Location via proxy:   [ UP ]  
[Report a bug]   [Manage cookies]                
SlideShare a Scribd company logo

Coastal subsidence

•
•
Md Asif Hasan
Md Asif Hasan

This document was submitted as an assignment as a part of the partial fulfillment of my master's degree.

1 of 2
Download to read offline
Assignment on “Coastal Subsidence” Submitted by Md. Asif Hasan Roll: SH-037 Date: September 03, 2019
COASTAL SUBSIDENCE Coastal subsidence is the motion of a coastal surface as it shifts downward relative to a datum such as sea-level. It is a major concern in cities near the Coastal region. Subsidence can cause decreases in property values, changes in surface-water drainage patterns that lead to flooding, and large areas of inundation caused by tropical storm surges. Processes acting behind Coastal Subsidence Several natural and human-related processes are known or suspected to be causing subsidence in coastal Louisiana today and in the recent geologic past. Almost all previous studies, however, have provided qualitative insights rather than quantitative measurements of actually how much sinking has occurred. It can be said with reasonable certainty that modern subsidence is the integrated effect of multiple natural and anthropomorphic processes that operate at several different spatial and temporal scales. It follows that the motion at any point on the Earth’s surface is thus dependent on a unique set of local and regional conditions. Therefore, to more fully understand subsidence, we must be able quantify the relative contributions of each process operating at a locality. Because these processes do not occur everywhere and at all times, hard evidence must be produced to rule in or rule out specific causes at any particular place. A list of processes proven or hypothesized to be responsible for coastal subsidence: Natural processes ➢ Sediment compaction; ➢ Sediment consolidation; ➢ Compaction of semi-lithified rock ➢ Regional faulting associated with gravity spreading and/or salt evacuation; ➢ Sediment load-induced down-warping; ➢ Salt evacuation; ➢ Cap-rock collapse due to natural pressure reduction above salt domes. Human-induced processes ➢ Organic sediment decomposition due to drainage or agricultural projects ➢ Groundwater extraction-compaction of shallow aquifers (sands) ➢ Oil/gas extraction related-compaction of sediment layers (sands) ➢ Fault motion-induced by shallow groundwater withdrawal ➢ Cap-rock collapse due to induced pressure reduction above salt domes. Sea level Rise and Coastal Subsidence Sea level has risen approximately 120 m since the last glacial maximum about 20,000 years before present. Accelerated sea level rise is considered one of the most certain and most costly impacts of greenhouse gas enrichment and associated atmospheric warming. During the 21st century, the global average rate of sea level rise is expected to increase 2- to 4-fold over the 20th century rate (1-2 mm per year). The added stress of accelerated sea level rise on sinking coastal environments portends an increase in flood damages in the Gulf Coast that is greater than that projected for many other coastal regions. The relative rate of sea level rise along any coastal segment is dependent upon both the rate of sea level rise (which varies among ocean basins due to depth, basin geometry, and other factors) as well as the vertical motion of the land surface.

More Related Content

Coastal subsidence

  • 1. Assignment on “Coastal Subsidence” Submitted by Md. Asif Hasan Roll: SH-037 Date: September 03, 2019
  • 2. COASTAL SUBSIDENCE Coastal subsidence is the motion of a coastal surface as it shifts downward relative to a datum such as sea-level. It is a major concern in cities near the Coastal region. Subsidence can cause decreases in property values, changes in surface-water drainage patterns that lead to flooding, and large areas of inundation caused by tropical storm surges. Processes acting behind Coastal Subsidence Several natural and human-related processes are known or suspected to be causing subsidence in coastal Louisiana today and in the recent geologic past. Almost all previous studies, however, have provided qualitative insights rather than quantitative measurements of actually how much sinking has occurred. It can be said with reasonable certainty that modern subsidence is the integrated effect of multiple natural and anthropomorphic processes that operate at several different spatial and temporal scales. It follows that the motion at any point on the Earth’s surface is thus dependent on a unique set of local and regional conditions. Therefore, to more fully understand subsidence, we must be able quantify the relative contributions of each process operating at a locality. Because these processes do not occur everywhere and at all times, hard evidence must be produced to rule in or rule out specific causes at any particular place. A list of processes proven or hypothesized to be responsible for coastal subsidence: Natural processes ➢ Sediment compaction; ➢ Sediment consolidation; ➢ Compaction of semi-lithified rock ➢ Regional faulting associated with gravity spreading and/or salt evacuation; ➢ Sediment load-induced down-warping; ➢ Salt evacuation; ➢ Cap-rock collapse due to natural pressure reduction above salt domes. Human-induced processes ➢ Organic sediment decomposition due to drainage or agricultural projects ➢ Groundwater extraction-compaction of shallow aquifers (sands) ➢ Oil/gas extraction related-compaction of sediment layers (sands) ➢ Fault motion-induced by shallow groundwater withdrawal ➢ Cap-rock collapse due to induced pressure reduction above salt domes. Sea level Rise and Coastal Subsidence Sea level has risen approximately 120 m since the last glacial maximum about 20,000 years before present. Accelerated sea level rise is considered one of the most certain and most costly impacts of greenhouse gas enrichment and associated atmospheric warming. During the 21st century, the global average rate of sea level rise is expected to increase 2- to 4-fold over the 20th century rate (1-2 mm per year). The added stress of accelerated sea level rise on sinking coastal environments portends an increase in flood damages in the Gulf Coast that is greater than that projected for many other coastal regions. The relative rate of sea level rise along any coastal segment is dependent upon both the rate of sea level rise (which varies among ocean basins due to depth, basin geometry, and other factors) as well as the vertical motion of the land surface.