Quantifying the Effects of Combined Waves and Tides on Deltas: An Experimental Study

Abstract:

The classical Galloway diagram captures qualitatively the dramatic effect waves and tides have on reshaping river deltas. Here we investigate these controls in a series of laboratory experiments in which the relative energetics of river, wave, and tidal forcing could be controlled and systematically varied. The delta is fed from a single source of water and low-density, sand-size sediment in one corner of a 5m x 5m basin. Experimental tides are produced by transferring water back and forth between the main experimental basin and an auxiliary holding basin. The tidal period is 60 s and a typical tidal range is 30 mm. Waves are produced using a floating, oscillating paddle placed opposite the sediment feed location. They typically have a period of 1 s and an amplitude of 10 mm. The total energy flux associated with waves and tides is controlled by varying the temporal intermittency of each process, while river energy and sediment fluxes are held steady. The experiments show a variation in delta morphology as a function of wave and tidal forcing that parallels that observed in the field: increasing wave strength redistributes sediment and flattens the shoreline; increasing tidal strength creates well defined tidal channels as well as inlets through the wave-worked shoreline. Both waves and tides reduce the mobility of the main fluvial channel. Quantitative morphologic measures of these effects vary systematically as a function of dimensionless relative wave and tidal strength. The image below shows typical experimental delta morphology associated with mixed wave-tide forcing.