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Poulaphouca lakeside |
A question was asked on the Blessington Blog today to the effect, “Will the lake get as low as this again over the next weeks?” It showed a photo of the Poulaphouca Reservoir taken during a previous drought, in which neat linear piles of rocks could be seen along the shoreline, clearly marking the locations of the walls of houses that were submerged when the valley was flooded in the 1940s.
The lower operating range of the reservoir is 179.9 mOD (metres over datum, the datum or reference level being Poolbeg), according to a Wicklow County Council document, so there's a long way to go before we run out of water. Whether we will face a crisis obviously depends on the inflow from the River Liffey and Kings River which feed the lake, and whether they'll continue to top up it up sufficiently. Yesterday, the water level in the lake was 184.37 mOD.
Is there much water in the Golden Falls lake to keep the Liffey alive downstream, without drawing from Poulaphouca? I haven't been up there for a couple of months, but it should be easy to check from ESB hydrometric graphs. Currently, the ESB are discharging from Golden Falls at a rate of approximately 2 m³/s, averaged over a day (Their graph is wonky, and I haven't been able to contact anyone about it, but the value is from a table and it's been constant for weeks). The Ballymore Eustace WTP currently outputs approximately 308,000 m³ per day. Each 10 cm drop in the level of the 22.3 km² Poulaphouca lake corresponds to 2.23 million m³ of water. Even if the current River Liffey flow is maintained downstream of Golden Falls, with no water flowing into the Poulaphouca lake to replace it from the two feeding rivers, and water is routed through the Golden Falls lake, without its level increasing, that's 2 x 60 x 60 x 24 = 172,800 m³ for the river, plus 308,000 m³ for the treatment plant = 480,800 m³ daily discharge, corresponding to only a 2 cm drop daily.
Looking at the drop in level from 12th July to 5th August, it's about half a metre, so that tallies with those calculations.
At the current rate of flow in the River Liffey, the lake at Golden Falls would drain in approximately 10 days. However that lake is tiny in comparison with the Poulaphouca Reservoir which supplies it with water.
Turlough Hill is the hydro control centre for all the dams in the country. Unlike thermal power stations that have to be "fired up", hydro stations such as Poulaphouca, Golden Falls and the pumped storage station at Turlough Hill can be brought online rapidly, when there's a demand for power.
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| Water level in Poulaphouca Reservoir Image courtesy ESB |
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