Third Chinese-German Joint Symposium on Coastal and Ocean Engineering
National Cheng Kung University, Tainan
November 8-16, 2006
A Field Study on Long Period Oscillation in Hualien Port
Yung-Fang Chiu
Center of Harbor and Marine Technology
Institute of Transportation, Taichung
yfchiu@mail.ihmt.gov.tw
Jaw-Guei Lin
Department of Harbor and River Engineering
National Taiwan Ocean University, Keelung
jglin@mail.ntou.edu.tw
Yu-Feng Lin
Department of Civil Engineering
Dahan Institute of Technology, Huaien
linyf@ms01.dahan.edu.tw
Abstract
In order to fulfill the information about long period oscillations in Hualien Port, this
study extended the sampling interval from 1200 seconds to 15000 seconds for the
synchronous wave measurements in both Yen-Liao fishery port and Hualien Port.
By using the digital filter analysis and FFT, the wave records are found, in both time
domain and frequency domain, to contain the component waves with periods
longer than 1000 seconds. Such phenomenon leads to a new possibility about the
mechanism of harbor resonance in Hualien Port.
1 Introduction
Hualien port is the only commercial harbor along the middle east of Taiwan coast.
Except for the Ho-Ping Industrial Port, the nearest commercial port is Su-Ao Port
which is around 60 Km away from the Hualien Port at north side, and at the south side,
the nearest fishery port is around 50 Km in distance. Therefore, from the point of view
of either the safety of marine transportations or nearshore recreations, Hualien port
plays an important role along the East Taiwan. Due to its long-narrow and parallel to
the coastline layout and steep bathymetry outside the port, Hualien port (see Fig. 1)
suffered from long period oscillation, especially during the typhoon intrusions, ever
since the current layout was constructed, and seriously affects the safety of anchored
ship that causes the evacuations at about 48 hours before typhoon landing alert.
1
3
9
2
4
5
8 7
6
1～9：IHMT
：current study
Fig.1 Layout of Hualien Port and wave station locations
In order to solve the long period oscillation problem in Hualien Port, a large amount
of studies was done since 1989 that includes numerical simulations, hydraulic
model tests and field measurements. Except for a conference entitled with “Hualien
Port Harbor oscillation and its improvement” had been held in 1996, some relevant
publications are Chang and Tzeng (1993 and 1995), Su and Chen (1995), Tzeng
and Chien (1996), Su et al. (1996), Chien and Chiu (1996), Chang (1996),
Tzeng(1998), Chien and Tzeng (1999), Juang (2000), Juang and Jian (2000),
Hsiao et al. (2000), Lee et al. (2001), Lee (2001), Kuo et al. (2002), Chiu et al.
(2003), Chang and Lin (2003), Chiu et al. (2003), Su (2003), and Chiu et al. (2004).
In the last paper, Chiu et al. had made a detail review of the studies between 1989
and 2003, this section only briefly discuss their achievements.
1-1 Numerical Simulations
Su et al. (1996) used nearshore wave model - WP21 to discuss the wave motions
in Hualien Port, and found the strong oscillations might occurred when the incident
waves have periods of 42, 87, 92, 118 and 155 seconds. Juang (2000) and Juang
and Jian (2000) used MIKE21-EMS to discuss the mechanism of harbor
resonance during typhoon intrusion period, and addressed that the infragravity
waves are the major causes and incident waves with periods of 100, 130, 160, 190
and 195 seconds can induce large degree of oscillations. Lee et al. (2001) studied
by mild slope equation and boundary element method, and found that the incident
waves with periods of 48, 56, 96, 116, 140 and 160 seconds may cause the long
period oscillations in the harbor.
Although all these studies indicate that Hualien Port has several oscillation modes,
but their results are different. The reasons that cause such differences should be
the wave theories, numerical methods, or the grid sizes of computational mesh
used in their studies are different. It should be pointed out that all these studies are
focus on the natural frequency of Hualien Port, i.e. they treated the long period
oscillations in the port as harbor resonance problem which are related to their
natural frequencies.
1-2 Hydraulic Model Tests
In order to improve the harbor resonance problem, the Center of Harbor and
Marine Technology (hereafter, IHMT) had conducted three major projects in 1996,
1997 and 2000. Chien et al. (1996) designed 4 new layouts by constructing jetty
outside the harbor, or extending the east breakwater, or create a new opening at
the middle of the east breakwater. Fig. 2 shows one of the layouts. Chien et al.
(1997) designed another 11 new layouts coupled with the improvement of south
shore protection and the reconstruction of east breakwater. Fig. 3 shows one of the
layouts. Chiu et al. (2000) discussed the improvements of long period oscillations
by the new construction of a coastal structure outside the harbor entrance (Layouts
1 and 2 in Fig. 4) or the extension of Hualien Fishery Port (Layout 3 in Fig. 4).
Concluding the results from these three major projects, one can find that only
limited improvement can be found in all experiments, the long period oscillations
still exist in the harbor. Chiu et al. (2004) concluded that once the major part of
harbor layout was constructed, the improvement of harbor resonance or long
period oscillation problem is highly difficult. Huge budget and time are needed, but
the achievement is limited. Any harbor problem is better to be evaluated and be
prevented at the earlier planning stage.
Fig. 2 Layout concept in IHMT 1996 project (Chien, 1996)
Fig. 3 Layout concept in IHMT 1997 project (Chien, 1997)
Layout 3
Layout 1
Layout 2
Fig. 4 Layout concepts in IHMT 2000 project (Chiu, 2000)
1-3 Field Observations
Tzeng (1996) analyzed the wave records inside/outside the harbor in 31 typhoon
period between 1900 and 1996, and found that the typhoon wave energies were
concentrated between wave periods of 11 and 15 seconds. The wind wave
energies declined very fast as propagating into the harbor, but long period wave
energies increased. Long period component wave energies have evidently
increased outside the entrance, but are concentrated at 137, 147 and 185 seconds
in outer basin, and 147 and 185 seconds in inner basin. Free long waves induced
by wave breaking or forced long wave accompanied with wave group are the
possible mechanisms of Tzeng (1996) proposed for occurrences of such long
waves in the harbor. Tzeng and Chien (1996) analyzed the field data between 1989
and 1994, and pointed out that during the Typhoon Tim in 1994 intrusion period,
the dominant wave energies are concentrated at component waves with period of
47, 82, 98, 114 and 158 seconds. Kuo et al. (2002) analyzed 5 typhoon wave
records between 1994 and 1997 and stated that the mechanism of long period
oscillations in the harbor is directly related to the low frequency portion of wave
energy outside the harbor, but not definitely related to the principal wind direction.
Chang and Lin (2003) used least square method to develop a principal component
wave period and related amplitude identification model, and showed that the
principal wave periods were 87.6 and 152 seconds in harbor and 15, 52, 80 and
130 seconds outside the harbor during the Typhoon Tim intrusion in 1994.
1-4 Discussions of Previous Studies
From the above relevant studies, one can find that the characteristics of long period
oscillations in Hualien Port had been widely discussed for many years, including
field measurements, hydraulic model tests, and numerical simulations. The
mechanism that causes such phenomenon, however, is still unclear: infragravity
waves, forced long waves, edge waves or other sources. Su (2003) physically
discussed the possible mechanism and stated that the infragravity waves with
periods lie between 80 and 90 seconds or between 130 and 160 seconds can
hardly exist in typhoon waves. Unless the regular surge, the field irregular wind
waves also have less possibility to induce regular group waves, and conceptually
inferred that edge waves are the most possible mechanism without any proof.
With one step further of discussion, due to the artificial input of incident wave
condition and simplifications of the physical boundaries, the numerical simulations
and the hydraulic model tests for harbor planning and design are actually tools only
to find the optimal harbor layout or to investigate the characteristics of some
specific layout. Field observations are actually the most suitable tool on fulfilling the
wave information around the harbor in practice.
2 Evidence of the Existence of Long Period Oscillations
In this section, the field wave records measured inside the Hualien Port between
2000/9/6~2000/10/11 measured by IHMT and some of their analyses were used to
show the existence of long period wave components. Each record was sampled 17
minutes in 2 Hz sampling rate with pressure wave gauges. The locations of wave
stations are shown in Fig. 1. During that period, from the official records of Central
Weather Bureau, Taiwan, a typhoon Bopha intruded between 2000/9/8 and
2000/9/10. Fig. 5 shows two wave records as examples, and the long period
oscillations with period more than 100 seconds can be seen in the time series.
Short period waves riding on long period waves was found in the figure, and such
Wave Profile (m)
phenomenon always causes the analysis problem that zero-up-crossing method is
not available on identifying the individual waves because of the difficulty of de-trend
process.
Time (sec)
Fig. 5 Wave record samples inside the Hualien Port (2000/9/6~2000/10/11)
Figure 6 shows the time sequences of total wave spectral energy (m0) of several
wave stations. Within two periods, from 2000/9/9 to 2000/9/17 and from 2000/9/25
to 2000/9/30, Hualien Port was subjected to large wave action. Fig. 7 shows the
time sequence of stacked occurrence ratio of component wave energy with respect
to the total wave energy of each wave record. The wave energies are summed up
in different period intervals as shown in the legend of Fig. 7 to investigate the
energy distribution in each wave record (in y-direction) and the variations in time (in
x-direction). Together with Figs. 6 and 7, we can see that during the large wave
action period, the short wave portion contains large wave energy, but in ordinary
wave action period, wave energies were concentrated at wave periods larger than
100 seconds. Due to the short waves dissipated faster than long waves, and the
filtering effect of connected channel between two basins, the waves in inner basin
contains more long wave energy than outer basin. Such phenomenon leads to a
thought that very long period waves can exist at any time, not particularly occur in
typhoon period, and a question is arose that if the long period oscillation that
affecting the ship anchorage safety problem in Hualien Port is the problem of the
accumulation of wave energy in the natural frequencies of the harbor (resonance
problem), or in very long period waves (oscillation problem)? The harbor resonance
problem can be improved by changing the harbor layout or increasing the wave
absorber in the harbor. However, if the very long period waves exists all the time
and is coming from offshore then the problem cannot be solved easily.
Considering Hualien Port is around 3000 m in length, from a simple calculation that
the wave length of first resonance mode is 4 times the harbor length (12000 m), the
related wave periods lie between 87 seconds (in deep water) and 903 seconds (in
18 m depth at harbor entrance). Such large difference is because of the fast
variation of the sea bottom around Hualien Port. This conceptual range will be used
in identifying the long period components.
m0
0.05
0.04
0.03
0.02
0.01
0.00
m0
0.05
0.04
0.03
0.02
0.01
0.00
m0
0.05
0.04
0.03
0.02
0.01
0.00
0.05
0.04
0.03
0.02
0.01
0.00
Station 2
Station 5
Station 6
Station 7
Station 8
9/7/00
9/8/00
9/9/00
9/10/00
9/11/00
9/12/00
9/13/00
9/14/00
9/15/00
9/16/00
9/17/00
9/18/00
9/19/00
9/20/00
9/21/00
9/22/00
9/23/00
9/24/00
9/25/00
9/26/00
9/27/00
9/28/00
9/29/00
9/30/00
10/1/00
10/2/00
10/3/00
10/4/00
10/5/00
10/6/00
10/7/00
10/8/00
10/9/00
10/10/00
10/11/00
m0
0.05
0.04
0.03
0.02
0.01
0.00
m0
In general, field wave observations can be done by many kinds of instrument, but
the data sampling is always sampled 20 minutes in 2 Hz sampling rate within every
2 hours, or sampled 10 minutes in 4 Hz sampling rate within every 1 hour.
Date (Year 2002)
Fig. 6 Time sequence of total wave spectral energy (m0) (2000/9/6~2000/10/11)
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.4
Period Range
0s~ 15s
15s~ 30s
30s~ 45s
45s~ 60s
60s~ 75s
75s~ 90s
90s~105s
105s~120s
120s~135s
135s~150s
165s~180s
195s~210s
255s~270s
330s~345s
510s~525s
1020s~1035s
0.5
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0
0.0
Date (Year 2000)
Station 7
09/07
09/08
09/09
09/10
09/11
09/12
09/13
09/14
09/15
09/16
09/17
09/18
09/19
09/20
09/21
09/22
09/23
09/24
09/25
09/26
09/27
09/28
09/29
09/30
10/01
10/02
10/03
10/04
10/05
10/06
10/07
10/08
10/09
10/10
10/11
10/12
Occurrence
1.0
09/07
09/08
09/09
09/10
09/11
09/12
09/13
09/14
09/15
09/16
09/17
09/18
09/19
09/20
09/21
09/22
09/23
09/24
09/25
09/26
09/27
09/28
09/29
09/30
10/01
10/02
10/03
10/04
10/05
10/06
10/07
10/08
10/09
10/10
10/11
10/12
Occurrence
Therefore, there are 2400 data points for each wave record which can be used in
wave statistics. When using the Fast Fourier transform (FFT) for spectral analysis,
2048 data points are usually employed and the longest wave period can be
resolved is 512 seconds for 2 Hz case and 256 seconds for 4 Hz case. With such
limited length of data, the very long period component waves cannot be fully
identified. So, the question stated above is not easy to answer.
Date (Year 2000)
(a) Station 2 (at outer basin)
(b) Station 7 (at inner basin)
Fig. 7 Time sequence of stacked non-dimensional wave energy (2000/9/6~2000/10/11)
3 Long Time Field Wave Observations around Hualien Port
In order to enrich the long period wave information of Hualien Port, our study using
the pressure wave gauges and extending the sampling interval from 1200 seconds
(20 minutes) to 15000 seconds (250 minutes) in 2 Hz sampling rate continuously
measured the wave motions in Hualien Port and in Yen-Liao Fishery Port for a
short period. As shown in Fig. 8, Yen-Liao Fishery Port is a simple port which
located around 19 Km south from Hualien Port. Due to steep bottom slope and
Wave station
Hualien Port
Yen-Liao Fishery Port
Fig. 8 Location of Yen-Liao Port
deep water depth outside the Hualien Port that causes the difficulty on setting a
wave station at offshore sea bottom, the measurement of Yen-Liao Port was
treated as a reference of waves outside the Hualien Port. The observations
discussed in this paper were done in April, 2006 in Hualien Port with 2 wave
stations (one at outer basin and one at inner basin, measured synchronously, see
Fig. 1), and in June, 2006 in Yen-Liao Fishery Port with 1 wave station (see Fig. 8).
Within these two periods, weak frontal lows had approached, but the sea surfaces
are still calm.
Totally, there are 11 records obtained in Hualien Port, and 7 records obtained in
Yen-Liao Port. Each record contains 30000 data points for 15000 seconds. Figs. 9
to 11 show two of the wave records in each wave station. The records in Figs. 10
and 11 are respectively measured at the same time. The long period wave motion
is not so conspicuous in Yen-Liao Port (Fig. 9), but is dominant in the inner basin of
Hualien Port (Figs. 10 and 11), wave motions with period more than 1000 seconds
exist. In inner basin, the short period waves quickly decay and make the long
period wave motions noticeable.
After the de-mean and de-trend processes to eliminate the tidal fluctuation for each
wave record, digital filter was used to extract the time series of component waves
with wave periods shorter than and equal to 30 seconds, between 20 and 200
seconds, and longer than 200 seconds from the original. Fast Fourier Transform
was also involved to find their power spectrum. In order to compare the variation of
wave energies between different wave groups, the wave period interval was
divided into four parts: (A) below 30 seconds; (B) between 30 and 200 seconds; (C)
between 200 and 2000 seconds; and (D) above 2000 seconds. The results are
discussed as follow.
η (m)
3
2
1
0
0
5000
10000
15000
Time (sec)
(a) Record 1
4
η (m)
3
2
1
0
0
5000
10000
Time (sec)
(b) Record 2
Fig. 9 Wave record measured in Yen-Liao Fishery Port in June, 2006
15000
η (m)
14
13.8
13.6
13.4
0
5000
10000
15000
10000
15000
Time (sec)
(a) Record 1
η (m)
14.8
14.6
14.4
14.2
0
5000
Time (sec)
(b) Record 2
Fig. 10 Wave records measured in outer basin of Hualien Port in April, 2006
η (m)
8.6
8.4
8.2
8
0
5000
10000
15000
Time (sec)
(a) Record 1
η (m)
8.8
8.6
8.4
8.2
0
5000
10000
15000
Time (sec)
(b) Record 2
Fig. 11 Wave records measured in inner basin of Hualien Port in April, 2006
3-1 Waves in Yen-Liao Fishery Port
Figure 12 shows the results related to the wave records shown in Fig. 9. Fig. 12(a)
shows two original and related filtered component wave time series, Fig. 12(b)
shows the spectra, and Fig. 12(c) shows the distributions of non-dimensional
accumulated spectral energy. In order to investigate the variations of wave energy
in wave periods, instead of wave frequencies, the x-axes in Figs. 12(b) and 12(c)
are all expressed by wave period. In Fig. 12(a), after the isolations by digital filtering,
the long period wave motions for periods longer than 200 seconds can be found in
the time series. Although the water fluctuation is small (below 10 cm), but their
existences are affirmative. For the time series of wave periods longer than 200
seconds, we may also found a single oscillation act as carrier wave that almost
covers the whole time interval with around one cycle. Considering the period of
semidiurnal tide is around 44700 seconds, this component wave should not be
tide-related. All these evidences show that the wave motions in Yen-Liao Port does
contain very long period component waves. Fig. 12(b) show that the spectral
densities of long period component sometime will higher than the gravity waves.
The spectral densities in interval B are smaller than in interval A in general, the
difference is around O(10) to O(102); the spectral densities in interval C lies
between intervals A and B; and the spectral densities in interval D have same level
as in interval A. The long period wave components of wave period longer than 1000
seconds are dominant in our measurement. The dominant periods above 200
seconds are 500, 600, 800, 900, 2500, 3750, 5000, and 7500 seconds. The
spectral peaks between 30 and 200 seconds can hardly be identified. From Fig.
12(c), the component waves in interval A (gravity waves) contains the largest
portion of spectral wave energy, and interval D takes the most of the rest, only
small portion of wave energy is contained in intervals B and C.
In general spectral analysis by FFT, the low frequencies portion with frequency
nears to two frequency units ( 2Δf , where Δf = 1 NΔt and Δt is sampling time) are
always be treated as numerical error and be neglected. According to such concept,
in our measurement, the sampling interval is 15000 seconds, and the sampling rate
is 2 Hz, the frequency unit ( Δf ) should be 0.000067 Hz (= 1 15000 Hz). The longest
resolvable period is 1 2Δf = 7500 seconds and is suggested not to put into
discussions. From the time series in Fig. 12(a), however, these components do
definitely exist and should be taken into account.
As mention before, Yen-Liao Fishery Port is a simple and small fishery port, the
harbor layout has ‘F’ form and facing to the south. Due to it is fully connected to the
open sea, its long period wave components can be assured to be affected by the
offshore waves, and should have directly relation to the long period wave
components outside the Hualien Port.
2
2
1
η (m)
η (m)
1
0
-2
0
-1
-1
De-Mean & De-Trend
0
5000
10000
-2
15000
Time (sec)
0.05
De-Mean & De-Trend
0
5000
10000
15000
Time (sec)
0.04
η (m)
η (m)
0.02
0
0
-0.02
>200s
-0.05
0
5000
10000
-0.04
15000
Time (sec)
0.1
η (m)
η (m)
-0.05
15000
0
5000
10000
0
-0.05
200s~31s
15000
-0.1
Time (sec)
2
200s~31s
0
5000
η (m)
0
-1
10000
2
0
<31s
<31s
0
5000
10000
15000
Time (sec)
4
1
η (m)
10000
Time (sec)
0.05
0
-2
5000
0.1
0.05
-0.1
>200s
0
15000
-2
0
5000
10000
Time (sec)
Time (sec)
(a) Time series of wave profile
15000
10
10
10
sdf
10
10
10
10
10
10
10
2
10
1
10
0
10
-1
10
-2
10
-3
sdf
10
-4
10
-5
10
-6
10
-7
10
-8
10
10
0
10
1
2
10
Period (sec)
10
3
10
4
10
2
1
0
-1
-2
-3
-4
-5
-6
-7
-8
10
0
10
1
2
10
Period (sec)
10
3
10
4
(b) Power spectrum
100
Energy Ratio (%)
Energy Ratio (%)
100
80
60
40
20
0
10
100
1000
Period (sec)
10000
80
60
40
20
0
10
100
1000
10000
Period (sec)
(c) Non-dimensional accumulated wave energy distributions
Fig. 12 Analyzed results for wave records shown in Fig. 9 (measured in
Yen-Liao Fishery Port in June, 2006)
3-2 Waves in Hualien Port
Figures 13 and 14 show the results related to the wave records shown in Figs. 10
and 11. In each figure, Fig. (a) shows two original and related filtered component
wave time series; Fig. (b) shows the spectra; Fig. (c) shows the distributions of
non-dimensional accumulated spectral energy; and Fig. (d) shows the distributions
of dimensional accumulated spectral energy.
In Fig. 13(a), the wave motions for periods longer than 200 seconds are found in
outer basin of Hualien Port with small water fluctuation (below 4 cm). Fig. 13(b)
shows that long period components near 7500 seconds are still exist, and more
peaks occur between 200 and 2000 seconds. The spectral densities in interval B
are still smaller than in interval A with difference of O(10), but is increased in
interval C to the order of in interval A; spectral densities in interval D are still
dominant but smaller than in interval C. The dominant periods above 200 seconds
are 700, 900, 1200, 1500, 2500, 3750, 5000, and 7500 seconds. The spectral
peaks between 30 and 200 seconds can hardly be identified. From Fig. 13(c), the
wave energies are still concentrated in intervals A and D, intervals B and C only
contain a small portion of wave energy. Such tendencies also appear in the results
0.2
0.2
η (m)
0.1
0.1
η (m)
0
-0.1
-0.2
0
5000
10000
-0.2
15000
Time (sec)
0.1
0
-0.1
De-Mean & De-Trend
De-Mean & De-Trend
0
5000
10000
15000
Time (sec)
0.02
0
-0.1
0
η (m)
η (m)
0.05
-0.05
-0.02
>200s
0
5000
10000
>200s
-0.04
15000
Time (sec)
0.15
0
5000
10000
15000
Time (sec)
0.05
η (m)
η (m)
0.1
0.05
0
-0.05
0
200s~31s
0
5000
10000
200s~31s
-0.05
15000
Time (sec)
0.2
10000
15000
Time (sec)
0.1
η (m)
η (m)
5000
0.2
0.1
0
-0.1
-0.2
0
<31s
0
5000
10000
0
-0.1
-0.2
15000
<31s
0
5000
Time (sec)
10000
15000
Time (sec)
(a) Time series of wave profile
10
10
10
sdf
10
10
10
10
10
10
10
2
10
1
10
0
10
-1
10
-2
10
-3
sdf
10
-4
10
-5
10
-6
10
-7
10
-8
10
10
0
10
1
2
10
Period (sec)
10
3
10
4
10
2
1
0
-1
-2
-3
-4
-5
-6
-7
-8
10
0
10
1
2
10
Period (sec)
10
3
10
4
(b) Power spectrum
100
Energy Ratio (%)
Energy Ratio (%)
100
80
60
40
20
0
10
100
Period (sec)
1000
10000
80
60
40
20
0
10
100
1000
Period (sec)
(c) Non-dimensional accumulated wave energy distributions
10000
-2
-2
10
Spectral Energy
Spectral Energy
10
-3
10
-4
10
-5
10
-3
10
-4
10
-5
10
-6
-6
10
10
100
1000
10
10000
10
Period (sec)
100
1000
10000
Period (sec)
(d) Dimensional accumulated wave energy distributions
Fig. 13 Analyzed results for wave records shown in Fig. 10(a) (measured in outer
basin of Hualien Port in June, 2006)
η (m)
0.1
0.05
η (m)
0.1
0.05
0
-0.05
-0.1
-0.1
0
5000
10000
15000
Time (sec)
0.1
η (m)
η (m)
0
0
5000
10000
>200s
0
5000
0.05
0
10000
15000
Time (sec)
0.04
η (m)
η (m)
15000
0
-0.04
15000
Time (sec)
0.1
0.02
0
200s~31s
200s~31s
0
5000
10000
-0.02
15000
Time (sec)
0.1
0
5000
10000
15000
Time (sec)
0.1
0.05
η (m)
0.05
η (m)
10000
Time (sec)
-0.02
>200s
0.15
0
-0.05
-0.1
5000
0.02
-0.05
-0.05
De-Mean & De-Trend
0
0.04
0.05
-0.1
0
-0.05
De-Mean & De-Trend
0
5000
10000
0
-0.05
<31s
-0.1
15000
<31s
0
5000
10000
15000
Time (sec)
Time (sec)
(a) Time series of wave profile
2
10
10
1
10
10
0
10
10
-1
10
10
-2
10
-3
10
sdf
sdf
10
-4
10
10
-5
10
10
-6
10
10
-7
10
10
-8
10
10
10
0
1
10
2
10
Period (sec)
10
3
4
10
10
2
1
0
-1
-2
-3
-4
-5
-6
-7
-8
10
0
(b) Power spectrum
10
1
2
10
Period (sec)
10
3
10
4
100
Energy Ratio (%)
Energy Ratio (%)
100
80
60
40
20
0
10
100
1000
80
60
40
20
0
10000
10
Period (sec)
100
1000
10000
Period (sec)
(c) Non-dimensional accumulated wave energy distributions
-2
-2
10
Spectral Energy
Spectral Energy
10
-3
10
-4
10
-5
10
-4
10
-5
10
-6
-6
10
-3
10
10
100
1000
10000
10
10
Period (sec)
100
1000
10000
Period (sec)
(d) Dimensional accumulated wave energy distributions
Fig. 14 Analyzed results for wave records shown in Fig. 11(a) (measured
in inner basin of Hualien Port in June, 2006)
in inner basin in Fig. 14. The dominant periods above 100 seconds in inner basin
are 170, 200, 280, 700, 1200, 3750, and 7500 seconds.
As mentioned above, two wave records and their analyses of outer basin in Fig. 10
and of inner basin in Fig. 11 were measured synchronously. Comparing the results
of record 1 (left column) in Figs. 13 and 14, the total energies in inner basin and
outer basin seem to be equal, the energies for short periods declined in inner basin
but long period wave (more than 1000 seconds) energies increased rapidly. The
results of record 2 (right column) in Figs. 13 and 14 show that short period waves in
outer basin contain more energy than that in inner basin. On the contrary, long
period waves in inner basin contain more energy than that in outer basin. The wave
energies for wave periods lie between 20 and 1000 seconds between outer and
inner basins have very little difference.
4 Results and Conclusions
From the above discussions, some conclusions are made:
1.
The extension of sampling interval of field wave observation is much helpful on
collecting the full information of wave motions. Very long period wave
components can be found around the harbor, which are always neglected in
traditional observations. The extension is suggested.
2.
Long period waves exist not only inside the Hualien port, but also in Yen-Liao
Fishery Port. Large wave energies appear in wave periods more than 100
seconds, especially in wave periods more than 1000 seconds. However, since
the data only collected in two days, the characteristics of long period waves
can not be fully described. Further long term and large area observations are
suggested for future studies.
3.
Due to the long period component waves found in this study might not be the
harbor scale or the tidal scale, the long period oscillations in Hualien Port exist
not only in typhoon intrusion period, but also in ordinary days. Therefore, the
mechanism of oscillations, caused by natural periods of the harbor, or by the
intrusions of offshore long waves, should be clarified before discussing the
improvements of the oscillations.
5 Acknowledgements
This research was financially supported by the Institute of Transportation, Minister
of Communication and Transportation, Taiwan under the project entitled “A Study
on Harbor Resonance of Hwa-Lien Harbor (I) & (II)” with project numbers of
MOTC-IOT-94-H3DB003 and MOTC-IOT-95-H3DB002, and was done with the
cooperation of Center of Harbor and Marine Technology and Hualien Port Bureau.
The authors wish to express their sincere appreciation to all of them.
6 References
Lin, J.G., Y.F. Lin and Y.P. Tai. Water Level Observations inside the Keelung and
Hwa-Lien Harbors, Proceedings of the 27th Ocean Engineering Conference in
Taiwan, pp. 949-956, 2005.
Lin, J.G. and H.S. Chen. The Effects of Offshore Bottom Slope on the Numerical
Simulation Wave Behavior Around Harbor, Proceedings of the 18th Ocean
Engineering Conference in Taiwan, Republic of China, pp. 55-62, 1996.
Chiu, Y.F., J.G. Lin and C.C. Chien. A Discussion on Experimental Studies of Long
Period Oscillations in Hwa-Lien Harbor, Journal of Coastal and Ocean Engineering,
Vol. 4, No. 1, pp. 89-121, 2004.
Tai, Y.P.. The Influence of Tidal Station Allocation on Harbor’s Tidal Data and
Bathymetry Data, Master Thesis, Department of Harbor and River Engineering,
National Taiwan Ocean University, 99p, 2005.