JP6356037B2 - Diving work method - Google Patents

Description

  The present invention relates to a diving work method for performing a diving work in a turbid water area where turbid water in water is stagnant. More specifically, the flow field of fresh water by continuing the supply of fresh water from the entire surface of the water discharge surface formed on the front end side of the fresh water supply means for supplying fresh water to the diving work area in a direction substantially perpendicular to the water discharge surface. This is related to a diving work method in which a clear water area is formed in a muddy water area and a diving work is performed in the clear water area.

  In underwater surveys, underwater construction, underwater environment conservation, maintenance of underwater structures, work in the water and at the bottom of the water, that is, diving work is indispensable.

  In addition, in recent years, the number of sites suitable for dam construction has decreased, and social and economic factors have made it difficult to construct new dams. On the other hand, existing dams are also aging, and concerns about earthquakes and abnormal flooding have begun. Therefore, rather than constructing a new dam, the water control and water utilization functions are strengthened by redeveloping existing dams, such as repairing, renovating and enhancing dam bodies and discharge facilities, and excavating the bottom of dam lakes. Newly added and existing dams are used effectively for a long time. In this dam redevelopment project, diving work is indispensable, and it is expected that the importance of diving work will increase further in the future.

As a prior document, for example, Patent Document 1 describes an underwater working machine provided with a water flow generating device that secures a water area from the cockpit to the work attachment by discharging clear water from the vicinity of the cockpit. ing.
JP-A-8-311920

  For example, when it is necessary to perform diving work in a stagnation area of turbid water in a survey or construction work, the visibility required for diving work cannot be sufficiently secured due to turbidity, making underwater work difficult and inefficient. Turn into. Therefore, this turbidity in water is one of the causes of process delay and cost increase in the entire business.

  Accordingly, an object of the present invention is to provide a means for easily, effectively and continuously ensuring the field of view in a work area when diving work is performed in an area where turbid water in water is stagnant.

  The present invention is a diving work method for performing a diving operation in a turbid water area where turbid water in water has stagnated, and the water discharge surface from the entire surface of the water discharge surface formed on the front end side of fresh water supply means for supplying fresh water to the diving work area. A diving work method for forming a fresh water flow field in the muddy water area by continuing the supply of the fresh water in a substantially vertical direction to the muddy water area and performing a diving work in the fresh water area is provided. .

  In the present invention, the flow of fresh water is formed by continuing the supply of fresh water from the entire surface of the water discharge surface of the fresh water supply means in a direction substantially perpendicular to the water discharge surface. The field is maintained with little mixing with muddy water, and a highly transparent water area (fresh water area) of a certain length and size can be formed in muddy water. As a result, it is possible to effectively realize the visibility in the diving work area, so that it is possible to reduce the burden and danger when performing the diving work, and to perform the diving work easily and efficiently.

  Then, for example, by continuing to supply fresh water to the diving work area during diving work, the fresh water area can be formed in muddy water for a long time. In addition, for example, even if the flow field of fresh water is primarily disturbed by diving work, the flow is restored by continuing to supply fresh water to the diving work area, so a clear water area of a certain length and size is formed. Can be maintained.

  In addition, for example, even when diving work in a deep water area, the supply of fresh water is continued from the entire surface of the water discharge surface in a direction substantially perpendicular to the water discharge surface, and a flow field of fresh water is formed to form a diving work area. Since a highly transparent water area can be formed, it is possible to secure visibility in a diving work area relatively easily and inexpensively without requiring complicated devices and processes. As a result, process delays and cost increases in the entire business can be suppressed.

  According to the present invention, when diving work is performed in an area where muddy water in water is stagnant, the field of view in the work area can be secured simply, effectively, and continuously.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to FIG. Note that the present invention is not limited to these embodiments.

  FIG. 1 is a schematic longitudinal sectional view showing an example of a diving method according to the present invention.

  In FIG. 1, the water W is blocked by the construction of the civil engineering structure A, the water W is stored up to the height of the water surface W1, and the muddy water region R1 in which the muddy water W2 is stagnant is formed.

  For example, as shown in FIG. 1, when diving work is performed near the wall surface A1 at a certain depth of the civil engineering structure A, the fresh water W3 is continuously supplied to the diving work area R2 by the fresh water supply means 1, and the flow of the fresh water W3 By forming the field, a fresh water area R3 is formed in the muddy water area R2.

  By forming the clear water area R3 so as to include the diving work area R2, it is possible to secure visibility in the diving work area R2, thereby reducing the burden and danger when performing the diving work, and making the diving work easy and Can be done efficiently. In addition, during the diving operation, by continuing to supply the fresh water W3 to the diving work area R2, the fresh water area R3 can be formed in the muddy water W2 for a long time.

  The present invention only needs to be a diving operation performed in a stagnation area of turbid water in the water. For example, in addition to a civil engineering structure such as a dam as shown in FIG. 1, for example, a diving operation in a port facility, a dike, a pier, or the like. Is also widely applicable. In addition, even when diving work is performed in a place where the flow of water is relatively large, such as a wave, for example, by installing an enclosure or the like around the entire diving work area R2 to reduce the flow of water, The clear water area R3 can be formed in the muddy water W2, and the visibility in the diving work area R2 can be ensured. In addition, the diving work in the present invention includes a wide range of underwater work using heavy machinery / equipment as well as work by the diver P or the like.

  In FIG. 1, the fresh water supply means 1 includes a storage means 11 for storing fresh water W3, a pipe line 12 for introducing the fresh water W3 into the water, and a water discharge surface that is formed on the tip side and discharges the fresh water W3 into the water W. 13.

  The fresh water W3 to be used is not particularly limited as long as it has higher transparency than the region R1 in which the turbid water W2 in the water is stagnant and can secure a field of view in underwater work. For example, spring water in the vicinity of the area where the diving work is performed may be used, or water in the water area where the diving work is performed may be used. In addition, the collected water may be used as it is, or may be used after increasing the transparency by performing a purification treatment or the like. In addition, fresh water W3 may be collected and supplied using power means such as a pump.

  The storage means 11 is a part for storing the fresh water W3 supplied to the diving work area R2. The fresh water W3 to be used is stored in the storage means 11, and the fresh water W3 is supplied to the water discharge surface 13 via the conduit 12. In addition, installation of the storage means 11 is arbitrary. For example, without storing in the storage means 11 etc., fresh water W3 is discharged from the fresh water collection position such as spring water directly or after purification treatment, and then the fresh water W3 is discharged through the pipe 12 or the like. The structure supplied to 13 is also widely included in the present invention.

  For example, by setting the installation position of the storage means 11 higher than the water surface W1, the fresh water W3 can be supplied from the storage means 11 to the diving work area R2 with no power due to a difference in water level. Moreover, the water supply flow rate can be adjusted by adjusting the water level difference. In addition, the fresh water W3 may be supplied to the diving work area R2 using a power unit such as a pump, or the water supply flow rate may be adjusted using a water amount adjusting unit such as a valve.

  The water discharge surface 13 is a part that discharges fresh water W3 into the water W. The water discharge surface 13 has a structure capable of forming a flow field of the fresh water W3 by continuing the supply of fresh water from the entire surface in a direction substantially perpendicular to the water discharge surface 13 as a whole (for example, the same direction as the reference numeral X1 in FIG. 1). Any known technology can be used widely, and there is no particular limitation. For example, water is distributed almost uniformly over the entire water discharge surface 13 with a current plate, punching metal, or the like, or a structure that discharges water from each gap such as a net-like member, and a single pipe or bundle. In addition, by forming the pipe line 12 with a large number of thin tubes and supplying the fresh water W3 to the water discharge surface 13, the fresh water W3 is supplied from the entire surface of the water discharge surface 13 in a direction substantially perpendicular to the water discharge surface 13. Can be configured.

  The direction of the water discharge surface 13, that is, the flow direction X1 of the fresh water W3 can be appropriately designed according to the purpose and is not particularly limited. For example, as shown in FIG. 1, the water discharge surface 13 may be installed upward at the lower side of the diving work area R2, or may be installed downward at the upper side of the diving work area R2. For example, when the water temperature of the fresh water W3 is lower than the water temperature of the turbid water W2, the specific gravity of the fresh water W3 is higher, so it is easier to install it downward on the diving work area R2 and make the flow downward. A stable flow field of fresh water can be formed. Similarly, for example, when the water temperature of the fresh water W3 is higher than the water temperature of the turbid water W2, the specific gravity of the fresh water W3 becomes lower, so that it is installed upward below the diving work area R2 and the flow is directed upward. Thus, a flow field of fresh water that is more stable and stable can be formed. In addition, the water discharge surface 13 may be installed sideways to form a flow field of fresh water W3 in a substantially horizontal direction.

  When the flow field of the fresh water W3 is formed, it is preferable that the fresh water W3 is supplied in a substantially vertical direction with respect to the water discharge surface 13 and substantially uniformly from the entire surface of the water discharge surface 13. At that time, for example, the supply of fresh water W3 may be performed at a substantially uniform flow rate. Thereby, the flow field of the fresh water W3 substantially perpendicular to the water discharge surface 13 can be stably formed. Further, for example, when the flow velocity of water near the periphery of the water discharge surface 13 is set to be different from the flow velocity of water near the same center, the flow field of the fresh water W3 in a direction substantially perpendicular to the water discharge surface 13 is formed. As long as the fresh water W3 is supplied from the entire surface of the water discharge surface 13 in a substantially uniform manner. By appropriately setting so that the fresh water W3 is supplied in a substantially vertical direction with respect to the water discharge surface 13 and substantially uniformly from the entire surface of the water discharge surface 13, a flow field of the fresh water W3 that is difficult to mix with muddy water can be formed.

  For example, as shown in FIG. 1, by forming the fresh water area R3 along the wall surface A1 of the civil engineering structure A, the influence of the flow in the surrounding water area can be limited. Can be secured.

  In addition, for example, a plurality of water discharge surfaces 13 may be installed in parallel in the muddy water region R1. By arranging a plurality of water discharge surfaces 13 in parallel in the muddy water region R1, a water region with high transparency can be formed more widely, and a wider range of diving work regions R2 can be secured.

  In the present invention, since the fresh water W3 is supplied from the entire surface of the water discharge surface 13 of the fresh water supply means 1 toward the water discharge surface 13 in a substantially vertical direction, in principle, the size of the cross section with respect to the flow direction of the fresh water area R3. (Code X2) is substantially the same as the size of the water discharge surface 13.

  Therefore, the shape of the water discharge surface 13 can be appropriately designed based on the shape of the cross section of the fresh water region R3 to be formed, and is not particularly limited. For example, by making the shape of the water discharge surface 13 into a substantially circular shape, a substantially elliptical shape, a substantially rectangular shape, a substantially polygonal shape or the like, a fresh water region R3 having a substantially cylindrical shape, a substantially elliptical column shape, a substantially rectangular parallelepiped shape, or a substantially polygonal column shape can be formed. For example, when the water discharge surface 13 is installed, the shape of the water discharge surface 13 is preferably substantially rectangular from the viewpoint that the gap with the wall surface A1 or the water bottom of the civil engineering structure A can be reduced. The longest width of the water discharge surface 13 (for example, the diameter when the water discharge surface 13 is substantially circular) is preferably in the range of 0.5 to 20 m, and more preferably in the range of 0.8 to 15 m. And most preferably in the range of 1.0 to 10 m.

  The muddy water area R1 refers to an area where the muddy water in the water is stagnant, and is a water area where the transparency is not sufficient and the field of view necessary for performing the diving work cannot be sufficiently secured. The turbid water area R1 is not limited to a narrow area only when the transparency is not sufficient over the entire water area or the entire water depth, but widely includes a case where the transparency is not sufficient in a specific water area or a specific water depth where the diving operation is performed. The present invention is applied when a diving operation is performed in a specific area within the muddy water area R1.

  The diving work area R2 refers to an area where diving work in water is performed. In the present invention, the diving work area R2 is set to a specific area in the muddy water area R1, and the flow field of the fresh water W3 is formed so that the diving work area R2 is included in the fresh water area R3.

  The fresh water area R3 is formed in the muddy water area R1 by forming the flow field of the fresh water W3 by continuing the supply of the fresh water from the entire surface of the water discharge surface 13 of the fresh water supply means 1 in a direction substantially perpendicular to the water discharge surface 13. It is a water area with higher transparency than the muddy water area R1.

  The fresh water area R3 is formed by the supply of the fresh water W3, and since some mixing of the fresh water W3 and the muddy water W2 occurs, the boundary between the fresh water area R3 and the surrounding muddy water area R1 is not necessarily clear. It may not be. However, in the present invention, the continuous turbid water area is formed by forming the flow field of the fresh water W3 by continuously supplying the fresh water from the entire surface of the water discharge surface 13 of the fresh water supply means 1 in a direction substantially perpendicular to the water discharge surface 13. A water area with a different transparency from R1 is actually formed, and at least the range of the water area where transparency is high compared to the surrounding muddy water area R1 and a field of view necessary for diving work is secured. It is assumed to be a clear water area R3.

  In principle, by supplying fresh water from the entire surface of the water discharge surface 13 of the fresh water supply means 1 in a direction substantially perpendicular to the water discharge surface 13, a direction substantially perpendicular to the water discharge surface 13 (for example, reference numeral in FIG. 1). A flow field of fresh water W3 in the same direction as X1 is formed, and a fresh water region R3 is formed from the water discharge surface 13 in a direction substantially perpendicular to the water discharge surface 13 (for example, the same direction as X1 in FIG. 1). In the present invention, it is only necessary that the fresh water W3 is supplied in a direction substantially perpendicular to the water discharge surface 13 as a whole. For example, the flow of the fresh water W3 due to the influence of the flow of water in the water area where the fresh water W3 is supplied. The case where the direction in which the field or the fresh water area R3 is formed varies is also widely included.

  The longest width X2 of the cross section (surface perpendicular to the flow direction X1) with respect to the flow direction of the fresh water area R3 is determined by the shape of the water discharge surface 13 of the fresh water supply means 1 and the flow velocity of the supply water W3. Although it may vary slightly from the size, in principle, it is substantially the same as the size of the water discharge surface 13 as described above. Therefore, it can be adjusted by appropriately designing the size of the water discharge surface according to the content and purpose of the diving work.

  The length X3 in the flow direction X1 of the fresh water area R3 (the length of the range in which the degree of mixing of the fresh water W3 and the muddy water W2 is low and high transparency compared to the muddy water area R1) is maintained. It can be set as appropriate according to the purpose and is not particularly limited. For example, from the water discharge surface 13, at least, preferably 0.5-8 m, more preferably 0.8-10 m, and most preferably 1.0-15 m, and high transparency is maintained as compared with the muddy water region R1. Is preferred.

  The length X3 in the flow direction X1 of the fresh water area R3 can be adjusted by appropriately setting the flow velocity and the Reynolds number when supplying the fresh water W3 to the diving work area R2. For example, when the fresh water W3 is supplied to the diving work area R2, the fresh water area R3 can be formed in the range near the water discharge surface 13 even if turbulent flow is discharged from the water discharge surface 13. Further, for example, by reducing the Reynolds number of the flow of the fresh water W3 discharged from the water discharge surface 13, the fresh water W3 and the turbid water W2 are hardly mixed, and the length X3 of the flow direction X1 of the fresh water area R34 can be increased. it can. Further, by further reducing the Reynolds number, it is possible to form the fresh water region R3 longer in a strip shape. In addition,

  For example, the fresh water W3 and the turbid water W2 can be supplied by adjusting the Reynolds number to 6,000 to 16,000, more preferably by adjusting the Reynolds number to 8,000 to 14,000. It can be made difficult to mix, and a fresh water area R3 having a length and size necessary for diving work can be formed and maintained effectively. In general, the Reynolds number increases as the water discharge port width (for example, the longest width X2 of the water discharge surface 13) or the flow velocity is increased. For example, when the outlet width is increased until the Reynolds number becomes larger than the above range at a predetermined flow velocity, it is necessary to increase the supply amount of the fresh water W3 to the diving work area R2, and thus the diving work area R2 The burden of securing and supplying fresh water W3 becomes excessive beyond the range where the initial purpose of securing the visibility in can be sufficiently achieved. In addition, for example, when the flow velocity is increased until the Reynolds number becomes a value larger than the above range at a predetermined outlet width, in addition to the increase in the above-described load, the flow of the diving work area R2 becomes faster. The work burden on the person P also becomes excessive.

  In addition, as described above, even when the flow of the water area R2 is primarily disturbed by the diving work, the flow is restored by continuing the supply of the fresh water W3 to the diving work area R2, so that a certain length of time is maintained during the diving work. It is possible to maintain the fresh water area R3 having the size (reference X3) and the size (reference X2).

  In Example 1, a verification experiment was performed on a 1/10 scale.

  As a model of the dam lake, 500 L of water was put into the aquarium.

  The following were produced as fresh water supply means. First, colored water was put into a water container and used as a storage means. Next, 11 hoses with an inner diameter of 15 mm were bundled to form a pipe line, one end was immersed in a water container, and a head member having a water discharge surface was attached to the other end. Here, the head member has three punching metals inside, and a mesh is stacked on it, and the mesh side is used as a water discharge surface so that water is discharged almost uniformly from the entire surface of the water discharge surface. It is a thing. The water discharge surface was formed in a circular shape with a diameter of about 80 mm.

  After fixing the water discharge surface of the fresh water supply means downward in the water tank, the colored water in the water container is slowly discharged from the water discharge surface to the water tank due to the difference in water level by raising the position of the water container. It was.

  As a result, the colored water formed a water area having almost the same width as the size of the water discharge surface while being hardly mixed with the water in the water tank. Further, by continuing to discharge the colored water from the water discharge surface, the water area of the colored water was maintained for a long time with little mixing with the water in the water tank.

  According to the present embodiment, for example, in the turbid water area where the turbid water is stagnant as in the dam lake, the clear water is supplied to the muddy water so that the flow field of the clear water is formed almost uniformly from the entire surface of the water discharge surface. The area of high water is maintained with little mixing with muddy water, and a highly transparent water area of a certain length and size can be formed in muddy water. It was proved that a highly transparent water area can be formed in muddy water.

  In Example 2, the Reynolds number suitable for forming a highly transparent water area in muddy water was examined.

  As a small scale model, water was put into a 1,600 mm deep water tank and suspended so that kaolin had a concentration of about 50 to 60 mg / L to form a muddy water region.

  As a means for supplying fresh water, lay a sheet of 200mm square punching metal on the bottom side of a 200mm square, 300mm high rectangular container, and form the same size punching metal with a notch for inserting the hose up to the top surface After aligning the notch portions and further overlapping, a hose was inserted into the notch portion from the upper surface side of the container so that the tip portion reached the surface of the punching metal on the lowermost side. In this fresh water supply means, the upper surface of the container (that is, the upper surface portion of the uppermost punching metal) was the water discharge surface, and the water discharge surface was formed into a 200 mm square. In the preliminary experiment, this fresh water supply means was installed on the bottom surface of the model water tank, and the fresh water was supplied to the fresh water supply means. As a result, the fresh water passed through a plurality of punching metals and was discharged from the water outlet (upper side of the container). At that time, by making the flow velocity substantially uniform, a flow of fresh water in a substantially vertical direction is formed from the entire surface of the water discharge port, and the size of the cross section with respect to the flow direction of the fresh water area is the size and shape of the surface of the water discharge port. It was able to form and maintain a fresh water area that was almost the same and maintained a constant length.

  Subsequently, in this experiment, a flow of fresh water having a predetermined Reynolds number was formed by adjusting the supply flow rate of fresh water or the average flow velocity at the outlet. The Reynolds number was a value calculated by multiplying the flow velocity by the representative length (water outlet width) and dividing the value by the kinematic viscosity coefficient. And the degree of turbidity in each measurement height when the height at which the water outlet was installed was used as a base point was measured. Here, the degree of turbidity represents the degree of turbidity at each measured height after the start of fresh water supply when the turbidity in the water tank before the start of fresh water supply is used as a reference, and at each measured height after the start of fresh water supply. The turbidity was obtained by dividing the turbidity by the turbidity in the water tank before starting the supply of fresh water and multiplying by 100.

  The results are shown in FIG. FIG. 2 is a graph showing the degree of turbidity after starting fresh water supply at each measurement height, that is, the vertical distribution of turbidity when the flow of fresh water is adjusted to each Reynolds number. The vertical axis of the graph is the value obtained by dividing the measured height when the outlet is installed at the base point by the width of the outlet (200 mm) (the value representing the measured height as a ratio to the width of the outlet. ), The horizontal axis represents the degree of turbidity (%) based on the turbidity in the water tank before starting the supply of fresh water, and each broken line represents the result at each Reynolds number.

  As shown in Figure 2, when fresh water is supplied with the Reynolds number adjusted to 8,000 to 14,000, at a measurement height of 5 times the outlet width (measurement height of 1,000 mm in this model) However, the degree of turbidity could be suppressed to 30% or less before the start of supply of fresh water, and a highly transparent water area could be formed and maintained in turbid water. In addition, when fresh water is supplied with the Reynolds number adjusted to 6,000 to 16,000, turbidity is observed at a measurement height of 5 times the outlet width (measurement height of 1,000 mm in this model). The degree was reduced to about 40% before the start of fresh water supply. On the other hand, when fresh water is supplied with the Reynolds number adjusted to 4,000 or less, the degree of turbidity tends to increase even at positions where the measured height is relatively low, forming a highly transparent water area in turbid water And could not be maintained sufficiently.

  This result shows that while the Reynolds number is adjusted to 6,000 to 16,000, more preferably 8,000 to 14,000, a flow of fresh water in a substantially vertical direction is formed from the entire surface of the outlet, thereby substantially vertical from the outlet. It shows that a water area with high transparency can be formed in muddy water up to about 5 times the width of the outlet.

  And, as a result of this experiment, for example, the supply of fresh water was continued while adjusting the Reynolds number within this range from the entire surface of the water discharge surface with a water discharge width of 2,000 mm toward the direction substantially perpendicular to the water discharge surface. In this case, a clear water area with a width of about 2,000 mm can be formed in muddy water up to a length of at least about 10,000 mm in the flow direction (5 times the width of the outlet), that is, ensuring a field of view necessary for diving work. Suggest.

  As described above, for example, when redeveloping an existing dam, diving work is essential. At that time, underwater work in a water area below a depth of 50 m may be necessary, and a great amount of money may be required for diving work. On the other hand, especially in the case of diving work in deep water areas, if the visibility required for diving work cannot be sufficiently secured due to turbidity, underwater work becomes difficult and it is difficult to complete diving work within the scheduled time. Overall process delay and cost increase.

  Conventionally, as a means of securing visibility in diving work in dam lakes, etc., a method such as setting up a fence around the work area, and adding a flocculant into it to precipitate turbid components and increasing the transparency of the water area May be used. However, this method has problems such as being resuspended by diving work and not being able to maintain the transparency of the water area, and requiring a large amount of aggregating agent when the work area is large, which is very expensive.

  On the other hand, in the present invention, it is possible to easily, effectively, and continuously ensure the visibility in the diving work area by supplying the clear water to the muddy water so that the flow field of the clear water is formed from the entire surface of the water discharge surface. Therefore, it is industrially useful in that it can reduce the work burden and danger at the time of diving work, can reduce the cost burden, and can perform the diving work easily, inexpensively and efficiently.

Schematic of the longitudinal cross-section direction which shows the example of the diving work method which concerns on this invention. About Example 2, when adjusting the flow of fresh water to each Reynolds number, a graph showing the degree of turbidity after starting fresh water supply at each measurement height, that is, the vertical distribution of turbidity.

DESCRIPTION OF SYMBOLS 1 Fresh water supply means 11 Storage means 12 Pipe line 13 Water discharge surface A Civil engineering structure A1 Wall surface of civil engineering structure P Diver R1 Muddy water area R2 Submersible work area R3 Fresh water area W Water W1 Water surface W2 Turbid water W3 Fresh water X1 Flow direction of fresh water W3 X2 Longest width of cross section of fresh water area R3 X3 Length of flowing direction X1 of fresh water area R3

Claims (4)

A submersible work method for performing a submersible work in a turbid water area where turbid water in water is stagnant,
Supplying fresh water while adjusting the Reynolds number to be 6,000 to 16,000 from the entire surface of the water discharge surface formed on the tip side of the fresh water supply means for supplying fresh water to the diving work area in a direction substantially perpendicular to the water discharge surface The diving work method which forms a clear water area in the muddy water area by forming the flow field of the clear water by continuing, and performs a diving work in the clear water area.   The diving work method according to claim 1, wherein the fresh water is supplied at a substantially uniform flow rate. The diving work method according to claim 1 or 2, wherein a size of a cross section with respect to a flow direction of the fresh water area is substantially the same as a size of the water discharge surface. The diving work method according to any one of claims 1 to 3 , wherein a longest width of the water discharge surface is in a range of 0.5 to 20 m.

Images