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EM 1110-2-1601 1 Jul 91 these criteria. Important junctions in rapid flow designed to reduce wave effects should be model tested at all prob- able flow combinations as well as at desi gn flow. d. Confluence design crireria. (1) The results of several model studies in USAED, Los Angeles, indicate that some general guides can be adopted for the design of contluence junctions. Gildea and Wong (1967) have summarized some of these criteria: (a) The design water-surf3Ce elevations in the two joining channels should be approximately equal at the UpStre:lm end of the confiuence. (b) The angle of junction intersection should be preferably zero but not greater than 12 deg. (c) Favorable now conditions can be 3Chieved with proper expansion in width of the main channel below the junction. (d) Rapid flow depths should not exceed 90 percent of the critical depth (Froude number should be greater than 1.13) to maintain stable rapid flow through the junco tion (paragraph 2-2d(1)). (2) Model tests of many confluence SO"Uctures indi- cate very little crosswave formation and turbulence at the junction if these criteria are followed. Moreover, experi- ence has shown that the momentum equation approach given in Appendix E can be used for junctions involving small angles and equal UpStre:lm water-surf3Ce elevations. (3) Typical confluence layouts model tested by USAED, Los Angeles. and proven to have good flow characteristics are shown in Plate 56. The design with the offset in the main channel center line is normally used (plate s6a). WIlen the main channel center-line alignment cannot be offset. a layout with a transition on the wall opposite the inlet side should be used (plate 56b). The proper amount of expansion in the main channel down- Stre:lm of the contluence is very important in maintaining good flow conditions. Plate 57 gives the USAED, Los Angeles. empirical curve for the required iRCre:lSC in channel width. A~, as a function of the discharge ratio. If the junction angle is zero, the width of the channel at the confluence will be equal to the sum of the widths of the main and side channels plus the thickness of the dividing wall between the channels. If a reduction in width is required dOWRStre:lm from the connuence, the transition should be made gradually. 4-6 e. Design procedure. The design procedure for the typical open channel connuence shown in Plate 56 in- volves the followin g steps: (I) Determine side-channel requirements relative to discharge. alignment. and channel size. (2) Select junction point to obtain an enlrance angle less than 12 deg. This angle requirement may necessitale a long, spiral curve for the side channel upslream from the junction. (3) Determine the increase of channel width ~ from the QZfQ3 ratio curve in Plate 57. Compute the required downStre:lm channel width ~ = bl + ~ and the confiuence width be" bl + ~. (4) Make the confluence layout on a straight-line basis by setting the main channel walls parallel to and at distances of (1/2)~ and be' (I/2)b3 from the center line as shown in Plate 56a. (5) Connect the left walls of the side and the main channels by a curve determined by the apex angle , and a radius rL given by 4V2b2 rL= +400 gy (4-7) Equation 4-7 results from a study of a number of con. fluences built by USAED, Los Angeles. The term (4y2bz)fgy is the same as that used in Equation 2.34. (6) Make the right wall of the side channel concen- tric with the left wall and locate the junction intersection point. The right wall radius rR is given by rR = rL + b2 (4-8) (7) Determine the average depth of now at midpoint of the confluence by the momentum method (Appen- dix E) assuming bm = (1/2) (bl + bz + be> ' (8) Set the side-channel invert elevation so that the design water-surface levels in both channels approximate each other. A stepped invert in either of the channels may be required.