EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 10, 143-159 (1985)

CHANGES IN RIVER REGIME AFTER THE CONSTRUCTION OF UPSTREAM RESERVOIRS

NING CHIEN Department of Hydraulic Engineering; Tsinghua University; Beijing; China

Received 5 February 1984 Revised 10 July 1984

ABSTRACT

This article presents and analyses many years of investigations in China on the fluvial processes downstream of impounding and detention reservoirs. The study covers the change in hydrograph, the recovering of sediment concentration along the river course, the degradation of stream bed, the adjustment of longitudinal profile, the coarsening of bed material, the change in channel width, and the trend of channel pattern variation for alluvial streams downstream of impounding reservoirs. Without confluence of major tributaries, the degradation may extend to a great distance below the dam. In the process of reducing the sediment carrying capacity of the flow to match the diminished sediment supply, the coarsening of bed material is a factor of equal, if not greater, importance as compared with the flattening of channel gradient. In places where the flow has not been sufficiently cut down and the bank is erosive non-resistant, a receding of banklines may take place in concurrence with the deepening of the river bed. Below detention reservoirs, even if the total runoff and sediment supply remain essentially unchanged, the modification of the hydrograph is sufficient to enhance the deterioration of the downstream channel.

KEY WORDS Fluvial process River channels Equilibrium River-regulation Reservoirs

INTRODUCTION

The construction of reservoirs on alluvial streams upsets the state of equilibrium of the river, leading to a series of changes in fluvial processes and bringing about problems with flood control, navigation, and irrigation. The subject had been thoroughly reviewed by Petts (1979) and studied in detail more recently by Williams and Wolman (1984). In this article examples from sites in China will be drawn to illustrate the changes in the river regime after the construction of an impounding reservoir and a flood-detention reservoir respectively.

THE FLUVIAL PROCESSES DOWNSTREAM FROM AN IMPOUNDING RE3ERVOIR

A storage reservoir, built as an integrated part of a multipurpose development project in a water basin, often keeps the water level at a rather high elevation. The reservoir intercepts all the sediment coming from upstream, while holding up the water, and releases clear water to the downstream channel. In certain cases, facilities are provided for discharging density current from the dam. Since the sedimentary particles carried by the density current are so fine in size, they behave essentially as waskload* and play no part in the channel formation.

Sediment in transport can be classified as bed load and suspended load according to the mode and mechanism of motion. It can also be classified as bed material load and wash load according to the source of supply of the material. Bed material load is that part of the sediment load which consists of grain size represented in the bed. Since it is available in abundance in the river bed, it usually moves in capacity. Any oversupply or deficiency of this type of material will lead to aggradation or degradation of the stream channel. Wash load is that part of the sediment load which consists of grains finer than those of the bed. It comes essentially from the basin upstream and has nothing to do with the deformation of the main channel. For more detail please refer to Chien and Wan (1983).

01 97-9337/85/020143-17$01.70 0 1985 by John Wiley & Sons, Ltd.

144 NING CHIEN

Thus, the changes in regime downstream from a storage reservoir mainly reflect the re-establishment of equilibrium in the course of degradation, except below the confluence of major tributaries which bring large amount of sediment into the main stream (Kellerhals and Gill, 1973; Simons, Li, and Associates, 1982).

Modification of’ the hydrograph Because of the differences in the mode of operation and size of the reservoir, there is a great variability from

one dam to another in the magnitude and duration of flow released and amount of sediment sluiced out from the dam. Two typical cases related to Sanmenxia Reservoir on the Yellow River and the Danjiangkou Reservoir on the Han River will be cited here for illustrative purposes.

The Sanrnenxia ReserGoir on the Yellow River. The Sanmenxia Reservoir was completed in 1960 and was operated as a storage basin for one and a half years. Only 7 per cent of the incoming sediment was released from the dam in this period. In March of 1962, it was decided to lower the water level as much as possible because of excessive reservoir sedimentation. However, from that time until October of 1964,60 per cent of the sediment entering the reservoir was still trapped in the latter because of the inadequacy of the outlet discharge capacity, and the downstream channel kept degrading. Reconstruction works, aimed at enlargement of the sluicing capacity, were launched in stages starting from 1965. Since 1974, it was manipulated as a flood detention reservoir from July to October and served to store water for irrigation and power generation for the rest of the year. The loss of reservoir capacity was reduced to a trivial amount with almost all the sediment coming from the upstream watershed discharged again to the Lower Yellow River.

Due to the regulation of the reservoir, flood peaks were cut down to a considerable extent. On August 13 of 1964, a flood rushed into the lake with peak discharge of 12,400 m3/s. The maximum outflow was 4,870 m3/s only, a reduction of peak discharge of 60per cent. On the other hand, the medium flow was lengthened after the construction of the dam. Under natural conditions, on an average 130 days each year, the mean daily flow falls within the range of 1,OOO to 3,000 m3/s. It was extended to 204 days in the year 1961, an increase of 57 per cent.

Figure 1 is the plan view of the Lower Yellow River. Above Gaocun is a typical braided river of 270 km with a high wandering intensity. The distance between levees is 5 to 20 km. It is 146 km from Gaocun to Taochangpu, and this is a transitional reach from a braided to a meandering pattern. Below Taochangpu, the width of the channel contracts to a distance of 500 to 2,000 m, and systematic river bends develop. The channel bed of the Lower Yellow River has an alarming rate of aggradation, making the water level much higher than the adjacent lands. The river course becomes a dividing ridge across the plain, and receives practically no more water from the region downstream of Huayuankou.

The Danjiankou Reservoir on the Han River. The Danjiangkou Reservoir was completed in November of 1967. Table I gives the alteration of the flow regimen by closure of dam at gauging station Huangjiagang 6 km

laolonndl

G: Hill 0 100 km. - Dyke - ~ sand bar -A Direction of flow Scale

Figure 1. The plan view of the Lower Yellow River

RIVER REGIME CHANGES 145

Table I. Change in flow regimen at Huangjiagang after the closure of the Danjiangkou Reservoir

Items

Before After the the

closure closure

Characteristics Period used of peak discharge exceeds 10,OOO cu. m/s

No. of years in which the peak discharge

Av. peak discharge in cu. m/s

Period used Av. discharge in Jan., Feb. and

Period used Total runoff in lo9 m3 No. of months in which the

av. monthly flow in cu. m/s falls within the range of

Low water Dec. (cu. m/s)

Duration of medium flow

1954-66 1968-80

10 2 16,600 7,840

1956-67 1968-79 328 714

1955-56 1974-75 90.8 89.6

< 800 13 0 800- 2 20

1,500 > 1,500 9 4

Period used 195&58 1974-79 Sediment concentration Av. sediment concentration* in kg/m3 2.92 0.03

* Av. sediment concentration is obtained by dividing the total sediment load in the period with the total runoff.

below the dam site (Tong and Han, 1983). In addition to the cessation of the sediment supply, the flow conditions are modified by the storage reservoir in the following aspects: (1) attenuation of flood peaks; (2) increase in low water; and (3) extension of the duration of the medium flow.

The Han River below Danjiangkou flows a distance of 649 km before it enters the mighty Yangtze River at Wuhan (see Figure 2). Between the dam site and Huangzhang, for a total length of 247 km, the channel gradient varies from 3.2°/000 upstream to 1~3°/oo,downstream. Above Quanghua the width of thevalley is to 2 to

Tangbai River

Scale

hlver and Ter race Rocky hills f loodplain

Figure 2. The plan view of the Han River below the Danjiangkou Reservoir

146 NlNG CHlEN

\ 1 9 9 6

3 km which expands to 10 km at Huangzhang, and the river gives every appearance of braided stream. At Huangzhang, the river emerges on to the great Yangtze-Han Plain, and a meandering channel pattern begins to develop. Two subreaches can be distinguished in the last 402 km stretch of the Han River. Downstream from Zekou, the river is confined within two dikes, and the distance between the levees is 1 to 2 km. The meandering coefficient of the river course varies from 1.1 to 2.7, with an average of 1.5. All the bends are closely controlled by the bank protection works, and there is not much room left for free migration. Between Huangzhang and Zekou, the river behaves as a transition from a braided to a meandering pattern. Here, the distance between dikes is 2.5 to 5 km. The meandering coefficient modulates from 1.1 to 1.7, with a mean value of 1.47. The river bends are left more or less uncontrolled.

Reduction in sediment concentration of the flow In analysing the variation in sediment content of the flow downstream from a reservoir, two different aspects

should be considered, namely, whether there is a sufficient sediment supply and whether the flow is able to carry the sediment (Chien and Mai, 1962). Regarding wash load, even if practically no limitations have been imposed on the flow to carry this type of material, an ample supply of sediment is not available in the downstream channel after the yield from the upstream basin is blocked. On the other hand, there is not a lack of bed material in the channel, where the flow can pick up enough load; yet, the capacity to transport this type of material is greatly reduced due to the attenuation of flood peaks and the diminution in average discharge. For instance, the levelling-off of flood peaks reduces the transport capacity of the Han River downstream from Danjiangkou Reservoir by as much as 41 per cent (Han and Tong, 1982). This effect is further enhanced by the coarsening of the bed material by the flow’s selective process in eroding the bed. Consequently, the sediment concentration of the flow is greatly reduced. Figure 3 gives the sediment concentration measured at several gauging stations

Distance f r o m SWbol Station the d-,

6

2 29

- Huangjiagang ___ Huangzhang

x-x Xlantao 480

RIVER REGIME CHANGES 147

downstream from the Danjiangkou Reservoir before and after construction of the dam. It can be seen that the sediment content of the flow decreases progressively along the water course under natural conditions. After the completion of the reservoir, the amount of sediment carried by the flow is reduced to a minimum in Huangjiagang immediately below the dam. At Huangzhang and Xiantao, the concentration of sediment after 1967 reduced to 29.7 per cent and 396 per cent, respectively, of what had been observed before the completion of the project. At Huayuankou on the Lower Yellow River, the sediment concentration decreased 64 per cent and 82 per cent immediately after the Sanmenxia Reservoir was put into operation, discharging 1 , W 2 , 0 0 0 m3/s and 3,000 m3/s, respectively.

Extent of erosion and distance of concentration recovery The degradation of the river bed below the reservoir covers a great distance and extends progressively

downstream. After 13 years of operation of the Danjiangkou Reservoir on Han River, the bed became completely stabilized without any supply of sediment from the dam site to Guanghua, a length of 26 km. From Guanghua to Taipingdian, a reach 40 km in length, the bed suffers further erosion only during rare floods. Below Taipingdian to Xiangyang (43 km in distance) sediment moves in the main channel as bed load. The most extensive downcutting reach at present is located between Xiangyang and Huangzhang with a distance of 138 km. Even at Xiantao, 480 km below the dam, noticeable degradation is observed (Han and Tong, 1982). AS there are no major tributaries entering the Lower Yellow River below Huayuankou, ifclear water of 2,500 m3/s in discharge is released from the Sanmenxia dam over a sufficient length of time, the whole river course, with a total length over 800 km, will be degraded.

The clear water discharged from the reservoir picks up sediment from the channel while travelling downstream. After a certain distance, the incoming bed material load becomes equal to the sediment transport capacity of the flow, and the concentration of bed material load reaches saturation. This distance is called ‘the distance ofconcentration recovery’and is denoted by the symbol L , . From the dispersion theory of suspended load, the distance of concentration recovery cannot be very long. On the other hand, the length of the reach in which degradation takes place, or in other words, the distance from the dam at which the concentration of bed material load reaches a maximum (denoted by L2), is very long indeed, as shown above. The question that arises is how to unify the contradiction of a short L, against a long L2.

Let us assume that the river aggrades at a slow rate under natural conditions, as is the case in the Han River and in the Lower Yellow River. The solid line in Figure 4 is the variation in transport capacity of bed material load of the flow with distance. At the beginning of operation of the reservoir, the recovery of concentration of bed material load downstream from the dam follows the trend indicated by the dashed line in Figure 4a. These two lines intercept at point 0, the location designating the distance of concentration recovery. At this stage, it also represents the lower limit of the reach in which the channel is degraded. The coarsening of the river bed upstream from point 0 in the course of erosion makes the transport capacity of bed material load decrease by a large margin. The sediment carried through point 0 is not only smaller in quantity but is also coarser in composition as compared with the preceding stretch of time. On the other hand, below point 0, plenty of fine particles are still available in the bed and the transport capacity of the bed material load of the flow is larger than the incoming load. This will result in further erosion downstream from point 0. After a time interval, At, the capacity of the flow to carry the bed material load reduces substantially above point Band still maintains its original level below point B, as illustrated in Figure 4b. The clear water released from the reservoir, passing over the upper reach of a reduced transport capacity and a limited supply of sediment, takes a longer distance to recover the same sediment concentration. This implies that the location of the dashed line in Figure 4b must shift slightly towards the right as compared with that in Figure 4a. Meanwhile, the line of transport capacity of bed material load is also lower than before, making the interception point A of the solid line and the dashed line shift upstream. In spite of the fact that below point A the incoming bed material load is already equal to the ability of the flow to transport it, degradation still persists up to point B since both the supply of fine particles from the bed and the sediment transport capacity increase progressively in the direction of flow. In this way, point 0 in Figure 4a dissolves into two points in Figure 4b, with the position of the concentration recovery (point A) shifting upstream and that of maximum concentration (point B) shifting downstream. If the flow conditions remain unchanged, the process as depicted above will steadily continue, causing the degradation to

148 NING CHlEN

- Sediment transport / I --- oncoming load I

(a)

-

capeci t y

Distance from the dam

Figure 4. Changes, over time, in distance of concentration recovery and distance of maximum concentration below the dam (refers to bed material load only)

extend further downstream (Figure 4c). The whole process develops at a rather swift pace as indicated by the data gathered thus far.

In consequence of the degradation extending for a great distance below the dam, the absolute amount of downcutting at any one point along the river course cannot be very large. Four years after the completion of the Sanmenxia Reservoir, the water level at a discharge of 3,000 m3/s of the Lower Yellow River was lowered between 0.6 to 1.3 m for a stretch of 490 km in length from Huayuankou to Luokou. Downstream from the Hongshan Reservoir, the bed of the Laoha River (a tributary of the Liao River) degraded 2 m in eight years for a length of 50 km. Between Huangjiagang and Xiangyang the low water level dropped between 0.7 to 1.6 m after the construction of the Danjiangkou Reservoir. Numerous other examples can be cited from data all over the world (Williams and Wolman, 1984).

Coarsening of the river bed In this section the bed coarsening in the process of degradation will be discussed which plays an important

role in stabilizing the bed against further erosion. Mechanism which causes the formation of erosion pavement. It is generally conceded that the selective process

of the flow in the course of erosion is responsible for the coarsening of the river bed. This is indeed the case; however, it is not the complete picture. Following the gradual formation of the erosion pavement at the bed surface, the material moved out from the bed becomes coarser and coarser. These coarse particles, while entering the lower reaches of the river, might be unable to be kept in motion by the Row. Thus, for reaches

RIVER REGIME CHANGES 149

Table 11. Annual erosion and deposition rate in lo4 tons/year of material of different sizes in the middle and the lower reaches of the Han River after the completion of the Danjiangkou Reservoir in 1967

Particles size in mm*

Reach Year

1974 Xiangyang 1975

to 1976 Huangzhang 1977

1978 1979

< 0.01 0*014*025 0.0254-05 005-010 0.10-025 0.254-50 0.50-1.0

- 77 - 250 - 390 - 570 - 576 + 20 + 19 -458 -891 -631 - 674 - 364 + 692 + 90 -157 -113 - 146 - 270 - 354 + 30 + 18 -205 -177 - 55 - 85 - 124 + 205 + 33 -51 - 63 - 79 -112 - 63 + 36 +6 -325 -157 - 142 - 191 - 197 + 231 + 12

1974 -266 -251 - 157 - 121 +64 + 144 - Huangzhang 1975 -101 -35 - 50 - 19 + 99 + 148 -

to 1976 -48 - 80 -91 - 14 + 75 + 107 - - Xiantao 1977 -55 - 78 - 18 - 130 + 53 + 72

1978 -36 - 33 - 29 -61 - 37 + 43 1979 -31 - 57 - 70 - 77 +81 + 47

- -

* + denotes aggradation and - denotes erosion.

farther away from the dam, the exchange process between the material in motion and that in the bed manifests itself in a simultaneous erosion of fine particles and deposition of coarse particles. Table I1 gives the amount of erosion and deposition of material with different sizes between Xiangyang and Xiantao of the Han River from 1974 to 1979. It can be seen that, with 1975 as an example, 3,018 x lo4 tons of particles finer than 0.25 mm are eroded from the bed between Xiangyang and Huangzhang, and, in the mean time, 782 x lo4 tons of particles coarser than 0.25 mm are replaced to the bed. The replenishment of coarse particles (> 01 mm) to the bed is even greater than the loss of fine particles ( < 0.1 mm) from the bed in the reach immediately below. Such a process naturally induces the coarsening of the river bed to occur at a highly accelerated pace.

Types ofbed armouring. Three different types of coarsening of river bed can be distinguished. In some of the mountain streams or rivers which enter the plain from the gorge, there exists a layer of gravel or a number of covered debris cones sliding from the valley sides within a short vertical distance from the present river bed. Once the gravel layer or the debris cones are exposed through the removal of the surface material by the flow, the bed composition suddenly becomes so coarse that further degradation is completely inhibited. Figure 5a is the change in bed composition 1.6 km downstream from the Hoover Dam of the Colorado River after the reservoir was put into service for one year. The composition of the sample obtained in 1936 was so coarse that it had nothing in common with that of the preceding year. In fact, the sample represented deposits laid down at different times and under very different circumstances.

In some rivers, the bed is composed of sand and gravel. The gravel, which can be moved freely under natural conditions, becomes insusceptible to motion when the incoming flow is completely modified by the regulation of the reservoir. The gravel will thus be accumulated at the bed surface in the course of downcutting of the channel and will form a pavement resistant to further erosion. Figure 5b gives the change in bed composition of the Yong-ding River downstream from the Guanting Reservoir in which material coarser than 5 mm can no longer be moved by the diminished flow.

In many alluvial streams, there exists no deep-laid gravel layer or covered debris cones, and the composition of the bed material is so fine that the velocity of the flow released from the reservoir is quite sufficient to move all of the available sediment in the bed. Even in this case, coarsening of bed surface still exists due to the differential transport capacity for particles of different sizes. Figure 5c gives the modification of the bed composition in the Colorado River 25 km from the Imperial Dam after six years of persistent scour. Within this period, the median diameter of the bed material increased from 0125 to 0.32mm.

!

150 NlNG CHIEN

100

60

k O 0 100

20

0 0.04 0.1 0.4 1 10

Diameter, D, mm.

Figure 5. Three different types of coarsening of the river bed

Coarsening ofthe river bed composed of sand and gravel. Based on the process of formation of erosion pavement, the following formula can be used to evaluate the maximum depth of degradation

in which h,-maximum depth of degradation Po-the percentage of non-moving particles in the original bed material within the depth of degradation PI-the percentage of non-moving particles available in the erosion pavement h,-the thickness of erosion pavement eo, el-the void ratios of the original river bed and the erosion pavement, respectively Harrison ( 1 950) is the first one to find from a flume study on sediment segregation in a degrading bed that p1

does not necessarily have to be 100 per cent, and h, can be taken as the size of the non-moving particles of which 65 per cent by weight is finer. His findings are substantially verified by datacollected from the Yong-ding River downstream of the Guanting Reservoir (Yin, 1963). Here, p1 varies from 55 to 75 per cent, with an average of 65 per cent. The thickness of the erosion pavement, h,, is equivalent to Ds0 to Dg0 of the composition of the bed armour, including all the moving and non-moving particles presented in that layer.

Once a bed armour is formed, the roughness will be significantly increased, and the sediment transport capacity will be lowered to a great extent, owing to an increase in depth of flow and a decrease in velocity. This is well illustrated by the field data of the Yong-ding River, as shown in Table 111.

Coarsening ofthe sand bed. Even a bed of fine sand can be coarsened in the process of degradation, a fact often overlooked in the past. Figure 6 illustrates the change in composition at Huayankou of the Lower Yellow River in accordance with the mode of operation of the Sanmenxia Reservoir.

From 1960 to 1964, when the reservoir functioned essentially as a storage lake and most of the incoming sediment was trapped by the reservoir, the bed of the Lower Yellow River became progressively more coarse.

RIVER REGIME CHANGES 151

Table 111. Changes in flow conditions of the Yong-ding River after the formation of erosion pavement* ~ ~~~

Discharge Velocity of flow Depth of flow Manning (m3/s) Year (m/s) (m) coefficient

50 1952 1.04 0.78 0.0157 1959 0.76 1.37 0.0290

100 1952 1-21 1 .00 0.01 52 1959 1.02 1.72 0.0220

~~ ~

*The Guanting Reservoir was built in 1955.

%i 0.26 Period of Period of flood

$: 0.22 -water storage detention

1% 0.18 - - 6 0.d

dc, P 6

g; 0.14 - G Z 0.10 - f

0.06 1961 62 63 6 4 65 66 67 68 69 70 71 72

Y e a

Figure 6. Change in bed composition at Huayuankou of the Lower Yellow River in accordance with two different modes of operation of the Sanmenxia Reservoir

After reconstruction of the dam and sluicing out of sediment from the reservoir, the bed downstream again silted up and resumed its former composition under natural conditions.

Coarsening of the sand bed results in a significant increase in the roughness, as shown in Figure 7. This must be chiefly attributed to an increase in the bar resistance. Such a change, together with a reduction in the supply of fine material, decreases transport capacity of the flow. From the data collected at Huayuankou, an increase in the median diameter of the bed material from 0.10 to 0.1 3 mm can reduce the transport capacity of the bed material load by as much as 65 per cent.

The process of coarsening of a river bed composed of sand can be predicted by the Einstein bed load function which permits the transport rate of each size fraction of the bed material to be calculated. This method was employed to forecast the change in bed composition at Huayuankou of the Lower Yellow River after the construction of the Sanmenxia Reservoir; the results agreed reasonably well with the field data collected after the reservoir was actually put into operation (Chien, 1959). The key point is how to determine the thickness of the effective bed material zone in which the particles are agitated by the flowing water. In Chien (1959), this zone is assumed to be twice the amplitude of the fluctuations of the bed elevation during high flows in normal state.

Adjustment of longitudinal profile Construction of a reservoir in the river reduces the sediment supply downstream, and changes in the fluvial

processes should tend to reduce the sediment transport capacity in order to re-establish the equilibrium. According to Mackin’s (1948) concept, the slope should be flattened considerably following the dam construction, and the whole process would proceed downstream at a slow rate. Taking into full account the long distance of degradation, it became evident that a large reduction in slope is next to impossible since enormous amounts of sediment must be removed from the bed for such achange to become a reality. Both field

152 NING CHIEN

c c, C

0.025 I I

v. YL" -.

0.010 1 1 4000 6000 boo Goo 1000 2000

Discharge, m3/sec.

c 0.025 c, (b) Huayuankou 8 0.020 t o Xingzai -

.r( :: 0.015

2 % d o e o , 1959 0.013

b00 000 1000 2000 40~0 6000 Discharge, m 3 /sec.

Figure 7. Change in roughness of the Lower Yellow River in the course of degradation by clear water

data and laboratory experiments seem to indicate that if the bed material is coarse enough for the formation of an erosion pavement, the reduction in transport capacity is achieved mainly through coarsening of the bed, and change in slope is usually of minor importance. If formation of bed armour is impossible, adjustments of bed composition and slope take place concurrently until the sediment transport rate is sufficiently cut down to match the incoming sediment load.

In Harrison's (1950) laboratory study of which erosion pavement is eventually formed, no apparent change in slope is noticed during the course of degradation. Experiments carried out at Kyoto University of Japan essentially tell the same story (Ashida and Michiue, 1971). The bed degradation occurs parallel to the initial bed slope if an armour coat is formed, and the slope becomes progressively flatter in the course of erosion if the flow is able to keep all the sediment in motion.

The adjustment of slope of the Yong-ding River downstream from the Guanting Reservoir behaves practically the same as that of the type of parallel degradation. Figure 8 illustrates the profile of the thalweg after the river emerges from the Guanting gorge. Six years after the reservoir started to detain floods, the deepening of the thalweg, although varying from place to place, manifested itself as a whole, parallel with the original bed. The water surface slope during low and medium flows from Huangjiagang to Xiangyang downstream from the Danjiangkou Reservoir ranged from 2.86°/0,0 in 1960 to 2.68°/00, in 1978, a reduction of only 6.3 per cent.

Change in rate of degradation with time Owing to the progressive coarsening of river bed and, in some cases, the flattening of the bed slope, the

degradation rate at any one site will decrease progressively with time. Chien (1958) had found that the variation of degradation rate with time follows

AE = a/Tb

in which AElAT-degradation rate in m3/yr T-duration of degradation, in years a-coefficien t

RIVER REGIME CHANGES 153

Dlatance below the outlet of Ouantlng Gorge, km.

Figure 8. Profile of the thalweg of the Yong-ding River before and after the construction of the Guanting Reservoir

The exponent b seems to vary with the percentage of non-moving particles in the original bed, as demonstrated by the data of Harrison’s (1950) experiment:

Percentage of non-moving particles in the original bed 1.6 5.0 10.5

The exponent b 0.75 1.19 1.92

The degradation rates of the Colorado River downstream from Hoover Dam and Parker Dam, if converted into rates under constant discharge, seem to follow equation (2) too, with exponents of 1.42 and 1.0, respectively. It is interesting to point out that equation (2) is quite similar in form with the hyperbolic equation

AEIAT = l/(c1 + c , T )

as developed by Williams and Wolman ( 1 984).

(3)

Adjustment of cross-sectional shape Two different mechanisms are involved in determining the transverse shape of a degraded channel. The

downcutting power of the clear water makes the channel deeper and narrower in the progress of erosion. On the other hand, wandering of the river course, although much diminished in intensity, still exists after erection of the dam. This leads to bank caving if the material is not erosion-resistant. In its natural state, the collapse of the bank line in one place is compensated for by the deposition of sediment and rebuilding of high banks in

154 NING CHIEN

other places. In the long run, there is a balance between the erosion of high banks and the building of new valley lands, and the width of the channel, although fluctuating from time to time, is maintained at a certain level. With the construction of a reservoir upstream, the process of bank erosion continues and the rebuilding process ceases because of the elimination of both floods and sediment. This results in the widening of the river channel at the cost of the loss of valuable lands in floodplain. Table IV shows the widening of the stream channel that occurred in the Yong-ding River below the Guanting Reservoir. This widening, occurred very rapidly at the beginning of the new epoch, is gradually stabilizing as time passes. Harrison and Mellema (1982) have reported that the same phenomenon has also been observed in the Missouri River since construction of six reservoirs in the upstream basin.

The relative strength of downcutting and augmentation of width depends upon a number of factors, among which are the position of the reach relative to the dam site, the intensity of the flow to scour the bed, the height of the banks, and the cohesion of the bank material. Figure 9 gives the width of the main channel and the

Table IV. Widening of the Yong-ding River below the Guanting Reservoir*

After dam construction Length Before dam construction April Sept. Sept.

Reach (km) Item December 1950 1956 1957 1958

Lugouqiao Area of flood plain, 294 21.3 16.8 16.7

Jinmenzha Distance between 790 1,060 1,210 1,214 to 30 sq. km.

banklines, km.

Jinmenzha Area of flood plain 18.6 12.8 11.2 11.0 to 30 sq. km.

Shifosi Distance between 420 600 650 655 banklines, km.

*The reservoir began to retain floods in 1953 and was completed in 1955.

100 150 200 250 d 3

d X

Distance, km.

Figure 9. Change in width and depth of flow of the Lower Yellow River after the construction of the Sanmenxia Reservoir

RIVER REGIME CHANGES 155

difference in elevation between the bed and the floodplain of the Lower Yellow River before and after the building of the Sanmenxia Reservoir. It can be seen that four years after the installation of the dam and within a distance of 50 km below the emerging point of the river, from the gorge to the plain, the deepening of the channel is the dominant feature in the adjustment of cross-sectional shape. The width of the main channel even reduces somewhat in this part of the river. In the reach from the confluence of the Yi and Lo Rivers to Gucheng with a length of nearly 150 km, downcutting and widening take place in concert. Below Gucheng, the river more or less maintains its original state.

Change in channel pattern It takes a much longer time before any change in channel pattern begins to take shape. Although 22 years

have elapsed from the time that the Danjiangkou Reservoir began to detain floods, the time is still too short for any definite trend in the development of channel pattern to become apparent. At present, all we can do is make a rough sketch of what might be proven significant in the long run.

With a careful study of the field data, it is discernible how the change in flow regimen affects the development of channel pattern (Tong and Han, 1983). Such a development takes different form for reaches of different characteristics.

An attenuation in flood peaks implies that the time is shortened for flow to spill over the floodplain and to inundate the mid-channel bars. The growth of floodplain is thereby inhibited to a certain extent, and the mid- channel bars tend to stabilize. The flow diverted into secondary branches is also reduced both in quantity and in duration, an occurrence injurious to the maintenance and growth of the side channels. The pools in the meandering reach, which has not yet been subjected to the general degradation of the clear water, will be raised.

A strengthening of the medium flow is beneficial for the development of medium flow channel. The effect of this strengthening is felt in many different ways: deepening and decrease in the width of the main channel, levelling of bed undulations in both the transverse and longitudinal directions, silting up of the secondary branches, enlargement of the mid-channel sand bars, and a reduction in the radius of curvature of river bends.

A hydrograph distributed in a more even manner leads to a smaller variation of flow direction, a more fixed ratio between the flow in the main course and that in the secondary branches, and a fixation of the point at which the low and medium flows impinge against the bank lines.

All of these produce a combined pushing effect on the transformation of the channel pattern. In the braided reach, the wandering intensity of the river coarse is distinctly reduced. Some of the mid-channel sand bars are combined together, and some are transformed into point bars (see Figure lo*). There are 26 large branches from the dam site to Huangzhang, and 15 of them are blocked at present. Some are completely silted up and the river course there tends to take a more curved path with the meandering coefficient increasing from 1.25 to 1.50. Some branches are blocked only at their entrance and may rejuvenate if rare floods are discharged from the reservoir.

Figure 10. Change in channel pattern at Tongmuling Reach of the Han River after the construction of the Danjiangkou Reservoir

Figures 10 and 1 1 are obtained through range-line survey at low flow season and measured up to the banklines.

156 NING CHIEN

Figure 11. Change in channel pattern at Shayang Reach of the Han River after the construction of the Danjiangkou Reservoir

In the free meandering reach between Huangzhangand Zakou, the noted changes are (1) a shiftingaway and a moving downward of the main current from the concave side of the bank; and (2) a cut of the point bar on the convex side. This happens in 11 out of 18 bends; a typical example is shown in Figure 11. No apparent changes are noticed in the confined meandering reach below.

THE FLUVIAL PROCESSES DOWNSTREAM FROM A FLOOD-DETENTION RESERVOIR

A flood-detention reservoir is used not for the purpose of storage but for relieving the downstream area from the threat of flood by whittling down the peaks of the flood. As soon as the flood recedes, the stream channel in the reservoir will resume its natural state and most of the sediment deposited in the preceding stage will be sluiced out from the reservoir.

Modijication of hydrograph Figure 12 is the inflow and outflow hydrograph of Guanting Reservoir on Yong-ding River from August 24

to September 10 of 1953 when an incoming flood was backed up by the dam under construction. The regulation of the reservoir caused a peak inflow of 3,700m3/s to be levelled off to 800m3/s. When the backwater reached a certain distance from the dam site, density current began to form and moved downstream through the reservoir. The sediment concentration of the density current vaned from 50 to 80 kg/m3 (August 26 to 29). Following the lowering of the water level, retrogressive erosion of the new deposits took place bringing a great amount of sediment out from the dam (September 2 to 13). The ratio between the sediment concentration of the outflow and that of the inflow was kept above 2 at this period and could be as much as 11.4. The concentration remained above 100 kg/m3 for flows of 100 to 200 m3/s, with a maximum of 350 kg/m3.

A positive correlation between the discharge and concentration usually exists for an alluvial stream in its natural state. The larger the flow, the greater is the sediment content. However, such a natural trend is completely reverted below a flood-detention reservoir. The sediment content of high flows is either reduced to a trifling amount or consists only of wash load; that of medium to low flows can be very high in concentration. A modification of a hydrograph such as tfiis naturally brings about a profound and farreaching effect on the fluvial processes.

RIVER REGIME CHANGES 157

Figure 12. Inflow and outflow hydrograph of the Guanting Reservoir on Yong-ding River Basin during natural flooddetention period

Essential features of the fiuvial processes downstream from a fiood-detention reservoir The loss of adaptability between the concentration and discharge during flood-detention causes a braided

stream to deteriorate further, if the total amount of sediment released from the dam is not significantly reduced. Many of the braided streams in China, including the Lower Yellow River and the Liu River (a tributary of the Liao River in northeastern part of China), have been subjected to aggradation for a considerable length of time. During the high flow season, the floodplain is raised whenever inundated by flood water, and even the main channel degrades to a degree. In non-flood seasons, the banklines recede under the attack of the ever-shifting main current and the bed is silted to an extent more than sufficient to make up for the erosion of the preceding period. In the long run the river channel, including the bed and high banks, rises as a whole.

After the construction of a flood-retention reservoir, the low and medium flows, already saturated with sediment, are loaded with an additional supply of sediment eroded from the reservoir. The surplus material, exceeding the carrying capacity of the flow, will deposit in the main channel, particularly in the upper part of the reach. The high flows, after unloading their burden in the lake, become quite clear and cause the downstream bed to degrade. However, as their ability to transport sediment is greatly reduced by the attenuation of flood peaks, the amount of erosion is generally smaller than the additional accretion resulting from the non-flood seasons. Quite often the high flows merely shift the previous deposits from the upper part of the reach to the lower part. The levelling off of the floodpeaks also reduces the chance of the water to overflow the floodplains. Only a limited amount of aggradation will occur even when the water does spill over the high banks, as the sediment content is much smaller now. The combined effect of all these processes aggravates the disadvantageous situation already existing in the downstream channel by raising the bed and reducing the difference in elevation between the bed and the floodplain further and making the appearance of the flow even more disorderly.

The two following examples will be cited to illustrate our points.

Fluvial processes downstream from the Naodehai Reservoir The Naodehai Reservoir was built in 1942 across the abovementioned Liu River with a storage capacity of

168 million cu. m. The annual average sediment concentration of Liu River is 52 kg/m3, which is even higher

158 NING CHIEN

than that of the Lower Yellow River, and the mean discharge is merely 11.5 m31s. The reservoir was used for flood-detention before 1970.

Liu River leaves the gorge at Dabanqiao. It is confined by high banks and sand dunes between Dabanqiao and Dahanden and is able to migrate freely on the alluvial fan below Dahanden. Before the construction of the Naodehai Reservoir, the Liu River was subjected to a slow rate of accretion, especially in the reach below Xinmin, downstream from Dahanden. After the reservoir was put into operation, the sedimentation rate of the upper reach became greatly intensified. The reach with outcropped rocks in the neighbourhood of Dabanqiao was completely covered with sand. At Damiao, midway between Dabanqiao and Dahanden, the bed rose 1-5 m in 10 years. In the meantime, the high banks kept receding, making the river channel much wider and shallower. The planform of the river took a decisive change for the worse and became more disorganized than ever.

Fluvial processes downstream from the Sanmenxia Reservoir As mentioned previously the mode of operation of Sanmenxia Reservoir was changed from storing water to

detaining floods only since the end of 1964. From November, 1964 to October, 1973 43,700 million cubic metres of water and 1,630 million tons of sediment were brought annually to the Lower course of the river, with a mean sediment concentration of 37.3 k@m3 which is very close to the average concentration of 37-4 kg/m3 between 1950 and 1960 before construction of the dam. And yet, the average rate of aggradation increased from 368 million tons per year between 1950 and 1960 to 438 million tons per year between 1965 and 1973. Between 1952 and 1960, there were seven floods with a peak discharge over 10,000 m3/s. The rise of the river bed overtook that of the floodplain, causing the difference in elevation between these two parts of the river channel and the bankfull discharge to decrease progressively, as shown in Table V. This, in turn, led to the rise of the flood water level on an unprecedented scale.

The storage of water in the non-flood seasons, starting from 1974, relieved the upper reach of the Lower Yellow River from the pressure of excessive deposition to a certain degree as the clear water released from the dam brought about scour above Gaocun. On the other hand, the discharge of clear water seldom exceeded 1,500 m3/s, and the degradation could not extend beyond Gaocun. In fact, some of the material removed from the reach above was deposited again below Aishan. In a natural state, the repeated inundation of the floodplain by large floods above Taochangpu enables the flow to relieve part of its sediment load on the high banks; this has an advantageous effect of maintaining the river channel below Aishan. This favourable situation no longer exists because most of the sediment sluiced out from the reservoir in the flood season is carried by flows with discharges of 2,000 to 4,000 m3/s which are confined mostly to the main channel. In spite of the fact that the downstream channel benefits in certain ways from the reconstruction of the Sanmenxia Reservoir, the combined effect of these two processes as described above causes the reach below Aishan to silt up at a rate much faster than before. From 1953 to the mid-season of 1960, 3.6 per cent of the sediment deposited in the Lower Yellow River was distributed in the reach between Aishan to Lijing. This percentage rose to as much as 29.3 per cent from September of 1973 to October of 1980 (He et al., 1981).

Table V. Changes in difference in elevation between the bed and the high banks, Az, and the bankfull discharge, Qm, of the Lower Yellow River after the construction of the Sanmenxia Reservoir

Gauging A Z

station (m)

Before dam After dam construction Before dam After dam construction construction Storage Flood-detention construction Impounding Flood-detention

period period period period Oct. 1960 Oct. 1964 June 1973 July 1958 Oct. 1964 June 1973

Huay uankou 1.51 2.24 0.57 6,300 9,000 3,500 Jiahetan 0.97 2.34 095 6,000 11,500 2,600 Gaocun 1.64 2.44 1.02 5,600 12,000 3,000 Luokou 5.86 9.61 4.03 8,800 8,400 5 ,000

RIVER REGIME CHANGES 159

CONCLUSIONS

Changes in regime will take place after the construction of upstream reservoirs the scale of which depends very much on the extent that the delicate balance between the flow and the sediment load is upset.

The regulation of an impounding reservoir results in a reduction of peak discharge, increase in low water, and lengthening of the duration of medium flow. The bed material load released from the dam is often cut down to a trivial amount. The alluvial channel will be degraded and, if there are no major tributaries entering the main stem, the degradation can be extended hundreds of kilometres below the dam. This implies that the absolute amount of downcutting at any one point along the river course and the flattening of the channel gradient cannot be very large. Even in sandy rivers the selective process of the flow and the winnowing of the finer particles from the bed, although not sufficient to form an erosion pavement, still play an important role in stabilizing the bed against further erosion. The disruption of equilibrium between the formation and destruction of the floodplain leads to a widening of the stream channel, if there is a lack ofcohesive constituents or vegetative protection of the banklines.

The operation of a reservoir on flood-detention basis transforms part of the sediment load carried by the floods to the low- and medium-flows. This will aggravate the siltation of the main channel and enhance deterioration of the stream channel downstream.

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