Chapter 8 Open Channels Contents 1.0 Introduction ...................................................................................................................................... 1 2.0 Natural Stream Corridors ............................................................................................................... 2 2.1 Functions and benefits of Natural Streams ................................................................................................ 3 2.2 Natural Stream Corridors Prior to Urbanization ....................................................................................... 4 2.3 Urban Stream Corridors ............................................................................................................................ 5 2.3.1 Impacts and Constraints ................................................................................................................ 5 2.3.2 Stream Degradation....................................................................................................................... 5 3.0 Preserving Natural Stream Corridors ............................................................................................ 7 3.1 Preserve Natural Streams and Riparian Vegetation .................................................................................. 7 3.2 Provide Ample Space for Stream and Floodplain ..................................................................................... 8 3.3 Manage Increased Urban Runoff ............................................................................................................ 11 3.4 Monitor and Proactively Address Channel Instability ............................................................................ 12 4.0 Stream Restoration Principles ...................................................................................................... 13 4.1 Understand Existing Stream and Watershed Conditions ......................................................................... 14 4.2 Apply Fluvial Geomorphology Principles to Manage Sediment Balance ............................................... 17 4.2.1 Aggradation, Degradation, and Equilibrium ............................................................................... 17 4.2.2 Dynamic Equilibrium and Threshold Principles in Stream Restoration ..................................... 19 4.2.3 Support the Stream’s Natural Capacity to Sustain Itself ............................................................. 20 4.3 Establish Effective Cross-Sectional Shape .............................................................................................. 21 4.3.1 Maintaining Floodplain Function ................................................................................................ 21 4.3.2 Sizing of Bankfull Channel ......................................................................................................... 22 4.3.3 Addressing Incised Channels ...................................................................................................... 24 4.3.4 Floodplain Terraces..................................................................................................................... 25 4.4 Maintain Natural Planform Geometry ..................................................................................................... 28 4.5 Develop Grade Control Strategy to Manage Longitudinal Slope ............................................................ 30 4.6 Address Bank Stability ............................................................................................................................ 35 4.6.1 Bioengineering Techniques ......................................................................................................... 35 4.6.2 Bank Protection Approaches ....................................................................................................... 35 4.7 Enhance Streambank and Floodplain Vegetation .................................................................................... 42 4.8 Evaluate Stream Hydraulics of over a Range of Flows ........................................................................... 44 5.0 Naturalized Channels ..................................................................................................................... 46 5.1 Understand Existing Stream and Watershed Conditions ......................................................................... 46 5.2 Apply Fluvial Geomorphology Principles to Manage Sediment Balance ............................................... 47 5.3 Establish Effective Cross-Sectional Shape .............................................................................................. 47 5.4 Maintain Natural Planform Geometry ..................................................................................................... 48 5.5 Develop Grade Control Strategy to Manage Longitudinal Slope ............................................................ 48 5.6 Address Bank Stability ............................................................................................................................ 49 5.7 Enhance Streambank and Floodplain Vegetation .................................................................................... 49 5.8 Evaluate Stream Hydraulics over a Range of Flows ............................................................................... 49 6.0 Swales .............................................................................................................................................. 50 6.1 Design Criteria for Swales ...................................................................................................................... 51 January 2016 Urban Drainage and Flood Control District 8-i Urban Storm Drainage Criteria Manual Volume 1 Open Channels Chapter 8 6.1.1 Stability Charts ........................................................................................................................... 51 6.2 Grass Swales ........................................................................................................................................... 52 6.2.1 Soil and Vegetation Properties .................................................................................................... 52 6.2.2 Construction ................................................................................................................................ 52 6.3 Soil Riprap and Void-Filled Riprap Swales ............................................................................................ 52 7.0 Hydraulic Analysis .......................................................................................................................... 57 7.1 Preliminary Channel Analysis................................................................................................................. 57 7.2 HEC-RAS Modeling ............................................................................................................................... 58 7.2.1 Cross Section Location ............................................................................................................... 58 7.2.2 Cross Section Geometry ............................................................................................................. 59 7.2.3 Roughness Coefficients .............................................................................................................. 60 7.2.4 Design Storms ............................................................................................................................. 63 7.2.5 Output Variables ......................................................................................................................... 64 7.3 Evaluation of Erosion at Channel Bends ................................................................................................. 68 8.0 Rock and Boulders .......................................................................................................................... 69 8.1 Riprap Sizing .......................................................................................................................................... 71 8.1.1 Mild Slope Conditions ................................................................................................................ 71 8.1.2 Steep Slope Conditions ............................................................................................................... 71 8.1.3 Design Safety Factor ................................................................................................................... 72 8.2 Boulder and Riprap Specifications .......................................................................................................... 72 8.2.1 Boulders ...................................................................................................................................... 72 8.2.2 Soil Riprap .................................................................................................................................. 72 8.2.3 Void-Filled Riprap ...................................................................................................................... 73 9.0 References ........................................................................................................................................ 83 Tables Table 8-1. Maximum prudent values for natural channel hydraulic parameters ........................................ 45 Table 8-2. Minimum dimensions