Laserfiche WebLink
<br />of boulder-filled streams by Herbich and Shul its (1964) and Richards <br /> <br />(1973) indicated that there is a spill-resistance component with in- <br /> <br />creasing flow. Spill resistance is a result of increased turbulence or <br /> <br />roughness resulting f,.om the velocity of water striking the large area <br /> <br />of protruding bed roughness elements and eddy currents set up behind the <br /> <br />larger boulders. Aldridge and Garrett (1973) believe that the effect of <br /> <br />the disturbance of the water surrounding boulders and obstructions <br /> <br />increases with veloci:y and may overlap with nearby obstruction disturb- <br /> <br />ances and further increase turbulence and hence roughness. Bathurst <br /> <br />(1978) noted supercri:ical flow over boulders in natural channels and <br /> <br />hydraul ic jumps occurring just downstream, but these were very 1 imited <br /> <br />in areal extent. Very local ized areas of supercritical flow were ob- <br /> <br />served during the collection of data for this study where flow went over <br /> <br />" <br />m:. .. <br />) <br />,.. ..., <br /> <br />boulders. The relation of Froude number to friction slope for t~e d~ta <br /> <br />in table 1 is shown i" figure 8. <br /> <br />The Froude number is computed as: <br /> <br />F=~ <br />j gO ' <br /> <br />where F is the Froude number and the other variables are as previously <br /> <br />( 13) <br /> <br />The Froude number computed above is an index of the average cross- <br /> <br /> <br />defined. Froude numbers less than 1, or. subcritical f~ow. occurred at <br /> <br />al I sites. There also does not appear to be any tendency for Froude <br /> <br />numbers to increase with friction slope. <br /> <br />section flow conditions. Froude numbers can also be determined for <br /> <br />individual subareas of the cross section at which the discharge measure- <br /> <br />ment was made. Both the Froude number computed from hydraulic proper- <br /> <br />ties of the cross section and the Froude numbers computed from the <br /> <br />individual subareas of the discharge measurement section were much less <br /> <br />than unity, or subcritical. <br /> <br />'/1 <br />