Flood prevention on the lower reaches of the Rellow River

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Flood

Prevention on the Lower

Reaches of the Yellow River

Gong Shiyang

Wu Zhiyao

(The Yellow River Conservancy Commission)

Abstract

. The paper deals briefly with the floods, sediment regime, main features of the river as well as the engineering projects and measures for flood protec-tion on the lowe1; reaches of the Yellow River.

The Yellow River is the second largest river rn China, taking its source in the Yaoguzonglie Basin on the Xizang (Tibet) - Qinghai Plateau. It flows through Qinghai, Sichuan, Gansu, Ningxia, Nei Monggol, Shaanxi, Shanxi, Henan and Shandong province and autonomous region to empty itself into Bohai sea. The total length of the main river is 5, 464 km and the drainage area 752,000 km2

• The upper reaches of the river lie above Tuoketuo county

in the Nei Mongg;ol Autonomous Region, the middle reaches between T'uoketuo and Taohuayu (near Zhengzhou) in Henan province, and the lower reaches downstream of Taohuayu (Fig. 1).

Fig, 1. Map of the Yellow River Basin (including different yield zones of floods)

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The lower course of the Yellow River runs across the North China Plain over a total length of 785 km. Owing to channel acer~. over a long period

. . : £ ~

of time, the river bed is higher than the ground surface on both banks, thus forming a "suspended river", which depends on dykes to contain the water. The river is actually a watershed of the Huaihe and Haihe river.

The Yellow River is known all over the world as a "harmful river" as it frequently overflowed the floodplains along the lower course down the ages. In the 2000 odd years before 1946, there occurred more than 1500 dyke-breaches along the lower reaches, and the river changed its course 26 times. The area affected was 250,000 km2, extending from Tianjin in the north to the Changjiang (Yangtze)-Huaihe River basin in the south. Untold miseries were brought about to the people each time the dyke bursted. Take for instance the flood in August 1933, with a peak discharge of 22,000 m3_{/s, which caused 54 breaches on both}

banks along the lower reaches, flooding a vast expanse of 10,000 km2 _inhabited

by over 3. 6 million people, more than 10,000 of them being killed thereof. Since the founding of new China, great importance has been attached to the work of flood prevention along the lower reaches as a major task in harnessing the Yellow River. While the existing works for flood protection have been steadily improved and reinforced, systematic observations and studies have been made on the particular features of the floods and sediment regime of the Yellow River. Planning for combatting the floods were worked out, a number of engineering projects for flood protection were built, the administrative work of flood prevention have been improved and, in particular, a:n enormous force of a mass character has been organized, achieving successes in controlling the floods and safeguarding the main dykes during the past 30 consecutive years,

I ,

Main Features of the Floods and Sediment Regime on the Lower Reaches

of the Yellow River.

1. Floods.

There are four periods of high water each year on the lower reaches of the Yellow River, namely the sprin~ flood, summer f'lg"Q~J autumn flood and ~ : Spring flood occurs in Mir~h~ April, ~;hen the peachtree;~ are blossoming (hence it is called "peach flood" here along the Yellow River). This is due to the melting of ice and snow on the upper reaches so that both the peak discharge and total runoff are relatively small, the daily mean discharge not exceeding 4,000 m3_{/s, so that the dykes on the lower course are not menaced,}

The ice runs generally occur in February, when the ice cover in the sections in Henan province begins to thaw and the ice floes are drifted downstream to the reaches in Shandong where the river

pheric temperature prevailing there. ice dams may frequently be formed,

• 2 •

is still frozen up due to the lower atmos-As the masses of floe ice are jammed up,

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main dykes.* The summer floods occur in July and August, during the dog days, as we call it here, frequented by rainstorms which give rise to major floods. Unbroken spells of wet weather often prevail in September and October, which may also lead up to comparatively large floods, known as autumn floods, The occurrence of summer and autumn floods are actually in succession and the people have the habit of calling it "the period of major floods in summer and autumn". Most disastrous floods on the lower reaches of the Yellow River occur in this period, so did most of the dyke-breaches, overflowing and flooding in history.

A hydrometric station was first established rn Shaanxian county near Sanmenxia as early as 1919. At present there are altogether 474 hydrometric stations and 53 gaging stations on the main river and its tributaries as well as 1, 400 rain-gauge stations in the entire basin. Besides, we are in possession of a great deal of historical documents and cultural relics and data regarding rainfall, river regime and disastrous floods, which are of value to the know-ledge of the characteristics of floods on the lower reaches of the Yellow River.

With respect to geographical location, .the middle reaches of the Yellow River contributes most to the major floods during summer and autumn, The floods are formed by runoff from three zones: (1) Catchment for the reach between Tuoketuo and Longmen, with an area of 111,691 km2

; (2) That for the

reach between Longmen and Sanmenxia, including those of the tributaries Jinghe, Luohe, Weihe river etc., with a watershed of 161,671 km2 _{and (3)}

Drai-nage basin of the main river between Sanmenxia and Huayuankou and the tributaries Yihe, Luohe and Qinhe river, covering an area of 41,687 km2 _(see

Fig, 1). The regions above Tuoketuo contribute a discharge of 2, 000-3, 000 m3_{/s during this period in general. This constitutes the base flow of the flood}

downstream. Rainstorms frequently occur at the same time over the first two areas mentioned above, giving rise to major floods, such as those in 1843 and 1933. They are characterized not only by high peak flows, but also by hyper concentration of sediment due to severe soil erosion on the loess plateaus on the middle reaches, which brings about tense situation in respect of flood pre-vention on the lower reaches. The third area is another storm area, 1 • 1 W IllCil also yields major floods often, as in the years 1761 and 1958. Floods from this area concentrate rapidly and there is:very little time available for carrying out flood prevention work downstream after getting the flood forecast, so that they constitute a serious threat on the lower reaches. Floods from area (1) and (2) and from area (3) do not generally meet, but the runoff from the * Ice runs constitute a serious menace to the si.fety of dykes along the lower reaches, particularly

in Shandong province• This has been dealt with in "Ice Dams on the Lower Reaches of the Yell ow River" by Bao Xicheng, Zhao Guanming and Wang Hongxiang •

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area above Sanmenxia mostly comprises over 50o/o of the total quantity flowing through the section at Huayuankou for whatever type of flood (see Table 1). There are no large tributaries feeding the Yellow River below Huayuankou except Wenhe river, and the peak flow diminishes gradually all the way down-streams owing to channel storage.

Table 1. Characteristic Values of Floods in 1843, 1933, 1761 and 1958

Sanmenxia station Huayuankou station

Peak Total Total Peak Total Total

Origin Year run off run off runoff runoff Remarks

flow 1n 12 in 5 flow in 12 in 5

days days days days

(m3_{/s) (10°m}3

) (l09m3) (m3/s) (l09m3) (109m8)

Mainly through

1843 36,000 11. 9 8.4 33,000 13 .6 9.0 investigation

from area & analyses

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observed

(2) 1933 22,000 9.18 5.18 20,400 10.05 5.96 _values

Mainly through

1761 6,000 5,0 2.6 32,000 12.0 8.56 investigation

from & analyses

area (3) 1958 ,6, 400 5.15 2.65 22,300 8.68 5.63 observed values

The major floods mostly happen in the period from mid July to the end of August. During this period, there occur a number of storms of high intensity but short duration, so that the floods thus formed have high peak discharges but small total runoff. The rise and fall of each flood usually take 12 days, most of the runoff being concentrated in 5 days. The total runoffs of floods in excess of 10, 000m3_{/s are not very large (see Table 2 and Fig. 2).}

I

;,~·,

.

,., ',,>O l ' '' < I ,1 " o ,) u r II 1'7 !'-., ',lJ •

Fig. 2. Flood hydrographs for the years 1933, 1954 and 1958. 2. Sediment,

The sediment problem 1s the basic cause of the complexity and arduousness • 4 •

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Table 2. Characteristic Values of Different Floods of the Yellow River.

Sequence

I

Characteristic values Frequency of occur-rence of floods Item N n mean c,/cv 1% 0.1% value Cv Peak flow 600 39 9,780 0.50 4 27,400 38,700 (m3_/s) Total runoff

I

in 5 days 51 2.65 0.49 3.5 7.13 9.84 (109_m3 ) Total runoff

I

I in 12 days 51 5.29 0.41 3 112.2(3.3)* 15.8(6.2)* (109_m3 )

Remarks, *Figures in brackets refer to quantity of runoff for the discharge in excess of l0,000m3_/s.

of the work of flood protection on the lower reaches of the Yellow River. The upper and middle reaches of the Yellow River lie on a vast plateau of loess, where the soils are loose and vegetative cover sparse, The land is criss-crossed by numerous gullies and the slopes are rather steep, Severe ero-sion of soil takes place every year during the rainy seasons in summer and autumn, resulting in heavy sediment transport as much as 1. 6 billion tons per annum on the average carried to the lower reaches, of which some 400 million tons are deposited in the channel on an average every year, thus raising the river bed 0 .1 m per year averagely. Of the rest, another portion of almost 400 million tons is carried to the deep sea, and the remaining part is deposited in the river channel, delta and along the coast near the mouth of the estuary below Lijin. Each year, there is a gain of 23. 5 km2 _{of land thanks to}

sedimen-tation in the sea, and the delta grows at a rate of O. 42 km/yElar.

The channel accretion and the extension of the delta at the river mouth

~ee p the "suspended river" on the lo,ver reaches to develop steadily, vri th

consequent rise in flood stage, adding to the burden of flood protection. At present, the river bed on the lower reaches of the Yellow River is generally 3-5 m higher than the adjoining land behind the levees on both banks. At some place the maximum difference in elevation is 10 m. In 1978, the flood stage at all sections corresponding to a discharge of 3,000 m3_{/s is}_{2 m higher than that}

in 1950 (see Table 3). If a flood of the same magnitude as that in 1958 should occur at Huayuankou, the anticipated high water stage will be 1-2 m above that which occurred in 1958.

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Table 3. Rise in Water Level (t.H, in metres) at Various Sections through the Years 1950-1978, When Passing the Same Discharge of 3,000 m3_/s.

Station Hua- Jiahetan Gaocun Sunkou Aishan Luokou Lijin

yuankou

t.H 1. 9 2.55 2.4 2.0 2.0 2.0 1. 75

Owing to the deposition of sediment, there are some reaches where not only the river bed is higher than the floodplain, but the latter is also higher than the land surface behind the dykes on both banks, Such is rather unfavou-rable to flood prevention (see Fig, 3). It can be seen from the figure that the river bed at this locality is on the average O. 8 m higher than the land near the banks and 1. 6 m higher than the floodplain in the vicinity of the main dykes (within a distance of 600-1000 m). The difference in elevation may be as much as 3 m at the lowest stretches of land. The transverse slope of the flood-plain is as large as 1/3, 000 or so, which is much steeper than the longitudinal gradient of the river in the same section ( about 1/6; 000). Once the flood discharge exceeds the bankfull capacity, the flood water may rush across the banks to attack the main dykes and then pass by alongside them, bringing about tense situation with respect to the safeguarding of the embankments.

Fig. 3. Cross-section of the Yellow River at Mazhai.

d,dw,a of

5bt, r,e·11t

/40IJD . }E!M)

Moreover, the variation of sediment transport within a year and from year to year are both considerable. The annual sediment transport was as high as 3. 9 billion tons in 1933, whereas in 1965 it was only 450 million tons .

. A.bout 85% of the sediment transported every year concentrate in lhe rainy

season (from July to October),most of the sediment being carried by the flood waters. Take for instance the two floods in July and August 1977, which carried a total quantity of 1'. 8 billion tons of sediment downstream, comprising 90% of the total quantity transported in that year. The maximum instantaneous value of sediment concentration at Xiaolangdi was 898 kg/m3 _{and that at}

Hua-yuankou 546 kg/m3_• _{Owing to the high concentration of sediment load,} _there

occurred very intensive scouring and depositing in the channel and radical changes rn the regime of flow. Some vulnerable spots of the dykes were severely

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scoured, or dangerous situation appeared at otherwise ordinary sections, Severe deposition took place on several reaches and, as a result, the flood stage suddenly rose, leading up to high tension in the work of flood prevention, Cite the instance of severe deposits on the reach between Huayuankou and Dongba-tou, in Lankao county, where the river stage corresponding to a discharge of 5,000 m3_{/s was raised 0.2-0.4 m higher than that during the flood in}₁₉₅₈ _with

•

a peak flow of 22,300 m3_{/s, was a result of a single flood in} _1973,. _Another

example is the flood in August 1977 which brought about an abnormal pheno-menon of a sudden drop in the water level followed by an abrupt rise in stage within a comparatively short time interval on the reach between Xiaolangdi and Huayuankou, The stage at Jiabu dropped 0.8 m in 6 hours, followed by an abrupt rise of 2. 84 m in 1. 5 hours, leading up to dangerous situation· at many vulnerable spots (see Fig. 4).

QJ, ~!• i:.s ;:z 1;;) r.;_,J I I

~

(>lr.,,l I ~(tt.tl \ / 1/ ('-' 1\ !H1a.a.,,J ': HIS ' \ ,.,

-s

'

!--, ::: 91 \ a-:1; l r D-:icfarj? 1 P-{'ii1. r5dr-,1~1 ,,,,r,:,,.t, H-7: '.:'.: !Sl~f•'! ., Hn?,•~ 'AJJ 19771 ,~, . / ')'

Fig, 4. H'<1 drographs of flood stage observed at various stations between Xiaolangdi and Huayuankou in 1977.

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H •

General Features of the Channel on the Lower Reaches of the Yellow River.

After changing its course for many times, the river has been keeping to its present course above Dongbatou, in Lankao county, for more than 500 years, whereas the course below Dongbatou was taken in 1855 after a major dyke breach, The river is contained between two dykes and covers an area of over 4,200 km2

•

On account of the particular features of the Yellow River with respect to flood flow and sediment regime and the effects of human activities, sections on the lower reaches of the river become narrower and narrower further downstream. Transversely, the cross-sections are mostly compound ones, formed by the channel (including the main channel wh'ich continuously contains moving water) and terraces of one or more stages, called the flood-plain. The channel is the principal part of the cross-section for passing the floods, contributing to over 70-90% of the discharge capacity of the entire section. The floodplains a·re less capable to pass the flood waters because of the high coefficient of roughness and relatively low velocity of flow, the main function being that of detaining the floods, A list of the geometry of the channel on different reaches along the lower course is given in Table 4.

Table 4. Geometry of River Channel on the Lower Reaches.

Reach Taohuayu-Dongbatou Dongbatou-Gaocun Gaocun-Taochengpu Taochengpu-Yuwa Yuwa-River mouth I

I

Length Spacing (km) of dykes 135 5~14 71 5rv20 165

I

1~8.5 I 350 I 0.4~5 64

Width (km) Mean

longi-I

I

tudinal Channel Main gradient

width channel (0/000) 1~3 1. 44 1. 81 1.6~3.5 1. 3

I

1. 80 0.5~1.6 0.73 1.15 0.4~1.2 0.65

I

1.0 Remarks

I

As the floods on the lower reaches of the Yellow River are characterized by high peaks and small total runoffs, with no feeding from large tributaries all along the way, the channel storage in the wide sections of the river up-stream of Gaocun has much to do with cutting down the flood peaks and total

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runoffs along the reaches downstream. This is also exfremely favourable to flood protection. The maximum flood peak of 22,300 m3 _{/s at Huayuankou in}

1958, for instance, was reduced to only 15,900 and 10,400 m3 /s at Sunkou and Lijin respectively, The effect of channel storage in detaining the flood and cutting down the peak depends further on the magnitude of the flood. The higher the peak, the more obvious the effect. When the peak discharge is below 5, 000-6, 000 m3_{/s (equal to the bankfull capacity), the influence becomes rather}

inappreciable (see Fig, 5),

~ l----l---l----+-200//0 # - > - - - - J - - - ' - - - ' - ' (iaocun zoooo ~) )jt Li -~

Fig, 5. The effect of channel storage in cutting down the flood peak,

With respect to morphology, the lower reaches can be further divided into wandering sections ( braided channel above Gaocun), transitional sections (between Gaocun and Taochengpu) and meandering sections (between Taocheng-pu and Yuwa), Below Yuwa, there is the estuary, In the braided channel, the river is extremely wide and shallow, and there are a number of small inter-laced channels separated by bars through which the flow passes, rather scattered. The deep portion of the channel shifts frequently even during a single flood. On the reach below Dongbatou, the floodplains are low, with plenty of interla-cing channels, often connected to low-lying areas near the:dykes, In case of overflowing of the banks, there is the danger of main current being diverted through the cross channels in the transverse direction and flowing further downstream along the dyke, constituting a menace to the latter, as mentioned before, On the other hand, the discharge capacity and channel storage are also larger here thanks to the broad floodplain, so that the rate of rise in stage is rather low ( the water level rises only 6 - 8 cm for an additional flow of 1,000 m3_{/s), and thus the range of river stage for the high and the low water}

is only 2-3 m in general, The high terri;lces formed above Dongbatou as a result of headward erosion after the river changed its course in 1855 are

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ally' the front lines of defence of the dykes on both banks and are therefore favourable to the work of flood control, However, the effect is being lessened due to channel accretion.

Through the construction of a large number of river training works on the transitional reach between Gaocun and Taochengpu and the meandering reach between Taochengpu and Yuwa, the channel to pass the dominant discharge of medium floods has been put urider control, and both the floodplains and the channel are relatively stable, But as the sections are comparatively narrow and the discharge capacity small, · the flood stage rises more rapidly here than in the broader sections upstreams, The reach below Taochengpu is particularly so, where the river stage rises or drops O. 5 m for an increase or decrease of 1,000 m3_/s, _{within a wider range of}_6-7_{m in general for the high and low}

water. The duration of the high water stage is also longer than that above Gaocun. Moreover, · scouring in the vertical direction generally to a depth of 4-5 m are often observed in the main channel during floods. This adds to the troubles in protecting the river training works, Besides, the existence of sharp bends in narrow sections often give rise to jamming of ice during the winter seasons, leading sometimes even to the forming of ice dams and, consequently, tense situation during the ice run.

][ . Flood Protection Works on the Lower Reaches of the Yellow River.

In compliance with the particular features of the floods, sediment regime and channel pattern on the lower reaches of the Yellow River, and by summa-rizing the experiences in the work· of flood protection both in past history .as after liberation, the policy of "retaining the floods upstream and discharging them downstream, diverting the floods and detaining them on either bank if needed" has been adopted to cope with the floods on the lower reaches. · The concrete measures are: streng!,heJLtnlLJ!.niL.hci.ilittninK the J!ykes, constructing rivet' trairijnfLwgrks, discharging the water and sediment into the sea; setting urt;;;;~:--ks

£

or diversion ~Tffood~vat;r i~t~-approprTaTectefe~tT;;~ 1:i~;r;;_;,-t;-be

operat;d necessaryJ buildi~g--~f-;;;ei:voirs 'on the ·its

tributa-ri-e-;--~-the middle reaches to retain the floods and to turn harms into benefits

(se-e

Fig.

6:).

It is demanded tha t-th;-dyke~ should be safe under all

circum-stances when the observed maximum peak discharge of ·22, 000 m3 _{/s is not}

excee-dedJ and that there should be ways and appropriate measures to deal with still larger floods.

1. Dykes and river training works. (1) Dykes.

The total length of the dykes along both banks of the Yellow River is 1,369.5 km ( not including those along the tributaries and second lines of clef ence). The dykes have mostly been built through renovation of the old

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[

jf:,,,·.1;

' l I

',l

~"I

Fig. 6. Sketch map showing the flood control works on the lower reaches of the Yellow River.

embankments. Keeping pace with the steady accretion of the river bed, the dykes have been heightened and strengthened three times entire reaches in the past 30 years and a total volume of 500 million m3 _{of earthwork has}

been accomplished, At present, the dykes are generally 6-lOm hgih, leaving freeboards of 3. 0 m, 2. 5 m and 2 .1 m respectively for the upper, middle and lower sections. The crest widthis7-15m. The front slopes are 1:2.5andl:3 and the back slope 1: 3, generally provided with additional berm behind.

In order to raise the capability of the dykes to resist the floods, beside planting grass and willows to make the dykes green, measures have been taken throughout the years to place clay blankets and sloping cores, as well as imper-meable clay cores, and to remedy hidden defects in the dykes by probing with pointed steel rods and subsequent grouting, The method of probing and grouting has been proved most satisfactory. Aitogether more than 92 miliion holes were made by probing to remove over 330,000 hidden defects.*

The foundations of the dykes along the lower reaches are mostly sandy soils or alternate layers of sandy and clayey soils, susceptible to piping. Besides, the soil materials used for' placing the embankments are rather uneven, so that it was not infrequent that piping occurred behind the dykes during floods. Such was a frequent cause of dyke-breaches in the history, bringing

* "To Remedy Hidden Defects in Dykes by Probing with Pointed Steel Rods and Subsequent Grouting", by Cheng Zidao and Liu Yuli.

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about catastrophes. After the founding of new China, the method of adding berms to the front or back slopes by manual labour for dyke-strengthening was first used to assure safety of the main dykes, later, the particular feature of hyper concentration of sediment was made use of by drawing water from the river during the dry seasons and letting the sediment deposit on either side of the embankment to strengthen the dyke proper and to increase the creep length. A number of suction dredgers of simple construction were made in recent years to lift the sediment from the bottom of the river or draw water of hyper con-centration during flood seasons, to be laid in prescribed zones to reinforce the embankments, thus speeding up the strengthening of dykes by means of warping, These methods are not only more economical than that of adding berms to the front and back slopes with manual effort, but are also of advantage to farm irrigation and amelioration of soils along both banks.* Up to the end of 1979, altogether 160 million m3

of earthwork' had been accomplished, strengthening the dykes by warping, Sections of the main dyke to a total length of 245 km gained at least 50 m in width with a layer of soil deposited at least 2 m thick, Sub-stantial improvements were made at dyke sections which previously shown serious signs of piping or leaks. Besides, nearly 3 million mu (200,000 ha) of former saline and alkaline land have been ameliorated through warping with highly silt-laden water of the Yellow River.

(2) Training works.

River correction works fall under two groups, namely "vulne9b~~ts"

~ .

and "const 'nts for bank protection". Spur dykes, "stack" dykes (short groins) and revetments are built at dyke sections which come into direct contact with the current to prevent scour. Works as such are carried out at the "vulnerable spots", as they are called here. When there are enough floodplains wide in front of the dykes, "constraints for bank protection" are made to keep them from being scoured, so as to give the dykes a good shelter. Besides, constraints as such are used to direct the current in an attempt to achieve a more or less stable channel for the dominant discharge for medium floods. The people along the river used to make "brush-and-soil works" down the ages to resist scour, by placing al tern at!~ layers of willow or sorghun stems and soil, which proved to be effective in protecting the dykes. But the disadvantage is that works as such cannot stand long use. More than 5,000 spur dykes, "stack" dykes (short groins) .and revetments have been all rebuilt and the brush-and-soil works replaced by permanent roe.le and earth structure*%. Also over 3,000 new groins have been added, The total amount of rock used was 10 million m3 _{or more. At}

*

"Strengthening of Dykes by Warping", by Bao Xicheng, Zhang Mingde and Wang Rusiu.

**

"Engineering Structure for River Correction on the Lower Reaches of the Yellow River", by Shen Hongxin and Xu Fulin.

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present, the meandering reach below Taochengpu is already under control, the transitional sections between Gaocun and Taochengpu have been practically controlled, whereas only a few constraints have been laid in the braided channel above Gaocun, which should be further strengthened and replenished.

2. Diversion works and detention basins.

There are waterlogged areas along the lower reaches of the Yellow River which were overflowed naturally during major floods through uncontrolled diver-sion of water and subsequent detention. Dongpinghu on the right bank and the floodplain between Jindi River and the main dyke to the left of the Yellow River are the comparatively larger ones. Since the founding of new China, a number of diversion gates, outlet sluices as well as enclosing dykes etc. have been constructed there to cope with extraordinarily large floods, to raise the capacity of diverting and detaining floods, to safeguard the dykes and to keep the lowlying land from being overflowed too frequently when not absolutely necessary. In doing so, these areas have been converted into detention basins which can be operated in time of need. The former is called the "Dongpinghu Detention Reservoir"* and the latter the "North Jindi Detention Basin", the main parameters for the two projects being shown in Table 5.

Table 5. Main Parameters of the Dongpinghu Detention Reservoir and the North Jindi Detention Basin. Item

I

m

8

/s 10,000 10,000

diversion

Length of enclosing dykes km 85.11₎ ₁₂₃2₎

Maximum height of enclosing

8.5 13

dykes m

Area of cultivated land in

103

mu 512 2,420

the basin

Inhabitant 200,000 1,255,000

1) Only the exterior enclosing dykes are taken into account. 2) Only the North Jindi dyke is taken into account,

· * "A Brief Account of the Dongpinghu Detention Reservoir,".

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The order of operating the detention basins is as follows: When a flood peak of 22,000 rn.3_{/s appears at Huayuankou, the Dongpinghu reservoir is to}

take in 7, 000-8, 000 m3 _{/s through the diversion gates,} _{and the rest is to ,be}

passed in the river as usual, but keeping the discharge at Aishan from exceeding 10,000 m3_/s. _{In case of an extraordinarily large flood with a peak of over}

22,000 m3 _{/s, the North Jin di Detention Basin can be operated at an opportune}

moment.

3. Reservoirs on the main river and its tributaries.

In order to control the floods as well as to fully utilize the water resources of the Yell ow River, the Sanmenxia Reservoir was built on the main river in 1960 and the Luh~n Reservoir on the tributary Yihe River in 1965. The Guxian Reservoir on the, tributary Luohe River is now under construction.

The catchment of the Sanmenxia Reservoir comprises 92% of the total drainage area of the Yellow River, keeping the floods from the two main source areas from overrunning the lower reaches, At first, the reservoir was operated with the aim of storing water and as a result, sedimentation in the reservoir area became very serious. After being rebuilt in two stages, 2 tunnels and 8 bottom outlets were added and four of the penstocks for power generation were converted into outlets for discharging flood water and sediment. The reservoir is then operated to store water in the dry seasons~ while letting the water to discharge freely during the flood season, exercising some sort of control over the discharge only when necessary. The rate of sedimentation rn the reservoir is thus greatly reduced. A state of equilibrium of scour and fill has been reached in the portion of the reservoir area in the gorges below Tonggu!ln, In using the present way of operating the reservoir, the maximum outflow will not exceed 15,000 m3

/ s in case of a major flood above Sanmenxia,

When a major flood occurs in the catchments for the reach between Sanmenxia and Huayuankou, the reservoir can be operated at the opportune moment to hold 3 or 4 billion m3

of water. This is of much significance to the relieving of burden on the lower reaches. By retaining water during the dry seasons to a total volume of 1, 4-2. 6 billion m3

, the menace of ice run along the lower

course can also be alleviated, while adding to the storage in the dry weather, Besides, turbines and generators totalling 250,000 kw in capacity were also in-stalled, which marked the beginning of multipurpose utilization and drawing of benefits thereof,

The catchments of Luhun and Guxian reservoir comprise 21

o/o

of the water-sheds for the reach between Sanmenxia and Huayuankou, A total capacity of 600 million m3 _{is available for retaining a major flood with a recurrence period}

of 100 years, to cut down the flood peak at Huayuankou by 2,000-6,000 m3

/s. The main parameters of the three aforementioned reservoirs are listed in

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Table 6.

Table 6. Main Parameters of the Sanmenxia, Luhun and Guxian Reservoir.

Item Unit Sanmenxia Luhun Reservoir Guxian Reservoir project

Location on the main on the tributary on the tributary _river _{Yihe River,} _{Luohe River.}

Catchment km2 _688,421 _3,492 _5,370

Type of dam gravity con- earth dam gravity concrete

crete dam dam

Maximum height _m ₁₀₆ ₅₅ 118 of da·m Total storage

I

109 m3

I

36.0

I

1. 29 L16

Capacity for flood l09 m3 I 6.04 0.64 0.46 retaining ,, Capacity of outlet m3/s 15,000 16,150 12,000 structure .,

4. Reconstruction work and prov1s1ons for removing and rehabilitating the inhabitants on the floodplains and in the detention basin.

There are altogether over 1. 2 miHion people living on more than 2. 7 millio,n mu (180,000 ha) of cultivated land on the floodplains between _the main dykes along the lower reaches of the Yellow River, . and, many other people live on the vast expanse of farmlands in the detention basins mentioned al:iove. In order to safeguard the life and property of the masses during flood diversi9n and detention and to utilize the land resources fully, the following measures are being taken in the main:

The state grants support in farm production to the local inhabitants by carrying out irrigation and drainage projects and supplying agricultural

machi-neries etc. As there are more occasions for the water to overflow the

flood-plains and a part of the detention basin of the Dongpinghu Reservoir, only wheat is planted there every year in view of the scarcely evitable flooding in autumn. Diversified economy · is also developed here, appropriate to local conditions.

As regards the problem of security and protection against flood, temporary shelter platforms and terraces for raising the elevation of the houses, as well as levees around the villages, are built in the relatively shallow water of the detention basins where the velocity of flow is rather low, whereas the

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tants of other areas in the basins are to _be temporarily transferred to safe spots in case of need, In order to organize the masses for a safe retreat, highways and bridges were constructed for the particular purpose, and boats and trucks are provided. Every family in every village is notified beforehand of the destination and route to the safe spot, Every year, the work should undergo inspection to see

if

the plan is practicable and preparation adequate, The work is divided up and each part is assigned to a person who assumes full responsibilty for it.

Concluding Remarks

Flooding by the Yellow River on its lower course had brought about untold miseries and calamities to the Chinese people down the ages. After the founding of new China, an end was put to the distressful situation of "two breaches every three years". Successes in fighting the floods and in safeguarding the main dykes have been won in the past 30 years on end. The achievements are no doubt gigantic, But as the floods are not yet under complete control and, in particular, the sediment problem has not yet been solved, the work of flood protection on the lower reaches of the Yellow River still remains to be arduous and protracted. Henceforth, beside further sophisticating the measures of "retai-ning the floods upstream and discharging them downstream, diverting the floods and detaining them on either bank if needed" by means of a complete system of engineering structures for flood protection, improving the method of flood forecasting, raising the level of management and dispatching in the work of flood prevention etc., it is necessary to seek new ways and to study appropri-ate methods of solving the sediment problem, in close coordination with the ~nified planning for the harnessing of the Yellow River and the course of its enforcement.

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Strengthening of Dykes by Warping

Bao Xicheng Zhang Mingde Wang Rusiu

(Shandong Provincial Yellow River Bureau)

Abstract

In this paper, analyses have been made of the necessity of strengthening the dykes along the lower reaches of the Yellow River, a comparison has been made of the different methods of dyke renovation and the typical cross sections to be attained throught reinforcing are demonstrated, These are all based on the actual conditions of the dyke proper and foundation. Strengthening of dykes by means of warping has been proved to be of beneficial result, after being subjected to severe tests of floods and ice runs.

I .

The necessity of Strengthening the Dykes

The total length of the dykes on both banks on the lower reaches of the Yellow River is 1369. 5 km, of which 550. 7 km lie in Henan province and 818. 8 km in Shandong. The standards for the design of the dykes are that they should be capable of coping with a peak discharge of 22,000 m3_{/s at Huayuankou,}

and 10,000 m3_{/s at Aishan, after taking into consideration the channel storage}

and flood diversion into the detention basin at Dongpinghu; that the free board should be consistent with the width of river, 2 .1 m, 2. 5 m and 3. 0 m being adopted for different sections (larger values used for the upper sections); that the crest should be 7-15 m wide and that the front slope should be 1 :2. 5 or 1:3 and the land-side slope 1:3, one or two berms 4 m in width and with side slope of 1: 5 being usually added to the back slope.

Before liberation, the main dykes along the Yellow River were mostly 3-6 m in height, with crest width of from 4 to 8 m. The earth fill was generally not of good quality. According to statistics made on hidden defects in dyke sections in Shandong province, it was shown that there existed 131 cavities and leaks due to lack of compaction, 652 undetected blockhouses, trenches and dugouts, 544 caves of burrowing animals (badgers, foxes and rats), 428 cavities due to rotten roots, tombs, wells etc. in the dyke proper. There were also remains of decayed straw mattress revetments and rotten stems used during gap closures in the foundation over a length of 53. 2 km. 1t was found by tests

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made during excavation that the dry density of the soil in the old embankments was only 1.1-1. 3 t/m3

• Some of the fills were made by using lumps of mud

or frozen soil. The dykes were therefore rather incapable of resisting the floods.

Since the beginning of the work to harness the Yellow River by the people and for the people, the main dykes have been heightened and widened extensively three times. The crest was raised 1-2 m each time. All the work was done by renovating the old dykes. A total amount of earthwork. of 500 million m3

was carried out in the years 1947-1949. The change and development of the dyke cross-sections are shown in Fig. 1. Owing to channel aggradation, the floodplains in front of the dykes in Shandong are on the average 2. 9 m higher than the land surface behind the dykes (the maximum difference in elevation being 5. 6 m). The dykes are generally 10 m high (reaching a maximum of

13.8m).

I 1.90

1949Jf).,UJ5 .

old emb<mKme,d ,· 1949

Fig. 1. The development of dyke section at Beidianzi, rn the city of Jin an, through renovation.

1.90

The main dykes of the Yellow River were built through heightening an.cl strengthening the old embankments many a time and oft. Owing to the diversity of soils, insufficient stripping, lack of compaction in individual sections and improper and inadequate treatment of the junctures between sections assigned to different work teams during construction, the quality of the embankments

is uneven. The main parameters of the soils In the dyke pro per are on the

average as follows:

The main dykes along the lower reaches of the Y ell~w River are built on alluvial soils, the soil layers along the dyke lines

faU

under the following types, in accordance with the geological formation: (1) alternating layers of highly pervious silty sand and sandy loam; (2) a layer of clay and clay loam, 0. 5-3. 0 m thick, sandwiched in the silty sand and sandy loam; (3) clayey subsoil which becomes fissured near the ground surface, permeable, but does not deform appreciably under the action of seepage force. The soil .condition of the dyke foundation also varies greatly here and there, the physical and

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Item Unit Sandy Silt Sandy Loam Remarks soil loam Dry density t/m3 a minimum of 1. 5 is required - - - , ~ - - -

~"--·-18-26

coefficient

mechanical properties of some of the soils being as follows: very fine sand, dry density 1. 45-1. 67 t/m3

• angle of internal friction

31. 0-36 .1

°,

non-uniformity coefficient 3. 5 sandy loam, dry density 1.37-1.59 t/m3

, angle of internal friction

20. 0-34. 6

°,

cohesion 0-0 .12 kg/cm 2, non-uniformity co-efficient 3. 6-3. 8

silty sand, dry density 1.47-1.58 t/m3_, _{angle of internal friction}

32. 2-32. 4

°,

cohesion 0-0. 08 kg/cm2_, _{non-uni£ ormity}

coef-ficient 3. 04-3. 40.

There occurred frequent dyke-breaches along the Yellow River in the past. As many as 143 gaps were formed in the dykes during floods in the period of 1855-1938 in Shandong province alone. According to statistics, 53% of the breaches were due to inferior quality of the dyke proper or damages in the foundation, 10

%

of which were caused by scouring, 29

%

were result of over-flowing due to insufficient height and 8% breaching on purpose. It is obvious from the above figures that failure due to clef ects in the dyke pro per and its foundation constitutes the majority of cases.

Since the founding of the People's Republic of China, the main dykes along the Yellow River have been made higher and broader for many times, but still there occurred dangerous situation during major floods, as shown in the following table:

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Imminent dangers Peak

River Duration of

Year flow at stage high stage Length of Length of

Luokou _(m) above 29.0m _Leaks Pi- _Slip leaking cracked

(m3

/s) (days) ping sections sections

(km) (km)

1958 11,900

the diameter of the sand boils being O. 5-15cm.

Cavities which leads from the front to the back slope with diameters of from 0.15-1.0 m, occurred in dyke sections in the city of Jinan and Qihe and Dong'a counties in 1958. As these had been discovered in time, they were quickly blocked up through rush repairs so that disasters were headed off.

It is therefore ne~essary to further strengthen the dykes in order to ensure safety in preventing flood. Moreover, a particular feature of the Yellow River is its high sediment content. There are 400 million tons of sediment deposited each year in the channel on the lower reaches, raising the flood stage accor-dingly. The high water level at Jinan for the same discharge of 8,000 m3/s was 2. 26 m higher in 1976 than in 1958. A rise of O .126 in per annum is manifested on the average. Viewing the problem from the angle of the progress of the work of harnessing the Yellow River which cannot be very quick in the nearest future, it is obvious that the deposition of sediment in the river bed on the lower reaches will take place continually. Hence, the main dykes should be constantly heightened and strengthened.

]I . Methods of Strengthening the Dykes.

1. Measures taken to strengthen the dykes ·of the Yellow River· are as follows: (1) Intensive probing and subsequent grouting; (2) Use of sloping clay core; (3) Addition of sand and gravel inverted filters and (4) Enlarging the cross sections. It has been proved by practice that probing intensively with steel rods and, subsequently, pressure grouting is effective in eliminating

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cavities and cracks, but not of much good to the unconsolidated or loose soils in the dyke proper or the foundation. The upstream clay core is well impermeable, but a very large quantity of clay will be needed if it is adopted on the entire dyke line. For the 800 odd km of main dykes in Shandong province alone, some 40 million m3 _{of clay will be needed. As there is only}

a rather small clay deposit along the dyke lines of the Yellow River, too much farmland would be destroyed as borrow-pits. The application of filter zones of sand and gravel is limited, because the dykes need further heightening. There are also difficulties in supplying the materials for the entire dyke lines. Moreover, the filter zones are difficult to inspect and maintain, as they get clogged up readily, and a large amount of work would be needed to keep them in good order. By comparing the different methods in a comprehensive way, it has been concluded that strengthening the dykes by enlarging the cross-sections is an acceptable way, because of the abundance of soil material, low cost and adaptability to the need of further heightening, and in that the work may be carried out in stages appropriate to the concrete conditions.

2. Standard profile for dyke-strengthening.

(1) The dykes should be strengthened so that the hydraulic gradient of the seepage flow is kept well below the permissible limit for the particular foun-dation soil, with the aim of preventing any deformation of founfoun-dation soils due to seepage, and making the dyke stable enough, with a sufficient cross-section for resisting the flood in normal cases as well as during earthquakes etc.

The safety of flood protection on the Yellow River has a most important bearing on the national economy of our country. The main dykes are the key elements for assuring the security. Hence the strengthening of the dykes should be adequate even under comparatively unfavourable conditions. The parameters

of soil materials used for dyke-strengthening are shown on the next page, The following points are taken into consideration in determining the additional width required to prevent any move~ent of soil due to undersee-page:

(a) Sandy soil, silt and light susceptible to

piping, the non-uniformity coefficient being smaller than 10.

( b) The permissible exit gradient of the seepage flow. From the results of 15 tests, it has been found that sand boils occur at a gradient of 0. 7-1. 0. It has been observed that boils occur in the sandy loam and silty sand

in the foundation of the Dongpinghu detention basin in Shandong, province if

the hydraulic gradient lies between 0.5 and 1.0. The actual gradient which caused local failures due to underseepage at the Jingjieng dyke, also in China, which is rather similar to the dykes along the Yellow River, is known to be

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Relative density 0.576 Non-uniformity _coefficient 2.3

0.73-0.77, according to observed data. Hence, a critical gradient of 0.75 may be chosen, which gives a tolerable gradient of 0.25 with a factor of safety equal to 3.

( c) Computed values of exit gradient for typical sections (Fig, 2). The results of computation for different additional widths of levee by the method of flow net and by using empirical formula are compared as follow:

Exit

~

- - ~ W i d t h

Original Increased 30 m Increased 50 m

Method used ~

Method of flow net 0.342 0.289

I

0.250

Empirical formula 0.354

I

0.260 0.223

( d) The location of sand boils landward of the levee of the Yellow River 1n Shandong during rainy seasons over the years is mostly within 50 m of the teo, At places where the dyke was breached in the past, near abandoned diversion canals and at vulnerable spots where rotten materials remain in thick layers in the foundation, boils due to underseepage may be within the

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,rt ~i .±. )l.

cl°1/e# soil

larr-50 _, oi 0 I ~ ICO =5....,.,0=='0==2=0=""'30 .if, 12'. : ~ Unit : M

Fig. 2. Typical cross~sect1o-n and location of $eepage Tine

range of 70 to 90 m from the toe, The ratio of head to horizontal creep length is from 1: 13 to

U

15, according to analyses made on statistical data. Hence the additional width required at these places should exceed 50 m.

From the above-mentioned, it is stipulated that the additional width required for strengthening the normal dyke sections should be 50 m, and that for vulnerable spots 100 m, in order to control piping. The top of the berm added for dyke-strengthening should be at least 1 m higher than the outcrop of the seepage line. Due regard should be paid

to;

problems of embankment quality, such as the existence of hidden defects, and :the need to further heighten the levee. In such cases, the additional fill on the back slope should be level with the flood stage and at the front 0. 5 m higher than the flood stage. At present, additional fills of sandy soils have been placed to an elevation 0.5 m above the outcrop complying with the predicted flood stage in 1983, above which loam is placed as high as the flood stage in 1983.

(2) When earthquakes of intensity 7-8 are to be considered in the design, sandy soils with relative density D, smaller than 0. 70-0. 75 are susceptible to liquefaction, according to the specification adopted in China for the design of hydraulic structures to sustain shocks.

A part of the levees along the lower reaches of the Yellow River is located in regions where earthquakes of intensity 7 may occur. The foundations are mostly sandy soils, Some of the soil materials used for dyke-strengthening are very fine sand, It is stipulated that the stability of the embankment should be checked to see if it could sustain earthquake shocks of intensity 8.

The method of limiting equilibrium ( total stress method) has been used in the analyses for the stability of. the embankment during earthquakes, the side forces between slices being taken int~ .consideration. The results of the computations are as follows:

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Location Crest width Under static condition K value, of slip

Ni!.

within the

I

when

failure subjected

surface

"fr/or

oi/o~

K to shocks

i 2. 7.7

I

1.06

Down-I

Remarks: (1) or, 'rr--eff ective initial normal stress and shearing stress respectively;

cii-ci3--maximum and minimum principal stress respectively;

K--factor of safety.

(2) Computations were made for each sliding surface both for the high

stage and the medium stage. The minimum K-values are tabulated, which need. be only larger than unity, because the foundation strata have heen simplified in such a manner as to reflect the most unfavourable condition.

10 S 0 10 20 30 m

Fig. 3. Rough sketch of the location of slip surfaces 1n the computations.

It has been shown by the results of computation that the dyke proper as well as the foundation will lose their stability and slide during the earthquake. Something like 20 m or. more, including the dyke crest or the top of the additional fill will be possibly included in the sliding mass, and there will still

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be 30 m or so left (in case the levee is widened 50 m) to resist the floodwater, so that rush repairs may be carried out in time to assure the safety of the main dyke,

3. Method of dyke-strengthening by warping.

If the dykes are to be strengthened in conformity with the aforesaid standard, a total amount of earthwork as large as 430 million m3 _{will have to}

be accomplished in Shan dong province alone. The following points have been studied in order to complete the work which embraces such a large quantity of earthfill.

(1) A comparison of the method of construction. Three methods can be used under the present conditions in our country, namely, by manual labour, using earth-moving machineries and by means of warping, A comparison has been made of the methods as shown· in the tabl~:

Labour force Farmland

Method Main required used as Investment

equipments borrow-pits (10° yuen)

(103 _workdays) (103 ha) Manual 430,000 47.7 602 labour Machines 1434 used for 38 31. 8 860 placement scrapers 200 dredgers Warping of simple 20 240 comtruction

It can be seen that placement by manual labour or by using earth-moving equipments costs more and needs a bigger labour force, while large tracts of land have to be spoiled due to excavation, On the other hand, practically no farmland will be spoiled when warping is applied, and the tail water can be

used for irrigation,1_{l[e therefore adopt the}_{last-mentioned for the strengthening} of dykes.

(2) Different ways of strengthening the dykes by warping, Warping makes full use of the particular feature of the Yellow River-that of hyper concentra-tion of sediment, The sediment of the Yellow River is made use of to strengthen the dykes by means of warping, There have been three different stages of development in the method of warping,

(a) Warping by gravity flow as a means of dyke-strengthening, The water from the Yellow River found its use first in irrigating farmlands 1n the

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fifties, As the flow is highly silt-laden, sedimentation basins were needed to make the water clear before being applied to the fields. The low-lying alkaline and saline lands along the Yellow River were mostly chosen to be desilting basins. After a few years' practice, it was found that the ground surface became raised through sedimentation, and at the same time the soils were improved, which is beneficial to agricultural production. Such a practice was afterwards developed and the depositing of sediment was carried out according to plan, making use of deep pools formed at the gaps and waterlogged areas landward of the levees. The pools were soon silted up, and the depressions raised. In doing so, the dykes of the Yellow River were actually strengthened. In recent years, some 5 billion m3 _{of water have been diverted every year}

from the Yellow River in Shandong province alone for warping, and 32 million m3 _{of earthwork within the scope prescribed for dyke-strengthening}

have been accomplished by means of gravity flow carrying the sediment which subsequently deposits.

(b) Warping by means of pumping, As a part of the dykes was strengthend through sedimentation of silt in the sixties to the extent that further warping by gravity flow became impractical because of the rise in ground elevation, pumping had to be resorted to. The water is mainly lifted through pumping stations built at the outlets of culverts, sluices or siphons, for irrigating the farmland as well as reinforcing the dykes by means of warping in prescribed areas. There are 47 pumping stations in Shandong province along the Yellow River for these purposes, with a total discharge of 220 m3 _{/s, altogether}

accom-plishing 20 million m3 _{of earthwork within the prescribed zones for}

dyke-strengthening over the years.

(c) Using suction dredgers of simple · construction for warping, The above-mentioned methods are used mainly in the dry seasons for irrigation as :vell as warping, The tempo of the work to reinforce the dykes is rather slow on account of the low sediment content during the seasons of low water. Irrigation is not required during the rainy season, and if water is diverted during the floods, too much excessive moisture has to be drained away after depositing the silt, This will bring about troubles in draining off the local discharges from the waterlogged land and is therefore cliff icult to realize. Suet.ion dredgers of simple construction were built by the staff members of the Shandong provincial Yellow River Bureau to quicken the steps. These are used in combination with diversion of water for irrigation and warping by the usual practice, but then the sediment content of the water is increased and the quantity of water to be drained away greatly reduced, By using dredgers, the work of dyke-strengthening by warping can be carried out all the year round, The process of silting by means of suction dredgers is shown in Fig, 4. The

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sediment at the river bed is first dislodged by water jets under high pressure, so that the sediment content will reach a definite level before being extracted by mud pumps or mixed-flow · pumps. The mud flows through a pipe-line leading to the landward area prescribed for warping, After depositing the sediment to strengthen the dyke, the clear water is drained off. In Shandong province alone, 200 dredgers of simple construction have been built in 1974-1979, accomplishing 91 million m3

of earthwork in total for the strengthening of dykes.

Suction dredJer of simple wns/rud;Oll

oo.q,oJ!UliA~

(J..J..I.Hfi>>

Pontoon ( bu;(/ bJ ,na,wal Labour)

Pif)e-lrne

itJ,., t·t

®iti.nUt

depwts

o/

3"JVi~ Ji-Ow

Fig. 4. Rough sketch of the use of suction dredgers of simple construction for warping as a means of dyke-strengthening

]I. Benefits Drawn from Warping,

1. Reinforcing the dykes to resist the floods better. Pools and low-lying land behind 460 km of dyke in Shandong province have been silted up in recent years through warpipg by gravity flow. The ground surf ace has been rais·ed ·and the difference in elevation of the land in front of and behind · the dyke has been reduced, thus improvirig the condition of the dykes during the rainy season when both sides were formerly overflowing with water. The use of suctio~ dredgers has rendered 220 km of main dykes safer, to different extents. ·The creep length has been increased, the dyke proper reinforced and the capability to resist the floods raised. The function of removing defects and hidden perils, such as cavities, sand boils, leaks and cracks, is manifest. It has been proved,· after undergoing several rigorous trials of several floods that the hoped-for results have been achieved.

During the flood in 1976, the highest ~tage at Luokou, near Jinan, Shandong

province, was 32.14m, that is, 0.05mhigher than that in 1958 (which was 32.09 m). The river remained at its high stage over a longer period of time than before (in 1976, the stage remained higher than 29. 0 m for 60 days on end, and in 1958 18 days only). There was practically no dangerous situation all the way along the 80 km stretch of dyke with 3 m of deposits, whereas 1n 1958 there were many places where the dyke was in danger.

During the ice run in 1978, jamming occurred at Lijin, where the river stage in a 50 km reach above Wangzhuang exceeded the flood stage in 1958 and