Proceedings of CSAW'04 - FTP Directory Listing - University of Malta

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16 Andrew Calleja volume of water to do so, until it encounters a drop in elevation (like the edge of a table) after which it all drains out from the boundary leaving only a thin film due to water tension properties. This is how Simulo Tempestas models the flow over a flat terrain. Whenever water is introduced to a small part in a flat piece of land, the water spreads throughout uniformly contributing to a rise in the water level throughout the entire area. Then, at the boundary of the flat land two different scenarios are possible: 1. The flat land might be part of a pit and thus all boundary pixels have a higher lying neighbour (discussed under the Catchments right after), 2. or else at least one boundary cell has a lower lying neighbour. Since these group of cells are now treated as one object there is no need to make any alterations to the individual cells. During the simulation, when modelling precipitation, the entire flat land is treated as one piece therefore any water introduced is contributing to a global rise and any water flowing out is contributing to a global lowering of the level. Catchments When surface run-off is generated and made to flow down slopes it might end up in some areas where there is no escape. Water flowing into parts on the terrain that are contained start to fill up and thus change completely the dynamics of the flow. Catchments, as defined in the context of Simulo Tempestas, are areas on the terrain that take water in to form ’pools’. This filling process happens because of the existence of pixels that are lower than the rest. The discretised terrain provides height values to the nearest metre, therefore when catchments start filling up, changes occur only at intervals of one metre. At these intervals, with incoming water flow increasing the height level, the surface area of the catchment grows bigger as it consumes new cells on the catchment brim. These cells are not regarded anymore as a separate pump with the previous slope gradients to lower pixels, but as cells that share the same water level with all the neighbouring cells that are part of this new catchment surface. At one point, when the water level reaches the ’brim’ of the catchment and makes contact with the rim of other neighbouring catchments, the water will start to overflow accordingly. Once these neighbouring catchments fill up to the brim too they merge into one bigger catchment forming a bigger surface area and once again the dynamics of the flow changes as they now share common boundaries. Mimicking such interconnectivity between catchments is possible through the use of a tree structure. These catchments are interconnected in such a way that help us keep track of the water in these pits. The purpose of such a tree is to store a hierarchy of these catchments as they fill up with water. The idea behind this approach came from a paper by Lee Vincent and Pierre Soille [1]. The authors of this paper discuss simulating a flooding process as if water is coming out of the ground, moving upward. They do this to delineate the watershed areas on the terrain without any pre-processing of the pits on the ground. To establish these catchment areas they process the pixels one altitude at a time starting from lowest going up to flooding the entire terrain. 3.2 Climate Just as nature has its clouds and wind, our system too has its virtual clouds and wind. These clouds are stored in a climate layer in the layering system mentioned before. The precipitation process is responsible for feeding the hydrological model with rain. The rain data is fed into each individual cell on the ground by the simulation engine. Due to the nature of our DEM data i.e. the fact the cells have a large surface area of 50m by 50m, an assumption must be made that water is dropped
Simulo Tempestas: A Simulation Tool for Hydrological Modelling 17 Fig. 5. Simplified catchment tree at work
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96 Sandro Spina and John Abela 2.1
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114 Nikolai Sultana A variety of to
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116 Nikolai Sultana This visualisat
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120 Christine Vella 1. SharpHDL, wh
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130 Wallace Wadge Grid technologies
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Author Index Abela, Charlie, 86 Abe
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