Abstract. Vegetation in wetlands is a large-scale nature-based
resource providing a myriad of services for human beings and the
environment, such as dissipating incoming wave energy and protecting coastal
areas. For understanding wave height attenuation by vegetation, there are
two main traditional calibration approaches to the drag effect acting on the
vegetation. One of them is based on the rule that wave height decays through
the vegetated area by a reciprocal function and another by an exponential
function. In both functions, the local wave height reduces with distance
from the beginning of the vegetation depending on damping factors. These
two damping factors, which are usually obtained from calibration by measured
local wave height, are linked to the drag coefficient and measurable
parameters, respectively. So the drag coefficient that quantifies the effect
of the vegetation can be calculated by different methods, followed by
connecting this coefficient to hydraulic parameters to make it predictable.
In this study, two relations between these two damping factors and methods
to calculate the drag coefficient have been investigated by 99 laboratory
experiments. Finally, relations between the drag coefficient and relevant
hydraulic parameters were analyzed. The results show that emergent
conditions of the vegetation should be considered when studying the drag
coefficient; traditional methods which had overlooked this condition cannot
perform well when the vegetation was emerged. The new method based on the
relation between these two damping factors performed as well as the
well-recognized method for emerged and submerged vegetation. Additionally,
the Keulegan–Carpenter number can be a suitable hydraulic parameter to
predict the drag coefficient and only the experimental setup, especially the
densities of the vegetation, can affect the prediction equations.