Large-scale flume tests in the Grosse Wellenkanal
Analysis of wave parameters and cross-shore transport and comparison
with UNffiEST-TC
J.A. Roelvink', A . J . H . M . Reniers and Th.J. Meijer
1. Introduction
In this abstract we discuss the analysis of wave and profile measurements in the Grosse Wellenkanal, reported in Dette & Uliczka (Coastal Sediments, 1988) of which the raw data were made available by J. Oelerich in January 1992. This analysis provides us with accurate data not only on the profile development, but also on the variation in time and space of the sediment transport, various wave height parameters and the water level set-up. Comparison of these data with our predictions enables us to pin-point the most important flaws in our models.
2. Measurements
The measurements were conducted at the Grosse Wellenkanal in Hannover in May 1987, and refer to random wave tests on an initial profile with slopes of 1:20 for the underwater beach and 1:4 for the dune and foreshore; depth at the wave maker was 5 m and sediment size D50 was 0.225 mm. The test programme was subdivided into 45 runs of 780 s each, where for each test exactly the same time-series of waves was applied to the profile left after the previous test. The rms wave height, defined as sqrt(8 mO), was 1.24 m and the peak period was 6 s. After each test, the profile was measured at approx. 1 m intervals. The model length was approx. 190 m, where the slope started at approx. 80 m from the wave maker. Ten wave gauges were installed; the raw data from each wave gauge over each test were stored at a frequency of 5 Hz. A data report on the measurements as well as diskettes containing the raw profile and wave data were prepared by the Leichtweiss Institut.
3. Data analysis 3.1 Profile data
The profile data were used to obtain sediment transport distributions at each test, according to the procedure described in a note sent to the Coastal Profile group on 30 January 1992. Due to the high resolution in time and space of the profile data, correction and filtering techniques could be used, so the resulting transport distributions are quite accurate.
3.2 Wave data
Time series analyses were performed on the wave data. The main parameters of interest to us were the rms wave height of the wind-wave frequency band, the wave set-up and the correlation between low-frequency waves and the short-wave variance. The pro-cedures used are briefly described in the note mentioned above. The results of the analyses have been sent to the members of the Coastal Profile group.
4. Comparison with model predictions.
Two kinds of model runs were carried out: one, where the model was run in morpho-dynamic mode starting at the profile of test 14, up to test 45; and one where the model was run for a fixed bottom, every sixth test using the measured bottom profile at each test. The first run can be seen as a test of the overall predictive ability of the model; parameters to test are the total erosion quantity and the shape of the equilibrium profile. The second set of runs enables us to make a direct comparison of the 'internal' parame-ters Hrms, set-up, correlation long waves-short waves and sediment transport. I n the figures below, the results for the second set of runs are shown.
T e s t 02 T e s t 08 T e s t 14
For the first three runs, there is a very large discrepancy between measured and computed transport rates. The wave height is underpredicted in the most erosive area, which may be an important cause for the discrepancy. Probably, long-wave effects and reflection are important factors. The measured wave set-up lags behind the computed set-up; this is a transition zone effect which has not yet been corrected in this version of the model. The crude model of the long-wave effects incorporated in the present model does not predict
the correlation very well; this parameter is also heavily influenced by the wave generation in the tests, which is first-order steering without reflection compensation.
In the next three runs, the discrepancies in the wave height and transport decrease, as the beach becomes more dissipative and less reflective.
D . a o 1 0 0 . 1 5 0 . T r a n s p 100. 1 5 0 . D.000 -0.O01 • 0 . 0 0 2 100. 1 5 0 . T r a n e p . 1 0 0 , 1 5 0 , T e s t 20 T e s t 26 T e s t 32
For the last two runs (not shown here), the results are similar to test 32: the agreement between measured and computed wave heights and transports is much better. This may explain why in the morphodynamic run the total erosion volume is underpredicted, whereas the shape of the final profile is predicted reasonably well.
5. Conclusions
The analyses of the measurements presented here clearly point at some weaknesses in the model and provide objective standards against which the models must be validated. The weak points in the model are most severe for strongly erosive circumstances, which is not surprising since the model primarily aims at representing average conditions for profiles near equilibrium.
