Floodplains are land areas adjacent to rivers and streams that are subject to recurring inundation. 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 satellite remote sensing method presented in this chapter is one of many flood hazard assessment techniques that are available. 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.
Floodplains can be looked at from several different perspectives: 'To define a floodplain depends somewhat on the goals in mind. Traditional as well as more recent approaches to gathering and analyzing the necessary information are discussed in Section C., Flood Hazard Mapping Techniques and Application of Satellite Data. Floodplain width is a function of the size of the stream, the rates of downcutting, the channel slope, and the hardness of the channel wall. In moderately small streams the floodplain is commonly found only on the inside of a bend (meander), but the location of the floodplain alternates from side to side as the stream meanders from one side of the valley to the other. Channel mobility can be an important characteristic when trying to delineate the potential floodplain. A major flood in a humid region is less likely to cause channel widening and floodplain destruction, because vegetation inhibits erosion.
Generally, only annual floods are used in a probability analysis, and the recurrence interval-the reciprocal of probability-is substituted for probability. The floodplains of some streams, however, are inundated infrequently, at intervals of 10 years or more. The length of time that a floodplain is inundated depends on the size of the stream, the channel slope, and the climatic characteristics. People have been lured to floodplains since ancient times, first by the rich alluvial soil, later by the need for access to water supplies, water transportation, and power development, and later still as a relegated locus for urbanization, particularly for low income families. In a humid climate during a major flood, a considerable part of the flow of a stream with a wide floodplain is carried by that floodplain. Drainage and irrigation ditches, as well as water diversions, can alter the discharge into floodplains and the channel's capacity to carry the discharge. Urbanization of a floodplain or adjacent areas and its attendant construction increases runoff and the rate of runoff because it reduces the amount of surface land area available to absorb rainfall and channels its flow into sewers and drainage ways much more quickly. In summary, floodplain dynamics are basic considerations to be incorporated in an integrated development planning study. Delineating floodplains and other areas subject to flooding is valuable input for proposing compatible development activities.
Development planners need to know how often, on the average, the flood plain will be covered by water, for how long, and at what time of year. Flood inundation and floodplain maps have been prepared from satellite data for more than a decade by hydrologists all over the world. In order to integrate flood plain information into a planning study, the definition of floodplains and flood-prone areas and the probability of a given event occurring during the lifetime of a development project should be determined. A variety of mitigation measures can be identified and selected which will reduce or minimize the impact of flooding. All available flood-related information should be gathered during the preliminary mission of the planning study. Landsat TM and SPOT HRV imagery can be effectively used to map floodplains accurately at scales as large as 1:50,000 and to convey the idea that the river meanders across the floodplain. Information from floodplain maps can be used in the preparation of land-use and land-capability maps at this stage (see Chapter 3).
Modify Hazard: dams, catchment areas, retention ponds, high flow diversions, cropping patterns, reforestation.
Modify Land Use; use zones, subdivision regulations, sanitary and water-well regulations, development restrictions, easements and setbacks, floodplain management taxation.
Data products such as photographs, film positives, and slides derived from satellite imagery are also used in the implementation stage of floodplain-related projects. It should also be emphasized that once the implementation stage is reached, information generated from field studies and engineering design activities should include a flood frequency analysis, if it is not already available by this time.
Traditionally, gathering and analyzing hydrologic data related to floodplains and flood-prone areas has been a time-consuming effort requiring extensive field observations and calculations. With the development of remote sensing and computer analysis techniques, now traditional sources can be supplemented with these new methods of acquiring quantitative and qualitative flood hazard information. Conventional dynamic flood frequency analysis techniques have been developed to quantitatively assess flood hazards over the past half century. This dynamic approach requires extensive long term field surveys, with a network of gauging stations that can develop the data needed for precise risk assessments. Flood-inundation and flood hazard maps have been prepared by many hydrologists all over the world from aircraft and satellite data, mostly from the visible and infrared bands (Deutsch, 1974). Satellite data can be used to find indicators of floodplains, and may be easier to use than aircraft images in delineating floodplains (Sellers et al., 1978). Floods, hydraulic forces, engineering structures, and development on the floodplain can and do result in physical changes in the river channel, sedimentation patterns, and flood boundaries, as discussed earlier in this chapter. It should be noted that delineation of floodplains using remote sensing data cannot, by itself, be directly related to any return period.
A critical but generally underestimated requirement for effective use of satellite imagery in flood hazard assessments is the selection of data. NOTE: Because the repeat cycle of the Landsat and SPOT systems is greater than 15 days, it is not always possible to collect imagery during peak flooding stages. Integrated regional development planning studies do not traditionally include original flood hazard assessments but rather depend on existing, available information. Figure 8-10 presents a diagram of the steps involved in the preparation of Landsat data for use in a flood hazard assessment. In mapping floodplains, black-and-white positive film transparencies of Landsat imagery in 70mm format are especially useful for floodplain delineation. 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. The method presented in this chapter can be used in sectoral planning activities and integrated planning studies, and for damage assessment. 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. If such dynamic data have been collected for many years through stream gauging, models can be used to determine the statistical frequency of given flood events, thus determining their probability.
As a result, flood hazard assessments based on direct measurements may not be possible, because there is no basis to determine the specific flood levels and recurrence intervals for given events. Composed of unconsolidated sediments, they are rapidly eroded during floods and high flows of water, or they may be the site on which new layers of mud, sand, and silt are deposited.

Floodplains are uncommon in headwater channels because the stream is small, the slopes and rate of downcutting are high, and the valley walls are often exposed bedrock.
Widening of a river channel and destruction of part of the floodplain by major floods is common and has been observed in semiarid regions. While mobility is not much of a problem in areas with dense vegetation and consolidated soil types, in areas where the vegetation is sparse and soil types are coarse and erodible, mapping of the floodplain must include anticipation of the possibility of channel migration in addition to the existing channel configuration.
However, the flood may cut secondary channels through a floodplain and deposit sand and gravel over large areas, particularly those dedicated to agricultural production.
In fact, some terraces may have been floodplain boundaries prior to renewed downcutting or tectonic activity.
On small streams, floods induced by rainfall usually last from only a few hours to a few days, but on large rivers flood runoff may exceed channel capacity for a month or more.
On the wide floodplains of large rivers bordered by natural levees, the water may drain back slowly, causing local inundation or pounding which may last for months.
Clearing the floodplain for agriculture permits a progressively higher percentage of a large flood discharge to be carried by the floodplain. It is essential that the study recognize that changes brought on by development can and will affect the floodplain in a multitude of ways. With remote sensing methods, the extent of floodplains and flood-prone areas can be approximated at small to intermediate map scales (up to 1:50,000) over entire river basins. Failure to understand the nature of flood hazards and to comprehend that they are not necessarily random in time and space, but are in fact roughly predictable conforming to statistical probability, can bring about increased flood risk. Natural changes as well as changes brought on by development activities affect the floodplain and must be understood to identify appropriate development and natural resource management practices. The chosen acceptable frequency of a particular flood event should be appropriate for the type of development activity.
Many traditional techniques are dynamic: they monitor the continuous change in river or stream flow and require considerable field work and maintenance of long-term records. The relationship of the region's natural goods, services, and its hazards and current natural resource management practices should be put in the context of affected ecosystems (OAS, 1984). It is expected that the initial information collected would be general and based on existing hydrologic and precipitation data, satellite imagery, aerial photography, damage assessments, and scientific and engineering studies. Satellite imagery is especially useful to update existing floodplain and flood hazard maps, particularly for those areas which are highly dynamic in nature. Natural resource management planning should include a precise delineation of floodplains and related hydrologic hazards at map scales suitable for the formulation of projects.
Such information is a critical component of a risk analysis, and without it the usefulness of floodplain delineation information is greatly diminished. This traditional approach uses historical data of flood events to delineate the extent and recurrence interval of flooding.
This static approach uses indicators of flood susceptibility to assess an area's flood proneness (Sellers et al, 1978). These traditional techniques yield dynamic historical flood data which, when available, is used to accurately map floodplains. Figure 8-9 lists the suggested bands and spectral composites of the various satellite systems for analysis of floodplains and related hydrologic features.
However, when it is used in conjunction with other information, the delineated floodplain can be related to an estimated or calculated event. A number of sensors on board Earth observation satellites have provided data suitable for mapping floodplains and areas inundated by floods. However, data collected within a period of as much as a month following the flood commonly reveal the extent of the flooded area, due to reflectance differences between the inundated and non-inundated areas.
As emphasized earlier in this chapter, if such information is needed but is not available, an assessment should be undertaken as part of the study.
In the next section, two case studies demonstrate how Landsat data was actually used for flood hazard assessment. 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. 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.
A combination of these [characteristics] perhaps comprises the essential criteria for defining the floodplain" (Schmudde, 1968). In regions without extended periods of below-freezing temperatures, floods usually occur in the season of highest precipitation. Hazard assessments based on remote sensing data, damage reports, and field observations can substitute when quantitative data are scarce. As such, the river may change its course and shift from one side of the floodplain to the other. If a river carries fairly coarse sediment during a flood, it tends to be deposited along the channel bank as a natural levee. As is the case with these regions having a high erosion potential, the phenomenon of channel migration during flooding events will often cause a large portion of flood waters to be carried in a channel that did not exist prior to the onset of the flooding event. A terrace can usually be distinguished from an active floodplain by the type of vegetation and the surface material present. The 10-year flood, for example, is the discharge that will exceed a certain volume which has a 10% probability of occurring each year.
In some climates, several years of intense flood activity are followed by many years in which few floods occur. In 1982-83, the Parana River Basin in Brazil, Paraguay, and Argentina was subject to extensive flooding from late November 1982 through mid-1983. While some activities can be designed to mitigate the effects of flooding, many current practices and structures have unwittingly increased the flood risk.
Some parts of the floodplain are eroded and other parts are built up by deposition of coarse sediment, while the channel capacity of the river channel is gradually reduced.
Artificial fill in the floodplain reduces the flood channel capacity and can increase the flood height.
Early review of available flood hazard information and the programming of complementary flood hazard assessments are prudent and allow the planner to foresee and evaluate potential problems related to river hydraulics and floodplain dynamics.
Flood hazard maps can be prepared early in a development planning study to aid in defining and selecting mitigation measures for proposed sectoral development projects. A detailed discussion of the application of various remote sensing technologies to natural hazard assessments can be found in Chapter 4. Changes in floodplain utilization-such as urbanization and more intensive agricultural production-can increase runoff and subsequent flood levels.
For example, it may well be worth the risk of occasional flooding to plant crops in the floodplain where soils are enriched by cyclical flooding and the deposition of sediments.
While a dynamic long-term flood history is desirable, such static techniques are capable of yielding useful information for flood hazard assessment, especially in the diagnostic and preliminary stages of an integrated development planning study.

Also, the small-scale resolution but synoptic regional coverage provided by the NOAA satellite series carrying the Advanced Very High Resolution Radiometer (AVHRR) provides a highly informative aid to planners in determining the extent of flood events. It is important to bear in mind that floodplain and flood hazard maps are not intended to be substitutes for, but rather precursors to, engineering design studies.
Figure 8-7 outlines the relationship of flood information and a flood hazard assessment to general development activities. Satellite image maps provide clear, visible evidence to managers that floodplains are dynamic areas and should be studied in conjunction with other thematic maps to identify applicable mitigation measures. Proposed crop production and construction of irrigation infrastructure, culverts, bridges, roads, and other permanent structures must be studied to evaluate their flood risk. Floodplain management, flood prevention, and flood mitigation measures (both structural and non-structural) should be included if they are not already part of the project formulation activities.
The SPOT HRV and Landsat 4 and 5 TM sensors are currently the best available sources of high-resolution data and should be considered for use as the basic data in preparing large-scale maps for flood risk assessments.d.
In addition to a record of peak flows over a period of years (frequency analysis), a detailed survey (cross sections, slopes and contour maps) along with hydraulic roughness estimates is required before the extent of flooding for an expected recurrence interval can be determined. Digitized color-infrared aerial photographs to classify vegetation that correlates with floodplains have also been used (Marker and Rouse, 1977). This static method can reveal an area's flood proneness and yield information useful for a flood hazard assessment.b. SPOT and Landsat program film product costs are such that the cost of producing thematically enhanced photo-optical data products for specific applications such as floodplain delineation and flood mapping now approaches the cost of digital image processing. If time and budget constraints do not permit a detailed, large-scale assessment to be carried out, a floodplain map and a flood hazard assessment can be prepared using the photo-optical method, using Landsat data and the planning study information which is usually available (see Figure 8-6). Where most floods are the result of snowmelt, often accompanied by rainfall, the flood season is spring or early summer.2.
They present mapped information defining flood-prone areas which will probably be inundated by a flood of a specified interval (Riggs, 1985).
Figure 8-3 portrays this dynamic pattern whereby the river channel may change within the broader floodplain and the floodplain may be periodically modified by floods as the channel migrates back and forth across the it. This phenomenon occurs all too frequently in arid regions, where high velocity flood waters make drastic changes in the channel configuration during the flooding event. The reduction in channel capacity, although it may be temporary, can result in more frequent inundation of the floodplain and contribute to its modification.b. The floodplain may be developed and occupied during the years with the least flood activity. The duration of a flood from tropical storms or snowmelts may inundate a floodplain several times during a single month. Where channels are perched due to repeated deposition of sediment, flood waters may never drain back to the channel since that channel bottom is higher than the adjacent floodplain.d. Where floods are seasonal, crops may be selected that can withstand floods of short duration and low volume during the flood season.
Then, mitigation measures can be identified to avoid or minimize these hazards and can be incorporated into the formulation of specific sectoral investment projects. In addition to discerning the risks of flooding, the same satellite data can be used to assess other hydrologic and atmospheric hazards as well as geologic and technological hazards. In the absence of information from dynamic techniques, it is possible to estimate the probability of a flood event occurrence when information from static techniques is combined with historical flood observations, disaster reports, and basic natural resource information, particularly hydrologic data. Selected critical study sub-areas should be identified, and the preparation of additional flood hazard information should be designed into subsequent study activities. Small scale satellite image maps complement traditional thematic maps with synoptic spatial information that can be used as a basis for a regional assessment of the hydrologic regimen, including floodplain definition for major river valleys.
Landsat MSS data, which have been collected over most land areas of the world intermittently since 1972, provide the best and most readily obtainable record of floodplains and land-use changes caused by floods, sediment deposition, and human activity. Similarly, the flood hazard information is critically important in planning urban, industrial, recreational, tourism, and parkland development.c. These static techniques provide pictures of an area that can be analyzed for certain flood-related characteristics and can be compared to images from an earlier or later date to determine changes in the study area. Therefore, the focus of the method presented here is to provide a technique which uses original or raw film data for floodplain mapping and floodplain hazard assessments. Landsat digital data have also been combined with digital elevation data to develop stage-area relationships of flood-prone areas (Struve, 1979).
Floodplains are, in general, those lands most subject to recurring floods, situated adjacent to rivers and streams. This condition can result in surface water elevations contained within the channel being considerably higher than the land surface elevations immediately outside these levees, which results in a flooding potential that is much worse than that in the typical situation where the channel is at the bottom of a U-shaped cross section of the floodplain. As a result, this development is subject to the risk of flooding as the cycle of flooding returns. Early consultation with water resource and management specialists during the planning study is prudent, for it enables the planner to foresee and evaluate potential conflicts between present and proposed land use and their relationship to flood events and the hazards they may pose.
On the other hand, it is more appropriate to site a large agroindustrial or housing project in an area with a very small probability of a large flood occurring each year (see Chapter 2). In any event, the principal objectives of using dynamic techniques are to calculate the return period or frequency of particular flood events and to determine stream flow and flood-level characteristics. Flood event frequency estimates, particularly for an extreme event, is valuable information to the planning study. Similarly, in mapping a flood using satellite imagery, the inundated area can be compared with a map of the area under preflood conditions. 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. Development activity, particularly deforestation and intensive crop production, may drastically change runoff conditions, thereby increasing stream flow during normal rainfall cycles and thus increasing the risk of flooding.
These are important for the planner to know in order to adequately weigh the risk of development in a floodplain. Figure 8-6 shows the relationship of satellite remote sensing data and other flood hazard information to the information used in the integrated development planning process.
What is more readily available is information derived from static techniques which are capable of yielding information on flood hazard assessment. More intensive use of the floodplain, even under strict management, almost always results in increased runoff rates.

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