Calculating particulate entrainment from riprap

[fernley]

I've been using an equation from Izbash(1970) to help predict stable stone sizes for riprap which could undergo river flow rates of up to 10m/s, unfortunately, the resultant Dn sizes seem rediculously large. Is there a better more realistic formula for estimating the size at these high flow rates?

 

[Ron]

Think about it a minute...at that velocity, you need large stone sizes! How large are you getting from the equation? Maybe the equation is more realistic than you think.

 

[cvg]

10 m/sec is over 30 feet/sec which is very high, supercritical flow. riprap is not a particularly good method to protect river banks against such high velocity flows, it requires very large sizes with resulting very thick layers and high cost. It is usually much more economical to use concrete or to employ methods of reducing the velocity.

USACE has a design method you can use to verify your calculations, it is available online.

Publication Number: EM 1110-2-1601
Title: Engineering and Design - Hydraulic Design of Flood Control Channels

http://140.194.76.129/publications/eng-manuals/em1110-2-1601/toc.htm

 

[fernley]

Unfortunately my hands are tied with the requirements of the Environmental agency, because apprently there are 'tiny creatures' that like to burrow into the bank and they are looking for a 'natural' way of solving the problem. Even a gabion solution is unacceptable to them.
With regards to the calculation, the formula requires larger rock sizes in a linear fashion relative to flowrate. However, as I understand it, the rocks are sized at a nominal diameter and are assumed to be spheres for the convenience of the calc. This does not produce a linear increase in weight relative to the linear requirements for larger diameter rocks, therefore at 10m/s an average rock size of 7m dia is required, weighing in at 511te, which seems a tad heavy to me...I've added a pic. of the bank in question

 

[Ron]

Where does the 10 m/s come in? Flood stage? Looks rather placid and the rocks that are there are cobbles or smaller.

 

[fernley]

Its due to a bit of clever design by the Environmental agency. A hydralic dam further upstream diverts water into a reservoir which then overflows back into the river directly on the outer radius of our river bend. At full flow the water is about 12" below the top of the bank so it rises about 8'. The outflow washes soil and debris down which effectively narrows the river at this point increasing flow and exacerbates scouring on our bank. I've uploaded a short film, though looking at it, I'm wondering if my 10m/s flow is right...

 

[fernley]

I've just been down to measure the flow on the river after the levels have been elevated 2' by the snowmelt and the river's still only doing 1m/s, looking at it now 10m/s seems ridiculous. I can't afford to wait for another flood event, the river could potentially rise another 4' any ideas on how to work out maximum flow?
PS I've calculated a flowrate of 2.7m/s gives a stone Dn of o.4m

 

[Ron]

Yes...you might want to check that velocity....that would be shooting a canoe going downstream at 36km/h (20+ mph)... Wouldn't there be more erosion of the bank than is currently showing?

 

[fernley]

Hi Ron, Unfortunately, flood events are unpredictable and I only have a narrow time window to work with live willow, I really need to carry out remedial work before the spring

 

[cvg]

measure the depth, width, and slope, estimate mannings N and calculate a normal depth. If I had to guess, you are probably not exceeding 10 feet/sec