Tuesday, September 13, 2011

Dam Failure of a Coal Slurry Impoundment

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © 2011. All rights reserved.

I was recently asked my opinion on a good way to model the following dam breach event:
I …recently completed three consulting projects where I simulated the breach of three proposed coal slurry impoundments.  The permitting agency required a RAS model of an “instantaneous” hypothetical breach (over full depth, almost 80 ft for one of the impoundments).  I was able to achieve a stable model with brief breach formation time and satisfy the permitting agency.  The client (a coal company) considered the results to be unrealistic due to the rapid failure time and the fact that much of the impoundment is very viscous slurry; they have asked me to revisit the problem.  They asked me to model a partial breach of the top 10 ft, which they estimate is the distance from the top of slurry to the top of impoundment and occupied by water for the failure scenario, followed by the viscous slurry.  I was wondering if HEC-RAS could model such a complex situation.  I was thinking it might be modeled using the sediment transport capabilities within RAS.  I do have properties of the slurry, including particle size distribution, etc.  I suspect a more complex model is needed, but wanted to get your opinion, since I frequent your blog and have seen many complex issues addressed with RAS.
Thanks to Jason Hill, Ph.D., P.E. for sending in this interesting problem.  I don’t know if it is ultimately the best solution, but one that I think may work is as follows:
to model the breach of a partial water, partial slurry impoundment, you’re going to have to get creative.  First of all, RAS technically cannot model highly viscous fluids, like mud or slurry flows.  Really your only option for a “RAS-Only” model is to bump up Manning’s n values to account for the highly viscous flow.  Without a means of calibrating these high n values, you really are just guessing when you increase them. 
Here’s my suggestion:  Not sure if this would work, but what I would explore is the use of a combination of HEC-RAS, NWS BREACH, and FLO2D.  First, assume the first “pulse” of flow (water flow) will be separate and distinguishable from the second pulse (slurry flow).  The initial (water) part of the breach and the first pulse can be modeled and mapped using HEC-RAS exclusively.  For the second pulse of flow, I would model the remainder of the breach using NWS BREACH.  This model will simulate the breaching process and will generate a breach outflow hydrograph for you.  An advantage of NWS BREACH over RAS is that it provides an input for sediment concentration of the breach flow.  Once BREACH has provided you with a breach outflow hydrograph, use that as the inflow to a FLO2D model.  I say FLO2D only because I’m familiar with it and it can model highly concentrated mudflows.  But any model that you can find that models mudflows will work in this case. 
In summary, you’ll end up with two hydrographs to route downstream and to map independently:  the water hydrograph, and the slurry hydrograph.  The “water” breach will be modeled, and the water hydrograph will be routed using HEC-RAS.  The “slurry” breach will be modeled with NWS-BREACH, and the slurry hydrograph will be routed using FLO-2D (or other model capable of simulating mudflow).”
Although I know a little bit about NWS BREACH and FLO2D, I freely admit I haven’t tried this before. I think it can be made to work but I can also foresee a few hurdles.  Namely, what happens when/if the slurry flow and the water flow ultimately mix together somewhere downstream?  How do you map that condition?    If any of you out there have other suggestions, please feel free to comment to this post.  

Friday, September 2, 2011

Overflow Gates

Written by Brian Wahlin, Ph.D.,  P.E., D. WRE | WEST Consultants
Copyright © 2011. All rights reserved.
In modeling irrigation canals in HEC-RAS, a typical structure that is encountered is a check gate. Check gates are designed to back the water up behind them in an effort to keep the water level immediately upstream of these gates at a constant level. As the flow rates change in the canal, the gate openings on the check gates are adjusted (i.e. opened or closed) in order to pass the new flow rate while maintaining the water level upstream of the check at the desired elevation. Why go through this effort? Farmers usually get their water from these irrigation canals via orifices just upstream of the check gates. The flow through these orifices is dependent on the head (or the water surface elevation) acting on it. Since farmers want a constant flow rate delivered to their fields, the check gates in the main canal are adjusted when the flow changes to make sure the water level upstream of the check (and hence the rate of flow delivered to the farmers) remains constant.
Check gates can take a wide variety of forms, but typically they fall into two categories: undershot gates and overshot gates. Undershot gates are things like radial gates and sluice gates. As the name implies, water shoots “under” these gates. Inline weirs are typical examples of overshot gates. For these structures, water does not pass “under” the gate but instead flows over the top of the structure. Since many irrigation districts operate on limited budgets, it’s not uncommon to see an overshot gate made simply of several 2x4 wood boards that slide into groves in the canal walls. The irrigation canal operator “opens” these types of gates by manually removing one or more of the wood boards. In a similar manner, these gates are “closed” by adding one or more wood boards.
Modeling undershot gates has been straightforward in HEC-RAS for many years. You simply select whether you have a radial gate or a sluice gate and then enter the appropriate input data for the gate. The gate openings are then set through the flow editor-either steady state or unsteady state-depending on your situation.
Modeling overshot gates in HEC-RAS has been a little more challenging. Unlike the undershot gates, there really wasn’t a gate type that allowed water to flow over the top like a weir. Thus, you had to model overshot check gates using the geometry of the inline structure. An example is shown in the figure below. The four long white rectangles in the middle of the structure are undershot types of gates (in this particular case, they happened to be sluice gates). The two short white rectangles at the far right and the two at the far left (the ones with the open top) are overshot gates. Because in previous version of HEC-RAS, there wasn’t a particular “overshot” type of gate, you were stuck with coding these gates using the weir/embankment button. While this is perfectly fine for steady state mode where you only have one weir height setting, it becomes problematic if you run the model in unsteady mode and the weir height changes during the simulation. Because you have to model the weir as part of the geometry, there is no easy way to adjust the weir height as a function of time in unsteady mode.
Starting with HEC-RAS version 4.0, there is a new type of gate called an “overflow gate.” As the name implies, this type of gate allows water to flow over the top of the gate as shown in the schematic below. There are two types of overflow gates in RAS: open air and closed top. Both of these gates allow water to flow over the top of the structure. The difference is that the closed top gate is kind of like an elevated orifice. At some water levels, water will flow over a closed top gate like a weir. At higher water levels, the closed top gate will function as an orifice.
For irrigation canals, open air overflow gates are exactly what we need to model overshot check structures. To use this option, enter the inline structure information exactly as before. Now, select “Overflow (open air)” as the gate type. There are three types of weir shape methods: Broad Crested, Sharp Crested, and Ogee. For modeling irrigation checks, using the Sharp Crested option is probably most appropriate. With this option, there are three ways to enter the discharge coefficient for the weir equation: User entered coefficient, Rehbock equation, or Kindsvarter-Carter equation.
The gate opening is still set through the flow editor (either steady state or unsteady state). For overflow gates, the gate opening is now from the top of the gate rather than the bottom. The figure below shows a check structure modeled with open air overflow gates. As can be seen, the gate opening on the far left is 1.56 feet. But this distance is measured from the top of the gate frame rather than the bottom. Now, if you are running this model in unsteady mode and the gate opening changes (that is, another weir board is added or removed), you can reflect this in the gate opening boundary condition.image