Environmental geology of urban and urbanizing areas : a case study from the San Marcos area, Texas

Grimshaw, Thomas Walter, 1945-
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A major focus of environmental geologic work for the foreseeable future will be on the growing urban areas of the world. A systematic methodology for investigating the environmental geology of such areas can be developed by dividing the geologic environment into three major components (substrate, processes, and landform) and the urban system into four major components (situs, input, output, and transportation). These conceptual tools can then be used to develop a three-step procedure: 1) generation of the data source maps; 2) investigation of environmental geologic conflicts of existing urban systems; and 3) incorporation of geologic considerations in planning for future growth. The data sources should be prepared in map form and should comprise two types of data, natural and cultural data. A conventional geologic map is the primary document used in the derivation of the five natural data source maps: 1) engineering geology; 2) soils; 3) resources; 4) processes; and 5) landform. The necessary cultural data source maps are a current land use and a land use control map. In the investigation of environmental geologic problems of existing urbanization the urban system concept provides an excellent means of organizing the various facilities and activities of a city. If this scheme is used, the facilities can be systematically considered and their associated conflicts disclosed. Environmental conflicts of future urbanization can be prevented by evaluating the ability of land to sustain the demands that will be imposed by projected urban facilities. The urbanization can then be fitted to the land by siting the facilities where the land can best meet their demands. The procedure for determining land suitability for an urban facility consists of three steps: 1) screening; 2) evaluation; and 3) verification. Areas that are totally unsuited for the facility are eliminated in the screening step. The remaining candidate areas are rated for their relative suitability in the evaluation step, which consists of an eight-step algorithm developed by adaptation of a formal decision-making technique. The results are confirmed in the verification step, and the final product, the suitability index map, should serve as the primary basis for determining where the facility will be located. The San Marcos case study area comprises two 7-1/2 minute quadrangles in south-central Texas between Austin and San Antonio. The area is almost ideal for testing a methodology for environmental geologic investigation of growing urban areas. The methodology developed in this study was shown to be highly effective when applied to this area. In its present state of development the methodology appears to be a conceptually sound and effective approach to optimizing the interaction between cities and their geologic environments. Future work should concentrate on adaptation of key sections for automation and on the application of the three-step procedure to a variety of different kinds of cities and geologic environments in order to refine and test it.