(click for larger image-109K)

Purpose:

• Determine the effects of disturbance & recovery histories on wind-erosion vulnerability.
• Determine the roles of different desert surfaces (e.g., natural pavement, biologic soil crust) in providing soil-surface stability.

Methods:

Threshold friction velocities (TFV) that move sediment particles are determined using a portable, open-bottom wind tunnel, 150 mm x 150 mm cross section by 2.4 m length; 5:1 contraction. Wind speeds that correspond to particle movement across the surface are recorded from a tube anemometer at several heights above the surface midway across the working section (photo).

Threshold friction velocity = kz(dU_t / dz)

where U_t is wind speed at the particle movement threshold, z is roughness height characteristic, and k is Von Karman's constant.

Particle Size Analysis (PSA). Standard sieves were used for particles larger than coarse sand. A laser-light scattering method is used for particles between 0.03 and 850 m m (see diagram). Samples were treated to remove carbonate and organic matter.

Data are normalized for <850 µm fraction. PSA will also be done on sediment trapped in a filter at the send of the wind tunnel to compare moved particle size with the population of particles in the soil.

Example:

Studies at Greenwater Townsite (a mining town, built in 1906 and abandoned in 1908), Greenwater Valley, Death Valley National Park. Plats of roads, buildings, and undisturbed areas outside town were used to select sites for wind-tunnel experiments (see Webb, R. and others, USGS Bulletin 1793). The experiment shown here illustrates the effects of modern footprint disturbance on soil-surface stability.

Threshhold friction velocities (TFV) for sites of differing soil type, inside and outside the Greenwater townsite. Footprint disturbance decreases soil stability at the sites.

Particle size analysis of samples from disturbed and undisturbed sites. data are normalized for <850 µm fraction. PSA will also be done on sediment trapped in a filter at the end of the wind tunnel to compare moved particle size with the population of particles in the soil.

Studies elsewhere in the Mojave Desert (near Valjean, CA, and at Joshua Tree, CA) show that increasing disturbance results in (1) lower wind speeds to move particles and (2) greater sediment erosion (higher sediment production).