Analysis of Coastal Morphology and Sedimentation
for Jingtang Excavated-in Port

Sun Ziyu

Professor, Senior Engineer, Deputy President of China Communications

First Design Institute of Navigation Engineering (FDINE)

1    Introduction

Ever since the earliest civilisations, the seacoast has held great appeal to man - ancient and modem. “Those living on a mountain live off the mountain, those living near the sea live off the sea” - make use of local resources. As an old Chinese folk adage, it has been proved by the humanity history.

The typical element of the coastal morphology in the Jingtang Port area is barrier islands (see the outline picture). The sediment from Luanhe River laid a foundation for the development of the barrier island coast during the different historical time and the different estuary position. It is essential to understand the regularity of the coastal evolution and the sediment transport for the construction of an excavated-in port on the typical barrier island coast. The morphological processes at work on coastal landforms are influence by a number of environmental factors, notably geological, climatic, hydrodynamic, sediment transport, even human activities. Modification of coastal landforms by human activities may be either direct or indirect. The building of sea wall, groynes, and breakwaters, the dredging of harbour entrances, and reclaiming or dumping material on the coast and offshore. The last two aspects like Jingtang Port did, are all direct influences of coastal topography. Consequently, the interaction of coastal morphology and human activities will re-build the natural equilibrium exerting a great influence on the harbour siltation and shoaling.

Fig.1    Coastal morphology in the Jingtang Port area

2    Objective and Methodology

The principal objective of this study is to find a reasonable and economical solution to prevent or minimise the siltation of Jingtang Port's entrance. To do so, require an understanding of the factors and processes at work in the coastal morphological system: the origin of the barrier island coast, the sources of sediment, the pattern of change, the paths of sediment flow, the sediment accumulation of the approach channel, and the quantities involved under given engineering conditions and periods of time. The scale concepts will be drawn into this study.

l       Scale level 1: macro-analysis of a relative large range of coast focused on coastal morphological processes.

l       Scale level 2: studies at the intermediate level, which involve the sediment transport and the harbour shoaling.

l       Scale level 3: small-scale studies of port engineering problems focused on optimum and development of layouts to minimise the entrance channel sediment accumulation.

Studies natural processes of coastal morphology play an important role to reduce and to minimise the negative effects of building breakwaters on the coast in the around conservation of natural resources. The prediction of sediment accumulation on a navigation channel is an extremely important, but difficult goal. Some of methods used to calculate and estimate the siltation of the channel will be discussed and recommended.

3    Coastal Development and Evolution

Coastal morphological processes differ from subaerial processes in that water movements in the nearshore environment are more complex, the relationship between the processes and responses in the coastal zone are particularly complex. 70 km long coast along Jingtang Port is the typical barrier island coast in China; it can be called the Luanhe Delta coast, which compose the interface between the delta plain of Luanhe River and the Bohai Gulf.

It is clear from researching works that barrier islands originated in a variety of ways. The two main modes for barrier origin are by long-shore growth of spits and by development of emergent beaches offshore. But many barriers have had a composite origin. Barrier islands are best developed where the tidal range is relative low, littoral regime of moderate wave energy, and gently sloping coasts. The plentiful supply of sand-size material can reach the barriers either from along-shore or offshore.

Along the Luanhe Delta coast, there is the moderate wave energy with tidal range less than 1-2 m, coastal setting of low sloping plain (slope about 0.3%), and an abundant supply of sediment of sand-size (annual average about 20 million tons/year). These conditions laid an essential foundation for the development of barrier islands.

Zenkovitch (1957) who analysed the prevailing and strong wind direction and the orientation of spits along the Luanhe delta coast deduced that the origin of these barrier islands was long-shore sediment transport generating. However, his viewpoint is full of contradictions to the practical features of the development and distribution of local barrier islands, which are analysed as following.

(1) The barrier islands expresses a discrete segment distribution characteristic, they extend interlocking but not touching in proper order from the Southwest to Northeast. The ends or hooks of SW barriers ordinarily flank the landside of the ends of NE barriers. It means that barrier islands originated neither due to the long-shore sediment transport nor forming at same period. The barrier formed much later the further from the SW, this is well agreement with the migration of the Luanhe River basin from SW to NE.

(2) It is obviously that barrier islands located on the seaward edge of the modem Luanhe delta formed due to cross-shore sediment transport. The Luanhe River outlet started at Daqinghe mouth, HulinKou and LaomiGou after 1453, since then, the meteorological and sediment regime of the Luanhe River drainage and coastal areas has not distinctly changed. It can be deduced that when Luanhe River flowed into the sea via these outlets, the formation of the old deltas should be the same as the modem one. Barriers were formed bulges of the shoreline about some distance into the sea.

(3) From SheGang to CaofeiDian, there are abandoned old outlets on the mainland just opposite the barriers. These barrier islands were formed during Luanhe River flowing into the sea via the different outlet. Following figure shows the developing process of the alluvial plain-delta of Luanhe River after the post-glacial period.

 (4) Although the wind-wave from various directions, waves are almost perpendicular to the shoreline due to the refraction, producing the dynamic condition for the barrier islands formation.

To sum up, it is not reasonable and suitable to explain the origin of local barrier island coast by using the long-shore sediment transport processes. The formation of the local barrier can be considered as the result of the cross-shore sediment drifting by readjusting the sand deposited on the shallow sea bottom during Luanhe River flowing into the sea at different location and in different time period. Wave action should be the main generating force for forming barrier islands. CaofeiDian was formed when Luanhe River flowed into the sea via ShouHe and XiaoqingHe River, and then, Luanhe River shifted to the Northeast flowing out via Daqinghe River forming DawangGang, ShijiuTou and YueTou barrier islands. Later, Luanhe River migrated to NE further forming HulinKou, DawangPu, DenglongPu and SheGang barrier chain. Froml915, Luanhe River started flowing into the Bohai Gulf at present location, the modem barrier island chain with arc-shape was formed.

In the early stages of development, the barriers are commonly formed adjacent to wave breaking zone, and these are referred to as offshore bars. The barriers became emergence due to the storm waves action; finally, a chain of barrier islands has formed bulge of the shoreline about few kilometres into the Bohai Gulf, enclosing lagoons. Luanhe River supplies plentiful sand-size sediment for building the barrier islands. After the Luanhe River migrated and new delta growth had become well established at the new outlet, the old delta coast began to be cut back re-modified nearly as straight because the sediment supplies became insufficient. The origin of local barrier islands produces a deep influence to the developing layouts for Jingtang Port.

The barrier island coast along the Luanhe mouth to Daqinghe mouth is the edge of the Luanhe Delta. Since Holocene epoch, Luanhe River frequently swung in this area forming a fun-shape delta having the top of Luanxian, and a long chain of barrier islands was formed in front of the delta. The old coast has been cut back after Luanhe River shifted to another position. Even along the modem delta coast, barrier islands are been eroding due to a great amount reducing of sediment supply from the river.

The local barrier islands shifted landward about 450 m in average from 1976 to 1994, the shifting velocity is about 25 m/year. As a result of building a reservoir in upstream of Luanhe River, a great quantity of sediment was intercepted since 1970s; the sediment supply to the coastal area has been massively decreased. But the long-shore sediment transport capacity is no more difference from 1970s to 1990s, the erosion of barrier island is the inevitable outcome of the coastal dynamic actions, and also affected by cross-shore sediment transport, especially during storm period. Barrier islands seaward side and offshore sea bottom near them is eroded, the eroded sediment re-deposit on the top or the landslide of them; therefore. Barrier islands are shifting landward year by year meanwhile the erosion process of them is ongoing.

4    Analysis of the Sedimentation

The effective protection against sediment accumulation is the key to the design and construction of an excavated-in harbour in the barrier island coastal segment with active sediment transport, composting long-shore and cross-shore.

After the Jingtang Port started operation and the access channel was opened for navigation, regular bathymetric surveys were carried out until 1994 to monitor the sediment accumulation inside the channel. The annual siltation behavior of the access channel from 1992 to 1994 is given in following table.

    Annual siltation of the access channel

Isograph of Siltation Pattern  (1992,5—1992,11)       (1992,11—1993,3)      (1993,3—1993,12)

Fig.3    Isographs of the siltation pattern

Isographs of the siltation pattern are shown in above figures. The heavy siltation occurs in the entrance channel part, especially concentrating on the area of intersection of the breakwater extension line and the entrance channel segment. The investigation reveals that the heavy siltation usually occurs in autumn and winter, just after storm surge processes.

Since an access channel is dredged, the local environmental condition must be changed. It necessarily leads to the re-adjusting of the seabed landform adapting to the hydrodynamic and sediment conditions. It is contended that there are two main sources of sediment resulting in the sediment accumulation in the channel. One is the long-shore sediment transport, which depends on the on-shore wave energy generating the long-term influence on the sedimentation of the channel. Other one is local eroded material due to the effect of breakwaters, especially the eastern breakwater, generating relative short-term influence on the channel sedimentation. The prediction or estimation of the sedimentation of the dredged channel is very important for the designing a harbour, especially a harbour with the open dredged channel. When currents cross the dredged channel, the sediment transport capacity decrease due to the increase of the water depth, and the bed-load material and a certain amount of the suspended sediment will be deposited in the channel. The most relevant processes to sedimentation of the channel include: the convection of the particles by the horizontal and vertical fluid velocities; the diffusion or mixing of the particles due to the current-related and wave-related mixing processes; the settlement of the particles due to gravity; and the pick up of the particles from the seabed by the flow.

Basically, a relatively accurate sedimentation prediction requires a detailed field survey to determine the boundary conditions such as: current velocity; streamline pattern; wave characteristics; salinity; size, composition and porosity of bed material; sediment concentration; particle fall velocity of suspended sediment and effective bed roughness. Further improvement of the accuracy can be obtained by carrying out a trial dredge investigation. Such an investigation can be considered necessary when the costs of the capital and maintenance dredging values of the planned channel are relatively large compared with those of the trial dredge channel. The sedimentation rate that observed in the trial dredged channel could be used to check or calibrate the available prediction methods resulting in a more accurate sedimentation prediction. Here, the practice survey data of sedimentation of the channel will be used to check the prediction accuracy, and to calibrate the empirical coefficients. The calculating results are given in following table by using the formulas ofC.V.Gole (1971), Bijker (1980), Liu Jiaju (1984), and Van Rijn (1986).

  Calculation of the sedimentation rate of the entrance channel (m/9 months)

As seen in table, the predicted sedimentation value that calculated by four formulas is well agreement with the measuring data because the relatively accurate measuring data are used for the calculation. These four formulas can be divided into two types, C.V.Gole formula and Liu Jiaju formula belong to the empirical formula, which accuracy is large degree depended on the measuring data used for calculation and calibration. Bijker and Van Rijn formulas should be the theoretical formula, which consider almost all-main factors affecting the sedimentation processes, and relatively high accuracy is obtained by using Bijker and Van Rijn formulas.

5    Development and Optimum of Layouts for Jingtang Port

The Comparison of the total costs for three scenarios for improvement of the entrance channel is as follows:

  Comparison of the total costs for three scenarios (* million U.S. dollar)

Note: analysis based on “Present-Day-Value” has not been carried out, but it will have a lower effect on scenario 1 and 2.

The costs of the breakwater usually make up for a quite large percentage of the total costs of a harbour's infrastructures. Therefore, it is necessary to determine the reasonable length of breakwaters, which can be implemented by comparison of the breakwater constructing costs and maintenance dredging costs. The design life of breakwater is taken 50 years and maintenance costs of breakwater make up 1% of the total capital investment. The result is shown in following figure. As seen in figure, the head of the eastern breakwater is located at -2.0 m depth contour while the breakwater length is 600 m, the same, the tip at -4.0 while the length is 1500, and the tip at -8.0 m while the length is 3400 m. The lowest point of integrated curve indicates that the length of breakwater is about 2,500 m, which implies the head of breakwater reach to -6.5 m depth contour. The present situation of the eastern breakwater of Jingtang has been agreement with the analysed result that is reasonable and further extension will not be economical. Finally, the strengthening maintenance dredging is recommended for improvement the sedimentation of the entrance channel of Jingtang Port.

6    Conclusions

According to the scale concepts, the local environmental conditions, the morphological features of development and evolution, and sediment transport along the typical barrier island coast, have been discussed. The regional topographic change of seabed due to construction of the port of Jingtang, the sedimentation of the approach channel and its solution has also been emphasized.

l       Along the Luanhe delta coast, existing conditions of moderate wave energy with relatively small tidal difference, gently slope, and an abundant supply of sediment of sand size, laid an essential foundation for the origin of barrier island coast. Barrier islands of the study area can be considered as the result of cross-shore sediment drifting by readjusting the sand deposited on the shallow sea bottom during Luanhe River flowing into the sea at different locations and in different historical periods.

l       In the early srtages of development, barrier islands are commonly formed adjacent to the wave breaking zone, and these are referred to as offshore bars. The barriers become emerged due to storm wave action forming a chain of barrier islands bulged the shoreline, enclosing lagoons. Analysis of bathymetric data and interpretation of aerial photographs and satellite images, reveals that the old delta coast has been cut back extensively; re-modified nearly straight because the sediment supplies become insufficient, after Luanhe River migrated and a new delta growth had become well established at the new outlet.

l       Analysis of the sedimentation pattern of the approach channel reveals that the siltation threshold occurs in the entrance channel. The sedimentation is acted by a combination of long-shore and cross-shore sediment transport depending on the on-shore wave energy for the long-term influence on it, and local erosion due to the effect of breakwaters for the relatively short-term influence on the channel siltation. Comparing the prediction formulas of sedimentation for the dredged channel, indicates that both Bijker formula and Van Rijn method are preferable to predict sedimentation of the approach channel for Jingtang Port.

l       Based on the situation of Jingtang Port, three scenarios for minimising of the siltation of the channel have been presented, including maintenance dredging only, maintenance dredging "strengthening", and the extension of breakwaters. The result of the comparison of three scenarios indicates that all of them are feasible, but the maintenance dredging "strengthening" solution is more economical, which is finally recommended for improvement of die sedimentation of the approach channel for the Jingtang excavated-in port.