The Impact of Woody Vegetation on Levees Research Experiences from Austria

Transcription

1 EGU 2012 LEVEE VEGETATION RESEARCH SYMPOSIUM 2012 Sacramento, California, August 28 30, 2012 The Impact of Woody Vegetation on Levees Research Experiences from Austria Walter LAMMERANNER Institute of Soil Bioengineering and Landscape Construction University of Natural Resources and Life Sciences, Vienna, Austria

2 Institute of Soil Bioengineering and Landscape Construction Department for Civil Engineering and Natural Hazards University of Natural Resources and Life Sciences (BOKU) Vienna, Austria Head: Prof. Dr. Florin Florineth

3 2006 floods in Austria

4 March River VIENNA Lower Austria border river to Slovakia ~ 50 km from Vienna

5 Levee failures Poor condition of levees (construction ) Overtopping of levees

6 Levee failures But also only poor reinforcement and protection through grass-vegetation

7 Soil bioengineering techniques using woody plants These techniques have a long history within riverbank stabilization and restoration works Why do not use them on levees as well? Source: Florineth, 2004 Source: Florineth, 2004

8 Decision to test willow-brush-mattresses on levees What impacts do these vegetation structures have on levees?

9 Potential negative impacts of woody vegetation Facilitated water movement along root penetration paths Decaying roots lead to voids, threaten structural integrity and facilitate percolation Additional load through trees which transfer forces (wind, water, snow) into the levee Windthrow of trees can lead to damage or even collapse of the levee

10 Potential negative impacts of woody vegetation Surface erosion along stems through water turbulences Attractive habitat for burrowing animals Hindering effective levee inspections and defense operations

11 Vegetation grown from brush-mattresses potentially can: Protect the surface trough dense and flexible stems Reinforce the upper soil layer and protect the surface from erosion by a dense rooting system Transpire moist from the soil which aids to soil stability (even below the rooting zone) Reduce the flow velocity near the levee surface

12 And potentially do not: Led to additional load (which is minimized) Endanger the levee through windthrow plants stay shrubby and if getting too large they can be cut to the ground and grow again afterwards

13 And potentially do not: Led to decaying roots Vegetation is kept vital through repetitive cutting to the ground

14 And potentially do not: Led to surface erosion along stems Dense stand of flexible and thin stems

15 And what about:? Attracting burrowing animals Are burrowing animals attracted by woody plants? And if not which species? Especially if the vegetation is very dense?

16 And what about:? Hindering levee inspection and defense Woody vegetation has to be limited to particular areas of the levee

17 Objectives Test willow-brush-mattresses on levees compacted to modern standards Investigations regarding to: Plant performance and development (above- and below ground) Impact on seepage and soil water balance Impact on the structural performance of the levee

18 Site layout

19 Cross section and measurement devices

20 Grain size distribution silt-sand-gravel composite

21 Compaction Proctor density

22 Compaction tests Dynamic plate load tests Sampling rings Isotope probe

23 Willow-brush-mattresses Using Purple-Willow (Salix purpurea) a shrubby willow with a large ecological amplitude and very good propagation properties

24 Plant performance 4 th growing season After implementation

25 Plant performance above-ground diameter and length

26 Seepage tests

27 Seepage tests

28 Seepage line water-side slope face 160,0 160,0 159,5 159,5 159,0 159,0 158,5 158,5 158,0 158,0 157,5 157, water level bassin willows grass-herb-vegetation water level bassin willows grass-herb-vegetation Test 1 Test 2 (two weeks later)

29 Seepage line middle of the crest 160,0 160,0 159,5 159,5 159,0 159,0 158,5 158,5 158,0 158,0 157,5 157, water level bassin willows grass-herb-vegetation water level bassin willows grass-herb-vegetation Test 1 Test 2 (two weeks later)

30 Root investigations

31 Root investigations Measurable maximum rooting depth ~ 80 cm in the 3 rd growing season Concentration of roots in the upper 30 cm

32 Root investigations Fleece-like dense network of fine roots

33 root mass 1. vegetation period above-ground biomass 1. vegetation period biomass per m² [g] ,9 194,0 biomass per m² [g] ,2 211, ,0 0 2,1 0 roots <2 mm roots 2-5 mm total roots branches leaves total biomass

34 Root-shoot ratio: 0,9 root mass 1. vegetation period above-ground biomass 1. vegetation period biomass per m² [g] ,9 194,0 biomass per m² [g] ,2 211, ,0 0 2,1 0 roots <2 mm roots 2-5 mm total roots branches leaves total biomass

35 root mass 2. vegetation period above-ground biomass 2. vegetation period biomass per m² [g] ,4 294,9 biomass per m² [g] ,4 313, , ,0 0 0 roots <2 mm roots 2-5 mm total roots branches leaves total biomass

36 Root-shoot ratio: 0,9 root mass 2. vegetation period above-ground biomass 2. vegetation period biomass per m² [g] ,4 294,9 biomass per m² [g] ,4 313, , ,0 0 0 roots <2 mm roots 2-5 mm total roots branches leaves total biomass

37 root mass 3. vegetation period above-ground biomass 3. vegetation period ,2 601, ,8 557,3 biomass per m² [g] biomass per m² [g] ,8 17,9 roots <2 mm roots 2-5 mm roots 5-10 mm total roots ,5 branches leaves total biomass

38 Root-shoot ratio: 1,1 root mass 3. vegetation period above-ground biomass 3. vegetation period ,2 601, ,8 557,3 biomass per m² [g] biomass per m² [g] ,8 17,9 roots <2 mm roots 2-5 mm roots 5-10 mm total roots ,5 branches leaves total biomass

39 Research on soil water balance Lysimeter like design Impact of vegetation on soil water balance Comparison of rooting systems between willows and grass-herb vegetation

40 Research on soil water balance Lysimeter like design soil moisture probes tensiometers soil material drain weighing device

41 Research on soil water balance Lysimeter like design

42 Soil water moisture 30 cm depth 1 st vegetation period without vegetation grass-herb-vegetation willows 0,25 0,20 0,15 0,10 0,05 0, volumetric water content [Vw/Vt] time Axis

43 Pore water pressure 30 cm depth 1 st vegetation period 1000 without vegetation grass-herb-vegetation willows pore water presure [hpa] time axis

44 Soil water moisture 30 cm depth 2 nd vegetation period without vegetation grass-herb-vegetation willows 0,25 0,20 0,15 0,10 0,05 0, Volumetric water content [Vw/Vt] Times axis

45 Pore water pressure 30 cm depth 2 nd vegetation period 1000 without vegetation grass-herb-vegetation willows pore water presure [hpa] Time axis

46 Root investigations

47 Root mass comparison root mass per soil volume 2. vegetation period root mass/soil volume [mg/cm³] 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 root mass per soil volume 2. vegetation period root mass/soil volume [mg/cm³] 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 depth [cm] Drain Willow vegetation depth [cm] Drain Grass-herb vegetation

48 Root length comparison willow vegetation root length per soil volume 1. vegetation period root length/soil volume [cm/cm³] root length per soil volume 2. vegetation period root length per soil volume [cm/cm³] depth [cm] depüth [cm] Drain 0<.L.<= <.L.<= <.L.<=1 1<.L.<=2 2<.L.<=5 5<.L.<=10 0<.L.<= <.L.<= <.L.<=1 1<.L.<=2 2<.L.<=5 5<.L.<=10 1 st vegetation period 2 nd vegetation period

49 Overall Preliminary conclusions Shrubby willows grown from brush-mattresses seem to be an appropriate vegetation form for well compacted levees They develop a dense and near surface root system They form a dense stand of thin and flexible stems Up to now they don t have a significant impact on seepage They have a significant positive impact on soil water balance But: Research work has to be continued on the long term run

50 Many thanks to all staff members and students who have been involved in the project!

51 Thanks for your attention!