Natural bank protection
Natural bank protection
(OP)
Hi there,
I'm currently involving in a river bank (slope stabilization). The proposed river is to be widen by cutting back the river bank 1.5 m back and 1 m depth from the ground level. The angle of the slope is 35 degree. The scheme is to increase the capacity of the river to prevent flooding.
As part of the slope stability check, i also wanted to check the cutting against scour and softening effect due to the rise in water level.
Can anyone help with some technical info or any useful design manual and textbook which talk about the natural bank protection.
Your help will be very much appreciated.
I'm currently involving in a river bank (slope stabilization). The proposed river is to be widen by cutting back the river bank 1.5 m back and 1 m depth from the ground level. The angle of the slope is 35 degree. The scheme is to increase the capacity of the river to prevent flooding.
As part of the slope stability check, i also wanted to check the cutting against scour and softening effect due to the rise in water level.
Can anyone help with some technical info or any useful design manual and textbook which talk about the natural bank protection.
Your help will be very much appreciated.





RE: Natural bank protection
RE: Natural bank protection
RE: Natural bank protection
I concur with the comments made by VAD and BigH - good advice.
In slow moving streams, I've gotten good performance with gabion mats and 75 mm stones. If you go this route, I'd pass on the vinyl coated wire and only use the galvanized product - it lasts longer. The gabion mats trap soil over time, and vegetation grows in them. You will have a very natural setting within a few years of the original construction with the gabion approach.
RE: Natural bank protection
There is a slight change in the design now. The existing river bank will be cut back 2m from the crest and the slope angle is around 40. The height of the cutting is 3m. The FoS of the stability i found is marginally close to 1.
The shortcoming of the design is that the cutting must complies with the given geometry because of limited land use.
RE: Natural bank protection
I am taking a wild guess that the FOS near to one is not the the only value you have determined from your slope stability evaluation. Often one obtains a range of values depending on the values of the parameters used. I would also venture to say that I will be comfortable with the nearly 1 FOS. I have seen too many nearly FOS 1 slopes that stand for years. Alot depends on choice of parametric values. Here is where you agonize and make your judgement from gut feelings etc. Look at the pessimistic and optimistic range of values and invoke your experience and judgement.
Focht3 has provided you with the use of gabions. Depending on the observations made of the existing river banks, this is a good solution. You may also concieve that the gabions may be required at the toe and that riprap and larger rock can be used on the slope. The gabions could be more attractive depending on costs, materials procurement etc. There are aslo some proprietary products that are handy in the event that rock is hard to obtain.
Remember slopes are strong in the short term but weak in the long term hence calculations must invoke both coditions. The effects of rapid drawdown must be looked at as well. The safest condition is when the slope is submerged but this event has other effects as well.
I am looking for a paper on river bank performance which may be useful to you, but cannot locate it at this time. I think it was from the International Erosion Control Association (if I have the name right) Proccedings perhaps between 1986 and 1993. If I do I will provide the reference.
Good Luck
RE: Natural bank protection
What are the possible consequences of failure of the cut - and protected - slopes? Is your client willing to take the risk?
VAD's comment about FOS=1 not being the 'only' FOS you have calculated is on target, and I have also seen slopes with "theoretical" FOS's near unity that have remained stable for many years. But I have also seen the converse occur: "theoretically" stable slopes that failed - often spectacularly. You have not told us anything about the site location, soils, parameters you used in your calculations, etc. And a 40 degree slope concerns me - a lot. That's 1.2:1 (H:V) - a very steep bank.
Anyway, a good thread. Let us hear from you!
RE: Natural bank protection
This may be too steep to place gabion mattress - usually this is limited to approximately 2h:1v or flatter. same with riprap, only the slope should be closer to 3h:1v. you could use stacked gabion baskets which can be designed with a vertical face.
RE: Natural bank protection
For scour- that is from my first comment and Focht3 rightly points out that gabion baskets work too. On my current job we have "slope protection" by dry stone masonry - why? I can't figure it out other than it "looks good". There is no flows (gradiants of 1m in 2km) so what scour would we get. In your case, with river - you need the velocities like I said. Gabions are a good choice and, in my case, I would recommend that such blankets be used as there is no way that even a diligent contractor will be able to construct a dry stone pitching to the degree to pass project auditors (read that Indian or Chinese auditors). So, a blanket with "correct" thickness would be best policy.
As for your slope stability - if your bank is only 3m high - this is not high. What you are probably the most worried about is erosion/scour of the bank - get it lined with stone, good vegetation. Your existing bank is probably standing okay - if cut at similar "angle" the new one will probably behave similarly given same water flow characteristics, etc.
Best regards to all - sorry I've been off for a while.
RE: Natural bank protection
RE: Natural bank protection
RE: Natural bank protection
RE: Natural bank protection
Gabion structures are also easily designed and constructed as soil bioengineering structures (Green Gabions, EnviroLogs, etc.) These are fully functional gabion structures, as well as functional riparian habitat. They have proven to be a very useful for the sort of project described. Depending upon the project, vegetated structures can be constructed with slopes up to 1H:1.7V (not a typo!).
Focht3
Your comments on the choice of galvanized wire over PVC coated galvanized wire concerns me greatly. Unless there was something wrong with coating on your projects, PVC coated galvanized wire will last much longer than straight galvanized wire. I would be very interested in discussing this further.
RE: Natural bank protection
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Natural bank protection
RE: Natural bank protection