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  • Dams are structures that hold water back

  • and control its elevation.

  • For thousands of years, and in nearly every civilization,

  • they have been used to store water,

  • facilitate irrigation,

  • control flooding,

  • and provide the elevation changes needed

  • to extract power from flowing water.

  • As the first video in this series showed,

  • water exerts pressure on any surface that it contacts,

  • and the magnitude of that pressure is proportional to the depth, h.

  • For a dam to do its job, it must resist the forces generated by these pressures.

  • Common strategies for doing so include buttress dams,

  • embankment dams

  • and arch dams.

  • To make ponds for mills

  • and to control water elevation in rivers,

  • another strategy is often used.

  • It consists of vertical piers

  • with straight, horizontal members between them.

  • The water pressure that pushes against

  • these horizontal members is carried by bending,

  • and so these members function as beams.

  • We can build a dam that works in this way

  • using a flat sheet of cardstock

  • to represent the horizontal beams.

  • We insert the ends of the sheet

  • into the grooves of the Plexiglas box

  • so that they can support it, like the piers of a real dam.

  • For simplicity, we use marbles to represent the water.

  • Which part of the dam do you think will collapse first?

  • The top?

  • The middle?

  • The bottom?

  • As the depth of the model water increases,

  • the cardstock bends, until the dam collapses,

  • unable to resist the lateral loads.

  • The marble depth that produced failure is 4 centimeters,

  • and as you can see, failure started at the bottom.

  • That should not be surprising,

  • since the pressure is greatest there.

  • Another strategy for making a dam

  • is to use an arch laid on its side, like this.

  • If you watched our "Arches and Chains" video,

  • you will know that arches can be very effective in load carrying

  • and that they carry load by compression.

  • To build an arch dam,

  • we make the cardstock panel a little longer

  • than the distance between the grooves.

  • Then when we install it,

  • it automatically curves into an arch shape.

  • As we pour in the marbles, raising the model water level,

  • the cardstock resists the resulting pressure

  • by compression, just like an arch.

  • As you can see, an arch shape like this

  • can carry a greater depth of marbles

  • than our earlier flat design.

  • Even so, the arch dam also eventually collapses

  • when the model water level

  • reaches a depth of 9 centimeters.

  • As before, failure begins at the bottom of the dam

  • because the pressure is greatest there.

  • As this model has demonstrated,

  • an arch-shaped dam can support

  • a much greater depth of marbles or water

  • than a corresponding flat one.

  • That is why arch dams are so often used for big, tall dams.

  • As a side note, you might be interested to know

  • that the concrete in tall arch dams

  • is usually made thicker toward the bottom

  • so that it can better resist

  • the higher water pressures there.

  • We hope that this video helped you

  • to better understand how flat

  • and arch-shaped dams work.

  • To learn more about how soil and water interact

  • with other kinds of structures,

  • we hope you will view more of the videos in this series.

Dams are structures that hold water back

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B1 US dam arch water pressure elevation horizontal

7. Dams

  • 19 1
    Jeff posted on 2020/06/23
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