3. Site History.

3.1. Llyn-y-fan Fach lake is situated on the Black Mountain, in Brecon Beacons National Park (BBNP), at an elevation of 506 m, directly under 'Bannau Sir Gaer' (750 m). It is the source of the river Sawdde. Llyn-y-fan Fach is living history, the birth-place of the old Welsh Legends of the 'Lady of the Lake' and 'The Physicians of Myddfai'.

3.2. From 1918 until 1993, the lake was used as a direct potable water supply reservoir. Lake and nearby stream water was treated on site and piped to Llanelli. Built   initially by Llanelli Rural District Council, the system passed through the control of several local and national authorities before privatisation in 1989. 

4. Catchment Hydrology

4.1. The Lake

The capacity is 818 Megalitres (Ml) - or 818,000 cubic metres; with a depth of 29 m. Water is contained by an embankment comprising of concrete, reinforced by earth and clay, which raises the natural water level by 3 m.

Two stone/concrete impoundment weirs are in good repair. Seepage water from the embankment is examined weekly for turbidity which could indicate earth movement. None has been detected in many years. The catchment has been enlarged by diverting water from the upper reaches of the adjacent Nants Sychlwch and Coch, along the contours, via weirs, pipe and culvert. If the lake is not full, the river bed can be virtually dry for the first 0.5 - 1 km or so.

4.2. Until 1993, potable water was extracted from the lake by syphon, and piped (bore 300mm) a distance of 1.2 km downhill through filter beds at 358m, and on down 1 km to a treatment plant at 264 m. Some 8.2 megalitres per day (Ml/d) were abstracted from the catchment.

4.3. The Sawdde is joined by the Nant Sychlwch (Lower), upstream from the filter beds. Downstream, the river is joined by the Nant Coch (Lower). The more substantial Nant Melyn, with its confluence also upstream from the ex-treatment property, made no contribution to water supply.

4.4. At the filter beds, the estimated annual Average Daily Flow (ADF) is 15.5 Ml/d; the total catchment, Sawdde and Melyn, is 26.4 Ml/d. Subtracting the lake catchment from the total leaves 14.9 Ml/d.

4.4. Seatrout regularly spawn in the catchment, migrating upstream almost as far as the Filter Beds, and some distance up Nant Melyn. On 1 November 1994, several seatrout and a pair of salmon, estimated to be ranging in weight from 1 to 15 lbs were observed trying to ascend a 2 m weir some 200 m downstream from the Filter Beds. One salmon succeeded, during a 30 minute period. Small (5 - 7") trout were leaping, even further upstream. Given the gradient, strong tumbling current, and large sized bedload, fish are forced to drop back. Spawning success in these uppermost reaches must be, at best minimal.

5. Property Description

5.1. DCWW own (vice-WWA and Llanelli RDC) the lake and its catchment, a tract of land containing the Sawdde, catchment feeder streams, filter beds and buildings, the unmetalled road, and the treatment complex. The scheme originated under a Parliamentary Order laid in 1912.

5.2. The location is approached, initially from Llangadog, via Llanddeusant and a minor unclassified single-lane road. The immediate unmetalled approach roadway (lane) is , owned and maintained by DCWW. The owner of the adjacent Blaenea Farm has a right of way. Members of the local Graziers Association use the roadway for access to the Mountain.

The public, with no formal right of way, are allowed access, but are required by BBNP to park nearby.

6. Present Ownership and Hatchery Management

6.1. DCWW continues its ownership, and maintenance of the lake, road, catchment aqueducts, impoundments and outfall pipework

6.2. The Hatchery is situated, mainly within the ex-treatment property; and the filter beds

7. The Lake

The lake supports a small population of under legal size  brown trout.  Although LLanelli RDC sold fishing day-permits (for one shilling each in 1931), WWA and DCWW never continued this practice.  At one time, local farmers used to set basket-traps in the Sawdde to catch eels descending from the lake. According to folklore, these could have been as much as 8 feet long. More recently, water-engineers reported one which they estimated to have the circumference of a 'saucer'.

7.1.1. Upstream from the filter beds, a series of weirs and settlement pools, were at one time used to control stream turbidity.

7.2. Two filter beds are contained in a rectangular concrete basin, roughly 40m x 25m. Free water depth is about 1.5 m, plus up to 1.0 m depth of underlying filtering stones. The stream flows through a central channel, in which the water level is partially controlled by an adjustable otterboard weir. At the inflow, adjustable side-shutters allow a permutation of river and/or lake water. This can be discharged back into the river, through the filtering layers and/or piped on. The piped inflow is valve-controlled and can also bypass the beds.

7.3. Downstream from the Filter Beds, nearer the Ex-treatment Property, lying between the property and the Sawdde, is a boulder strewn area of land, bearing evidence of several old river channels, marking an initial, but temporary piedmont zone. Near the property, water emerging into one such channel suggests a continuing underground flow, possibly through upstream leakage from the river's present channel. The land is probably also owned by DCWW. NRA rainfall gauges are sited in an enclosure, and connected by telemetry cable to an external control box on the ex-treatment building. A boggy area outside the property boundary, drains via culverts under the roadway, into an  emergent stream.