Amir Etemad-Shahidi
Dept. of Civil Eng., Iran Uni.of Sci. & Tech.
Tehran, Iran, P.O.Box 16675-163
E-mail:etemad@sun.iust.ac.ir
also JWRC Co.
Abstract: Simultaneous measurements of wave, current and tide has always been of interest in field studies. The performance of the instruments and their predeployment, deployment and data retrieval and processing are the main concerns for the field engineers. This paper attempts to discuss about the abovementioned issues during a field study in port of Bushehr, Persian gulf. The instrument used was a S4ADW electromagnetic current meter deployed on the sea floor in a depth of about 18 m. The instrument was set up to collect directional wave and tidal data simultaneously but with different sampling frequencies for a period of one month during a moderate wind condition in summer. The collected wave data was processed by using Wave for windows software on a Pentium 433 Hz PC. The results and the experience obtained during this field trip are presented and the advantage and disadvantages of the S4ADW both from theoretical and practical points of view are discussed. It will be shown that this instrument and the accompanying softwares are very user friendly and can be used easily in the field.
Keywords: wave
measurement, S4, field study, directional wave and current, bushehr
Coastal and ocean engineers are concerned with waves and currents because of their effects on natural phenomenon, on operation and works of men and etc. The engineers need to have reliable information about sea condition in the design and operational phase of marine and coastal projects such as ports and offshore structures. The most important factors are often the waves and currents. To simulate and analysis of structures and their effects on the environment, usually simultaneous wave and tidal data is required. These information are used as the boundary condition in numeric and physical models to predict and compare different proposed scenarios. In this way the managers can choose the best solution. The collection of the abovementioned data needs a field study. One of the major concerns in the field study is the instrument used. In brief, it should be user-friendly and cost-effective in all phases of the campaign i.e. predeployment, deployment and post deployment.
The wave data can be nondirectional or directional which the latter is more required nowadays and is more expensive, probably by a factor of five [WMO, 1989]. Instruments which are deployed below the sea surface are equipped with pressure sensors which measure the pressure variation due to the passage of waves. Since the wave induced pressure attenuates with depth, the first order wave theory is used to estimate the attenuation rate and the pressure variations are modified based on the wave length. Simultaneous surface and sea bed measurements of Draper [1957] showed that the first order wave theory is valid specially where the attenuation is large (deep water deployment of the sensor) and in the worst case the error encountered in calculating wave height is about -15% where the attenuation is small (shallow deployment). This is not a disadvantage for this method since mounting the instrument at the bottom is easier and safer, specially during hurricanes [Taylor and Trageser, 1990]. These instruments, if equipped with velocity sensors, can measure the direction of the waves as well. A good example of this kind of instruments is S4 family made by Interocean systems. In the following parts of this paper S4 is briefly described and its advantages is mentioned. Then the study area and the motivation of the campaign is given. Finally the obtained results and experiences are presented and discussed.
The S4ADW is a solid state electromagnetic current meter equipped with a piezoelectric pressure sensor. It is a sphere with 11 kg weight, 25 cm diameter and no protruding sensor (Figure 1). Therefore is it very easy to deploy in the field. Directions of the waves are measured using the current speed and direction record to determine the orbital velocity components of particular frequency bands. Combining that information with wave height as measured by the pressure sensor with automatic depth attenuation correction will result in directional wave data. For a more detailed description, reader can refer to Lawson et al [1983]. S4ADW has 20 MB (7 million samples) non volatile memory. Its sampling frequency is 2 Hz and can be used in depth up to 70 meters. In addition to wave and tidal data, it can optionally measure, temperature, conductivity, turbidity, PH, D.O. and OBS.
The port f Bushher is one of the main ports of Iran in the Persian gulf (Figure 2). It is located in the north west of the gulf at 50° 50¢ E and 28° 59¢ N. The tidal variation in the area is about 2 m and mostly diurnal. Wind direction and speed varies in time and is mainly moderate SW monsoon in mid summer [Niyati and Maraghei, 1995]. The approach channel for the port is dredged to –10m and therefore. Ships up to 20,000 tons with a draft of 9 m can berth in this port (Figure 3). The sea bottom is a large muddy flat area consisting of fine mud with grain sizes from 5 to 20 microns (Niyati and Maraghei). Both inner and outer channels suffer from siltation and on average 1.2 million m3 material is dredged annually to maintain the approach channel depth. Port authorities have decided to apply some remedy measures to reduce the dredging costs. Delft hydraulics as the supervisor of the projects, has proposed an extensive field campaign to study the siltaion regime of the port. The S4 was one of the instruments used which its data will be later used for calibration and validation of the numerical models.
Two different sampling strategies were used in this study. For directional wave analysis, the instrument was setup to sample the velocities and pressure at 2 Hz for 20 minutes every one hour. For the tidal data, the instrument recorded one minute averages of the half second sample rate for every 15 minutes. On the shore, with these sampling strategies, the battery life and memory capacity were estimated to be about 2 months. The instrument was set to start data collection at 2400 hours on 14 June, 2000 for a month. A diver dove into the calm sea to deploy the S4 in a depth of about 16 m. The mooring eye of the instrument was bolted to a nonmagnetic vertical steel frame connected to a cement base plate with a weight of 50 kg. For additional safety, the frame was also secured by a plastic rope to a 70 kg anchor 20 m away and also an immersed buoy 1 m above. In this way the S4 was seated about 0.5 above the sea bed vertically. The orientation of the S4 was checked visually every two weeks by a diver. The position of the S4 was marked by GPS and also by a small guide buoy on the surface. Since the area was used for fisheries, a small boat was always present around the site to prevent accidental trapping of the instrument in the nets. At 1100 hour of 16th of July, 2000 the instrument was retrieved by the diver in a choppy sea. The instrument and the base plate were intact but some growth of foulants was observed on the sphere. S4 was rinsed and then transferred to the coast for data retrieval by using a RS 232-C interface unit to an IBM pc in binary format. The data set (11 MB) was also copied on CD for safety reasons. The quality of the data was checked by using the S4 application software provided by Ineterocean system on the shore. This was done by plotting and scanning of the tidal velocities and elevations and comparing them with the tide table. In this way, the integrity and validity of the velocity and pressure signals were verified prior to the further processing of directional wave data. Since no spike, gap or suspected data was seen no interpolation or editing were necessary for the raw data. The sphere was later cleaned in the lab by a plastic brush after immersing in HCl solution for half an hour.
Collected raw directional wave data was
processed by using Wave for windows version 2.2 provided by Interocean systems
on a Pentium 433 Hz pc in less than 10 minutes. This software analyses and
displays all wave climate static parameters such as significant height, period
and spectral band width. This software also displays the raw data. Data is
presented in tabular and graphical (2D and 3D) format. In brief, the linear wave
theory is invoked in this software and the statistics of sea surface elevation
is obtained by applying frequency dependence depth correction to the Fourier
coefficients of the pressure time series. While the lowest frequency of the wave
spectrum is determined by the record length, its highest frequency depends on
the sensor depth, the depth of water and the noise in pressure sensor [Taylor
and Trageser, 1990]. The significant wave height is estimated as four times
standard deviation of surface elevation. The direction of the waves is
determined from the phase difference between obtained surface elevation and the
two horizontal components of velocities.
A full analysis of the data and its engineering applications is out of the scope of this paper and will be published later. Here, some parts of the collected data are shown and discussed to show the advantages and disadvantages of the S4 and its accompanying softwares When two different sampling strategy is defined for the S4, It will record the data in two different files. The data for tidal set up is shown in Figure 4. The first and last parts of the data are not shown since the S4 was out of water then. As seen, the tidal level (depth) record shows a more or less diurnal tide with a maximum amplitude of 2 m. The neap and spring tides are also depicted. The S4 current meter application software calculates the mean of the depth record (16.5 m) which is actually the depth of the pressure sensor.
Directional wave data of the same data collection period (but with higher sampling frequency) consisted of about 800 bursts with 2400 samples each. Figure 5 shows the wave parameters for the first burst. For processing this burst the default cutoff frequency of 0.33 Hz were used. As shown the noise in the pressure signal starts at about 0.29 Hz and varying direction, resulting in a significant wave height of 0.6 m which is fake. The spectrums of other bursts in calm wind conditions showed the same trend and therefore the abovementioned cutoff frequency was used for further data processing. A spectrum of a burst in windy condition is shown in Figure 6. In contrast to the previous burst, the pressure signal is not dominated by noise and shows the passage of real waves. Furthermore, the direction of the waves is nearly constant at peak of the spectrum. Note that the period of the peak wave energy (4.5 S in this burst) is close to the cutoff period (3.5 s) and care must be taken in selecting the cut off frequency specially in short fetch or low wind speed conditions.
A S4ADW directional wave and current meter was deployed successfully for a period of one month in a depth of 18 m to study the siltation regime of port of Busher, Persian gulf, Iran. The predeployment, deployment and post deployment phases were found to be easy and cost effective considering the power consumption and battery life of the system. Hence, the S4ADW was recommended and used for other field studies where simultaneous measurements of wave and tide are required for numerical modeling. The only encountered disadvantages of the system were: (a) The determination of the cut off frequency for the wave data processing is not trivial, specially when short period waves are present and needs some engineering judgment and experience. (b) There is no point in sampling the wave with frequencies higher than about 0.3 Hz (S4ADW allows you to sample up to 2 Hz) since the noise to signal ratio is very high at high frequencies. (c) The graphical software is limited and can not plot 3D figures of temporal variation of parameters such as direction and wave height which gives a general image of wave climate. If required, data should be saved in ascii format and plotted by another graphical software.
Acknowledgements
This study was supported financially by Port and Shipping Organization of Iran. I am also grateful to JWRC company for its support, assistance and facilities used in this study.
References
Draper, L.,1957, Attenuation of sea waves with depth, La Houille Blanche, 6, 926-931.
Lawson, Lemiux, Luck and Woody, 1983, The development of a spherical, electromagnetic current meter, Oceans 83 conf., paper no. AB 1137.
Niayati,M.F. and A. Maraghei, 1995, Morphological aspects for Bandar Nowshahr and Bandar Bushehr, Delft Hydraulics.
Taylor,G. and J.H. Tragser, 1990, Directional wave and current measurements during hurricane Hugo, Marine instrumentation 90 conf., 118-140, San Diego.
WMO,1988, Guide to wave analysis and forecasting, WMO no. 702, Geneva.
Fig. 1 The
picture of the S4. Note the size and the weight of the instrument. Photo
courtesy of Interocean systems.
Fig. 2 Persian gulf and Port of Bushehr which is located in the north west and marked with a circle (sc: 1∶140000).
Fig. 3 Bushehr channel Which is dredged to CD –10 m since 1974. The outer and inner channels are marked with straight lines (sc:1∶175000).
Fig.
4 Tidal data recorded every 15 minutes (one minute averaged)
for a month. (a) Tidal current speed. (b) Tidal level
Fig. 5 Pressure (detrended), spectral density and Direction of the waves vs. Frequency at 2400 Hrs on 15/7/2000 in a calm sea. Note the noise at high frequencies.
Fig. 6 Pressure (detrended), spectral density and Direction of the waves vs. Frequency at 0500 Hrs on 18/7/2000 in a sea with moderate wind. Note the constant direction at spectrum peak.