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OR <br />Figure 22. Stream reach with large, irregularly spaced bed elements. <br />Photo courtesy of Don Kelly, aquatic biologist, Sacramento, <br />CA. <br />entire cross section. However, when attempting to calibrate the model <br />to measured velocity distributions, it becomes difficult to control the <br />water surface elevation at the measured level. Thus, for boulder strewn <br />rivers, Manning's equation may be calibrated to the water surface <br />elevation or to the velocity distribution. Calibration to both parameters <br />simultaneously proves to be quite difficult. Again, in using the rating <br />curve approach, this problem is not hard to overcome. <br />Channel Stability <br />Changes in channel configuration occur regularly in nature in <br />response to changes in flow regime,- sediment yield, and-chance events <br />such as large storms or runoff from unusually deep snowpacfc_ Short- <br />term and nonpersistent channel changes are termed scour and fill. <br />Scour and fill occur with some regularity in all streams at one <br />time or another. The primary factor influencing the technique selected <br />for hydraulic simulation is the periodicity with which scour and fill <br />occurs. This phenomenon is most active and the periodicity shortest in <br />alluvial streams with sandy beds. Most alluvial streams experience at <br />least one cycle of scour and fill each year, resulting in a rating curve <br />1ocp (=+gore 3). As long as the field measurements do not overlao t^2 <br />rising and falling limbs of the hydrocr_ph, the rating curve approach <br />works well in this type of stream. However, in streams which exhibit <br />J ?