Deltas’ responses to fluvial and marine forces
PhD: University of Colorado Boulder, 2015.
This thesis addresses the responses of river deltas to both fluvial and marine environmental changes. Fluvial sediment flux, the predominant factor providing sediment to deltaic systems, can vary significantly over time, affecting delta formation and evolution. Hurricanes and cold fronts, which cause significant changes in coastal hydrodynamics, also greatly influence delta morphology. This thesis incorporates two areas of interest: a) the Ebro River in Spain, for which numerical simulations of long-term fluvial sediment fluxes were conducted using the hydrological transport model HydroTrend, and b) the Wax Lake Delta (WLD) in Louisiana, for which the morphological changes were investigated under different magnitudes of coastal storms by applying the numerical model Delft3D.
The 4,000-years history of fluvial sediment flux for the Ebro delta was reconstructed based on climate changes and anthropogenic factors. The results demonstrate that long-term fluvial water discharge to the river mouth is controlled by changes in precipitation, which have a high annual variability but no long-term trend. Modeled suspended sediment load, however, has an increasing trend over time, which is closely related to anthropogenic land cover variations. The recent significant decrease in sediment flux (more than 99%) is also attributed to human influences (dam emplacements).
Hurricanes and cold fronts are the two major weather systems influencing the morphology of coastal wetlands along the Gulf of Mexico. Here, simulations show that Hurricane Rita, which made landfall 120 km to the west of WLD as a Category 3 hurricane in 2005, had a significant impact on the delta, with more than 500,000 m3 of bottom sediment eroded, while the simulated largest cold front in the 2008-2009 season (mean wind speed of 11.4 m/s) caused net erosion of 100,000 m3. However, the cumulative impact of cold fronts in 2009 caused 1,900,000 m3 of erosion on the WLD during 29 events, much higher than the erosion caused by the two hurricanes in 2008 (Gustav and Ike caused erosion of 500,000 m3). Winds and waves, hurricane tracks, and aboveground vegetation are the major driving forces of the morphological changes, while fluvial input and roots have a minor influence on delta morphology during hurricane events.