With this flash flood experiment, you will get an idea of how an area can flood due to intense rainfall or a rainfall long in duration.

Materials:

Adjustable spray bottle
3 - Dry Sponge
1 - Wet Sponge
Large dishpan
Water

Procedure:

1) Place one dry sponge in the dishpan and tilt the dishpan so the water would run to one end.

2) Set your spray bottle to spray a mist.

3) Spray the highest end of the sponge a few times.

4) Place another dry sponge in the dishpan.

5) Adjust stray bottle to shoot a steady stream of water.

6) Spray the second dry sponge at its highest point numerous times.

7) Place the wet sponge in the dishpan and spray its highest point with a mist until the sponge is completely saturated with water and water is no longer absorbed.

What's Going On?

The first part of the experiment (Steps 1-3) shows a slow delivery of water to the sponge. The sponge has no problem absorbing the few sprays of mist.
During steps 4 and 5, the steady stream of water is too much for the sponge to handle, hence the water runs off without being absorbed by the sponge.
Lastly, steps 6 and 7 shows us that the sponge will absorb as much water as it can. Eventually the sponge can't hold any more water and the water runs off.
      The sponges represent our earths soil.  When the rain is of low intensity, the ground has little trouble absorbing the water. When the rain is too hard, the soil can't absorb the all the water and we see the excess water run off.  If the rain event is of long duration, the ground will absorb as much as it can, but eventually, it becomes saturated the extra water will begin to runoff. This extra water can cause flooding in streets, near rivers, and in fields.