Sediment Erosion and Deposition Within Mangrove Forests
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...
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| Format: | Thesis |
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Massachusetts Institute of Technology
2023
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| Online Access: | https://hdl.handle.net/1721.1/152741 https://orcid.org/0000-0003-1173-597X |
| Summary: | 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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