DYNAMIC CHARACTERISTICS OF SEA CURRENTS AND SOME ISSUES FOR SUSTAINABLE DEVELOPMENT IN THE YELLOW RIVER DELTA

 

HU Chunhong ¹ and Ji Zuwen²

 

(China Institute of Water Resources and Hydropower Research)

Dept. of Sedimentation Engineering, IWHR

P. O. Box 366

Beijing 100044

China

Tel. 86-10-68415522-6659

Fax. 86-10-68416371

 

 

ABSTRACT

Dynamic characteristics of sea currents in the Yellow River Estuary, such as tidal current, residual current and storm surges are studied and the direction and quantity of sediment dispersion are analyzed in the paper .The resultant direction of sediment dispersion is found northeast. The sea currents transport 40%of the total sediment load from the Yellow River to the mid sea. Optimum direction of the Yellow River outlet for full utilizing the sea currents to transport sediment to the mid sea, and strategies for stabilizing the Yellow River outlet are proposed. Some important factors for the Yellow River delta sustainable development are studied.

 

1 INTRODUCTION

The Yellow River Estuary is a weak tide estuary. The Yellow River carries annual average of 1.0 billion tons sediment to the estuary region, 50 per cent of sediment deposit on the estuary region, 20 per cent of sediment on rive course and delta,30 percent of sediment are carried into deep sea. Such a great amount results in a rapid seaward extension of the estuary with a mean rate of 2.6 km/a and a recent rate of 1.1 km/a. It also results in a rapid change in the location of the outlet with a large amplitude, which is a serious threat to flood control. The fluvial processes in the estuarine area are caused by the action of both the river and sea, because the estuary is where the river interacts with the sea. Therefore, one of the key issues in the study of the evolution of the Yellow River Estuary is to analyze both the river and ocean dynamics. Many studies in this field have yielded fruitful results [1-6]. Due to the lack of the field data, however, those studies mainly focused on river dynamics and the resulting fluvial processes. From the viewpoint of fluvial processes and regulation of the Yellow River Estuary, ocean dynamics is of vital importance for the regulation of the Yellow River Estuary. This paper is an attempt to investigate the regulation of the estuary and to suggest the utilization of ocean energy on the basis of the analysis.

On the other hand, Since the Shengli oil field was discovered and developed on the Yellow River delta in 1960¢s, the society and economics of the Yellow River delta have developed rapidly. For this reason it is required that the river course at the estuary is never diverted freely as past. If not , it will exert heavy losses of economics. Even if diverting the course at the estuary is required, it must be conducted under the control of mankind. Stabilizing the river course at the estuary has been the objective requirement of sustainable development of the Yellow River delta, Therefore some important factors for the regional sustainable development are studied.

 

2 TIDES AND TIDAL CURRENTS

2.1 Tides

Tides in the Yellow River Estuary are complex and have three distinguished, features, i.e. multiple tidal patterns, swaying of the channel outlet, and wandering of the exit channel. After the tide from the open sea enters the Bohai Strait, it splits into two branches in the Bohai Sea under the influence of the Coriolis force and local topography. One branch propagates northward, rotating clockwise in the Liaodong Bay, and forming an amphidromic point in the west of the Liaodong Bay. The other branch, however, first propagates westward, rotating counterclockwise in the Bohai Bay, then it propagates eastward, forming an amphidromic point at Wuhaozhuang near the Yellow River mouth. In the amphidromic region, the tidal range of the M₂ component is almost nil while the velocity of tidal currents is the highest (Fig.1).

 

Fig.1 Location of tidal stations in the Yellow Rive

 

Tides in the Yellow River Estuary are of multiple patterns. The irregular semidiurnal tide exists in most of the estuarine area while the irregular diurnal tide only occurs in a small region outside the Shengxiangou outlet. The diurnal tidal area coincides with the amphidormic area of the M₂ component. However, the farther away from the Shenxiangou outlet, the more obvious the features of semidiurnal tide. Two types of tides exist in the irregular semidiurnal tidal area due to tidal range difference: neap tide in the region west of the amphidormic point, and high tide in the south of that point.

The existing outlet of the Yellow River is 35 km south of the amphidromic point. Tides change rapidly because of the amphidromic point. The characteristics of tides, tidal ranges and duration of rises and falls in the two regions on the two sides of the outlet are different. The data collected at Gudong and Hekou tidal stations (see Fig.1) in 1991-1992 indicate that the tidal levels vary slightly in this region; from Gudong to Nanlanni stations the tidal range is less than 2 m. The tidal range at the amphidromic point outside the Shengxiangou outlet is smallest, 0.8 m, and increases to 1.6-1.8 m at the apexes of the neighboring bays (Bohai and Laizhou Bays). The distribution of tidal levels along the delta shoreline is in the shape of a saddle, which is low in the middle and high at the two ends. Table 1 presents the harmonic constants of the principal tidal components at various tidal stations. From Gudong to Hekou

 

Table 1 Harmonic constants at the tidal stations

 

and Nanlanni tidal stations, the percentage of diurnal components K₁+O₁ decreases gradually while that of semi-diurnal, components M₂+S₂ increases gradually, therefore, the ratio of (K₁+O₁)/( M₂+S₂)changes from 2.47 to 0.28. This change indicates that from the north to the south of the existing outlet, the tide pattern changes from an irregular diurnal tide to an irregular semi-diurnal tide, along with a tidal range increase from 1.0 to 1.3 m. Moreover, the local topography and shallow water have resulted in an unequal duration of rise and fall on both sides of the existing outlet. In the north, the duration of rise is longer than that of fall; for example, the rise at Zhonglanni tidal station lasts 7 hours while the fall lasts 5 hours. In the south, however, the fall lasts longer than the rise; for example, at Nanlanni tidal station, the duration of rise is 4 hrs 50 min while that of fall is 7 hrs 40 min. This unequalness in duration has a significant impact on the sediment transport in the estuarine area; it also helps in maintaining a relatively stable direction of the existing outlets.

As the Yellow River Estuary is a weak tide estuary, the tidal reach of the Yellow River is short, 17 km in the dry season and 9 km in the flood season. The tidal current reach is less than 6 km and sometimes does not exist at all. Within the tidal current reach, saline wedge appears in the channel. The front of the wedge reaches 0- 6 km upstream in the mouth during the flood season and 4-7 km during the dry season. This shows that only a very short river reach is affected by the sea currents. Ocean dynamics mainly manifest themselves in the river mouth region.

 

2.2 Tidal Currents

In the Bohai Sea the tidal currents are rotational. The elliptically of tidal current in mid sea is 0.3; however, due to the shore restriction, it declines to 0.1 in the offshore area. Generally, the long axis of the tidal ellipse is parallel to the shoreline. At the apexes of the two bays tidal currents are almost perpendicular to the shoreline. Fig. 2 shows the velocity field of tidal currents in the offshore region of the Yellow River delta after Qingshuigou outlet became

 

Fig.2 Measured fields near offshore of the Yellow River delta

the outlet channel of the Yellow River in 1976. Two enclosed regions of high tidal currents velocity exit; one at the exit of Shengxiangou outlet, the other at the exit of Qingshuigou outlet. However, the change in the maximum tidal current is more obvious at the center of both regions, and is ever more significant at the Qingshuigou outlet exit. The velocity was 1.87-1.90 m/s in 1984, 1985 and 1987, but has increase to 2.2 m/s in 1990 under the combined actions of sand spits extension and sea bed increase at the exit. Both the protrusion of spits and the slope increase are beneficial to the seaward transport and dispersion of the Yellow River sediment.

Within a certain distance from the exit of the channel outlet, the velocity of ebb tidal current is larger than that of flood tidal current; beyond this region the former is smaller than the latter. Correspondingly, the sediment concentration of ebb tidal current in the outlet channel is 2-4 times that of flood tidal current.

 

3 RESIDUAL CURRENTS AND STORM SURGES

3.1 Residual Currents

Residual current is the difference between the actual sea current and the periodic tidal current. They are induced by wind, local topography, density difference, river runoff, atmospheric pressure difference, etc. The complexity of factors results in great variation in the residual current¢s magnitudes and directions. In the sea off the Yellow River delta, wind is the prevailing non-periodic force to move sea water; thus the wind currents are much larger than the residual currents induced by other factors. In spring and summer, the surface wind current propagates northward under the southern monsoon impulsion. Whereas in winter, it moves southward under the northern monsoon. The magnitude of wind current is generally 0.1-0.3 m/s. In the region near the delta front the residual current is mainly affected by riverine runoff, therefore its direction outside the outlet exit coincides with that of the riverine flow direction. Table 2 presents residual currents data obtained in October 1992. The farther from the exit of the outlet, the smaller the magnitude of the residual current. The current direction was southward, same as that of the river flow.

 

Table 2 Residual currents measured in October 1992

 

3.2 Storm Surge and Its Role on Sediment Transport

According to the wave data at Dongying Harbor the occurrence probabilities of H1/10>2.2 m wave in NNE-E and H1/10>1.0 m wave in NNE-SSE are 1.5% and 9.4%, i.e. they occur 5.5 and 34.5 days in a year, respectively. The annual sediment load, under the normal flow conditions (water depth over the mouth bar is 1 m, H1/10>1.0 m, wave steepness 0.044 and mean bed sediment diameter 0.035 mm) and the sediment load during a storm surge were calculated using the formula of sediment load due to wave action. For the storm surges, the sediment load in two consecutive days were calculated for setup heights of 1 m and 2 m (water depths over the mouth bar were 2 m and 3 m respectively, H1/10=2.5 m, wave period T=6.0 s). The sediment load for two days, using the 1 m setup accounts for 57% of the sediment load in 34.5 days under normal conditions. The sediment load for two days using the 2 m setup is 2.18 times the sediment load for 34.5 days under normal conditions. The storm surge on April 5,1964, for example, had a storm current of 1.0 m/s, strong winds, and a mean velocity on a vertical during the ebb tide of 1.0 m/s. As the suspended sediment transport capacity is proportional to velocity, a storm surge will increase the sediment transport capacity by 4 times and the sediment discharge by 2 times of those under normal conditions. It is estimated that the sediment discharge over the mouth bar, during a setup by a storm surge, may be many tenfold of that under normal conditions.

 

4        DIRECTION AND AMOUNT OF SEDIMENT TRANSPORT IN THE YELLOW

RIVER ESTUARY

4.1 Direction of sediment Transport

When a large amount of sediment carried by the Yellow River enters the nearshore area, it deposits rapidly due to the decrease of flow velocity resulting from planar diffusion and retardation by the sea. This sorting effect of waves, coarse silt deposits near the exit of the outlet, while fine silt and clay are transported further to the outlet sea. In the end of a dry season, sediment is transported to the southern region outside the outlet by the action of northwest winds and southward tidal currents. The high sediment concentration diffuses like a belt while the low sediment concentration diffuses like a plume along the shoreline. In the flood season under the combined action of southwest winds and tidal currents, sediment diffuses northward; high and medium sediment concentration still diffuses like a belt while the plume of low sediment concentration diffuses further seaward. At beginning of the dry season with weak winds and sea currents, sediment diffusion manly follows the direction of a diffused riverine flow in the sea, i.e. a transitional pattern of sediment diffusion of high, medium and low concentrations.

Fig.3 shows the paths of sediment transport during the flood and dry seasons in 1989,indicaing that the direction of sediment transport in both seasons is northeast, entering the middle of the Bohai Sea. The satellite images in the tow seasons in 1989 show the same trend of sediment transport.

 

 

Fig.3 Path of sediment transport in the Yellow River estuary

 

4.2 Amount of Sediment Transported to the Deep Sea

In addition to deposition of sediment in the river channel, on the floodplains, and in the nearshore region, a part of sediment from the Yellow River is transported to the deep sea. From June 1976 to September 1985,the mean annual sediment load at Lijin station(the entrance station of the estuary) was 0.861 billion tons, of which 17.6% deposited in the river channel and on the flood plains (above 0 m of Dagu datum), 57.6% deposited in the delta region, and 24.8% was transported to the deep sea. Since 1988, experimental dredging and river training have been carried out, resulting in a single eastward outlet channel and the increase of sediment transport capacity of the outlet channel. As the direction of the river flow was perpendicular to the ocean current, it is also favourable to sediment transport to the deep sea. From 1988 to 1993 the amount of sediment deposition in the delta region was 55% of the total incoming sediment load to the estuary, a bit smaller than that during the previous period. During the periods from September 1988 to October 1989 and October 1989 to September 1990, the amounts of deposition in the river channel and on the floodplains were 0.0429 billion tons and 0.0138 billion tons, i.e. they accounted for 7.1% and 2.7% of the total incoming sediment load of the estuary, respectively. As a rough estimation, 5% of incoming sediment load deposited in the river channel and on the floodplains; this amount was smaller than that in the previous period. Based on the above-mentioned results it can be estimated that about 40% of the total incoming sediment load was transported to the deep sea (See Table 3); this amount is larger than that of the previous period.

 

Table 3 Amount of sediment deposition in the shallow delta region

 

5        DISCUSSION ON KEY POINTS OF SUSTAINABLE DEVELOPMENT OF THE

YELLOW RIVE DETLA

5.1 A stable flow course and mouth harnessing is the major presupposition of the sustainable development with the industrial and agricultural development in the delta region, especially the exploitation of the Shengli Oil field, and the stability of the flow course of the Yellow River mouth has become the restricted factor for developing regional economy on the delta. A stable river flow course to the Baohai Bay based on the reasonable treatment of sediment deposition will result in a relatively stable distribution of industry and agriculture. This is a significant presupposition of the sustainable development of the Yellow River delta.

5.2 To ensure the water resources supply is a reliable foundation of sustainable development . Both the daily life of local people and the development of industrial and agricultural production on the delta can not deviate from the supply of water resources, but the local water resources is very deficient with an average personal water of 314 m³/yr. According to analysis, if the sustainable development need to be assured, the annual water supply for the delta should be increased to 1´10⁸m³. Therefore, a dependable supply of water resources is a foundation of the sustainable development for the delta region. Only the mud water from the Yellow River, the industrial waste water, living waste water, ground water, light salt solution near ground surface and surface water can be fully utilized, then the shortage of water resources can be effectively resolved.

5.3 Environment protection, sedimentation control and risk prevention are the guarantee of the sustainable development. The menace of environmental conditions of the Yellow River delta is caused by the waste water and waste gas from the oil field, and the natural disaster. Meanwhile the development of the delta also is restricted by floods, Storm tides, ice-floods and waterlogging. Therefore the weakened environment seriously affects the development in the delta region, and the environment curing and risk prevention is the important guarantee for the sustainable development of the Yellow River delta.

5.4 Exploitation and multi-utilization of Oil is the motive power of the sustainable development. The oil exploitation of shengli Oil field is a supporting industry in the delta. The exploitation and multi-utilization of Oil will strengthen the economical potential and promote overall development in the delta region.

5.5 Protecting the natural wetland is another restricted condition of the sustainable development of the delta. The Yellow River delta with a total area of 6000km² has a wetland area of 4500km² located in the intertidal zone. The formation of the wetland is closely related to the deposition of sediment carried by the Yellow River. It is a world famous marshland with good ecological system. The wetland is not only of benefit to the existence and multiplying of living beings, but also provides a rarely good environment to the life of local people and development of soci-economy. Mankind must control themselves, reasonably take advantage of the natural resources, coordinate the relation between economical development and the eco-environment protection, maintain the basic ecological process, life system and organism diversification. Any damage of the natural wetland would inevitability affect the basic conditions of the existence and development of local people and become an important restricted condition of the sustainable development of the Yellow River delta region.

 

6 CONCLUSIONS

6.1 The Yellow River estuary is located in a sea of weak tide. The tidal reach is short, 17 km in the dry season and 9 km in the flood season; the tidal current reach is only 4-7 km in the dry season and 0-6 km in the flood season. There are many patterns of tide in the estuary, and most of them are irregular semidiurnal tides. An amphidromic point exists at the Shengxiangou outlet. The average tidal range in the estuarine region is 1.0-1.3 m.

6.2 There exist two high flow velocity regions in the estuarine region: one at the exit of the Shengxiangou outlet, the other at the existing outlet.

6.3 The average residual current is about 0.1-0.3 m/s and larger on the sea surface than on the bottom. Storm surges occur 3 to 4 times each year, which plays a key role in sediment movement and the evolution of river mouth.

6.4 The yearly resultant direction of sediment dispersion in the Yellow River estuary is northeast; About 40% of the total incoming sediment load to the estuary is transported to the deep sea under the existing conditions of ocean dynamics.

6.5 Tidal currents, residual currents and storm surges play an important role in sediment transport to the deep sea and stabilization of the river channel outlet, therefore, the role of such agents should be fully considered during the regulation of the Yellow River estuary.

6.6 Some important factors for the sustainable Development of the Yellow River Delta include that the mouth harnessing and river channel stability are the prerequisite; the water resources support is the foundation; the environment, improvement and the risk prevention are the guarantee; the oil production its integrated utilization are the driving forces; the natural wetland reserve is an important constraint condition.

 

REFERENCES

1. HOU Guoben, SHI Maochong and WANG Tao 1993,The Dongying Harbour, Ocean Press.

2. HU Chunhong et al 1995,Evolution of the Yellow River mouth bar and its regulation, IWHR.

3. Institute of Oceanography, Ministry of Geology and Mineral Resources, Survey on Sedimentary Dynamics in the Yellow River Estuary, 1993.

4. JI Zuwen, HU Chunhong et al 1994,Analysis of recent evolution of the Yellow River estuary by landsat images, Journal of Sediment Ressearch,No.3.

5. JI Zuwen, HU Chunhong et al 1995, Analysis on recent evolution of mouth bay of the Yellow River, Journal of Sediment Research, No.3.

6. LI Zegang 1991,Flow fields and their variation offshore the Yellow River delta, Research on the Yellow River Delta, First Issue.