Comparing Flood Frequency Analysis Using Annual Peak Rainfall Data Of Ibadan, Oyo State

Project and Seminar Materials for Civil Engineering CE

Comparing Flood Frequency Analysis Using Annual Peak Rainfall Data Of Ibadan, Oyo State


Abstract


This study is aimed at comparing flood frequency analysis using annual peak rainfall data of Ibadan, Oyo State, Nigeria. The study is limited to the study area and will be analyzed using different methods in order to determine flood frequency and return period. The results of the study areas will make certain predictions on effectiveness or accuracy of the method used. The three methods (Califonia, Allen Hazen, and Weibul ) used to estimate flood frequency were compared scientifically by statistical tools. Both statistical comparison and graphs representation confirmed the uniforms of the result obtained from the three methods. The result of Califonia is highly recommended for further flood analysis.


Table of Content


Chapter One:

Introduction

  • 1.1 Background to the Study
  • 1.2 Study Area
  • 1.3 Statement of the Problem
  • 1.4 Aims and Objectives
  • 1.5 Scope of Work

Chapter Two:

Literature Review

  • 2.1 General Introduction
  • 2.2 Classes of Flood
  • 2.3 Causes of Flood
  • 2.4 Effects of Flood
  • 2.4.1 Loss of Human Lives
  • 2.4.2 Loss of Properties and Farmlands
  • 2.4.3 Destruction of Public Utilities
  • 2.4.4 Epidemic
  • 2.5 Factors Affecting Urban Flood
  • 2.5.1 Geographical Factors
  • 2.5.2 Geomorphological Factors
  • 2.6 Flood Control and Alleviative Measurement
  • 2.6.1 Structural / Mechanical Measures
  • 2.6.2 Non-structural Measure
  • 2.6.3 Public Education
  • 2.7 Analysis of Storm and Runoff Estimate
  • 2.7.1 Probability or Statistical Method
  • 2.7.2 Unit Hydrograph Method
  • 2.7.3 Empirical Formula

Chapter Three:

Methodology

  • 3.1 General
  • 3.2 Collection of Data
  • 3.3 Analysis of Data
  • 3.4 Statistical Comparison of the Data
  • 3.4.1 Descriptive Statistics
  • 3.4.2 Analytical Statistics

Chapter Four:

Result and Analysis

  • 4.1 Monthly Rainfall Variation Analysis
  • 4.2 Estimation of Flood
  • 4.2.1 Selection of Peak Values of Rainfall Depth
  • 4.2.2 Estimation of Recurrence Interval
  • 4.2.3 Estimation of Rainfall Intensity
  • 4.2.4 Estimation of Probability
  • 4.3 Comparison of Recurrence Interval
  • 4.3.1 Descriptive Statistics
  • 4.3.2 Analytical Statistics

Chapter Five:

Conclusions and Recommendations

  • 5.1 Conclusions
  • 5.2 Recommendations
  • References

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.

Ibadan is located in southwestern Nigeria in the southeastern part of Oyo State about 120 km east of the border with the Republic of Benin in the forest zone close to the boundary between the forest and the savanna. The city ranges in elevation from 150 m in the valley area, to 275 m above sea level on the major north-south ridge which crosses the central part of the city. The city’s total area is 3,080 km2.She is located in coordinate 70 23’47’’N and 3055’0’’E in Nigeria.

The city is naturally drained by four rivers with many tributaries: Ona River in the North and West; Ogbere River towards the East; Ogunpa River flowing through the city and Kudeti River in the Central part of the metropolis.

Map Of Ibadan

  • Figure 1.1: Map of Ibadan
  • Source: google map

Ibadan has a tropical wet and dry climate, with a lengthy wet season and relatively constant temperatures throughout the course of the year. Ibadan’s wet season runs from March through October, though August sees somewhat of a lull in precipitation. This lull nearly divides the wet season into two different wet seasons. November to February forms the city’s dry season, during which Ibadan experiences the typical West African harmattan. The mean total rainfall for Ibadan is 1420.06 mm, falling in approximately 109 days. There are two peaks for rainfall, June and September. The mean maximum temperature is 26.46 C, minimum 21.42 C and the relative humidity is 74.55%.

Table 1.1: Climatic data for Ibadan

MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Record high °C (°F)37
(99)
39
(102)
38
(100)
38
(100)
35
(95)
33
(91)
31
(88)
31
(88)
36
(97)
33
(91)
34
(93)
35
(95)
39
(102)
Average high °C (°F)33
(91)
34
(93)
34
(93)
33
(91)
32
(90)
29
(84)
28
(82)
27
(81)
29
(84)
30
(86)
32
(90)
33
(91)
31
(88)
Average low °C (°F)21
(70)
        22
(72)
23
(73)
23
(73)
22
(72)
22
(72)
21
(70)
21
(70)
22
(72)
22
(72)
22
(72)
21
(70)
22
(72)
Record low °C (°F)10
(50)
12
(54)
18
(64)
18
(64)
18
(64)
18
(64)
16
(61)
16
(61)
17
(63)
18
(64)
14
(57)
14
(57)
10
(50)
Rainfall mm (inches)8
(0.31)
23
(0.91)
76
(2.99)
125
(4.92)
145
(5.71)
163
(6.42)
132
(5.2)
74
(2.91)
170
(6.69)
152
(5.98)
43
(1.69)
10
(0.39)
1,121
(44.13)
humidity76717578828688888684807681
Avg. precipitation days12591112121015124194
Mean monthly sunshine hours1701981701701701418557851411981981,783
Source: BBC Weather

1.3 Statement of the Problem

The economic losses, in addition to loss of lives, resulting from flood and erosion are enormous and the amount spent on the control of the phenomena by the communities, states and federal governments in alarming. For instance, between 1987 and 1989 the federal government of Nigeria disbursed N 368.9 million to 12 states to fund 12 erosion and flood control projects. Similarly, in recent times the ecological fund running to several millions of US dollars have been released to execute flood control projects in various urban centers in Nigeria. Among such project is the Ogunpa channelization at Ibadan.

One may marveled at such a hung amount of money spent in order to control flood. Therefore, the research attempts to look into how frequency this problem occurs in Southwest Nigeria by comparing Ibadan with Abeokuta data and infer appropriate solutions peculiar to the problems In order to guard against future recurrence of flood.


1.4 Aims and Objectives

The general aim of the study is to collect, analyze and compare frequency occurrence of flood in the study area using annual peak data.

Specifically, stated bellows are the major objectives of the study:

  1. Collect and analyze data – annual rainfall data of the study area will be analyzed using different method in order to determine flood frequency and return period.
  2. Compare the results of the study areas to make certain predictions on effectiveness or accuracy of the method used.
  3. Suggest best method of analyzing flood frequency.

1.5 Scope of Work

The study is limited to flood frequency analysis using annual peak rainfall data of Ibadan and comparison of the results obtained by using various methods selected.


Chapter Five


5.0 Conclusions and Recommendations

5.1 Conclusions

Subsequent to the analysis and comparison of the results obtained from the flood estimate of the study area, the following conclusions are arrived at:

  1. The rainfall is heaver in September and June with average rainfall depth of 187.71mm and 180.72 mm respectively. Tendency of flood occurrence is higher in those months.
  2. Previous occurrence of flood in the city also point at May and July, August and October (months before and after June and September) as likely months of flood occurrence. In other words, May to October is the likely months of flood occurrence in Ibadan city.
  3. The results of flood estimate by Califonia, Allen Hazen and weibul’s methods did not have any significant difference. Both statistical comparison and graphs representation confirmed the uniforms of the result obtained from the three methods.
  4. It was observed that the result of Califonia’s method has the least variance and standard deviation. It is then considered as the most appropriate result.

5.2 Recommendations

From the foregoing conclusions on comparison of flood frequency analysis, the following courses of action are recommended:

  1. Adequate caution or preventive measure should be adopted by both Government and individual against likely occurrence of flood in May to October each year.
  2. Any of Califonia, Allen Hazen, Weibul and other probability/statistical methods of flood estimate are recommended okey.
  3. The result of Califonia’s method of flood frequency analysis is highly recommended for further flood analysis. This simply means that Califonia’s method is the best from other methods.

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