Mapping intertidal sediment distributions using the RoxAnn System, Dornoch Firth, NE Scotland

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1 Mapping intertidal sediment distributions using the RoxAnn System, Dornoch Firth, NE Scotland J. HULL & R. NUNNY Ambios Environmental Consultants Ltd, The Ferns, Kilkenny Ave, Taunton, Somerset TA2 7P J, UK Abstract: The RoxAnn system (manufactured by Marine Micro Systems Ltd) is a remote sensing technique available for mapping sediment distributions. The acoustic returns from a standard boat-mounted echo sounder are electronically processed and the output stream logged together with accurate position data from a Differential Global Positioning System (or similar high accuracy positioning systems). The RoxAnn output comprises depth and indices of bed roughness (El) and bed hardness (E2). The combination of these indices require to be translated into traditionally used parameters (such as visual appearance and sediment particle size) to provide useful mapping data. The most reliable means of achieving this is through a ground-truthing exercise, where grab samples and video footage are collected at sites of known RoxAnn values. Two problems are encountered in this process: firstly RoxAnn acoustic signatures often do not uniquely represent a specific bed type as described in traditional terms, secondly positioning errors can lead to poor calibration of the RoxAnn signatures in the ground truthing process. The first problem is difficult to correct, and can only be mitigated against through good survey design and execution. The second problem can be addressed in the data processing, and procedures for this are discussed. The RoxAnn data collected from the Dornoch Firth, used in conjunction with qualitative analysis of colour aerial photographs of the upper intertidal areas, provided a detailed classification of substrate types, including a classification of sands which allowed detailed insight into active processes of transport. Mapping distributions of sediment characteristics is a long-practised art, providing data basic to any appreciation of sedimentary processes, and often yielding much detailed information about the nature of the processes themselves. Remote sensing technologies available today enable us to move away from a process of guesswork around a matrix of spot samples into the realm of Spatially Continuous Mapping using spot sampling for ground truthing. Satellite imagery, aerial photography, swath bathymetry, side-scan sonar and RoxAnn-type systems are regularly in use to provide effectively Spatially Continuous Data on seabed characteristics. Each remote-sensing system maps its own suite of optical or acoustic parameters, and each varies in efficiency according to the conditions of the local environment and the objectives of the survey. This paper is concerned with the practical aspects of the use of the RoxAnn system, an acoustic methodology (Chivers et al. 1990; Collins & Voulgaris 1993), through reference to a study undertaken in 1996 of an estuary in the north-east of Scotland, the Dornoch Firth. This study was undertaken for Scottish National Heritage for conservation purposes. The aim of the study was to produce a biotope map of the intertidal and subtidal areas within the Firth. Biotopes, in the marine context, may be defined as 'the physical habitat together with its associated community of species'. The physical habitat can be broadly defined as a function of: (1) Depth: intertidal drying heights (times) and subtidal water depths. (2) Salinity. (3) Water column energies: tidal currents, riverdriven currents, wave energy. (4) Substratum: rock outcrops and sediments. A RoxAnn survey was employed to provide information about depth, water column energies (through resultant sediment characteristics, namely compaction and bed forms) and substratum. The Dornoch Firth (Fig. 1) has 44kin of tidal watercourse, although the survey was restricted to the downstream 25-30km. The width of the waterbody varies from 1 to 5 km. It is an area of strong tides, with a 4m tidal range, and much of the Firth is dry at low water springs and has large areas only shallowly inundated at high water springs. The water is turbid, due both to sediment suspension and peat discolouration of the fresh waters feeding the Firth. Field methods Mapping of the estuary posed logistical problems: it is large, some 25% of the area is always inundated with turbid water and the extensive HULL, J. & NUNNY, R Mapping intertidal sediment distributions using the RoxAnn system, Dornoch Firth, NE Scotland. In: BLACK, K. S., PATERSON, D. M. & CRAMP, A. (eds) Sedimentary Processes in the Intertidal Zone. Geological Society, London, Special Publications, 139,

2 268088, ,08 278~ t88, , , , ,88 ~ AR KINCARD BRIDGE AREA 1. e~50~0,00 Fig. 1. The Dornoch Firth, showing survey areas and RoxAnn trackplot. The National Grid is shown.

3 SEDIMENT MAPPING USING ROXANN 275 Source pulse F~st return Energw backsco, tered From seabed 9econd return,- "r~e Fig. 2. Graph of acoustic energy level (y) vs time (x) for an echo sounder output, showing the portions of the acoustic return quantified by the RoxAnn system. upper fiats are only shallowly inundated for short periods of time. The approach adopted was to: (1) use the RoxAnn system mounted in a very shallow draft vessel (capable of operating in lm of water), to survey the bulk of the Firth and (2) fly a 'qualitative' set of aerial photographs of the intertidal areas to provide a complementary data source, in particular providing information about the extremely shallow areas where a boat could not reach. This approach provided photographic data of the uppermost intertidal levels, photo and acoustic data of the middle intertidal levels, and acoustic data of the subtidal channel floor areas. Position fixing was by public broadcast Differential GPS throughout the exercise, giving an accuracy of +8 m. Ground truthing was achieved by visiting 80 intertidal sites, and collecting grab samples and underwater video footage from a further 64 intertidal and subtidal sites. Particle-size analyses (dry sieving at 0.5~b intervals) were undertaken of sediment samples taken from the grab. RoxAnn is an echo-sounder signal processing system (Nunny 1995). In this particular survey a 200kHz echo-sounder was used, this being highly sensitive to signals returning from the seabed sediment/water interface. With the RoxAnn system the following aspects of the returning acoustic signal (Fig. 2) are examined in an analogue fashion, and digitally recorded: (1) the time to first return, giving water depth; (2) the length of the tail associated with the first return, giving a seabed roughness index, annotated E 1; (3) the volume of the second return (first multiple), giving an index of the strength of the returning signal, and hence the seabed hardness, annotated E2. These three pieces of data, together with the time and the DGPS position, were logged at 2 s intervals. Survey lines were run at 200 m spacing (Fig. 1). Data analysis The successful utilization of RoxAnn data in seabed mapping requires care in two critical areas: (1) Positioning-errors in ground-truthing. The need for a high level of positioning accuracy during data collection and the taking into account of the effects of positioning errors when relating the acoustic data to the ground-truth observations. Table 1. Potential positioning errors Acoustic Grab/video Grab/video Potential data position vessel compound position DGPS location error (sum DGPS error error of individual error errors) ±8m +8m +10m 26m

4 i Downloaded from at Pennsylvania State University on March 4, J. HULL & R. NUNNY COMPOUNDING OF POSITIONING ERRORS i i i I I GROUND-TRUTH SAMPLING POSITION I! I APPARENT POSITIONS I ACCEPTABLE SAMPLING POSITION Fig. 3. Illustration of maximum potential compound error involved in relating ground-truth observations to the RoxAnn trackplot. I!... I I I ,9- I~ 0.8- == ~, 0.7- o.s o11 o12 o13 -~ 0.4 o '7 o18 o19 1'o 11 ' 1 ' E2 (Hardness) Fig. 4. Scatter plot of the means of RoxAnn E1 and E2 values in the vicinity of each ground-truth sampling site. One standard deviation error-bars are also plotted.

5 SEDIMENT MAPPING USING ROXANN 277 (2) Uniqueness of acoustic signal. The recognition that RoxAnn does not uniquely categorize sediments in terms of traditionally used parameters, e.g. particle-size characteristics. These are now dealt with in further detail. Positioning-errors in ground-truthing Attempts to calibrate RoxAnn prior to a survey by successively running the instrument over several areas of apparently known and uniform sediment types have met with little success (Broffey 1996). The alternative approach Stn 10 Stn ~.~ +o i _~ zo BLI E1 Cel 4O I~ 0, :If I}I Ill I I n I 0 ~ ('~ e') "q" M') SO r'.- CO 0"~ ','- ~- N ~ o o o o o ~; o c; o.... E2 Stn 41 Stn o ++ so i 4o ~ Ill El00,.~ , 20 0 o o o o o o o o 0 E2~ Fig. 5. Examples of frequency distribution histograms for two ground-truthing sites with large standard deviations. In processing, Station 10 was rejected as an unsuitable calibration site, and the mean E1 value at Station 41 was adjusted to equal one of the modal values. Station 41 Station I , +, ~, i i ~,,,i, i i !.6 El I I ', ~ ' 'I Fig. 6. Examples of the variation between succesmve E1 and E2 values recorded at two ground-truthing sites with large standard deviations (see Fig. 5).

6 278 J. HULL & R. NUNNY involves producing an xy scatterplot of the E1 and E2 values associated with each of a large number of grab/video ground-truthing sites visited during the survey, and then producing sub-divisions of the graph area based upon the observed groupings of the various types of substratum. The inter-relation of acoustic and groundtruth data in this fashion relies heavily upon accurate position fixing. Compound errors may build during sampling ( Table 1, Fig. 3). Thus, when a position is located on the acoustic trackplot, and ground truth data collected from that location, the actual truthing position may be up to m away from the originally identified point. To take account of this potential variability, the RoxAnn values along the vessel's course for an equivalent distance either side of the selected location should ideally be examined. This procedure gives insight into the potential effect of positioning errors in the data calibration process, and the heterogeneity of the sediment body at each ground-truthing site. During the Dornoch survey acoustic variability at each sampling site was quantified by determining the mean and standard deviation of the twenty data points (~30m) centred on the theoretical sample location (Fig. 4), by producing a histogram showing the frequency distribution of these twenty E1 and E2 values (Fig. 5), and by plotting the succession of the twenty individual E1 and E2 values as an xy graph (Fig. 6). These three types of plot provided a more informed basis from which to embark upon the calibration process than simply the raw E1 and E2 values from the theoretical sampling location. A scatter plot of the mean E1 (y-axis) and E2 (x-axis) values from each ground-truthing station was produced, coded according to particle-size content (%mud, %sand, %gravel, modal grain size of sand population), and/or visual description in the case of rock, boulder or coarse 'lag' deposits. These plots were contoured or blockcategorized accordingly, using a contouring package with a kriging routine for the former. Each point on this calibration scatter-plot was then examined individually. Where the standard deviations of the El/E2 values were small, and where the histogram frequency distribution showed a 'normal' situation, confidence was given to the individual point, and particle-size content isolines and/or block categories consistent with these data points accepted. Where, however, standard deviations were large, the mean values of El/E2 could not be used with confidence in the calibration process. Two situations become apparent when examining the histograms for calibration points with large standard deviations: (1) A wide scatter of RoxAnn values existed, suggesting a highly variable seabed on a small scale. Data points of this nature were not considered suitable for calibration purposes, and were abandoned. (2) The data indicated the presence of two (or more) seabed reflector types within the sampled area, i.e. the presence of two (or more) modal values for El/E2. In this instance it was considered realistic to adopt the El/E2 modal value that best fitted the calibration pattern established using data points with an associated high level of confidence (low standard deviations). A t, t.e t ,, 1} 0.8- e 0.6- I A W % MUD % GRAVEL 6O5o,o.,o /~ o- ~,~,~ _1~ 2.2 OI :1 o.e- ze ' ~ 0.6- IL[]I " 0.2- SAND 2.8 MODE (phi) 0,0..., ' ' ~4 118 E2 (Hardness) Fig. 7. RoxAnn EIE2 scatter plots contoured for %gravel and %mud values (upper) and sand grain population mode in phi units (lower) for data from sedimentary ground-truthing sites in the Dornoch Firth.

7 SEDIMENT MAPPING USING ROXANN 279 Through an iterative process, the original calibration scatter plots and fitted seabed-type distributions were reworked, removing or modifying rogue points and generally allowing a simpler pattern to emerge (Fig. 7). Finally, the El/E2 scatter plot was 'boxed' into seabed-type classification based on a combination of particlesize and visual appearance (Fig. 8). Uniqueness of acoustic signal A problem basic to all Spatially Continuous Mapping is a traditional reliance upon particlesize and visual appearance in classification of seabed types, whereas remote sensing methods map other parameters. In the case of RoxAnn, aspects of the acoustic reflectivity of the seabed are being mapped, viz the local variability in backscatter (El, equating to physical roughness) and sound-absorptive properties (E2, equating to physical hardness). Although grain-size and bed morphology play a role in determining roughness and hardness of the seabed, other factors such as sediment compactness (history of disturbance) play an equal role. Thus it frequently happens that sediments with a similar RoxAnn signature have very different visual appearance and particle-size content. There are three steps which can be practically applied to mitigate this problem. (1) The collection of a large number of ground truth samples, with several samples from each acoustic type, thus giving scope for identification of differences. (2) The initially production of calibration diagrams for areas of similar water energies, e.g. upper estuary, lower estuary, sea coast. If these prove to be compatible they may be amalgamated at a later stage, but it is commonly found that parts of the calibrations will vary between such areas i i I tn (1) t" t- t~ o V LU 0.8~; "! i o.o~o.o E2 (hardness) Fig. 8. RoxAnn calibration diagram showing estuary-bed categories mapped in the Dornoch Firth.

8 280 J. HULL & R. NUNNY I ~ DORNOCH _ MIDDLE,ESTUARY RUBHA~NAN~SOHARBH:TO,NEW,BRIDGE 88~-gSg I.J t ~ @ Fig. 9. Example of substrate map produced for Area 3 (see Fig. 1) in the Dornoch Firth. Classification key is provided in Table 2. (3) Acceptance that it may be necessary to revisit sites to further ground truth areas of acoustic signal where ambiguity of classification may exist. In the Dornoch Firth two situations of acoustic signature 'overlay' were encountered. The first posed little problem as the sediment areas were clearly geographically separate: areas of very soft (mobile) medium/fine sands on the sea beaches outside the estuary had signatures similar to sandy muds within the estuary. The other situation involved three substrate types with similar acoustic signatures (Fig. 8) and sometimes overlapping geographical extents: (1) Channel floor mussel/gravel areas. (2) Upper intertidal rock/brown algae areas. (3) Fans of fine-gravels located in upper mudflat areas where small rivers discharge into the estuary, thought to be introduced during winter spates. Fortunately area (1) could be differentiated from areas (2) and (3) on the basis of bathymetry, and areas (2) and (3) were differentiable with reference to the aerial photographs. On completion of the calibration process, the RoxAnn El and E2 data from all the surveyed tracks were contoured (using a contouring package with a Kriging routine) to produce maps of blocks of the estuary, and these maps then combined according to the E1 and E2 groupings identified in Fig. 8, to produce a map of sediment distributions. Results From the RoxAnn data, complemented in the upper intertidal areas by colour aerial photographs, it was possible to produce a quite detailed substratum classification system and map of the Firth. The classification system related quite clearly to water energies and sedimentary processes, as identified in the Table 2. An example map of a portion of the estuary is shown in Fig. 9, illustrating the level of detailed mapping possible using this system.

9 SEDIMENT MAPPING USING ROXANN 281 Table 2. Estuary bed classification system derived for the Dornoch Firth based on aerial photography and RoxAnn survey information Aerial photo data Stable sediments. Characterized by a dark coloration relating to fines/ organic content and importantly a surface biological film. Generally smooth texture but often algal patches where cobbles or other hotdfasts available. Commonly exhibits range of drainage rill patterns. Muds, muddy sands and sands. Mobile sands without bedforms. Characterized by light colour. Commonly contain small-scale rippled surfaces below the resolution of the camera. RoxAnn data! Mud. >30% sediment finer than 63#m Muddy sand % mud, 70-90% sand. II ~ / z Sand. >90% sand (2mm - 63~m) / ~ ; Suspension / V/ / sands (mean <200#m) Fallout zones Low energy stable sand areas Recirculation zones High tidal energy sand shoals Mobile sands with bedforms. Waveforms indicating regular (tidal) transport of sediments. Irregularly mobile sands with bedforms. Light coloured but less well-defined bedforms with dark trough floors of stable sediment/biological film. (not visible) As Stable sediments above. Rock. Structure visible. (not visible) \\ \ \\ \\ \\ \ "evq ~ o 1 Io( o o Medium sands (mean #m) Coarse (bedload) sands (mean >500txm, channel floor sands) X ~ Gravelly sand % gravel, 60-90% sand. X N ;~ X (Channel floor deposits, intertidal fluvial fans XX and gravel spreads). XX K Rock. - Scoured cobble pavements. m (Channel floor areas) m (not visible) ~ Mussel beds Conclusions The RoxAnn seabed mapping system was originally marketed principally as a tool for the marine fishing industry working in coastal and offshore waters. Studies such as this work in the Dornoch Firth show that the system can be used to provide detailed maps of substrate characteristics in shallowly inundated (intertidal) areas. To enable translation of the acoustic parameters measured into traditionally recognized descriptions of the seabed careful groundtruthing is required, which relies upon very accurate position fixing and the application of a rigorous approach to the calibration process. Through reference to the characteristics of

10 282 J. HULL & R. NUNNY grain-size populations, the map data can reveal much about sand transport processes active within the mapped area. References BROFFEY, M A Study of Coastal Processes and Material Transportation at Kingsdown, Kent. Dept of Ocean Sciences, University of Plymouth, Year 3 Dissertation. CHIVERS, R. C, EMERSON, N. & BURNS, D New acoustic processing for underway surveying. The Hydrographic Journal, April COLLINS, M. B. VOULGARIS, G Empirical Field and Laboratory Evaluation of a Real-Time Acoustic Seabed Surveying System. Proceedings of the Institute of Acoustics, 15(2). NUNNY, R. S The Potential for Small-Boat Mounted Acoustic Seabed Mapping Systems in Tropical Marine Habitats. University College of Belize Marine Research Centre Technical Report Series 1.