Summer 2009
part 5 - some notable downpours and another A470 landslip!
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Through
most of August and into the first week of September, unsettled
and profoundly unsummerlike conditions have prevailed with a
near-constant mobile Westerly airflow
affecting the western UK. It has brought us, on a regular basis,
frontal rainbands followed by squally showers, and at times some
fairly impressive thunderstorms, torrential falls of rain, hail and
even a landslide that closed a major trunk-road. Plenty for me to get
my teeth into in other words. That's the useful thing about
storm-chasing - it can be done in bad weather! So we'll get the good
weather over and done with straight away - this was taken above
Aberdyfi in mid-August on a day when storms affected many areas but for
once left us alone:
This
entry features two successful storm-intercepts (out of a total of four
"chase-days"), the first of which involved an early start on August
22nd. A strong-looking thunderstorm was evident on the rainfall radar,
crossing Cardigan Bay: a quick step-back through the 5-minute plots
gave a time for its landfall and a place - Borth beach. Upon arrival,
it was still way out to sea, so I was well in time, and a heavy shower
over the hills above Aberdyfi provided a nice surprise by throwing out
a lateral gust-front southwards (the prevailing wind being westerly):
The target storm duly
advanced on my position, and I was a bit disappointed to see that it
was a mess, structure-wise - this was the one I had hoped might sport a
spectacular shelf-cloud but all I had was a darker and darker sky.
Lightning flickered closer and closer so I came down from the top of
the beach to find somewhere less exposed to sit it out and then see if
the sight of the anvil retreating up the Dyfi Valley might offer any
opportunities....
The sight of the storm retreating certainly beat the sight of it
advancing, but quickly the morning sun appeared over the top of the
anvil, to blow out the highlights of subsequent images....
To work around this bit of natural inconvenience, I concentrated on the
foreground:
It was a huge tide, covering almost all of the flat, sandy parking-area:
It's
rare for a storm to catch me by surprise but the next one did! August
28th had been a day of mostly mundane showers and overcast conditions,
but this cell exploded into life. The radar screengrab below shows how
heavy it was, with one pixel plotting a mighty 90-120mm/hour rainfall
rate - that's tropical!
The cell in question had moved in from the NW, so as the radar shows it
had been raining heavily for a while before the torrential stuff
started to arrive with huge raindrops hitting the road with almost
explosive force:
In no time, a blinding downpour was in progress:
Ducking in and out of the front door of the house I was in, I was able
to grab a series of images, drying off the skylight filter in between
shots, when a distant roaring sound began and got louder. Suddenly, a
violent squall of even heavier rain and hail swept up the street,
tearing apart the
minor river of water flowing down it, bouncing inches into the air:
Gutters were not coping and the water was literally streaming from the
rooftops until it passed as suddenly as it had begun - a wild five
minutes indeed!
The hailstones were
still lying in drifts where the floodwaters had left them an hour
later. These were up to large pea-size, many showing a classic "birds'
eye" structure indicative of several stages of growth as the storm
updraughts had lifted them up through the cloud repeatedly, allowing
several stages of hailstone growth:
September
1st was a notably thundery day here, and justified a day out with the
camera. I stayed local (time and again this has proved a good tactic)
and watched the rainfall radar to identify targets. The first one was a
line of storms moving up from the SW. ETA - ~1130 BST north of Tywyn.
Getting there in good time, it duly arrived and the obviously heavy
precipitation looked to be spawning a strong gust-front:
However, this was as
good as it got:
The next storm was heading straight in to my south so a move was called
for. I headed back south-east to meet its tail-end, spitting out the
occasional cloud-to-ground lightning bolt, with the thunder booming
from side to side of the Dysinni valley while the sun lit up my
vantage-point:
A while later, the same storm towering high over Cadair Idris, with the
sun penetrating up-valley as far as Bird Rock. Vigorous updraught
towers on the storm's rear flank indicate it to be a backbuilding
multicell:
I then headed to my favourite spot above Aberdyfi to watch a mature
storm drifting up over Plynlimon and developing cells between me and
it, before activity seemed to die off a bit:
It was time
for a forecast/radar update, so I nipped home to do that and have a
brew. The radar and satellite images showed one final band of storms
along a surface trough so I headed down to Borth for the finale,
arriving at a windswept beach, the incoming tide washing great clumps
of eel-grass, wrack and kelp ashore. One couple were braving the
elements and the car-park was almost deserted - amazing given that the
previous day had been August Bank Holiday Monday!
As the rough weather swept in I resorted to the same tactics as on the
previous trip and headed to an almost deserted Ynyslas to ride the
storm out and see what its other side looked like. It was a humdinger,
with hail lashing across the sand: the photo below was taken not long
after it started:
No spectacular anvil this time, but it produced this rainbow as a final
flourish!
Some big rainfall totals must have occurred in places and the following
day I received an email from Richard Street who had been travelling
along the A470 between Dinas Mawddwy and Dolgellau. He said: "I found
your site completely by accident, looking if there was any news on the
latest landslide at Dinas Mawddwy last night around 6.30. I arrived
there not long after on my way home....." and attached a series of
images, of which I have posted two, below, with my thanks for his
permission to use them:
This area is prone to landslips during heavy rain for geological
reasons: the last major slip occurred in January 2005 a few hundred
metres further up the valley. A couple of days later I drove up to the
Bwlch from the Dolgellau side and parked (the road had been blocked-off
here), and walked down to the site of the slip. The council people had
cleared the debris from the road but the slip's path and the damage
caused to the crash-barriers were still evident:
This
type of landslip is known as a bog-burst, which typically occurs in
areas where gleyed tills (the grey clay in the images) are present, sat
atop the bedrock. The tills are geologically young, remnant deposits
associated with the deglaciation 12,000 years ago when the most recent
ice-age ended. In many areas they have been eroded away by water
action. In this landslip-prone area, quite large tracts remain, and
being impermeable they create poorly-drained areas which in turn allow
bogs to develop, with matted vegetation of reeds, sedges, mosses etc.
Over centuries, this in turn allows peat to accumulate between the
vegetation and the till.
Under conditions of oversaturation due to prolonged heavy rainfall, the
peat can become supercharged with water which can then force its way
gravitationally downslope, along the interface between the till and the
matted bog-vegetation above. As the water builds up, it can
hydraulically jack-up the matted bog-vegetation so that a large bulge
appears in the hillside. This eventually fails when the hydraulic
pressure exceeds the tensile strength of the matted vegetation - and
whoosh!
Because the initial burst strips away the bulging vegetation, exposing
the till beneath, the ensuing high-speed debris-flow will readily erode
down into the till so that the resultant debris consists of a liquefied
mixture of clay and peat, dense and fast enough to transport large
clumps of reeds, rocks and so on, and to demolish things like
crash-barriers in this case. Had a car been it its path it would have
stood no chance - it would have been flung downslope along with
everything else. The final image shows a rather sorry fence-post:
The road will remain closed for a few days, according to reports,
whilst safety assessments are carried out and repairs made, In the BBC
Wales report, local councillor John Pughe Roberts is quoted: "It is
nature - what can you do about it?"
Indeed! It's just as much a part of natural things as is sunshine - and
ourselves. It's just that sometimes, Nature does something to remind
any of us who might have forgotten the point that it calls the shots at
all times!
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