Natural bank protection 4

[Geolearner]

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.

 

[VAD]

The answer to your concerns could lie with the observation of the condition of the existing river bank rather than with slope stability calculations perse. The book Protection of River and Canal Banks - a guide to selection and design by RW Hemphill and MEBramley would be a useful reference.

 

[BigH]

See US Army Manuals as well. India has a Guideline available as well. The size of river bank protection will depend to a large degree on the velocity of the river. Large flow velocities will require larger rock. I seem to remember (offhand) that for flows of up to 1.5m/sec, 75mm dia stone is sufficient for protection. I can find out IRC's guideline values if you wish. Just advise.

 

[Focht3]

You referred to this as a slope stabilization project. Why? Are we missing something, or am I reading too much into your original message?

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.

 

[Geolearner]

Sorry guys, the question is confusing indeed. However, what i am doing now is: first to check the stability of the cutting before implementing the river 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.

 

[VAD]

Hello geolearner:

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

 

[Focht3]

What is the total height of the embankment - from the toe (below the water) to the top-of-bank?

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!

 

[cvg]

have you considered the rapid drawdown case in your stability analysis. Assume the river flows at flood stage and then the flood passes. The phreatic line within the river bank is quite high and high pore pressure causes the FOS to go down. Will there be a chance of this happening prior to the bank protection being constructed?

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.

 

[BigH]

cvg - your comment on rapid drawdown is a good thought. I agonized over this once in Toronto when I didn't put in any significant barrier between the river bank and the site's admin building. Why - because if the riverbank is clayey, and you are talking of a quick flood (freschet), then the duration is so very short that there is no real siginficant inflow of water (therefore phreatic surface) inside the bank. If the river was long time flooded (like a reservoir), then this is different. - Just a thought on your comments.

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.

 

[cvg]

depending on the channel geometry and the water velocity and flow characteristics, scour should be carefully considered. Don't forget to protect the "toe" of the bank. This is a critical point, if you get a scour hole at the toe, the bank lining may slide into the hole, or be undermined. This will fail the lining. A cutoff trench filled with rock or concrete or alternatively, a horizontal mat of gabions extending some distance towards the center of the river will protect against the undermining.

 

[bartshippee]

I have a corrolary question to CVG's latest comment. How does one determine the "extension" distance of the mat in lieu of an anchor trench? I have a large sewage lagoon (800 ft across) that is scouring one bank from wave action. The level is relatively constant until the summer months, where it drops severely as the water is used in a spray irrigation application. The existing embankment is quite steep (and long) and presents a problem for construction equipment trying to dig an anchor trench. I would like to recommend a gabion mattress with a long runout at the base rather than the conventional anchor trench at the toe. 10 feet past the lowest water level is my gut feeling. Guidelines or suggestions, anybody?

 

[cvg]

We recently designed a flood control channel with 9 foot apron at the toe of the bank. This was convenient because the gabion mat comes in a 9 foot width x 100 foot roll. However, the length was based on scour estimates. Our calculated scour depths were in the range of 6 - 9 feet for most of the channel. Flow rates ranged up to 6,000 cfs

 

[GabionGuy]

The "extension length" for gabion mattresses is typical 1.5 to 2 times the expected scour depth.

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.

 

[Focht3]

Theoretically, the PVC lasts longer. But practically it doesn't always. I suspect the problem has to do with the installation...



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?"

 

[layfieldman]

You could also consult with a CPESC on the options for bioengineering. I know that they've had some success with using natural vegetation for bank protection.

 

[Prepakt1]

You can also use concrete filled mattress or concrete filled bags for river slope protection probably up to 60 degrees .We used them on the Thames in London on very steep banks. Concrete mattress 200mm thick is rated for flows of upto 5M/sec.