International Levee Handbook (ILH) significance for vegetation management. Jonathan Simm HR Wallingford, United Kingdom

Transcription

1 International Levee Handbook (ILH) significance for vegetation management Jonathan Simm HR Wallingford, United Kingdom

2 Presentation summary 1. Motivation and drivers for ILH 2. Understanding Failure Structure of International Levee Handbook (ILH) 3. ILH approach to vegetation Page 2

3 Handbook motivation > Concern about recent international levee failures during severe flooding events causing loss of life and severe property damage: Katrina (2005) Xynthia (2010) - Atlantic coast of France Summer 2007 fluvial floods UK > Address poor understanding and interface between geotechnics and H&H communities of practice in levee design and management > Desire to produce good practice handbook for levee management and design target audience: owners, operators, designers & constructors. Page 3

4 Partner countries / organisations France scientists (IRSTEA, CETMEF) and practicing engineers Germany scientists (Univ Dresden, Hanover) involved in German guidance and standards Netherlands STOWA (research arm of Dutch water boards, private companies United Kingdom Environment Agency, CIRIA, HR Wallingford, private companies United States USACE, private companies, etc, support from National Levee Safety Ctte. Page 4

5 > UK probabilistic flood systems analysis uses a frequency-based approach to levee failure (fragility curves), as well as overtopping. > This allows back-attribution of the residual risk in the flood plain to levee segments. > Relative residual risk helps to prioritise investment Failure frequency/probability Probability of defence failure Standard of protection provided by defence Difference relates to factor of safety in design 1.0 Typically assumed fragility curve True fragility 0 0 Severity of load event Page 5

6 Levee form / components To establish risk of failure of a levee segment, we need to understand failure mechanisms, including contributions of various levee components Page 6

7 Functions of levee components > Asdasd Page 7

8 Failure definitions (& difficulties) Types of structural mechanism failures > Deterioration Generally slow process by which resilience or standard of protection is reduced. Deterioration is best managed by good maintenance. > Damage Rapid deterioration such as scour and slippage (commonly during a flood) that does not cause a breach but which requires immediate repair or emergency action. > Breach Catastrophic collapse (often at high water level). Results in significant loss of crest and causes substantial loss of water. Time and causation chains are significant in distinguishing between the above. Hydraulic performance mechanisms > Performance Failure Overtopping or through-flow at an hydraulic elevation or load below the design standard which does not cause damage or breach Page 8

9 Scenarios of mechanisms Breach Key Loss of crest Animal borrowing Event Overtopping Erosion AND Gate OR Gate Overtopping Lack of resistance to surface erosion Lack of scour protection Loose noncohesive soils Animal borrowing Deterioration Extreme high water level Local overtopping Desiccation cracking Poor vegetation control Overtopping Erosion Damage BREACH Breach Local Settlement High water level High water level External Event Consolidation of foundation soils Compaction of fill materials Internal erosion of fill or foundation soil Animal borrowing Trafficking by animals or humans Overtopping Local Event Page 9

10 Failures and levee management life cycle > Levee deterioration and damage in routine levee management (O&M, assessments) > Managing damage and breach before, during and after severe events > Allowing for external change (loadings, land-use) and effect on probability or consequence of failure Page 10

11 ILH structure Deterioration: 4,5,9 Overtopping: 2,6,8 Damage: 3,8,9 Breach: 6,8 Page 11

12 Vegetation analysed > To be consistent with rest of handbook, we must consider positive/negative impact of vegetation in terms of: Failure processes: Deterioration, damage, breach, crest lowering Functions of levee components: stability, impermeability, surface protection, filtration, drainage > Levee handbook considers the following principles in detail for vegetation (without prescribing solutions in conflict situations): Levees should be protected from external erosion (by grass cover) Adequate access and visibility should be maintained Development of vegetation-induced damage or defects should be avoided Existing woody vegetation should be managed to minimize environmental impacts. Page 12

13 Adverse impacts of woody vegetation Deterioration mechanism Blowover/Overturning of large tree Root Penetration (and subsequent decay) Woody Vegetation Weight and Wind Loading Potential role of woody vegetation Removal of large section of a levee during a flood event. On the waterside, levee susceptible to scour. Change of soil permeability or concentration of seepage Uplift/cracking of rigid levee protection elements Adverse transfer of loading to levee slope. Potential levee damage modes affected Overtopping, External erosion, Slope instability, Throughseepage, Under-seepage, and Internal erosion and piping Through-seepage, Underseepage, Internal erosion and piping, and External erosion Slope instability Scour flows Flow concentrations or eddies in waterside or overtopping flows. External erosion Burrowing Attraction of burrowing animals into a levee. Internal erosion and piping Discouraging growth of grass and turf (sod) resulting in bare patches Blocking sunlight, absorbing nutrients and moisture or releasing herbicidal chemicals External erosion Page 13

14 Proposed rules for primary dikes (Netherlands, Deltares, 2011) Page 14

15 Proposed primary dike deviations (Netherlands, Deltares, 2011) Page 15

16 Existing vegetation: secondary dikes (Netherlands, STOWA, 2011) It is inherent in evaluating existing vegetation that the presence of the vegetation entails an extra risk, but that this (for the lifespan of the vegetation) is acceptable if certain conditions are met. The presence of vegetation could lead to damages to the flood defence, but these damages do not immediately result in a failure of the flood defence. This implies that the presence of vegetation is undesirable from a safety perspective, but that existing vegetation can be tolerated if it does not immediately result in a failure of the flood defence. Page 16

17 Managing large and wide levees (France, IRSTEA, 2011) Page 17

18 French selective cutting Page 18

19 Trimming/thinning for access & visibility and for levee integrity (United Kingdom, Environment Agency, 2011) Page 19

20 Levee with trees Boston, UK Levee being monitored as part of EU FP7 project UrbanFlood Page 20

21 Tidal variation; landside slope Page 21

22 Variation of relative strain 1, m Relative strain - TOP 167 m 1,000.0 µstrain m/m , Section B:B Section A:A Section C:C Distance along the sensor from Towerstreet to Grand Sluice bridge "Y" Top/Bottom trench 6/14/2011 6/29/2011 8/11/2011 8/16/2011 Page 22

23 European middle-ground positions > Perform an initial evaluation or assessment that would lead to removing woody vegetation that poses an obvious or serious threat to levee integrity (e.g. Deltares, 2011). > Avoid roots penetrating into critical parts of levees, but allow woody vegetation or retained tree roots on wide levees or on seepage or stability berms (e.g. Environment Agency, Rijkswaterstaat, STOWA) if a seepage barrier is added (e.g. slurry cutoff wall from Pohl, 2011 and Pinhas, 2011). > Trim and/or thin woody vegetation in some places on the levee to provide visibility and access (e.g. Environment Agency Guidelines for England and Wales, 2010). Wait and see Boston example > Consider staged efforts over time that is, phasing of vegetation removal over long periods of time (Irstea, 2011). Maintain a sufficient grass cover to control invading fast-growing and light-demanding invasive species. Page 23

24 Outstanding scientific questions > Woody vegetation has environmental benefits. But does woody vegetation have any positive benefits in resisting some levee failure modes, e.g. slope instability? How are these balanced against the adverse effects? > Is failure caused by woody vegetation (a) (slow) deterioration which can be effectively managed, or (b) (fast) failure which may lead to breach in a flood event? > What is relative significance of failures due to woody vegetation in relation to other causes (e.g. burrowing animals) > If woody vegetation can (sometimes) be managed rather than removed, what evidence is needed to develop appropriate management plans? > If woody vegetation has to be removed, what procedures and timescales can be adopted whilst maintaining levee integrity (e.g. with regard to existing tree roots)? Page 24

25 > Draft for review by September 2012 > Technical work complete March 2013 > Editing / printing complete September 2013 > Will be available as hard copy and pdf Questions? See also HR Wallingford Howbery Park, Wallingford, Oxfordshire OX10 8BA, United Kingdom tel +44 (0) fax +44 (0) info@hrwallingford.com