Coastal Monitoring. Heather Ann Nicholson

Transcription

1 Heather Ann Nicholson

2 Abstract Coastal monitoring is just one of the many uses for information that has been gathered through hydrographic surveying techniques. Hydrographic surveying equipment has evolved from a simple graduated pole and sextant to state-of-the-art remote sensing techniques. As equipment has evolved and post processing techniques have been developed, accuracy has increased. Introduction The International Hydrographic Organization (IHO) defines hydrography as: the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary purpose of safety of navigation and in support of all other marine activities, including economic development, security and defense, scientific research, and environmental protection. Accurate hydrographic surveying for coastal monitoring is essential for a wide range of scientific endeavors including: coastal flood zone modeling, estimating storm and tidal surges, quantifying volumes of sand movement due to erosion and accretion, studying marine currents and surface circulations [3][14][15]. Coastal monitoring provides quantitative data that can be used to make better informed decisions [15]. A simple example to illustrate this deals with tracking coastal erosion and accretion. Coastal erosion and accretion are naturally occurring processes that have only recently become a major concern because of the number of people who inhabit the coastal regions of the world [26]. Simple procedures like dredging and beach reclamation affect the natural rates of erosion or accretion [12]. Rising ocean levels, due in part to climate change, have compounded the need for coastal monitoring [18]. Without accurate data that compares the elevations of the land to the rise in ocean tides, emergency agencies could not plan evacuation routes for coastal communities in case of natural disasters. This case exemplifies the vitality of continuous coastal monitoring in areas of extreme vulnerability not only for environmental concerns, but also for the safety and well-being of coastal inhabitants. P a g e 2

3 Figure 1 Sextant ( Historic Procedures In 1834, the first official hydrographic survey in the United States was conducted [28]. Historically, depths were measured by sounding poles, in shallow regions, and weighted ropes, in deeper areas, and positions were determined by three-point sextants [28]. A sextant is shown in Figure 1. Coastal environments are especially challenging to surveys using traditional surveying equipment and procedures [26]. Some regions are difficult to survey because they are hazardous and difficult to access due to sheer cliffs or dangerous tides [26]. Traditional surveying methods also require fixed control points, which do not lend themselves to dynamic coastal environments [15]. To monitor historic trends, large areas, both from the shoreline to a fixed base line and nearshore hydrographic regions, are required to be surveyed at regular intervals [31]. These are just a few of the reasons that hydrographic surveyors are relying more and more on developing technologies to accurately monitor coastlines. Current Procedures Overview Coastal monitoring is a complex process that requires the combination of spatial data from a wide range of remote sensing and traditional surveying techniques [23]. It often involves the marriage of geophysics, physical oceanography, ocean acoustics, cartography, and marine geology [37]. Coastal monitoring and mapping is becoming a critical aspect of a wide range of issues like safe navigation, resource management, environmental protection, and coastal planning [2]. One of the major complications of coastal monitoring is determining where the P a g e 3

4 water ends and the land begins, this is partly due to the dynamic nature of shallow coastal waters [15]. To make sound determinations on coastal change, it is necessary to separate land surface height changes and sea level changes and then place both in the same global reference frame [25]. By comparing historic data with current data, trends can be documented [15]. The ever changing, dynamic nature of shallow coastlines also requires constant resurveying in order for nautical charts to be up to date [12]. These nautical charts are created from bathymetric surveys, the equivalent of an underwater topographic survey [12]. Hydrographic surveys require both precise horizontal positioning and vertical depth to the ocean floor [28]. Horizontal position is often calculated relative to the position of a surface vessel with GNSS equipment [8]. If ocean based, it is also necessary to know the motion of the vessel (roll, pitch, heave) and the bottom depth, which is needed to calculate required frequency [8]. Vertical position is established using a wide variety of systems including SONAR and RADAR. Data Processing Coastal surveying requires the merging of data from where the land meets the ocean. It is essential to connect data about landslipping, erosion, and climate change because what effects the coastal land formations will also affect the near-shore environment, and vice versa [15]. Frequently, water depth are represented as the lowest mean water level that is further reduced to allow for a margin of error, because of the dynamics of the ocean, that is necessary for safe navigation [12]. The shallowest likely depth is combined with the estimated rate of change, the shoaling rate, to give a better overall picture of the ocean bottom [12]. Shoaling is the term used to describe the process when a wave enters a shallow area the wave increase in height [12]. One pressing challenges in integrating a variety of surveying techniques is transforming the data into a common datum [26]. All hydrographic data must have its vertical position referenced to a common datum, and its horizontal position recorded, most often in latitude and longitude recorded from GNSS [27]. Surveying precision and accuracy is of the utmost importance because nautical charts and maps are only as good as the data used to make them [13]. It is also necessary to quantifying all uncertainties [11]. Least squares are utilized to calculate the unknown parameter and to preform statistical computations [11]. One example of the many uses of standard statistical testing of hydrographic surveying data is to monitor deformation of the ocean bottom [12]. Deformation analysis is performed by using least squares and statistical analysis techniques. Morphological parameters, ie. wave amplitude, are denoted by u and their associated errors û are used to create a stochastic model [12]. The associated covariance matrix is denoted Cu [12]. Therefore, the overall deformation analysis is modeled using the equation N = U + V [12]. Statistical analysis is then used to create confidence intervals. P a g e 4

5 Routinely, computer programs are used to process the vast amounts of hydrographic data. Surface matching is a software solution that uses least squares to register datasets, and then it performs 3D coordinate transformations to align all the dataset into one 3D coordinate system [26]. In order to do this, the software performs three rotations, three translations, and three scale factors [26]. Utilizing least squares allows for the comparison of residuals. Higher residuals are found in areas where the datasets are not in agreement [26]. Least squares also allows for the use of a weight matrix, which would put less accuracy on less reliable observations [26]. Figure 2 DEM of the Elizabeth Islands, Massachusetts. Computer Models Once all the data has been processed, visualization is just as important because often a picture can convey a complex procedure better than a one-hundred page paper. Digital elevation models (DEM), are created by combining all available relevant data (Figure 2). The DEM is created by interpolating data between points of different density [5]. A DEM is used to determine an accurate model of the bathymetry, and requires accurate positional data and corrected depths [5]. Data is geo-referenced by using known ground control points and then imported into photogrammetric mapping software [37]. Today s complex models require accurate data about: rock types, water table levels, storm frequency, beach thickness, wave height, wind direction, which are collected by a wide range of equipment and surveying procedures [15]. Improvements in coastal monitoring have allowed scientists to accurately model and predict geological changes and hazards that effect coastlines [26]. P a g e 5

6 Geographic Information Systems (GIS) are used for storing, integrating, analyzing and visualizing hydrographic data that has been compiled from a wide variety of sources [32]. GIS is also an invaluable tool used to model the dynamics of coastlines and monitor historic trends [2]. With its every increasing capability, GIS software is also able to predict coastal changes by using Demographic Spatial Analysis and Modeling (DSAS) [2]. DSAS is a cutting-edge technology that allows for three-dimensional georeferenced models. Vertical and Horizontal Positioning Systems Global Positioning System GPS One of the first modern ocean navigation systems called TRANSIT, designed in 1960, used the Doppler Effect, which measures the change in frequencies on transmission signals [34]. Building on this technology, a system called TIMATION was built, which grew into modern day Global Positioning System (GPS) [34]. GPS works on the simple mathematical principle of threedimensional trilateration. GPS has significantly improved from its original design, which was only accurate to the tens of meters [34]. Today, GPS is the primary system used to determine horizontal position. GPS is also being utilized for the continuous monitoring of areas with high deformation [26]. Remote methods, which include a combination of LiDAR and GPS, are being used because of their accuracy, flexibility, and overall ability to collect data in a highly mobile coastal environment [15]. Currently, real-time kinematic GPS (RTK-GPS) is also used to establish ground control [9]. Figure 3 Single and multibeam sonar. (Aykut, Akpmar and Aydin 2013) Sound Navigation and Ranging Sonar SONAR was first used in 1906 and quickly advanced as a result of the Titanic disaster and WWI [34]. SONAR has now developed into a precise remote-sensing system. It became evident, during P a g e 6

7 the Cold War, that in order to effectively use sonar specific water conditions need to be monitored [34]. To compute the corrected distance the velocity of sound propagation in the water column and the difference in position between the SONAR and the datum must be known [5]. In order to increase the accuracy of depth measurements, proper calibration is essential because the speed of sound in water is affected by temperature, chemical composition, and depth [34]. As shown in Figure 3 there are two major types of sonar, single beam and multibeam. Single beam sonar was used from , some applications still use single beam today [27]. Single beam sonar measures the strength of the returning signal to create a picture of the ocean bottom [28]. The picture is black and white, black for areas of little return and white for areas of high return [28]. Single beam sonar is more effective in relatively shallow and flat areas [28]. It is effective at locating underwater obstructions but cannot determine the water depth [28]. Multibeam sonar has been used from the 1990 s to the present [27]. The U.S. Geological Survey (USGS) often maps coastal regions using multi-beam SONAR systems [31]. Multibeam sonar measures and records the time it takes the signal to return [28]. The image created by multibeam sonar looks like a color picture because the return time is color coded by a multispectral scale [28]. 3D multi-beam sonar systems are capable of generating a photo quality picture [34]. Today it requires a combination of navigation and orientation sensors to link the SONAR data with a specific point upon the ocean bottom. GPS is connected above the water to the sonar system and to the ship so that the sonar s position relative to the ship can be calculated [34]. One major drawback is that positions are only georeferenced to the sensor on the boat [34]. Another drawback of any SONAR system is that they are ineffective in very shallow water [31]. Light Detection and Ranging LiDAR Since 1990, airborne LiDAR systems have revolutionized the way coastal surveys are completed [31]. Two government agencies that utilize LiDAR are the National Oceanic and Atmospheric Administration and the U.S. Army Corps of Engineers. NOAA uses airborne LiDAR to survey rugged and complex coastlines [28]. The U.S. Army Corps of Engineers utilizes SHOALS (Scanning Hydrographic Operational Airborne LiDAR Survey) systems to map coastal areas [31]. The SHOALS system was built by Optech Inc., of Toronto, Ontario, Canada to provide a completely seamless and rapid survey that bridges the gap in data where the shore meets the water [31]. The SHOALS system is capable of separating the water surface from the water bottom [23]. P a g e 7

8 Figure 4 Principles of LiDAR for hydrographic surveying. (National Oceanic and Atmopsheric Administration 2013) LiDAR has the capacity to scan one of the hardest areas to survey, where the land meets the water. The LiDAR point cloud is used to develop a digital elevation model, which joins bathymetric and topographic data into a common datum. Bathymetric LiDAR use laser pulses, one infrared, another green, and measures the time delay in the return signal [28]. As shown in Figure 4, the infrared signal gets reflected off the water where the green signal penetrates to the ocean bottom [28]. From these two measurements, water depth and coastal elevation can be determined [28]. LiDAR has the capability to pinpoint areas of very small variations on the ocean floor [17]. It is also an invaluable tool for delineating coastal wetlands [9]. By integrating data from LiDAR with aerial photography scientists are able to monitor changes in bathymetry as well as calculate the amount of sediment shifting due to erosion and accretion [31]. It has become common to combine airborne and boat-mounted LiDAR to map both shore and near-shore environments [34]. Linking Global Positioning Systems (GPS) with Internal Navigation Systems (INSP) allows for the accurate 3D location of LiDAR data on the ground [23]. LiDAR provides a point cloud that can be used to create a true 3D model of the area. The model can be used to calculate distances, areas, and volumes and well as calculating the slope stability of cliffs [15]. When using LiDAR it is essential to avoid gaps in the data coverage. One way to avoid these holes is to scan the same area from multiple viewpoints [15]. LiDAR has its limitations, it is faster than sonar and less expensive, but can only be used in shallow-water areas when the skies are clear [24]. Satellite Systems Technology has revolutionized the ability to use remote sensing to accurately monitor coastlines [4]. Since the 1970 s, significant research has been done on the ability to use satellite imagery for bathymetric surveying [30]. Satellite sensors are able to provide a near real-time capability to monitor the biochemical, optical, and physical processes of coastal and open oceans [4]. By P a g e 8

9 using multiple satellite images, a surveyor is able to calculate the rate of coastal change [3]. Satellite observations, taken over the last 20 years, have shown that most of the world s oceans are rising at by 3mm/yr [6]. The latest innovation in satellite altimetry uses high frequency signals that are capable of monitoring small ocean level changes along coastlines [6]. However, this requires the proper classification of multispectral images [23]. There are several satellite systems that provide data that is useful for coastal monitoring and hydrographic surveying in general. These include IKONOS, Landsat, Gravity Recovery and Climate Experiment (GRACE), and Jason/Poseidon. IKONOS, a commercial satellite remote sensing system owned by Lockheed Martin, combines images and LiDAR elevation data to allow for more accurate coastal monitoring [22]. Jason/Poseidon is an oceanographic system that monitors ocean conditions. Jason/Poseidon satellites are able to map 95% of the Earth s oceans every 10 days by bouncing microwave pulses off the water and measuring the time it takes to return the signal [6]. The GRACE satellites measure arctic ice, which can help to predict changes in ocean levels due to melting [6]. Landsat, Ikonos, and several other satellites have the necessary equipment on board to provide an accurate picture of ocean conditions [30]. Like more traditional systems, satellites have several limitations. Satellite data are vulnerable to cloud cover and have inferior resolution when compared with aerial photography [26]. Satellite derived bathometric data is only accurate in areas with high water clarity/low turbidity [30]. The data is not accurate enough to meet hydrographic surveying standard established by the International Hydrographic Organization (IHO), but they are useful for planning and establishing areas of specific concerns for resurvey [30]. Synthetic Aperture Radar SAR One major area of research in coastal monitory is rooted in climate change. Synthetic Aperture Radar (SAR) is being used to track and monitor polar ice sheets [24]. SAR allows for large areas, up to 500 km wide swaths, to be imaged at high resolutions [33]. SAR is either mounted on aircrafts or spacecraft and can operate in all types of weather, both day and night [24]. SAR works similar to ordinary radar, the distance to an object determined by measuring the time it takes for the pulse to be returned [33]. What sets SAR apart is its ability to add another dimension, called azimuth, which is perpendicular to the range (Sandia National Laboratories 2013). With repeated coverage, high-resolution SAR instruments provide the most efficient means to monitor and study the changes in important elements of the marine environment. [24, p.210] P a g e 9

10 Figure 5 Tracking coastal changes with aerial photography. ( Complementary Systems Aerial Photography and Video Aerial photographs have been used, since the early 1960 s, as an effective way to monitor coastal change [26]. To accurately estimate shoreline migrations, a combination of topographic maps, nautical charts, and aerial photographs are used [37]. The number of geological hazards including rock falls, landslides, and coastal erosion are better understood through comparing historic and current survey data [26]. As shown in Figure 5, aerial photographs can provide the historic records used to monitor coastal changes [26]. Video monitoring of nearshore environments are being used to determine beach slope, nearshore bathymetry, wave period and direction, and rip current dynamics [29]. Current video monitoring will also be useful for historic comparisons in the future. However, like any surveying technique aerial photography has some limitations including; proper camera calibration, adequate ground control, and weather related issues that can diminish the clarity of the photograph [26]. Figure 6 Hydrographic buoy. (National Oceanic and Atmopsheric Administration 2013) P a g e 10

11 Buoys and Tidal Gauges To study the dynamic of the coastline, data on waves, currents, and metamorphic sedimentation transportation is needed [29]. Buoys can include sensors to collect oceanographic parameters including: current direction and speed, wave frequency and speed, and wind speed and direction [14]. The latest technology in monitoring buoys are capable of monitoring turbidity, current direction and speed, temperature, coupled with GPS [16]. A monitoring buoy is shown in Figure 6. There is current research in the testing of GNSS-based tide gauges that would measure the ocean height from multiple satellites, which should provide a highly accurate picture of the average ocean surface height at the given position [25]. Combined Systems Often it is impossible to adequately monitor coastal dynamic environments using one technique. Historically, surveyors have used aerial photography, field surveying, and remote sensing to monitor dynamic coastlines [3]. By the very nature of the environment, it is necessary to combine data from a variety of sources to accurately model coastal areas [35]. There is the capabilities to seamlessly merge data from traditional hydrographic surveys, using sonar, with a wide range of remote sensing data collected using ground and airborne LiDAR [34]. Rear Admiral Gerd Glang of the US National Oceanic and Atmospheric Administration (NOAA) sees the future of hydrographic surveying relying more on an extensive network of trusted partners, volunteers who collect and upload bathymetric data, to provide much of the data necessary to update nautical charts [13]. Figure 7 An autonomous unmanned surface vehicle for hydrographic surveys. P a g e 11

12 Emerging Technology There is a major push to establish a real-time remote-sensing coastal monitoring system. Several companies have designed and developed autonomous robots capable of gathering water samples and uploading the data remotely [34]. The latest developments combined all the most cutting-edge technologies into small, autonomous vehicles that are capable of collecting data at nearly the same capacity as large hydrographic surveying vessels but at a fraction of the cost [34]. Figure 7 shows an autonomous robot with these capabilities. These systems can be equipped with SONAR for mapping close to shore as well as in hard to reach places that may be hazardous [34]. CDL engineering, out of Aberdeen Scotland, has developed the world s first 3D scanning subsea robotic system [7]. This technology may soon be used on all hydrographic vessels. Figure 8 Phase-differencing bathymetric sonar swath. ( Phase-Differencing Bathymetric Sonar (PDBS), also called interferometric sonar, is able to calculate precise position by measuring the differences between several phases and combine them with range calculations [27]. PDBS utilizes wide-swath sonar that is effective in shallow areas, less than 20-m deep [27]. As shown in Figure 8, PDBS is capable of generating an accurate three-dimensional model of the seafloor [27]. PDBS has been tested and proven that it can provide high-quality data in an efficient manner [27]. However, PDBS is still in the testing phase. Conclusion Hydrographic surveying for coastal monitoring has evolved from a low-tech, labor intensive process to a state-of-the-art, ever changing procedure. It is becoming more and more reliant on the latest engineering developments in order to lower costs while increasing effectiveness. While new technology is constantly being developed and tested, it is important to remember that all hydrographic surveying is the simple location of a finite point in three-dimensional space. P a g e 12

13 Coastal monitoring is a complex and costly endeavor, but one that is vitally important. Social scientists have estimated that around 300 million people live in coastal and low-lying areas that will be directly affected by flooding from sea level variations in the coming century. A rise in ocean levels, as small as 2 mm per year, will cause increased coastal flooding, and more powerful waves that will cause an escalation in erosion rates. Like most technology-based processes, coastal monitoring advances exponentially. The sky is truly the limit. In the not so distant future all hydrographic surveys will be compiled in real-time, remotely, and visualization will be accessible over the web. However, traditional surveying methods will more than likely still be used to validate control points. References [1] Ackermann, F. "Airborne Laser Scanning - Present Status and Future Expectations." ISPRS Journal of Photogrammetry and Remote Sensing, 1999: [2] Ahmad, Sajid Rashid, and V. Chris Lakhan. "GIS-Based analysis and Modeling of Coastline Advance and Retreat Along the Coast of Guyana." Marine Geodesy, 2012: [3] Alhin, Khaldoun Abu, and Irmgard Niemeyer. " Using Remote Sensing and Geoinformation Systems: Estimation of Erosion and Accretion Rates Along Gaza Coastline." IEEE, 2009: [4] Arnone, Robert A., and Arthur R. Parsons. "Real-time use of Ocean color Remote Sensing for." Chap. 14 in Remote Sensing of Coastal Aquatic Environments, edited by Richard L Miller, Carlos E. Del Castillo and Brent A. McKee, [5] Aykut, Nedim Onur, Burak Akpmar, and Omer Aydin. "Hydrographic data modeling methods for determing precise seafloor topography." Comput Geosci, 2013: [6] BBC News. "Satellites trace sea level change." September 24, [7] CDL Inertial Engineering. Inertial Engineering. October (accessed October 22, 2013). [8] Clarke, John E. Hughes. "Introduction to Geomatics: Hydrography/Ocean Mapping." University of New Brunswick, October [9] Corbley, Kevin. "Where the Water Ends." Point of Beginning, May [10] Cowell, Peter J., and Thomas G. Zeng. "Integrating Uncertaintly Theories with GIS for Modeling Coastal Hazards of Climate Change." Marine Geodesy, 2003: P a g e 13

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