Sea level tools only tell half a story

Tools used to measure relative sea level rise in low-lying coastal areas like south Louisiana tells but half a story, according to fifth-year Tulane University PhD student Molly Keogh’s published research in Ocean Science. Researchers have indicated sea level rise estimates don’t account for the primary contributor to higher water levels: Sinking marshes. Relative sea level rise is the sum of rising seas and sinking land generally measured with tide gauges. In areas with no bedrock, gauges are fixed to rods nearly 60 feet deep. It’s a problem when it comes to measuring subsidence near the surface, wrote Keogh. Subsidence is used to describe settling/sinking land. Subsidence of the delta is caused by leveling off of the Mississippi River, faulting and the withdrawal of water, oil and gas. Some 60 percent of subsidence occurs within the top 16 feet, Keogh continued. Sediment at the surface is newer; and over time compacts under its weight and further buried. This “shallow subsidence” tide gauge measurements miss the relative sea level rise, she told NOLA.com. Tide gauges don’t account for shallow subsidence and underestimate sea level risings. “We could be looking at only half the true rate,” Keogh said. There is a solution: An instrument called a surface elevation table that can be used to record shallow subsidence. Unlike tidal gauges, SETs have an arm that extends horizontally with pins on the ground to track surface elevation change. (Source: NOLA.com 02/05/19)