the farmer's crop has been destroyed in the flood - make expression into flood
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Floodplains are land areas adjacent to rivers and streams that are subject to recurring inundation. Owing to their continually changing nature, floodplains and other flood-prone areas need to be examined in the light of how they might affect or be affected by development. This chapter presents an overview of the important concepts related to flood hazard assessments and explores the use of remote sensing data from satellites to supplement traditional assessment techniques.
The primary objective of remote sensing methods for mapping flood-prone areas in developing countries is to provide planners and disaster management institutions with a practical and cost-effective way to identify floodplains and other susceptible areas and to assess the extent of disaster impact. The method presented in this chapter can be used in sectoral planning activities and integrated planning studies, and for damage assessment.
The satellite remote sensing method presented in this chapter is one of many flood hazard assessment techniques that are available. This method has the following characteristics:
- It uses remote sensing data covering single or multiple dates or events.
- It permits digital (by computer) or photo-optical (film positive or negative) analysis.
- It is best used as a complement to other available hydrologic and climatic data.
- It is useful in preliminary assessments during the early stages of a development planning study because of the small-to-intermediate scale of the information produced and the ability to meet cost and time constraints. The data may also be applicable to other aspects of the study.
A. FLOODPLAINS AND THEIR RELATIONSHIP TO INTEGRATED REGIONAL DEVELOPMENT
1. Floods, Floodplains, and Flood-Prone Areas
2. Flood Hazard Assessment
3. Land Surface Characteristics Related to Floods
This section is designed to provide the planner with background information on the nature of floods and the terms and concepts associated with assessing the risks from this natural hazard.
1. Floods, Floodplains, and Flood-Prone Areas
Flooding is a natural and recurring event for a river or stream. Statistically, streams will equal or exceed the mean annual flood once every 2.33 years (Leopold et al., 1964). Flooding is a result of heavy or continuous rainfall exceeding the absorptive capacity of soil and the flow capacity of rivers, streams, and coastal areas. This causes a watercourse to overflow its banks onto adjacent lands. Floodplains are, in general, those lands most subject to recurring floods, situated adjacent to rivers and streams. Floodplains are therefore "flood-prone" and are hazardous to development activities if the vulnerability of those activities exceeds an acceptable level.
Floodplains can be looked at from several different perspectives: 'To define a floodplain depends somewhat on the goals in mind. As a topographic category it is quite flat and lies adjacent to a stream; geomorphologically, it is a landform composed primarily of unconsolidated depositional material derived from sediments being transported by the related stream; hydrologically, it is best defined as a landform subject to periodic flooding by a parent stream. A combination of these [characteristics] perhaps comprises the essential criteria for defining the floodplain" (Schmudde, 1968). Most simply, a flood-plain is defined as "a strip of relatively smooth land bordering a stream and overflowed [sic] at a time of high water" (Leopold et al, 1964).
Floods are usually described in terms of their statistical frequency. A "100-year flood" or "100-year floodplain" describes an event or an area subject to a 1% probability of a certain size flood occurring in any given year. For example, Figure 8-1 shows this frequency in terms of flood levels and floodplains. This concept does not mean such a flood will occur only once in one hundred years. Whether or not it occurs in a given year has no bearing on the fact that there is still a 1% chance of a similar occurrence in the following year. Since floodplains can be mapped, the boundary of the 100-year flood is commonly used in floodplain mitigation programs to identify areas where the risk of flooding is significant. Any other statistical frequency of a flood event may be chosen depending on the degree of risk that is selected for evaluation, e.g., 5-year, 20-year, 50-year, 500-year floodplain.
Frequency of inundation depends on the climate, the material that makes up the banks of the stream, and the channel slope. Where substantial rainfall occurs in a particular season each year, or where the annual flood is derived principally from snowmelt, the floodplain may be inundated nearly every year, even along large streams with very small channel slopes. In regions without extended periods of below-freezing temperatures, floods usually occur in the season of highest precipitation. Where most floods are the result of snowmelt, often accompanied by rainfall, the flood season is spring or early summer.