What Role do Hurricanes Play in Sediment Delivery to Subsiding River Deltas?

The Mississippi River Delta (MRD) has undergone tremendous land loss over the past century due to natural and anthropogenic influences, a fate shared by many river deltas globally. A globally unprecedented effort to restore and sustain the remaining subaerial portions of the delta is now underway, an endeavor that is expected to cost $50–100B over the next 50 yr. Success of this effort requires a thorough understanding of natural and anthropogenic controls on sediment supply and delta geomorphology. In the MRD, hurricanes have been paradoxically identified as both substantial agents of widespread land loss, and vertical marsh sediment accretion. We present the first multi-decadal chronostratigraphic assessment of sediment supply for a major coastal basin of the MRD that assesses both fluvial and hurricane-induced contributions to sediment accumulation in deltaic wetlands. Our findings indicate that over multidecadal timescales, hurricane-induced sediment delivery may be an important contributor for deltaic wetland vertical accretion, but the contribution from hurricanes to long-term sediment accumulation is substantially less than sediment delivery supplied by existing and planned river-sediment diversions at present-day river-sediment loads

The mineral sediment loading of the modern Mississippi River Delta: what is the restoration baseline?

A restoration baseline for river deltas establishes a framework for achieving goals that can be thwarted by choosing an improper historical background. The problem addressed here is identify the size of the modern Mississippi River delta that restoration should use as that baseline. The sediment loading to the Mississippi River main stem delta fluctuated over the last 160 years with a consequential dependent plasticity in delta size. A visual time series of the delta size is presented, and the area: sediment loading ratio is calculated. This ratio ranged from 1.8 to 3.9 km(2) per Mmt sediment y(-1) during the pre-European colonization of the watershed in the 1800s, a maximum size in the 1930s, and then lower after soil conservation and dam construction decades later. This land building rate is similar to the 1.3 to 3.7 km(2) per Mmt sediment y(-1) for the Wax Lake and Atchafalaya sub-deltas located to the west, which receives some of the Mississippi River sediment and water from the main channel below St. Francisville, LA. The significance to restoration of delta land lost since the 1930s is that the baseline for the 1930s was conditioned on previous sediment loading that has since declined. Most sediment is trapped in the delta, and so the existing situation is close to a zero-sum land balance. The restoration potential should be based on the delta land area that could be built from the current sediment loading, not from those of the era during peak agricultural expansion and soil erosion in the watershed. Sediment diversions upstream will, therefore, deplete sediment supply downstream where delta land will be lost. The choice of which baseline is used can be seen as a choice between unrealistic perceptions that leads to unachievable goals and agency failures, or, the realism of a delta size limited by current sediment loading

Vulnerability of Louisiana’s coastal wetlands to present-day rates of relative sea-level rise

Coastal Louisiana has lost about 5,000 km(2) of wetlands over the past century and concern exists whether remaining wetlands will persist while facing some of the world's highest rates of relative sea-level rise (RSLR). Here we analyse an unprecedented data set derived from 274 rod surface-elevation table-marker horizon stations, to determine present-day surface-elevation change, vertical accretion and shallow subsidence rates. Comparison of vertical accretion rates with RSLR rates at the land surface (present-day RSLR rates are 12±8 mm per year) shows that 65% of wetlands in the Mississippi Delta (SE Louisiana) may keep pace with RSLR, whereas 58% of the sites in the Chenier Plain (SW Louisiana) do not, rendering much of this area highly vulnerable to RLSR. At least 60% of the total subsidence rate occurs within the uppermost 5–10 m, which may account for the higher vulnerability of coastal Louisiana wetlands compared to their counterparts elsewhere