1. International Levee Handbook Overview of the handbook Chapter 9 Design

2. www.ciria.org Chapter 9 - Context

3. www.ciria.org Chapter 9 - Design Typical cross-section, (courtesy Peter Buck, Sacramento Area Flood Control Agency)

4. www.ciria.org Section 9.1 – Principles of levee design Sets out the principles which underpin levee design. Provides design principles and considerations applicable to all levees. Guiding Principles: –Impermeability –Stability –Durability –Resilience –Serviceability –Whole life planning and sustainability Section 9.1 provides a list of important engineering considerations related to common modes of failure.

5. www.ciria.org Section 9.1 – Principles of levee design Key issue – balancing conflicting requirements A balance must be found between conflicting engineering, environmental, social and economic factors at the earliest possible stage in the development of a project. For example: –wall vs view –natural vegetation vs grass –leisure access vs maintenance Flood wall Sunbury, Pennsylvania, USA (courtesy Baltimore District, USACE)

6. www.ciria.org Section 9.2 – The levee design process Design process starts with a decision that change is needed. Identifies roles and responsibilities of key players and their interactions: –Asset owner (and maintainer) –Designer –Constructor Sets out key stages in the design process from identification of the need to operating and maintaining the new or modified asset. Highlights importance of phasing larger projects to meet statutory planning and environmental requirements.

7. www.ciria.org Section 9.2 – The levee design process Key issue – staging the design Flow charts are provided for guidance (see next slide). Flow charts are provided to guide readers through the design process and the key technical stages. Flow charts from Chapter 9 of ILH

8. www.ciria.org

9. Section 9.3 – Reporting and documentation Predicting future performance of existing levees requires a knowledge and understanding of ground conditions, levee construction and historical response to flooding. Historically, this information has not always been acquired, recorded or archived. It is important that this situation is reversed and that modern levees record relevant data whenever a levee is constructed or adapted or when subjected to flooding. Section 9.3 provides recommendations for good reporting practice throughout the key stages of levee design, construction and operation.

10. www.ciria.org Section 9.3 – Reporting and documentation Key reports and documents: Risk register – highlights major risks and shows how these are managed through the design process. Basis of design report – records key design requirements and criteria; is updated throughout design process. Site investigation report – records site investigations and ground investigations; includes interpretative data. Design report – documents design assumptions, calculations and conclusions; sets basis for design documentation. Detailed design output – drawings and specifications setting out detailed requirements for construction and construction records. Operations and maintenance manual – sets out operations and maintenance requirements on which design is based.

11. www.ciria.org Section 9.4 – Levee layout and alignment Levee alignment is an important early consideration. Can be controlled by many considerations (safety, risk, social, environmental, hydraulic, geomorphological, geotechnical and practical considerations, etc). Selection of final alignment may require careful hydraulic and geomorphological study in order to balance the risks and benefits associated with new levee construction. Issues such as set-back levees, no-construction zones, vegetation management zones and interior drainage systems are discussed.

12. www.ciria.org Section 9.4 – Levee layout and alignment Benefits of set-back alignment: Better soil for levee foundations Widens river channel to improve flow at choke points Replaces old, defective levee without increasing risk during construction Moves levee from erosional points Expands floodplain to make space for river in flood and benefit threatened species through habitat creation Example – Set-back levee on Feather River Levee realignment on Feather River (courtesy Three Rivers Levee Improvement Authority, Yuba County, California, USA

13. www.ciria.org Section 9.5 – Levee geometry Describes how the geometry of the levee is defined Setting crest levels for riverine levees –Hydraulic modelling –Freeboard –Superiority considerations –Spillways Setting crest levels for coastal levees –Sea levels, tides and waves –Storm surge Levee cross section design (crest width, side slopes, etc) starts with environmental and operational requirements and is modified as necessary as the design progresses

14. www.ciria.org Section 9.5 – Levee geometry Geometry established to: Preserve mature waterside vegetation Meet technical objectives for safety, impermeability, durability and resilience Create habitat without compromising on engineering performance. Example – Natomas Levee Improvement Programme Typical cross-section, (courtesy Peter Buck, Sacramento Area Flood Control Agency)

15. www.ciria.org Section 9.6 – Surface protection measures Surface protection of levees is required to protect the levee against: –Waterside erosion against current velocities and wave action –Crest and landside erosion from overflow and wave overtopping Section 9.6 identifies a range of potential measures to protect against external erosion. –Grass and reinforced grass –Armourstone –Gabions –Flexible concrete block systems –Soil cement mixing (roller compacted concrete)

16. www.ciria.org Section 9.6 – Surface protection measures Reinforces turf to provide greater resilience to overflow without the use of heavy construction materials (eg concrete) Provides greater resilience to external loads such as wheel loading Particularly effective in short term before grass cover is well established on new levee surface. Example – Turf reinforced mattress, New Orleans Turf reinforced mattress (courtesy USACE, 2011)

17. www.ciria.org Section 9.7 – Control of seepage and uplift The primary function of a levee is to provide a barrier to the passage of water during a flood. Levees should be designed to control: –The flow of water through the levee body –The flow of water through the ground beneath the levee –The flow of water along interfaces between the levee and incorporated structures In addition to the transmission of flood water, seepage can lead to levee instability through various mechanisms. Section 9.7 identifies methods of seepage control and outlines the various benefits and disadvantages of each.

18. www.ciria.org Section 9.7 – Control of seepage and uplift Relative benefits and disadvantages of various seepage control measures Images courtesy Richard Bird, URS

19. www.ciria.org Section 9.8 – Control of internal erosion The issue of internal erosion is critical to the design of large dams. The important mechanisms are: –Backward erosion –Concentrated leak erosion –Suffusion –Contact erosion Exactly the same processes can occur in levees but the periods of seepage are generally much reduced. Section 9.8 provides guidance on evaluating the potential for internal erosion, assessing the potential impact and designing to manage the risk.

20. www.ciria.org Section 9.8 – Control of internal erosion Backward erosion Concentrated leak erosion Suffusion Internal erosion mechanisms

21. www.ciria.org Section 9.9 – Mass stability throughout levee life Discusses the importance of designing the levee to resist instability during critical stages of a levee’s life, including: –During construction –During a flood –After a flood (rapid drawdown failures) –During an earthquake Section 9.9 highlights important issues to be considered when designing levees to resist the failure mechanisms relevant to each of the situations described above. Possible approaches to improving stability by design are set out in Section 9.9.

22. www.ciria.org Section 9.9 – Mass stability throughout levee life During a flood Examples of potential failure mechanisms to be considered during the design During construction Rapid drawdown Image courtesy Mary Perlea, USACE Image courtesy USACE, 2003 Image courtesy STOWA, NL

23. www.ciria.org Section 9.10 – Analysing failure mechanisms This section sets out the steps necessary to perform stability calculations for cross-sectional design of a levee. Reference is made to Chapter 7 (for site characterisation) and Chapter 8 for the detail of the methods of calculation. Commonly adopted safety factors are provided for various design approaches including the “global factor” approach and the partial factor approach adopted by the Eurocodes. These safety factors are related to the various failure mechanisms discussed in Section 9.9 and to the risk level. Particular consideration of the approaches adopted by different European countries to Eurocode design is given.

24. www.ciria.org Section 9.10 – Analysing failure mechanisms Target global factors of safety and allowable hydraulic gradients for levee design (after DWR, 2012)

25. www.ciria.org Section 9.11 – Transitions Many levee failures that have occurred during flooding are a result of overtopping, overflow and internal erosion. It is estimated that at least 50% of failures caused by internal erosion are linked to transitions. Transitions can include: –Change in levee type or structural form (eg embankment and wall) –Part buried structures and encroachments (eg culverts) –Buried structures (eg pipes and cables) –External structures (eg drainage ditches and boundary walls) Section 9.11 provides a discussion on the principles of transition design and examples of established techniques.

26. www.ciria.org Section 9.11 – Transitions Careful thought should go into the design of all transitions. In each case, the objective should be to lengthen and disrupt potential seepage paths by carefully controlling the construction of the interfaces between material types to prevent seepage and hydraulic separation. The example provided is taken from the levee at Comps close to the Gard and the Rhône rivers (courtesy Thibaut Mallet).

27. www.ciria.org Section 9.12 – Design for serviceability This section details how levees can be designed to manage serviceability problems such as settlement, desiccation and animal burrowing. Emphasis is placed on proactive assessments of the impact of settlement, desiccation and animal burrowing but remedial works options are also discussed. The importance of managing the serviceability issues is highlighted through a discussion of how levee failures can be initiated by serviceability related problems. Examples of methods that have been used to manage potential serviceability problems are provided.

28. www.ciria.org Section 9.12 – Design for serviceability Section 9.12 provides examples of designs and methodologies used to control the damage caused by burrowing animals. The method shown in the top image uses a wire mesh buried beneath the topsoil. The lower image shows how geosynthetic clay liners can be used in conjunction with layers of locally available gravel. The examples provided are taken from remedial works at Arles in France (upper image) and new levee construction method used to ward off burrowing animals in an area of Germany where animal burrowing is common (lower image). Courtesy SYMADREM and EGISeau Courtesy Heerten and Werth, 2006

29. www.ciria.org Section 9.13 – Levee Earthworks This section sets out how the performance of a levee can be affected by selection, deposition and compaction of earthworks materials. The section discusses material selection and specification and provides examples of how both good and poor quality materials can be incorporated into a levee. The effect of material selection and compaction on key performance characteristics such as strength, permeability, compressibility and density are considered. This section provides detailed discussions on levee construction, the use of geofabrics and innovative techniques for construction and ground improvement.

30. www.ciria.org Section 9.13 – Levee Earthworks Section 9.13 highlights the importance of compaction control during the construction of levees. Levees are prone to additional settlement when they first impound water. This potential problem can be controlled through careful control of the air voids ratio of the compacted fill during construction. Basis of design for engineering fill using five per cent air voids (after Trenter and Charles, 1996)

31. www.ciria.org Section 9.14 – Spillways This section covers the design of levee spillways: –hydraulic design to establish location, length, level and shape –design of the spillway structure to provide resilience to overflow The hydraulic design process is iterative and depends on both the impact of the spillway on the flood hydrograph and the characteristics of the flow over the spillway itself Design principles and examples are presented for: –Massive spillway structures –Protected earth embankments –Grass spillways –Variable spillways (erodible, removable, inflatable and collapsible). –Gated spillways

32. www.ciria.org Section 9.14 – Spillways Example: Spillway on the left bank of the Rhône at Vallabrègues, France The spillway (coloured yellow) discharges into a flood expansion zone bounded by levees and high ground. Water starts to overflow in a 1 in 10 year event and peaks in a 1 in 50 year event. The maximum storage capacity is reached in a 1 in 1000 year event. Use of a spillway on the left bank of the Rhône at Vallabrègues (Degoutte et al, 2012)

33. www.ciria.org Section 9.15 – Associated Structures The introduction of rigid structures into relatively flexible earthen levees can create interface problems which can lead to rapid and catastrophic failures if they are not designed appropriately. This section covers the design of associated crest structures including: –Crest walls –Embedded walls –Pipes, conduits and culverts Critical design principles are established and examples of both good practice and historical failures are provided.

34. www.ciria.org Section 9.15 – Associated Structures Example: T-Wall design, New Orleans The failure of the “I” walls in New Orleans during Hurricane Katrina caused catastrophic flooding. Subsequent investigation has identified the causes of the failures and provided improved design principles. Section 9.15 provides a description of the failure mechanisms and sets out the new design principles. Examples are provided. Typical T-wall cross-section in LPV 105 (courtesy Barry Fehl / URS)

35. www.ciria.org Section 9.16 – Design input – design and construction Section 9.16 highlights the potential design input that may be required during the construction of a levee. Technical direction on site may be required: –To check for compliance with the design assumptions –To react to ground conditions encountered during construction and modify or adapt the design if necessary –To modify the design if the assumed materials are unavailable –To monitor earthworks behaviour before, during and after construction, particularly when constructing on soft clay foundations –To update the design report and the operations and maintenance manual on the basis of observations made during construction. Section 9.16 also considers the use of the observational method for levee construction.

36. www.ciria.org Section 9.16 – Design input – design and construction Use of the observational method for levee design The observational method is an emerging method for the delivery of civil engineering projects; it trades conservative design for site observations during construction. The use of the observational method for levee design and construction is limited because the key design decisions relating to performance during a flood cannot easily be tested during construction. The observational method can however be used effectively to optimise the rate of levee construction when building on soft clay foundations.

37. www.ciria.org Summary The design of levees can involve a complex balancing act between constraints and objectives. In producing a design, the designer must not lose sight of the principal objective of creating a sustainable barrier to the passage of water. The design process for levees (Source: ILH Chapter 9)

38. www.ciria.org Thank you Chapter 9 Team Lead: To download the International Levee Handbook go to: www.ciria.org/ILHwww.ciria.org/ILH Dr Philip Smith Royal HaskoningDHV E: philip.smith@rhdhv.com