U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska
Date of this Version
2009
Abstract
Total-load sampling has been a perpetual problem in sediment monitoring. Usually a combination of bed-load sampling devices, suspended load suction samplers, and some kind of flume, for total flow rate, is used. Total-load, sediment-sampler-design concepts that can perform all three of these functions are proposed. The resulting designs would require installation at sites that can provide a step-overfall height about equal to the maximum channel flow depth. The concepts are simple, but appear to have been overlooked or ignored for the past many decades, and are based on a moving conveyor belt that is long and wide, with many slots, all of the same size, onto which the stream to be sampled discharges. All flow drops through the slots, and with equal sized slots each must catch a similar proportion of the total flow. Hence, only one slot needs to be collected. As a practical extension it is proposed to replace the conveyor belt with a rack having several slots that represent a short section of the total conveyor belt that is large enough so that the flow does not notice the missing belt parts. This rack is then traversed back and forth on a track through the falling nappe. Laboratory tests of this proposed sampling-assembly rack indicated that the number of the required slots is related to the channel depth and the sum of the slot openings. When the rack is composed of sufficient slots so that the slot-width sum is more than half the channel overfall depth, the system undersampled from 0 to 2% but when there are insufficient slots whose sum represents less than one-third of the overfall depth, the system undersampled by over 8%. The concepts are extended to the condition with a stopped belt where several sampling-slot groups are equally spaced beneath the overfall. A “test of concept” sampler assembly of the stopped-belt idea was built and tested. The sample catch across the stream was within about 4% of expected, offering a total load sampling system where motorized equipment is difficult to install, or electric power is not available. Design and construction suggestions are presented. The catch rate can be small enough to facilitate convenient flow measurement of the catch, which can be converted to total streamflow without the need for separate channel flow measurements.
Comments
Published in the JOURNAL OF HYDROLOGIC ENGINEERING, January 2009.