Unique drainage problem 3

[longers]

I am working on a drainage problem that presents a number of interesting challenges. In a nutshell, it involves designing a system of drains to capture and convey runoff in a large open-cast mine. Due to the nature of this type of operation, the slopes along the ramps/haul roads are such that any drains aligned with them would likely convey a supercritical flow.

As usual, there are a number of options, constraints and trade-offs.

Obviously, in this applications physical space is a significant constraint. Therefore there are advantages to designing a system that operates under supercritical flow conditions (reduced channel dimensions). However, I feel slightly uncomfortable with the relatively high velocities - erosion and scour are considerations (the rock material of the pit is not overly competent) and safety of personnel must be given priority. Initial calculations indicate that flow velocities throughout the system would be in the order of 4 - 6 m/s, with corresponding Froude numbers in the order of 1.6 - 3.0 (for the 10% AEP design storm).

I am considering including some hydraulic manipulation of the flow (such as energy dissipation devices and hydraulic jumps) in the design in order to induce sub-critical flow conditions. Obviously this would require larger drains, but the reduction in flow velocity would result in less intensive erosion protection measures and better safety outcomes.

Currently I am developing a hydraulic model of the system to better understand the channel geometries required for various flow conditions.

I would appreciate any thoughts, opinions or comments on the different options outlined above. In particular, can anyone comment on the flow velocities expected under supercritical conditions? Do you think these could be managed, or should I avoid them at all costs?

Thanks for reading, and thanks in advance for any replies.

 

[LincolnPE]

Please provide your channel rock size, channel slope, design flowrate, and channel cross section dimensions.

 

[beej67]

Sounds like a pretty reasonable application for these things, to me:

http://www.mining-technology.com/contractors/waste-management/smartditch

..Especially since you can pick them up and move them around.

If you've got a ditch drain on the inside face of a haul ramp, I seriously doubt you're going to come up with a good way to keep that flow subcritical. If an HDPE ditch liner is off the table, I'd do what they do in landfills and provide diversion berms to downdrain structures.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

waste rock from overburden should be plentiful so large riprap might be a better option. MSHA will probably require a berm anyway, so safety is not to much of an issue

drainage down haul roads is always a problem, I have not seen an adequate solution. due to active mining, frequent maintenance of the roads is not a big deal. Keep it simple, graded channels, rock lining if necessary, check dams etc. those HDPE channels will get ripped to shreds in no time. and by all means, divert as much runoff at the perimeter of the pit as possible.

 

[longers]

Thanks everyone for the replies. Much appreciated.

Sounds like my efforts would be better spent on erosion protection, rather than forcing subcritical flow.

The waste rock material on site is not overly competent, so I am thinking that shotcrete might be an option as it is plentiful on site. I think it is worth at least trialling the HDPE liners and even keeping some on hand for tactical placement during development of the pit.

I should have mentioned that the project is located in a tropical region that receives around 4m of annual rainfall!

Thanks again for your replies.

 

[LincolnPE]

Sounds like my efforts would be better spent on erosion protection, rather than forcing subcritical flow.

Think about that. You might have a happier client in the long run if you flip that. How happy will they be always chasing and maintaining erosion control? Designing that way may be exposing your system to potential catastrophic failures. The other approach would be safer.

I like a series grouted riprap rundowns, no more than about 4' high, stepping things down nice and safely, due to the gently sloped runs in between the rundowns.

 

[cvg]

lincoln, sounds great in theory until you run the numbers
i'm looking at a pit right now with two haul ramps, each about 10,000 feet long and total drop of about 1500 feet deep. 15% slope. To construct a stepped channel, 4' drops every 27 feet would require 750 grouted drop structures plus a stepped channel excavated into solid rock, 25 feet wide. Now if I maintain the 15% slope, use large, readily availble rock for erosion protection and no concrete, the channel is only 5 feet wide. Mine planner will be completely unwilling to give up an extra 20 foot wide bench

 

[LincolnPE]

To construct a stepped channel, 4' drops every 27 feet would require 750 grouted drop structures plus a stepped channel excavated into solid rock, 25 feet wide.

You have no channel stability worries on your site...you're in solid rock

 

[cvg]

well, everything below the soil overburden is rock, some competent, some not so much...

But probably the same situation with most mines and really the only portion needing any significant erosion protection is the elevations of the pit above the bedrock level or where rock quality is extremely low.

 

[longers]

Lincoln you make a good point - the client will have the system for around 20 years, and having to constantly fix erosion problems is certainly not a good outcome. I would feel more comfortable reducing the flow velocity.

Since we're going to line the channels with shotcrete, I was thinking of embedding some concrete blocks in the bottom to increase its roughness. Also like the idea of the stepped rundowns. Might be a good solution as our ramp slopes are slightly lower than in Cvg's application - around 8-11%. I'll run some numbers and see how it stacks up.

Thanks again for the replies - much appreciated.

 

[LincolnPE]

Yeah, go ahead and rough it up..sounds good.

Ahhh...you're going to sleep so much better now.

 

[civilman72]

the client will have the system for around 20 years, and having to constantly fix erosion problems is certainly not a good outcome

While some owners may find long-term maintenance as undesirable, a mining company will probably not consider ongoing maintenance of drainage infrastructure (i.e. roadside ditch) as a major problem. They should have the equipment and manpower to handle it, and will already be doing periodic road maintenance and repairs. I would assume that the mining company will consider the time-value of money and will find it easier to budget for the minor annual costs associated with periodic erosion control maintenance and repairs, rather than trying to budget for a large up-front cost for construction of significant erosion control infrastructure. And remember, no matter what is installed (i.e. grouted riprap rundowns), it will all require repairs/maintenance at some point.

I design roads in steep mountain areas and have found that trying to maintain subcritical flow in a roadside ditch is usually impractical, both from a cost and construction standpoint.

 

[longers]

Thanks for your input, civilman72. I'm very interested in your experiences in steep mountain areas. This is particularly valuable:
I design roads in steep mountain areas and have found that trying to maintain subcritical flow in a roadside ditch is usually impractical, both from a cost and construction standpoint.
It seems like we will need to balance the trade offs between achieving acceptable erosion levels and the practical realities on site. I appreciate your comment about the inevitability of significant maintenance of this system, but I'd also like to make the system as low-maintenance as possible (the site has a number of other issues such as security, and I can see maintenance of the drainage system being neglected).

I've also put together a hydraulic model of the system under the design flow rate, and I'm seeing less extreme velocities than I'd originally feared. Am now thinking that some well-placed rundowns as suggested by Lincoln, plus some small retarding basins at disused bench levels to attenuate the flow are worth pursuing.

Thanks again

 

[LincolnPE]

Either way you go (sub or super) make sure you design a hydraulically stable channel, staying a good factor of safety away from the minimum specific energy for the channel design, so the channel is not capable of going very erratically unstable on you, bouncing between its sequent depths.