Wednesday 20 August 2008
Friday 25 July 2008
Because I know you all want to see some overwash ...
The final run of E6 (24/07) - sea level 3.625m, lagoon level 3.25m, 6s 1m waves
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
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
Wednesday 23 July 2008
The D series - cumulative and incremental change in profile
From top to bottom, series DD1, D2 and D3. The upper panel in each figure is cumulative change relative to initial profile (every 15 minutes of waves at varying tidal heights, always low to high and back to low). Red indicates relative accretion and blue indicates erosion. Y axis increase landwards, and profile number corresponds with increasing time.
Qualitatively, a similar pattern emerges - morphological change is assymetric with the tide, and becomes more so when the lagoon levels become high or low. In addition, the consistent theme is one of sediment convergence, especially on the ebb tide (presumably when the groundwater table is super-elevated?), and positive feedback - once features form, they grow. Series DD1 and D2 have nice examples of berms stranded by the ebbing tide. Series D3 has the best example of sedimentation through sediment convergence
Profile response D2 and D3
Tests D2 and D3 looked at profile response with a tide and fixed low and high lagoon level, respectively. The major point of note is that whereas initial and end profiles on D2 were similar, the same was not the case for D3 (high lagoon). It seems that the high lagoon promoted substantial net offshore sediment transport, qualitatively similar to both C6 and C4, although here the zones of erosion and accretion were more substantial. I think it could be conclusively said that high lagoon levels promote offshore sediment transport, as expected.
Profile response D1 - first tidal run; and DD1 - the same experiment repeated with a 15min longer flood and 15min longer ebb tide
We saw significant differences in profile response between D1 and DD1. D1 and DD1 were essentially the same experiment (2.5m lagoon, tide between 1.75 and 3.25m), 0.8m waves, 4.5s period. The only difference between them was the rate at which the tide flooded and ebbed, being a 75 minute flood and ebb on D1 and a 90 minute flood and ebb on DD1.
As you can see, the profile responses were quite different. The profile of DD1 at the beginning was very much the same as at the end (see the next post for the differences over the tidal run) - apart from some crestal accretion, the two profiles were virtually identical. However, the profile at the end of D1 was very different to the beginning, with offshore sediment transport in the region of wave breaking and profile accretion/flattening offshore. The tide pushed up a similar lens of sediment at the crest
Sensitive to the initial profile, or the shape of the tidal curve? It is likely that the initial profile DD1 had equilibriated to the wave/tide conditions, which explains the lack of subsequent net change
Profile response C5 and C6
The profile response (before/after) of series C5 and C6 (longer period waves, low and high lagoon respectively). As with the runs with shorter wave periods, a low lagoon level promoted beachface accretion. The profile change associated with a higher lagoon level (C6) was less substantial, with some crestal accretion and relative depletion in the region of wave breaking (exit point effect?). C6 destroyed the subtidal ripple field
Profile Response C3 and C4
Sorry guys, realised that I've been a little thin on outputs recently. This and the following few posts have the profile change after the tests.
This is the difference between C3 and C4 (short period waves, low and high lagoons, respectively). The low lagoon level promoted onshore sediment transport and beachface accretion, whereas the high lagoon level promoted more offshore sediment transport and accretion in the region seaward of wave breaking
BARDEX LOGBOOK - 18/07 to 22/07
Friday 18/07/08
Another D-Day in the Deltagoot. The water required for today’s run (D3 – high lagoon plus tide) was pumped in over night.
Major concerns today over first the level of the buffer, then the rates at which we can achieve pumping out of the sea (so concerns over first the time it takes to achieve a low sea level to start the run, and then the length of the ebb tide later on the experiment). The pump which pumps water from the buffer to sea was working against the pump which pumps the water from the sea to buffer (because the buffer was so high the head above the pump was prohibitively high), so the ramp down to low tide was very long, and concerns that the system was at stretching point for the ebb tide to happen within the desired 90 minutes. An option was to lower the level of the lagoon, or to increase the length of either the whole tidal curve to keep it symmetric but within the limits of the pumping system, or to have an asymmetrical tide. It was decided that none of these options was desirable, because it would mean re-runs of at least yesterday’s test (and possibly more). Thus we asked for another solution to be found – the pump from buffer to sea was disabled, which helped the sea level drop, but still it was not fast enough for our ebb tide rate. Other options were considered including opening the release gate at the end of the flume to drain water out on the ebbing tide, thus aiding the speed at which we could achieve the ebb, using smaller pumps. It was felt that these pumps were not strong enough to make a noticeable difference.
By this point, there were fears over the stability of the front of the barrier because of a long delay in the start of the experiment, with high lagoon levels and low sea levels. So it was decided to start the run (pumping issue not a problem on the flood tide), and in the meantime wait for the pumping company to bring an extra submersible pump, to aid the ebb tide. At some point on the flood tide the pump would have to be reactivated in order to achieve a smooth flood tide, and we hope that the extra pump on the ebb would make the difference.
Pump reactivated on flood tide, and it worked well to keep the sea level stable.
Did first half of D3 (until high tide, until which point the problems with pumping out fast enough would not be realised), then installed 2 new pumps, ready for the ebb tide on Monday morning. In consequence, overwash rigs would have to be reconfigured on Mon afternoon, in order to record the swash on the remainder of D3.
Jon and Saul still trying to clear data from the srp using dos commands, problem is there are permissions set – no-one at the moment knows how to remove the directories, and Jon is awaiting response from Marine Electronics. Discovered that files can be accessed and deleted through internet explorer (ftp to there rather than through windows explorer or DOS, as before) – all instruments reprogrammed to sample from tues morning at 9am.
Monday 21/07/08
Because some water had to be drained from the flume in order for the pump installation on Friday, and because the technicians forgot to refill, the first part of Monday morning was spent reintroducing the lost water to the flume, before the rest of D3 could begin. Then delay when wave paddle overheated. In meantime, we put the offshore rig back in. Started run at 10:45 approx (series D33)
Computer in control room was not switched on for timings file (seq) for series D33 – clocks reset, and new run started – ‘Series D333’ approx 13:15. At the start of D333, there’s 2 mins of ‘accidental waves’ at the start, and camera not in position. Afterwards, no problems to the end of the run.
Response of the profile was to cut back from the beachface and accretion offshore. The elevated groundwater table made a significant difference to the dynamics of the beachface.
Meanwhile, offshore rig moved – landward side rotated off the wall – will have to be replaced.
In the afternoon, the swash rigs were reconfigured. The two suspect EMs (red cyclinder) were removed. The remaining 4 EMs were configured on their own at 4 different locations from barrier crest to lagoon mid-slope, each with a PT. The central logger was turned around and moved ~2m landwards. The two EM cyclinders were moved to over the break in slope at the lagoon side. None of the bed level sensors were touched. All instruments were cleaned and surveyed in using a total station. The offshore PT was moved ~3m landwards, and the 3 infiltration/exfiltration PTs were moved landwards to the crest. The atmospheric PT was repositioned higher, on the top of the flume wall. The remote control was also strapped higher to avoid submersion.
The video camera was tilted downwards, and the profiler carriage was moved to a new position for the runs, some 10m or so back, in order to catch the back of the barrier. This video camera now has a field of view stretching from the lagoon water line at 2.5m, to the barrier crest. The second video camera will be logging for the shorelines, and activity seawards.
Torsten from Hamburg installed 2 high frequency (1kHz) pressure transducers near the likely breakpoint, mounted on a scaffold pole which could be easily moved between wave runs if necessary to catch the plunging face of the wave
Tuesday 22/07/03
Series E1 – varying sea level (2.5 to 3.625 in increments of 0.125m). Hs=1m, Tp=4.5s
The response of the beach was cutback of the crest face, and accretion of the crest of the barrier. The barrier is now thinner than at any point previously. A decision was taken not to reset the barrier because it would be impossible by hand, and long period waves would now only serve to overwash the barrier. Crestal accretion was achieved through infiltration-enhanced runup limit, but overwash sensu-stricto was not achieved.
Series E2 – constant sea (3m) and lagoon level (2.5m), varying wave height (1.05 in 0.05 to 1.2 m). The run was stopped when waves breaking off the paddle, and therefore dissipating energy before they arrived at the beach. Afterwards it was realised that probably the maximum wave height acheived was 1.15 +/- 5cm.
Sediment photos taken at end.
Still gaps in the video record during runs because the video camera keeps going onto ‘demo mode’ and i haven’t figured out how to turn that off. I can’t be there to tend to the video camera all the time, so asked the profile technicians to keep an eye on it for me, but still not perfect
Another D-Day in the Deltagoot. The water required for today’s run (D3 – high lagoon plus tide) was pumped in over night.
Major concerns today over first the level of the buffer, then the rates at which we can achieve pumping out of the sea (so concerns over first the time it takes to achieve a low sea level to start the run, and then the length of the ebb tide later on the experiment). The pump which pumps water from the buffer to sea was working against the pump which pumps the water from the sea to buffer (because the buffer was so high the head above the pump was prohibitively high), so the ramp down to low tide was very long, and concerns that the system was at stretching point for the ebb tide to happen within the desired 90 minutes. An option was to lower the level of the lagoon, or to increase the length of either the whole tidal curve to keep it symmetric but within the limits of the pumping system, or to have an asymmetrical tide. It was decided that none of these options was desirable, because it would mean re-runs of at least yesterday’s test (and possibly more). Thus we asked for another solution to be found – the pump from buffer to sea was disabled, which helped the sea level drop, but still it was not fast enough for our ebb tide rate. Other options were considered including opening the release gate at the end of the flume to drain water out on the ebbing tide, thus aiding the speed at which we could achieve the ebb, using smaller pumps. It was felt that these pumps were not strong enough to make a noticeable difference.
By this point, there were fears over the stability of the front of the barrier because of a long delay in the start of the experiment, with high lagoon levels and low sea levels. So it was decided to start the run (pumping issue not a problem on the flood tide), and in the meantime wait for the pumping company to bring an extra submersible pump, to aid the ebb tide. At some point on the flood tide the pump would have to be reactivated in order to achieve a smooth flood tide, and we hope that the extra pump on the ebb would make the difference.
Pump reactivated on flood tide, and it worked well to keep the sea level stable.
Did first half of D3 (until high tide, until which point the problems with pumping out fast enough would not be realised), then installed 2 new pumps, ready for the ebb tide on Monday morning. In consequence, overwash rigs would have to be reconfigured on Mon afternoon, in order to record the swash on the remainder of D3.
Jon and Saul still trying to clear data from the srp using dos commands, problem is there are permissions set – no-one at the moment knows how to remove the directories, and Jon is awaiting response from Marine Electronics. Discovered that files can be accessed and deleted through internet explorer (ftp to there rather than through windows explorer or DOS, as before) – all instruments reprogrammed to sample from tues morning at 9am.
Monday 21/07/08
Because some water had to be drained from the flume in order for the pump installation on Friday, and because the technicians forgot to refill, the first part of Monday morning was spent reintroducing the lost water to the flume, before the rest of D3 could begin. Then delay when wave paddle overheated. In meantime, we put the offshore rig back in. Started run at 10:45 approx (series D33)
Computer in control room was not switched on for timings file (seq) for series D33 – clocks reset, and new run started – ‘Series D333’ approx 13:15. At the start of D333, there’s 2 mins of ‘accidental waves’ at the start, and camera not in position. Afterwards, no problems to the end of the run.
Response of the profile was to cut back from the beachface and accretion offshore. The elevated groundwater table made a significant difference to the dynamics of the beachface.
Meanwhile, offshore rig moved – landward side rotated off the wall – will have to be replaced.
In the afternoon, the swash rigs were reconfigured. The two suspect EMs (red cyclinder) were removed. The remaining 4 EMs were configured on their own at 4 different locations from barrier crest to lagoon mid-slope, each with a PT. The central logger was turned around and moved ~2m landwards. The two EM cyclinders were moved to over the break in slope at the lagoon side. None of the bed level sensors were touched. All instruments were cleaned and surveyed in using a total station. The offshore PT was moved ~3m landwards, and the 3 infiltration/exfiltration PTs were moved landwards to the crest. The atmospheric PT was repositioned higher, on the top of the flume wall. The remote control was also strapped higher to avoid submersion.
The video camera was tilted downwards, and the profiler carriage was moved to a new position for the runs, some 10m or so back, in order to catch the back of the barrier. This video camera now has a field of view stretching from the lagoon water line at 2.5m, to the barrier crest. The second video camera will be logging for the shorelines, and activity seawards.
Torsten from Hamburg installed 2 high frequency (1kHz) pressure transducers near the likely breakpoint, mounted on a scaffold pole which could be easily moved between wave runs if necessary to catch the plunging face of the wave
Tuesday 22/07/03
Series E1 – varying sea level (2.5 to 3.625 in increments of 0.125m). Hs=1m, Tp=4.5s
The response of the beach was cutback of the crest face, and accretion of the crest of the barrier. The barrier is now thinner than at any point previously. A decision was taken not to reset the barrier because it would be impossible by hand, and long period waves would now only serve to overwash the barrier. Crestal accretion was achieved through infiltration-enhanced runup limit, but overwash sensu-stricto was not achieved.
Series E2 – constant sea (3m) and lagoon level (2.5m), varying wave height (1.05 in 0.05 to 1.2 m). The run was stopped when waves breaking off the paddle, and therefore dissipating energy before they arrived at the beach. Afterwards it was realised that probably the maximum wave height acheived was 1.15 +/- 5cm.
Sediment photos taken at end.
Still gaps in the video record during runs because the video camera keeps going onto ‘demo mode’ and i haven’t figured out how to turn that off. I can’t be there to tend to the video camera all the time, so asked the profile technicians to keep an eye on it for me, but still not perfect
Tuesday 22 July 2008
Monday 21 July 2008
Subscribe to:
Posts (Atom)
