Streambank erosion:Processes, prediction, and the surprising role of enthalpyTESS THOMPSON, ASSOCIATE PROFESSOR
B IOLOGICAL SYSTEMS ENGINEERING
Acknowledgements
• Former graduate students• Marc Henderson• Waverly Parks Garnand• Akinrotimi Akinola • Barbra Utley
• Laura Lehman – lab manager
• Siavash Hoomehr – post-doc
• Matt Eick – collaborator
• Marty Rabenhorst (UMD) and SGS Minerals Services for clay testing
1. Required for channel meandering
2. Critical part of evolution of incised channels
3. In some watersheds is the dominant source of
suspended sediment to streams
4. Impacts human infrastructure and property
Why is bank retreat important?
Chesapeake Bay, USA (NASA)
How does streambank retreat (typically) occur?
Freeze-thaw and wet-dry cycling weaken soil
Soil entrained during high flows
Mass failure from slope instability
SubaerialProcesses/Erosion Fluvial Entrainment Bank Failure
Freeze-thaw and wet-dry cycling weaken soil
Soil entrained during high flows
Mass failure from slope instability
SubaerialProcesses/Erosion Fluvial Entrainment Bank Failure
My research focuses on the fluvial erosion of cohesive streambanks
Physical and chemical factors affect cohesive soil erosion
Physical
Soil texture◦ Amount and type of clay
◦ Clay mineralogy
Organic matter content
Soil stress history
Subaerial processes
Chemical
Size and valence of interlayer cations
Stream ◦ Electrical conductivity
◦ pH
◦ Temperature
If streambanks are cohesive, is urban stream chemistry important?
1. Stream temperature can increase 7°C during summer thunderstorms (Palmer)
2. Salt concentrations during winter can be as high as 19 ppt
3. Stream temperature may increase 2-5°C due to climate change
4. Will stream pH decrease due to climate change?
We conducted flume studies to quantify changes in fluvial erosion rates with changes in stream chemistry
➢ Temperature
➢ pH
➢ Deicing salt
➢ Subsequent study on soil and water temperature
Flow straightener
Flow direction
Flume insert
100 cm
40 cm5-cm dia. soil core, flush w/flume wall
Screw-type advance mechanism
Remolded, 5-cm diameter cores of two natural soils were tested in an 8-m recirculating hydraulic flume
Water temperature, pH, and salt concentration were varied
➢ Water temperatures of 10, 20, and 30˚C
➢ pH of 6 and 8➢ NaCl concentrations of 0 and
5000 mg/l➢ 3 replicates for each soil-T-pH-
salt combination
Soil type
Summer(25oC water)
Ambient (25oC soil)
Cooled (15oC soil)
Heated (40oC soil)
Winter (15oC water)
Cooled (0oC soil)
Ambient (15oC soil)
Heated (25oC soil)
n=90
A follow-up study evaluated soil and water temperature effects on the erosion of cohesive soils
Combined effects of soil and water temperatures
No statistically significant
difference in erosion rates at
conditions of equal
temperatures (Wilcoxon test)
𝐑𝟐 = 𝟎. 𝟔𝟑
𝐑𝟐 = 𝟎. 𝟗𝟎
𝐑𝟐 = 𝟎. 𝟖𝟎
Temperature difference appears to influence erosion rather than just either soil or water temperatures.
Erosion rate increases with increasing soil enthalpy, depending on clay type.
Wrapping it up…1. Clay type and stream chemistry play a significant role in the fluvial erosion of
cohesive streambanks
2. Stormwater regulations should require temperature control in addition to peak flow and volume control to maintain channel stability
3. The change in erosion rate with temperature is directly related to the change in enthalpy of the streambank soils
x=0
x=10 x=20
x=30x=32
x=34
x=44
x=54 x=64
Longitudinal distributionx
Vertical (bank) distribution
} 30 cm
x30 cm
Simpson, 2006
Over 250 erosion pins were installed along a 400-m reach of Stroubles Creek in Blacksburg, VA and measured monthly for 20 months.
1 2 3 4 5 6 7 8 9 10 12 14 16 18
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Vo
lum
e (
m^3
)
Study Month
Temporal and spatial variability in predicted volume eroded for 2-m horizontal erosion pin spacing
Jan `06 Mar `07
Erosion per bank area calculated by two different methods summed over the entire study periodTotal Erosion per Block for the study period calculated by two methods
-12
-10
-8
-6
-4
-2
0
1 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Block #
Ero
sio
n (
m3)
Spatial Interpolation Averaging
How do we model these processes?
Freeze-thaw and wet-dry cycling weaken soil
Soil entrained during high flows
Mass failure from slope instability
SubaerialProcesses/Erosion Fluvial Entrainment Bank Failure
Excess shear stress equation models the erosion rate of fine grain soils due to fluvial forces
a
cadr KE )( −=Er = Erosion rate (L/T)
Kd= Erodibility coefficient (L2.T/M)
a = Actual shear stress (M/L.T2)
c = Critical shear stress (M/L.T2)
a = Exponent, assumed equal to 1