Scaling and Analysis and New instrumentation for Dynamic bed TestS (SANDS)
The Sands Project (scaling and analysis and new instrumentation for dynamic bed tests) is a research project financed by the European Commission within the I3 project HYDRALAB III. It deals with the performance of Mobile Bed Tests looking at the flume and paddle characteristics but also at the sedimentary body behaviour and the corresponding instruments deployed in the flumes or basins. The main research topics shall try to answer the questions: Are there issues such as how to design a Mobile Bed Test or how to interpret the obtained observations.
Sands has been structured in three blocks: 1º Instrumentation, 2º Performance and 3º Morphodynamics. Within the first block we look at the instruments to recover: a) bottom dynamics, b) swash zone morphodynamic fluxes, c) sediment transport within the water column and d) fluxes through the granular medium. These measurements are obtained by means of advanced state of the art gear and also by means of newly instrumentation both of optic and acoustic type, developed within the project.
This instrumentation is tested within a series of carefully designed Mobile Bed Tests which are included in the second block of the project dedicated to performance. For this purpose the tests have been executed at three scales trying from the very beginning to reproduce exactly the same geometric and hydraulic conditions in all cases. These tests have been carried out at the different flumes at Hannover, Barcelona and Delft. The last block of Sands deals with the interpretation of these results and therefore the associated test morphodynamics. It includes a critical review of analysis and protocols to interpret the data including the quality control and error bounds arising from those analyses. The logic structure of the project is schematized in figure 1.
Figure 1. Schematization of relations between tasks.
The tests designed for analyzing the performance limits of Mobile Bed experiments were done using undistorted models and similar Froude number in the three facilities (Hugues 1993). The experiments were done in the Large Wave Channel (GWK) of the Coastal Research Centre (FZK) in Hannover, considered as prototype and with a length of 300 meters, a depth of 7 meters and a width of 5 meters. For these tests the mean diameter of the sand bed was selected as 0.28 mm. The tests in the Large Wave Flume CIEM of the Maritime Engineering Laboratory (UPC) in Barcelona, with a length of 100 meters, a depth of 5 meters and a width of 3 meters have been performed with a scale of 1:1.9 to the prototype and with a sediment median grain size of 0.25 mm. In the Scheldt flume of Deltares in Delft with a length of 50 m, a depth of 1.2 m and a width of 1 m, the tests have been performed in a scale of 1:6 related to the prototype and with a sediment grain size of 0.128 mm.
The Delft flume has performed experiments with a beach slope of 1 in 10, 1 in 15 and 1 in 20, while the flumes of Hannover and Barcelona have performed experiments of 1 in 15. This represents the normal availability of flumes for carrying out tests, in which the size plays a critical role in determining cost and therefore the number and days of experiments carried out.
The wave conditions being tested reproduce erosive (Hs 1 m and Tp of 5.7 s at prototype scale) and accretive (Hs 0.6 m and Tp of 7.5 s also at prototype scale) wave conditions. The wave time series of the three facilities have been generated by scaling down the prototype time series by Froude law scaling (scaling also the generation frequency). The time series reproduce a Jonswap spectrum (gamma 3.3). To avoid uncertainties with second order generation and absorption, which depend of the kind of paddle, the time series have been generated using 1st order approximation. Every time series has 500 waves.
First the erosive conditions were investigated and without reshaping the beach slope this were followed by the accretive wave conditions. The duration of tests for the 1:15 slope are summarized in Table 1. It should be mentioned that the duration of erosive tests for the Delft case is longer than what would correspond to a Froude scaling of the duration. This is because we decided to carry on with the tests for a “very long” comparatively time interval to analyze the corresponding profile evolution.
|Table 1. Tests sequences and durations in “clock” hours (Froude-scaled).|
Among the different measuring equipment used during the experiments there is standard equipment: Wave gauges (Resistance or Acoustic), Beach profiler, Velocimeters (Electromagnetic and Acoustic) and different equipment to measure the sediment concentration (Optical Backscatter Sensors or Transverse Suction System). An important effort has been done to develop and tests the limit of new measuring equipment in sediment transport experiments. To do that different equipment has been tested within this project:
- Advanced acoustic probe for bed shape mapping: The miniature echo sounder UltraLab® UWS works with an ultrasonic-impulse-run time procedure. It was developed for highly time- and position-resolved measurements of distances in fluids. With optical sensors, measurements through container walls are also possible. Measurement objects may be the bottom, a target within the fluid or the surface of the fluid. In respect to its special property of the directly proportional distance/voltage value, the import of the analogue output signal into an external data acquisition system is facilitated. This means, for example, that the output 0-10 voltage signal is proportional to the water depth 0-10 meter (5.32 volts are equal to 5.32 meters).
- Integrated 3-component acoustic Doppler velocity and sediment flux profiler: Combining the measurement of suspended sediment concentration and the 3D velocity field within Benthic Boundary Layers of flows and / or waves above mobile sediment beds. Both measurements are done simultaneously in a co-located measurement volume using one piezo-electrical emitter and several (up to 7) mono- or bistatic receivers all being connected to a single instrument hardware unit. The originality of this new instrument is the combination of the principles, technologies and inversions of Acoustic Backscattering Systems (ABS) with those implemented in Multi-bistatic Pulse-to-Pulse Coherent velocity profilers. The spatio-temporal resolution is in the range of O(1mm) and O(10ms) to allow the fine-scale studies of hydrodynamics and coupled sediment processes (including the turbulent processes) in most of the flows within Hydralab facilities such as wave channels, U-tubes or current open-channels.
- Advanced optic probe for bed shape mapping: Two techniques to measure the sediment transport in the swash zone have been developed. The first uses a structured light approach (laser) and the second uses a white-light irregular/regular pattern projected by a video projector. Both techniques use a group of cameras to recover several points within the image to reconstruct the bed-shape of the studied area. The images are acquired inside the intra-wave frequency to recover the sediment transport information wave per wave while also allowing studies of the sediment transport in an entire series. The acquired data are taken at well-defined intervals of the total SANDS time series, allowing also the study of the bottom changes from one time series to the next one. Both techniques are non-intrusive, capable to yield information of the sediment transport. A major objective was to develop the techniques such that they can be applied at the large-flume scales with accompanying constraints without loosing measurement precision.
- System for imaging velocimetry and suspended sediment tracking: The goal is to measure local sediment transport using an imaging method. A slice of fluid is illuminated by a laser sheet, and the sediment particles are individually visualized and tracked. The whole system, involving the laser source and the endoscopic imaging system, can be introduced in water like a local probe, to minimize optical paths inside the turbid medium.