Geology: Floods
Because 100 Acres: The Virginia B. Fairbanks Art & Nature Park is located within a meander of the White River, flooding is a common and characteristic event. Have you ever wondered why certain areas flood frequently? Let's take a closer look at flooding within the Park.
Flooding occurs in the Park when there is heavy rainfall or extensive snow melt in the upstream region of the White River watershed. As increasing amounts of water flow through the watershed and into the river as runoff and groundwater, the level of the river rises and approaches bankfull stage (as indicated above). When the level surpasses this point (also known as flood stage) water begins to spread throughout the Park, which is an active flood plain.
As the level and speed of the river increase during a flood, erosion of the banks is accelerated as new portions come in contact with the water. As natural levees are breached, they become eroded as well. These processes can change the shape of a river, the most extreme cases occurring when a river cuts a new channel, turning a meander into an oxbow lake.
The increased levels of sediment in the river are deposited in flood plains, as the flood waters are slowed down by plants and rough terrain. You might notice looser, more sandy soils within 100 Acres composed of sediment that has been left by previous floods.
Flood Magnitudes
How far the flood waters spread through 100 Acres depends on the flood magnitude. Geologists express flood magnitudes as recurrence intervals. The recurrence interval for a flood is the average interval of time within which it is equaled or exceeded once. For example, a 100-year flood will occur once per century on average. Another way to think about it is that there is a 1% chance that a 100-year flood will occur in any year. The image on the left below shows what the Park would be like during such a flood. On the right is a flood simulation that has a 10% chance of occurring in a given year.
| Recurrence | Probability |
|---|---|
| 10 year | 10% |
| 25 year | 4% |
| 50 year | 2% |
| 100 year | 1% |
Historic Floods
Below is a simulation of the historic 1913 flood if it were to happen in 100 Acres today. Six to twelve inches of rain falling over much of northeast, central and southern Indiana in three days caused this event. Recent flooding in June 2008 broke some of the records set in 1913. For example, at Newberry the flood waters in the White River rose to 27.50 feet in 1913, but the new record was set in 2008 at 28.04 feet as 748,000 gallons of water gushed past that point every second. Thousands of Hoosiers evacuated their homes in both events.
Inundation Maps
As a result of collaboration with the USGS, the IMA now has set of inundation maps, which can be used to estimate the extent of flooding using the stream gauge installed in the Park. The hydrograph generated by the gauge reports the current level of the river, and the corresponding map shows a simulation of the flooding that would occur, based on digital elevation models. The map below shows the levee breaching when the stream gauge reads 16 feet.
