CIVE 530 - OPEN-CHANNEL HYDRAULICS LECTURE 17C: SEDIMENTATION ENGINEERING III

17.5  SEDIMENT MEASUREMENT TECHNIQUES

The measurement of fluvial sediments is often necessary to complement sediment yield and sediment transport studies. The accuracy of the measurement depends on the equipment and techniques, but also on the application of basic principles of sediment transport. As sediment enters a stream, it separates itself into bed-material load and wash load. In turn, the bed-material loal is transported either as bed load or suspended load. The suspended bed-material load plus the wash load constitutes the total suspended load of the stream or river. The term "sampled suspended sediment discharge" is used to describe the fraction of suspended load that can be sampled with available equipment. It excludes the "unsampled suspended sediment discharge," i.e., the fraction of suspended load that is carried too close to the streambed to be effectively sampled. The suspended sediment discharge is the sum of sampled and unsampled suspended sediment discharges. SEDIMENT-SAMPLING EQUIPMENT Sediment-sampling equipment can be classified as follows: suspended sediment samplers, which measure suspended sediment concentration; bed-load samplers, which measure bed load; and bed-material samplers, which sample the sediment in the top layer of the streambed. Suspended sediment samplers Suspended sediment samplers can be classified as follows: depth-integrating, point-integrating, single-stage, and pumping samplers. Depth-integrating samplers accumulate a water-sediment sample in a pint-size milk bottle as they are lowered to the streambed and raised back to the surface at a uniform rate of transit. They are designed so that the velocity in the intake mozzle is nearly equal to the local stream velocity. Samples may be collected by wading in a stream, by hand from a suitable support, or mechanically with a cable-and-reel setup. The DH-48 depth-integrating suspended wading type sampler is shown below. DH-48 depth-integrating suspended wading type sampler. With wading-rod suspension, this sampler is used in shallow streams where the product of flow depth (ft) and mean velocity (fps) does not exceed 10. The U.S. DH-59 sampler (24 lb) with handline suspension is used in streams with low velocities but with depths that do not permit samples to be collected by wading. DH-59 depth-integrating suspended hand-line sampler. The U.S. D-49 sampler (62 lb) with cable-and-reel suspension is designed for use in streams beyond the range of hand-operated equipment. D-49 suspended sediment sampler. Depth-integrating samplers improve sample accuracy and reduce the cost of collecting suspended sediment data. Field data of questionable quality. Point-integrating samplers accumulate a water-sediment sample at any selected point in a stream. A rotary valve is operated by a solenoid to open and close the sampler. These samples can also be used to collect depth-integrating samples by moving it through the vertical. This permits depth integration in streams that are too deep to be sampled with a depth-integrating sampler. The U.S. P-46 and P-61 (100 lb), P-63 (200 lb) and P-50 (300 lb) point-integrating samplers are in current use. P-61 suspended sediment sampler. The single-stage sampler was developed to obtain suspended sediment data in flashy streams, particularly those located in remote areas. The pumping sampler is designed to obtain a continuous record of sediment concentration by sampling at a fixed point at specific intervals. This sampler does not require an operator. Bed load samplers Bed load samplers are of three types: basket type, pan type, and pressure-difference type. The basket and pan types canuse an increase in resistance to flow and a reduction in stream velocity at the sampling location. The reduction in stream velocity interferes with the rate of bed-load transport, compromising the accuracy of the measurement. The pressure-difference bed-load sampler is designed to eliminate the reduction in velocity, increasing accuracy. The efficiency of a bedload sampler is the ratio of sampled bed load to that actually transported. Efficiency varies with sample type, method of support, particle size, and bed configuration. Calibration of bed-load samplers has indicated a mean efficiency of about 45 percent for the basket and pan types, and 70 percent for the pressure-difference type. Bed-material samplers Bed-material samplers are of three types: drag bucket, grab bucket, and vertical-pipe, or core sampler. The drag bucket sampler consists of a weighted section of cylinder with an open mouth and cutting edge. As the sampler is dragged upstream along the bed, it collects a sample from the top layer of bed material. The grab-bucket smapler is similar to the drag bucket, consisting of a cylinder attached to a rod, and used primarily in shallow streams. The core sampler consists of a piece of metal or plastic pipe that can be forced into the stream by hand. Generally, the drag-bucket and grab-bucket samplers do not obtain fully representatiove samples because of the loss of fine material. The core smapler is satisfactory for use in shallow streams. The U.S. BMH-53 sampler consists of a 9-in long 2-in diameter brass or stainless steel pipe with a cutting edge and suction piston attached to a control rod. BMH-53 piston-type bed-material hand sampler. The piston is retracted as the cutting edge is forced into the stream bed. The partial vaccum that develops in the sampling chamber as the piston is withdrawn assists in holding the sample in the cylinder. The sampler can be used only in streams shallow enough to be waded. The U.S. BMH-60 bed material sampler with both handline and cable suspension is designed to scoop up a sample of bed sediment about 3 in wide and 2 in deep. BMH-60 handline bed-material sampler. At the close of sampling, the cutting edge rests againts a rubber stop, which prevents any sediment from being lost. The aluminum sampler weighs 30 lb, and the brass sampler 40 lb. It is used to collect bed-material-sediment samples in streams with low velocities but with depths beyond the range of the BHM-53 sampler. The U.S. BM-54 bed-material sampler (100 lb) with cable suspension is similar in design to the BMH-60 sampler. BM-54 bed-material sampler. It is used in deep streams where a heavier sampler is necessary. New samplers The U.S. DH-2 is a bag-type suspended-sediment/water-quality sampler, capable of being used as a hand-line sampler. DH-2 suspended sediment sampler. The sampler is designed to meet the requirements of the U.S. Geological Survey for a "clean" suspended-sediment sampler. The sampler will collect at least 1-liter of sample isokinetically to 35 ft. The U.S. D-99 sampler is a bag-type suspended-sediment/water-quality sampler to be used with a suspension reel. D-99 suspended sediment/water quality sampler. The sampler weighs 285 pounds and has a streamlined body. The bronze body casting is coated with plastic to reduce the potential for contamination when used for trace element sampling. SUSPENDED SEDIMENT DISCHARGE MEASUREMENTS Suspended sediment discharge measurements are used to determine sediment concentration at a point in a stream, except for a small unmeasured zone near the streambed. With wading equipment, measurements can generally be made down to within 0.3 ft of the streambed. For cable-supported equipment, the unmeasured zone varies between 0.5 and 1 ft, depending on the size of the sampler used. Suspended sediment discharge measurements include: suspended sediment concentration, specific gravity, temperature of water-sediment mixture, water discharge, and distribution of flow in the stream cross section. The streamflow depth and velocity and the facilities at the sampling site (bridge, cableway, and so on) have an influence on the choice of sampler. Stream depth determines whether hand samplers, such as the DH-48 or DH-49, or a cable suspended sampler, such as the D-49, are used. Flow depths over 15 ft require the use of point-integrating samplers to avoid overfilling of the sampling bottles. The larger the product of flow depth times mean velocity, the heavier the sampler required for proper measurement. The number of sampling verticals depends on the desired accuracy and the variation of sediment concentration across the stream. For streams with a stable cross section and essentially uniform sediment concentration across the channel width, sampling at a single vertical is usually adequate. Depth-integrating samplers produce a suspended sediment concentration, which can be measured in parts per million (ppm) and converted to mg/L. The suspended sediment discharge is given by the following formula: Qs = 0.0027 Cs Q in which Qs = suspended sediment discharge, in tons/day; Cs = suspended sediment concentration, in mg/L; Q = water discharge, in cfs; and The quantity 0.0027 is conversion factor for the indicated units (U.S. Customary). Measurements of suspended sediment discharge There are two techniques to measure suspended sediment discharge: EDI, or equal-discharge increment, and ETR, or equal-transit rate. In the EDI method, sampling is done at the centroid of equal-discharge increments. In the ETR method, sampling is done at the centroid of equal-length increments. The EDI method requires a knowledge of the lateral distribution of streamflow prior to the selection of sampling verticals. The ETR method is applicable to shallow streams where the cross sectional distribution of streamflow is not stable. Generally, the EDI method requires fewer sampling verticals that the ETR method. However, the ETR method does not require a prior discharge measurement. In the EDI method, the suspended sediment concentration is the average obtained from several depth-integrating samples. In the ETR method, the suspended sediment concentration is that of a composite sample encompassing several depth-integrating samples. The error in suspended sediment discharge provided by the measurement varies with the depth of the unsampled zone and the size distribution of suspended load. The error tends to be smallest in the cases where the vertical concentration gradient in the unsampled zone is small. The concentration gradient near the bed is small for silt and clay particles, and large for the coarser sand particles. Corrections in sampled suspended-sediment discharge to account for the unsampled portion are usually obtained through appropriate sediment transport predictors such as the Colby 1957 method or the Modified Einstein Procedure.

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