6.3, BEDLOAD-DOMINATED TRANSPORT MODELS
273
Ntm
~ morphological time scale
Nq - sediment transport rate scale (per unit width)
Nx
~ horizontal length scale
Nz
- vertical length scale
N(i_p) - material porosity scale (p is porosity)
Modeling Beach Accretion Processes
Storm erosion of beaches is known to be a sediment transport process dominated by turbulence and transport of material in suspension. However,
there is increasing interest in understanding the beach recovery process that
occurs during calm conditions. Scale model investigations of the beach recovery process may be hindered by incorrect similitude of water percolation
on the upper portion of the beach profile.
Part of the recovery process involves longer-period swell waves carrying
sediment onto the upper portion of the beach and depositing it there as the
slow moving water percolates through the beach. Regardless of whether
the sediment is reduced in size according to bedload or suspended load
transport criteria for correct reproduction of transport below the water
surface, percolation through unsaturated beach sand in the model will be
relatively slower because the intergranular spaces are smaller. Because
less water percolates through the beach, there is a surplus of water that
flows back down the beach, sweeping sediment with it. Additionally, the
percolating flow through most beaches has very low flow Reynolds numbers,
and we already known that viscous flows are not in similitude in Froudescaled coastal models.
The question remains as to whether or not percolation is an important
aspect of the beach recovery process. If it is significant, then improper similitude of percolation will cause difficulties in those models not conducted at
prototype scale. Further investigation of this topic is needed.
Modeling Longshore Transport
Modeling longshore sediment transport is an important application for
coastal movable-bed models. It is also perplexing because of the certainty
that sediment may be moved by suspension and by bedload in different
parts of the littoral zone. Probably the best advice about conducting longshore transport models comes from descriptions of past modeling efforts.
Kamphuis and Kooistra (1986) used scaled movable-bed models to study
the evolution of conical artificial sand-gravel islands intended for use as oil
drilling platforms. They conducted 52 model studies using 4 different scales,
273
Ntm
~ morphological time scale
Nq - sediment transport rate scale (per unit width)
Nx
~ horizontal length scale
Nz
- vertical length scale
N(i_p) - material porosity scale (p is porosity)
Modeling Beach Accretion Processes
Storm erosion of beaches is known to be a sediment transport process dominated by turbulence and transport of material in suspension. However,
there is increasing interest in understanding the beach recovery process that
occurs during calm conditions. Scale model investigations of the beach recovery process may be hindered by incorrect similitude of water percolation
on the upper portion of the beach profile.
Part of the recovery process involves longer-period swell waves carrying
sediment onto the upper portion of the beach and depositing it there as the
slow moving water percolates through the beach. Regardless of whether
the sediment is reduced in size according to bedload or suspended load
transport criteria for correct reproduction of transport below the water
surface, percolation through unsaturated beach sand in the model will be
relatively slower because the intergranular spaces are smaller. Because
less water percolates through the beach, there is a surplus of water that
flows back down the beach, sweeping sediment with it. Additionally, the
percolating flow through most beaches has very low flow Reynolds numbers,
and we already known that viscous flows are not in similitude in Froudescaled coastal models.
The question remains as to whether or not percolation is an important
aspect of the beach recovery process. If it is significant, then improper similitude of percolation will cause difficulties in those models not conducted at
prototype scale. Further investigation of this topic is needed.
Modeling Longshore Transport
Modeling longshore sediment transport is an important application for
coastal movable-bed models. It is also perplexing because of the certainty
that sediment may be moved by suspension and by bedload in different
parts of the littoral zone. Probably the best advice about conducting longshore transport models comes from descriptions of past modeling efforts.
Kamphuis and Kooistra (1986) used scaled movable-bed models to study
the evolution of conical artificial sand-gravel islands intended for use as oil
drilling platforms. They conducted 52 model studies using 4 different scales,