Quantifying hydraulic roughness in a riparian forest using a drag force-based method

Abstract

Flow resistance through riparian forests due to drag on trees is often expressed in hydraulic models with an increase in a boundary resistance factor such as Manning's n. However, when Manning's n is used as a proxy for vegetation drag, this parameter is dependent on flow conditions and a single, uniform value may be inadequate for simulating a broad range of flood magnitudes. To investigate this issue, flow resistance, and the commensurate Manning's n values through a riparian forest were computed using measured drag forces and estimates of the forest structure and tree morphology. The computed Manning's n values were applied to a 2D hydraulic model (TUFLOW) to simulate an observed flood and a range of design floods. Modelled peak flood levels for the observed flood were 0.16 m lower on average than that recorded at debris marks. There was little difference in modelled flood levels when using the computed Manning's n compared to a traditional, uniform Manning's n. Reassuringly, the traditional method appears adequate when reliable calibration data is available. Otherwise, the method developed here provides a useful alternative in cases where calibration data is limited or for testing reforestation as a nature-based solution in river or flood management.