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

© HR Wallingford 2012

Presentation summary

1. Motivation and drivers for ILH

2. Understanding ‘Failure’

3. ILH approach to vegetation

Structure of International Levee Handbook (ILH)


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.


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.


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


Levee form / components

To establish risk of failure of a levee segment, we need to understand failure mechanisms, including contributions of various levee components


Functions of levee components

> Asdasd


‘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


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


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


ILH structure

Overtopping: 2,6,8 Deterioration: 4,5,9

Damage: 3,8,9

Breach: 6,8


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.


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


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


Proposed primary dike deviations

(Netherlands, Deltares, 2011)


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)


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


French selective cutting


Trimming/thinning for access & visibility and for levee integrity

(United Kingdom, Environment Agency, 2011)


Levee with trees – Boston, UK

Levee being monitored as part of EU FP7 project UrbanFlood


Tidal variation; landside ‘slope’


-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


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.


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

http://www.leveehandbook.net/

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