This paper analyses sixty-nine field measurements to show that habitats have a significant influence on wave reduction, demonstrates the influence of specific engineering parameters on wave reduction effectiveness, reviews the costs and benefits of fifty-two nature-based defence projects (Fig 2), and demonstrates the cost-effectiveness of some of these projects relative to structures that provide the same wave reduction.
Natural defences and wave reduction
Meta-analyses of sixty-nine studies, among five habitats world-wide (coral reefs, mangroves, salt-marshes, seagrass/kelp beds), show that these habitats reduce wave heights significantly (see Methods) and this reduction varies with the habitat and the site. This is in line with findings from  and . On average, coastal habitats reduce wave heights between 35% and 71%. Coral reefs reduce wave heights by 70% (95% CI: 54–81%), salt-marshes by 72% (95%CI: 62–79%), mangroves by 31% (95% CI: 25–37%) and seagrass/kelp beds by 36% (95% CI: 25–45%). Across all habitats, coral reefs emerge as having the greatest potential for coastal protection: they are highly effective at reducing wave heights and are also exposed to higher, more powerful waves. Salt-marshes are almost as effective in terms of wave reduction but occur in more sheltered environments. Mangroves and seagrass / kelp beds are about half as effective, with mangroves occurring in the most sheltered environments (see S1 Table). The high reduction by coral reefs agrees with the results of , and the ordering of the other habitats is generally in agreement with the review by  which considered similar parameters in their re-analyses of field evidence for these habitats. There is also a strong positive, linear correlation between the extent of reductions in wave height, and the wave height before the habitat, in the order coral reefs > salt-marshes ~ mangroves > seagrass / kelp beds (see S1 Fig).
The influence of design parameters commonly used in engineering such as habitat width, the ratio of habitat width to the wavelength, and the ratio of habitat height to the water level (see Introduction, Fig 1) were also examined. Wave reduction in each habitat is influenced by different parameters. In coral reefs, wave reduction is influenced by a) reef width (S2 Fig); b) reef depth relative to the wave height and; c) reef width relative to the average wavelength (S3 Fig). The most effective reefs are at least twice as wide as the wave-length, and located at depths that are at most, half the incoming wave height. There is limited data in salt-marshes that suggests that wave reduction is linearly correlated with the relative height of the marsh, i.e. the submergence of the vegetation relative to the water level (S4 Fig). Wave reduction in salt-marshes is highest when the canopy is close to the water surface. This suggests that designs of ‘greenbelts’ for coastal protection, rather than only relying on width-based criteria [24,25], should also account for the relationships between habitat and hydrodynamic variables at each site. is also emphasised by Koch et al.,  who demonstrate spatial and temporal variations in wave reduction capacities across habitats. These analyses were performed only for coral reef and salt-marsh habitats. Mangroves and seagrass/kelp beds are not discussed due to the lack of comparable data on design parameters for these habitats.
Nature-based defence projects: costs, benefits and cost effectiveness for coastal protection
Table 1 summarises the costs, coastal protection benefits, objectives and exposure of fifty-two nature-based defence projects in coral reef, oyster reef, mangrove, and salt-marsh habitats. A sizeable proportion of salt-marsh and mangrove projects state that such habitats provide improved protection from storm events (41% in salt-marshes and 50% in mangroves; see Table 1). Other coastal protection benefits include savings in damages during storm events, reductions in erosion and reductions in the costs of engineering for coastal protection, reflected, in a few cases, by positive benefit-cost ratios (e.g. also see ). Restoration objectives vary across habitat types, with most mangrove and marsh habitats reporting coastal protection as a primary objective. Among the coral reefs a majority of projects are targeted primarily at habitat restoration and only a small number for coastal protection, even though many of these reefs are situated in highly exposed regions. Unit restoration costs are lowest for marshes and mangroves, and coral and oyster reefs show higher, and more variable, costs (Table 1).
Analyses of the costs and wave reduction of thirteen nature-based defence projects (see Methods, S4 Table) in mangroves and salt-marshes show that these projects can be several times cheaper than alternative submerged breakwaters (Fig 3) for the same level of protection. Together with their ability to keep pace with sea-level rise  this suggests that nature-based defences can become increasingly viable on sheltered coastlines. Depending on the water depth, mangrove projects in Vietnam can be three to five times cheaper than a breakwater, and salt-marsh projects across Europe and the USA vary from being just as expensive, to around three times cheaper (Table 1, Fig 3). Fig 3 plots the total restoration costs of mangrove and marsh projects along with breakwater construction costs at these sites for a range of depths and wave height reduction values. The cross-shore width of habitat and the degree of wave height reduction are also indicated for each project. Water depth is a crucial factor, with both habitats showing an increase in cost effectiveness at higher depths, due to the relatively steep increase in breakwater construction costs. Habitat width is found not to be a sufficient indicator of cost effectiveness. Also, these nature-based defences are limited to wave heights less than half a metre and are not always cost effective.
Fig 3. Costs versus water depth and wave height reduction extents of Nature-based Defence (NbD) projects and alternative breakwaters.
Costs of NbDs and cost curves of alternative breakwater structures plotted versus water depth are plotted for a) mangroves (n = 7) and breakwaters in Vietnam and; b) salt-marshes (n = 6) and breakwaters in Europe/USA. Circles represent NbDs and lines represent submerged breakwaters cost-curves in both panels. NbDs that fall below breakwater cost curves are cost effective in comparison. Breakwater cost curves are for an incident wave height Hsi of 0.2 m. All costs are represented on a per-metre coastline length basis (see Methods). Fig only shows mangroves and marshes as these were the only habitat types and locations for which project information was found in close proximity to field measurements.
Based on existing literature it was assumed that breakwater construction costs are uniform across the sites in Europe/USA and ten times lower for the sites in Vietnam . Such regional differences are also reflected in the reported habitat restoration costs in these countries. While accurate estimates of construction costs require detailed information on structure profile, material and labour costs, etc., water depth is often a critical driver of construction costs  and therefore the main influence on cost effectiveness. Only total project costs and habitat extents were used, given the high variability in the relationship between restoration project costs and sizes (see Methods). The study does not explicitly account for increases in restoration costs due to adverse ecological or geomorphic site conditions which can significantly increase these values .
In the cost comparisons we look for structures that are equivalent to marshes and mangroves in function–i.e. wave reduction, as well as location–i.e. within the near-shore zone, and choose submerged breakwaters as the best alternative. Submerged breakwaters provide wave reduction to varying degrees, similar to coastal habitats and can be located within the near-shore zone. Though seawalls are a common substitute for mangrove and marsh habitats [20,31], these are often located at the shoreline and, in addition to blocking waves, also protect against flooding from high water levels. While there are some indications that mangroves and marshes can offer protection from high water levels (Table 1), we do not find enough evidence on this for a comparison of effectiveness, and as such, focus on their wave reduction function. While coral reefs are also very similar to breakwaters in structure and wave reduction function, we do not find enough information on reef restoration projects for a direct cost comparison. It is important to note that coastal habitats are usually one of several structural, nature-based and non-structural measures for coastal protection .
This study focuses on coastal protection by wave reduction, though habitats often provide other ecosystem services such as biodiversity, fish production, recreation and many other social, economic and cultural values . The addition of these benefits, over and above their coastal protection value should make these natural approaches more appealing to coastal managers and decision-makers . Also, the loss of existing coastal habitats and their replacement by man-made structures can result in loss of these ecosystem services . In any case, policy decisions on where and how to conserve or restore habitats, rather than focusing on a single service, should consider multiple objectives for best allocation of available resources [36,37].
The data for the wave reduction analyses are all obtained from field observations of wave heights and hydrological variables. The datasets used in this study vary in terms of the type of data available for analysis, and these are described in S2 and S3 Tables. The wave reduction data are all field observations of wave heights through habitats (S2 Table). Almost all the studies provide information on habitat width, and most measurements in reefs also provide information on reef depth. Only a few studies–all in marshes, provide information on vegetation heights. The restoration project data are a mix of primary–i.e. observed and secondary–i.e. estimated costs and benefits (S3 Table). The coastal setting and exposure data for each project location are derived from other sources (see S1 Methods). Cost reporting by projects is highly variable (see Methods). All costs are reported on a per-m2 basis, and use total project costs for the cost comparison analyses. Ideally, in future, cost reporting in projects should be consistent and report both unit and total restoration costs. More such comparisons with hard alternatives, along with detailed and consistent data on the extents, costs and coastal protection benefits of existing restoration projects, are needed to inform the design and implementation of future nature-based defences.
We are interested in general conclusions about the parameters that influence wave reduction across multiple habitats and physical conditions. Therefore, the study uses average values of vegetation height and water depth for the parameter analyses. It is worth emphasising that the measurements of waves in the analysed studies are all under ‘normal’ conditions of low waves. Mean wave height values are used for the meta-analyses. Variations in wave height measurements at each site are accounted for within the analyses (see S1 Methods). However, when analysing extreme value measurements, it will be necessary to include analyses of variances to assess the effect on wave reduction. Also, site-specific variations in all these parameters will need to be considered when designing a nature-based defence project. For instance, the slope of a coral reef can influence variations in wave reduction over that reef  and hence, its effectiveness as a nature-based defence. Wave height is the response variable for the meta-analyses, following a number of the reviewed studies that report reductions in terms of wave heights. Field measurements and analyses of wave energy, rather than wave height, may provide a better picture of the processes that drive wave reduction at each site .
Field evidence of the protection offered by habitats is generally difficult to obtain. However, clear differentiation of measured parameters–i.e. physical reduction of wave heights or storm surges, versus economic savings in damage costs during extreme events–is essential to understand the extents to which, and conditions under which, different habitats offer protection. For instance, the review of nature-based defence projects suggests that mangroves are effective protection measures against flooding from storms (Table 1, S3 Table). The meta-analyses of wave heights however show that wave height measurements in mangroves have so far been limited to lower waves than in salt-marshes (Table 1, S1 Table).
Future studies of effectiveness and cost-effectiveness would also be strengthened by paired measurements of wave height reduction with and without habitat  accompanied by information on habitat parameters such as height, density and roughness [40,41,42]. A small but growing number of field observations, laboratory experiments and numerical models suggest that reefs and wetlands can act as buffers against extreme waves and water levels [8,43,44,45,46], though the observed data for extreme events is scant. It will also be critical to get similar field measurements of wave and water level reductions by habitats during extreme events . When evaluating restoration projects for coastal protection, it would be useful to follow monitoring and evaluation procedures set out within established coastal engineering frameworks. These could usefully include demonstrations of projects implemented in different physical settings , theoretical design frameworks [48,49], or even, evaluations of nature-based defences within national accounts . Such evaluation typically involves a before-after comparison of the coastal hazard at the site. However, a restoration project can typically have multiple objectives, the evaluation of which will require monitoring of outcomes at multiple impact and reference sites.
Coastal Management: Sitges Case Study
Sitges after the storms: contemplating the disappearing sand?
Many Catalan beaches are losing their sand. The November storms have starkly revealed this problem and the long term solution is a complex one involving a range of environmental, economic and social issues. The estimated costs of 1.690 million ptas to restore the damaged beaches and promenades are overshadowed by concerns regarding future sustainability, not just of the beaches, but of the coastal fringe in general and the tourist industry in particular. Both money and beach sediment are scarce resources in Catalunya, and conflict between competing resorts all looking to bolster their share of the tourist cake may be difficult to reconcile.
Sea levels are rising. Maximum wave heights have been steadily increasing during the last decade, from 8.22 metres in 1991 to 9.92 metres on 9th November 2001. The best defence against coastal erosion and rising sea levels is the humble beach, cheaper than traditional hard defences in both the short and long term. But the creation of sustainable artificial beaches is more complex than simply dumping sand or shingle on the shore andthe organisation responsible for these operations estimates annual sand loss at 10% of the amount spread.
Beaches act as the natural sustainable defence system against coastal erosion. The secret to their success lies in the fact they can adapt their shape very quickly to changes in wave energy and also dissipate this energy in minor adjustments of the position of each sand or shingle grain. The beach is therefore able to maintain itself in a dynamic equilibrium with its environment due to the mobility of its sediments. The beach tends to adopt different profiles according to the season. Sand lost during winter storms tends to be deposited off shore forming protective submarine banks, to be transported back on to the beach during low-energy summer sea conditions. Material is lost from the beach due to coastal currents (see below), but a constant source of new sediments is normally supplied from river deposition and coastal erosion.
Alarmingly, the Catalan supply appears to be running out. Rivers such as the Llobregat and Bes�s now barely reach the sea due to water abstraction. Where they do, damming, reforestation and increasing urbanisation upstream all act to reduce sediment input. Marina, breakwater and sea wall developments prevent the natural erosion of the coast and inhibit the transport of sediments in a southerly and south-westerly direction along the shore by coastal currents. The result is the same. The supply of sand is drying up, witnessed by several resorts including Sitges.
The key to the transport of sediment in a south and south-westerly direction along the Catalan coast is the dominant wave direction, mainly from the east (charts 1 and 2).
This dominant easterly wave direction is linked to the distance of open sea over which the wind can generate waves, (called the fetch). The longest stretch of unbroken sea facing the Catalan coastline lies some 500 kilometres to the east and south east (see map 1). This has been long-recognised by coastal engineers, and many sea defences are aligned approximately north-south to protect against storm waves from the east.
The arrival of wave fronts on the coastline at an oblique angle leads to sediment being carried up the beach at an angle approximately perpendicular to the wave crest, but gravity will cause the material and the backwash to take the steepest gradient seawards which in an oblique wave will be a different course from that taken by the swash. Consequently, material may be seen to drift south and south-westwards along the Catalan coast, a process called longshore drift. (See diagram 1 ).
The interruption of the supply of new sediment carried by longshore drift has been one of the factors leading to the loss of sand from the beaches at Sitges. The construction of the Aiguadol� marina updrift in the late 1970s blocked sediment transfer to the main beaches in the south west. Hard engineering schemes involving a number of breakwaters and eight rock islets were then constructed to help protect and build beaches between Punta de la Torreta and Punta de les Anquines. (See map). However, the loss of sand is continuing, with the alarming total losses on some beaches (see table 1), particularly from those beaches aligned south-east or lying within bays protected by the islets.
The data shows a general decline to the south-west in beach width, probably linked to the impact of the marina and breakwaters in preventing the movement of sediment by longshore drift. The beaches which have suffered least erosion (la Fragata, Anquines) are those protected by optimum breakwater alignment to the easterly waves. The beaches most seriously affected by erosion, (10, 12 and 13) are those within bays protected by the rock islets.
Photo 3: The effect of the rock islets: a negative one?
Photo 4: The November storms: the value of harbour walls (Punta de les Anquines)
Photo 5: Sitges: the disappearing beaches
The rock islets absorb wave energy but interfere with the pattern of waves as they enter the bays. The waves refract round the islets, (see diagram 2) with wave trains crossing each other in the lee of the island. This would leave some areas within the bays with waves approaching parallel to the shore and others with waves approaching obliquely. Waves breaking at an angle on some areas of the the bays would thus lead to the removal of sediment by longshore drift (photo 3).
There are three projects proposed by the Sitges authorities aimed at solving this problem of sand loss. They all involve the import of sand from up-drift, from a coastal zone between Premi� and Vilassar and the replenishment of the beaches between the breakwaters of La Frageta and Les Aquines. The eight artificial islands would also be eliminated. The most expensive option is costed at 551 million ptas. Alternative proposals include changing the orientation of some breakwaters (Manel Carbonell), constructing submerged breakwaters (Llu�s del Cerro) or beach replenishment from sea bed dredging (Oriol Pascual).
While the Sitges authorities anxiously await a decision on their proposals, Barcelona is planning the immediate construction of semi-submerged breakwaters off the beaches of Bogatell and Mar Bella, those most damaged in the recent storms. The costs are estimated at 525 and 375 million ptas respectively. The need for urgency in restoring these beaches is because of their great tourist value, a position taken by the Environment Minister, Jaume Matas.
Hard engineering schemes may be the only short-term solution to the loss of Catalan beaches, but they have a history of unexpected effects and may have huge consequences to the areas down the coast. The consequences of beach replenishment are equally uncertain. The grain size of the sediment is critical in affecting beach permeability and the absorption of wave energy, as the Weymouth, U.K. authorities discovered in 1996. A replenished beach protecting the Preston coast road was transformed from relatively permeable to relatively impermeable due to the smaller grain size of the imported material. The result? Waves simply ran up the beach on to the road.
Perhaps the best solution, at least for the present, is a shoreline management plan for the whole Catalan coast. This would offer a strategic approach to shoreline management and attempt to co-ordinate activities between coastal authorities and address the conflicts between competing interests. The long-term approach may be to rethink the intense development of the coastline, and work with the natural processes rather than against them. It may be unrealistic, however to hope for the return of the salt marshes to the Maresme!
|Sitges Beaches Management Case Study (S.W. Barcelona)|
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