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Water: WARSSS

Calculate Sediment Yield for Baseline Existing Condition: Flow Related Sediment Yield


This is accomplished by taking the converted dimensionless ratio flow-duration curve and multiplying increments/duration of time in days by the sediment yield associated with that flow. The computations for total annual suspended and bedload sediment yield are shown in Worksheet 24 (xls).

Procedural steps for computations of the FLOWSED model are presented in Table 19.

Table 19. FLOWSED Model. To calculate annual suspended sediment and bedload sediment yield for Streamflows under hydrologically restored, existing, and/ or proposed conditions.

Procedural Steps:
  1. Measure stream cross-section (on riffle), profile, pattern and materials.
  2. Measure bankfull width, mean depth, velocity and compute discharge.
  3. Measure suspended sediment at the bankfull stage, separate wash load in lab.
  4. Measure bedload sediment at the bankfull stage, sieve particle sizes, largest size.
  5. Compute average water surface slope.
  6. Collect point bar sample, weigh by size fraction, record (D50), largest size (Di).
  7. Collect pebble count on active riffle bed: obtain D50, D84 sizes (mm).
  8. Determine stream type.
  9. Conduct channel stability assessment procedure, including Pfankuch Channel Stability Rating.
  10. Obtain reference dimensionless bedload sediment rating curve for appropriate stream type/stability rating.
  11. Obtain reference dimensionless suspended sediment rating curve for appropriate stream type/stability rating.
  12. Determine ratio of washload/suspended sediment by Q/Qbkf relation.
  13. Construct a bedload rating curve (enter range of Q/Qbkf ratios into the reference bedload relation from step 10 and multiply by the measured bankfull bedload (step 4)).
  14. Construct suspended sediment rating curve in the same manner as (step 13), using reference dimensionless sediment relation, (step 11), and bankfull suspended sediment (step 3).
  15. Construct a suspended sediment rating curve less wash load (silt/clay) for potential settable sediment by multiplying ratio of wash load/suspended sediment for appropriate Q/Qbkf.
  16. Convert suspended sediment less wash load from mg/l to tons/day on rating curve, tons/day = 0.0027* cfs* mg/l.
  17. Convert suspended sediment less wash load from mg/l to tons/day as in step 16.
  18. Convert bedload in lbs/sec to tons/day, where: tons/day = (lbs*86,400)/2000 (if metric, convert kg/s to lbs/sec by multiplying by 2.205).
  19. Obtain dimensionless flow-duration curve from either water yield model or regionalized relation.
  20. Convert dimensionless flow-duration curve to actual flow by multiplying bankfull discharge (step 2) times the Q/Qbkf ratios from dimensionless flow-duration curve (step 19).
  21. Calculate total annual sediment yield for suspended sediment, suspended sediment less wash load, and bedload from sediment rating curve/flow duration curve procedure. Obtain flow from water yield model for hydraulically recovered condition to compare departure from existing/proposed condition (step 22). This represents the pre-treatment flow duration/sediment relation.
  22. To determine flow related increase in sediment, multiply post-treatment flow duration curve times appropriate sediment rating curves for suspended, bedload and total sediment rating curves to calculate total annual sediment yield using same procedure as (step 21).

FLOWSED is a model developed for the WARSSS objectives. It predicts annual suspended sediment and bedload sediment yield in tons/year. It utilizes both dimensionless suspended and bedload sediment rating curves by stream type/stability and regionalized dimensionless flow-duration curves. The suspended sediment dimensionless sediment rating curve used for FLOWSED is shown in Figure 56. The dimensionless bedload rating curve is shown in Figure 57 for "good" and "fair" stability stream types. An expanded data set involving measured suspended and bedload sediment rating curves were used to test significance between "poor" stability stream types from "good" and "fair" (Troendle, et al, 2001). Validation tests using one data point for bankfull discharge, bankfull suspended sediment and bankfull bedload sediment were used to predict sediment rating curves on independent data sets of the U.S.Geological Survey data in Wyoming, Idaho, Nevada, and Colorado. The resultant relationships depicting predicted versus observed values are shown in in Figure 58 (PDF, 75 kb, 1 p.), Figure 59 (PDF, 54 kb, 1 p.), Figure 60 (PDF, 59 kb, 1 p.) and Figure 61 (PDF, 50 kb, 1 p.). These predictions were made using the Pagosa Springs, Colorado data set in Figure 56 and Figure 57.

The prediction of total annual sediment yield is accomplished by measuring the bankfull values of discharge (CFS), suspended sediment (mg/l) and bedload sediment (kg/s), then converting the dimensionless relation to dimensioned values in order to obtain the rating curves.

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