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Mangrove-forest sustainability hinges upon propagule recruitment and seedling retention. This study evaluates biophysical limitations to mangrove-seedling persistence by measuring anchoring force of two mangrove species (Rhizophora mangle L. and Avicennia germinans (L.) L.). Anchoring force was measured in 362 seedlings via lateral pull tests administered in mangrove forests of two subtropical estuaries and in laboratory-based experiments. Removal mechanism varied with seedling age: newly established seedlings failed due to root pull-out while seedlings older than 3 months failed by root breakage. The anchoring force of R. mangle seedlings was consistently and significantly greater than A. germinans (p = 0.002); however, force to remove A. germinans seedlings increased with growth at a faster rate (p < 0.001; A. germinans: 0.20–0.23 N/g biomass; R. mangle: 0.04–0.07 N/g biomass). Increasing density of surrounding vegetation had a positive effect (p = 0.04) on anchoring force of both species. Critical velocities at which seedlings become susceptible to instantaneous uprooting estimated from anchoring forces measured in the field were 1.20 m/s and 1.50 m/s, respectively, for R. mangle and A. germinans. As estimated critical velocities exceed typical flow magnitudes observed in field sites, removal of established seedlings likely occurs following erosion of sediments from the seedling base.

Bank erosion in a sinuous alluvial channel is a continuous phenomenon resulting in bank instability and migration of sediment. In this study, flume experiments were conducted in a sinuous channel to investigate its morphological changes and hydrodynamics. High-order velocity fluctuation moments are analyzed at outer and inner banks to explain the morphological variation in a sinuous river channel. The variance of streamwise velocity fluctuations on both banks of the sinuous channel follows a logarithmic law from a particular depth. In the outer bend region, the magnitude of velocity fluctuation moment is significant, indicating erosion. The trend of velocity fluctuation at higher even-order moments is similar to the variance of streamwise velocity fluctuations where the outer bend magnitude is greater than the inner bend. The premultiplied probability density functions (PDFs) and the flatness factor show greater magnitude in the outer bend of the channel as compared to the inner bend.

The porous boundary of alluvial channels allows water to interact with the surrounding groundwater. With the reduction in groundwater level, the transfer of water from the main channel to the groundwater is significant and referred to as downward seepage. The action of downward seepage at the boundary zones of sinuous bend causes morphological alteration along the sinuous alluvial channel. Laboratory experiments were conducted for no-seepage and seepage conditions to study the effect of downward seepage on turbulence and bed morphology in rectangular and trapezoidal sinuous channels. The deformation along the streambed and bank of the sinuous channels showed remarkable alterations with seepage. The banks are eroded, and the bed elevation is lowered in the presence of downward seepage as observed from the cross-sectional profiles of the channel. This infers the excess sediment transport with downward seepage. With increased flow rate and downward seepage, prominent deposition zones near the inner region and erosion near the outer region of the bend were observed in the rectangular sinuous channel. The trapezoidal channel showed much less change as the side slope was near to the angle of repose of the sand. However, with the application of downward seepage, the streambed of the channel has lowered in elevation, indicating sediment movement. Streamwise velocity at the center of the bend showed an average percentage increase of 26% in the rectangular channel and about 20% in the trapezoidal channel. The Reynolds shear stress estimated with downward seepage has increased near the channel bed, indicating high momentum flux.

The presence of a small seepage velocity in streams and rivers induces transfer of mass and momentum near the sediment boundary, causing bank failure and instability of hydraulic structures. This experimental study is intended to report near-bed hydrodynamics and morphological characteristics under the impact of downward seepage. A laboratory investigation was conducted in a sinuous rectangular alluvial channel with rigid sides and movable uniform sand bed. From the distribution of Reynolds shear stress (RSS), the transverse or radial motion of the sediment across the bend was identified. High values of RSS in the outer region relate to higher momentum flux, eroding the outer bend. The contribution probability of ejection and sweep events are highest in the bend region. For seepage, the contribution probability of the events has increased. Moreover, the altered flow with seepage affected the transport of sediment and caused noticeable modifications along the channel bed. With seepage, the erosion rate has been enhanced along the outer bend of the channel.