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Flash Flood. 3/7/01

The triangle of land lying between Dolgellau, Bala and Trawsfynydd suffered some really intense weather recently. Rain began to hit Dolgellau in extremely heavy bursts from about 2.00 pm onwards and continued for much of the night as three areas of low pressure converged. The rainfall was accompanied by a severe electrical storm which knocked out lots of modems and computer equipment. A weather station on the Dolgellau-Bala side of the triangle recorded 79.9mm of rainfall in just three hours beginning from 3.00pm; that's over 3.5 inches, half the monthly average in just three hours.

The upland area of the triangle is largely sheep grazing with little vegetation to slow run off and infiltrate rainwater into soils. The soils themselves can be very thin anyway and down to zero on the eroded flanks of Arenig. The upland marshes, potentially a fantastic water buffer and storage facility, have largely been drained by ditching. So with little infiltration or storage this led to the very rapid rise in the water levels of the main rivers that feed from this area, particularly the Afon Lliw, Afon Prysor and the Mawddach tributaries, the Cain, Wen, Babi and Union.

The situation was worsened by the amount of timber lying close to the banks on certain stretches of these rivers, particularly the upper Mawddach and the Afon Wen. Some of this was sawn timber in stacks but from our own observations of the debris, a considerable amount was the stumps of felled trees and partially cleared windfalls, the latter being recognisable by the sawn top on long butts still attached to the roots. The wood was swept downstream and piled up against bridges. These formed dams until the bridge collapsed, allowing a surge of water and debris to race downstream to the next bridge. Most of the footbridges on the rivers were swept away and damage was caused to some of the main road bridges.

debris against bridge supports

Here's Lyn on the bridge over the Mawddach at the T'yn y Groes picnic site, Coed Y Brenin. Note the debris packed against the bridge support and the bent railings where logs crashed into them as the flood water came over the top of the bridge.

This is the same bridge from the bank. Note the height the river reached in order to flow over the top of the bridge. The local paper mentioned water levels at 25 and 35 feet above normal!

The banks of the rivers suffered severely in the flood, in some places being completely removed along with trees which further contributed to the damage lower down. The rivers ran brown with mud, all except the Afon Wen which lived up to its name and ran white for about a week. The huge amounts of stone and grit carried at such force by the water peeled the bark from any timber exposed to the flow and sandblasted the rocks, removing lichens and moss to produce a bright, clean appearance.

Above left is the Afon Wen showing considerable erosion to a forestry track; the tarmac road on the other bank suffered equally. Above right is debris on the Afon Babi, peeled ash and combed grasses. Merfyn, from Ganllwyd, told me of the thunder of boulders being rolled down the Mawddach and the fact that he could feel his house shaking. Caravans were washed away above Llanwchlyn and cattle and sheep were swept down the Union, some managing to emerge alive at Dolgellau and Penmaenpool.

The bulk of the debris was deposited on the lower reaches of the Mawddach, literally hundreds and hundreds of tons of timber, caught up against surviving trees in huge, tightly packed piles. Large quantities of water borne soil settled out on the old water meadows, as below; them monks knew what they were doing, eh? Note also large trees just dumped in the middle of fields.

All seriously exciting and scary stuff. Most people agree it was a miracle that no one was killed or even injured. Once again, nature points out our errors in water management.

Various patterns pointed out by Schauberger and Mollison (among others) were plainly visible. In particular, narrowing of the rivers produced higher water velocities and water levels and deep scouring of the bed. As soon as the river widened and slowed, deposition takes place (as above). Extreme erosion took place on the outside of bends with consequent landslides and tree fall, while considerable deposition is obvious on the inside of bends with clear sorting of materials from large stones through various grits to fine silt. Any branches which had dipped into the flood water showed die back of leaves within a week; why?

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