GIS for Coastal Zone Management - Chapter 10

CHAPTER TEN Decision-Making in the Coastal Zone Using Hydrodynamic Modelling with a GIS Interface Jacques Populus, Lionel Loubersac, Jean-François Le Roux, Frank Dumas, Valerie Cummins, and Gerry Sutton 10.1 INTRODUCTION AND CONTEXT There are many kinds of coastal water quality issues. They arise from various sources of inputs to the coastal zone that are either chronic or accidental. Pollution of seawater mostly results from a) point source and diffuse pollution originated from agricultural, industrial and urban activities, or b) pollution from maritime activities, e.g. waste oil as well as all types of toxic substances being dumped into the sea, including radioactive ones (Kershaw, 1997). Toxic phytoplankton blooms are a new plague. Although they are not the direct result of human behaviour, they are probably linked to human activities, and particularly contaminated ballast water. These harmful disruptions have severe effects on shellfish stocks, entailing long production shutdowns. Problems also currently exist with open sewers discharging into places designated for shellfish production or into recreational waters (Boelens et al., 1999). After flooding, there is run-off pollution from agriculture and urban areas and nitrate concentrations and bacterial counts increase remarkably. The increase in nitrate concentrations can lead to dramatic eutrophication, whereas pathogenic bacteria can make aquaculture products hazardous to human health (Lees, 2000). Other activities or phenomena such as dredging, deep water sludge disposal or landfill seepage are concerns for water quality and marine living resources (Sullivan, 2001). While hydrodynamic modelling results and the handling of geo-referenced information are becoming more readily available to coastal management stakeholders within GIS (Geographic Information Systems), there is still a lack of direct interfacing of model results with baseline mapping data (BASIC, 2001). This paper discusses solutions to bridge this gap and illustrates them with two case © 2005 by CRC Press LLC studies where effective model outputs are being used for improved environmental management and decision-making. 10.2 HYDRODYNAMIC MODELLING BASICS Mathematical models (Garreau, 1997) can solve geophysical fluid mechanics equations using a number of simplifying assumptions. In essence, knowledge of the bathymetry, wind tide and currents are used to predict water levels and concentrations of conservative elements over space and time. The ability of these models to accurately and reliably reproduce the important features of complex systems has improved considerably along with rapid growth in understanding of the underlying physical phenomena, and the ability to quantify them in terms of valid mathematical formulations. The widespread and affordable availability of high-powered computing facilities has also contributed to this development, given the heavy processing loads involved in running all but the simplest models. Confidence in the predictive capacity of models is governed by the quality of fit between modelled output data and real field measurements. Standard practice in building a typical coastal model entails a number of discrete steps, the first of which involves reconstructing the morphology of the area of interest in the form of a digital terrain model. The next step involves adding water to the system, and then setting it in motion through replication of vertical tidal fluctuations and horizontal wind friction. Together these forcing factors create gradients, which in turn drive horizontal current flows. A range of assumptions is usually made in order to simplify the system, and allow it to run effectively. It is recognised that models do not strive to exactly reproduce in detail all the features of a natural system; however the aim is to arrive at a situation where the model is capable of reliably reproducing the principal features through an iterative process of validation and calibration. 10.3 GIS FOR COASTAL ZONE MANAGEMENT A Geographic Information System (GIS) is a computer-based information system used to digitally represent and analyse geographic features. It is used to input, store, manipulate, analyse and output spatially referenced data (Burrough and McDonnell, 1998). A GIS can be distinguished from database management systems or from visualisation packages through its specialised capability for spatial analysis. The use of GIS for coastal zone management has expanded rapidly during the past decade and references are numerous (Durand, 1994; Populus, 2000; Wright and Bartlett, 2000). For optimum efficiency, geo-referenced data should be properly stored in geo-databases built on spatial data model design (Prélaz Droux, 1995). Some of the greatest challenges currently faced by those handling coastal zone data are a) the land-sea interface, with different mapping references in both horizontal and vertical modes, b) water dynamics and the related temporal issues, and c) 3D display requirements. © 2005 by CRC Press LLC 10.4 TOOLS AND DATA 10.4.1 Technical details of regional and local models MARS-2D is a bi-dimensional model using a finite difference method called ADI (Salomon, 1995). A broad regional model, extending between 40˚N and 65˚ N and from 20˚ W to 15˚ E with a 5 km grid, is used as a framework in which to embed further models of smaller extent for areas of interest along French Atlantic and English Channel coasts. Commonly, the embedding system has up to 5 levels allowing phenomena to be examined at resolutions from 1km down to 50m. This type of model is suitable for applications in areas whose waters are typically well mixed (e.g., coastal or mega-tidal areas). The model is designed to solve for tidal and wind driven currents and the transport of dissolved materials. MARS-3D (Lazure, 1998) is a fully finite difference model, in both vertical and horizontal orientations, which uses a time splitting method based on MARS-2D for the barotropic mode. Good coupling between 2D and 3D modes has largely been achieved through the use of iterative methods. MARS-3D is used at resolutions ranging from 5 km over regional areas of epicontinental seas, down to 100 metres over detailed areas, narrow bays and estuaries. It is currently run operationally at the regional scale (with a 5 km mesh) on all French seaboards. Table 10.1 gives an overview of MARS model features. Table 10.1 Comparison of MARS-2D and MARS-3D main features Model Area Grid and time step Period of time Applications Type MARS-2D English Channel, Bay of Biscay From 50m up to 10km From days to decade Tide currents, dissolved matter, salinity under homogenous conditions 2D Finite difference MARS-3D English Channel, Bay of Biscay ~ 5km, 5 – 20 minutes Year to decade Tide, currents, temperature, salinity, transport of dissolved matter 3D Finite difference 10.4.2 The modelling system A modelling chain has been developed at Ifremer over a period of many years. The system has a series of pre-processors and tools for the graphic display of results produced by the MARS computation kernel. © 2005 by CRC Press LLC This mathematical model uses some simplified hypotheses to solve the equations that govern how marine currents and sea levels evolve. In order to function, the process requires an input water level along the edge of the area of interest. These boundaries are usually unknown locally, since they are dependent on tidal and weather conditions, which are themselves usually derived from modelling over a much larger area. Thus, the modelling process advances through the generation of a series of sequentially nested models. An initial general model covering a large area of the continental shelf and the Channel is followed by a succession of intermediate models of increasingly smaller scope, but higher resolution. Boundary conditions for the wide-area model are resolved using world tide models, into which modelled meteorological forcing factors have been assimilated. The modelling chain can be summed up as follows: x A generated link calculates the position, extent and resolution of each sub-model, from the large-area model to the detailed high resolution model based on computational and hydrodynamic design criteria. Computational efficiency is optimised by maintaining a maximum resolution ratio (mesh size ratio) of between four and five between any two consecutively nested models. Hydrodynamic criteria are observed as far as possible in the design through avoidance of islands or zones with violent currents, although at present the system only works when model boundaries are strictly aligned to parallels and meridians. This link of the chain has a user-friendly graphic interface and generates a descriptive file of the entire nesting process. x The second link in this computation chain is a software program which calculates an interpolated bathymetry for each nested model. The link also has a graphic interface developed in UNIRAS, which restores a depth for each calculation link in a file. The bathymetry used for the large-area model has been validated, and is essentially taken as fixed. However, it is updated on an occasional basis, as new information becomes available. The MARS-2D computation kernel is used in both case studies below, where prevailing tidal currents have a relatively homogenous vertical structure, providing a good approximation of the mean current fields pertaining in the study areas. x Lastly, a range of graphic tools are used to display the resulting modelled outputs which are written in NETCDF format (Rew and Davis, 1990). NETCDF is a widely used self-documenting format, which also provided a suitable platform upon which the ArcView portal was subsequently developed. 10.4.3 Reference mapping data Currently, coastal practitioners must refer to common baseline or reference data, i.e., primary data to which secondary (or more application-related) data will be subsequently linked (Allain, 2000). The coastline, bathymetric data, and major administrative boundaries are examples of such baseline data that could be readily © 2005 by CRC Press LLC provided to a wider public under optimal conditions of accuracy, updating, and scalability to suit various needs. However, it is noted that the bathymetry of inter-tidal areas, and other near-shore zones, is often less easily available or poorly defined. Such paucity of data is usually associated with the high cost of acquisition and restricted accessibility. Bathymetric data for the two study sites under consideration was available from the French hydrographic service at a scale of 1:50, 000. ArcView™ was the main GIS platform used for the studies, within which the Spatial Analyst™ extension facilitated a number of operations on raster images such as recoding, resampling, changing extent, computing statistics and the use of algebraic image combination functions. Existing ArcView functionality also facilitated interactions between raster and vector data layers drawing on the attributes of file features attaching to vector data sets. However a major hurdle remains in the efficient handling of large numbers of raster data layers that typically accrue as the output from multiple model runs. 10.4.4 An Integrated GIS/model interface The primary rationale behind the creation of a GIS/model interface was to allow a) consultation and display of results contained in the output files produced by the MARS hydrodynamic model; and b) extraction of these results to import them to ArcView. The concept was initially tested through the development of ModelView (Loubersac et al., 2000), a prototype interface the functionality of which was successfully demonstrated in the case of hydrobiological contamination in the Bay of Marennes-Oléron, France. As a further development, in order to broaden the scope of application a platform-independent stand-alone interface, MODELCON, was designed for use in conjunction with a range of GIS packages. In order to ensure optimum compatibility with standard software packages and other GIS (e.g. Excel, MapInfo™ etc.), MODELCONV extracts NETCDF files to a standard ASCII format. The MODELCONV interface was developed in JAVA, with Microsoft’s Visual Studio 6 environment and the JAVA library to access NETCDF files (NETCDF JAVA version 2), ensuring maximum portability in anticipation of future use on Unix systems or via the Web. 10.4.5 A Geographic conversion module An additional processing module was developed in order to address specific geoprocessing requirements beyond those available in the standard version of ArcView. This module operates as an ArcView extension and was implemented in Avenue. It enables the user to perform a range of geodesic processing operations on point, multipoint, polyline and polygon data, as well as 2-D and 3-D related measurements (pointM, multipointM, polylineM, polygonM, pointZ, MultipointZ, PolylineZ, PolygonZ). © 2005 by CRC Press LLC ... - tailieumienphi.vn