Sinking Deltas due to Anthropogenic Activities

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Sinking deltas due to human activities from James P. Syvitski, et al

Worldwide, deltas are one of the most populated zones due to their rich soils but high potential of irrigation and energy production. That high demographic growth combined with poor land use management, indeed endanger the sustainability of deltas. Another negative effect of that malpractice is that it increases the risk of flooding after sinking of the sediments. In fact, the vulnerability is a result of sediment removal from anthropogenic activities such as oil, gas and water removal from the delta that underlies sediments. Other natural factors exist but mankind activities are just worsening it up. From those natural causes, we can list, storm surges, intense precipitation directly onto deltas, river overbanking, and hurricane induced storm surges.

To have a better understanding of the sinking phenomenon, a team of researchers lead by Dr Syvitski assessed 33 deltas in different continents. They used time-series of global high-resolution satellite data. Additional to that, topographic maps of deltas, as well as historical maps were used to determine the topography in relation to global mean sea level. That methodology helped georeferenced to see how the river channels changed their location and paths across each delta. Furthermore, visible and near infrared images from MODIS helped ascertain the origin of the flooding, whether it’s from the river runoff or coastal surges or to see if floodwaters carried sediments or not. The purpose was to determine whether the plains are keeping up with rising sea level by adding new sediments layers to surfaces during periods of flooding. The main equation used in this study was the vertical change in delta surfaces relative to local mean sea level:

ΔRSL = A – ΔE – CN – CA ± M.

Here, A is the delta’s aggradation rate and is determined from the volume of sediment delivered to and retained on the subaerial delta surface. ΔE is the eustatic sea-level rate gotten from shifts to the volume of the global ocean over time. CN is the natural compaction and CA the accelerated compaction, those two factors reduce the volume of deltaic deposits. And M is the downward vertical movement of the land surface as influenced by the redistribution of Earth’s masses.

The conclusions of the study were that sediment delivery to deltas has been lowered or removed at all scales. Additionally, deltas witnessed coastal inundation from surges, floods from rivers overbanking their levees, flooding from intense rainfall within the delta, or all three sources of flooding combined. Also, creating channels super-elevated above delta’s surrounding flood plains increases the flood risks. And that sinking rates are faster than global sea level rising.

Thus, it is a must that human come up with mitigation technics. In fact local authorities and scientists should be working together to put in place mitigations technics, like some engineering and institutional controls to eradicate that phenomenon.