Lock design

A navigational lock is a structure which enables the rising/lowering of the water level at a specific point on a stretch of water (canal, river, estuary) to allow the passage of watercrafts (boats, ships, etc.).

The two main uses of a lock are to:

  1. Enhance the navigability of a river or canal by raising the upstream water level (water retention)
  2. Connect two waterways in different catchment zones.

Other common uses include flood prevention/control within a waterway system and tide control, connecting a river/canal with an estuary by means of a sea lock.

Locks are part of the sluice family, structures which are built for water retention but also for the passage of water and vessels.
The following types of sluices can be distinguished:

Dewatering gates

Dewatering gates are used to discharge a surplus of water, for example for water management of polders or for flood control areas. They can also be used to separate salt water and fresh water or to retain water at high external water levels.

Stop locks

Stop locks refresh water in a channel or closed water basin and control the water level. They are used to clear the canals or basin from sediments and pollutants while the water is discharged. These locks are opened when the water in the basin or cannel is sufficiently higher causing a sufficient discharge to flush out the sediments.

Guard locks and Storm Surge Barriers

Guard locks and Storm Surge Barriers have two conflicting functions: the passage of vessels and retention of water. The gate of a guard lock will close rarely and only when the water level difference is too high. This allows a maximum passage of vessels. A Storm Surge Barrier is designed to protect the land for extreme water levels due to a storm surge and to allow safe passage of vessels meanwhile.

A navigational lock

A navigational lock connects parts of a waterway with different water levels. It allows waterways to be used by vessels independently from tides. In fact, a navigational lock combines water retention and ship passage as well, but it is more considered as a transport infrastructure. There are different types: the traditional lock for inland navigation and coastal areas, a lift lock and an inclined plane.

The size (length and width) of a lock is mainly dependent on the design vessel size and traffic projections.
The depth of a lock is dependent on the up- and downstream water levels, the draft of the design vessels and other factors such as inlet and outlet opening size and silt volume in the waterway.

  • The main components of a lock can be summarised as; lock gate, lock chamber, operations system and the filling and emptying equipment. Every lock is unique in terms of its size, water level difference, geology, location, design vessel, and the waterway’s associated flow and flood hydrology. Different technologies are available for each component depending on the boundary conditions at the lock in question.
  • The type of lock gate gives a lock its unique appearance. To retain the water level difference the gate needs to prevent water from flowing between the different water levels. Around the wet perimeter of the gate leakage and seepage will occur. In the coastal area, there is an additional strict requirement to separate salt and fresh water. The resulting hydraulic force due to the difference in water level needs to be resisted and transferred to the supports at the lock head.
  • The sealing provides many engineering challenges. It is therefore necessary that the type of gate system is decided in the early stage of the design process. For each type of gate, basic principles exist which can be altered to fit the specific situation. Each of the typical systems has advantages and disadvantages, which need to be considered on site.

Mitre Gate

Mitre gates are a pair of two standard hinged doors each having a width slightly larger than half of the width of the dock. They meet in the middle, so they form a chevron style shape in plan view. They open and close by rotating around a vertical axis. The reaction forces from water head difference are directed to the lock walls. The mitre gate is probably the most common lock gate type for narrow to normal width locks. From a width of about 30 m, forces increase to such an extent that it becomes uneconomical to have this type of gate. It should be noted that mitre gates are only suited to withstand excess water pressure from one side.

These gates don’t require a high superstructure. For smaller locks they are an effective solution with a low cost because the equipment doesn’t need to carry the weight of the gate and the opening and closing time is short. In that way the travel time through the lock is reduced. Furthermore, the mechanical components that move the gate are not complex. The disadvantages of mitre gates include the optimal closing of the gates; the dimensions should be very accurate in order to prevent leakage. Other drawbacks are its sensitivity to debris and ice and potential damage when a vessel impacts the gates.

Rolling gates

A rolling gate or sliding gate opens by sliding the gate perpendicular to the lock’s longitudinal axis into a gate chamber next to the dock. The gate moves on underwater rails or sliding tracks. This solution is used on the world’s largest locks as those of the New Panama Canal. Maintenance of these gates is straightforward as the gate chamber itself could be set dry. The mechanical systems for opening the gate are complex, however. Also, during opening and closing, the gate rolls or slides on its track while the upper edge of the gate is unsupported. This represents a stability challenge. In many cases, a one-step lock complex has four rolling gates; at each side of the lock the gate is backed up by a spare one. In case of damage or maintenance of the one gate, the spare one will ensure operability of the lock complex.

Roller gates have several advantages. When locks have a large width and a superstructure that limits vertical clearance is undesirable, the rolling gate is a good solution. Additionally, it is possible to retain water on both sides, and maintenance of the gate is easy. However, there should be sufficient space next to the lock head for the gate chamber. Both the sliding mechanism and the chamber are more expensive to design and build.

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Vertical lift gates

A vertical lift gate is a gate that is pulled up in vertical direction. Two towers at each side of the lock inlet and a lifting mechanism operates the gates. The ships pass below the hanging door. This solution is more common where design vessels are smaller and space around the lock is very limited. The gate can retain water from both sides.

Some advantages include the small footprint of the lock (no lock chambers as is the case with roller gates), easy inspection and maintenance and the insensitivity to ice and debris. Disadvantages are the limited vertical clearance (this type of gate won’t be used to lock Panamax vessels) and the complicated and expensive moving mechanisms and towers.

Other lock gates

Other gate types include:

Lock gates with rotation around a vertical axis:

  • Single leaf gates: which are in fact half a mitre gate and transfers the load by bending moments. This type of gate is rather rare and only used in locks with a small width.
  • Radial sector gates

Lock gates which are stored above the water level:

  • Double leaf gates
  • Lifting pivoted gates
  • A caisson style gate, more common in dry docks, is basically a movable steel box that is placed in front of the dock opening. This box has chambers that get filled with air which allow it to float when it must be moved. A tug boat is used to push the gate to its position. It is the most robust solution, however opening and closing of the lock takes time and a lot of equipment. Caisson gates are therefore mostly used as stoplogs for maintenance purposes.

Lock gates which are stored below the water level:

  • Submersible gates
  • Tainter gates (or flap gate) has a horizontal hinge positioned on the floor of the lock. When opened, the gate is rotated down to a horizontal position on the floor of the lock and the ship can sail over this gate. Therefore, a lowered zone is required to store the door below or equal to the bottom level of the remaining area of the lock. Flap gates are sensitive to debris or sediment that may accumulate in its action radius.

Lock Chamber

The Lock chamber could be conceived as a monolith “U” shaped structure or as two retaining walls and a base slab.

The three main types of retaining walls associated with a lock are:

  • Counterfort retaining wall
  • Anchored diaphragm retaining wall
  • Diaphragm wall without anchors

The counterfort retaining walls are generally favoured (depending on the geotechnical conditions) for the following reasons:

  • No special equipment is required for constructing this type of retaining walls, whereas other options need specialized machinery.
  • It is easier to construct the counterfort retaining wall as compared to other types of walls.
  • It is cheapest among all the above options (but case specific, since important earthworks and dewatering may be required during construction).

Gate operation system

The drive systems of lock gates can be very diverse and are dependent on the type of gates used in the lock. On small gates even manual operation is possible.

Mitre gates are mostly driven by oleo hydraulic cylinders which are actuated by an oleo hydraulic pump unit.  Other systems for mitre gates are spindle drives, rack and pinion combinations or panama wheels and connecting rods. These systems are powered by electric motors.

Rolling gates are opened and closed by pulling steel wire ropes attached to the gate.  The steel wire rope is wound on the one or the other cable drum driven by electric motors.

Vertical lift gates can be directly driven by oleo hydraulic cylinders or can be moved by a steel wire rope and sheaves assembly. That assembly can in its turn be driven by an electric motor or a cylinder.

The other types of gates (flap gate, lifting pivoted gates,  ..) are also driven by one of the above mentioned drive systems.

The choice on type of drive system is dependent on several conditions:

  • Available space
  • Forces to be developed
  • RAMS (Reliability / Availability / Maintainability / Serviceability)
  • Maintenance requirements and lifetime
  • Energy requirements
  • Environment

To increase reliability redundant electric motors may be installed.
The electromechanical equipment of the lock does not only cover the power transmission of the gates. The filling and emptying system of the lock has to be powered as well. Similar systems as on gates are used

Control system

Control systems equipment includes the instruments for measuring water levels, sensing open and closed positions of the gates, traffic lights, CCTV (television circuit), communication systems and appropriate lighting of the lock area. This all is regulated by PLC and the SCADA-control system.

Sometimes special systems are needed as well: systems to move away silt and de-icing systems.

Nowadays it is very common to remotely operate all the locks on a river or canal system from one centralized control room.  This control centre will avail of all the sensing and control I/O from all the locks, in addition to communication and CCTV connection.

Finally, for all this the necessary power supply has to be in place and emergency power supply also has to be foreseen.

Filling/emptying arrangement

When the vessel enters the lock and the lock gates are closed, the water level in the lock is lowered or raised. Depending on the size of the lock and the requested locking time (based on traffic forecasts) and many other factors (i.e. salt intrusion or hawser forces on the vessels in the lock chamber), the most optimal filling/emptying arrangement is selected.

Some systems are mentioned below:

  • Valves in lock gates;
  • By using the lock gates (some lock gates may be opened under differential water head);
  • By using short separate culverts;
  • Wall culvert side port system;
  • Wall culvert bottom lateral system;
  • Wall culvert bottom longitudinal;
  • Longitudinal culverts under the chamber slab;
  • Symmetrical distribution of flow in relation to both axes of the lock. Dynamically balanced lock filling system.

Water saving basins may be required as part of the filling and emptying arrangement when loss of water or salt intrusion occur in river systems with insufficient net water influx from precipitation.

Associated structures in a lock complex

In many cases, a lock is part of a river lock complex.
Associated structures include water diversion structures, fish ladders and hydropower turbines.

  • Water diversion structures, like a weir or dam, are built to change the flow characteristics of a river, make waterways more navigable, prevent inundations or save water upstream. Many structures exist ranging from low-head dams made from rubble to large dams with a retaining height of more than 100 meters. Many structures have movable parts like a weir with radial gates.
  • Fish ladders are structures found along dam/weir structures and facilitate fish migration. If these structures aren’t included in the lock-weir complex, fish would otherwise not be able to safely pass this barrier. A handful of fish ladder types exist, ranging from simple pools to mechanical fish elevators.
  • Hydropower turbines are basically structures which generate electrical power resulting from the difference in water level upstream and downstream of the lock-dam complex (head). The kinetic energy of the falling water is converted into mechanical energy which is converted to electricity. The output of a hydropower plant is function of the hydraulic head, the number of turbines and their efficiency. Small turbines next to low-head lock-weir structures can be sufficient to power the lock-weir-hydro-bridge complex while large hydropower plant along a high-head dam can provide power for tens of thousands of families.

Reference: Lock Harelbeke

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