THE FLUVIAL GEOMORPHOLOGY OF

BANK EROSION IN URBAN RIVERS

2014 CLEAN RIVERS, CLEAN LAKE CONFERENCE - MILWAUKEE, WI

Marty Melchior

Regional Director

THE MANY FACETS OF RIVER RESTORATION Modern river restoration encompasses (or will

encompass) many disciplines

Geomorphology Stream ecology

Hydraulic engineering

Hydrology

Botany

Social sciences

Cultural resources

Civil engineering

Geology

Sediment

transport

DOMINANT GEOMORPHIC PROCESSES IN

URBAN RIVERS

Lateral migration

Base level change

Valley modification – floodplain encroachment

Local scour associated with infrastructure

Sediment starvation

Sediment deposition/aggradation

Dams

Major changes/damage from large floods

DOMINANT GEOMORPHIC PROCESSES IN

URBAN RIVERS

Lateral migration

Base level change

Valley modification – floodplain encroachment

Local hydraulics associated with infrastructure

Sediment starvation

Sediment transport/movement

Sediment deposition/aggradation

Dams

Major changes/damage from large floods

Stream stability

Cross-section remains constant, only the

location changes

“Equilibrium” with incoming sediment and water

Floodplain formation = erosion + deposition

From Thorne et al. 1997

Understanding our place Floodplains are flat and an easy place

to build roads, cities and farms

Humans can’t perceive geologic time

This leads to problems

Water likes to

meander

Why do rivers meander?

Sinuosity

From L. Leopold

Why do rivers meander? Sinuosity and the dissipation of energy

Skiing 101: How to dissipate energy evenly

Translation of meanders

PLANFORM GEOMETRY

λ

Meander wavelength (λ)

Sheboygan River, WI

Rc

Radius of curvature (Rc)

BW

Belt width (BW) = amplitude of meanders, roughly equal to the floodplain width

Riffles and pools also dissipate energy

CHANNEL EVOLUTION MODEL

Schumm (1977)

Simon

Pattern of channel processes

LIMITED GEOMORPHIC ACTIVITY

Channelized ditches, even if not dredged, can

remain static for hundreds of years

If left alone, both of these systems would return to

their potential meandering plan and profile

CHANNEL EVOLUTION Stage II

• Active headcutting

• Unstable riffles

• Base level changes

• Aggradation

• Degradation

CHANNEL EVOLUTION Stage II

• Outfalls

• Suspended infrastructure

CHANNEL EVOLUTION

Grade control

• Road crossings have concrete or stone bases

that often arrest incision

• These perched crossings may be fish passage

barriers at certain times of the year

CHANNEL STABILITY

Stage III

• Stable bed

• Active widening (different from meandering)

Stage IV

• Incipient floodplain formation

• Floodplain expansion may continue for decades

Kinnickinnic River

Regulated flow

It’s important to be aware of dam releases or other

influencing regulated discharges

Example – channel forming discharge

Turkey River, IA

DEFORMABILITY

Alaska

Lincoln Creek

Most urban rivers have their

belt width defined already

How much migration is

acceptable?

DEFORMABILITY

What’s a normal rate of bank erosion?

Sometimes, lateral erosion is perceived

as being worse than it is

DEFORMABILITY

Actual erosion is easily measured. The Impact of

erosion is scale dependent and personal.

30 feet may not seem like much on the aerial photo,

but if you live next to the river, it is a big deal

1999 (yellow) bank against 2010 aerial

Bank erosion Major modes of soil loss

From Thorne 1993

BANK MORPHOLOGY

Erosion rates are influenced by:

• Bank soil type

• Stratigraphy

• Vegetation

• Bank height

• Soil moisture

Soil moisture Pore pressure

can increase soil

loss

Water pore pressure

Matric suction

Gravity vs veg Greater root density = soil

protection

Gravity weighs heavily

Soil loss vs. Root Density (RD)

EROSION RESISTANCE

Grasses

• Generally good to 2 ft bank height

• Reed canary grass dominates

EROSION RESISTANCE

Tree roots

• Generally good to 4 ft of

bank height

• Many evolved for river or

floodplain life:

• Flood tolerant

• Plastic root systems

• Vegetative

reproduction

• Wood and local scour vs.

global erosion

EROSION RESISTANCE

Willow/Cottonwood

• Riverine species

• Black willow – 40-60 ft root mass

• Low stem density = low

roughness

Bluff erosion processes

Bluff

Bank

Bluff erosion is when the

bank is actually an

older terrace wall

OTHER EXAMPLES OF EROSION RESISTANCE

CHANGING TIMES Conveyance was almost always the primary goal of

urban channel stabilization from 1940-1990

Economic decisions of the past did not always

include consideration of environmental costs

Are there natural analogs for non-deformable

streams? Yes!

Geomorphic control

Many natural streams have limited

deformability/migration

These typically flow over steeper slopes with larger bed material

Massachusetts

Wildcat Creek, Milwaukee

Limited lateral migration

Self armoring of toe

(launched stone)

GEOMORPHIC CONTROL IN URBAN RIVERS

We can build in artificial geomorphic control

(immobile sediment)

Grade control can be designed to form

drops, riffles and pools (vertical complexity)

Residual pools are better than no pools at all

Photo Interfluve – Jordan, MN

Show several

examples and

discuss

Geologic Control

Some river stability is controlled

by bedrock geology

Amnicon Falls SP, WI

This bed is not entirely immobile, but

deformability is limited by large

material (glacial lag deposits)

Duluth, MN

GEOLOGIC CONTROL IN URBAN RIVERS

We can impose artificial

geologic control (grade

control, boundary control)

Minneapolis

Milwaukee Plymouth, MA

ARTIFICIAL GEOLOGIC CONTROL ALREADY EXISTS

IN URBAN RIVERS Bridges, walls and other structures

act as geologic controls

These controls can influence

local hydraulics…

…which in turn affect shear,

sediment and stability

Milwaukee USGS

CONCLUSION

Streambank geomorphology is just one aspect of a many

faceted world. A complex world awaits you!

Don’t be too hard on the engineers and planners. They are

trying to do the best they can given some pretty tough

conditions.

Thorough understanding of the geomorphic processes at

work on your project will make you a happier person.

Thank you!

mmelchior@interfluve.com

920-354-8260