Physical inter-relationships between bed sorting and armoring, and
prediction of the active layer thickness and bed forms.
Separation of bed load and suspended load for application to
sediment transport modeling of reservoirs and estuaries.
Differences in the rate of transport on the bed and in suspension
require the two modes of transport to be modeled separately in some
situations. The difficulty lies in determining when a particle
ceases to move on the bed and becomes suspended. Research is needed
on the fundamental processes of particle entrainment and deposition,
including evaluation of the methods proposed by Bennet and Nordin,
Van Rijn, and the developing concept of turbulent bursting.
- Physical process description of bed sorting and
Nearly all of the numerical sediment transport models that
predict riverbed evolution in natural rivers over time make
simplifying assumptions of bed armoring and sorting. A better
understanding of the physical processes affecting bed sorting and
armoring is needed to improve the predictive capability of these
- Prediction/definition of active layer (or mixed layer)
thickness. Several methods have been proposed for calculating
the bed sediment thickness readily available for transport.
Modeling results can be highly dependent on this active layer
thickness. Research into bed form prediction would be useful for
active layer definition.
- Bed form prediction based on local hydraulic and sediment
conditions. None of the bed form predictors existing in the
literature are reliable. An analysis of existing field data and
an exhaustive evaluation of the bed form predictors would be
extremely useful. A reliable way of including bed forms into the
bed roughness is necessary. A better understanding of the
relationship between bed forms and sediment transport is highly
Sediment transport and erosion of cohesive beds and cohesive bed
mixtures. Functional relationships among hydraulic parameters (such
as shear stress, unit stream power, etc), cohesive soil properties
(such as percent clay, plasticity index, unconfined compressive
strength, etc), and erosion rates are needed.
Fall velocity of small particles (diameter < 0.0625
factors influencing the fall velocity of silts and clays (e.g.,
particle concentration, water temperature, shape factor,...) and
provide functional relationships that can be used in the computation
of the transport rates. Experimental data and analysis is necessary
to obtain the desired functional relationships. An extension of the
U.S. Inter-Agency Committee on Water Resources (1957) fall velocity
graph for particles in the silt and clay ranges is desired.
Sediment-laden flow. There is an overwhelming amount of research that
can be done to provide a better understanding of the processes behind
the transport in sediment-laden flow (hindered settling velocities,
viscosity of the mixture,...) Specifically, "What adjustments in
sediment transport equations need to be made for flows with high
concentration of wash loads?" Current practice is to use Colby's
(1964) method, which is based on a limited amount of data. The use of
modified dimensionless unit stream power (Yang, et al 1996) to rivers
other than the Yellow River needs to be explored.
Riparian vegetation and its effect on bed roughness and controlling
channel width and soil erosion. It is necessary to quantify the
effects of friction losses in flows through flexible and rigid
vegetation. What are the effects of vegetation to bank erosion rates
and to the onset of sediment transport.
Sediment transport in unsteady and non-uniform flow. In non-steady
and non-uniform flow, even when the system is not supply limited,
the sediment transport rate may be different from the transport
capacity. In rivers, this is an important factor in flash flood
waves and in the tide-influenced lower reach of the river. The
existing formulas for the suspended sediment transport have severe
limitations and more accurate relationships are necessary. Total
load bed-material formulas are also necessary (to avoid having to
distinguish between bed load and suspended load).
Turbulence in open channel flow. There has been an increase in
research into the turbulent structures of fluid flows using
laser-doppler-anemometry (LDA) and advanced numerical techniques,
such as large eddy simulation and direct numerical simulation. This
has shed new light about the nature of coherent structures and
turbulent bursts (at the channel bed), however there hasn't been any
application of that information to sediment transport. It is
desirable to quantify these aspects of turbulence in sedimentation
phenomena such as the sediment entrainment rates and the incipient
motion of sediment particles.
Development of rapid techniques and equipment to map bed sediment
size and thickness in rivers and reservoirs. Testing and
development of measuring equipment for mapping river and reservoir
bottoms. Technology and instruments are available that can map
reservoir and river bed sediments along with determining sediment
thickness and size gradation. Testing and further development of
this technology is needed on how reliable it is for different
environments. Development of this technology will provide a more
cost effective means of measuring existing river and reservoir
Influence of reservoir sediments on water quality, especially if
reservoir sediments are eroded. There are thousands of reservoirs
of all shapes and sizes that will fill with sediment from single
storm events or after hundreds of years of inflow retention.
Research is needed on the handling of these sediments from:
Width adjustment of alluvial channels. We often assume that channel
width is given or fixed in sedimentation and river hydraulic
studies. The fact is that the width of an alluvial channel is a
variable. Theoretical, laboratory, and field studies are needed to
determine how channel width will adjust under different flow and
- Cost effective methods of reservoir sediment removal and
redeposition, addressing the environmental impacts on the
reservoir and deposition site.
- Methods of flushing sediments downstream and the impacts on
the river water quality and environmental impacts.