Field Measurement of Particle Settling Velocity

Many stormwater management practices depend on sedimentation as their primary removal mechanism, but the settling velocity distribution of particles in runoff from a specific watershed is rarely known. A simple and effective method to characterize settling velocity distribution is needed to improve the design of stormwater management practices. Elutriation devices have been used to characterize the settling velocity distribution of river sediments (Walling and Woodward 1993) and combined sewer overflows (Krishnappan et al. 2004). The elutriation device has been modified and tested to determine its effectiveness in measuring the settling velocity distribution of particles in urban runoff. The device has a series columns with increasing diameters. Stormwater is pumped vertically through the columns, and the upward velocity through each of the columns can be calculated. Particles with settling velocities greater than the upward velocity settle in the column, and particles with settling velocities less than the upward velocity are flushed through the column. Initial experiments have shown particles can be predictably separated with the elutriation device. Using different materials and methods, an elutriation device to measure particle size distribution in the field and laboratory is being developed and tested. The application of the elutriation device will provide practitioners with a new tool to better design stormwater management practices.