博士 / 國立成功大學 / 水利及海洋工程學系 / 89 / Taiwan itself is a mountainous island over two-thirds consisting of foothills and mountains. River deltas in estuaries are one of the most heavily utilized regions for industrial, commercial, recreational, and municipal activities. About 80% of the total precipitation within a year in Taiwan concentrates from May to September. In this period, fluvial sediment dominates the formation of river deltas. In the other period from October to April of next year, the development of river deltas is highly affected by the waves generated by strong northeast monsoon. To investigate the development of river deltas under these two quite different dynamic environments, movable-bed experiments from a flume into a basin were conducted to study the development of river deltas in the fluvial-dominated period and wave-dominated period, respectively.
Experimental results of river deltas development in the fluvial-dominated period showed that the development of the delta could be divided into three stages: the delta develops mainly in its length in the first stage, and in its width in the second stage. At the end of the second stage, the delta has the same order of magnitudes in width and length, and then the formation of delta goes into the third stage in which a varying channel exists on the top of delta. When the development of river deltas transferred from the fluvial-dominated period to the wave-dominated period, the reduction of river deltas was strongly related to the waves of basin. The predominant flow after wave breaking was in the onshore direction under the wave accumulative affection. The plane outline of deltas converted from an elliptical shape in the fluvial-dominated period to a Gaussian function shape in the wave-dominated period. As the wave condition changed from accumulative type to erosive type, the predominant flow after wave breaking was in the offshore direction. The deltas were strongly eroded by the offshore current, and the storm beach was turning to the original delta formed in the fluvial-dominated period at the equilibrium state.
In the first stage of development in the fluvial-dominated period, the flow patterns were similar to those of the jet flow. The plane shape of deltas can be predicted by a formula based on the concept of incipient motion of particles. In the second stage, the delta developed mainly in its width. The length, width, front thickness, and central thickness of the delta were used to scale the geometric similarity. The plane geometry of deltas can be described using Gaussian functions. Transverse and longitudinal profiles fit the hyperbolic and linear function, respectively. The length, width and thickness of the delta deposition at the end of second stage have their own maximum values, , and , respectively, for each case. The total volume V of sediment deposition could be simply related to these maximum values by with shape parameters and was around 0.178 and 174. Following the second stage, the rest of the development of the delta was named as the third stage. In the third stage, channelization occurred on the top of a well-grown delta. As affected by the stream channels, the shape factor oscillated with time average near 1.0, and resulted in a non-symmetrical shape. The first stream channel occurred when the slope ratio (maximum slope on the delta surface along the centerline / average slope of the channel bed) approached to a minimal. The stream channel, after it occurred, tended to migrate toward the centerline of the delta accompanied with processes including side erosion and retrogressive deposition, which was named as the straightening process. After the straightening process, another channel emerged and repeated a similar cycle.
Furthermore, a numerical model was developed based on shallow water equations, a sediment transport equation and a sediment continuity equation. This model was applied to simulate the first and the second stages of the delta formation process in the fluvial-dominated period. Experimental results were adopted as objects for model verifications and model simulations. The simulation results show that the geometries of deltas are in good agreement with that of the experimental deltas. The sensitivity of parameters in the numerical model, such as the rough height, the critical shear velocity of particle and the eddy viscosity, to the simulation results was also discussed. Results show that the rough height and the critical shear velocity have more significant influence than the eddy viscosity to the simulated topography. Fifty-percent increase (or decrease) in the rough height or the critical shear velocity can result in about 5% variation in the simulated topography.
| Identifer | oai:union.ndltd.org:TW/089NCKU0083008 |
| Date | January 2000 |
| Creators | Chih-Ming Tseng, 曾志民 |
| Contributors | Chjeng-Lun Shieh, 謝正倫 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 172 |
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