Topic Page: Floodplains

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1 Topic Page: Floodplains Definition: f loodplain from The Penguin English Dictionary a low-lying area beside a river that is composed of sedimentary deposits and is subject to periodic flooding. Summary Article: Floodplain from Encyclopedia of Geography Image from: The setting sun glints off the Amazon River and... in Encyclopedia of Geography channel. 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 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). Floodplain Development 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.

2 The setting sun glints off the Amazon River and numerous lakes in its floodplain in this astronaut photograph from August 19, Source: NASA. Landsat 7 image showing floodplain of a section of the White Volta River in Ghana Source: Author.

3 Figure 1 Floodplain and its related features Source: Retrieved from Redrawn by J. M. Insaidoo. Satellite image showing land use in the floodplains of the Nile River (A) and the Mississippi River (B) Source: Author. 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.

4 Types of Floodplains 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. Figure 2 A sketch showing different floodplain water networks in uplifted block (A) and sunken block (B) sectors nodal points (1 and 2) Source: Based on Paira, A. R., & Drago, E. C. (2007). Origin, evolution and types of floodplain water bodies, the middle Parana River: Limnology of a subtropical wetland (pp ). Berlin, Germany: Springer. Redrawn by J. M. Insaidoo.

5 Figure 3 Hydrologic and topographic floodplains. The hydrologic floodplain is defined by bankfull elevation. The topographic floodplain includes the hydrologic floodplain and other lands up to a defined elevation. Source: Federal Interagency Stream Restoration Working Group. (1998, October). Stream corridor restoration: Principles, processes, and practices. Retrieved December 3, 2009, from Figure 4 Event hydrograph of water height in a river for the period from August 29, 2005, to September 5, 2005 Deposit s and Flow Over Floodplains Source: Author. 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. Met hods of Floodplain Delineat ion 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.

6 Information required for these models includes the peak flow rate or event hydrograph (Figure 4), topography, and field data on channel and floodplain characteristics. Uses of Floodplains 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-waterlevel 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. Figure 5 Cycle of land use on floodplains of the White Volta River Source: Nyarko, B. K. (2007). Floodplain wetland-river flow synergy in the White Volta river basin, Ghana (Ecological and Development Series No. 53). Bonn, Germany: Hausdruckerei der Universitat Bonn. 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. See also Coastal Erosion and Deposition, Floods, Rivers Further Readings Anderson, M. G., Walling, D. E., & Bates, P. D. (Eds.). (1994). Floodplain processes. New York: Wiley. Bridge, J. S. (2005). Rivers and floodplains: Forms, processes and sedimentary records. Oxford, UK: Blackwell. Nevidimova, O. Quantitative regularities in floodplain formation. Geophysical Research 11 :, Paira, A. R., & Drago, E. C. (2007).. Origin, evolution, and types of floodplain water bodies. In M. H.

7 Iriondo, J. C. Paggi, & M. J. Parma (Eds.), The Middle Paraná River: Limnology of a subtropical wetland (pp ). Berlin, Germany: Springer. Nyarko, Benjamin Kofi APA Chicago Harvard MLA Nyarko, B. K., & Nyarko. (2010). Floodplain. In B. Warf (Ed.), Encyclopedia of geography. Thousand Oaks, CA: Sage Publications. Retrieved from Copyright 2010 by SAGE Publications, Inc. Copyright 2010 by SAGE Publications, Inc.

8 APA Nyarko, B. K., & Nyarko. (2010). Floodplain. In B. Warf (Ed.), Encyclopedia of geography. Thousand Oaks, CA: Sage Publications. Retrieved from Chicago Nyarko, Benjamin Kofi, and Nyarko. "Floodplain." In Encyclopedia of Geography, edited by Barney Warf. Sage Publications, Harvard Nyarko, B.K. and Nyarko. (2010). Floodplain. In B. Warf (Ed.), Encyclopedia of geography. [Online]. Thousand Oaks: Sage Publications. Available from: [Accessed 19 August 2018]. MLA Nyarko, Benjamin Kofi, and Nyarko. "Floodplain." Encyclopedia of Geography, edited by Barney Warf, Sage Publications, 1st edition, Credo Reference,. Accessed 19 Aug