Keywords

1 Introduction

Some ports located at the estuary or inner bays in Japan suffer from river-derived sediment. For example, a large amount of sediment from rivers flows into estuary of Ariake Sea in Japan when the water rises due to seasonal rain or typhoon. Tideland at the mouth of the river stores sediment and contribute to the deposition of the port for a long term (refer to Fig. 1). To consider effective countermeasures, it is necessary to elucidate sediment dynamics due to waves and tidal currents of the target port. Therefore, field observation methods have been improved, and numerical simulation models for evaluating countermeasures have been developed for multiple ports in Japan. The first practical study of countermeasures against the port siltation in Japan was carried out in Kumamoto Port in Ariake sea. Tsuruya et al. (1990) developed the multi-layer numerical model that be able to predict the effect of submerged breakwater against the siltation. In the maintenance of navigation channels and basins, both to reduce dredging costs and to create disposal sites are issues. Furthermore, the procedure of environmental assessment for the construction of new disposal site takes long time. Therefore, it is necessary to predict medium- and long-term volume of dredging sediment, make a plan about the disposal of sediment and advance the preparation of construction of the disposal site.

This paper introduces our knowledge about sediment transport obtained through multiple operations.

Fig. 1.
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Tideland at the mouths of Shirakawa River and Tsuboi River in Ariake Sea in Japan

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2 Field Observation

2.1 Basic Surveys

It is classified the field survey as basic survey to grasp the present situation of sedimentation and application survey to specify the mechanism of sedimentation. For the basic survey, bottom sounding using a single-beam or multi-beam will be conducted to reveal the actual situation of navigation channels and basins. In recent years, the multi-beam sounding has been applying to the bathymetry survey for the progress of 3D visualization in the design and construction of facilities. Our company introduced Multibeam Echosounder (Sonic2024/Sonic2026), is able to carry out a wide range / high-resolution sounding depending on a purpose with an item (refer to Fig. 2).

Fig. 2.
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Multibeam echosounder

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2.2 Application Surveys

Application surveys are conducted in characteristic areas identified through the basic survey to elucidate suspended sediment transport and their deposition processes. Within a time limit and budget, it is important to make a plan of survey with building an appropriate hypothesis to get the high priority information.

Sediment transport is classified as suspended load and bed load. The suspended load travels a long distance and is often subject to sedimentation in navigation channels and basins. Current velocity/direction and SS concentration are investigated to know the suspended load transportation. Figure 3 shows survey method and Fig. 4 shows results of current velocity and SS concentration. It is recognized that the flood tide current includes a lot of sediment. Integrating temporally and spatially those, we can know sediment transport amount to inside the port.

Acoustic Doppler Current Profiler (ADCP) measures the current velocity and direction of multiple vertical or horizontal layers in the subsection. SS concentration is able to be calculated from echo intensity obtained by ADCP. Depending on a purpose and budget, single layer observation is conducted by using an electromagnetic velocity meter and a turbidity meter. Furthermore, there are some cases using sand level gauge to measure height of sedimentation.

There is a restriction in the selection of survey point because of precedence of using navigation channels and basins for usual activity in the port. If the survey point is not able to be located inside navigation channels and basins, it should be located on the course of sediment transport. Alternatively, it can be located in small pocket area which is dredged for the particular survey.

Fig. 3.
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Schematic diagram of current velocity survey method

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Fig. 4.
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An example of survey results

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2.3 Mechanism of the Port Sedimentation

Sedimentation mechanism is considered by results of field survey and the hypothesis which has been built before is verified by observation data. At that time, the relationship between metrological phenomena and actual situation of sedimentation is important. In Japan, we can use some public information, NOWPHAS (wave data by MLIT (Ports and Harbors Bureau, Ministry of Land, Infrastructure, Transport, and Tourism)), AMEDAS (meteorological data by JMA (Japan Meteorological Agency)) and Water Information System (river data by MLIT).

3 Numerical Simulation

3.1 Building of the Practical Numerical Model

In practice, customers demand speed and comprehensibility. Therefore, we focus on simply expressing the phenomenon to get quick and easy-to-understand results.

Numerical simulation model of the port sedimentation has 3 main components, current, wave and topographical changes (refer to Fig. 5). This method is based on Tshuruya et al. (1990). When necessary, topographical changes component is taking account of fluid mud layer near the bottom. Modeling of the fluid mud based on Odd and Cooper (1989) has been improved by various field and laboratory survey.

Because the phenomenon of port sedimentation is very complex and to perfectly verify it by computing is very difficult, the calibration of model parameters is conducted focusing on the primary factor of sedimentation and the specified phenomenon.

We have been recently focusing on the pickup and diffusion of bottom sediment by ships on the waterway and improving the numerical model including those effects. Our numerical model has been already applied at several ports in Japan.

Fig. 5.
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Overview of numerical simulation model of the port sedimentation

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3.2 Prediction and Estimation of the Effect of Countermeasure Against the Port Sedimentation

The effect of countermeasure against the port sedimentation is estimated by the difference between the case of existence of the installed countermeasure and the case of the uninstalled. The countermeasure is selected from the proposed in Fig. 6. In above-referenced Kumamoto port, the inverted-T submerged breakwater was applied as the countermeasure against the waterway sedimentation. The pocket dredging has been often applied in recent years because it doesn’t give influence on the ship navigation. The pocket is constructed at the adjacent area of waterway or the upstream of the suspended load. It alternatively catches sediment and reduces sedimentation rate on the waterway.

Fig. 6.
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Main Types of countermeasures

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4 Dredging and Sediment Disposal Plan

4.1 Dredging Efficiency

To reduce the dredging cost, it is necessary not only to reduce the amounts also to improve the work efficiency. In case that the dredger ship can work on one spot, the dredging efficiency is good (Fig. 7). Therefore, the dredging at narrow pocket area is economically more reasonable than the slight dredging at wide area in the waterway or basin. However, it is necessary to estimate the effect with a long-term viewpoint due to the increase of sedimentation rate at pocket area which are deeper than surroundings. Therefore, we evaluate the effects of pocket dredging and countermeasure facilities using a numerical simulation model.

Fig. 7.
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Work status of dredging

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4.2 Disposal Planning of Dredging Sediment

Dredging Sediments are deposited at the disposal site near the port and are appropriately treated. It is necessary to elaborate on a sediment disposal plan with estimation of sedimentation amount which will generate in future because the lack of disposal space affects activities in the port. But it is difficult to estimate long-term amount of dredging sediment because the sedimentation rate in the waterway or basin changes depending on typhoon coarse year by year. On that account, we have developed below methods.

  1. i.

    To estimate sedimentation amount from wave and bathymetry data (for approx. 30 years)

  2. ii.

    To estimate annual sedimentation rate by extreme value statistical analysis and draw out a probability density function

  3. iii.

    To calculate 10,000 cases of sedimentation rate by Monte Carlo method based on the probability density function

  4. iv.

    To estimate the expectation value of sedimentation rate as ensemble average of all cases.

Here, observation wave data by NOWPHAS can be used. When there is not it around the port, it is necessary to estimate wave height by using wind data.

4.3 Adaptive Management

In some cases, there is a gap between the sediment disposal plan and the actual results depending on the occurrences of meteorological event like a typhoon. Moreover, there is another case that the unexpected phenomenon which cannot be explained by assumed sedimentation mechanism occurs. Therefore, we recommend preparing “Clinical Chart to Consider Measures against Port Sedimentation” by each port. Historical records of field survey, dredging and countermeasure construction are organized in the chart. Moreover, the present situation about validation of the sedimentation mechanism and problem to be solved are also written in it. The aim of creation of the chart is to optimize maintenance of waterway and basin by means of handing over it from predecessor to successor for the maintenance.

5 Conclusions

For the consideration of measure against port sedimentation, to accurately grasp phenomenon which occurs on site, to estimate with numerical simulation based on the phenomenon and to plan an accommodative measure taking into account of uncertainty of the estimation are required. This paper introduces technical attempts from those 3 perspectives. A problem to be solved is to improve the measure against port sedimentation including domestic and overseas technical progress. We have been conducting overseas project about port sedimentation with technology which has been cultivated in Japan and continuing to contribute improvement of port accessibility in the world.