CHANNEL PATTERNS

A. Introduction

• location of erosion & deposition (bed/banks) affects channel shape

• channel pattern: planform of channel; shape as seen from a bird's eye view

• classification: straight, meandering, braided, anabranching

  • sinuosity: stream channel length divided by length of meander belt axis

  • sinuosity of 1.5 is dividing point between straight & meandering

• DISCUSSION

  • what are the defining characteristics of these four channel patterns?

  • under what discharge and load conditions do these channels form?

B. Straight Channels

C. Meandering Channels

D. Braided Channels

E. Anabranching Channels

F. Summary

1. Four basic channel patterns form a continuum

2. Conditions affecting formation

a. balance between hydraulic characteristics & sediment load affects channel planform and vice versa

b. threshold values for controlling variables

• threshold between meandering & braided: increase slope at a given Q or increase Q at a given slope and pattern will change from meandering to braided

c. ability to do work changes over time and space

• system maintains equilibrium by

  • eroding material when excess energy is available - degradation or lateral migration

  • depositing material when insufficient energy is available - aggradation or lateral deposition

• result: channel shape & position may change

3. Changes in channel pattern may be associated with:

• changes in slope (gradient) due to tectonic activity, changes in base level

• changes in available energy due to climatic change or contribution of tributary streams

• changes in resistance of banks to erosion

• changes in sediment supplied to stream due to land use change, glacials - interglacials