Coastal cities face dual challenges of rising sea levels and sinking land | Opinion

The Invading Sea invited students from the IHE Delft Institute for Water Education, who attended the annual International Water Professionals Program at Florida Atlantic University, to compare environmental challenges in their native countries to how Florida is addressing similar problems. This op-ed is part of that series.

As global temperatures increase, ice sheets melt and sea levels rise, coastal cities around the world are experiencing more frequent and severe flooding. Some of these cities are facing an additional challenge of land subsidence, which is occurring simultaneously.

Land subsidence is characterized by a gradual sinking of the surface, which can be caused by human activities such as excessive groundwater extraction, mining, oil and gas extraction, and the construction of heavy structures. Subsidence, when combined with sea-level rise because of climate change, significantly exacerbates the risk of high-tide flooding in coastal cities, such as Miami Beach, Florida, and Cartagena, Colombia.

The impacts of such flooding can include infrastructure damage, ecosystem degradation, loss of life, displacement of communities, contamination of freshwater sources, disruption of services, adverse effects on economic growth and public health risks. To effectively address the risk of such compound flooding, it is crucial to identify the primary contributor to the relative sea-level rise.

Relative sea-level rise is calculated by combining the rate of sea-level rise with the rate of subsidence. From 1993 to 2016, the National Oceanic and Atmospheric Administration determined the global mean rate of sea-level rise to be about 0.11 inches per year. NOAA projects that sea levels will rise about 9.84 inches to 1.41 feet above the 2000 mean sea level by 2040 and by about 1.74 feet to 4.49 ft) above that level by 2070.

On average, Miami Beach experiences an average rate of land subsidence of about 0.08 inch a year. Compared to the global average sea-level rise rate of 0.11 a year, subsidence doesn’t represent the most significant hazard to Miami Beach, as only 40% of the relative sea-level rise can be attributed to subsidence. Conversely, in Cartagena the average rate of land subsidence is 2.72 inches a year, accounting for 70% of the relative sea-level rise, which substantially increases the flood risk in the city.

Measures to mitigate coastal floods include a range of approaches. Structural measures such as seawalls, flood barriers and breakwaters, and non-structural measures such as building regulations, flood early-warning systems, and land-use planning could be considered in the city of Miami Beach, where the flood risk arises mainly from the sea-level rise be because of climate change.

In Cartagena, measures to mitigate the land subsidence should be prioritized before implementing coastal flood measures, as the flood risk is mainly associated to the local subsidence phenomenon. Non-structural measures such as limiting groundwater extraction, controlling oil and gas extraction, regulating building construction and land-use planning can be more effective in this case, for they directly address the main driver of flooding.

Analyzing the type and driver of floods is a vital step in flood risk assessment and management. It provides a comprehensive understanding of flood causes and dynamics, enabling the identification of different measures that could be implemented to mitigate flood risk in specific flood-prone areas. Consequently, decision-makers, stakeholders and communities would be able to allocate resources and efforts more efficiently before, during and after a flood event.

Juliana Vargas, a Colombian researcher and environmental advocate, is pursuing a master’s degree in flood risk management at the IHE Delft Institute for Water Education.

The Invading Sea

This story was originally published August 14, 2023 at 7:51 PM.