approach the shore they shoal (height increases), refract (bend towards shallower water), diffract (spread behind obstacles such as breakwater ends), reflect (from vertical walls) and eventually break.

Tides

Tides are periodic rise and fall of the sea surface caused mainly by the gravitational attraction of the moon and sun.

• Spring tides — highest high tides and lowest low tides, at new moon and full moon (sun and moon aligned).
• Neap tides — smallest tidal range, at the first and last quarters of the moon.
• Most coasts have semi-diurnal tides (two highs and two lows a day); some have diurnal or mixed tides.
• Tidal range influences quay heights, dock gate design (lock/wet docks where the range is large, e.g. the Gulf of Khambhat) and navigation windows.

Currents and littoral drift

• Currents are caused by tides, winds, river discharge and density differences.
• Littoral drift — movement of sand along the shore by longshore currents generated when waves approach the coast obliquely. Breakwaters and jetties interrupt the drift: sand accretes on the updrift side and the coast erodes on the downdrift side; harbour entrances silt up.
• Control: proper location and orientation of entrances, groynes, sand bypassing (dredging sand from the updrift side to the downdrift side), maintenance dredging.

Breakwaters

Breakwaters are structures built to protect the harbour area from waves, creating calm water for berthing and cargo handling. They also reduce siltation and protect the shore.

Types