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<br />Conversion to Metric Units <br /> <br />For those readers interested in using the metric system, metric <br />equivalents of English units of measurements are given in parentheses. <br />The English units used in this report may be converted to metric units <br />by the following conversion factors: <br /> <br /> From Multiply by <br />Square miles (mi2) 2.590 <br />Miles (mi) 1.609 <br />Feet (ft) .3048 <br />Square feet (ft2) .0929 <br />Inches (in) 2.54 <br />Cubic feet per second (ft3/s) .02832 <br /> <br />To obtain <br /> <br />Square kilometers <br /> <br />Kilometers <br /> <br />Meters <br /> <br />Square meters <br /> <br />Centimeters <br /> <br />Cubic meters per second <br /> <br />Physical Setting <br /> <br />The Dallas metropolitan area is in north Texas about 250 miles <br />(400 kilometers) from the Gulf of Mexico (fig. 1). The city has grown <br />rapidly in recent years, and presently (1973) very little land remains <br />undeveloped. <br /> <br />The altitude ranges from about 500 to about 700 feet (150 to 210 <br />meters) above mean sea level. In the upland areas, outcrops of lime- <br />stone, chalk, and marl are surrounded by a thin mantle of soil. The <br />soil mantle becomes much thicker and more extensive in the lowland areas. <br />The soils are mostly clays that are dark colored and extremely sticky <br />when wet. As with most clays, they show marked changes in volume with <br />changes in the moisture content. During dry periods, numerous cracks <br />develop in the clay and it becomes highly permeable; but almost immedi- <br />ately after wetting, the clay expands and the cracks rapidly close, <br />thereby reducing the permeability. In residential areas and public <br />parks, extensive watering throughout most of the year generally reduces <br />the cracking. <br /> <br />-3- <br />