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Former filter beds on Afon Sawdde below Llyn y Fan Fach Reservoir, including weirs

A Grade II Listed Building in Llanddeusant, Carmarthenshire

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Latitude: 51.8928 / 51°53'34"N

Longitude: -3.7409 / 3°44'27"W

OS Eastings: 280301

OS Northings: 223019

OS Grid: SN803230

Mapcode National: GBR Y6.QYJC

Mapcode Global: VH5FJ.3PQ9

Plus Code: 9C3RV7V5+4M

Entry Name: Former filter beds on Afon Sawdde below Llyn y Fan Fach Reservoir, including weirs

Listing Date: 16 February 2022

Grade: II

Source: Cadw

Source ID: 87840

Location: The filter beds are approximately 1 kilometre N of the reservoir dam, reached by a private track 3 kilometres SE of Llanddeusant village.

County: Carmarthenshire

Community: Llanddeusant

Community: Llanddeusant

Locality: Llyn y Fan Fach

Traditional County: Carmarthenshire


Llyn y Fan Fach is a lake within a glacial ‘cwm’ that was converted to a reservoir by Llanelli Rural District Council to supply clean water to Llanelli via water works at Llannon. Llanelli had grown rapidly in the C19 through industrial development and was suffering appalling sanitary conditions. A number of sites were assessed in the late C19 / early C20 and Llyn y Fan Fach was identified as the most suitable for a water supply for the town. An Act of Parliament was required, and obtained in 1912.

Engineers were appointed and contractors put in place. Work on the site above Llanddeusant began in 1914, initially employing 175 Irish workmen who were accommodated in two wooden on-site barrack blocks at a camp near the site of what is now the Llyn y Fan Fach car park. By 1916, the project was encountering difficulties in retaining the original workforce in this challenging and isolated upland environment. Llanelli Rural District Council applied to the War Office and Home Office for labour from prisoners-of-war and interned aliens, which was refused.

The project was completed using conscientious objectors. Under The Military Service Act of 1916 all men between the ages of 18 and 41 were conscripted into military service, with some exemptions, including for those deemed by local tribunals to be conscientious objectors – men, who for ideological or religious reasons, refused to fight. In practice few men received an absolute exemption from military service and most applicants were forced into army regiments. The men who refused to sign army papers, or to wear the uniform when sent to barracks were imprisoned. In July 1916 the Home Office introduced schemes whereby these conscientious objectors could be released from prison to undertake civilian work, mostly hard manual labour at specified work camps.

The Home Office established 4 work camps in Wales: Llanddeusant; Llannon (also part of the Llanelli Rural District Water Works project and which closed in August 1917); Penderyn (Rhondda Cynon Taf), where a reservoir was constructed to supply water to Mountain Ash; and at Talgarth (Powys), where conscientious objectors were engaged in laying a water main for a new sanatorium.

Conscientious objectors started arriving at Llanddeusant in September 1916, mostly men from north-west England and the Midlands who had been incarcerated in Cardiff and Llanelli prisons. In all a substantial cohort of around 200 conscientious objectors was employed at various times at Llanddeusant, most of whom lived in the pre-existing barracks, supplemented by a hospital (all dismantled on the completion of the scheme, with the remains recorded by archaeological fieldwork in 2017). The men were engaged in construction of the dam and associated water works, with the skilled work undertaken by professional masons. Some 30 men based at Llanddeusant also worked on the reservoir and valve house at Llannon.

On the signing of the November 1918 Armistice, UK military demobilisation started in January 1919 and was largely complete by February 1922. Conscientious objectors were at first detained by the Home Office to give demobbed soldiers first opportunity to find regular work after the war, but most had been released by May 1919.

While the bulk of the hard manual labour was completed at Llanddeusant by 1918, it is likely that the programme of works was actually finished (in 1921) by newly demobbed civilians. The scheme operated until 1967, and it is now managed by Dŵr Cymru as a reserve water supply.

The works comprise an integrated system of water management with a variety of structures and engineering works to capture, hold and release water. To supplement natural sources of water flowing into the glacial lake, a leat channels flowing ground water from higher level from the dammed Afon Sychlwch. The western section of the leat where it reaches the dam is a stone-lined channel and flows into the lake at a point where the level is controlled by a separate header dam.

The height of water in the glacial lake was raised by 10 feet by the construction of a concrete dam. A stone building survives on the N face of the dam, formerly used for the operational management of the water works, other concrete bases for buildings of unknown use are located nearby.

Water level is controlled by a slipway on the main dam and an outfall c.75m to the N of the dam. Just below the outfall there is a valve house, a chambered stone structure designed to further control the flow of water downstream. It was made redundant when the water flow was fully piped. Further works are located downstream to control the water flow before it reaches the filter beds and valve house. At this point aggregates were filtered out of the water and the flow further regulated.

There are also other associated landscape features surviving that were created as part of the water management system, including: roads, stone and concrete revetments, spoil tips, workshop and store structures, concrete bases for buildings, weirs, tanks, stillage basins, bridges and culverts located from the dam downwards. These were identified by Dyfed Archaeological Trust during fieldwork in 2017 and are recorded on the Historic Environment Record.

Contemporary with the dam and valve house.


Afon Sawdde flows into the filter beds on the upstream (S) side through a shallow stone arch. There are 2 filter beds, with concrete side walls, either side of a central channel. On the downstream side is a dam wall of quarried rock-faced stone to the outer beds, and smoother dressed stone to the central weir.

On the upstream side there are 3 weirs of stone and concrete, and a rubble-stone parapet alongside the track on the E side, which has openings with slots for stop planks.

Reasons for Listing

Listed for special architectural interest as part of a significant civic engineering scheme to supply water to Llanelli and which has survived intact; and special historical interest for its employment of conscientious objectors during World War One on nationally-important, but not war-related, work; and for group value with the associated reservoir dam, valve house and bridge over Nant Melyn.

External Links

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Recommended Books

Other nearby listed buildings

  • II Valve House on former filter beds on Afon Sawdde below Llyn y Fan Fach Reservoir, including weirs
    In the north east corner of the Afon Sawdde filter beds, approximately 1 kilometre N of the reservoir dam, and reached by a private track 3 kilometres SE of Llanddeusant village.
  • II Bridge over Nant Melyn
    Near the confluence with Afon Sawdde approximately 1.8 kilometres N of Llyn y Fan Fach reservoir dam, reached by a minor road and private track 2.5 kilometres SE of Llanddeusant village.
  • II Llyn y Fan Fach Reservoir Dam
    In a remote setting on the N side of a natural lake approximately 3.7km SE of Llanddeusant village and part of a complex of structures in a line from the lake alongside Afon Sawdde.
  • II Gorsddu
    Situated near head of upper Sawdde valley, some 700must W of car-park for footpath to Llyn y Fan Fach.

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