Discussion: what is a flood?

Too much water in too little time forces the water into places that are normally dry. Floods are the most deadly of all natural disasters, including tornadoes and hurricanes.

Pennsylvania has always been especially vulnerable to floods

Ask students: Did you know that Pennsylvania is more prone to floods than any other state? Why do you suppose that is?

Then lead a discussion to elicit some of the reasons. The following are reasons found on various Pennsylvania state sites, the Army Corps of Engineers, and the National Weather Service.

Why Pennsylvania is more prone to floods than other states:

• Our hilly terrain creates thousands of creeks and rivers in every valley.
• Because of all the hills, heavy downpours drain quickly into the streams and rivers before they have a chance to sink into the soil.
• Our soils contain a lot of clay, which absorbs water slowly, making more run-off.
• When clouds coming from the west hit the mountains, the lift they get “squeezes” rain out so that the ridges — including the Laurel Ridge — receive more rain and snow than towns farther west receive.

To make matters worse, Pennsylvania suffers more damage when floods do happen:

• Steep hills and easy river transportation makes our flat floodplains the best place to locate towns and factories, which means they are right in the way of floods (that’s why they call them floodplains!)
• Do you remember a flood ever occurring in your community? What kind of damage occurs in a flood?

Floods do damage by:

• Producing moving water with tremendous power
  • People who have not experienced a flood often don’t realize the dangers of moving water.
  • The power of the moving water is magnified by the debris that the water carries: trees, vehicles, boulders, buildings, etc.
  • Fast-moving water can sweep up everything in its path, throwing it against everything downstream and leaving terrible destruction behind.
• Leaving behind filthy dirt and debris
  • The water itself can ruin things, like books, furniture, photographs, and electronic equipment that shouldn’t get wet.
  • The debris in floodwaters also contain chemicals and disease germs. This means that floods usually pollute drinking water supplies.
  • The mud and debris floods leave behind not only cost a lot to clean up, but they are also a health hazard.

Man-made ingredients that can make floods worse:

• Obstructions in the rivers (bridges, natural obstructions like sand, silt, rocks, fallen trees);
• Changes in the shoreline;
• Changes in how fast the water runs off the land by building, mining, lumbering, etc.

Things that change the way water runs off the land:

• Buildings, roads, and development that cut away parts of hills and cover the soil with pavement. Water runs off quickly without being absorbed;
• Taking away vegetation, which absorbs tremendous amounts of water, keeps soil in place, and reduces erosion;
• Conduits that carry water into pipes below roads, etc., can get clogged with debris; without natural banks for extra water to escape, it becomes a dam instead of a conduit; water backs up and causes flooding behind it;
• Draining wetlands leaves fewer places for water to lay while it slowly soaks into the ground.

Overview of the Johnstown Flood of 1889

The worst flood in United States history happened when all the ingredients of floods combined in one big recipe for disaster.

Reading

As you read the story of the Johnstown Flood of 1889, make a note of every flood ingredient mentioned.

Activity

Activity