Abstract
Mangrove forests with complex root systems contribute to increased
coastal protection through drag effects. Previous flume studies proposed
a predictive model of drag in Rhizophora mangrove forests based on
quadratic drag law. However, its general applicability on mangrove
forests in the field has not been tested. To fill this knowledge gap,
this study quantified drag in a 17-year-old planted Rhizophora mangrove
forest using a comprehensive measurement of hydrodynamics and vegetation
morphology. The vegetation projected area density, a, showed an
approximate exponential increase towards the bed, mainly due to root
branching. This vertical variation led to enhanced vegetation drag per
unit water volume relative to velocity with decreasing water depth.
Alternatively, the drag per vegetation projected area solely depended on
the square of velocity, indicating association with the quadratic drag
law. The derived drag coefficient (CD) was 1.0 ± 0.2 for tide-driven
currents, consistent with previous flume studies. By using the mean
value of derived CD (1.0), it was confirmed that the quadratic drag
model expresses well the field-measured drag. We also presented a method
for predicting a value for a, another unknown parameter in the drag
model, using an empirical Rhizophora root model, and confirmed a
successful prediction of a and drag. Therefore, the drag in a Rhizophora
mangrove forest can be accurately predicted only using the input
parameters of the Rhizophora root model – stem diameter and tree
density. This provides insights into effectively implementing the drag
model in hydrodynamic models for better representation of mangroves’
coastal protection function.