Images from each field deployment are available in Appendix III. From visual
observations made during all the deployments, the WireWall field rig seemed to be an appropriate height to collect data for typical winter (windy spring tide) overtopping conditions, while also allowing ease of transportation (Figure 3.8). It is these conditions for which the alert threshold for hazardous overtopping conditions to promenade users could be usefully refined. During many of the events people may have got a little wet, but the overtopping was not considered hazardous to pedestrians. However, for the events when the vertical plume exceeded the top railing there was potentially a hazard as water and debris (including house bricks) were carried landward by the overtopping water. These conditions caused a higher volume of water to flow onto the promenade and created a return flow. Figures 4.19 and 4.20 show examples of what the conditions looked like as they passed through the field rig from the rear and side view cameras.
Figure 4.19 shows dense spray comes through the rig lower down as the vertical plume collapses under gravity, eventually causing an inland rush of water on the promenade.
Having the wires as close to the tarmac as possible is therefore critical to capture this overtopping flow. Figure 4.20 shows how a flow on the promenade develops and returns after being reflected by the splash wall. The momentum of the flow under this event enables some of the water to flow up and over the splash wall. In some cases the return flow had not fully drained before the next wave started to overtop. For this event the wave sequence often meant only the largest wave caused such a return flow and the next wave was often smaller not adding to the flow. In a higher energy storm more waves would cause flow on the promenade, which may not have time to drain between waves.
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Figure 4.19: Example photos from the rear rig mounted GoPro collected during the January 2019 deployment.
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Figure 4.20: Example photos from the side mounted GoPro (south of the rig) collected during the January 2019 deployment.
The camera footage allows us to identify the processes that occur that are not captured by the EA’s flood forecasting service. Figure 4.21 illustrates how a sea state that is not particularly rough results in a forecast of low waves that are, under normal spring tide conditions, depth limited and break on the toe of the defence, and are therefore forecast to result in runup on the stepped revetment and not overtopping. The photos show that while this is the case the runup still causes a (< ~ 2.4m) vertical plume of dense spray after impacting the vertical sea wall. The recurve works well deflecting the spray
seaward. However, when an onshore wind is present some of this plume is carried over the crest of the defence before it collapses under gravity. The influence of wind on overtopping has been incorporated into a new engineering tool developed by
Manchester Metropolitan University. To enable future collaborative research we added a weather station on to the rear of the rig for some of the field deployments to collect an initial dataset of wave overtopping and wind conditions. Although quite limited, the data are the first wave overtopping and wind data to be collected simultaneously at exactly the same location in the field.
At Crosby waves propagating from the west arrive at Crosby seafront with a slight angle due to the coastal orientation causing a southerly component to the wave direction that often carried the overtopping plume along the sea wall from north to south. During this period of travel the wind continually acted on the plume carrying more water over as the plume travelled along the sea wall. A clear example is available on YouTube,
https://youtu.be/BgvVIpV9Gag.
The camera footage collected as part of the project was vital to understand the behaviour and characteristics of the overtopping water in order to develop the data analysis techniques. By knowing the direction of travel through the rig we could identify when water coming in from the side (perpendicular to the expected land-sea direction) appeared to result in unrealistically high speeds (along the land-sea direction). We could identify when a flow of water along the promenade was coming under the rig from waves overtopping to the north of the rig. Under some conditions, water passed underneath the
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front row of wires (that projected seawards of the wall) and appeared first in the second row of wires: such events may have gone undetected if we had assumed that all
overtopping would appear first on the seawards most wires. There were also events where a violent jet went vertically upwards and was seen at almost the same time on both the first and second row of wires (resulting in the speeds calculated between the first and second row to approach infinity!). However, in the majority of such cases the water returns vertically downwards back into the sea and was not detected on any of the other rows of wires further inland: in the cases when the water was detected on inland wires, then the speed of travel at the inland wire was assumed to be the correct speed.
We have taken both concerns into consideration in the data analysis (Section 4.10) and captured uncertainty by presenting the data for different wire pair combinations. It is thought that using wire pair 13 is most accurate at calculating the volume of water that passes over the crest of the Crosby sea defence (see Section 4.10 for more details).
Figure 4.21: Example photo sequence to explain the site-specific processes during a typical windy spring tide that potentially require an alert to be issued for pedestrian safety, but are not represented within generic hazard warning tools. The position of the weather station is also shown (bottom right).
Summary
The size of the WireWall field rig seemed appropriate to collect data for typical winter (windy spring tide) overtopping conditions and allowed it to be transported in a long wheelbase high-top van.
The camera footage shows windy spring tides can pose a hazard to pedestrians on the promenade, even though hazard alert thresholds are not met.
Small waves breaking onto the stepped revetment run-up and on impact with the sea wall create a dense vertical plume of water, which can exceed 2.4 m and over top the defence crest when an onshore wind counteracts the effect of the recurve of the sea wall.
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