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

How do we measure and model bank retreat?

Measurement method

depends on the time

scale of interest

From Lawler

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

Noncohesive soil erosion

Cohesive soil erosion

Cohesive soils are dominated by inter-particle attraction

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

150C soil 250C soil

Results –water temperature effects

Er/U*- Nondimensional erosion rate

Results –soil temperature effects

250C water150C water

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

Questions?

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

Bank failure is modeled using slope stability analyses

From Langendoen, USDA-ARS