SEA-LEVEL AND SEA-STATE MEASUREMENTS WITH RADAR LEVEL SENSORS Dr. Ulrich Barjenbruch 1 and Jens Wilhelmi 2 The German Federal Institute of Hydrology (BfG) developed a cost-efficient method to monitor the water level and the wave condition in coastal waters. This measuring method has proved its functionality for several years in practice. The technology introduced here needs hardly any maintenance and its results show very good correlation in measuring accuracy with those of the conventional methods used so far in sea-state monitoring, such as the wave-rider buoy. INTRODUCTION The dimensioning and the safety of coastal-defence structures and port facilities presuppose good knowledge about the state of the adjacent sea areas. This knowledge also allows to derive data for the precise modelling of extreme seastate situations. Moreover, in harbours incoming and leaving ships need exact information about the wind sea and the prevailing swell. These requirements are confronted with shrinking budgets that are available for the monitoring of the sea state. Figure 1: Radar wave gauges (from the lef)t: Gauge Borkum, Lighthouse Alte Weser, research platform FINO, and the wave gauge in the lagoon of Venice (Italy) RADAR SENSOR FOR SEA STATE MEASUREMENTS Against this background, the German Federal Institute of Hydrology (BfG) developed a cost-efficient method of measuring the sea level and the sea state on the basis of a commercial radar level sensor that is normally used to indicate the filling-level in tanks. Such systems are at present in operation e.g. in the North Sea at the lighthouse "Alte Weser", at the gauge "Borkum", on the research platform FINO and in Italy in the lagoon of Venice in conjunction with the MOSE flood-defence system (see Figure 1). 1,2 Department: M1 Hydrometry and Hydrological Survey, Federal Institute of Hydrology, Am Mainzer Tor 1, Koblenz, 668, Germany 1
Ma g. ( db) f (Hz) 2 Figure 2: Radar-Sensor test in wave flume Radar Reference The radar level sensor was tested intensively in a wave flume for its measuring uncertainty. Figure 2 shows the measuring device and below the hydrographs of the radar sensor and the reference with a breaking wave of nearly 4 metres height. The principle of the radar wave gauge is the rapid measurement (scanning rate 2Hz) of the distance between the sensor (mounted on a building in the sea, such as a lighthouse) and the water surface. A post-processing of the raw data from the Magnitude (db) 2 1-1 -2-3 -4 A stop Magnitude Response in db -. 1 1. Frequency (Hz) radar sensor is strictly recommended for the exact monitoring of the sea state. For this purpose, the BfG developed special routines. First, the data should be checked for outliers. In the outlier test, all measured data are deleted which deviate too much in speed and in acceleration of the water body. Then, interpolation of the deleted outliers and resampling of the data follow. Finally, the data are subjected to a bandpass filter to Figure 3: Resampling of the raw data with hermite interpolation and band-pass filter with a 2nd order Chebyshev filter exclude artefacts. Then the microcontroller can compute in real-time from these measurements the sea-state variables "water level", "significant wave height" (H S ), "highest wave", and "mean wave period". Post-processing routines allow to determine additional characteristics of the sea state, such as the "spectral energy density" (S f ) or a spectrogram of the sea state as it is shown in Figure 6. An example of the spectral energy density (S f ) for a 1-minute period is given in Figure 4. On the left-hand side, the swell and the A stop F F Fs /2 stop1 stop2
3.1.8 fp1 =.21 fp2 =.88 fp3 =.23 2 1 fp1 =.96 fp2 =.17 fp3 =.19 S(f) (m 2 s).6.4 S(f) (m 2 s) 1.2.1.2.3.4..1.2.3.4. Figure 4: Spectral energy density on 19 Oct. and 1 Nov. 26. In the left-hand graph a clear distinction can be made between swell and wind sea, while the right-hand graph highlights the prevailing intensive wind sea. wind sea are clearly separated, whereas the right-hand graph shows a prevailing strong wind-sea situation. water level deflection [m] 4 2-2 -4 BfG wave gauge -6 21/9/28 28/9/28 /1/28 time from: 2.9.28 Figure : Wave deflection corresponding to the event shown in Figure 6 In Figure 6, one can clearly recognise the further temporal development of the sea-state spectrum. In the lower frequency range, close to 1 Hz, one sees the temporal development of the swell. On 24, 29, and 3 September, a strong sea state was measured, which is also seen in the recorded wave deflection in Figure. The spectrogram is computed with a 1-minute wide sliding window of a Fast Fourier Transformation (FFT)..8.7.6..4.3.2 2 2 1 1.1 2 21 22 23 24 2 26 27 28 29 3 1 2 3 4 BfG wave gauge / time [days] from: 2.9.28 Figure 6: Spectrogram of the sea state for 1 days, calculated with a sliding window FFT
4 platform. 1 m Figure 7: The research platform FINO 1 with a wave buoy in approximately 1 metres distance. A commercial wave-rider buoy (see Figure 7) is anchored in the immediate vicinity of the research platform FINO 1. A comparison of the data from these two systems is given in Figures 8 and 9. There is good agreement between the measurements regarding the basic structures of the significant wave height H S. The data gap in the middle of the Figure 8 originated from a power breakdown on the research WaveRider buoy BfG wave gage 4 Significant Wave Height H m [m] 3 2 1.8 power breakdown on the research platform 28/4/16 28//6 28//26 28/6/1 28/7/ 28/7/2 Figure 8: Comparison of measurements of the BfG wave gauge and the wave-rider buoy..6.4.2 1 2 3 4 6 7 8 9 1 11 12 13 14 1 16 BfG wave gauge / time [days] from: 1..28.8.6.4.2 2 2 1 1 1 2 3 4 6 7 8 9 1 11 12 13 14 1 16 wave rider / time from: 1..28 Figure 9: Comparison of the spectrograms of the BfG wave gauge and the wave-rider buoy.
S(f) [m 2 s] S(f) [m 2 s].1..1. Spectral density.1.2.3.4..6.7 BfG w ave gauge / frequency[ Hz] Spectral density fp1 =.82 [Hz] fp2 =.13 [Hz] fp3 =.44 [Hz] fp4 =.2 [Hz] fp1 =.84 [Hz] fp2 =.13 [Hz] fp3 =.43 [Hz] fp4 =.2 [Hz].1.2.3.4..6.7 w ave rider / Frequency [Hz] Figure 1: Spectral energy density with double peak swell and a slight wind sea. Figure 9 imparts an impressive overview of the temporal characteristics of the sea state over more than 1 days. At a closer look, one can also recognize that the BfG method has a somewhat lower signal-to-noise ratio (S/N) compared with the waverider buoy method. This is perhaps related to the supporting structure on which the radar sensor is mounted. Nevertheless, the spectral energy density values measured by both systems show the good correlation between the data of the two systems. In Figure 9 one can identify three separate sea-state condition at the end of 12 May 28. This is also visible in the diagram of the spectral energy density of this time period (Figure 1). The spectrum shows a double peak swell and a slight wind sea (.43 Hz). The match of the measuring data is very good, particularly as both test points are 1 metres distant. In conclusion, we can state that both systems have their advantages and drawbacks: The wave buoy is an established and approved system. The BfG system is a relatively new development. The wave-rider buoy can measure the wave direction. With the BfG system this is not yet possible in this stage of development. The measuring accuracy is quite the same with both the systems. With its low total cost of ownership and maintenance requirements, the BfG system has decisive advantages. The investment for the wave-rider buoy amounts to approximately 8,; that for the BfG system to about,. In contrast to the wave-rider buoy, the radar sensor is able to measure the water level. The BfG system needs a supporting structure. The wave-rider buoy is at higher risk of getting lost totally during storm. ACKNOWLEDGMENTS We thank the German Federal Maritime and Hydrographic Agency (BSH) for the good cooperation in testing of our measurement technology on the research platform FINO 1.
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