Tides Measurement and Tidal Analysis at Jakarta Bay

Tides observation conducted for these purposes such as real-time depth of water, determination mean sea level and other tidal datums to establish a system of tidal benchmarks and data for production of tide and tidal current predictions. Center for Marine and Coastal Mapping – Geospatial Information System used water level and tides data mainly to correct the water depth measurement to chart datum. This study uses sea level observation data conducted from 20th February 2018 until 4th April 2018 at Marina Batavia, Jakarta. This study found that tidal types at this location is mixed diurnal using formzahl number. Astronomical and shallow water possible constituent were derived from the harmonic analysis. Fourier analysis gives clearly visual interpretation in frequency perspective. Several constituents inseparable because of short duration records.

Investigation of bias in traditional grouping by means of regularization

<p>Harmonic tidal analysis bases on the presumption that since short records and close frequencies result in an ill-conditioned matrix equation, a record of length T is required to distinguish harmonics with a frequency separation of 1/T (Rayleigh criterion). To achieve stability of the solution, tidal harmonics are grouped. Nevertheless, if any additional information from different harmonics within the assumed groups is present in the data, it cannot be resolved. While the most information in each group is carried by the harmonic with the largest amplitude, time series from other harmonics is properly taken into account in estimated amplitudes and phases. However, if the signal from the next largest harmonic in a group is significantly different from the expectation, the grouping parametrization might lead to an inaccurate estimate of tidal parameters. That might be an issue since harmonics in a group do not have the same admittance factor, or if the assumed relationship between harmonics degree 2 and 3 is false.</p><p>The bias caused by grouping tidal harmonics can be investigated with methods used for stabilizing inverse problem solutions. In our study, we abandon the concept of groups. The resulting ill-posedness of the problem is reduced by constraining the model parameters (1) to reference values and (2) to the condition that admittance shall be a smooth function of frequency. The mentioned regularization terms are present in the least-squares objective function, and the trade-off parameter between the model misfit and data residuals is chosen by the L-curve criterion. We demonstrate how this method may be used to reveal system properties hidden by wave grouping in tidal analysis. We also suggest that forcing time series amplitude may be more relevant grouping criterion than solely frequency closeness of harmonics.</p>

A synthetic study of tidal analysis for satellite altimeter in shallow water

<p>Tidal constituents obtained from satellite radar altimeter derived water levels are widely used for ocean-related applications. However, in coastal waters, the tidal signal's complexity increases due to non-linear interactions between tidal constituents and other dynamics such as surge, giving rise to higher harmonics. A higher number of constituents increases the chance of pairwise frequency proximity, which creates retrieval time constraints using the typical series' length requirement criterion (Rayleigh criterion). Another issue is that with the lower observation frequency of altimeters, aliasing frequencies have to be considered. These lead to more challenges in shallow waters than its ocean counterpart since it is currently unfeasible to meet the series's time length requirements. In tidal analysis software, the Rayleigh criterion is often defined as fixed default harmonic selection condition. Therefore, many potentially important harmonics are left-out of the satellite radar altimeter based tidal analysis in shallow-waters, limiting derived usage.</p><p>To gain more insight into the accuracy of altimeter-derived tidal analysis, we extended the tidal analysis to include a more realistic correlation model for the surge. This model is implemented as a Kalman filter allowing us to obtain information about how the estimates' accuracy improves as more data becomes available. The improved correlation model aims to obtain realistic accuracy estimates for various strategies using synthetic data, i.e., before applying the method. An analysis of the condition number of the covariance information matrix was carried out alongside a twin experiment with simulated data. We demonstrated that the Rayleigh criterion is associated with the condition number of the information matrix and the effects of noise in the retrieval times. It shows that the accumulation of information is constant and proportional to the decrease of uncertainty. Depending on the amount of certainty one is after, the Rayleigh criterion is dispensable. Careful consideration has to be made for the signal to noise ratio of retrievals, especially when a constituent's amplitude is smaller than the variability introduced by noise, in our case, non-tidal variability. Overall, the analysis brings benefits on-top of traditional tidal analysis because it allows testing theoretical retrieval times and tidal analysis accuracy with multiple pairwise proximity issues and aliasing considerations. It also gives a straightforward way of analyzing the retrieval characteristics of semi-regular and irregular observation periods.</p>

Kajian Perubahan Garis Pantai Muara Gembong, Bekasi

Abstrak. Secara geografis, wilayah Pesisir Muara Gembong berbatasan secara langsung dengan Laut Jawa sehingga sangat rentan tehadap bencana abrasi yang serius dan perlu untuk dilakukan pemantauan secara kontinu. Tujuan dari penelitian ini adalah untuk mengetahui luasan abrasi yang terjadi dan mendapatkan alternatif penanggulangan abrasi di lokasi penelitian. Penelitian ini menggunakan metode tumpang tindih (overlay) citra satelit Landsat tahun 2012 dan tahun 2020 dengan menggunakan program ER Mapper untuk memperbaiki kualitas citra sebelum diolah selanjutnya dan program ArcGIS untuk tumpang tindih (overlay) citra serta prediksi perubahan garis pantai untuk tahun 2030 menggunakan program GENESIS. Penelitian dilakukan pada bulan Februari – Juli tahun 2020 di Pantai Muara Gembong, Bekasi. Hasil tumpang tindih Citra Landsat tahun 2012 dan tahun 2020 menunjukkan bahwa luasan abrasi di Muara Gembong adalah 252.071,71 m2. Tinggi muka air laut rata-rata adalah 0,60 m dengan surut terendah 0,49 m dan pasang tertinggi 0,62 m. Berdasarkan hasil analisis pasang surut maka dapat diketahui tinggi rencana bangunan pantai yang akan dibangun yaitu 2,87 m. Berdasarkan hasil simulasi yang dilakukan maka bangunan pantai yang tepat untuk mengatasi persoalan abrasi yang terjadi di lokasi penelitian adalah breakwater.Coastline Change Study in Muara Gembong, BekasiAbstract. Geographically, the Muara Gembong Coastal area is directly bordering to the Java Sea, so it is very vulnerable to serious abrasion disasters and it is necessary to carry out monitoring continuously. The aim of this research is to determine the extent of abrasion that occurs and to find alternatives to abrasion control at the research location. This research using the overlay method of Landsat imagery in 2012 and 2020 using the ER Mapper program to improve image quality before further processing and the ArcGIS program to overlay images and predict changes in coastlines for 2030 using the GENESIS program. The research was conducted in February - July 2020 at Muara Gembong Beach, Bekasi. The overlay results of Landsat imagery in 2012 and 2020 show that the area of abrasion in Muara Gembong is 252,071.71 m2. The mean sea level is 0.60 m with the lowest tide is 0.49 m and the highest tide is 0.62 m. Based on the results of the tidal analysis, it can be seen that the planned height of the coastal buildings to be built is 2.87 m. Based on the results of the simulations carried out, the beach protection building suitable for abrasion problems at the study location is the breakwater.

The research of Meinong earthquake induced hydrological anomalies and increased vertical permeability in southwestern Taiwan

<p>Hydrological anomalies induced by the earthquakes are valuable research data to understand the hydrogeology structure. At the same time, a complete hydrogeological data is the key to the study of earthquake hydrology. In this research, we collected the anomalous hydrological data after the M<sub>w</sub> 6.4 2016 Meinong Earthquake in Taiwan. The main purpose is to know the mechanism of hydrological changes triggered by earthquake and understand the local hydrogeological characteristics in the southern Taiwan.</p><p>From the distribution of the groundwater level change in the same location but different depths of aquifer, as well as the location of the rupture and liquefaction, it could be found that the co-seismic groundwater level change is large in Chianan Plain in the northwest of the epicenter and accompanied with a lot of ruptures and liquefactions located along the Hsinhua Fault. However, the observations in several wells around the Hsinhua Fault show a different water level change pattern compared with the other wells in Chianan Plain. Actually, these wells show that the co-seismic groundwater level decreases in the deep aquifer and increase in the shallow aquifer. It is shown that the Meinong Earthquake may enhance the connectivity between different aquifers near the fault zone and produce an increased vertical pressure gradient. The anomalous hydrological phenomenon also reflected in the river flow. Based on the river flow data we collected from five stations in the Zengwun River watershed, the river flow at two stations in the upstream dose not change after earthquake. There is a little increase at the midstream station. However, a large river flow increase is observed at the downstream station. After excluding the influence of rainfall, we think that the large amount of anomalous flow is caused by the rise of the co-seismic groundwater level between the middle and downstream sections, and a large amount of liquefaction in this area can prove this hypothesis.</p><p>The hypothesis of connectivity changes between different aquifers can be verified by analyzing the tidal response of different aquifers. Many studies have used the tide analysis to obtain the aquifer permeability and compressibility, and compared the changes in the analysis results before and after the earthquake. We think that if different aquifers are vertically connected after earthquake, the tidal analysis results should show a consistent permeability. Tidal analysis is executing now and the results will be provided at conference.</p>