My research program examines various aspects of erosion and sedimentation primarily in fluvial
systems through the use of experimentation, theory, field studies, and numerical modeling. The
overall goals of the research are to quantify flow and sediment transport processes using an
interdisciplinary approach and to determine the impact of these processes on river form and
function, water quality and ecology, landscape evolution, and watershed infrastructure and
integrity. This work is fundamentally important because sediment is one of the leading causes of
water quality impairment nationwide, and soil erosion is the leading cause of soil degradation
worldwide. Excessive sedimentation also reduces the performance of flood-control reservoirs
and increases risks to flooding within river corridors. Below is a brief description of these
activities, divided into four categories.
1. Flow and sediment transport in rivers
For uniform and steady flows in rivers, the entrainment and transport of sediment of various
sizes, shapes, and densities depend on the gravitational and frictional forces keeping the grains in
place, the time- and space-varying fluid forces acting to move and suspend the particles, and the
development of bedforms that further modulate boundary layer characteristics. Work to date,
utilizing state-of-the-art instrumentation, has focused on the following topic areas.
The entrainment, transport, and deposition of sediments with varying size, shape, and density.
The discrimination of fluid and sediment phases within sediment-laden, open channel flows
The modulation of turbulence by bedload and suspended load transport in rivers
Bedform development, stability, and transition in rivers, their interaction with turbulent flow,
their growth from initially flat-bed boundaries, and the effect of sediment gradation on bedform
characteristics and mass flux.
Some representative papers recently published on these topics include the following
My Ph.D. work focused on the processes that cause the entrainment, transport, and deposition of
sediment mixtures as well as the characteristics of low-relief bedwaves termed bedload sheets.
The presence of sediment in geophysical flows, either as bedload or suspended load, has a
measurable effect on the turbulence characteristics of such boundary layers. State-of-the-art
instrumentation enables the discrimination of fluid and sediment phases in sediment-laden openchannel flows, and these work can now define, verify, and challenge many of the concepts,
theories, and equations used for describing clear-water and sediment-laden geophysical flows
including velocity distributions, turbulence modulation, and suspended sediment load.
Bedform development, stability, and transition are intimately coupled to the turbulent flow field
present over these topographic features. The turbulent flow fields over fixed ripple and dune
bedforms have been quantified, providing new quantitative evidence for the fluid dynamical
differences of flow over such bedforms, new conceptualizations for the turbulent entrainment
and suspension of bed sediment, and a new hydrodynamic basis for bedform transition. Recent
work as examined the spectral signatures of flow and sediment transport over dune bedforms and
the development and evolution of bedforms from an initially flat bed.
2.
Soil erosion by upland concentrated flows
Rill and gully development in upland concentrated flows occur as a discrete soil erosion process
intimately related to the occurrence and upstream migration of headcuts. These areas of intense,
localized erosion often are the primary cause of soil loss and the dominant source of sediment
yield from agricultural landscapes. The formation, growth, and migration of headcuts has been
linked to the concentration of overland flow, rill and gully erosion and development, erosion of
bedrock channels, the initiation of drainage systems and landscape evolution, and the response of
landscapes to land use change and extreme hydrologic events. Work to date, utilizing speciallydesigned experimental facilities and state-of-the-art instrumentation, has focused on the
following topic areas.
Systematic behavior of headcut growth, development, and upstream migration with varying
boundary conditions,
Turbulent flow structure and bed pressure distributions within the plunge pool region
Application of impinging jet theory to soil erosion processes due to headcut migration
Integral time- and length-scales for headcut development
Analytic modeling of soil erosion due to plunge-pool scour
Modeling gully development, migration, and widening on agricultural fields in spatially-varied,
unsteady flows and its integration into AGNPS
3.
Reservoir sedimentation and assessment
There are more than 75,000 dams in the continental U.S., and these structures provide
opportunities for navigation, hydroelectric power, irrigation, flood reduction, and hazard
protection. Yet these dams divide or segment watersheds with respect to water, sediment,
nutrient transfer, and ecosystem dynamics. Because reservoirs effectively trap sediment, these
impoundments have proven to be important environmental “archives” of changes in water
quality parameters, land use, and sediment yield. The overall goals of the current research
program are to assess the sediment impounded by these dams in terms of the structure’s ability to
regulate floodwaters and the potential hazard these sediments may pose to the environment.
Work to date, utilizing various techniques, has focused on the following topic areas
Discriminating post-impoundment sediment accumulations from pre-existing or parent materials
using stratigraphic, geochronologic, and geophysical techniques
Determining the textures, spatial distributions, and rates of reservoir sedimentation at local and
regional scales and to relate these to geomorphic processes and land use change
Use of reservoir sediments as environmental, geomorphic, and hydrologic archives
Quantifying the chemical characteristics of the sediment impounded within reservoirs,
specifically sediment-associated trace elements, and to ascertain if these trace elements display
any trends temporally or spatially since dam construction
Use of acoustic, sub-bottom profiling to map impounded sediment and its comparison to
vibracoring and stratigraphic analysis
4.
Riparian vegetation and fluvial geomorphology
Because of increased popularity of vegetation in stream restoration and streambank stabilization
programs, there is renewed interest in examining the interactions between river flow, riparian
vegetation, and large woody debris. There are instances, however, when bank instability causes
excessive recruitment of large woody debris, which can effectively choke downstream river
corridors as witnessed along the Yalobusha River, MS. The overall goals of the current research
program are to assess the interactions between open channel flow and rigid vegetation and to
determine how river flow processes and forms are modulated such vegetation. Work to date,
utilizing various techniques and facilities, has focused on the following topic areas.
Using managed vegetation plantings to transform a straight, degraded stream corridor into a
meandering planform
Examining the turbulent-flow structures created by riparian vegetation and their effects on
mixing processes
Determining the morphologic and hydraulic adjustment of rivers to vegetation using physical and
numerical models
Assessing the impact of large woody debris on river form and function
Quantifying the drag on individual vegetal elements as a function of density and submergence
I designed an experimental stream-restoration program to use managed vegetation plantings
analogous to the willow-post technique (emergent, rigid vegetation) to transform straight,
degraded stream corridors typical of the loess region of the U.S. into biologically functional,
meandering streams (T. Pirim, student; #11). The sinuosity of the meandering thalweg, as
deduced using particle image velocimetry, and the magnitude of the lateral mixing and flow
resistance were shown to depend on the position and density of vegetation zones. Competitive
funds (USDA-CSREES) were secured to continue this work to address (a) alluvial adjustment to
rigid, emergent vegetation using a movable-bed Froude-scale model (on-going) and (b) flow
resistance within vegetated stream corridors (S. Vittalam, student; recently completed).
Additional collaborative studies have examined the geomorphic and hydraulic response of
alluvial channels to large woody debris (#12, #13), and the modeling of flow and sediment
transport processes within vegetated stream corridors (#1).