Culverts and how to survive them

With the ever increasing storms that we are facing in both town and country we need to rethink how we do things. This is particularly so in the culvert and stormwater management area. It is most interesting to see that the front page article in "Straight Furrow", dated 14th August 2007, talks about "Culvert Chaos". This article talked about the results of blocked culverts causing chaos and a near death experience for a Taranaki couple.

We were lucky enough to study roading, drainage, crossing and culvert construction at university and this has always stayed with me. Our monsoon type rainstorms are becoming very severe and at an increasing regularity. We need to be more aware of the issues and problems this can cause.

The flooding damage at Matata township was substantially reduced thanks to a topdressing pilot spotting the burst dam behind the township and alerting the authorities. It was the burst dam that caused the domino effect, causing so much damage.

My family trust owns large commercial buildings in Hamilton with equally large roofs. We have had to repitch the roof, increase spouting sizes, open overflows, all to carry away excess water in these monsoon type rains.

The hydraulic pressure of water is unbelievable. A raging river will pull big boulders downstream. You can actually stand beside a big raging river and hear the rocks rolling downstream in the water.

So what can we do?

Firstly, don't build houses anywhere near an area that may flood as a result of torrential rains further up the catchment or valley. If you do have an existing vulnerable building, get a removal truck in and shift it on to higher ground. This may sound expensive, but the cost of shifting a house is not that great, compared to losing everything you own. Look above any potential house site to ensure there are no slips that may demolish it. Keep the house site away from any depressions that could form a bed of a water-laden earth slump.

Secondly, think carefully about dam construction. Dams are logically placed in gullies to collect water. Could it collect more than you bargained for? Is the land above generally stable or could it be wet, sodden land ready to slip in adverse conditions and demolish your dam.

Thirdly, ensure that you have more than one culvert pipe in your dam and make sure the pipes are big enough. If one blocks, the other can take over and so on. If you think a 250mm pipe is big enough, put in a 500mm and put in two.

Fourthly, compact the dam and construct it properly. Plant trees strategically to help stabilise the dam in its initial years. Do the job properly the first time and consider all your options.

Fifthly, cut an overflow into hard ground. In other words, cut an area lower than the dam top to allow extreme water to flow across hard cut land and on down the valley. If your culverts block and you get a flash flood going over your earth dam, it could be sucked out. You must have it on hard cut land. Now this may cause slight inconvenience for crossing tractors and vehicles, but believe me it is better to have the dam there, than have it washed away.

Lastly, stop and think about what you are doing. What could happen further up and so on? Go for a walk up the valley; think about the catchment area, other dams that might fail, other areas that could run water into the valley and so on. It's not just about the dam or dams you have on your property. It's like a snowball. If one dam lets go further up it may wipe out several other dams, collecting water, debris and speed, creating absolute mayhem further down. These water bombs are a relatively new phenomenon and we all need to look at our existing structures and make sure they are up to the rain dumps that we seem to be experiencing.

The reason for this article is not to frighten you but to remind us about water, its power and the damage that it can do. Apparently silt laden water is even heavier and far worse than just water for doing damage. It is also about being proactive with better dam construction, bigger outlets, better planning and hand cut spillways for those unexpected storms.

It is easier to do the work now and have preventive maintenance because prevention is better than cure.


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