All the critical elements and forces driving the current crisis will converge at the narrow stretch best known for the Curragower Falls opposite Limerick City Hall.
Kieran Ryan, a boat builder who navigates the Shannon on a regular basis, explained that the red alert times will be at 7.30am on Sunday and 8.20am on Monday. “There will be a high tide of 6m at these two times. If there is a strong south westerly blowing up the Shannon Estuary, this will drive the tide back into Limerick and it will form a barrier and there will be nowhere for water coming down river to go.
"The main Shannon will be flowing down into that stretch as well as the Abbey and also the water from the Ardnacrusha Power Station tailrace [discharge channel].”
This, he said, could create an “immovable object meeting the irresistible force” scenario, with consequences for water levels upstream.
Mr Ryan said: “That stretch between the two bridges in Limerick would be like a washing machine spinning.”
Meanwhile, the huge protection operation at Springfield, Clonlara, has been completed with 14 homes given tonnes of sandbag armour to keep out rising waters. Two families have already left their homes.
Local county councillor Michael Begley said: “Unfortunately the weather forecast is a matter of great concern. Even if we manage to keep houses dry, there is the ongoing problem of access. It could become a major safety issue getting people in and out of their homes by boat when the water is a metre high and there are strong winds. This is a hazardous exercise.”
Upriver, residents in Montpelier are hopeful that an 2.5m-high embankment built over the past year along a 200m stretch between the village and Parteen Weir will yield big dividends by keeping flood waters at bay.
The ESB kept the discharge spill into the main Shannon from Parteen Weir at 375 cubic metres per second yesterday. However, it will have to increase the spill from Parteen Weir due to the amount of water flowing down from the upper reaches of the Shannon.
Heavy rains forecast for this weekend have heightened fears that the flooding crisis will persist into next week.