PERSPECTIVE OF COASTAL VEGETATION PATCHES WITH TOPOGRAPHY VARIATIONS FOR TSUNAMI PROTECTION IN 2D - NUMERICAL MODELING

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1 PERSPECTIVE OF COASTAL VEGETATION PATCHES WITH TOPOGRAPHY VARIATIONS FOR TSUNAMI PROTECTION IN D - NUMERICAL MODELING N. A. K. NANDASENA ), Norio TANAKA ) and Katsutosi TANIMOTO ) Member of JSCE, B.Sc.Eng (Hons), Graduate Scool of Science and Engineering, Saitama Universit ( Simo-okubo, Sakura-ku, Saitama 8-87, Japan) Member of JSCE, Dr. of Eng., Professor, Graduate Scool of Science and Engineering, Saitama Universit ( Simookubo, Sakura-ku, Saitama 8-87, Japan) Member of JSCE, Dr. of Eng., Professor, Graduate Scool of Science and Engineering, Saitama Universit ( Simookubo, Sakura-ku, Saitama 8-87, Japan) Effectiveness of two tpes of densel grown coastal vegetation, namel Pandanus odoratissimus and Casuarina equisetifolia were investigated wit different ground slopes, against a tsunami event in -D numerical simulation. Dimensions of vegetation patces and gap between tem were canged rationall in order to represent possible vegetation sapes available on interland. Bot vegetations sowed similar trend of draulic properties variations. It was observed tat reduction of maimum current velocit is iger tan te reduction of maimum water dept beind vegetation patc. Horizontal run-up lengt is reduced wit te patc size increases for ver mild ground slope, but wen slope becomes steep, gravit effect is dominant tan vegetation effect on run-up lengt. Relativel big patces wit narrow gap are identified as destructive forms. In fact maimum current velocit is increased at gap-outlet, gap-inlet and middle of te gap wen tsunami flows landward, towards offsore and in bot directions respectivel compared to te bare land. Ke words : vegetation patc, tsunami, topograp effect, D numerical modeling, bare land. INTRODUCTION Coastal vegetation is being introduced as a softcountermeasure against te tsunami impact. Tis tecnique can be promisingl applied to developing countries as it involves initial less capital investment in comparison wit oter sopisticated coastal protection measures like artificial structures. Researcers identified coastal vegetation can retardate considerable amount of violent energ carried wit gigantic tsunami waves in istoric tsunami field surves. In fact, not all coastal vegetation species, but densel grown vegetation tpes, like Pandanus odoratissimus, Casuarina equisetifolia and tpes of Mangrove species were beind of tose vegetation, damaged considerabl reduced, Tanaka et al. ), Mascarenas and Jaakumar ). Nandasena et al. ) conducted - D numerical simulation on selected coastal species and it was identified tat densel grown coastal vegetation species can reduce current velocit tan water dept remarkabl beind tose vegetation compared to sparsel grown coastal vegetations (or plantations). Tanaka et al. ) also investigated draulic properties troug coastal vegetation and teir breaking conditions in a tsunami event, eplored tat Casuarina equisetifolia and Pandanus odoratissimus eibited a strong potential to resist tsunami force in a two-laer arrangement. Above all calculations were confined to - D numerical simulations wit te assumption of infinitel long vegetation belt in te transverse direction. Tis paper address te effectiveness of coastal vegetation patces and gap (between vegetation patces) effect more compreensivel in - D numerical simulation to bridge te gap between - D and real-ground situation.. MATERIALS AND METHODS () Governing equations To describe te flow troug te vegetation, twodimensional dept integrated Continuit and Momentum equations in two perpendicular directions are used. Nandasena et al. ) improved governing equations wit te full effect of porosit due to solidit of vegetation. Here te governing equations () continuit () and () momentum equations in

2 and directions respectivel, are replaced b discarge and water dept terms in order to facilitate te simulation codes. Q Q θ + + = () t Q Q Q ( ) ( ) Q θ θg t / / k z τ θ + θ g + θb θ + f i = () ρ ρ i = QQ Q Q ( ) ( ) θ θg t / / k z τ θ + θ g + θb θ + f i = () ρ ρ i= Were, Q = discarge in direction, Q = discarge in direction, Q = Q + Q, = water dept, z = bed elevation measured from te selected datum, τ, τ = bed resistance per unit area in and direction respectivel, k k = resistance induced b f i, f i i = i = vegetation per unit area (drag and inertia) in and direction respectivel, ρ = sea water densit (= kg/m ), g = gravitational acceleration, θ, θ = dept b averaged and bed aerial porosities respectivel. πb m θ ( = ) () πb m θ ( b b = ) () Were, m = tree densit (number of trees per unit area), b = dept averaged tree diameter at water dept, b b = equivalent tree diameter on te ground. Bed resistance (τ ), drag force (F d ) and inertia force (F i ) s and components per unit area are reformulated to compatible wit governing equations, respectivel as follows. 7 / τ = ρgn Q Q + Q /( θ ) () F F d d 7 / τ = ρgn Q Q + Q /( θ ) (7) = m ρc d allbref Q Q + Q / θ (8) = m ρc d allbref Q Q + Q / θ (9) d( Q / θ ) Q Fi = mρc M () dt Q + Q d( Q / θ ) Q Fi = mρ C M () dt Q + Q Were, C d-all (for more details refer, Tanaka et al. ) ) is defined as follows. b( ) Cd ( ) Cd all = Cd ref d () bref C d ref Were, b() = tree diameter at eigt,, C d () = drag coefficient at eigt,, b ref = reference tree diameter at eigt,. m, C d-ref = reference drag coefficient (=.) C M = reference inertia coefficient (=.), n = Manning s rougness coefficient (=.), = submerged volume of a tree due to water dept. () Selection of parameters. Vegetation species ), ) Following te outcomes of Tanaka et al. and Nandasena et al. ), most suitable coastal vegetation species to mitigate te tsunami impact, Pandanus odoratissimus and Casuarina equisetifolia (oung specie) were selected. Psical caracteristics of Pandanus and Casuarina are, average tree eigt, 8m and m, average tree densit,. and. number of trees per unit area (in square arrangement) and average tree diameter at.m above ground level,.m and.m, respectivel.. Topograp, vegetation patces and tsunami condition Fig. sows te cross section of bed profile along te tsunami direction, was selected for te simulation. Tree different slopes, ver mild (:), tpical (:) and relativel steep (:) etending from te soreline, were cosen to identif te draulic properties canges on te topograp variations. Fig. sows te plan area of te vegetation patces arraignment in wic line A-A indicates te vegetation front and B-B limits te rear boundar. Lateral boundaries and distance from tem to near boundar of patc were fied ( m) as sown in Fig. in wic smmetr is establised. It is assumed te distance is sufficient to minimize te disturbance to te results b lateral boundaries reflection. Tsunami wave eigt = m and period = min were assumed at m water dept, respectivel. Discretization lengt was m for bot and directions and simulation was restricted to a complete inundation on te ground. Water dept and current velocit (U - tsunami direction, or X component, V perpendicular to tsunami direction, or Y component) were computed at five locations (

3 X m X : m m to ), of wic locations and were selected to eplore te effectiveness of te vegetation patc wile te locations, and were emploed to ceck vegetation gap effect on te tsunami flow. W (widt of te patc normal to te tsunami direction), L (lengt of te patc along tsunami direction) and G (gap) between tem were altered as follows in Table.. RESULTS AND DISCUSSION B : B : A : A : m Fig. Sketc of bed profile wit te vegetation boundaries A - A B - B Y W G W m m L L m Fig. Sketc of vegetation patces arrangement Table Vegetation patc dimension L = m W = m G = m Remarks Case L W G Wide gap - small patces Case L W G/ Narrow gap - small patces L W G Wide gap - big patces Case L W G/ Narrow gap - big patces : A - A B - B Pandanus odoratissimus can be broken or bent, wen tsunami water dept is iger tan 8% of tree eigt and Casuarina equisetifolia is stronger tan Pandanus, Tanaka et al. ). Hence, it was assumed tat te vegetation was ardl an broken in tis stud. Inertia and drag coefficients were constant and dnamic beavior (bending and saking of trees) of te vegetation during a tsunami event was not considered. Table, and sow te maimum water dept variation for eac case compared to bare land (witout vegetation). For eac case, maimum water dept for bare land and ten te ratio of maimum water dept wit vegetation to maimum water dept for bare land, i.e. (Vegetation/) are tabulated. Table, and sow maimum current velocit variation, (+ve) means landward, (-ve) indicates te offsore direction respectivel. For eac case, maimum current velocit for bare land (two maimums, one per landward and oter per offsore direction) and ratio of maimum current velocit wit vegetation to maimum current velocit for bare land (Vegetation/); first ratio, landward and te second, offsore direction represented, respectivel. Table and sows te maimum water dept and maimum current velocit variation for te : ground slope respectivel. Note tat onl location is Table Maimum water dept variation for : slope, P = Pandanus, C = Casuarina, locations and cases are as sown in Fig. and Table respectivel. Location Case P C P C P C P C P C ) Case..... ) ) Case ) Table Maimum current velocit (U) variation for : slope, P = Pandanus, C = Casuarina, locations and cases are as sown in Fig. and Table respectivel. (-) indicates te offsore currents velocit. Location Case P C P C P C P C P C.8, -.9., -..8, -.9., -.., -..89,.9,.8,.8,.98,.98,.99,.97,.9,., ) Case.8, -.9., -..8, -.9., -.., -..89,.9,.,.8,.98,.98,..,.,., ) , -.9., -..8, -.9., -.., -..89,.9,.,.,.98,.98,.7,.,.,.9, ) Case.8, -.9., -..8, -.9., -.., -..89,.9,.,.,.98,.98,.,.,.,., )

4 Water dept Water dept te same in eac case, but locations,,, and are different even in te bare land case because of te different patc size in corresponding comparison case. It was observed tat relativel big vegetation patces wit narrow gap (case ) reflected ig water dept at te vegetation front compared wit oter cases for ver mild slope condition. Toug Casuarina as ig tree densit tan Pandanus, maimum water dept recorded at vegetation front belonged to later, as its total projection area was ig and tus blockage was ig. Tere was no significant difference of maimum water dept variation troug vegetation and along te gap for small patces wit wide gap. However, wen te patc size increased and gap became narrow, reduction of maimum water depts beind te vegetation and gap-outlet (loc ) were remarkable as Fig. Temporal water dept variation for Case of : slope, P = Pandanus, C = Casuarina, B = and numbers represent locations as indicated in Fig. - P - C - B - B - C - P - P - C - B - B - C - P : bed slope - Case 8 : bed slope - Case - P - C - B - B - C - P - P - C - B - B - C - P 8 Fig. Temporal water dept variation for Case of : slope, P = Pandanus, C = Casuarina, B = and numbers represent locations as indicated in Fig. sown in Table. Maimum current velocit toward land was increased at gap-outlet (loc ) for inflow and te maimum current velocit offsore direction was magnified at gap inlet (loc ) for reversed flow in comparison wit bare land. Tis penomenon was dominant in big patces wit narrow gap. Contrar to small patces wit gaps, big patces wit gaps sowed maimum current velocit increment in te mid of te gap for bot inflow and out flow compared to bare land. Fig. and sow te temporal water dept variation at front of (loc ) and beind (loc ) te vegetation and troug te gap (i.e. gap - inlet (loc ), mid of te gap (loc ) and gap - outlet (loc )) respectivel. It was noticed tat ver mild slope can dela te seawater retrieving to te offsore and it would remain until net tsunami wave comes. Tis is due to vegetation resistance and also effect of feeble gravitational support to accelerate te backflow. Fig. and illustrate corresponding current velocit variation at vegetation area and troug te gap. It was dominant tat sudden dropped down of current velocit profile wen te wave collided on vegetation Current velocit Current velocit (m /s) : bed slope - Case - P - C - P - C - B - B - 8 : bed slope - Case - P - C - B - B - C - P P - P - C - C - B - B Fig. Temporal current velocit (U) variation for Case of : slope, P = Pandanus, C = Casuarina, B = and numbers represent locations as indicated in Fig. - P - C - B - B - C - P Fig. Temporal current velocit (U) variation for Case of : slope, P = Pandanus, C = Casuarina, B = and numbers represent locations as indicated in Fig.

5 front as sown in Fig. ( - P and - C) wile tis effect transferred to gap - inlet were current velocit fell down at te begin as sown in Fig. ( - P and - C). It was noticed tat current velocit at te gapoutlet (loc ) increased tan bare land at flow in landward and similar penomenon occurred at gapinlet (loc ) wen flow reversed. Tese results reveal tat big vegetation patces wit narrow gap eacerbate te damage situation troug and te vicinit of gap area as current velocit increment causes to magnif te damage force on te structures and properties. Table and sows te maimum water dept variation and maimum current velocit variation for : ground slope wit te different vegetation Table Maimum water dept variation for : slope, P = Pandanus, C = Casuarina, locations and cases are as sown in Fig. and Table respectivel. patc arrangements respectivel. Inundated maimum water dept of : slope were considerabl iger tan te : slope condition for bare land case. Reduction of maimum water depts were insignificant for small vegetation patces wit wide gap on tpical slope ( : ). But reduction of current velocit beind te small vegetation patc was considerable. Tere was no an difference between calculated maimum water dept at bot ends of vegetation and troug te gap. However cange of maimum current velocit sows similar trend as observed in Table. Table and 7 sows te maimum water dept variation and maimum current velocit variation for :, relativel steep ground Table Maimum water dept variation for : slope, P = Pandanus, C = Casuarina, locations and cases are as sown in Fig. and Table respectivel. Location Case P C P C P C P C P C ) Case ) ) Case ) Table Maimum current velocit (U) variation for : slope, P = Pandanus, C = Casuarina, locations and cases are as sown in Fig. and Table respectivel. (-) indicates te offsore currents velocit. Location Case P C P C P C P C P C., -.7.9, -.., -.7., -..9, -..9,.98,.,.,.,.,.,.,.,., ) Case., -.7.9, -.., -.7., -..9, -..9,.98,.9,.8,.,.,.,.,.9,.8, ) , , -.., -.7.9, , -..9,.98,.,.8,.,.,.,.,.8,., ) Case., , -.., -.7.9, , -..9,.98,.,.9,.,.,.7,.,.9,.8, ) Location Case P C P C P C P C P C ) Case ) ) Case ) Table 7 Maimum current velocit (U) variation for : slope, P = Pandanus, C = Casuarina, locations and cases are as sown in Fig. and Table respectivel. (-) indicates te offsore currents velocit. Location Case P C P C P C P C P C.99, -..88, -..99, -.., -..8, -..9,.99,.,.,.,.,.,.,.,., ) Case.99, -..88, -..99, -.., -..8, -..9,.99,.,.7,.,.,.,.,.8,.7 ) , -..9, -..99, -.., -..9, -..9,.99,.,.8,.,.,.,.,.97,.9, ) Case.99, -..9, -..99, -.., -..9, -..9,.99,.,.,.,.,.,.,.,., )

6 slope. It was observed wen bed slope increased inundated maimum water dept increased wile te maimum current velocit decreased. Cange of te maimum water dept was insignificant for all patc arrangements. It was presumed tat slope effect (or gravitational effect) was dominant tan vegetation effect for maimum water dept variation. However vegetation patc effect remained for maimum current velocit variation for te relativel steep slope. It was observed tat current velocit perpendicular to tsunami direction (V) was less tan. m/s for all simulated cases. Contour lines were parallel (uniform slope) in tsunami direction. Hence tis current velocit was generated due to resultant drostatic pressure between patc and gap. Table 8 sows te etreme orizontal run-up distances for different slopes wit different patc arrangement. It was understood tat Table 8 Horizontal run-up calculated from sore line, B, P Pandanus and C - Casuarina Case Case Case : - B 99 : - P : - C : - B 7 : - P : - C : - B : - P : - C wen te slope become steep run-up distance reduced and vegetation effect was feeble. Tis was due to dominance of gravitational effect.. CONCLUSION Two-dimensional numerical simulations were carried out to eplore te vegetation patc effect on a tsunami event wit different ground slope conditions. It was attempted to represent vegetation patc sizes available on te ground b canging teir dimensions rationall. Additionall it was included gap between vegetation patces were it is common in real ground (gap can be represented in real case due to deforestation, access, etc). Simulation results can be summarized as fallows. ) Out of selected vegetation species, Pandanus sows te ig capabilit to resist a tsunami event tan Casuarina. But difference is not so large. Tis was sown b D simulation too, Nandasena et al. ). Tus bot vegetations would be introduced as a soft countermeasure for tsunami protection. ) Vegetation patc can reduce maimum current velocit tan maimum water dept beind te vegetation irrespective of ground slope. Tis was confirmed b one-dimensional numerical simulation wit te assumption of infinitel long vegetation belt. Nandasena et al. ). ) Wen te ground slope becomes steep, reduction of maimum water dept beind te vegetation is reduced in comparison wit bare land. Tis is due to dominant of gravit effect tan vegetation effect. ) Increment of patc size reduces te maimum water dept beind te vegetation on ver mild slope, but for relativel steep slope, difference is negligible. ) Horizontal run-up lengt is reduced wit vegetation patc size increasing on ver mild slope, but wen te ground slope increases gravit effect beond te vegetation effect on run-up lengt. ) Relativel big vegetation patces wit narrow gap can increase te maimum current velocit at te gap outlet during tsunami flow landward and magnif at te gap-inlet wen tsunami flow reversed in comparison wit bare land. Maimum current velocit at te middle of te gap is alwas iger tan te bare land case for bot flow directions (landward / offsore). Tis indicates narrow gaps can be more destructive at a tsunami event. REFERENCES ) Nandasena, N. A. K., Tanaka, N., and Takagi, T.: A new derivation of orizontal two dimensional dept averaged momentum equation and continuit equation, wic include total effect of porosit inside te vegetation, Annual Journal of Hdraulic Engineering, JSCE, VOL, pp.-8, 7. ) Nandasena, N. A. K., Tanaka, N., and Tanimoto, K.: Capabilit of coastal vegetation species dominant in Asian region to retardate te tsunami impact, 9 t International Summer Smposium, JSCE, pp.-8, 7. ) Tanaka, N., Sasaki, Y., Mowjood, M. I. M., Jinadasa, K. B. S. N.: Coastal vegetation structures and teir functions in tsunami protection: Eperience of te recent Indian Ocean tsunami, Landscape and ecological engineering, pp.-, 7. ) Tanaka, N., Nandasena, N. A. K., Jinadasa, K. S. B. N., Sasaki, Y., Tanimoto, K., and Mowjood, M. I. M.: Effective bio sields structure for tsunami b coastal vegetation on sand dune, Civil Engineering and Environmental Sstem, 7, (Submitted). ) Mascarenas, A., and Jaakumar, S.: An environmental perspective of te post-tsunami scenario along te coast of Tamil Nadu, India: Role of sand dunes and forests, Journal of Environmental Management, 7, (In press). (Received September, 7)