Beach monitoring with GPS William F Price Department of Civil Engineering, University of Brighton, BRIGHTON, BN1 4GJ, UK

Transcription

1 Beach monitoring with GPS William F Price Department of Civil Engineering, University of Brighton, BRIGHTON, BN1 4GJ, UK Abstract In common with many other countries that have an extended coastline, sea defence work in the UK involves much time and expense. For these projects, topographical information is usually required to represent beach and other areas for modelling and design purposes and to record the response of a sea defence once implemented. This survey information often has to be collected in a hostile environment in a relatively short time and this can be difficult to do with conventional surveying equipment such as levels and total stations. However, with the advent of the Global Positioning System (GPS), surveying equipment is now available that offers the ability to gather survey data at a fast rate that suits coastal and other inshore work. This paper gives details of some GPS surveying carried out by the University of Brighton for beach monitoring work and describes the field procedures and data processing carried out. hi addition, the results obtained are discussed and future recommendations given. 1 Introduction In recent years, many flood defence schemes have been planned for protecting Britain's coastline and one such scheme is the Shoreham to Lancing Sea Defences [1], which are situated between Brighton and Worthing on the south coast of England (Figure 1). The cost of these works is expected to be 14 million over the next ten years. Under the direction of the Environment Agency in the UK, Phase 1 of these sea defences has involved the design and construction of three 50 m rock groynes on the beach at Lancing, one of which is shown in Figure 2. In order to monitor the effect on the beach area surrounding these groynes after

2 Computer Methods and Experimental Measurements Figure 1: Location of Lancing Beach between Shoreham and Worthing Figure 2: Typical rock groyne

3 Computer Methods and Experimental Measurements 553 construction, monitoring surveys were carried out by the consultant responsible for the construction of Phase 1 during the period October-December From January 1996, these surveys were carried out by the Department of Civil Engineering, University of Brighton (UoB). The specification for the UoB stated that for consistency, the same crosssections that the consultant had been monitoring should continue to be monitored and that all survey data was to be collected with a tolerance of ± 100 mm both horizontally and vertically. 2 Data acquisition and processing The original method used by the consultant for monitoring at Lancing involved taking measurements directly along a number of cross-sections specified for an 800m segment of the beach as shown in Figure 3. Figure 3: Positions of cross-sections monitored along Lancing Beach This involved locating the start position of each section and then taking measurements from these, with conventional surveying equipment, along the direction of each section. Although this is capable of producing satisfactory results it is a rather slow process and it limits the data available, at each survey, to the beach cross-sections only. Because of this, the method of data capture was changed by the UoB and XYZ coordinates (plan and height information) are now obtained using GPS equipment such as to cover the entire beach at random but close intervals that do not follow any predetermined cross-sections.

4 554 Computer Methods and Experimental Measurements This approach has the advantage of providing a complete record of the beach every time it is surveyed from which it is possible to produce a Digital Terrain Model (DTM). The DTM is used to generate the required cross-sections and other spatial data can be extracted if required at any point and scale on the beach. For all surveys, GPS rather than conventional surveying equipment is used as this enables a large number of points to be surveyed which in turn gives rise to an improved and more reliable DTM. All the XYZ data for each survey has been acquired using a Wild GPS- System 200. This uses two single frequency, 6-channel receivers and has a nominal baseline accuracy of 5 mm to 10 mm + 2 ppm for static surveys and 10 mm to 30 mm + 2 ppm when used in kinematic mode. The surveys themselves comprised two parts: the transfer of control from Ordnance Survey (OS) stations to establish reference points next to the beach at Lancing and the surveys of the beach at regular time intervals. In order to establish the first GPS station at Lancing, the System 200 was used in static mode and GPS satellite data was collected with the reference receiver located next to the beach as in Figure 4 and with the rover located at three nearby OS stations in turn. Figure 4: Wild GPS System-200 at reference station In common with all other GPS equipment, the initial XYZ coordinates obtained for any point surveyed with the System 200 are on the WGS84 coordinate system and it is necessary to transform these to local coordinates in

5 Computer Methods and Experimental Measurements 555 the UK in the form of National Grid (NG) eastings and northings and heights relative to the Ordnance Datum (or mean sea level). At Lancing, this was made possible by knowing the OS coordinates and heights of the three rover stations used in the static survey as well as their observed WGS84 values. Some care is needed when deriving transformation parameters for a NG or any other survey and it was decided to use a Bursa-Wolf 4-parameter transformation to shift the origin of the WGS84 system to the NG origin and to allow for a scale change between the two systems. Using these transformation parameters and the relevant GPS baseline data, the NG coordinates for the GPS reference station at Lancing Beach were computed. In addition to this, a second GPS station had to be established at Lancing, and this was done by taking further static readings, but between the two beach stations in this case. A check on the elevations obtained for the GPS stations at Lancing was carried out by levelling to these from two nearby OS bench marks. The results obtained by levelling agreed exactly with the GPS results. Full surveys of the beach at Lancing have been carried out at regular intervals since January Because of the need to gather as much detail as possible of the beach topography, the surveys are always completed within a period of about two hours before and after low tide. All satellite data is observed using the System 200 in kinematic mode: this has been chosen as it enables surveys to be carried out at walking pace with the equipment backpacked as shown in Figure 5, this being the most efficient and comfortable method for manoeuvring on foot around the beach. Figure 5: GPS equipment back-packed for kinematic survey of beach

6 556 Computer Methods and Experimental Measurements As the longest baseline from the GPS stations to any other points to be surveyed on the beach is less than 1 km, the accuracy for the GPS coordinates is expected to be 10 mm to 30 mm horizontally and up to 2-3 times this for heights, figures within the tolerance of 100 mm specified for these for the contract. For each survey, the reference receiver is located throughout at one of the GPS stations and the other is used by the roving (back-packed) receiver as a start point to initialise each GPS kinematic chain. The sampling rate for the kinematic chains is set at 4 seconds and, at a steady walking pace, this seems to provide the optimum rate for obtaining data on the more uniform parts of the beach accepting that it is sometimes necessary to wait at changes of slope and at irregular points for each 4 second epoch to occur. As the System 200 relies on post-processing of results, all the field data is processed immediately following each survey and a separate data file is created with lines comprising the NG Easting, Northing and height relative to Ordnance Datum of each point obtained by GPS and transformed from WGS84. It is worth noting that each data file contains between 3000 and 5500 points per survey. Occasionally, a kinematic chain does not compute and has to be re-observed as soon as possible at the next low tide. The reason why this occurs has not always been established but it is usually caused by a deterioration in satellite coverage and geometry whilst surveying. In case of problems with the GPS software not being able to resolve ambiguities, no kinematic chains longer than about one hour are normally observed. 3 Surface modelling The datafilesproduced from the GPS readings are used to create a DTM for each survey. This is done using the SURFER for Windows contouring and 3D surface mapping program. SURFER interpolates, from an irregular spread of XYZ data, Z values on a regularly spaced XY set of coordinates in grid files. Irregular in this sense means that XY (and Z) values have been obtained along random cross-sections and at other points where the beach surface was irregular. There are many methods available in SURFER for generating grid files and Triangulation with Linear Interpolation was chosen. The reasons for this are that the triangulation method creates a patchwork of triangles by drawing lines between data points and it then interpolates heights at grid nodes within these individual triangles. Because the original data points are used to define the triangles in this way, the data representation is good and break lines are also preserved, both of which were necessary as the beach data is gathered by surveying as many changes of slope as possible. When creating gridfilesfrom GPS datafileswith SURFER, a 1 m grid density has been chosen in order to best represent the beach surface for producing cross-sections and it took the computer less than a minute to create a grid file with this density. This was another reason for choosing triangulation as with Kriging, another of the

7 Computer Methods and Experimental Measurements 557 recommended gridding methods, the estimated time for generating a grid file would have been several hours with the size of datafilein use. 4 Cross-sections For the project, the NG coordinates of 19 cross-sections to be plotted along Lancing Beach were specified by the Environment Agency. To produce the required cross-sectional data for these with SURFER, a blanking file is first created for each section specifying the start and finish coordinates, and the slice command generates the cross-sections which are normally stored in separate data files. Following this, the cross-sections are plotted from the data files using Microsoft EXCEL and some sample plots are shown in Figure 6. 5 Discussion The survey method used for the monitoring of Lancing Beach has been successful and cross-sections have been produced. During the initial stages of setting up the measurement method a number of minor operating difficulties were experienced but were soon resolved and after this each survey has progressed with few problems except when the GPS satellite coverage tends to be poor at the same time as low tide. Regarding accuracy, it is estimated that after being processed by SURFER, the data presented on the cross-sections is probably known to about ± 100 mm, the limit specified in the contract. This accuracy depends on many factors, including the accuracy of the original GPS data, the number of survey points taken, the grid spacing in the DTM, and so on. The original GPS results will have a better accuracy than this, and are estimated to be known to about ± mm. All of these estimates refer to the accuracy of horizontal points on the NG and the accuracy of levels relative to the Ordnance Datum. It is emphasised that the measurement method at Lancing was proposed and implemented bearing in mind the tolerance specified of ± 100 mm. With a different accuracy requirement, the method could be modified to accommodate this. In conclusion, it is suggested that the GPS/DTM approach to beach monitoring is a valuable tool available to the Environment Agency and others. It is capable of producing a large amount of three-dimensional data in the same time it would take to survey a limited number of cross-sections along a beach using conventional methods and is easier to use in adverse weather as it does not involve the use of optical instruments. Furthermore, the operator(s) are always moving. In these respects for beach surveys, GPS surveying is certainly more 'user-friendly' as far as the surveyors on site are concerned! Further work is being undertaken by the Department of Civil Engineering, University of Brighton, to find the optimum way in which to carry out beach monitoring surveys with GPS and it is hoped that these results will be published at some time in the near future.

8 Computer Methods and Experimental Measurements Lancing Beach Section O s 1 o Chainage (m) Figure 6a: Change in section 11 during Winter 1996 Lancing Beach Section E G> -* ^^V\ V \ \ \ \^N,^v- ' Chainage (m) ' 12-Mar Apr-96 Figure 6b: Change in section 11 during Spring 1996

9 Computer Methods and Experimental Measurements A 5 T* Q ^A o < 3 E. ZT o :; o _1-9 _. ^^^ ^^^X\ \, \\V Lancing Beach Section 11 V \ v v ^v. ~* iii Chainage (m) I 1 i-jun Jul-96 Figure 6c: Change in section 11 after beach regrading in June 1996 Lancing Beach Section 03a I 2!' 0-1 >^^ \\ "\i V, \ ^^,^^=5^r^z -2 () ( )0 Chainage (m) 19-Sep Oct-96 Figure 6d: Response of beach to storm on 28 and 29 October 1996

10 560 Computer Methods and Experimental Measurements Acknowledgement The author wishes to thank the Environment Agency for giving permission for this paper to be published. Reference 1. Shoreham and Lancing Sea Defences, Environment Agency, Sussex Area Office, Worthing, BN11 1TF, UK, 1996.