Tidal analysis and prediction of the flow characteristics around Abu Dhabi Island

Transcription

1 Tidal analysis and prediction of the flow characteristics around Abu Dhabi Island K.A. Mohamed & M.E. El-Dahshan Research Center, Abu Dhabi Water and Electricity Authority Abstract In coastal seas and estuaries the astronomic tide is the main driving force of water motion. At times the motion induced by meteorological phenomena like wind and storms is equally important. For almost all activities along the coast and offshore a better understanding of the behavior of water levels and currents is essential. Tidal analysis and prediction are of a great help in this. They aim at predicting the water levels and currents induced by the astronomic tide at the desired locations based on the available measurements at these locations. The predicted water levels and currents are used as boundary conditions in the hydrodynamic physical and numerical models. They are used also as a calibration and verification data for these models. Tidal analysis and prediction techniques were applied m the tidal waters around Abu Dhabi Island, which is a complex and unique tidal system. Predicted astronomic tidal water levels m the tidal waters were obtained with high accuracy using the mentioned techniques. The tidal prediction enables a set-up of the hydrodynamic and dispersion model simulating the tidal waters around Abu Dhabi Island. It also enables the description of tidal wave propagation and flow characteristics in the tidal system. 1 Introduction The Set-up of hydrodynamic and dispersion models developed for tidal seas and estuaries is very much dependent on the astronomic tide forcing in the model boundary conditions. Astronomic tide is also very important in the oil spill models applied for tidal seas and estuaries. These models predict the spill trajectory, which helps in fighting the spill and enables the concerned authorities to take the necessary actions m the right time. Tidalanalysis and prediction techniques can provide the hydrodynamic and oil spill models with the predicted astronomic tidal water levels and currents, which are used in the boundary conditions and model calibration and verification. Predicted water levels enable

2 620 Advmces irl Fluid MechmksW the description of tidal wave propagation in the tidal seas and estuaries. Tidal water around Abu Dhabi Island is an example of a unique tidal system. Data collection program was carried out m 1997to obtain water levels for a period of at least one-month at selected locations in the tidal system around Abu Dhabi Island. The locations of the measuring stations were selected to cover the tidal system. The measurements were used m tidal analysis technique to obtain the characteristics of the tidal constituents describing the local astronomical tide at the stations. The prediction technique was used to predict the astronomical water levels m at the location of the measuring station. Predicted water level was compared with the measurements, which were carried out at the same measuring stations m August The paper will present the tidal analysis and prediction techniques used in the study and the obtained tidal constituents describing the astronomical tides in the system. The standard deviation of the astronomic tide from the measurements will be presented. A comparison between the predicted and measured water levels in August 2001 will be discussed. 2 Description of the tidal system Tidal water around Abu Dhabi Island is a complex tidal system. It consists mainly of North and South lagoons and open sea, which is part of Arabian Gulf. Figure 1 shows the general layout of the tidal system around Abu Dhabi Island. V B -a1layout of the tidal system around Abu Dhabi

3 Ad1wzcc,s ill Fluid Mechunics IV 62 I From the figure it can be seen that the exchange of tidal flow between North lagoon and open sea is done mainly through a relatively deep channel, which is Umm A1 Nar channel. The South lagoon is fed from open sea mainly through a deep channel, which is South Peripheral channel. The two lagoons are interconnected through a small channel, which is Maqta channel, where meeting point of the tides from North and South lagoons is located. 3 Water level measuring stations An extensive hydrographic survey was carried out m 1997 with the aim of collecting hydrodynamic data for calibration and verification of the physical scale model simulating the tidal water around Abu Dhabi Island. Water levels, current velocities and discharge measurements were carried out. Continuous measurements of water levels were camed out for at least one-month period. The locations of the measuring stations and cross sections were selected to cover the tidal water around the Island as it can be seen in Figure 2. r.... ', 1 Figure

4 622 Advmces irl Fluid Mechunks W From the figure it can be seen that Mina Zayed and Khalidia stations present the tidal characteristics at open sea, which is apert of Arabian Gulf. Umm AI Nar channel, IJmm a1 Nar Intake and Airport channel stations are located at the North lagoon. Futaisy and Mussafah stations are located at South lagoon. 4 Tidal analysis of water levels The local water level measurements for at least one-month period at each measuring station, which are shown m Figure 2 were analyzed by the tidal analysis technique. In the analysis the astronomical part was separated from the meteorologically induced part of the observation. The amplitudes and phases of the tidal constituents describing the local astronomic tides were determined. The so obtained tidal constants fully determine the local tide and can be used to predict the astronomical water level for any period mthe past or future. In the tidal analysis technique the measured tidal water levels can be described in terms of a series of simple harmonic constituents, each with its own characteristic frequency CO.The amplitudes A and phases G of the constituents vary with tie positions where the tide is measured. Compound and higher harmonic constituents may have to be added where large amplitude to depth ratio and bottom friction gives rise to non-linear interaction. The general formula for the astronomical tide is as follows: r=l h which: H(t) = water level at time t A, = mean water level of the period of measurements k = number of relevant constituents i = index of a constituent A = local tidal amplitude of a constant F, = nodal amplitude factor 0, = angular velocity (V, + U) = astronomical argument G, = improved kappa number (= local phase lag). F and (V, + U) are time dependent factors which, together with CO,can easily be calculated and are generally tabulated in the various tidal years books and built in the mathematical models of tidal analysis and prediction. A,, Aiand G,are position-dependent and present the local character of the tide. Tidal analysis aims at obtaining the position-dependent parameters, which are A,, A,and G, based on the analysis of the measured water levels at the measuring stations usmg the least square technique. The key part of the analysis is the proper selection of the set of constituents, which is assumed to give a proper presentation of the tide. Knowledge and information about the nature of the local tide, together with the sampling rate and duration of the measurements are essential to develop a good mathematical model for tidal analysis. Fourty tidal constituents describing the

5 Ad~mcc~s ill Fluid Mdxznics IV 623 local water levels due to tide in the tidal water around Abu Dhabi Island was selected based on the Fourier analysis. The duration of the measurements should be long enough to resolve the main constituents accurately, which is called Rayleigh criterion. The duration of the measurement at the measuring station was at least one-month period, which satisfies the Rayleigh criterion. The measurement interval was 10 minutes, which satisfies the Nyquist condition. This condition requires that the measurement interval should be less than the smallest wave period that is present in the signal. Getijsys tidal analysis and prediction numerical model of Delft Hydraulics was used in the analysis. The model Applies the least square technique on the measured water levels to obtain A,, Aiand G;m Equation 1. From the tidal analysis, the main tidal constituents at the measurmg stations in the tidal water around Abu Dhabi Island are M2, S2, Kl, 01, Pl, K2, QI, N2 and L2. These constituents present the three types of tides, which have big effect on the amplitude and phase of tidal harmonics. These types are principle, declination and Elliptical tides. From the obtained amplitude of the tidal constituents the type of tide at these stations was obtained by determining the so-called Form Number. The Form-Number (F) is defined as: Ho~+HKI F= Hnnz+fh In which: H= Amplitude of the specified tidal constituents 02,KI = Main declination diurnal tidal constituent M2J2 = Main principle semi-diurnal tidal constituents m And if: F< >F >F<3 P 3 the tide is semidiurnal the tide is mixed with mainly semi-diurnal tide the tide is mixed with mainly diurnal tide the tide is diurnal. Table 1 presents the amplitudes of the main tidal constituents at the measuring stations, which were obtained fi-omtidal analysis. The obtained Form Number is also presented m the table. From the table it can be seen that Form Number is between 0.86 and 1.2, which means that the tide in the tidal system around Abu Dhabi Island is mixed with mainly semi-diurnal tide, but with a significant diurnal component. This means that two high and low waters per day occur, showing inequalities in height and time. From tidal analysis technique the slandered deviation of the astronomic tide was obtained. The standard deviation is a measure for the quality of the water level measurements at the measuring stations. This deviation is mainly caused by the meteorological effect and measuring error by the tidal gauges, which affect the measured tidal signal. Table 2 shows the slandered deviation at the measuring stations. The table shows that, generally, the standard deviation of the measurements at the measuring stations varies between 5 and 9 cm, which is an acceptable range as mentioned in literature and previous studies relevant to this topic.

6 624 Advmces irl Fluid Mechunks W Table 1: The main tidal constituents at the measuring stations Table 2: The standard deviation of constituents at the measuring stations Measuring station Mina Zayed UAN ch UAN int. Airport ch Mussfah Bahrani South pc Futaisy Khalidia Maata ch Standard deviation (cm) Tidal prediction around Abu Dhabi Island Based on the obtained local values of Tidal constituents, A o,aiand Giwhich are obtained fiom tidal analysis technique, the prediction of tidal heights for any given period can be made. The time-dependent F and U over the considered period are accounted for based on the tabulated form available in the literature. These tables are built in the available tidal analysis and prediction mathematical models. The tables of time dependent F and U is included m Getijsys mathematical model, which was used m the study. New water level measurements in the water around Abu Dhabi Island was carried out for one- month period at four stations in August These stations are Mina Zayed, UAN Ch, Khalidia and Mussafah. Water level prediction at the mentioned stations during August 2001 was also obtained using the tidal prediction technique. The predicted astronomic water levels at the stations were compared with the measurements in August Figures 3 to 6 show the comparison

7 - ~~~~ ~~ ~~ 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Ad~mcc~s ill Fluid Mdxznics IV 625 between the measured and predicted water levels at the measuring stations for spring/neap tidal cycles in August g oo I Date ~~Iir measuredw.l - ~ Figure 3: Measured and predicted water levels at Mina Zayed station -..predictedw.l Date ~-MeasuredW-L Figure 4: Measured and predicted water levels at UAN channel station predictedw.l

8 ~ ~ _ ~~ _ ~ ~ p ~ ~~ ~ ~ ~ LP~ ~~~ ~~ ~~ ~ ~ ~~~~ I~ ~ Date m e dw.l predicted W.L Figure 5: Measured and predicted water levels at Khalidia station Figure 6: Measured and predicted water levels at Mussafah station The figures show that generally, the tidal prediction technique gives a good prediction of water level compared with the measurements. The difference between the predicted and measured water levels is due to the meteorological effect (i.e. wind conditions), which may occur. It can be seen that the trend ofthe difference between the measured and predicted water levels m all the stations is the same because the tidal water around Abu Dhabi is exposed to the samewind conditions. In Mussafah station, the difference between the measured and predicted water levels is bigger than in the other stations because this station is

9 Ad~mcc~s ill Fluid Mdxznics IV 627 located in shallower waters, where the wind conditions have more effect on water levels than in deeper waters. 6 Tidal propagation in the tidal system Prediction ofthe wave characteristics, which includes amplitude and phase has a big advantage in predicting the tidal wave propagation in the system. Figure 7 shows an example of predicting the tidal wave propagation. The figure shows the wave propagation in the North lagoon during the days of extreme neap tide of August Mina Zayed station presents the tidal wave at open sea and located at the entrance of the North lagoon. UAN intake station is located inside the lagoon at the Umm A1Nar Power and Desalination plant. The figure shows that the maximum time lag for the wave propagation fi-omopen sea to reach Umm A1 Nar basin where Umm A1Nar power and Desalination plant is located is about 1 hour m the neap tide. Generally, wave damping occurs when tidal wave propagates into the North lagoon and it depends on the bed roughness, geometry of the lagoon and the main channel and the existing tidal flats. Datc m n a a t a y e d ~~~~ ~ ~~ W.L. at UAN intake ~~~ ~ ~~ Figure 7: Tidal wave propagation fiom Mina Zayed to Umm AI Nar basin 7 Conclusion Tidal analysis and prediction techniques are a powerful tool for predicting the flow characteristics in tidal seas and estuaries. They are used in predicting the water levels in the tidal waters around Abu Dhabi Island. The local tidal constituents describing the local astronomic tide was obtained at the measuring stations in the tidal system around Abu Dhabi Island based on tidal analysis technique. These tidal constituents were used by tidal prediction techniques to

10 628 Advmces irl Fluid Mechunks W predict the water levels at the measuring stations. The predicted water levels proved to be in a good agreement with the measurements. The difference between the measured and predicted water levels is due to the meteorological effect induced by wind conditions. From the analysis of tidal constituents the type of the local tide around Abu Dhabi Island was described and it is mixed with mainly semi-diurnal tide, but with a significant diurnal component Tidal prediction enables studying the tidal propagation in the tidal waters around Abu Dhabi Island. The predicted water levels were used in the set-up of hydrodynamic and environmental models for tidal waters around Abu Dhabi Island. References Analysis and Prediction of Tides, Technical Report, Delft Hydraulics, 1992 Tidal Analysis and Data Validation, Technical Report, Research Center, Abu Dhabi water &L Electricity Authority, April 2000 Hydrodynamic Data from the Measuring stations, Technical report, Research Center, Abu Dhabi water & Electricity Authority, November 200 1