Contribution of tropical cyclones to the sediment budget for coastal wetlands in Louisiana, USA

Abstract

The storm surge from a single hurricane can deposit tens of millions of tons of sediment on coastal wetlands within 100 km of landfall, but the distribution and cumulative amount from hurricanes at a centurial timescale is unknown. Here we use a model calibrated by three storms to estimate the average deposition on the deteriorating Louisiana coast from 1851 to 2008. The total deposition on Louisiana coastal wetlands, exclusive of open water, averages 5.6 million tons of inorganic sediment per year, equivalent to 3.8 % of the modern annual Mississippi River sediment load. Seventy nine percent of this sediment is deposited in a 20 km strip along the Gulf of Mexico (7,400 km 2 wetlands) comprised primarily of salt marshes, and this distribution matches spatial and temporal patterns described in modern surficial deposits and sediment cores. We estimate that surge-induced deposition of sediment is attributable to at least 65 % of the inorganic content of the top 24 cm of soils in abandoned delta lobes, and 80 % in the chenier plain. While the most sedimentation from a given event results from the most intense storms, 78 % of the long-term hurricane sedimentation results from moderate storms (930-990 mb) that comprise 51 % of tropical cyclone events.Furthermore, we estimate that the 47 % of storms that make landfall with an internal barometric pressure above 990 mb account for only 7 % of the tropical cyclone sedimentation on wetlands.

Wetland Accretion Rates Along Coastal Louisiana: Spatial and Temporal Variability in Light of Hurricane Isaac’s Impacts

“…Sediment deposition from hurricanes Katrina, Rita, Gustav, and Ike totaled 68, 49, 21, and 33 million metric tons, leading to maximum accretion rates of 21, 20, 7, and 17 g/cm 2 , respectively [19]. An average yearly total of 5.6 million tons of inorganic sedimentation was determined from tropical cyclones on Louisiana wetlands, deemed an important component in coastal land-building [39]. In addition to the role and contribution of inorganic sediments toward increasing wetland surface elevation and providing stability to marsh substrate, Baustian and Mendelssohn [40] discovered their positive effects on wetland primary production and resilience, partly from the decline in sulfide due to a lessened influence of relative sea level rise.…”

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

“…The spatially averaged event sedimentation rate in Breton Sound for hurricane Katrina (sampled weeks after landfall) was previously estimated to be 26,000 ± 13,000 g cm −2 34 (see Methods), comparable to regionally averaged 8,9 . Accumulation rates per unit area for Katrina in this study were an order of magnitude less.…”

“…In order for this program to succeed, we must fully understand both natural and anthropogenic controls on sediment supply and delta geomorphology 2,4,5 . In the MRD, hurricanes have been paradoxically identified as substantial agents of both widespread land loss 6 , and vertical marsh sediment accretion [7][8][9] . 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-supplied sediment contributions to vertical delta accretion.…”

“…This land loss has been linked to diversion inflows of nutrient-rich river water, associated with weakened soils and plant growth [17][18][19] . Paradoxically, hurricanes have also been identified as sources of sediment for vertical wetland accretion, via surge inundation, with estimates of 116-131 million metric tons (Mmt) of Louisiana coastal wetland sedimentation from hurricanes Katrina and Rita in 2005, prompting the suggestion that hurricanes are more constructive than river-sediment supply to MRD wetland growth [7][8][9] . The applicability of these findings to coastal restoration has been debated 20,21 , but tropical cyclones are clearly important drivers of coastal geomorphology, and may play an important role in wetland sedimentation and the ability of marshes to keep pace with sea-level rise.…”

Discussion of: Olea, R.A. and Coleman, J.L., Jr., 2014. A Synoptic Examination of Causes of Land Loss in Southern Louisiana as Related to the Exploitation of Subsurface Geological Resources.Journal of Coastal Research, 30(5), 1025–1044.

“…Further, the amount of sediments flowing overbank before flood-protection levees were built was about 2% of the river's load (Kesel, 1988), and its accumulation would be concentrated no more than a few kilometers from the riverbank (hence the formation of the riverbank levee). Hurricanes, in contrast, bring a larger amount of sediment, which is spread across the coast in a more democratic manner (Tweel and Turner, 2014).…”