Browsing by Subject "River delta"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item The kinematics of distributary channels on the Wax Lake Delta, coastal Louisiana, USA(2013-05) Shaw, John Burnham; Mohrig, DavidThe Wax Lake Delta (WLD) is a sandy, modern river delta prograding rapidly into Atchafalaya Bay. This dissertation uses field data to improve the understanding of channel kinematics that dictate river delta geometry and stratigraphy, while providing a framework for coastal restoration efforts. The studies presented here show that the distributary channel network of the WLD is erosional. In the first study, analyses of the feeder channel to the WLD and the channel network within the sub-aerially emergent delta show that the channel bed has incised into the consolidated muds that act as bedrock. The large (>62%) fraction of bedrock exposure found in multi-beam surveys is related to the under-saturation of suspended sand measured during the flood of 2009. The second study concerns the delta front beyond the emergent delta Distributary channels extend 2 – 6 km into the delta front. Four bathymetric surveys of one bifurcating distributary channel – Gadwall Pass – show that the majority of bed aggradation occurs during floods, but the majority of channel extension of each bifurcate channel occurs during low river discharge. In the third study, field measurements of fluid flow during a tidal cycle indicate that tidal augmentation of during periods of low river discharge is responsible for channel extension during low river discharges. Flow direction measured from streaklines present in aerial photomosaics is combined with bathymetric evolution data to quantify spatial velocity changes on the delta front. These data show that flow spreading is insufficient to prevent acceleration at channel margins, providing an explanation for observed erosion. Flow divergence is limited on the delta front by the proximity of neighboring channels, even though they are separated by 10-30 channel widths. The associated convergence of flow in inter-distributary bays occurs along “drainage troughs”. These channel-forms collect flow that has been dispensed from distributary channel network. Finally, ambient currents in Atchafalaya Bay (0.06 – 0.2 m/s) caused by tides and the proximity to the neighboring Atchafalaya Delta appear to alter flow patterns on the delta foreset, and are responsible for channel curvature on the delta front.Item A network-based analysis of river delta surface hydrology : an example from Wax Lake Delta(2013-05) Hiatt, Matthew R.; Passalacqua, PaolaRiver deltas are dynamic ecosystems of environmental, ecological, and societal importance. In coastal Louisiana, land loss and increased nutrient loading are altering the eco-geomorphic equilibrium, raising concern for the environmental concerns associated with climate and anthropogenic change. Over the last 100 years, nearly 5000 km2 of wetlands have been submerged due to a variety of environmental and anthropogenic forces. Wetland drowning and costal retreat is predicted to continue, threatening both human and environmental interests. As a result, mitigation efforts in the form of planned river diversions designed to build new land by natural delivering sediment to the once-leveed floodplains have been proposed. Denitrification in coastal wetlands has the potential to limit the risks of hypoxia and related eco-geomorphic issues by reducing the nitrogen export to receiving waters. The goal of this research project is to understand and quantify to propagation of environmental fluxes through a delta distributary system. Water fluxes in the delta distributary network are not solely propagated within the distributary channels and interdistributary areas, taking the form of inundated island interiors surrounded by subaerial levees, are hydrologically important. At Wax Lake Delta (WLD) in coastal Louisiana, roughly 50% of the flow is exchanged to the island swamps, suggesting that significant portions of nutrients are transported to the island interiors. The hydraulic residence times (HRTs) of the islands are estimated to be 15 – 29 hours and 11 – 18 hours on Mike and Pintail Islands, respectively; both being well below HRT estimates for significant denitrification. Spatial variability in network structure, specifically channel width and frequencies of confluences and bifurcations, influences the transport dynamics within the delta. However, flow partitioning in major distributary channels at WLD is relatively constant with time, supporting the hypothesis that network structure controls flux dynamics.Item Validation of a reduced-complexity numerical model for resolving deltaic dynamics : internal consistency and morphodynamics(2015-05) Van Dyk, Corey John; Passalacqua, Paola; Mohrig, DavidRiver deltas are fragile ecosystems that have immense ecological, economic, and social importance. The ability to understand them is facilitated by numerical models that can resolve the complex hydrodynamics and morphodynamics of deltas. DeltaRCM is one such model, and to validate its behavior, internal consistency is tested with variable input parameters; results indicate realistic growth with predictable patterns. The morphodynamics are tested against experimental and real deltas with the use of metrics: specifically, delta growth metrics like shoreline-to-area ratio and relative shoreline roughness, channel overlap, and avulsion behavior. DeltaRCM performs very well when compared to real systems with growth rate and relative shoreline roughness, and fairly well for shoreline-to-area ratio. The channel overlap metric suggests DeltaRCM displays a slightly higher degree of channel stability than an experimental delta, though the general trend of memory decay remains the same. A similar link exists between DeltaRCM and reality for the wetted fraction, in that general trends are similar but comparison breaks down at finer scales. Furthermore, based on DeltaRCM results, wetted fraction is an imperfect tool for determining avulsion timescale. A new metric, the sedimentograph, is introduced as a way of describing delta growth at the subsurface level; DeltaRCM gives reasonable results for this metric, though comparison to real systems is difficult.