The influence of Hurricane Andrew on sediment distribution in Louisiana coastal marshes

The areal extent and temporal patterns of sediment deposition in delta marshes and shallow water bodies of Louisiana associated with the passage of Hurricane Andrew were determined from sediment traps, marker horizons, and benchmarks deployed for other studies. Data were collected over different time scales from 11 sites representing salt, brackish, and intermediate marshes, shallow water substrates, and regularly-flooded and partially impounded marshes located in Terrebonne, Barataria, and Pontchartrain basins. Storm tide data were collected at 6 hydrograph stations located in the vicinity of the sediment sampling sites. Significant sediment deposition and surface elevation changes were associated with the occurrence of storm tides both near and far (up to 130 km away) from the track of the storm. Short-term sediment deposition indicated an increase of up to 1–3 orders of magnitude compared to pre-storm rates. This increased rate remained high for up to 12 weeks, suggesting that material brought in by the storm was available for resuspension and deposition. But short-term sediment deposition decreased precipitously after passage of the first major cold front across Louisiana, suggesting that the sediment made available by the storm was removed or consolidated. The storm-associated sediments were characteristically coarser with lower organic matter content than the pre-storm marsh soils. Marsh vertical accretion and surface elevation change were enhanced on both semi-annual and annual time scales for all sites we studied. This single storm generated 2–6 cm more vertical accretion than occurred in either the year before or after the storm. Sediment was both introduced from outside the coastal marsh system and redistributed as the marsh substrate eroded. At shallow water sites, elevation increases were greater and were maintained near wave barriers constructed to baffle water movement while unprotected areas had smaller increases and quickly lost whatever gains had occurred. These findings indicate the potential importance of hurricanes in providing a pulse of sediment on a broad geographic scale to balance the effects of subsidence in this rapidly subsiding delta. These results further suggest that hurricanes play a large role in coastal marsh survival, particularly for marshes remote or isolated (i.e., impounded) from a riverine source of sediment.