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International Levee Handbook (ILH) –significance for vegetation management Jonathan Simm HR Wallingford, United Kingdom
© HR Wallingford 2012 Page 2
Presentation summary
1. Motivation and drivers for ILH
2. Understanding ‘Failure’
3. ILH approach to vegetation
Structure of International Levee Handbook (ILH)
© HR Wallingford 2012 Page 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.
© HR Wallingford 2012 Page 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.
© HR Wallingford 2012 Page 5
Failure frequency/probability
Standard of protection provided by defence
Difference relates to factor of safety in design
Severity of load event
Probability of defence failure
00
1.0
‘True’ fragilityTypically assumed fragility
curve
> 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
© HR Wallingford 2012 Page 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
© HR Wallingford 2012 Page 7
Functions of levee components
> Asdasd
© HR Wallingford 2012 Page 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
© HR Wallingford 2012 Page 9
Key
Consolidation of foundation soils
Compaction of fill materials
Internal erosion of fill or foundation
soilAnimal borrowing
Trafficking by animals or humans
High water level
Extreme high water level
Local overtopping
LocalSettlement
Overtopping
Desiccation cracking
Poor vegetation control
Lack of resistance to
surface erosionLoose non-
cohesive soilsLack of scour
protection
Overtopping Erosion
Breach
Loss of crest
AND Gate
OR Gate
Animal borrowing Event
Animal borrowing Deterioration
BREACH
Overtopping Erosion
Breach
Damage
High water level External Event
Overtopping Local Event
Scenarios of mechanisms
© HR Wallingford 2012 Page 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
© HR Wallingford 2012 Page 11
ILH structure
Overtopping: 2,6,8 Deterioration: 4,5,9
Damage: 3,8,9
Breach: 6,8
© HR Wallingford 2012 Page 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.
© HR Wallingford 2012 Page 13
Adverse impacts of woody vegetation Deterioration mechanism Potential role of woody
vegetation Potential levee damage modes affected
Blowover/Overturning of large tree
Removal of large section of a levee during a flood event. On the waterside, levee susceptible to scour.
Overtopping, External erosion, Slope instability, Through-seepage, Under-seepage, and Internal erosion and piping
Root Penetration (and subsequent decay)
Change of soil permeability or concentration of seepage Uplift/cracking of rigid levee protection elements
Through-seepage, Under-seepage, Internal erosion and piping, and External erosion
Woody Vegetation Weight and Wind Loading
Adverse transfer of loading to levee slope.
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
© HR Wallingford 2012 Page 14
Proposed rules for primary dikes (Netherlands, Deltares, 2011)
© HR Wallingford 2012 Page 15
Proposed primary dike deviations
(Netherlands, Deltares, 2011)
© HR Wallingford 2012 Page 16
Existing vegetation: secondary dikes
“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’.”
(Netherlands, STOWA, 2011)
© HR Wallingford 2012 Page 17
Managing large and wide levees (France, IRSTEA, 2011)
© HR Wallingford 2012 Page 18
French selective cutting
© HR Wallingford 2012 Page 19
Trimming/thinning for access & visibility and for levee integrity
(United Kingdom, Environment Agency, 2011)
© HR Wallingford 2012 Page 20
Levee with trees – Boston, UK
Levee being monitored as part of EU FP7 project UrbanFlood
© HR Wallingford 2012 Page 21
Tidal variation; landside ‘slope’
© HR Wallingford 2012 Page 22
-1,000.0
-500.0
0.0
500.0
1,000.0
1,500.0
0.0 50.0 100.0 150.0 200.0 250.0
µstr
ain
- 10-
6 m
/m
Distance along the sensor from Towerstreet to Grand Sluice bridge
Relative strain - TOP
6/14/2011
6/29/2011
8/11/2011
8/16/2011
Sect
ion
C:C
Sect
ion
B:B
Sect
ion
A:A
"Y"
Top/
Bott
om tr
ench
47 m
167
m
Variation of relative strain
© HR Wallingford 2012 Page 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.
© HR Wallingford 2012 Page 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)?
Questions? See also www.leveehandbook.net
HR Wallingford Howbery Park, Wallingford, Oxfordshire OX10 8BA, United Kingdom tel +44 (0)1491 835381 fax +44 (0)1491 832233 email [email protected]
> Draft for review by September 2012
> Technical work complete March 2013
> Editing / printing complete September 2013
> Will be available as hard copy and pdf