Research Collection

Conference Paper

Landslide generated impulse waves: assessment and mitigationof hydraulic hazards

Author(s): Evers, Frederic M.; Schmocker, Lukas; Fuchs, Helge; Schwegler, Benno; Fankhauser, Andres U.; Boes,Robert

Publication Date: 2018-08-21

Permanent Link: https://doi.org/10.3929/ethz-b-000276308

Rights / License: In Copyright - Non-Commercial Use Permitted

This page was generated automatically upon download from the ETH Zurich Research Collection. For moreinformation please consult the Terms of use.

ETH Library

Research Collection

Conference Paper

Landslide generated impulse waves: assessment and mitigationof hydraulic hazards

Author(s): Evers, Frederic; Schmocker, Lukas; Fuchs, Helge; Schwegler, Benno; Fankhauser, Andres U.; Boes,Robert M.

Publication Date: 2018-06

Rights / License: In Copyright - Non-Commercial Use Permitted

This page was generated automatically upon download from the ETH Zurich Research Collection. For moreinformation please consult the Terms of use.

ETH Library

COMMISSION INTERNATIONALE DES GRANDS BARRAGES

------- VINGT-SIXIEME CONGRES DES GRANDS BARRAGES

Vienne, Juillet 2018 -------

LANDSLIDE GENERATED IMPULSE WAVES:

ASSESSEMENT AND MITIGATION OF HYDRAULIC HAZARDS *

Frederic M. EVERS1, Lukas SCHMOCKER2, Helge FUCHS3, Benno SCHWEGLER4, Andres U. FANKHAUSER5, Robert M. BOES6

1 Doctoral researcher, 2 Senior researcher, 3 Senior research assistant, 6 Director of the Laboratory of Hydraulics, Hydrology and Glaciology (VAW),

ETH ZURICH 4 Project manager, 5 Department head, KRAFTWERKE OBERHASLI AG

SWITZERLAND

1. INTRODUCTION

Rapid mass wasting into reservoirs, including subaerial landslides and rock avalanches, may generate large water waves as shown in Fig. 1 [1]. Starting from a limited impact zone, the generated wave train propagates radially in all directions, depending on the reservoir bathymetry. The waves run-up the shoreline and potentially overtop a dam, thereby endangering its structural integrity and adjacent infrastructure. For assessing whether these impulse waves represent a threat to reservoir infrastructure, it is of crucial importance to understand the hydraulic processes of impulse wave propagation as well as wave-shore and wave-structure interaction.

A comprehensive hazard assessment of landslide generated impulse waves requires a multidisciplinary approach. Expertise in geotechnics and glaciology is needed for the identification of unstable slopes as well as potential ice and snow

* Ondes d’impulsion générées par glissements de terrain : Evaluation et atténuation des risques hydrauliques

avalanches, respectively. The quantification of key slide characteristics at impact, including slide velocity and mass, are essential for predicting the wave train magnitude. Based on these input parameters, a hydraulic analysis yields spatial wave height information depending on the wave propagation distance and direction. With the predicted wave height at a certain location within the reservoir, the run-up and potential overtopping volumes are estimated. Among other approaches, generally applicable prediction equations developed from systematic multi-year scale model tests provide a straightforward approach to determine the magnitude of impulse wave events and their impact along the shoreline. Recent research projects advanced the overall process understanding particularly with regard to spatial impulse wave propagation as well as wave run-up, overland flow and wave overtopping [2], [3], [4].

Fig. 1 Phases of a landslide generated impulse wave event: (1) wave generation, (2)

wave propagation, and (3) wave impact including wave runup, dam overtopping, overland flow ([5], adapted from [6])

Phases d’un événement d'ondes d’impulsion générées par les glissements de

terrain : (1) génération des vagues, (2) propagation des vagues et (3) impact des

vagues avec jet de rive, débordement de la digue et écoulement de surface ([5],

adapté de [6])

Official regulations require dam operators to take impulse wave related

hazards into account [7], [8]. While increasing the freeboard by a temporary water level drawdown represents an effective ad hoc mitigation measure, a structural dam heightening along with physical protection for critical infrastructure may be more cost-efficient in the long term. Adequate mitigation measures have to be assessed on a case-related basis. Ongoing glacial retreat allows for new reservoirs in locations previously covered by ice. In addition to hydropower and flood protection, these new periglacial reservoirs are capable of substituting former glacier functions, e.g. allocation of the annual runoff. However, the formerly ice-covered steep mountain flanks become exposed and potentially unstable and are particularly subject to mass wasting. The assessment of impulse wave hazards is therefore crucial to dam safety.

The present work discusses the advantages and limitations of current methods for the prediction of the hydraulic features of landslide generated impulse waves. The focus is on generally applicable equations from model tests describing spatial impulse wave propagation. Based on the case study of the planned Trift reservoir, mitigation measures are discussed.

2. HAZARD ASSESSMENT

2.1. PREDICTION METHODS

Table 1 summarizes the main methods for predicting the hydraulic features of impulse waves generated by subaerial landslides. These methods vary both in their requirements regarding the experience of the user and the extent of the base data as well as the uncertainty of the results and their comprehensibility. In general, the quality of the results increases together with the costs and time of their production [4]. The following overview of the methods focuses primarily on the prediction of the hydraulic features.

Table 1

Methods for the prediction of landslide generated impulse waves (modified from [6])

Méthodes pour la prévision des ondes d'impulsion générées par les

glissements de terrain (modifié de [6])

Qu

ali

ty o

f re

su

lts

Tim

e r

eq

uir

em

en

ts

Co

sts

User

exp

eri

en

ce

Cla

rity

an

d

co

mp

reh

en

sib

ilit

y

Eff

ort

fo

r g

overn

ing

para

mete

rs

Generally applicable equations

from model tests Estimation Low Low Engineer Medium Medium

Prototype-specific model tests Exact Very high Very high Engineer High High

Numerical simulations Estimation

– exact High

Medium

– high Expert Low High

Empirical equations from field

data Rough Low Low Engineer Medium Medium

Analytical investigations Rough Low Low Engineer Low Medium

Generally applicable equations from model tests are a straightforward

approach for predicting the magnitude as well as the impact of landslide generated impulse waves at the shore or at dam structures. The quality of the results is sufficient for an estimation of the magnitude of the hydraulic target value, e.g. wave amplitudes. Since the time and cost requirements are low, this method is especially advantageous when an event is imminent or as a first assessment to determine, whether further more detailed and accurate investigations are needed. The equations are derived from experimentation in hydraulic laboratories for simplified geometrical dimensions. By applying the principles of similitude, these equations

are transferable to prototype dimensions, given the scale effects in the model tests are negligible. In this regard, [9] found that the stillwater depth of the model test has to be at least 0.2 m and the generated waves should have a period larger than 0.35 s. The parameter range of the model tests limits the application range of the equations. [10] discuss various existing equation sets and applied these to predict the wave features of hydraulic experiments. It was found that strong over- and underestimation up to an order of magnitude may arise, if the equations are applied outside their parameter range or if the slide model does not reproduce the slide characteristics at prototype scale (e.g. rigid slide body vs. free granular slides). In addition, the correct definition and implementation of the parameters influence the prediction results. While some studies include the velocity of the slide front, others are based on the centroid velocity [5]. If the slide parameters were derived e.g. from geotechnical simulations, this aspect has to be carefully taken into account. Another limitation of this prediction method is the representation of effects due to the reservoir shape. In compact reservoirs, the wave train is undisturbed on its propagation path. In narrow reservoirs with dendritic shape, the wave propagation is affected by reflection and diffraction of the waves. In general, these effects have a damping effect. However, they may also lead to superposition of waves causing an unpredictable increase of amplitude.

Prototype-specific model tests allow for the representation of complex reservoir geometries taking into account the effects of wave reflection and diffraction. Therefore, the quality of the results is quite high. Another advantage of hydraulic models in general is their high clarity and comprehensibility. The requirements in terms of scale effects are identical to the model tests described in the previous paragraph. This may lead to very extensive model setups, especially if the wave generation location is in the shallow part of the reservoir and the governing stillwater depth has to be at least 0.2 m. This aspect produces very high requirements of this prediction method regarding time and costs. Another challenge of this method is the reproduction of the dynamic impact parameters of the landslide. While the dynamic parameters of the water are inherently reproduced in the model if scale effects are adequately considered, the slide composition is not. [11] used granular plastic material with densities comparable to snow and rock avalanches. These materials feature rheologies very different from the prototype and a sufficient reproduction, i.e. slide shape and velocity at impact, is not inherently given. Therefore, the target slide parameters have to be matched in an iterative way. Studies applying this prediction method include [12], [13], [14], [15].

Numerical simulations also offer the possibility of representing complex reservoir geometries. The elimination of scale effects by resolving the physical processes directly at prototype scale is another advantage of this method. Due to the exponential growth of computing power (Moore’s law), the application of this prediction method benefits from a reduction of its time and in particular of its cost requirements. Nonetheless, its clarity and comprehensibility is low, as only very experienced users may interpret its results properly. Expert knowledge on how and to which degree the fundamental physics are correctly implemented in the numerical model is necessary to avoid significant model effects. These model effects may be caused e.g. by an improper model discretization or an

oversimplified mathematical representation of both the landslide and wave dynamics. Therefore, the quality of results may range between estimation and exact.

The applicability of empirical equations from field data and analytical

investigations for practical hazard assessment purposes is very limited. The first method generally lacks a sound data basis regarding the wave propagation, as wave magnitudes are indirectly determined by back-calculation from measured runup heights after an event. Moreover, only a small number of events has been sufficiently documented regarding slide characteristics and runup features to draw general conclusions. This limits the application range of the derived equations. The latter method is only applicable to idealized scenarios with simple initial conditions.

2.2. HYDRAULIC MODEL TESTS

Model tests in hydraulic laboratories are conducted to derive empirical equations for describing a physical system, which represents a hydraulic phenomenon. In dimensionless form, these equations are applicable also to prototype dimensions, if scale effects are negligible (see Section 2.1). A set of independent parameters, which describe a target variable, defines a physical system. Figs. 2 and 3 show the parameters and one selected target variable for the spatial generation and propagation of landslide generated impulse waves. The governing dimensionless parameters include the slide centroid velocity Vs, the slide volume Vs, the bulk slide density ρs, the slide thickness s, the slide width b, the slide impact angle α, the stillwater depth h, the water density ρw, the gravitational acceleration g, the time t, the wave propagation distance r, and the wave propagation angle γ. The first wave crest amplitude ac1 is selected as the target variable. A polar coordinate system describes the spatial evolution of the wave features with r and γ.

Fig. 2 Definition plot of impulse waves with selected independent governing parameters

and first wave crest amplitude ac1 as dependent target variable [5] Schéma de définition des ondes d’impulsion avec des paramètres principaux

sélectionnés et l’amplitude de crête de la première vague ac1 comme variable de

consigne dépendante [5]

Fig. 3 Definition plot of spatial impulse wave propagation [5]

Schéma de définition de la propagation spatiale des ondes d’impulsion [5]

[5] conducted impulse wave experiments in a 4.5 m by 8 m wave basin. The application of a videometric measurement system with four spatially referenced cameras (AICON 3D Systems GmbH, Braunschweig, Germany) allowed for tracking the wave propagation within the basin. The videometric system yielded up to 6000 measurement points within a measurement area of nearly 14 m2 at a frame rate of 24 Hz. The tests were conducted within the following test parameter ranges: Vs, 0.72 - 4.76 m/s; ms = Vsρs, 10 - 40 kg; s, 0.06 - 0.12 m; b, 0.25 - 1.00 m; α, 30 - 90°; and h, 0.2 - 0.4 m.

Fig. 4 shows an interpolated representation of the water surface during an experimental run. The center of the slide enters the wave basin at x = y = 0 along the y-axis at t = 0 s (not displayed). At t = 0.375 s, the slide displaces the water and generates the first wave crest featuring a laterally decreasing crest amplitude from γ = 0° (y-axis) to γ = 90° (x-axis). Close to the impact location, the videometric system is not capable of tracking the water surface. The propagating first wave crest was subject to amplitude decay at t = 0.750 s. The first wave trough forms following the crest. At t = 1.125 s, both first crest and trough amplitudes were subject to further decay. The second wave crest clearly emerges at t = 1.500 s. The contours at t = 1.875 s and 2.250 s show the further propagation of the second wave crest. Its decay rate is smaller than for the first wave crest. In addition, the wavelengths of the second wave as well as the following wave train are substantially shorter than for the first wave.

Compared to conventional measurement methods with locally fixed wave gauges, the contours obtained with the videometric system provide a more complete insight into the spatial wave propagation process. While gauges yield wave information only point-wise at their respective location, the contour plots allow for independently tracking the location of specific wave features.

Fig. 4 Contour plots for laboratory experiment with Vs = 1.06 m/s, ms = 20 kg, s = 0.06 m,

b = 0.50 m, a = 45°, and h = 0.40 m [5] Graphiques de contours pour l'expérimentation de laboratoire avec Vs = 1.06 m/s,

ms = 20 kg, s = 0.06 m, b = 0.50 m, a = 45° et h = 0.40 m [5]

2.3. IMPACT RADIUS

The quasi-continuous representation of the water surface presented in Section 2.2 allows for an adaptive tracking of the impulse wave features including crest and trough amplitudes close to the impact location. [5] derived the impact

radius r0 as the boundary between the impact zone, characterized by the turbulent collapse of the impact crater, and the wave propagation zone:

γγ 22

90,0

22

0,0

2

90,0

2

0,0

0cossin °°

°°

+=

rr

rrr . [1]

The impact radii r0,0° and r0,90° included in the elliptic function of Eq. [1] are

computed based on the slide impact velocity Vs, the slide thickness s, the slide width b, the slide mass ms, and the slide impact angle α (Figs. 1 and 2):

hh

b

bh

V

h

s

gh

Vr

w

sss

125.025.00625.0

2

125.025.0

0,07

6cos5.2

=° α

ρ

ρ [2]

hbh

V

h

s

gh

Vbr

w

sss

125.00625.0

2

125.025.0

90,07

6cos5.1

2

+

=° αρ

ρ. [3]

The lower application limit of these equations regarding the wave

propagation distance is r > 1.1h. Further limitations of Eqs [1] to [3] are detailed in [5]. Compared to the limitation r > 5h for the equations by [6], the application of the impact radius r0 allows for a prediction of impulse wave features closer to the impact location. This is especially advantageous for reservoirs with small ratios of significant propagation distances to the water depth.

3. HAZARD MITIGATION

3.1. CASE STUDY TRIFT RESERVOIR

The retreating Trift glacier, Switzerland, uncovered a large trough filled by a proglacial lake. Fig. 5 shows the state of the glacier in 1948 and its lake in 2008. The hydroelectric company KWO AG is planning to build a reservoir, which would create a total storage volume of 85 million m3. A photomontage of the future Trift reservoir is also shown in Fig. 5. The planned arch dam has a height of 167 m. The turbines of the corresponding powerhouse will harness a water head of 440 m featuring an average annual energy production of 145 GWh at an installed capacity of 80 MW.

Fig. 5 Trift glacier in 1948 (left), Trift glacial lake in 2008 (middle), and photomontage of

the future Trift reservoir in 2028 (right) Glacier du Trift en 1948 (à gauche), lac glaciaire du Trift en 2008 (au centre) et

photomontage du futur réservoir du Trift en 2028 (à droite)

The Trift reservoir will be located in a mountainous environment. While

geotechnical investigations of the surrounding mountain flanks yielded no increased susceptibility to significant slope failures, snow and ice avalanches might generate impulse waves posing a risk during the operation of the future reservoir. Despite its lower density, snow and ice avalanches are similar in mechanism to very rapid landslides. Therefore, this case study is transferable to other types of subaerial mass wasting. To assess the hazard related to these impulse waves, their damage potential at the dam site requires quantification. Fig. 6 shows four selected snow avalanche scenarios, which may affect the reservoir. These scenarios were assessed with generally applicable equations from model tests by [6] and [5]. Both, the direction of the slide centroid (γ = 0°) and the respective wave propagation angles γ directed to the center of the dam crest are included. The reservoir shape is compact and the propagation path to the dam is undisturbed. The significant water depths h required for the prediction of the wave magnitude range between 100 m and 150 m. While scenarios C and D satisfy the lower limitation of the wave propagation distance r > 5h by [6], scenarios A and B undercut this threshold. For the latter, [6] yield more conservative results for the first wave height than [5], incorporating the impact radius r0.

[16] computed the required freeboard as shown in Fig. 7 by applying the approach by [5] for the height of the first wave and by [6] for the runup height. The seasonal reservoir level will be fluctuating and will have its minimum and maximum in April and September, respectively. The available freeboard is lower than the required freeboard from August to October. In these months, the present likelihood of snow accumulation leading to an extreme avalanche is very low. In the future however, changes of the hydrological regime will shift the peak reservoir level towards the winter. Therefore, an advanced planning of mitigation measures is advisable.

Fig. 6 Plan of the Trift reservoir during operating phase at a reservoir level of 1767 m

a.s.l. with selected snow avalanche scenarios and their respective wave propagation angles γ (orthoimage reproduced by permission of swisstopo

(JA100120)) Plan de la retenue du Trift en période d’exploitation au niveau de retenue

1767 msm avec des scénarios d’avalanche de neige sélectionnés et leurs angles

respectifs de propagation de vagues γ (orthoimage reproduite avec l'autorisation

de swisstopo (JA100120))

Fig. 7 Reservoir level and freeboard of future Trift reservoir [16]

Niveau et revanche de la retenue du Trift [16]

0

20

40

60

80

100

120

140

160

1620

1640

1660

1680

1700

1720

1740

1760

1780

Fre

ebo

ard [

m]

Res

erv

oir

lev

el [

m a

.s.l

.]

Reservoir level

[m a.s.l.]

Dam crest level

[m a.s.l.]

Freeboard

[m]

Required. freeboard

[m]

3.2. MITIGATION MEASURES

Mitigation measures include temporary as well as permanent intervention techniques. While a short-time reservoir draw down or the intentional triggering of avalanches are active temporary measures, an increase of the dam crest elevation is a permanent intervention.

Temporary interventions require the definition of a threshold from when they have to be initiated. Therefore, the potential sliding mass has to be adequately monitored. [17] present an early warning system for an instable rock slope and its performance prior to an actual failure event. The alarm threshold was based on the displacement velocity of the instable rock slope and the system permitted a lead time of several days ahead of the event. A system for monitoring and signalizing potential ice avalanches into the proglacial lake of the Trift glacier is presented by [18]. They conclude, that the early warning signs of glaciers are subject to high uncertainties. A self-evident mitigation measure after an early warning alert is a precautionary draw down of the reservoir level to increase the freeboard. However, this measure may take several days depending on the outlet structures and the reservoir volume. Moreover, it has to be taken into account, that a rapid draw down may trigger partially-submerged slides [19] or lead to inadmissible pore pressures in embankment dam bodies. Consequently, the successful application of temporary measurers against rock and ice avalanches is dependent on the lead time achievable by the installed early warning system. For snow avalanches, a reservoir draw down is evitable, if the accumulation of extensive snow accumulations is avoided by precautionary avalanche blasting [16]. The evolution of snow depths is comparatively easy to monitor.

Permanent interventions generally involve higher costs. These are caused by either the construction of additional structures, e.g. dam crest elevation, or financial losses due to continuing restrictions on the reservoir operation, e.g. the reduction of the maximum reservoir level. The costs of these measures are substantially higher in comparison to the installation of an early warning system including sensors for monitoring the instable slopes. However, the permanent measures allow for a later increase of the maximum water level after a slope failure or a risk reduction due to glacier recession.

In a preliminary study, [16] identified the following mitigation measures as viable strategies for the future Trift reservoir: avalanche blasting as a temporary intervention against snow avalanches and the lowering of the maximum water level or the increase of the dam crest elevation, respectively, as a permanent intervention against ice avalanches from the Trift glacier. For avalanche blasting, a return period for snow accumulations of 10 years was estimated. Due to this long return period, the installation of permanent blasting infrastructure turned out to be economically unviable in comparison to the operation of helicopter flights. As shown in Fig. 5, the Trift glacier retreats at a high rate. Therefore, the risk of ice avalanches is continuously decreasing during the projected operation period of the reservoir. A continuous adjustment of the maximum water level to the current risk potential allows for limiting the costs of the related permanent interventions. In a further step, the feasibility of these two measures has to be analyzed in detail.

4. CONCLUSIONS

Landslides pose a hazard to the operation of reservoirs, especially in high-altitude mountain regions. Ongoing glacial retreat in these regions allows for new reservoirs in locations previously covered by glaciers. Among various methods for the prediction of hydraulic hazards related to impulse waves generated by subaerial landslides, engineers may apply generally applicable equations derived from model tests as a straightforward and time-saving option. This is particularly advantageous for investigating scenarios with various boundary conditions. The determination of the impact radius shows how the application of state-of-the-art measurement techniques in hydraulic experimentation extends the application range of existing equations. The implementation of mitigation measures to reduce the hazard of impulse waves requires an assessment on a case-related basis. The case study of the future Trift reservoir demonstrates the advantages of generally applicable equations for preliminary feasibility studies.

ACKNOWLEDGEMENTS

The authors would like to acknowledge Dr. Annina Sorg, IMPULS AG, Thun, Switzerland, for conducting the avalanche simulations. The project is embedded in the framework of the Swiss Competence Centre of Energy Research – Supply of Electricity (SCCER-SoE).

REFERENCES

[1] ICOLD (INTERNATIONAL COMMISSION ON LARGE DAMS). Reservoir landslides: Investigation and management – Guidelines and case histories. Bulletin 124, Paris, 2002.

[2] FUCHS H. Solitary impulse wave run-up and overland flow. Doctoral Dissertation 21174, VAW-Mitteilungen 221 (R. Boes, ed), ETH Zurich, Switzerland, 2013.

[3] FUCHS H., EVERS F.M., BOES R. Impulse waves in reservoirs: recent research at VAW. Proc. HYDRO 2016, Int. Conf. Hydropower & Dams, Montreux, Switzerland, 2016.

[4] KOBEL J., EVERS F.M., HAGER W.H. Impulse Wave Overtopping at Rigid Dam Structures. Journal of Hydraulic Engineering, 143(6): 04017002, 2017.

[5] EVERS F.M. Spatial propagation of landslide generated impulse waves. Doctoral Dissertation 24650, ETH Zurich, Switzerland, 2017.

[6] HELLER V., HAGER W.H., MINOR H.-E. Landslide generated impulse waves in reservoirs: Basics and computation. VAW-Mitteilung 211 (H.-E. Minor, ed.), ETH Zurich, Switzerland, 2009.

[7] ACHTERBERG D., GOTZMER J.W., SPATH R., TSENG M., WOODWARD D.E., MILLER N., SHIPMAN S.A. Federal guidelines for dam safety: selecting and accommodating inflow design floods for dams. Dept. of Homeland Security, Federal Emergency Management Agency, Washington DC, 1998

[8] POUGATSCH H., AMMANN E., HAUENSTEIN W., LOOSLI D., MOUVET L., MÜLLER R.W., RECHSTEINER G. Sicherheit der Stauanlagen. Basisdokument zur konstruktiven Sicherheit (‘Dam safety. Base document on structural safety‘). Federal Office for Water and Geology, Biel, Switzerland, 2002.

[9] HELLER V., HAGER W.H., MINOR H.-E. Scale effects in subaerial landslide generated impulse waves. Experiments in fluids, 44(5), 691–708, 2008.

[10] EVERS F.M., HAGER W.H. Spatial impulse waves: wave height decay experiments at laboratory scale. Landslides, 13(6), 1395–1403, 2016.

[11] FUCHS H., BOES R.M., PFISTER M. Impulse waves at Kühtai Reservoir generated by avalanches and landslides. Proc. ICOLD Symposium „Dams

under changing challenges“ (A.J. Schleiss & R.M. Boes, eds.), 79th Annual Meeting, Lucerne. Taylor & Francis, London, 701–708, 2011.

[12] DAVIDSON D.D., MCCARTNEY B.L. Water waves generated by landslides in reservoirs. Journal of the Hydraulics Division ASCE, 101(12), 1489–1501, 1975.

[13] SLINGERLAND R.L., VOIGHT B. Occurrences, properties, and predictive models of landslide-generated water waves. In: Voight B. (ed) Developments

in geotechnical engineering 14B, rockslides and avalanches 2, engineering

sites. Elsevier Scientific Publishing, Amsterdam, 317–397, 1979. [14] CHAUDHRY M.H., MERCER A., CASS D. Modeling of slide‐generated

waves in a reservoir. Journal of Hydraulic Engineering, 109(11), 1505–1520, 1983.

[15] HUANG B., YIN Y., WANG S., CHEN X., LIU G., JIANG Z., LIU J.A. physical similarity model of an impulsive wave generated by Gongjiafang landslide in Three Gorges Reservoir, China. Landslides, 11(3), 513–525, 2014.

[16] DÄHLER T. Impulswellenprojekt Trift (‘Impulse wave project Trift‘). Master

Thesis, ETH Zurich, Zürich, Switzerland, 2017. [17] LOEW S., GSCHWIND S., GISCHIG V., KELLER-SIGNER A., VALENTI G.

Monitoring and early warning of the 2012 Preonzo catastrophic rockslope failure. Landslides, 14(1), 141–154, 2017.

[18] DALBAN CANASSY P., BAUDER A., DOST M., FÄH R., FUNK M., MARGRETH S. MÜLLER B., SUGIYAMY S. Hazard assessment investigations due to recent changes in Triftgletscher, Bernese Alps, Switzerland. Natural Hazards and Earth System Sciences, 11, 2149–2162, 2011.

[19] PARONUZZI P., RIGO E., BOLLA A. Influence of filling-drawdown cycles of the Vajont reservoir on Mt. Toc slope stability. Geomorphology, 191, 75–93, 2013.

SUMMARY

Rapid mass wasting into reservoirs, including subaerial landslides and rock avalanches, may generate large water waves. The assessment of hydraulic hazards related to these waves is important for dam safety. For hazard assessment studies, wave features including wave amplitudes and runup heights are predicted based on the slide characteristics at impact, including slide velocity and mass. General applicable equations from model tests, prototype-specific model tests, and numerical simulations are practical prediction methods. These methods involve differences regarding their respective quality of results, time requirements, costs, user experience, clarity and traceability, as well as effort for determining the governing parameters. Engineers may apply generally applicable equations from hydraulic model tests in a straightforward and time-saving way based on simplified input parameters to get an estimation of the wave magnitude and related runup heights. Novel measurement techniques allow for the establishment of improved prediction equations from hydraulic experimentation. The application of a videometric measurement system at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, yields spatial wave propagation patterns with a high resolution. Based on these new equations, mitigation measures are evaluated in a preliminary feasibility study to reduce the impulse wave hazard at the future Trift reservoir projected by the Swiss hydroelectric company KWO AG.

RÉSUMÉ

Le glissement rapide de masse dans les retenues, y compris les glissements de terrain subaériens et les avalanches rocheuses, peut générer de grandes vagues d’impulsion. L'évaluation des risques hydrauliques liés à ces vagues est importante pour la sécurité du barrage. Pour les études d'évaluation des dangers, les caractéristiques des vagues, y compris leur amplitude et leur hauteur, sont prédites en fonction des caractéristiques de glissement au moment de l'impact, y compris la vitesse et la masse en glissement. Les équations générales applicables à partir des essais de modèles, les essais de modèles spécifiques aux prototypes et les simulations numériques sont des méthodes de prédiction pratiques. Ces méthodes impliquent des différences quant à la qualité des résultats, aux exigences de temps, aux coûts, à l'expérience de l'utilisateur, à la clarté et à la traçabilité, ainsi qu'à l'effort de détermination des paramètres directeurs. Pour obtenir une estimation de l'amplitude des vagues et des hauteurs de jet de rive correspondantes, les ingénieurs peuvent utiliser des équations généralement applicables à partir d'essais de modèles hydrauliques de manière simple et rapide, basées sur des paramètres d'entrée simplifiés. De nouvelles techniques de mesure permettent d'établir des équations de prédiction améliorées à partir d'expériences hydrauliques. L'application d'un système de mesure vidéométrique au Laboratoire d'hydraulique, d'hydrologie et de glaciologie (VAW) de l'EPF de

Zurich permet d'obtenir des modèles de propagation spatiale des ondes à haute résolution. Sur la base de ces nouvelles équations, les mesures d'atténuation sont évaluées dans le cadre d'une étude de faisabilité préliminaire visant à réduire le risque de vagues d’impulsion dans la future retenue du Trift, projetée par la société hydroélectrique suisse KWO AG.

KEYWORDS ANALYSIS EMERGENCY PLANNING FREEBORD HEIGHTENING HYDRAULIC MODEL TEST LANDSLIDE OVERTOPPING RESERVOIR OPERATION RESERVOIR SLOPE RISK ASSESSMENT SAFETY OF DAMS WAVE MOTS-CLÉS CALCUL PLAN D'ALERTE ET DE SECOURS REVANCHE SURELEVATION ESSAI SUR MODELE HYDRAULIQUE GLISSEMENT DE TERRAIN DEVERSEMENT SUR LE BARRAGE EXPLOITATION DU RESERVOIR VERSANT DE RETENUE ANALYSE DE RISQUE SECURITE DES BARRAGES VAGUE