| 引用本文: | 贾建军,高抒.建立潮汐汊道P-A关系的沉积动力学方法.海洋与湖沼,2005,36(3):268-276. |
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| 建立潮汐汊道P-A关系的沉积动力学方法 |
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贾建军,高抒
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1.国家海洋局海底科学重点实验室 杭州310012;2.南京大学海岸与海岛开发教育部重点实验室 南京210093
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| 摘要: |
| 在均衡状态下,潮汐汊道系统的纳潮量(P)与口门面积(A)之间存在着较稳定的关系,通常用幂函数形式来表达。计算P-A关系的传统O’Brien方法因其统计性质无法给出单一潮汐汊道的P-A关系。根据沉积动力学原理,每个潮汐汊道的PA关系都受涨落潮历时、断面平均流速、口门形态、纳潮量、淡水径流量、沿岸毛输沙量和沉积物粒度等因素的控制,对上述因素的每一种组合,都有对应的均衡态P-A关系。作者在前人工作的基础上,尝试着建立了计算单一潮汐汊道P-A关系的沉积动力学方法,并以山东半岛月湖为算例对该方法的应用进行了讨论。结果表明,均衡态潮汐汊道的P-A关系的指数n稳定在1.15左右,而系统C的变化较大。 |
| 关键词: 潮汐汊道 纳潮量 P-A关系 沉积动力学 山东半岛月湖 |
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| 基金项目:国家杰出青年基金资助项目,49725612号 |
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| A DYNAMIC SEDIMENTOLOGICAL APPROACH TO P-A RELATIONSHIPS IN TIDAL INLETS |
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JIA Jian-Jun1,2, GAO Shu3
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1.Key Laboratory of Submarine Geosciences, State Oceanic Administration, Hangzhou, 310012;2.Ministry of Education of P.R.China Key Laboratory for Coast and Island Development, Department of Geo & Ocean Sciences, Nanjing University, Nanjing, 210093;3.Ministry of Education of P R China Key Laboratory for Coast and Island Development, Department of Geo & Ocean Sciences, Nanjing University, Nanjing,210093
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| Abstract: |
| Tidal inlets are common geomophological features in sandy coasts in the world. Their conditions, identification, and maintenance are important in mariculture, tourism, science research, and marine engineering. It is believed that the relationship between a spring tidal prism (P) and its cross-section area of inlet (A) represents the equilibrium condition of tidal inlets, and can be expressed in a power-law form mathematically. A conventional O. Brien method of determining P-A relationships depends upon data sets from a number of inlet systems. However, it has two shortcomings. Firstly, many P-A relationships for tidal inlets are located in different geological and hydrological settings with varying power n and coefficient C that difficult to decide which one should be chosen. Secondly, O’Brien method cannot be used to define the equilibrium status of a single tidal inlet.
In terms of sediment dynamics, a tidal inlet system is related with many hydrological and sedimentological factors, such as flood and ebb durations, mean cross-section current speed, tidal entrance shape, freshwater discharge, longshore sediment transport rate, and sediment property, etc. An equilibrium P-A relationship for the tidal inlet varies with changes in any of these factors. Therefore, what the O’Brien method figures out is not the equilibrium status, but an average one for overall tidal inlets.
In this paper, a dynamical sedimentological approach to the P-A relationships for single tidal inlet is proposed. The approach can be described as an 8-steps procedure:
1) To input known parameters, including fresh water discharge, tidal prism, durations of ebb and flood tides, ratio of width to depth (R) of the entrance channel, grain-size of sea-bed sediment, and gross volume of longshore drifts. These data can be obtained through in situ observation and calculation.
2) To initialize mean ebb current velocity at a small value, providing the value is smaller than that under equilibrium conditions.
3) To calculate mean flood current velocity on the basis of the law of mass conservation.
4) To calculate the volume of bedload sediment transported through the entrance channel during flood and ebb tides.
5) To calculate the change of cross-sectional area at the entrance channel caused by bedload sediment transport.
6) To calculate the ratio of width to depth of the entrance channel due to the change of cross-sectional area.
7) On the basis of observation, the ratio of width to depth of inlet is assumed to be stable under equilibrium conditions. Therefore, if R derived in step 6) equals to that input in step 1), a status defined by parameters as input in step 1) can be treated as of equilibrium. Otherwise, rewind the procedure from step 2) with mean ebb current velocity being increased (say, increasing Ue by 0.005 m/s in each cycle) until step 7) is satisfied. To derive P-A relationships with data satisfying Step 7).
30 experiments were carried out in Yuehu Inlet, a small inlet-lagoon system in Shandong Peninsula. Results show that the value of power n should be larger than 1, which implies that, in order to maintain the status of stability, the inlet width would narrow, and the current speed at the entrance would increase as tidal prism become smaller. For the reason of power n<1, as many researchers argued before, is that the influence of tidal prism was over-valued. Meanwhile, the variation range for coefficient C is one order of magnitude higher than that for power n, implying that the coefficient C is almost independent on many factors such as longshore drift, and freshwater discharge, etc. It should be pointed out that P-A relations given by the approach are just a representative one for tidal inlets in equilibrium in average status. As tide, wave, freshwater discharge, and tidal inlet form constantly change, the real P-A relationships would fluctuate also.
Solutions should be made before using our approach to P-A relationships. One is to get the cross-section distribution pattern of current speed at inlet, and the other is to know the relationship between the water level and the cross-section mean current speed at inlet. Unfortunately, no such solution is available. In a simplified way, depth-averaged current speed can be used to substitute cross-section current speed if the ratio of width to depth of inlet is large enough, as shown in the experiments in Yuehu Inlet. For the second problem, the mean water level is used to represent the whole processes of up-and-down tides, resulting in decreased accuracy of calculated sediment transport rate. |
| Key words: Tidal inlet, Tidal prism, P-A relationships, Sediment dynamics, Shandong Peninsula |
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