Why do our coasts need protecting?
Around 700 million people currently live within the low elevation coastal zone (LECZ). LECZ is the area within the first 10 m of mean sea level (and below) and within 10 km of the coastline. This means more people than ever are susceptible to the impact of waves. Climate change is increasing sea level rise and the frequency and intensity of storms, which means we are getting larger waves more frequently. These larger waves are creating two critical hazards for coastal communities, increased erosion and increased flooding.
What does coastal erosion look like?
It is important to understand that all coastlines are dynamic and constantly changing, and erosion is a part of this natural process. However, climate change is exacerbating natural erosion, which is putting coastal communities at risk. You might be familiar with beach erosion in the form of sand cliffs that form after storms – this is when a large storm occurs and quickly removes tonnes of sediment from the beach. These sand cliffs are prominent around Australia after storms, especially at Surfers Paradise (in the image below) and at Byron Bay Main Beach. The other type of erosion (which is less obvious) is beach narrowing. Narrowing occurs over longer periods when the net sediment transport moves offshore. It does not happen overnight, so it can be hard to identify, but the beach will progressively lose sediment and become narrower.
So how does erosion increase flooding?
The height of a wave is affected by the water depth such that deeper water will contribute to larger wave heights. Therefore, if sediment is constantly being eroded from beaches, the water depth increases, creating larger waves. As wave heights increase, more ‘power’ in the wave allows them to move further inland. Consequently, the further inland they move, often the longer the floodwater water will ‘sit’ there. Stagnant floodwater is a significant problem for coastal communities globally, especially for communities whose sewer systems connect with their stormwater systems.
What can we do about it?
Coastal Engineering Solutions
There are two main types of coastal infrastructure; hard solutions and soft solutions. Hard solutions refer to ‘solid’ structures such as groynes, breakwaters, and seawalls, while soft solutions do not require infrastructure and include solutions such as beach nourishment. Coastal engineers have been designing hard solutions for centuries, but they are not without their problems.
Groynes are hard structures that run perpendicular to the coastline and trap sediment as it drifts down the beach. While this is great for the groyne’s updrift side, which now has a much wider beach, the groyne has stopped sediment moving downdrift, causing the beach to erode downstream. These negative consequences are typical and occur everywhere for many different types of hard solutions.
In this video you can clearly see a groyne preventing longshore drift on Queensland’s Gold Coast.
Nature-based solutions (NbS)
Coastal ecosystems, including mangroves, saltmarsh, tidal flats, seagrass, oyster and coral reefs, can help protect our coasts from flooding and erosion. These ecosystems can provide coastal protection in three ways:
Attenuating waves: Reducing the height of waves
Stabilising sediment: Reducing the ability of sand on the seafloor to be resuspended (and relocated elsewhere)
Accreting sediment: Increasing the amount of sand on the seafloor
The above-ground biomass (tree trunk, branches, leaves, and above-ground roots) obstructs the waves, reducing wave energy and wave height. The reduction in wave height also reduces the velocity of the water flowing through the ecosystem. If velocity in the flow is reduced, the sediment suspended in the water column has more time to settle onto the seafloor. As more and more sediment settles, the seafloor, beach or creek bed accretes (increases in height).
The below-ground biomass (roots and rhizomes) form dense root mats that trap and bind sediment and reduce resuspension. In specific environments like seagrass meadows, layers of biological matter called ‘biofilms’ can also reduce sediment resuspension. A reduction in resuspension also encourages sediment accretion.
If sediment can settle within and around an ecosystem, by default, erosion is reduced.
Mangroves
Seagrass
Saltmarsh
Tidal flats
Oyster reef
Coral reef
Other benefits of NbS
Utilising coastal ecosystems to enhance coastal protection also comes with many co-benefits referred to as ecosystem services. Ecosystem services are the benefits and commodities produced by these environments that society can directly or indirectly use. They are divided into three categories:
Provisioning services: products that are directly produced by the ecosystem e.g. food, compost fertiliser, pharmaceuticals, raw materials, genetic resources
Regulating services: services that control other processes e.g. coastal protection, geomorphology, water quality, nutrient cycling, biological control, sediment stabilisation, carbon sequestration
Cultural services: non-tangible benefits from the ecosystem e.g. recreation, tourism, research, education, cultural artefacts, spiritual, bequest value
Supporting services: services that support all other ecosystem services e.g. soil formation, primary production, fishery habitat
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