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September 2010 (vol. 5, issue 4)
Measurement of Particle Settling Velocity for Stormwater Runoff
Contributed by Eric Hettler and John Gulliver. University of Minnesota, St. Anthony Falls Lab
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. An elutriation device has been developed to characterize settling velocity distribution in stormwater runoff and subsequently improve the design of stormwater management practices. Elutriation is a method to directly measure settling velocity distributions of particles, where water flows upward through a column and a suspension of particles is introduced. Particles with settling velocities less than the upward fluid velocity in the column will eventually be flushed out of the column, and particles with settling velocities greater than the upward fluid velocity will be retained in the column. If a series of columns with varying diameters (thus varying upward velocities) is used, a settling velocity distribution of the particles can be calculated based on the upward velocity of water in the columns and the mass of particles retained in each column. Elutriation devices have been applied to measure the settling velocity distributions of sediments in a river (Walling & Woodward 1993) and particles in combined sewer overflows (Krishnappan et al. 2004, Marsalek et al. 2006). Modifications have been made so the device is easier to use, cheaper and easier to manufacture, and more appropriate for implementation to stormwater runoff.
The modified elutriation device developed at St. Anthony Falls Laboratory (shown in Figure 1) consists of three columns with diameters of 2.47-in. (63 mm), 3.9-in. (99 mm), and 6.0-in. (152 mm). Experiments have shown the modified device can predictably separate silica particles down to 17 micrometers and up to 140 micrometers. This range includes the most important particles when considering the performance of a traditional settling pond or a proprietary underground device. Using different materials and methods, an elutriation device to improve design of management practices by measuring particle size distribution in the field and laboratory is feasible. The application of the elutriation device will provide practitioners with a new tool to better design stormwater management practices.
Figure 1. Elutriation device for measuring the particle settling velocity of sediment in stormwater. Water flows from left to right, into the bottom and out of the top of each column sequentially.
- Krishnappan, B. G.; Marsalek, J.; Exall, K.; Stephens, R. P.; Rochfort, Q.; Seto, P., A water elutriation apparatus for measuring settling velocity distribution of suspended solids in combined sewer overflows. Water Quality Research Journal of Canada 2004, 39, (4), 432-438.
- Marsalek, J.; Krishnappan, B. G.; Exall, K.; Rochfort, Q.; Stephens, R. P., An elutriation apparatus for assessing settleability of combined sewer overflows (CSOs). Water Science and Technology 2006, 54, (6-7), 223-230.
- Walling, D. E.; Woodward, J. C., Use of a Field-Based Water Elutriation System for Monitoring the In-Situ Particle-Size Characteristics of Fluvial Suspended Sediment. Water Research 1993, 27, (9), 1413-1421.