River Bank Erosion

[FishFinger]

Hi all,

I have a 4 rai piece of land located on the Ban Pakong River in Ban Sang district of Prachinburi province. My wife and I intend to build a house there very soon, but we have a problem with river bank erosion.

Just over a year ago we filled in soil and landscaped the area in order to make an elevated area for our house above the floodline. We were horrified to see that part of the bank subsided and collapsed in one spot due to a combination of removing bushes and the extra weight of soil too near the river. We got some local advice at the time and put in a zig zag line of 8 m pilings a few meters back from the edge. Within a few days most of them had fell over and took more soil with them. You can still see a few pilings still sticking out along the river's edge in one of the photos and a pump house for sprinklers that was originally 2 m higher than it is now.

Yesterday I went with my wife to the Civil Engineering office in Prachinburi. They told us that a proper dyke can cost up to 100,000 baht per m and the land is 58 m wide! This involves using 18 m pilings. The only other option would be to keep a gradual slope to the river and to plant bushes and trees to prevent any further erosion (I was thinking "Don Sai" which forms a rigid structure of tap roots reminiscent of mangroves. The soil in this area is clay and building anything is problematic here.

Are there any experts out there who have any ideas that could be of help? We worry that if there are any problems after the house is built, then it might be difficult to get heavy machinery in and it would be best to sort the problem out now.

Any help would be appreciated.

Best regards

Fish Finger

[chownah]

A couple of ideas for the construction.

1. Build the house near the river but build it on piles big and deep enough so that even if the river bank eroded from under your house it would still stand and be structurally solid.

2. Drive piles at the existing water's edge and drive them to existing water level(more or less depending on constructibility and aesthetics....then excavate behind them making an inlet from the river back far enough that you feel comfortable building the house....the house is back from the river proper but has the appearance of being on a small inlet of the river.....you could even build your own little side channel if you wanted to spend a few more bucks and be creative...then you'd have your very own artificial island!!!!

Just a couple of ideas. Sounds like that anything you do at that site should be professionally engineered unless you can stay well back from the river and build with conventional piles more or less at grade...and even then the piles should be engineered.

Chownah

[macan]

Sheet metal pilings and rock gabions are the usual 'hard' approaches to bank stabilisation. 'Soft' approaches include, as you suggested, using vegetation to provide stability in the remaining bank; vetiver grass is used in the Mekong Delta and I believe that the Asian Institute of Technology has done a lot of work with this plant in various applications. Another soft approach is to use structures, including living trees to promote siltation; live willows can be woven into hurdles in a process known as spiling or bundles of sticks can be anchored to the bank.

Good luck!

[FishFinger]

Thanks for the help on and off the list with ideas to prevent river bank erosion. To be honest, I am no nearer a solution than I was before. If I do go for growing plants, then I need to be careful that they can survive underwater certain periods, as the water level is very high in September and October. If I go for pilings, then it seems pretty obvious that it is a big job that has to be done right.

Interestingly there was an article in the Daily News which showed how to prevent sea erosion and reclaim back land. They use triangular posts arranged to make a curtain. Silt flows easily one way, but not so easily back and so sediment builds up inside the barrier.

Going over a bridge on the way to Bangkok last week I noticed that many landowners have postioned lines of posts about 1 m apart outside their properties. They are positioned just along the water's edge. I will try and find out if this is related to preventing erosion of the bank.

Best regards

Fish Finger

[Gary A]

Rocks, LOTS of rocks. They don't wash away and although it would be expensive, probably the best long term solution.

[NotYetArahan]

Chownah seems to be always right: build the house away from the river on properly engineered piles. Let the river and the banks play with eachother or protect with vegetation. A hard protection is not so beautiful and indeed expensive.

If you go for a rock protection it is large: it should protect between low and high water, the clay should not wash out underneath, so you need a geotextile layer, small rock and larger rock. Or mostly more expesive concreting. Also slopes of rock berm 1:2 steepest. Thickness count on 1.0 meter approx. Also you probably need to protect the toe: extra tonnes rock. Also calculations should be made for slip circles possibly resulting in more rock at the toe or shallower slopes.

Sheetpiling can be done, but probably the toe need to be protected against scouring. Good anchoring far landwards is required. calculations and some soil investigation required.

Gabian rock is possible but the difference between low and high water will require large quantities as per rock berm

What is the scouring tendency at that location? Are you at an inner or outer bend or a straight? If the tendency is your land will be eaten away you should go for expensive solutions. Probably sheetpiling

[chownah]

Rocks, LOTS of rocks. They don't wash away and although it would be expensive, probably the best long term solution.

The old school rule of thumb for river bank erosion control in engineering circles is to use rock...and if that doesn't work use bigger rock...and if that doesn't work use even bigger rock...and if that doesn't work then construct a concrete channel.

In using rock you must be sure that it is heavy enough to not get moved in the high velocity flow of flood conditions....this often means big rock....consult an engineer to find out how big is necessary for your location.

Also, in using rock the rock must sit on something solid enough to support its weight and the weight of all the rocks using it for support and anything else that gets constructed on top of the pile of rocks. In areas where the soil is soft like alot of river banks in Thailand when you put a large rock at the bottom of a river it will just slowly sink into the mud until it reaches a layer solid enough to support it. This means that it might take alot more rock than you might think...and after it is constructed it might destabilise after awhile as the rocks sink into the mud over time....again an engineers advise can be very helpful.

Also, if there is no source for large rock close by then rock can be very expensive.

About gabions......gabions placed in water will deteriorate over time and it must be considered that after a number of years they will turn into a loose pile of rocks so this must be considered in the design of both the gabion and of the installation overall.....once again an engineer's advise will help.

You can try to do these things without the engineer.....your success may vary.....be sure to check out what the locals have done and try to find a way to evaluate its success. Just because your neighbor did something last year to stabilize his river bank doesn't mean that it will work and even if it does work there it doesn't mean that it will work on your river bank as conditions do vary.

[NotYetArahan]

Indeed,

carefull engineering is required after a soil investigation (CPT is sufficient). Having soil info and topography survey, the calcs and coming with solutions take only a few days for an experienced geo engineer.

Brace yourself for the rock structure which will be engineered. Anyhow, for each soil type there is a solution. Soft clay is not too suitable for a rock berm protecting a slope (rock package sliding down). In that case sheetpiling is probably better.

[FishFinger]

I should have mentioned that the river is about 8 m deep and the soil here is clay. Most building use pilings that simply work through friction. I am sure the bedrock is very deep here. There are no boreholes round here, as even people who have gone very deep have had difficulty finding much water. I guess it is difficult finding sediment with porous quality that allows water passage.

Our land is about 100 m before the start of a bend and so erosion shouldn't be too bad. We have thought it over and have decided to plant "don sai" (there are a few varieties) and "don lampoo". The first plant sends down air roots from the branches that form a matted and very rigid structure. "Don lampoo" looks a bit like mangroves as roots grow out of the trunk above ground and give it a very stable base. Both plants can survive extended periods of flooding and are recommended for river banks. I have aquired a few plants already and will make a trip to Klong 15 on the Nakhon Nayock - Rangsit road which is a major trading point for all sorts of plants.

Concerning the use of rocks, a landowner on the outside of a bend just used this method, but he put the rocks behind a barrier of pilings. The river is like a black hole of silt and it would take an enormous amount of rocks to get up to the level of the river bank if we just threw them in. We are talking 58 m of river bank as well.

Thanks for all the ideas.

Best regards

Fish Finger