HR Wallingford overtopping tests
HYDRALAB+, RECIPE Task 8.2: HR Wallingford overtopping tests
The tests were intended to generate overtopping measurements to develop guidance to simplify / accelerate model testing for analysis of wave overtopping for critical coastal / shoreline structures. The first step for the design of the tests was the definition of possible wave heights, wave periods and respective steepnesses, all of which must fit the wave maker capacity (viable tests).
2D model tests at HR Wallingford have measured overtopping on simple (smooth) 1:2 slopes, scale 1:20, with crest levels 24m (structure A1) and 20m (structure A2). No approach slope or bathymetry was used, so the depth at the structure toe was the same as at the wave paddle, see Figure 1. The tests measured wave-by-wave overtopping volumes, and mean discharges. The collection chutes from the test section to the measurement tanks were varied between 0.8m to 6.7m width to accommodate a wide range of discharges with three tank sizes. Most tests were run at water levels of 14m and 15m (19 and 35 tests respectively), and nine tests at 16m water level. Target wave conditions were Hs ≈ 0.8 - 3.7m, and extreme tests at Hs ≈ 4.8m. Wave periods (Tm) ranged from 5.4 - 13.4s giving wave steepnesses of som ~0.06 (storm sea), 0.035 (ocean waves, PM), and 0.01 (swell). Tests were run for 500 or 1000 waves. One test used multiple simulations to give 10 different samples each of 1000 waves, particularly important to establish the statistics of tests for very low discharges.
Figure 1 Flume layout with 1:2 slope (after calibration)
Mean overtopping discharge
The second design step involved estimating overtopping discharges with the intended crest and water levels for all suggested wave conditions using empirical formulae from the EurOtop 2 manual (EurOtop, 2016). Those results then helped us prioritise viable and interesting test conditions (mainly low / very low discharges or volumes).
The crest level of 24m was selected to measure overtopping discharges for the larger wave conditions. For this set of tests (crest at 24m, water levels 14m and 15m) wave heights of Hs = 1.6m, 1.4m and 0.8m were omitted since no overtopping was expected. A lower crest at 20m was chosen to measure overtopping from the smaller wave conditions, and still have a reasonable overlap with conditions tested for the 24m crest. For this set of tests (crest at 20m, water levels 14m, 15m and 16m) it was decided to exclude wave heights of Hs = 4.8m and 3.7m since very high overtopping would be very difficult to measure. Following the same reasoning, for the water level of 16m, only the smaller wave conditions were tested, Hs =2.8m was excluded but Hs = 0.8m is included even though only a very small discharge was expected.
Individual overtopping volumes
The third step taken in the design of the tests was to estimate the number of overtopping waves and individual maximum overtopping volumes expected for the combinations to be tested. These estimates again used empirical formulae from EurOtop 2 (EurOtop, 2016). Wave-by-wave overtopping volumes and the number of waves expected to overtop in each test were the basis for identifying test durations and where it could be interesting to use multiple simulations changing the seed.
Test conditions were calibrated in the flume before construction of the test section, to minimise corruption of the incident waves by reflections. Calibration was an iterative process: the amplitude of the signal driving the wave generator was adjusted until the spectral significant wave height measured at the calibration point was within ±5% of the target significant wave height. Incident and reflected wave spectra were determined using a four point reflection wave gauge array. The waves were non-repeating wave sequences, with durations equal to 1,000 times the mean wave period, required for a statistically significant sample for wave calibration analysis.
Following calibration of the wave conditions, the expected overtopping discharges were re-calculated with the empirical formulae from EurOtop 2 (EurOtop, 2016). This guided the selection of the overtopping measurement tanks and chutes to be used for each test.
All tests were initially carried out for 500Tm, except for the conditions where the predicted number of overtopping waves was less than 3%, where 1,000Tm were used.
During testing all wave conditions were recorded by the same wave gauges used for the wave calibrations. Another wave gauge was installed in the overtopping tank and an event detector on the crest of the structure. Overtopping discharges were quantified by collecting the overtopping water in a calibrated tank and measuring the volume collected in a known time. The discharged water was collected by a chute leading to calibrated collection tanks, so mean overtopping discharges could calculated directly from the duration, chute width, and depth of water in the tank.
Subsequent to testing, wave conditions were analysed and processed to determine incident wave heights, Hm0, removing the reflections through spectral analysis. The incident spectra obtained during testing were then used to update the prediction of the mean overtopping discharge with the EurOtop formulae (EurOtop, 2016). Mean discharges during testing were then compared with those predicted by the empirical formulae.
Finally the number of overtopping waves, Now, and individual overtopping volumes, Vow, were determined by analysing the signal of the wave probe inside the overtopping tank together with the event detector.
It was always expected that the main part of the data would be fully explained by the standard EurOtop 2 (EurOtop, 2016) empirical methods, and indeed in Figure 2 much of the data are within or in very close proximity to the 90% confidence band of the EurOtop 2 empirical formula.
A significant proportion of test conditions had however been chosen to give low to very low overtopping discharges. The objective of producing test data in this space has been effectively achieved.
Figure 2 Wave overtopping data
EurOtop, (2016). Manual on wave overtopping of sea defences and related structures. An overtopping manual largely based on European research, but for worldwide application. Van der Meer, J.W., Allsop, N.W.H., Bruce, T., De Rouck, J., Kortenhaus, A., Pullen, T., Schüttrumpf, H., Troch, P. and Zanuttigh, B., www.overtopping-manual.com
N,W.H. Allsop / E. Silva
Rev 1: 22 December 2016