Hydraulics, sedimentation, and nutrient deposition within the middle Mississippi River Batture, USA

Abstract

The Mississippi River batture (the land between the channel's low-water elevation and the flood mitigation levee) is an important sink for fluvial sediments and nutrients. However, the batture's role in sediment and nutrient storage remains poorly understood. To gain a better understanding of sediment and nutrient deposition within the batture, a 2D hydrodynamic model for a 30 km segment of the middle Mississippi River (MMR; between the confluences of the Missouri and Ohio Rivers) was developed to quantify flood extent and shear stresses for batture inundating discharges. Comparing modeled water surface elevations and shear stress to sedimentation estimates revealed which discharge conditions were conducive for deposition. Batture areas that experienced sedimentation during the 2019 flood were sampled for analysis of total organic carbon (TOC) and total nitrogen (TN) to gain insights into where these nutrients were deposited.Hydraulic modeling revealed MMR batture inundation includes six phases across both the rising and falling limb of the flood hydrograph: (1) back flooding (i.e., downstream to upstream inundation), (2) mixed back and overbank flooding, (3) overbank flooding conducive for deposition on the rising limb, (4) overbank flooding conducive for erosion on both the rising and falling limbs, (5) overbank flooding conducive for deposition on the falling limb, and (6) a floodplain draining phase. Phases two, three, and five (discharge conditions >20 to 86 % annual exceedance probability [AEP]), are most suitable for fine-grained deposition because of direct transfer of sediment-laden water from the river to batture, and large areas of the batture experiencing shear stress below the critical threshold for deposition. During the transition from phases three to four, flood depths increase, and hydraulic conditions rapidly favor erosion over the entire batture suggesting the batture may transition from sink to source of sediment for discharges that are ≤20 % AEP.Flood sediment TOC and TN concentrations were more than three times greater in backswamp clay and silt deposits, than in levee and splay sand. Within the backswamp, the most favorable discharge conditions for fine-sediment deposition are >20 to ≤86 % AEP on the rising limb, and >20 to ≤90 % AEP on the falling limb of the hydrograph. However, up to 40 % of the backswamp does not experience shear stress conditions below the threshold for fine-grained deposition. These findings highlight the impact hydraulic conditions have on sediment and nutrient retention in this important river connected landscape.