Integrated Flood Risk Analysis and Management Methodologies

For the engineer, vegetation is a highly variable material hence calculations with vegetation will carry a high degree of uncertainty. The type and condition of vegetation on an embankment can have both positive and adverse effects on embankment performance. Embankments considered within this report comprise sea, river and reservoir flood defences. Why is there vegetation on an embankment? In practice it is important for an embankment to maintain integrity and stability up to (and preferably beyond) the maximum design load conditions. To cope better with these loads, the surface layer is typically protected by different means, including vegetation. As well as the main goal of providing surface protection, the design of embankments may also consider the creation of a more natural river environment and habitats and to promote recreational areas for the surrounding population. In this role, the selection of appropriate vegetation is fundamental. What is the influence of vegetation? Vegetation can have a number of effects beyond the provision of resistance to erosion. Vegetation may affect the conveyance of the river and the performance deterioration processes of embankments. Erosion resistance Vegetation increases the resistance of the embankment surface to erosion because it reduces flow velocity and shear stresses near to the embankment soil surface. Vegetation can also provide direct surface protection to erosion if it is long and flexible enough to lay and cover the soil surface when it is affected by a flow. Further protection from vegetation is provided by the root mat which develops below the bed surface of the embankment providing additional strength to the soil structure. Conveyance Conveyance is a quantitative measure of the discharge capacity of a watercourse. Water levels related to a certain discharge are mainly influenced by the resistance to flow due to surface roughness. In channel vegetation modifies the roughness surface depending on its type (flexibility, size, etc), the season (the expected biomass and the percentage covered will vary during the year), and other factors such as maintenance. Deterioration Vegetation can lead to embankment deterioration in several ways. One of the main types of deterioration relates to changes in soil moisture. In certain soils, prolonged extraction of moisture by plant roots can lead to desiccation and thus to cracking and seepage. Trees on an embankment can also promote local erosion around its trunk causing narrowing of the embankment crosssection. Ultimately, the failure of a tree and its roots could cause severe erosion of the soil. In addition, as vegetation promotes the creation of habitats, it can also encourage burrowing within embankments by vermin. Management actions also affect the performance of vegetation. For example, the eventual strength of the grass is governed by the management regime. Species that are not managed tend to have weak and sparse roots which results in a grass layer far less resistant to erosion.

Integrated Flood Risk Analysis

and Management Methodologies

Grass Erosion on Embankments

AN OVERVIEW

Revision Number 9_1

Task Leader HR Wallingford

FLOODsite is co-funded by the European Community Sixth Framework Programme for European Research and Technological Development (2002-2006) FLOODsite is an Integrated Project in the Global Change and Eco-systems Sub-Priority

Start date March 2004, duration 5 Years Document Dissemination Level

Co-ordinator: HR Wallingford, UK

Project Contract No: GOCE-CT-2004-505420

DOCUMENT INFORMATION

Title Erosion of Embankments - A Review of Current Knowledge

Lead Author Marta Roca Collell, Mark Morris

Contributors Georg Petersen

DISCLAIMER

This document reflects only the authors’ views and not those of the European Community This work may rely on data from sources external to the FLOODsite project Consortium Members of the Consortium do not accept liability for loss or damage suffered by any third party as a result of errors

or inaccuracies in such data The information in this document is provided “as is” and no guarantee or warranty is given that the information is fit for any particular purpose The user thereof uses the information at its sole risk and neither the European Community nor any member of the FLOODsite Consortium is liable for any use that may be made of the information

© FLOODsite Consortium

is necessary to understand the processes and is a key element in managing failure risks

Past and present research on the erosion of surface vegetation has been reviewed and this report provides a brief summary along with listings of relevant projects and research findings Based upon these results, gaps in knowledge were identified and research needs described

Document Information ii

Document History ii

Acknowledgement ii

Disclaimer .ii

Summary .iii

Contents v

1 Introduction 1

1.1 General framework 1

1.2 Aims and objectives 2

1.3 Structure of this report 2

2 Summary of existing knowledge 3

2.1 Fundamentals about erosion resistance 3

2.2 Current and Recent knowledge sources 4

2.2.1 Research Projects 4

2.2.2 Reference Institutions 5

2.2.3 Journal Papers 6

3 Recommended actions and initiatives 7

3.1 Determination of time to failure 7

3.2 Vegetation management 7

3.3 Develop a wider management framework linking vegetation type, management, soil moisture, soil fissuring and soil erodibility 8

3.4 Improve basic knowledge and understanding of vegetation-erosion processes 8

3.5 Defining failure modes and mechanisms 9

3.6 Experimental and field data 9

3.7 Estimation of vegetation roughness 10

4 Summary and prioritisation of recommended actions 11

5 References 13

Appendix 1: Current Research Details 15

Tables Table 2.1 Selected research programmes and projects showing main research topic areas 4

Table 4.1 Summary and prioritisation of recommended actions 11

Figures Figure 1 General framework of vegetated embankments 2

1 Introduction

For the engineer, vegetation is a highly variable material hence calculations with vegetation will carry

a high degree of uncertainty

The type and condition of vegetation on an embankment can have both positive and adverse effects on embankment performance Embankments considered within this report comprise sea, river and reservoir flood defences

Why is there vegetation on an embankment?

In practice it is important for an embankment to maintain integrity and stability up to (and preferably beyond) the maximum design load conditions To cope better with these loads, the surface layer is typically protected by different means, including vegetation

As well as the main goal of providing surface protection, the design of embankments may also consider the creation of a more natural river environment and habitats and to promote recreational areas for the surrounding population In this role, the selection of appropriate vegetation is fundamental

What is the influence of vegetation?

Vegetation can have a number of effects beyond the provision of resistance to erosion Vegetation may affect the conveyance of the river and the performance / deterioration processes of embankments

Erosion resistance

Vegetation increases the resistance of the embankment surface to erosion because it reduces flow velocity and shear stresses near to the embankment soil surface Vegetation can also provide direct surface protection to erosion if it is long and flexible enough to lay and cover the soil surface when it

is affected by a flow Further protection from vegetation is provided by the root mat which develops below the bed surface of the embankment providing additional strength to the soil structure

Conveyance

Conveyance is a quantitative measure of the discharge capacity of a watercourse Water levels related

to a certain discharge are mainly influenced by the resistance to flow due to surface roughness In channel vegetation modifies the roughness surface depending on its type (flexibility, size, etc), the season (the expected biomass and the percentage covered will vary during the year), and other factors such as maintenance

Deterioration

Vegetation can lead to embankment deterioration in several ways One of the main types of deterioration relates to changes in soil moisture In certain soils, prolonged extraction of moisture by plant roots can lead to desiccation and thus to cracking and seepage Trees on an embankment can also promote local erosion around its trunk causing narrowing of the embankment cross-section Ultimately, the failure of a tree and its roots could cause severe erosion of the soil In addition, as vegetation promotes the creation of habitats, it can also encourage burrowing within embankments by vermin

Management actions also affect the performance of vegetation For example, the eventual strength of the grass is governed by the management regime Species that are not managed tend to have weak and sparse roots which results in a grass layer far less resistant to erosion

Figure 1 summarises the general framework of points highlighting how vegetation influences embankment performance

Figure 1 General framework of vegetated embankments

The objective of this review is to provide a brief overview of knowledge, and hence gaps in knowledge, relating to erosion of embankment surface protection The review does not repeat literature reviews and work already done by others, but instead references these and forms a concise overview of the current state of art, practice and, in particular, identifies where the gaps in knowledge are and what needs to be done to address these

1.3 Structure of this report

A summary of fundamental concepts about erosion resistance and existing research projects is presented in Chapter 2 Recommended actions and initiatives are presented in Chapter 3 (taking into account work done) and a suggested prioritisation of these actions is presented in Chapter 4

Supporting material on Current Research Details may be found in Appendix 1

2 Summary of existing knowledge

This review does not repeat literature reviews and work already done but instead, references these and forms a concise overview of the current state of the art to act as a base to identify where gaps in knowledge exist and how these may be addressed

A brief summary of key issues regarding erosion resistance is presented in Section 2.1 An overview

of recent and ongoing research projects is given in Section 2.2 In collating the information, outputs from Actions 1-3 of FLOODsite Task 4 are also incorporated, along with past and present research work undertaken in the UK, Germany, the Netherlands, the US and from EC and European national research programmes

The surface layer of an embankment needs to cope with a number of different conditions which can occur solely or in combination with each other Loading conditions, for which the intensity may vary include:

• Wave breaking impacts on the exposed (seaward / river ward) front face:

• Wave run up on the front face

• Lateral flow currents along the front face

• Overtopping flows over the crest and down the rear (landward) face:

• Overflowing - when water levels rise above the crest level and flow passes continuously over the crest

• Seepage flows through the embankment (developing into piping)

• Rapid drawdown, generating a risk from uplift pressures in the surface cover layers

In addition climatic factors have an effect on the embankment condition with, for example, rain and sun leading to wetting and drying of the embankment, combined with frost and ice causing further surface damage

Erosion of an embankment occurs when bank material is displaced by the effect of these loads causing friction, drag, lift and pressure forces Erosion can occur on any exposed face, for example the front face, inner toe, crest, rear face and outer toe

The initiation and speed of erosion is controlled by the difference between loading forces and resistance forces Whilst the loading forces are dependent on the environmental conditions, the resistance forces are dependent on the embankment design, its shape, type and size of materials used and quality of work

The erosion process generally starts when loading forces exceed the resisting forces of particles or elements As a first step, the resistant surface protection is attacked Only when this fails does the embankment itself starts to erode, which is normally a faster process than erosion of the surface protection Exceptions exist in the case of internal erosion where the soil matrix within the embankment body is eroded and particles start migrating internally causing stability loss and voids, eventually leading to piping

The effect of repetitive loading and stress concentration around non-homogeneities in vegetation is related with the concept of progressive collapse It can be seen that vegetation (e.g turf) can be weakened by repetitive loading Hence, discontinuous loadings as wave overtopping flow or wave impact has the potential to generate a more critical condition than steady overflow

2.2 Current and Recent knowledge sources

2.2.1 Research Projects

A considerable number of research programmes have been undertaken to improve knowledge about breach initiation and development but only a few of these projects specifically consider the performance of vegetation in cover layers Some of the current or recent larger projects and programmes are summarised in Table 2.1 below A more detailed description of the projects and research programmes is available through the summaries provided under Appendix 1

Table 2.1 Selected research programmes and projects showing main research topic areas

Type of dike

Type of vegetation

Failure modes/actions Research approach

Engineering tools for safe, efficient hydraulic structures and channels (2008)

Improved methods for predicting earthen embankment erosion and failure, and development of generalised hydraulic guidelines and tools for roller compacted concrete spillways used to protect earthen structures from erosion and to increase discharge capacity This will consider quantification

of the protective capabilities of vegetation

FLOODsite Task 4 (2006-2009)

Understanding and predicting failure modes This research collected existing information on defence failure mechanisms and extended knowledge in a number of critical areas by reviewing current international projects and detailed failure mode analysis on the basis of hydraulic model testing and numerical modelling (Allsop et al., 2007)

Wave Overtopping Simulator (Infram - 2007)

A wave overtopping simulator was developed in response to the need to test wave overtopping performance of grass covered dikes A prototype was developed and tested by Infram as part of the FLOODsite and ComCoast projects (van der Meer, 2006a) The simulator is placed on top of the embankments and flow surges released to simulate wave overtopping processes The performance of grass and sub soil may be assessed through to destruction

Studies were also undertaken into development of grass reinforcement to strengthen grass resistance to wave overtopping (van der Meer, 2006b)

IPET - Interagency Performance Evaluation Taskforce (2005-2007)

Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System The project comprised an intense performance evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System during Hurricane Katrina IPET applied some of the most sophisticated capabilities available in civil engineering to understand what happened during Katrina and why Their purpose was not just new knowledge, but application of that knowledge to the repair and reconstitution

of protection in New Orleans as well as improvement to engineering practice and policies The results

of much of the IPET work are largely already in the ground, having been transferred and applied prior

to the formal completion of the project report

• The System: What were the pre-Katrina characteristics of the hurricane protection system (HPS) components; how did they compare to the original design intent?

• The Storm: What was the surge and wave environment created by Katrina and the forces incident

on the levees and floodwalls?

• The Performance: How did the levees and floodwalls perform, what insights can be gained for the effective repair of the system, and what is the residual capability of the undamaged portions? What was the performance of the interior drainage system and pump stations and their role in flooding and dewatering of the area?

• The Consequences: What were the societal-related consequences of the flooding from Katrina to include economic, life and safety, environmental, and historical and cultural losses?

• The Risk: What were the risk and reliability of the hurricane protection system prior to Katrina, and what will they be following the planned repairs and improvements

RIMAX, Subject 3 Protection and Control (2005-2007)

Risk Management of Extreme Flood Events The aim of RIMAX was to develop and implement improved instruments of flood risk management by the integration of different disciplines and several participants Research focused on extreme flood events in river basins with mean events with a return period of more than a 100 years and a highly destructive potential Next to other tasks, Subject 3, Protection and Control, deals with:

• Dyke safety, monitoring and dyke protection

• Management of dams and retention systems

• Management of urban infrastructure (water supply, sewage etc.) during floods

• Risk-based reliability analysis of flood defence system

COMCOAST (2004-2007)

ComCoast was a European project that developed and demonstrated innovative solutions for flood protection in coastal areas ComCoast created multifunctional flood management schemes with a more gradual transition from sea to land, which benefits the wider coastal community and environment whilst offering economically sound options The ComCoast concept focused on coastal areas comprising embankments Smart grass reinforcement was extensively researched and tested with a wave overtopping simulator on real dikes (van der Meer, 2006a, van der Meer, 2006b)

Most of these projects are based upon, or use as a reference, previous investigations which focus on different aspects of the breaching process but not specifically vegetation (for example IMPACT, DSIG Breach, HR Breach, EurOTop, PRODEICH, CLASH, etc.)

2.2.2 Reference Institutions

Some research institutions are considered as a reference on this topic:

• CIRIA Is a member-based research and information organisation in the UK dedicated to improvement in the construction industry It has published grass performance curves relating total exposure time to failure under a steady flow velocity (Hewlett et al., 1985) These are widely used

in the UK both for design and performance assessment It has also recently published an updated version of the manual “Use of vegetation in civil engineering”(Coppin and Richards, 1990)

• TAW Technical Advisory committe for Flood Defence and its continuating platform ENW (Expertise Network for Flood Protection) sited in The Netherlands bring together specialist in the area of flood protection They have published several documents about grass cover as dike revetments (TAW, 1997, TAW, 1999) However, despite a programme of research from 1986 until at least 1996, detailed guidance on erosion resistance to overtoping and overflowing water is missing

• USACE / USDA / USBR The US Army Corps of Engineers, US Department of Agriculture and

US Bureau of Reclamation are all federal agencies that publish guidance documents to support embankment (levee) and dam design, construction and management The USDA (ARS-HERU at Stillwater) has a long history of research into grass-erosion performance aimed at aiding the construction and maintenance of small farm reservoirs

Another important topic covered by papers is the influence of vegetation in compound-channels flows

In compound channels the momentum transfer between the main channel and the floodplain strongly influences water levels In natural rivers, floodplains are often home to many kinds of vegetation that increases flow resistance Laboratory tests considering vegetation in compound channels are also found in literature

Bank erosion caused by hydraulic forces acting on the bank surface has also received a lot of attention The most commonly observed bank erosion phenomena in nature are the failure of banks due to geotechnical instability of the bank The rate of bank erosion (taking into account streamline flow and secondary currents) is linearly related to the excess near-bank velocity An erosion coefficient is included to account for variations in bend geometry and properties of bank material In this case vegetation is considered to influence bank roughness and cohesion

However, despite this wide range of ‘associated’ research into vegetation in and around the river channel, research that directly addresses the issue of bank vegetation preventing erosion induced by overtopping or overflowing is very limited

3 Recommended actions and initiatives

Despite many research studies being undertaken during the past three decades, the engineering role of vegetation in resisting erosion under various overtopping and overflowing conditions is still poorly understood Whilst some aspects of vegetation maintenance and management have been well researched and documented, others aspects relating to erosion performance are understood only qualitatively and application of knowledge is based on engineering experience and judgement

Seven actions are recommended to help address this gap in understanding vegetation performance:

Since existing guidance on vegetation performance is limited and over 20 years old, review, integration and analysis of more recent performance data (worldwide) might allow a quick advance in the provision of more reliable design / performance guidance for industry

Actions

• A detailed review of the basis of any current erosion models and a detailed review of the availability of new field or laboratory test data (last two decades)

• Analyse combined data and / or extend data sets through large scale / prototype testing

• Further develop and extend performance curves consistent with original CIRIA curves

Actions

• Review existing knowledge regarding embankments (vegetation) management

• Collect field data demonstrating the impact of different management approaches

• Produce updated guidance for the management of vegetated embankments

• Implementation a programme for ongoing monitoring and data collection

management, soil moisture, soil fissuring and soil erodibility

Objective

The way in which vegetation helps or hinders embankment performance depends upon complex interactions between vegetation type and state and soil type and state The objective here is to clarify those relationships in order to provide more reliable guidance on how to optimise performance from both vegetation and soil

Justification

The type and condition of vegetation affects its erosion resistance This performance is closely linked with the type of soil that the embankment is built from In turn the condition (and erodibility) of the soil depends upon factors such as composition (clay content), moisture content and compaction The condition of the outer layer of the soil is directly affected by the vegetation, both in terms of integrating with the root mesh and moisture content These can also directly affect fissuring within the soil layer In order to manage the grass-soil layer most effectively, it is important to recognise and quantify these complex interactions so that optimal combinations of vegetation and soil type and state may be achieved

Actions

• Review and bring together existing knowledge on vegetation performance and management, soil erodibility and soil fissuring Identify common factors and develop a framework for optimising performance

• Validation of proposed approaches is likely to require field and / or laboratory testing

• Provide industry guidance covering simultaneous management of soil type and state and vegetation type and state

processes

Objective

The improve knowledge and understanding in key process areas relating to the performance of vegetation against erosion Specifically, investigating root performance, response to different load types, cumulative effects and performance on steep slopes

Analysis of response to different load types and slope - Lateral currents, overflow and waves are the main loading types on vegetated surfaces, hence knowledge about these processes should be improved, especially in relation to wave overtopping and impact, and unsteady and discontinuous

needs to include grass behaviour under flow on flat, gently sloping and steeply sloping embankment face.

Cumulative loading effects – The impact on vegetation performance of cumulative loading from a combination of different loads may be more substantial than individual events For example, prolonged high steady water, followed by drawdown and then wave impact may result in a more rapid failure that simply wave impact

Actions:

Root performance:

• Review of existing knowledge leading to improved understanding and predictive models on root performance Potential development and justification of discrete root loading models if appropriate

Loading types and slope:

• Identify specific influences that load types have on surface erosion
basic failure modes

• Detailed R&D to improve the accuracy of performance predictions for wave impact and overtopping

• Assessment and improved understanding of the impact of slopes (including steep slopes) on vegetation performance

Cumulative loading:

• Review and analysis of the potential effects of different cumulative loading

• Identification of likely correlations between loading types for a range of seasonal and storm conditions, but also including climate change trends Guidance on optimal management action and measures to maintain embankment performance

Actions

• Identify main factors that contribute to failure of the surface vegetation

• Analyse failure modes leading to development of limit state descriptions suitable for use in reliability and system risk models

Actions

• Investigation into roughness coefficients under typical erosion flows (overtopping / overflowing) leading to guidance on parameter calculation including allowance for seasonal variations

4 Summary and prioritisation of recommended actions

The actions identified in Section 3 are summarised in Table 4.1 below and assessed to provide an indication of priority (based upon judgement) for the various actions Actions are categorised according to:

Nature of Initiative

Field
Fieldwork

Res
Research

BP
Best Practice/Guidance documents

Data
Investigation/Data collection

Priority of Initiative

H
High

M
Medium

L
Low

Table 4.1 Summary and prioritisation of recommended actions

1 Determination of time to failure

1.1 Review of current erosion models and available data Res H 1.2 Analyse data and extend data through testing Res/Field H 1.3 Develop improved performance curves (such as CIRIA 116)

3 Framework linking vegetation type, management, soil

moisture, soil fissuring and soil erodibility

3.1 Review and develop framework linking vegetation

performance, management action, soil erodibility and soil

H

3.3 Guidance covering simultaneous management of soil type

and state and vegetation type and state

Data

M

4.6 Cumulative loading correlations / climate change Res / Data M

5 Failure modes and mechanisms

5.1 Identification of main factors affecting failure; failure modes Res / Data M 5.2 Analysis of failure modes leading to limit state equations Res / Data M