Layer Thickness Estimation of Density Current from Plunge Point
Date Issued
2010
Date
2010
Author(s)
Yang-Ting, Lin
Abstract
In recent years, density current is formed due to reservoir inflow contents large amount of silt during typhoon seasons. The deposits entered reservoirs is difficult to remove, thus heavily reduced the life span of the reservoirs. Reservoirs in Taiwan have multiple objectives, including flood decreasing, water depositing, electricity generating, also are local tourist attractions. Using empty reservoir strategies is not suitable for most of reservoirs in Taiwan. How to take advantage of density current phenomena to discharge silts entering the reservoirs, thus extends life span of the reservoirs, is an important research topic for reservoir management in Taiwan.
As muddy inflow entering the reservoir, depositing delta is formed in the upstream of the reservoir. Then at a point of suitable condition of plunging, the muddy inflow plunge under clear water and density current is formed. As the head of density current flowing across the reservoir, clear water is absorbed into density current, thus the volume of density current is increased. This research is focused on the change of layer thickness of density current from the plunge point to the stable density current behind the head of the current. Referred to [Parker , 2007] using momentum equations estimating layer thickness of muddy inflow from plunge point to the turning point where volume of density current staring to increase. This research uses momentum equation on estimating the increase of layer thickness of density current because of absorbing water into turbidity layer. The effects of slope and energy dissipation on the plunge point are also take into consideration. Experiment results of [俞 , 1991] and [徐 , 2002] is used to verify the estimation of turbidity layer thickness of this research. The turbidity and flow rate measurement data of density current in Shimen Reservoir during typhoon seasons is also used to verify the estimation of this research. The comparison of experiment data from [俞 , 1992] and estimation of this research shows, with the amount of absorbed water is under half the amount of muddy inflow ( denote by γ< 0.5), the experimental data can fit with the estimation in this research. The comparison with experimental data of [徐 , 2002] shows, the estimation of this research is also applicable to the type of density current forced to form by releasing the turbidity water on bed of the experiment tank, and density current layer thickness of low γ value (less than 0.1). With the measurement data in Typhoon Jangmi, Typhoon Sinlaku and Typhoon Matsa compared with theoretical estimation, the result shows the estimation of turbidity layer thickness contenting slope and energy dissipation on plunge point modifications fit the measurement data better. The measurement data also shows that on the same cross-section of Shimen Reservoir, the value of γ is within close range on different typhoon.
This research shows that using simple momentum equations is capable of estimating layer thickness of density current. Also the comparison with theoretical estimation between experimental data and density current measurement data in Shimen Reservoir shows, the theoretical estimation of this research is capable of estimating density current layer thickness and cross-sectional flow rate in every specific cross-sections in reservoirs.
Subjects
momentum equations
plunge point
density current layer thickness
entrainment
Type
thesis
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