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rapidly developing melropolilan area which includes Salt Lake City and several suburban communities in Salt Lake County, Utah. Because of rapid urban growth and the consequent effects on runoff and potential damage, flood control is of increasing concern 10 city and county officials. The Salt Lake Valley is in the Great Basin. It is a nUn shaped valley bordered on the easl and wesl by mountains and by lower spurs of these mountain ranges to I.he soulh. The valley, which is about 15 miles wide (east and west) and 25 miles long, is bi- sected by Ihe Jordan River which flows northward and discharges into Ihe Great Salt Lake. The average elevation of the valley floor is approximalely 4,000 feet above mean sea level. The valley is semiarid with an annual average of 15 inches of rainfall. The Wasatch Mountains, with peaks up to 11,000 feet above sea level, rise abruptly on the east side of the valley. Because of this sharp rise, much of the pre. cipitalion is produced by the orographic lifting of air masses moving In an easterly direction and falls in the mountains. The Wasalch Mounlain Range Ihus pro- vides a large portion of the waler supply for the valley beiow. Several small streams run weslward from moun. lain canyons inlo the valley and discharge Inlo the Jordan River. The site selected for this sludy is limiled to the part of the eastern side of the valley shown In Figure 1.1. This specific area was chosen because of the his- tory of flood conlrol proposals that would affect the inhabitants. This area is bordered on the west by the Jordan River, on the easl by the Wasatch Mountains, on the north by the heavily urbanized Parley's Creek drainage, and on the south by the less urbanized but developing lillie Collonwood Creek watershed. Al- together, the area contains about one-half of the east- ern section of the Salt Lake Valley. The population within the 1970 census tracts of the study area, was 131,882. It is of varying den- sity and growing rapidly. From the 1970 figure of 383,035 people, the population of Salt Lake Counly is expected to grow 10 about 785,000 people by 1985. The area's proximity to the cen tral business dislricl of Salt Lake City and present rapid developmenl sug- gest that a large partof lhis expected growth will oc- cur In the sludy area as Indicated by Ihe masler plan for the county (A Master Plan for Salt Lake County, 1965). The area has a long history of flooding (Corps of Engineers, 1969A: 11-19), and continuing urban development is Increasing the urgency of the flood problem. Some of the presenl development is occur. ing in the flood plains and moun lain canyons and "new residential developments are rapidly expand' ing. . ." (Corps of Engineers, 1969 A: 5). This urban growth nol only allers run-off relationships by pro- ducing higher peak flows, but also increases Ihe dam. age potential from a flood. The Process of Model Development Computer Simulation Models In a computer simulation model, one uses an underslanding of the fundamenlal processes and the coupling relationships among them to predict the con- sequences of possible changes. For example, one could predict changes which result from increasing urbanization and the consequenl flood damage and demand for flood control. Mathematical simulation is achieved by using arithmetic relationships to repre- sent Ihe various processes within the prototype sys- lem and by linking these equations inlo a model that represents the functioning of the system as a whole. Thus, compuler simulation uses a model developed to behave like a prolotype system 10 predicl probable responses to situations for which protolype response has not been recorded or responses one mighl expect from the prololype to be altered. Steps in Developing a Working Model A computer model simplifies the real world in representing it and thus becomes an abstraction from reality. The degree of simplification is a function of bolh modeling inlent and knowledge aboullhe real world. Verbal information and conceptualization are translated into mathematical form for use in a com. puter (Forrester, 1961). Model development proceeds from verbal symbols which result from theoretical and empirical studies to the mathematical symbols which compose the model. The development of a working malhematical model thus requires two major sleps. The firsl is the creation of a conceptual model of the various elements of the system and Ihe interrelalionships among them. In practice, the hypotheses necessary for Ihe real world for a parlicular study area are formulated from the most pertinent and accurale dala available. As addi- tional informalion is obtained, the conceptual model is improved and revised to more closely approximale reality. The second step is the operationallzation of the conceptual model inlo a working compuler. During this slep, an allempt is made 10 express the various processes and relationships identified or hypothesized in Ihe conceptual model in mathematical form. This step usually requires further simplification and thus means thallhe resulting model is further removed from reality. The loss of information, first, between the real world and the conceptual model and, second, between 3