Transportation Engineering Vol. II

Transportation Engineering Vol. II
Volume II C Venkatramaiah Transportation Engineering Vol. II

Dock and Harbour Engineering
Part III Dock and Harbour Engineering Dock and Harbour Engineering

Chapter 16: Introduction
Water transportation or transportation on water means transportation of passenger and goods using ships, steamers and boats, moving on water bodies. This has great significance in view of the availability of vast stretches natural waters in the form of streams, rivers, seas and oceans. Waterway: Any navigable water of sufficient depth, free of obstructions. These may be rivers, lakes, canals, streams, seas and oceans. Canals and channels are man-made, while the others are natural. Classification of waterways (a) Inland waterways-rivers, lakes, canals and channels within the mainland. (b) Ocean waterways-seas and oceans. Dock and Harbour Engineering

Primary advantages and disadvantages of water transportation Advantages: Heavy and bulk cargo can be carried. Cheapest mode using natural water bodies. Disadvantages: Relatively slow Feeder modes of transport such as roads and railways are essential. Accident-prone during storms and cyclones. Certain relevant definitions Harbour: A place of refuge for ships, well protected from the action of wind and waves in seas or oceans, in close proximity to the shore. Provision for supplies, transfer of cargo, refuelling of vessels and repairs is available. Dock and Harbour Engineering

Port: A harbour with terminal facilities for loading and unloading of cargo, storage areas, and handling equipment such as cranes. Ships may be berthed at piers of wharves for facilitating loading and unloading. Thus, a port is a commercial harbour with all the necessary infrastructure. Satellite port: A small port with minimum facilities for berthing of ships and loading and unloading of cargo. Historical aspects of water transportation The very first mode to have been developed since millennia. Egypt and Greece thrived on shipping even before the Christian era. Romans could flourish through shipping by extending their trade with countries like China at the beginning of the Christian era. Europeans used ships for sea voyages; Columbus discovered America in 1492 AD. Dock and Harbour Engineering

Inland waterways in India Developed on major river such as the Ganga, Brahmaputra, Godavari, Krishna and Mahanadi. Ocean waterways in India Ships were used in ancient days in India for trade with Persia, Arabia, East Africa, and Malaga. Significant developments in India during the British rule, and more so, after independence. Ports and harbours were developed. Further expansion has been taking place continuously. Elements of water transportation Waterways Terminals or ports Vessels or ships and carriers Dock and Harbour Engineering

Planning of water transportation Systematic procedures have been laid down for planning of the infrastructure for ports and phased development. Modern trends in water transportation Use of systems approach and computers for planning and data analysis and storage. Planning of integrated development with other modes of transport. Energy conservation techniques. Dock and Harbour Engineering

Chapter 17: Planning and Layout of Harbours and Ports
Definitions of important terms Port of entry: A designated area from where foreign nationals and foreign goods are cleared through customs for entry into the country. Free port: An enclosed and policed area adjacent to port of entry, with facilities for handling cargo and supplying fuel for ships; there is no intervention of custom authorities. Passenger terminal: The area where passengers embark or disembark along with their personal luggage. Marine terminal: The area where docking facilities are provided along with those for cargo. Cargo terminal: The area meant for handling bulk cargo, such as cement, grains and petroleum. Dock and Harbour Engineering

Anchorage area: A place where ships are anchored for inspection, to avoid docking space, fair weather conditions, or for quarantine to prevent the spread of infectious diseases. Offshore moorings: Anchorages at places where it is not feasible to build a dock or a harbour; anchors, sinkers and buoys are provided to which mooring lines of a ship may be attached. Turning basin: An area of water inside a harbour, or an enlargement of a channel, meant to allow the turning of a ship. The radius of the area should be at least twice the length of the ship for free turning. If the ship is turned warping around the end of a pier, the turning basin can be of a much smaller area. Plimsoll mark or load line: Mark or line indicating the level up to which a ship may be loaded in different bodies of water during the various seasons of the year, conforming to the maritime laws prevalent in the area. Beam: Width of a vessel. Dock and Harbour Engineering

Draft: The depth of the keel of a ship below the water level for a particular condition of the loading; thus , the ship rests in the water up to this depth when fully loaded. Fairway: It is merely the open water of navigable depth. Channel lines: The limits of navigation channels which are maintained at specified depth by dredging, if necessary. Ballast: The weight added to the ballast compartments of a ship, in order to increase the draft, after the cargo is discharged; this is needed to improve the stability of a ship and is expressed in tonnes of water. Displacement tonnage: Actual weight of the vessel, or that of the water displaced when a float; this may be in the loaded or light condition. Displacement light: This is the weight of the ship in tonnes, without cargo, fuel and stores. Dock and Harbour Engineering

Dead weight tonnage (DWT): Weight of cargo and fuel loaded to the Plimsoll line. This is the carrying capacity of the ship; it varies with the season and latitude, and the salinity of water. The mean of the values in different seasons is considered. Gross tonnage: Carrying capacity in terms of volume Net tonnage: Gross value less the space for the crew, machinery, fuel and engine room. Cargo tonnage: This is the basis of freight charge. Classification of harbours Depending on the nature of protection Natural harbour Semi-natural harbour Artificial harbour Dock and Harbour Engineering

Based on the purpose or function Refuge harbour: For quick shelter for ships during storms Commercial harbour: For import and export of merchandise cargo Fishery harbour: For the use of fishing vessels Military harbour: For the Navy Marine harbour: Provide facilities for small vessels. (small harbour or boat basin for 100 to 200 or more boats.) Based on location Sea or ocean harbour River harbour Lake harbour Canal harbour Dock and Harbour Engineering

Natural harbours These are formed naturally by the configuration of the land, resulting in creeks or basins, usually in bays and river mouths. Bombay (now Mumbai) and Kandla ports are examples of natural harbours in India. When the area within the harbour provides good anchoring and berthing conditions in the form of a deep navigable channel with a protective natural shoal, it is called a 'natural roadstead'. Dock and Harbour Engineering

Semi-natural harbours This type has protection on two sides by headlands, requiring artificial protection only at the entrance. Visakhapatnam is an example of this type in India. Artificial harbours These are formed by constructing what are called breakwaters to provide shelter to ships. Madras (now Chennai) harbour is a good example of an artificial harbour in India. Dock and Harbour Engineering

Site selection for a harbour Factors governing site selection: Availability of natural protection from waves, tides and cyclones Availability of land transport feeder routes. Strategic aspects (especially for Naval bases.) Site investigation This involves the following: Topographic surveys Hydrographic surveys Meteorological data Oceanographic data Geological conditions Soil investigation Seismic data Model testing Dock and Harbour Engineering

Salient features of a harbour Approach channel Anchorages Size, shape and depth Turning basin Harbour basin Breakwaters Docks Wharves, quays, jetties and piers Layout of a harbour Several combinations are involved in achieving a good layout. Madras harbour and Visakhapatnam harbours are good layouts. A schematic of Madras (Chennai) harbour is shown on Slide 17. Dock and Harbour Engineering

Schematic of Madras (Chennai) harbour Dock and Harbour Engineering

Port layout The following are the types of port layouts: (a) Square layout (b) Rectangular layout (c) Rectangular layout with a central pier (d) Machicolated layout Better than rectangular layout. Quays are provided perpendicular to the longer sides in a staggered manner. Greater quayage is obtained and turning becomes simple because of the provision of a turning basin at the centre of the layout. Dock and Harbour Engineering

(e) Tridentine layout A modification of machicolated layout, in which the longer sides are built to flare out; turning basin is also provided. (f) Digital layout This is semi-circular layout, with two channels-one for entry and the other for the exit of the vessel. Hence, no separate free-turning space needed be provided. The berths are provided radically, and huge quayage is obtained. Tridentine Layout Digital Layout Dock and Harbour Engineering

Facilities required at a port Facilities for guiding the vessels: Fixed signals, floating signals and light houses. Protection facilities: Either by natural formation or artificially by constructing breakwaters. Docking facilities: Wharves, quay walls, or piers are built with aprons to form wet docks in which vessels can be berthed with minimum oscillation. Cargo handling facilities: Craves on pavement track, supported on quay walls of wet docks. Conveyors and vacuum pumps are used for certain special kinds of cargo. Repair facilities: Graving or dry docks in which vessels can be berthed and lifted, is necessary, for carrying out repairs. Dock and Harbour Engineering

Storage facilities-transit sheds for temporary storage, warehouses for extended storage, and cold storages for perishable goods. Offshore anchorage facilities Quarantine facilities Turning facilities-turning basins Dredging facilities-for excavating silt to maintain the necessary draft. Administrative facilities-offices Miscellaneous facilities-fire protection, and other service units. Dock and Harbour Engineering

Major ports of India Nhava Sheva – Mumbai – Kochi – Kandla – Chennai – Kolkata – Haldia – Visakhapatnam – Marmugao – Paradeep – Mangalore – Tuticorin There are also over 200 minor and medium-sized ports. Environmental Impact Assessment (EIA) Assessment of existing conditions-flora, fauna, water quality, etc. Prediction of future effects-on aquatic life and on social and economic factors Consideration of alternative proposals Resources for minimising adverse impact if any. Monitoring processes-for planning-remedial action. Dock and Harbour Engineering

Port Authorities and Bodies These are for administration and promotion of maritime interests. Important bodies in India National Association of Port Authorities National Bureau of Customs National Maritime Administration Waterfront commission National Coast Guard Immigration and Naturalisation Service Quarantine and National Public Health Service Security Association Dock and Harbour Engineering

Chapter 18: Natural Phenomena Affecting Harbour Design
(a) Meteorological phenomena: Winds, cyclones and storms (b) Oceanographic phenomena: Waves, tides and tidal currents, and silting erosion Wind: Air in motion Wind speed is expressed in 'knots' or 'nautical miles/hr'. One knot = km/h Besides the speed, wind direction is also important. Gradient speed Since surface friction encountered by wind over water is more than that over land, wind speed over the sea is about two-thirds of that at a height of 600 m, while over land it is about one-third the speed. This speed is known as the 'gradient speed'. Dock and Harbour Engineering

Beaufort scale Prior to the development of precise instruments, wind speed was estimated by its effects. Admiral Beaufort of the British Navy devise a scale for wind strength based on the speed; which ranges from 1 to 12, as the wind speeds and the adverse effects increase. 1.6 km/h ... calm .... Beaufort No. 0 43 km/h strong breeze ... No. 6 More than 120 km/h...Hurricane... No. 12 Wind rose Diagrammatic representation of wind direction and duration (in percent time) for a reasonably long period like 1 to 5 years, based on observed data. This diagram helps in deciding the best orientation of the various components of a harbour. 'Sea breeze' blows towards the land and 'land breeze' blows towards the sea. Dock and Harbour Engineering

Cyclones and storms Storm: Atmospheric disturbance accompanied by high winds, it may be accompanied by low-quantum precipitation and thunder and lightning (No. 11 on Beaufort scale) Cyclone: A violent tropical storm in which the air moves very fast in a circular manner. This is also sometimes called a 'cyclone storm'. Storm and cyclone are not differentiated on Beaufort scale. Waves on the sea – Water waves Caused by various disturbances – Artificial or natural. Artificial sources are moving vessels – Larger ships and ocean liners, and men-made explosions. Natural sources are winds, tides and earthquakes. These waves produced on bodies of water are 'water waves', differentiated from others such as different kinds of energy waves like sound waves, light waves and electromagnetic waves. Dock and Harbour Engineering

Origin of waves: Water waves are generated by the transfer of wind energy to the water surface. Types of water waves: These are: Factors affecting water waves Wind velocity Wind duration Bed friction Depth of water Wave characteristics Wave height – Depends on the 'Fetch', or the straight stretch of open sea available for uninterrupted wave growth. Wave length Wave velocity Dock and Harbour Engineering

Water Wave Significant wave height – taken to be one-third of the highest wave height. Modification of water waves – the phenomena are: (a) Decay (b) Breaking (c) Diffraction (d) Refraction (e) Reflection Dock and Harbour Engineering

Wave action on harbour structures Waves exert pressure on harbour structures, while breaking against them; the effects of this have to be accounted for in their design. Wave pressure on vertical walls comprises hydrostatic as well as hydrodynamic pressure Tides Periodical variations in the water level of seas and oceans Hydrographic surveys Conducted as part of shore-line surveys, and soundings for the determination of water depth. These have many applications in harbour engineering. Agents causing tides The moon-being the nearest to the earth it is the principal tide-producing agent. The sun-owing to its relatively large distance from the earth, solar tides are only times the lunar tides. Dock and Harbour Engineering

Newton's equilibrium theory This is used to explain the phenomena of tides. Spring tides and Neap tides: Result from the combined effect of lunar and solar tides. Spring tide: High tide produced when the sun, the moon and the earth are such that the lunar and solar tides add to each other; high water is higher than normal and low-water is lower than normal. Dock and Harbour Engineering

Neap tide: Tide produced when the sun, the moon and the earth are so situated that the tide-producing forces of the moon and then sun are perpendicular to each other; in this case, high water is below the average while low water is above the average, Two spring tides occur during a lunar-cycle one at new moon and the other at full moon. Two neap tides also occur – one at the first quarter of the lunation and the other at the third quarter. Dock and Harbour Engineering

Tidal range The difference between the water levels of the high tide and the low-tide. Several factors affect the tidal range: Variation in the relative positions of the sun and the moon. Ellipticity of the orbit of the moon around the earth and of the earth around the sun. The departure of the earth's shape from a spheroid. The effects of land masses in the ocean bed. Prediction of tides is a complex process requiring harmonic analysis of past data for a long period. Total currents are also to be considered in the design of harbour structures. Dock and Harbour Engineering

Silting and erosion Silting at some points and erosion at some other locations near the coast line and within the harbour area are due to the phenomenon of 'Sediment transport'; this is also a complex process in the shallow zone of the continental shelf of the sea-bed near the shore. Littoral drift: This is the process of movement and deposition of land, drifting in a zig-zag manner in the proximity of the cross-line. This is caused by the effects of wind, waves and tides. Dock and Harbour Engineering

Erosion Erosion of the coast line is caused by the forces of nature and sediment transport. Silting and littoral drift necessitate dredging operations for maintaining the necessary depth of water for navigation in and around the harbour. Also, the effects of erosion are to be considered for preventing the weakening of the foundations of harbour structures. Dock and Harbour Engineering

Chapter 19: Facilities for Ports and Harbours
Facilities for ships at ports and harbours (a) Protection facilities – Breakwaters. (b) Loading and unloading facilities for cargo and for passengers to embark and disembark ships – Quays, wharves, piers, dolphins, jetties, fenders, trestles, and moles. (c) Cargo-handling and storage facilities – Cranes, bulk cargo equipment, container cargo equipment, transit sheds for temporary storage, including cold storages for perishables. (d) Navigation facilities – For safe navigation of vessels from the high seas into the harbour and out of it or further travel-navigation aids and navigation lights. (e) Docks and repairing facilities for vessels. Dock and Harbour Engineering

Breakwaters A 'breakwater' is a structure meant to reflect and dissipate the energy of the wind-generated waves; this results in the necessary protection to the vessels inside the harbour area in relatively calm waters. This is mandatory in the case of artificial harbours. A suitable approach channel with a well-designed entrance is created. Considerations in the design of breakwaters: Alignment: Straight converging arms are preferred with an angle of intersection not exceeding 60, for a narrow-entrance to the harbour. Design data: Nature of coastal currents Wind characteristics Wave characteristics Properties of sea-level material Dock and Harbour Engineering

(3) Design considerations: Maximum wind and wave forces for design, Protection measures to prevent scour of bed. Height of breakwater to be more than that of the highest wave, the difference is the 'freeboard'. (4) Forces on breakwaters: Hydrostatic force on the inside Wind and wave forces on the outside. Chemical action of sea-water and the effects of marine life on the materials of construction have to be considered. Classification of breakwaters (a) Mound breakwater (b) Composite breakwater (c) Vertical wall type breakwater (d) Special types of breakwaters. Dock and Harbour Engineering

Considerations in the choice of the type: Depth of water Nature of sea-bed material Availability of construction materials Availability of equipment Resources and time constraints Mound breakwater Also called 'Rubble-mound breakwater, it is popular for its simplicity of construction. It usually consists of three zones: Hearth or the core: It increases the bulk; the stones may weigh 10 to 20 kg each. It is the least pervious of the zones. Filter on secondary consumers: Much bigger tones than those in the core are used – 0.5 to 1.5 t. The function is to destroy the energy of the waves penetrating the armour layer above it. Armours: Stone blocks weighting up to 30 tonnes each are used. Resist most of the wave energy-dumped stones gives rise to maximum voids in the zone. Dock and Harbour Engineering

Rubble-mound breakwaters have good energy dissipation characteristics. Preparation of foundation is not required. Sudden failures do not occur because of the flexibility due to high porosity. But huge quantities of stones are required and frequent maintenance is needed due to settlement. Design is only empirical, strengthened by patented concrete. Dock and Harbour Engineering

Rubble-mound breakwaters strengthened by patented concrete blocks or simple concrete blocks Concrete blocks of the desired shape and size can be cast with a suitable mix proportion to achieve the desired strength. These blocks are laid around armour stones on the seaward side of the mound to act as additional armours. Patented shapes are tetrapods, tribars and quadripods. Dock and Harbour Engineering

Composite breakwater Solid concrete superstructure at low water level, to serve as a platform for cargo. Methods of construction of mound breakwaters Barge method Staging method Low-level method Dock and Harbour Engineering

Vertical wall breakwater Concrete block gravity walls, concrete caissons, steel sheet pile walls with solid concrete wall at the top are a few types. Loading and unloading facilities Components required for embarkation and disembarkation of passengers, and loading and unloading of cargo fall into this category. These are- (1) Quays and quay walls (2) Wharves (3) Piers (4) Dolphins (5) Jetties (6) Fenders (7)Trestles (8) Moles Dock and Harbour Engineering

Quays and quay walls Quays: Stationary platforms, built along or parallel to the shore, to serve the purpose of loading and unloading of vessels brought near them. Quay walls: These are constructed to retain the filling and embankment needed for quays. Design is similar to that of retaining walls. Types of quay walls Solid and dwarf walls, the later are supported on piles. Quay walls supported on rubble mound, timber crib or lattice. Dock and Harbour Engineering

Piers Structures built perpendicular to the shore, and serve a similar purpose as quays. Piers may be solid structures, or may be supported on piles. Solid pier can be with masonry or concrete. Dolphins A closely spaced cluster of piles, the tops of which are pulled together and tied by means of a cable, is called a 'dolphin' Dolphins are used for mooring vessels, as also for transferring cargo from one ship to an adjacent one, when both are moored. These are designed to resist water current forces, impact force of vessels, and wind forces. Dock and Harbour Engineering

Breasting dolphins – To ensure a vessel along the dock on hold it against water current, Mooring dolphins – To hold a vessel against its broadside wind force. Jetties 'Jetty' is a narrow-structure projecting from the shore into the waters, with berths on one or both sides. Fenders Buffers or cushions to prevent damage by vessels hitting the structures; springs or rubber may be used. Dock and Harbour Engineering

Trestles Timber structures on wood piles, which serve as piers. These are lighter and more economical for use on temporary basis. Moles These are rock-fill structures extending from the shore towards the water to serve as piers. Wharves Platforms near the shore for vessels to be berthed. Open or framed structure with posts and bracings, or supported on piling, or a sheet-pile wall. Wharves are built parallel to the shore and abutting it. These are similar to quays. Dock and Harbour Engineering

Cargo-handling and storage facilities Cargo-handling equipment: General cargo: Fixed crane (capacity 50 t or more) Mobile crane, Hand crane (Some may be stationed on quays and some on ship deck) trucks, tractors and trailers. Bulk-cargo: Grain, ore, coal Liquids and oils need special equipment, designated for them. Container cargo: Packaged cargo containers are transferred to heavy trucks/wagons from ships to be transported on highways/railways. Transit sheds and aprons: For temporary storage of cargo. These structures are specially designed for convenience in handling and moving cargo into and out of transit sheds. Cold storages: These are meant for storing perishables such as fruits and vegetables. Dock and Harbour Engineering

Navigation facilities Navigation aids are essential to ensure safe, efficient and comfortable negotiation of vessels, not only in harbours and inland waterways, but also in the open sea,; this is especially true during nights and bad weather conditions. Navigation lights: (a) General lights (b) Local lights General lights: Navigation aids along the coast, which may serve more than one port. These are 'lighthouses' or 'lightships'. Government of India is responsible to build lighthouses; the Director General of light houses and lightships looks after the maintenance. Local lights: Aids to guide vessels in approach channels and harbour entrances and to anchorages and berths Light houses Channel makers Leading lights On the shore, piers, wharves, and breakwaters. Dock and Harbour Engineering

Another classification: Fixed light stations: Lighthouses on the shore, beacon lights on breakwaters, piers, wharves. Floating light stations: Moored light vessels or lightships, buoys Types of buoys: (i) Mooring buoys (ii) Wreck buoys (iii) Buoys for special indications Modern trends in navigation Use of: Plan-position indicator Continuous-wave hyperbolic system Automation of all operations using computers Global Positioning System (GPS) Dock and Harbour Engineering

Chapter 20: Docks and Repair Facilities
A 'dock' is defined as a sheltered basin or enclosure for berthing ships; this facilitates maintaining a reasonable uniform water level for loading and unloading of cargo. Classification of docks: (a) Wet docks or harbour docks (b) Dry docks for repair facilities Wet docks Designed to maintain uniform level of water to facilitate the handling of cargo. Storms in the open sea and disturbances in the harbour basin do not affect the waters in the dock enclosure. Dock and Harbour Engineering

Classification of wet docks Wet docks in tidal basins Wet docks in enclosed basins Water level varies significantly due to tides in tidal basins. A wet dock may be constructed perpendicular to the shore for permitting vessels to enter it and discharge the cargo by berthing inside, alongside its walls. Although water level variations are high for large tidal range, unrestricted entry and exit of vessels without expensive locks is an advantage. Dock and Harbour Engineering

Wet docks in enclosed basins In this case, the dock is enclosed and away from the open sea or harbour basin, with controlled entry and exit via an entrance basin and locks. Uniform water level can be maintained for stability during loading and unloading of cargo. Locks and lock gates are expensive; further, more time is required for entry and exit of vessels through locks. Dock and Harbour Engineering

Planning and design of wet docks Criteria in planning Available area Maximum length of quay in relation to the dock basin Rectilinear in shape and not curvilinear Common shapes of dock basins: Rectangular Diamond Inclined quay Depth of water in docks and dock basins must be adequate for the largest vessel permitted. Dock and Harbour Engineering

Design of wet docks Loads expected on dock walls Dead load (self-weight) Live load Earth pressure including surcharge effect Static and dynamic water pressure Loads due to ship impact Uplift pressure at the base, if any Stability conditions Designed as a gravity retaining wall Factors of safety should be adequate in bearing, sliding and overturning for the max loads. Dry docks and repair facilities Dry docks are meant to provide repair facilities for ships by allowing them inside with water, and emptying the dock by pumping out water. Thus the entire ship, including its keel, is exposed for inspection and repairs, as required. Thus, a dry dock is also sometimes called a 'repair dock'. Dock and Harbour Engineering

Classification of repair facilities Dry dock or a graving dock This is the most popular type. Side walls are built of masonry or concrete. The size can be up to 300 m long and 30 m wide. Dock and Harbour Engineering

Forces on a dry dock Weight of vessel on dock floor Weight of water when the dock is full. Earth pressure and hydrostatic pressure on the side walls. Surcharge pressure on side walls due to heavy equipment such as cranes. Uplift pressure on the floor when the dock is being emptied. Design considerations Dock empty condition Dock full condition Dock floor should rest on hard rock/firm soil; otherwise a 5-metre-thick concrete bed should be laid. Sequence of construction should be carefully planned for ensuring stability during all the stages of construction. Dock and Harbour Engineering

Marine railway or slipway Also called a 'slip dock', it is simply a railway laid at a uniform slope, starting from some distance under water to a point on the shore at which the longest vessel to be accommodated is completely above the range of the tide. A 'cradle', or a platform of steel which is mounted on a system of rollers, can move on an inclined track. Keel and bilge blocks of timber are provided on the cradle to receive the ship on to it. Cross-section of cradle Typical profile of a slipway used along slipway Dock and Harbour Engineering

Locks and lock gates 'Locks' are devices which are employed to overcome differences in water level, in order go provide access to enclosed chambers such as docks. Similar to river or canal locks. 'Lock gates' are used to control entry into and exit from locks Dock and Harbour Engineering

Timber gates or steel gates may be used. Usually two pairs of gates are adequate. The gates may be operated manually or by means of power; the latter is preferred for large gates. A typical lock Wooden lock gate Dock and Harbour Engineering

Chapter 21: Maintenance and Dredging of Harbours
Maintenance of a harbour Maintaining the necessary draft or depth of water for the vessels using a harbour on a continuous basis is an important aspect of maintenance, besides others. (Navigable depth of water means the sum of the draft and the vertical clearance between the bottom of the ship and the sea-bed.) Dredging This is the technique of removal of silt and other materials from the bed to increase the depth of water, in order to restore the navigable depth. Since the cycle of erosion and silting of sea-bed is inevitable as a result of natural phenomena, disturbing the natural regime, periodical dredging becomes necessary for the maintenance of a harbour. Dock and Harbour Engineering

Classification of dredging (a) Capital dredging (i) Improvement dredging (ii) Sundry dredging (b) Maintenance dredging Capital dredging Initial dredging that is needed to provide sufficient depth of water for navigation at a particular site; this is also known as 'Primary dredging'. Dredging carried out to increase the depth of water so as to make it sufficient for larger vessels is called 'improvement dredging'. Dredging for reclamation of new areas for navigation is known as 'sundry dredging'. Dock and Harbour Engineering

Maintenance dredging As the depth of water initially created gets affected regularly because of the natural phenomenon affected regularly because of the natural phenomenon of sediment transport, which is a complex process, periodic dredging to maintain navigable depth of water in the vicinity of a harbour is a necessity. Such dredging is known as 'maintenance dredging'. This is also called 'secondary dredging. In the case of predictable cycles of silting, the annual cycle of dredging work may be planned. Otherwise, dredging may have to be carried out at short notice. Equipment for dredging-Dredges The primary equipment for dredging are known as 'Dredgers' or 'Dredges'. Categories of Dredgers: (1) Mechanical – Dredging by means of mechanical action (2) Hydraulic – Dredging by means of suction Dock and Harbour Engineering

Mechanical type Grab or clamshell dredger (b) Bucket-ladder dredger Endless chain of buckets, running around a ladder. Suitable for sand, Hard clay, gravel or even broken stone (c) Dipper dredger Heavy-duty equipment – can be used Even for boulders and rocks Dock and Harbour Engineering

Hydraulic type The main component is 'sand pump', which sucks the sludge mixed with water and sends into a discharging pipe line. Pure suction is adequate for loose material; but rotating cutters at the end of the suction pipe, or a system of water jets may be needed to deal with harder material. Special equipment for dredging (a) Rock breaker (b) Hopper barge Dock and Harbour Engineering

Execution of dredging jobs Methods of execution: Direct labour method Contract system Each method has its own advantages and disadvantages. Disposal of dredged material may be on to the shore through larges or into the deep sea if suction method is used. Coast protection A coastline continuously changes because of natural phenomena such as winds, waves, storms and cyclones, which act incessantly. As a result of this, either erosion or deposition of earth material on the shore occurs. Erosion may lead to exposure of foundations of harbour structures. Thus, the need for coast protection is well recognised, even for the protection of beaches. Dock and Harbour Engineering

Shore protection works The following are commonly used: (1) Seawalls (2) Revetments (3) Bulkheads (4) Sand dunes (5) Groynes (6) Offshore breakwaters Seawalls: Heavy structures which derive their stability through their own weight. They may be vertical, battered or retractive (concave towards the sea). Revetments: Pitched stone revetments with rubble stones are used to protect the seaside of earth embankments. Dock and Harbour Engineering

Bulkheads: Along the shore line-sheet pile walls or flexible earth-retaining structures. Sand dunes Formations of sand along the coast, which prevent the movement of waves and tides. Vegetation stabilises dune sand, which dissipates wave energy. Dock and Harbour Engineering

Groynes: A groyne helps in building up a protective beach, and traps littoral drift. Groynes protect also the toes of seawalls or bulkheads. Groynes may be built perpendicular to the shore or inclined to it. A 'system' of groynes protects the shore line effectively. Offshore breakwaters: Used to protect vulnerable areas from wave action. It can act as a tap for littoral drift. Generally, they are of the rubble-mound type and provide protection to harbour entrances. Only small vessels can take refuge in an area protected by an offshore breakwater. Dock and Harbour Engineering

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