Ask A Scientist

How can eight inches of rain cause such a huge flood and destroy so many peoples homes?

Asked by: Riley Jane Petcosky/Sue Kim
School: Vestal Hills Elementary School
Grade: 5
Teacher: Mrs. Morgan
Hobbies/Interests: Soccer
Career Interest: Riley, an ER doctor; Sue, a teacher

Answer from Steve Rebello

Math/Science Academic Counselor

Before Binghamton University: 
Taught grade Physical Science and Biology in the Johnson City School district, and High School Biology and Physics in the Upward Bound program at BU.
Graduate School Research Area: Mycology, Phylgenetics
Degrees:
AA - Liberal Arts - Broome Community College
BS - Biology Education - SUNY Cortland
MSEd - Environmental Education - SUNY Cortland
Family
Wife, Emily - 5th grade teacher in UE School district
Son, Brayden - Just born last month - 9/18/11
Hobbies/Interests
Photography, camping, skiing/snowboarding, teaching
 
 
   

Answer: Eight inches of rain doesn't sound like that much water, in fact eight inches is only about as high as your pencil is if you stood it on end. We get snowstorms that leave us with feet of snow and they don't cause any major damage so what makes this so different?  

When rain hits the ground some of it sinks into the ground and becomes groundwater. The rest runs across the surface of the earth and is called runoff. As the ground becomes saturated with groundwater (like a sponge that won't hold any more water) the amount of rain that becomes runoff is even greater. Groundwater levels were already high from the rain that came from Hurricane Irene so it didn't take much for the ground to become completely saturated.  

Water flows from areas of high elevation to areas of low elevation. Rain that falls on the hilltops flows downhill into the valleys where there are usually streams. These streams all run down hill and small streams connect with larger streams and those streams eventually flow into the rivers. Some of the area was affected by flash flooding from the quickly rising streams from all of the runoff. Most areas were flooded from the river, which didn't crest until the after rain had stopped. The reason the river crests later than the streams is because it takes a while for all of that water to make its way into the river.

 

So even though we only had 8 inches of rain from Tropical Storm Lee, the Susquehanna River crested at 35.26 feet in Vestal!  This 18 inches above the previous record and over 17 feet above the flood stage (stage at which the river first starts overflowing its banks in low-laying areas).  All of that water came from the runoff water that fell over the entire 4,520 squre miles of the Upper Susquehanna River watershed.  If you calculated the volume of the water that fell over the area (4,520 square miles x 8 inches deep) it comes out to about 846,000,000,000 cubic feet of water!  Some of that became groundwater, but much of it was runoff that ended up in our streams and river causing them to flood.

Last Updated: 9/18/13