Tan Kin Lian's Blog: Dam and flood control

Saturday, July 17, 2010

Dam and flood control

Hi Mr. Tan

I was very surprised to see a news clip from CNA a few hours ago today (16 Jul 2010). It said that to prevent floods, the Chinese government used explosives to blast open a dam. I saw the video footage and it was
awesome!! The dam must have been incorrectly constructed, i thought in my naive mind!!

I went to internet to try to research on flood control, it seems to be a very technical and academic subject, i have difficulty understanding what is a dam, what is a levee, what is a barrage, and especially why blast open a dam, and how floods can happen. (i didn't see anything to say that massive floods occur because of blocked culverts)

I wonder if there are any experts reading your blog who could lend some help to interprete the reasons for the floods in singapore recently, and also how come a dam in china has need to be destroyed if floods are meant
for flood control? CNA news clip didnt elaborate on the issue, from the layman point of view, it seems like the dam is blocking flow of water out to sea, so blast it up. Isn't our Marina Barrage a dam too? What's the difference? Are there independent experts willing to share some knowledge?

rex

Any knowledgeable person can contact rex at rex@humbleideas.com

4 comments:

Vincent Sear said...: I heard this from chatting with a neighbourhood uncle who's being fishing up and down the rivers for decades.

Generally, a barrage is built at river mouth to control seawater from flowing inwards. In Singapore's case, many people don't realise that our rivers inland were filled with seawater up to around Kallang area (hence a.k.a. Kallang Basin). With Marina Barrage, all inland rivers are freshwater now. It's basically engineering the shift of the basin area from Kallang inland to Marina at seamouth.

A dam is built across a river of a certain size for flood control, harness hydropower or change river course, e.g. for irrigation, navigation etc.

A levee is built parallel along a riverbank. Think of it as an artificial riverbank or an embankment.

All these structures should come with various systems of drainages and floodgates to regulate the tidal levels according to weather conditions.; July 17, 2010 10:38 AM
Unknown said...: There is definetly a problem with Marina Barrage.
Common sense prevails, you are blocking the out flowing water at the rivermouth.
Nobody dares to investigate further because it is Lee Kuan Yew's personal project?; July 17, 2010 10:38 AM
C H Yak said...: Hi Rex

A dam and a barrage is built essentially for the same function ... to raise the water level behind them.

It is a dam if built across a stream of water ... e.g. like the 3Gorges dams in China. A barrage is built at the mouth of the water channel or river mouth ... e.g. Marina Barrage. Hence, for the 3 Gorges dams they cannot be barrages because the river mouth is far far away as the YangTze crosses few provinces and empties into the Pearl Delta near Shanghai.

The water level is raised upstream. In the 3 Gorges Project, it was raised by more than 100 metres at the deepest point. If there is too much water behind the dam wall, the gates are opened to let go the water ... hence a contant level is maintained ... the gates are also used to generate electricity by using the rushing water to turn turbines.

I had the experience to cross one in China on a cruise along the 3 Gorges many years back when they were building the biggest one then.

To cross the dam upstream, the vessel must enter a drydock al the lower level downstream ... water is pumped into the drylock until it is same level as the upstream water level ... the drydock gate then open and all the vessels can continue to travel upstream. And the reverse happens if you are travelling downstream.

For the barrage, since it is at the river mouth, it can also prevent water to flow in during high tide. Our Marina Barrage is used as a reservoir.

Hence, I believe if it is high tide and the dam still holds back more than required level of water, the upstream will be affected. In our case the drainage system empty into the reservoir before the barrage. Hence, the water level of the reservoir must be controlled well.; July 17, 2010 1:33 PM
rex said...: rex comments as follows,

thanks for the educational comments. i think that dams are workable in countries with terrain of proper topography. The gradient of the drainage towards the sea should be correctly calculated, because the dam will obstruct the flow and "increase the level of water upstream". This is a trade off which has to be accepted unfortunately.

I think that in constructing a dam, you need very very strong pillars to support the structure and the dam gates, many of them. Therefore the width of the pillars plonked into the sea at regular intervals, maybe staddling 20% of the width of the river mouth, WILL OBSTRUCT the free flow of water EVEN WITH GATES OPENED.

Hence i think that if the gradient of the land and the topography is not suitable it is SUICIDE to build a dam. Like Singapore. Very flat, why we have a dam?

The govt response that the dam helped to allow the flow of water is a one step forward two steps back approach. Whilst it is true that the open of flood gates will release the water, never forget that if the dam was not there in the first place, all the water would have flowed out even better, because of the pillars and superstructure obstruction and the possibly insufficient gradient of the terrain.

rex; July 18, 2010 8:31 AM