Page:The New International Encyclopædia 1st ed. v. 10.djvu/814

* INUNDATION. 724 INUNDATION. lower Nile, tlie acciiiiiiilatiiin liaviii;; taken weeks to tlow down the river elianiiel from the upper waters. It cinitiniies till S<>ptcinher, and at Cairo the water readies a lieifjht of 2.> feet alinve the normal river level. A vast tract of tlo<id plain and delta is inundated, and this water makes agriculture ])ossil)h' in the des- ert, and is the basis upon which the ancient Eg)'ptian ajiriculturc was fiiunilcd. The sedi- ment deposited by the Hoods fertilizes the land, and is raisin;; the level of the Hood plain at a rate of over four inches a century. The Mississippi also supplies an illustration of jfreat river Hoods. An area of liO.OOO square miles, inhabited by a million ])eople, is liable to Hoods. Low natural embankments, built by the Hoods, and called natural levees, keep the rivi'r in its channel, exceptin;; during; very hi;;h water, and levees added by man ar<' built from the tJulf to Cairo in the ell'ort to hold the extraordinary floods. But even these artificial levees arc sonic- times inetTcctive. (.'icvasses 50 to 500 f»rl wide break throii^'h the levees and a vast area is inundated. These great floods come usually be- tween Kebruary and May, and are caused by the coincidence of heavy rains, oV rapidly melting snows, or both, in the Missouri, Mississippi, and Ohio valleys. During tlii' great Hoods tlicdiscliargc of the Mississippi reaches 2.000,000 cubic feet per second, and is far in excess of the capacity of the channel. The water-level then rises -40 or 50 feet, and great destruction of life and prop- erty results. (Jreat inundations bv the Missis- sippi are recorded in 1S2S. 1S44. 184!l, I.H.50, 1853, 1S59, 18,3, 1807, 1870. 1874. 1882, 1884. 1800, 1892, 1803, and 18!I7. The lloangho of China flows out of its mountainous course over a low alluvial fan delta which is rapidly growing out into the sea. The town of Putai, which was on the seacoast about 2000 years ago. is now over 40 miles inland. Xot only is the alluvial fan delta growing outward, but upward, and when the river bed is built up above the level of the -surrounding plain the cmirsc of the stream be- comes exceedingly unstable and liable to shifting in time of flood. The river then sometimes bursts tlirougli levees fully 70 feet high, erected more than half a mile from the banks. It has shifted its course nine times in 2500 years, laying waste a region as large as tireat Britain. Its mouth has changed position fully 200 miles by these shiftings. In the Hood of 1887, which covered an area of 50.000 square miles, densely jiopulated, a niillicm people were drowned, and still more lives were lost by the famine and disease that followed the disaster. The river has been used as n weapon of war as far back as the year 1200. Hundreds of thousands of people have thus been drowned by the turning of the river floods against them. SmallcV river Hoods are of common occurrence, and arc frequently caused by ice jams, which form a temporary dam and hold the water back. The bursting of the dams of artificial ponds and lakes also causes de.structive floods. Inundations of the sea are far less widespread than those of rivers, because some coasts arc tor high, and others are not frequently visited by high sea or 'tidal waves.' The great nuniljer of river lowlands, and the frequency of the causes of a rise in the river water, makes these river floods more general than those of the sea. But where low eoast^ arc exposed to inundations of the .sea, the results are terribly destructive, cs|)ecially since the lowland coasts are usually densely populated. There are various causes of inundations by the sea; the simplest are illus- ■ Irated by those countries which are actually below the level of the highest tides, and arc pro- lectcil from iniindalinn by saml iliiiic hilK and dikes. The Netherlands ofl'er the best illustra- tion, i'liis plain is tiie delta of the Khinc and -Meusc. and is open to Hoods from river and ocean, and parts of it lie well Ixdow the level of the sea. At various limes the dikes, which were formerly less solidly constructed than at (iresent, have given way, and large' tracts have been submerged, causing a vast lied. During the Sjianish wars the people voluntarily admittcil the sea to protect their cities from the attacks of the Sjianiards. Kveu dry lanil sevi'ral fivt above sea-level is subject to inundaticms by the .sea when there is some unusual ri.se in the water, comnionly called a "tidal wave.' doubtless because it reaches its greatest height at high tide. The lowlands of the Netherlands, Denmark, anil England have suirercd from such floods when high win<ls have driven water u))iin the shore and piled it up at ])eriods of unusually high tide. At such times the surface of the sea may rise 10 or 15 feet higher than the normal reach of the high tide. The West Indies, the (lulf Coast, and the coast of the .Middle Atlantic States arc subject to such inundations during the passage of the fierce y'op- ical hurricanes. The Hooding is experienced even as far north as Xew York and Boston; and the destruction at (lalveston. Sept4'nibcr 8, 1000, when thousands of lives were lost, was due to the pas- sage of one of these hurricanes. The Se,a Islands on the (Jcorgia coast are occasionally Hooded dur- ing these storms, and any low point ahnig the coa.st south of New York is liable to such 'tidal waves' at any time, but especially in August and the autumn months, when tropical hurricanes develop most commonly. High water followed the hurricane of September 3-12. 188!t. all the way from the West Indies to New .lerscy. The rise ill the level of the ocean during the passage of these storms is due to the combination of ordinary tides with the efTect of the steady violent winds which drift the water before them and |)ilc it up on the shelving shore. As illustrated in Oalveston, this wave rises over ordinarily habitable land, nnilcrmining houses, and thus aiding in the destruction done by the winds themselves and the wind waves which these winds raise on the surface of the sea. Far greater destruction of life is accomplished in the Pacific than in the Atlantic by these tropical storms, there called typhoons. The sea wave washes over low coral atolls. In the case of the Samoan hurricane of March 15. 1880, a typhoon washed ashore 21 vessels that were anchored in the harbor of .Vpia. Some 150 of the sailors on these vessels were drowneil, and only two of the ships that were in the harbor were afloat after the hurricane. Typhoon waves advance on the low delta coasts of India anil .'Southern China. There has lieen case after ease of destruc- tion of life to the number of many thousands in the delta lowlands of Asia ; and in at least one case, in 187fi, it is estimated that 150,000