A12. Sediment sampling

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Figure 1: Gravel bed

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1. Purpose and applicability[edit]

Sediment sampling is primarily used to:

  • To ascertain the prevailing sedimentary process (e.g. erosion, transport, deposition) within a system and its current and future conditions in order to inform the development of restoration designs.
  • To access restored channel forms/systems, post restoration.
  • Changes in hydrology impact on channel forms
  • Impact of changes in sediment characteristics on habitats (veg smothering eg.)
  • Sediment transport dynamics
  • Movement of nutrients and contaminants associated with fine (cohesive) sediments

2. Method summary[edit]

Sampling before and after a restoration project, or setting up a monitoring scheme to continue to sample over time, will provide a temporal observation of how the channel sediment is changing.

  • Characterising the properties of sediments and the prevailing sedimentary processes can be technically challenging and time consuming
  • Differing levels of complexity
  • At a minimum, a qualitative assessment of the size distribution of sediments and sedimentary process should be undertaken rapidly in a day by 2 people.
  • At the other end of the spectrum more thorough studies entail detailed sampling and analysis may be required in order to understand long term trends
  • Long-term data can be used to formulate sediment rating curves for numerical simulations and, in some cases, physical modelling
  • This is undertaken over two or more years with multiple site visits
  • Forethought is required to ensure the correct methods are applied to answer the research objectives to avoid time/fund wasting

3. Sediment Characterisation[edit]

Several sediment characteristics are of interest for monitoring purposes, including clast size, shape, settling velocity and mineralogy.

Fine sediments

  • Qualitative assessment including visual inspections.
  • Quantitative analysis requires the collection of sediment samples for laboratory analysis.
  • Laser diffraction particle size analyser traditionally used to determine size range of suspended particles in a solution
  • Data from laser diffraction not accurate for modelling transport, since fine suspended materials often found in an aggregated state in suspension
  • Suggest use of LISST, video imaging

For sand-bed rivers, samplers which utilise a pressure difference to capture and retain sediment samples are more efficient. These samples exert a different pressure at the entrance to the trap, and the exit, to allow sediments to be retained, and water to continue to pass through (Peters, 2003).

Stacked sieves are an efficient method for analysing gravel-pebble samples collected in the field. Sieve sampling uses a sieving motion to separate sediment into size categories using stacked sieves in descending size order. Sediment is grouped as it moves down the sieve stack.

A variety of bed load samplers are available, although not all are suitable for every river site (Singhal et al. 1981). Bed sediment samples can be collected using samplers placed on the river bed to capture any rolling substrate. Box or basket samplers are more suited to mountainous coarse-bed streams. These sediments can be collected and recorded bank-side, or analysed back in a laboratory.

For coarser material such as pebbles and gravels, random sediment sampling can be carried out in the field using a gravelometer for the Wolman pebble count procedure. This method involves pacing across the width of the channel, in 10 even paces. At each pace, use your index finger to randomly select a pebble from the riverbed. Using the gravelometer, record the smallest size-graded hole that the pebble will fit through. Randomly survey 10 pebbles at each pace. This will provide a dataset of 100 records for each channel cross section, demonstrating the range of sediment sizes at this site.

4. Site selection[edit]

Selecting a suitable site. The site should satisfy the following criteria (Singhal et al. 1981):

  1. It should be in a straight reach of length at least 4 times the width of the channel, but not less than 150 m.
  2. The chosen reach should be stable, i.e. neither silting nor scouring.
  3. A normal section should be located in the middle of the selected reach.
  4. It should not be adjacent to hydraulic structures.
  5. It should be accessible, and preferably located near a village or town.

5. Selecting a suitable sampler[edit]

Figure 2: Sand bed

This will depend on the type of channel sediment. A sampler should fulfil the following requirements:

  1. It should be streamlined to avoid disturbance of the flow.
  2. The velocity of inflow at the mouth of the sampler should be equal to the velocity of the stream.
  3. The mouth of the sampler should always face the direction of the current.
  4. The mouth should be outside the zone of flow disturbance set up by the body of the sampler, and filling should be smooth without sudden inflow or gulping.
  5. It should be able to collect samples at desired depths.
  6. should require minimum care, maintenance, and repair.

For any method of sampling, repeat monitoring should be carried out after river restoration to determine how the hydromorphological features have altered and how the channel substrate has been impacted by the intervention.

6. Advantages[edit]

  • Sediment sampling can be carried out for fine and coarse material
  • Some equipment can be cheap, simple & reusable
  • Sediment dependent, samples can be recorded in the field, reducing laboratory time and allowing more time for analysis
  • In some cases low labour needed – 2 people (1 to carry out collection, 1 to record values)
  • Lots of data can be collected, allowing for representability of a site, and comparability spatially & temporally

7. Limitations[edit]

  • Random selection can be accidently bias – human error – for example using the Wolman pebble count, we naturally select larger gravels on the top of the bed surface. Larger cobbles can sometimes overlay smaller gravels beneath.
  • Bypassing the first large cobble which is touched, & searching for smaller gravels in order to demonstrate the variety of sediment on the river bed, introduces significant bias into the technique

8. Recommendation for method application[edit]

  • Think about study comparison with other sites/years

9. Costs[edit]

Approximately £60 (Source: https://www.coleparmer.co.uk/p/gravelometer/58940)

10. References[edit]

  1. Wolman Pebble Count Procedure - https://andrewsforest.oregonstate.edu/sites/default/files/lter/data/studies/gs002/Wolman_Pebble_Count.pdf
  2. Craig Fischenich et al. 2007. Sediment Sampling and Analysis for Stream Restoration Projects
  3. Galia, T., Škarpich, V., Gajdošová, K. & Krpec, P. (2017) ‘Variability of Wolman pebble samples in gravel/cobble bed streams’, Acta. Sci. Pol. Formatio Circumiectus, 16(1), pp 237-246. Available at: http://www.formatiocircumiectus.actapol.net/pub/16_1_237.pdf
  4. Kondolf, G. M., Lisle, T. E., and Wolman, G. M. (2003) Bed sediment measurement, in Kondolf, G. M., and Piegay, H. (eds.), Tools in fluvial geomorphology, John Wiley and Sons Ltd., Chichester, UK, 347-395.
  5. Peters, J. J. (2003) ‘Sediment sampling in sand-bed rivers: methods and instruments’, AGORA: Hydraulic Instrumentation, IAHR Congress, pp 41-48
  6. Saud Afzal, M. (2013) ‘3D Numerical Modelling of Sediment Transport under Current and Waves’. Available at: https://www.researchgate.net/publication/263208172_3D_Numerical_Modelling_of_Sediment_Transport_under_Current_and_Waves
  7. Singhal, H. S. S., Joshi, G. C. & Verma, R. S. (1981) ‘Sediment sampling in rivers and canals’, Erosion and Sediment Transport Measurement (Proceedings of the Florence Symposium, June 1981), IAHS Publ. no. 133. Available at: https://iahs.info/uploads/dms/iahs_133_0169.pdf