Non-linear wave attenuation quantification model improves the estimation of wave attenuation efficiency of mangroves

Abstract

The wave attenuation function of mangroves follows the Dalrymple model but changes nonlinearly with vegetation structure in various places, which has not been fully addressed. Here, taking the Da Guan Sha of Beihai city, China, as a case study, the landscape metrics characterizing mangrove structures, including the diameter at breast height (DBH), crown breadth, crown density, and rhizome and stem systems, were obtained through remote sensing and field measurements. Then, the nonlinear spatiotemporal pattern of the wave attenuation function (decreasing rate of wave height) was explored and revealed among six 20х200 m parallel transects passing through mangroves along the direction of the current. The results showed the following: (1) The DBH and crown breadth first increased and then decreased from the seaward to landward side of each transect, showing a nonlinear inverted "U" pattern of distribution. (2) The wave attenuation function of mangroves was dominated by the canopy at high tide but significantly affected by the structure of the rhizome or stem system at low tide. The wave height during a specific storm surge could decrease by nearly 97% among the six transects at high tide but by only 87% at low tide on average if the wave crossed through the mangroves over a distance of ≥100 m from the seaward side of the mangroves edge. (3) The wet surface width of a major part of mangrove trees with wave attenuation functions would change with the distribution of the DBH, crown breadth, and tree height along the transects, which would also change with the dynamics of the water level. The wave attenuation of mangroves ranged from 85% to 90% at low tide and from 96% to 97% at high tide if the wave crossed through mangroves at a distance of ≥100 m from the seaward side of the mangrove edge. (4) The predicted practical decreasing rate of wave height based on the forward differencing method (a method that considered the varied vegetation structure along transects in the model during the calculation of wave attenuation, which would be close to reality) would be 12% and 9% higher at low and high tide, respectively, than those estimated by the simplified calculation method (a method of taking the average value of the vegetation structure of each transect to calculate wave attenuation) if the wave crossed through mangroves at a distance of ≥200 m from the seaward side of the mangrove edge. The integration of mangrove structures into the established empirical models improves our understanding of the nonlinear wave attenuation function of mangroves and may provide decision makers with more valuable information on restoring mangrove ecosystems to protect against storm surges.