Soft engineering & Ecosystem-based coastal defence

Delta ecosystems provide storm protection, nutrient and pollution removal, and carbon storage. Yet, in Pakistan, 1/5th of the Indus delta plain has been eroded since it was dammed in 1932; the Yellow River delta in China has retreated 300m in 35 years (Giosan et al., 2014), and it is thought that delta areas will increase a risk of flooding by 50% - the poorly preserved ones at least.

These regions are starved lands, and The Nile carries 98% less mud than it did a century ago. The naturally high subsistence rates of deltas are greatly exacerbated by human activities; the Chao Phraya delta in Thailand sinks by up to 15cm per year due to intensive groundwater exploitation. Methane extraction in the Po delta of Italy causes the same reaction. As the marshes weaken, the vegetation dies and soil formation slows significantly, speeding up land loss.

Natural deltas may rise in balance with sea level through sediment accumulation in regularly inundated wetlands. Conventionally engineered deltas do not see this land rise through sedimentation, since that process is cut off by flood-protecting structures such as dikes or dams. The land sinks due to human activities including soil drainage, and wetland loss causes severe water level rise.

It is postulated that nature-based engineering solutions might maintain sedimentation processes that drive land-level maintenance/accretion and flood storage through wetland conservation and restoration, chiefly by removing conventional engineering methods.

Conventional Coastal Engineering

Conventional engineering methods include building sea walls, dykes and embankments, but these constructions require continuous and costly maintenance, to a point where, with increasing flood risk, it may be becoming unsustainable. This, alongside the negative impact such structures have upon land subsidence, soil drainage and hindering natural sediment accumulation, has led to the introduction of ecosystem-based approaches. Conventional engineering approaches may protect the Hinterland (presumably), but does very little for the coastline itself. Which would be expected to last longer against changing sea levels and storms?

Conventional engineering often disturbs natural processes in ways that can accelerate local sea-level rise and increase long-term flood risk. When wetlands are disconnected from the rivers, the land no longer builds up with sea-level rise; deltas are incredibly susceptible to subsidence. Relying on conventional engineering poses severe risk to local populations, and unintentionally exacerbates long-term flood risk, compromising the sustainability of local communities. 

Restoration

The Future Earth and Sustainable Deltas 2015 is one example of an inititative whose findings are not being implemented, and hard engineering methods prevail with regards to Hinterland protection. Gaps in knowledge slow down the implementation of generic solutions, and it is often forgotten that the health of wetlands is crucial to delta resilience. It is surmised by Giosan et al., 2014 that biophysical and biogeochemical research on wetland processes must be expanded to address the wide range of deltaic conditions. Further, agricultural and industrial practices should be assessed to select sustainable practices.

Wetland reclamation leaves nowhere for flood waters to go, meaning storm surges that do occur will rise higher and propagate faster/further inland. Consider the Scheldt estuary in Belgium, where high water levels have increased by 1.3m from 1930.

Nature-based engineering largely involves restoration in order to provide greater water storage, improve the capacity to build up sediment, regain elevation and maintain a cost-efficient, self-sustaining environment in the future, which conventional engineering cannot offer.

Soft engineering methods

Beach nourishment is a positive feedback loop that does not stop the erosion process. It is only really implemented on developed coastlines, not under the motivation of ecological conservation. When shoreline erosion is compensated with sand from outside the system, it must be continuously repeated as the sand rectangle spreads down the shoreface at an exponential rate. There are numerous problems and benefits of beach nourishment. Interestingly, you are allowed to suck up three sea turtles in the U.S dredging schemes before it becomes punishable - naturally there are big enforcement problems regarding this cutting edge rule.

 Mismatching coastal material is a problem from a residence/tourism angle, particularly when fine sand is replaced with rocky cobble. Further, changing the material alters the heat absorption capacity of the sand, which impacts turtle egg development and the ratio of males:females produced. False crawls can also be initiated when a beach is so fine-grained it is too compact for the female turtles to dig. 

Ecosystem-based flood defense

Cities located in estuaries or deltas such as London are thought to benefit enormously from the restoration of tidal marshes/mangroves between the city and sea, since they act to provide extra water storage areas and friction to attenuate the propagation of storm surges. It generally reduces flood risk inland. The marshes are constructed by the landward displacement of historical dykes, or purposeful, controlled flooding of chosen regions. Marshes improve water quality, sequester carbon, produce fisheries, work with conservation and create recreational space. The delivery of scarce nutrients such as silica by tidal wetland water suppresses the growth of toxic algae but stimulates the growth of phytoplankon. There is often a worry that these constructed wetlands will soon become submerged however. A cost-benefit analysis for Humber estuary stretching over 25 years showed it is more cost effective to restore tidal marshes than maintain dykes.

Behind sandy coastlines, beach and dune barriers are essential, and produced by artificial sediment placement, and in the Netherlands, a 17km stretch of coastline has combatted coastal erosion through careful planning and sculpting of the shore. This has avoided the need for constant dredging. The construction of oyster reefs from gabions on sand flats further work to reduce waves, currents and erosion.

Limitations 

Ecosystem-based flood defenses generally require more space than conventional structures, which makes them difficult to implement in urban areas such a s New York or Tokyo. The more space there is between the sea and urban area, the more efficient these approaches become. For this reason, cities closer to the coastline would benefit more from a combination of conventional and ecosystem-based engineering methods. Where there is little space at all along the beach, seaward ecosystem creation (e.g. off-shore reef development) may be an option, but could destroy or disturb the existing ecosystems. 

Locations with low elevation and high tidal inundation are often far less successful at growing tidal marshes, and this is a problem that may be transferred across to mangrove forests, although very little data is known about tropical projects in the long-term. Engineering structures such as weirs or sluices can help with this, but will also rack up the total price. Soil properties and their influence over water filtration, wind waves, bioturbation, grazing and seed dispersal can also hamper wetland development. 

Public perception may be an extremely strong blocker to implementing ecosystem-based management strategies; chiefly regarding the idea that laboriously reclaimed valuable land should be given back to the sea. In the Netherlands, there is significant cultural and political objection to such projects, where 50% of the population live below sea level. Their long-time struggle against the seas has become a part of their cultural heritage, however perhaps societal opposition may be overcome with clear communication of the benefits of such defense schemes.

Further public resistance may arise from concerns that wetlands may facilitate mosquito breeding and disease transmission, particularly in tropical regions amongst the more vulnerable areas e.g. Asia.