Evaluation of Bridge Scour Research: Geomorphic Processes and Predictions (2011) / Chapter Skim
Currently Skimming:
Pages 10-22
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 10... ...
2.1 2. REVIEW OF CURRENT PRACTICE For this project, current practice is defined as the guidance that is contained in the three FHWA documents that address the topics of geomorphology, aggradation, degradation, lateral migration and channel widening.
|
From page 11... ...
2.2 Table 2.1. Current Practice Based on Type and Level of Analysis.
|
From page 12... ...
2.3 Figure 2.1. Geomorphic factors that affect stream stability (adapted from Brice and Blodgett)
|
From page 13... ...
2.4 The characteristics include: Stream size Flow habit Bed material Valley setting Floodplains Natural levees Apparent incision Channel boundaries Tree cover on banks Channel sinuosity Channel braiding Channel anabranching, and Variability of channel width and development of point bars Some of these factors indicate that a channel is more susceptible to stream lateral or vertical instability. For example, a channel with well-developed natural levees tends to have lower rates of lateral migration.
|
From page 14... ...
2.5 2.1.3 Rapid Assessment HEC-20, Section 4.5 and Appendix D present an approach that incorporates several factors (channel characteristics, hydraulic and sediment transport characteristics) to quickly rate a channel from poor (highly unstable)
|
From page 15... ...
2.6 2.2 Geomorphology Level 2 Topics 2.2.1 Channel Response HEC-20, Section 4.4 and HDS 6, Section 5.5 provide a general discussion of channel response to change. Each document indicates that there are a large number of variables that are involved in channel response and that predicting river response is a complex task.
|
From page 16... ...
2.7 that was not intended for detailed use. Once distortion has been removed from the photography, detailed measurements of bank location can be made and rates of channel migration can be determined.
|
From page 17... ...
2.8 2.4 Aggradation and Degradation Level 1 Topics 2.4.1 Bridge Inspection Records There are two instances where bridge inspection is mentioned in HEC-20 related to the topic of stream instability. The first reference is in Section 3.3.1 (Data Needs for Level 1 Analyses)
|
From page 18... ...
2.9 2.5 Aggradation and Degradation Level 2 Topics 2.5.1 Rating Curve Shifts HEC-20, Section 3.6.7 provides a detailed explanation of using rating curve shifts (specific gage analysis) as Step 7 of a Level 2 analysis for evaluating channel degradation.
|
From page 19... ...
2.10 2.5.3 Equilibrium Slope A channel that has a sediment transport capacity equal to the sediment supply is in a state of equilibrium. If the sediment supply, flow rate or some other variable is altered, then the channel will respond.
|
From page 20... ...
2.11 Level 3 procedure. As with simpler methods, sediment transport modeling is also limited by the uncertainties inherent in any sediment analysis.
|
From page 21... ...
2.12 2.7.2 Field Evidence Field evidence of channel migration and channel widening are discussed in HEC-20, Sections 2.3.9 and 3.5.4, and in HDS 6, Section 5.8.1. Field evidence includes bank condition, bank erosion and mass wasting, and also the presence of large point bars that often indicate channel migration.
|
From page 22... ...
2.13 2.9 Lateral Migration and Channel Widening Level 3 Topics 2.9.1 Sediment Transport Modeling Including Geotechnical Stability Sediment transport models have primarily focused on aggradation and degradation of the channel because this is the part of the channel that is directly involved in the transport of sediment. Channel banks are sources of sediment and bank stability is impacted by channel degradation and through the fluvial entrainment of bank toe materials.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.