2014 clean rivers, clean lake -- fluvial geomorphology of bank erosion
TRANSCRIPT
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.
