Toward understanding the deposition of sediment and nutrients within the Mississippi River batture

Keywords: Mississippi River, hydraulics, sedimentation, nurients, floodplains
Session Type: Virtual Paper Abstract
Day: Friday
Session Start / End Time: 2/25/2022 08:00 AM (Eastern Time (US & Canada)) - 2/25/2022 09:20 AM (Eastern Time (US & Canada))
Room: Virtual 19

Abstract

Sediment and nutrient deposition and storage are important but poorly understood functions of the Mississippi River batture (the land between the channel's low-water elevation and the flood mitigation levee). To understand deposition within the batture, a 2D hydrodynamic model for a 30 km segment of the Mississippi River was developed to quantify inundation and critical shear-stress for the range of discharges in which the batture is inundated. Flood sediments were analyzed for total organic carbon (TOC) and total nitrogen (TN) for insight on these nutrients’ distribution throughout the batture. Hydraulic modeling showed inundation of the batture begins through back flooding of the backswamp through side channels starting at the ~30% daily exceedance probability (EP). At the transition between backflooding to overbank inundation which occurs between the 15% and 6.5% EP are where the largest percentage of the batture (up to 35%) is inundated and shear stresses are below the critical value for deposition of fine-grained sediments. TOC and TN concentrations in flood sediments were found to be more than two times greater in the clays and silts deposited in the backswamp than the sands deposited on the levees and splays (0.10 vs. 0.04 g N/cm3 and 1.1 vs. 0.5 g C/cm3, respectively). Within the back swamp, concentrations of these nutrients tended to be higher where hydraulic conditions were more conducive for the deposition of fine grain sediments. These findings underscore the importance of understanding hydraulic conditions for the assessment of sediment and nutrient retention in this important river connected landscape.