Pathways for sediment transport and retention in a vegetated, mid‐channel island: Connecting sediment dynamics to morphology in Meinmahla Island, Ayeyarwady Delta, Myanmar

Abstract

Vegetated mid-channel islands play an important though poorly understood role in the sediment dynamics and morphology of tide-dominated deltas. Meinmahla Island is a mangrove-forest preserve at the mouth of the Bogale distributary channel, in the Ayeyarwady Delta, Myanmar. In this relatively unaltered mid-channel island, sediment dynamics can be directly connected to morphology. Field measurements from 2017 to 2019 provide insight into the pathways for sediment transport and resulting morphological evolution. Water depth, salinity and turbidity were monitored semi-continuously, and velocity profilers with turbidity and salinity sensors were deployed seasonally in single-entrance (dead-end/blind) and multi-entrance tidal channels of the island. The morphological evolution was evaluated using grain size, 210 Pb geochronology, remote sensing and channel surveys. The data show that ebb-dominant, single-entrance channels along the island exterior import sediment year-round to the land surface. However, these exterior channels do not deliver enough sediment to maintain the observed ca 0.8 cm/yr accretion rate, and most of the sediment import occurs via interior, multientrance channels. Interior channels retain water masses that are physically distinct from the water in the Bogale distributary, and estuarine processes at the tidal-channel mouths import sediment into the island. Sediment is sourced to the island from upriver in the wet season and from the Gulf of Mottoma in the dry season, as the location of the estuary shifts seasonally within the Bogale distributary. The salinity and biogeochemistry of the distributary water are affected by interactions with sediment and groundwater in the island interior. The largest interior channels have remained remarkably stable while the island has aggraded and prograded over decadal timescales. However, the studied multi-entrance channel is responding to a drainage-network change by narrowing and shoaling. Overall, mid-channel islands trap sediment and associated nutrients at the river-ocean interface, and these resilient landscape features evolve in response to changes in drainage-network connectivity.

Can Unleveed Agricultural Fields in Deltas Keep Pace With Sea‐Level Rise?

“…Unfortunately, much of the velocity data from the low‐flow deployment in the forest and the high‐flow deployment in the field were corrupted due to instrument disturbance (e.g., movement by farmers or local fauna). The remaining data suggest similar patterns during the wet and dry seasons, which is consistent with previous studies of the area (Glover et al., 2022), so only one deployment is shown for each site. Water velocities in both locations were generally <0.5 m/s.…”

“…The sediment‐accumulation rate adjacent to the dead‐end channel in Meinmahla Island, 0.72 cm/year, was consistent with previously published rates of 0.26–1.78 cm/year from the exterior of the island (Glover et al., 2022). A comparably robust accumulation rate could not be calculated for the agricultural field.…”

“…At the agricultural site, the core was collected within the rice field. On Meinmahla Island, cores were collected within the mangrove forest (Glover et al., 2022). Cores were subsampled in the field at 2–5‐cm intervals.…”

“…The total 210 Pb activity was calculated by measuring the activity of the granddaughter 210 Po in relation to a known 209 Po spike. Activity was normalized by mud fraction and the supported activity, 0.7 dpm/g, was determined from the average of the deepest samples (Glover et al., 2022). Excess activity was calculated by subtracting supported activity from the total activity, and the accumulation rate, S , in cm/year was determined from, $$S=\lambda {\Delta }z/\mathrm{ln}\left(\frac{{A}_{0}}{{A}_{z}}\right)$$, where λ is the decay constant of 210 Pb, 0.0311 years −1 , A 0 is the excess activity at the top of the log‐layer, A z is the activity at depth z , and Δ z is the depth difference between the points.…”

“…The total 210 Pb activity was calculated by measuring the activity of the granddaughter 210 Po in relation to a known 209 Po spike. Activity was normalized by mud fraction and the supported activity, 0.7 dpm/g, was determined from the average of the deepest samples (Glover et al, 2022). Excess activity was calculated by subtracting supported activity from the total activity, and the accumulation rate, S, in cm/year was determined from,…”