Contents

Cover

Spillways on River Levees

Foreword

Introduction

1. Background, definitions, various configurations

Typology of flood protection levees on rivers

Protected areas and flood expansion areas

Typology of spillways on levees

2. The historical background of spillways in France

1584 to 1891: The first spillway constructions on the Loire and Vidourle Rivers

20th century: Several scattered constructions and major development of the Rhône River

21st century: A relative acceleration in construction

3. The hydraulic design of spillways

The spillway’s impact on the flooded river’s waterline

Impact of the spillway on the waterline in the protected area

The spillway’s flow law

Spillway location

Protection, safety, and danger floods

The principle of flood attenuation or flood hydrograph flattening

General hydraulic design principles

The required modelling

4. Spillways and river geomorphology

Overview of sediment transport

Overview of morphological changes in watercourses

Morphological changes in leveed watercourses

The spillway’s influence on sediment transport (riverbed)

Sedimentation behind spillways

The influence of bed change on spillway performance

Torrents

5. Civil engineering design for levee spillways

Safety criteria for levee spillways

Principles of fuse plugs or movable devices

The plan shape of the levee spillway

Types of straight weirs and constituent materials

Fuse plugs or movable devices

Continuous maintenance tracks

Overflow erosion on earthen levees

6. Emergency management of the levee system

The benefits and limitations of spillways in emergency management

The types of emergencies to manage

Considerations before drafting an Emergency Management Plan

Emergency management and levee managers

Local Emergency Action Plan (LEAP)

Special Emergency Management Plan for Flooding

7. Economic aspects

8. Summary and Conclusion

The benefits and limits of a spillway in a protected area

Spillway features

Is a spillway always required in a protected area?

Concluding remarks

Appendix 1 - Spillway types on river levees, according to their hydraulic function and their purpose in flood management

Appendix 2 - A history of spillways on the Loire River

Appendix 3 - Overflow levees and spillways in the CNR’s Rhône River development

General principles

Example 1: The Chautagne plain and Lac du Bourget

Example 2: The Printegarde plain in Livron-sur-Drôme

Glossary

Reference List

Spillways on River Levees

Gérard Degoutte and Rémy Tourment
Editors

This book was translated by Diana Huet de Guerville from the French edition entitled Les déversoirs sur digues fluviales.

Cover photos
Left, Jargeau spillway on the left bank of the Loire (P. Royet)
Middle, Lunel spillway on the Vidourle River (G. Degoutte)
Right, masonry spillway on the right bank of the Giessen River (DDT Bas-Rhin)

Gérard Degoutte is an engineer in the Corps des ponts, des eaux et des forêts (France’s Bridges, Waters and Forests Corps) and member of the French Standing Technical Committee for Dams and Hydraulic Works.

Rémy Tourment is a civil engineer, chairman of the Technical Committee on Levees within the International Commission on Large Dams (ICOLD) and vice-president of the Comité français des barrages et réservoirs (National Dams Committee associated with ICOLD).

They have both dedicated their careers at INRAE to research, teaching, expertise and auditing activities in the field of hydraulic structures, dams and levees, and river morphology.

To cite the book: G. Degoutte, R. Tourment (eds), 2021. Spillways on River Levees. Versailles, éditions Quæ, 184 p. doi:10.35690/978-2-7592-3285-7 undescribed image

© éditions Quæ, 2021

ISBN papier : 978-2-7592-3284-0
ISBN PDF : 978-2-7592-3285-7
ISBN epub : 978-2-7592-3286-4

Éditions Quæ
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78026 Versailles Cedex

www.quae.com undescribed image
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E-books licence CC-by-ND-NC 4.0

For any questions or suggestions: quae-numerique@quae.fr undescribed image

Foreword

This handbook was produced at the initiative of the French Ministry in charge of the environment and written by a working group led by INRAE and supervised by the DGPR/SRNH/STEEGBH (General Directorate for Risk Prevention/ Natural and Hydraulic Risks Department/Technical Department for Electrical Energy, Large Dams, and Hydraulics).

The handbook was first published in French in 2012 and initially translated into English the same year, but not published. Gérard Degoutte, Paul Royet (INRAE, retired), and Aline Leclerc (Agora Traductions) worked on the first version of the translation. In 2021, Rémy Tourment (INRAE, chairman of the ICOLD Technical Committee on Levees) updated the English version of the handbook with the help of Adrien Rulliere (INRAE) and translator Diana Huet de Guerville.

The working group in charge of the French version, initially led by Rémy Tourment (INRAE) and then by Gérard Degoutte (CGAAER and INRAE), also included the following members:

Nicolas-Gérard Camphuis (CEPRI then the Loire-Brittany Water Agency), general reviewer

Gérard Degoutte (CGAAER and INRAE), group leader and general reviewer

Patrick Deronzier (MEDD/DEEEE), during the first meetings

François Dols (Director of the Rhône-Méditerranée Water Agency) during the first meetings

David Goutx (Agro ParisTech then Météo France), co-writer of Chapter 4

Frédéric Grelot (INRAE), during the first meetings and the review of Chapter 8

Xavier Martin (CGEDD), during the first meetings and reviewer of an initial version

Nicolas Monié (MEDDTL/DGPR/SNRH/STEEGBH), co-writer of Chapter 2

Jean Maurin (DREAL Centre), co-writer of Chapters 3 and 7, writer of Appendix 1, and general reviewer

Paul Royet (INRAE), main writer of Chapter 6 and general reviewer

Rémy Tourment, group leader, general reviewer, and coordinator of the second English version

Gilles Trapatel (CNR), c-writer of Appendix 2

Yves Le Trionnaire (MEDD/DE), during the first meetings

Thibaut Mallet (Symadrem), co-writer of Chapters 4 and 7

The following people also helped write the French version of this handbook:

Francis Fruchart (CNR), Chapter 4 and Appendix 2

Frédéric Laugier (EDF) for the PK weir paragraph update in Chapter 5

Patrice Mériaux (INRAE) and Yann Quefféléan (ONF/Direction technique RTM), for the torrential river paragraph in Chapter 5

Stéphane Bonelli (INRAE) and Daniel Puiatti (DPST consulting), for the lime treatment paragraph in Chapter 5

We thank the following people for their proofreading:

Jean-Marc Kahan (MEDDTL/DGPR/SNRH, Director of STEEGBH) and Patrick Ledoux (CETE Méditerranée), for the entire handbook

Christophe Peteuil (CNR), for Chapter 5

We would like to thank the following people for their illustrations or the information they provided:

Stéphane Barelli (VNF Nancy), Arnaud Bollery (CU Montbéliard), Pierre Cadoret (DDTM11), Benoît Cortier (Hydratec), Rémy Croix (Egis Eau), Georges Darbre (Swiss Committee on Dams), David Di Dio Basalmo (DDT 67), Francis Fruchart (CNR), Jérôme Gondran (SMAV Durance), Olivier Manin (Symbhi), Olivier Overney (Ofev Bern), Franck Rangognio (BRL Ingénierie), Burkhard Rosier (EPFL), Akim Salmi (ISL), Xavier Suisse de Sainte-Claire (Egis Eau), and Éric Vuillermet (BRL Ingénierie).

This handbook was funded as part of the RESBA project, financed by the Interreg V-A France-Italy (ALCOTRA) Cooperation Programme.

Le guide a été financé dans le cadre du projet RESBA, financé par le programme de coopération INTERREG V A France-Italie ALCOTRA.

Introduction

The likelihood that overflow will occur on flood protection levees is far from insignificant. When water overflows an earthen levee, it greatly increases in velocity and erodes the levee slope or toe. This erosion extends backwards and opens up a breach that provokes sudden flooding in the supposedly protected area, often with significant economic impact. The suddenness of this process can lead to human casualties in areas with inhabitants or transport infrastructure.

Levees are a double-edged sword: they offer protection against medium-sized floods if properly built, but can create a hazard during high floods if no provisions are made to secure them against overtopping.

Dams almost all have flood spillways to prevent even extremely rare floods from overflowing over their crests. Can we systematically apply this technique to levees, particularly earthen ones?

This technical handbook outlines the benefits and limitations of spillways on flood protection levees. Some spillways can protect leveed areas by limiting the adverse consequences of overflows. They are meant to be used infrequently. Other spillways are specifically designed for flood control and are used on a more regular basis. Since these two types of spillways have very different objectives, we will refer to the former as safety spillways and the latter as diversion spillways. Some structures, especially the oldest, actually play both roles. The hydraulics remain the same regardless, and this handbook covers all types of levee spillways.

What types of levees? This handbook examines flood protection levees on rivers, including mountain torrents. We will not address levees along coasts or canals. We focus on existing levees that already have spillways or the possibility of adding them, as well as new levee projects that could incorporate spillways into their construction design.

1

Background, definitions, various configurations

Gérard Degoutte

This chapter provides an overview of river levees, levee systems, and their potential spillways. We will first describe the various configurations before addressing their purpose. A spillway’s function is closely tied to the purpose of the area it protects or fills with water, with a broad range of scenarios we will examine in closer detail.

Typology of flood protection levees on rivers

Definition

Levees are typically very long structures built above the natural ground level to either channel water or prevent it from flowing through. Excluding those built along canals, levees are designed to protect certain areas against river or marine flooding. This handbook only addresses riverine flood protection levees that provide partial or total protection against floodplain inundation.

As we will see later, levees may protect against certain floods while only delaying stronger ones. But this is a consequence of the definition rather than part of the definition. The main issue is that the presence of a river levee is a double-edged sword: it offers protection against small- or medium-sized floods but creates a hazard during strong or extreme floods or when it is poorly maintained. If the levee fails, the resulting flood wave may cause even more damage in the floodplain than if there had been no levee at all. Earthen levees offer particularly low resistance to overflows. They may also breach before overflowing because of internal erosion, toe scour, or sliding.

These levees (sometimes called “dry levees”) are seldom subject to hydraulic loading, much like flood retention dams or basins.

River levees are generally made from earthfill taken from the riverbed or the surrounding area. They are sometimes built of masonry or concrete, particularly on urban sites, in which case they are sometimes called dikes.

Earthen dams are also sometimes called levees, but this term is incorrect and should be avoided. A dam is built across a river to block at least the riverbed, and frequently the floodplain and beyond. A levee, on the other hand, never blocks the riverbed.

Levees may connect several natural topographical features such as hillsides, promontories, or terraces. Although these natural features are not considered levees, we should analyse their resistance in the same way as human-made levees. We must clearly define the area that is being protected from flooding by one or more levees and potentially some natural features. All these components taken together form what we call a leveed system.

Longitudinal levees

In leveed valleys, the goal has often been to prevent water from entering most of the floodplain. The levee runs parallel to the watercourse, either at the riverbank level or some distance away. In the latter case, the space between the levee and the riverbank is called an unprotected floodplain or ségonnal in French (Figure 1.1). As its name suggests, this area is not protected by the levee and will be inundated by even higher water levels if flooding occurs.

Figure 1.1. Longitudinal levee and unprotected floodplain.

Close protection levees

Levees that prevent large areas from being submerged will worsen overtopping downstream and on the opposite bank. This is why close protection levees are built as near as possible to the stakes to be protected, thereby expanding the flooded area (Figure 1.2). If the stakes to be protected are next to the hillside, the close protection levee will close onto it. If all the stakes to be protected are in a flood-prone area, the close protection levee may be called a ring levee.

Figure 1.2. Close protection levees completely or partially surround areas to be protected.

Levee rank

A close protection levee may complement a longitudinal levee: a primary levee protects mainly rural areas and a secondary close protection levee protects mainly urban areas. The rank of protection refers to the order in which levees go into effect in the event of a strong flood. No other consideration, either in terms of volume or height should be inferred. In Figure 1.3, on the right bank, the “secondary levee” designation is clear. The area on the left bank is protected from overflows by the longitudinal levee. A peripheral levee prevents discharge into the floodplain from a tributary, a spillway, or an upstream breach. In this configuration, it is hard to say which levee will be activated before the other, so no particular rank of protection is given.

Figure 1.3. Primary and secondary levees where applicable.

Transverse levees

Transverse levees are built perpendicular to the general direction of the valley. They complement longitudinal levees and protect against upstream or downstream inflows. In Figure 1.4, on the left bank, two transverse levees close onto the hillside and isolate a protected area. The upstream levee prevents water from flowing into the flood plain and the downstream levee prevents water from flowing from downstream due to the backwater effect. On the right bank, the transverse levee does not enclose a protected area but splits it into two parts. This may help protect the downstream section if there is a breach upstream. This type of levee may also separate two leveed areas with different purposes: a flood expansion area upstream and a flood protection area downstream. One example is the La Montagnette levee between Vallabrègues and Tarascon, shown in Figure 1.14, or the canal levee between Tours and Saint-Pierre-des-Corps.

Protected areas and flood expansion areas

We define two types of leveed areas in very different contexts: protected areas in urban environments and flood expansion areas that are preferably non-urbanised. As we will see later, an area can be either a protected area (up to a certain water level) or a flood expansion area (above this level). We use the term “leveed system” to mean either of these. A leveed system is an entire area in a floodplain that has some level of flood protection thanks to one or more levees or natural topography (such as the valley slope).

Figure 1.4. Transverse levees.

Protected area

The term “protected area” is quite standard in France. It was used in French Decree 2007-1735 of 11 December 2007 on the safety of hydraulic works, without being properly defined. The Environnement circular of 8 July 2008 on this decree includes the following definition:

“A protected area is the area that will not be inundated by a flood that reaches the structure’s protection level. It is not the more limited area in which residents would be endangered by high or fast water levels in the event of a levee failure. Neither is it an area that is inundated for the design flood level in the PPRI (Flooding Risk Prevention Plan) by known high water levels, a 100-year flood, or the maximum floodable area.”

The more recent French Decree 2015-526 of 12 May 2015 also mentions protected areas; a protected area is associated with a levee system and a protection level. A protected area can also include specific parts associated with different protection levels.

A protected area is a contiguous surface that is protected from flooding by a set of levees or other structures (road embankments, etc.) or a raised topographical feature such as a hillside or a terrace. It is an area that is likely to flood without a levee and is protected from flooding as long as the levee plays its protective role — that is until the levee is overtopped or damaged. See Figure 1.5. For close protection levees, as shown in Figures 1.2 and 1.3, the protected area is clearly defined.

Figure 1.5. Area protected by a longitudinal levee, a hillside, and a transverse levee. This configuration will be effective whatever the direction of the river current.

In Figure 1.5, the protected area is closed and must include a drainage system. Another option is for the protected area to be partially open downstream. See the example in Figure 1.6. This area is closed by the levee on one side and cut off by a tributary flowing on the other side through the hillside. This will allow common floods to fill in the area from downstream through the backwater effect. The entire area is not a protected area since a small section at the junction is flood-prone, while the rest is protected by the levee.

Most of the protected areas that have been built recently or are currently planned are in highly urbanised areas. After all, there is no need to protect an area in which flooding will have limited impact, or in which there are no stakes. In the past, however, mainly rural areas were protected. Camargue Island is probably the best example, along with the Isère River Valley upstream from Grenoble, the Save River Valley in Gers, the Vidourle River Valley, and many more.

Flood expansion area

...