Large Wave Flume Experiments
The CIEM wave flume of the Maritime Engineering Laboratory at the Catalonia University of Technology is 100m long, 3m wide and 5m deep. The experiments in the CIEM wave flume consist of three separate measurement campaigns, each of 8 week duration. In the first two campaigns a mobile sediment bed will be placed in the flume and the measurements will focus on the hydrodynamics and sediment processes under regular breaking and irregular non-breaking waves. The first campaigns (co-funded by EU-FP7 Hydralab IV) took place in Nov-Dec 2013 and the second campaign will take place in May-June 2014. For the third measurement campaign (Sep-Oct 2014) the bed profile in the flume will be made rigid by constructing a layer of reinforced concrete on top of the sand bed. Rigidizing the bed allows for controlled and systematic measurements of the hydrodynamics under breaking waves. Due to the absence of the mobile sands, optical instrumentation can be used to measure the hydrodynamics (Particle Image Velocimetry, Laser Doppler Anemometry).
A new mobile measurement frame was purpose-built for the SINBAD experiments. The frame was designed and manufactured at the University of Twente, from where it was transported in parts to UPC in Barcelona where it was assembled and attached to the flume’s existing measurement carriage (see figures on the left). Attached to the trolley the frame can be positioned at any cross-shore location along the flume. Vertical positioning is achieved through a spindle screw, allowing the frame to be positioned over a 1.4 m vertical range with an accuracy of 1mm.
Barcelona Mobile Bed 1
This experiment (co-funded by EU-FP7 Hydralab IV) took place from November 2013 until January 2014. The objective of this experiment was to measure the hydrodynamics and sediment transport processes under 1) irregular non-breaking waves and 2) under regular breaking waves. The bed profile consisted of a raised 20m long horizontal test section which on the offshore side was flanked by a slope from the flume floor and on the onshore side continued in a rigid parabolic shaped dissipative beach to absorb incoming waves (see figure below). Two different initial slopes were considered, a 1:10 slope and a 1:20 slope, in order to obtain varying breaking-wave conditions (Iribarren number).
A wide range of high resolution instruments were deployed in the wave flume allowing detailed measurements of hydrodynamics and sediment concentrations. Two CCM+ tanks, developed in an early stage of this project (Van der Zanden et al., 2013), were buried at two different cross-shore locations in the horizontal test section. Above each CCM+ tank a high-resolution Acoustic Concentration and Velocity Profiler (HR-ACVP, based on Hurther et al., 2011) was deployed from a fixed frame mounted to the flume sidewall (see figure left). The mobile frame was equipped with a range of equipment enabling detailed velocity and concentration measurements within the wave boundary layer (HR-ACVP, a Nortek Acoustic Doppler Velocity Profiler (ADVP), Transverse Suction System (TSS), Optical Backscatter Sensor (OBS)) and higher up in the water column (Acoustic Backscatter System, two OBS’s, several single point Acoustic Doppler Velocimeters (Nortek ADV), TSS nozzles, and a 2D Sand Ripple Profiler (SRP) was mounted to monitor the local bed elevation (see figures below). The complete bed profile was frequently measured using CIEM’s bed profiler, these bed profiles give insight into the bar development and enable determining the net sediment transport rate along the beach profile. The third figure below shows the bed profile development for the regular breaking wave experiment with the 1:10 initial slope. The yellow markers indicate the measurement positions of the mobile frame. The dataset of this experiment can be downloaded here.
Barcelona Mobile Bed 2
During the first mobile-bed experimental campaign, sand transport rates and processes under a wide range of irregular and breaking wave conditions were examined. During the second campaign (May & June 2014), the aim is to capture the spatial variation of the breaking-wave-induced processes on sediment transport rates. Therefore, only one experiment is scheduled, which is repeated numerous times. During each of these repetitions, the mobile measuring frame is positioned at another cross-shore location around the wave-breaking area. After each experiment, the flume is drained and the sand bed is brought back to its original configuration. The experimental set-up is similar to that of the first campaign.
The images below can be clicked to see some movies of the experiment; the top two show movies of breaking waves in the flume, the bottom two link to timelapse movies of a typical two day experimental procedure per measurement location, with on the left bed reshaping and preparation on the first day and on the right the measurement campaign on the following day. This two day sequence was repeated for 12 different locations.
Here it can be seen how the measurement equipment is dismantled from mobile frame at the end of the experiment (not in realtime!).
New CCM system
A new conductivity-based measuring system for sheet-flow concentrations (CCM system) was developed at the University of Twente in 2012. The CCM system is similar to earlier versions and consists of two large tanks with a total of four measuring probes. When applied in flume studies, the tanks are buried under the sand bed. The probes enter the sheet-flow layer from below, ensuring minimum flow disturbance.
Compared to earlier CCM versions, a new ‘bed-level tracking mode’ was added. This feature is essentially a calculation loop that compares measured concentrations with a certain target value, and translates the concentration offset to vertical probe movement. In this way, the probe can closely follow the interface between bed and water and can be used to study bed level motions. Moreover, the position of this ‘bed-level tracking’ probe provides a reference bed level for other CCM probes in the tank.
More details about the CCM tank and results of its first application during a series of swash zone experiments can be found here. The system will be applied during the SINBAD experiments to study sheet flow processes under breaking and irregular waves.