Barrier island evolution, middle Atlantic shelf, U.S.A. Part I: Shoreface dynamics
Barrier island evolution, middle Atlantic shelf, U.S.A. Part I: Shoreface dynamics
复制标题
美国大西洋中部陆架障壁岛演化第一部分：岸面动力学
DOI：
10.1016/0025-3227(85)90089-1
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发表时间：
1985
期刊：
影响因子：
2.9
通讯作者：
T. Hopkins
中科院分区：
文献类型：
--
作者：
D. Swift;Alan William Niederoda;C. E. Vincent;T. Hopkins
在本文的第一部分中，分析了大西洋中部屏障（长岛海岸）岸面近底流体速度和沉积物浓度的测量结果，以深入了解侵蚀岸面退缩的动力学。在第二部分*中，这些数据与海岸地层学观测相结合，以构建大西洋中部屏障的演化和行为模式。长岛海岸上的波浪运动往往会把沉积物推到岸上。对不对称浅滩波驱动的陆上沉积物通量的计算表明，在大西洋中部大陆架上常见的波态，在大约10米水深（上岸面）的陆地方向上，值变为强烈的正值，但在等深线的近岸方向上，值降至非常低的值。在较低的滨面和内陆架底板上，尽管近底速度场的波轨道分量几乎是对称的，但晴朗天气下的波流相互作用往往会导致沉积物向陆地蠕动。这种运动的发生是因为波浪接近方向通常位于沿岸风力流方向（上海岸或下海岸）的90°范围内。因此，作用在沉积物上的流体剪切应力在波轨运动的滨向行程中有增强的趋势，在向海行程中有部分消除的趋势。在大风暴期间，沉积物的输送速率至少增加了一个数量级，风驱动的洋流的作用变得重要起来。更多的沉积物被波浪轨道流成分重新悬浮，因为风暴波比晴天的巨浪更强大。但风力驱动的水流成分也更强，现在可用于输送再悬浮的沉积物。岸面上的风暴流发生在不同的动力区（冲浪区、摩擦主导区、过渡区、地转区）。这些区域是根据动力学因素来定义的，并随着引起风应力的强度而扩大或缩小。在水流高峰期，以摩擦为主导的地转带的特征长度尺度倾向于与滨面和内陆架的形态带相对应。风暴加剧的、风力驱动的、下海岸的流动在本质上经常是喷射状的，可能是上涌的，也可能是下涌的。在长岛海岸的大多数风暴中都出现了带有强烈下涌成分的风驱动的海岸流，这对海岸的沙收支尤其重要。在这些事件中，沙粒被风暴增强的由波浪驱动的沿岸流和离岸流组成的上表层环流系统所携带，并被送入主要的风驱动的沿岸流。由于底部流动的近海成分，砂被冲刷到较低的滨面，并进入邻近的内大陆架。晴朗的天气过程可能无法将风暴沉积的沙子从这样的离岸位置送回海滩。因此，长岛屏障海岸的岸面运输机制由长时间（数月）组成，在此期间，沙子缓慢地向海滩移动，在此期间，沙子从岸面转移到邻近的内大陆架的短时间（数小时或数天）被打断。无论是后退（长岛的情况）还是前进，岸面的长期运动感觉都必须取决于相对于内大陆架的岸面沙的损失或增加。数据表明，海岸砂预算不仅受上海岸面循环（砂从海滩棱柱中取出，储存在断点柱中，随后返回海滩）的控制，而且受…
In part I of this paper, near-bottom fluid velocity and sediment concentration measurements from the shoreface of a middle Atlantic barrier (Long Island coast) are analyzed to provide insight into the dynamics of erosional shoreface retreat. In Part II∗, these data are combined with observations of coastal stratigraphy in order to construct a model for the evolution and behavior of middle Atlantic barriers.Wave motions on the Long Island shoreface tend to drive sediment onshore. Calculations of the onshore sediment flux driven by asymmetrical, shoaling waves show that for the wave states commonly encountered on the middle Atlantic shelf, values become strongly positive landward of approximately 10 m water depth (upper shoreface) but drop to very low values shortly seaward of that isobath. On the lower shoreface and inner shelf floor, fair-weather wave-current interaction tends to cause a landward creep of sediment, even though the wave-orbital component of the near-bottom velocity field is nearly symmetrical. This movement occurs because the wave approach direction generally lies within 90° of the direction (up-coast or down-coast) of the alongshore wind-driven current. Therefore, fluid shear stress acting on the sediments tends to be reinforced during the shoreward stroke of the wave orbital motion and partially cancelled during the seaward stroke.During major storms, the rate of sediment transport increases by at least an order of magnitude and the role of wind-driven currents becomes important. Much more sediment is resuspended by the wave orbital current component because storm waves are more powerful than fair-weather swells. But the wind-driven flow component is also much stronger and is now available for transporting the resuspended sediment. Storm flows over the shoreface occur in distinct dynamic zones (surf zone, friction-dominated zone, transition zone, geostrophic zone). The zones are defined by dynamical considerations and expand or contract with the intensity of the causative wind stress. During peak flow events, the characteristic length scales of the friction-dominated and geostrophic zones tend to correspond with the morphologic zones of the shoreface and inner shelf.The storm-intensified, wind-driven, alongshore flows of the lower shoreface are frequently jet-like in nature and may be upwelling or downwelling. Wind-driven coastal flows with a strong downwelling component occur during most storms on the Long Island coast and are particularly important to the coastal sand budget. During these events, sand is entrained by the storm-intensified upper shoreface circulation system of wave-driven alongshore currents and rip currents, and is fed into the main wind-driven coastal current. Because of the offshore component of bottom flow, sand is swept down the lower shoreface and onto the adjacent inner shelf. Fair weather processes may be unable to return storm-deposited sand to the beach from such an offshore position.The shoreface transport regime of the Long Island Barrier coast thus consists of long periods of time (months) during which sand moves slowly toward the beach, punctuated by short intense periods (hours or days) during which sand is transferred from the shoreface to the adjacent inner shelf. The long-term sense of movement of the shoreface, whether retreating (the Long Island case) or prograding, must depend on the loss or gain of sand by shoreface with respect to the inner shelf. The data shows that the coastal sand budget is controlled not only by the upper shoreface cycle (withdrawl of sand from the beach prism, storage in the breakpoint bar and its subsequent return to the beach), but by a cycle of …