Yeah, like I said, those elevations really don't change. My experience is that locations are removed because of fill placed there to raise it above the flood elevation, but only when some compensating storage is provided elsewhere tbat is directly connected to the flood zone. Because if you fill in a portion of a flood zone that will cause the flood elevation to rise and FEMA and the permitting agencies won't allow you to affect other areas. That typically means building a pond or using other land adjacent to the flood zone and digging out some area.
But, I don't have any experience with levees and dams to say how those are modeled. Though I do think FEMA is invested in making their maps as accurate as they can.
Don't forget about subsidence.
https://www.houstonchronicle.com/news/houston-texas/houston/article/Brownwood-The-suburb-that-sank-by-the-Ship-4379765.phpBrownwood: The suburb that sank by the Ship ChannelAccording to the U.S. Geological Survey, between 1943 and 1973, roughly 4,700 square miles of land southeast of downtown Houston, chiefly Baytown and Pasadena, dropped by at least 6 inches. The area near the Ship Channel sank around 9 feet, and an estimated 100 acres of the San Jacinto Battleground - about a quarter of the original historic park's acreage - disappeared underwater.
What was happening? In the late '60s, hydrologist Robert Gabrysch, with the U.S. Geological Survey, began explaining to neighborhood groups, Rotary Clubs and anyone else who'd listen. Our area's gooey land, a clay-ey gumbo, lacks the firmness of bedrock or even sand. And as water-hungry industries pumped enormous quantities of groundwater out of that land, he explained, the land compacted. Subsidence, the process is called.
"It happened slowly," Mason tells me, the same as he tells the ninth-graders, as we hiked through Baytown Nature Center's tall trees. "Nobody noticed at first. But those incremental changes added up. Pretty soon people's houses started flooding - not just in hurricanes, but in regular storms, and even windy days and high tides."
Some of Brownwood's houses sank into the bay. Others, merely flood-prone, were rented to shaggy young people willing to cope with the occasional inundation. Residents kept their appliances atop concrete blocks or tables, and stowed important papers on the second or third floor. They watched the weather, and could pack and flee at a moment's notice. And they learned that when wading through floodwaters, it's best to avoid snakes and floating balls of fire ants.
"They built levees," Mason says. "They bought pumps. And look around."
Not a house survives.
