Flood hazards along the Balcones escarpment in central Texas; alternative approaches to their recognition, mapping, and management
The public tends to dismiss floods as somewhat unreal catastrophes or occasional inconveniences that usually affect others. When a flood disaster strikes at the local level, the magnitude of the event is appraised in terms of the damage to human works on the river-valley floor or, in some cases, in terms of the loss of life. Resources are mobilized to combat the disaster, and discussions ensue concerning flood control plans and projects. The affected communities may then learn that they have experienced a "100-year flood," or a flood discharge that has a "1 percent probability of being equalled or exceeded in a given year." They may further learn that the unpleasantness of this event can be prevented and controlled by various combinations of levees, dikes, dams, reservoirs, and channels. Rarely does the public hear that floods are a natural part of a river's activity, really an essential part of a river's long-term task of conveying water and sediment down gradient from an evolving landscape to a base level, such as the Gulf of Mexico. Flooding is a great natural hazard because people occupy river-valley bottoms, flood plains, and other flood-prone areas. The term "flood" is variously defined depending on the concerns of its user. To all, it is an overflow of a stream channel that exceeds certain limits. To the flood-plain manager, these limits are those at which life and property are damaged or threatened. To the hydrologist, the limits are arbitrarily defined on the basis of magnitude-frequency studies of stream flow. The geomorphologist and the geologist view floods relative to the natural features associated with the stream or river. Clearly, the study of floods and the mapping of their potential occurrence require an interdisciplinary approach. The accelerating demand for flood-plain information makes desirable an evaluation of alternative techniques to standard engineering flood line and regional flood analyses (Wolman, 1971). Different mapping techniques may be appropriate to different localities depending on the local hydrologic regime, the level at which planning is being performed, and the funds available to finance the study. A geologic approach to flood hazard mapping can be used effectively at the state or regional scale to provide interim flood hazard information prior to detailed hydrologic and hydraulic studies on a local basis. If included within an overall program of regional environmental geological mapping, morphological flood-plain mapping can provide a relatively inexpensive byproduct of a general program of environmental inventory. It is a well-known fact that, despite immense public expenditures for flood protection, flood losses remain substantial, potentially costing an average of $2 billion (1966 dollars) per year nationally (U.S. Water Resources Council, 1968). Assuming established trends in the increased use and development of hazardous flood plains, this figure will increase to $5 billion by 2020. In 1966 the estimated annual flood damage for Texas rivers draining more than 250,000 acres, exclusive of the Rio Grande, was $28.2 million (U.S. Water Resources Council, 1968). Total damage to smaller basins was estimated at $55.9 million. Despite the current total investment of over $400 million in flood control works, the total damage figures are projected to rise to $59.3 and $125.3 million respectively by the year 2000. An increasing percentage of the annual national flood loss is the result of so-called catastrophic floods (Holmes, 1961), i.e., floods which either (1) have a return period of 100 years or more, or (2) cause failure of a flood protection project by exceeding the project design flood. The average amount of flood loss from floods of moderate frequency is decreasing relative to these catastrophic events. The estimated $3 billion damage produced by Hurricane Agnes flooding in the eastern United States during the summer of 1972 may represent the pattern for most future flood losses. Approximately 40 percent of the damage from Agnes flooding occurred in areas which had received federally funded flood protection benefits. The message for Texas, where flooding occurs because of what is perhaps the most catastrophic rainfall regime in the conterminous United States, is that flood-plain managers must consider alternative approaches to reservoirs, levees, floodwalls, and channels. Flood-plain management requires that an all-out effort be made to (1) increase basic knowledge of floods and flood hazards, (2) define and outline major flood areas, and (3) improve methods of flood-frequency analysis (U.S. Congress, 1966, p. 18-19). White (1964) has shown that from the theoretically broad range of choice for the flood-plain manager, only a few choices are generally considered in decision-making. This results in far less efficiency than could be achieved by considering the whole range of possible choices. Two main factors seem to limit choice: the flood-plain manager's perception of the nature and magnitude of the flood problem, and his perception of alternative responses. This report is a preliminary attempt to describe the flood problems of Central Texas, to suggest alternative approaches to their evaluation, and to relate these scientific goals to the managerial goals of the State and of local communities.