Vegetation effects on experimental Holocene estuary morphology

Abstract

On a small scale, effects of vegetation on tidal systems are well studied. On a larger scale, however, it remains unknown how vegetation influences long-term estuarine morphological development. For Holocene estuaries, it is hypothesized that estuaries filled up under influence of vegetation. During my thesis, a landscape experiment was conducted to study the long-term influence of vegetation on estuarine morphology in a relatively short time period. First, a drowned river valley was created in a titling flume. Subsequently, sediment and vegetation were introduced to fill up available accommodation space. In combination with constant river discharge and tidal currents, an estuary evolved. One experiment lasted for 3000 tidal cycles. Medicago sativa and Lotus pedunculatus seeds were introduced after every 500 tidal cycles, to represent riparian vegetation and salt marshes. Two different vegetation densities were formed, to study the effect of either a sparse or dense vegetation cover. Generally, the introduction of vegetation led to a decrease in channel and bar number, accompanied by a reduction in braiding index, and an increase in channel depth and width. Dense vegetation captured a majority of the sediment at the bayhead delta, which elevated drastically. Sediment transport inhibition was clearly visualized by the decrease in ebb tidal delta sediment volume. Bar width reduced, in contrast to an increased bar width in presence of sparse vegetation. Sparse vegetation allowed sediment transport throughout the basin. The estuary clearly evolved in a tide-dominated and a river-dominated part. Focussing of the main river channel towards the tidal inlet occurred in presence of sparse vegetation. As experimental estuaries can be related to modern-day systems, obtained results can be beneficial in protecting natural estuaries and their inhabitants. Overall, this research shows vegetation plays a major role in long-term estuary development.