Mangroves Protect the Coast. What Protects the Mangroves?

Two modular cellular structures for the years when a planted shoreline is most likely to fail

A mature mangrove forest is one of the few coastal defences that becomes more effective with age. Its roots bind the bed, its stems and prop roots attenuate incoming waves, and the sediment it traps raises the ground beneath it. On paper, planting mangroves is the obvious answer to an eroding tropical shore.

The record is less encouraging. After the 2004 Indian Ocean tsunami, Sri Lanka put roughly 1,000-1,200 ha of mangroves under restoration across 23 project sites. A later nationwide survey found successful restoration on about 200-220 ha, and at nine of the 23 sites no planted mangroves had survived at all. Survival was linked to post-planting care and to planting at the wrong topography, where soil and hydrology did not suit mangroves (Kodikara et al. 2017).

In most of these cases, the nursery stock was not what failed. The site conditions were.

What a seedling has to get through

Flume experiments and field observations on Avicennia alba, a pioneer species, show that a stranded propagule has to pass three thresholds before it can be called established (Balke et al. 2011). It needs a period free of flooding long enough to put down roots. Those roots then have to grow long enough to resist dislodgement by waves and currents, and the length required rises with the force acting on the plant. Longer roots still are needed to survive higher-energy events that strip away the surface layer of the bed. A young plant that meets rough water or a mobile bed before it has passed all three is washed out.

Young mangroves also give the least protection when they need it most. Field measurements in the southern Andaman region of Thailand found wave attenuation rates of about 0.002 per metre in the sparse fringe vegetation of Avicennia and Sonneratia, against about 0.012 per metre in the dense Rhizophora stands further inland, a sixfold difference (Horstman et al. 2014). Hydraulically, a new planting behaves like a sparse fringe.

Elevation is the other constraint. Mangrove species are distributed along the shore according to how long and how often they are flooded (Krauss et al. 2008). A seedling planted outside its band struggles, however well it was raised in the nursery.

A planted shoreline therefore needs calmer water at the plant, a bed that stays in place around the roots, and ground at the right level, and it needs all of this during the first years, before the forest can provide any of it for itself.

Borrowing time from structures

Engineers working on mangrove-mud coasts have tried to supply these conditions with low permeable structures. Over about fifteen years, engineers built permeable dams of vertical poles and brushwood in Guyana, Indonesia, Suriname, Thailand and Vietnam. They dampen waves and trap sediment in the basins behind them, and at some sites mangroves recolonised the raised ground (Winterwerp et al. 2020).

The same review is candid about the materials. At Demak, Indonesia, shipworms attacked the local timber used for the poles, and it deteriorated within a year. Bamboo performed better but lasted less than two years when permanently submerged (Winterwerp et al. 2020). The dams were conceived as temporary works, which in practice means repair and maintenance for as long as the forest takes to establish.

The principle held. The materials did not.

Two structures for the establishment years

Hydrantula has developed two modular structures for this task. Both follow the construction principle of our system: standard HDPE pipes act as permanent formwork, purpose-built rotomoulded polyethylene fittings join them, and the assembly is filled in place with concrete. No timber is available for shipworms to consume, and no steel reinforcement can corrode; where reinforcement is required, it is glass-fibre composite.

The first is a low cellular field. Its walls are pairs of stacked pipes, about a third of a metre high, joined into an open grid of six-sided cells roughly 4 m across. The field lies on a sandy or silty foreshore and can be extended in any direction by adding cells. The walls slow water moving across the protected area so wave-borne sand settles inside the cells, and where sediment supply allows, the bed builds up toward the top of the wall. Mangroves are planted inside the cells, in ground that the structure holds around their roots.

A cellular field with mangroves planted in the cells. Indicative visualisation; not a record of measured performance.

The second is a chain of lens-shaped cells, each about 1.5 m long and 0.9 m wide, with walls of six stacked curved pipes rising to about 0.6 m. Neighbouring cells are set at an angle to one another, so the chain runs in a zigzag and can be extended at either end. Each cell is lined with geotextile and filled either with fertile substrate, making it a planting pocket, or with rock, making it a permeable barrier. Because the fill is placed cell by cell, planting pockets can sit at different bed levels to match the flooding regime each species prefers.

A chain of lens-shaped cells used as planting pockets. Indicative visualisation; not a record of measured performance.

Both structures' fittings carry vertical channels. A pipe driven through a channel

works as a pile, anchoring the structure in the bed, and further tiers can be stacked on the same piles.

How they are combined

The layout follows the task and the configuration of the shore, but in both typical arrangements the field forms the base, covering the eroding shore in plots.

In the first, a line of chains stands seaward of the fields and takes the first wave. The chain stands markedly higher than the field wall and meets the incoming energy before it reaches the plantings behind.

In the second, chains run between the fields, perpendicular to the shoreline. They divide the shore into compartments and hold back the sand that would otherwise move along it.

Either way, the plantings sit inside cells rather than on open mud. The intent is to give seedlings sheltered water and a stable bed from the day they are planted, instead of waiting for a forest dense enough to provide those conditions itself.

Why permeability matters

Both structures are deliberately leaky. Narrow gaps between the pipe rows let water and fine sediment through, so the cells slow the flow rather than block it.

A solid vertical wall behaves very differently. It reflects the incoming wave, and in the limiting case of full reflection, the bed shear stress next to the wall rises by a factor of four, with a high risk of scour at its foot (Winterwerp et al. 2020). Toe scour is exactly the kind of bed instability a young mangrove cannot survive. Mangroves also depend on regular tidal exchange, which a sealed barrier would cut off.

Growing with the shore

A modular structure can change as the shoreline changes. The field gains cells, the chain gains links, and where the site checks allow, a second tier can be placed on the existing piles if water levels rise or the bed builds up faster than expected. None of this requires removing what is already in the ground.

What has not yet been shown

Neither structure has yet been tested hydraulically. Both are intended for sheltered, low-energy shores, and waves of up to about 0.4 m are a design assumption rather than a demonstrated limit. Mangrove coasts are often soft mud, and the field in particular is light for its plan area, so its resistance to sliding depends on the piles. Site-specific checks set pile layout and embedment, settlement, and local scour. Sand builds up in the cells only where the shore still receives sediment, and some eroding coasts no longer do. The survival and growth of plantings are targets to be monitored, not outcomes we can promise.

These limits are stated plainly for a reason: the problem these structures address is itself a record of planting programmes that looked convincing until they met the site.

Where this applies

The structures are intended for eroding mangrove shores and tidal flats, restoration and compensation planting, the green edges of reclaimed land, and estuary and lagoon banks where plantings have repeatedly failed.

Hydrantula operates as a technology licensor and supplier of proprietary fittings. The contractor carries out construction under licence, with our technical supervision and a project-specific verification package.

If you are planning a mangrove restoration or a living shoreline and have watched plantings wash out on a mobile bed, we would be glad to talk.

References: Balke, T., Bouma, T.J., Horstman, E.M., Webb, E.L., Erftemeijer, P.L.A. & Herman, P.M.J. (2011), Marine Ecology Progress Series 440:1-9. Horstman, E.M., Dohmen-Janssen, C.M., Narra, P.M.F., van den Berg, N.J.F., Siemerink, M. & Hulscher, S.J.M.H. (2014), Coastal Engineering 94:47-62. Kodikara, K.A.S., Mukherjee, N., Jayatissa, L.P., Dahdouh-Guebas, F. & Koedam, N. (2017), Restoration Ecology 25(5):705-716. Krauss, K.W. et al. (2008), Aquatic Botany 89(2):105-127. Winterwerp, J.C. et al. (2020), Ecological Engineering 158:106078.