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Llynyfan (cont) APPENDIX No. 2. CATCHMENT HYDROLOGY, WATER DEMAND, QUALITY A2.1. Hydrology A2.1.1 The lake capacity is 818 Ml. The total water depth is 29m. 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. It is the source of the river Sawdde, but if the lake is not full, the river bed can be virtually dry for the first 0.5 1 km or so. A2.1.2. Until recently, 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. A2.1.3. 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 just upstream from the ex-treatment property, made no contribution to water supply. A2.1.4. At the filter beds, the estimated annual 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. A2.1.5. Little detailed hydrological data is readily available, other than rainfall statistics. From these, long-term monthly averages have been employed to produce estimated average daily flow (ADF) within sectors of, and out from the catchment. Rainfall available for run-off is calculated by percentage reductions in average recorded monthly amounts. These are:- 20% January - April and September - December; 30% May and August; 40% June and July - much greater than the 17% rainfall > runoff reduction used by DCWW in an Operational Yield Calculation, dated 29 Nov 90. The adjusted figures are applied to sector areas, to produce flow regimes. A2.1.6. Subsequent comparisons indicate a reasonable degree of accuracy in the area assessment methodology (Jones (1967) and DCWW (1990). Table A2.2. Comparison of catchment area selections Jones (1967) DCWW (1990) Roberts (1994) Ha Ha Ha Lake and catchment 216.9 226.0 216.55 (inc. Sychlwch U. ) ( Coch U. ) Sawdde to filter beds 87.0 83.0 86.4 (inc. Sychlwch L. ) ( Coch part L.) A2.2. Water Demand The estimated flow regimes indicate that sufficient water is available within the catchment to provide a supply to a large capacity unit. The Canadians have a scheme where parr are reared in semi-natural conditions. An initial, rough calculation based on their use of water, suggests that well in excess of 200,000 parr could be reared (DFO, Canada 1993 A2.3. Water Temperature and Quality A2.3.1. At such high elevations, low temperatures could have posed a problem, especially in freezing conditions. Near the ex-treatment property, ground water emerges and creates a small stream (amount not included above). Its temperature is warmer than stream water in winter, staying around 10 deg C all year. A2.3.2. Scientific opinion is divided on the value of the rapid growth associated with optimum temperatures. Such growth to smolting in salmon, can result in fish returning mainly as grilse. Symons (1979) suggests that juvenile salmon which smolt at 2 years, would require water temperature to exceed 7 deg C for some 450 days. Edwards et al (1979), while equating brown trout growth-rate to temperature, found homothermous conditions in chalk streams to be more important in high growth rates, than calcium and food-web effects. Also that in trout generally, the rates are 60% - 90% of the computed maximum for a temperature regime. Table No. A2.3. Mean Water Quality Determinands Unit Lake Filter Beds |
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