Research measures coastal wetlands' incomparable flood defense
Coastal wetlands provide even more flood protection than previously thought, a new study reveals.
While wetlands have long been considered a nature-based solution, the authors of a paper in Environmental Research Letters noted that the evidence of their role in estuaries remained scarce. The team’s Wales-based hydrodynamic modelling found that, for 100 year storms, estuary saltmarshes mitigated flood extent by an average of 35%.
The researchers' simulations showed that wetlands that grow in estuaries, such as salt marshes, can reduce water levels by up to 2 meters and provide protection far inland up estuary channels. This subsequently saved up to $38 million in avoided flood damage costs per estuary during a large storm thanks to the wetlands' role in preventing storm floods. Even across all the modelled storm scenarios, the wetlands still prevented an average of $2.7 million annually.
The research is timely as wetlands are facing growing threats from continued urban development. 22 of the largest 32 cities in the world - including London, New York and Tokyo - are built on low-lying land around estuaries, which puts them at increasing risk of flooding in a warming climate.
At the same time climate change is driving increases in the magnitude and frequency of storms, and sea level rise.
Previous research has focused on wetlands along open coastlines, where the plants absorb wave energy and stop waves pushing inland. However, the new study focuses on estuarine environments, including looking at what happens in upstream estuary areas where waves tend to be much smaller.
Taking this fuller picture into account, the researchers demonstrate that wetlands can have a much bigger storm flood prevention role because they not only absorb wave energy, but simultaneously reduce storm surges as they move up estuary channels.
This happens because the marshes along the estuary edges cause drag or friction, slowing down surges of water caused by storms, and protecting vulnerable upstream areas from flooding.
The research team included experts from Swansea University, with colleagues from the universities of Bangor and Exeter, Plymouth Marine Laboratory and from CSIRO in Tasmania.
The team used hydrodynamic models of eight estuaries of very different size and character in various parts of Wales. They simulated storms of different strengths and modelled the damage they would be likely to cause.
They found that coastal wetlands:
- Reduced flooding across all eight estuaries in the study
- Lowered storm water levels by as much as 2 metres in upstream areas
- Made the biggest difference when faced with the most powerful storms - reducing average flood extents by 35% and damages caused by 37% ($8.4M).
