A floodplain is one of the ubiquitous fluvial features found along major rivers. It is a wide, flat plain of alluvium on either side of a river extending to the base of a valley that is seasonally inundated. Inundation may originate from overbank flow due to the high amount of rainfall or spillage from dams (see Amazon and White Volta photos). Geomorphologically, actively inundated areas are the composite fringing floodplains of rivers located near the main channel.
A floodplain is a complex assemblage of landforms (Figure 1) including channel features such as bars; channel edge features such as banks, benches, knickpoints, and levees; and features such as old channels (oxbow lakes), old levees, back swamps, and crevasse splays. The existence, development, and spatial structure of a floodplain and its features are, in effect, a record of the past history of the river and its current activities. Floodplains have natural qualities that provide sites for agricultural production, urban expansion, industrial location, recreational activities, and conservation of nature and sites of cultural and historic value (see Nile and Mississippi photos).
Along the longitudinal river profile, tectonic interventions contribute to the formation of floodplains, creating different channel densities and patterns that influence hydraulic connections between lotic and lentic water bodies. Tectonic deformation of a river course leads to raised and subsided blocks displaying different hydrologic patterns (see A and B in Figure 2). Uplift blocks develop large and small lakes, whereas subsided blocks display complex anastomosing drainage patterns with a lot of irregular interconnected lakes.
Base level and climate changes to some extent influence floodplain development. The base level at the coast or valley mouth controls the upstream extent of horizontal and vertical channel migration, creating a landmass that can contain seasonal overbank flow and sediment deposition. Floodplain development is stabilized if there is a final standstill of the base level.
The scientists Wolman and Leopold proposed in 1957 a model of floodplain formation based on geomorphologic reasoning and observation of streams. A floodplain is constructed entirely of horizontal layers of fine-grained sediments interrupted by coarse-grained channel deposit. It is dominated by braids and meanders migrating back and forth over the valley floor, leaving behind a sandy bar deposit.
According to Nevidimova, floodplains are formed due to the simultaneous horizontal and vertical displacement of the river channel. A displaced river in a horizontal plane leaves behind a low surface that gets inundated during high waters but changes with time.
Geomorphologically, two types of floodplains can be distinguished, namely, hydrological flood-plains and topographic floodplains (Figure 3). A hydrological floodplain is the land adjacent to the base flow channel residing below the bankfull elevation, which is inundated once every year. Not every stream has such a floodplain. A topographic floodplain is the land adjacent to the channel, including the hydrologic floodplain and other land up to an elevation base reached by a flood peak of a given frequency.
Floodplain dynamics are determined by the amount of sediment deposited during the period of flooding and the amount of material removed during the time between floods. Water flow and sediment transport are complicated and difficult to evaluate due to variability in floodplain width and surface topography. Understanding the flow and deposition of sediments involves theoretical approaches and numerical modeling. These approaches are based on experiments carried out in flumes rather than on field approaches, largely due to the unpredictability and difficulty in collecting samples and making observations during flood events.
The sedimentation on floodplains consists of millimeter-thick to decimeter-thick strata sets, produced by discrete overbank flooding events. The grain size and internal structures are difficult to specify as they depend on local flow conditions and sediment availability. Many floodplain deposits have varying quantities of fine to very fine sand, silt, and clay. Floodplain deposits closest to the main channel are comparable with the main channel deposits but decrease in grain size with distance from the main channel.
Several methods have been developed to help delineate floodplains. These methods include remote sensing techniques and modeling within the GIS (geographic information system) platform using applicable algorithms. The hydraulic techniques use backwater modeling, based on losses along floodplain reaches. Backwater modeling uses hydrographs of complete events rather than the peak flow rate. Examples of the model include HEC-RAS, TELEMAC, SOBEK, and LISFLOOD-FP. Information required for these models includes the peak flow rate or event hydrograph (Figure 4), topography, and field data on channel and floodplain characteristics.
Floodplain resources provide a wide range of benefits to the surrounding communities. Communities use resources found on floodplains for the improvement of their livelihood, for example, firewood, tree stumps for shelter construction, and wood for fencing. Subsurface-water-level fluctuations of floodplains have potential effects on sustenance of stream flows, groundwater recharge, availability of water for livestock, and availability of water for dry-season irrigation. Floodplain use for crop cultivation during the season and off-season exposes the land to environmental hazards, altering its hydrodynamics and affecting the flora and fauna. The increasing use of agro-chemicals has the potential of polluting water bodies, changing the species composition, and entering the food chain. The human occupation of floodplains of major rivers such as the Nile, Mississippi, Ganges, Rhine, and Amazon has led to substantial impacts.
Qualitative characterization of the observed cycle of land use activities on floodplains (Figure 5) and parameters such as the magnitude, frequency, areal extent, spatial distribution, and predictability of floods within a floodplain help avert land degradation to some extent.
Coastal Erosion and Deposition, Floods, Rivers
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