A small exploratory grant is proposed to collect sediments deposited on wetlands by Hurricanes Katrina and Rita. The amount of sediments that hurricanes leave in wetlands has been documented at a few discrete locations along this coast, but there are no coast wide measurements of this deposition, and no post-deposition studies of their stability. The paleo-chronology of sediment cores from this coast demonstrates the coincidence of peaks in inorganic concentration when hurricanes occur. A conservative estimate is that the net annual accumulation of sediments from a hurricane storm surge is >30X higher than once arrived from overbank flooding of the formerly levee-free Mississippi River, and >200 times greater than from either of the two river diversions meant to restore wetlands (built @$100 million each). This is a timely opportunity to quantify the amounts delivered by strong storms in a systematic way and to contribute to the science infrastructure necessary to efficiently and effectively restore this highly degraded coast.
The general purpose of the field sampling is to: 1) estimate the sediment volume brought to wetland environment by these hurricanes, 2) collect sufficient material to characterize (with other funding instruments) its content and subsequent movement into soils or re-mobilization; and, 3) discern relationships, if any, between sediment deposition and both plant composition and wetland geomorphology.
Samples will be collected using a helicopter for up to 4 days, for approximately 6 hours daily (weather permitting). Data collection will consist of site photographs, measurements of sediment depth, various sediment collections, and archiving material of representative plants and easily accessible invertebrates such as snails and crabs. The sample collection volume will be sufficient to conduct analyses of particle size, CNS stable isotopes in soils and plants, and routine soil and plant analyses (e.g., salinity, % organic, C/N ratios, trace metals, exchangeable cations, etc.
(2) non-technical explanation of the broader significance
A small exploratory grant is proposed to collect sediments deposited on wetlands by Hurricanes Katrina and Rita. The tidal surges moving inland through the St. Bernard estuary and into the New Orleans area left significant amounts of sediments in the urbanized portion of the watershed, and unknown amounts elsewhere. These sediments may be of significant factor in wetland stability because of either their volume or the nutrients attached to them. The amount of sediments that hurricanes leave in wetlands has been documented at a few discrete locations along this coast, but there are no coast wide measurements of this deposition amount, or post-deposition studies of their stability. A rapid response sample collection is necessary before visual identification of the layer is much more difficult, if not impossible. The main purposes of this proposed sample collection are to quantify the amount of sediments introduced to these wetlands by these hurricanes, to understand how the deposition rate varies across the landscape, and to estimate their importance to wetland stability. The archived samples must be analyzed using other funding instruments.
An analysis of dated sediment cores from this coast demonstrates the coincidence of peaks in inorganic concentration when hurricanes occur. A preliminary analysis of these data, and using conservative parameter values, suggests that the net annual accumulation of sediments from a hurricane storm surge is much higher than once arrived from overbank flooding of the formerly levee-free Mississippi River or from either of the two river diversions (built @$100 million each) meant to restore this severely degraded coast. The available restoration approaches bringing sediments and nutrients to wetlands are neither trivial nor homogenous endeavors; they are a heterogeneous mixture of size, cost : benefit ratios, predictability and consequences. There are, therefore, significant management implications of this proposed work.
These results will have an immediate application to the evaluation of restoration strategies, which are currently focused on the large scale diversions of sediments in the Mississippi River to theoretically 'sediment deprived' and 'nutrient stressed' wetlands. Additional benefits of this sample collection and subsequent analyses include being able to trace the movement of sediment-bound nutrients into plants, to offer data to calibrate sediment transport models and carbon sequestration in wetlands, and to establish permanent plots to follow soil evolution.
