BARDEX LOGBOOK: 23/07 - 24/07

Wednesday 23/07/03

Series E3 – tide (3.25 upwards in increments of 0.125), plus constant lagoon (3.25m) . Hs=1m, Tp=4.5s

Water pumped into the system to achieve the high sea and lagoon levels. Lagoon level not steady – have to keep manually overrirding it to pump out faster than is achieved on automatic - +/- 8cm of desired level – no-one sure why. Water levels being topped up during the run – during the profiling/instrument adjjustements the water can be pumped in from the canal.

3.25, 3.375, 3.5, 3.625, 3.75m

Saul realised that the reason why the vectrino slave has consistently less data than the master, even though their settings are the same and they are properly synchronised, hitherto unrealised. The reason is that the new Toshiba laptop that the instrument logs into turns its hard drive off after 10 minutes of ‘inactivity’ – he set it to a longer time, so hopefully from now on the problem is solved...

Series E4 – lengthy discussions as to how to proceed. The initial idea was to start with the profile configuration as it is, set the lagoon to 3.5 and sea level to 3.5, and change the wave height (steadily increasing in 5cm increments from 0.8m to 1.3m – the new achievable height considering the elevated sea level).

However, it was noted that this is a juncture at which it would seem prudent to test the reproducibility of the test sequence with respect to observed morphological changes. The changes over E3 were classic cut and fill from low to high, resulting in sedimentation near the crest landwards of the breakers, and erosion near and seawards of the breakpoint. Thus it would be interesting to allow the tide to fall back to 3.25m, with the same wave conditions as before and the same lagoon level. It would be very interesting to see if the profile change observed over E3 would reverse. The idea then would be to repeat E3, pushing the tide back up again to 3.75m. Scientifically speaking, this would give us an opportunity to test a classic hydrodynamic versus morphodynamic hypothesis – because we are sending the same waves down every run, and controlling the sea and lagoon levels, if the profile change occurs exactly in reverse on the ebb, and indeed the same as E3 on the subsequent flood, then hydrodynamic and hydraulic conditions are controlling the profile development. If not, then profile change is very sensitive to morphodynamic feedbacks in the form of sensitivity to the sequence of change. It may also be interesting to overlay the hydrodynamic time series from the instruments on the offshore rig, to see how well they match

The material overwashed in the previous run is lost from the system, and we can carry on with overwash experiments with long period waves tomorrow.

E4 started approx 13:00 with sea=3.625.

Cilia noticed that on one of the pumps at high lagoon level, when the pump becomes submerged and draws in air, the flow meter stops working momentarily. This is only a problem when the lagoon is high, and means that the data for these runs will be noisy – we will need to try to filter this signal out at a later date. In the meantime, the technicians will try to fix the problem for tests from now on, and also try to measure the level at which it starts to become a problem.

Finished the run. The low tide profile (3.25m) was remarkably similar to the start profile at the start of E3, and on the subsequent rising tide (a repeat of E3), the sequence of change was remarkably and satisfyingly similar, except for some more overtopping at high tide which was inevitable.

Still big problems with the video record – very interrupted from the second camera – tomorrow will stand by it all day to keep an eye on it, and log using avi capture software rather than wmv.

Noted that there was quite strong alongshore sediment size grading – significantly finer on the west (rig) side compared with the centre and east side, so did 2 lines of sediment photos (west and centre) instead of the usual 1.


Thursday 24/07/08

High pressure sensors redeployed so one is on top of the other – it was noted l;ast night that the pressures measured are not as high as expected – Torsten believed it was because, even though the sensors are sited correctly, that the sediment and water are buffering the pressure. So this time we are trying one PT slightly out of the still water level, and one slightly below at the same location, to compare the pressure records from each

The offshore rig is still a little bit far from the action, but not in closure depth. The slope just seaward of where the waves are breaking is too steep to have the rig stable, but we will reassess later today

Middle rung of bed level sensors (24 to 39) raised slightly so they are more than 30cm from bed again. Resurveyed in, but large errors on resectionning so will redo after E5 run

Series E5 – SWL=3.5m, lagoon=3.25m, Hs increasing from 0.8m in increments of 0.1m to a possible 1.3m

Note that a few waves (2-3 mins?) were sent down the flume in error at the start before E5 began
Run finished approx 12:30. Max wave height achieved 1.3 – started to break off the paddle so decided to stop. Response of the beach was to steepen more – waves breaking and destroying themselves so little overtopping.

Landwards EM reconfigured so has some vertical flexibility
Tracer deployed. BLS and HF PTs resurveyed

Series E6 – lagoon=3.25, tide 3.25-3.625, Hs=1m, Tp=6, start 13:42

For all these overwash runs, the EMs get buried very quickly during the runs, but if we had them too high they may not record any velocities at all. Its a tough call to make, but we consistently place them at 3cm above the bed

3.25, 3.375, 3.5, 3.625 – significant overwashing on run 4 (3.625) – to back of barrier. Seepage also significant at the back despite high lagoon level, because of the waves, so deciosion taken to stop there

Beach was very 3d – overwash occurred primarily on the western (rig) side. So we did a full 3D total station survey from the level of the seawards benchmark to the lagoon. Also a 3D sediment photo survey was undertaken at the same time