ABSTRACT
The high water level, HWL, for a design flood is the most important parameter in flood control and management. It is a critical issue how to determine Manning’s roughness coefficient in vegetated reach, since channelized rivers tend to be densely vegetated during previous decades. Although there are several sophisticated numerical models to analyze the flow structure in vegetated channels, flow resistance due to vegetation is usually formulated in terms of a drag coefficient which is not always a convenient parameter in determining HWL in a flood control design. HWL is usually analyzed by a one dimensional non-uniform flow model in which Manning’s coefficient is required in describing channel perimeter’s roughness such as vegetation. In this study, Manning’s coefficient n for canopy drag was theoretically estimated by using a two-dimensional two-layer (2D2L) hydrodynamic model that was developed by the authors. The model’s performance was already confirmed in laboratory and field experiments. A functional dependency of nv on vegetation properties such as vegetation structure, stem diameter and height of vegetation, etc. was investigated. In the case of emergent vegetation, nv increases with increasing discharge, since flow blockage due to vegetation proportionally increases with water depth. On the other hand, https://www.w3.org/1998/Math/MathML"> n v https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429069246/b59e18fc-9e1d-4389-b000-a1069c1cd27e/content/eq3111.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> takes approximately a constant value independently of water depth and discharge in the case of submergent vegetation, which is one of characteristic findings in the present study.
