I don't know if anyone remembers me. I'm not a physicist but I tend to do pretty well at understanding some of the basic principles of classical physics, and that's recently created food for thought on my part. There's a message board I've found that is devoted primarily to debunking popular myths and conspiracy theories, but there's plenty of sensible talk about current topics there too. There's a discussion there about the Oroville Dam incident, and some ideas have been floated regarding what might have led to the failure of the floor of the main spillway. One or two people are suggesting that it was the Bernoulli Effect, where as water flowing over the floor slab of the structure speeds up due to the steepening downward slope, the pressure applied to the slab decreases so much that the slab is lifted from its base by the pressure of non-moving water underneath the slab (and yes, the soils below that slab must be perpetually saturated - there's no way to prevent it). I'm not accepting that the Bernoulli Principle applies in such a case. For what it's worth, I spoke with an engineer who remembers having had a number of classes regarding the design of open structures for water flow, and the Bernoulli Effect was never mentioned in that context. Anyway, I'd like some help either supporting or modifying/disproving some ideas I have for why Bernoulli's Principle was not a cause for this failure. My First Reason For Not Believing This: Okay, the idea that the slab of the spillway might lift due to increasing the travel speed of the water flowing across it is not too outrageous on the face of things, given that virtually everyone knows that the reason that some kinds of flat roofs on buildings are weighted down with a layer of gravel is to reduce the tendency of the roof to lift due to differential pressure during extreme wind. In fact, one apartment building I lived in had a 50-pound trap door to the roof, held down only by gravity over a set of guide rails, and on one very windy day, that door lifted up and flipped over, hitting the roof with a terrible bang. But I think there's a key point here. In a building, whatever pressure is exerted against the bottom of the roof by still air contained within can expand to maintain that pressure as a section of the roof (or heavy trap door) rises due to the reduction of air pressure on the top side, and due to the rather large amount of reserve volume, the air pressure on the bottom side will remain essentially the same as it does so (the reduction in pressure as that inside air expands very slightly is negligible). Thus, the air in the building essentially "maintains contact" and "keeps applying pressure" as the roof rises. Now, can the same happen with water on the higher-pressure side of a concrete slab? For all practical purposes, water is the same volume at all pressures, and conversely, it has no ability to expand when pressure exerted on it by an outside force is reduced, so even if the pressure applied by flowing water on the top side of the slab were reduced to zero (and even if the slab had no weight of its own), the water beneath the slab would be unable to expand and cause a lifting force. Right? Correct me if I'm wrong. My Second Reason For Not Believing This: I understand that any fluid being forced through a pipe or hose will experience a reduction in pressure at locations where the cross-sectional area becomes less and the velocity must therefore increase. Though I have long since forgotten how to do the math, I understand that the principle is that the total energy of the moving material cannot change, so if velocity increases, pressure must decrease, and visa versa. However, the dam spillway in question is just an open trough, and as such, the pressure exerted by water on the *sides* and *floor* of the vessel is at least in part (and perhaps entirely) a function of the mass of water that is present, and the force applied on that material by gravity. I don't see that simply increasing the speed of the water and changing nothing else can reduce the force applied to the trough because that would be the same as making the water weigh less. Taking this a step further, in this spillway situation, when the water increases speed as the slope becomes steeper (and perhaps due to its downhill acceleration), the volume of flow per unit of time (usually expressed as cubic feet per second in such cases) must remain the same since "what comes in" to any given section of the trough equals "what goes out". With that in mind, if the velocity of the water were to double within a certain section of the trough, the depth of that water would be reduced to half of that prior to the velocity increase, so volume passing per unit of time remains the same. Now, here's what I'm thinking. I don't know if there might be other ways in which the pressure of the water might be reduced when its velocity in an open trough increases, but there should be pressure (or some component of the total pressure if things are more complex than I know) applied to the floor of the trough no matter what happens, and that pressure should be the weight of the water per unit of floor area. For example, if water weighs roughly 62 pounds per cubic foot, a six-foot thickness of flowing water will apply a pressure to the floor of 372 pounds per square foot, and if the velocity were to double so that the thickness of the flowing water were cut in half, the pressure applied to the floor would be 186 pounds per square foot (for simplicity, I'm ignoring the fact that the normal force applied to the slab is not vertical, and thus is somewhat less than the full gravitational weight of the material). Is that correct? If so, it seems to me that there would be no way to reduce the pressure applied by flowing water to the floor of the spillway to less than that which is due to the water's actual weight. Further, recalling Reason #1 above, the water beneath the slab would never be able to "push back" to lift the floor in the first place, since all it can do (assisted of course by the soil who's pore space the water occupies) is support the force of the slab that is due to gravity, whereby any reduction in that applied weight (such as mechanical lifting of the slab by some outside force) will simply result in an equal reduction in applied force from below, the same as what happens if you don't apply all your body weight to your bathroom scale. So, am I totally wrong in my assessment of this problem, or am I missing something? Thank you.