Modelling geomorphic responses to human impacts and extreme floods: Application to the Kander river, Switzerland

Jorge Alberto Ramirez, Andreas Paul Zischg, Stefan Schürmann, Markus Zimmermann, Rolf Weingartner, Margreth Keiler Source: https://en.wikipedia.org Historical background • In 1714 Kander river flowed into the Aare river: • Causing massive flooding in the region of Thun

Source: Google maps, Wirth et al. (2011) Historical background • In 1714 Kander river flowed into the Aare river: • Causing massive flooding in the region of Thun • Kander river was deviated to lake Thun by engineering works

Kander correction

Source: Google maps, Wirth et al. (2011) Historical background • In 1714 Kander river flowed into the Aare river: • Causing massive flooding in the region of Thun • Kander river was deviated to lake Thun by engineering works • Four years after Kander correction eroded ~30 m

1714 River bed Kander correction

30 m

1718

Source: Google maps, Wirth et al. (2011) Aims • Can we model geomorphic effects of human intervention in fluvial systems?: • River restoration • River engineering • Test landscape evolution model (LEM) on Kander correction • Determine sensitivity of LEM to extreme flood events (climate change)

Restoration Engineering Extreme floods

Source: https://mostlyaboutmayflies.wordpress.com, http://www.theadvocateproject.eu, www.gettyimages.ch CAESAR-Lisflood

• Landscape evolution model simulating erosion and deposition within river reaches (CAESAR) • A hydrodynamic 2D flow model (based Lisflood FP model) that conserves mass and partial momentum

Source: https://sourceforge.net/projects/caesar-lisflood/ Model test using Kander Correction

• Erosion: incision of channel

1714 (year 0) River bed

30 m • In ~4 years the Kander correction eroded ~30 m • Afterwards the river eroded less and ‘stabilized’ 1718 • Channel erosion propagated upstream (year 4) 5 m 2016 (present day)

Source: http://media.web.britannica.com Model test using Kander Correction

• Deposition: development of delta in lake Thun

Lake Thun

Kander Correction Topography Lake Thun

• Present day topography was used to represent the river banks Kander Correction • Historical data was used to develop river channel and Kander correction

elevation high

low Discharge Simme

12 year simulation Hourly Discharge 1986-1998 ) 1 - s 3

Kander Discharge (m

time (hr) Sediment inputs Simme • 20,000 m3 yr-1 were added to both the Simme and Kander

• High flows were ≥ 30 m3 s-1 and we assumed upstream ) sediment transport occurred 3 above this threshold Kander • Amounts of sediment were proportionally added over time (m Sediment based on the discharge that was above the threshold

time (hr)

Source: Geschiebehaushalt Kander, 2014 Grainsize distribution • 6 grain size classes (silt to boulder) were estimated from Kander and Simme • Each gird cell in the model initially contains the same grainsize percentages

Source: Geschiebehaushalt Kander, 2014 Initial Conditions Lake Thun • Kander without correction • 1986 discharge and sediment inputs for Kander and Simme were repeated • Grainsize mixing occurred, channel erosion and deposition • Model ran for a total of 8 years until the reach was in equilibrium: 3% difference between sediment coming in and out of the reach Erosion (m) 1-1.7 0.5-1 0.05-0.5

Deposition (m) 1.5-2.6 1-1.5 0.5-1 0.05-0.5 Kander correction: 1714 Lake Thun Lake B Ramp A Correction

Correction

A Elevation (m)Elevation B Lake

Distance (m)

• The correction Length: 340 m, Width: 32 m, Slope: 0.8%. • A ramp connected the correction to the lake • Lake Thun was added to the DEM at the location of the shoreline. The lake was set as a non-erodible plane. Source: Geschiebehaushalt Kander, 2004 Kander correction: 1714 Lake Thun Lake B Ramp A Correction

Correction

Wall A (m)Elevation Lake Thun Lake B

Distance (m) elevation (m) 668

556

Source: Geschiebehaushalt Kander, 2004 Kander correction model Lake Thun

• Simulated 12 years of movement of water and sediment • Every year topography was recorded (1714-1726) Kander Correction

water & sediment inputs water depth high

low Model test: Kander erosion Elevation Profile Kander correction Lake year 0 Flow

Lake

Correction Elevation (m)Elevation

Distance (m) Model test: Kander erosion Elevation Profile • 29 m of erosion within 4 years Kander correction Lake year 0

Lake

year 1 Correction Elevation (m)Elevation year 2 year 4

Distance (m) Model test: Kander erosion Elevation Profile • 29 m of erosion within 4 years Kander correction Lake • 2 m difference with todays river year 0

Lake

year 1 Correction Elevation (m)Elevation year 2 year 4 year 12

present day

Distance (m) Model test: Kander erosion

900 m of upstream incision within 12 yrs

Lake Thun Erosion (m) 0-2 2-5 5-10 10-15 15-20 20-25 25-35

Year 1 (1715) Model test: Kander erosion

900 m of upstream incision within 12 yrs

Lake Thun Erosion (m) 0-2 2-5 5-10 10-15 15-20 20-25 25-35

Year 1 (1715) Year 12 (1726) Model test: Delta formation

Year 0 (1714) Lake Thun

Lake Thun outlet

Elevation (m) 558-559 563-564 Delta 1860 559-560 564-565 560-561 565-566 561-562 566-567 562-563 567-614 Model test: Delta formation

Year 0 (1714) Year 6 (1720) Lake Thun

Lake Thun outlet

Elevation (m) 558-559 563-564 Delta 1860 559-560 564-565 560-561 565-566 561-562 566-567 562-563 567-614 Model test: Delta formation

Year 0 (1714) Year 6 (1720) Year 12 (1726) Lake Thun

Lake Thun outlet

Elevation (m) 558-559 563-564 Delta 1860 559-560 564-565 …more time needed 560-561 565-566 561-562 566-567 562-563 567-614 Response to extreme floods Determine sensitivity of LEM applied to steep rivers and extreme flood events

Kander river ) 1 - s 3 Discharge (m

Date

Source: http://www.bafu.admin.ch Extreme hydrographs

36 scenarios Sediment added proportional to discharge Hydrographs 600 2005 flood Duration (hr) 6 Peak discharge (m3 s-1) 500 12 50 100 200 300 400 500 24 48

) 400 )

6 1 - 72 s hr 12 3 144 300 24

duration ( duration 48 200

72 X Discharge (m Flood 144 100

Time (hr) Response to extreme floods Lake Thun

• Determine how much incision occurs with flood events of different magnitude and duration

1714 (year 0) River bed incision

? m water & sed inputs

water depth high

low Geomorphic change Model produces plausible erosion and deposition under extreme flood conditions Flood duration has greater effect on change in elevation than peak discharge Single extreme flood events can produce up to 6 m of erosion, 4m of deposition Peak discharge (m3 s-1) discharge 500, duration 144 50 100 200 300 400 500 discharge 50, duration 6

) 6 X deposition hr B 12 24

duration ( duration 48 A 72 erosion A Flood 144 X (m) elevation in Change Correction

Absolute change in elevation (m) Distance (m) 0-0.5 Elevation (m) 0.5-1.0 B 1.0-1.5 1.5-2.0 Distance (m) 2.0-2.5 Geomorphic change

Flood that is 3 times longer, and 10 times lower in peak discharge produces similar change in elevation Long duration floods (6 day) with relatively low discharge are geomorphically important

Peak discharge (m3 s-1) discharge 50, duration 144 50 100 200 300 400 500 discharge 500, duration 48

) 6 B deposition hr 12 24 A

duration ( duration 48 X 72 erosion A Flood 144 X (m) elevation in Change Correction

Absolute change in elevation (m) Distance (m) 0-0.5 Elevation (m) 0.5-1.0 B 1.0-1.5 1.5-2.0 Distance (m) 2.0-2.5 Conclusions

CAESAR lisflood can replicate geomorphic effects of human intervention in fluvial systems, this includes: River bed incision Upstream incision Delta formation

Model produces plausible erosion and deposition under extreme flood conditions

Long duration floods with relatively low discharge are geomorphically important