The Effects of Channelization

Christin Cruz

Overview

• Types of Channelization

• History

• Morphological Effects

• Biological Impacts

Types of Channelization

• Definition – includes “all processes of river engineering for the purposes of flood control, drainage improvement, maintenance of navigation, reduction of bank erosion ot relocation for highway construction” (Brookes, 1988).

History

• Long history – Yellow River, China, 600 BC

• Most prolific in developed countries

• In past 150 years in the US 320,000 km of river modified (reference: Mississippi is 3,705 km) as of 1988

• Denmark – almost 100% of drainage network has been modified

Types of Channelization

• Straightening

• Resectioning

• Clearing and Snagging

Types of Channelization

• Straightening– River is shortened by

artificial cutoffs

Yalobusha River, Mississippi (Downs and Gregory, 2004)

Types of Channelization

• Resectioning (widening and/or deepening)– Increase capacity to

reduce overbank flooding

(Iowa DNR website)

Types of Channelization

• Clearing and Snagging– Removal of

obstructions

Morphological Effects

Morphological Effects

Morphological Effects

• Example• Willow Drainage Ditch, Iowa• Began modification in 1919 with a trapzoidal

cross-section with a 1:1 side slope• Gradient increase, low reaches 1.0 m/km to 1.5

m/km, upstream 1.4 m/km to 1.7 m/km• Maximum increase in channel size of 440%• Depth increase of 3.4 m to 13 m

Morphological Effects

Morphological Effects

• Resectioning– Widening reduces stream power, reducing

sed. discharge– Low flows deposit sed. Because of reduced

velocities– Can form structures such as mid-channel bars

or berms

• Resectioning• Example:• River Tame, England• Reverted back to

original capacity in less than 30 yrs

Morphological Effects

Morphological Effects

• Clearing and Snagging– Few studies with before-and-after effects– Cause bank erosion and widening of channel

Biological Effects

• Bank cover is eliminated

• Pools are lost

• Flow approaches laminar

• Homogeneous substrate

• Habitat diversity and niche potential are reduced

Biological Impacts

Biological Effects

• Macriinvertebrate– Species are confined to specific ranges of

velocity– Require specific a specific substrate– Temperature and dissolved oxygen

Biological Effects

• Fish– Reduced habitat (lack of cover)– Sediment blankets effects spawning areas– Overhanging veg. effects temp. and potential

food sources

Biological Effects

Biological Effects

Conclusions

• Channelization has negative affects on stream morphology and organisms within the stream