Deitrick
Autumn R.
Deitrick
Autumn R.
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ThesisSediment erosion and deposition within mangrove forests(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2023-09) Deitrick, Autumn R. ; Nepf, Heidi M. ; Ralston, David K.Mangroves are highly productive ecosystems that sequester carbon in their own biomass and by trapping carbon-rich sediment imported from outside the forest and deposited in the forest. Aboveground biomass, like mangrove pneumatophores (i.e., aerial roots), creates conditions that facilitate sediment deposition by enhancing drag and slowing currents near the bed. However, pneumatophores also generate turbulence that enhances turbulent kinetic energy (TKE), which can promote sediment resuspension. Two studies were conducted to better understand the impacts of pneumatophore-generated turbulence on sediment transport. The first study investigated whether pneumatophore-generated turbulence impacted the erosion threshold and rate of natural cohesive sediment collected from a black mangrove habitat. Sediment cores with intact belowground and aboveground biomass were placed in a recirculating channel. Pneumatophores were removed from one side of each core. Each side of the core, with and without pneumatophores, was separately exposed to the same sequence of channel velocities. Although the presence of pneumatophores significantly enhanced the turbulence in the channel, the bed stress, threshold for sediment resuspension, and rate of sediment erosion were similar for the bare and vegetated sides of each core. This result differs from non-cohesive sediments, for which pneumatophore-generated turbulence has been found to increase erosion rates. The second study considered deposition. Laboratory experiments measured TKE and net deposition of non-cohesive sediment in bare and vegetated channels. For the same velocity, as pneumatophore density increased, TKE increased and net deposition decreased. The impact of TKE on deposition was described in terms of a deposition probability model. This model was used to predict deposition over a range of typical mangrove field conditions, which indicated that pneumatophore-generated turbulence can facilitate the delivery of sediment farther into the mangrove forest. Understanding how pneumatophores impact the balance of the competing processes of deposition and erosion is critical for improving the assessment and modelling of sediment retention and carbon storage in mangrove forests.
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ArticleThe influence of vegetation-generated turbulence on deposition in emergent canopies(Frontiers Media, 2023-10-10) Deitrick, Autumn R. ; Hovendon, Erin H. ; Ralston, David K. ; Nepf, Heidi M.Laboratory experiments measured sediment deposition and turbulent kinetic energy (TKE) in bare and vegetated channels. The model vegetation represented a mangrove pneumatophore canopy. Three solid volume fractions were considered (f = 0.01, 0.02, and 0.04). For the same channel-averaged velocity, the vegetated region had elevated near-bed TKE compared to the bare region. Net deposition in both regions was measured by adding a sediment slurry of 11-micron solid glass spheres to the flume and collecting the deposited sediment from the flume baseboards after a 4-hr experiment. The elevated near- bed TKE in the vegetated region resulted in lower deposition compared to the bare region. A model for deposition probability written in terms of near-bed TKE (TKE model) more accurately predicted the measured deposition than a model based on bed shear stress (tb model). Application of the model to field conditions suggested that, by inhibiting deposition, vegetation-generated TKE facilitates the delivery of sediment farther into the mangrove forest than would be achieved without vegetation-generated TKE.
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ArticleVegetation-generated turbulence does not impact the erosion of natural cohesive sediment(American Geophysical Union, 2024-07-10) Deitrick, Autumn R. ; Ralston, David K. ; Esposito, Christopher R. ; Baustian, Melissa M. ; Burgos, Maricel Beltran ; Courtois, Andrew J. ; Nepf, Heidi M.Previous studies have demonstrated that vegetation-generated turbulence can enhance erosion rate and reduce the velocity threshold for erosion of non-cohesive sediment. This study considered whether vegetation-generated turbulence had a similar influence on natural cohesive sediment. Cores were collected from a black mangrove forest with aboveground biomass and exposed to stepwise increases in velocity. Erosion was recorded through suspended sediment concentration. For the same velocity, cores with pneumatophores had elevated turbulent kinetic energy compared to bare cores without pneumatophores. However, the vegetation-generated turbulence did not increase bed stress or the rate of resuspension, relative to bare cores. It was hypothesized that the short time-scale fluctuations associated with vegetation-generated turbulence were not of sufficient duration to break cohesion between grains, explaining why elevated levels of turbulence associated with the pneumatophores had no impact on the erosion threshold or rate.