Data from the sky inform flood planning

by Tiffany Jolley, Prairie Research Institute

Hydrologic models based on lidar data can be created to see how water flows over a landscape, under bridges, and through culverts. Modeling can predict where bottlenecks might occur and where that potential overflow water would flood into. Modeling can also reveal the locations of flood plains, indicating what structures would be affected by a flood event and help to map out evacuation routes that would not likely be underwater.

The Illinois State Water Survey’s the Coordinated Hazard and Assessment Mapping Program (CHAMP) is at the helm of one of the largest 2D models in Illinois, spanning five southern Illinois counties – Johnson, Pope, Massac, Pulaski, and Alexander. The extremely flat topography of this region, known as the Cache River Valley, follows the Cache River system and the historic path of the Ohio River that’s been heavily manipulated by humans over time.

“Water moves in different directions during the course of a flood and that makes the hydraulic flow patterns really complicated to study flooding in this area,” said Chris Hanstad, a CHAMP project engineer. “The Water Survey works with regulatory groups in this region a lot because of this reason.”

Pictured in the model above are the complex hydraulics and hydrology of the Cache River. Flow paths and auxiliary channels adjacent to the Cache River can flow in the opposite direction of the Cache River. At other times, flow can all be moving in the same direction, sometimes even the main Cache River can backup and flow to the East.

The models rely on detailed light detection and ranging (lidar) data captured by the Illinois State Geological Survey (ISGS). Lidar data is acquired from a sensor that is attached to the belly of a plane that flies an average of 2,000 to 1,500 meters above the ground. The sensor emits pulses of light and measures the time it takes for each pulse to return to the sensor. That measurement is then used to compute distances to objects (latitude, longitude, and elevation) with accuracy within centimeters both vertically and horizontally. As the light returns to the sensors several times, it travels through soft targets such as trees, power lines, and bushes until it encounters the ground or a building.

In flood mitigation, lidar data allows the researchers to “see beneath the trees.” Scientists are able to look at only the bare earth returns and triangulate specific points to create a seamless mosaic representation of the ground.

“The Cache River valley has an interesting geologic and man-made history that has affected flooding,” said Hanstad.

At the turn of the 19th century, agriculture interests, and then later in the 1950s the Army Corps of Engineers built levees that forced the Upper Cache and Lower Cache River to become separated.

CHAMP and Hanstad are still waiting on input for the modeling of the Reevesville Levee in southern Illinois. As their analysis work winds down, FEMA will begin to produce new floodplain maps for the area.

FEMA requires floodplain maps to be based on current risks and current conditions, developed using data from recent climate assessments, which has been a challenge for communities in emphasizing today’s risk and future risk 25 to 50 years from now.

Flood model of a storm in March 2008 that produced between 7 and 12 inches of rainfall in about 40 hours over the region. This event was the flood of record for the Upper Cache River.

“A lot of floodplain maps were based on older rainfall data, but our new studies are using Bulletin 75 rainfall data which was published in 2020,” said Hanstad.

Models and simulations can also aid in post-disaster recovery. Ongoing work will help bolster mitigation efforts by looking at damages and prioritizing high-risk areas for mitigation against future disasters.

This story first appeared on the Prairie Research Institute News Blog. Read the original story.

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