Development of an automated method for continuous ...

EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 36, 347–362 (2011) Copyright © 2010 John Wiley & Sons, Ltd. Published online 13 July 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.2045

Development of an automated method for continuous detection and quantification of cliff erosion events Pascal Bernatchez,* Yvon Jolivet and Maude Corriveau Center for Northern Studies, Department of Biology, Chemistry and Geography. Université du Québec à Rimouski (UQAR), Rimouski, Québec, Canada Received 2 July 2009; Revised 22 April 2010; Accepted 27 April 2010 *Correspondence to: Pascal Bernatchez, Research Chair in Coastal Geoscience, Center for Northern Studies, Department of Biology, Chemistry and Geography, Université du Québec à Rimouski (UQAR), 300 Allée des Ursulines, Rimouski, Québec, Canada, G5L 3A1. E-mail: [email protected]

ABSTRACT: Three intrusive systems of detection and quantification of coastal erosion events (using thermocouples and thermal pins) were developed and tested from 2005 to 2008 in different regions of the Gulf and maritime estuary of the St Lawrence (Quebec, Canada). The 3-m-long thermal pins inserted inside unconsolidated deposits allow the monitoring of erosion for a time period sometimes extending over several seasons. The thermocouple or thermocable method allows not only the instrumentation of unconsolidated deposits but also of rocky and cohesive substrate to a depth of 85 cm. An autonomous microclimatic station located near the experimental sites simultaneously samples temperature parameters, precipitation, snow cover, wind speed and direction as well as global radiation. The differential analysis of cliff thermal regime performed simultaneously with an analysis of air temperature makes it possible to determine the activation periods of coastal erosion processes. The results also make it possible to establish with precision the actual influence of rapid variations of certain climatic and microclimatic parameters (radiation, presence of snow cover, precipitation, etc.) on the physical state of surfaces and also on the activation of certain physical processes connected to coastal erosion events. The automated thermal erosion pin system (ATEPS) allows high temporal resolution (i.e. continuous) monitoring, enabling a real coupling of coastal erosion rates and climatic parameters. Preliminary results with the ATEPS system indicate that mild winter temperature and direct solar radiation are significant factors controlling cliff retreat rates. Moreover, the melting of segregation ice during the spring thaw contributed for more than 70% of cliff retreat against only 30% for frost shattering. Copyright © 2010 John Wiley & Sons, Ltd. KEYWORDS: automated thermal erosion pin system (ATEPS); coastal monitoring; microclimatology; cliff erosion; thermocouples; thermal regime; weathering processes

Introduction Most decisions concerning the regional development of coastal territories and the management of natural hazards within coastal zones are based on shoreline retreat rates (Dolan et al., 1991). In addition to the use of multidate aerial photography which is the most frequently used method to calculate shoreline movement over a long-term period (Grenier and Dubois, 1992; Suanez and Simon, 1997; Moore and Griggs, 2002; Boak and Turner, 2005), new technological advances have been developed over the last decade. Quantification of coastal evolution has been achieved using a range of modern technologies, such as digital aerial images and videography, high resolution satellite images (Ikonos, QuickBird), airborne lasers (Marfai et al., 2008; Stockdonf et al., 2002; Robertson et al., 2004; Zhang et al., 2005; Young and Ashford, 2006; Boak and Turner, 2005; McCulloch et al., 2002; Moore, 2000; Leatherman et al., 1995), terrestrial laser scanning (Gulyaev and Buckeridge, 2004) as well as differen-

tial global positioning systems (DGPSs) which are used either on foot or mounted on an all-terrain vehicle (Baptista et al., 2008; Stockdonf et al., 2002). Monitoring pins have also been implanted on top of cliffs (Bernatchez and Dubois, 2008) or inserted horizontally directly into the cliffs (Manson, 2002; Greenwood and Orford, 2007). Although these methods perform well on the whole, not enough information is provided to allow effective understanding, management, prevention and vulnerability mitigation with respect to coastal erosion risks. The poor temporal resolution of the monitoring process is in most cases inadequate to identify the exact moment of the erosion event and to quantify the connections between erosion events, the intensity of those events, meteorological phenomena and the geomorphological processes responsible for the erosion events (Lawler, 2005a). At best, the temporal resolution of the measurements of retreat rates is monthly, but in this case requires considerable physical and financial effort for on-site field monitoring, resulting in readings generally done over a short time period (Manson,

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Location of the experimental sites.

2002; Bernatchez and Dubois, 2008). In the context of anticipating the geomorphological response of coastal systems to climatic changes, it becomes necessary to acquire data with high temporal resolution (i.e. continuous) in order to realize a real coupling of coastal retreat rates and climatic parameters. The establishment of coastal surveillance systems using fixed digital cameras (Holman and Stanley, 2007) has allowed continuous monitoring of wave climate and environmental conditions in order to establish the causes of morphosedimentological coastal changes (Morris et al., 2001; Turner et al., 2004; Turner et al., 2006). The most noteworthy limitation of optical captors has to do with climatic conditions (intense rainfall, snow and fog) that make images unusable in periods during which morphological changes are susceptible to being very significant (storms). Night vision technologies are also usually faulty, making it impossible to detect coastal changes at night (Holman and Stanley, 2007). Coastal cliffs are also affected by numerous erosion processes conditioned by the fluctuations of the internal thermal regime of materials that compose the cliffs (Trenhaile and Mercan, 1984; Ødegård and Sollid, 1993; Lewkowicz, 2001; Hall, 1999, 2004; Hall et al., 2005; Bernatchez and Dubois, 2008), but cameras do not permit the measurement of such changes. This paper presents three methods for detecting the moment of cliff erosion events that were developed over a three-year period of experimentation. Specific examples are presented to illustrate the potential of such high-temporal-resolution monitoring systems, not only for quantifying cliff erosion but also for quantifying the influence of thermal regime on the weathering processes of cliffs.

Location and General Description of the Experimental Sites The experimental sites are located in the maritime estuary and Gulf of St Lawrence and the Chaleur Bay in Quebec, eastern Canada (Figure 1). The instrumented cliffs represent a variety of lithologies (Figure 2). The spring tides vary on average from Copyright © 2010 John Wiley & Sons, Ltd.

1·2 m to 4·9 m (Canadian Hydrographic Service, 2009) and therefore cover a tidal regime from microtidal to macrotidal. Storms mainly occur during winter. According to Dfb (snow, fully humid, warm summer) and Dfc (snow, fully humid, cool summer) of the Köppen–Geiger updated classification (Kotteck et al., 2006), the region is characterized by a continental humid climate with cold winters and cool to warm summers without a dry season. Summer is relatively short, and only in July does the monthly mean maximum daily temperature slightly exceed 20 °C. On the north shore of the St Lawrence maritime estuary and gulf, the maximum and minimum daily temperature averages vary between 20·9 °C to 8·0 °C for July and −7·9 °C to −20·9 °C for January (Environment Canada, 2009). In the second part of December, the drop in temperature causes the formation of an ice foot on the upper foreshore. The ice foot period lasts approximately four months until the end of April and completely protects coastal cliffs from wave action (Figure 3). Even during storms, waves do not reach the base of cliffs. This natural coastal protection allows us to quantify the erosion induced by weathering processes and to separate the effect of basal wave sapping in the coastal retreat (Bernatchez and Dubois, 2008).

Distinctive Physical Aspects of the Microclimatologic Study of Cliffs as a Detection Method for Coastal Erosion Cold regions are subject to very significant variations in temperature (Bland and Rolls, 1998; Lewkowicz, 2001) and are often associated with a precipitation regime composed of rain, snow, and sometimes a mix of both. This variability in the climate is in large part responsible for the complex physical state of surfaces (bare, iced or snow-covered). Climate variability operates along with the highly dynamic processes of steep slopes (running water, groundwater flow, landslide, rockfall, suffosion, surface desiccation, etc.) to modify the physical properties of ground surfaces, in some cases at a very high frequency (daily, hourly). Also, during mild winter Earth Surf. Process. Landforms, Vol. 36, 347–362 (2011)

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Figure 2. Photographs showing the different lithological structures where the experiments took place: (A) sandstone; (B) conglomerate; (C) carbonate; (D) silt, coarse sand and clay; (E) fine sand and argillite; (F) closer view of argillite cliff. This figure is available in colour online at wileyonlinelibrary.com/journal/espl

Due to the effects of such varied climate, steep surfaces and the speed of annual cliff retreat (> 1 m/yr) may pose a real challenge to the study of coastal microclimate. This challenge involves, among other things, the development of an instrumentation adapted to this very active type of environment in order to characterize the coastal microclimate linked with the active processes that induce variations in the thermal regime of cliffs.

Material and Methods

Figure 3. Ice foot protecting coastal cliff from wave action at Ragueneau, March 10, 2008. This figure is available in colour online at wileyonlinelibrary.com/journal/espl

periods and seasons of transition, factors such as wind direction, intensity of sun radiation, type of precipitation, presence or lack of ground snow cover and snow thickness on cliff surfaces (blowing snow) can vary even more rapidly than in cold weather periods, where temperatures nearly always maintain themselves under 0 °C. The interpretation of rapid variations of cliff temperatures along with the identification of thermal signatures (thermal regime specific to the ground/ice interface) during periods of high climatic variability must necessarily proceed with thorough knowledge of the physical state of surfaces. Copyright © 2010 John Wiley & Sons, Ltd.

Three systems for recording temperatures inside the cliff were tested (Figures 4A and 4B). The automated thermal erosion pin system (ATEPS) uses temperature sensors of the Thermochrons (DS1922L-F5) type; the semi-autonomous thermocable system (SATS) is controlled from an acquisition system Smart Reader 6 Plus that can be installed in close proximity to thermocables and its functioning does not require any external power source; the thermocable relay system (TRS) is connected to a relay box type AM16/32 which is controlled by the data acquisition system of type CR10X or CR800 of the reference meteorological station.

Reference Meteorological Station These systems are completed by a portable reference meteorological station. Each station includes a data acquisition system of type CR10X to which are connected several measuring instruments whose results are used for the quantification Earth Surf. Process. Landforms, Vol. 36, 347–362 (2011)

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Figure 4. (A) Schema illustrating the environmental setting of the three systems of cliff instrumentation; (B) diagram illustrating the three systems of cliff instrumentation. This figure is available in colour online at wileyonlinelibrary.com/journal/espl

of the different climatic parameters of the local climate and of the microclimate on experimental sites. The precision of the measuring instruments and the frequency of data acquisition are listed in Table I. To complete basic climatic data (temperature, relative humidity, wind speed and direction) information from meteorological stations belonging to the Environment Canada network is sometimes used to validate and verify weather status (cloudiness, rain, snow, etc.).

Measuring Systems of Cliff Internal Temperature using Thermocouples (SATS and TRS) Two data acquisition systems, which differ by type of data registration system, were used to gather temperature data Copyright © 2010 John Wiley & Sons, Ltd.

taken from thermocouples installed on a thermocable. The method of temperature measurement by thermocouples of type T (copper-constantan) is well adapted to environmental conditions since its functioning range (Table I) amply covers the range of temperatures to which natural surfaces are submitted. Each wire of a 0·5 mm diameter is individually covered by a Teflon® sheath whose results in durability tests with respect to abrasion and water submersion are considered excellent (OMEGA, 2004). Teflon® also retains a good flexibility when used in a cold environment (OMEGA, 2004). Finally, an external envelope of heat-shrink tubing maintains and protects both united conductors. The fabrication method of thermocouples consists in twisting the hot junction, which represents the meeting point between the two wire conductors of the thermocouple, over a distance of half-a-centimeter and then welding this junction to tin (ASTM, 1993). In order to protect the extremity of the Earth Surf. Process. Landforms, Vol. 36, 347–362 (2011)

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Table I. Technical specifications of data acquisition systems and measuring instruments Operating temperature −55 °C to + 85 °C

Datalogger CR10X (Campbell Scientific)a Datalogger Smart Reader 6 Plusb Datalogger Thermochronc (DS1922L-F5)

−40 °C to +70 °C −40 °C to + 85 °C

Sensors Thermistor NTC (Internal reference temperature)b Thermocouple reference with thermal Shield (CR10XTCR)a Thermocouples (type T)d Temperature sensor (Thermochron DS1922L-F5)c Relative Humidity (RH) and Temperature Probea (Vaisala HMP45C)

Accuracy

Data collection frequency

Analog entry ±0·1% of FSR (−25 °C to +50 °C);

−40 °C to +70 °C

±0·2 °C from 0 °C to +70 °C

30 minutes

−55 °C to + 85 °C

< ± 0·1 °C (−24 °C to +45 °C)

5 minutes

−200 °C to +350 °C −40 °C to +85 °C

Typical precision 0·5 °C or 0·75% ±0·5 °C

−40 °C to +60 °C

±2% RH (0−90% RH) ±3% RH (90−100% RH) ±0·2 °C to +30 °C ±0·5 °C to −40 °C Typical ± 3% of natural diurnal radiation

Silicon Pyranometera (LI-COR LI200S)

−40 °C to +65 °C

Tipping Bucket Rain Gauge (TB4)a Sonic Ranger 50 KHz (Campbell Scientific SR50M)a Wind Monitor for Marine Applications (RM Young 05106–10)a Barometric Pressure Sensor (RM Young 61205V)a

0 °C to +70 °C −45 °C to +50 °C

±3% from 25 to 500 mm/h ±1·0 cm

−50 °C to +50 °C

Speed: ± 0·3 m/s Direction: ± 3°

−50 °C to +60 °C

±0·5 hPa

a

Campbell Scientific Corp., 2009;

b

ACR, 2009; c Maxim Dallas Semiconductor, 2009;

thermocouples from humidity and abrasion, a transparent thermal retractable sheath, whose opening is afterward coated with polish, covers the sensitive end of the sensor (Figure 5A). Each temperature captor is then plunged into a bath of water and melting ice in order to evaluate its precision. The sensors whose temperature exceeded the margin of −0·5 °C and +0·5 °C were rejected from the experiment. In the TRS an aluminum plate covers the analog inputs of the CR10X and the AM16/32 relays to reduce temperature variation at the junctions, thus minimizing errors in the readings of the thermocouples. This system requires an external power source (battery or alternative electric current) and in the case of battery use, an additional power system (solar panel) is necessary to ensure its functioning over a long period of time. The thermocouples that must be introduced into rocky cliffs (Figure 5B) are inserted beforehand in a sheath of polyethylene foam with a 2·5 cm diameter. The expansion of the insulating foam inside the rock tends to prevent the displacement of air and sensible heat in the proximity of captors. In unconsolidated deposit cliffs, thermocouples are mounted on a rigid rod covered by a heat shrinkable tube that can be pushed inside sediments. Where the compaction of unconsolidated materials poses a hindrance to the insertion of the thermocouple line, a rigid metallic rod with a dimension slightly superior to the one of the thermocouple line is pushed in, in order to create the space necessary for its insertion. The insertion holes for the thermocables excavated in the rocky cliffs were made with the help of a percussion power drill with a 2·5 cm diameter drill bit. The maximum depth of the insertion holes is 85 cm, in accordance with the maximum length of the power drill rod. Finally, the external opening is sealed by watertight cement which prevents water infiltration. Copyright © 2010 John Wiley & Sons, Ltd.

30 minutes 30 minutes 5 minutes

Reading every five seconds and recorded reading average of readings every minute 15 minutes 60 minutes Reading every five seconds and recorded reading average of readings every minute 15 minutes

d

OMEGA, 2004, 2009.

Measuring System of Cliff Internal Temperature by Thermal Pins (ATEPS) The ATEPS uses temperature sensors of the Thermochrons (DS1922L-F5) type. They were inserted in receptacles (DS9098P) allowing their serial assembly and then placed in a sheath of polyethylene foam (Figure 5C) already prepared to receive each Thermochron. The distance interval between the sensors can be adjusted according to the type of material or the spatial resolution necessary for the types of processes studied. In the case of pins to be inserted in unconsolidated deposits, the temperature sensors are separated by a 10 cm interval and their depth varies from 0 cm (surface) to 300 cm. The insertion of pins inside the cliff is done with the help of an auger with a diameter slightly superior to the pin. The insertion hole is then gradually emptied out until the insertion of the pin can be completed. In the case of rocky cliffs, a sensor was added at a depth of −5 cm from the surface. The pin’s maximum length is generally 85 cm (the maximum length of the drilling bit of the percussion power-drill used). The foam sheath and the temperature sensors are then thrust inside a coal-colored polyvinyl chloride (PVC) tube 3 m long and with an internal diameter of 2·5 cm. The temperature sensors are placed at the surface of the sheath to brace the sensitive part of the sensor against the internal wall of the PVC tube. The internal temperature of the ground can thus be rapidly transferred through the PVC tube to the temperature captor, and if the tube is located in open air, solar radiation can be rapidly absorbed by the dark side of the PVC rod and thus rapidly increase the sensor’s temperature. This process allows the increasing of the temperature variations between internal and external sensors, and thus identifying more easily Earth Surf. Process. Landforms, Vol. 36, 347–362 (2011)

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Figure 5. (A) Type T thermocouples (copper-constantan); (B) installation of the data acquisition system SATS including two thermocables in sandstone at L’Anse-à-Beaufils (Gaspésie, Québec); (C) network assembly of temperature sensors of the Thermochron type on a base of insolating foams; (D) data collection of the ATEPS system with a laptop. This figure is available in colour online at wileyonlinelibrary.com/journal/espl Table II. Thermal properties of soils and constituents Water content (m3/m-3) Water (0·01 °C) Ice (0 °C) Air (0 °C) Materials of thermal pins Polyvinyl chloride (PVC) (25 °C) Polyethylene foam Unfrozen soils Sandy soil (40% porosity)

Clay soil (40% porosity)

0·0 0·2 0·4 0·0 0·2 0·4

Density (kg/m3)

Mass heat capacity (J/kg K)

Thermal conductivity (1/Wm K)

Thermal diffusivity (× 10−6 m2/s)

999·8 916·2 1·29

4210 2050 1005

0·56 2·22 0·024

0·14 1·18 18·51

1400 29

580 1500

0·19 0·033–0·036

0·23 0·76–0·83

1600 1800 2000 1600 1800 2000

800 1180 1480 890 1250 1550

0·30 1·80 2·20 0·25 1·18 1·58

0·24 0·85 0·74 0·18 0·53 0·51

Source: Monteith (1973); Engineering toolbox (2009); Tremco-Illbruck (2009).

moments of erosion events. Communication wires crossing a waterproofed plug allow data collection from a laptop (Figure 5D). Strips of paint are applied on the external wall of the tubes at 10 cm intervals in order to facilitate visual reading of cliff retreat.

Thermal Properties of Ground and Thermal Pin Materials The ground temperature is governed by climatic and microclimatic conditions of the site and the thermal state of this surface is a major factor which determines the deep ground Copyright © 2010 John Wiley & Sons, Ltd.

thermal regime (Musy and Soutter, 1993; Oke, 1987; Williams and Gold, 1977). Any changes in ground temperature are governed by physical properties like conductivity, diffusivity, heat capacity and solar radiation (Jalota and Ghuman, 2006). The rate of heat transport always varies according to the thermal properties (Table II) of the soil composition which can sometimes vary greatly in density, water content, ice, vapor, organic content and mineral composition. Generally, higher mass heat capacity means that more energy will be required to vary temperature. Thermal conductivity indicates the ability of soil to conduct heat into the ground. The thermal properties of the pins system, especially with its low thermal mass of polyethylene compared to the soil Earth Surf. Process. Landforms, Vol. 36, 347–362 (2011)

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Figure 6. The principle of the method used to identify the moments of cliff retreat based on the thermal regime. This figure is available in colour online at wileyonlinelibrary.com/journal/espl

thermal mass, can enter rapidly (