Winter 2010-11 part 3:
Aspects of Inundation - Dyfi floods and a look at Sarn Cynfelyn
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This
post
has
a
flooding
theme,
as
the
first
half of January 2011 saw the
Atlantic take the weather over with conviction. Gone was the prolonged,
numbing cold of December, to be replaced with overnight minima of more
than 10C, gales and rain. Cheap on the heating-bills but incredibly
damp and, for some, a bit more serious than that.
Perhaps this was an omen - at sunset on January 6th, sun and clouds
conspired together to produce a rather plausible imitation of an
incoming Independence Day
spaceship!
The weather set-up during the second week of January was a classic
"warm conveyor" - that's a long-fetch warm south-westerly airflow with
its roots down towards the tropics, bringing with it stacks of
moisture. This is illustrated vividly below with the January 12-13
midnight atmospheric sounding (taken with a weather-balloon) from
Camborne in Cornwall. Notice how close together the two heavy black
lines are: these are air temperature and dewpoint with height. If a
sounding has the dewpoint line way over to the left of temperature, it
means the air is relatively dry. In this case, the two lines run pretty
much together up to the 450 millibar height, at over 6000m (~20,000
feet). This means that the air is almost completely saturated from
ground level for an awful long way up. That in turn means lots of
potential rain: the value "PWAT" (precipitable water) at the bottom of
the column at the RHS of the diagram is very high for January at 28.82.
It means that if a mass of air was lifted from near ground level to the
tropopause, 28.82mm of rain would fall out of it. Not only that, but
the unidirectional strong winds (arrows to the R of the diagram) would
keep replacing the air and its moisture, so that an awful lot of rain
would result - hence the term "conveyor".
Incidentally, the tropopause is easy to spot on soundings. It is the
point at which the temperature and dewpoint-lines abruptly change
direction and head rightwards, denoting an increase in temperature with
height (the opposite of what occurs lower down). In other words, it's a
dirty great temperature-inversion. Here, it is situated at just above
the 200 millibar height, at over 11800m above surface, or way above the
height of Everest. That's very high for January, a clear indication
that this is a warm airmass of tropical origin.
The result - in winter - of such a moisture-laden airmass conveyoring
its way across the Atlantic and over the UK is days of very low
cloudbases and continuous moderate rainfall: the air trundles in and
meets the western mountains of Wales, the Lake District and Scotland
and is cooled as it is forced to rise over them. That cooling reduces
the air's moisture-carrying ability (air can carry an extra 7% of
moisture with every extra degree Celsius of warmth), and continuous
rain is the
result.
By the morning of the 13th, a major Dyfi Valley flood was underway.
I've illustrated several of these before so this time I thought I'd try
to get some different angles. I headed south-west towards Derwenlas:
the road was deep underwater shortly beyond there, but I found a few
vantage-points en-route; the main channel is somewhere in here:
At one of the railway-crossings, the swirling waters were pretty deep!
What does that notice on the gate say? Zooming in - aha!
That might sound harsh, but you don't want a train meeting a herd of
stray cattle. It has happened...
By now the flood-plain was one vast sheet of water, with just the tops
of gates poking out above its surface....
To the north of Machynlleth, from the end of the platform at the
railway station, I could see blue flashing lights in the distance. It
turned out that water had poured down the bank behind the cottages,
flooding the one on the end to a depth of about a foot and requiring
the evacuation of its elderly resident...
The total from this one event was probably getting on for 100mm, or 4
inches. Warm conveyors are the main cause of significant floods in this
area. It is just as well that the hill-snow had all but gone by this
date or it would have been a lot worse!
Over the following weekend the conveyor persisted with two further
instances of flooding. By the 17th it had finally cleared away and I
went down to Borth to see if anything interesting had washed ashore in
the accompanying gale. It hadn't, but it was a good excuse for a tramp
along the shoreline as the sun attempted to emerge from behind the
great banks of cloud:
It briefly broke through, giving the surf a mercury-like appearance:
On this occasion, all the ebbing tide left behind was masses of foam!
High pressure
then quickly built across the UK, giving clear, crisp days and cold
frosty nights, though nothing like the deep-cold of December. On the
20th I had a couple of spare hours in the afternoon, so I headed over
to Clarach, to walk to Wallog and back via the excellent Ceredigion
Coastal Footpath. The light was rather diffuse, especially out to sea
where a few fog-patches were present, but it was a large tide so the
feature I wanted to look at would be well-exposed at low water. I'd
been inspired to write about it by recently reading Alastair McIntosh's
excellent book, "Hell and High Water", in one chapter of which he takes
an in-depth look at the Flood Legends from around the world. More about
that below, but I'd heartily recommend the thoughtfully-written book to
all. If you Google the title, it comes up on the first page of results
at Amazon and at the author's own website. Anyway, back to the
storyline:
On reaching the cliff-tops, this was the view to the north:
Despite the calm conditions, some underwater feature is clearly
disturbing the ebbing tide. To get a handle on what it might be, here's
a shot of the old Admiralty chart that adorns one of my walls:
The chart shows that, in the main, the depths (numbers in italic) at
low tide in this part of Cardigan Bay are around 12-14m, but jutting
out from the coast is a narrow belt of shallower ground in which depths
are
often just 1-3m. This feature is called Sarn Cynfelyn, or locally "The
Causeway". Nearing Wallog, its landward part came into view:
It does look like a causeway of sorts....
Here it is from Wallog, with the old limekiln in the foreground - the
easiest way to get agricultural lime in years gone by was to bring the
limestone in by boat and burn it at the drop-off point. There isn't any
outcropping limestone as such in this part of Wales.
From Wallog, the coast path winds its way
steeply up the high clifftops on its way to Borth. Aberdyfi is the town
in the distance....
I left the path and headed seaward towards the high water mark, with
Sarn Cynfelyn stretching out ahead. The tidal disturbance was still
visible although it would soon be slack water....
Out onto the Sarn:
So what is it? Well, if you have ever been on a glacier, you'll have
noticed that on either side there are belts of rocky debris - lateral
moraines that, snowplough-like, the glacier has pushed aside in its
slow downhill progress. That's what this is - it dates back to the last
Ice-Age when valley-glaciers from the Welsh mountains pushed out across
the lowland plain to merge with the main Irish Sea Glacier. The
boulders are all of local rocks from the nearby hills....
Towards the end on a big low tide like this, the kelp-zone, underwater
most of the time, is entered. Further out to sea, this gives way to
coralline red seaweeds. I know every mile of the Sarn well, having
fished it from Aberystwyth-based boats for over 25 years. It holds a
wide variety of reef-dwelling fish, the main attraction being the tasty
black bream that can be caught in the summer months. As well as being a
great area for recreational angling, the reef is a mainstay of the
local lobster and-crab-potting industry.
This was as far
as I could go - but as the image shows, it clearly continues way into
the distance, where some shallow fog lingers to blur the horizon....
Google Earth provides a useful zoom-out at this point:
The light looking back inland was a little better. Here are the high
cliffs northward with Borth on the far left:
This is the view looking back east towards Wallog. The house is a noted
landmark, visible for several miles out to sea - so long
as nobody paints it green!
To the south,
part of Aberystwyth is visible, with the Castle and War Memorial
standing out....
The view westwards is best appreciated by
returning to the cliff-top. Sarn Cynfelyn's end is marked by the large
Patches Buoy, a navigational aid to prevent larger boats running
into trouble on its shallows. 6.5 nautical miles out from Aberystwyth,
it's just visible in the image below:
Here is a clearer view obtained by digital cropping. Beyond the Buoy
there are sandbanks, and then in another mile or two another extensive
but deeper-water area of boulder-reef, which we simply call the "Big
Rough"!
Finally, here is a retrospective on the return journey. The group of
distant mountains on the RHS is the Snowdon Horseshoe, with Tywyn
visible across the Bay:
At the time of the formation of Sarn Cynfelyn, all of this was land. At
the height of the last glaciation, sea levels were about 120m lower
than at present and, as the chart shows above, that's more than 100m
below much of Cardigan Bay. The entire Bay would have been a low-lying
plain.
Interestingly, coastal West Wales has its very own Flood Legend, which
goes as follows:
Cantre'r Gwaelod (also known as Cantref Gwaelod or Cantref y Gwaelod)
translates into English as the Lowland Hundred. It is the legendary
sunken kingdom that is said to have long ago occupied a tract of
fertile land in what is now Cardigan Bay. Cantre'r Gwaelod's capital
was Caer Wyddno, seat of the ruler Gwyddno Garanhir. The land is
usually described as being defended from the sea by a dyke, complete
with sluice-gates, managed by one Seithennin, who is often described as
a bit of a party-animal. One one drunken and stormy night, the gates
were neglected and as the tide rose, so the sea swept in, ruining the
land.
That there are Flood Legends from so many parts of the planet is a
feature suggestive of a common event experienced by many cultures, and
a eustatic (= global) rise in sea level is a very neat solution. The
diagram below (reference 1 below) depicts what happened. The first
thing that catches the eye is that the rise was not smooth, but had
periods when it was very rapid indeed, such as Meltwater Pulse 1A,
during which the sea rose some 20m in a period variously estimated to
be between 200 and 500 years. It also shows that the rise tailed off
about 7000 years ago, which would have been in the late Mesolithic or Middle
Stone Age, that lasted from 10,000 to 6000 years before present or
8000-4000 BC.
Further
back in time, peat samples dredged up from the floor of Cardigan Bay
18.5m below present sea-level have yielded a date of 8740+/- 100 years
before present, or 6740BC, which is again within the Mesolithic. This
we can fix on the diagram by outlining a simple rectangular marquee in
Photoshop:
It suggests that sea-levels back then were just over 20m lower than at
present. Significant areas of Cardigan Bay would still have been
low-lying land. From a post a couple of years back, here is a
description of this time:
The
environment
was
populated
by
pine,
birch
and
alder
woodland
inhabited by wild boar,
wild cattle (aurochs), red and roe deer. The life of Mesolithic Britons
seems to have been a complex picture of part-nomad, part-seasonal
occupier and possibly in some areas in some cases, permanent occupier,
with attendant land and food source management under favourable
environmental conditions. Increasing population and the need for
reliable sources of food led to a transition to agriculture, ushering
in the Neolithic age.
They also had the final 20m of post-glacial sealevel rise to contend
with!
Is the tale of Cantre'r Gwaelod an account of this slow but steady
inundation of a great fertile plain over a couple of millennia? The
more fertile it had been, the greater the loss, if one equates fertile
ground to food and shelter. Just how far back does the oral tradition
go? Science seeks its answers via empirical observation, but in some
cases that is not entirely possible. On the one hand, we have a old
flood legend, quite specific to this area, embellished no doubt through
time but there anyway; on the other we have the hard evidence for at
least the more inshore parts of Cardigan Bay having been land during
the Mesolithic and having been subsequently inundated. Will the two
threads ever be joined? Walk out along Sarn Cynfelyn, despite it being
a glacial moraine and not a road, on a quiet, still winter's afternoon
while the waves gently sway the kelp-fronds, and you are sent
momentarily back down through the ages to contemplate such things.
References:
1) Fleming, Kevin, Paul Johnston, Dan Zwartz, Yusuke Yokoyama, Kurt
Lambeck and John Chappell (1998). "Refining the eustatic sea-level
curve since the Last Glacial Maximum using far- and intermediate-field
sites". Earth and Planetary Science Letters 163 (1-4): 327-342.
doi:10.1016/S0012-821X(98)00198-8
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