COMPARING FLOOD FREQUENCY ANALYSIS USING ANNUAL PEAK RAINFALL DATA

CHAPTER ONE 

INTRODUCTION    

  1.1 BACKGROUND TO THE STUDY 

  Flood is a basic phenomenon of nature, it occurs during or immediately after an intensive rainfall or large and sudden snow-melt due to increase in temperature (in the glacial/temperate countries). Also, flood is a high water stage in which overflows its natural or artificial banks onto normally dry land such as a river inundated its flood pain. When flood spreads to area of steep slope, it accelerate the runoff, the potential energy immediately takes on an additional and high kinetic energy due to increased velocity. The effects of flood on human well being range from unqualified blessings to catastrophes. The regular seasonal spring flood of the Nile River prior to construction of the Aswan high dam, for example were depends upon to provide moisture for the fertile flood pains of its delta. The uncontrolled flood of the Yantze River and the Huang Ho, have however, repeatedly wrought disaster when these rivers habitually re-chart their courses. Uncontrollable floods likely to cause considerable damage commonly result from excessive rainfall over brief period of time, as, for example the Omiyale flood of the Ogunpa river in Ibadan, Nigeria (1958, 1973 and 1980), the flood of Paris, France (1658 and 1910), of wars haw, England (1861 and1964), potentially disastrous floods may, however, also result from ice jams during the spring rise, as with the Danube, Switzerland (1342, 1402, 1501 and 1830), from storm tides such as those of 1099 and 1953 that the coast of England, Belgium, and the Netherlands; and from tsunamis, the mountainous sea waves cause by earthquakes, as in Lisbon (1755) and Hawaii, U.S.A (Hilo, 1946). Flood can be measured by height, peak discharge, area inundated, and volume of  flow, these factors are important to judicious land use, construction of protective levees and storage reservoirs, and, indirectly, the implementation of programs of soil and forest conservation to retard and absorb runoff from storm and more recently monitoring of river flow by computers using specifically designed software. The discharge volume of an individual storm is often highly variable from month to month and year to year. A particularly striking example of this variability is the flash flood, sudden unexpected torrent of muddy and sporadic rainfall: it is uncommon, of relatively brief duration and generally the result of summer thunderstorms in mountains. A flash flood can take place in a singles tributary while the rest of the drainage basin remains dry. The suddenness of its occurrences causes a flash flood to be extremely dangerous.   

1.2    STUDY AREA

Ibadan is the capital city of Oyo State and the third largest metropolitan area in Nigeria, after Lagos and Kano, with a population of 1,338,659 according to the 2006 census. Ibadan is also the largest metropolitan geographical area. She is located in south-western Nigeria, 128 km inland northeast of Lagos and 530 km southwest of Abuja, the federal capital, and is a prominent transit point between the coastal region and the areas to the north.