Mapping Mountains – Trimble Surveys – Moel Hebog

24/25 / 26.12.13  Search for the critical bwlch of Mynydd Mawr (SH 539 546)

Mynydd Mawr (SH 539 546) is listed as a Marilyn, Hump, Hewitt, Nuttall, Sim, 600m Twmpau, Welsh Highland P15 and a Dominant hill with 462m of drop and its critical bwlch listed at SH 558 539.  The position of this bwlch had long intrigued me, as the more I studied the map, more possible positions for it emerged.  Over three days I investigated a number of areas and took data at six different positions.  The only position that was visually dismissed is the current listed one at SH 558 539.

The position for the critical bwlch is problematic due to the land between Mynydd Mawr and its parent hill; Trum y Ddysgl (SH 544 516) now having a reservoir placed on it.  This reservoir is Llyn y Dywarchen, and to complicate matters, the reservoir was once a natural lake and when enlarged it eventually became two lakes with the addition of the crescent shaped Bwlch-y-Moch, each was connected by a strip of water.  The northerly of these two lakes no longer exists and is a reed infested wet land that descends from the earthen embankment at SH 561 535 to the old breached dam at SH 556 539.

My search began in the darkening hours of Christmas Eve with five minutes of data collected at the low point of the hill to hill traverse between Mynydd Mawr and Trum y Ddysgl.  This low point is now on a road at SH 55866 53334 and below one of the stone dams of the lake.  I placed the Trimble on the grass verge beside the road and hoped no car would run it over.  This point is where a 232m spot height appears on the Ordnance Survey enlarged mapping on the Geograph website.

The Trimble beside the road at SH 558 533 beneath the main dam of Llyn y Dywarchen.

At 8.10am the following morning before the parsnips had been cooked I was back beside the lake gathering data on a concrete path that is part of the headwall of the dam at SH 55884 53342.

The concrete path that crosses the outflow of Llyn y Dywarchen above the stone head wall of the main dam.

After visiting the critical bwlch and summit of Bryn Gwyn (SH 556 534) I headed toward another dam at SH 56091 53229 and gathered more data.  This dam is smaller than the main one at SH 558 533.  The Trimble was placed on the dam wall and not on the thick wooden sleepers that formed a path over the outflow as this point was judged to be below the lakes water level.

Gathering data on the dam at SH 560 532 with Mynydd Mawr and Y Garreg in the background.

My last point for the morning’s data gathering before unwrapping Santa’s pressies was on what might be the natural bwlch.  This is an extensive bog with its high point at SH 563 530.  As the Trimble was achieving its 0.1m accuracy before the time when the screen is pressed to start gathering data, the sky toward the south started to close in and envelop the hills with ominous looking showers.  As the accuracy was achieved and the screen pressed the rain was falling and waterproofs were quickly put on.

 The walk back through the bog, over a field and down to the road and up to the car was a soggy squelch.

The weather quickly closed in whilst at the only natural bwlch that remains; SH 563 530.

I now needed to investigate the northern section of Llyn y Dywarchen, and if necessary venture to where the critical bwlch is currently listed.  The indulgence of Christmas was weighing heavily as I made my way around the northern section of the lake.  Surprisingly I wasn’t the first to be out beside the lake on Boxing Day morning as a photographer and assistant were quietly going about their business as dawn was breaking.  The far side of the lake now has an earthen embankment where once the water channel connecting Llyn y Dywarchen with the second lake of Bwlch-y-Moch once existed.

I took data at the low point of the earthen embankment (SH 56160 53573) close to where a 236m spot height appears on the Ordnance Survey enlarged mapping on the Geograph website.

The earthen embankment at SH 561 535.

The Trimble now needed to go for a swim, so I placed it as close to the surface of the water as I risked and gathered more data to establish the height of the surface water.  It rested snuggly on a rock as water lapped beside it.  Every minute or so I took a glance toward it and hoped it hadn’t slowly slipped in to the lagoon.

The Trimble placed on a rock about 2cm above the surface water of Llyn y Dywarchen.

This was the sixth point I had gathered data from as I tried to pinpoint the critical bwlch position of Mynydd Mawr.  I only had one more place to investigate and this was the land between the earthen embankment and where the critical bwlch is currently listed at SH 558 539.  I’d looked down on this land from the summit of Y Garreg two days ago and had thought the land descended from the lake toward the old dam wall at SH 556 539.  However, if the land went up from the earthen embankment toward a high point at approximately SH 561 537 it would mean this was a contender for critical bwlch position.

From the vantage point of the earthen embankment the land to the north looked as if it was descending, if true this would confirm the water drainage as indicated on larger scale Ordnance Survey maps.  I walked a few hundred metres on the western side of what was once the lake known as Bwlch-y-Moch.  To my eye the ground was descending, I then ventured in to the bottom, reedy land and looked back to the earthen embankment; the land seemed to be going up to the bottom of the embankment.  Looking back to the north-west the swing of land heading toward the old dam at SH 556 539 looked as if it was also consistently descending.  Although eyesight alone is not ideal to judge gently descending land, something that was definite is that this land is lower than the surface water in Llyn y Dywarchen.  Satisfied that I’d achieved as much as I could with the equipment at hand I headed back to the car and the prospect of a walk on the beach followed by Boxing Day bubble and squeak.

Beside Llyn y Dywarchen.

There seems to be two current options for a natural bwlch connecting Mynydd Mawr to its parent hill.  One was visually dismissed as being lower than the level of surface water in Llyn y Dywarchen, whilst the other is higher and is a natural connecting bwlch.  But although the land at SH 563 530 is a natural bwlch, is it the natural bwlch or did a lower natural bwlch exist near the main dam wall at approximately SH 558 533 or near the earthen embankment at approximately SH 561 535?  This is hard to answer, but there is only one inflow in to this lake from a point that is a bwlch between Mynydd Mawr and its parent hill, this is at SH 563 530.  All other current inflows are either coming down from adjacent hillsides or from the old leat that once straddled the road on an aqueduct and fed water towards Beddgelert.  The old stone stanchions for this aqueduct are still present either side of the high point of this road.  The present direction of this water has been reversed since the aqueduct no longer exists. 

If another inflow existed it would be an indicator toward where the possible old critical bwlch once lay.  However, if one did exist it may now be under the raised waters of the lake.  But as the land at SH 563 530 is higher than the surface water of the lake it implies that the reservoir has been constructed over the old critical bwlch.  In cases like this, when the natural critical bwlch is thought to be under the waters of a reservoir the surface water of the lake is deemed as the height of the bwlch. 

Postscript: 

Since the bwlch survey of this hill full LIDAR coverage is now available.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height and positional data that is now freely available for England and Wales.  Consequently the numerical details for this hill has been analysed using this technique, resulting in the LIDAR height and position being prioritised.  The full details for the hill are given in the 7th option below.

Survey Result:

Mynydd Mawr – bwlch 1^st option – low point on road below dam wall

Height:  231.9m (converted to OSGM15)

Grid Reference:  SH 55866 53334

Mynydd Mawr – bwlch 2^nd option – concrete path over stream on dam wall

Height:  235.0m (converted to OSGM15)

Grid Reference:  SH 55884 53342

Mynydd Mawr – bwlch 3^rd option – small dam wall

Height:  235.1m (converted to OSGM15)

Grid Reference:  SH 56090 53229

Mynydd Mawr – bwlch 4^th option - natural ground

Height:  238.0m (converted to OSGM15)

Grid Reference:  SH 56373 53017

Mynydd Mawr – bwlch 5^th option – earthen dam

Height:  235.6m (converted to OSGM15)

Grid Reference:  SH 56160 53573

Mynydd Mawr – bwlch 6^th option – surface water level

Height:  234.7m (converted to OSGM15)

Grid Reference:  SH 56154 53572

Mynydd Mawr – bwlch 7^th option - LIDAR

Summit Height:  697.6m (LIDAR)

Summit Grid Reference:  SH 53971 54692 (LIDAR)

Bwlch Height:  234.1m (LIDAR)

Bwlch Grid Reference:  SH 56156 53605 (LIDAR)

Drop:  463.5m (LIDAR)

Dominance:  66.44% (LIDAR)

For further details please consult the Trimble survey spreadsheet click {here}