L M Lalicata

A Desideri

F Casini

L Thorel

Experimental observation on a laterally loaded pile in unsaturated silty soil

Keywords: Centrifuge modelling, unsaturated soils, soil-structure interaction, piles, lateral loading

An experimental study has been carried out to investigate the effects of soil partial saturation on the behaviour of laterally loaded piles. The proposed study has been conducted by means of centrifuge tests at 100×g, where a single vertical pile has been subjected to a combination of static horizontal load and bending moment. The study has been conducted on a silty soil characterized with laboratory testing under saturated and unsaturated conditions.

During flight, two different positions of water table have been explored. The influence of density has been investigated compacting the sample with two different void ratio. Finally, the effects of a variation of saturation degree on the pile response under loading have been studied rising the water table to ground surface. Data interpretation allows drawing different considerations on the effects of partial saturation on the behaviour of laterally loaded piles. As expected, compared to saturated soils, partial saturation leads always to a stiffer and resistant response of the system. However, the depth of the maximum bending moment is related to the position of water table and the bounding effects induced by partial saturation appears to be more important for loose soils.

Introduction

The significant soil volume interested by the kinematics of piles foundation under lateral loading is typically limited vertically in the first meters (several diameters of pile) of depth from ground level.

A number of studies available in literature [START_REF] Banerjee | The behaviour of axially and laterally loaded single piles embedded in nonhomogeneous soils[END_REF][START_REF] Randolph | The response of flexible piles to lateral loading[END_REF][START_REF] Krishnan | Static and dynamic lateral deflexion of piles in nonhomogeneous soil stratum[END_REF][START_REF] Higgins | Elastic solution for laterally loaded piles[END_REF][START_REF] Di Laora | Kinematic Bending of Fixed-Head Piles in Nonhomogeneous Soil[END_REF] point out that, in this class of problems, the system response is mainly influenced by relative pile soil stiffness ratio, opportunely evaluated in the significant volume of soil, while for short and rigid piles response is affected both by stiffness and slenderness ratio. Most relevant parameters such as head displacement and rotation, maximum bending moment and its position along pile length are also strongly influenced by non-linear stressstrain relationships of soils [START_REF] Budhu | Nonlinear analysis of laterally loaded piles in cohesionless soils[END_REF][START_REF] Budhu | Nonlinear analysis of laterally loaded piles in cohesionless soils[END_REF][START_REF] Russo | A method to compute the non-linear behavior of piles under horizontal loading[END_REF].

The behaviour of pile under lateral loading depending, among others factors, from the stiffness of the pile and of the soil sourroundings. It is well known that a flexible (or long) pile is characterized by no variation of load or displacement at the tip level during lateral loading at the head. While, a perfectly rigid (or short) pile should behaves with a constant rotation all along its length. The reality is often in between those idealised two extremes cases.

In many applications, the significant volume of soil can be above the water table, hence in partial saturation conditions. Nowadays, the effects of suction and saturation degree on the mechanical behaviour of soils have been widely investigated by laboratory studies (Fredlund et al 1978;[START_REF] Cui | Yielding and plastic behaviour of an unsaturated compacted silt[END_REF][START_REF] Escario | The shear strength of partly saturated soils[END_REF][START_REF] Vassallo | Effects of net stresses and suction history on the small strain stiffness of a compacted clayey silt[END_REF][START_REF] Casini | Effetti del grado di saturazione sul comportamento meccanico di un limo[END_REF][START_REF] Casini | Consequences on water retention properties of double-porosity features in a compacted silt[END_REF][START_REF] Biglari | Modelling the initial shear stiffness of unsaturated soils as a function of the coupled effects of the void ratio and the degree of saturation[END_REF][START_REF] Sivakumar | Influence of compaction procedure on the mechanical behavior of an unsaturated compacted clay Part 1: Wetting and isotropic compression[END_REF]Salager et al 2013, Hamid & Miller 2009). In the last years, some studies have been published on the influence of partial saturation in engineering problems such us bearing capacity of shallow and deep foundations [START_REF] Georgiadis | The influence of partial saturation on pile behavior[END_REF][START_REF] Vanapalli | Shear strength of unsaturated soils and its applications in geotechnical engineering practice[END_REF]. However, in soil-structure interaction problem, potential benefits of suction are often neglected by practical applications.

Physical centrifuge modelling represents a valid methodology to experimentally investigate soilstructure interaction problems. The small scale model is linked to the full scale prototype, following scaling laws (e.g. [START_REF] Corté | Model testing-Geotechnical model tests[END_REF]Schofield ,1990 ). When the scale of the model is 1/N, the model has to be subjected to a centrifuge g-level of N. Studies were so far conducted on fully saturated or completely dried soil model. Only recently, due to a better understanding of the behaviour of compacted soils [START_REF] Thorel | Physical modelling of wetting-induced collapse in embankment base[END_REF][START_REF] Caicedo | Centrifuge Modelling of Unsaturated soils[END_REF] for the preparation of soil model and to the new instrumentation available to measure suction and saturation degree during the tests [START_REF] Caicedo | Centrifuge Modelling of Unsaturated soils[END_REF][START_REF] Soranzo | Face stability of shallow tunnels in partially saturated soil: centrifuge testing and numerical analysis[END_REF], unsaturated soil behaviour has been explored by centrifuge modelling [START_REF] Casini | Effetti del grado di saturazione sul comportamento meccanico di un limo[END_REF][START_REF] Thorel | Physical modelling of wetting-induced collapse in embankment base[END_REF][START_REF] Soranzo | Face stability of shallow tunnels in partially saturated soil: centrifuge testing and numerical analysis[END_REF]. Scaling laws for unsaturated soils have been experimentally investigated by [START_REF] Depountis | Centrifuge modelling of capillary rise[END_REF] and [START_REF] Caicedo | Validation of time scale factor of expansivesoils in centrifuge modeling[END_REF]. The authors found that capillary rise and diffusion time in the centrifuge could be scaled of 1/N and 1/N 2 respectively. Details of analytical formulation can be found in [START_REF] Caicedo | Centrifuge Modelling of Unsaturated soils[END_REF] [START_REF] Soranzo | Face stability of shallow tunnels in partially saturated soil: centrifuge testing and numerical analysis[END_REF]. The scaling factors adopted in this study are listed in Table 1 The current experimental study is focused on the influence of partial saturation on the behaviour of laterally loaded pile. The aim of the experiment is to study the response of a single vertical freehead pile, embedded in a homogenous fine graded soil, subjected to a combination of lateral loading and bending moment, under different hydraulic condition and initial void ratio. The study has been developed by means of physical modelling in macro-gravity (N=100×g) using the IFSTTAR centrifuge facilities of Nantes [START_REF] Rosquoët | Lateral cyclic loading of sand-installed piles[END_REF].

The study is organized as follows. First, model and instrumentation set up are reported. Secondly, the soil hydro-mechanical characterization is briefly presented together with the results of flooding test in oedometer apparatus conducted to explore volumetric behaviour during wetting. Thirdly, soil model preparation and test procedures are presented and described. Finally, the results of the study are discussed focusing the attention on the load-head displacement relationship and flexural pile response, exploring the influence of the different structure and soil saturation conditions. In the last section, the effects of a variation of saturation degree on the pile behaviour are analysed.

Experimental facilities

SOIL PROPERTIES

The material used is a commercial kaolin named B-Grade kaolin. The soil has a fine silt fraction of about 90% and a clay fraction of 10%. As reported in Table 2, the material has a liquid limit (wL) of 42%, a plastic limit (wP) of 28% and therefore a plastic index (IP) of 14%. The main hydromechanical saturated parameters are summarized in Table 2.

In order to define the after compaction conditions (w, e) for centrifuge models, a number of flooding experiments have been performed on samples with different compaction features with a constant vertical stress v=150 kPa. This load corresponds approximately to the vertical stress exerted at half of pile embedded length (z  8m).

Tests are carried out in oedometric apparatus of 70 mm in diameter and 18 mm thick. Following the standard procedure proposed by several authors [START_REF] Wheeler | An elasto-plastic critical state framework for unsaturated soil[END_REF][START_REF] Tarantino | Compaction behaviour of clay[END_REF][START_REF] Casini | Effetti del grado di saturazione sul comportamento meccanico di un limo[END_REF], the soil has been preliminarily dried at 105 °C for 24 hours then demineralized water has been added to reach the desired water content. The material has been kept in sealed bags for 24-48 hours. The specimens have been statically compacted (v=1.5 mm/min) directly in the oedometric ring, allowing to reach quite accurately the required void ratio.

The tests consist of two steps:  Load at constant water content;  Soaking at constant load;

The grid of water content and void ratio has been chosen on the base of the results of the Standard Proctor test, black line in Figure 1, from which results an optimum water content (wopt) of 0.21 and an optimum void ratio (eopt) of 0.74 (optimum dry density d,opt=1.528 gr/cm 3 ). Three voids ratio are taken into account (e0 = 0.77, 0.92 and 1.12), for water content ranging from 10% to 26%, with steps of 4 %, in order to cover both dry than wet side of the Proctor curve, Figure 1. For each grid point at least two samples are tested. Moreover, samples conditions after soaking, are presented in Figure 1 with grey symbols, grouped for initial void ratio. All the samples have elevated values of saturation degree, from 0.95 to 1.0. A significant influence of initial void ratio can be recognized: more compacted (e0=0.77) soil shows a little swelling during wetting while the others exhibit collapse deformation during soaking increasing with initial void ratio. A negligible influence of compaction water content on the deformation upon wetting has been found for this material.

Based on the experimental results obtained, reported in Figure 1, in order to have a collapsible and a swelling soil sample upon wetting, two initial void ratio are selected (0.93 and 0.75) respectively with the same water content w (0.15).

The soil water retention curve (WRC), obtained using the suction controlled oedometric cell [START_REF] Romero | Development of a new suction temperature controlled oedometer cell[END_REF], is presented in Figure 2 in terms of suction and saturation degree relationship.

Experimental data refer to the main wetting curve obtained for two different void ratios, 0.93 and 0.75 respectively; both of them has been fitted using [START_REF] Van Genuchten | A closed-form equation for predicting the hydraulic conductivity of unsaturated soils[END_REF] equation the parameters of which are reported in Table 3. In the suction range experimentally studied, as porosity decrease air entry value (1/) increases from 50 to 166 kPa and the slope of the transition (n) zone reduces from 1.4 to 1.3 in the Sr-s plane. The findings are consistent with literature results [START_REF] Gens | Effect of structure on the volumetric behaviour of a compacted soil[END_REF][START_REF] Romero | Characterization and thermo-hydro-mechanical behaviour of unsaturated Boom clay: an experimental study[END_REF][START_REF] Romero | Retention curves of deformable clays[END_REF][START_REF] Fredlund | Equation for the soil-water characteristic curve[END_REF][START_REF] Tarantino | Compaction behaviour of clay[END_REF][START_REF] Romero | An insight into the water retention properties of compacted clayey soils[END_REF]. Neglecting for the sake of simplicity the hysteresis of WRC, the as-compacted suction, according to Van Genuchten (1980) equation, is respectively 900 kPa and 3400 kPa for e0=0.93 (Sr0=0.43), and e0=0.75 (Sr0=0.52).

MODEL AND INSTRUMENTATION

For the purpose of the experiment, 180 mm of soil are statically compacted imposing a constant displacement rate (v=1.5 mm/min), under one-dimensional condition, in a rigid cylindrical container of 300 mm of diameter. A 10 mm thick sand layer, surrounded by geotextile, is laid as drainage layer at the bottom of the model. A 2 mm thick plastic sheet is placed at the border to reduce the container's roughness and shear stresses developing during model preparation. To prevent water evaporation in the upper part of the model, a plastic film covers the soil surface. The bored pile is installed at 1×g, the pre-hole has been realized by means of a manual screwing system of the IFSTTAR facilities [START_REF] Khemakhem | Static and cyclic lateral pile behavior in clay[END_REF] for an embedded length of 150 mm.

The bottom of the model is connected to a water reservoir the level of which is governed by an electro-pneumatic valves system directly controlled by the operator in the centrifuge control room.

A laser sensor measures the water height in the tank.

The model has been extensively instrumented in order to follow both the equalization phases and pile loading, a schematic view of the instrumentation used is proposed in Figure 3. Five LVDT sensors measure soil settlements, two of them are far from the expected interaction zone and they measure the settlements due to flight and consolidation only, the other three are in line with applied load, in the passive area, and can measure also the soil movement during pile loading. The pore water pressure (negative and positive) in the soil is measured with three tensiometers, placed at different depth at the model border. The sensor's range is -500 kPa to 500 kPa. The sensor calibration and saturation procedure of the tensiometer and of the porous stone are described by [START_REF] Mancuso | Messa a punto di un nuovo minitensiometro ad alta capacità[END_REF].

The load has been applied at 35 mm from ground level by a hydraulic actuator. The loading phase was displacement-controlled (v=0.003 mm/s at model scale), and a load cell (full scale 2500 N) provided the measure of lateral load. One LVDT built in on a rotational joint gives pile's vertical displacement and rotation.

At the end of each test, undisturbed specimen were sampled all along the model height in order to obtain water content and void ratio distribution with depth.

MODEL PILE CHARACTERISTICS

The 1/100 model pile is a close-ended tube, instrumented with 10 pairs of strain gauges arranged every 15 mm, Figure 4. After a calibration in the lab, the set of gauges deliver the bending moment profile along the pile length. At the prototype scale (N=100×g) the model represents a full circular pile of 1.2 m of diameter, 15 m of embedded length with a bending stiffness of 3.9 GNm 2 , subjected to a lateral load applied at 3.5 m from ground surface. The combination of high bending stiffness and relatively small slenderness ratio (L/D = 15/1.2 = 12.5) led to a substantial rigid behaviour of the pile.

Centrifuge test and procedures

MODEL PREPARATION

The soil model used in the centrifuge model has been statically compacted in six layer, with the same procedure followed for the laboratory tests, with w=15% and e0=0.93 or 0.75 and it is located on 'dry side' of the Proctor curve. The distribution of vertical compaction stresses, measured during compaction and reported in Figure 5, shows a reasonably good homogeneity for any sample and a very good repeatability of the results in different tests. The compaction stress increases with dry density from a mean value of 500 kPa in the looser state (e0=0.93) to 1400 kPa in the denser state (e0=0.75). The homogeneity and repeatability are confirmed also by cone penetration tests (with a dc= 12 mm) at 1×g reported in Figure 6, referring to low compacted soil. The curves have a similar trend with an increase with depth up to z~50 mm and then are characterized by a mean constant value of qc~ 3.6 MPa.

EXPERIMENTAL PROGRAMME

The tests performed in centrifuge are nine , four of team are presented in details, focusing the attention on two parameters: Initial void ratio (2 cases)

 Elevation of the water table (2 cases)

The testing programme is resumed in Table 4 where zw/L is the water table elevation to embedded length ratio and e0, w0, vc and Sr0 are, respectively, the initial void ratio, the gravimetric water content, the compaction stress and the degree of saturation. These values have to be intended as mean values of soil properties.

PROCEDURE

In the main tests (T_06 and T_08) the pile is loaded in unsaturated conditions, until a normalized lateral displacement, y/D, of 30-40% was reached. Then, the water table zw, has been imposed at 7 m (prototype scale) from ground level. The water table elevation over the embedded length ratio is close to 0.5 (zw/L = 7/15 = 0.46).

In the following step, the actuator control has been switched from displacement to force control and the water table level has been raised quickly to ground level. The instrumentation on the pile allowed continuous monitoring of head displacement, rotation and bending moments along pile during water table rising.

For the sake of comparison, a pile load has been conducted up to soil failure in fully saturated condition, zw=0 (T_05 and T_09).

TEST STEPS

The following steps characterize the main tests (T_06, T_08):

1) 1×g imbibition: a zero pore pressure is applied at the model base in order to reduce after compaction suction;

2) Flight and application of hydraulic condition at model base: the system has been left in equalization for at least 3 hours;

3) Pile load at displacement control (v=0.003 mm/s); 4) Increase of water table and equalization.

For reference tests in fully saturated condition (T_05 and T_09), only the first three steps are needed; the water table has been directly imposed at ground level.

Results and discussion

In this section are presented selected results to illustrate the influence of soil state condition and partial saturation on the response of laterally loaded piles. The complete results of the experimental program are detailed in [START_REF] Lalicata | Effect of saturation degree on the mechani-cal behaviour of a single pile subjected to lateral forces[END_REF].

General consideration about soil state at the end of the reference tests on saturated soil for e0=0.93 (T_05) and e0=0.75 (T_09) can be deduced by the analysis of the water content and the void ratio distribution with depth reported in Figure 7. Data shows that for e0=0.93 (T_05) void ratio decreases with depth (typical of soil NC) from a value of ~0.9 at surface to a value of 0.76 for a depth of 17 m; on the other hand, in T_09 (e0=0.75) the void ratio is mostly constant over the entire range of depth.

Moreover, the comparison of the experimental data with the oedometric normal consolidation line, grey line in Figure 7, highlights how the loose soil lays on the NCL below 4 meters of depth, while the denser ones intercept it at 12 meters from ground level. These differences, even for the same stress history prior to pile loading (1×g imbibition, increase of total stress and in-flight equalization), may be ascribed both on the different initial void ratio and the different shape of the WRC that controls the variation of mean effective stress and the preconsolidation pressure during hydromechanical stress paths [START_REF] Lalicata | Effect of saturation degree on the mechani-cal behaviour of a single pile subjected to lateral forces[END_REF].

LATERAL LOAD AT CONSTANT WATER TABLE LEVEL

The load-displacement curves for samples in fully saturated conditions and different void ratio are shown in Figure 8, at the prototype scale. The lateral displacement measurement, y, refers to the displacement at the load application point (nominally 3.5 m above ground level). In the range of lateral displacement explored, the load-displacement relationship is highly non-linear [START_REF] Rosquoët | Lateral cyclic loading of sand-installed piles[END_REF][START_REF] Mayne | Laboratory Modeling of Laterally-Loaded Drilled Shafts in Clay[END_REF][START_REF] Russo | A method to compute the non-linear behavior of piles under horizontal loading[END_REF].

The experimental data can be adequately fitted by means of a hyperbolic function [START_REF] Mayne | Laboratory Modeling of Laterally-Loaded Drilled Shafts in Clay[END_REF]:

lim 11 y H y KH      (1)
Where K [MN/m] is the initial stiffness and Hlim [MN] is the asymptotic load. In Figure 8, the fitting of hyperbolic function with the load-test data appears to be quite satisfying: dashed and continuous lines are used for e0=0.75 and 0.93 respectively. The numerical values of lateral stiffness and ultimate load are reported in Table 5.

The strength mobilized, under the applied lateral displacement, is different in asymptotic value and shape for denser and looser samples: for every y value, the H/Hlim ratio is higher for e0=0.93 compared to e0=0.75. At the end of the test (y=1.6m), for the loose soil (e0=0.93, T_05) the measured load of 0.7 MN is about the 80% of ultimate capacity Hlim. On the contrary, the denser soil, e0=0.75 (T_09), is still far from the ultimate capacity showing a value of 3.2 MN for the maximum lateral displacement applied (y=1.4 m), giving a load ratio, H/Hlim, of 0.55. Initial lateral stiffness increases more than three times as initial void ratio decreases as well. In addition, for low initial void ratio, the load increases almost linearly for a significant displacement range, up to 0.2 m, suggesting that a small amount of yielding occurs in the soil. Furthermore, in the looser state the load-displacement relationship exhibits a non-linear behaviour from low values of displacement, indicating that the soil develops significant plastic strain even for very low load level [START_REF] Russo | Piles under horizontal load: an overview[END_REF]. The findings are consistent with the deduced over-consolidation ratio induced by different initial void ratio already commented.

The influence of partial saturation on the load-displacement behaviour is analysed in Figure 9,

where the load-displacement curves for looser (Figure 9a) and denser (Figure 9b) samples are reported. In both cases, the partial saturation induces higher stiffness to the soil above the water table, with more appreciable effects for high void ratio. Moreover, for loose soil in presence of partial saturation the load-displacement relationship exhibits an initial linear branch, which cannot be found in saturated condition, consistent with the well-known increment of preconsolidation stress in unsaturated soils [START_REF] Gens | Soil-environment interactions in geotechnical engineering[END_REF].

It is known that stiffness in fine graded saturated soil depends on mean effective stress p', void ratio e and/or OCR in a non-linear way [START_REF] Viggiani | Stiffness of fine-graded soils at very small strains[END_REF][START_REF] Rampello | The dependence of G0 on stress state and history in cohesive soils[END_REF]. Most

recently other researchers have proposed a modified formulation to take into account the effect of saturation degree and suction on the small strain stiffness [START_REF] Biglari | Modelling the initial shear stiffness of unsaturated soils as a function of the coupled effects of the void ratio and the degree of saturation[END_REF], in which stiffness variations are mainly attributed both to the increase of mean effective stress p' and to increase the yielding pressure induced by partial saturation [START_REF] Jommi | Remarks on the constitutive modelling of unsaturated soils[END_REF][START_REF] Laloui | On the use of the generalised effective stress in the constitutive modelling of unsaturated soils[END_REF][START_REF] Tamagnini | An extended cam-clay model for unsaturated soils with hydraulic hysteresis[END_REF].

Following the generalized effective stress framework [START_REF] Laloui | On the use of the generalised effective stress in the constitutive modelling of unsaturated soils[END_REF][START_REF] Bishop | Some aspects of effective stress in saturated and partly saturated soils[END_REF], in unsaturated conditions the main effective stress increases (T_06 and T_08), compared to saturated ones (T_05 and T_09), of the quantity

  '1 w r w w r p z S z S          .
On the other hand, as expected, the stiffness increment due to capillary forces, is better appreciated on the loose soil (e0=0.93, T_06) which starts from lower values of stiffness. Those increments become less important as the initial structure increases due to the decreasing of initial void ratio.

In addition, the different distribution of saturation degree with depth for the two materials, illustrated in Figure 10, plays an important role in the understanding of the observed behaviour. For sake of simplicity, the plots are evaluated supposing that pore pressures were in hydrostatic conditions ( ww uz  ); the values of degree of saturation deduced from the SWRC, Table 3, are very different.

At ground level, where the differences are more pronounced, the reduction of Sr compared to saturated conditions for the looser soils are 4 times higher than that corresponding to the denser soil.

This difference in the distribution of Sr, give an increases in the ratio p'cunsat/p'csat between the preconsolidation pressure in unsaturated conditions p'cunsat and in saturated conditions p'csat, which is properly described with a exponential function of the degree of saturation (e.g. [START_REF] Jommi | Remarks on the constitutive modelling of unsaturated soils[END_REF][START_REF] Tamagnini | An extended cam-clay model for unsaturated soils with hydraulic hysteresis[END_REF]Gallipoli et al 2003). As well as the stiffness and the strength increase with the decreasing of Sr.

Table 5 summarizes the relative influence of the position of water table, zw/L, and soil state for lateral stiffness K and ultimate load Hlim respectively, evaluated by means of eq. ( 1). Passing from zw/L = 0 to zw/L = 0.46 it can be observed a stiffness increment of more than 300% for high initial void ratio and of 50% low initial void ratio. Therefore, structure effects induced by partial saturation are significantly important for soil with initial open structure because of lower values of saturation degree which allows developing higher bonding effect induced by meniscus. A direct connection between lateral stiffness K and soil stiffness Es cannot be easily recognised because of the pile behaves almost like a rigid pile, hence the head displacement is the sum of a deflection and a rigid rotation [START_REF] Lalicata | Effect of saturation degree on the mechani-cal behaviour of a single pile subjected to lateral forces[END_REF] and elastic solutions [START_REF] Randolph | The response of flexible piles to lateral loading[END_REF][START_REF] Higgins | Elastic solution for laterally loaded piles[END_REF][START_REF] Di Laora | Kinematic Bending of Fixed-Head Piles in Nonhomogeneous Soil[END_REF] are not directly applicable. In comparison with lateral stiffness, Hlim appears to be less affected by partial saturation: compared to saturated condition, gains are 12% for e0 = 0.75 and 220% for e0 = 0.93.

The comparison of bending moment profiles, at 0.1 MN of applied load, for T_08 and T_09 (e0 = 0.75) is presented in Figure 11 (a). The pile does not behaves as a purely flexible pile, since bending moment propagates all along the entire embedded length then the active length of the pile is equal, or even greater of, to the actual length L [START_REF] Randolph | The response of flexible piles to lateral loading[END_REF]. The stiffness increment in the shallower seven meters from ground level, induced by partial saturation, led to a reduction of maximum bending moment of more than 20% for H=0.1 MN. Thanks to partial saturation, the position of maximum bending moment slightly moves upward passing from 5 to 4 meter from mudline. The small differences of the moment measured at the ground level in the two cases may be ascribed to the different settlement of the soil during the consolidation phase [START_REF] Lalicata | Effect of saturation degree on the mechani-cal behaviour of a single pile subjected to lateral forces[END_REF].

Double derivation of moment profile allows calculating the soil pressure distribution along the pile for every load increment. Soil pressure distribution is shown in Figure 11 (b) for the same soil condition and load level of Figure 11 (a). In the shallower part, the unsaturated soil hold higher pressure compared to the saturated one. Pressure remains almost constant in the first 6 meters from ground level and then it reduces smoothly towards the pile tips changing sign at 12 meter of depth.

Referring to almost rigid piles, the findings reported in the present study seems to indicate that the unsaturated zone above water table can change significantly the soil reactions distribution all along the pile length.

The ratio of maximum bending moment obtained for unsaturated condition and saturated condition remains at the constant value of 0.76 for relatively low lateral load, up to 0.5 MN, then it gradually increases to 0.94 measured for H=2.1 MN, as with the increase of the applied load the rigid behaviour becomes dominant compared to the flexural one in both cases (Figure 12).

The bending moment profiles of T_06 and T_08, relative to 0.1 MN of lateral load, are pointed out in Figure 13. This comparison allows to highlight the influence of soil stiffness, here mainly related to initial void ratio and different Sr distribution, on bending response of the pile: as for softer soil (T_06, e0 = 0.93) the interaction involves greater volumes of soil compared to stiffer ones (T_08, e0 = 0.75). As expected, the bending moment increases as soil stiffness decreases, and as for the looser sample (T_06), the bending moment distribution is more homogeneous along the entire pile length and the maximum bending moment takes place at greater depth. It is worth noting that all these results are significantly affected by the moment at ground level induced by load eccentricity that significantly increases the values of the maximum bending moment and the downward load transfer [START_REF] Budhu | Nonlinear analysis of laterally loaded piles in cohesionless soils[END_REF].

WATER TABLE RISING AT CONSTANT LOAD

For T_06 and T_08, when the lateral head displacement has reached the 30-40% of the diameter, the loading phase has been stopped and the actuator has been switched from displacement control to loading control. By the electro-pneumatic valves system, the pore pressure at the bottom of the model has been raised quickly from 120 kPa to 190 kPa, in order to simulate the water table raising from 7 m of depth to 0 m. The model has been left in equalization for the remaining time test (320 min for T_06 and 380 min for T_08, at model scale), the analysis of soil settlements and pore pressure evolution indicates that the stationary condition has not been reached in none of the tests [START_REF] Lalicata | Effect of saturation degree on the mechani-cal behaviour of a single pile subjected to lateral forces[END_REF].

Suction decreases, due to imbibition, led to a reduction of mean effective stress and of the preconsolidation pressure (where the collapse for saturation occurs), as well as of the soil stiffness and strength. As a result, the pile head displacement has increased in both tests (Figure 14).

Displacement increment resulted higher for open soil structure, T_06, than for closer one, T_08. The global response reported in Figure 14 (a) and (b) is the result of a combination of multiple factors such as differences in saturated permeability, different soil stiffness prior to loading and load ratio level between initial load and ultimate load in saturated condition. First, since saturated permeability decreases with void ratio and, in this phase, the hydraulic equilibrium was far to be reached, lower values of pile lateral displacement are expected for the model with e0=0.75. Secondly, for the same perturbation applied, higher displacement is expected for loose soil (e0=0.93) because of lower stiffness. Finally, looking at the comparison between the load-displacement relationship for saturated and unsaturated condition, it seems reasonable to suppose that for e0=0.93 (Figure 14 (a)) a lateral load of 1.2 MN it is not sustainable in fully saturated conditions; while, for e0=0.75 even with no suction (Figure 14 (b)), a value of 2.1 MN appears to be sustainable by soil, then the system could have advanced faster to collapse in the first case. However, it is to be noted that the load-deflection curve obtained in saturated conditions (i.e. T_09) not necessarily represents a lower bound solution,

given that soil behaviour is strongly nonlinear and dissipative, therefore the final point measured during saturation (grey triangles in Figure 14 and, finally, small dashes has been used for z=14m. In order to better emphasize the phenomena, the results are represented in a semi-logarithmic plane and time has been replaced to zero just before the increase of water pressure. Instead of elapsed time, the Terzaghi non-dimensional formulation for time factor has been used:

2 v c Tt H 
, where cv is the coefficient of vertical consolidation, t is the elapsed time and H is the drainage layer. For sake of simplicity the same reference values of cv and H are adopted for all the tests and are 1.0×10 -6 m 2 /s (Table 2) and 180 mm (initial soil model height) respectively. It is interesting to note that, even if no stabilization occurs, the slope of the pore pressure curves gradually reduces, while the increment of lateral displacement keeps growing; moreover, in the last part of the test, a slight increment of displacement rate can be observed for T_06.

During saturation, the strength reduction in upper part produced the reduction of soil pressure against the pile in the first meters of depth from ground level and an increase in the last meters.

Observing the bending moment profiles prior and post water table rising, it can be observed (Figure 16) a general increment of the moment below the position of the maximum bending moment (from 6 m to 15 m of depth). Since the pile undergoes a rigid body motion during saturation the maximum bending moment variation is less than 10%.

Conclusion

In the present work, the soil pile interaction under lateral loading in unsaturated condition, by means of centrifuge tests has been studied. The models have been statically compacted at two different void ratios and the same water content. The initial conditions have been chosen based on a wide set of flooding tests at different void ratios and water content with the aims to cover a range of void ratios representative of in situ soil conditions.

The pile has been loaded with two positions of water table: one at half of pile's embedded length (zw= 7 m) and the other at ground level (zw= 0 m). Compared to saturated conditions, a significant increase in lateral stiffness on the load-deflection relationship: 300% for loose soil (e0=0.93) and 50% for dense soil (e0=0.75) has been observed in both cases. The effect of partial saturation is stronger for the looser state as, for the same suction, bonding effects due to capillary forces gives a major increment in soil structure. Moreover, the major changes of Sr in the looser sample is given to the different shape of WRC with void ratio.

The present study focus on the behaviour of an almost rigid pile and additional tests are required to investigate the effect of partial saturation varying the soil-pile relative stiffness and the slenderness ratio. In the particular case of rigid pile, that involves a significant volume proportional to the entire embedded length, the increase of soil stiffness in the shallower soil layer leads to a reduction of maximum bending moment more than 20% that moves towards ground surface. For flexible piles that have a critical length lower than the embedded length, the pile response is strongly dependent from the stiffness of the soil close to the surface. Therefore, it is possible that stiffness variation induced by partial saturation may will have more visible effects for flexible piles compared to the rigid pile of this study, which represents a lower bound solution in the understanding of the role of partial saturation in horizontally loaded piles During saturation, due to the combination of excessive load level and strong variation of water table level, the system goes towards soil collapse with a close to rigid body motion. Findings appear to show that neglecting the presence of partial saturation is not necessarily a safety solution when significant variation of saturation degree takes place. Reduction of soil strength and stiffness due to saturation can lead to greater pile head displacement than those predicted by fully saturated analysis.

The increase of lateral displacement depends both on of the value of the load and the initial water table position.

. 482

  (b)) did not represent the end of the process. The evolution of head displacement with the centrifuge time and the pore pressure for the two tests are presented in Figure 15 (a) and (b). In both Figures, black line refers to e0=0.93 (T_06), grey colour is used for e0=0.75 (T_08); in Figure 15 (b) different tensiometers position with depth is illustrated with various style line: a continuous line has been used for z=4m, large dashes for z=9m

Figure 1 :

 1 Figure 1: Flooding test results in the compaction plane.

Figure 2 :

 2 Figure 2: Main wetting Soil Water Retention curves for B-grade Kaolin.

Figure 3 :

 3 Figure 3: Experimental set-up and instrumentation.

Figure 4 :

 4 Figure 4: Model pile instrumented with 10 pairs of strain gauges.

Figure 5 :

 5 Figure 5: Vertical compaction stress with depth for different models.

Figure 6 :

 6 Figure 6: CPT profiles for e0=0.93.

Figure 7 :

 7 Figure 7: Water content and void ratio with depth after test T_05 and T_09.

Figure 8 :

 8 Figure 8: Effect of soil state condition on Lateral Load-Displacement curves (prototype scale).

Figure 9 :

 9 Figure 9: Load-displacement curves for (a) loose soil (e0=0.93); (b) dense soil (e0=0.75).

Figure 10 :

 10 Figure 10: Different distribution of saturation degree due to different porosity.

Figure 11 :

 11 Figure 11: Comparison of bending moment distribution along pile for highly compacted soil for zw/L =0 (T_09) and zw/L =0.46 (T_08).

Figure 12 :

 12 Figure 12: Maximum bending moment ratio, in unsaturated and saturation condition, for highly compacted soil, e0=0.75.

Figure 13 :

 13 Figure 13: Comparison of bending moment distribution along pile for zw/L =0.46 and different initial void ratio.

Figure 14 :

 14 Figure 14: Load-displacement curves during saturation for (a) loose soil (e0=0.93); (b) dense soil (e0=0.75).

Figure 15 :

 15 Figure 15: Evolution with time during rising table of tests T_06 and T_08: a) lateral displacement; b) pore pressure at different depth.

Figure 16 :

 16 Figure 16: Bending moment profiles after and before saturation for a) e0=0.93; b) e0=0.75.

Figure 1 :

 1 Figure 1: Flooding test results in the compaction plane.

Figure 2 :

 2 Figure 2: Main wetting Soil Water Retention curves for B-grade Kaolin.

Figure 3 :

 3 Figure 3: Experimental model: (a) cross section in the load plane, (b) aerial view of the basket, (c) perspective of valve system and soil container, (d) detail of the loading system and LVDT.

Figure 5 :

 5 Figure 5: Vertical compaction stress with depth for different models, each point is located in the middle of one layer.

Figure 6 :

 6 Figure 6: CPT profiles for e0=0.93.

Figure 7 :

 7 Figure 7: Mean values of water content and void ratio with depth after tests T_05 (e0=0.93) and T_09 (e0=0.75).

Figure 8 :

 8 Figure 8: Effect of soil state condition on Lateral Load-Displacement curves (prototype scale).

Figure 9 :

 9 Figure 9: Load-displacement curves: (a) loose soil (e0=0.93), (b) dense soil (e0=0.75).

Figure 10 :

 10 Figure 10: Different distribution of saturation degree due to different porosity for zw=7 m.

Figure 11 :

 11 Figure 11: Influence of water table elevation, zw =0 (T_09) and zw =7 m (T_08), for highly compacted soil (e0=0.75): (a) bending moment distribution along pile, (b) soil reaction distribution along pile.

Figure 12 :

 12 Figure 12: Maximum bending moment ratio, in unsaturated and saturation condition, for highly compacted soil, e0=0.75.

Figure 13 :

 13 Figure 13: Comparison of bending moment distribution along pile for zw =7 m and different initial void ratio.

Figure 14 :

 14 Figure 14: Load-displacement curves during saturation: (a) loose soil (e0=0.93), (b) dense soil (e0=0.75).

Figure 15 :

 15 Figure 15: Evolution with time during water table rising of tests T_06 (e0=0.93) and T_08 (e0=0.75): (a) lateral displacement, (b) pore pressure at different elevations.

Figure 16 :

 16 Figure 16: Bending moment profiles before and after saturation: (a) T_06 (e0=0.93), (b) T_08 (e0=0.75).

Table 1 : Scaling laws for centrifuge modelling.

 1

	Parameter	Scaling law model/prototype
	Length	1/N
	Density	1
	Unit weight	N
	Stress	1
	Strain	1
	Force	1/N 2
	Bending moment	1/N 3
	Seepage velocity	1/N
	Consolidation time	1/N 2
	Capillary rise	1/N

Table 2 : Index properties and hydro-mechanical properties of B-Grade kaolin.

 2

	wL	wP	IP	s	CC	CS	N0	'	ksat	cv
	(%)	(%)	(%)	(gr/cm 3 )	(-)	(-)	(-)	(°)	(m/s)	(m 2 /s)
	42.2	28.2	14.0	2.66	0.26	0.078	1.36	22	4.0•10 -9	1.0•10 -6

Table 3 : Van Genuchten parameters for different void ratio.

 3

	e0	Srsat	Srres		n	m
	(-)	(-)	(-)	(1/kPa)	(-)	(1-1/n)
	0.93	1.0	0.16	0.02	1.4	0.2501
	0.75	1.0	0.16	0.006	1.3	0.2307

Table 4 :

 4 Testing programme.

	Name	e0	zw/L	w0	vc	Sr0
		(-)	(-)	(%)	(kPa)	(%)
	Test 05 T_05 0.93	0.0	15.03	580	42.02
	Test 06 T_06 0.93	0.46	14.67	559	41.01
	Test 08 T_08 0.75	0.46	14.72	1395	51.03
	Test 09 T_09 0.75	0.0	14.72	1395	51.03

Table 5 : Lateral stiffness and asymptotic load.

 5

	Test	zw/L	K	Hlim	e0
		(-)	(MN/m)	(MN)	(-)
	T_05	0.0	1.5	0.9	0.93
	T_06	0.46	6.2	2.9	0.93
	T_09	0.0	5.0	5.6	0.75
	T_08	0.46	7.4	6.3	0.75

Acknowledgments

This research has been performed in the framework of the GEO-TRANSALP-PILE-UNSAT agreement developed between DISG, the "Department of Structural and Geotechnical Engineering" of "Sapienza" University of Rome, "Department of Department of Civil and Computer Engineering" University of Rome "Tor Vergata" and IFSTTAR, the "French Institute of Science and Technology for Transport, Development and Networks". They are greatly acknowledged. The authors would like to thank the technical team of the Ifsttar's geo-centrifuge, the technical staff of DISG geotechnical laboratory and also Dr. M. Blanc for his advices.


Jérôme Le Coz

Antoine Patalano

Daniel Collins

Nicolas Federico Guillen

Carlos Marcelo Garcia

Graeme M Smart

Jochen Bind

Antoine Chiaverini

Raphaël Le Boursicaud

Guillaume Dramais

Nicolás Federico Guillén

Carlos Marcelo García

Isabelle Braud

Crowdsourced data for flood hydrology: feedback from recent citizen science projects in Argentina, France and New Zealand

Keywords: Crowdsourced data, citizen science, flood hydrology, flood mapping, LSPIV

Introduction

New communication and digital image technologies have enabled the public to produce and share large quantities of flood observations. Such observations are often authored, time-stamped, georeferenced and eventually shared through social media [START_REF] Fohringer | Social media as an information source for rapid flood inundation mapping[END_REF]. Social media actually convey flood information of diverse nature including flood hazard and damage reports, but also rational discussion (understanding), public debate, appeal and remark to the government and local authorities, and emotional messages and expression of feelings [START_REF] Al-Saggaf | Social media in Saudi Arabia: Exploring its use during two natural disasters[END_REF].

As for other fields, initiatives for crowdsourcing flood data have emerged in the recent years, with a main focus on rapid, near real-time mapping of the reports of flood damages and emergencies, generally to support disaster management [START_REF] Fohringer | Social media as an information source for rapid flood inundation mapping[END_REF][START_REF] Koswatte | SDI and crowdsourced spatial information management automation for disaster management[END_REF]. Efficient tools for collecting, filtering, reviewing and analysing massive amounts of data in social media have to be developed. Typically, Fohringer et al. (2015) implemented the PostDistiller tool for the data mining of Twitter posts and applied it to map the inundation extent and depths of the June 2013 flood in Dresden, Germany. As done for other types of volunteered geographical information, interactive maps have also been developed for crowdsourcing flood data and reports, for instance:  PetaJakarta a , an open source flood map of Jakarta, Indonesia, to share real-time flood information using social media (Twitter);  The QLD Flood Crisis Map, an interactive map based on the open-source Ushahidi b platform and operated by Australian Broadcasting Corporation (ABC) to allow citizens report information during the 2011 Queensland floods [START_REF] Koswatte | SDI and crowdsourced spatial information management automation for disaster management[END_REF];  Flooding Points c , a collaborative flood map for São Paulo, Brazil, also based on the open-source Ushahidi platform and the concept and prototype elaborated by Hirata et al. (2016, in press).

Another way to enhance flood data crowdsourcing is through dedicated mobile phone applications such as, for instance:

 Flood Patrol (Philippines), an Android mobile phone application developed for allowing people send flood reports to NOAH d (Nationwide Operational Assessment of Hazards) for mapping;

 SIGNALERT e (France), a smartphone application to report various situations of natural hazards including floods; a https://www.petajakarta.org/banjir/en/ b https://www.ushahidi.com/ c http://g1.globo.com/sao-paulo/mapa-do-alagamento/platb  mPING f (Meteorological Phenomena Identification Near the Ground, NOAA, USA), a free mobile phone application used to collect public weather reports, including flooding across the USA as contributions to the Flood Observations -Citizens As Scientists using Technology Project (FLOCAST g ) launched in 2013.

Most of these projects and the related research focussed on volunteered geographic information, usually flood damages rather than quantitative hydraulic data. Despite of quality and credibility issues related to crowdsourced data, their filtering and mapping allows for unprecedented spatio-temporal analyses of the flood hazard and flood damages. Combining crowdsourced data and authoritative data, [START_REF] Schnebele | Road assessment after flood events using non-authoritative data[END_REF] were able to assess the spatio-temporal dynamics of the damages to the transportation infrastructure in New York City flooded by Hurricane Sandy (29-30 October 2012). Using telephonic reports of flood incidents in Rotterdam from 2004 to 2011, [START_REF] Gaitan | Spatial distribution of flood incidents along urban overland flow-paths[END_REF] analysed the spatial distribution of flood damages and their (lack of) relation with the subcatchments and flow paths derived from a DEM of the urban area.

Quantitative hydraulic data such as the extent and depths of inundated areas [START_REF] Fohringer | Social media as an information source for rapid flood inundation mapping[END_REF] or flow rate estimates [START_REF] Fujita | Image analysis and reconstruction of the 2008 Toga River Flash Flood in an urbanised area[END_REF], Le Boursicaud et al. 2016) can be computed using messages, photos and videos from eyewitnesses and help improve the understanding and modelling of flood hazard. This way, ordinary citizens or some enthusiastic flood chasers can contribute to hydrological science in the same way the so-called storm chasers have historically contributed to meteorological science since the Tornado Intercept Project (1975) of the National Severe Storm Laboratory (USA).

Projects encouraging the public to act as citizen scientists in flood hydrology still appear to be scarce.

The main well-established initiative of that kind is the CrowdHydrology h project [START_REF] Lowry | CrowdHydrology: Crowdsourcing Hydrologic Data and Engaging Citizen Scientists[END_REF], which encourage citizens to read and text the station number and the water level to the phone number listed on the gauge of gauging stations in the USA. The water level is then added to a publicly available database.

To our best knowledge, besides the CrowdHydrology project there is no available feedback on such specific projects, and more generally there is a lack of feedback and guidance on the failure and success factors of data crowdsourcing and citizen science projects for flood hydrology.

We introduce three recent citizen science initiatives which have been launched independently by research organisations to document floods in some catchments and urban areas of Argentina, France and New Zealand. These projects were specifically designed to derive quantitative hydraulic data from digital photos d http://noah.dost.gov.ph/ e http://www.signalert.eu/ f http://www.nssl.noaa.gov/projects/ping/ g http://flash.ou.edu/flocast/ and videos from the public for further retrospective analysis and modelling of the flood processes. While the three projects differ in their objectives, methods and hydrological situations, they provide convergent feedback on the potential and limitations of such initiatives for flood hydrology.

The three projects

Flood Chasers (Argentina)

Objectives

The mountainous rivers of the Córdoba province, Argentina, are characterised by the occurrence of flash floods with very high volumes of fast flows during the rainy season (October to April). Due to the sudden nature of flash floods, it is very unlikely to be able to survey several river sections of interest using instruments and techniques suitable for recording the peak flow and its temporal evolution during the flood. Even when it is possible to visit the study sites in time, flow velocities and floating river debris endanger both the instruments and operators. Considering this issue, the researchers of the National University of Córdoba have developed and implemented the Flood Chasers Project ("Cazadores de crecidas" in Spanish, [START_REF] Patalano | Desarrollo e implementación del proyecto "Cazadores de crecidas" en ríos de la provincia de Córdoba [Development and implementation of the Cazadores de crecidas project in rivers of the Córdoba province, in Spanish], IV Simposio sobre métodos experimentales en hidráulica[END_REF] to populate a database of videos and photos of flash floods in rivers of the province of Córdoba recorded and shared by citizens using advanced digital technology (cell phones, digital cameras, tablets, etc.) on the basis of the willingness of people filming these extreme hydrological events to share their footage in social networks, websites, forums, etc.

Flood videos recorded by citizens are then processed to estimate river flow velocity and discharge using image velocimetry techniques such as Large Scale Particle Image Velocimetry (LSPIV, [START_REF] Patalano | Evaluación experimental de la técnica de velocimetría por seguimiento de partículas a gran escala para la determinación de caudales en ríos serranos [Experimental evaluation of large scale particle tracking velocimetry technique for flow discharge measurements in mountains rivers[END_REF].

The implementation of LSPIV using non-professional videos appears as a valuable alternative or supplemental technique to traditional post-flood discharge estimation methods such as the slope-area method, and proves great potential in the Córdoba province, Argentina [START_REF] Patalano | Desarrollo e implementación del proyecto "Cazadores de crecidas" en ríos de la provincia de Córdoba [Development and implementation of the Cazadores de crecidas project in rivers of the Córdoba province, in Spanish], IV Simposio sobre métodos experimentales en hidráulica[END_REF]. During the last rainy season (2014)(2015), extreme hydrological events occurred in the Córdoba province and in some cases the data obtained with such technique are the only available information to characterise the observed hydrological events.

Implementation

The first step of the Flood Chasers Project was to create a website with domain of the University, then perform an intensive dissemination of the existence of this webpage in major newspapers and some television channels. On the Project website (Figure 1) people can upload their flood videos along with metadata (recording date and time, location of the river section, etc.). In case the recording conditions are favourable, the videos are manually screened, analysed and processed using the Large Scale Particle Image Velocimetry technique (LSPIV, [START_REF] Fujita | Large-scale particle image velocimetry for flow analysis in hydraulic engineering applications[END_REF]. This technique allows quantifying the surface velocity field of the rivers remotely. The website provides guidelines for users without prior knowledge of hydraulics about the best way to make contributions that are useful for quantifying flood discharges. Users who are interested in participating in the Flood Chasers Project can send or upload their videos using different platforms (i.e. Dropbox, Mega and WeTransfer) following basic tutorials generated by the scientists. There is also a webpage listing all the recorded floods and including the flow results if the video has been suitable for analysis. In turn, there is a YouTube channel called "Cazadores de Crecidas" in which the Project leaders upload their own collection of videos, which are also published on the website. The images are analysed following the methodology described by [START_REF] Patalano | RIVeR -Towards affordable, practical and user-friendly toolbox for Large Scale PIV and PTV techniques[END_REF] of the videos with respect to the flow surface. The rectification is made using distances between fixed points (ground reference points) observed in the images that were surveyed in situ after the flood event. Combining the surface velocity profile in the area of interest with the bathymetry profile subsequently surveyed and a relationship between depth-average and surface velocity, the flood discharge at the time of the video in a defined river cross-section can be estimated.

The relationship of the Flood Chasers Project leaders with government authorities is more than optimal.

The minister of water, environment and energy of the Córdoba province is a professor at Universidad Nacional de Córdoba (at leave) who is interested in the implementation of the technique. The results obtained in the project have been used by the government to hydrologically evaluate some of the recorded events. Due to this interest the project has received support from the government including logistics for conducting the required surveys (topography survey, fieldwork, etc.).

i http://www.cazadoresdecrecidas.unc.edu.ar/

Author-produced version of the article published in Journal of Hydrology ( 2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Examples of results

The methodology was applied to a video recorded by the witness of a flash flood that occurred in the Ceballos River in the city of Río Ceballos (31°10′S; 64°19′W), Province of Córdoba, Argentina, after an extreme rainfall event occurred on the morning of 15 th February 2015. The return period of the rainfall event was estimated to be more than 100 years with observed rainfall depth of about 250 mm during 12 hours. The witness uploaded the video to the project website (Figure 1) with the sufficient metadata so the scientists could reach the place where the video was recorded. Subsequently, the scientists went to the place and surveyed the river cross-section bathymetry using a topographic total station. The camera movement during the record due to the videographer holding the camera by hand without a tripod was preliminarily corrected.

Such movement created significant errors in the processing of the images (Figure 2a). Then, the surface velocity profile was computed (Figure 2b) and ortho-rectified.

(a)

(b)

Author-produced version of the article published in Journal of Hydrology ( 2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ The computed surface velocity data in the area of interest were used for the calibration of the hydraulic numerical model (HEC-RAS) implemented to simulate the flow conditions of the analysed event. The Digital Terrain Model of the whole reach of interest required to implement the hydraulic model was provided by the government of the Córdoba Province. Based on the surface velocity field and the water level data at the measured section, the bed and banks roughness could be estimated iteratively and the flow discharge computed. By the time that the video was recorded, the total discharge estimated using the calibrated model was 225 m 3 /s (Figure 3), which was not the peak discharge. The maximum water level (not observed in the video) was measured in situ using a post-event survey of the high water marks. Using the calibrated model, the peak discharge of the event was estimated to be 290 m 3 /s. The flooded area predicted using the calibrated flood model of the Ceballos River can be seen in Figure 4. The main problem faced in the Flood Chasers Project in Argentina is that many of the videos are not suitable for processing because they do not meet the required conditions: most people record videos with continuous movements of the camera and zooming, and in many cases it is not possible to detect the ground reference points necessary for image ortho-rectification. For the next rainy season, the scientists are looking for training groups of people who can be great partners. For example the civil defence workers (i.e. firefighters) will be contacted for training since they are usually the first to arrive to the rivers when flash floods occur. Advanced students (16 to 17 years old) from high schools that are located near rivers will also be trained. The youth of that age usually use cell phones which now have the minimum resolution required to implement the image velocimetry technique. A strong message of safety is also transmitted to all trained people because it is very important to avoid any life in danger and be sure there is no risk in recording a video.

Author-produced version of the article published in Journal of Hydrology ( 2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

FloodScale (France)

Objectives and background

As part of the ANR j FloodScale project (France) on Mediterranean flash floods [START_REF] Braud | Multi-scale hydrometeorological observation and modelling for flash flood understanding[END_REF] Chasers project, except that ortho-rectification was applied to every image pair before velocity computation.

Flood videos recorded by witnesses usually do not match the requirements of the traditional LSPIV technique. Using such videos raises several issues (Le [START_REF] Boursicaud | Guide méthodologique -Collecte et préparation de film amateur en vue d'une exploitation par analyse d'image [Methodological Guide -Collection and pre-processing of home movies for operations by image analysis[END_REF][START_REF] Boursicaud | Gauging extreme floods on YouTube: Application of LSPIV to home movies for the post-event determination of stream discharges[END_REF] that must be solved for the video to be successfully processed:

 Fixed points that can be located in the images and in the real world are needed to calibrate the orthorectification;

 Both river banks should be visible in the image so that a complete cross-section is monitored;

 The video should be recorded from a fixed point of view: a shake with limited amplitude was found to be acceptable;

 The precise location and timing of the video are necessary;

 Authors of the videos must be contacted in order to get their personal agreement for the use of their video materials and additional information.

Using flood home movies therefore requires some image pre-processing to be applied before the LSPIV analysis, such as:

 Correction of hand-held camera movement: image alignment;

 Correction of distortion due to non rectilinear camera lenses;

 Relative positioning of the free-surface.

The project followed a fully reproducible approach, based on free software only. LSPIV analysis was conducted using Fudaa-LSPIV (Le were sampled from the video at an adequate time rate using ffmpeg n , and image pre-processing was conducted using Hugin o . The complete procedure to collect and pre-process home movies for further LSPIV analysis comes as a short methodological guide (Le [START_REF] Boursicaud | Guide méthodologique -Collecte et préparation de film amateur en vue d'une exploitation par analyse d'image [Methodological Guide -Collection and pre-processing of home movies for operations by image analysis[END_REF].

Simple procedures for taking useful flood videos, highlighting safety warning (Figure 5) have been disseminated to the public through various media including the project website

Examples of results

Partly due to communication restrictions, few videos were received by the scientists, even after the series of significant floods which occurred during the 2014 autumn. And none of those were actually usable because they generally did not follow the recommendations. Some of them were not located in the study area. In turn, some useful videos were harvested by scientists after each flood of interest, especially drone videos (Figure 6) shared on YouTube r by a local company s specialising in UAV-borne image recording. The footage was also shown by local and national TV channels. Valuable sequences with remarkable stability and clear views of the Ardèche river channel upstream and downstream of the submersible Sampzon bridge (44°25'46.4"N; 4°21'25.6"E) were selected (Figure 6).

Some fixed ground reference points could be identified, especially road signs and marks and elements of the bridge structure. Their positions were topographically surveyed using a differential GPS and crosssectional bathymetry profiles were also measured using a hydro-acoustic profiler (ADCP). Then image sequences were extracted, converted in grey scale levels, ortho-rectified, and instantaneous velocity fields were computed. Time-averaged velocities were interpolated at each node of a transect used for discharge computation. As often, the water level estimation appeared to be the main source of uncertainty (Table 1).

However, discharge estimates were found to be consistent with the flow rated at the upstream gauging station when realistic time lags due to flood propagation are considered (between 30 and 60 minutes). Even with uncertainties possibly as large as 15% or 20% such discharge estimates remain highly valuable to reduce the uncertainties of rating curve extension and post-flood peak discharge assessment. The main perspectives are to reiterate communication campaigns with the twofold objective of mitigating issues with the local governmental authorities, and increasing the quantities of usable videos spontaneously provided by the public. This would certainly be reached with opportunistic calls to be launched through any media just after a flood in the affected area. Thus the hydrologic watch could focus on a broader region such as the French Mediterranean belt, or even at a larger scale as part of the HyMex project which develops postflash flood surveys throughout the catchments contributing to the Western Mediterranean Sea.

Engaging with the locals in preparation of future floods at research sites like the Ardèche catchment would remain feasible through poster campaign at key observation points with safe conditions, especially near gauging stations which provide the water level data that are critical for velocity and discharge computations.

Such sites could be pre-surveyed and permanent ground reference points could be marked. A best viewpoint for videos could be marked and suggested on an adjacent poster or sign, for instance. The public could be asked to read the staff gauge like in the CrowdHydrology project and provide the water level along with their videos. Specialised partners such as technical staff from local municipalities, firemen, civil defence, volunteering observers, storm/flood chasers, kayakers, UAV-borne image companies, etc. could be informed and trained.

RiskScape (New Zealand)

Objectives

The New Zealand national institute NIWA has called citizens to contribute to flood mapping during recent inundation events in the cities of Christchurch (43°31′S; 172°38′E) in 2014 and Dunedin (45°52′S; 170°30′E) in 2015. Residents were invited to send in their pictures of flooded areas, which were further used to build flood hazard maps, as part of the RiskScape t project.

The 2010-2011 Canterbury earthquakes changed surface elevations across the region and the city of Christchurch, in the South Island of New Zealand. This led to changes in flood risk, which was highlighted by the 5 th March 2014 flood as some suburbs of Christchurch experienced uncharacteristically severe inundation.

The flood not only highlighted the altered risk profile, but it also presented a rare opportunity to update flood hazard knowledge of the city. With the widespread availability of camera phones and photos of inundation, t www.riskscape.org.nz this research project sought to employ citizen science to supplement professional measurements of inundation.

Implementation

Two days after the flood peak, members of the public were invited, via press releases to mainstream media, a scientific blog u and outreach via social media (Facebook, Twitter), to send photos of maximum flood levels to NIWA, indicating when and where each photo was taken. The photos were assessed by professionals in terms of the ability to identify a clear water level around the time of the peak inundation.

Water surface levels were derived from the photos in conjunction with LiDAR scans of the city. In locations where photos did not permit clear identification of water levels, field visits were carried out to make supplemental measurements. The resulting point data were interpolated, along with additional survey data, to derive a RiskScape flood hazard model to calculate flood risk and potential losses across the city. During flooding in the city of Dunedin in 2015, the same request for photographs was made, but the project was discontinued due to a lack of responses.

u http://sciblogs.co.nz/waiology/2014/03/07/calling-for-christchurch-flood-photos/

Examples of results

Over 600 photos were received for the Christchurch study (e.g., Figures 7 and8). Not all photos were used or could be used, and for various reasons -the flood level may not have been discernible; the time or location was unknown; or flood data may have been available from other photos or another source. Ultimately, 300 photos were used to get a fix on 289 water levels around the city. Combined with field surveys, the resulting inundation map (Figure 9) indicates flooding along the Avon and Heathcote Rivers as well as various residential suburbs throughout Christchurch, including the severely impacted Flockton Basin. High media and public interest suggested that, in addition to providing useful data, the public engagement fostered a greater appreciation of flood hydrology and its scientific study. Author-produced version of the article published in Journal of Hydrology ( 2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

The resultant information has been invaluable in understanding the changed flood risk for postearthquake Christchurch. These data were used to validate hydrodynamic models and are being used to plan mitigation, response and evacuation plans and to calculate human and economic costs of flooding. The widespread use of camera phones provided data on flood levels and extent that could not have been obtained by field surveys by hydrology technicians due to insufficient resources to cover the wide extent of relatively short duration flooding.

3 Lessons learnt

Write a clear and simple procedure

Once the objectives and tools of the project have been defined, a clear and simple procedure has to be written for the attention of the public. This is a key driver of the success of all citizen science projects. The three projects have produced procedures in the form of simple instructions for recording the images and sending the data in good agreement with their distinct objectives, either flood mapping or flood discharge estimation. The type of images and metadata, the protocols to send them to the scientists and the safety and copyright aspects were dealt with in a concise and precise manner. However, the simple but more demanding requirements on flood videos for LSPIV applications in the Flood Chasers and FloodScale projects proved to be much less followed than those on flood mark photos for the RiskScape project. The rate of photos usable for flood mapping was much higher than the rate of videos usable for discharge estimation. For the capture of valuable videos, a written procedure may not be sufficient and additional training should be provided as tutorials and short videos, e.g. on collaborative websites like WikHydro in France v . Instructions displayed in situ may also be more efficient.

Crowdsourcing flood hydrology data makes the most of modern digital image technologies and social media. It is advisable to target the kinds of equipment most commonly used by people in the area of interest, smartphones typically but also drones (UAV) increasingly. Drone images show great potential for the LSPIV technique, through excellent viewpoint stability and negligible perspective effects to correct since images are often taken vertically from the sky. Then image ortho-rectification is not absolutely necessary, only estimation of the size of the pixels, which requires much less ground reference points to be surveyed. Aerial photos taken from drones could certainly be used in flood mapping projects as well, in conjunction with high-resolution DEM.

v http://wikhydro.developpement-durable.gouv.fr/

Author-produced version of the article published in Journal of Hydrology ( 2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/ S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Develop suitable tools for data collection and processing

From a technical point of view, suitable tools are requested to collect, process the image materials, and assess the quality of results, in order to reach the scientific objectives of the project. It is necessary to develop platforms able to collect large quantities of data coming with contrasted formats and quality levels. Website forms and uploads, as well as smart (apps) or basic (texts) phones, are efficient tools to collect crowdsourced data, as long as the effort required from the user is kept minimal. Developing dedicated smartphone apps could be very efficient for the data collection.

However, it sometimes remains more efficient for scientists to harvest adequate image materials from sharing platforms and social media, and further locate and contact the authors for more information. As exemplified by the three projects described in this paper, both bottom-up and top-down ways of collecting data should be explored.

The processing of the collected data generally requires specific techniques and tools that are not previously available off the shelf. Research groups need to be committed into the development of suitable imagery techniques, hydrologic and hydrodynamic models, GIS applications and flood risk assessment methods. It is important to carefully review the quality of the crowdsourced data, in particular through postevent field investigation and cross-checking of the results with other sources of information, including professional data. Locating precisely the photo/video viewpoints is often easy whereas their precise timing may be not, especially when the authors cannot be contacted. The determination of the water level is a major source of uncertainty in the discharge estimation.

The use of crowdsourced hydrologic data in models and scientific studies also requires suitable techniques that are able to assimilate data coming with contrasted and possibly large uncertainties. This is typically the case of the Bayesian techniques implemented in the FloodScale project for stage-discharge rating curve development (Le Coz et al. 2014b), hydrologic modelling and flood frequency analysis [START_REF] Kavetski | Understanding predictive uncertainty in hydrologic modeling: The challenge of identifying input and structural errors[END_REF]. The FloodChasers and RiskScape projects illustrate how such data can be successfully included in 1D/2D hydrodynamic models, post-flood surveys and flood risk and damage assessment.

Implement an efficient communication plan

Communication is key to efficiently mobilise flood observers, make them aware of flood-related dangers (and discourage them from risking their lives), avoid conflicts with local authorities, and provide feedback on the use of collected data. Simple and direct explanations are essential, even when it comes to science or legal terms. Such initiatives are usually well received by the locals and the media. Communication efforts should be directed at both the general public and specific community groups such as kayakers or storm/flood chasers. It is important to communicate with and train groups of more advanced operators (firemen, civil defence, hydrologic services, volunteers, etc.) so they can make better quality videos that fit the requirements of the data processing, in optimal safety conditions.

The reported experience highlights the necessity of adapting the communication vectors to each project.

Communicating through radio and TV allows to reach a wide audience. Local newspapers and scientific blogs can probably reach a more specific audience but are efficient to release punctual and periodic reminders of the projects which are useful, especially at the beginning of the project or flood season, during or after flood events, and when results and outputs are available. Distributing flyers and displaying posters and signs at key observation points and at places attended by the public when there is a flood cannot be done extensively throughout a large catchment, but it may dramatically increase the quantity of image materials at particularly strategic sites.

Social media like Twitter and Facebook are essential as they allow to reach a wide audience and to do it at any time, but you first have to communicate to make people `follow' you. Typically, the Flood Chasers Project received more videos when the minister of Water, Environment and Public Services of the Córdoba Province, Argentina, `re-twitted' the project on Twitter. Then, people forgot about it until an article was published in a local newspaper. Being constantly active in the whole range of available media is important.

Get the support of local stakeholders

As consistently illustrated in the polarised French and Argentinian experiences, getting the support of local authorities or not can significantly enhance or jeopardise your citizen science project. The local context and profiles of people in charge are of course important as given factors of the issue, but the communication strategy should account for this issue from the beginning. In the specific context of the FloodScale project, calling for images before rather than during or just after the flood appeared to be the main source of concerns for the administration. Though arguably hypocritical, you will not be blamed for encouraging people to unsafe behaviour if you call for existing images, as usually done by mass media. It also appears advantageous to focus on a recently flood-affected area rather than spending communication efforts over a large region in preparation of future floods. Still, the support of local stakeholders and authorities should be sought actively. There is always a chance that they realise the potential of citizen science initiatives for the success and acceptance of flood risk policies.

Public awareness and engagement matter

The call is much more successful when the public and the media are prepared and aware of natural hazards, as was the case in Christchurch, New Zealand, after the earthquakes of 2010 and 2011. During this time, both the public and media became more attuned to natural hazard science, and also became more receptive to citizen science-based initiatives such as the earthquake felt reports on GeoNet website w . This was not the case for Dunedin, hence one potential reason for the failure to receive photographs, although there are others (e.g., flooding was not as extensive and occurred during the end of the day).

Provided there is sufficient expert oversight during project design and data analysis, such citizen science seems a viable means of improving our understanding of flood risk. In turn, the reported projects highlight the value of citizen science initiatives as a means of both gathering data and engaging with the community. In some cases, the value of the obtained data is much less than the impact and engagement of the citizen community [START_REF] Lowry | CrowdHydrology: Crowdsourcing Hydrologic Data and Engaging Citizen Scientists[END_REF]. Feedback to the public is absolutely essential, through any available means: project website, online database, publication of results, follow-ups on scientific blogs xy and communication through the media.

Social studies could also profit from such crowdsourced feedback from people on how they have been affected by the flood and how they have reacted, as done through calls and interviews in social science projects like in HyMex z project [START_REF] Ruin | Human exposure to flash-floods-relation between flood parameters and human vulnerability during a storm of September 2002 in Southern France[END_REF]. Not only eyewitnesses of the flood but broader communities, which are not always visible in more formal reporting procedures, may use social media to express their feelings and contribute to the public debate (Al-Saggaf and Simmons 2015).

Conclusions

The reported three projects are typical of emerging citizen science initiatives for crowdsourcing flood hydrology data. These photo-based flood mapping (RiskScape, New Zealand) and video-based flow estimation (Flood Chasers, Argentina, and FloodScale, France) projects were specifically designed to produce georeferenced and time-stamped hydraulic quantities readily usable for flood hydrology studies and hydraulic modelling. Compared to other similar projects, they involved similarly simple procedures for the public, but more advanced data processing and reviewing by the scientists. An exciting perspective would be to combine such 'measurement-oriented' and 'citizen hydrologists' approaches with the powerful tools developed in other projects for data mining the social media contents and conducting the spatial analysis of volunteered geographic information.

The three projects illustrate the great potential of citizen science initiatives for improving flood risk assessment in interaction with the local communities. Key drivers for success appear to be: a clear and simple procedure, suitable tools for data collecting and processing, an efficient communication plan, the support of local stakeholders and authorities, and the public awareness of natural hazards. Beyond the technical and communication challenges, this is an efficient way to enhance the culture of flood risk and make people more engaged collectively. We hope that this feedback may help such initiatives emerge and develop successful strategies.

Figure 1 .

 1 Figure 1. Home page of the website of the Flood Chasers Project i

Figure 2 .

 2 Figure 2. Successive frames (a) of a video recorded during the flood in the Ceballos River in Río Ceballos, Province of Córdoba, Argentina, on 15 February 2015, and time-averaged surface displacement vector field (green vectors) in the area of interest (b). Red crosses show discarded grid nodes where the flow free surface is not visible.

Figure 3 .Figure 4 .

 34 Figure 3. Bathymetry of the cross-section of interest and depth-averaged velocity field estimated using a 1D hydrodynamic model (HEC-RAS). The velocity (1.47 m/s) on the right side is the velocity obtained by LSPIV and used for the model roughness calibration.

  , a similar action was independently launched throughout the Ardèche river catchment, South-East France. The FloodScale project, a contribution to the HyMeX k project (Hydrological cycle in the Mediterranean Experiment,Drobinski et al., 2014) was designed to make progress in the understanding and modelling of flash floods. The observation strategy combined a four year multi-scale data collection in two meso-scale catchments in South-East France with opportunistic measurements during the autumn season, when flash floods are more likely to occur. To decrease the uncertainty of stage-discharge ratings and streamflow records, innovative methods for gauging rivers during flash floods were implemented, including portable surface velocity radars[START_REF] Welber | Field assessment of non-contact stream gauging using portable surface velocity radars (SVR)[END_REF] and Large Scale Particle Image Velocimetry (LSPIV) at permanent video camera stations (Le[START_REF] Boursicaud | Gauging extreme floods on YouTube: Application of LSPIV to home movies for the post-event determination of stream discharges[END_REF] or using home movies from the public in post-flood surveys (Le[START_REF] Boursicaud | Gauging extreme floods on YouTube: Application of LSPIV to home movies for the post-event determination of stream discharges[END_REF]. Specific communication actions focussed on the determination of flood discharges within the Ardèche river catchment (France) using home movies shared by observers and volunteers. Safety instructions and a simplified field procedure (see flyer and poster in Figure5) were shared through local media and were made available in French and English on the project website l . Legal aspects including a copyright transfer form have been settled with the help of Irstea's (National Research Institute of Science and Technology for Environment and Agriculture) legal services. This way, simple flood observers or even some enthusiastic flood chasers were encouraged to contribute to hydrologic sciences.

Figure 5 .

 5 Figure 5. Instructions provided to the public as a flyer (a, b) and a poster (c) for the information campaign of the FloodScale Project q

Figure 6 .Table 1 .

 61 Figure 6. Applying Fudaa-LSPIV software to a drone video of the Ardèche River at Sampzon Bridge, France, shared on YouTube after 2014 floods: raw image sequence with ground reference points (a), same images converted to grey scales and ortho-rectified (b), time-averaged surface velocity field (up to 4 m/s) and depth-averaged velocities interpolated at the transect used for discharge computation (c), 3-D view of the transect (d)

Figure 7 .Figure 8 .

 78 Figure 7. Photos of the 5 th March 2014 inundation in Christchurch, New Zealand, received from the public. (a) Slater Street, Richmond. (b) Francis Avenue, Mairehau. (c) Buxton Terrace flow gauging site, Saint Martins. (d) Intersection of Oxford Terrace and Barbadoes Street.

Figure 9 .

 9 Figure 9. Interpolated flood depths based on citizen scientist photos and field surveys, for the 5 th March 2014 flood in Christchurch, New Zealand.

  Author-produced version of the article published in Journal of Hydrology (2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

h http://crowdhydrology.geology.buffalo.edu/

  Author-produced version of the article published in Journal ofHydrology (2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ 2.2.2 Implementation The processing of flood videos to extract surface flow velocities and discharges is based on the LSPIV technique originally proposed by Fujita et al. (1998). The cross-correlation analysis of visible movements in image pairs is similar to the conventional PIV technique used in hydraulic laboratory studies, but large-scale outdoor applications require suitable image ortho-rectification and discharge computation steps. The applicability of the LSPIV technique to flood home movies was investigated by Le Boursicaud et al. (2016), as already done by Fujita et al. (2013) using the STIV (Space-Time Image Velocimetry) technique, and is also the core of the Flood Chasers Project (Argentina). LSPIV steps are similar to the technique used in the Flood

j Agence Nationale pour la Recherche (National Agency for Research, France) k www.hymex.org l https://floodscale.irstea.fr/donnees-en/videos-amateurs-de-rivieres-en-crue/videos-amateurs-de-rivieres-en-crue

  Coz et al. 2014a), a free, user-friendly software available online m . FramesAuthor-produced version of the article published in Journal of Hydrology (2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

m https://forge.irstea.fr/projects/fudaa-lspiv/files

  Author-produced version of the article published in Journal of Hydrology (2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

r https://www.youtube.com/watch?v=4lgws8pvFyg s http://www.ardechevideo.com/

  Author-produced version of the article published in Journal of Hydrology (2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

  Author-produced version of the article published in Journal of Hydrology (2016) 541, 766-777 The original publication is available at http://www.sciencedirect.com/science/article/pii/S0022169416304668 , doi : 10.1016/j.jhydrol.2016.07.036 ©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

w http://www.geonet.org.nz/quakes x http://sciblogs.co.nz/waiology/2014/06/16/citizen-scientists-help-map-christchurch-flooding/ y http://sciblogs.co.nz/waiology/2014/09/04/full-citizen-science-flood-map-for-christchurch-march-2014/ z www.hymex.org and web form: http://goo.gl/forms/q80gfQbPNN

Acknowledgements

All the citizens who have provided images and data for the three projects are gratefully thanked, as well as the field hydrologists who conducted the supplemental surveys. The Flood Chasers project acknowledges the support of the Government of the Córdoba Province, Argentina, through the PROTRI project. The FloodScale project was funded by the French National Agency for Research (ANR) under Contract No ANR 2011 BS56 027, which contributes to the HyMeX program. Catherine Ambroise-Rendu and Thierry Fournier (Irstea) contributed to the FloodScale communication campaign. Collaboration between NIWA and Irstea was enhanced by the PHC Dumont D'Urville project 34185SH (2015-2016) and the Irstea grant and NIWA support for the one-year scientific visit of J. Le Coz at NIWA Christchurch (AWATERE Project).

Author-produced version of the article published in Journal of Hydrology (2016)