Docks and Harbour - Civil Engineering study

 Docks and Harbour

•      Harbour is a sheltered area of the sea in which vessels can be launched, built or taken for repair or could seek refuge in time of storm and provide facilities for loading and unloading of cargo and passengers.

•     Port is a harbour where terminal facilities like stores, landing of passengers and cargo are added.

TYPES OF HARBOURS

• 1. Natural harbour It is an inlet protected from storms and waves by natural configuration of land.Mumbai, Kandla and Karwar are examples of natural harbours.

• 2. Semi-natural harbour The harbour in which natural configuration of land prefects the harbour on sides only and needs man-made protection at the entrance. Vishakhapatnam is a semi-natural harbour.

• 3. Artificial harbour It is harbours protected by man-made structures like breakwaters or bydredging. Chennai is a man-made harbour.

 

Classification Based on Utility

• 1. Harbour of refuge: Used to anchor ships in storms.

• 2. Fisheries harbours.

• 3. Commercial harbours.

• 4. Military harbours: Seabird at Karwar is a military harbour.

Classification Based on Location

• 1. Canal harbour.

• 2. Lake harbour.

• 3. River harbour.

• 4. Sea harbour.

 

Natural Phenomenon Influencing Location of Harbour on Seashore

1. Littoral Drift 

    The process of carrying the drifting sand and depositing it on seashore is known as littoral drift. If a harbour is constructed in the path of the littoral drift, there will be accumulation of sand on one side and erosion on the other side of the harbour. Groynes are used to protect the shore by trapping littoral drift.

2. Tides 

    The artificial rise and lower of mean sea level periodically is known as tide. It is observed that the general water level rises and falls, approximately in a period of twelve hours and twenty-five minutes. It is also observed that much higher water levels and much lower water levels occur on new and full moons. These tides are called spring tides. Water levels which occur about seven days from new and full moons are moderate and are called neap tides. The total height of spring tides is about 1.5 to 2.0 times that of neap tides.

3. Waves and winds 

    Wind generates waves on water bodies. In maritime works, speed of wind is expressed in knots. One nautical mile is the length of one minute of arc of a meridian on earth surface. Since radius of earth of equator is about 6378 km, 1 nautical mile = 6378 >× pi/180 >×1/60  km,
    i.e., 1 nautical mile is 1.852 km. Hence, one knot is 1.852 km/hour.
In 1805, Admiral Beafort of British Navy classified wind into 12 categories starting from calm (Beafort No. 0) to Hurricane (Beafort No. 12) depending upon velocity is km/hour. A wind is said to be calm if velocity is about 1.6 km and is said to be hurricane if it is above 120 km/hour.Height and length of wave: Thomas Stevenson gave a formula for estimating wave height in meters as h = .34(F)^1/2 , where F is the fetch in km. However, waves cannot attain full height in shallow water. No wave can have a height greater than the depth of water.
    Berlin’s formula for finding the wavelength is L = 1/2pi × g t^2= 1.56 t^2

 

FEATURES OF A HARBOUR

 

1. Harbour entrance 

    Depth and width required at the entrance are more than those required in the channel since it is more exposed to waves. Entrance should be wide enough for navigational requirement. However, it should not be too wide to increase the wave height within the harbour.

 

2. Approach channel 

    The dredged fairway through which ships proceed from the open sea to the harbour basin is known as approach channel. The depth should be sufficient for navigation of design vessels at all the time.

3. Turning basin 

    It is the area required for mano-euvring the ship when it enters or leaves the berth, so that it moves head on.

4. Sheltered basin 

    It is the area protected by shore and breakwaters.

5. Breakwaters 

    The protective barrier constructed to enclose harbours and to keep the waters undisturbed by the rough sea are called breakwaters. Types of breakwaters used are

• 1. Heap or mound breakwater
• 2. Mound with superstructure
• 3. Upright wall breakwater.

    Breakwaters should be designed to resist hydro-static forces, action of wind and waves, solvent action of seawater and sea insects. The breakwater height is 1.2 to 1.25 times the height of the waves expected, above the mean sea level.

6. Wharves and quays 

    The platforms provided along the shore or breakwater for ships to come close enough for loading or unloading are called wharves. Wharves along and parallel to the shore are generally called quays. Wharves and quays have back-fill of earth or other materials and have wide platform at top.

7. Jetties 

    A jetty is a narrow strip structure projecting from the shore into water with berths on one or both sides.

8. Lock and locked basin 

     Locked basin is an enclosed basin wherein a number of vessels can be berthed and has an entrance controlled by lock gate.

9. Fender 

    The cushion provided on the face of a jetty for absorbing shocks by ships is known as fender. They may be rubber strips, timber grills, concrete or rubber fenders.

10. Slip 

    The space of water between two adjacent piers where ships are berthed is known as slip. For safe navigation and provide enough space for barges to load and unload, the slip should be three to four times the beam of largest ship to be accommodated.

11. Docks 

    Docks are enclosed areas for berthing ships, to keep them afloat at a uniform level. There are two types of docks:

• 1. Wet docks: Docks used for berthing of vessels to facilitate loading and unloading of passengers and cargo.
  

• 2. Dry docks: The docks used for repairs of vessels are known as dry docks.
  

12. Aprons 

    The open space left immediately in front of the berth is known as apron. It is used for loading, unloading and for installing railway track, road or conveyors. The width of apron usually varies from 10 m to 25 m, depending upon the traffic.

13. A dock requires transit sheds, warehouses, cold storage and guardhouse, etc.

Maintenance Dredging

    Dredging is defined as excavation under water. To maintain navigation depth dredging is required.

Types of dredges:

• 1. Dipper dredger
• 2. Grapple dredgers
• 3. Bucket elevator or ladder dredgers
• 4. Suction dredgers

Navigational Aids

    For safe, efficient and comfortable travel of ships into harbour navigation aids are required. They are in the form of illuminated signals or lights. They may be classified as the following.

• 1. General lights They guide the ships along the coast and through rivers serving as approaches to more than one harbour. They may be light houses, or light ships.
  

• 2. Local lights They guide the ships to harbours through approach channels to the berths. They are light houses, fixed structures, channel markers, floating buoys, lights on shore, wharves, breakwaters, etc.