M.W.L. Fluctuations Inside a Cooling Water Tank Set Inside a Coral Reef Julio Monroy Department of Civil Engineering, Kagoshima University -2-4 Korimoto, Kagoshima-shi, Japan 89-65 Ryuchiro Nishi, Michio Sato, Kazuo Nakamura Department of Ocean Civil Engineering, Kagoshima University Sadao Fujihata, Kouji Tsunoue Civil Engineering Section, Kagoshima Branch Office, Kyushu Power Generation Co. Ltd., 89-65, Kagoshima-shi, Japan ABSTRACT This paper is part of a study on sustainable cooling water supply from a coral reef shore to a power plant during a storm. Typhoon waves break at the entrance of the China Port (-m D.L.), Okinoerabu, and cause a significant mean water level fluctuation inside the port. The lowering of the water level affects the intake of cooling water for a power plant. Wave data taken at the entrance of the harbor and mean water level data taken inside the intake tank are decomposed into infragravity wave and wind wave component. It can be observed that the infragravity wave component, which is not normally considered in the cooling water tank design, is significantly large during a typhoon and possibly affects for one half of the surf beat period. KEY WORDS: coral reef, storm waves, infragravity waves, power plant, cooling water, field observation. INTRODUCTION Severe typhoon waves breaking at a reef edge, and propagating over a reef flat, present a highly nonlinear wave transformation problem. Coral reef grows up to about mean low tide level, therefore at low tides waves will break over the reef edge and no significant wave energy propagates inshore. However, waves are able to pass across the reef flat and break on the beach at high tide. During the occurrence of storms, the breaking of severe typhoon waves at or over the reef edge generates low frequency fluctuations of the mean water level across the reef flat. Nakaza and Hino (99) analyzed wave data from the coral reef zones of the Okinawa Island in Japan and showed that long-period bore-like waves have a major role on those fluctuations. Gerritsen (98) showed, based on field measurements on Ala Moana reef in Honolulu, Hawaii, that wave set up on the reef is modulated by a long wave component. The transformation of short period waves on a coral reef differs from that in normal sloping beaches due to the steepness of the front face. This fact was proved experimentally by Gourlay (994) and by field experiment conducted by Massel (994). For a shallow reef, waves break on or past the reef edge and propagate over the shallow reef flat as bore-like structures during low tide conditions. The bores lose energy due to bottom friction and breaking and reform as oscillatory waves. The waves breaking on coral reef are bound to produce long-period oscillations that are different from those of waves breaking on a normal beach slope due to this distinct breaking mechanism,. Karunarathna and Tanimoto (996) developed a numerical model for the calculation of the long period fluctuations on a horizontal shelf with steep seaward face and found that: when short-period waves break on the reef face, the damping of them produces small long-period waves. However, there is no theory available to predict the wave heights and water level fluctuations over the reef flat for the case of intensified storm waves. Furthermore, there has not been much research on the low-frequency fluctuation during storm wave conditions for harbors set in this kind of reef configuration. In this paper, the result of wave observations carried out at the China Port in Okinoerabu Island, Kagoshima, Japan is presented. The main aim of the study is to understand the phenomena of low-frequency fluctuation of the mean water level during storms conditions. High amplitude waves break at the entrance of the port (on the lowered reef edge) and propagate inside the port causing irregular low frequency water oscillations. It is of special interest to understand the causes of the exceptional low mean water levels at the reef flat where a cooling water intake for a power plant is located. The intake is connected through a pipe to a water supply tank from where the water is pumped to the generators. The water level in this tank is monitored every second. If the water level inside the cooling water tank goes bellow a lower limit, the power plant operation should be terminated. The mean water level fluctuation in a coral reef is generally larger during a storm. In fact, the water level inside the cooling water tank tends to reach a warning level at least once a year during storms. Therefore, wave and mean water level observations were necessary to study the character of the low-frequency mean water level fluctuation over the coral reef, and to design an appropriate cooling water tank system. This paper mainly shows the results of the observations during 2 and 2, conducted in China Port, Kagoshima Prefecture, Japan. FIELD OBSERVATION Study Area China port is located at the south of Okinoerabu Island in Kagoshima Prefecture, Japan. The study area is surrounded by a fringing reef that 22-TA-5 Monroy Page of 5
has a reef flat extending for nearly 5m from the shore. The port was constructed by dredging the coral reef. The depth at the entrance of the port is nearly m(m.w.l). The navigational channel over the reef is 65m wide. Fig. depicts the bathymetry just in front of the cooling tank system. Fig 2. is a top view of the study area. In Fig. 2 the waves are breaking on the reef edge around the port entrance of the reef. As it can be seen in the photograph, the waves pass over the entrance of the port without breaking during normal conditions. On the other hand, high amplitude waves break at the entrance of the port causing long period oscillations inside the port under storm conditions. Depth(m - -2-3 -4-5 the year 2 was taken from September 27 th until October 4 th. Unfortunately the weather conditions changed and the forecasted storm did not occur, consequently the data for this run termed TK2 (for the tank data) and WH (for the incident wave data) are for normal wave conditions. Year 2. Measurements were conducted for a longer period in order to obtain at least one storm event. Measurements were carried out from October 5, until December 7. Data Acquisition Setup Data was collected at the entrance of the port and in the intake tank. A pressure gage ( Wave Hunter 94) was installed on the bottom at the entrance of the harbor (Fig. 2). The first set of data collected by the Wave Hunter gage is termed WH. This data was taken from 3:, September 26 th, 2 to 3:4 October 4 th, 2 with a sampling rate of.5 seconds. The total interval is 92 hours and 4 minutes. The second data set termed WH2 was collected from 2: October 5 until 4:5 December 6, 2. The data was sampled in 2 minute bursts every 2 hours with a sampling rate of.5 seconds to make the period of data acquisition longer in order to sample at least one storm event. The sampling rate was set to be.5 seconds. -6 75 5 225 3 375 45 525 6 Distance from the shore 675 75 825 9 975 5 25 2 275 65 Fig. Reef configuration at the entrance of China Port Fig. 3, Schematic diagram of the intake pipe and water tank. Fig. 2 Aerial photograph of the study area at China port. Data Acquisition Schedule. The data acquisition was scheduled to obtain a storm wave data during the typhoon seasons of 2 and 2. Year 2. The first trial was accomplished from August 24 th until August 3 th. Unfortunately failure of the equipment did not allow to obtain the incident wave data for this run and only the data for the water level inside the intake tank (TK) is available. The second data set for The arrangement of the intake head and tank is shown in Fig. 3. The mean water level (h) at the intake head is nearly 3m. The water level inside the intake tank is obtained by an ultrasonic sensor. This analog data is digitazed with an AD converter. The first set of data, termed TK was taken from 4: August 24 th to 23: August 3 th, 2 with a sampling rate of second. Storm data were measured from August 27 to August 29. The second set of data in the tank, termed TK2 was taken from September 27 until October the 5 th 2 with a sampling rate of.5s. No storm event was observed during this period. Table summarizes the information for all the data sets. DATA ANALYSIS The data was analyzed by using the wave by wave method and the spectral method. First a 3point moving average filter was used to separate the tide component from original water elevation data. A low pass windowed-sinc filter (Smith 997) was used to separate the data in the infragravity frequencies (.3Hz-.3 Hz) from the gravity waves frequencies. The data was divided in 2 minutes segments for which the characteristic wave height and period were calculated. A power spectrum was calculated for each 2 minutes segment. 22-TA-5 Monroy Page 2 of 5
Table. Conditions for the different data sets. Data Set Label Storm Measuring Starts Measuring Ends Measuring interval TK Storm 8/24/ 8/3/ Continuous. TK2 No Storm 9/27/ /4/ Continuous.5 TK3 Storm /5/ //2 Continuous.5 WH No Storm 9/26/ /4/ Continuous.5 WH2 Storm /5/ 2/6/ 2min. every 2 hrs RESULTS Sampling rate(s) Water Level Inside the Tank The water fluctuations inside the tank are dominated by the infragravity oscillations. Fig. 4 shows a one hour record of the surface elevation for both normal and storm conditions. It is observed that only the long period oscillations are of significance. The significant wave period is shown in Fig. 5. No explicit correlation is observed between infragravity and gravity components. The infragravity wave period is in the range of 8 to 22 seconds. Surface elevation (m).4.2. -.2 -.4. 8. 8.2 8.4 8.6 8.8 8..5. -.5 -. Storm conditions Normalconditions 3. 3.2 3.4 3.6 3.8 3. Fig. 4 One hour plot of the surface elevation for the tank Gravity component Period(s) 6 5 4 3 2 TK T /3 Infragravity Gravity 5 5 Fig. 5 Significant wave period for the Tank (TK)..5 22 2 8 6 4 2 8 6 4 2 Infragravity compontne Period(s) The amplitude of the infragravity fluctuations inside the tank during storm conditions is intensified. Fig. 6 shows the significant wave height for the TK data. The wave height of the infragravity waves can reach more than one meter. The power spectra for the TK data during normal and storm conditions are shown in Fig. 7. The resemblance of both spectra is noticeable. The peak energy is observed at.6 Hz (66.7 s). This plot suggest that the power at all frequencies is amplified during storm conditions and that energy peaks are amplified more than 4 times. Wave Height (m).2..8.6.4.2. Infragravity wave Gravity wave Inside the tank. 5 5 Fig. 6 Time series of significant wave height ( ) for gravity and infragravity components inside the tank. Power (m 2 s).. E-3 E-4 E-5 E-6 E-7 E-8 E-9 Storm conditions No storm conditions E-3.. Frequency (Hz) Fig. 7 Spectrum for the tank data for normal and storm conditions. The distribution of significant wave heights versus period is shown in Fig. 8. The solid lines represent upper limit for the fluctuation heights inside the tank. The gravity wave upper limit is controlled by the diameter of the intake pipe. The upper limit for the infragravity component is nearly.2 meter. This limit is produced by frequencies much greater than the resonant frequencies of the tank and is not normally considered in the cooling water tank design. 22-TA-5 Monroy Page 3 of 5
WH Gravity. (m ). Infragravity E-3 Fig. 8 Distribution of and T /3 for TK. Incident Wave Conditions Normal Conditions. The incident wave data (WH) were obtained at the entrance of the harbor. Fig. 9 shows a typical example of the surface elevation data where the wave groupiness can be clearly seen. Fig. shows a time series of the significant wave heights ( ) of the gravity and infragravity components. The y-axis is in logarithm scale to compare the wave heights. The gravity wave component is at least one order of magnitude bigger than the infragravity component. No good correlation was observed between the short period waves and the infragravity waves during normal conditions. The infragravity component ( ) is plot versus of the gravity wave component in Fig.. The solid line is a linear fit with equation: Y=-5.39+.3*X. The correlation factor is.43. A power spectrum for the whole data set is shown in Fig. 2. There is a clear energy peak in the gravity wave portion of the spectrum at.8 Hz (T=2.5s). There a peak in the infragravity range at.57 Hz (T=75s). There an additional peak at.2 Hz (T=5s). The spectrum for the tank during normal (no-storm) conditions has also been plot in Fig. 2 for comparison. Interestingly the energy peak for the ingragravity range of the spectrum is at the same frequency (.57Hz) for the tank and incident wave under normal (no-storm) condition. The peak for the tank is 25% bigger than for incident waves. 5 5 2 Fig., Time series of for gravity and infragravity components for incident wave under normal conditions (WH). Fig. Correlation between of Infragravity vs. gravity component during normal conditions... E-3 Incident no-storm Power (m 2 s) E-4 E-5 E-6 E-7 E-8 E-9 Tank no-storm E- E-3.. Frequency (Hz) Fig. 2 Spectrum for Incident wave conditions and tank during nostorm conditions. Fig. 9 Plot of time series of surface elevation for incident wave during normal conditions. Storm Conditions. For the storm conditions significant waves heights greater than 4 meters where obtained. Fig. 3 shows the time series of the significant wave height for both the gravity and infragravity component. The y-axis is in logarithm scale for a clear view of the relationship between the gravity wave height and the infragravity wave 22-TA-5 Monroy Page 4 of 5
heights at the entrance of the port during storm conditions. The significant height of the infragravity component is plot versus the one of the gravity wave component in Fig. 4. A good correlation between the short period and long period wave heights can be seen. The solid line is the linear fit: Y=-.2+.8X with a correlation factor of.95. According to this plot the infragravity wave height is 8.5% of the gravity component at the entrance where the waves tend to break during a storm. The fraction is expected to increase inshore. (m).. WH2 Gravity Infagravity 2 4 6 8 2 4 Fig. 3 Time series of significant wave heights( ) for incident wave during storm conditions (WH2). H /3 Infragravity com ponent.4.3.2.. WH2..5..5 2. 2.5 3. 3.5 4. 4.5 Gravity wave component Fig. 4 of Infragravity wave component vs. H/3 of gravity wave component for incident wave during storm conditions (WH2). Power (m 2 s).. E-3 E-4 E-5 E-6 S pectrum for Incident w ave (W H ) Storm conditions Normalconditions E-3.. Fig. 5 Power spectra for incident wave. Frequency (H z) The power spectrum of the incident waves is shown in Fig. 5. The top spectrum corresponds to the average of 5 individual spectra corresponding to storm conditions during the pass of a Typhoon. The lower spectrum is an average spectrum for the WH data (normal conditions). For the normal conditions the energy peak of the infragravity component is at a frequency of.6 Hz (T=66 s). For the storm conditions the peak is at.7hz (T=59s). The gravity wave section of the spectrum is shifted to higher frequencies during storm conditions. CONCLUSIONS The data analyzed in this study have shown that large fluctuations of the water level inside the cooling water intake tank set in a coral reef shore are caused by infragravity waves. The infragravity waves are predominant in the power spectrum for the tank data since the upper limit for the gravity waves is regulated by the diameter of the intake pipe. Therefore, the infragravity waves should be considered in the design of a cooling intake tank set in this kind of coral reef area. During normal (calm) conditions the water level inside the tank experiences long period fluctuations for which the energy peaks of the power spectrum are at the same frequency as the peak infragravity wave energy for the incident waves. The incident wave heights for the gravity and infragravity components showed good correlation during storms. The infragravity wave energy inside the tank is amplified during a storm. An specific correlation between the incident waves and the infragravity fluctuations inside the tank can not be given at this time because the data for the tank during storm conditions (TK3) that was measured at the same time as the incident wave data WH2 is not available at the time this report is being written. The analysis of these data will give us more knowledge of the relationship between the incident wave action and the infragravity fluctuations inside the tank during a storm. ACKNOWLEDGEMENTS Mr. Asazaki and Mr. Yamauchi at Kyushu Power Generation Company offered general advice and comments. Mr. Kubo from Kubo Construction Company provided support with the installation of the wave gages. Mr. Yamamoto helped with the gage installation as a diver. REFERENCES Gerritsen, F., (98). Wave attenuation and wave set-up on a coastal reef. In: Proc. 7 th Coastal Eng. Conf., Sydney, 98. Am. Soc. Civ. Eng., New York, Vol., pp. 444-46. Gourlay, M. R.,(994) Wave transformation on a coral reef. Coastal Eng., Vol 23, pp 7-42. Karunarathna, H., and Tanimoto, K (996). Long-period water surface fluctuations on a horizontal coastal shelf with a steep seaward face, Coastal Eng, Vol 29, pp 23-47. Massel, S. R. (994). Measurements and modeling of waves incident on steep islands or shoals. In: Int. Symp. Waves. IAHR, Univ. of British Columbia, pp. 982-99. Nakaza, E. and Hino, M., (99). Bore-like surf beat on reef coasts. In: Proc. 22 nd Int. Conf. Coastal Eng., pp. 743-756. Smith, S.W.(997), The Scientist and Engineer s Guide to Digital Signal Processing, California Technical Publishing, pp 285-296. 22-TA-5 Monroy Page 5 of 5
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