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Study on Response of Salinity Distribution Around the Yellow River Mouth to Abrupt Change in River Discharge with in-situ Observation and Numerical Simulation

Author: WangYuCheng
Tutor: GaoHuiWang
School: Ocean University of China
Course: Physical oceanography
Keywords: Yellow River water-regulation abrupt change of river discharge river plume flushing time numerical simulation
CLC: TV68
Type: Master's thesis
Year: 2010
Downloads: 56
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Abstract

River plume is a general phenomenon on the continental shelf sea, and it not only impacts on water temperature, salinity and other characteristic features, but also the transport of nutrients, sediment, pollutants,migration and distribution of plankton, spawn, etc.During the development and evolution, the river plume is affected by many factors such as river discharge, wind, tide, topography and thermal stratification which make it complex and non-linearity. Among these factors mentioned above, temporal variation of river discharge is an important one which includes the long period of annual and seasonal variability, and short period of abrupt changes only in a few days. From 2002, the Yellow River Conservancy Commission (YRCC) carried out a series of water-regulation experiments;since 2005,the experiment had been normal at the beginning of every flood season. The duration of the water-regulation is only about two or three weeks, during which the discharge of the Yellow River changes abruptly. The aim of this thesis is to investigate the variational character of the Yellow River plume during the period of water-regulation in 2009 through intensive in-situ observation and then examine the influence factors by model simulation.Based on in-situ observation of salinity and temperature during the period of Yellow River water-regulation event, we found that the mean salinity in the Yellow River estuary area during the event varied～1.21,which is much larger than its seasonal variation (～0.50) and inter-annual variation (～0.05),respectively. The distinction was very obvious between surface and bottom layer. Before the event, a small size of plume was observed near the river mouth. During the event, the plume extended over 24 km offshore in the surface layer in the direction of outflow of river water. After the event, the plume retreated to the river mouth in the surface layer but still kept a larger size than that before the event.The downstream propagation of the plume (as in a Kelvin wave sense) was apparent in the bottom layer during the second survey and in the entire water column during the third survey. Based on the conservation of volume and salt within a simple box model, we found that the flushing time is proportional to the river discharge. Using some theoretical formulas, it was found that the tidal mixing was stronger than the wind mixing at the area close to the river mouth where the water depth is shallow and tidal currents are strong. However, as leaving away from the river mouth, the water becomes deep and tidal currents get weaken; the intensities of two mixing processes became comparable.Based on a simple 3D rectangle model and POM (Princeton Ocean Model), we simulate the development of river plume considering the influence of buoyancy force, topography, wind and tide; after that, we delete one forcing to investigate the influence of that factor. With small runoff, the river plume is controlled by the community of topography, wind, and tide and mainly appeared at the upshelf direction of the river mouth. With abrupt increase of the river runoff, buoyancy flow is the main flow; with the maintenance of larger discharge, the river plume kept growing; after some days of water-regulation, it could carry through a relative strong state. Topography is helpful for alongshore transport of the diluted water and the influence is different in different zones.The influence of tide is very important when the river discharge is small;due to stronger mixing, more diluted water is transported to low layer.By this research, a basic understanding of the Yellow River plume during water-regulation period is presented which established a basis for the future study on short-term change of river plume with consideration of local topography and other influence factors.

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