Technical Guidelines for Sampling Bed Material Supplement 13A (210–VI–NEH, August 2007) Technical Supplement 13A Guidelines for Sampling Bed Material Part 654 National Engineering Handbook Issued August 2007 Cover photos: Top—Bed-load sediment may be poorly graded or well graded. Armoring layers may also be present. Advisory Note Techniques and approaches contained in this handbook are not all-inclusive, nor universally applicable. Designing stream restorations requires appropriate training and experience, especially to identify conditions where various approaches, tools, and techniques are most applicable, as well as their limitations for design. Note also that prod- uct names are included only to show type and availability and do not constitute endorsement for their specific use. (210–VI–NEH, August 2007) Technical Guidelines for Sampling Bed Material Supplement 13A Contents Purpose of bed-material sampling TS13A–1 Site selection to representative sampling TS13A–1 Sand-bed streams TS13A–2 Cross-sectional approach .............................................................................TS13A–2 Reach approach .............................................................................................TS13A–3 Gravel-bed streams TS13A–3 Surface sediment sampling ...........................................................................TS13A–6 Sediment intrusion into spawning gravels ..................................................TS13A–8 Selection of a sampling procedure TS13A–8 Step-by-step field sampling procedures TS13A–9 Other bed-material characteristics TS13A–10 Bank material TS13A–10 Conclusion TS13A–10 Tables Table TS13A–1 Bed-material sampling sites TS13A–2 Figures Figure TS13A–1 Gradation pattern on a point bar TS13A–3 Figure TS13A–2 Bed sampling locations for sand-bed TS13A–4 streams Figure TS13A–3 Gravel-bed sediment profile showing vertical TS13A–5 variation Figure TS13A–4 Coarse-bed stream sampling hierarchy TS13A–5 Figure TS13A–5 Gravelometer held above stream TS13A–6 Figure TS13A–6 Piston sampler TS13A–7 (210–VI–NEH, August 2007) TS13A–i Technical Guidelines for Sampling Bed Material Supplement 13A Purpose of bed-material transport calculations, critical shear stress de- terminations, determining potential for particle sampling sorting and armoring, and determining hydrau- lic roughness. The characteristics of a given stream are linked to the composition of the material that comprises its channel Complex studies may need to secure data to meet a bed, bank, and sediment flow. Knowledge of stream- combination of objectives and purposes. However, bed material is necessary for a variety of engineering sediment data collected for one purpose will not nec- and environmental purposes. The size and gradation essarily be applicable for another. While the issues and of the streambed material may affect the source, recommendations presented here are generally appli- transport, and fate of pollutants; fish habitat; resource cable, the focus is on bed sampling for stream assess- management; morphological trends; and stream resto- ment and design. rations. Bed-material sampling programs must be carefully Site selection for representative designed to meet the particular needs of a specific study. Studies may include objectives related to the sampling following: • Contaminants—Typically attach to cohesive Sufficient sampling of the streambed should be con- sediment and, therefore, are distributed over a ducted to determine the spatial variability, size, and wide area, especially in areas where flow veloc- gradation of the bed material. No simple rule exists ity is low. Sampling for a contaminant concen- for locating representative sampling sites or reaches. trates on depositional zones in the stream and The general rule is to carefully select sampling loca- overbank. tions and avoid anomalies that would bias either the calculated sediment discharge or the calculated bed • Aquatic habitat—Fish habitat studies may stability. Sampling locations must be representative of focus on the suitability of the streambed for the hydraulic and sedimentation processes that occur spawning. Sampling for this type of study is in that reach of the river. The site should be morpho- often extensive, identifying lateral, longitudi- logically stable. To ensure data reflect reach-averaged nal, and temporal variations in the surface layer river conditions, there should be no tributary inflow in over a wide area of the stream. An assessment the proximity of the site, as it may interfere with the of vertical variations may also be of critical homogeneity of the section by supplying sediment for importance, as the composition of the mate- deposition. The site should not be located adjacent to rial immediately below the surface, especially a zone of active bank erosion, as the material depos- the fines content, may be of importance in the ited in the channel near the eroding area may not be evaluation of spawning habitats for some spe- representative of the reach. Although bridges provide cies. good access, bridge crossings are typically not appro- • Gravel mining—Resource management stud- priate sampling sites because either they are located ies are frequently concerned with the need or at natural river constrictions or their abutments and feasibility of sand and gravel mining. Core or piers create constrictions and local scour. Dead-water substrate sampling that identifies vertical varia- areas behind sand bars or other obstructions should tion of the streambed is essential for this type be avoided, as these are not representative of average of study. flow conditions. • Stream assessment and design—Morphologic The location of the bed sample should be chosen with and engineering studies are concerned with the target analysis in mind. Table TS13A–1 provides changes in the character of the river over time. guidance for where a bed-material sample might be These studies require knowledge of the grain taken as a function of the type of geomorphologic or size distribution of both the bed surface ma- engineering analysis to be conducted. This list is not terial and subsurface material for sediment inclusive, exhaustive, or absolute. Ideally, bed-material (210–VI–NEH, August 2007) TS13A–1 Technical Supplement 13A Guidelines for Sampling Bed Material Part 654 National Engineering Handbook samples should be taken at different times during the Representative bed-material sampling in sand-bed year to account for seasonal variations. streams may be accomplished by one of two meth- ods. Employing the cross-sectional approach requires selecting a site and time for sampling where and when Sand-bed streams the bed characteristics are typical. This method re- quires considerable experience. Unanimity of opinion about where and when the typical condition occurs Sand-bed streams have relatively homogeneous bed- cannot be expected, even among experienced river sci- material gradation. Vertical and temporal variability entists. Frequently, judgment is influenced by the type are normally insignificant in stable sand-bed streams. of streams the sampler has experienced and by the in- Longitudinal variability typically occurs over distances tended use of the data. Employing the reach approach, of many kilometers. However, lateral variability, where samples from several systematically selected especially in bends, can be significant. In sand-bed cross sections are averaged to obtain a representative rivers, sampling of bed material is most frequently sample, may eliminate some uncertainty associated done in the low-flow channel. The sampling equip- with the cross-sectional approach. ment and methodology used depend on the river depth and velocity. The task can be accomplished in flowing streams either by wading or from a boat or in Cross-sectional approach ephemeral and intermittent streams in the dry. Vertical variations in the bed material are usually insignificant This approach requires the selection of a representa- in flowing water, and samples are collected from the tive cross section for a reach. In streams with rela- surface. However, in standing water or on dry beds, a tively uniform depths, between three and five samples layer of fine material is sometimes found deposited on should be taken across the section to account for the bed surface during the recessional part of a flood lateral variations. In streams with variable depths, hydrograph. It is standard practice to remove this fine more samples are required. Twenty verticals are surface layer before collecting a bed-material sample commonly taken along the cross section in braided in this location. streams. Taking bed-material samples at crossings where flow distribution is more uniform reduces the Einstein (1950) recommended using only the coarsest lateral variation in the samples. However, at low flow, 90 percent of the sampled bed gradation for computa- crossings may develop a surface layer gradation that tions of bed-material load. He reasoned that the finest reflects sediment transport conditions at the lower 10 percent of sediment on the bed was either material discharge, which may be coarser or finer than the bed trapped in the interstices of the deposit or a lag depos- gradation at bankfull discharge. Also, crossings are it from the recession of the hydrograph and should not typically submerged, and more elaborate sampling be included in bed-material load computations. equipment is required than at exposed bars, where a Table TS13A–1 Bed-material sampling sites Purpose of analysis Sample location