2002 Annual Report
Research
Impact of Roughness Elements on Reducing the Shear Stress Acting on
Soil Particles
Anita Thompson, Research Fellow
Bruce Wilson, Associate Professor
Bradley Hansen, Assistant Scientist
Udai Singh, Research Associate
Funding Source
USDA National Needs Fellowships; Minnesota Local Road Research Board
Objective
The goals of the project are to develop a better understanding of the
detachment of soil with vegetal cover. The specific research objectives
are to:
- Determine the effects of vegetation on reducing surface shear stress over a range of flow conditions.
- Measure the surface shear stress resulting from variations in vegetal
density, size, placement, and shape in a laboratory flume.
Project Description
Surface roughness in the form of vegetation, micro-relief, and soil aggregates
provides resistance to and reduces the velocity of overland flow. As water
moves across a rough surface, part of the total force acts on the roughness
elements, and the remainder acts on the intervening soil surface. Flow
resistance has been empirically characterized by Manning’s roughness
coefficient as a function of the velocity and depth of flow. Although
these relationships provide information on average resistance to flow,
the shear stresses acting on the soil are unknown and are the driving
force for particle detachment. Although studies have been conducted to
investigate the fraction of the total shear acting on the soil, the process
is still poorly understood.
The drag partition theory proposed for wind erosion will be applied and
evaluated for surface runoff. With this theory, the drag force acting
on the element per unit area and per unit volume is used to partition
total shear. The impact of a “shelter” area and volume is considered.
The resulting relationships are easy to use. They require an estimate
of the roughness density and the ratio of the drag coefficient for an
isolated roughness element and the non-vegetated surface.
Results
This study quantifies the shear partition (ratio of particle shear to
total shear) for idealized vegetation. Instrumentation were designed and
constructed to measure the components of the partition. A unique flume
and hot-film anemometry were used to measure detailed spatial and temporal
variations of particle shear. Instrumentation was developed to measure
drag force on idealized shapes representative of vegetal elements.
For idealized vegetation, the shear partition is shown to decrease with
increasing vegetative density. For the densities investigated, particle
shear accounts for 13 to 89% of the total shear. Shear partitioning theories
developed for wind erosion by Raupach (1992) and Wooding et al. (1973)
are shown to adequately represent the observed data for water flows. The
application of shear partitions to field design is possible using the
theory of Raupach (1992). For example, the vegetation density (the ratio
of plant upstream projected area to surface area) can be determined for
a given threshold particle shear.