LOWER COURSE
Facilitating the outflow of water, out of the river system and into the ocean is a vital part of preventing floods
Ensuring that rivers can quickly discharge their waters into the ocean will reduce the risk of flood. Ocean surge during and after a typhoon adds to flooding by surging ocean water into river mouths and over the coast. This surge of ocean water blocks the river water from discharging.
This page provides information on the following nature based solutions to help protect the flow of water out of the river system.
Establishing Mangroves Forests
Establishing Mangrove forests along the coast can reduce waves and storm surges. Cross-shore width and forest thickness provide benefits as the greater the density of roots and canopy the greater the reduction of wave strength. It is very important to plant and maintain long, deep mangrove forests. Thick
Plant local species in shallow water, above mid tide levels.
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Things to Consider (from Mangroves for coastal defence Guidelines for coastal managers & policy makers, Wetlands International and The Nature Conservancy. )
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• Maintain wide mangrove belts. To significantly reduce everyday waves you need a belt of hundreds of meters wide – the more the better. A few trees won’t help very much.
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• The more obstacles the better, so mangroves of various species of different age and size are most valuable in reducing wave heights. In a dense mangrove forest packed with aerial roots and low branches, a wave may be reduced to half its height after 100m passage through the mangroves, while in a more open forest, it might take 500m of mangroves to reduce such a wave.
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• Even relatively young or small mangrove forests can reduce the height of wind and swell waves. Hence, reforested areas become effective at reducing waves in just a few years.
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• Protection of adjacent ecosystems is important as these can also help attenuate waves. They include sandbanks, seagrasses and coral reefs, as well as landward dunes and saltmarshes.
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$$ Cost of Mangrove forest restoration can be in the hundreds of thousands, however these costs are consistantly seen to be cost effective and have long term financial savings when it comes to damage prevention.
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Further Reading
Mark Spalding, Anna McIvor, Femke Tonneijck, Susanna Tol and Pieter van Eijk, (2014) Mangroves for coastal defence Guidelines for coastal managers & policy makers, Wetlands International and The Nature Conservancy.
Menéndez, P., Losada, I.J., Torres-Ortega, S. et al. The Global Flood Protection Benefits of Mangroves. Sci Rep 10, 4404 (2020).
Establish Dunes
Establishing dunes protects against storm surge. There are several ways dunes can be established along the shore.
Geotextile Sand Bags
Filling large hessian bags called geotextile sand bags with sand will allow for structure and protection for a new dune. Local wind and salinity resistant grasses can be planted through holes in hessian.
​Thing To Consider
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New sand dunes must be planted with appropriate vegetation to ensure stability of the dune.
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Geotextile sand bags must be placed in a terraced shape to replecate natural dunes to ensure strength and stability.​
Dune Fences
Dune Fences and thatching of plant debris and branches will stabilise and promote capture of sediment blown in on the wind.
​ Thing To Consider
The length of the fence is the most important characteristic when it comes to sand accumulation. A longer fence will accumlate more sand and create a greater storm surge barrier.
Fences may be stright or zig zagged along the beach.
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$$ Compared to rock or concrete barriers, construction of fence or sand bag barriers is very cost effective.
Establish Oyster Reefs
Oyster Reefs help to protect coastlines. Serving as natural breakwaters, they reduce the force of waves.
Oyster Reefs also serve as shelter for fish and other marine creatures.
Carefully placed rock, concreate shapes and sterile shells can attract oyster breeding. In some areas oysters may need to be introduced manually.
Oyster reefs also serve to clean water, store CO2 and provide food and shelter for marine life.
Things To Consider
- areas with moderate wave action are ideal. Consistant strong waves could damage reefs.
-Shallow water of 0.5 to 2 meters is best.
- Water temperature of 20-30 degrees.
- Use local oyster species
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$$ Similar to mangrove resteration, developing oyster reefs can be in the hundreds of thousands, however these costs are consistantly seen to be cost effective and have long term financial savings when it comes to damage prevention.