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Field Methods for Measurement of Fluvial Sediment

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Field Methods for Measurement of Fluvial Sediment WSGS science for a changing world Techniques of Water-Resources Investigations of the U.S. Geological Survey Book 3, Applications of Hydraulics Chapter C2 Field Methods for Measurement of Fluvial Sediment By Thomas K. Edwards and G. Douglas Glysson This manual is a revision of “Field Methods for Measurement of Fluvial Sediment,” by Harold I? Guy and Vernon W. Norman, U.S. Geological Survey Techniques of Water-Resources Investigations,‘Book 3, Chapter C2, published in 1970. SEDIMENT-SAMPLING TECHNIQUES 61 depth limit of the depth-integrating sampler, the sampling should be much greater during these periods observer should try to obtain a sample by altering the than during the low-flow periods. During some parts technique to collect the most representative sample of these critical periods, hourly or more frequent possible. The best collection technique under these sampling may be required to accurately define the conditions would be to depth integrate 0.2 of the trend of sediment concentration. During the remainder vertical depth (0.2& or a lo-foot portion of the of the year, the sampling frequency can be stretched vertical. These samples then can be checked and out to daily or even weekly sampling for adequate verified by collecting a set of reference sampleswith a definition of concentration. Hurricane or thunderstorm point-integrating sampler. By reducing the sampled events during the summer or fall require frequent depth during periods of high flow, the transit rate can samplesduring short periods of time. Streams having be maintained at 0.4 V, or less in the vertical, and a long periods of low or intermittent flow should be partial sample can be collected without overfilling the sampled frequently during each storm event because sample container, even under conditions of higher most of the annual sediment transport occurs during velocities that usually accompany increases in these few events. discharge. During long periods of rather constant or gradually varying flow, most streams have concentrations and Sampling Frequency, Sediment Quantity, quantities of sediment that vary slowly and may, Sample Integrity, and Identification therefore, be adequately sampled every 2 or 3 days; in some streams,one sampling a week may be adequate. Sampling Frequency Several samplings a day may occasionally be needed to define the diurnal fluctuation in sedimentconcentra- When should suspended-sediment samples be tion. Fluctuations in power generation and evapotrans- taken? How close can samplesbe spacedin time and piration can cause diurnal fluctuations. Sometimes still be meaningful? How many extra samples are diurnal temperature fluctuations result in a snow and required during a flood period? These are some ques- ice freeze/thaw cycle causing an accompanying fall tions that must be answeredbecause timing of sample and rise in stage. Diurnal fluctuations also have been observations is as important to record computations noted in sand-bed streams when water-temperature (see Porterfield, 1972) as is the technique for taking changes cause a change in flow regime and a drastic them. Answering such questions is relatively easy for change in bed roughness (Simons and Richardson, those who compute and assemblethe records because 1965). they have the historical record before them and can The temporal shape of the hydrograph is an easily see what is needed. However, the field person indicator of how a stream should be sampled. frequently does not have this record and certainly Sampling twice a day may be sufficient on the rising cannot know what the conditions will be in the future. stage if it takes a day or more for a stream to reach a Observers should be shown typical hydrographs or peak rate of discharge.During the peak, samplesevery recorder charts of their stations or of nearby stationsto few hours may be needed. During the recession, help them understand the importance of timing their sampling can be reduced gradually until normal samplesso that each sampleyields maximum informa- sampling intervals are sufficient. tion. The desirable time distribution for samples The sediment-concentrationpeak may occur at any depends on many factors, such as the seasonof the time relative to the water discharge; it may coincide year, the runoff characteristics of the basin, the with the water-discharge peak or occur several days adequacy of coverage of previous events, and the prior to or after it. Hydrographs for large rivers, accuracy of information desired or dictated by the especially in the Midwest, typically show water- purpose for which the data are collected. discharge peaks occurring several days after a storm For many streams,the largest concentrationsand 70 event. If the sediment concentration has its source to 90 percent of the annual sediment load occur during locally, the sediment peak can occur a day or more spring runoff; on other streams, the most important prior to the water-discharge peak. In this case, the part of the sediment record may occur during the receding limb of the sediment-concentration curve period of the summer thunderstorms or during winter will nearly coincide with the lagging water-discharge storms. The frequency of suspended-sediment peak. In this event, intensive sampling logically should 62 FIELB METHODS FOR MEASUREMENT OF FLNVL4L SEDIMENT be done prior to the water-discharge peak. Detailed Stream-sediment stations may be operated or sampling of hydrograph peaks during the initial stages sampled on a daily, weekly, monthly, or on an of a monitoring program will help determine when the intermittent or miscellaneousschedule. Usually, those sediment-samplingfrequency should be increasedand operated on a daily basis are considered adequate to decreasedin order to optimize the sediment-sampling yield the continuous record. One should be mindful effort relative to peak-flow conditions. that each sample at a specific station costs about the Intermittent and ephemeral streams usually have same amount of money, but the amount of additional hydrograph traces in which the stage goes from a base information obtained often decreases with each flow or zero flow to the maximum stage in a matter of succeeding sample after the first few samples are a few minutes or hours, and the person responsiblefor taken. Sometimes samples obtained on a monthly obtaining the samplesfrequently does not know when basis yield more information for the money than those such an event is to occur. A sampling scheme should from a daily station, although there is a danger that too be designedto define the sediment dischargeby taking little information may be of no value or may even be samples during the rising stage, then the peak stage misleading. For a given kind of record, the optimum and the recession.Generally, adequatecoverage of the number of samples should be a balance between the peak is obtained if sampleson the rising limb are four cost of collecting additional samples and the cost of a times as frequent as samples collected during the less precise record. recession. For example, if the recession is best The frequency of collection of bed-material sampledon a bi-hourly basis, the rising limb should be samplesdepends upon the stability of the streambedat sampledevery one-half hour. the sample site. In many cases,seasonal samples may Elaborate and intensive sampling schedulesare not be adequate to characterize the distribution among required for each and all events on small streams that particles comprising the bed. However, samples drain basins of rather uniform geologic and soil should be obtained whenever possible during high- conditions becausesimilar runoff conditions will yield flow eventsin order to describe the composition of bed similar concentrations of sediment for the different material as compared to its composition during runoff events. Once a concentration pattern is periods of normal or low flow. Particularly important established,samples collected once or twice daily may is the collection of bed-material samples following suffice, even during a storm period (Porterfield, 1972). high flows that have inundated the flood plain and Streamsdraining basins with a wide variety of soils greatly altered the streambedconfiguration. and geologic conditions and receiving uneven distribu- tions of precipitation cannot be adequately sampledby !Sediment Quantity a rigid, predetermined schedule. Sediment concentra- Previous sectionsdiscussed the number of sampling tion in the stream dependsnot only on the time of year, verticals required at a station to obtain a reliable but also on the source of the runoff in the basin. Thus, sediment-dischargemeasurement or a sample of the each storm or changing flow event should be covered cross-sectional concentration. The number of cross- as thoroughly as possible, in a manner similar to that sectional samplesrequired to define the mean concen- described for intermittent and ephemeral streams. tration within specific limits also has been discussed. The accuracy needed in the sediment information The requirements in terms of quantity of sediment for also dictates how often a stream should be sampled. use in the laboratory to determine particle-size The greater the required accuracy and the more gradation may at times exceed the other requirements complicated the flow system, the more frequently it for concentration. The size range and quantity of will be necessaryto obtain samples. This increase in sediment needed for the several kinds of sediment sampling frequency-with the added costs of labora- analyses in the laboratory are given in table 3. The tory analysis-greatly increasesthe cost of obtaining desirable minimum quantity of sediment for exchange the desired sediment information. Often, however, the capacity and mineralogical analyses is based on the record may actually cost less when adequatesamples requirements for radioactive cesium techniques are collected than when correlation and other synthetic describedby Beetem and others (1962). means must be used to compute segmentsof a record To estimate visually the quantity of sediment becauseof inadequatesampling. entrained in a sample or series of sample bottles SEDIMENT-SAMPLING TECHNIQUES 63 Table 3.
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