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7/14/2009 5:02:31 PM
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UCREFRP
UCREFRP Catalog Number
7930
Author
Ketcheson, G. L.
Title
Sediment Rating Equations
USFW Year
1986.
USFW - Doc Type
An Evaluation for Streams in the Idaho Batholith.
Copyright Material
NO
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were supply-limited prior to management, the <br />introduction of accelerated sediment could produce a <br />more continuous supply of sediment and actually <br />improve the sediment-discharge relationship. <br />Channel sediment storage may impact the quality <br />of the sediment-discharge relationships because stor- <br />age was not accounted for in the equations. Van <br />Sickle and Beschta (1983) discussed the importance <br />of channel bed forms and large organic debris as <br />storage sites for both suspended and bedload sedi- <br />ment. Noting this, they proposed a supply-based <br />model for small streams that uses the suspended <br />sediment rating curve and a supply depletion func- <br />tion. The supply function expresses a declining sus- <br />pended sediment concentration to reflect storm hys- <br />teresis and seasonal decline of sediment supply. <br />Megahan (1982) found an average of 15 times more <br />sediment stored behind obstructions in channels <br />than was delivered to the mouths of the drainages. <br />This suggests that sediment movement is dependent <br />on the dynamics of channel storage components and <br />not just streamflow. It is likely that fluctuations in <br />these storage components during high flows limit <br />the effectiveness of the bedload sediment rating <br />curves. <br />A more appropriate bedload sediment rating equa- <br />tion should include a constant to define at what <br />streamflow bedload sediment movement is initiated. <br />This plus the introduction of a sediment storage fac- <br />tor might significantly increase the success rate of <br />bedload sediment rating equations. <br />As mentioned, automatic pumping samplers were <br />used in Silver Creek in addition to the hand sam- <br />ples. Sediment rating equations were developed for <br />these data as well. Six streams with 4 years of rec- <br />ord resulted in 23 rating equations. A total of 74 <br />percent or 17 of the equations were significant; only <br />five equations had an R2 greater than 0.60. The <br />large number of automatic samples made it possible <br />to divide the samples into groups. This was done for <br />rising and falling limbs of the hydrograph around <br />the annual snowmelt peak, for all peaks, and on a <br />seasonal basis. The seasons used were fall (September <br />to mid-November), winter (mid-November to mid- <br />March), spring (mid-March through May), and sum- <br />mer (June through August). These divisions will not <br />be discussed in detail. However, the success rate for <br />these rating equations was: of 278 equations, 62 per- <br />cent were significant; 24 percent had an R2 greater <br />than 0.60. <br />Figure 2 is an illustration of the equations from <br />grouping the data by rising and falling limbs for <br />each peak over a year. The lines labeled 4R and 4F <br />are the rising and falling limb rating curves, respec- <br />tively, for the snowmelt peak. Curve IF is a falling <br />limb at the beginning of the water year and is not <br />statistically significant at the 95 percent level of <br />confidence. Curves 2R and 2F represent an early <br />winter rain, 3R and 3F represent a February thaw, <br />and 511 and 5F result from spring rains. <br />3.5 SC-1 MANNING, 1982 <br />3.0 <br />2.5 a? >? <br />J Q <br />rn 2.0 h <br />E 3a 5F <br />3f <br />0 1.5 <br />w <br />N a ti? <br />O 1.0 tiQ <br />J <br />0.5 <br />0.0 <br />-0.5 <br />-1.0 0.0 1.0 <br />LOG 0, ft3/s <br />Figure 2-Suspended sediment rating curves <br />based on individual rising and falling limbs <br />of an annual hydrograph. R = rising; F = <br />falling; numerals indicate chronological order <br />of the hydrograph peaks. The extent of the <br />curves indicates the limits of the data. <br />Documenting Management Effects <br />Previous analyses have indicated that floods and <br />management activities cause shifts in suspended <br />sediment rating equations (Flaxman 1975; Rosgen <br />1980). In the handbook, "An Approach to Water <br />Resources Evaluation of Non-point Silvicultural <br />Sources," Rosgen (1980) presented one such analysis <br />for the Needle Branch watershed in Oregon. Rating <br />curves shifted following the 1964 flood and subse- <br />quent clearcutting operations. No statistical informa- <br />tion was presented regarding the significance of the <br />observed changes in the rating equations. In the fol- <br />lowing examples from the Idaho batholith, the <br />influence of management on both suspended and <br />bedload sediment rating equations will be discussed <br />in terms of statistical significance. <br />In Silver Creek, 38 ha were harvested on south <br />aspects in watershed SC-6 (163 ha). The units were <br />clearcut and yarded by helicopter in the fall of 1976. <br />The following year was extremely dry and no <br />instream sediment was sampled, so 1978 is the first <br />posttreatment year for which sediment rating equa- <br />tions were developed. The suspended sediment rat- <br />ing equations (fig. 3a) indicate that a shift to a <br />steeper slope may have occurred between 1976 and <br />1978. However, the two curves are not statistically <br />different at the 95 percent level of confidence. No <br />statistical change in sediment yield occurred at the <br />sediment dam following the timber harvest. <br />The only curve in figure 3a that is statistically <br />different from any of the others is the 1982 curve. It <br />differs from both the 1975 and 1980 curves in slope
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