Field Measurement And Simulation Of Wave Attenuation Effects In Mangroves

The quantitative study of wave attenuation effect of mangrove is very important toevaluate the mangrove function, such as disaster prevention. It is urgent to start the monitoringand evaluation in South China. This study put forward the quantitative wave attenuationindexes of mangrove and analyzed the correlation between attenuation effects and influencingfactors through long-term field monitoring on11different types of mangrove forest on SouthChina coast, and based on this, simulation model of wave attenuation effects of mangrovewere established, and the preliminary structure standards of mangrove for wave attenuationwas proposed to provide reference and basis for the comprehensive evaluation on waveattenuation function of mangrove in South China. The main research results are as follows:(1) Quantitative wave attenuation indexes of mangrove were obtained from fieldobservation of11different types of mangrove forests. Wave attenuation effects were differentamong the11mangrove types due to the difference of forest structure. The biomass volumedensity below breast height was the most representative and comprehensive parameter of foreststructure for wave attenuation effect of mangrove, in that the wave attenuation effect of themangrove became intensified with the increment of biomass volume density.(2) Among11types of mangrove forest, from the forest edge to25m inside the forest, thelargest wave height attenuation rate was77.3%(plot013, Kandelia obovata artificial forest atQuanzhou Bay, Fujian), and the smallest wave height attenuation rate was13.0%(plot010,Sonneratia apetala artificial forest at the coastal area of Lianhua Mountain, Panyu,Guangdong); from the forest edge to50m inside the forest, the largest wave height attenuationrate was84.8%(plot013, Kandelia obovata artificial forest at Quanzhou Bay, Fujian), andthe smallest wave height attenuation rate was25.8%(plot011, Sonneratia apetala artificialforest at Dongzhai Harbor, Hainan); from the forest edge to75m inside the forest, the largestwave height attenuation rate was86.9%(plot013, Kandelia obovata artificial forest at Quanzhou Bay, Fujian), and the smallest wave height attenuation rate was45.7%(plot009,Sonneratia apetala artificial forest at the coastal area of Lianhua Mountain, Panyu,Guangdong).(3) A nonlinear positive correlation was shown between the wave attenuation rate and thedistance of wave propagation in the mangrove. Within a short distance across the forest standfrom the forest edge, the wave attenuation extent was most significant, and along with thedistance increase into mangrove forest, wave height kept decreased gradually, while the extentof attenuation became weakened. Take plot012, Kandelia obovata artificial forest at DongzhaiHarbor, Hainan as an example, within the scope of0-25m from the forest edge, wave heightattenuation rate was13.0%for each10m distance; from25m to50m, the wave heightattenuation rate was5.0%for each10m distance; from50m to75m, the wave heightattenuation rate was1.4%for each10m distance; within the scope of75-100m, the waveheight attenuation rate was3.9%for each10m distance.(4) Water depth had a significant effect on wave attenuation rate. The wave attenuationeffect declined gradually as the water depth at forest edge grew deeper. When the water depthgrew deeper, the obstacle that the wave encountered became smaller as the diameter of thetrunk got to be smaller upward from the ground, so the resistance of the trunk to the wave grewweaker and the wave attenuation rate fell. Take the study area at Dongzhai Harbor, Hainan asan example, the ground section area of Sonneratia apetala artificial forest-plot011was1.6times the section area at breast height, and samely the ground section area of Kandelia obovataartificial forest-plot012was13.4times the section area at breast height.(5) Wave height at forest edge affected the wave attenuation rate of mangrove as well. Atthe same water depth, the wave attenuation rate of the studied mangrove forest tended to go upwith the increase of wave height at forest edge. For example, at50m across plot001,Sonneratia apetala artificial forest at Qi-ao Iland, Zhuhai, Guangdong, for the water depth of0.8-1.0m, the wave attenuation rate was26.5%with the wave height of5-10cm,45.5%withthe wave height of20-25cm, the latter was1.71times the former; and for the water depth of 1.0-1.2m, the wave attenuation rate was17.3%with the wave height of5-10cm,37.3%withthe wave height of20-25cm, the latter was2.16times the former.(6) Similar to the wave attenuation effect of mangroves, the vegetation structure ofspartina also influenced its wave attenuation effect. The higher the density, the greater thewave attenuation rate. At25m across the spartina vegetation, the wave attenuation rate of thestudied plots (plot003and plot008, spartina vegetation at Qi-ao Iland, Zhuhai, Guangdong)was53.3%-58.1%, and at50m across the spartina vegetation, the wave attenuation rate was84.6%(plot008, spartina vegetation at Qi-ao Iland, Zhuhai, Guangdong).(7) Wave height changed slightly after the wave transmitted across the mudflat for50m.Among the studied4types of mudflats, the maximum wave attenuation rate was5.1%, and themaximum wave increasing rate was9.0%. At different water depth, wave height variation wasdiversified, no regularity could be shown. And at the same water depth, the variation of waveattenuation rate with changing wave height showed no regularity either.(8) Among all the bare mudflat types, plot014at Dongzhai Harbor, Hainan had thebiggest wave attenuation rate,5.1%for a distance of50m, while plot011, Sonneratia apetalaforest and plot012, Kandelia obovata forest in the same area had a wave attenuation rate of25.8%and45.0%respectively. Take it for granted that the wave attenuation rate of mudflatwas equal to the bed friction influence in the mangrove forest, it could be speculated that thebed influence only accounted for11.3%-19.8%of the whole wave attenuation rate of mangroveat most, indicating that the vegetation structure of mangrove played the dominant role in thewave attenuation.(9) Simulation equation for50m wide mangrove belt was established through regressionmethod between wave attenuation rate of mangrove and its biomass volume density belowbreast height, model expression is as follows:y=-2.36338*e-(x/341.42152)+2.3333This model could be used to provide theory basis for predicting wave attenuation effect ofmangroves and determining the establishing standard of high-efficient mangrove of disasterprevention. Based on this model, the standard of forest structure with different wave attenuation expectations was proposed, which could be used for evaluating wave attenuationand disaster prevention effect of mangrove. In accordance with this structure standard, thedensity of the mangerove forest stand with DBH of15cm must be at least4419/hm2to realizean attenuation rate of80%.