Disturbance of sedimentary processes in tidal salt marshes invaded by exotic vegetation.

In situ moorings were conducted at salt marsh and bare flat to reveal the environmental disturbance of sedimentary processes in the intertidal flat. Spartina alterniflora (Spartina), an invasive species, grew up to 156 cm in the Ganghwa tidal flat from June to November 2019. This rapid growth has resulted in the dense salt marsh, which complicated hydrodynamics and associated sediment processes. Stems and leaves could effectively trap fine-grained sediments (17.24-20.42 µm) at the bed, increasing the differences in bed elevation between the two sites to up to 5.11 cm. The cohesive sediments accumulated in Spartina communities were resuspended differently by stem-scale turbulence generated from the disturbance of stems and leaves, depending on wind forcing and vegetation conditions. The vegetated sediments, under low wind speeds (<4 m s-1), were hardly resuspended in the water column, compared to those in the bare flat, resulting in sedimentation. Under high wind speeds (>6 m s-1), stem-scale turbulence was sufficiently strengthened to surpass the sedimentation of suspended sediments; thus, it resuspended additional bed sediments without a loss of the trapped sediment, unlike in the bare flat. The flocculation of suspended sediments in Spartina communities was mainly controlled by stem-scale turbulence. The flocs were confined to a size of 40 µm (settling velocity: 0.17 mm s-1) and developed an approaching spherical shape. After the cut-off of Spartina, a favorable condition for the flocs to grow by 57 µm (settling velocity: 0.23 mm s-1) was established with decrease in stem-scale turbulence. These larger flocs were able to develop into a ramified spherical structure. Despite diminishment of stem-scale turbulence, the disturbed sediments were outflowed, resulting in an abrupt decrease in bed elevation (0.12 cm day-1). The results suggest that the sediment accumulated in Spartina communities remained erodible due to frequent disturbances without sufficient consolidation.

Assessment of potential impact of invasive vegetation on cohesive sediment erodibility in intertidal flats.

In-situ erodibility experiments were conducted to reveal the effects of vegetation on sediment stability in an intertidal flat. Spartina alterniflora (Spartina), one of the most widespread types of vegetation, led to complexity in sediment erodibility. The long stems and leaves of Spartina, which grew to approximately 156 cm from May to November 2019, were effective in trapping suspended sediments in the water columns, eventually promoting the deposition of approximately 2.3 cm within its communities. Sediments eroded by increasing bed shear stress (τb) mainly originated from sediments that were adhered to the stems and leaves of Spartina (May: 76%; November: 54%). They protected subsequent bed erosion against τb. However, this was only an apparent effect because the Spartina caused the erosion rate (E) to stagnate by suppressing the outflow of eroded mass from the bed. As the protective effect of the stems was removed, the uppermost sediment layers in the Spartina communities became more vulnerable to erosion by τb, with the initial erosion thresholds lowered to 0.1 Pa (May) and 0.05 Pa (November). Despite continuous sedimentation by sediment trapping, the sediment bed in Spartina communities had not been consolidated under repeated tidal inundation, showing no distinct development of the critical shear stress for erosion. Thus, the differences in E between cases with or without Spartina's stems reached approximately 1.22 × 10-6 (May) and 1.83 × 10-6 kg m-2 s-1 (November) at τb = 0.6 Pa. Results suggest that the increase in thickness of erodible layers mainly contributed more than sediment volume fraction to the enhancement of erosion potential with τb. This study highlights the necessity to assess both positive and negative effects of Spartina on the stability of sediment beds in intertidal flats.

Effects of vegetation and fecal pellets on the erodibility of cohesive sediments: Ganghwa tidal flat, west coast of Korea.

Although the Korean tidal flats in the Yellow Sea have been highlighted as a typical macrotidal system, so far, there have been no measurements of the sediment erodibility and critical shear stress for erosion (τce). Using the Gust erosion microcosm system, a series of field experiments has been conducted in the Ganghwa tidal flat to investigate quantitatively the effects of biogenic materials on the erodibility of intertidal cohesive sediments. Four representative sediment cores with different surficial conditions were analyzed to estimate the τce and eroded mass. Results show that τce of the "free" sediment bed not covered by any biogenic material on the Ganghwa tidal flat was in the range of 0.1-0.2 Pa, whereas the sediment bed partially covered by vegetation (Phragmites communis) or fecal pellets had enhanced τce up to 0.45-0.6 Pa. The physical presence of vegetation or fecal pellets contributed to protection of the sediment bed by blocking the turbulent energy. An inverse relationship between the organic matter included in the eroded mass and the applied shear stress was observed. This suggests that the organic matter enriched in a near-bed fluff layer is highly erodible, and the organic matter within the underlying sediment layer becomes depleted and less erodible with depth. Our study underlines the role of biogenic material in stabilizing the benthic sediment bed in the intertidal zone.

Voltage-controllable wavelength-selective optical switching based on multiply cascaded long-period fiber gratings.

A novel wavelength-selective optical switching device based on multiply cascaded long-period fiber gratings is proposed and experimentally demonstrated. The on and off states of each channel in the optical switching device can be effectively switched by voltage-controllable coil heaters. The device has advantages of multichannel operation, multiwavelength selectivity, and bandwidth controllability. It can be useful for applications in multiwavelength operational signal gating, optical switching devices, routers, and multiplexers in optical communication systems.