CHAPTER 5 OPEN CHANNELS 22 February 2000 Chapter Five - Open Channels Table Of Contents 5.1 Overview ..............................................................................5 - 1 5.1.1 Introduction.....................................................................5 - 1 5.1.2 Channel Types...................................................................5 - 1 5.1.2.1 Natural Channels..............................................................5 - 1 5.1.2.2 Grass-lined Channels...........................................................5 - 1 5.1.2.3 Trickle Channel Linings.........................................................5 - 2 5.1.2.4 Rock-lined Channels...........................................................5 - 2 5.1.2.5 Concrete Channels.............................................................5 - 2 5.2 Symbols And Definitions...................................................................5 - 3 5.3 Hydraulic Terms.........................................................................5 - 3 5.3.1 Introduction.....................................................................5 - 3 5.3.2 Steady And Unsteady Flow..........................................................5 - 4 5.3.3 Uniform Flow And Normal Depth.....................................................5 - 4 5.3.3.1 Uniform Flow And Normal Depth Example...........................................5 - 4 5.3.4 Critical Flow.....................................................................5 - 8 5.3.5 Gradually Varied Flow .............................................................5 - 8 5.3.6 Rapidly Varied Flow.............................................................. 5 - 10 5.3.6.1 Hydraulic Jump.............................................................. 5 - 10 5.3.6.1.1Storm Sewers............................................................ 5 - 10 5.3.6.1.2Box Culverts ............................................................ 5 - 10 5.3.6.1.3Vertical Drop Structures.................................................... 5 - 11 5.4 General Open Channel Design Criteria........................................................ 5 - 11 5.4.1 Introduction.................................................................... 5 - 11 5.4.2 Channel Transitions .............................................................. 5 - 11 5.4.3 Return Period Design Criteria....................................................... 5 - 12 5.4.3.1 Approximate Flood Limits Determination ............................................... 5 - 12 5.4.4 Velocity Limitations .............................................................. 5 - 12 5.4.5 Grade Control Structures .......................................................... 5 - 14 5.4.6 Streambank Protection ............................................................ 5 - 14 5.4.7 Construction And Maintenance Considerations .......................................... 5 - 14 5.5 Natural Channel Design Criteria............................................................. 5 - 15 5.6 Grass-Lined Channel Design Criteria......................................................... 5 - 16 5.6.1 Design Velocity and Froude Number.................................................. 5 - 16 5.6.2 Longitudinal Slopes .............................................................. 5 - 16 5.6.3 Roughness Coefficients ........................................................... 5 - 16 5.6.4 Freeboard...................................................................... 5 - 17 5.6.5 Curvature...................................................................... 5 - 17 5.6.6 Cross-sections.................................................................. 5 - 17 5.6.7 Grass Species................................................................... 5 - 18 5.7 Wetland Bottom Channel Design Criteria...................................................... 5 - 21 5.7.1 Design Velocity ................................................................. 5 - 21 5.7.2 Longitudinal Slopes .............................................................. 5 - 21 5.7.3 Roughness Coefficients ........................................................... 5 - 21 5.7.4 Design Depth................................................................... 5 - 21 5.7.5 Freeboard...................................................................... 5 - 22 5.7.6 Curvature...................................................................... 5 - 22 5.7.7 Cross-sections.................................................................. 5 - 22 5.8 Rock-Lined Channel Design ............................................................... 5 - 25 5.9 Concrete Channels ...................................................................... 5 - 26 Chapter Five - Open Channels Draft Drainage Criteria Manual Table Of Contents 5.10Grade Control Structures.................................................................. 5 - 27 5.10.1 Vertical Riprap Drops............................................................. 5 - 27 5.10.1.1 Approach Depth............................................................. 5 - 27 5.10.1.2 Trickle Channel.............................................................. 5 - 27 5.10.1.3 Approach Apron............................................................. 5 - 27 5.10.1.4 Crest Wall.................................................................. 5 - 27 5.10.1.5 Stilling Basin ................................................................ 5 - 28 5.10.1.6 Exit Depth.................................................................. 5 - 28 5.10.1.7 Design Example.............................................................. 5 - 28 5.11 Stability And Bank Protection .............................................................. 5 - 31 5.11.1 Channel Stability Guidelines ........................................................ 5 - 31 5.11.2 Rock Riprap.................................................................... 5 - 31 5.11.2.1 Edge Treatment.............................................................. 5 - 32 5.11.2.2 Construction Considerations .................................................... 5 - 32 5.11.2.3 Design Procedure ............................................................ 5 - 36 5.11.3 Wire-enclosed Rock.............................................................. 5 - 37 5.11.4 Pre-cast Concrete Blocks.......................................................... 5 - 37 5.11.5 Grouted Rock .................................................................. 5 - 38 5.11.5.1 Construction................................................................ 5 - 41 5.11.6 Bioengineering Methods ........................................................... 5 - 42 References................................................................................ 5 - 47 Drainage Criteria Manual Open Channels 5.1 Overview 5.1.1 Introduction Consideration of open channel hydraulics is an integral part of projects in which artificial channels and improve- ments to natural channels are a primary concern. Open channels are encouraged for use, especially in the major drainage system, and can have advantages in terms of cost, capacity, multiple use (i.e., recreation, wildlife habitat, etc.), and flow routing storage. Disadvantages include right-of-way needs and maintenance requirements. Where natural channels are not well defined, runoff flow paths can usually be determined and used as the basis for location and construction of channels. In some cases the well-planned use of natural channels and flow paths in the development of a major drainage system may obviate the need for an underground storm sewer system. For any open channel conveyance, channel stability must be evaluated to determine what measures are needed so as to avoid bottom scour and bank cutting. This chapter emphasizes procedures for performing uniform flow calculations that aid in the selection or evaluation of appropriate channel linings, depths, and grades for natural or man-made channels. Allowable velocities are provided, along with procedures for evaluating channel capacity using Manning's equation. Even where streams retains a relatively natural state, streambanks may need to be stabilized while vegetation recovers. To preserve riparian characteristics of channels, channel improvement or stabilization projects should minimize the use of visible concrete, riprap or other hard stabilization materials. Hydraulic analysis software such as the Corps of Engineers HEC-RAS program may be useful when preparing preliminary and final channel designs. For any open channel conveyance, channel stability must be evaluated to determine what measures are needed to avoid bottom scour and bank cutting. Channels shall be designed for long term stability, but be left in as near a natural condition as possible. The use of open, natural channels is especially encouraged in the major drainage system and can have advantages in terms of cost, capacity, multiple use (i.e., recreation, wildlife habitat, etc.) and flow routing storage. It shall be demonstrated that the natural c o ndition or an alternative channel design will provide stable stream bed and bank conditions. Where this cannot be demonstrated, a concrete low flow liner with a nonerosive crossection may be required by the Director of Public Works and Utilities. Even where s treams retain a relatively natural state, streambanks may need to be stabilized while vegetation recovers. To preserve riparian characteristics of channels, channel improve- ment or stabilization