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.