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Diyala

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22

ATKINSON AND CARDIACOS ON

I'apcr No. 5443.I'

The Reconstruction of the Diyala Weir."

By JOHN DEKEYNE ATKINSON, B.A., M. Inst. C.E., CARDIACOS,E.T.P. (Paris). and GEORGE

(Ordered by the Council to be published .with written discussion.?)TABLE O F CONTENTS The rivcr and the sitc . . . The original weir . . . . The first reconstruction . . . The 1937 repairs . . . . . The final design . . . . . The first season'swork, 1939 . The 1 9 3 9 4 0 flood . . . . The second season's work . . Subsequent operation of the weir The 1940-41 flood . . . . Conclusions . . . . . . Appendix: costs . . . . .

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THERIVER

SITE.

THEDiyala river rises in the mountain country on the Iran-Iraq boundary and flows in a south-westerly course to join the river Tigris 32.5 kilometres (20.2 miles) downstream from Baghdad. In the upper two-thirds of its course it passes through hilly country, emerging on to the Iraq plain a t Table Mountain, a name given to thisplace in theMesopotamian campaign during the last war, when the thirteenth and fourteenth Indian divisions were operating there and left memorials on two small hills on opposite sides of the river a short way above the site of the weir, which is 171 kilometres (106.25-miles)from the junction of the river with the Tigris. I n 1917 a gauge, known as the Flying Bridge gauge, was erected a t a site 1-428 kilometre (0.88 mile) above the weir, which has been regularly recorded ; a permanent flood discharge measuring station was established there in 1933, and is the only one of its kind on the river. There is another regularly recorded gauge a t Jalula, 38.5 kilometres (23.9 miles) above Table Mountain, The Alwan river, the last perennially contributing~~

Correspondence on this Paper can be accepted until the 16th March, 194G, and will be published in a Supplement to the Institution Journal for October l94fi. Contributions should be limited to about 600 words.-SEc. INST.C.E.

t

TTIE RECONSTRUCTION O F THE DIYALA WEIR.

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tributary to the Diyala, joins the main stream about 3 kilometres (2.86 miles) abovc Jalula (Fig. 1, Plate 1 ) . Thc main river below Table Mountain flows in a well-defined chnwel through the alluvial plain and,.with theexception of the last 30 kilometres (18.6 miles), does not rise above the level of the plain in high flood. It is by nature a torrential stream in flood and is liable to very rapid rises and falls ; on 2 February, 1939, a rise of 3.97 metres (13 feet) was recorded a t Flying Bridge in 24 hours. The main flood normally occurs in March, but peaks may occur a t any time during the months of January to May inclusive ; the normal lowstage period is from June to December. Fig. 2, Plate 1, shows the timestage and time-discharge relations a t Flying Bridge and may be taken as representing average conditions ; the highest and lowest recorded gauges and the corresponding discharges are :Dt. ae3 February, 1939 3 July, 1932

Flying Bridge Gauge. metres.70.50 * 65.24 t

Estimated D s h r e icug. cumecs. 3,08015 .

Table Mountain is the name that was given to one height in the Gebel Hamrin, which is a long range of hills extending in anorth-west-south-east direction across the northern part of Iraq ; it is pierced by three rivers, namely the Tigris at Fatha gorge, the Adhaim at Injana, and the Diyala a t Table Mountain, where the Gebel is composed of sandstone and conglomerate containing some gypsum and is of little value for constructional purposes. The bed of the river here is gravel and sand, which occurs mixed, segregated, and layered, the former being mostly fine with a small proportion of larger gravel up to4 inches and 5 inches. The landson both sides of the river are irrigated by a system of canals ; all those on the left bank take off in the immediate vicinity of the weir, but the head of the right bank canal, the Khalis, is about 5 kilometres (3.1 miles) upstream of the weir and outside its influence. The head of the Khalis was formerly just below the weir site, and the intention to make a is new offtake for the canal between the right flank of the weir and the hills, when the presenthead, which gives considerable troublewith gravel deposits in the head reach and requires theannual construction of a temporary bund in the river to ensure its water-supply, will be abandoned. Gravel deposits have also caused some trouble in the left bank canals, particularly the Ruz, which is the uppermost one. The layout of these canals is shown in Fig. 1, Plate 1. The levels in the river during the low period were insufficient to supply the canals and .the discharge of the river a t this period is such that even

*

t

After collapse of old weir and before reconstruction commenced. While original weir was in operation.

24

ATKINSON AND CARDIACOS ON

if all of it were diverted it would not meet the potential demands of the area for summer cultivation. Clearly then a weir had t o be provided to divert the summer supply to the canals as the first stage in the improvement of summer irrigation, and a t some future period storage reservoirs will have to be provided farther up the river. Prior to the construction of the first weir a bund of brushwood and gravel was constructed each year by thelocal people after theflood had subsided, but thiswas destroyed by the first freshet of the following rainy season, which usually arrived a t the time of high water demand for winter sowings, which were thus often delayed, to the greatloss of the cultivators. This unsatisfactory state of affairs was first remedied in 1928 by the construction of a weir across the river just below the offtake of the Mahrut canal where the channel of the river broadens out very considerably. THE ORIGINAL WEIR. This consisted of a simple mass concrete core wall of trapezoidal form, with a crest-level of 66.00 metres (216.5 feet) ; the foundation-level was 63.00 metres (206.7 feet) throughout, whilst the bed of the river varied from 66.50 metres to 63.30 metres (218.1-207.7 feet). The main channel at that time wasin the centre of the river ; it was about 100 metres (328 feet) wide and its average bed-level was about 63.4 metres (208 feet). The original design included gravel filling on both sides of the wall, that on the upstream side having a slope of 4 : 1 and that on the downstream 12 : l ; the former, however, was never made, but the latter was covered with a slab of reinforced concrete 0.10 metre (3.9 inches) thick divided by bitumen-filled jointsinto twelve sections, each measuring 36 metres by 36 metres (118 feet by 118 feet). The slab in each section was pierced by short lengths of 3-inch pipe, twenty to a section, to deal with upthrust. The length of the crest of this weir was 437 metres (1,434 feet), which is considerably more than the average width of the Diyala river ; the flanks were formed of concrete slabs laid on a 1 : 1 slope ; the layout and a cross-section of the weir are shown in Fig. 3. It was completed on 24 September, 1928, a t a cost of ;E17,628,having taken 49 months to construct ; the quantities of work done were as follows :Excavation and filling . . . 29,663 cubic metres (38,797 cubic yards) . 2,765 ,, ,, (3,616 ,, ,, ) Mass concrete main in wall Brickwork . . . . . . 145 ,, ,, ( 189 ,, ,, ) R.C. slab in D.S.' apron and flanks 18,018 square metres (21,549 square yards)

. .

Good clean shingle and sand are obtainable in any quantities at the site from the bed of the river. It was stated at the time of construction that the weir was to be considered as only a temporary structure, to replaced by a more permanent be one at a later date. After it,s construction a new gauge was erected just

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SECTION A-A.

I

Scale 1 :600

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ATKINRON AND CARDIAC08 ON

upstreamfrom the weir on the right flank, which has been regularly recorded sincc 1929. Although there was no cutoff on the upstream side, and 110 protective works at the downstream end of the apron, the weir functioned satisfactorily until 18 December, 1935, when it collapsed and was very largely destroyed. The river conditions preceding the collapse were that during November there were no violent fluctuations, the mean of the weir gauge for the month being 65.84 metres, that is, no water passing over the weir except on one or two days. There were rises of 0.46 metre and 0.41 metre on 1 and 2 December, or a total of 0.87 metre ; thereafter. it dropped back to 66-20 metres on the 17th, rising to 66.75 metres on the 18th, the dayof the collapse, and to66.95 metres the following day, after which it dropped right back again. It had previously sustained more severe conditions than these without apparent damage, as may be seen from the following Table of maximum flood levels since the time of construction :Year.1929 1930 1931 1932 1933 1934 1935 Weir Gauge. metres feet 66.64 218.58 220.74 67.30 66.62 218.51 66.78 21944 220.22 67.14 66.99 219.73 66.95 219430'

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This seven-ycar period was one of low floods, whilst the following sevenyear period was one of high floods ; the mean maxima, a t Flying Bridge gauge, for the two periods were 67.50 metres (221.46 feet) and69.26 metres (227.23 feet) respectively. For four out of the seven years of the latter period the weir was breached and therefore levels at the we