The calculations have been described in the Introduction to Sediment & River Stability section, however a field procedure for bar sampling, pavement/sub-pavement sampling, and wet sieving on-site is presented in Table 21 (below). The user is advised to review additional detailed work of particle size sampling by Bunte and Abt, (2001). Bar samples are field sieved using the procedure shown in Figure 125 (PDF, 732 kb, 1 p.) and recorded in Worksheet 26 (xls).
The entrainment computations are completed and summarized in Worksheet 27 (PDF, 33 kb, 1 p.). This method is a very good tool and has shown consistency comparing actual bedload/scour chain data verification with predicted values.
If the protrusion ratios as described in Equation II-7 or Equation II-8 are outside the range for both the relations as indicated in Worksheet 27, the user is advised to utilize the shear stress equation (Equation II-2) and apply it with a revised Shield's relation using Colorado data (or local data as available) (Figure 126 (PDF, 28 kb, 1 p.)). A grain size is selected corresponding with the shear stress to determine sizes that the river can potentially move. Based on measured bedload sizes in a heterogeneous mixture of bed material comprised of gravel and cobble, the previous published Shields relation underestimates particle sizes in the shear stress range of 0.1 lbs/ft² to 1.0 lbs/ft².
In order to determine the ability of the existing stream reach to transport the largest clast size of the bedload sediment, it is necessary to calculate the critical dimensionless shear stress (). This calculation determines depth and slope necessary to mobilize and transport the largest particle made available to the channel. The dimensionless shear stress at bankfull stage is used as the assessment basis with a similar entrainment analysis of the reference, stable condition for comparison to the potentially disturbed reach. To maintain stability, a stream must be competent to transport the largest size of sediment, as well as have the capacity to transport the load (volume). These calculations provide for a prediction of sediment competence. The procedure for the bar and pavement/sub-pavement sampling is presented in Table 21.
|Table 21. Field procedures for Bar and pavement, sub-pavement samples.
Bar Sample: Collect sediment core samples from point bars along the project and reference reaches. At least one sample should be collected from each reach associated with a change in stream type. Conduct a critical shear stress analysis using the following procedures:
- Locate a sampling point on the downstream one-third of a meander bend. The sample location on the point bar is halfway between the thalweg elevation (the point of maximum depth) and the bankfull stage elevation. Scan the point bar in this area to determine the sampling location by observing the maximum particles that are on the surface of the bar.
- Place the five-gallon bottomless bucket at the sampling location over one of the representative larger particles that are observed on the lower one-third of the point bar. Remove the two largest particles from the surface covered by the bottomless bucket. Measure the intermediate axis for each particle and individually weigh the particles. Record these values. The largest particle obtained is Di, the largest particle from the bar sample. Push the bottomless bucket into the bar material. Excavate the materials from the bottomless bucket to a depth that is equal to twice the intermediate axis width of the largest surface particle. Place these materials in a bucket or bag for sieving and weighing.
- For fine bar materials: Follow the directions above, except that when the bottomless bucket is pushed into the bar material, excavate materials from the bucket to a depth of 4 to 6 inches. Place these materials in a bucket or bag for sieving and weighing.
- Wet-sieve the collected bar materials, using water and a standard sieve set with a 2-millimeter screen size for the bottom sieve. Weigh bucket with sand after draining off as much water as possible. Subtract tare weight of bucket to obtain the net weight of the sand.
- Weigh materials sieved and record weights (less tare weight) by size class. Be sure to include the intermediate axis measurements and individual weights of the two largest particles that were collected.
- Determine a material size class distribution for all of the collected materials. The data represents the range of channel materials subject to movement or transport as bedload sediment materials at bankfull discharge.
- Plot data; determine size-class indices, i.e., D16, D35, D50, D84, D95. The D100 should represent the actual intermediate axis width and weight (not the tray size) when plotted. The largest size measured will be plotted at the D100 point. (Note: D100 = Di). The intermediate axis measurement of the second largest particle will be the top end of the catch range for the last sieve that retains material (use procedure in Figure 125 and record in Worksheet 26).
- Survey a typical cross section of a riffle reach at a location where the stream is free to adjust its boundaries. Plot the survey data. Determine the hydraulic radius of the cross-section.
- Conduct a Wolman Pebble Count (100 count in riffle) of the bed material in the coarsest portion of the wetted riffle area. The pebble count should be conducted at multiple transects that represent the riffle. Plot data and determine the size-class indices.
Pavement/sub-pavement sample. (Alternate procedures for obtaining a pavement/sub-pavement sample when you are unable to collect a bar sample):
- Locate a sampling point in the same riffle where cross-section survey was conducted. The sampling point should be to the left or right of the thalweg, not in the thalweg, in a coarse grain size portion of the riffle.
- Push a bottomless bucket into the riffle at the sampling location to cutoff the streamflow. The diameter of the bucket (sample size) should be at least twice the diameter of the largest rock on the bed of the riffle.
- Remove the pavement material (surface layer only) by removing the smallest to the coarsest particles. Measure the intermediate axis and weight of the largest and second largest particles. Record these values. Place the remaining pavement materials into a bucket or bag for sieving and weighing.
- Remove the sub-pavement material to a depth that is equal to twice the intermediate axis width of the largest particle in the pavement layer, or at least 150mm depth.
Caution: If a coarser bed material persists under the sub-pavement, it generally is material remnant of the previous bed. Stop at this condition and do not excavate deeper, even if the depth is not at twice the maximum pavement particle diameter. This residual layer is generally not associated with the size distribution of bedload transported at the bankfull stage. Collect the sub-pavement materials into a separate bucket or a bag. Measure the intermediate axis and weight of the two largest particles in the sub-pavement sample. Record these values. Sieve and weigh the remaining sub-pavement materials. The sub-pavement sample is the equivalent of the bar sample; therefore, you use the largest particle from the sub-pavement sample in lieu of the largest particle from a bar sample in the entrainment calculations. Note: If the largest particle collected from the sub-pavement is larger than the pavement layer, the largest rock should be discarded from the sub-pavement layer. Drop back to the next largest particle size to determine the largest particle size to be used in the entrainment calculation.
- Wet-sieve the collected pavement materials and then the sub-pavement materials, using water and a standard sieve set with a 2-millimeter screen size for the bottom sieve. Weigh bucket with sand after draining off as much water as possible. Subtract tare weight of bucket to obtain the net weight of the sand.
- Weigh materials sieved and record weights (less tare weight) by size class for both the pavement and sub-pavement samples. Be sure to include the mean intermediate axis width and individual net weights of the two largest particles that were collected (Worksheet 26). g). Determine a material size class distribution for the materials. The sub-pavement data represents the range of channel materials subject to movement or transport as bedload sediment materials at bankfull discharge.
- Plot data; determine size-class indices, i.e., D16, D35, D50, D84, D95. The D100 should represent the actual intermediate axis width and weight (not the tray size) when plotted. The largest size measured will be plotted at the D100 point. (Note: D100 = Di). The intermediate axis measurement of the second largest particle will be the top end of the catch range for the last sieve that retains material.
- The pavement material size class distribution may be used to determine the D50 of the riffle bed instead of doing the 100 count in the riffle bed.
- Determine the average bankfull slope (approximated by the average water surface slope) for the study reach from the longitudinal profile.
- Calculate the critical dimensionless shear stress required to mobilize and transport the largest particle from the bar sample (or sub-pavement sample). Use the equations and record the data in the form shown in Worksheet 27.
Use the value of the largest particle in the bar sample (or sub-pavement sample), Di, in millimeters and the revised Shields Diagram to predict the shear stress required to initiate movement of the largest particle in the bar and/or sub-pavement (Di).
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