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<br />Table 2. Hydraulic properties and channel geometry of streams as a function <br />of channel slope (adapted from table 1 in volume 1 of this report). <br /> <br />Average value of variable <br />for sample (N varies <br />from 44 to 763) <br /> <br /> Water-surface slope (ft/ft) <br /><0.001 >0.001-<0.005 >0.005 All slopes <br />19.77 114.8 116.7 16.7 <br />3.97 4.94 7.4 4.4 <br />150.4 123.4 119.2 10.3 <br />14..8 4.1 4.1 6.9 <br />0.22 0.45 .0.68 0.36 <br />1.56 1.62 1.71 1.61 <br />1.61 1.68 1. 73 1.55 <br />19.4 27.8 19.5 19.8 <br />0.979 1.03 0.965 0.975 <br /> <br />Maximum point velocity, V (ft/s) <br /> m <br />Average velocity, V (ft/s) <br />a <br />Maximum depth, d (ft) <br /> m <br />Average depth, d (ft) <br /> a <br />Froude number, F <br /> <br />V /V <br />m a <br />d /d <br />. m a <br />T/d (T=top width) <br />m <br />R/d (R=hydraulic radius) <br />a <br /> <br />1Maximum value for sample. <br /> <br />Application of the hydraulic factors in equations 8 and 9 indicates that <br />accurate estimates of Manning's n and velocity are needed. The hydraulic radius <br />or mean depth may be defined by measuring the cross sectio~ <br /> <br />There are difficulties in applying the concepts of permissible velocity or <br />shear stress to determine the rip rap material required to resist erosion. Per- <br />missible velocities (such as given in figure 2) and shear stresses are usually <br />expressed as mean values for th~ cross section. Estimates of shear stress based <br />on gradient are not considered reliable because in localized areas of turbu- <br />lence, the gradient may be negative, and at channel banks, the gradient along <br />each bank may be dissimilar. ~problems in analyzing. boundary stresses based <br />on shear stress are discussed in detail in later sections of the report. The <br />actual point values that effectively contribute to erosion of the bank material <br />are difficult to determine awl are es.ti-~"..rl fJ:Dm J:.el.atblnsltips established <br />using laboratory data. These data are then extended to accommodate the. magni- <br />tude of hydraulic conditions that occur in the field. <br /> <br />CHARACTERISTICS OF RIPRAP FAILURE <br /> <br />Inadequate recognition of the type of erosion process that is occurring or <br />improper riprap design may lead to failure of the riprap, as shown in figure 1. <br />Types of erosion that can be successfully controlled by riprap include channel <br />degradation, bank erosion, scour, and changes in alinement associated with <br />meandering, branching, and braiding of streams. The rate of channel erosion <br />varies with time, but is primarily a function of the magnitude of streamflow. <br />Other factors that affect channel erosion are stream control works, sand and <br />gravel pit operations, and land-use developments. <br /> <br />9 <br />