Hi, I’m James LaRosa, the Service Hydrologist
at the National Weather Service Office in Nashville, Tennessee.
Here we have a Ward’s Flood Plain Model. It uses different types of trays to simulate
different effects of flooding. The trays that are used in the presentation will act as a
wetland, a parking lot, or a retention pond. Each one will have a different effect on the
river in the model. It’s a
good exercise to show how development practices and changes to a watershed effect flooding
in similar rain events. This tray contains sponges to simulate a wetland.
Essentially what a wetland does is soak up a lot of water. The clear tray acts as our
“rain maker”. So we’re going to pour some water in the tray, which is going to simulate
rainfall. And as expected the sponges soaked up a lot of the rain and we didn’t see any
of the runoff in our river. But what happens over the course of heavy rains is that the
ground can only soak up a certain amount of water. When the ground becomes saturated water
begins to run off. So when we add more rain, we see water begin to run through our river…which
may overflow into the flood plain. In this case water approaches the houses in our flood
plain, but comes up just short of them. This is a typical rainfall event. The wetland soaked
up as much water as it could, then over time the excess water will make its way through
the creeks, streams, and rivers. When cities become more developed they will
put in shopping centers and parking lots. This tray represents a parking lot, where
we will put our shopping center and our cars. We place our rain maker over the area again
and pour water into our model. Even with a minimal amount of rain, water
immediately begins to runoff into the creeks and streams. Without the wetland there, there
is nothing to absorb the water, so the river rises and begins to approach houses in the
flood plain and covers other low lying areas near creeks and streams. Of course if we continued
to pour water through it we would have more flooding. Dams can control most of the water along larger
rivers and streams. But what we have started to do is add retention ponds to developed
areas. Water collects in the pond and is slowly released through the outlet. If you live in
a big subdivision you probably have one of these. They can be pretty inconspicuous and
covered with trees. It acts similar to the way a wetland acts. So we put our parking lot over our retention
pond and add our rain maker. When it rains in this situation the rain collects in the
retention pond, then comes out the outlet slowly when the retention pond gets full.
The water releases at a slow enough pace that it doesn’t overflow into the low lying areas.
It is released at a controlled rate so as to minimize flooding. Thank you for watching! And make sure to come
visit us at the Wilson County Fair or another event!