Dynamic Revetment - Response to Sea Level Rise!

Hello everyone! We have a new team here with a lot of expertise and the experiment is running on schedule! We are on the second phase of the DynaRev experiment - testing the response of a 'dynamic revetment' to both sea level rise and storm conditions.

Figure 1 - The new team!

To refresh: the revetment is composed of 9m^3 of pebbles, placed directly on top of the sandy profile in the wave flume. Initial thickness varied from 30 cm (at the toe and crest) to 15cm in between toe and crest and landward of the crest. The revetment is called 'dynamic' because it is meant to respond to storm or sea-level rise forcing by 'rolling' landward. Within the revetment 97 pebbles with embedded RFID tags were placed, with focus on the revetment toe and crest. The location of these pebbles is measured after each sea level rise implementation to observe how the revetment is moving.

Figure 2 - The initial revetment.

The water level in the flume started at 4.5m. We have raised the water level in 4 steps of 0.1m to a final level of 4.9m. For the first 2 steps, little change was observed of the revetment and there was minimal overtopping.

Figure 3 - Revetment toe after SLR step 2.

During step 3 (water level 4.8m) the revetment began to be affected. Pebbles were lost seaward from the revetment, but not a significant amount. The revetment toe mostly stayed in the same location as where it was first constructed, however the toe and front face of the revetment has lowered significantly (e.g. a lowering of 0.4m has occurred at the toe). This is most likely due to underlying sand being lost through the porous pebble revetment. The result is a revetment face that is steeper, longer and thinner. RFID surveys of the tagged pebbles revealed that there has been mostly movement of pebbles seaward.  Overtopping was also occurring at this step 3 water level (4.8m) at approximately 30 overtopping waves per hour. We are now currently testing the final water level of 4.9m. The revetment continues to change with the same trend (toe staying fixed but revetment face lowering) and overtopping rates are significant at about 300 overtopping waves per hour. About 95% of the pebbles remain part of the revetment but they have moved around a lot. We have now begun to see noticeable movement of pebbles landward, i.e. waves pushing pebbles up the revetment to or beyond the crest. While this has resulted in the revetment crest increasing in thickness, it has not increased in elevation due to underlying sand loss. It remains unclear whether or not the lowering of the revetment due to underling sand loss will continue, if it does, the revetment will continue to thin and potentially fail. We wonder if we had made the revetment thicker initially whether or not sand loss through it could have been minimised. On the other hand, it could be that to mimic the natural version of these 'dynamic revetments' (that have evolved over thousands of years) we need a well mixed composite profile of sand and pebbles (as evident in nature) on which to place our protective structure.

After this final sea level rise test, we move on to testing storm waves. Bring on the destruction!

Figure 4 - Revetment after step 3.

Figure 5 - Revetment after step 4. Clusters of rock are only just visible in the pools of water seaward of the revetment toe.