Plant engineering for riverbanks, a solution destined to be developed

Etude du comportement des fascines sur un modèle réduit de cours d’eau © S. de Danieli /Irstea
Etude du comportement des fascines sur un modèle réduit de cours d’eau © S. de Danieli /Irstea

Today, riverbanks are degraded oases of biodiversity. However, plant engineering seems to be an increasingly promising solution to restore them. At the Irstea Grenoble Center, specialists in plant engineering structures applied to riverbanks are analyzing their behavior when subjected to significant stress. Their aim is to improve design methods for these structures to eventually provide designers with efficient solutions, suitable for even extreme situations.

Riverbanks fulfill some essential ecological functions, such as providing a habitat for both terrestrial and aquatic biodiversity and creating corridors that encourage animal and plant movements. They also play other roles, such as flood protection or providing economic and social services (landscapes, fishing, etc.). Despite this clear abundance, riverbanks and associated riverside vegetation 1 are now very degraded environments. As humans have set up activities and built infrastructure on their edges, they have immobilized many river sections, protecting themselves from the river within by stopping all changes.

The principle of plant engineering is to copy natural models that work well

Plant engineering that involves using local plants to stabilize banks is an interesting solution for mitigating the consequences of these developments. In addition to preserving banks against erosion and therefore protecting the housing and infrastructure located behind them from flooding - in a similar way to civil engineering structures (rock filling, concreting, etc.) - plant techniques can also be used to restore the ecological functions of riverbanks. "Furthermore, plant engineering reflects a growing social desire to move closer to nature. Global changes clearly highlight past mistakes, such as the "all concrete" rush of post-war decades, and ensure we are aware of the need to fix these mistakes," notes André Evette, an ecological engineer at Irstea Grenoble.

restoring banks using plant engineering

Restoring banks using plant engineering
Restoring banks using plant engineering: before and after © S. de Danieli /Irstea

Pushing plant engineering to its limits

Increasingly used in lowlands, where they are well understood, plant engineering techniques still raise many questions, particularly in relation to their resistance. Specializing in plant engineering for around 30 years, teams from the Irstea Grenoble Center are currently looking at riverbanks subject to more extreme conditions than those found in the lowlands, such as on lakes or waterways (banks battered by waves), in mountain regions (slopes and specific hydrological flows), or even in Quebec (frozen banks).

As part of the Resibio project 2 supported by the French Biodiversity Agency, scientists have been looking at creating better defined resistance limits for the techniques in order to refine design criteria for these structures. "Although design methods for civil engineering structures, based on mechanical and physical properties, are perfectly understood, this is not the case for plant engineering, which uses a living material whose properties are much more difficult to control," clarifies Evette.

Historical techniques analyzed on site and in the lab

Using results from the previous Geni'Alp project, which aimed to show the feasibility of these techniques in mountain areas by implementing new structures on steep slopes (5 to 10%), scientists have been following two leads:

  • Studying the behavior of existing structures to determine the conditions (hydraulic power, flood levels, sediment transporting, etc.) that they have resisted or failed to resist, and identifying the precise causes of their destruction;
  • Using a scale model of a river developed in the lab to analyze the mechanical and physical processes that lead to erosion of the most common structure: fascines (willow bundles placed in the riverbed at the base of the bank to stabilize them, due to their ability to grow from cuttings and develop easily).

Although researchers are looking to complete the project by analyzing other frequently used techniques before putting forward specific recommendations to structure designers, the Resibio project has already provided significant results. These include highlighting higher resistance values than those tested and reported in literature up to now, and acquiring a detailed understanding of erosion processes such as scouring (erosion of the riverbed under fascines), fascine degradation methods, and the technical elements to reinforce them. These results open the way for an improved understanding of structure design for extreme situations as well as an improved estimate of how long they will be able to fulfill their role. It is enough to widen the scope of application for these more environmentally friendly techniques.

For more information

1- All plants that grow on riverbanks.

2- Resibio - Improving design methods for plant engineering structures using a systemic and mechanistic multi-scale approach (2017-2018). Project led by Alain Recking, Stream Erosion, Snow and Avalanches Laboratory (ETNA), Irstea.