FLOOD HAZARD MAPPING AND RISK MANAGEMENT

 FLOOD HAZARD MAPPING AND RISK MANAGEMENT

CHAPTER ONE:

INTRODUCTION

1.1 BACKGROUND TO THE STUDY

Flooding is a general temporal condition of partial or complete inundation of normally dry areas from overflow of inland or tidal waters or from unusual and rapid accumulation of runoff (Jeb and Aggarwal, 2008). Floods are the most common natural disasters that affect societies around the world. Dilley et al., (2005) estimated that more than one-third of the world‟s land area is flood prone affecting some 82 percent of the world‟s population. The reason lies in the widespread geographical distribution of river flood plains and low-lying coasts, together with their long standing attraction for human settlement. Floods are natural phenomena, but they become a cause for serious concern when they exceed the coping capacities of affected communities, destroying lives and damaging property. They affect settlement of all types, from small villages and mid-sized market towns and service canters to major cities and metropolitan. In many regions of the world, people moving from rural areas to cities, or within cities, often settle in areas that are highly exposed to flooding thereby making them highly vulnerable if there is no flood defence mechanism (Jha et al., 2012).

Urban flooding can be coastal, fluvial or pluvial or even a combination of these types of floods. Coastal flooding is caused by extreme tidal conditions that occur because of high tide levels, storm surge and wave action. Fluvial (River related) flood occurs when the discharge of a river exceeds the capacity of the river channel to contain it. While pluvial flood takes place when the rainfall rate exceeds the capacity of storm water drains to evacuate the water and the capacity of the ground to absorb water (Ball et al., 2008). Pluvial flooding often occurs unexpectedly in locations not obviously prone to flooding and with minimal warning and is not well understood by the general public – hence the term „invisible hazard‟ (Houston et al., 2011). Pluvial flooding is a characteristic of urban areas where large areas of impervious ground exist and inadequate drainage systems abound. As urban growth increases, the impervious surface area also increases; thereby rendering populations vulnerable to water inundation as natural streams and human-made drainage fails to cope with increased runoff subsequent to heavy rainfall (Youssef and Pradhan, 2011). Urban pluvial flooding frequency is expected to increase not only due to urbanization but also to expected climate changes (Ugarelli et al., 2011; Simes et al., 2014). This type of flooding can happen virtually anywhere and has the potential to cause significant damage and disruption in highly urbanized areas, where the density of properties, critical infrastructure and population is usually high. The volumes involved and the risk related to pluvial flooding often result in consistent economic losses and consequent damage in the long term due to the high frequency of this kind of event (Freni et al.,2010). Related consequences of pluvial flooding mainly consist of economic losses such as damage to buildings and their contents and to infrastructure and intangible damages due to traffic delays, road, public and commercial function closures, and evacuation of people. Globally, the economic cost of extreme weather events and flood catastrophes is severe, and if it rises owing to climate change, it will hit poorest nations the hardest consequently; the poorest section of people will bear the brunt of it. It is therefore, urgent that the vulnerability of developing countries to climate change is reduced and their capacity to adapt increased at national, regional and community levels (UNFCC, 2007).

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FLOOD HAZARD MAPPING AND RISK MANAGEMENT