Philosophy

THE RECENT SURVEY AND MONITORING TECHNIQUES OF THE SHORELINE EROSION

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During the past five years, both Electronics and survey technology have developed rapidly. Many techniques - especially positioning and echo survey systems - are improving very quickly. The present paper reviews some of the most recent survey and
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   THE RECENT SURVEY AND MONITORING TECHNIQUES OF THE SHORELINE EROSION Associate Prof  .   Mahmoud El-Mewafi , Associate Prof  .   Z. Zeidan   and   Dr.   A. Awad   Public Works Dept., Faculty of Engineering, Mansoura University, Egypt  ص :   ا د ظ و   تاوإ ا٥   ي   ع   ر   ثد   ا  ت ٥ ي .    دد   ع   ر   ثد   دف تا و   ا٥ راأ م د ي   اخو   ظأ ٦ه .     ا ق ثد    ض    ه   مدوا٥ راأ و   ا٥غو تا و    تا م دا   ئ  ى   ب و و   ٥ .     ةد      و   ج ز   و    س ر   ئ ش   ى   ا   ق   ض      و ( 1986 - 2000 ) . لد   ن دو   ه       ر   ا ئ  ي   ٥ 10 م / ي   ر   و   د و   ج    د ز طا٥ ب   كذو   ٥٥ ٦ه .    ا٥ ز    م د ةرو       اآ خ      ئ  ٦ه   ا  . ماظ   و دوئ  ٦ه   ى   ثد   دف    و      ن ئاش      ح   . Abstract During the past five years, both Electronics and survey technology have developed rapidly. Many techniques - especially positioning and echo survey systems - are improving very quickly. The present paper reviews some of the most recent survey and monitoring techniques commonly used today, such as Echo sounder, multibeam echo sounding,   total stations, Trisponder , and positioning systems. The rapid erosion rate in the some of Egyptian coasts has created a need for monitoring methods to design adequate and efficient protection technique for the shorelines. Three sites along the Egyptian coasts named Ras El-Bar, El-Gameel and Ezbet El-Borg coasts were studied for along time since 1986 to 1997. The erosion rate for the Egyptian coasts is estimated at 10.00 m/yr because of increased wave action and tidal effects. It makes the utilization of breakwaters, as a wave dampening technique, very favourable for the project sites. Monitoring of shoreline and bed topography after breakwaters construction showed that sediment was accumulating in nearly all the shoreline.  1.   INTRODUCTION Recently the increasing numbers of coastal communities, the governments, as well as forward-looking politicians, are recognizing that sandy beaches are good for the economy, as well as being beautiful. The beaches are the essence of Egypt, and the basis for much of the tourism income and jobs. The aim of this study is to discusses solutions of the coastal erosion and protection on retreating coastlines. The position of the shoreline along Coasts and around inland waters (lakes) varies over a broad spectrum of time scales in response to shoreline erosion (retreat) or accretion (advance). Shoreline position reflects the coastal sediment budget, and changes may indicate natural or human-induced effects alongshore or in nearby river catchments. The detailed shape and sedimentary character of a beach are highly sensitive to deep-water wave-energy, shallow water wave transformation, wave setup, storm surge, tides, and near shore circulation. Changes in the position of the shoreline affect transportation routes, coastal installations, communities, and ecosystems [Reid, 1984 -Fox & Davis, 1978]. The effects of shoreline erosion on coastal communities and structures can be drastic and costly. It is of paramount importance for coastal settlements to know if local shorelines are advancing, retreating or stable. Using survey techniques the following parameters are commonly monitored: 1-   Width of the dry beach, position of the mean water line, the high water line, or the base of the beach where well defined. However, measurements are subject to local variations in water level and sand storage. 2- Beach profiles along sequential transects normal to the shoreline. Best for evaluating seasonal or other short- term shoreline movements, and beach morphology. To help in understanding why shoreline change is occurring, it can be helpful to measure; Water levels, wind speed and direction, storm waves, and coastal currents; these can be related to shoreline change.    3- Losses or gains of sediment (sediment budget) in specific coastal compartments or cells. A sediment surplus is typically associated with an advancing shoreline, whereas a deficit may lead to shoreline retreat. 2. SURVEYING AND MONITORING TECHNIQUES OF THE SHORELINE EROSION To document annual shoreline movement, total stations, Trisponder & Echosouner and differential GPS approaches were used at specific points along the shoreline   (Coastal profiles)  and unspecific points under water to identify changes in coastal and seabed land elevation. The procedure of these methods, the difficulties encountered, and investigations of the reliability of the measurements will be discussed and explained. 2.1 TOTAL STATIONS METHOD Field mapping is the basic requirement for studying monitoring of shoreline and bed topography. Topographic mapping can be accomplished using total stations. Points can be collected offshore to depths of approximately 1 m to provide overlapping coverage with the bathymetry survey. The bathymetric survey points (data of water depths and coordinates of all points under water to depth of approximately -7 m) were also collected using a total stations. The measuring process consists of a shore-based total station, a boat-mounted transducer, a receiving unit, and a computer that controls the digital data collection process. The shore station data is radio-telemetered to the boat computer where depth-position data is calculated and automatically stored. The location of the boat is determined by targeting a reflective prism mounted directly above the transducer. Digital depth records are checked by comparison with the analogy sonar recording [Craenenbroeck & et al, 1998]. 2.2 TRISPONDER & SINGLE BEAM ECHO SOUNDER METHOD The measuring process of trisponder & single beam echo sounder method consists of shore- reflectors (receivers), a boat-mounted echo sounder, transducer, a digital remounting unit of trisponder, and a computer that controls the digital data collection  process. The depth-position data is calculated using echo sounder and automatically stored. The location of the boat is determined using resection technique directly by targeting a reflective prisms mounted above the fixed points on the shoreline [Reid, 1984]. 2.3 MULTIBEAM ECHO SOUNDING METHOD As is the case with the commonly used single-beam echo sounders, multibeam echo sounders also use short acoustic transmission pulses to scan the seabed. Each seabed element produces its own echo, which is received at the receiving part of the acoustic transducers. These echoes, together with sound travelling time and other system parameters, are processed into calculations of angle between the element on the seabed and the transducer normal. In this way, the horizontal distance and depth to each seabed element can be calculated. With multi-beam echo sounding, where the transducers are mounted in the bow of the boat lines were surveyed parallel to the axis of the boat. Moreover, with the multi-beam echo sounder, the progress of the shoreline could be observed clearly, which enabled the daily re-definition of the sedimentation areas. When a multi-beam system is built into the bow of the vessel, or mounted in the bottom, the wide coverage of the multi-beam makes it possible to "view" the seabed accurately on both sides. Making use of dedicated software, the cross-profiles of the seabed, can be shown on the computer three-dimensionally. Early results with multibeam echo sounding however identified a very small systematic error in the system, which resulted in areas of seabed, which were known to be flat taking on a very slight convex shape across the width of the multi-beam swath. The error was corrected by applying a very small adjustment in software of approximately seven thousandths of a degree (0.007°) [Jorgen, 1999]. 2.4 DIFFERENTIAL GPS METHOD   With the important increase of the accuracy of positioning systems for many civilian applications, the accuracy of the positioning of the shoreline monitoring increased drastically and was even more improved by the development of systems to monitor online the position of the shoreline monitoring with regard to the sediment (Figure 1).    Figure (1): Differential GPS rover receiver, combined single-beam echo sounders transducer . Global Positioning Systems (GPS), based on satellite transmission of microwave signals to surface receivers, whose positions can be determined with accuracies as great as 3-5 mm for short baselines (100 km) and 10-15 mm over longer distances (1000 km). To document annual shoreline movement, differential GPS approaches were used at specific points along the shoreline and unspecific points under water. Intervals between points will be determined based on shoreline configuration. It is recommended that at least 25 points be used, when possible, within a land type. The measuring process consists of a shore-based fixed receiver, a Zodiac (Surveying vassal) - mounted rover receiver unit, combined single-beam echo sounder transducer, and a computer that controls the data collection process. A particular positioning system was developed, where a high frequency echo-sounding system was mounted in an acoustic mirror mode at both sides of the Zodiac [Wu, 1993] During the monitoring operations, the positioning of the specific points along the shoreline and unspecific points under water DGPS systems, combined single-beam echo sounders are used. Datum conversion enables us to combine GPS measurements with the existing conventional measurements. It has advantages in establishing minor control points and finding outlier points in local datum since they can be done with efforts much less than when the conventional method is used. This has led us to establish converting processes from a global datum to local ones[Wu, 1993].
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