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Source: https://erdc-library.erdc.dren.mil/jspui/ One of the most comprehensive compilations of wave information is the hindcast wave estimates provided by the US Army Corps of Engineers' Wave Information Studies (WIS). This report compares summaries of the 1956-1975 Atlantic coast Phase III shallow-water WIS estimates with two measured data sets from the Coastal Engineering Research Center's Field Research Facility (FRF) at Duck, NC. The first consists of 5 years of energy spectra from a Waverider buoy, while the second is 1 year of directional spectra from a high-resolution linear array. Results of the study provide the engineer with a clear understanding of the differences between the WIS estimates and the FRF measurements that will help ensure the appropriate application of the WIS wave information. Following a description of the WIS estimates and the FRF measurements, wave period, height, and direction summaries are compared. The wave periods did not compare well, whereas wave heights did. The wave direction comparison is less conclusive; however, net longshore sediment transport estimates made using the WIS estimates are very close to that using the FRF measurements. An attempt is made to reconcile the differences in the wave periods with only limited success. In addition to the use of different definitions of period, it is suggested that questions about wave generation and propagation will have to be answered before close agreement can be expected. Wave height distributions agree well for waves over 2m. However, only 44 percent of the heights exceed 0.5 m for the WIS estimates, compared with 76 percent for the FRF measurements. This difference is attributed, in part, to the coastal orientation, which greatly reduces the fetch for winds blowing near shore-parallel at the gage site. Also, the Phase III hindcast method does not consider additional wind-wave growth from the input Phase II points. Both the WIS and FRF directions tend to be primarily shore-normal. A clear difference between the distributions is that the WIS estimates have many more shore-parallel low waves. This difference arises because the hindcast method allows for the transformation of wind-sea energy derived from wave conditions propagating in all directions. Longshore sediment transport rate estimates are computed to investigate the consistency between using the WIS estimates versus the FRF measurements for engineering applications. Although the gross northward and southward values differed by as much as a factor of 2, the net values are very similar. When annual gross northerly and southerly values for each of the 20 years of WIS estimates are compared with the FRF measurements, the FRF transport rates fall well within the variation of the WIS estimates. The consistency in the WIS estimates is due in part to the transport rate computation being more sensitive to wave height and direction, for which WIS estimates and FRF measurements generally agree, versus wave period for which the WIS estimates are low. |
