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ISBN: 978-1-925627-03-9

Experimental Testing of Flood Hazard Curves for a Partially Submerged Vehicle Grantley Smith1, Ben Modra2, Stefan Felder3 1

Principal Engineer and Manager, Water Research Laboratory, UNSW Sydney, Australia E-mail: [email protected] 2

Principal Engineer, Water Research Laboratory, UNSW Sydney, Australia

3

Lecturer, Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Sydney, Australia

Abstract Every year floods cause enormous damage and loss of life on a global scale including fatalities that occurred in vehicles becoming unstable in floodwaters. Historically, threshold conditions for vehicle instability in floodwaters have been investigated at model scale and have relied on correct scaling of the immersed vehicle weight and tyre friction, and assumptions about the buoyant behaviour of the vehicle. Instability criteria have been determined through measurement of the flow hydrodynamics, which initiated movement, of the model scale vehicle. Novel, prototype scale testing was conducted at the UNSW Water Research Laboratory measuring the threshold forces required to move a partially buoyant, full-scale vehicle. The flow hydrodynamics required to reproduce these instability threshold forces were then determined using a 18:1 scale model. Several novel results were obtained from the testing, including the measurement of the coefficient of drag of a vehicle under partially submerged conditions, and characterisation of the vehicle’s buoyant behaviour in water and subsequent traction available at the tyres. This paper provides an overview of the laboratory investigations, presents the measured data and discusses the relevance of the new tests for flood hazard curves.

1. INTRODUCTION Every year floods cause enormous damage and loss of life on a global scale. An analysis of global statistics for loss of life showed that inland floods (river floods, flash floods and drainage floods) caused 175,000 fatalities and affected more than 2.2 billion people between 1975 and 2002 (Jonkman, 2005). In a recent detailed analysis of flood fatalities in Australia, Haynes et al. (2016) noted that 1,859 people have died in floods since 1900 and that 178 of these flooding related deaths have occurred since 2000. The study noted that while flood fatality rates are generally falling per capita, the number of fatalities that occurred in vehicles, has increased in the last fifteen years. When discussing flood fatalities, Haynes et al. (2016) also noted:  

that the greatest proportion of flood related fatalities occurred while the person was attempting to cross a bridge causeway or road, whether the person was in a vehicle or on foot; most victims were capable of independent action and aware of the flood, however the speed and depth of the water took them by surprise.

Beyond the social behaviour and decision making of drivers in flood emergencies, there is a clear need to understand the mechanisms by which vehicles become unstable in floodwaters. The safety of people during flood events can be compromised when the vehicles they are travelling in are exposed to flood flows which cause the vehicle to become unstable by losing traction (frictional instability through sliding), to topple because of uneven surfaces or to become buoyant (floating). These mechanisms can lead to a complete loss of control over the vehicle resulting in the vehicle being swept downstream (Shand et al., 2011; Haynes et al., 2016). The weight distribution in most vehicles is uneven with a greater proportion of the weight located near the engine. In most vehicles, this means that the rear of the vehicle floats first with the front wheels below the engine remaining on the ground for longer. Vehicles may

Experimental Testing of Flood Hazards Curve for a Partially Submerged Vehicle

Smith

experience flow conditions ranging from subcritical (deep, slow moving water), through critical (on a causeway or bridge), to supercritical (shallow and fast). A number of studies have considered the flow conditions in which a vehicle may become unstable, and have resulted in various safety guidelines. Shand et al. (2011) highlighted that guidelines provided for vehicle stability in floods have generally been based on the product of depth and velocity (v D), as derived by scaled experimental investigations of stationary vehicle stability. Martínez-Gomariz et al. (2016) provided a compilation of all previous literature including a graphical comparison of all available experimental and theoretical results. The graphical comparison is reproduced in Figure 1. A comparison with the findings of these experimental studies shows the vehicle stability threshold data proposed by Shand et al. (2011) and adopted by Australian Rainfall and Runoff for flood hazard estimates are conservative compared to the compiled data sets.

Figure 1 Representation of all the results from literature (instability points and limit functions) obtained in experimental and theoretical studies (after Martinez-Gomariz et al., 2016)

Past experimental research was conducted at laboratory scale (Figure 1) and there is currently no fullscale testing of the stability of vehicles in floodwaters. The orientation of the vehicle in some studies is also not particularly relevant to the exposure of the risk to life of people in vehicles which encounter floodwaters. Evidence presented shows that vehicles driven into floodwaters most commonly do so perpendicular to the direction of travel. Herein new experiments with a prototype scale vehicle were conducted at UNSW Water Research Laboratory to better understand and quantify the mechanisms by which vehicles become unstable in floodwaters. The study comprised three phases including the measurement of the traction forces which induced vehicle movement at prototype scale, detailed smallscale tests of the drag forces in a range of flow conditions and based on these measurements, a review of the flood hazard vulnerability criteria for flood exposed vehicles.

2. METHODOLOGY The test methodology was designed to separate the response of the vehicle exposed to floodwaters to both friction and hydrodynamic forces. Two series of tests were completed at UNSW Water Research Laboratory: a) Full-scale measurement of the force required to overcome the friction force of a vehicle’s tyres on the roadway surface in various water depths; and b) Knowing the full-scale forces required to initiate vehicle instability, scale model testing of the hydrodynamic forces to reproduce the vehicle instability forces.

HIWE2017 Smith, Modra and Felder

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The full-scale tests were conducted by winching the test vehicle relative to a water body at various depths in a tow tank. This allowed full scale testing of buoyant friction forces under varying water levels. Moving of the full-scale vehicle to determine the vehicle instability force was achieved using a high powered winch to drag the vehicle sideways as illustrated in Figure 2. The tested vehicle was a Toyota Yaris which represented a typical small-size car.

Figure 2 - Winch assembly of the prototype tests including the support framework with winch and dynamometer (left hand photo), and the pulley and wheel attachment (right hand photo).

A second series of tests was conducted at a model scale of 1:18 scale (Figure 3) to determine the equivalent hydrodynamic forces required to reproduce the prototype scale vehicle instability forces. The tests were conducted with the same vehicle type (Toyota Yaris) for a range of flow conditions comprising supercritical, subcritical and near-critical flows to represent the full range of flow conditions a vehicle may be exposed to in flood waters. The testing was conducted for a range of flow depths by adjustment of an upstream sluice gate and a downstream weir. The flow depth was measured with a pointer gauge providing the average flow conditions upstream of the vehicle (Figure 3).

Figure 3 - Test assembly for testing of the horizontal hydrodynamic forces The point of loss of traction, and therefore vehicle stability, is that point at which the hydrodynamic force FH applied to the vehicle overcomes the friction force FF of the wheels:

FH > FF

(1) The friction force is dependent on the downward force on the wheels, and may be considered as a vehicle as a whole, at an individual wheel, or at an axle: FF = µ (W – B – L) (2) where W is the weight of the vehicle; B is the buoyancy force which is dependent on the water depth d, L is the uplift force due to water impacting the vehicle which is dependent on the water depth and velocity; and µ is the coefficient of friction between the tyres and the road. Note that once the tyres are


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sliding, the coefficient of friction µ decreases and it becomes more difficult for the vehicle to regain traction. The hydrodynamic force can be expressed in the form of a drag force as: FH =0.5.ρ.A.CD.v2 (3) Where ρ is the water density, A is the projected (profile) area of the vehicle impacted by water, CD is the coefficient of drag which is dependent on the water velocity and depth, and v is the depth-averaged flow velocity.

3. FULL SCALE VEHICLE TRACTION TESTS The aim of the traction tests was to measure the force required to break vehicle traction at varying water depths. The testing procedure was designed to, as closely as possible, mimic the conditions of a vehicle entering a flood flow. By flooding the test tank quickly to ensure minimal water ingress into the vehicle cabin, and then towing the vehicle sideways (perpendicular to the direction of travel) using a winch assembly attached to the wheel hub, it was possible to measure the force required to break traction for the entire vehicle or specifically at the rear axle. In this study, a Toyota Yaris was tested representing a light modern vehicle that would be more easily swept away in flood waters. The specifications for the vehicle are provided in Table 1. Figure 4 shows the Toyota Yaris in the experimental facility before a winch test. Table 1 - Vehicle specifications of prototype vehicle in present study Vehicle Weight Length Width Floorpan Height Tyres

Toyota Yaris Sedan 2006 1045kg 4.3m 1.69m 0.155m Front: Bridgestone Potenza195/60R1 5 Rear: Hankook Optimo 185/60R15

Figure 4 - Toyota Yaris in WRL’s test facility, preparing for a winch test from both axles.

The coefficient of friction (C f) between the tyres and the concrete floor of the laboratory was measured to be 0.78. Note that these are considered best possible conditions for traction (tyres at rest on concrete), and a suitable derating of Cf is required when considering available traction in actual flood conditions, as discussed in Section 5. An example time series of the traction force is provided in Figure 5 for a test series with one driver occupant winched from the rear axles only. Note that the forces have been converted to kN, while the data recorded by the dynamometer was in metric tonnes. Further tests were conducted with 4 occupants and with 1 occupant winched from both rear and front axles.


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Figure 5 - Traction force time series for a test on the Toyota Yaris with one driver occupant, winched from the rear axle only, on a wet floor, showing the phases of loading the axle (tyre deformation), breaking traction, and dragging the vehicle. The point of breaking traction was recorded for further analysis.

Results of the breaking traction force for three test cases for the Toyota Yaris (rear axle, 1 passenger and 4 passengers; both axles, 1 passenger) are provided in Figure 6. Points on the vertical axis (traction force ≈ 0 kN) indicate those depths where the vehicle could be pushed by hand.

Figure 6 - Measured peak horizontal traction force for a Toyota Yaris in varying static water levels. For this case, the vehicle was loaded with a single occupant, and towed from the rear axle. A salient observation from these test results was the clear distinction between the stability of the vehicle when the water level was below the vehicle’s floor pan and once it reached a point just above the floor pan. The buoyancy force of the air trapped in the vehicle cabin is analogous to a bubble floating on the water surface. As soon as floodwaters were deeper than the floor of the vehicle and began to engage with the air trapped in the vehicle cabin, the force required to move the vehicle sideways rapidly decreased. For the Toyota Yaris, this threshold depth for force reduction was approximately 0.2 m. As the flood water depth increased above 0.2 m, the force required to move the Toyota Yaris sideways rapidly decreased until the rear wheels were floating off the ground and effectively zero force was required to move the car sideways at a depth of 0.6m.


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4. HYDRODYNAMIC FORCE TESTING (1:18 SCALE) Hydrodynamic force testing was conducted on a 1:18 scale model vehicle to determine the coefficient of drag CD for the vehicle by direct measurement of hydrodynamic drag forces on the vehicle. The tested flows were typical of those capable of destabilising a vehicle and ranged from subcritical to supercritical flows with a range of inflow Froude numbers 0.3 < Fr < 4.2. The model vehicle was suspended by fine wire cables to a rigid frame and an adjustment mechanism that allowed the vehicle tyres to be held just above the surface of the flume (Figure 3). The cables were attached to each of the four wheels. The vehicle was oriented so that the flow was directly perpendicular to the vehicle (90 degrees to the direction of vehicle travel). The weight of the model was such that the vehicle was never buoyant under any flow condition, so that only the hydrodynamic forces were measured. Buoyancy, tyre friction and other forces were not measured in the model. The vehicle was held at a fixed level, such that loading and unloading of the springs due to buoyancy or rolling of the vehicle from side impact forces was not considered. The drag forces were measured directly by attaching fine wire from load cells to the upstream wheels, and logging to a computer at 500Hz and then averaging over 1 minute to give the result. Water depths were measured with a point gauge, and flows with a magnetic flow meter. The coefficient of drag was calculated for all flow tests using Equation (3). The experiments allowed the direct measurement of FH and flow depth d. The average flow velocity was calculated based upon continuity considerations and the cross-sectional area of the vehicle based upon the flow depth. The resulting coefficient of drag is presented in Figure 6. The drag coefficients ranged between 1.0 < CD < 1.8.

Figure 6 - Coefficient of Drag for a Toyota Yaris based on horizontal hydrodynamic force tests.

5. FLOOD HAZARD CURVES The testing conducted for this investigation provided the opportunity to review the flood hazard curves for vehicles derived for floodplain planning and emergency management, such as the hazard curves published in Australian Rainfall and Runoff (Shand et al., 2011) and AEMI Handbook 7 (2014). Consideration of the prototype instability force testing and model scale hydrodynamic testing in combination enabled flow instability threshold conditions for vehicles to be developed and compared to existing flood hazard curves. This assessment required estimation of suitable values for coefficients in the governing equations. In particular, the analysis required the adoption of values for the coefficient of drag CD for the partially submerged vehicle and the coefficient of friction µ for the vehicle’s tyres, as could be conservatively estimated for actual flood conditions. A conservative effective coefficient of friction µ = 0.3 was adopted using similar reasoning to the findings of Bonham and Hattersly (1967) who considered a braking force coefficient of µ = 0.5 on a worst case scenario of “a smooth causeway surface on a wet country road” with adjustments made for a reduced sideways force compared with breaking force (10%), the reduced slipping force compared with peak breaking force (20%), and a further reduction due to debris that may become caught under the wheels


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(20%). This resulted in an adopted coefficient of friction of 0.3. The measured traction results were scaled by 0.3 / 0.78 (idealised available Cf in a flood / measured Cf for testing), for inclusion in the flood hazard curves. An adopted coefficient of drag of CD = 1.8 provided a conservative value based on the range of measured values. The uplift of the vehicle was assumed to be zero, but can be considered to be accounted for in the conservative value of the coefficient of drag. A comparison of data derived from the testing on the basis of these adopted coefficients with the currently adopted curves from Australian Rainfall and Runoff is provided in Figure 7 for the Toyota Yaris (representative of small cars). At face value, the test results indicated that the Toyota Yaris was more stable than the interim vulnerability threshold condition for a ‘small passenger vehicle’ from Australian Rainfall and Runoff expressed in terms of a flow velocity times depth product (v.D).

Figure 7 - Stability Curve for a Toyota Yaris based on full scale traction tests, upper limits for the drag coefficient CD = 1.8 based on scale testing, and points scaled to a flood condition coefficient of friction µ = 0.3.

6. DISCUSSION There are a range of limitations in the testing regime which need to be carefully considered when interpreting the test results from this investigation into a form suitable for classifying flood hazard behaviour for floodplain planning or emergency planning and operations. Firstly, when considering the vulnerability classification for small passenger vehicles, the Toyota Yaris, while small, was far from the smallest, most vulnerable vehicle on Australian roads. Secondly, there were a number of mitigating factors within the testing program that made it sensible to apply a conservative interpretation of the results for a real-world application. These relate predominantly to the interpretation of the hydrodynamic flow conditions from the 1:18 scale modelling to real world conditions and include: 

   

The laboratory testing was conducted in ideal conditions, with regular channel dimensions and steady flow. These conditions would rarely be encountered in real world conditions. In the real world, flows are often sediment laden and sometimes contain large debris both of which would increase the flow force on an exposed vehicle; Road surfaces in flood affected areas are also often uneven with potholes and loose gravel making full contact of all wheels on the submerged road surface problematic; Laboratory testing was completed with stationary vehicles. In real flood conditions, most cars will be driven into floodwaters and the act of rolling into the floodwaters may contribute additional uplift forces and an associated reduction in friction not accounted for in the testing; The impact of turbulent, pulsating or varying flows has not been considered in the testing but might often be prevalent in real world conditions, especially in flash flood conditions, and is likely important in initially destabilising a vehicle; The changes to vehicle height and roll angle (loading and/or unloading of the suspension) due to buoyancy and side impact forces were not considered in the scale hydrodynamic tests;


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Experimental Testing of Flood Hazards Curve for a Partially Submerged Vehicle 

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Uplift forces on the model scale stationary vehicle were not well represented in the testing, largely due to the magnitude of surface tension forces compared to the weight of the vehicle. In other words, when considering the vertical forces, the weight of the water raised by surface tension was significant compared with the uplift forces in the model.

In due consideration of both these aspects of the testing program i.e. i) that the test vehicle is not representative of the most vulnerable vehicles in their class on Australian roads; and ii) that there are limitations with the absolute accuracy of the results in the testing program, it is appropriate that flood hazard thresholds for planning purposes remain conservative compared to the test data presented here. On this basis, the authors recommend that the interim flood hazard threshold curve from Australian Rainfall and Runoff summarised here as Table 2 for the vehicle class tested in this investigation be adopted for use in future floodplain management and emergency management. Table 2 - Proposed flood hazard thresholds for vehicle stability Class of Vehicle

Length (m)

Kerb Weight (kg)

Ground clearance (m)

Limiting still water depth1

Limiting high velocity flow depth2

Limiting velocity3

Equation of stability

Small passenger

< 4.3

< 1250

< 0.12

0.3

0.15

3.0

D.V ≤ 0.3

1

At velocity = 0 m/s;

2

At velocity = 3 m/s;

3

At low depth

7. CONCLUSIONS The test program and analysis presented here describes a novel approach to the derivation of vehicle stability thresholds in flood flows. Flood hazard thresholds were derived by dissecting the flood forces that impact a vehicle, evaluating vehicle stability directly form the stability balance equation, and applying appropriate factors of safety. The experiments provided novel results for traction threshold forces and drag coefficients of vehicles in flood waters. The present data confirmed existing threshold values for vehicle stability. The experiments were conducted with a Toyota Yaris and further tests are required with a larger range of vehicle classes. Further improvement of the stability estimates would require an extensive test program involving a detailed study of tyre friction in flood conditions, and fullscale testing of hydrodynamic forces across a broader range of vehicles.

8. REFERENCES AEMI. (2014). Australian Emergency Management Handbook: Managing the floodplain: best practice in flood risk management in Australia, (AEMI Handbook 7, 2014). Bird, G. and Scott W.J.O. (1936) Studies in Road Friction, I. Road Surface Resistance to Skidding D.S.I.R., Road Research Tech. Pap. No.1 H.M.S.O., 1936 Bird, G. and Scott W.J.O. (1936) Studies in Road Friction, II. An Analysis of the Factors Affecting Measurement D.S.I.R., Road Research Tech. Pap. No.2 H.M.S.O., 1936 Bonham, AJ. and Hattersley, RT. (1967) Low Level Causeways, University of New South Wales, Water Research Laboratory, Technical Report No. 100, August 1967. Haynes, K., Coates, L., Dimer de Oliveira, F., Gissing, A., Bird, D., van den Honert, R., Radford, D., D’Arcy, R, Smith, C. (2016). An analysis of human fatalities from floods in Australia 1900-2015. Report for the Bushfire and Natural Hazards CRC Hoerner, S. F., (1965). Fluid-dynamic drag: practical information on aerodynamic drag and hydrodynamic resistance. Midland Park, N.J.: Dr.-Ing. S.F. Hoerner. Jonkman, S. N. (2005). Global perspectives on loss of human life caused by floods. Natural Hazards, 34 (2), pp.151-175.


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Keller, R.J., and Mitsch, B. F., (1992) Stability of Cars and Children in Flooded Streets. International Symposium on Urban Stormwater Management: Preprints of Papers. Barton, ACT: Institution of Engineers, Australia, 1992: 264-268. National conference publication; no. 92/1. Kramer M., Terheiden K., and Wieprecht S. (2016) Safety criteria for the trafficability of inundated roads in urban floodings. In: International Journal of Disaster Risk Reduction, 17 pp. 77–84 Martínez-Gomariz, E., Gómez, M., Russo, B. and Djordjević, S. (2016), Stability criteria for flooded vehicles: a state-of-the-art review. J Flood Risk Management. doi:10.1111/jfr3.12262 Munson, NSW SES (2016) Flood safety warning following 300 flood rescues. http://www.ses.nsw.gov.au/news/76365/77179 accessed 27 September 2016.

Web

article:

Shand, T. D., Cox, R. J., Blacka, M. J. and Smith, G. P. (2011). Australian Rainfall and Runoff Revision Project 10: Appropriate Safety Criteria for Vehicles - Literature Review. Australian Rainfall and Runoff Revision Project 10. Stage 2 Report. Prepared by the Water Research Laboratory. P10/S2/020. February 2011. Smith, G. P., Davey, E. K., and Cox, R. J. (2014) Flood Hazard. UNSW Australia Water Research Laboratory Technical Report 2014/07. Taborek, J.J. (1957) "Mechanics of Vehicles," Machine Design, May 30-Dec. 26. Wong J.Y, (1993) Theory of ground vehicles, 2nd ed.


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