Weather Banter - Weather effects on the English Channel (and presumably similar channels)

On 24/10/2020 13:30, N_Cook wrote:
Weather effect on the English Channel
Its a pity the NTSLF site does not have a page about the lumps and bumps
on its plots from the seaiching or sloshing of water in confined
channels . The only time any of their plots is near enough flat is after
a few days of high pressure.

A mysterious windless and no low passing (inverse barometer effect),
0.6m NTSLF surge about midday tomorrow for the Pompey area, what is its
origin?
https://www.ntslf.org/storm-surges/latest-surge-forecast?port=Portsmouth
After previous otherwise mysterious stealth surges, it transpired
(confirmed by a number of repeats ofsuch wind and time delays) that when
strong WNW wind passed over the Isle of Wight area, the wind induced
windstress on the sea surface produces a surge that then reflected
neatly and focussed by the regular curve of the French Pas de Calais
coast, back to the IoW area.
This time its probably the 47mph or so sustained SSW wind, about
22:00 tonight into the straight Brighton-Eastborne coast, induces a
surge, that then reflects off that coast, crosses the Eng Channel to
France and focussed reflection off the Cherbourg-le Havre coast, back
to us, despite that coast being only very roughly curved.

Analysis emailed to the local Southampton marine flood action group.
The significance of 0.6m surge on a 5.0m highest astronomic spring tide
locally is that without property protection measures in place, a 5.6m
combined tide would be 0.2m of water above floor level of the lowest houses.
I could place the image referred to an a website if anyone here is
interested.

Stealth Surges
Another one to add to the ones bouncing off the Pas de Calais from
strong WNW wind passing over us.
Even more stealthy , although this wind did pass over us early evening,
it would not be necessary next time and so a chance of a fully stealth
0.6m surge on a 5m tide sometime, with NTSLF down.
From strongish, 47mph average, SSW wind passing over Brighton to Beachy
Head coast, slightly curved so maybe a bit of focussing and the Seine
Bay very crudely a curve.
My analysis for Ouisterham in the bay of the Seine north France.
Firstly I forgot to say before, NTSLF had the timing of the mystery
surge 25 Oct as noon both at Pompey and Bournemouth suggesting the surge
was coming from the perpendicular bisector direction of the line between
Bournemouth and Pompey, so to that bay on the French coast.
All times below are GMT, using the IOC tidegauge plot and superimposing
a tide curve. Times and heights from a www source, Meteo365.com ,
seems good enough for this purpose, the blue circles on the attached
graphic. 3 on the IOC curve of 24 Oct and the fourth a bit above the IOC
curve as the wind off-shore SSW , would have locally depressed the tide
a bit then. Blue lines and the intercepts to the red tide gauge plot
scaled for the intercepts on 25 Oct and 4 simple "elastic " green cuves
fitted beteen those points to look like the curve of 24 Oct.
Required locally 1.28m adding to the heights , for the IOC GPS Geoid
offset or whatever for their world standardised curves.
From Brighton tide gauge peak a very unconvincing 0.39m at 20:50 24 Oct
, as lot of storm jitter, probably more like 0.3m .
Newhaven 0.29m peak at 20:50
Ouisterham peak from my plot 0.5m at 06:15 25 Oct
Pompey peak 0.62m 12:20
Lymington peak 0.6m 12:10 to 12:40
Bournemouth peak 0.5m 12:00
So 6 to 7 hours for each crossing of the Eng Channel.
Its a pity NTSLF does not have an explainer of some of the less obvious
positive and negative lumps and bumps on its outputs.

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm