Office of Structures Manual for Hydrologic and Hydraulic Design Chapter 14: Stream Morphology and Channel Crossings August 2007 Preface Federal and state policies and practices that apply to the hydraulic design of structures are pre- sented throughout the various chapters of the Manual for Hydrologic and Hydraulic Design. These policies and practices have been developed to meet the objectives of applicable federal and state laws and regulations regarding streams and floodplains while achieving the transportation objectives of a safe, efficient, and cost-effective structure that is compatible with the environment of the stream being crossed. This chapter presents guidance developed and implemented by the Office of Struc- tures (OBD) specifically for Maryland streams. These procedures outline the approach to be used in evaluating the morphology of a stream reach in the vicinity of a waterway crossing. Similar guidance on several provided procedures has not been found to be available in federal manuals or other publications accessible to the public. Most of the guidance in Chapter 14 is based on the results of studies and investigations conducted since the mid-1990s in Maryland. The investigations from which these techniques were developed targeted wadeable gravel-bed streams that generally maintain a pool-riffle morphology and have channel slopes of 0.2% to 4%; many of the techniques, however, may be found to be applicable to streams of other morphologies. Likewise, while the guidance focuses on bridge and culvert crossings of streams, it is equally rele- vant to other investigations of stream stability. Occasionally, for example, the OBD conducts stream morphological studies to evaluate stream stability with regard to highway embankments, retaining walls, or adjacent non-highway infrastructure such as utility lines that may parallel or cross a highway or stream corridor. This is a working draft of Chapter 14. The guidance will continue to be modified and expanded as additional information becomes available regarding the morphological behavior of Maryland streams and as improved methods are developed for assessing this behavior. Contents List of Figures ...................................................................................................................................... vi List of Tables ...................................................................................................................................... vii List of Symbols .................................................................................................................................. viii Glossary ................................................................................................................................................ x 14.1 Introduction ............................................................................................................................ 1 14.1.1 QUALIFICATIONS .................................................................................................... 1 14.1.2 ELEMENTS OF THE STREAM MORPHOLOGY STUDIES ................................ 2 1. Existing Crossing ................................................................................................... 3 2. Long-Term Changes in Channel Bed Elevation .................................................. 3 3. Channel Lateral Movement ............................................................................... 3 4. Sediment Dynamics ............................................................................................... 4 5. Debris ..................................................................................................................... 4 6. Structure and Bend Scour ...................................................................................... 4 7. Environmental Considerations .............................................................................. 4 8. Historic and Contemporary Modifications to Channels and Valleys .................. 5 14.2 Preliminary Morphology Study ........................................................................................... 6 14.2.1 BACKGROUND DATA COLLECTION .................................................................. 6 Existing Land-Use and Existing and Ultimate Hydrology ......................................... 6 Bankfull Flow and Channel Geometry Estimates ...................................................... 7 Physiographic Region and Geology of Site ................................................................ 7 Historic and Contemporary Modifications to Channels and Valleys......................... 7 Valley and Channel Planform Characteristics ............................................................ 8 14.2.2 VISUAL ASSESSMENT ............................................................................................ 9 Summary of Field Procedures ..................................................................................... 9 Equipment and Mapping ....................................................................................... 9 Photographic Documentation ............................................................................... 9 Organization of the Visual Assessment ............................................................... 10 Key Features and Observations ................................................................................. 12 Existing Crossing ................................................................................................. 12 Channel Classification at the Crossing .............................................................. 20 Base Level Points................................................................................................. 20 Low-Flow High-Gradient Features .................................................................... 22 Stream Banks ....................................................................................................... 22 Pools .................................................................................................................... 26 Bars, Riffles, and Surface Particle-Size Characteristics .................................... 27 Debris ................................................................................................................... 29 Channel Confluences and Tributaries ................................................................ 30 Other Structures and Flow Obstructions ............................................................ 32 iii Terraces, the Active Floodplain, and Other Valley Bottom Features ............... 32 Channel-Valley Orientation and Channel Planform.......................................... 33 14.2.3 RAPID CHANNEL MEASUREMENTS ................................................................ 33 Pebble Count .............................................................................................................. 34 Bulk Bar Sample ........................................................................................................ 34 Soil and Bed Load Materials for Scour Studies ........................................................ 34 Potential for Long-Term Degradation ....................................................................... 36 Cumulative Degradation Method ....................................................................... 37 Pool Base Level Method ...................................................................................... 38 14.2.4 ANALYSIS AND DEVELOPMENT OF RECOMMENDATIONS ..................... 39 Development of the Detailed Study Scope ............................................................... 39 14.2.5 PRELIMINARY MORPHOLOGY REPORT ......................................................... 40 14.3 Detailed Morphology Study ................................................................................................ 43 14.3.1 PRELIMINARY STUDY REVIEW AND SITE RE-EXAMINATION ................ 43 Development of the Detailed Study Scope ............................................................... 44 Extent of the Channel Profile Survey .................................................................. 44 Sediment Assessment Reach ................................................................................ 46 Lateral Channel Movement ................................................................................. 46 Soil and Bed Load Materials for Scour Studies ................................................. 46 Selection of Locations for Data Collection ............................................................... 46 14.3.2 DATA COLLECTION .............................................................................................. 46 Valley Longitudinal Profile ....................................................................................... 46 Channel Profile Survey .............................................................................................. 47 Channel Cross Sections ............................................................................................. 47 Bed Sediments ........................................................................................................... 48 Bankfull Flow Indicators and Channel Characteristics ............................................ 51 Subsurface Sampling: Site Borings, Geoprobe® Samples, and Trenches ................ 51 Bank Geometry, Bank Materials, and Stratification ................................................. 51 Lateral Channel Movement