Alongshore Advective Acceleration in the Surf Zone

概述：本项目主要研究由于表层水深变化导致的岸线流动变化和顺流加速度。在均匀入射波的海滩上，沿岸水流变化与地表形态变化之间的反馈关系可能对地表形态的演变至关重要。确定由表层特征引起的沿岸平流加速和流动变异性的大小，对于开发精确的沿海演变和污染输送模型非常重要。现场证据表明，平流加速对动量平衡有重要贡献，也可能为岸线变化提供新的理论和模型。理论价值：分析和数值研究表明，沿岸平流加速可能对表层离岸流、沿岸流曲流、分散、沉积物运输和形态演化很重要。最近的数值模型模拟表明，在岸线水深变化小至10厘米的海滩上，岸线平流加速度相对于其他方面的深度和时间平均表层动量平衡可能很大。然而，现场测量表明，使用忽略平流加速度的简化方程可以很好地预测沿岸流动。此外，除了在强烈的离岸流附近外，实地观测显示，沿岸表面带流动的沿岸变化相对较小。简化的方程可以准确地再现观测到的速度，因为平流加速度项被岸上压力梯度项平衡，或者因为平流加速度是由参数化的底部摩擦系数计算的。也有可能基于观测的岸线流梯度估计偏差较低，因为由于原位传感器之间的空间距离相对较大，无法解决流量最大值和最小值。另外，数值模型对摩擦系数很敏感，可能会高估平流加速度。将获得密集分布的波浪强迫、压力梯度和地表流动观测资料，以校准数值模式的模拟结果。现场测量将用于估计平流加速度，现场测试的模型将用于探索在沿岸不均匀地表测深存在的情况下平流加速度的原因。特别是，观察和模拟将解决以下问题：沿岸平流加速重要吗？模型对沿岸水流梯度和平流加速度的预测准确吗？是什么导致了沿岸平流加速？通过将现场观测设计与数值模型模拟相结合，以测量小海面水深变化的空间尺度上的流量变化，研究人员将能够解决这些问题，即在入射波接近均匀的海滩上，岸线平流加速的重要性和原因。更广泛的影响：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，向本科生、应届毕业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：为目前在海岸咨询和工程公司工作的年轻专业人员提供2至3周的实地实习，以及为有兴趣从事海岸工程职业的学生提供为期6个月的奖学金。已有十多家公司表示对这些实习项目感兴趣，并表示愿意支持其员工参加拟议中的项目。学生奖学金将类似于之前由首席研究员劳本海默（Raubenheimer）主持的Swashzone奖学金，该奖学金已使几名学生在沿海工程公司获得了职位。实习生和学生将与调查人员一起进行实地观察，并改进近岸过程的模型，这反过来将改善沿海管理，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.