Alongshore Advective Acceleration in the Surf Zone

概述：本项目的重点是近岸水流的变化和平流加速由于冲浪带的水深变化。近岸水流变化和冲浪区形态变化之间的反馈可能是近岸均匀入射波浪海滩冲浪区形态演变的关键。确定由冲浪带特征引起的近岸平流加速和水流变异性的大小，对于发展准确的海岸演变和污染输送模式很重要。平流加速对动量平衡有重要贡献的现场证据也可能导致新的岸线变化理论和模型。智力优势：分析和数值研究表明，近岸平流加速可能对冲浪带撕裂流、近岸水流曲折、扩散、泥沙输送和地形演变具有重要作用。最近的数值模式模拟表明，在近岸水深变化小至10厘米的海滩上，近岸平流加速度相对于深度和时间平均的冲浪区动量平衡中的其他项可能较大。然而，现场测量表明，用忽略平流加速的简化方程可以很好地预测近岸流动。此外，除了在强撕裂流附近，现场观测显示近岸冲浪带气流的近岸变化相对较小。简化的方程可以准确地再现观测速度，因为平流加速项由近岸压力梯度项平衡，或者平流加速度由参数化底摩擦系数考虑。也有可能的是，基于观测的沿岸流动梯度估计偏低，因为由于现场传感器之间的相对较大的空间间隔，流动的最大值和最小值没有得到分辨。另一方面，对摩擦系数敏感的数值模式可能会高估平流加速度。将获得密集分布的波浪强迫、压力梯度和冲浪带流动的观测结果，以校准数值模式模拟。现场测量将被用来估计平流加速度，现场测试的模式将被用来在近岸非均匀表面带测深的情况下探索平流加速的原因。特别是，观测和模拟将解决以下问题：近岸平流加速重要吗？近岸气流梯度和平流加速的模型预测准确吗？近岸平流加速的原因是什么？通过将现场观测与数值模型模拟相结合，研究人员将能够解决近岸平流加速在近乎均匀入射波的海滩上的重要性和原因。广泛的影响：美国海岸和海滩保护协会(ASBPA，http://www.asbpa.org))担心，对海岸工程感兴趣的美国人无法获得现场经验。因此，将与ASBPA和感兴趣的工程公司建立伙伴关系，为本科生、新近毕业的学生和从事沿海职业的年轻专业人员提供实地培训。特别是，将提供两个项目：为目前受雇于沿海咨询和工程公司的年轻专业人员提供为期2至3周的以实地工作为重点的实习，以及为对沿海工程职业感兴趣的学生提供为期6个月的奖学金。已经有十多家公司表达了对实习的兴趣，并提出支持他们的员工参与拟议中的计划。学生奖学金将类似于之前由首席研究员劳本海默主持的Swashzone奖学金，后者导致几名学生获得了沿海工程公司的职位。实习生和学生将与调查人员合作，获得实地观察，并改进近岸过程的模型，这反过来将改善海岸管理，帮助保护沿海社区，并维持沿海资源。

Overview: This project focuses on the alongshore flow variations and advective accelerations owing to surfzone bathymetric variations. Feedbacks between alongshore flow variations and surfzone morphological changes may be critical to the evolution of surfzone morphology on beaches with alongshore uniform incident waves. Determining the magnitude of the alongshore advective accelerations and flow variability resulting from surfzone features is important to developing accurate models for coastal evolution and transport of pollution. Field evidence that the advective acceleration contributes significantly to the momentum balance also may lead to new theories and models for shoreline change.Intellectual Merit: Analytical and numerical studies suggest that alongshore advective acceleration may be important to surfzone rip currents, alongshore flow meanders, dispersion, and sediment transport and morphological evolution. Recent numerical model simulations indicate that alongshore advective acceleration can be large relative to other terms in the depth- and time-averaged surfzone momentum balance on beaches with alongshore bathymetric variations as small as ten centimeters. However, field measurements have shown that alongshore flows are predicted well with simplified equations that neglect advective accelerations. Furthermore, except near strong rip currents, field observations show relatively small alongshore variations in alongshore surfzone flows. The simplified equations may reproduce observed velocities accurately because the advective acceleration term is balanced by an alongshore pressure gradient term, or because advective acceleration is accounted for by the parameterized bottom friction coefficient. It also is possible that the observation-based estimates of alongshore flow gradients are biased low because flow maxima and minima are not resolved owing to the relatively large spatial separations between in situ sensors. Alternatively, the numerical models, which are sensitive to the friction coefficient, may overpredict the advective acceleration. Densely distributed observations of wave forcing, pressure gradients, and surfzone flows will be obtained to calibrate numerical model simulations. The field measurements will be used to estimate advective acceleration and the field-tested model will be used to explore the causes of the advective accelerations in the presence of alongshore inhomogeneous surfzone bathymetry. In particular, the observations and simulations will address the following questions: Is alongshore advective acceleration important? Are model predictions of alongshore flow gradients and advective acceleration accurate? And what causes alongshore advective acceleration? By combining field observations designed to measure flow variations at the spatial scales of small surfzone bathymetric variations with numerical model simulations, the investigators will be able to address these questions about the importance and causes of alongshore advective accelerations on beaches with near uniform incident waves.Broader Impacts: The American Shore and Beach Preservation Association (ASBPA, http://www.asbpa.org) is concerned that Americans interested in coastal engineering are not able to obtain field experience. Thus, a partnership with ASBPA and interested engineering firms will be established to provide field training to undergraduates, recent graduates, and young professionals embarking on coastal careers. In particular, two programs will be offered: 2- to 3-week-long fieldwork-focused internships for young professionals currently employed in coastal consulting and engineering firms, and 6-month-long fellowships for students interested in coastal engineering careers. More than a dozen companies already have expressed interest in the internships, and have offered to support their employees to participate in the proposed program. The student fellowships would be similar to the Swashzone Fellowships previously conducted by the lead investigator, Raubenheimer, which have led to several students receiving positions in coastal engineering firms. The interns and students will work with the investigators to obtain field observations, and to improve models for nearshore processes, which in turn will improve coastal management, help protect coastal communities, and sustain coastal resources.