JP4492363B2 - Design method for fixed sand bypass system - Google Patents

Description

  The present invention relates to a method for designing a fixed sand bypass system for appropriately moving sand drifting along a coast.

  In an artificial structure such as a harbor facility provided on the shore of a beach or an estuary, a dike or the like is usually provided and the shape is different from a normal natural coastline. Therefore, the flow of drifting sand carried on waves and beach currents is affected by these structures, and the flow stops and the drifting sand accumulates, the ship's route is buried, and conversely, the drifting of sand drifts. The phenomenon that the existing coastline is eroded and decreases tends to occur. In order to cope with these problems, for example, a dredger is often used for dredging a place where drifting sand is accumulated and transporting the drifting sand trapped in an eroded place by a dump truck or the like. However, such methods have been pointed out as problems such as continuous cost generation, environmental deterioration due to noise, exhaust gas, and the like.

  Therefore, an attempt was made to continuously transfer dredging and underwater objects on the sea floor using a fixed sand bypass system consisting of a fixed pump, a pressure-feed pipeline, a belt conveyor, etc. without using dredgers for dredging work. In Gold Coast (Queensland), etc. (see, for example, Non-Patent Document 1).

However, in the design of such a fixed sand bypass system, the number of pumps, pump capacity, installation position, etc. are only determined based on approximate estimation, and the design is adapted to the flow of sand drift. That is not true. In addition, dredging operation after installation is only performed by the operator measuring the amount of sand accumulation. For this reason, even when trying to construct a similar system, it has been pointed out that the installation cost and operating cost of the system are unclear, and that once installed, it is difficult to change the system.
"Export Sand Bypass system", published by Queensland Government, [Searched on December 16, 2004], Internet <http://www.transport.qld.gov.au /Home.nsf/index/sandbypasssystem>

  The present invention provides a design method capable of optimizing the number of installed pumps and installation positions in advance when installing a fixed sand bypass system, and minimizing system installation cost and operation cost. Is an issue.

  The present invention relates to a design method for a fixed sand bypass system that performs dredging and movement of sand drift accumulated near the coast, a step of calculating the sand drift amount distribution near the coast, and a dredging corresponding to the sand drift distribution. Calculating an optimal arrangement of pumps, calculating a cost when the dredging pump is arranged in the optimal arrangement, and calculating an optimal number of the dredging pumps based on the cost. Is a design method characterized by

  Here, it is preferable that the optimal arrangement and the cost are calculated for a plurality of cases in which the number of installed units is equal to or less than a predetermined maximum number of installed units. In calculating the optimum position, it is preferable to use an optimization algorithm for a plurality of design proposals.

  Conventionally, a fixed sand bypass system, which was designed by rough estimation and similarly installed and operated, can be designed to allow dredging and movement of sand drift under optimal conditions. Also, installation costs and operating costs are minimized.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, as an example in which a fixed sand bypass system is provided, a case where a harbor facility 2 surrounded by a dike 10 is constructed on a beach 1 will be described as shown in FIG. FIG. 1A shows a state before the fixed sand bypass system is installed on the beach. The lower side is the land side and the upper side is the sea side. The beach current 30 is assumed to flow along the coastline 1 as indicated by the white arrow from right to left as viewed in the drawing.

  The drifting sand is carried by the beach stream 30 and moves. However, since the coastal stream 30 is blocked by the levee 10 on the upstream side of the harbor facility 2, a drifting sand accumulation portion 4 is generated in front of the levee 10. Similarly, sand drift accumulates near the port entrance due to the beach current 30, but this deposit enters the port due to the incoming waves, resulting in a bay port deposit 21, which fills the navigation route of the ship to the port. On the other hand, on the downstream side of the harbor facility 2, the drifting sand that has been deposited on the beach before the harbor facility 2 is provided is not supplied as appropriate, so that the coastline is eroded and the coastline is deformed. .

  Since it is not desirable to leave such drift sand accumulation and erosion for a long period of time, install a fixed sand bypass system as shown in Fig. 1 (2) to trap the drift sand that has accumulated on the upstream side. It is desirable to move it to the downstream side through the land side or the underwater pipeline, etc., and to bury it in the eroded portion 3. Such a fixed sand bypass system is configured such that, on the upstream side of the harbor facility 2, the sand 50 deposited by a fixed jet pump or the like is sucked to form the sand pocket 40 and dredged, and the dredged sand is pipelined or removed. For example, it includes a transfer unit 51 that moves to the downstream side through the land side via a belt conveyor, and a landfill unit 52 that throws the transferred sand drift into the erosion unit 3 on the coast.

  Here, the design conditions of the fixed sand bypass system are particularly problematic in the number of installed jet pumps for the dredger used in the brim section 50, the installation position, the lifting capacity, and the operating conditions. This is because the dredging position and dredging ability of the system are determined, and the installation cost and operating cost of the system are almost determined. For this reason, the design method described below is intended to determine the installation contents and operating conditions of the collar unit 50.

  FIG. 2 is a flowchart showing a schematic flow of a design method in installing the fixed sand bypass system. The system design method will be described below according to this flowchart. In principle, this design flow can be performed manually by humans, but it is usually performed using a computer because it requires enormous operations. . The following description will be made on the assumption that the calculation is performed by a computer.

  First, on the target beach, the maximum number of jet pumps to be used for dredging is set (step S100). The maximum number of installations may be set as appropriate so that the number of installations is larger than the number of installations that are empirically required, and need not be a highly accurate number. If this amount is too large, an extra computational load is applied to the computer, but if it is too small, an optimal design may not be obtained. Therefore, it is desirable to set a large number.

  Next, the terrain condition of the beach where the fixed sand bypass system is installed and the wave condition off the beach, which is the origin of the beach current carrying the sand drift, are set (step S200). Here, the terrain conditions include the position and shape of the existing coastline and harbor facilities, the topography of the seabed near the coastline, the soil quality of the seabed, and the like. In addition, the wave conditions include the statistical wave height, period, wave direction, etc. of waves analyzed by the spectral method, etc., based on the data of what waves have been hit in the past few decades off the coast. Data is included.

  Next, from the set terrain condition and wave condition, the target beach wave field and beach flow field are calculated (step S300). A wave field is composed of a scalar field in which the planar distribution of time-averaged wave energy in the vicinity of the beach is obtained, and a vector field in which the wave direction is obtained. For example, it can be calculated via the energy balance equation. From the wave field, time-averaged wave height and wave direction values can be obtained for each beach location.

  Further, the beach flow field is a vector field that specifies the speed (velocity) and direction (flow direction) of the beach flow averaged for each location near the target beach. The coastal flow field is premised on the average wave height and wave direction distribution obtained from the wave field, and the beach topography is one of the boundary conditions. Assuming boundary conditions in the left and right direction), and further assuming a friction term on the seabed, the calculation is performed using a continuous equation and an equation of motion.

Next, using the distribution of the flow velocity and direction and the distribution of wave height and direction obtained from the beach flow field, the sand distribution on the beach is calculated (step S400). The amount of sand drift at any point is defined as a function of the shear stress caused by the flow near the bottom, and this shear stress can be calculated by the wave height, wave direction and beach current at that point. Fig. 3 shows the distribution of the amount of sand drift Q _{by in} the coastal direction passing on the straight line in the coastal direction. The horizontal axis in FIG. 3 indicates the distance from the shoreline to the offing. Here, j = 1 indicates the case where the significant wave height of the incoming wave offshore is 50 cm, j = 2 indicates that the significant wave height is 1 m, and j = 3 indicates the case where the significant wave height is 2 m. The significant wave height means an average of the wave heights of the selected waves, in which the number of waves of 1/3 of the entire wave is selected in order from the highest wave height of a certain wave. Based on the sand drift distribution on the upstream side of the beach current obtained in this way, past topographic change data, etc., the target sand bypass amount for one year to be dredged in the upstream part is calculated.

  For specific formulas and calculation methods for calculating wave fields, beach flow fields, and sand drift distributions, see, for example, Shinzo Shimizu, Takahiro Kumagai, Nobuo Mimura, Jun Watanabe, “Three-dimensional beach transformation considering shoreline changes” Long-term prediction model ”, Coastal Engineering Papers, Vol. 41, pp. Mathematical formulas and calculation methods described in 406-410, published by Japan Society of Civil Engineers (1994) can be given. The calculation may be performed in accordance with a normal numerical solution obtained by discretizing the differential equation.

  Next, as shown in FIG. 1 (2), the installation position of the ridge part 50 in a direction substantially parallel to the beach current is set in advance in the drifting sand accumulation part 4 on the upstream side of the beach flow. The number of pumps optimized for the above, the position of the pump in the direction substantially perpendicular to the beach current, the depth of the sand pocket, and the required pump capacity are determined (steps S500 to S800).

  First, the figure which looked at one of the installation examples of the collar part 50 from the upstream is shown in FIG. In FIG. 4, the right side is the offshore side and the left side is the land side, and the earth and sand suction system 60 including five jet pumps 72, the ground liquefaction pump 73, and the slurry suction pipe 74 is sent three times. They are connected in series with a tube. First, seawater (or river water) is sucked by the high-pressure water pump 70, and high-pressure seawater is sent to each sediment suction system 60 by the seawater pipe 71. Seawater (or river water) is sucked by the water pump 61 and sent to the sediment diluted water flow pipe 62, and the seawater mixed with drifting sand trapped in the drain receiver 64 is conveyed via the sediment flow pipe 63. In the middle of the sediment transport pipe 63, an outlet from the slurry suction pipe 74 of the sediment suction system 60 is connected. The jet ejected from the jet pump 72 entrains surrounding earth and sand to form a slurry, and the slurry is sucked from the inlet of the slurry suction pipe 74 and sent to the earth and sand flow pipe 63 by the negative pressure generated by the jet. In addition, the sediment transport pipe 63 has a constant gradient descending from the sea side toward the land side.

  In addition, each earth and sand suction system 60 is provided with a ground liquefaction pump 73 that ejects high-pressure seawater supplied from the seawater pipe 71 toward the seabed, and winds up the earth and sand accumulated on the seabed with a water flow. The slurry wound up by the ground liquefaction pump 73 is sucked into the slurry suction pipe 74 by the negative pressure generated by the jet pump 72 which is similarly supplied with high-pressure seawater from the seawater feed pipe 71.

FIG. 5 shows an example of a cross section when the seabed is dredged by such a brim 50. The original sea bottom 81 indicated by the broken line is gently descending from the coast toward the offshore. As a result of drifting sand drifting on the sea bottom by the suction pipe 74, the sea bottom 82 indicated by the thick solid line is sucked. It is in a state where mortar-shaped sand pockets are lined up around the tube. The portion sandwiched between the original seabed 81 and the seabed 82 that has been trapped is the maximum trapping amount of sand drift, and is the upper limit of the sand bypass amount per operation. In this example, five suction pipes 74 are numbered 1 to 5 from the land side, and the distance from the sea surface 80 to the original seabed 81 is d for each suction pipe, and the seabed drowned from the original seabed 81 The distance to 82 is h, the distance from the sea surface 80 to the sediment transport pipe 63 is H, and the corresponding suction pipe number is assigned and displayed. Further, the distance from the shoreline to the offshore is represented by X, for example, the position of the _first suction pipe is represented by X1, and so on.

Next, in order to optimize and design such a buttock 50, assuming the number of installed jet pumps, the installation position and installation depth of each jet pump in the left-right direction (land ← → off-shore direction) in FIG. Is determined (step S500). First, assuming that k pumps (starting from k = 1) are used in the flange 50, the optimal installation position, installation depth, and required pump capacity of the pump are calculated. The calculation in step S500 may be started from k = k _{max or} may be started from the number expected to be the best.

  Various optimization algorithms are known for determining the optimum installation position and the like, and which one is adopted is arbitrary. Here, an example of calculation using a so-called genetic algorithm will be described. According to this algorithm, it is possible to determine an optimum design with a relatively light calculation load corresponding to the required accuracy. This flow is shown in FIG.

In the flow of FIG. 6, N-th generation design proposals A _{1 to} A _M are read with respect to the assumed number of installed jet pumps k. Here, the design plan varies the installation positions X _{1 to} X _k and the depth of the sand pocket formed by the jet pump (corresponding to the installation depth) while maintaining the order of the _{first to kth} of the jet pump. This is considered to be a group of design proposals that have been changed. That is, it is considered to be a group of proposals that variously assume the shape of the drowned sea floor 82 of FIG. It is assumed that the angle of repose of the sand pocket is obtained by conducting an experiment in advance at the place where the fixed sand bypass system is constructed. Then, arbitrary M design plans are read out from the group of such design plans, and set as the Nth generation (starting from N = 1) design plan (step S510).

Next, the required pump capacity is calculated for each of these Nth generation design proposals (step S520). A specific flow of this step is shown in FIG. First, using the selected one of the design plan A _m from the N-generation design proposal previously read (S521 step), the depth of the jet pump mount location and sand pockets included in the design plan data Then, the plane area of the sand pocket is calculated (step S522). That is, the plane area | region which looked at the sand pocket which should be formed in a mortar shape according to a design plan from the upper side of the perpendicular direction is calculated | required. The calculation is performed using the depth data of the design plan and the angle of repose obtained experimentally in advance. In addition, the part which the plane area | region of the sand pocket adjacent to each other overlaps is excluded.

  Next, the volume of such a sand pocket is calculated (step S523). The calculation may be performed using the already obtained plane area and angle of repose. The volume determined in this way is the capacity of the sand pocket.

  Next, a certain operation cycle period, for example, one day is considered as a unit time, and the amount of sand drift flowing into the sand pocket during this unit time is calculated using the sand amount distribution for each wave height stage calculated in step S400. (Step S524). Specifically, the sand drift distribution is integrated along the sand pocket position. However, when the integrated value is larger than the previously obtained sand pocket capacity, the sand pocket capacity is set to the maximum value. Note that that the operation cycle period is one day means that the fixed sand bypass system is operated every day. The operation cycle period may be appropriately determined in consideration of the convenience of system operation management.

  Next, the optimum operating pressure of the jet pump is determined (steps S525 to S528). First, the operating pressure of the jet pump is assumed to be a certain value, and the maximum amount of earth and sand that each jet pump can suck as the performance of the pump itself at that pressure is determined using the graph illustrated in FIG. 8 (step S525). The graph of FIG. 8 shows the relationship between the pump pressure, the sediment suction flux of the pump, and the height at which sediment can be sucked up, and is obtained in advance by experiments under standard conditions. The plurality of curves mean that the pump pressure (water head) is 110 m, 120 m, 130 m, 140 m, and 150 m from below.

  Next, the operation time of each jet pump is obtained (step S526). As a general rule, the operating time is a value obtained by gradually reducing the amount of inflow sand drift by the sediment suction flux, but it is considered that the operating time does not exceed the value obtained by dividing the sand pocket capacity by the sediment suction flux. Furthermore, the operating time does not exceed the operating cycle period. In addition, when a plurality of jet pumps are used, the jet pump on the shore side should be operated as much as possible. That is, the operation time of the jet pump on the shore side is secured in preference to the operation time of the offshore pump. This is because the shore side jet pump requires a lower suction height and the sediment suction flux becomes larger, so that the use of the shore side jet pump improves the efficiency of the entire system as much as possible.

  Next, the maximum sediment suction amount for each sand pocket is calculated from the sediment suction flux of the pump and the operation time (step S527). Here, it is assumed that the maximum amount of soil suction does not exceed the capacity of the sand pocket. Next, the annual sediment suction amount Q of the entire system is calculated using this maximum sediment suction amount (step S528).

  Next, it is determined whether or not this annual earth and sand suction amount Q has reached the target sand bypass amount for one year obtained in step S400 (step S529). If the annual sediment suction amount Q has not reached the target sand bypass amount, the flow branches right from the step S529, the pump pressure is changed as appropriate (step S530), and the calculation is repeated after returning to the step S525.

In step S529, if the annual sediment suction amount Q reaches the target sand bypass amount, the flow branches downward and proceeds to step S531, until calculation is completed for all of the selected M design plans. The calculation from step S521 to step S531 is repeated. When the calculation for all M is completed, step S520 in FIG. 6 is completed, and the jet pump pressures P _{1 to} P _M required for each design plan are obtained. Next, the design proposals are arranged in order with this necessary pressure (step S530). This is because a design plan with a smaller required pressure is considered to have better cost performance for system operation.

Next, it is determined whether the number of generations of the design plan has reached a preset maximum generation number N _max (step S540). If not, the flow branches to the right from step S540, leaving a plurality of designs in order starting from the one with the lowest required pressure among the design plans ranked in step S530, while the plurality of designs with the higher required pressure. The plan is excluded (step S550). Next, in addition to the remaining design plans, a plurality of design plans different from the design plan selected in step S510 are selected, and next-generation design plans A _{1 to} A _M are selected.

Next, using the next-generation design plan, the calculations from step S510 to step S540 are repeated, and selection is repeated so that only the design plan with a small required pressure remains. When the number of generations reaches the predetermined number of generations N _max in step S540, the flow branches downward, and the design plan with the smallest necessary pressure is selected as the best design plan (step S570). By doing so, a design plan having the performance necessary for the system and the smallest necessary pressure is determined. This completes step S500 in FIG. 2, and the optimum design for the arrangement and capacity of the pump has been determined.

  Here, the example of the calculation result about various design proposals in case of one pump is shown. First, assuming the beach and port structure shown in FIG. 1, specific numerical conditions are assumed as shown in Table 1.

  Based on this premise, FIG. 9 shows the result of calculating the necessary pump pressure at the pump installation position and installation depth obtained by various design proposals. In FIG. 9, the vertical axis represents the depth h from the original seabed of one sand pocket generated by one jet pump, and the horizontal axis represents the distance X from the shoreline to the offshore direction. Although FIG. 9 is divided into two areas A and B by a solid line, the area A is an area where the target sand bypass amount cannot be obtained, and the area B is an area where the target sand bypass amount can be obtained. It can be seen that when the number of pumps is one, the minimum depth of the sand pocket is 13 m. It can also be seen that the necessary sand bypass amount cannot be obtained even if the installation position of the pump is too close to the shore side or too far from the shore side.

  Further, the numbers in the broken line and the parentheses described in the region B indicate the pump pressure (in this case, indicated by the pump head) necessary to obtain the target sand bypass amount. It can be seen that the pump pressure is distributed in order from the smallest 60 m to the largest 140 m.

Next, on the premise of such an optimal design, the construction cost and operation cost of the fixed sand bypass system when the number of pumps is k are calculated (step S600). Next, it is determined whether or not the number of pumps has reached the preset maximum number (step S700). If not, the flow branches to the right, and the number of pumps is increased by one, and steps S500 to S700 are performed. Repeat the operation. In this way, the optimum design for each case is determined, and the construction cost and operation cost for each case are calculated. Here, on the assumption of the conditions shown in Table 1, an example of calculation results in which the optimized arrangement position X, installation depth h, and pump pressure P ₀ are optimized when the number of pumps is 1 to 5 is used. It shows in Table 2.

When the number of pumps reaches the maximum number k _max in step S700, the flow branches downward, and a design plan for the number of pumps that minimizes the sum of the construction cost and the operation cost is selected (step S800). If the number of jet pumps is large, the construction cost will increase, but the required operating time of each jet pump will decrease and the operating cost will decrease. Conversely, if the number of jet pumps is small, the construction cost decreases, but the required operating time of each jet pump increases and the operating cost increases. Therefore, there is a value of the number of pumps in which the total cost including the construction cost and the operation cost becomes a minimum value. For example, FIG. 10 shows a graph in which the construction cost and the operation cost in the case of Table 2 are plotted. FIG. 10 shows that the minimum value is obtained when the total cost is k = 3. In step S800, the minimum value of the number of pumps is determined in this way. Thus, all design conditions are determined and the flow ends.

  In this way, the fixed sand bypass system can be designed in consideration of the ocean wave conditions offshore and the flow of drift sand caused by the beach conditions, so it is possible to construct the most efficient and minimal system, The total cost is also minimized. In addition, since the calculation for optimizing the installation position and installation depth of the pump is performed by a genetic algorithm, the calculation processing load is small, and an optimum solution can be obtained relatively easily.

  Next, the fixed sand bypass system (I) [Example] optimized by the design method of the present invention, and the fixed sand bypass designed by the safety side over-spec without optimization, as in the past. The simulation result regarding the difference in dredging performance with system (II) [comparative example] will be described. Assuming the beach shown in Fig. 1, a fixed sand bypass system will be constructed on the upstream side of the beach flow. As a result of optimization design for the fixed sand bypass system (I), the pump as shown in Table 3 It is assumed that it is optimal to use 6 units. The six pumps are arranged from the shoreline to the offshore, with the interval being narrowed, and the initial digging depth from the seabed is set to be shallower offshore. The shape of the sand pocket formed thereby is shown by the solid line in FIG.

  On the other hand, as shown in Table 3, the fixed sand bypass system (II) designed with excessive specifications without being optimized as described above has a wider range of six pumps equally spaced and deeper. All six units were arranged with the same installation depth of 15 m. The shape of the sand pocket formed by this arrangement is indicated by a broken line in FIG. The pump pressure was the same for (I) and (II). When the pump installation depth becomes deep as in (II), the capacity of the formed sand pocket increases, but the amount of sand that can be dredged due to insufficient pump pressure is reduced.

  Table 4 shows the results of calculating how much earth and sand can be sucked in the case of the incoming wave level (j) as shown in FIG. At any level, it can be seen that the optimized system (I) has a higher sediment suction probability than the conventional system (II).

  Moreover, the simulation result about the change of a shoreline when these two systems are each applied to the beach shown in FIG. 1 (1) is demonstrated. This is shown in FIG. A shoreline 100 indicated by a thick solid line in FIG. 12 indicates a beach shape in the case where sedimentation and erosion of sand drift due to the beach current have occurred before the shoreline is performed. A shoreline 110 indicated by a thin solid line indicates a beach shape that occurs 10 years later when the optimized fixed sand bypass system (I) is installed. It can be seen that the port facility 2 has almost returned to the state before the construction. On the other hand, a shoreline 120 indicated by a broken line indicates a beach shape that occurs 10 years later when a conventional fixed sand bypass system (II) is installed. It can be seen that although the beach shape has been restored to some extent, the sand bypass amount is insufficient and the restoration is insufficient.

  In addition, regarding sediment accumulation at the port entrance, when the optimized fixed sand bypass system (I) was installed, the sediment volume at the port entrance decreased to about 15% of the case when it was not installed. On the other hand, when the conventional fixed sand bypass system (II) was installed, it recovered only to about 35%.

  Although the embodiments have been described above, the present invention is not limited to the specific embodiments described above, and various modifications are possible. For example, the optimization method is not limited to the genetic algorithm, and for example, a random search method such as a Monte Carlo method can be arbitrarily used. In the above explanation, the position where the jet pump is arranged offshore from the shore side is optimized, but the distance between the port facility and the shoreline in the direction parallel to the shoreline is also optimized. You may make it do.

It is the figure which showed the model of the beach where a fixed sand bypass system is installed, (1) is the figure which showed before installation, (2). It is the flowchart which showed the whole system design flow outline. It is the figure which calculated | required the amount of sand drift which flows to a coast with respect to the offshore direction distance. It is the conceptual diagram which showed schematic structure of the collar part. It is the conceptual diagram which showed the example of the shape of the sand pocket formed in the seabed. It is the flowchart which showed the detailed flow of S500 step. It is the flowchart which showed the detailed flow of step S520. It is the figure which showed the relationship between the sediment suction capability of the jet pump itself, the suction height, and the pump pressure. It is the figure which showed the intermediate result of the optimization calculation in case there is one jet pump. It is the figure which showed the concept of the calculation which minimizes the number of jet pumps. It is the figure which showed the sand pocket shape produced by simulation. It is the figure which displayed the simulation result regarding a shoreline change.

Explanation of symbols

50 Saddle 60 Sediment suction system 61 Sediment dilution water feed pump 62 Sediment dilution water feed pipe 63 Sediment feed pipe 64 Drain receiver 65 Slurry pump 66 Slurry discharge facility 70 Seawater suction pump 71 Seawater feed pipe 72 Jet pump 73 Ground liquefaction Pump 74 Slurry suction pipe 80 Sea surface 81 Sea bottom 82 before dredging 100 Sea bottom 100 after dredging Dredging 110 before dredging 10 years after optimized fixed sand bypass system (I) The resulting shoreline 120 The shoreline produced after 10 years by the conventional fixed sand bypass system (II)

Claims (3)

A computer-based design method for a fixed sand bypass system that performs dredging and movement of drifting sand that accumulates near the coast, where the terrain conditions of the beach where the fixed sand bypass system is installed and the beach from which the beach current that carries the drift sand is generated Offshore wave conditions are set, from the topographic and wave conditions set above, a target beach wave field and beach flow field are calculated, and the calculated beach flow field by using the distribution and the wave height and wave direction distribution of the determined flow rate and flow direction from the steps of sediment transport distribution near the coast Ru is calculated,
One of a plurality of design proposals prepared in advance is selected, and the amount of sand that flows into the sand pocket within a unit time is calculated from the assumed number of dredging pumps installed and the calculated sand distribution. Using the dredging pump of the selected design plan, a step of calculating a pump pressure necessary to suck the amount of sand drift flowing into the sand pocket, and the necessity repeatedly with respect to the plurality of design plans a step of optimal placement of dredging pumps Ru is calculated by calculating the pump pressure, resulting required pump pressure is preferentially minimal design plan,
A step of the based on the number of installed assumed dredging pump and selected optimal arrangement of the at the cost of the case of arranging the dredge pump Ru is calculated,
Said assumed the costs vary within predetermined ranges the number of installed dredge pump repeatedly calculated, step seeking minimum point of the cost for the installed base, the optimum number of installed the dredge pump Ru is calculated The design method characterized by having.   The design method according to claim 1, wherein the optimal arrangement and the cost are calculated for a plurality of cases in which the number of installed units is equal to or less than a predetermined maximum number of installed units. 3. The design method according to claim 1, wherein a genetic algorithm for a plurality of design proposals or an optimization algorithm based on a random search method is used for calculating the optimum position.

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