1.7.4 Chapter 5 - River Morphology and River Response
Rivers have different alignments and geometry. There are meandering rivers, braided rivers,
and rivers that are essentially straight. In general, braided rivers are relatively steep and
meandering rivers have more gentle slopes. Meandering rivers that are not subject to rapid
movement, are reasonably predictable in behavior; however, meandering rivers are generally
unstable with eroding banks which may result in destruction of productive land, bridges, bridge
approaches, control works, buildings, and urban properties during floods. Bank protection
works are often necessary to stabilize reaches of many rivers and to improve them for other
aspects of flood control and navigation.
Chapter 5 presents the fluvial geomorphology of rivers and methods to predict river response
to external forces such as a bridge crossing or other natural or human induced changes.
Terms such as fluvial cycles, meandering, alluvial fans, geomorphic thresholds, nick points,
and head cuts are described. A simple river classification scheme is presented. A variety of
methods to predict a river's response to change are given. Additional information on stream
stability at highway structures can be found in HEC-20 (Lagasse et al. 2001). Example
problems to determine river classification and response are given at the end of Chapter 5.
1.7.5 Chapter 6 - River Stabilization and Bank Protection
Numerous types of river control and bank stabilization devices have evolved through past
experience. Concrete, brick, willow and asphalt mattresses, sacked concrete and sand, riprap
grouted slope protection, sheet piles, timber piles, steel jack and brush jetties, angled and
sloped rock-filled, earth-filled, and timber dikes, automobile bodies, and concrete tetrahedrons
have all been used in the practice of training rivers and stabilizing river banks.
The study of river morphology and river response in Chapter 5 makes it clear that both short-
and long-term changes can be expected on river systems as a result of natural and human
influences. Recommended structures and design methods for river control are presented in
Chapter 6. The integrated and interactive effects of these structures with the river are
discussed in Chapter 9. Detailed guidelines for selection and design of stream instability and
bridge scour countermeasures are presented in HEC-23 (Lagasse et al. 2001). Example
problems related to riprap are solved at the end of Chapter 6 in SI and English units of
measurement. However, HEC-23 has detailed design guidelines for river stabilization and
1.7.6 Chapter 7 - Scour at Bridges
Scour at highway structures is the result of the erosive action of flowing water removing bed
material from around the abutments and piers which support the bridge and bed and bank
material of the stream the structure crosses. Both scour at highway structures and stream
migration (instability) can cause a bridge failure.
All material in a streambed will erode with time. However, some material such as granite may
take hundred's of years to erode, while sandbed streams may erode to the maximum depth of
scour in hours. Sandstone, shales, and other sedimentary bedrock materials can erode to the