Roles of the riparian vegetation: the antagonism between flooding risk and the protection of environments

Since the beginning of the 20th century, man has domesticated his environment and caused the modification of hydraulic conditions during floods. In parallel, civil engineering has strongly progressed in the domain of hydraulic structures but especially the construction of dams and dikes has also massively increased and part of the population has lost the culture of risk by thinking they were completely out of danger from flooding. Events of hydrometeorological origin over the last 25 years has reminded man that the hydraulic infrastructures in place for a few centuries now, are not unalterable. An unmanaged vegetal colonization along the edge of watercourses (protection dikes, retention dams, appointed river banks, ect three types of inconvenience as (i) overflow in the case of watercourse containment, (ii) formation of woody jams which generate a risk of bridge obstructions or water retention, (iii) the presence of trees and their roots which damages the containment systems protecting the territories. It is important to manage the development of this vegetation in order to conserve the positive effects on the area while also limiting the negative impacts. The current boom in vegetation engineering techniques shows that man is relearning how t


Background
Historically, man lived in harmony with watercourses, cultivating their land and managing their habitat according to the benefits and the whims of the rivers (Fig. 1.a, b, c).Since the end of the 19th century, man has domesticated with determination his environment (Fig 1.d).The rural exodus, the agricultural depreciation and river disturbance have resulted in a dynamic of closure areas and in the modification of hydraulic conditions during the flow of flood waters [1].In parallel, civil engineering has strongly progressed in the domain of hydraulic structures but especially the construction of dams and dikes has also massively increased (Fig 1. e,f), part of the population has lost the culture of risk by thinking they were completely out of danger from flooding [2].Events of hydrometeorological origin over the last 25 years has reminded man that the hydraulic infrastructures in place for a few centuries now, are not unalterable.More than 70% of these structure ruptures are linked to premature aging generating their failures, of which 20% ascribe to factors of external degradation such as burrowing animals or woody vegetation [3].Riparian vegetation brings a lot of ecological functions along the rivers [4].But an unmanaged vegetal colonization along watercourses can present different types of inconveniences [5,6].The figure 1 shows vegetation place along river since Antiquity.From the 19 th century, human activities pressured rivers and riparian environments and all vegetation has disappeared on river bank in a lot of the towns.After the second world war, with the advent of electricity, the intensive consumption of wood was reduced.As a result, woody vegetation reappeared along the rivers and started to grow on dikes.Nowadays, some river beds are completely invaded by vegetation.surface area (Fig. 9) or to an overflow in the case of watercourse containment (Fig. 10).

Log-jams risk on river
During flood when vegetation has not been managed, rivers carry a high volume of dead wood.The abundant senescent vegetation along watercourses favours the formation of woody jams which generate a risk of water retention or bridges obstruction.In the latter case, flood water can overflow the top of the bridge (Fig. 11).Log-jams rupture can worsen flooding risk, with dangerous fast flowing water.

Risk linked to vegetation on earth dikes and dams
Earth dikes constitute a dense network in many territories with major human and economic stakes.Woody vegetation induces several risks which are not compatible with dike safety and tree root systems generate a premature aging of embankments [17].Unmanaged vegetation development on earth dikes and dams induces a low visibility and may mask disorders.As seen previously, these wooded areas attract a multitude of wild fauna such as burrowing animals causing underground galleries and holes which impact the permeability and stability of dikes (Fig. 12).

Figure 12. %DGJHU ¶V burrow in dike (Source: CZ)
In addition, the presence of trees and their roots damages the containment systems protecting the territories.In fact, root systems of most of the trees like locust (Robinia pseudiacacia), willow (salix.sp)and poplar (populous.sp)have some big and long horizontal roots which can penetrate the dike body.Poplar and willow also have big and long taproots which can grow deeply in the dike.Most of the damages on hydraulic structures are mainly due to erosion phenomena which could be ranked in two types: internal and external erosion.
-External risk is linked to live roots (decompacting soil, cracking concrete, soil uprising).A dike is weakened by tree uprooting (Fig. 13), an important part of the dike can be removed with root system.There is a high risk of piping due to root decay inside the dike and sinkholes creation with stumps decomposition (Fig. 14).To synthetize, vegetation on dikes increases risks of degradation which can be the origin of dangerous flood by dike breaching.Risks linked to vegetation are higher when human infrastructures (bridges, dams, dikes) are numerous.

Case study: Isère riparian forest
The Isere is an Alpine river in the Rhône-Alpes region of southeastern France.The Isère's course measures 286 kilometers and runs through a wide variety of landscapes.From its source near the Italian border in the western Alps, it crosses the Pays de Savoie and the Tarentaise Valley.Then it cuts between the Chartreuse and Belledonne mountain ranges, follows the Vercors Massif, passes through the Dauphiné province and finally meets with the Rhône at the foot of the Vivarais.Isere river crosses Grenoble city.This town is located in a depression and is confronted by air pollution problems and flood risk.The Isere river is almost completely contained by dikes.The vegetation control and management on this site must take into account many contradictory social pressures and technical issues [17].Ten years ago, Isère dikes were totally covered by trees (Fig. 16).

Conclusion
It is therefore important to manage the development of riparian vegetation in order to conserve the positive effects on the area (shade, water decontamination, bank stabilization, biodiversity) while also limiting the negative impacts.

Figure 13 .-
Figure 13.Dike degradation due to a falling pine tree (Source: CZ)

Figure 14 .
Figure 14.Sinkhole induced by stump decomposition and generating risk of internal erosion (Source: PM) Internal erosion risk is also due to live roots penetrating rigid protection, like stone facing riprap (Fig.15) or concrete slab joints.During flood, water can infiltrate the embankment.

Figure 15 .
Figure 15.Stump development inside a handmade stone protection (source: PM)

Figure 17 .
Figure 17.Result of conciliation between safety and riparian stakes between concave and convex banks (Source: CZ)