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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.

 

Date: 2015-10-19; view: 503; Нарушение авторских прав; Помощь в написании работы --> СЮДА...



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