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Water: WARSSS

Bedload Transport (Stream Power)


These calculations use the Bagnold equation (Equation II-13), the POWERSED model (Flowchart 12 (PDF, 25, 1 p.) and Table 20 and Worksheet 25), or a user-specified bedload transport model. The objectives of the POWERSED model are to determine changes in bedload transport due to alteration in the dimension, pattern and profile of the river with corresponding changes in hydraulic relations. This model was also developed for WARSSS in order to predict the effects of channel instability and sediment supply changes in sediment transport. If desired, other bedload transport models can be utilized by the user based on familiarity with and validation of the model for application to the particular stream types being analyzed. If necessary, the user can review the model assumptions, calibration, detailed discussion and associated references presented in the Introduction to Sediment and River Stability section.

flowchart12 worksheet25

Table 20. Prediction of Bedload Transport Changes Due To Alterations of Channel Dimension and/or Slope (Same stream with different bankfull discharges)

  1. Select a reference reach.
    1. Survey a stable cross section, measure the stream gradient, and bed material.
    2. Measure bankfull discharge (cfs).
    3. Measure bankfull bedload (kg/s).
  2. Obtain an appropriate dimensionless bedload rating curve.
    1. Construct a dimensional bedload rating curve for the defined range of flow using the bankfull discharge and bankfull bedload transport.
  3. Obtain the drainage area of the reference reach.
    1. Predict bankfull discharge and cross section dimensions using regional curves.
    2. Validate the regional curves using the measured bankfull discharge and cross section dimensions.
  4. Use RIVERMorph or Dimensionless Hydraulic Geometry by Stream Type to predict the hydraulic geometry of the stable cross section for a full range of discharge (baseflow to above bankfull).
    1. Construct hydraulic geometry curves from the RIVERMorph output.
    2. Check predicted versus measured bankfull velocity.
    3. Obtain hydraulic geometry for each discharge value within the defined range of flow.
    4. Calculate unit stream power for each discharge value within the defined range of flow.
  5. Select an impaired reach on the same stream.
    1. Obtain the drainage area.
    2. Predict bankfull discharge from the validated regional curve.
    3. Survey the cross section, measure the stream gradient, and bed material.
  6. Obtain the stable (potential) dimension, pattern, and profile for the impaired reach:
    1. Slope = valley slope/sinuosity.
    2. Obtain appropriate cross sectional area from regional curve
    3. Obtain width/depth ratio (w/d) from reference dimensionless ratios by stream type.
    4. Calculate appropriate width: .
  7. Use RIVERMorph to predict the hydraulic geometry of the impaired and potential cross sections for a full range of discharge (baseflow to above bankfull). Follow the step below for the impaired and potential cross sections.
    1. Construct hydraulic geometry curves from the RIVERMorph output.
    2. Obtain hydraulic geometry for each discharge value within the defined range of flow.
      * If channel has multiple channels, divide the channels into 1/3's and treat as a separate channel (see RIVERMorph).
    3. Calculate unit stream power for each discharge value within the defined range of flow.
  8. Plot unit stream power vs bedload transport for the stable cross section.
  9. Construct a unit stream power versus bedload transport curve for the impaired and potential cross sections using the relationship constructed in step 8.
  10. Obtain a dimensionless flow duration curve for the appropriate region.
    1. Create a dimensional flow duration curve using the bankfull discharge for the stable reach.
    2. Create a dimensional flow duration curve using the bankfull discharge for the impaired reach.
  11. Calculate total annual sediment yield (tons/year) for all three cross sections using the appropriate flow duration curve.
    1. Convert the predicted bedload transport for each discharge value within the defined range of flow from kg/s to tons/day.
    2. Multiply the predicted bedload transport (tons/day) by the percent time factor from flow duration curve.
    3. Sum the time adjusted bedload transport and multiply by 365 days to obtain annual bedload yield in tons/year.
    4. Divide the annual yield by the drainage area to obtain the annual unit area bedload yield (tons/year/mi²).
    5. Compare the annual unit area bedload yield predicted for all three conditions (stable, impaired, and potential).

The Bagnold relation, FLOWSED, TR-55 and POWERSED models are programmed in RIVERMorph, which applies the suspended and bedload sediment rating curve/flow duration/revised unit stream power-transport curves or a comparable model selected by the user to predict the same outputs. The prediction includes river stability and total annual bedload sediment yield in tons/year. The equations or computer programs will generate a change in coarse bedload transport that will be influenced by change in channel cross-section and/or slope. Also changes in streamflow/velocity/unit stream power/critical dimensionless shear stress, and other variables due to land use changes are used to predict changes in river stability and total annual bedload sediment yield. These changes are compared to stable reference conditions for a departure comparison.

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