Fp1a.jpg (54565 bytes)Meandering rivers indicate depositional environments were sediment transported in approximately equals sediment transported out.  Floodplains of such rivers usually include at least all of the land between meandering channels.  Photo taken from window of commercial aircraft, north coast of Honduras, July, 1995.

FP.jpg (94594 bytes)In this cartoon, the rich green is likely to become flooded as the river (blue) rises out of its banks.   The flat land nearby is also floodplain, although it is covered with water less frequently.




Fp2a.jpg (49807 bytes)More north coast  meanders, 1995.  A similar perspective just after Mitch would show almost the entire foreground under water.  Sediment of the floodplain is also mobile.   Erosion is lateral, particularly along bends in the channel.  The outside part of the bend erodes, with deposition on the inside of the bend.  Meanders get increasingly contorted until outsides of two bends meet - and is about to occur just left of center of this image.  The abandoned channel forms an oxbow lake, which gradually fills with sediment.

Floodplains in mountainous areas (like Tegucigalpa) can also be identified from their shapes and other clues, if human activities have not obliterated too much of the geological evidence.  The floodplain hazard is rated in terms of the probability that a flood will cover a particular part of the floodplain in any given year.  If the probability is 5% (1 in 20) that, during any given year, water will cover where you are standing, you are on the 20-year floodplain.  This does not mean floods are 20 years apart.  The floodplain has no memory.

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