ON THE THRESHOLD OF RIVER CHANNEL SHRINKAGE[1]

Chen Dong

China Institute of Water Resources and Hydropower Research,

Beijing, 100044, China, 86-10-68415522-6633

Tang Chun-Ling

Beijing Lili New Technology Development Corporation,

Beijing, 100055, China, 86-10-63455836

Cao Wen-Hong    Hu Chun-Hong

China Institute of Water Resources and Hydropower Research,

Beijing, 100044, China, 86-10-68415522-6633

Abstract: The development and utilization of water resources in a river basin become more and more intensive, leading to hydrological changes and river channel shrinkage. Based on the principle of geomorphic thresholds and analyses of large amounts of field data, the author set up a new conceptual model of channel shrinkage. The determination of the dominant discharge of channel formation under shrinkage conditions has been presented and the relationship of dominant discharge with channel morphology under shrinkage conditions applied. The external threshold of the channel shrinkage in the lower Yellow River is also determined with the mathematic model.

Keywords: shrinkage, river channel, dominant discharge, channel formation, threshold

1    INTRODUCTION

Since the concept of thresholds was firstly introduced into Geomorphology in 1970s^[1], extensive works have been undertaken by researchers all over the world ^[2-4]. However, as a significant branch, the study of channel morphologic thresholds has not been paid due attention until recent years. Water conservancy projects built in last century showed that, reduction in runoff does not mean that shrinkage is taking place. The impact of a high dam on the downstream channel gradually ran into severity only after a long period of time ^[5]. However, development and utilization of water resources in a river basin become more and more intensive in recent years, leading to the cumulative variation of water runoff and sediment load. Correspondingly, in the river channels also occurs some variation, mainly the decrease of conveyance capacity of floodwater ^[6-7].

Shrinkage of river channel is a problem internationally concerned. After the completion of the Aswan High Dam, the maximum daily average discharge during flood season reduces from 10,000 m³/s to 2,500 m³/s. Intensive change of the hydraulic conditions leads to the readjustment of downstream channel as the reduction of the cross sectional area ^[8-9]. G. E. Petts summarized the results of some researchers and also pointed out that in the UK and USA many reservoirs function to flatten the peak flow by regulation which results in the decreasing of conveyance capacity of floodwater ^[5]. Starting from 1986, the lower Yellow River in China has experienced a longer period of low water with insufficient sediment load than previous years. Because of the lack of channel formation flood, the channel cannot be recovered after the flood season, which is quite unfavorable for flood control. In Aug. 1996 a medium-sized flood occurred in the lower Yellow River. The first flood peak at Huayuankou hydrometric station was 7,860 m³/s, but the corresponding flood level was 94.73 m, which exceeded the historical record of the maximum flood level at the station. In August 1998, the first flood peak at the station was only 4,700 m³/s, yet the corresponding flood level raised up to 94.38 m.

2    THRESHOLDS AND EQUILIBRIUM IN FLUVIAL PROCESSES

The threshold of river channel should not be confused with the equilibrium. They are different stages in fluvial processes. The relationship between them can be analogized with the mechanical movement of a sphere in gravitational field (Fig 1).

Assume the intermittent tractive force acting on the sphere horizontally to be small or with a short duration. The sphere will roll right and left near the bottom of concave (Position 1 in Fig.1). Once the impact of the tractive force exceeds a certain critical value, the sphere will roll through the vertex of the convex (Position 2 in Fig.1) and then into another concave (Position 3 in Fig.1).

The river system is a complex open system with unsteady boundary conditions, however it follows an analogous physical law as the above mechanical movement. According to the hypothesis of minimum energy dissipative, a system under its equilibrium has its maximum entropy. A system has a tendency to its equilibrium state at most cases, i.e., so-called the inertia of system. For example, the self-recovery function of the river channels in fluvial process. On the other hand, the field of outside forces and boundary conditions vary all the time, leading to the fluctuation of the system state. The deviation from the equilibrium is absolute and permanent, whereas the equilibrium state is relative and transient. If the variation of outside forces or boundary conditions exceeds a certain critical value, the fluctuation of the system can become so large that the system will break away from the original inertia and run into another equilibrium range.

3    VARIATION OF OUTSIDE BOUNDARY CONDITONS OF RIVER  SYSTEM¡ª¡ªTHE MUTAGENESIS OF ONCOMING RUNOFF AND SEDIMENT LOAD IN THE LOWER YELLOW RIVER.

Channel shrinkage, similar to any other phenomenon in fluvial processes, stems fundamentally from the imbalance of sediment transportation. The disparity between channel shrinkage and normal deposition is due to the change in conditions of oncoming runoff and sediment load, which presents a trend called mutagenesis. The mutagenesis of oncoming runoff and sediment load in the lower Yellow River is characterized by the following features:

(1) The total amount of runoff and sediment load decreases markedly after 1985. The dominant causes were human activities, which were irreversible and non-periodic compared to natural factors ^[10] and hence play a cumulative role in the evolutionary tendency of shrinkage.

(2) The comparison of field data at Aisan hydrometric station between the period of 1975-1984 and 1985-1994 was shown in Fig.2 and Fig.3. It can be seen that the recurrence interval of floods larger than 3500 m³/s has varied from one year to 3 or 4 years. Because of the lack of channel formation flood, the deposits occurring during the lower water period cannot be recovered in the flood season. The same regulation has been gained from the filed data at Huayuankou and Lijin hydrometric stations.

(3) The peak value of Q_iSP~Qi curve (here S is the sediment concentration, Q_iS is the sediment discharge, P is the weight percentage of duration, Q_iSP stands for the dominant sediment discharge) has mutated markedly.

Using the data series of 1965-1970, the dominant discharge for channel formation was computed and the results were shown in Fig. 4. In Fig. 4 only one peak appears near the discharge of 3400 m3/s, which approaches the bankful discharge at Aishan Station. As the diversion of water is relatively less, no channel shrinkage takes place. Using the observed data at Aishan Station in the period of 1986-1994, the dominant discharge for channel formation under shrinking was also computed and the results were shown in Fig. 5, where two peaks appear. The second peak is near the discharge of 4800 m3/s, which is the dominant discharge for channel formation under flood, whereas the first peak is within the range of 1000~1400 m3/s, which is called the dominant discharge for channel formation under shrinking condition [6-7]. The two dominant discharges and corresponding morphological relations cause the continuous repeated cycles of ¡°shrinkage - partial recovery - reappearance of shrinkage¡± in the channel of the lower Yellow River.

4    VARIATION OF INTERNAL ATTRIBUTES OF RIVER SYSTEMS¡ª¡ª THE SHRINKAGE OF CHANNEL OF THE LOWER YELLOW RIVER

The internal attributes of the channel shrinkage of the lower Yellow River will be illustrated as follows:

4.1    Deposition in the main channel

Starting from 1986 through 1994, the total amount of aggradation in river channel from Tiexie to Lijin in lower Yellow River is only 60% of that in the 1950¡¯s ^[12-13], yet the total amount of the deposits in the main channel increases by 2.3 times. (see Table 1). 

Table 1    Amount of deposition and scouring in the lower yellow river for different period (10⁸ t)

4.2    Incomplete recovery of channel in flood season

The incomplete recovery of the shrunken channel in the flood season has been discussed above. The cumulative amount of deposition and scouring in the main channel of lower Yellow River during the period of 1975-1984 and 1985-1994 are shown in Fig. 6 and Fig. 7 respectively. It can be seen from Fig.6 that with the scouring in flood seasons and deposition in non-flood seasons, equilibrium can be found in the reach during a long period. Whereas the finding from Fig.7 is the weakening in scouring and gradual transformation to deposition in the lower Yellow River.

5    COMPUTATION OF EXTERNAL THRESHOLD

The software used is the mathematical model for non-equilibrium transport of sediment in multi-systems ^[12]. The lower Yellow River is taken as the computed area, from Aishan to the estuary, totals more than 400 km.

5.1    Effects of changes in hydrography

The hydrological condition adopted in the computation consists of two ¡°two-extreme¡± sets of series in Aishan Station. One is the observed data series during 1985-1994. This is hydrological series of low water, which is unfavorable for the river channel. The other is the predicted series^[13] of 1965-1975 after the balance of sedimentation in Xiaolangdi Reservoir. This is a period with abundance of water and sediment load under the regulation of reservoir, which is favorable for the river channel. Table 2 shows the relation between computed depth of deposition in various parts of the lower Yellow River and the two sets of series.

Table 2    Computed depth (m) of deposition in various parts of the lower yellow river for different
                period

It can be seen from Table2 that for the two hydrological series, the mechanism of deposition in the estuaries reach differs. Unlike the deposition extending upward from the estuarine delta in the series of 1965-1974, the deposition in the series of 1986-1994 develops mainly from the upstream to the downstream. For the estuarine reach, the influence of estuary always exists (the steeper the slope, the less the influence), while the deposition proceeding from upstream to downstream due to hydrological changes just dictates the breakthrough of a threshold of river evolution -- the channel shrinkage.

5.2    Effects of reduction in runoff

The relation between depth of deposition in various parts of the lower Yellow River and percentage of reduction in runoff is shown in Fig. 8. It can be seen from Fig. 8 that when the reduction in runoff increases, the value of the slope ratio of curve in Fig. 8 turns from negative to positive; the river channel displays a trend of weakening in the deposition extending upward and gradually turns to the deposition from the upstream to the downstream. The turning point, i.e., the external threshold is the reduction percentage 30-40%.

5.3    Effects of reduction of sediment load

Reduction of sediment load has significant effects on channel shrinkage. The relationship between depth of deposition in the main channel of the lower Yellow River and the percentage of synchronous reductions in runoff and in sediment load is shown in Fig. 9. For the case of simultaneous reductions in runoff and sediment load, the essence of channel shrinkage will not change, i.e., the effects of reduction in runoff are greater than that in sediment load on the channel shrinkage. It can be seen the external threshold is the reduction percentage 60-70%. The depth of deposition in the reach from Aishan to Luokou is on the small side due to the readjustment of the channel to the reduction of sediment load.

6    CONCLUSIONS

(1) Shrinkage of river channel is a problem internationally concerned, which represents the transformation process of river systems from quantitative to qualitative.

(2) The threshold of river channel should not be confused with the equilibrium. They are different stages in fluvial processes.

(3) The mutagenesis of oncoming runoff and sediment load in the lower Yellow River is characterized by the following features:

¢Ù The total amount of runoff and sediment load decreases markedly after 1985. The dominant causes were human activities.

¢Ú Not only the peak, but also the frequency of occurrences of channel formation floods decreased, which lead to the deposition during the lower water period cannot be recovered in the flood season.

¢Û The peak value of Q_iSP~Qi curve has mutated greatly. Appears the dominant discharge for channel formation under shrinking condition.

(4) If the runoff of the predicted series of 1965-1975 after the balance of sedimentation in Xiaolangdi Reservoir is to be reduced, the external threshold is the reduction percentage 30-40%. If the runoff and sediment load reduce synchronously, the external threshold is the reduction percentage 60-70%.

(5) It should be noted that the external threshold varies according to the characteristics of the drainage basin, oncoming runoff and sediment load, etc. It should be determined with physical or numerical models.

References

[1]  Schumm, S.A. 1973, Geomorphic Thresholds and Complex Response of Drainage Systems. In Marie Morisawa (ed.), Fluvial Geomorphology, Publications of Geomorphology, State University of New York, Binghamton, pp.299-310.

[2]  YIN Guo-kang, 1984, The application of thresholds of geomorphic processes in erosion control, Journal of sediment research, No. 4, pp. 25-36 (In Chinese).

[3]  LU Zhong-chen et al, 2000, Geomprphic thresholds of the channel process in the lower Yellow River, Journal of sediment research, No. 6, pp.1-5 (In Chinese).

[4]  NI Jing-ren, MA Ai-nai, 1998, Dynamics of River Geomorphology, Peiking Univ. Press (In Chinese).

[5]  PETTS G.E. 1988, Impounded River, translated into Chinese by Z.Y. WANG et al, China Environmental Science Proc.

[6]  CHEN Dong, Zhang Qi-shun 1997, Study on channel shrinkage, Journal of sediment research, No. 4, pp. 25-36 (In Chinese).

[7]  CHEN Dong, CAO Wen-hong and ZHANG Qi-shun 1998. On the Shrinkage of River Channel. International Journal of Sediment Research Vol. 13, No. 2, pp.27-39.

[8]  MOATTASSEM M. EI and ABDELBARY M.R. 1993, Bed degradation and channel shifting in the Nile after Aswa High Dam ICOLD, 61st Executive Mesting, Cairo-Egypt.

[9]  CAO Wen-hong, CHEN Dong 1998, Sediment response and enlightenment from the Aswan High Dam, Journal of sediment research, No. 4, pp. 79-85 (In Chinese).

[10]  QI Pu et al 1997, Hydrological Change and Deposition Downstream the Yellow River and Mitigation Measures, Yellow River Press. (In Chinese).

[11]  HU Chunhong et al 1995, self readjustment of longitudinal profile of the Lower Yellow River, Research Report, IWHR  (In Chinese).

[12]  CAO Wen-hong, Zhang Qi-shun 1997, Mathematical Model for non-equilibrium transport of sediment in multi-systems, Journal of sediment research, No. 2, pp. 60-63  (In Chinese).

[13]  Qu Shao-jun, Zhang Qi-wei 1995, Mathematical Model for the sedimentation in the reservoirs at the Middle Yellow River and for the transport of sediment in the lower Yellow River. (In Chinese).

Fig. 1    Difference between threshold and equilibrium

Fig. 2    Field data at Aisan Hydrometric station in the period of 1975~1984

Fig. 3    Field data at Aisan Hydrometric station in the period of 1985~1994

       Fig. 4    Computation of dominant discharge          Fig. 5    Computation of dominant discharge
                    using the data series of 1965~1970                      using the data series of 1986~1994

Fig. 6    Cumulative amount of deposition and scouring in the main channel of lower Yellow River during the periods of 1975~1984

Fig. 7    Cumulative amount of deposition and scouring in the main channel of lower Yellow river during the periods of 1985~1994

Fig. 8    The relation between depth of deposition and the reduction in runoff

Fig. 9    The relation between depth of deposition and the reduction in runoff and sediment load

[1] Foundation Item: Key project of fundamental research of China, No. G1999043604

Project of the National Natural Science Foundation and the Ministry of Water Resources of China, No. 59890200