What can communities in the United States, such as Miami, do to reduce the damage of increased high-tide flooding caused by rising sea levels?
The incidence of high-tide flooding (HTF) has doubled in the past two decades and is expected to increase five to 15 times more in the next 30 years, according to a new report released by the National Oceanic and Atmospheric Administration (NOAA).
The State of High Tide Flooding and Annual Outlook examines the incidence of HTF, a phenomenon also known as “nuisance” or “sunny day” flooding, that occurs when tides reach anywhere from 1.75 to 2 feet (53cm to 61cm) above the daily average high tide and start flooding streets and storm drains.
According to the report, “as sea-level rise continues, damaging floods that decades ago happened only during a storm now happen more regularly, such as during a full-moon tide or with a change in prevailing winds or currents.”
NOAA gathers data from 97 tide gauges located along the US coast. Coastal communities saw twice as many high tide flooding days between May 2020 and April 2021 than they experienced 20 years ago. In the same period, 14 locations tied or broke their records for the number of HTF days along the US Southeast Atlantic and Gulf coastlines, a dramatic increase from 2000.
“High-tide flooding impedes the use of roads and increases wear and tear on stormwater and wastewater systems,” writes Renee Collini, a Coastal Climate Resilience Specialist at Mississippi State University, in The Conversation. “The impact can seem minor, but as the frequency increases, these seemingly inconvenient flood days can have long-lasting impacts.”
These effects include decreased property values, increased insurance premiums and a loss in revenue for businesses affected by floodwater. Flooded roadways also pose a danger to road users and impede emergency vehicle access.
How can communities protect themselves against high-tide flooding?
Collini outlines several ways communities can reduce damage from HTF:
- Upgrade infrastructure, such as raising roadways and installing backflow preventers in stormwater systems.
- Modify building standards, such as increasing freeboard – the distance required between the first floor and base flood level – or designating base flood elevations outside current Federal Emergency Management Agency (FEMA) flood zones to help prepare communities to withstand higher seas.
- Preserve and restore coastal habitats that provide natural flood protection, such as marshes and barrier islands.
Other solutions include the construction of seawalls to protect flood-prone coastal communities such as New York, Charleston and Miami. The US Army Corps of Engineers recently published a proposal to build a six-mile seawall in New York for A$162 billion.
This hefty price tag is too high for some, who consider billion-dollar seawall infrastructure an expensive band-aid solution to a problem set to become more frequent and severe in future decades as sea levels rise due to climate change.
“This approach is almost certainly a short-term strategy that will protect only a few cities and will protect only selected portions of those cities effectively,” argues Gary Griggs, director of the Institute of Marine Sciences at the University of California.
Griggs believes cities should consider other longer-term alternatives before investing in seawalls, citing Boston as an example. Flooding is a major issue in Boston, where sea levels rose 23 centimetres in the 20th century and are expected to increase another 23 centimetres by 2030. The city’s economic losses due to coastal flooding are estimated at A$322 million per year between now and 2050.
In 2019, the city released its Coastal Flood Resilience Design Guidelines to raise awareness of future coastal flooding risks and promote strategies to reduce flood damage and disruption.
“Boston recently considered flood barriers for either its outer or inner harbour, but rejected these options in favour of softer options like climate-resilient zoning with special requirements for new projects in flood-prone areas,” writes Griggs.