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<br />5. LOCATION - Ideally, a short weir should be placed a distance (S) upstream from the <br />location where the midstream tangent flow line (midstream flow line located at the start of <br />the curve) intersects the bankline (PI). Additional bendway weirs are then located based on <br />the site conditions and sound engineering judgment. Typically, the weirs are evenly spaced <br />a distance (S) apart. <br /> <br />6. SPACING - Bendway weir spacing is influence by several site conditions. The following <br />guidance formulas are based on a cursory review of the tests completed by WES on <br />bendway weirs and on tests completed by MRD on underwater sills. Based on the review, <br />bendway weirs should be spaced similarly to hardpoints and spurs. Weir spacing is <br />dependent on the streamflow leaving the weir and its intersection with the downstream <br />structure or bank. Weir spacing (S) is influenced by the length of the weir (L), and the ratios <br />of weir length to channel width (W) and channel radius of curvature (R) to channel width. <br />Spacing can be computed based on the following guidance formulas: <br /> <br />( R )0'8( L )0.3 <br />S=1.5L - - <br />W W <br /> <br />(LaGrone 1995) <br /> <br />S = (4 to 5)L <br /> <br />(Saele 1994) <br /> <br />Maximum Spacing (Smax) is based on the intersection of the tangent flow line with the <br />bankline assuming a simple curve. The maximum spacing is not recommended, but is a <br />reference for designers. In situations where some erosion between weirs can be tolerated, <br />the spacing may be set between the recommended and the maximum. <br /> <br />( 2)0'5 <br />Smax = R 1-(1- ~) <br /> <br />(LaGrone 1995) <br /> <br />Results from the spacing formulas should be investigated to determine that the weir spacing, <br />length, and angle will redirect the flow to the desired location. Streamlines entering and <br />exiting the weirs should be analyzed and drawn in planform. <br /> <br />7. LENGTH OF KEY - Bendway weirs like all bankline protection structures should be keyed <br />into the bankline to prevent flanking by the flow. Typically the key length (LK) is about half <br />the length of the short weirs and about one fifth the length of the long weirs. Tests <br />conducted by MRD found that lateral erosion between spurs on nearly straight reaches <br />could be estimated by using a 20 degree angle of expansion (see Figure 1.3). The <br />following guidance formulas for LK were therefore developed. These formulas compute <br />minimum LK and should be extended in critical locations. The need for a filter between the <br />weir key and the bank material should also be determined. Guidelines for the selection, <br />design, and specification of filter materials can be found in Brown and Clyde (HEC-11) <br />(1989) and Holtz et al. (FHWA HI-95-038) (1995). <br /> <br />When the channel radius of curvature is large (R > 5W) and S > Utan(200) <br /> <br />LK = S tan(200) - L <br /> <br />(LaGrone 1995) <br /> <br />1.6 <br />