EVALUATION OF THE WAVE ABSORBER AT
PENTWATER, MICHIGAN
U.S. Army Corps of Engineers Engineering Research & Development Center
Vicksburg, MS, 39180
January 20, 2004
This discussion compares incident and transferred energy for gages located within
the Pentwater, Michigan boat channel. Comparisons are made primarily by examining
the differences in the incident and transferred siginificant wave height, (Hm0). Other
comparisons use spectral results. Spectral analysis allows the energy of the total wave
record to be broken down into discrete frequency bands. Energy inside and outside the
wave absorber may then be compared and a transfer factor for each discrete frequency
can be determined.
Incident, (lakeside of absorber) and transferred (harborside of absorber) wave data
were collected within the Pentwater, Michigan boat channel. Non-directional, inter-
nally recording gages, each mounted about 6 inches off the bottom were deployed
as indicated in figure 1. The incident gage is MI002 and is indentified as Northwest
Channel. This gage was deployed in 11 feet of water and was located 121 feet west of
west edge of north wave absorber. The harborside gage is MI004 and is designated as
Northeast Channel. MI004 was deployed in 8 feet of water, 72 feet east of east edge
of north wave absorber. The purpose of this data collection effort was to determine
characteristics of the wave absorber located at the western end of the channel. Wave
gages were set to collect hourly using subsurface pressure sensors, however, MI004
collected every 26-32 minutes throughout the study. The sample rate for these sensors
was 2 Hz and the burst length was 1024 seconds.
An internally recording directional wave gage was deployed in the open lake at 43
deg 47.101 N 86 deg 27.203 W in 10 meters of water. No data was received from this
gage.
Additional directional wave data was available from NDBC buoy 45007 located at
42.67 N, 87.02 W. This gage has a nominal depth of 164 meters and has a burst length
of 20 minutes with a burst interval of 1 hour. A comparison of wave heights between
offshore NDBC buoy (45007) and the incident channel gage (MI002) is depicted in
figures 7 and 8.
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