Model versus nature: Hydrodynamics in mangrove pneumatophores

Water flows through submerged and emergent vegetation control the transport and deposition of sediment in coastal wetlands. Many past studies into the hydrodynamics of vegetation fields have used idealized vegetation mimics, mostly rigid dowels of uniform height. In this study, a canopy of real mangrove pneumatophores was reconstructed in a flume to quantify flow and turbulence within and above this canopy. At a constant flow forcing, an increase in pneumatophore density, from 71 m⁻² to 268 m⁻², was found to cause a reduction of the within-canopy flow velocities, whereas the over-canopy flows increased. Within-canopy velocities reduced to 46% and 27% of the free-stream velocities for the lowest and highest pneumatophore densities, respectively, resulting in stronger vertical shear and hence greater turbulence production around the top of the denser pneumatophore canopies. The maximum Reynolds stress was observed at 1.5 times the average pneumatophore height, in contrast to uniform-height canopies, in which the maximum occurs at approximately the height of the vegetation. The ratios of the within-canopy velocity to the free-stream velocity for the pneumatophores were found to be similar to previous observations with uniform-height vegetation mimics for the same vegetation densities. However, maxima of the scaled friction velocity were two times smaller over the real pneumatophore canopies than for idealized dowel canopies, due to the reduced velocity gradients over the variable-height pneumatophores compared to uniform-height dowels. These findings imply that results from previous studies with idealized and uniform vegetation mimics may have limited application when considering sediment transport and deposition in real vegetation, as the observed turbulence characteristics in nonuniform canopies deviate significantly from those in dowel canopies

Deposition gradients across mangrove fringes

Observations in a mangrove in the Whangapoua Harbour, New Zealand, have shown that deposition rates are greatest in the fringing zone between the tidal flats and the mangrove forest, where the vegetation is dominated by a cover of pneumatophores (i.e. pencil roots). Current speeds and suspended sediment concentrations dropped substantially across this zone. Near-bed turbulence within the fringe was substantially lower where the pneumatophore canopy was denser, facilitating the enhanced deposition in this zone. However, the near-bed conditions were not the primary control on the instantaneous sediment concentrations at this site. The total deposition across the different zones was the combined result of the reduced near-bed turbulence inside the vegetation and the larger-scale dynamics over the spatially variable vegetation cover, along with other confounding factors such as changing sediment inputs

Exploring Strategies To Promote Engagement And Active Learning Through Digital Course Design In Engineering Mathematics

This research explores the strategies and techniques used to foster and promote the engagement and active learning of engineering students within a digital course. This digital course has been developed to address the varying levels of understanding of fundamental mathematics among first-year engineering students, who often have disparate levels of prior knowledge at their high school completion. We observe an increasing need to bridge the widening gap between high school and university mathematics in order to prevent engineering students from being hindered in their academic successes due to a lack of prior mathematical understanding. With a team of engineers and mathematicians, both researchers and educators, we are developing a mathematics Bridging Course including the use of digital tools, such as videos, online interactions and technology-based assessments. These sources were created, investigated and/or modified to develop an engaging learning environment in which students are made aware of and guided through misconceptions and mistakes in their understanding of fundamental mathematics. In the development of this Bridging Course, we consider the importance of interactive learning and timely feedback for student learning. We investigate the impact of digital course design on students’ performance and learning outcomes using a qualitative approach. Students feedback within the first stage of the implementation of the course offered a positive assessment of the course, accentuating its inherent advantages and attributes. The students’ feedback proved to be an invaluable source of insights, specifically concerning the enhancement of question distractors, thus prompting revisions and augmentations in the assessment items employed

Mangrove forest drag and bed stabilisation effects on intertidal flat morphology

Mangrove trees influence their physical environment by exerting drag on tidal flows and waves while also stabilising the sediment bed of intertidal flats. These processes influence sediment accretion, the mangrove habitat and their resilience to sea level rise. However, little is known about the magnitude and spatial extent of the effects of mangrove forests on sediment transport and the morphology of the intertidal flat. We use manipulated simulations with an extended process-based numerical model, to study the influence of mangrove forests on intertidal flat morphology on a yearly timescale. The model includes the influence of mangrove trees on tidal flows, waves and sediment dynamics. The model is calibrated and validated with a comprehensive set of measurement data including hydrodynamics, sediment transport and morphological processes from an expanding mangrove forest in the sediment-rich Firth of Thames estuary in Aotearoa New Zealand. Sediment accretion on the upper intertidal flat is predominantly influenced by the characteristic morphology of the established mangrove forest, with increased bed stability at higher mudflat elevations related to prolonged aerial exposure and drying of the bed. Our results show that, in comparison to the situation without mangroves, sediment accretion increases in the most seaward fringe area of the forest. The unvegetated intertidal flat fronting the mangrove forest captures less sediment compared to the situation without mangroves. The mangrove forest drag triggers the development of a steeper, convex-up-shaped, upper intertidal flat profile, especially during periods with higher water levels and waves. These effects are expected to influence the development and storm-recovery of natural and restored mangrove forests and may contribute to the resilience and persistence of mangrove-vegetated intertidal flats for coastal flood risk reduction.</p

Are flow-vegetation interactions well represented by mimics? A case study of mangrove pneumatophores

Arrays of real mangrove pneumatophores (i.e. aboveground pencil roots) and artificial dowel mimics were constructed in a laboratory flume to examine differences in canopy flow dynamics. Compared to the uniform-height dowel canopy, the non-uniform height of the pneumatophores significantly reduced the intensity of the canopy shear, and shifted the turbulence maxima observed directly above the dowels upwards by approximately the standard deviation of the pneumatophore heights. Consequently, bed shear stresses were up to two times greater in the uniform-height dowel canopy than in a pneumatophore canopy of similar density. At the same time, ratios of the within-canopy velocity to the free-stream velocity above the canopies were not significantly altered by the heterogeneous height, shape and spatial distribution of the pneumatophores. Our results emphasize that uniform dowels are poor proxies of real pneumatophore canopies and may lead to underestimations of sediment-trapping efficiency