The sun shone through the occasional rain shower and the Amite River crested Monday, but water continued to rise in many areas along the river and rescues continued throughout the Baton Rouge area.
While it might appear to be time for the water to start receding, once the complex web of bayous, drainage canals and land development are plugged into the equation, it becomes apparent why the massive amount of water trying to flow down the Amite River is slow to move.
The widespread flooding through and around East Baton Rouge, Ascension, Livingston and Iberville parishes can be blamed on river flooding and a sometimes counter-intuitive notion of backwater flooding.
“It’s complicated,” said Dennis Demcheck, supervisory hydrologist with the U.S. Geological Survey’s Lower Mississippi – Gulf Water Science Center in Baton Rouge.
The Amite River at Bayou Manchac and Port Vincent crested at record-breaking levels Monday m…
People have an image of rain filling up tributaries which in turn causes flooding in a main river. Under usual circumstances, once the water in those tributaries goes down, the main river levels also recede, Demcheck said.
“Backwater flooding is not like that at all,” Demcheck said.
In the recent storm, water flowed into the Amite River from all of the rain upstream which essentially made the tributaries to the Amite River lower in elevation. At the same time, water flowing into the tributaries increased the water level, but they continued to be below the level of the Amite River. Water is looking for the lower elevation so as the Amite rises, that water will get into the tributaries which will start overflowing as well.
“Water is flowing away from that bulge (of water in the Amite River),” Demcheck said. “It’s still flowing downhill. It’s flowing the way gravity wants it to flow.”
Someone watching a tributary to the Amite River, many of which flow through East Baton Rouge, might see water flowing in an abnormal direction.
“So a homeowner is going to see it flowing the opposite direction, but it’s not going backwards, it’s going downhill,” Demcheck explained.
Todd Baumann, data chief with the USGS Lower Mississippi – Gulf Water Science Center in Baton Rouge, drove over Bayou Manchac on his way to work Monday morning and saw that effect.
“Bayou Manchac was flowing east to west this morning,” he said. “And that causes that whole system to back up.”
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Baumann said the historic level of flooding caused difficulty with some gauges along the Amite River going off line, probably because they went under water and no way to reach some of the site to verify readings.
“Everything from Tickfaw (River) west to Vermilion (River) exceeded anything we’ve seen before,” Baumann said, including the Amite, Comite, Tickfaw, Tangipahoa, Natalbany and Vermilion rivers.
“We’re seeing stuff we’ve never seen before and not by a small margin,” Baumann said.
With helping coming in from other USGS offices, the big effort next week will be in trying to quantify how much water was flowing through these rivers during this flood.
The Amite River at Bayou Manchac and Port Vincent crested at record-breaking levels Monday morning, but it will take time for the massive amount of water to move through the system.
Even after the crest of water passes through the Amite River, the water levels are so much above normal that the river is still above some of the tributaries and the flooding continues.
Demcheck said unlike the 2011 Mississippi River flood where the water released into the Atchafalaya River basin was sucked up parched soil caused by a drought, this flooding came at a time when the ground was already saturated.
So many variables go into where a bayou or creek will get some of this overflow water that there is hard and fast numbers to tell exactly where the backwater flooding will occur or how high it will get. There’s also no set stage for the Amite River at which it appears water will start to drain from different areas.
“Case and point, with 17 feet at Port Vincent, that’s going to go everywhere,” Baumann said.
Water is already falling in the rivers with the Comite River expected to get below flood level Tuesday morning.
Along the Amite River, the water will get below flood level at Denham Springs Wednesday morning, at Bayou Manchac early Thursday morning, at Port Vincent Thursday evening and the river should be down to minor flooding at French Settlement by Thursday evening.