Downstream changes of meandering styles in the lower reaches ofthe River Vecht, the Netherlands

Abstract

The morphodynamics of the lower River Vecht, the Netherlands, and theinfluence of geomorphological setting and bank composition on meandermigration were studied by means of reconstructing the pre-channelizationlandform configuration on a scale of 1 : 25,000, using historical maps from1720, 1850 and 1890 A.D. and other data.

A downstream sequence of reaches was observed, each with a typical fluvialstyle and channel migration rate: (a) a narrow meander belt and a highlysinuous channel with intermediate migration rate, in the middle of anextensive floodbasin; (b) a wide meander belt and high rates of lateralchannel migration, especially where large meanders impinged upon valleybluffs, as part of an incised setting; (c) a low sinuosity, embanked channelwith low rates of downstream migration because of confinement by dikes,occurring in an inland delta with sandy sediments.

Local variation in meander migration rates was observed within reach B. Thiswas caused by the spatial variability of bank resistance as reflected by thewidth-depth ratio of the channel and the silt-clay ratios of deposits. Riverbanks are: 1) very erodible when consisting of channel deposits, aeoliandune deposits or when coarse fluvio-periglacial deposits occur at theirbase; 2) erodible when dominated by overbank deposits or aeolian sand sheetdeposits; 3) resistant when a plaggen-layer is exposed; and 4) veryresistant when dominated by floodbasin deposits.

These implications of meander variability enable to assess the effects ofthe rehabilitation of the meandering process.