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~. I, I . . I I , Design Guidelines 1. HEIGHT - The height of the weirs, H, is determined by analyzing the depth of flow at the project site (Refer to Figures 1.1 and 1.2). The bendway weir should be between 30 to 50 percent of the depth at the mean annual high water level. The height of the structure should also be below the normal or seasonal mean water level and should be equal to or above the mean low water level. The weir must be of adequate height to intercept a large enough percentage of the flow to produce the desired results. For applications relating to improved navigation width, the weir must be at an elevation low enough to allow normal river traffic to pass over the weir unimpeded. 2. ANGLE - The angle of projection, 9, between the bendway weir axis and the upstream bankline tangent typically ranges from 50 to 85 degrees. Experience has indicated that it is easier to measure this angle from the chord between two weirs in the field rather than using the bankline tangent. The chord is drawn from the points of intersection with the weirs and the bankline (see Figure 1.1). The angle of projection is determined by the location of the weir in the bend and the angle at which the flow lines approach the structure. Ideally, the angle should be such that the high-flow streamline angle of attack is not greater than 30 degrees and the low flow streamline angle of attack is not less than 15 degrees to the normal of the weir centerline of the first several weirs. If the angle of flow approaching the upstream weirs is close to head-on, then the weir will be ineffective and act as a flow divider and bank scalloping can result. If the angle of flow approaching the upstream weirs is too large then the weir will not be able to effectively redirect the flow to the desired flow path. Ideally, the angle should be such that the perpendicular line from the midpoint of an upstream weir points to the midpoint of the following downstream weir. All other factors being equal, smaller projection angles, e, would need to be applied to bends with smaller radii of curvature to meet this criteria and vice versa. Experiments by Derrick (1994) resulted in a weir angle of 60 degrees being the most effective for the desired results in a physical model of a reach on the Mississippi River. Observations by laGrone, indicate that the angle, e, of the upstream face of the structure is most important in redirecting flows. The upstream face should be a well defined straight line at a consistent angle. 3. CROSS SECTION - The transverse slope along the centerline of the weir is intended to be flat or nearly flat and should be no steeper than 1 V:5H. The flat weir section normally transitions into the bank on a slope of 1V:1.5H to 1V:2H. The structure height at the bankline should equal the height of the maximum design high water. This level is designed using sound engineering judgment. The key must be high enough to prevent flow from flanking the structure. The bendway weir should also be keyed into the stream bed a minimum depth approximately equal to the 0100 size. 4. LENGTH - The bendway weir length (L) should not exceed 1/3 the mean channel width (W). A weir length greater than 1/3 of the width of the channel can alter the channel patterns which can impact the opposite bankline. Weirs should be long enough to cross the stream thalweg. Weirs designed for bank protection need not exceed 1/4 the channel width. A length of 1.5 to 2 times the distance from the bank to the thalweg has proven satisfactory on some bank stabilization projects. The length of the weir will affect the spacing between the weirs. Maximum Length L = W/3 (typically: W/10 < L < W/4) I 1.4