deteriorated the ground in the vicinity of the piles and beneath the dike.
3.2 Waterway Submerged Dike
In the sea area around Kumamoto Port soft bottom materials, consisting of silt and clay, are distributed over a wide range. Such bottom materials are carried by waves and currents, sedimenting at waterways and anchorages, this phenomenon is called siltation, which may possibly obstruct smooth navigation and docking and undocking. Hence, efforts have been made since 1985 to develop a siltation prevention work method in a soft ground sea area.
3.2.1 Observation at Trenches
From 1987 to 88 three sand traps (30 mW × 50 mL × 2 mD) were dredged in the Kumamoto Port sea area to observe-siltation. The results are as shown in Fig. 12. Trenches not surrounded by submerged dike siltated 150 or so cm for one year whereas those surrounded by 1-m high submerged dike did not show noticeably distinctive siltation even in rough sea. This confirmed that the installation of submerged dike is highly effective to prevent siltation.
3.2.2 Forecasting Siltation
?@ Development of a siltation forecasting simulation model
For the purpose of estimating siltation preventation effect of submerged dike, a numerical simulation program was developed to study the effective arrangement of submerged dikes.
Simulation was executed based on findings about siltation obtained from laboratory experiments on the physical characteristics of bed material and from field observation at sand traps. As a result, two models were established : 4th District Port Construction Bureau model and Port and Harbour Research Institute model. The general description of these models is as follows.
<4th District Port Construction Bureau model>
In this model siltation in the dredging area is considered to be caused by density current flow of the high-concentration SS layer on the bottom. Hence, the density of the suspended mud layer in the vicinity of the sea bottom is fixed to a given level, so that increase and decrease in the amount of suspended mud are expressed in increase and decrease in the layer thickness. The model is thus a 2-layer model.
<Port and Harbour Research Institute model> 7)
This is a 7-layer model, for three dimensional calculation of tidal current, wave and bed material diffusion.
?A Consideration of the optimum arrangement of submerged dikes 8)
To obtain the most effective arrangement of submerged dikes for waterways and anchorages at the time of temporary service of Kumamoto Port in 1992, four settings of dike arrangement and height were studied as shown in Fig. 13, comparing with the case without such dikes and calculating the estimated amount of siltation using the two models.
Data of wave and sea level correspond to those recorded during a storm on August 31, 1987.
Shown in Fig. 14 and Table 2 are the results of calculation using the 4th District Port Construction Bureau model. The installation of submerged dike greatly reduces siltation in all cases, namely to 1/4 to 1/13. The case with the least siltation indicated a curve which was determined in consideration of the flush effect that suspended mud is forcibly flushed toward offshore during ebb tide. Case D as well, in which the height of the dike on the coast side, where a strong mud flow exists due to the shallowness of water, increased from 1.0 to 1.5 m, was least subject to siltation. Hence, case D was chosen for the basic arrangement of submerged dike since this was capable of preventing as well as discharging siltation most effectively.
