TY - JOUR
T1 - Flow separation and roughness lengths over large bedforms in a tidal environment
T2 - A numerical investigation
AU - Lefebvre, Alice
AU - Paarlberg, Andries
AU - Ernstsen, Verner Brandbyge
AU - Winter, Christian
PY - 2014/12/1
Y1 - 2014/12/1
N2 - This study characterises the shape of the flow separation zone (FSZ) and
wake region over large asymmetric bedforms under tidal flow conditions.
High resolution bathymetry, flow velocity and turbulence data were
measured along two parallel transects in a tidal channel covered with
bedforms. The field data are used to verify the applicability of a
numerical model for a systematic study using the Delft3D modelling
system and test the model sensitivity to roughness length. Three
experiments are then conducted to investigate how the FSZ size and wake
extent vary depending on tidally-varying flow conditions, water levels
and bathymetry. During the ebb, a large FSZ occurs over the steep lee
side of each bedform. During the flood, no flow separation develops over
the bedforms having a flat crest; however, a small FSZ is observed over
the steepest part of the crest of some bedforms, where the slope is
locally up to 15°. Over a given bedform morphology and constant water
levels, no FSZ occurs for velocity magnitudes smaller than 0.1 m s−1; as the flow accelerates, the FSZ reaches a stable size for velocity magnitudes greater than 0.4 m s−1.
The shape of the FSZ is not influenced by changes in water levels. On
the other hand, variations in bed morphology, as recorded from the
high-resolution bathymetry collected during the tidal cycle, influence
the size and position of the FSZ: a FSZ develops only when the maximum
lee side slope over a horizontal distance of 5 m is greater than 10°.
The height and length of the wake region are related to the length of
the FSZ. The total roughness along the transect lines is an order of
magnitude larger during the ebb than during the flood due to flow
direction in relation to bedform asymmetry: during the ebb, roughness is
created by the large bedforms because a FSZ and wake develops over the
steep lee side. The results add to the understanding of hydrodynamics of
natural bedforms in a tidal environment and may be used to better
parameterise small-scale processes in large-scale studies.
AB - This study characterises the shape of the flow separation zone (FSZ) and
wake region over large asymmetric bedforms under tidal flow conditions.
High resolution bathymetry, flow velocity and turbulence data were
measured along two parallel transects in a tidal channel covered with
bedforms. The field data are used to verify the applicability of a
numerical model for a systematic study using the Delft3D modelling
system and test the model sensitivity to roughness length. Three
experiments are then conducted to investigate how the FSZ size and wake
extent vary depending on tidally-varying flow conditions, water levels
and bathymetry. During the ebb, a large FSZ occurs over the steep lee
side of each bedform. During the flood, no flow separation develops over
the bedforms having a flat crest; however, a small FSZ is observed over
the steepest part of the crest of some bedforms, where the slope is
locally up to 15°. Over a given bedform morphology and constant water
levels, no FSZ occurs for velocity magnitudes smaller than 0.1 m s−1; as the flow accelerates, the FSZ reaches a stable size for velocity magnitudes greater than 0.4 m s−1.
The shape of the FSZ is not influenced by changes in water levels. On
the other hand, variations in bed morphology, as recorded from the
high-resolution bathymetry collected during the tidal cycle, influence
the size and position of the FSZ: a FSZ develops only when the maximum
lee side slope over a horizontal distance of 5 m is greater than 10°.
The height and length of the wake region are related to the length of
the FSZ. The total roughness along the transect lines is an order of
magnitude larger during the ebb than during the flood due to flow
direction in relation to bedform asymmetry: during the ebb, roughness is
created by the large bedforms because a FSZ and wake develops over the
steep lee side. The results add to the understanding of hydrodynamics of
natural bedforms in a tidal environment and may be used to better
parameterise small-scale processes in large-scale studies.
U2 - 10.1016/j.csr.2014.09.001
DO - 10.1016/j.csr.2014.09.001
M3 - Journal article
SN - 0278-4343
VL - 91
SP - 57
EP - 69
JO - Continental Shelf Research
JF - Continental Shelf Research
ER -