An Examination of Breakwater Performance at Burns Harbor, Indiana [report]

James P. McKinney, Margaret A. Sabol
2003 unpublished
This discussion compares incident and transferred spectral results for gages located at Burns Harbor, Indiana. Comparisons are made by examining the differences in the incident and transferred harbor energy spectrums. Spectral analysis allows the energy of the total wave record to be broken down into discrete frequency bands. Energy inside and outside the harbors may then be compared and a transfer factor for each discrete frequency can be determined. Incident and harbor wave data were
more » ... within and offshore of the breakwater at Burns Harbor, Indiana. The purpose of this data collection effort was to determine characteristics of the rubble mound breakwater located there. Wave records were collected hourly using subsurface pressure sensors. The sample rate for these sensors was 1 H z and the burst length was 2048 seconds. The analysis utilized the Welch, [1], spectral analysis method with 50% overlapping segments. Since the raw time series were obtained using sub-surface systems, a depth determined high frequency cutoff was applied. The averaged co-and quad-spectra from each analyzed record were used to calculate significant wave height (H m0 ), peak period (T p ), and mean wave direction at T p (D p ) and energy spectrums. The time period 6 January 2003 0700 thru 7 January 2003 0 was selected for detailed analyses. Figure 1 , top plot, shows six successive hourly energy spectrums for IN001, located in the open lake. The lake is relatively flat at 0700 with H m0 0.53 m and T p about 4.3 seconds. Over the next 5 hours the H m0 builds to 1.86 m and the T p moves to 6.6 seconds. The energy at the long period end of the spectrum is very small. Examining simultaneous analysis results from within the harbor provides a description of the breakwater's performance. Figure 1 , bottom plot, shows spectrums for the same six hours at the inside gage, IN002. There is almost no energy at 0700 GMT 1/6/03, with H m0 0.05m and T p 4.5 sec. In subsequent hours, bimodal spectrums develop as more and more energy is transferred through the breakwater, with peaks >20 seconds and between 6 and 10 seconds. The magnitudes of these peaks are small when compared to energy for those frequencies outside the harbor. To provide a more direct comparison of incident and transferred energy, a transfer coefficient (xfer) can be calculated by dividing the transferred energy at each frequency by the corresponding outside energy, eqn.1.
doi:10.21236/ada459002 fatcat:t57rxfp52reihmekn24r56yk7a