EXPERIMENTAL STUDY OF SOLITARY WAVE RUNUP REDUCTION BY COASTAL LAGOON, CORAL REEF AND FOREST

• Material Type: 博士論文
• Title: EXPERIMENTAL STUDY OF SOLITARY WAVE RUNUP REDUCTION BY COASTAL LAGOON, CORAL REEF AND FOREST
• Author/Editor: TALPE, LIYANAGE CHANAKA VINODH
• Author Heading: TALPE, LIYANAGE CHANAKA VINODH
• Publication Date: 2021
• Publication Date (W3CDTF): 2021
• Alternative Title: 潟湖、サンゴ礁、海岸林による孤立波の遡上高減少量に関する実験的研究
• Degree grantor/type: 埼玉大学
• Date Granted: 2021-03-25
• Date Granted (W3CDTF): 2021-03-25
• Dissertation Number: 甲第1208号
• Degree Type: 博士(学術)
• Text Language Code: eng
• Subject Heading: solitary wave; runup; wave breaking; coastal lagoon; coral reef; forest
• Target Audience: 一般
• Note (General): type:text; EXPERIMENTAL STUDY OF SOLITARY WAVE RUNUP REDUCTION BY COASTAL LAGOON, CORAL REEF AND FORESTThe 2004 Indian Ocean Tsunami and the 2011 Great East Japan Tsunami emphasise the importance of Ecosystem-based Disaster Risk Reduction (Eco-DRR) to minimise the incredibly challenging impact of future tsunamis, to protect the living community, environment, and infrastructure.As a preliminary study, a statistical and geospatial analysis was conducted for coastal lagoons in Sri Lanka that affected by tsunami using collected field surveying data, statistical data, DEM data and land use GIS data. The spatial variability of the settlement, forest and lagoon mouth was discussed. Influence by existing coast protective structures, drainage structures and the connection to the sea was investigated. The physical dimensions such as beach slope, dune height, barrier length, lagoon length in cross-shore and longshore direction and the area were investigated and how such parameters affect the damage ratio was illustrated.Based on the statistical and geospatial analysis, it was found that the building located close to the lagoon mouth and on barrier land were found to be extremely vulnerable to tsunami wave. The presence of a narrow channel connecting to a lagoon observed high tsunami damage ratio. The lagoons with the closed mouth also exhibited more severe damage in the east part of Sri Lanka. The location of the forest does not make much difference in tsunami damage, but the dense mangrove could reduce the damage, whereas patchy, scattered and swamps did not reduce the tsunami damage. Existence of coast protective structures and the drainage structures did not exhibit an effect on impact to tsunami damage. The offshore bathymetry (i.e., fringing reef or reef lagoon) did not influence on tsunami damage ratios. When the length of barrier length in opening direction was less than 650 m, cause extensive damage, and the effectiveness of lagoon could not be observed. When the summation of lagoon length and barrier length reached beyond 750 m the damage in the upstream of the lagoon was minimised which implies that the distance to the coast is a major governing factor in evaluating damage ratio. The beach slope, barrier height and area of a lagoon is found to be less influence on the damage ratio.Limitations of the statistical and geospatial analysis were presented. Hence the laboratory experiments were conducted at Saitama University to understand how a tsunami-like solitary wave vertical runup can be reduced in one horizontal dimension against the effects of the beach slope, dune height, depth and length of the lagoon in the cross-shore direction and also with and without forest. The rigid emergent circular cylinders in a staggered arrangement were used as the forest model for all wave conditions. The maximum runup of a compound slope was measured to investigate the influence of onshore slope and offshore slope. Besides, the solitary wave runup on coral reef system has been tested. The bathymetry of reef profile was tested as reef-flat, reef-lagoon and reef-crest. The rectangular strips at regular intervals representing the roughness of a more simplified coral reef system, along the cross-shore direction were tested for the case of reef-flat with roughness.The solitary wave runup on a plane beach having the slopes of 1/4, 1/7 and 1/10 were tested. The compound slopes consist of 1/7 and 1/10 as the foreshore slopes and 1/4 as the onshore slopes were tested. The forest model on a sloping beach having slopes of 1/4 and 1/7 were investigated. The maximum runup of a lagoon by changing the foreshore slopes as 1/1, 1/2, 1/4, 1/7 and 1/10, dune height and lagoon inside depths, was measured. Also, the forest model was placed on a sand dune by changing the position (front, middle and back of sand dune) and investigated. A coral reef platform consists of the fore-reef slope of 1/7 and landward slope of 1/4 were tested.The plunging breaking, surging breaking and nonbreaking type of waves were observed on the beach slope and the fore reef slope. The undulations with no breaking and leading wave breaking and turbulent bores were observed inside lagoon with the change of wave transmission height to lagoon water depth ratios. The multiple wave reflection and backwater rise could be observed with the introduction of forest model. The progressive and growing, progressive and dissipative, and resonant waves could be observed in coral reef platforms. The turbulent bores and spilling breaking on the reef flat, surging breaking (bore type) on the landward slope and nonbreaking waves were observed. The resultant wave transmission height and maximum runup were measured varying incident wave characteristics as well as the dimensional physical properties of each model.The wave height to depth ratio from 0.1 to 0.4 was used. The breaking criterion was discussed. The dimensional analysis was employed to pick up essential parameters for comparing the maximum runup effect. The present laboratory data and the previous researchers’ work were also employed in the study. Hence the results of maximum runup were used in conjunction with previous research works to derive empirical formulas for solitary wave runup on a plane beach, forest on a sloping beach, sand dune coastal lagoon, reef flat and reef lagoon, individually.The maximum runup found to be most sensitive to the bathymetry profile change and the location of wave breaking and then to the wave formation inside of the sand dune coastal lagoon or reef platform. At the near-breaking condition, the highest runup was observed. The runup effect corresponded to change of physical dimension was discussed in detail. In the case of the coral reef system, the length of reef-flat was found to be a dominant factor when it equals the one-fourth of the incident wavelength of a solitary gives the highest runup. The incident wave height to reef water depth ratio also found to be a determining parameter for estimating runup as it describes the breaking limit.The forest on a sloping beach effectively reduces solitary wave runup from 4% to 28%. The higher runup reduction occurred on mild slopes with highly nonlinear waves. By increasing the forest width by twice, the runup reduction can be further increased from 6% to 27%. The runup on a sand dune coastal lagoon with a forest of finite width is reduced effectively by 17% to 45% depending on the slope parameter of front beach slope which is associated with the breaking phenomenon. At near-breaking condition, the effectiveness of coastal trees is found to be comparatively less. However, by introducing a larger forest width, the runup can be further reduced by 55% to 81%. The runup reduction by reef crest as compared to the reef flat was observed up to 31% and higher runup reduction was seen in shallow depths. In reef flat with dense roughness (‘d’ type) and shallow depths, runup was reduced up to 66%. In deep reef water depths, there was no reduction could be observed, and some cases runup with reef crest and reef flat with dense roughness was higher than in the case of the reef flat due to the resonance factor. For immediate roughness case (‘k’ type), the runup reduction was recorded between 3% to 81%. The highest runup reduction was observed in shallow water depths. However, the difference in the runup reduction by changing the ratio of spacing to roughness height (pitch ratio) among intermediate roughness (‘k’ type) cases was not significant. The effect of runup reduction by the roughness of coral reef roughness was found to be less significant than the forest of finite width. The maximum runup on a reef lagoon was 5% to 59% higher than the reef flat case except for weakly nonlinear waves on deep water.Thus, a coastal lagoon having larger dune and more considerable length in the cross-shore distance with a forest width helps to increase more resilience against tsunami attack even when energy reduction at the beach slope is not sufficient enough. A coral reef platform with wide width, shallow reef water depth and intermediate roughness can be considered as useful in tsunami energy dissipation but found to be not so effective compare to the sand dune coastal lagoon with coastal forest.The most critical parameters which can be used to evaluate tsunami damage is highlighted in this study to design coastal landscapes based on Eco-DRR concept.; Declaration ...................................................................................................................................... ivAcknowledgements .......................................................................................................................... vAbstract ........................................................................................................................................... viContents ........................................................................................................................................... xList of Figures ................................................................................................................................xiiiList of Tables ................................................................................................................................ xxiiNomenclature ............................................................................................................................... xxiii Roman symbols ........................................................................................................................ xxiii Greek symbols .......................................................................................................................... xxv Other symbols .......................................................................................................................... xxviAbbreviations ........................................................................................................................... xxviChapter 1 Introduction ............................................................................................................. 1 1.1 Background ............................................................................................................. 1 1.2 Objective of the thesis ............................................................................................. 5 1.3 Outline of thesis ....................................................................................................... 5Chapter 2 Vulnerability analysis for coastal lagoons in Sri Lanka .......................................... 7 2.1 Introduction ............................................................................................................. 7 2.2 Materials and Methods ............................................................................................ 9 2.3 Results and Discussion .......................................................................................... 14 2.3.1 Effect of settlement area in the lagoons from West to South ................................ 15 2.3.2 Effect of settlement area in the lagoons from South to East ................................. 21 2.3.3 Effect of mangrove ................................................................................................ 25 2.3.4 Effect of land/barrier length in the opening direction ........................................... 25 2.3.5 Effect of lagoon length in the opening direction ................................................... 27 2.3.6 Effect of lagoon width in the longshore direction/perpendicular to openingdirection ................................................................................................................. 29 2.3.7 Effect of the area of the lagoon ............................................................................. 29 2.3.8 Effect of beach slope ............................................................................................. 302.3.9 Effect of sand dune/barrier height ......................................................................... 312.3.10 Effect of tsunami height ................................................................................. 32 2.4 Conclusion ............................................................................................................. 33Chapter 3 Runup on a plane beach ......................................................................................... 36 3.1 Introduction ........................................................................................................... 36 3.2 Literature review ................................................................................................... 37 3.3 Laboratory experiments ......................................................................................... 45 3.4 Model derivation ................................................................................................... 47 3.5 Results and discussion ........................................................................................... 48 3.6 Summary and conclusions ..................................................................................... 53Chapter 4 Runup on compound slopes................................................................................... 55 4.1 Introduction ........................................................................................................... 55 4.2 Materials & Methods ............................................................................................. 57 4.3 Results and Discussion .......................................................................................... 60 4.4 Conclusion ............................................................................................................. 63Chapter 5 Runup with Forest ................................................................................................. 64 5.1 Introduction ........................................................................................................... 64 5.2 Materials and Methods .......................................................................................... 67 5.3 Results and Discussion .......................................................................................... 69 5.4 Conclusion ............................................................................................................. 73Chapter 6 Runup with Sand Dune Coastal Lagoon ............................................................... 74 6.1 Introduction ........................................................................................................... 74 6.2 Materials and Methods .......................................................................................... 75 6.3 Results and Discussion .......................................................................................... 8 1 6.3.1 Wave transformation and runup on a sand dune coastal lagoon without a forest . 81 6.3.2 Wave transformation and runup on a sand dune coastal lagoon with a forest ...... 86 6.4 Conclusion ............................................................................................................. 91Chapter 7 Runup with coral reef ............................................................................................ 93 7.1 Introduction ........................................................................................................... 93 7.2 Materials and Methods .......................................................................................... 96 7.3 Results and Discussion ........................................................................................ 101 7.3.1 Wave transformation along the Reef-crest, Reef-flat with and without roughness ......................................................................................................................... 101 7.3.2 Wave runup with the Reef-crest, Reef-flat with and without roughness ............. 105 7.3.3 Wave transformation along the Reef-lagoon ....................................................... 111 7.3.4 Wave runup with Reef-lagoon............................................................................. 115 7.4 Conclusion ........................................................................................................... 122Chapter 8 Conclusions & Recommendations ...................................................................... 124 8.1 Summary ............................................................................................................. 124 8.2 Direction of future research ................................................................................. 125References .................................................................................................................................... 126; 主指導教員 ： 田中規夫
• DOI: 10.24561/00019574; https://doi.org/10.24561/00019574
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