A sequence of highly effective tidal surges battered coastal areas in South Africa’s Western Cape, Jap Cape and KwaZulu-Natal provinces over the weekend of 16 September. One individual died; automobiles, properties and companies have been broken. The Dialog Africa requested Jasper Knight, a geoscientist who researches coastal processes, to clarify what occurred.
What drove the flooding?
A crucial mixture of three elements resulted on this vital flooding. First, a storm surge related to low-pressure climate methods coming onshore. These occur very generally however usually don’t lead to excessive quantities of flooding by themselves.
Second, low-pressure cells usually lead to robust onshore winds, and these can whip up the ocean floor and create massive waves which may doubtlessly run additional inland, particularly when the ocean floor is raised.
Third, the coincidence of the storm with the interval of excessive tide (a month-to-month occasion) and equinoctial high-high tide (a seasonal occasion). It’s this mixture that’s the trigger right here, not these particular person elements in isolation.
What’s a storm surge?
That is the place the extent of the ocean floor close to the coast is quickly raised up and ends in flooding alongside that coastal stretch. Storm surges are attributable to a low strain (cyclone) climate system approaching the shoreline from the ocean. Low strain causes the ocean floor to bulge upwards beneath the centre or eye of the cyclone, and the magnitude of this disturbance is said to the severity of the low strain system – the deeper the low strain cell, the upper the elevation of the ocean floor. That is often on the order of tens of centimetres to 1 metre or so.
Because the cyclone approaches land, the water floor alongside the coast rises.
What’s a spring tide?
Regardless of their title, spring tides are common occurrences all year long. They happen when the solar, Earth and moon are in alignment, and this occurs as soon as each (lunar) month. As well as, there are additionally occasions of the yr, across the equinoxes, the place spring tides are greater than common.
We’re very close to the spring (vernal) equinox within the southern hemisphere (which is on or about 22 September). This can be a interval when the solar is aligned overhead of Earth’s equator and so exerts a much bigger tidal drive on the oceans. This will likely have been a contributing issue to the upper water ranges across the coast.
Had been folks sufficiently warned?
Tidal patterns are extremely predictable and this knowledge is broadly accessible for ports or harbours alongside the coast. On this case, the South African Climate Service issued a warning. This info is especially helpful for boaters, fishermen and different coastal customers. Climate patterns are additionally pretty predictable, which is what climate forecasting is all about, so we all know when a giant storm could also be approaching.
Nonetheless, the web results of all of those elements together is much less predictable: though low-lying coastal areas are weak to flooding, forecasters could not know precisely when or how excessive.
The opposite situation right here is not only areas being coated by water but additionally the velocity at which the water waves are shifting, which is an element of their destructiveness.
What sort of emergency measures could possibly be put in place?
Native communities have to be warned extra clearly and successfully if there’s a menace of a storm surge and of coastal flooding. Folks and belongings (like automobiles, something that’s moveable) needs to be moved from the world or saved inside. Folks are likely to need to go to the ocean to observe the waves however this places them at extra threat. Roads needs to be closed off the place potential to maintain folks protected and away from the world. Floodwater administration by utilizing sandbags and comparable actions needs to be undertaken.
Precisely the identical measures utilized in locations like Florida within the US for decreasing hurricane threat needs to be utilized in South Africa, akin to boarding up home windows, maintaining belongings indoors, evacuating folks from excessive threat areas, and shifting furnishings in homes to the primary ground to cut back flood impacts.
Storm surges – and sea ranges rising in future – usually are not going to go away so we have to be ready for them.