Field observations and laboratory research have established several key parameters that determine the magnitude of tsunami mitigation offered by various types of coastal forests. These parameters include forest width, tree density, age, tree diameter, tree height, and species composition. Each parameter can be manipulated to produce the required level of mitigation. However, the relationship between the parameters is complex and characterized by co-dependence and interaction amongst them.
Forest width is one of the most important factors in mitigation. Over the width of the forest, energy is progressively dissipated by drag and other forces created by tree trunks, branches and foliage, as well as the undergrowth, as the tsunami passes through the forest. Even when energy levels are high, the width effect remains strong. Simulations show that a 3-fold and 6-fold increase in energy from increased wavelength (period) resulted in only a small increase in energy transmission for widths greater than 100 meters.8 However, for a narrower forest of 50 meters the loss in hydraulic force (drag force) reduction was more apparent. This suggests that the narrower the forest the greater the risk from a long period tsunami (i.e. far-field tsunami).9 As such, increasing forest width will progressively reduce risk and potential impact.
There is evidence that some coastal areas very close to the epicentre of the earthquake that caused the 2004 Indian Ocean tsunami were protected by extensive mangroves. In a few locations on the Aceh coast, Nicobar Islands and Andaman coast, mangroves were sufficiently wide to mitigate the massive near-field tsunami.
Width effect remains intact under a broad range of conditions. Simulations show a coastal forest of 200 meters width reduced the hydraulic force of a three meter tsunami by at least 80 percent, and flow velocity by 70 percent for all scenarios examined (Harada and Imamura, 2003). Despite increases in tsunami height, period and wave length and changes in forest density, the reductions in force appear robust for a forest of this width. However, the maximum tsunami height tested was only three meters. Larger waves may cause breakage and the percentage reduction would likely fall. On the other hand, smaller waves, although having less force and depth, may pass under the canopy with little mitigation afforded by the forest.
As forest width decreases, the importance of undergrowth and lower branches becomes apparent, particularly for shorter period tsunami (i.e. near-field tsunami). The lack of undergrowth allows much of the tsunami to pass below the forest crown with little reduction in force. Compared to the 70 percent reduction in velocity for a three meter wave at 200 meters width, for a one meter wave the reduction is only 43 percent. For small tsunami (around one-meter in height), which generally pass below the canopy, a doubling of forest width from 100 to 200 meters produced negligible additional velocity reduction.
Field evidence also shows that forest width is a critical parameter in mitigation. Japan’s coasts are frequently struck by tsunamis, and protection forests of Japanese pine (Pinus thunbergii and P. densiflora) planted in the 1930s and earlier – up to 200 meters in width – have reduced damage to houses, and stopped fishing boats and aquaculture rafts washing inland.10 Pine forest widths of at least 20 meters are needed to withstand flow depths of one to three meters. For larger waves, width (w) would need to increase according to the relationship w = 20(H/3)0.5, where H is wave height above ground, to maintain the mitigation effect (Shuto, 1987). For example, width was at least 26 meters for a five meter inundation height. Unfortunately, data do not exist to extrapolate the relationship beyond five meter heights with confidence. Though mitigation is said to occur if the forest is not destroyed, the amount of mitigation was not documented in the historical records.
Some plantations not specifically established for coastal protection also exhibited mitigation effects. In Thailand, a large grove (250-300 meter in width) of cashew nut trees (Anacardium spp.) protected a house situated 450 meters from the shore, while nearby houses 700 meters from the shore were destroyed by the 10-meter near-field tsunami.11 Also in Thailand, mangroves exhibited mitigation effect for 5-10 meter tsunamis if widths were sufficient to absorb wave energy through breakage. For example, only the first 50 meters of a Rhizophora mangrove was destoryed by an 8-meter tsunami in Phang Nga province. Similarly, in Sri Lanka, Rhizophora spp. and Ceriops spp. were severely damaged in the first 2-3 meters, while the remaining 3-4 meters were much less damaged by a 6-meter tsunami. Once the destructive forces are spent, the remaining forest will further mitigate the tsunami flow.
In beach forests, sufficient forest width is necessary to absorb enough of the tsunami’s energy to reduce flow velocity and depth before exiting the forest. In Indonesia, for example, 40 meters of beach forest was effective in the 2006 West Java tsunami in reducing 6-7 meter waves to just 1.6 meters (Latief and Hadi, 2006). In Sri Lanka, Pandanus spp. and Cocus nucifera arrested the 2004 tsunami at 100 meters for 4.5-5.5 meter wave (Ranasinghe, 2006), and elsewhere at 155 meters for a 6.0 meter wave (Tanaka et al 2007). However, it is likely that coconut trees contributed significantly less than the pandanus given the relative difference demonstrated elsewhere in Sri Lanka: pandanus forests, 10 meters in width reduced inundation distance by 24 percent while 110 meters width of coconut trees was necessary for an equivalent reduction. Similarly, a band of pandanus in front of a coconut grove 100 meters in width reduced the distance by another 30 percent. The difference in mitigation capacity is attributed to the greater density of the pandanus.
In other instances, forests failed to protect coasts during the 2004 tsunami. Insufficient width was one cause. For example, in Sri Lanka an area of highly populated settlements behind shelterbelt plantations of Casuarina equisetifolia were not protected. The shelterbelts were, however, only 10-15 meters wide and were themselves badly damaged, which indicates the trees were perhaps also not very large as maximum wave heights were only 6-9 meters. For other species, even a width of 200 meters may be insufficient. Evidence, also from Sri Lanka, documents that a 200 meter wide mangrove of Sonneratia spp. were uprooted or collapsed under the tsunami. Factors other than width, such as immaturity, stem diameter, or anchorage strength, may have contributed to the failure.
Consequently, width alone is not sufficient to protect coastal areas from moderate size tsunamis. Yet, when other factors are also in place, evidence shows that for waves less than 6-8 meters, width as little as 50-100 m can provide substantial mitigation. Even 10 meters of dense pandanus at the beach head can have a significant effect.
Coastal forest at Shizugawa Park (Miyagi Prefecture) on the Sanriku coast of Japan. Heavily populated, the coast is subject to frequent tsunamis. This forest is reported to have reduced damage from 1960 Chilean tsunami (Izumi 1961 in Harada and Imamura, 2003). Note the trunk deformations caused by storms and tsunamis. Such forests still serve additional uses, such as recreation
A coastal forest provides a permeable barrier. Spacing of trees (horizontal density) and the vertical configuration of above-ground roots,12 stem, branches and foliage (vertical density) define the overall density (also called vegetation thickness) or the permeability of a barrier.
Though forest density may have a less pronounced mitigation effect relative to width, density directly relates to the forest’s ability to reflect a tsunami, as well as absorb its energy. A wave encountering a permeable barrier of stems, branches and foliage (and above-ground roots with some species), is partially reflected and partially transmitted into the forest where its energy gradually adsorbed.
Moderate densities are the most effective in tsunami mitigation. If too sparse, like most coconut groves, waves will pass through unmitigated. On the other hand, if the forest is too dense, like some mangroves, a large wave may completely level the forest and pass over unmitigated.13
Vertical density, and not just horizontal density, is an important factor in determining a forest’s potential for mitigation. A forest with sparse undergrowth and trees with few branches at lower levels will provide less mitigation than a forest with high vegetation density from the ground to the canopy. Mangrove with high stilt roots or uneven-aged forests with multistoried, dense undergrowth, are examples of forests that have high densities in the lower strata.
In general, increasing the vertical and horizontal density will enhance the mitigation effect of a coastal forest. Increased reflection and energy absorption at higher densities are responsible for observed reductions in water depth and flow velocity (current), respectively. And because the hydraulic force is the product of flow depth, density of seawater, and the square of flow velocity14, it consequently drops as density increases. The mitigation effects for a simulated coastal forest of waru (Hibiscus tiliaceus) at Sissano, Papua New Guinea have shown a substantial reduction in inundation depth and hydraulic force. The maximum drop in hydraulic force for one location was 275 000 Newtons per meter to 90 000 Newtons per meter, or about 67 percent reduction, with a forest barrier of four large waru trees per 100 m2 (Hiraishi and Harada, 2003).
Evidence from the field also corroborates that vegetation thickness or density is an important mitigation parameter. Coconut trees (Cocus nucifera), for example, have been shown to be more effective when densely grown. In Kerala, India, densely planted coconut groves protected the coast (Chadha et al., 2005) and in Sri Lanka, damage extended to only 100 meters where spacing was about three meters between trees or about 14 stems per 100 m2.
In general, however, coconuts are planted with wide-spacing and also do not have low branches to reduce flow rates. Furthermore, village coconut groves typically lack understorey vegetation and thus drag at lower levels is limited. For example, where spacing between trees in the Sri Lanka case above was 4-40 meters the tsunami passed through the 500-meter wide coconut grove unmitigated (Tanaka et al., 2007). Similarly, in Sri Lanka and Indonesia, houses in and behind coconut groves were destroyed (Tanaka et al., 2007). Elsewhere in Sri Lanka where the tsunami was only 2.5 meters in height, widely-spaced coconut trees provided little mitigation. The lack of lower branches and understorey vegetation greatly reduce the mitigation potential of coconut groves. Significant protection from scouring and erosion by the extensive root mats of coconuts has, however, been documented.
Mangroves typically form denser stands and provide greater tsunami mitigation. Densities that are too high may increase the risk of catastrophic failure. Although, even if this does occur, the dense root system of an incompletely uprooted tree can still provide resistance to tsunami flow – a level of resistance that may even exceed the drag of the branches. Broken branches snared by standing trees at the front established as the forest progressively collapses provide additional resistance.
In India, dense mangrove (Rhizophora spp. and Avicennia spp.) was associated with low damage in 96 percent of surveyed cases (Danielsen et al 2005). Density was reported at between 14-26 trees stems per 100 m2. In Thailand, evidence shows a clear relationship between mangrove coverage and degree of damage to houses (Chang et al 2006). Exposed villages had the highest levels of damage and those behind mangroves the least with villages partly covered by mangroves exhibiting intermediate levels of damage – an experience also noted elsewhere.
The importance of undergrowth and resistant soil substrate is also revealed in case of plantations. Much higher forest widths are required for mitigation if a dense understorey of vegetation is absent. Also, poor soil resistance to scouring caused by the high flow velocities of the tsunami can result in uprooting trees and reduction of the mitigation effect (Shuto 1987, Tanaka et al. 2007, Dengler and Preuss 2003).
Two-layer beach forest structure of Panadus odoratissimus (foreground in photo) and Casuarina equisetifolia at Kalutara, Sri Lanka.
Forest age (the average age of trees of the dominant size class) is directly correlated with both tree height and diameter. Increases in age, diameter and height generally enhance the mitigation effects of coastal forests. Diameter growth also enhances the breaking strength of trunks and branches. It also raises the resistance of the forest being toppled, up to a point after which resistance falls. On more mature stems the rate of increase in strength, stiffness and diameter slows relative to accumulation of mass in the canopy such that mechanical failure is more likely if the tree is subjected to an external force (Niklas and Spatz, 1999).
Simulation exercises for forest widths of 200 meters show that forest growth or aging can have a significant effect on tsunami mitigation (Harada and Kawata, 2005). It is assumed that the branch-free understorey climbs by about 5 cm per year, with branch height equal to 0.5 meters at 10 years and increasing to 2.5 meters by 50 years. The initial tsunami height used in the simulation was three meters with a period of 60 minutes. Results reveal that the youngest forest, a 10-year old pine forest, provides the greatest mitigation effect, if it is not washed away. In the case of a large tsunami, however, a 10-year old pine forest with an average diameter of 7 cm would likely not withstand a massive wave or succession of waves (Shuto, 1987). As forests grow older the mitigation effect falls at a marginally decreasing rate, until there is little difference in mitigation between a 40- and 50-year old forest.
Post-tsunami field surveys in Sri Lanka and Thailand showed that older Casuarina equisetifolia shelterbelts withstood the tsunami, but failed to provide protection (IUCN 2005a, Tanaka et al., 2007). The tsunami passed through the shelterbelt without resistance from lower-level branches or undergrowth, a condition typical of the species. For a coastal forest of mature casuarina (e.g., 80 cm dbh) the mitigation effect is marginal and only slightly more than Cocos nucifera. Very young stands, on the other hand, less than 10-15 cm diameter were uprooted and washed away providing no mitigation. A similar forest-age effect was found for Manikara spp. in Sri Lanka (Tanaka et al., 2007).
On the other hand, slightly older plantations of C. equisetifolia (e.g. of 15 cm dbh and above) are more effective (Tanaka et al., 2007; Wetlands International, 2005). They are second only to Pandanus odoratissimus, which ranked the highest in mitigation effect (Tanaka et al., 2007). Besides the high drag resistance young casuarina trees provide over the full height of the tree (close to 10 meters), immature trees were not broken easily by the tsunami.
Typical widths of shelterbelts in Sri Lanka and India are in the range of 10 to 20 meters. In the face of the 2004 tsunami, either the trees were too young (2 years old) so that they were uprooted and swept away, or too old (50-100 cm dbh) and hence provided little resistance because of the moderately wide spacing and species’ minimal branching at lower levels. Shelterbelts of an intermediate age would have provided more protection. In India, houses situated within plantations were mostly protected by 35-year old shelterbelts with average diameters at breast height of about 10-20 cm and densities of 19-22 trees per 100 m2 (Danielsen et al., 2005). At 200 meters in width, however, they were 10 times the width of typical shelterbelts, which makes direct comparison with earlier mentioned cases difficult.
Moreover, examination of data from five tsunamis in Japan shows that diameter at breast height is an important determinant of stem breaking strength in coastal pine forests and that tree diameter of 10 cm or more is required to withstand an inundation depth of 4.65 meters. For larger waves, diameter (d) would have to increase at the rate of, d = 0.1H 3, where H is water depth; so for a 6-meter wave, a diameter of over 22 cm would be necessary (Shuto, 1987).
Because the mitigation potential declines with age for Pinus spp. and Casuarina spp., particularly for smaller tsunamis, management is required to produce uneven-aged stands with a range of tree sizes and branches at all levels to enhance mitigation potential. However, a trade-off exists between stand age and breaking strength and uprooting resistance. Older trees have stronger trunks and branches, and more extensive root systems to anchor the tree in the soil. However, beyond a certain age, older trees become prone to breakage, especially near the trunk base (Niklas, 1999).
Other vegetation types may show the opposite relationship whereby mitigation potential increases with age. Mangroves are a prime example, but other species and forest types that retain dense understorey growth would qualify equally. Mangrove species that exhibit stilt-rooting (along with the beach forest Pandan trees) are unique in that density increases at lower levels as they grow older. In Rhizophora spp., for instance, the density in the lower 0.3 meters of a young grove could equal 300-550 stems per 100 m2 (Massel, Furukawa and Brinkman,1999). As the grove ages, roots reach a height of 1.0 meter or more above the ground and increase in girth, thereby reducing porosity and increasing reflection of incident waves. Field research in the Tong King delta, in northern Vietnam supports these results and show that the hydraulic resistance of Kandelia candel mangrove forest increased with age (Mazda et al., 1997).
Height of the dominant and codominant trees in a coastal forest has a direct bearing on the forest’s frontal area projected towards a tsunami. The taller the forest the greater the reflective area of the barrier ‘wall’ and the lower the potential it will be overtopped by a tsunami. Assuming forest density is sufficient to resist the wave and the soil cohesion is strong enough to withstand additional leverage from force high in the tree, especially at the front edge of the forest, increasing tree height will enhance resistance to the tsunami.
Height of dominant and codominant trees is a function of tree age, tree species and growing conditions. Inadequate water supply, poor soil fertility or soil depth, etc. will stunt forest growth and reduce stand height. Heights of sub-dominant and suppressed trees depend on the rate at which the gaps or openings in the canopy are created either through tree fall or thinning. The manipulation of height at lower strata is important in maintaining a continuous barrier throughout the height of the forest.
Some mangrove species, even in the lower tidal areas, can reach considerable heights if left undisturbed. Trees heights between 30 and 50 meters have been reported in Latin America, Africa, and Asia (Dahdouh-Guebas, 2006). In Kenya, seaward Avicennia marina can reach 20 to 30 meters with stems several meters in circumference. In West Papua, Indonesia, 30-meter-tall mixed Camptostemon schultzii–Avicennia spp. are found, and in other parts of Indonesia Rhizophora spp. greater than 30 meters of height have been documented. In Panama, Rhizophora mangle and R. racemosa can grow up to 45 to 50 meters high (Dahdouh-Guebas, 2006).
Some beach forest species can also reach significant heights. For example, Terminalia catappa, is a large deciduous stately tree, and although branchless below the canopy, grows up to 25 to 40 meters in height with a spreading crown. Pongamia pinnata is another beach forest species that can grow up to 25 meters tall, but more commonly reaches only about 12 meters. Both Terminalia and Pongamia are frequently retained as ornamentals in altered forests. Casuarina equisetifolia, which is found in beach forests, altered forests and plantations, grows to heights of between 6 and 35 meters. The trunk is branchless up to 10 meters on large specimens and in older stands this reduces mitigation effectiveness at lower elevations.
Forest height is also important in relation to the risk of overtopping by a tsunami, which limits the mitigation capacity. Mitigation is a function of the volumetric occupancy of submerged forest15 (Latief and Hadi, 2006), but water passing above the forest canopy will flow relatively unabated. For narrow coastal forests, plunging of the water behind the forest can also impart larger inertia forces than normal because of acceleration and impact forces. Erosive scouring will also be more significant just beyond the coastal forest because of turbulence and strong downward forces. Furthermore, a tsunami can strike buildings at greater heights of because of the upward deflection by a barrier (Preuss, Radd and Bidoae, 2001).
Tall coastal forests are also subject to greater leverage force that increases the chances of breakage and uprooting (Niklas and Spatz, 1999). The tendency towards uprooting is, however, partly countered by greater stem weight, which lowers uprooting chances (Gardiner, Peltola and Kellomaki, 2000). Despite these caveats, coastal forests have a potential advantage over seawalls in that they are taller and less expensive for an equivalent height.
A 300-400 year old coastal forest established to protect agricultural land and community in a hazardous bay, Oki Bay (Kochi Prefecture), Japan.
The make-up of the coastal forest has important implications for the level of tsunami mitigation. Two critical aspects of tree species composition and forest type are the vertical configuration of roots, bole, branches and foliage, and understorey development. As discussed above, variation in vertical density affects drag resistance at different heights in the forest, and hence overall resistance to tsunami flow. Drag resistance at lower layers is determined primarily by the shade tolerance of plant species in the understorey and the rate of creation of canopy openings. Tree species that retain lower branches or have stilt rooting contribute significantly to density at lower layers.
Tree species have a characteristic profile projected towards the tsunami. The variation in projected area at different elevations in the tree directly affects the overall reflection and drag resistance properties of coastal forests consisting of these trees species. Forest types common to Asian and Pacific coastal areas can to a large extent be classified according to their vertical configuration characteristics. For example, moderate height and lack of branches below the canopy characterise one type of tree that has specific mitigation properties, and are represented by species such as Cocus nucifera and mature Casuarina equisitifolia. These species can be found in plantations, altered forests, and beach forests.
Other species of other forest types may have different profiles, and consequently different mitigation potential. Pandanus odoratissimus, representative of a beach forest species with stilt-rooting and dense foliage, and Rhizophora apiculata, representative of large tidal-zone mangroves, exhibit the greatest drag resistance, especially within the lower strata (Tanaka et al., 2007). Mature Casuarina equisetifolia and Cocus nucifera provide little resistance to tsunami at any elevation in the tree. On the other hand, while the plantation species, Anacardium occidentale, and a mangrove species of small tidal zones, Avicennia alba, provide little resistance at lower heights, their wide, multi-branching crowns provide significantly more drag resistance at greater heights. Such characteristics are of great importance in relation to larger tsunamis.
Changes in species composition resulting from colonisation by invasive species can affect the capacity of coastal forests to mitigate tsunami. Mangroves in particular are affected by a process called cryptic ecological degradation, in which introgressive mangrove-associated vegetation or minor mangrove species slowly start to dominate a forest of ‘true’ mangrove species (Dahdouh-Guebas et al., 2005). The invasive species do not have the same mitigation capacity as ‘true’ mangrove species. This slow degradation usually results from human activities. From a mitigation standpoint, it is dangerous because people assume that some degree of protection exists because mangroves are still present. Instead, coastal areas become more vulnerable because the degradation progresses largely undetected, when compared to loss of mangrove area, which is easily observable.
Tree species diversity also seems to be an important factor determining the degree of protection. According to WWF-India, the Machilipatnam port, located inland of the Krishna mangroves in Andhra Pradesh, India, was completely unaffected by the tsunami, despite its vulnerable location near the mouth of a canal. A survey discovered that the mangroves in the area are relatively rich in species, and that “these species-rich stands were considerably taller and denser than stands elsewhere that were dominated by just a few species” (Maginnis and Elliott, 2005).
With regard to altered forests, which are characterized by a significant proportion of tree species not native to beach forests, evidence from Sri Lanka suggests that introduced ornamental and fruit tree species broke more easily than native species when struck by the tsunami. Replacing species adapted to storm waves and winds, these introduced species diminish the overall mitigation capacity of the beach forest. However, this does not imply that all native species retained in the altered forest have high breaking strengths. For example, Borassus flabellifer (palmyra)—commonly kept as an ornamental tree—is more vulnerable to tsunamis than Cocus nucifera.
Density, diameter, height, age and the other parameters that effect mitigation are not independent of one another. They therefore have to be considered in an ecological context to assess what is and what is not possible in terms of tsunami mitigation. For example, it may not be possible to establish high density mangroves on beaches exposed to rough seas, or to have large diameter trees on shallow sandy soils.
Beach forests, mangroves and other forest types each have specific ecological requirements for successful establishment. Even within these forest types every species will require levels of freshwater, salinity, organic matter, etc. to be within certain limits. This is particularly the case with mangroves where some species are more tolerant of high salinity than others, such that the species associations gradually change according to inundation frequency, evaporation, freshwater flushing, etc. Consequently, ecological conditions will constrain the ‘design options’ for a coastal forest.
Once species suitable for a site are determined, the parameters of forest density, tree diameter, height and age can be manipulated to obtain the desired level of mitigation. Because different variables have similar effects on tsunami mitigation the same level of mitigation can result from raising or lowering the importance of a parameter and making a compensating change in one or more other parameters. For example, if the maximum width of a proposed coastal forest is constrained by existing land uses, a compensating increase in density (vertical and horizontal) may be able to achieve the desired level tsunami mitigation.16
Coastal forests can be designed in a number of possible combinations of width and density (or some other parameter combination) to deliver the required levels of mitigation in hydraulic force, flow velocity or depth. For example, one model shows that a width of at least 200 meters for a pine forest of 10 trees per 100 m2 would reduce hydraulic force to just 10 percent of the tsunami’s initial force (Harada and Imamura, 2006). Consequently, buildings constructed to withstand 20,000 Newtons per meter (N/m) of pressure and built behind such a coastal forest could survive a tsunami generating 200,000 N/m of force. If land was not available for the full 200 meters, the model shows that increasing density to 50 trees per 100 m2 would allow width to be reduced to 100 meters and still get only 20,000 N/m pressure as the tsunami exits the coastal forest.
Coastal forest (center of photo) established to protect low-lying plain of Tsukihama city (Miyagi Prefecture). The town is especially vulnerable to large tsunami created by the bay. A denser, 90 meter forest forms the first line of defence, while a less dense, mixed forest, 100-190 meters wide is found behind an intervening area limited to agricultural land and buildings.
8 Wavelength and period are related to energy. The mass of water set in motion by upward displacement caused by a submarine plate rupture, equal to width and length of the rupture zone and the height of displacement, determines the wavelength. The greater the mass, the greater the wave�s energy. Period relates to speed of tsunami, which is determined by depth to seafloor and not energy. Period indirectly measures energy because it is the time for one wave to pass a point. The longer the wavelength, the longer the period.
9 Far-field and near-field refer to the relative distance traveled by a tsunami from generation source to coastline. Far-field tsunamis have been generated far from the coastline, travel across oceans, and are characterized by long wavelengths (periods) of great energy. Near-field tsunamis originate much closer to the coast, and are characterized by shorter wavelengths, but can arrive without warning. They have great energy also, but because of greater wave height, rather than long wavelength.
10 Protection forests in Japan serve multiple roles including protection from storm waves and tsunamis, and from salt spray and sand abrasion which detrimentally affect agricultural crops, and recreation.
11 The plantation was fronted by a five-meter wide C. equesetifolia shelterbelt. By the time tsunami struck the cashew plantation wave height above ground level was six meters.
12 Above-ground roots also provide additional vertical density, in the case of some mangrove species and some beach forest species like pandanus.
13 Very high vegetative densities can provide too much resistance at the forest front, overcoming the ability of trees and soil to withstand the force. One of the most advantageous features of coastal forest over other types of coastal defences is its characteristic of allowing a portion of the tsunami to pass through the forest with its force gradually attenuated, where a solid wall may be broken apart, lifted up, or overtopped.
14 Strictly speaking 'hydraulic force' is pressure per unit length (breadth) on a building wall or some other obstruction (i.e. Newtons per meter). It is estimated by FD = hru2 where h is flow depth, r is density of seawater, and u is wave or flow velocity (Harada and Imamura, 2003). Force is the product of wave mass and its speed as it hits the wall.
15 The volume occupied by portion of the forest below the water level defines the volume of solid obstruction (the trees) relative to volume of water flowing through the forest. The greater is the ratio of solid to water the less the water able to pass, greater the obstruction, and greater the surface area creating drag resistance.
16 It is important to note that by including coastal forests in the mitigation strategy for a coastal area set-back widths can be greatly reduced where the appropriate safe distance was determined for a beach in the absence of barrier.
