Coastal Engineering 68 (2012) 1–5

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Short communication

Application of LiDAR technology for measurement of time-varying free-surfaceprofiles in a laboratory wave flume

Chris E. Blenkinsopp ⁎, Ian L. Turner, Michael J. Allis, William L. Peirson, Luke E. GardenWater Research Laboratory, School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052, Australia

⁎ Corresponding author. Tel.: +61 2 80599861; fax:E-mail address: c.blenkinsopp@unsw.edu.au (C.E. Bl

0378-3839/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.coastaleng.2012.04.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 September 2011Received in revised form 8 March 2012Accepted 17 April 2012Available online 16 May 2012

Keywords:Wave measurementLiDARFree-surface elevationWave transformationLaboratory wave flume

An experimental study was completed to investigate the ability of a fixed, two-dimensional LiDAR instru-ment to obtain detailed measurements of propagating waves within a laboratory wave flume. The resultsshow that this technology can be used to obtain synchronous free-surface measurements at a horizontal spa-tial resolution O(10 mm), with comparable vertical accuracy to that of more conventional, high-precisioncapacitance wave probes. The principal advantage of using the LiDAR is that a single, non-intrusive instru-ment can be utilised to measure the entire wave field at high resolution allowing detailed evaluation ofwave transformation throughout the experimental domain.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Reliable measurements of the time-varying free-surface elevationin both the laboratory and field are essential for the investigation ofwave processes such as the evolution of wave groups, wave heightdecay in the surf zone and swash hydrodynamics.

In the field, wave height data in the nearshore are commonlyobtained using fixed pressure transducers and wave staffs, whileultrasonic altimeters have been successfully used in the swash zone(e.g., Turner et al., 2008). Open ocean studies generally employtethered wave buoys to measure surface waves, though in recentyears remote-sensing techniques such as satellite altimetry (e.g.Queffeulou, 2004) and RaDAR (e.g. Nieto et al., 1998) have become in-creasingly common, primarily for obtaining time-averaged wave fieldstatistics. LiDAR (Light Detection And Ranging) is an optical remote-sensing technology which uses a scanning laser to provide accuratetwo or three-dimensional measurements of surface profiles. Thistechnology has become commonly used for coastal field researchover recent years as aircraft-mounted LiDAR allows rapid collectionof morphology data over large regions of the coastline. AirborneLiDAR has also been used to obtain wave measurements (e.g. Hwanget al., 2000), however as the instrument position is constantly moving,this technique cannot be used to obtain time-series wave heightinformation.

Laboratory measurements of water surface elevation in both 2Dwave flumes and 3D basins are typically obtained using capacitance- orresistance-type wave probes (e.g. Shepherd, 1997; accuracy O(b1 mm))

+61 2 99494188.enkinsopp).

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or ultrasonic altimeters (e.g. Gillespie et al., 1982; accuracy O(1 mm))all of which provide accurate measurements of the time-varying free-surface elevation but are restricted to single point measurements. Thuslarge arrays containing multiple instruments are required to capture thespatial evolution of a series of waves. Previous techniques designed tocapture wave measurements at a greater spatial resolution are restrictedto optical techniques undertaken in 2Dwaveflumes. For example She andCannings (2007) used a video camera installed at still water level lookinghorizontally through the glass flume-wall to record waves along a lengthof theflume. The free-surface profile at the sidewall was extracted using athresholding operation and edge detection algorithm. Similar techniqueshave been used by Govender et al. (2002) and Blenkinsopp and Chaplin(2008). However it is noted that by viewing the flow field from theside, the actual surface position is obscured by sidewall effects such asthe presence of ameniscus and boundary effects. The use ofmore sophis-ticated optical techniques which make use of a light sheet to avoid side-wall effects, such as Laser Induced Fluorescence (e.g. Shand et al., 2009;accuracy O(0.1 mm)) and Particle Image Velocimetry (e.g. Peirson,1997; accuracy O(1 μm)), has been shown to provide very high-qualitymeasurements. These techniques are however typically time-consumingto set up and require significant post-processing to obtain meaningfultime-series.

While large-scale applications of LiDAR for coastal research arerelatively common, the use of LiDAR to investigate coastal processesat smaller scales such as the measurement of wave height time-series, is limited. However the ability of a single instrument to mea-sure complete 2D profiles or 3D surfaces unobtrusively at highspatial and temporal resolution has obvious advantages over thepoint-measurement methods outlined above. Belmont et al. (2007)discussed the use of a vessel-mounted LiDAR system to obtain

Fig. 2. SICK LMS200-30106 LiDAR instrument. The LiDAR measures along a semi-circular 2D transect in a plane towards the reader.

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shallow-angle wave height measurements in the ocean. Blenkinsoppet al. (2010) demonstrated the ability of a single, fixed LiDAR instru-ment to obtain field measurements of the time-varying swash-surface elevation along a 9 m cross-shore profile at high temporal(sample rate of 37.5 Hz) and spatial (O(1 cm)) resolution with compa-rable accuracy to that obtained by an array of 10 ultrasonic altimeters.

This paper will examine the ability and accuracy of a single, fixedLiDAR instrument to obtain detailed measurements of the propaga-tion and shoaling of waves along a 6 m test section within a laborato-ry wave flume.

2. Instrumentation and methodology

LiDAR measurements are based on the time of flight principle. Theinstrument transmits a pulsed laser in the direction of a target. On in-teraction with the target, the transmitted light is partially reflected/scattered back to the instrument and the time of travel between theincident and reflected pulses determines the distance to target. To ob-tain measurements of entire 2D profiles (see Fig. 1) or 3D surfaces,the laser is deflected by an internal mirror which rotates in one(2D) or two (3D) planes to produce a fan (2D) or cone-shaped (3D)scan of multiple measurement points.

The experimental work described in this paper was undertakenusing an LMS200-30106 two-dimensional laser measurement systemmanufactured by SICK (2006). This instrument has a low cost of ap-proximately $5000 US and is shown in Fig. 2. The dimensions of thisparticular instrument are 185 mm×156 mm×155 mm and it weighs4.5 kg. The LMS200-30106 uses an eye-safe pulsed laser beam(λ=905 nm) and was configured to obtain measurements at an an-gular resolution of 0.5° within a 180° field of view. For the experimentdescribed here, the measurement range was fixed at 8 m and thesample rate was 37.5 Hz. The instrument manufacturer's specifica-tions indicate that at a range of 8 m the instrument has a resolutionof 1 mm and statistical error (defined as one standard deviation ofrecorded values over 100 measurement points) of 5 mm.

Laboratory testing of the ability of the LiDAR instrument to mea-sure wave profiles was conducted in a two-dimensional wave flumeat the Water Research Laboratory which measures 35 m in length,0.9 m in width and was filled with fresh water to a depth of 1.05 m.The wave flume is equipped with a paddle-type wave generator

Fig. 1. Principle of operation of a 2D scanning LiDAR instrument.

which was used to generate one irregular and five different regularwave test cases, each lasting 3 min in duration and with wave periodsin the range of 1.02 to 2.02 s and wave heights from 0.16 m to 0.3 m.A submerged planar slope with a 1:10 gradient and total height of0.90 m was installed in the base of the flume as shown in Fig. 3.This caused waves to shoal as they propagated along the flume andenabled the LiDAR instrument's ability to measure reshaping wavesto be assessed.

The LiDAR was mounted 3.55 m above the flume bed centreline,6.75 m upstream of the slope crest and orientated to scan along thelength of the flume. The raw data from the instrument was sampledat the maximum rate of 37.5 Hz. To provide additional measurementsof time-varying free-surface elevation at fixed points for verificationof the LiDAR measurements, four capacitance wave probes with anaccuracy of ±3 mm were installed along the flume at 1.5 m intervals,offset 100 mm from the flume centreline (so as to prevent interfer-ence with the scanning laser beam) as shown in Fig. 3 and sampledat a rate of 120 Hz. Note that a plate extended along the roof of theflume adjacent to the wave paddle and impeded the LiDAR line ofsight for angles smaller than α=−39.5° (x=4m from the paddle).

Detection of water surfaces using a laser measurement system canbe problematic because reflections from a smooth water surface arespecular and so a return is only achieved when the incident angle ofthe laser is approximately perpendicular to the surface. To improvethe number of signal returns obtained during the testing describedhere, approximately 1 kg of kaolinite, a clay mineral with a particlesize (D50) of 20 μm was mixed into the fluid. As previously demon-strated by Allis et al. (2011), the presence of particulates greatly in-creased the number of signal returns obtained from a flat watersurface and valid free-surface measurements were obtained overthe full test region between α=−39.5° (x=4.5 m from the wavepaddle) and α=75° and (x=10.5 m, near the crest of the submergedplanar slope).

The horizontal location xLID on the water surface at each value ofbeam angle α varies as the free-surface rises and falls and this

Fig. 3. Schematic detailing laboratory setup and instrumentation.

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variation gets larger with increasing beam angle. To remove this ef-fect and allow direct comparison with the point measurementsobtained from the capacitance probes, the LiDAR measurementswere spatially interpolated using linear interpolation to give surfaceelevation at 20 mm intervals along the length of the flume.

3. Results

A comparison between the time-series of free-surface measure-ments obtained from the LiDAR and capacitance probes, both smoothedusing a 3-point running average in the time domain is presented inFig. 4, where thewave height is 0.3 m andwave period of 2.02 s. It is ev-ident that where the wave form is approximately sinusoidal at waveprobe locations 1 to 3 (refer Fig. 3), the LiDAR correctly reproducesthe free-surface variation measured by the capacitance wave probes.Atwave probe 4,where thewave is asymmetric as it approaches breakinggood agreement is again achieved, though it is noted that the LiDAR signaldrops out as the steep front face of the wave passes the measurement

Fig. 4. Free-surface elevation measurements obtained by both the LiDAR (thin grey)and capacitance wave probes (thick dashed) in regular waves (H=0.16 m,T=2.02 s) at four cross-shore locations during a ten second interval. Note that thewave probe (WP) numbers refer to the positions defined in Fig. 2.

location. This occurs because at this position, the grazing angle betweenthe LiDAR beam and wave face becomes too acute as the wave facesteepens andno valid signal return is obtained. For the experiment geom-etry, instrument and seeding material used in this study, the limitingangle between wave face and LiDAR beam was 28.5°. This will howevervary with the geometric setup and potentially the characteristics of thewave field. It is noted that results comparable to those presented inFig. 4 were obtained for the other regular wave cases and a comparisonof the wave energy spectra for the irregular case showed that both mea-surement techniques recorded similar spectral characteristics.

In order to investigate further the agreement between the LiDARand capacitance probe measurements, Fig. 5. presents a scatter plotwhich compares the measurements from the LiDAR to those obtainedusing the four capacitance probes resampled at a frequency of 37.5 Hzover the period shown in Fig. 4. This figure confirms the agreementbetween the two instruments and can be used along with Fig. 4 tofurther investigate the differences between the LiDAR and capaci-tance probe measurements. Over the course of the 3 minute test du-ration, the RMS differences between the measurements obtainedfrom the two methods are 4.7 mm, 4.9 mm, 6.1 mm and 5.0 mm forprobes 1 to 4 respectively. These differences are equivalent to 1.6%to 2.0% of the wave height and are of the same order of magnitudeas the estimated accuracy of the capacitance wave probes. It isnoted that the maximum differences between the two sets of dataare typically no greater than ±11 mm at all locations. It is notedthat the LiDAR tends to systematically underestimate (overestimate)free surface elevation at points upstream (downstream) of the instru-ment location.

The observed differences between the LiDAR and capacitanceprobe measurements are attributed to four main reasons: (1) capaci-tance probes measure surface elevation at a fixed horizontal locationwhile the LiDAR measurements are based on a linear interpolationbetween the two closest adjacent measurement points which movewith varying free-surface elevation. (2) The angle between theLiDAR beam and the water surface is continuously changing as thewaves propagate along the flume. This will alter the amount of lightreflected back to the instrument and cause the shape of the measure-ment spot on the free-surface to deviate from circular, introducingsome uncertainty over the exact location of the measurement point.The results indicate that for the experimental setup used in thisstudy, the limiting grazing angle is 28.5°, below which no valid signalreturns are obtained by the LiDAR instrument. (3) The diameter of thecircular measurement spot (i.e. the area over which the distance mea-surement is averaged) increases linearly with measurement distance.

Fig. 5. Scatter plot of free-surface elevation measurements obtained by the LiDAR (zLID) and capacitance wave probes (zWP) in regular waves (H=0.16 m, T=2.02 s) at four cross-shore locations during a ten second period. Note that the wave probe (WP) numbers refer to the positions defined in Fig. 2.

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(4) There is a 100 mm offset along the wave crest between the LiDARand capacitance probe measurements, consequently some of thedifferences between the two instruments may be attributed to anelement of three-dimensionality in the wave fronts though therewas no obviously visible cross-tank variations in wave characteristics.

The ability of the LiDAR to measure time-series of wave profile in-formation rather than just point measurements is demonstrated inFig. 6. This figure shows a 6 m free-surface profile along the lengthof the wave flume at one second intervals as a 0.3 m high regularwave with a period of 2.02 s propagates along the flume from left toright. The LiDAR measurements clearly capture the form of the

Fig. 6. Free-surface profile measured by the LiDAR at one second intervals during thepassage of a single wave. Note that each cross represents a single measurementpoint, the high spatial density of the measurements obscures many of the individualpoints at the scale of this plot.

propagating wave at the different time intervals as it becomes asym-metric and approaches breaking. The high spatial density of the mea-surements is evident in Fig. 6, with horizontal resolution in the rangeof 15 to 55 mm. This technique is ideal for the investigation of wavetransformation as it does not interfere with the flow and can obtain‘snapshots’ of wave shape at high temporal resolution which cannotbe obtained with alternative methods. Note that greater profilelengths can be achieved by increasing the LiDAR elevation, but alower spatial resolution would be achieved.

4. Conclusions

The results presented in this paper demonstrate the use of a fixedLiDAR instrument to obtain free-surface measurements at a horizontalspatial resolution O(10 mm) along a six metre section within in a labo-ratory wave flume. The free-surface measurements obtained by theinstrument compared favourably (RMS differences smaller than 2% ofthewave height) with those from four fixedwave probes. The responseof the LiDAR is affected by the location of the instrument relative to themain area of interest and as such, careful consideration should be givento instrument positioning and initial verification of instrument accuracyshould be carried out on a case-by-case basis. However, these resultsprovide encouraging evidence that for applications where detailedmeasurements of wave profile are required, a single LiDAR instrumentcan replace large arrays of conventional wave probes and providemuch higher spatial resolution than can be practically achieved usingconventional laboratory instrumentation.

Acknowledgements

The authors would like to acknowledge the financial assistanceprovided by the Australian Research Council (ARC; DP110101176and DP0985602).

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