From the  History of Dam Construction

Less than thirty miles from Cairo, in the Wadi-el-Garrawi one can still see the abutments of а huge dam. Its failure was so catastrophic that nothing of the sort was attempted again ti11 over 3,000 years passed. It is supposed to have been built from 2950 to 2750 В.С. by some unknown Pharaoh with an architect farahead of his time.

For an initial experiment in dam construction the size of the structure is surprising. It is 348 feet long at the top and about 265 feet at the base. It originally had а height of 40 feet. The construction of so huge а dam must have occupied the energies of many hundreds of men and animals for а who1e season in this desert place. Quite possible the construction took placein summer to avoid interruption by floods. The dam was faced on the upstream side with а carefully laid covering of limestone b1ocks of about 50 1b weight.

Except for the carefully laid upstream face, the dam appears to have been built in haste, particularly the downstream portion or, rather, what remains of it. One obtains the impression that the architect was pressed for time. He didn't think that the impounded water might one day overflow the dam and ruin the structure.

One observes, too, no spillways were provided to carry off surplus water. This absence of spillways leads to the conclusion it was never intended to fillthe reservoir completely. The problem before the ancient engineer was to build а dam large enough to withstand and contain any flood that was likely to come down the Wadi-el-Garrawi. The ancient Egyptians had an empirical formula for computing the volume of а cylinder and may have had some rough idea of how much water their reservoir would hold, iffilled.

Evidently the dam had been erected to provide drinking water for the workers and beasts at alabaster quarries.

How much rain was it reasonable to expect? We, modern people, have an advantage over the designer of the Wadi-el-Garrawi dam. We know the area of the catchment upstream the dam and we have rainfalJ records for оvег forty years. То estimate the proportion of runoff to rainfall Ь always difficult since it depends on the intensity as well as the actual amount of the fal1. The slope of the catchment is very important, as wе11 as absorption on а parched desert surface, the latter being а diminishing factor.

А rainfall of 10 millimetres or over in а single day occurs on the average in three years out of four in that place. During forty years there were 20 such falls, 10 of which exceeded 20 mill1metres.

The ancient people were obviously ignorant of the fact that а single mi11imetre of runoff from the catchment area of the Wadi-el-Garrawi is represented by а volume of water of 185,000 tons.

And so we came to the catastrophe. Not long after the dam was finished, perhaps with the very first flood to come down the Wadi – there is по silting upstream the dam – а breach took place. А fall of 20 millimetres, of which there have been ten in forty years there, would be quite sufficient. Of that 20 millimetres, 8 would be immediately absorbed and а quarter of the remainder, between 500,000 and 600,000 tons would go rushing down the Wadi, fill the reservoir and overflow the dam. In а thundering cataract it was pouring down the downstream face and in а few hours thе dam was destroyed. The contents of the dam would then rush down the Wadi to the cultivated lands and the Nile. So, in а few hours and probably at night were destroyed all the results of the labour of many hundreds of men and animals and the reputation of the engineer.

For us, situated at the distance of 5000 years, it is hard not to feel sympathy with the unknown engineer who so boldly attempted the impossible – for that age. Had he made use of mortar, had he provided а spillway, had he chosen а wadi with а gentler slope, how different might have been the history of Egyptian irrigation.

Упражнения

I.Определите значения следующих слов по сходству с однокоренными словами в русском языке:

cylinder, empirical, formula, intensity, absorption, portion, reputation, factor, fact, catastrophe, sympathy, architect, energy, problem, idea, Egyptian

II. Найдите в тексте антонимы следующих слов.

to increase, to construct, to begin, the former, upstream, presence, ancient

III. Найдите в тексте синонимы следующих слов:

to destroy, to reduce, to get, to try, to finish, to calculate, evident, the rest, excess, initial

IV. Подберите правильные словарные определения к следующим понятиям:

flood, rainfall, dam, catchment, runoff

1.The amount of rain, usually expressed in millimetersor inches; depth of water on an area that reaches the surface of the earth. 2. А barrier across а watercourse for the purpose of impounding water barriers built to raise water-leve1, to divert water, to create а hydraulic head which can be used to generate power. 3. А relatively high flow in а river, markedly higher than usual; а body of water, rising and overflowing land. 4. The area from which а lake stream or reservoir receives surface fIow which originates as precipitation. 5. Portion of total precipitation from а given area that appears in natural or artificial streams.

V. Расскажите о возможных причинах разрушения плотины.

Vl. Сделайте краткий пересказ текста “From fhe Hisfory of Dam Construction”.

Methods of Irrigation

The methods by which irrigation is applied to the land should depend, under ideal conditions, on individual land features such as the slope of the land, the crops to be irrigated, the nature of the water-supply and the ability of the soil to absorb and hold water.

There are four general methods of applying water: 1) by flooding, thus wetting all the 1and surface; 2) by furrows, thus wetting only part of the ground surface; 3) by sprinkling, in which the soi1 is wetted with а spray; 4) by subirrigation, in which the soil is wetted on1y а little if at all, but in which the subsoil is saturated. The first three methods come under the general heading of surface irrigation.

Flood irrigation generally requires large streams or canals, gentle topography(ground slopesshould usually be nogreater than three per cent) and careful levelling of the land. In theory, it should be possible to ensure that every part of the area to be irrigated absorbs the predetermined amount of water; but in practice, although all parts usually receive an adequate amount, some receive too much. For this reason, flood irrigation is more suited to close-growing crops like rice.

Furrow irrigation is а method by which water is run in furrows, normally made by cultivating between crop rows. The earth is thrown up into ridges between the furrows and the seeds are planted in the centre of the ridges. Furrow irrigation is very common because it is adaptab1e to а great variety of land slopes and soi1 textures and can be used with either large or small streams of irrigation water.

Difficulties may arise with the use of furrow irrigation on unsuitable soils. If the soil is very pervious, the water running along the furrows may sink vertically into the soil without ever reaching the centre of the ridges where the seeds are sown. On the other hand, the soil may be so impervious that the water does not reach the centre of the ridge and the seeds do not germinate anyhow. А more general disadvantage of furrow irrigation is that to ensure that the whole of the irrigation area receives enough water, it is almost always necessary to overwater some parts.

Sprinkler or spray irrigation applies water to the surface of the soi1 in the form of а spray, and is а form of artificial rain. А common type of sprinkler head is the revolving one which distributes water radially. This form of irrigation has advantages over other surface irrigation. It can be adapted for use on almost a11 types of soil, especially sandy soils which absorb water rapidly. It enables erosion to be controlled on steep land and can often be used on soils that are too shallow too steep or too rolling to be irrigated by surface methods. Sma11 streams of water can be used efficiently with this method, and it is adaptable to all the major crops with the exception of those, 1ike rice, that require standing water. А uniform application of water is made possib1e, and the amount and timing of the irrigation can be easily controlled. Land is not needed for irrigation structures, and so larger areas аге available for cropping.

There are, however, а number of disadvantages of sprinlkler systems. The water distribution is easily affected by the wind, which may disturb the pattern of wetting so that some parts gettoo much water and some too little.

I. Найдите в тексте слово, которому соответствует следующее словарное определение:

The moving of soi1 in ргерагing land for irrigation to produce а leve1 or uniformly sloping surface