Sedimentation Potential in the Mississippi Delta Without Human Influences

Abstract:

Land loss poses a substantial threat to ecosystems and communities in the Mississippi river delta. Maintaining the current delta plain critically depends on the delivery and trapping of sediment within the delta to compensate for ongoing relative sea level rise (sum of subsidence and eustatic sea level rise). However, man-made levees, constructed for flood protection, inhibit the transport of sediment from the river to the delta plain. Simultaneously, the Mississippi’s sediment yield has decreased significantly due to the construction of dams in the river’s upstream tributaries. It is therefore generally assumed that low sedimentation rates on the delta and subsequent land loss occur because of human interventions in the landscape. However, deltas also go through natural cycles of delta lobe avulsion and growth, followed by decline and a new avulsion. Although human activity has undoubtedly influenced the balance between sedimentation and relative sea level rise, it is unclear whether the Mississippi delta could have sustained its size even in an undisturbed state. We test this by modelling sediment dispersal and deposition in the Mississippi’s Barataria Basin before widespread anthropogenic interventions. To do so, we make use of Delft3D FM, a numerical modelling software for hydrodynamics and sediment transport, and a reconstruction of the delta’s morphology and forcing conditions for the year 1890. Our model results show how much sediment dispersed from the river could have been trapped on the delta plain for a range of different flood magnitudes. We then compare modelled sedimentation rates to rates of relative sea level rise. This allows a first assessment on whether land loss of the Mississippi river delta is a natural process, or a direct consequence of river engineering.