"The Senior Parliamentary Secretary, Ministry of Environment and Water Resources, said that six out of the seven gates at the barrage were left open to manage the amount of rainfall during the heavy thunderstorms last month."
- Today 12 Jul 2010[Link]
Here's a set of pictures (courtesy of Kojakbt) that appeared in the Straits Times explaining how the crest gate works to alleviate floods:
You look at the middle diagram which shows the crest gate lowered to allow storm water to go into the ocean. The pumps in the barrage are not used in this situation. The gate which has been retaining water is lowered to allow fresh water to flow into the ocean. Can someone tell me how this alleviates floods? When the barrage did not exist, water flowed unhindered into the ocean. Now we have to wait for the crest gates to open...and when they are fully open the passage for the water is much smaller than before when the barrage did not exist. When the gates are closed there is a lot of back pressure under heavy rainfall causing water to accumulate in the drainage system behind the barrage. It was reported that the crest gates were lowered during the recent floods so we did not face the situation where the sea level was higher than the fresh water level i.e. the pumps were not used during the recent floods.
I can understand that the barrage can block water from the sea when tide rises but I really can't understand how the barrage 'alleviates flood' when rainfall is heavy and the fresh water level rises above sea level. Can someone explain to me how having the barrage helps (compared with not having it)?