Setup in the surfzone

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2007Surfzone wave height transformation and wave-breaking-driven increases in the mean sea level (setup) are examined on alongshore-uniform beaches with alongshore homogeneous and inhomogeneous wave forcing. While previously derived models predict wave heights adequately (root-mean-square errors typically less than 20%), the models can be improved by tuning a free parameter or by using a new parameterization based on the deep-water wave height. Based on a sensitivity analysis of the cross-shore momentum balance used to predict setup, a one-dimensional (1-D) model is developed that includes wave rollers and bottom stress owing to the mean offshore-directed flow. The model predicts setup accurately at three alongshore homogeneous field sites, as well as at a site where the incident wave field is alongshore non-uniform, suggesting that setup is driven primarily by the cross-shore (1-D) forcing. Furthermore, alongshore gradients of setup can be important to driving alongshore flows in the surfzone, and the 1-D setup model predicts these gradients accurately enough to simulate the observed flows.I would like to thank the National Science Foundation (OCE-0622844), the Office of Naval Research (G000783, G000782, N00014-99-10193, N00014-02- 10145), the MIT/WHOI Academic Programs Office, and the MIT Presidential Graduate Fellowship Program for their generous support of my research

Wave-driven setup and alongshore flows observed onshore of a submarine canyon

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C07025, doi:10.1029/2007JC004514.The effect of alongshore variations in the incident wavefield on wave-driven setup and on alongshore flows in the surfzone is investigated using observations collected onshore of a submarine canyon. Wave heights and radiation stresses at the outer edge of the surfzone (water depth ≈2.5 m) varied by up to a factor of 4 and 16, respectively, over a 450 m alongshore distance, resulting in setup variations as large as 0.1 m along the shoreline (water depth ≈0.3 m). Even with this strong alongshore variability, wave-driven setup was dominated by the cross-shore gradient of the wave radiation stress, and setup observed in the surfzone is predicted well by a one-dimensional cross-shore momentum balance. Both cross-shore radiation stress gradients and alongshore setup gradients contributed to the alongshore flows observed in the inner surfzone when alongshore gradients in offshore wave heights were large, and a simplified alongshore momentum balance suggests that the large [O(1 kg/(s2 m)] observed setup-induced pressure gradients can drive strong [O(1 m/s)] alongshore currents.Funding was provided by the Office of Naval Research and the National Science Foundation

A Systematic Review of Coastal Vulnerability Mapping

Coastal areas worldwide represent an aggregation of population and assets of growing economic, geopolitical, and sociocultural significance, yet their functions are increasingly challenged by worsening coastal hazards. Vulnerability assessments have been recognized as one way we can better understand which geographic areas and segments of society are more susceptible to adverse impacts from different stressors or hazards. The aims of this paper are to evaluate the state of coastal vulnerability assessment mapping efforts and to identify opportunities for advancement and refinement that will lead to more cohesive, impactful, and policy-relevant coastal vulnerability studies. We conducted a systematic review of the literature that addresses physical and social vulnerability to coastal hazards and contains corresponding mapping products. The content was analyzed for the scale of analysis, location, disciplinary focus, conceptual framework, metrics used, methodological approach, data sources, mapping output, and policy relevance. Results showed that most Coastal Vulnerability Mapping Assessments (CVMAs) are conducted at the local level using a range of methodologies, often with limited inclusion of social considerations and limited discussion of policy relevance. Based on our analysis, we provide seven recommendations for the advancement of this field that would improve CVMAs’ methodological rigor, policy relevance, and alignment with other vulnerability assessment paradigms

Benefiting All Users: Gender Equality and Inclusion

Women’s needs and priorities often differ from those of men, and their ability to access and use climate services is significantly more constrained. To address these disparities, and to ensure access to and benefit from these services, it is critical to use gender-informed approaches

The Case for Agricultural Climate Services in Africa

Ninety percent of the world’s farms are managed by small-scale farmers, feeding millions of people. Unfortunately, those farmers face significant impacts from climate variability and change. These impacts disrupt their ability to meet livelihoods and sustenance needs, and to produce enough food for a growing world. Climate services provide information about these impacts and aim to support agricultural decision-making for improved livelihoods, resilience, and food security

Making Climate Services Work for Africa's Farmers at Scale

The substantial body of knowledge about good practice in climate services suggests that making climate services work for farmers at a national scale requires managing tradeoffs between meeting farmers’ context-specific needs and providing cost-effective services at scale

Advancing Impact Evaluations of Agricultural Climate Services in Africa

The design and implementation of effective climate information services (CIS) requires understanding the extent to which they impact the decisions and lives of those who use them

Testing and calibrating parametric wave transformation models on natural beaches

Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Coastal Engineering 55 (2008): 224-235, doi:10.1016/j.coastaleng.2007.10.002.To provide coastal engineers and scientists with a detailed inter-comparison of widely used parametric wave transformation models, several models are tested and calibrated with extensive observations from 6 field experiments on barred and unbarred beaches. Using previously calibrated (“default”) values of a free parameter γ, all models predict the observations reasonably well (median root-mean-square wave height errors are between 10% and 20%) at all field sites. Model errors can be reduced by roughly 50% by tuning γ for each data record. No tuned or default model provides the best predictions for all data records or at all experiments. Tuned γ differ for the different models and experiments, but in all cases γ increases as the hyperbolic tangent of the deep-water wave height, Ho. Data from 2 experiments are used to estimate empirical, universal curves for γ based on Ho. Using the new parameterization, all models have similar accuracy, and usually show increased skill relative to using default γ.The Office of Naval Research and the National Science Foundation provided support

Effects of wave rollers and bottom stress on wave setup

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C02003, doi:10.1029/2006JC003549.Setup, the increase in the mean water level associated with breaking waves, observed between the shoreline and about 6-m water depth on an ocean beach is predicted well by a model that includes the effects of wave rollers and the bottom stress owing to the mean flow. Over the 90-day observational period, the measured and modeled setup are correlated (squared correlation above 0.59), and agree within about 30%. Although rollers may affect setup significantly on beaches with large amplitude (several meters high) sandbars and may be important in predicting the details of the cross-shore profile of setup, for the data discussed here, rollers have only a small effect on the amount of setup. Conversely, bottom stress (calculated using eddy viscosity and undertow formulations based on the surface dissipation, and assuming that the eddy viscosity is uniform throughout the water column) significantly affects setup predictions. Neglecting bottom stress results in underprediction of the observed setup in all water depths, with maximum underprediction near the shoreline where the observed setup is largest.Funding was provided by the Office of Naval Research, the National Science Foundation, and the Army Research Office