Submersible modular bridge

Abstract

A submersible modular bridge comprises at least one bridge member 110 and at least one tether 130 for holding at least a portion of the at least one bridge member below the surface 140 of a body of li

Description

Field of the disclosure The present disclosure relates to a submersible modular bridge, a method of constructing the same, and a remote controller for a modular submersible bridge. Background to the Disclosure Bridges are important infrastructural elements. During natural disasters or wars, bridges are often destroyed, and so in these scenarios it is often desirable to build new bridges quickly. Traditional bridges, such as brick bridges, take a long time to complete, and require structural work to be carried out, which may be infeasible in an emergency scenario such as a flood or when under enemy fire. Pontoon bridges were developed for quick deployment in such scenarios. These bridges consist of a number of floats connected together, which are strung across a body of water. An advantage of these bridges is that they may be constructed from low-cost modules. However, such bridges remain vulnerable to destruction, e.g. due to high winds, or due to airborne hazards. For example, during a war, pontoon bridges are visible from the air, and so may be fired at from the air to destroy the bridge. It is therefore desired to produce a bridge that is quick to construct but less vulnerable to destruction than prior pontoon bridge systems. Summary of the Disclosure According to a first aspect of the present disclosure, there is described a (pontoon) bridge comprising modules, wherein the bridge is configured for use under the surface of a liquid, or otherwise underwater. “Use underwater” herein indicates that the bridge runs under the surface of a body of water, such that to traverse the bridge requires travelling through the water, rather than above the water as with conventional bridges. Such bridges may be more stable than conventional pontoon bridges, while being less resourceintensive to construct than permanent bridges. Modules are units that may act as a basis for construction. Unlike bricks or beams, which are elements without sub-parts, and which are combined in resource-intensive operations at the desired building site, modules are substantially pre-fabricated, which may be done at any location, such as away from the desired building site, and may comprise sub-elements which are combined at a separate location to the desired building site, such as a factory. Assembly of modules at a desired building site is therefore much more straightforward than assembly of a bridge using conventional building materials such as bricks or beams. Preferably, the bridge is submersible. In an example, a module of the bridge comprises at least one platform, and at least one anchor connected to the platform. These modules may be combined sequentially to build an underwater / submersible bridge. A load may traverse the bridge by travelling across the platforms. According to second aspect disclosed herein, there is provided a (pontoon) bridge for traversing a body of liquid (such as water), comprising at least one bridge member; and at least one tether (configured) for holding at least a portion of the (respective) at least one bridge member below the surface of a body of liquid. Preferably, the at least one tether is arranged (in use) to hold at least a portion of the (respective) at least one bridge member below the surface of a body of liquid. The at least one tether preferably extends beneath the bridge towards a bed of the body of water (in use). The at least one tether preferably ‘holds’ the portion of the at least one bridge member in the sense of (continuously) retaining the portion of the at least one bridge member in position, more preferably while the bridge is in an unloaded state (that is, when the bridge is in position but nothing is crossing or resting on the bridge). The tension in the tether means that, when a load is placed on the bridge member, the load does not sink until the weight of the load exceeds the tension in the tether, which is caused by the upthrust of the bridge member. This means that, unlike a conventional floating bridge, the bridge members will not sink at all when they are subjected to a load below a threshold. Such bridges may therefore be more stable than conventional pontoon bridges. Such bridges will also have reduced volume requirements for the platforms compared to conventional pontoon bridges, because conventional pontoon bridges must compensate for the instability by a large platform area, to reduce the distance that the platform sinks when subjected to a load. Therefore, the bridge of this aspect may be constructed with smaller platforms than conventional pontoon bridges while having the same or improved stability. In examples, the bridge member is buoyant relative to the liquid. The bridge member may be a platform. In examples, the at least one tether is suitable for holding the at least one bridge member below the surface of a body of liquid. Preferably, the at least one tether holds the at least one bridge member below the surface of a body of liquid when the bridge is in an unloaded state (that is, when the bridge is in position but nothing is crossing or resting on the bridge). In examples, the bridge comprises a plurality of bridge members - preferably, in such examples, a plurality of respective tethers are provided. In examples, the at least one tether is suitable for holding the entirety of the at least one bridge member below the surface of the body of liquid. When the whole of the bridge member is held below the surface of the body of liquid, the tension in the tether is at a maximum for a given platform volume. Advantageously, this maximises the weight that the bridge member can bear without moving downwards, improving stability. In examples, the tether holds the respective at least one bridge member below the surface of the body of liquid when the bridge is in an unloaded state. In examples, the at least one bridge member comprises at least one vertical channel. This allows liquid to pass through the bridge, rather than only around the bridge, when the bridge is raised or lowered. This may reduce the force required to lower or raise the bridge. In examples, the bridge comprises at least one anchor attached to the (respective) at least one tether, preferably at an other end of the at least one tether to the end which attached to at least a portion of the (respective) at least one bridge member. In examples, the at least one anchor is at least one screw anchor. Screw anchors are lighter than weight anchors, but may provide a similar resistance to upwards or lateral movement. This means that screw anchors may provide suitable anchoring properties, while being easier to transport than weight anchors due to their lower weight and volume. Preferably, the screw anchor comprises features for cooperating with a tool for screwing the screw anchor into a material. In examples, the at least one anchor comprises a weight, wherein the weight is greater than the displacive capacity of the float. Weight anchors may be chosen where the bed of a body of liquid is not suitable for a screw anchor. Preferably, the anchor comprises a cavity, into which a weight may be inserted. Preferably, the anchor comprises at least one hollow body for receiving material. This allows an anchor to be transported with a lower weight, with the additional weight only added at or near the location. This renders transport of the anchor more efficient. More preferably, the anchor comprises a lid. This allows weight to be secured within the anchor. Preferably, the hollow body for receiving material is configured to be filled with water. Preferably, the anchor comprises a resealable outlet. Unsealing the resealable outlet allows water to enter the anchor, allowing weight to be added to the anchor when the anchor is already submerged at the location for construction of the bridge. Advantageously, this renders the anchor easier to transport to the location, and streamlines the installation process. In examples, each anchor is connected to a plurality of tethers, preferably wherein each anchor is connected to two tethers. In examples, the bridge further comprises a means for adjusting the tension in the tether. Adjusting the tension in the tether to tighten the tether further submerges the bridge. This may be advantageous in order to allow boats to pass over the bridge. It may also be advantageous where the level of the body of liquid varies significantly, as it allows the bridge to be held in the correct position even when the level changes. A further advantage of adjusting the tension in the tether is that it allows the bridge to be protected when conditions at the surface could damage the bridge. For example, if the body of liquid becomes stormy, the bridge may be lowered, and then subsequently raised when conditions clear. The bridge may advantageously be hidden from view or concealed to a certain extent by being lowered, which may be useful in some situations. Preferably, the means fortightening the tether comprises a ratchet and / or a snatch block. Such means reduce the force required to tighten the tether. Advantageously, this makes it easier to tighten the tether. Preferably, the means for adjusting the tether comprises a motor, more preferably a motorised reel, most preferably wherein the motorised reel may be remote controlled / may be activated by a remote control. A motor allows the bridge to be lowered without requiring human exertion. This makes lowering the bridge easier. Activating the motor by remote control allows the bridge to be lowered from a different location. This may be useful when the location is dangerous, due to storms or similar events, because it allows human operators to remain in safety while still controlling the bridge. Preferably, a single remote control may activate all the motors of the bridge. This allows a single remote control to lower the entire bridge simultaneously. Advantageously, this may ensure that there is no damage due to shear forces arising from modules descending at different rates, as well as rendering lowering or raising of the bridge easier. In examples, the at least one bridge member is connected to a plurality of tethers, preferably wherein the at least one bridge member is connected to four tethers, more preferably wherein each tether is connected to a respective anchor. Having four tethers on a single bridge member allows the roll and tilt of the bridge member to be adjusted, to ensure that the bridge member surface is level. Additionally, having four tethers allows a single tether to be broken without the module being destroyed, as the remaining tethers may hold the bridge member in place. Advantageously, this improves the stability and resilience of the bridge. Preferably, a tether is attached to the at least one bridge member at each corner of the bridge member. In this context, “at each corner” implies at or near each corner rather than at the outermost point of the corner. For example, at a corner may include a tether being positioned at any point within a quarter of the bridge member comprising the corner. Having the tethers at each corner allows the orientation of the bridge member to be more finely adjusted. In examples, the at least one bridge member comprises at least one tapered edge, preferably wherein the tapered edge is along a transverse side of the bridge. Where the bridge crosses a body of water which flows, the tapered edge streamlines the bridge. Advantageously, this may improve stability of the bridge, by reducing the lateral forces on the bridge, and may reduce the visibility of the bridge, by reducing visible obstruction to the water flow. In examples, the tapered edge may be a separate component configured to be attached to the at least one bridge member. In examples, the at least one bridge member comprises at least one upwardly extending barrier (also referred to as a ‘rail’), preferably wherein the upwardly extending barrier is positioned along a transverse side of the bridge. When a vehicle traverses the bridge, the upwardly extending barrier may prevent the vehicle from driving over the edge of the bridge, or may provide a physical warning that a driver may feel so that the driver knows that they have reached an edge of the bridge. Advantageously, this may ensure that even when a bridge is not visible or is only partially visible from above, a driver may still safely traverse the bridge. In examples, the bridge member comprises features for connecting to at least one other bridge member. In examples, the bridge comprises a plurality of bridge members, preferably wherein the bridge members are connected by an interlocking mechanism. Providing a bridge as a plurality of members allows a bridge to be pre-fabricated in a modular manner, and then assembled at location. Modular assemblies are known to be highly efficient, as economies of scale occur when producing the same module many times. Further, modular assemblies are adaptable to fit a variety of bridge size requirements. Connecting the modules by an interlocking mechanism allows the bridge to be constructed quickly and easily. Pre-fabricated modules also reduce the construction resources that are needed at the location, rendering construction of the bridge at the bridge location more straightforward. An interlocking mechanism further simplifies construction. Preferably, a plurality of modules are positioned across the width of the bridge. This allows the same modular elements to be used to construct bridges of different widths. It also means that, if there is a malfunction of one module, the bridge may not be adversely affected. Advantageously, this improves the reliability of the bridge. Further, the modules may be made smaller without restricting the bridge width, which may make them easier to transport to the location. In examples, the bridge further comprises a flexible membrane. This may provide a suitable traction for vehicles traversing the bridge, improving safety of the bridge. It may also smooth the interface between adjacent platforms, where one platform is not at the same level as the next platform, or where there is a small gap between platforms. This may also improve safety of the bridge. In some examples, this flexible membrane may span a plurality of platforms, and may smooth the interface between adjacent platforms. In other examples, the flexible membrane may not span adjacent platforms. The flexible membrane may protect the platform from damage during use, increasing the lifespan of the bridge. The flexible membrane may also be replaceable, to allow the lifespan of the bridge to be further extended. The flexible membrane may be suitably reinforced or armoured. The flexible membrane may be configured to be submersed with the platform(s) to which the membrane is connected. Preferably, the flexible membrane itself may be buoyant, such that it floats / does not substantially sink during installation, dismounting, or any other scenario in which the flexible membrane is disconnected from the platform(s), which may serve to improve ease of use. A plurality of flexible membranes may be provided. Preferably, each of the plurality of flexible membranes have consistent (or the same) dimensions. A first subset of the plurality of flexible membranes may be connected to / engaged with one or more platforms (and so may be submerged with the platforms) and a second subset of the plurality of flexible membranes may be configured to float in use (although the second subset may be connected to the first subset, such that the second subset may be submerged as the first subset is submerged). Optionally, the second subset of flexible membranes is not directly connected to / engaged with one or more platforms. Certain of the flexible membrane(s) may include voids with may receive material, such as water, thereby to adjust the buoyancy of the flexible membrane. Optionally, the flexible membranes having voids also include or are connected to pumps thereby to allow the voids to be filled or emptied. A combination of flexible membranes with and without voids may be provided. In examples, the bridge further comprises an end portion for connecting the bridge to an edge of a body of liquid. In examples, the end portion comprises an L-shaped member for receiving material so as to provide a surface on top of the material for connecting the bridge and an edge of the body of liquid. In examples, the L-shaped member comprises spikes for resisting movement of the L-shaped member, preferably wherein the spikes are positioned along one arm. In examples, the L-shaped member comprises two parallel L-shapes connected by struts. Such L-shaped members allow a bridge end to be constructed using readily available materials, such as pipes, without disrupting the water flow, while maintaining a good level of stability. Preferably, the end portion comprises a plurality of pipes and / or a plurality of sacks of particulate matter. Where a body of liquid has a direction of flow, positioning the pipes parallel to the flow direction reduces disruption of the flow. This reduces stresses on the end portion and reduces visibility of the bridge. Sacks of particulate matter provide a stable surface for a vehicle to traverse, and conform easily to an irregular lower surface, resulting in a smoother upper surface which is easier for a vehicle to traverse. Optionally, the end portion may be used apart from other components of the bridge so as to allow a small body of liquid to be crossed. According to a third aspect disclosed herein, there is provided a method of installing an underwater / submersible bridge across a body of water, the method comprising: installing an anchor in a bed of a body of water, wherein the anchor is attached to a bridge member by at least one tether; and tightening the tether to lower the bridge member below the surface of the body of water. The method allows a bridge to be easily installed in a modular fashion. It is possible to pause installation and resume when conditions are favourable, and only a limited number of tasks need to be done on location. This improves the safety of bridge installation in adverse conditions relative to conventional methods. In examples, the method further comprises attaching the anchor to the bridge member by the at least one tether. Preferably, attaching the anchor to the bridge member by the at least one tether is done in shallow water. In examples, the method further comprises connecting a plurality of bridge members. Preferably, a row of bridge members are connected in shallow water, and are then moved to their position in the bridge, where they are connected to another row of bridge members. Preferably, the method further comprises forming a bridge end as described in relation to the second aspect of the present disclosure. Preferably, the method further comprises attaching at least one float to at least one bridge member, moving the at least one bridge member into position, and then removing the at least one float. Preferably, the method further comprises attaching a tapered edge portion to at least one bridge member. According to a fourth aspect disclosed herein, there is provided a remote controller configured for use with the bridge of any preceding example. Advantageously, this allows a bridge to be lowered or raised from a distance, allowing safe operation of the bridge even when there are adverse conditions at the bridge location. As used herein, the terms “platform” and “member” should be considered to be broadly interchangeable. A body of liquid may be a body of water, such as a river, canal, or lake. A tether may be a rope, cable, or chain. Any feature in one aspect of the disclosure may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, features implemented in hardware may be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly. Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure, such as a suitably programmed processor and associated memory. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the disclosure can be implemented and / or supplied and / or used independently. The disclosure extends to methods and / or apparatus substantially as herein described with reference to the accompanying drawings. The disclosure will now be described, by way of example, with reference to the accompanying drawings. Description of the Drawings Figure 1A shows a first example module of a bridge; Figure 1B shows the first example module of a bridge subject to a load; Figure 2A shows a second example module of a bridge; Figure 2B shows a screw anchor for use with the bridge of Figure 2A; Figure 2C shows a screw rod for use with the screw anchor of Figure 2B; Figure 3A shows an exploded view of a third example module of a bridge; Figure 3B shows a view of the third example module of a bridge with the lid removed from view; Figure 4A shows an example wiring setup for the module of Figures 3A and 3B; Figure 4B shows an example reel with a tether; Figure 4C shows an example reel and gear system; Figure 5A shows an example weight anchor with the lid closed; Figure 5B shows the weight anchor of Figure 5A with the lid open; Figure 5C shows a further example weight anchor; Figure 6 shows an example edge portion for a bridge; Figure 7 shows an example end portion for a bridge; Figure 8 shows an L-shaped member for use in constructing an end portion for a bridge; Figure 9 shows an example bridge in cross-section; and Figure 10 shows a perspective view of a row of modules of a bridge; Figure 11 shows an example method for constructing an end portion for a bridge, such as the end portion shown in Figure 7; Figure 12 shows an example method for adding a row of modules, such as the row of modules shown in Figure 10, to a bridge under construction; and Figure 13 shows a second example method for adding a row of modules to a bridge under construction. Description of the preferred embodiments Referring to Figure 1A, there is shown a module 100 of a bridge according to the present invention. The module comprises a platform (also referred to herein as a ‘bridge member’) 110, an anchor 120, and a tether 130 connecting the platform and the anchor. The platform is buoyant relative to the water, and may for example comprise a hollow structure. The platform is held below the surface of the water 140 by the tether 130 linking it to the anchor 120, which comprises a weight large enough to exceed the upthrust generated by submersion of the platform, such that the platform sits beneath the surface of the water. The anchor rests on a riverbed 150. In other examples, as will be described, more than one tether 130 and anchor 120 per platform 110 may be used. Figure 1B shows the module shown in Figure 1A bearing a load 160 (which it will be appreciated is exemplary - a load such as a car would not generally be used with a single module). The load 160 rests on the platform 110. The weight of the load 160 provides a downward force on the platform 110. When this downward force is lower than the upward force of the platform due to upthrust, the forces are balanced through the tension in the tether 130. This means that, when the weight of the load 160 is below a threshold, the platform does not move downwards significantly when the load 160 is moved onto the platform 110, because the forces on the platform 110 remain balanced. As the weight of the load 160 is increased, the tension in the tether 130 decreases, until, when the upthrust and weight are balanced, there is no tension in the tether 130. Such a weight is referred to herein as the maximum loading capacity of the platform. Many vehicles are capable of driving across submerged surfaces provided that the surface is not submerged below a threshold distance from the water surface. For example, some vehicles are capable of driving across a surface submerged by 1 m. This means that a bridge below the surface of a river may be suitable for allowing the passage of vehicles across a river that otherwise would not be traversable. Similarly, humans are able to wade through water as long as the depth does not exceed a threshold, and so may be capable of crossing an submersible bridge. Advantageously, unlike conventional pontoon bridges, a bridge made up of modules as shown in figures 1A and 1B may not sink in the water when a load moves across the bridge. In a pontoon bridge, the only forces on each platform are the upthrust from the water and the weight from a load. When a load is applied, the platform sinks until the upthrust has increased sufficiently to balance the load. This may make it challenging for traffic to cross the bridge, as a load passing across the bridge will always be at the lowest point on the bridge, and so must always move uphill. Such a pontoon bridge may also be unstable due to the motion when traffic moves across the bridge. Additionally, any connections between platforms of a pontoon bridge require a great deal of flexibility in order to cope with the extremes of relative motion between adjacent platforms. Advantageously, a submersible bridge may be invisible from above, or may be only partially visible from above, as the surface of the water obscures the line of sight. This means that an submersible bridge may be harder for an enemy to detect in a hostile situation. Similarly, a submersible bridge has increased thermal contact with a body of water traversed by the bridge compared to a similar bridge above water, due to all surfaces of the submersible bridge being in thermal contact with the body of water (when submersed). Additionally, infrared emissions of a submerged bridge are partially cloaked by absorptions and emissions of water above the bridge. Forthese reasons, submersible bridges may be harder for an enemy to detect using thermal cameras. Further, a submersible bridge may be partially shielded from airborne hazards by water covering the surface of the bridge. In particular, any falling objects (e.g. resulting from high winds) will transfer a proportion of their energy to the water, rather than the bridge. Further, shells that explode on impact may be triggered to explode on contact with the surface of the water rather than on contact with the bridge itself, meaning that a proportion of the energy of the shells is transmitted to the water rather than to the bridge. This may result in reduced damage to the bridge when subjected to airborne hazards. Referring to Figure 2A, there is shown a module of a bridge according to a second embodiment of the present invention. The module comprises a float 110, a screw anchor 220, a tether 130, and barriers 270. The screw anchor 220 may be installed in a bed of a body of water (a seabed, riverbed, etc). An exemplary tool for installation is shown in Figure 2C. Where a bridge is to be installed over a body of water with a relatively hard bed, a screw anchor may be a viable alternative to a weight anchor of the type shown in Figures 1A and 1B. Screw anchors are capable of resisting an upward force greater than their weight, so a screw anchor may be lighter than a weight anchor capable of providing the same downward force on a platform. This means that modules comprising screw anchors may be easier to transport. Advantageously, screw anchors are more resistive towards lateral forces than weight anchors (because they are screwed into the bed). This means that screw anchors may be able to hold a module of a bridge in place despite currents in a body of water creating lateral forces on the platforms. Further, screw anchors are less disruptive to marine life, as they cover a smaller surface area of the bed of a body of water. However, where a bridge is to be installed over a body of water with a relatively soft bed, a screw anchor may not be suitable, because the bed may not provide sufficient resistance towards upward motion of the screw anchor when subjected to an upwards force from the platform via the tether. For example, where a bridge is to be installed over a mud or silt riverbed, a weight anchor would be more appropriate than a screw anchor. In such a situation, a weight anchor may also need to have a large surface area in contact with the bed to prevent sinkage. Similarly, where a bridge is to be installed over a body of water with a very hard bed, such as a stone or concrete bed, or where a body of water is too deep, it may be impractical to embed screw anchors in the bed. In these examples, a weight anchor may be more appropriate. Weight anchors may also be more resistant to loosening over time, and installation of weight anchors creates less noise that the installation of screw anchors. Barriers 270 are provided on either side of platform 110. Where a vehicle such as a car passes across the bridge, the barriers on the side of each platform may guide the vehicle. The barriers may be square with the top surface of the platform 110, or may have a steep angle. Advantageously, this may reduce the likelihood of a vehicle falling off the bridge by preventing a vehicle from driving over the barrier and off the platform, and may also guide the vehicle to maintain the centre of mass of the load closer to the tether, reducing tilting of the platform. In an example, the platform comprises at least one tapered edge 280. When the tapered edge is positioned perpendicular to the flow of the body of water, this reduces the forces on the bridge, and also reduces the disturbance to water flow by the bridge, which reduces visibility of the submersible bridge. Referring to Figure 2B, there is shown a screw anchor 220 for use with the module of Figure 2A. The screw anchor 220 comprises handle 224, hinge 223, head 222, fixed disc 225, rotating disc 227, thread 221, and bar 226. The head 222 is at a first end of bar 226. The head extends a short distance from the bar. Rotating disc 227 is positioned around the bar 226 at the first end of the bar, and is attached to the base of the head 222, and is able to rotate around the axis of the bar. The handle 224 is attached to the rotating disc 227 via hinge 223. Thread 221 is located around the bar 226, and extends from a second end of the bar most of the way along the bar. Fixed disc 225 is attached to the bar 226 below the rotating disc 227. In use, a cable or tether may be attached to handle 224. The screw anchor 220 may be lowered towards a bed of a body of water. A tool, such as illustrated in Figure 2C, may then grip the head 222 by engaging with the faceted shape of the head. Rotation of the head 222 causes the thread 221 to dig into the ground. The screw anchor 220 may be rotated until all or part of the thread is below the ground of the bed of the body of water. The thread provides resistance to upwards motion of the anchor. When the screw anchor has been fully screwed into the bed, the disc 225 impacts the bed, preventing the screw anchor from being further screwed into the bed. This reduces the likelihood of earth being introduced into screwdriver cavities. Advantageously, this improves installation and removal of the screw anchors. Advantageously, the hinges on the handle 224 may allow the handle to be moved so that the head 222 may be accessed axially, and then the handle may be moved back into an axial position. This means that a tether may be attached to the handle before installation of the screw anchor, without the tether obstructing axial access to the head, while ensuring the tether is still positioned axially with respect to the screw anchor when tension is applied to the tether. This ensures that forces are directed axially on the screw anchor during use, reducing the risk of damage to the screw anchor and improving the ease of installation. Advantageously, the rotation of the rotating disc 227 means that a tether attached to the handle 224 before installation of the screw anchor 220 does not become twisted during installation. The hinge 223 may be a single hinge extending through the disc 225. Alternatively, the hinge may comprise two pivot points, each provided at an end of the handle 224. In examples, the head may be hexagonal. Advantageously, such shapes allow a good grip to be achieved by a tool. In other examples, the head may have other shapes, such as triangular, square, and octagonal. In further examples, the head may comprise an indent for engaging with a tool. In examples, the disc 225 may be the same width as the thread 221. This may assist in positioning of the screw anchor. For example, in embodiments the screw anchor 220 may fit through an access hole in a module for a bridge, which may be sized to accommodate the screw anchor 220. Engagement of the disc 225 with walls of the access hole may help to position and / or angle the screw anchor as desired. Referring to Figure 2C, there is shown a tool 230 for use with a screw anchor, such as the screw anchor 220 of Figure 2B. The tool comprises body 228 and tool head 229. The tool head 229 comprises an aperture (not shown), which is shaped to receive the head 222 of the screw anchor 220. The body 228 has the same crosssection as the head 222 of the screw anchor 220. In use, the tool head 229 fits around and engages the head 222 of the screw anchor 220. Rotation of the tool head 229 therefore rotates the screw anchor 220, causing the screw anchor to be screwed into the bed of the body of water. As the screw anchor 220 is screwed into the ground, it moves lower, and so the tool 230 may no longer be easily rotated by a user at the surface of the body of water. A second tool 230 may then be placed over the tool 230, so that the tool head 229 of the second tool engages with the body 228 of the first tool. Additional tools may be used as needed until the screw anchor is fixed in location. In examples, the tool 230 may be magnetic. Advantageously, this may allow easy retrieval of multiple tools, where multiple tools have been used during installation. In examples, the tool may comprise a handle. This may provide a better grip for a user. In examples, the tool may be configured to be rotatable by a machine. In examples, the tool may be configured to be rotated by a motor within the module of the bridge, such as the motors 316 of module 300 shown in Figure 3A. In examples, the tool may comprise a ratchet. When the head 222 comprises an indent, the tool head 229 may comprise a protrusion that engages with the indent, and the body 228 may comprise a similar indent at its tip. Referring to Figure 3A, there is shown an exploded diagram of a third example module 300 of a bridge. The third example module 300 of a bridge comprises a platform 310, reels 312, reel cavities 314, motors 316, battery 318, circuitry 320, electrics cavity 322, tethers 324, reel cavity lids 326, electrics cavity lid 328, and weight anchors 330. Each of the tethers 324 is attached at a first end to a respective weight anchor 330, and at a second end to a respective reel 312. Two reels 312 fit within each reel cavity 314, each reel cavity extending between a top and bottom surface of the platform 310. The reel cavity lids 326 each close a respective reel cavity. Each reel 312 is connected to a respective motor 316. This may be a direct connection or connection via a gear system as shown in Figure 4B. Circuitry 320 connects each motor 316 to the battery 318. The battery 318 and motors 316 fit within electrics cavity 322. Electrics cavity 322 may be sealed by electrics cavity lid 328. The platform 310 is generally shaped as a cuboid or rectangular prism, where the reels 312 (and so the weight anchors 330 and tethers 324) are located generally towards the corners of the platform. This may assist in stably locating the platform. The platform 310 is buoyant, for example by being hollow (apart from the described cavities), such that it may operate in the manner described with reference to the platform 310 of Fig. 1A and 1B. In the alternative, the platform 310 may be connected to a separate buoyant component which provides the necessary buoyancy. Figure 3B shows this third example module 300 of a bridge with the components in their respective cavities, and with the reel cavity lids 326, electrics cavity lid 328, weight anchors 330, and tethers 324 removed from view. In use, the weight anchors 330 are installed on a bed of a body of water. For example, the weight anchors 330 may be installed on a riverbed. The battery 318 powers the motors 316. When the motors are supplied with power by the battery 318, they each rotate a respective reel 312, which tightens or loosens a respective tether 324. The electrics cavity lid 328 seals the electrics cavity from water, protecting the circuitry 320, motors 316 and battery 318 from water damage when the body 310 is submerged. Advantageously, providing a battery within the module may reduce the manual labour required to install a module of the bridge. The motors may be activated and deactivated by a remote control (not shown). This may increase the ease of installation, and may allow a plurality of platforms to be raised or lowered simultaneously. A plurality of platforms may be raised or lowered in response to events such as a change in water level of the body of water. This may allow a bridge formed from a plurality of such platforms to remain at a level suitable for use even during times of flooding. The platforms may also be lowered for protection from storm damage, as the currents may be lower near the bed of the body of water. Lowering the platforms may also be serve to temporarily render the bridge impassable. Advantageously, providing four weight anchors may reduce the minimum weight and size requirements for each anchor, which may render the weight anchors easier to transport to the body of water. Additionally, the provision of four weight anchors may improve stability where the bed of the body of water is uneven. The provision of four tethers may increase stability against currents, and may allow the angle of a platform to be adjusted to ensure that the surface is level, whatever the weight distribution on the platform, or gradient of the bed of the body of water. It will be appreciated that fewer or more than four weight anchors could alternatively be used (with a suitably adapted platform). The described arrangement (of multiple weight anchors connected to a platform by tensioned tethers) may be referred to as a ‘tension-leg’ arrangement, Although Figure 3A shows the platform being used with weight anchors, the platform could also be used with screw anchors. As the reel cavity 314 extends the full depth of the platform, when the reel cavity lids 326 are removed, access is provided through the platform to the bed of the body of water. This means that screw anchors may be installed through the reel cavities 314. In some examples, the reels 312 may be removed during installation of the screw anchors. In other examples, the reels 312 may not be removed during installation of the screw anchors. Although in embodiments, the reel lids 326 may be inserted before lowering the platforms, in other embodiments the reel lids 326 may be inserted after lowering the platforms. This may be preferable where the cables are manually tightened. Although Figures 3A and 3B show a single battery and four motors, any number of batteries and motors may also be provided. For example, each motor may be powered by an individual battery. The number of motors may for example be two. Each motor may turn more than one reel. Similarly, each reel may wind more than one tether. For example, two tethers may be attached to a single reel. In examples, the electrics cavity lid 328 may seal the electrics cavity 322 to give waterproofing of the electrics cavity 322 to the IP68 standard. The electrics cavity lid 328 may be opened for repair and maintenance purposes, and may be kept closed during installation and dismantling of the bridge. Alternatively and / or additionally, the motors 316 may be waterproof, preferably to the IP68 standard. In an alternative, the reel lids 326 may seal the reel cavities 314 to waterproof the reel cavities. This may protect the reels 312 from damage due to corrosion. Alternatively and / or additionally, the reel lids 326 may not seal the reel cavities 314. Instead, the reel lids 326 may be closed with a quick-release system. Advantageously, this may improve the ease of installation and / or assembly. In examples, the reel lids 326 may comprise a hole, meaning that even when the reel lids 326 are fixed over the reel cavities 314, there is a vertical channel allowing the passage of water between the top surface and bottom surface of the platform. Advantageously, this may render the platform easier to raise and lower when in use. In examples, at least one edge of the platform may comprise a mechanism for interlocking with another element, such as another platform or an edge piece as shown in Figure 6. In an example, the volume of the platform may be around 8 m3, and it may weigh approximately 2,250 kg. The maximum loading capacity of the platform will be the difference between the weight of the platform and the weight of an equivalent volume of water, which in this example would be roughly 5,500 kg. Figure 4A shows an example wiring setup for the module 300 of Figures 3A and 3B. The setup comprises motors 316, battery 318, circuitry 320, and power cable 340. The circuitry 320 connects the battery 318 and each motor 316. To reduce the footprint of the electrics cavity, the circuitry 320 extends from one side of the battery 318 only. This allows the remaining sides of the battery to be closed. The power cable 340 allows the circuitry to be connected between platforms, and / or for the circuitry to be connected to an external power source. Connecting the circuitry of multiple platforms allows power to be passed between platforms if needed. Connecting the circuitry to an external power source allows the batteries to be recharged if needed. In examples, the motors may be configured so that, when the platform is allowed to rise, electricity is generated. This may improve the energy efficiency of the module. In this regard, the platform may comprise one or more suitable turbine(s), being rotated by the relative movement of the platform and the surrounding water as the platform rises, and a generator powered by the turbine(s), the generator being arranged to supply power to the battery. Alternatively and / or additionally, turbines may be provided that are rotated by the water flow of the body of water. These turbines may power a generator to supply power to the battery. In an example, the battery capacity is 1 KWh. When the platforms have a maximum loading capacity of approximately 5,500 kg, such a battery is capable of submerging a platform to 1 m approximately 70 times. This allows the bridge to be reused over an extended period without the batteries requiring recharging. In an example, the battery may be removed and replaced when it has run out of charge or is approaching running out of charge. In an example, the motors 316 may each operate at 1 kW. With 4 motors operating at 1 kW, a 5,500 kg platform may be submerged by 1 m in about 12 seconds. Preferably, the circuitry comprises a control unit for controlling the motors. Alternatively, the circuitry may simply comprise an on / off switch, which may be manually operated. In examples, the circuitry comprises additional connections to an external power source. In such examples, the control unit may be controlled at least in part by power line communication. A remote controller (not shown) may be configured for communication with the control unit, where the control unit comprises suitable communication means for communicating with the remote controller. The remote controller may be able to activate the motors to raise or lower the platform. The remote controller may communicate with the control unit using any suitable communication method known in the art, such as infrared or radio. In examples, the remote controller may be a computing device such as a mobile phone (in which case the control unit may include a suitable GSM™ (Global System for Mobile Communications) standard communication systems). Software may be installed on the mobile phone in order to allow communication between the mobile phone and the control unit. Figure 4B shows an example reel 312 comprising end plate 401, reel shaft 402, and reel gear 403, and a gear system. The gear system comprises chain 404, shaft gear 405, and motor shaft 406. The end plate 401 and reel gear 403 are attached to the reel shaft 402 at opposite ends of the shaft to each other. The shaft gear is mounted at the end of the motor shaft 406. The chain 404 is held under tension between the shaft gear 405 and reel gear 403. The motor shaft 406 is connected to the motor (not shown), or to suitable gearing being powered by the motor. Figure 4C shows an example reel 312 with a tether 324 wound around it. The tether 324 comprises an end 325, and is wound around the reel shaft 402 (visible in Figure 4B). In use, the motor shaft 406 may be rotated by a motor in order to tighten or loosen the tether 324 by rotating the reel. Rotating the motor shaft 406 rotates shaft gear 405. Shaft gear 405 is connected to reel gear 403 by chain 404, which engages with the teeth of both the shaft gear 405 and the reel gear 403. Rotation of the shaft gear 405 therefore rotates the reel gear 403. This causes the reel 312 to rotate, drawing in or extending the tether 324, depending on the direction of rotation. Advantageously, the gear system may reduce the torsional force on the motor, improving reliability of the system. In examples, the reel gear 403 may have five times as many teeth as the shaft gear 405. This may reduce the torsional force required by the motor without requiring an excessive length of time for the reel to be wound. In an example embodiment, the reel shaft 402 may be 40 mm in diameter, and 400 mm in length. The end plate 401 may be 300 mm in diameter. There may be 315 mm between the end plate 401 and the reel gear 403. The tether may be 20 mm in diameter, and so may fit in 15 adjacent rolls around the reel shaft 402. Each roll may have the tether circuiting the reel 6 times, corresponding to a total of roughly 50 m of tether wound around the reel 312. In examples, the reel gear 403 may be attached to a second end plate 401. In examples, the end of the tether end 325 may comprise a clip, for example a carabiner clip. In other examples, the end of the tether end 325 may comprise a loop, and may be connected using a tie, clip, or equivalent means to an anchor. In other examples, instead of a chain, the gears may directly interlock, or may be connected by means of a belt, a plurality of gears, or any equivalent system as known in the art. Figure 5A shows a weight anchor 400 for use in a submerged bridge. The weight anchor comprises a base 410, outer walls 420, lid flaps 430, lid handles 440, anchor handles 450, resealable outlets 460 comprising a plug with handles 465, feet 470, and central strut 480. The outer walls 420 extend upwards from the edges of base 410. The lid flaps 430 are each attached to the central strut 480 by a respective hinge (not shown). The lid flaps 430 are movable such that when in a closed position, the lid flaps 430, walls 420, and base 410 define a closed volume. The central strut 480 comprises resealable outlets 460. The feet 470 protrude from below the base 410, and are generally shaped as square frustrums. The feet provide stability and reduce sinkage of the anchor into a bed of a body of water. Figure 5B shows the weight anchor 400 with the lid flaps 430 in an open position. Inside the weight anchor is material 490. When the weight anchor 400 is empty, it may have a relatively low weight. Advantageously, in the empty state the weight anchors are light and so easier to transport. At or near the location at which a bridge is to be installed, the weight anchors may be filled with material 490, which preferably has a high density, such as scrap metal, sand, or gravel. Once a required mass for the weight anchor has been achieved, any remaining space may be left empty and the anchor may be sealed by closing the lid flaps. In an example, a platform of a bridge may be attached to two anchors. If the platform has a volume of 7.8 m3, and a mass of 2250 kg, then the platform will require approximately 5550 kg of additional weight to submerge it fully, because this is the difference between the mass of water displaced by the platform and the mass of the platform. If steel is used to fill the weight anchors, then a volume of steel is required that is sufficient to provide this additional weight, bearing in mind that the steel will also displace a volume of water. The density of steel is approximately 7850 kg / m3, but given that the steel is displacing a volume of water, it has an effective density lower than this of 6850 kg / m3. Therefore, to achieve the 5550 kg of additional weight to submerge the platforms requires roughly 0.8 m3 of steel, which split between two anchors results in 0.4 m3 of volume capacity being required by each. Of course, scrap metal may be irregularly shaped, and so may not effectively pack within a weight anchor. Additionally, preferably the weight anchors (when loaded) should be heavier than the minimum weight needed to submerge the platforms, in order to ensure good stability. More preferably, the weight anchors (when loaded) should be around 25% heavier than the minimum weight needed to submerge the platforms. Therefore, it is desirable for the weight anchors to be larger than 0.4 m3, for examples 0.5 m3 or even 1 m3, so as to allow for irregularly shaped scrap metal to be effectively used as the filing material. Example dimensions for such a weight anchor could be 1.8 m in length and width, and 0.15 m in height, or 1.8 m in length and width and 0.3 m in height. It is desirable for the base of the weight anchor to be wide, in order to spread the weight, and for the height to be low, to reduce lateral forces on the weight anchor. It is also desirable for the weight anchor to be slightly larger than needed to ensure that the weight anchor is stable on the bed of the body of water. Higher density materials will reduce the size required for the weight anchor. Such materials may also be easier to transport, since they will be more compact. Additionally, a smaller weight anchor may, when empty of scrap metal and / or water, weigh less than the maximum loading capacity of a platform. This means that the weight anchor may be supported by the platform when moving the platform to the desired location, which simplifies the installation process. At the desired location, further scrap metal may be added to the weight anchors, and / or the resealable outlets may be unsealed in order to allow water into the weight anchor, which may result in the weight anchor exceeding the maximum loading capacity of the platform, and so submerging the platform. The resealable outlets 460 allow air to be retained within the weight anchor and then released when the weight anchor is in location. The outlet handles 465 allow the plugs of the resealable outlets 460 to be removed when the anchor is at location. The resealable outlets 460 are located on an upper region of the weight anchor to make sure air is not trapped in the container when the resealable outlets 460 are opened to fill the container with water. The resealable outlets 460 may be circular in shape. The plugs of the resealable outlets may be attached to the outlets by magnetism, so that applying an upwards force to the handles is sufficient to remove the plugs from the outlets. Advantageously, this may allow the plugs to be removed from above the water, by means of a cable or equivalent. In use, the weight anchor 400 may be attached to a platform (such as the platform shown in Figures 3A and 3B) via a tether attached to one or both of the anchor handles. The weight anchor 400 can then hold the platform (or at least part of the platform) below the surface of a body of water. In some examples, the weight anchor 400 may be designed to be transportable to a bridge location during installation by placing the weight anchor adjacent or beneath a platform. As the weight anchor is sealed with some air retained, its density is lower and so its effective weight may be lower than that required to submerge the platform. The platform may then be moved to the desired location with the weight anchor suspended by the platform, beneath the surface of the water. When at the desired location, the removable outlets may be opened. This allows water to enter the weight anchor and increases the effective weight of the weight anchor above the threshold required to submerge the platform. This allows the platform to be submerged. Example dimensions of such a weight anchor are 1.8 m length and width, and 0.5 m height. Alternatively, the weight anchor may be transported to the desired location on the platform. For example, the weight anchor may be transported empty, and may be filled with scrap metal or other weights at the desired location. In another example, fewer weight anchors may be transported on the platform than are required to submerge the platform, and an additional one or more weight anchors may be added to the platform at the desired location. While in the example above, the resealable outlets are sealed by magnetic plugs, in other examples, the resealable outlets may be sealed by a cork or other sealing mechanism. During installation, tethers such as ropes or cables may be attached to the weight anchor handles and / or the outlet handles. A distal end of each cable may be attached to a float (not pictured) for ease of access. Alternatively, a distal end of each cable may be attached to the platform. Advantageously, when the outlet plugs are secured by a mechanism such as a magnetic attraction, or by an elastomeric material such as a cork, attaching the plug to a cable secured to the platform may allow the outlet plugs to be removed automatically on submerging the weight anchor, simplifying the installation process. In examples, the feet may have any other suitable shape to reduce motion on the bed of the body of water. For examples, the feet may instead comprise a ridged surface to provide a grip on the bed of the body of water. In examples, the feet may be detachable. Advantageously, this may allow feet to be chosen on the basis of the precise conditions of the bed of the body of water, such as stony, muddy, or uneven. In examples, instead of filling the weight anchor with scrap metal 490, other weights may be used. Such weights may be shaped in order to fit compactly within a container, allowing the weight anchor to be designed with a smaller volume. In an example, 4 tons of scrap metal may be added to the weight anchor for use. In examples, additional water outlets may be provided lower on the weight anchors, such as on the bottom or sides of the anchor. These may allow water to drain from the anchor, which may be useful if the anchor is to be removed and reused elsewhere. Figure 5C shows a second embodiment of a weight anchor 500, comprising handles 450, feet 470, and body 510. The body 510 may be solid. The weight anchor 500 is squat relative to the weight anchor 400, and does not include a hollow portion. Instead, the weight anchor 400 is formed of solid material. As with the weight anchor 400, the feet 470 reduce lateral motion of the anchor on the bed of the body of water, and the handles 450 allow at least one tether to be attached to the weight anchor. Since the weight anchor 500 is solid, it may have a much lower volume than the weight anchor 400, while still having the same capacity to submerge a platform. In an example, two such anchors may be provided to submerge a single platform, and each anchor may have a volume of 0.5 m3 This provides some spare capacity and ensures that the platform will be submerged, but without requiring an excessive volume of metal. The base of the weight anchor may be wide. Advantageously, this spreads the weight and so reduces the pressure on the bed of the body of water. The dimensions may be chosen to avoid weight anchors overlapping when in position under a platform. For example, the base may be 1.8 m by 1.8 m, and the height may be 0.15 m. The weight anchors 400, 500 may be made of steel. Figure 6 shows a bridge edge piece 600. This may be attached to a platform as shown in Figures 3A and 3B. The bridge edge piece 600 comprises a tapered edge 602, a barrier 604, and connection formation 606 (it will be appreciated that the connection formation is shown schematically). The connection points 606 are on at least an opposite side to the tapered edge 602. In use, the bridge edge piece 600 is connected to a side of a platform using the connection formation 606. For example, the connection formation 606 may be reversibly connected to a corresponding formation on the side of the platform. The platform may have corresponding formations on all four sides, to allow the platform to be used with the bridge end piece 600 in any orientation. The barrier 604 then extends (a short distance) above the top of the platform, similarly to the barrier 270 of Figure 2A. When the platform is positioned along a side of a bridge, the tapered edge 602 then streamlines the platform. This reduces stresses on the platform, and may also reduce visibility of the bridge by reducing visible disruption to water flow around the bridge. The barrier 604 may guide vehicles to ensure they do not drive off the bridge. In embodiments, further elements may be added to the barrier 604, such as rubber elements protruding inwards, which may warn vehicles as they approach the bridge edge piece 600. Figure 7 shows an example bridge end for connecting a bank to a body of a modular submersible bridge comprising a module 100. The bridge end comprises a plank section 710 and a support section 720. The plank section 710 and a support section 720 may extend from the end of a bank to a platform 100, which, when in situ, is generally located in relatively deep water. In most natural bodies of water, the water near the bank is shallow relative the water further from the bank. In a river, the bed of the river is generally sloped or curved such that the water gradually gets deeper further from the bank (where, generally, the middle of the bank has a fairly constant depth). This may make it beneficial to provide specialised structures for connecting the bank and the main part of the bridge (being formed of platforms 100). The support section 720 comprises an L-shaped member 722, large pipes 734, small pipes 736, soft layer 742, and hard layer 744. The L-shaped member 732 comprises spikes 724 arranged along at least one arm (preferably along an outer portion of the longer arm of the L-shaped member - that is, the spikes extend away from the member perpendicular to the extension of the shorter arm from the longer arm of the member). The plank section 710 spans a portion of the bank where the water is shallow. The plank section may comprise multiple planks arranged end-to-end, and / or side-to-side. The number of planks end-to-end may be determined by the steepness of the bank, as the plank section may only be used where the water is relatively shallow, so that a vehicle traversing the plank surface is not submerged too far below the surface of the water. The number of planks side-to-side may be chosen based on the predicted width of a load. The L-shaped member 722 forms the base of the support section 720. The L-shaped member comprises spikes 724 along a first arm of the L-shaped member, which runs along the bottom surface of the bank. The spikes dig into the bottom surface of the bank and reduce the likelihood of the support section sliding further into the body of water. Pipes 734 and 736 lie across the L-shaped member, and are held from rolling further down into the body of water by a second arm of the L-shaped member, extending upwardly from a lowest point of the L-shaped member. The soft layer 742 spans the upper surface of the pipes across the L-shaped member. The soft layer 742 may comprise sacks of sand and / or gravel. The hard layer 744 is positioned directly above the soft layer, such that the soft layer supports the hard layer. The hard layer thereby forms a rigid path across the support section. To construct the support section, first the L-shaped member is positioned on the bottom surface of the bank, with the spikes pointing into the bank, and with the corner of the L positioned at its deepest point in the body of water, such that the second arm of the L-shaped member points upwards from this point. Next, the large pipe 734 are laid across the L-shaped member. Small pipes 736 are then laid above the large pipes to partially fill the remaining gaps. The bridge end allows a vehicle to travel from an edge of a body of water onto a platform of a module 100 of a floating submersible bridge. The bridge end could also be used on its own to cross a small body of water, such as a canal. The plank 710 provides a stable surface for a vehicle to traverse where the water is shallow. Where the water is deeper, the support section provides a basis to support the vehicle. The pipes are prevented from rolling further into the body of water by an upwardly extending arm of the L-shaped member 722. The L-shaped member 722 comprises spikes 724 to grip the bed of the body of water. Advantageously, the pipes cause minimal disruption to water flow because water is able to flow through the pipes. To construct the bridge end, first the L-shaped member may be placed on the bed of the body of water near the bank. Large pipes 734 may then be laid down parallel to water flow. Small pipes 736 may then be laid down above the large pipes 734. These may fill in any gaps left between pipes and / or the L-shaped member. On top of these, first a soft layer 742 is laid down to further smooth the surface of the bridge end. Above this, a hard layer 744 is laid down to provide a suitable surface for a vehicle to traverse. Finally, a plank 710 may be laid down on the bank. As will be appreciated, in use suitable alternative materials may be used in place of the large pipes 734, small pipes 736 and soft layer 742, depending on what materials is available in the area in which the bridge is being set up. Advantageously, the bridge end may be installed using common materials that are relatively easy to obtain. Using hollow pipes meansthatthe bridge end provides minimal obstruction if the body of water has a direction of flow perpendicular to the bridge. In examples, the pipes may be tied together using a rope or cable. Advantageously, this may reduce motion of the pipes when subject to lateral forces due to a load. In examples, instead of a single diameter of small pipes and a single diameter of large pipes, a plurality of pipes of differing diameters may be provided. In such examples, the pipes may be placed on the L-shaped members starting from the largest diameter pipes, and moving progressively smaller. In some such examples, a single layer of pipes may be used, and the diameters of the pipes may be chosen such that the single layer provides a substantially horizontal upper surface. The pipes may be specialised components produced for use with the L-shaped member 722, or may be off-the-shelf components (optionally being suitably adapted such as by being cut to size). In examples, instead of a soft and a hard layer being provided, a single flexible layer may be provided. For example, such a flexible layer may comprise caterpillar tracks (or a similar material). Such a layer may be capable of conforming with an irregular surface while still providing a suitable surface for a vehicle to traverse. Figure 8 shows an example L-shaped member 800 for use in constructing a bridge end, such as the bridge end shown in Figure 7. The L-shaped member comprises two side portions 802, 804, connected by struts 806. The struts 806 connect the side portions 802, 804 to create astable base. Advantageously, this may assisting in maintaining the L-shaped portion in the desired position. A plurality of such L-shaped members 800 may be used to construct a bridge end. In some examples, the L-shaped member may form an angle of 90 degrees between the upward facing arms and the base (i.e. between the shorter arm and the longer arm). In other examples, the angle may be smaller or larger than 90 degrees. Angles smaller than 90 degrees may be more suitable for use on steeper surfaces. In some examples, the angle between the arms may be adjustable. The L-shaped member shown in this example does not comprise spikes. In some environments, spikes on a base of the L-shape may be advantageous in order to grip the bed of the body of water. However, in other environments, this may not be necessary, as the weight of the components placed on the L-shaped member will improve grip of the bed of the body of water. In examples, the L-shaped member may be constructed from any number of parallel L-shapes. Although in the figure, only two are shown, in other embodiments three or more L-shapes may be connected by struts to form the L-shaped member. Figure 9 shows a view of a modular bridge in cross-section, cut along the length of the bridge. The modular bridge comprises L-shaped members 800 at either end of the bridge, pipes 734 on the L-shaped members 800, weight anchors 500, and platforms 300, and tethers 324 connecting the platforms 300 to the anchors 400. The bridge may be partially or fully submerged below the surface of a body of water. Advantageously, the tension in the tethers allows a weight to be applied to the platforms while reducing or even eliminating a displacement of the platforms, when the weight is below a threshold. This occurs because the weight causes the tension in the tethers to decrease. This renders the bridge much more stable than conventional pontoon bridges. A vehicle or a pedestrian may cross the bridge. When the bridge configured for pedestrian use, the bridge may be only partially submerged, or may be fully submerged but by less than a certain threshold. For example, the bridge may be submerged by less than 5 cm. When the bridge is configured for use for vehicles, the bridge may be submerged further. For example, when the bridge is used for cars, it may be submerged by less than 10 cm. For other vehicles, the bridge may be submerged by up to 1 m. It will be appreciated that the depth of submersion of the bridge is set at the time that the bridge is set up. Where the bridge is submerged and a vehicle or pedestrian crosses, such crossing may be described as ‘fording’. Figure 10 shows a perspective view of a row 1000 of modules spanning the width of a bridge. The row 1000 comprises platforms 300, bridge edge pieces 600, weight anchors 500, and tethers 324. The platforms 300 are positioned in a row, and are each attached to the neighbouring platforms. For example, a first side of a platform may comprise an extending digit, and a second side of a platform may comprise a recess, sized such that a digit of a first platform may fit within a recess of a second platform. Advantageously, this may increase the stability of the bridge when a single platform is subjected to a load. The outermost edges of outermost platforms may interlock with an innermost edge of the edge pieces 600. Similarly, the row 1000 may interlock with further rows 1000 to form the modular bridge. In other examples, other locking systems known in the art may be used. The platforms are attached to weight anchors 500 via tethers 324. Although in Figure 10 the other platforms (adjacent the bridge ends) are shown as not being attached to weight anchors, it will be appreciated that weight anchors could also be attached to these outer platforms. In particular, during installation the weight anchors may be attached starting from the innermost platforms and moving outwards, such that Figure 10 might be considered to show a late stage of installation (just before the final weight anchors are installed). When weight anchors are not attached to the outer platforms, these platforms act as floats and so may assist movement of the row to the desired location. These platforms may then be removed when at location, as is discussed in relation to Figure 12. Advantageously, forming a single row of the bridge from a plurality of platforms may mean that the same component parts may be used to form bridges of different widths. This means that a suitable width for the bridge may be chosen, and the parts may be installed as desired. Additionally, the use of a plurality of platforms may ensure that no single point of failure exists in the bridge, because the attachment between the platforms may ensure that they move together, compensating for any failure in the electronics for a single platform, or any damage to the tethers. In examples, the bridge may support a flexible membrane. This may improve traction on the bridge. Figure 11 shows an example method 1100 for building a bridge end, such as the bridge end shown in Figure 7. At a first step 1102, L-shaped members are placed into position. They are positioned in shallow water near a bank of a body of water, with a longer arm of the ‘L’ lying along the ground perpendicular to the desired direction of the bridge, and a shorter arm of the ‘L’ extending upwards through the water. A plurality of L-shaped members are placed parallel to each other in this position. At a second step 1104, pipes are placed on the L-shaped members. The pipes may be placed in order of diameter, with larger pipes placed on the L-shaped members before smaller pipes. The pipes are placed on the L-shaped members, so that the pipes are orthogonal to both the arm lying along the ground and the shorter arm extending upwards through the water. At a third step 1106, sacks of particulate matter are placed on top of the pipes, covering partially or fully the upper surface of the pipes. The particulate matter may be, for example, sand or gravel. At a fourth step 1108, a layer is placed on top of the sacks. This layer may be a flexible track, and may protect the sacks from wear, and provide a suitable surface for vehicles to traverse. Figure 12 shows an example method 1200 for adding a row of platforms with weight anchors to a bridge during construction of the bridge. At a first step 1202, the platforms are assembled into a row of platforms. This involves attaching the platforms together, side by side. At this point, the tethers may be retracted into the platforms by fully winding the tethers around the reels. Alternatively, the tethers may be loose and may hang below the platforms, or the tethers may be attached to each other across the top surface of the platform. For example, opposite tethers on each platform may be attached to each other. At a second step 1204, the weight anchors are lined up in shallow water, spaced as they are intended to be spaced in the row of the bridge. For example, the weight anchors may be lined up in two rows of anchors, so that each platform in the row of platforms may be attached to two anchors. For example, if the platforms shown in Figures 3A and 3B are used, and are connected along their longer sides, then two rows of weight anchors will need to be lined up to anchor a single row of platforms. If the platforms shown in Figures 3A and 3B are instead connected along their shorter sides, then only a single row of weight anchors, will be needed, provided that the weight anchors are similar to those in Figures 5A and 5C in having two handles, and provided that the weight anchors are positioned so that a handle lies under each reel of the platforms. For the weight anchors shown in Figures 5A and 5C, this positioning comprises a 90 degree rotation of each weight anchor compared to the required positions of the weight anchors when the platforms are connected along their longer sides. At a third, optional, step 1206, floats are attached to the platform row. These may be attached at the ends of the row. Alternatively and / or additionally, the floats may be attached along a front and / or a back of the row. The floats may be additional platforms, or an additional components. This step may be performed before or after the second step 1204. At a fourth step 1208, the platform row is moved over the weight anchors. Since the weight anchors are positioned in shallow water, it may be easier to move the platform row into position over the weight anchors than it would be outside the water, as the water reduces the effective weight of the platform row. At a fifth step 1210, each of the tethers of the platforms are attached to a respective anchor. In examples, one anchor may be attached to two tethers. At a sixth step 1212, the row is moved to the desired position. Since the platform row is supported by floats, the platform row is light enough to support the weight anchors. The weight anchors therefore hang suspended from the platform row by the tethers as the row is moved through the water to the desired position. At a seventh step 1214, the floats are removed. When the floats are removed, the platform row is no longer light enough to support the weight anchors. The platform row therefore sinks until the weight anchors are supported by the bed of the body of water. Alternatively, at this step, the weight anchors may be lowered to the seabed by loosening the tethers prior to the floats being removed, and then the tethers may be tightened to submerge the platforms. If the floats were attached to the ends of the row, optionally bridge edge pieces may be attached after removal of the floats. Alternatively, the bridge edge pieces may be attached between the platforms in the row during building of the row of platforms in step 1202, and the floats may be attached across the bridge edge pieces. At an eighth step 1216, the row of platforms is attached to a previous row of platforms in the bridge. These method steps may be repeated as many times as required until the bridge comprises enough rows to span a body of water. Advantageously, this method may not require use of electronic components in order to submerge the bridge. However, in examples an additional step may be required in which the tethers may be tightened or loosened in order to match the height of the platform row with the previous row of the bridge. In examples, the floats may not be removed until a plurality of rows are in position, and the step 1216 of attaching a row to a previous row may be done before removal of the floats for at least some rows of the bridge. This allows the attaching step to be done at the surface of the water, which may be easier than carrying out the process underwater. Alternatively, submerging each row before attachment to a previous row may be done where conditions at the surface are not good, and so there is a risk of damage to the bridge. In such scenarios, assembly of the row in steps 1212-1216 may be done at a distance from the bridge site. Preferably, where the body of water has a direction of flow, the assembly of the row is done upstream of the bridge site. In examples, the floats may comprise a plurality of additional platforms at each end of the row. In alternative embodiments, floats may be attached directly to the weight anchors, or no floats may be used, and instead boats may be used to carry the anchors into position. Figure 13 shows an alternative method for adding a row of modules to a bridge under construction. In a first step 1302, a row of platforms is built. This may comprise attaching platforms side by side, and attaching edge portions (such as the edge portion shown in Figure 6) to the outermost platforms in the row. In a second step 1304, the anchors are positioned on the platforms. The anchors may be weight anchors or screw anchors. Where the anchors are (hollow) weight anchors, preferably they are not filled and so may be supported in the water by the platforms. In a third step 1306, tethers are attached to the anchors. Tethers may be attached used a clip element, an element integral to the tether and / or anchor, or by tying the tether to the anchor. In a fourth step 1308, the row is moved to a desired position. This is preferably an incomplete edge of the bridge under construction. In a fifth step 1310, the anchors are installed. This may be by adding weight to the weight anchors, or by screwing screw anchors into the bed of the body of water. In a sixth step 1312, the row is attached to a previous row at an edge of the bridge under construction. In examples, a screw rod, such as the screw rod shown in Figure 2C, may be used in the fifth step 1310 to install the screw anchors. This may be done by operating the screw rod through the reel cavities of each platform, by operating the screw rod from a row already part of the bridge, using an installation platform, which may be moved along the bridge during installation, or by operating the screw rod through holes in the platform designed for this purpose. A number of screw rods as shown in Figure 2C may be used in order to reach the bed of the body of water. A screwdriver may be used to drive the screw rods, which in examples may be motorised. In examples, the platforms in this embodiment comprise four access holes, each sized to allow a screw anchor to fit through the hole. In these examples, instead of the screw anchors being installed by operating a tool through the reel cavities, the screw anchors are installed by operating a tool through these access holes. In such examples, the reel cavities may not open. In such examples, the platforms may comprise a plug for closing the access holes after installation. In such examples, step 1304 may comprise positioning the plugs for use with the access holes on each platform, and step 1310 may comprise closing the access holes after installation of the screw anchors. In examples, the tether may be fixed at a suitable length, such that installation of the anchors also submerges the platforms. In these examples, it may be possible to manually adjust the tether length by small amounts after the platforms have been submerged. In other examples, the tether may be long enough that installation of the anchors does not submerge the platforms. In these examples, an additional step is required of tightening the tethers to submerge the platforms before the row may be attached to a previous row in step 1312. This additional step allows the height of the row to be configured over a greater range. Optionally, instead of performing this step of tightening the tethers row by row, a plurality of rows may be connected together at the surface after the step 1310, and may be submerged together by tightening the tethers for the plurality of rows at the same time. In examples, the steps 1308 may be done before the step 1306. In such examples, the tethers may be releasably attached to each other when the row is being moved to the desired location, to avoid the tethers trailing in the water. The anchors (and optionally plugs for closing access holes) may be positioned between the tethers, so that the tethers hold these objects on the platform during transit. Optionally, the tethers may be passed through the access holes or reel cavities to a top surface of the platform, before connection to each other. Alternatives and modifications It will be understood that the present invention has been described above purely by way of example, and modifications of details can be made within the scope of the invention. For example, although in the above examples, the platforms are fully submerged, in other examples the platforms may be only partially submerged. Where there is no desire for a bridge to be camouflaged, this may be advantageous, because it allows transport to cross the bridge without being submerged below the surface of the water, while unlike a conventional pontoon bridge, the platforms do not move significantly further down in the water when a weight passes across the platform, unless the weight exceeds a threshold. This results in such a bridge providing greater stability than a conventional pontoon bridge. In some examples, some of the platforms of a bridge may be held below the surface of the water, while others, such as those near the bank, may be only partially submerged. Although in the above examples, the bridges comprise a plurality of modules, each comprising at least one platform, in other examples the floating bridge may be formed from a single module comprising a single platform. This may be advantageous where a body of water is narrow, and so only a single platform is needed to span the body of water. Although in examples, the platforms are powered by batteries, in other examples the platforms may be powered by an electricity grid. In such examples, the rows may be connected such that each platform is in series with a platform on a previous row and on a subsequent row, and in parallel with all other platforms in the same row. Advantageously, this means that any failure at one position in a row does not affect the remaining platforms in the row. Alternatively, the rows may be connected such that each platform is in parallel with a platform on a previous row, and in series with the other platforms on the same row. This allows a fault on one row to be compensated for by the electrics on the neighbouring rows. Where sufficient power is provided, such a failure may be compensated for by other platforms, allowing the bridge to continue to be lowered and raised as a whole. Further methods of wiring known in the art may be used in order to localise electrical faults in the system and ensure that such localised faults do not compromise the operation of the bridge. In examples, no electrical components may be enclosed within the platform, and instead separate external tools may be used to lower the platform. These tools may be electrical tools. In examples, the reels may be connected to a shaft for use with external tools. Preferably, the shaft is hexagonal in cross section. Advantageously, a hexagonal shaft may be suitable for use with common tools. In examples, the shaft may be connected to a ratchet mechanism. Advantageously, this may allow a user to turn the shaft in one direction, and may prevent the shaft from turning in the opposite direction when a tool is removed. In further examples, a gear system may be provided to allow tools above the platform to transfer rotation to the reels in the platform. In such examples, the gear system may change the direction of rotation, and / or the plane of rotation. In examples, the shaft may have a diameter of 40 mm and a length of 400mm. In examples, each anchor may be connected to multiple tethers. These tethers may each be connected to a different position on a single platform, or may be connected to different platforms. In examples, both weight anchors and screw anchors may be used in combination. Advantageously, this may improve the overall performance of the bridge by combining the benefits of screw and weight anchor systems, such as in the circumstances where only portions of the bed of the body of water are suitable for screw anchors. Alternative installation mechanisms to those discussed in relation to Figure 13 may be conceived of by the skilled person. For example, the screw anchors may be installed using an industrial drilling rod. In examples, instead of installing the anchors at the same time as the platform rows, the anchors may be installed first, and then the platform rows may be moved to the desired location. In such examples, tethers may be attached to the anchors, with a second end of the tethers attached to a respective float. These floats may be used to retrieve the second ends of the tethers. The floats may be removed when a platform row is in location and is ready to be attached to the tethers. In examples, a bridge may be constructed by first carrying out the method of Figure 11, followed by repeatedly carrying out the method of Figure 12 and / or Figure 13 until the platforms span the body of water, and then carrying out the method of Figure 11 in order to provide an exit from the bridge. Alternatively, a bridge may be constructed by first carrying out the method of Figure 12 or 13 to install a central row at the centre of the body of water, and then repeating the method of Figure 12 and / or Figure 13 simultaneously to add rows on either side of the central row. This may be faster because work may be carried out in parallel in both directions. The bridge ends may be constructed in parallel with the method of Figure 12, or after the method of Figure 12 has been completed. In examples, instead of the bridge end of Figure 7, other end structures may be used. For example, a conventional pontoon bridge may be used to connect the bridge to the banks of the body of water. Advantageously, where the level of the body of water changes significantly, a pontoon bridge may be more easily adapted to the current level, since the pontoon bridge platforms float on the surface of the body of water, and so will remain on the surface of the body of water even when the surface rises or falls. In examples, the platforms may each be substantially identical, and may each comprise tapered edges and / or barriers. Such platforms may be used individually to form a single row. In examples, the platforms are connected to further platforms which are not connected to anchors such that the further platforms are submerged as the platforms are submerged. This may allow the size of the bridge to be increased at low complexity. The buoyancy of the further platforms should be taken into account in the capabilities of the anchored platform and anchors. Preferably all the platforms should be floating even if it is not crucial, to ensure they do not sink during installation or dismantling. Preferably all the platforms should have consistent dimensions. In examples, the platforms include voids which may be filled with water or other material thereby to allow the buoyancy of the platforms to be adjusted. Optionally, pumps are provided to assist in filling or emptying the void, preferably where said pumps are provided with the platforms. A combination of platforms with and without voids may be provided (in which case the platforms are preferably arranged to have consistent dimensions). In examples, weight anchors for multiple platforms are consolidated into a single anchor or arranged as a series of long metal reinforcement bars, rods, or pipes attached to the platforms. In examples, the platforms have a 1:2 ratio of width to length. It will be appreciated that the described bridge could be appropriately scaled up or down (with the buoyancy being appropriately adjusted) so as to support pedestrians only, and / or larger vehicles and loads. It will be understood that aspects and embodiments are described above purely by way of example, and that modifications of detail can be made within the scope of the claims. Each apparatus, method, and feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

1. A bridge, comprising:at least one bridge member; andat least one tether for holding at least a portion of the at least one bridge member below the surface of a body of liquid.

2. The bridge of claim 1, wherein the at least one bridge member is buoyant relative to the liquid.

3. The bridge of claim 1 or 2, wherein the at least one tether is suitable for holding the at least onebridge member below the surface of a body of liquid.

4. The bridge of any preceding claim, wherein, when the bridge is in an unloaded state, the at least one tether holds the at least one bridge member below the surface of a body of liquid.

5. The bridge of any preceding claim, further comprising at least one anchor attached to the at least one tether.

6. The bridge of claim 5, wherein the at least one anchor is at least one screw anchor, preferably wherein the at least one screw anchor comprises features for cooperating with a tool for screwing the at least one screw anchor into a material.

7. The bridge of claim 5, wherein the at least one anchor comprises at least one hollow body for receiving material.

8. The bridge of claim 7, wherein the hollow body is configured to be filled with water.

9. The bridge of any of claims 5 to 8, wherein each anchor is connected to a plurality of tethers,preferably wherein each anchor is connected to two tethers.

10. The bridge of any preceding claim, further comprising means for adjusting the tension in the tether.

11. The bridge of claim 10, wherein the means for adjusting the tension in the tether comprises a motorised reel.

12. The bridge of claim 11, wherein the motorised reel is remotely controlled.

13. The bridge of any preceding claim, wherein the at least one bridge member is connected to a plurality of tethers, preferably wherein the at least one bridge member is connected to four tethers; more preferably wherein the at least one bridge member is connected at each corner by a respective tether to a respective anchor.

14. The bridge of any preceding claim, wherein the at least one bridge member comprises at least one tapered edge, preferably wherein the tapered edge located is along a transverse side of the bridge.

15. The bridge of claim 14, wherein the at least one tapered edge is a separate component configured to be attached to the at least one bridge member.

16. The bridge of any preceding claim, wherein the at least one bridge member comprises at least one upwardly extending barrier, preferably wherein the barrier is positioned along a transverse side of the bridge.

17. The bridge of any preceding claim, wherein the bridge member comprises features for connecting to at least one other bridge member.

18. The bridge of any preceding claim, wherein the bridge comprises a plurality of connected bridge members.

19. The bridge of claim 18, wherein a plurality of modules are positioned across the width of the bridge.

20. The bridge of any preceding claim, further comprising an end portion for connecting the bridge andan edge of a body of liquid.

21. The bridge of claim 20, wherein the end portion comprises an L-shaped member for receivingmaterial so as to provide a surface on top of the material for connecting the bridge and an edge of a body of liquid.

22. The bridge of claim 21, wherein the L-shaped member comprises spikes for resisting movement of the L-shaped member.

23. A bridge comprising a plurality of modules, wherein the bridge is configured for use under the surface of a liquid.

24. A method of installing a submersible bridge across a body of water, the method comprising: installing an anchor in a bed of a body of water, wherein the anchor is attached to a bridge member by at least one tether; andtightening the tether to lower the bridge member below the surface of the body of water.

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