Editor’s note: 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 write articles comparing environmental challenges in their native counties to how Florida is addressing similar problems. This piece is part of that series.
By Juliana Vargas, IHE Delft Institute for Water Education
As global temperatures increase, ice sheets melt and sea levels rise, coastal cities around the world are witnessing more frequent and severe flooding events. Some of these cities are facing an additional challenge as land subsidence is occurring simultaneously.
The land subsidence phenomenon 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 due to climate change, significantly exacerbates the risk of high-tide flooding in coastal cities such as Miami Beach, Florida, and Cartagena, Colombia.
The impacts of coastal 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 compound flooding driven by sea-level rise and subsidence, 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 2.9 mm (about 0.11 in) per year. NOAA projects that sea levels will rise 250 mm to 430 mm (about 9.84 in to 1.41 ft) above the 2000 mean sea level by 2040 and 530 mm to 1370 mm (about 1.74 ft to 4.49 ft) above that level by 2070.
On average, Miami Beach experiences an average rate of land subsidence of 2 mm (about 0.08 in) per year. Compared to the global average sea-level rise rate of 2.9 mm per 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 6.9 mm (about 2.72 in) per 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 wide range of ways to reduce flood risk and its impacts on communities and the environment. Structural measures such as seawalls, flood barriers, breakwaters and non-structural measures such as building regulations, flood early warning systems development and land-use planning could be considered in the city of Miami Beach, where the flood risk arises mainly from the sea-level rise due to climate change.
In the city of 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, as they directly address the main flood driver.
Analyzing the type and driver of floods is a vital step in flood risk assessment and management. This analysis allows for 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.