Quantification of the partition between bare soil evaporation and plant transpiration using stable water isotopes

Evapotranspiration from continental surfaces is one of the most important components of the global water cycle, but it is certainly the less known. The lack of knowledge is even larger when referring to the partition of evapotranspiration between its components: bare soil evaporation and plant transpiration. Isotopic biogeochemistry can provide useful information to progress in a better quantification of this partition. Assuming specific hypotheses of stationarity, it is possible to identify and quantify the different sources of the atmospheric water vapour (local and regional, vegetation and soil) Analysis of the heavy stable isotopic ratios of water in both the liquid and vapour phases : 18O and 2H can allow to determine the « history » of the water in the soil since the last rainfall event (infiltration, re-evaporation) or the root extraction depths. . The presentation will provide a synthesis of the theoretical basis for the interpretation of the isotopic composition of the various reservoir water (soil, plant, atmosphere) and an illustration of recent advances obtained within the framework of the PIETE (Isotopic partition of evapotranspiration between evaporation and transpiration) project. The project combines laboratory and field experiments with a modelling work in order to progress in the understanding of the coupled water, heat and isotopic transfer within the soil vegetation atmosphere continuum. Four types of experiments were conducted : (i) Laboratory Characterization of the water vapour released during plants transpiration, focusing on transient regimes and especially water stress; (ii) laboratory characterization of the isotopic signature of the water vapour released by soil evaporation; (iii) determination of the partition between evaporation and transpiration under controlled conditions using a soil monolith which was sawn with grass.; (iv) determination of the partition of evapotranspiration under field conditions, with an experiment conducted in Lusignan (France) during the whole growing cycle of a maize field in 2004. The experiments were complemented with a modelling of the experimental conditions using the SiSPAT_Isotope SVAT model (Braud et al., J. Hydrology, 2005) which couples water, heat and isotope transport within the soil -vegetation -atmosphere continuum. The modelling of the bare soil columns allowed some progress in the understanding of evaporation under dry conditions, and especially about the formation and evolution of the evaporation front. Isotope measures were also useful to better quantify the hydrodynamic behaviour of the soil. The modelling of the field and laboratory experiments with vegetation was more complicated and required simplifying hypotheses about isotope transport within the plants. Data and model were used to determine the conditions when these hypotheses can be considered as valid. All the conducted experiments allowed some progress in the understanding of the partition of evapotranspiration between bare soil evaporation and plant transpiration. They open perspectives for the improvement of root extraction modules and showed that isotopic data were providing additional information for a better understanding of surface processes, but also to evaluate existing SVAT models.

CemOA : archive ouverte d'Irstea / Cemagref


Jing Huang

Emmanuel Richaud

Pierre-Yves Le Gac

Adelina David

Renata Drozdzak

Gilles Recher

Multiscale study and kinetic modeling of PDCPD thermal oxidation

Introduction

PDCPD is one of the youngest members of the thermoset family. It was developed in the laboratories of BFGoodrich and Hercules and is now used on large scale in a variety of applications. Compared to other thermosets, PDCPD has excellent impact performance, superior chemical resistance, and very low water absorption (0.09% weight increase after 24 h in water). PDCPD areas of applications include large to very large parts, typically in small to medium series, possibly used in hostile environments, and where design freedom is favored. Some examples include body panels for agricultural and construction equipment, trucks and buses where PDCPD brings high impact resistance even at -40°C, and a high heat deflection temperature (typically > 100°C). PDCPD is obtained through Ring Opening Metathesis Polymerisation (ROMP) of the DCPD monomer. 'ROMP reaction' is a metal carbene catalyzed reaction using strained cyclic olefins to produce a wide range of polymers. The driving force is the relief of ring strain.

Today's commercial DCPD resin formulations based on Molybdenum and Tungsten are processed using Reaction Injection Moulding (RIM). In the RIM process two or three monomer streams are brought together in a mixing head and then injected in the mould where the polymerisation takes place. The RIM process is especially suitable to produce large plastic parts. The formulations based on Ruthenium catalysts are no longer limited to RIM but can be used in a variety of thermoset process such as resin infusion and RTM. The robust nature of the Ruthenium catalysts allows using reinforcements such as glass and mineral fibers, foaming agents, and other types of fillers. It has been demonstrated that PDCPD glass fibers composites exhibit superior fatigue resistance and damage tolerance without compromising on the ultimate strength.

PDCPD has excellent impact performance, superior chemical resistance, very low water absorption, very low dielectric constant, excellent thermal stability, and intrinsic fracturetoughness properties that are desirable for a variety of applications. The high density of carbon-carbon double bonds in the polymer backbone makes polydicyclopentadiene susceptible to oxidation. The oxidized layer which is formed spontaneously on PDCPD thick parts allows efficient part painting by using conventional paints.

The oxidation process, however, can also lead to the oxidative polymer damage which might have an important influence on overall mechanical properties. Polymer aging due to oxidation is considered as irreversible. Therefore, it is very important to understand the oxidation mechanism of PDCPD polymer and find a method to increase its oxidative stability.

Mechanical failure in PDCPD submitted to thermal oxidation

When PDCPD undergoes oxidation, large changes of the mechanical behavior occur as shown in Fig. 7.1. This figure plots tensile curves of 3 mm thick PDCPD for several aging duration in oven at 140°C. A large decrease in elongation at break clearly appears whereas the modulus is almost constant during oxidation. This large decrease in fracture properties is related to chemical modification of the network during the degradation. In fact, during polymer oxidation both chain scission and crosslinking occur. For PDCPD, a large crosslinking process occurs due to the presence of double bonds in the polymer backbone. This crosslinking process can be highlighted considering the modulus in the rubbery state that is directly proportional to the crosslink density in the material. For example, Fig. 7.2 shows DMA results for several aging durations in seawater at 90°C where degradation processes are similar as in air. A clear increase in the rubbery plateau is observed that clearly indicates a large increase in crosslink density during oxidation.

Previous results obtained with 2 mm thick samples revealed that the oxidation process is limited by oxygen diffusion and a degradation profile exists through sample [START_REF] Le Gac | Durability of polydicyclopentadiene under high temperature, high pressure and seawater (offshore oil production conditions)[END_REF] (as it will be developed in the next section). In order to understand mechanical changes during oxidation process of the PDCPD, thin films (i.e., 60 µm thick samples) were aged at 80°C in oven. In these conditions, oxidation is homogeneous through sample thickness. Changes in Young's modulus and maximal stress during oxidation are plotted in Fig. 7.3.

Both modulus and maximal stress increase during oxidation, which can be related to the crosslinking process induced by degradation. First, the modulus in glassy state is related to cohesion and segmental mobility [START_REF] Pascault | Thermosetting polymers[END_REF] and its changes will be studied in future work. Then let us consider the maximal stress increase. During crosslinking induced by oxidation, an increase of T g is expected. In fact, according to Eyring's equation based on free volume theory at a given strain rate it is possible to show that stress at yield is directly related to T g according to:

T C T y g σ ( ) = ⋅ -
where σ y is the stress at yield, C is a constant that depends on strain rate, T g is the glass temperature transition and T the absolute temperature. As a conclusion it appears that oxidation leads to a large increase in crosslink density. In terms of mechanical behavior, it leads to a drastic reduction of the elongation at break, a slight increase in Young modulus related to cohesive energy as well as an improvement in maximal stress. This latter increase can be attributed to an increase in T g that will be characterized in the future.

Molecular changes in oxidized PDCPD

Estimation of thickness of oxidized layer

The effect of sample thickness has been studied by TGA. Samples with different thicknesses were aged at 120 and 150°C under 1 bar O 2 so as to determine the maximal thickness below which oxidation is not controlled by oxygen diffusion. TGA curves are presented in Fig. 7.4. At 150°C, the rate of mass uptake reaches rapidly its maximal value (r m ). The maximal rate seems almost constant for samples having a thickness lower than about 20 µm (Fig. 7.4) which corresponds to twice the thickness of oxidized layer (TOL). Since it is well established that the TOL decreases with temperature [START_REF] Gijsman | Influence of temperature and stabilization on oxygen diffusion limited oxidation profiles of polyamide 6[END_REF][START_REF] Celina | Oxidation profiles of thermally aged nitrile rubber[END_REF], its value will, hence, be assumed higher than 10 µm for temperatures lower than 150°C so that oxidation will be studied for samples about 10 µm thick.

Detection and quantification of hydroperoxides

Infrared spectra shows a growth of a hydrogen bonded hydroxyl band at ca. 3400 cm -1 (Fig. 7.5A). This broad band consists of various species corresponding to hydroxyl groups being either hydroperoxides, alcohols or carboxylic acids, all of which being reported to be oxidation products in other hydrocarbon polymers [START_REF] Richaud | Hydroperoxide build-up in the thermal oxidation of polypropylene -a kinetic study[END_REF]. It is, however, difficult to really distinguish all these species.

However, hydroperoxides (being the key species of the oxidation mechanisms) can also be detected and quantified by other chemical or thermal methods [START_REF] Gardette | Advantages and limits of hydroperoxide titration methods in solid polymers[END_REF]. The following DSC procedure was used for the titration of hydroperoxides:

• equilibrate at room temperature • heating to 300°C under nitrogen at a 10°C min -1 rate

The DSC traces of oxidized samples reveal the growth of an exotherm centered at about 150°C which disappears upon treatment by sulfur dioxide (Fig. 7.6A). The same disappearance occurs after the second cycle of the same DSC procedure. That means that this titration of hydroperoxides is irreversible. This exotherm is hence attributed to the decomposition of hydroperoxides (POOH) formed during the propagation step (Scheme 7.1).

PDCPD exposed at 70°C under air [START_REF] Le Gac | Durability of polydicyclopentadiene under high temperature, high pressure and seawater (offshore oil production conditions)[END_REF]. Changes in hydroperoxides for PDCPD exposed at 50°C under air [START_REF] Pascault | Thermosetting polymers[END_REF].

Several kinds of hydroperoxides can be formed 7.7). POOH concentration was calculated (Fig. 7.6B) using a molar enthalpy (440 kJ mol -1 ) given by our previous study [START_REF] Richaud | Thermooxidative aging of polydicyclopentadiene in glassy state[END_REF]. At 50°C, it is observed to increase rapidly in the earliest exposure time to reach a value of about 1.5 mol l -1 after 10 h of exposure and then to reach a plateau. Hydroperoxides are an intermediate species of oxidation process and decompose later to generate stable products as presented below. ③ By the decomposition of tetroxides generated by termination process between two peroxy (POO°) radicals (Scheme 7.3B) [START_REF] Russell | The rates of oxidation of aralkyl hydrocarbons. Polar effects in free radical reactions 1, 2[END_REF].

Carbonyl build-up

It must be mentioned that POOP and POOOOP decomposition generate a pair of alkoxy radicals in each case, so that the mechanisms can be considered as close. However, the kinetics are not priori because POOP and POOOOP stabilities are not the same.

Their concentration was calculated using the Beer-Lambert law with a molar absorptivity of 300 mol l -1 cm -1 [START_REF] Richaud | Hydroperoxide titration by DSC in thermally oxidized polypropylene[END_REF]. The changes in concentration are depicted in Fig. 7.8B showing an increase of carbonyl concentration over time with the classic self-accelerating shape characteristic for the oxidation of hydrocarbon polymers.

Double bond consumption

On the basis of Infrared results (Fig. 7.5A and C), it is possible to distinguish three types of double bonds in the structure of PDCPD: cyclic double bond at 3049 cm -1 , acyclic cis double bond at 733 cm -1 and acyclic trans double bond at 973 cm -1 , which decrease with aging time. Similarly, double bonds concentration was calculated with using the Beer-Lambert equation with molar absorptivity of 19 mol l -1 cm -1 (calculated with the help of cyclopentene) at 3049 cm -1 , 60 mol l -1 cm -1 [START_REF] Calvert | Photochemistry[END_REF] at 733 cm -1 , and 100 mol l -1 cm -1 [START_REF] Mcmurry | Correlation of infrared spectra[END_REF] at 973 cm -1 . Fig. 7.8A shows that the changes of normalized concentration of different types of double bonds, the consumption rate divided by the initial concentration of these three types of double bonds are almost the same. The first hypothesis in kinetic simulation is hence that these three types of double bonds are very close (i.e., one rate constant is valid for each sort of double bonds).

According to Fig. 7.8B, the double bonds concentration decreases with aging time, certainly because of opening by alkyl of peroxyl radicals (Scheme 7.4):

Kinetic modeling

According to the above reported changes, it seems that the following mechanistic scheme might be used for modeling the kinetic curves for PDCPD: where POOH, P°, POO°, PH, F, P=O are, respectively, hydroperoxides, alkyl, peroxy radicals, abstractable C-H, double bonds and carbonyls, x is crosslinking, k i , and γ i are, respectively, kinetic rate constants and yield for each reaction.

(1u) POOH → 2P° + γ 1 P=O k 1u (1b) POOH + POOH → P° + POO° + γ 1 P=O k 1b (2) P° + O 2 → POO°k 2 (3) POO° + PH → POOH + P°k 3 (A-1) P° + F → x + P°k a1 (A-2) POO° + F → γ 1 P=O + P°k a2 (4) P° + P° → γ 4 x + (1 -γ 4 )F k 4 (5) P° + POO° → γ 5 POOP + γ 5 x + (1 -γ 5 )F + (1 -γ 5 )POOH
As published elsewhere [START_REF] Coquillat | Thermal oxidation of polybutadiene. Part 2: Mechanistic and kinetic schemes for additive-free non-crosslinked polybutadiene[END_REF] the mechanistic scheme leads to a set of differential equations. The numerical solution of this later leads to a simulation of experimental results using the following equations:

γ γ ( ) ( ) [ ] [ ][ ] [ ][ ] [ ] [ ][ ] = - °- °+ - °+ - °°d F dt k P F k POO F P sk P POO 1 1 a a 1 2 4 2 5 d P O dt k POOH k POOH k POO C C k POO u b a 1 1 1 1 2 1 2 6 2 γ γ γ [ ] [ ] [ ] [ ][ ] [ ] = = + + °= + °d POOH dt k POOH k POOH k POO PH k P POO 2 1 u b 1 1 2 3 5 5 γ ( ) [ ] [ ] [ ] [ ][ ] [ ][ ] = - - + °+ - °° FIGURE 7.8
Concentration changes in acyclic trans double bonds at 3050 cm -1 (■), acyclic cis double bonds at 973 cm -1 (), cyclic cis double bonds at 733 cm -1 (▲ ) for exposure at 50°C under air (A) and in carbonyl (■) and double bonds () for exposure at 50°C under air (B).

SCHEME 7.4 Possible processes of crosslinking (NB: the same can occur with POO°).

The main aim is to estimate each rate constant from accelerated aging tests is to use their extrapolation to any other given exposure conditions to perform a prediction of changes in chemical structure and later the change in crosslinking from:

dx dt k P k P F k P POO a 4 4 2 1 5 5 γ γ [ ] [ ][ ] [ ][ ] = °+ °+ °°It
is thus needed to propose a reliable method for estimating each involved kinetic parameter. There are schematically two options:

• Estimate the set of rate constant from best fitting approach. However, there is infinity of possible sets given the high number of parameters compared to the relative simplicity of kinetic curves for oxidation in the earliest exposure time. • Assess independently rate constants from specific experiments, as it will be developed below.

According to several papers, the rate constants for propagation obey well-defined structure-oxidizability relationships:

• k 2 is close to 10 8 l mol -1 s -1 for hydrocarbon compounds [START_REF] Denisov | Oxidation and Antioxidants in Organic Chemistry and Biology[END_REF] • k 3 is taken from the value of polybutadiene which is unsaturated thermoset like PDCPD [START_REF] Coquillat | Thermal oxidation of polybutadiene. Part 2: Mechanistic and kinetic schemes for additive-free non-crosslinked polybutadiene[END_REF] It remains to estimate k 1u , k 1b , k a1 , k a2 , k 4 , k 5 , and k 6 .

The thermolysis under vacuum of hydroperoxides previously generated during an exposure under air is described by the following equation:

d POOH dt k POOH k POOH 2 u b 1 1 2 [ ] [ ] [ ] = - -
However, when POOH concentration reaches the plateau in Fig. 7.6B, thermolysis data fit pretty well with a second order reaction kinetics, consistently with the relatively high concentration in POOH making the bimolecular decomposition much faster than first order one. The rate constant for bimolecular process k 1b can thus be directly estimated from the slope in in Fig. 7.9.

The unimolecular process is, however, expected to play a more significant importance at low hydroperoxides concentration, i.e., in the earliest conversion degrees. Its importance is also supposed to be greater under air than under elevated oxygen pressure (since POOH concentration increases with external oxygen pressure) [START_REF] Richaud | Effect of oxygen pressure on the oxidation kinetics of unstabilised polypropylene[END_REF].

We have thus chosen the following strategy:

• The value of k 1b is the slope of this bimolecular linear equation. The yield of carbonyl γ 1 was obtained by thermolysis of hydroperoxides (Fig. 7.10). • k 1u is adjusted from the kinetic curves to permit a good fit under air and elevated oxygen pressure.

Let us now turn to rate constants for termination (k 4 , k 5 , k 6 ) and double bonds opening (k a1 , k a2 ). Under very elevated oxygen pressure, P° radicals are almost instantaneously scavenged so that reactions (a1), ( 4) and ( 5) are almost negligible. It can thus be shown in a first approach that:

v d P O dt k POOH k POO k PH k 1 4 ox excess b b b 1 1 2 6 6 2 1 3 2 6 γ γ γ ( ) [ ] [ ] [ ] [ ] = = = + °= + v d C C dt k POO F k k F k C C excess a a 2 2 3 2 6 [ ] [ ][ ] [ ] = - = = °= =
With these approximate calculations, k a2 and k 6 were thus determined at 50°C. Using k 2 and k 3 from literature, and k 1b from thermolysis experiments, a first simulation run was obtained (dotted line in Fig. 7.11). Then the above complete mechanistic scheme (with k 6 , k 01 , and k 02 ) was used for adjusting k 4 , k 5 , k a1 , and k 1u to fit the available experimental data under oxygen excess and under air (full line in Fig. 7.11). All the oxidation rate constants utilized for simulation runs are summarized in Table 7.1. The model failure to simulate the degradation at longer exposure time suggests further improvements (taking in to account of other reactive sites than allylic sites) would be needed. However, our main aim was to simulate the gradation until embrittlement which occurs at relatively lower conversion degrees. It is shown that the set of kinetic constants obtained by inverse method makes it possible to obtain a satisfying simulation of the accumulation curves of carbonyls, hydroperoxides and decrease curves of double bonds. Moreover, the magnitude orders of these rate constants follow the following classification: [START_REF] Gillen | General solution for the basic autoxidation scheme[END_REF] 10 k k k > ≥ > which is general classification for most polymers. The order of magnitude of k a1 with k a2 for PDCPD is observed to be in line with the case of the other rubbery polymers (see Table 7.2) where:

k k a a 1 2

Conclusions

This chapter presents a methodological approach for predicting the lifetime of polymers applied in the case of PDCPD submitted to thermal oxidation. It is shown this polymer undergoes thermal oxidation leading mainly to an increase in T g and yield stress and a decrease of ultimate elongation. The rate of crosslinking can be predicted from a nonempirical model in which kinetic approach describes the chemical changes induced by thermal oxidation (mainly the appearance of oxygenated species such as unstable peroxides and stable carbonyls). The proposed model can reasonably fit the experimental data presented here for aging at 50°C. It remains now to extend those predictions in the case of bulky materials (i.e., by adding the diffusion coupling phenomena) together with adding to the mechanistic scheme the effect of commercial antioxidants to dispose of a nonempirical tool for polymers lifetime prediction.

FIGURE 7 . 1

 71 FIGURE 7.1 Tensile behavior of 2 mm thick commercial PDCPD after oxidation after several aging time at 140°C.

FIGURE 7 . 2 FIGURE 7 . 3

 7273 FIGURE 7.2 Change in modulus with temperature for different aging times in seawater at 90°C [1].

FIGURE 7 . 4

 74 FIGURE 7.4 Gravimetric curves for purified PDCPD films and aged at 150°C under 1 bar O 2 [5].

FIGURE 7 . 5

 75 FIGURE 7.5 Changes of FTIR spectra of purified PDCPD exposed at 50°C under air in hydroxyl (A), carbonyl (B), and double bonds (C).

Fig. 7 .

 7 Fig.[START_REF] Gardette | Advantages and limits of hydroperoxide titration methods in solid polymers[END_REF].5B displays a complex carbonyl band in the 1650-1750 cm -1 region with a maximum centered at ca. 1710 cm -1 corresponding to the aldehydes or ketones. There are at least three kinds of reactions likely to explain their origin: ① By initiation process (i.e., uni-or bimolecular decomposition of peroxides), for example as shown in Scheme 7.2.NB : hydroperoxides decomposition by uni-or bimolecular process generate the same kind of alkoxy radical :

FIGURE 7 . 6 FIGURE 7 . 7

 7677 FIGURE 7.6 DSC thermograms before (full line) and after (dashed line) SO 2 treatment for purified.

SCHEME 7 . 2

 72 SCHEME 7.2 Formation of carbonyls from hydroperoxides decomposition.

k 5 ( 6 ) 6 SCHEME 7 . 3

 56673 SCHEME 7.3 Formation of carbonyl from dismutation from dialkylperoxide (A) and tetroxide (B).

FIGURE 7 . 9

 79 FIGURE 7.9 Thermolysis curve of hydroperoxides at 90°C in N 2 (A). Second order plot for POOH decomposition (B).

FIGURE 7 .

 7 FIGURE 7.10 Carbonyl yield as a function of temperature.

FIGURE 7 . 11

 711 FIGURE 7.11 Simulation runs for carbonyl (P=O) (A) and double bonds (C=C) (B) and POOH (C) at 50°C under 1 MPa and under air.

Table 7 . 1

 71 Rate constants at 50°C for PDCPD, k 1u in s -1 , k 1b , k 2 , k 3 , k a1 , k a2 , k 4 , k 5 , k 6 in 1 mol -1 s -1 .

	Run for dotted line
	k 1u	k 1b	k 2	k 3	k a1	k a2
	1.2 × 10 -5	1.8 × 10 -6	1.0 × 10 8	0.048	0	0.37
	k 4	k 5	k 6	γ 1	γ 4	γ 5
	0	0	10	0.25	0	0
	Run for full line
	k 1u	k 1b	k 2	k 3	k a1	k a2
	1.2 × 10 -5	1.8 × 10 -6	1.0 × 10 8	0.048	9000	0.37
	k 4	k 5	k 6	γ 1	γ 4	γ 5
	5.0 × 10 8	1.7 × 10 8	10	0.25	0	0

Table 7 . 2

 72 Constant rates used for modeling oxidation at 50°C.

		Polybutadiene [13]	Polyisoprene [17]	Polychloroprene [18]
	k a1 (l mol -1 s -1 )	620	3.83 × 10 4	97
	k a2 (l mol -1 s -1 )	0.01	7.61	0.16


laboratoires publics ou privés.

In order to study the impact of land use change on the water cycle, distributed hydrological models are more and more used, because they have the ability to take into account the land surface heterogeneity and its evolution due to anthropogenic pressure. These models provide continuous distributed simulations of streamflow, runoff, soil moisture, etc, which, ideally, should be evaluated against continuous distributed measurements, taken at various scales and located in nested sub-catchments. Distributed network of streamflow gauging stations are in general scarce and very expensive to maintain. Furthermore, they can hardly be installed in the upstream parts of the catchments where river beds are not well defined.

In this paper, we present an alternative to these standard streamflow gauging stations network, based on self powered high resolution water level sensors using a capacitive water height data logger. One of their advantages is that they can be installed even in ephemeral reaches and from channel head locations to high order streams. Furthermore, these innovative and easily adaptable low cost sensors offer the possibility to develop in the near future, a wireless network application.

Such a network, including 15 sensors has been set up on nested watersheds in small and intermittent streams of a 7 km 2 catchment, located in the mountainous "Mont du Lyonnais" area, close to the city of Lyon, France. The land use of this catchment is mostly pasture, crop and forest, but the catchment is significantly affected by human activities, through the existence of a dense roads and paths network and urbanized areas. The equipment provides water levels survey during precipitation events in the hydrological network with a very accurate time step (2 min).

Water levels can be related to runoff production and catchment response as a function of scale. This response will depend, amongst other, on variable soil water storage capacity, physiographic data and characteristics of raining events.

Several raining events, with different characteristics, have been extracted from a large data set collected during two years. From these numerous hydrographs, analyses of flow dynamic through rising limbs, falling limbs and lag times show different behaviour pattern according to instrumented reaches. The paper will present first results from this synoptic survey. It shows the dynamic spatial pattern of stream flow along the water pathes. At finer scale, this synoptic survey could show the impacts of anthropogenic features on water concentration and their hydrological connectivity to natural river network.

A

  distributed water level network in ephemeral river reaches to identify the hydrological responses of anthropogenic catchments B. Sarrazin, Isabelle Braud, M. Lagouy, Jean-Stéphane Bailly, C. Puech, Hervé Ayroles To cite this version: B. Sarrazin, Isabelle Braud, M. Lagouy, Jean-Stéphane Bailly, C. Puech, et al.. A distributed water level network in ephemeral river reaches to identify the hydrological responses of anthropogenic catchments. EGU General Assembly, Apr 2009, Vienna, Austria. p. EGU2009-6103 -p. hal-00504204 A distributed water level network in ephemeral river reaches to identify hydrological processes within anthropogenic catchments B. Sarrazin (1), I. Braud (2), M. Lagouy (2), JS. Bailly (3), C. Puech (3), and H. Ayroles (4) (1) ISARA-Lyon, 23 Rue Jean Baldassani, 69364 Lyon Cedex 7, France (sarrazin@isara.fr), (2) Cemagref, UR HHLY, CP 220, 3bis Quai Chauveau, 69336 Lyon Cédex 9, France, (3) Cemagref, UMR TETIS, Maison de la Télédétection, 500 Rue Jean-François Breton, 34093 Montpellier Cedex 5, France, (4) IMFT 1 Allée du Professeur Camille Soula 31400 TOULOUSE, France

CemOA : archive ouverte d'Irstea / Cemagref


F Bigot

Stéphane Berger

Lambert

C Bigot

F Berger

S Lambert

An innovative type of rockfall protection fence on forested slopes specially designed for low energy events

An innovative type of rockfall protection fence on forested slopes specially designed for low energy events

C.

Forest management in mountainous regions implies sylvicultural interventions on slopes. But in such forested areas, engines, workers or falling trees can initiate rock falls and the site can temporarily turn to a potentially dangerous site. This presentation deals with a type of fence specifically developed for this context. In particular, this type of fence requires no heavy interventions neither than heavy engines such as for soil-moving or nailing. Moreover, it is a temporary structure, supposed to act for the duration of the interventions or as long as it is necessary to counterbalance the lack of protection within the growing period of the trees. The fence is composed of a wire mesh, three cables and two rigid rods. This fence is tightened between existing trees. The first real size experiments have been made during summer 2008. It consisted in impacting successively the fence in its centre with rocks of varying mass (from 30kg to 500kg) with the aim of evaluating its capacities. For this purpose, a cable was tightened between two trees. This cable allowed conveying a trolley supporting the rock. The rock was dropped just before the impact thanks to a snap shackle. The system allowed reaching a maximal rock velocity before impact of 17m/s. This velocity was calculated thanks to image analysis obtained with a numerical video camera and a numerical high speed camera. The main continuous measurement made was tension in the upper cable. In addition, an accelerometer was nailed on one of the fence supporting trees. A total of 17 impact experiments were performed on for different fences. The first results showed that (i) angular blocks can cut wires even for low mass rocks, (ii) the wire rupture hardly propagates in the mesh during another impact, (iii) supporting cables can be cut during the impact, (iv) the fence is not perforated by the direct impact even for rocks 500kg in mass, and (v) a reparation of a perforated mesh consisting in placing a patch is perfectly efficient. In parallel to these experiments, numerical simulations were performed based on the discrete element model developed by Bertrand (2008). It has allowed the estimation of the relevance of some technical options. The results obtained are encouraging and should lead to the development of efficient low energy fences to be used on forested slopes.

CemOA : archive ouverte d'Irstea / Cemagref