FLOODING AND GLOBAL WARMING

Flooding and Global Warming



Flooding and Global Warming

Introduction

Floods

Floods occur when streams and rivers overflow their banks and inundate the adjacent lands, called floodplains. In other instances, localized flooding occurs when drains cannot cope with heavy rainfall and along shorelines from sea surges that can be wind driven or associated with storms. Since all rivers flood, and human activities normally take place on the relatively flat flood-plains, the consequences of flooding can be disastrous. Thus, the simple definition of flooding belies a complex of causal mechanisms that involve physical processes of the natural environment and social, economic, and political forces operating in the human environment. It is these factors in combination that determine the magnitude, severity, and ultimately the impacts of flooding. Thus, globally, floods are the most widely experienced natural disaster, causing property damage, economic disruption, and loss of life. At the same time, there are positive outcomes from flooding. Floods provide soil moisture for regions in which flood recession agriculture, practiced and can deposit nutrient-rich sediments to fertilize farmlands. In the next section, we will critically examine the causes of floods, flood magnitude and severity and the process of flood mitigation.

Discussion & Analysis

Causes of Floods

Floods can be understood within the concept of the drainage basin (watershed), often referred to as the fundamental unit of hydrological research. This unit can be viewed as a system, with one part inextricably linked to the next. The basic input to this system is precipitation, and it is how precipitation moves through various storage components that determine downstream discharge, the output to the system. The speed of water moving through the system and the associated “losses” to evaporation, transpiration, and storages in groundwater and lakes will affect the timing and quantity of water in the river channel. If the outcome is flows exceeding bank full capacity, then there is a flood in that drainage basin (Figure 1).

Figure 1 Stream cross-section and floodplain

Source: New Hampshire Office of Energy and Planning, (2007) “Floodplain learning on demand” Retrieved December 3, 2009, from www.nhflooded.org.

Determining exactly how individual drainage basins respond to a given input of precipitation is a challenge to hydrologists. The characteristics of drainage basins, including their shape (especially at minor scales), geology, soil types, vegetation, and geomorphology, all play a role in determining discharge. Furthermore, when precipitation occurs, it is the antecedent environmental conditions within the drainage basin that control flood severity. For example, snow cover, frozen ground, wet or saturated soils (or even dry soils on occasion), pre-existing high river levels, ice-covered streams, and heavy precipitation can all exacerbate hydrological conditions and lead to floods. The response of a drainage basin to a given input of precipitation, therefore, may be relatively fast, or “flashy,” where flows can be high, peaking rapidly, often with a high velocity (Figure 2). In contrast, some systems may take days or weeks to reach peak discharge and inundate the floodplains, conditions that are termed sluggish. The former systems have a short precipitation-to-flood lag time, the latter, ...